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September 6, 2024

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Researchers advance new class of quantum critical metal that could advance electronic devices

by Marcy de Luna, Rice University

A new study led by Rice University's Qimiao Si has unveiled a new class of quantum critical metal, shedding light on the intricate interactions of electrons within quantum materials. Published in Physical Review Letters on Sept. 6, the research explores the effects of Kondo coupling and chiral spin liquids within specific lattice structures.

"The insights gained from this discovery could lead to the development of electronic devices with extreme sensitivity, driven by the unique properties of quantum-critical systems," said Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy and director of Rice's Extreme Quantum Materials Alliance.

Quantum phase transitions

At the core of this research is the concept of quantum phase transitions. Just as water changes between solid, liquid and gas states, electrons in quantum materials can shift between different phases as their environment changes. But unlike water, these electrons follow the rules of quantum mechanics, leading to much more complex behaviors.

Quantum mechanics introduce two key effects: quantum fluctuations and electronic topology. Even at absolute zero where thermal fluctuations disappear, quantum fluctuations can still cause changes in the organization of electrons, leading to quantum phase transitions. These transitions often result in extreme physical properties known as quantum criticality.

Moreover, quantum mechanics give electrons a unique property tied to topology, a mathematical concept that when applied to electronic states can produce unusual and potentially useful behaviors.

The study was carried out by Si's group in a long-term collaboration with Silke Paschen, study co-author and a professor of physics at the Vienna University of Technology, and her research team. Together they developed a theoretical model to explore these quantum effects .

The theoretical model

The researchers considered two types of electrons : some moving slowly, like cars stuck in traffic, and others moving quickly in a fast lane. Although the slow-moving electrons appear stationary, their spins can point in any direction.

"Ordinarily, these spins would form an orderly pattern, but the lattice they inhabit in our model doesn't allow for such neatness, leading to geometrical frustration," Si said.

Instead, the spins form a more fluid arrangement known as a quantum spin liquid, which is chiral and picks a direction in time. When this spin liquid couples with the fast-moving electrons, it has a topological effect.

The research team discovered that this coupling also triggers a transition into a Kondo phase, where the spins of the slow electrons lock onto the fast ones. The study reveals the complex interplay between electronic topology and quantum phase transitions .

Usual electrical transport

As electrons move through these transitions, their behavior changes dramatically, particularly in how they conduct electricity.

One of the most significant findings is about the Hall effect, which describes how an electrical current bends under the influence of an external magnetic field, Paschen said.

"The Hall effect contains a component that is enabled by the electronic topology," she said. "We show that this effect experiences a sudden jump across the quantum critical point."

Implications for future technology

This discovery advances our understanding of quantum materials and opens up new possibilities for future technology. An important part of the research team's finding is that the Hall effect responds drastically to the quantum phase transition, Si said.

"Thanks to the topology, this response happens in a minute magnetic field," he said.

The unusual properties could lead to the development of new types of electronic devices such as sensors with extreme sensitivity that could revolutionize fields like medical diagnostics or environmental monitoring.

Co-authors of the study include Wenxin Ding of Anhui University in China, a former postdoctoral fellow in Si's group at Rice, and Rice alumna Sarah Grefe '17 of California State University.

Journal information: Physical Review Letters , arXiv

Provided by Rice University

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New Semiconductor Technologies and Applications

Currently, Beyond CMOS technology is used in a few niche applications. But most Beyond CMOS technology exists more in the realm of possibility than reality. It will likely be a while before this technology is integrated in devices that are available to the general public. What is important, however, is that researchers are working toward new and exciting capabilities to meet market demands.

Change is hardly something new within the semiconductor industry. In line with Gordon Moore’s 1965 observation (known as Moore’s law), the number of transistors on an integrated circuit has doubled roughly every two years for more than fifty years. This constant technological development has facilitated the creation of a seemingly endless stream of new devices that have become central to daily life in the twenty-first century. While it may be easy to take change in the field of technology for granted, recent industry transformations tied to new semiconductor technologies and applications deserve recognition.

The semiconductor industry appears to be at or near an inflection point within its history. It is facing two discontinuities simultaneously. The industry is having to face the physical limits of existing semiconductor materials and the potential end of Moore’s law. At the same time, new applications are driving demand for even smaller, lighter, and more powerful semiconductors. While these changes could certainly threaten the industry’s steady rate of performance, the semiconductor industry is adapting to meet new challenges and opportunities.

New semiconductor applications are now driving technological development and spurring problem-solving and innovation. Amid new demand, semiconductor companies are responding with a combination of More Moore, More than Moore, and Beyond CMOS. In what follows, we explore these developments in semiconductor technologies as well as new applications that are driving transformation within the industry.

Trends in the application of semiconductor technology

Semiconductors are a pervasive though often unrecognized feature of everyday life in the twenty-first century. They are an important part of consumer electronics such as smartphones and televisions. But they also exist in a range of other items including washing machines and refrigerators as well as automobiles and airplanes. Because of this, increasing demands for everything from safer travel to more efficient housekeeping and more engaging entertainment are impacting the semiconductor industry.

Where does new semiconductor technology fit in the new digital ecosystem?

Technological advancements and the integration of technology in the new digital ecosystem have had a major impact on the semiconductor industry. Demand for the integration of maximum functionalities on systems on a chip (SOCs) has grown, increasing the number of semiconductor intellectual property (IP) cores.

Furthermore, demands for high-performance memory systems have directly impacted semiconductor memory IP. The market demand for faster and more efficient memory solutions is driving the development of increasingly complex semiconductors that can facilitate intense memory operations. Overall, dependence on IP solution providers is growing, which drives huge investments in the market.

Historically, strong investment—in combination with global cooperation—has enabled the development of semiconductors in line with Moore’s law and has facilitated steady industry advancement. Continued investment is likely to continue driving growth within the industry. For the next five years, from 2020 to 2025, the compound annual growth rate of the semiconductor memory IP market is expected to exceed 12 percent .

How does new semiconductor technology differ from earlier technologies?

Transistors and integrated circuits have come a long way to get to where they are today. They have played a major role in the development of the computer industry. But even this industry has an older history than most realize.

English mathematician and inventor Charles Babbage created the first machine (his “Analytical Engine”) that featured many of the elements of modern computers in 1837. The first fully electronic computer (the electronic numerical integrator and computer, or ENIAC), powered by vacuum tubes, was introduced in 1946.

In the following year, 1947, a group of scientists at Bell Laboratories created the first transistor. Transistors were much smaller, much lighter, more durable, and more efficient than vacuum tubes. Their invention, followed by the development of the integrated circuit in the late 1950s, paved the way for the revolution in personal computers and the rise of Silicon Valley.

While the personal computer industry was still small in the 1970s and 1980s, large computers—even with sophisticated cooling techniques—started pushing past the power limits of early transistor technology. This paved the way for the integration of complementary metal-oxide semiconductor (CMOS) technology. CMOS uses complementary and symmetrical pairs of two different types of metal-oxide-semiconductor field-effect transistors (MOSFET)—p-type and n-type. In comparison to earlier semiconductor technologies, CMOS technology came with the benefits of low power consumption and limited waste heat.

CMOS technology facilitated the development of new logic and memory products in the personal and commercial branches of the computer industry by the mid-1990s. Within a decade, however, power limits became an issue once again. This spurred the industry to adopt multicore processing to increase computational performance.

Innovation has continued to drive new semiconductor technology in the present. In order to achieve performance comparable with previous decades, the industry has started to depart from past architectures and devise new solutions to meet present and future problems.

Where is the future of the semiconductor industry headed?

In an effort to maintain its steady rate of performance, the semiconductor industry has embraced two key types of techniques in an effort to overcome existing semiconductor limits: More Moore (PDF, 2 MB) and More than Moore .

More Moore techniques attempt to further develop CMOS technologies to keep pace with scaling in accordance with Moore’s law. Demands for continued scaling and performance improvements have become especially important amid demand for new applications. High-performance computing, mobile computing, and autonomous sensing and computing are driving More Moore technologies. These technologies target more performance at constant power and cost.

In contrast to More Moore, More than Moore techniques, which are focused on improving the use of existing semiconductors, do not necessarily scale with Moore’s law. But new demands and applications are likewise driving these solutions. The integration of new nondigital functions into personal electronics systems, for example, are spurring More than Moore solutions. We can see examples of these solutions in miniature camera modules, motion sensing, biometric identification, and health monitoring systems. Future developments in nano- and biotechnology are also likely to continue spurring More than Moore technologies and products.

With these developments on the horizon, the future of the semiconductor industry is headed Beyond CMOS . Beyond CMOS refers to digital logic technologies that surpass current CMOS scaling limits. More Moore techniques have already extended current CMOS scaling limits past an order of magnitude in feature size and two orders of magnitude in speed. But Beyond CMOS technology is now progressing even further to achieve More than Moore.

A key goal of Beyond CMOS research and development is to replace commonly used static random-access memory and flash memory technologies with both new volatile and nonvolatile memory technologies. Researchers hope to meet the demand for electronically accessible memories that are high speed, high density, low power, and embeddable.

Access the IRDS™ Roadmap

Advancements in semiconductor technologies and applications

The growth of fabless design houses and foundries has completely transformed the business of the semiconductor industry in the last fifteen years. With fabless manufacturing, specialized manufacturers produce semiconductor devices while system integrators maintain control over the business model and design of semiconductor chips.

This specialization allows system integrators to establish system requirements for new products at the start of a design cycle. Semiconductor manufacturers, in turn, meet demands that arise from these system requirements and make their way down the fabrication production chain.

In the past, the creation of a newer, faster integrated circuit triggered the design of new personal computers. But today the relationship between new devices and semiconductor tech is reversed: the design of new smartphones triggers the creation of new semiconductor devices.

These recent transformations are part of a broader shift within the industry. The industry is moving away from being a monoculture and toward more diversity and innovation. This promises to eliminate some of the unfortunate consequences of previous industry development. Namely, this shift reverses the architecture and industry consolidation that has limited industry participation and innovation and caused insecurity within the industry in the past.

What emerging technologies will improve the semiconductor industry?

The rapid development of smartphones along with computers and other electronics has forced a significant downscaling in the physical size of devices. This, in turn, has necessitated constant improvement in the semiconductor industry. As we noted above, semiconductor technology has been advancing for decades in line with Moore’s law. However, the industry also appears to be reaching the limits of miniaturization with existing technologies.

These limits have led to innovation with new materials to complement existing CMOS technology and achieve More Moore. Graphene and related two-dimensional (2-D) materials, for example, have great potential to overcome the limitations of silicon technology. As such, they offer hope for improvements in both device component function and performance in computational and noncomputational applications.

The integration of 2-D materials with silicon technology in computational systems can reduce the challenges of doping, contact resistance, and dielectrics/encapsulation. In the realm of noncomputational applications, these materials can be integrated into future cameras, low-power optical data communications, and gas sensors and biosensors.

Compound semiconductors, which combine two or more chemical elements, are also at the forefront of developing semiconductor technologies. Companies are interested in compound semiconductors made of chemicals such as gallium nitride or gallium arsenide, for example, because of how they operate in comparison to silicon.

Compound semiconductors can operate at higher frequencies and higher temperatures and also emit and detect light more efficiently. As such, they have great potential value for applications involving power electronics, radio-frequency communications such as Wi-Fi, and photonics such as solar cells.

What introduced some recent advancements in semiconductor technologies and applications?

As suggested above, recent advancements in semiconductor technologies have largely been demand driven. As the demand for new capabilities has grown, researchers and academics around the world have needed to turn to new Beyond CMOS technology.

Recent Beyond CMOS advancements have also been made possible, however, due to the alignment of outside technological advancement and innovation as well as specialization within the industry. Research on More Moore and More than Moore techniques has preceded much of the current need for newer, more advanced technology.

Some companies have been key players in driving new semiconductor technology forward. Intel, for example, has created a magnetoelectric spin-orbit logic device . This device has the potential to reduce voltage by five times and energy between ten and thirty times current levels. Research institutes such as UCLA’s California NanoSystem Institute have also played a role in preparing for a future with Beyond CMOS technology through their efforts to create more flexible and scalable electronic devices .

How have semiconductors contributed to advancements in electronics?

Recently, Beyond CMOS technology has sparked major advances in cryogenic electronics. These are electronics necessary for operations at temperatures below four degrees Kelvin. Cryogenic technology can be applied to improve sensors, signal and media processing, and digital and quantum computing.

In the near future, semiconductors also have the potential to play a significant role in advances in electronics in the medical field. New sensors to monitor brain activity, new systems to deliver drugs and monitor exercise activities, and new communication networks to send data between patients and doctors will depend on semiconductors.

Projected developments in semiconductor technologies and applications

Although change has historically been a defining feature of the semiconductor industry, constant change can create challenges. Rapidly evolving technology and corresponding demand can leave industries scrambling.

As we have suggested above, the semiconductor industry has taken these challenges in stride through More Moore and More than Moore solutions. These developments have worked to reinvigorate the semiconductor market. Many more groups are entering the supply side of the market to meet demands for innovation. While large vertically integrated sellers previously dominated the semiconductor market, it is shifting to become much more of a buyer’s market. 

How is the semiconductor technology and applications industry doing right now?

Amid this broad transformation, the semiconductor industry has been forced to contend with a recent period of contraction. The year 2019 was not a favorable one for the industry. This stemmed in part from the US-China trade war, which threatened to disrupt existing semiconductor supply chain networks.

In response to the trade war, David French, the chairman of Silicon Power Technology, noted that China’s consumption of semiconductors is 40 to 50 percent of global supply and asserted: “That number is going to go up whether we sell to them or not.”

While French suggested that China could use its own technology to replace semiconductors purchased from the United States, other industry experts foretold industry struggles. Regardless of any actual outcome of the trade war, uncertainty and risk surrounding the trade war created difficulties for the semiconductor industry.

Nevertheless, the industry is poised for recovery and strong growth in 2020 and beyond. As technology continues to rapidly develop, the semiconductor industry plans to follow suit; total industry sales are projected to reach $66.8 billion USD by 2021.

How can new semiconductor technologies and applications improve further?

Technological innovation will be key to the recovery and growth of the semiconductor industry in the near future. We can see evidence of such innovation in the new chiplet approach to developing powerful processors quickly. This approach is part of the semiconductor industry’s plan to allow systems to continue improving in performance capability through More than Moore solutions to existing scaling limits.

Chiplets are small pieces of silicon that are designed to integrate with each other through package-level integration. Proponents of chiplets suggest that they will facilitate greater system specialization and higher yields. In an industry already transformed by fabless design, small, specialized chiplets designed to be combined with a general-purpose processor and other specialty chiplets might become the eventual end-product goal.

Currently, Intel is designing and connecting chiplets through the Embedded Multidie Interconnect Bridge (EMIB), which functions like a high-density bridge connecting two chiplets together. The company is using EMIB to create best-in-class products with significant improvements in form factor and size. Such improvements will likely prove critical to keeping up with demands for newer, smaller devices.

What’s on the horizon for new semiconductor technologies and applications?

Progress in artificial intelligence (AI) (discussed in more depth below) is creating new demand for semiconductors and thus new industry growth and innovation. However, the future also holds opportunities for new and exciting applications of semiconductor technology within existing industries.

The development of fifth-generation (5G) wireless cellular technology, for example, portends new applications for semiconductors within the communications industry. Rapidly developing demands for video game consoles and televisions with high-quality video streaming will also create more demand for semiconductors.

What emerging industries use semiconductors?

As market demands spur the creation of new semiconductor devices, new technologies and applications promise to have a major impact on the semiconductor industry. AI, for example, promises to drive major expansion of the semiconductor industry due to high demand.

Much of this demand will likely stem from the automotive market, which is expected to grow the fastest in the AI sector through 2022. Autonomous vehicles, or self-driving cars, are equipped with sensors that allow them to “see” the world around them.

Self-driving cars are already on the roads today, and they are likely to play a disruptive role not only in personal transportation but also in many other industries as well. The coming years will witness a huge spike in demand for AI-tailored semiconductors as connectivity, sensing, and instant computing become increasingly central to everyday devices.

Semiconductors already play a prominent role in the technology that people use every day. And as technology continues to expand and transform at a rapid pace, particularly through the Internet of Things, semiconductors promise to follow. Although change is nothing new to this industry, upcoming technological developments offer the promise of something new and exciting for semiconductors going forward.

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• v.24(7); 2021 Jul 23

Paper-based wearable electronics

1 Department of Biomedical, Biological & Chemical Engineering, University of Missouri, Columbia, MO 65211, USA

Margaret Page

2 Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO 65211, USA

Ganggang Zhao

Samuel b. stoll.

Skin-interfaced wearable electronics can find a broad spectrum of applications in healthcare, human-machine interface, robotics, and others. The state-of-the-art wearable electronics usually suffer from costly and complex fabrication procedures and nonbiodegradable polymer substrates. Paper, comprising entangled micro- or nano-scale cellulose fibers, is compatible with scalable fabrication techniques and emerges as a sustainable, inexpensive, disposable, and biocompatible substrate for wearable electronics. Given various attractive properties (e.g., breathability, flexibility, biocompatibility, and biodegradability) and rich tunability of surface chemistry and porous structures, paper offers many exciting opportunities for wearable electronics. In this review, we first introduce the intriguing properties of paper-based wearable electronics and strategies for cellulose modifications to satisfy specific demands. We then overview the applications of paper-based devices in biosensing, energy storage and generation, optoelectronics, soft actuators, and several others. Finally, we discuss some challenges that need to be addressed before practical uses and wide implementation of paper-based wearable electronics.

Graphical abstract

Biodevices; Bioelectronics; Electronic materials

Introduction

Emerging wearable electronics have achieved significant advancements in a wide spectrum of applications, spanning from healthcare and human-machine interface to soft robots and virtual and augmented reality ( Bariya et al., 2018 ; Kim et al., 2019 ; Ray et al., 2019 ; Someya and Amagai, 2019 ; Someya et al., 2016 ; Yang and Gao, 2019 ). Existing wearable electronics usually rely on clean-room-based fabrication techniques and/or nonbiodegradable polymer supporting substrates (e.g., polyimide [PI], polyethylene terephthalate [PET], and silicone elastomers) ( Liu et al., 2017 ; Sun et al., 2018 ; Xu et al., 2020a ; Yu et al., 2019 ). Its commercial translation is therefore limited as a result of the high manufacturing cost. Moreover, next-generation wearable electronics should be skin friendly (e.g., breathable), disposable, and one-time use to minimize the risks of inflammation and infections. Considering the wide distribution and implementation of wearable electronics in the future, the accumulation of disposed electronic wastes will require substantial demand for the landfill space and cause unfavorable environmental issues. Such requirements contradict the current materials selections of wearable electronics as they are generally nonbreathable, nonbiodegradable, and expensive. Therefore, researchers are motivated to seek sustainable alternatives with desired features that can address the aforementioned handicaps.

Paper is mainly composed of cellulose microfibers with diameters of tens of microns and lengths up to 5 mm, which can be further extracted into nanofibrils with substantially decreased diameters consisting of ordered linear cellulose molecular chains ( Figure 1 A) ( Mahadeva et al., 2015 ; Tobjörk and Österbacka, 2011 ). The abundance of raw materials from wood, together with scalable roll-to-roll (R2R) manufacturing process, enables the extensive use of low-cost cellulose paper (0.1 cent dm −2 ) in our daily life ( Tobjörk and Österbacka, 2011 ). Examples include packaging, display, and information storage. Owing to the hierarchically entangled cellulose fiber structures, paper features many intriguing properties in terms of rich surface chemistry (i.e., hydrophilic hydroxyl groups) and pore size tunability. Moreover, the superior tailorability of paper granted by kirigami and origami designs brings more possibilities for the development of paper-based three dimensional devices ( Ding et al., 2016 ; Lin et al., 2017 ; Xu et al., 2020b ). In addition, paper's intrinsically sustainable, breathable, flexible, biocompatible and biodegradable nature broadens its promising and versatile applications in wearable electronics ( Figure 1 A). Leveraging scalable manufacturing approaches (e.g., inkjet printing, screen printing) or highly customizable and easily accessible writing process (e.g., pencil or pen writing), myriads of applications have been explored for paper-based wearable electronics, including displaying ( Asadpoordarvish et al., 2015 ; Ha et al., 2018 ; Jeong et al., 2019 ), sensing ( Mahadeva et al., 2015 ; Tai et al., 2020 ), optoelectronics ( Ha et al., 2018 ), energy harvesting and storage ( Yao et al., 2017 ; Zhang et al., 2015 ), and many others ( Hamedi et al., 2016 ; Liao et al., 2020 ).

An overview of desirable merits of cellulose paper

(A) Schematic illustration of cellulose paper with high flexibility, breathability, biodegradability, and disposability.

(B) Scanning electron microscope (SEM) images of pristine and acid-treated commercial paper. Adapted with permission from Ahn et al. (2020) . Copyright 2020, American Chemical Society.

(C) Acid-base treatment and surface modification of cellulose paper with trichlorooctylsilane. Adapted with permission from Ahn et al. (2020) . Copyright 2020, American Chemical Society.

(D) A typical life cycle of the biodegradable, eco-friendly, and sustainable cellulose paper. Adapted with permission from Jung et al. (2015) . Copyright 2015, Nature Publishing Group.

(E) Incineration of a paper-based electronic device. Adapted with permission from Gao et al. (2019a) . Copyright 2019, American Chemical Society.

In this review, we discuss the potential, progress, and challenges of cellulose paper-based wearable electronics. Specifically, we first introduce the critical mechanical and chemical features of cellulose papers, which can meet the requirements of human- and eco-friendly wearable electronics. Next, we overview the recent research of paper-based electronic devices, including biosensing, energy harvesting and storage, soft actuators, optoelectronics, and several others. Finally, we discuss the opportunities and future challenges. We believe that paper-based wearable electronics can find a breath of potential applications in low-resource environments and home-based, personalized healthcare as a sustainable platform.

Property demand

Wearable electronics require mechanically flexible and stretchable yet robust supporting substrates. At the molecular level, the inter- and intramolecular hydrogen bonds among densely packed hydroxyl groups, together with Van del Waals interactions, in cellulose paper generate strong interfacial interactions ( Li et al., 2021 ). Moreover, randomly distributed fibrous networks provide physical entanglement that forms porous structures ( Figure 1 B), which further enhance the flexibility ( Ahn et al., 2020 ). These features offer lightweight, flexible, yet mechanically tough properties. Commercial copy paper is typically filled with mineral fillers such as calcium carbonate (CaCO 3 ), chalk, and clays to improve light scattering, ink absorbency, and surface smoothness ( Tobjörk and Österbacka, 2011 ). Removing these fillers can tune the porosity, pore size distribution, and mechanical strength. For example, acid and/or base treatments of a commercial A4 paper increase the pore size and porosity from 0.31 μm and 50.3% to 12.2 μm and 82.9% ( Figures 1 B and 1C) ( Ahn et al., 2020 ). As a result, the mechanical toughness along with fracture strain decreased somewhat, whereas the tensile strength increased from ∼20 to ∼30 MPa. In addition, the pristine porous fibrous network provides excellent breathability (>6,000 g m −2 day −1 ) ( Xu et al., 2020b ) that far surpasses that of human skin (204 g m −2 day −1 ) ( Chao et al., 2018 ), which is critical considering long-term and continuous skin-interfaced applications. Also, high stretchability can be achieved in paper with judiciously engineered kirigami designs.

The abundant hydroxyl groups of cellulose fibers are advantageous for microfluidics where liquid transportation is driven by capillary force. The rich surface chemistry has also been explored when integrating paper with conductive materials (e.g., metallic nanomaterials, carbonaceous nanomaterials) ( Zhang et al., 2018 ). However, the intrinsic hygroscopic nature of cellulose paper inevitably compromises the stability and robustness because of unfavorable mechanical degradation and irreversible deformations in wet environments ( Tobjörk and Österbacka, 2011 ). This handicap can be resolved by surface chemistry engineering. For example, silanization of paper results in substantially increased hydrophobicity ( Figure 1 C) ( Ahn et al., 2020 ). Particularly, an omniphobic “R F ” paper was fabricated by vapor-phase silanization of paper with fluoroalkyl trichlorosilanes, while preserving its intrinsic porous structure ( Glavan et al., 2014 ). This strategy enables a hydrophobic and oleophobic paper that repels both aqueous and organic solutions such as blood, significantly improving the stability and robustness in biological environments. Inkjet printing of conductive inks on the “R F ” paper also exhibits higher lateral printing resolution down to ∼28 μm ( Lessing et al., 2014 ). In addition, other device fabrication methods, which generally involve deposition of hydrophobic reagents on paper, are also demonstrated, including analog printing, wax printing, flexography printing, and screen printing ( Mahadeva et al., 2015 ).

Given the potential of green and environmentally friendly wearable electronics, cellulose paper is playing increasingly critical role as a sustainable supporting substrate. While most polymers take hundreds of years to decompose in natural environments, cellulose paper can be decomposed within several weeks by a variety of microorganisms (fungi, bacteria, and yeasts) that exist naturally in soil ( Coughlan, 1991 ). Figure 1 D presents a typical life cycle of cellulose nanofibrils that are originally extracted from wood, subsequently degraded by fungi, and recycled back for forestry fertilizer ( Jung et al., 2015 ). Specifically, two types of fungi, brown rot fungus Postia placenta and white rot fungus Phanerochaete chrysosporium , have demonstrated the capability of cellulose biodegradation. The disposability of paper-based electronics is also exemplified by a simple incineration process ( Figure 1 E) ( Gao et al., 2019a ). Moreover, cellulose is considered as a biocompatible material and has been used in numerous bioengineering-related fields. In addition to skin-interfaced electronic devices that are covered in this review, other application areas of cellulose materials include tissue engineering, drug delivery, medical microfluidic diagnostics, and wound healing ( Cate et al., 2015 ; Czaja et al., 2006 ; Gong and Sinton, 2017 ; Hickey and Pelling, 2019 ; Sun et al., 2019 ). Owing to the aforementioned properties, cellulose paper emerges as an appealing platform for the fabrication of wearable electronics.

Applications of paper-based wearable electronics

Wearable electronics can provide continuous, long-term monitoring of dynamic changes of physiological signals and hold great promise in fitness tracking, medical diagnostics, and human-machine interface ( Bariya et al., 2018 ; Kim et al., 2019 ; Ray et al., 2019 ; Someya and Amagai, 2019 ; Someya et al., 2016 ; Yang and Gao, 2019 ). Owing to the intrinsic nonconductive nature (10 11 –10 15 Ω sq −1 at a relative humidity of 20%–40%) of cellulose paper ( Tobjörk and Österbacka, 2011 ), conductive and semiconductive materials are required to make paper-based electronic devices. A number of solution printing techniques have therefore been developed for the fabrication of paper-based wearable devices as they are typically fast, inexpensive, and easily customizable. The existing techniques include inkjet printing ( Choi et al., 2016 ; Huang et al., 2014 ; Lessing et al., 2014 ), screen printing ( Adkins et al., 2017 ), spray coating ( Asadpoordarvish et al., 2015 ), vacuum filtration ( Li et al., 2014 ), pen writing ( Liao et al., 2015 , 2020 ; Russo et al., 2011 ; Xu et al., 2020b ; Zheng et al., 2011 ), and dip coating ( Ding et al., 2016 ; Gong et al., 2014 ). Also, in situ synthesis of functional materials by chemical reduction or polymerization is another alternative strategy ( Zhang et al., 2018 ). Selection of printing techniques should be based on the demands for lateral resolution, printed thickness, homogeneity, printing speed, materials, and ink properties. Although optimization by mixing with mineral fillers has already been made in commercial copy paper to minimize surface roughness, challenges still remain when printing nanomaterial-based inks. For example, ink absorption into the porous substrate typically occurs as a result of the capillary force from micro-sized pores ( Choi et al., 2016 ). Furthermore, nonuniform pore size distribution within the paper matrix causes random spreading of the ink droplets, together with its hygroscopic expansion, thereby leading to decreased lateral resolution ( Choi et al., 2016 ; Lessing et al., 2014 ). Some methods have been proposed to alleviate this issue, such as surface silanization of paper with fluoroalkyl trichlorosilanes ( Lessing et al., 2014 ) and surface coating with a primer layer where cellulose nanofibrils are deposited to form a well-developed nanoporous structure with minimized surface roughness ( Choi et al., 2016 ).

During the past several years, the studies on paper-based wearable electronics have grown substantially, aiming to collect a variety of information such as temperature ( Xu et al., 2020b ), strain (mainly bending strain) ( Hua et al., 2016 ; Liao et al., 2015 ), pressure ( Gao et al., 2019a ; Gong et al., 2014 ; Guo et al., 2019 ; Zhong et al., 2015 ), light ( Lin et al., 2017 ; Pataniya and Sumesh, 2020 ), biopotential ( Sadri et al., 2018 ; Xu et al., 2020b ), pH ( Xu et al., 2020b ), gas ( Huang et al., 2014 ; Lin et al., 2014 ; Liu et al., 2014 ; Mirica et al., 2012 , 2013 ), humidity and respiration ( Duan et al., 2019 ; Güder et al., 2016 ), and biochemical compositions ( Gong and Sinton, 2017 ; Pal et al., 2018 ). In these devices, paper can serve as either breathable and biocompatible supporting substrates or active materials. Leveraging the piezoresistive characteristic of many functional materials, such as carbon-based materials, metals, metal oxides, and conductive polymers, numerous paper-based strain and pressure sensors have been developed ( Mahadeva et al., 2015 ). The enabled devices can provide real-time and continuous monitoring of many vital signals, such as respiration, pulse waveform, and acoustic vibration ( Figure 2 A) ( Güder et al., 2016 ; Tao et al., 2017 ). Leveraging its hygroscopic nature, cellulose paper has been used to make humidity sensors for various applications in noncontact switch, skin humidity, breath rate, and baby diaper ( Figure 2 B) ( Duan et al., 2019 ). Moreover, paper-based VOCs (volatile organic compounds) sensors are emerging as an inexpensive and versatile alternative of conventional bulky and expensive gas chromatography mass spectroscopy (GC-MS) technique. The applications include medical diagnosis, environmental monitoring, and food quality assessment. For example, breath VOCs analysis has been employed for noninvasive diagnostic of a breadth of diseases including ovarian carcinoma, cancers, and end-stage renal disease ( Broza et al., 2019 ). By modifying paper with various functional materials (e.g., carbon nanotubes, graphene, polypyrrole, and pencil leads), numerous paper-based chemiresistive VOCs sensors have been developed for analysis of ethanol, NO 2 , and NH 3 ( Mirica et al., 2012 ; Tai et al., 2020 ). The enabled paper-based VOCs sensors generally demonstrate good reproducibility and sensitivity (down to ∼80 ppb) ( Liu et al., 2014 ), which largely depends on the active sensing materials.

Paper-based wearable biosensors

(A) Paper-based wearable pressure sensor for pulse waveform measurement. Adapted with permission from Guo et al. (2019) . Copyright 2019, American Chemical Society.

(B) Paper-based wearable humidity sensors for multiple biomedical applications. Adapted with permission from Duan et al. (2019) . Copyright 2019, American Chemical Society.

(C) Pencil-paper-enabled multimodal wearable device and its application in monitoring of the human mental state. Adapted with permission from Xu et al. (2020b) . Copyright 2020, National Academy of Sciences.

(D) Paper-based wearable smart bandage for electrochemical sensing. Adapted with permission from Pal et al. (2018) . Copyright 2018, Elsevier.

In addition to piezoresistive and chemiresistive sensors, biopotential sensors that monitor the electrical properties of biological tissues provide a wealth of valuable information. For example, electrocardiogram (ECG) represents the electrical activity of heart muscle and remains an essential role in cardiac assessments. To achieve high-fidelity recording of electrophysiological signals, an intimate contact between the sensing electrodes and skin is required. Although paper is generally not sticky, silicone adhesive can be spray coated on its surface to resolve this issue ( Xu et al., 2020b ). The resulting paper substrate exhibited reliable and robust adhesion with skin while preserving its intrinsic breathability. In addition, there are various types of commercial adhesive cellulose papers (e.g., Aquasol adhesive water-soluble paper, Post-it Notes paper) that can meet this particular requirement without needing additional coating of silicone adhesive. Owing to low-cost resources, ease of fabrication, and abundant potential designs, paper-based wearable electronics show great potentials in electrical and optical stimulation, temperature, ECG, EMG (electromyogram), and EEG (electroencephalogram) recordings ( Figure 2 C) ( Broza et al., 2019 ; Xu et al., 2020b ).

Given the rich surface chemistry, intrinsic hydrophilicity, and hierarchically porous structure, cellulose paper is particularly attractive for biochemical sensors. The hydroxyl groups together with micro-sized pores provide the capillary force that drives the transportation of biofluids, which provides inherent capability for microfluidic devices. By selective surface functionalization with hydrophobic moieties, microfluidic channels can be constructed on paper ( Mahadeva et al., 2015 ). Although continuous operation driven by the capillary force in a long-term manner has been rarely demonstrated by paper-based microfluidic devices, the physiologically relevant pressure from sweat glands (∼2 kPa) may enable continuous sweat transportation as demonstrated by elastomer-based microfluidic devices ( Reeder et al., 2019 ; Yang et al., 2020 ; Yu et al., 2020 ). Although colorimetric detection is widely studied, electrochemical sensing is highly desirable owing to its quantitative analysis and insensitivity to light, dust, and insoluble compounds. This technique is extensively explored for a number of analytes such as glucose, cholesterol, drugs, pH, uric acid, potassium ferricyanide, L-lactate, and alcohol in blood, urine, and sweat samples ( Nie et al., 2010 ; Pal et al., 2017 , 2018 ; Xu et al., 2020b ). For example, a paper-based wearable omniphobic smart bandage (OPSB) has been developed for the detection of pH, uric acid, and potassium ferricyanide in human open wounds, where carbon and silver/silver chloride inks are screen printed as working, counter, and reference electrodes, respectively ( Figure 2 D) ( Pal et al., 2018 ). Subsequent interfacing the OPSB with a wearable potentiostat, battery, and antenna completed the device fabrication, enabling a wearable system for the early detection of pressure ulcers ( Figure 2 D). Paper-based point-of-care diagnostic devices are well studied in terms of the sample collection and analysis. Such biosensing applications can be exemplified by modifying paper electrodes with ion-selective membranes for potentiometric ion sensors. Various ion-selective electrodes are adopted for the detections of Cd 2+ , Ag + , K + , NH 4 + , Cl − , and Na + ( Hu et al., 2016 ; Mensah et al., 2014 ; Novell et al., 2012 ; Sakata et al., 2020 ). Given the intrinsic high porosity of cellulose paper substrate, these devices usually possess enhanced active surface area and therefore exhibit higher sensitivity. Of note, direct pencil drawing has also been demonstrated for the fabrication of wearable sweat sensors ( Xu et al., 2020b ). Compared with other often-used microfluidic devices ( Bandodkar et al., 2019 ; Reeder et al., 2019 ; Yang et al., 2020 ), cellulose paper-based microfluidic devices exhibit distinct features, including (1) flexibility and breathability; (2) disposability; (3) inexpensive and scalable manufacturing process. However, challenges remain as encapsulation is required to minimize sample contamination and cross talk of different biosensors. Besides, the reproducibility and complicated calibration procedures are still main issues of paper-based biochemical sensors that need to be addressed by optimizations and innovations in the manufacturing process.

As compared with traditional wearable electronic devices, cellulose paper-based electronics typically exhibit comparable performances in terms of the sensitivity and signal quality (i.e., signal-to-noise ratio) since similar active materials are generally used in both paper-based and traditional wearable electronics. This can be exemplified by paper-based electrophysiological sensors, which exhibit negligible difference in signal quality with conventional gel electrodes ( Xu et al., 2020b ). In addition, surface modifications with hydrophobic moieties can substantially improve the stability of paper-based wearable devices. However, their long-term stability and robustness under wetting environments and repeated mechanical deformations are rarely studied, which are worth further investigations.

Energy harvesters

Wearable electronics require electric power to support sensing, data communication, and signal conditioning. Current incorporation of batteries and coin cells into skin-like wearable electronics pose challenges as a result of their mismatch in form factors ( Bandodkar et al., 2020 ). A self-powered wearable system that harvests energy from body motion, sweat, ambient light, and moisture therefore emerges as an economically viable, sustainable solution ( Bandodkar et al., 2020 ; Park et al., 2018 ; Song et al., 2020 ; Xu et al., 2020b ; Yu et al., 2020 ). Given wide availability, disposability, and biocompatibility of cellulose paper, much progress has been achieved for paper-based energy-harvesting devices. These power generators generally rely on triboelectric, thermoelectric, hygroelectric, piezoelectric, and electrostatic effects. For example, cellulose paper easily donates electrons when rubbed with materials that are likely to gain electrons such as polyvinylidene fluoride (PVDF). Leveraging this unique property, a large variety of triboelectric nanogenerators has been developed ( Feng et al., 2016 ; Guo et al., 2017 ; Xia et al., 2018 ; Zhang et al., 2017a , 2017b ). In addition to cellulose paper supporting substrates, nanogenerators generally require additional modifications or active layers. For example, conductive electrodes and triboelectric pairs are required in electrostatic and triboelectric nanogenerators ( Figure 3 A) ( Guo et al., 2017 ; Zhong et al., 2013 ). When gum wrappers are used, however, additional preparation of conductive electrodes is eliminated since aluminum foil coated on the backside can serve as conductive pathways ( Feng et al., 2016 ). The resulting mechanical energy harvesters can generate instant and pulsed voltage output (hundreds of volts), which requires voltage rectification before practical uses. A systematic integration of paper-based triboelectric nanogenerators with paper-based microfluidic biochemical sensors provides a proof-of-concept self-powered wearable device for point-of-care applications ( Pal et al., 2017 ).

Paper-based energy harvesters

(A) Schematic illustrations of the fabrication process of paper-based electrostatic nanogenerators. Adapted with permission from Zhong et al. (2013) . Copyright 2013, Royal Society of Chemistry.

(B) An electrochemical sensor powered by a paper-based triboelectric nanogenerator. Adapted with permission from Pal et al. (2017) . Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(C) Schematic illustrations of a paper-based humidity energy harvester for a self-powered transdermal drug-delivery system. Adapted with permission from Xu et al. (2020b) . Copyright 2020, National Academy of Sciences.

(D) Paper-based ionic conductors for thermoelectric heat harvesting. Adapted with permission from Li et al. (2019) . Copyright 2019, Nature Publishing Group.

The rich surface chemistry of cellulose paper has also been explored for moisture-induced energy generation. The abundant hydroxyl and carboxyl groups, which naturally exist on paper, provide exciting opportunities for ambient humidity-related applications. Water absorption by hydrophilic moieties induces the dissociation of surface functional groups, thereby releasing a large number of positive ions (i.e., protons) ( Bai et al., 2019 ). If there is an inherent concentration gradient of oxygen-containing groups, chemical potential difference spontaneously forms and drives proton movement ( Figure 3 C). This charge carrier transportation leads to charge separation and therefore generates voltages and currents in an external circuit. Leveraging this unique feature, hygroelectric generators prepared by either generating asymmetric moisture levels or constructing a heterogeneous hydrophilic functional groups have been reported ( Gao et al., 2019b ; Xu et al., 2020b ). Unlike mechanical energy harvesters, moisture-induced energy generators can produce continuous electrical output in the range of hundreds of millivolts that are suitable for direct use or storage. A proof-of-concept self-powered iontophoretic transdermal drug-delivery system was demonstrated using the moisture-induced electric energy ( Figure 3 C) ( Xu et al., 2020b ).

Besides, thermoelectric generators (TEGs) that can convert low-grade heat into electric energy is of considerable interest for wearable electronics. Like other energy generators, paper-based TEGs can be fabricated by integrating with conventional p- and n-type inorganic thermoelectric composites into a porous structure ( Rojas et al., 2017 ; Zhao et al., 2019 ). These TEGs generally exhibit output voltages in the range of tens of millivolts and output power of tens of nanowatts, which are, however, still far from practical wearable applications. Introduction of electrolyte into a cellulosic membrane with oxidized and aligned cellulose molecular chains results in sub-nanoscale confinement of ions and therefore induces selective ion diffusions that essentially enhance thermoelectric performance ( Figure 3 D) ( Li et al., 2019 ). Yet, this process requires a particular cellulose (i.e., type II cellulose) and does not occur in natural wood or type I cellulose.

Energy storage devices

Despite recent progresses, challenges still remain as current paper-based energy generators have numerous limitations such as inadequate power output, low energy efficiency, and intermittent energy generation. A wearable energy storage system is therefore required to provide stable and continuous power supply that can meet the demand for sensing, data communication, and signal conditioning. Various energy storage platforms have been developed to meet this requirement, among which batteries and supercapacitors stand out. Cellulose paper is promising to make lightweight, low-cost, and disposable self-powered wearable electronics. Moreover, hierarchically porous cellulose fibrous networks and hygroscopic features contribute to the penetration and absorption of conductive fillers, thereby enhancing their interface interactions and facilitating electrons/ions transfer, which is desirable for high-performance wearable batteries.

Cellulose paper-based supercapacitors, Li-ion batteries, Li-S batteries, Li-O 2 batteries, and biofuel cells have been extensively studied ( Yao et al., 2017 ; Zhang et al., 2015 , 2018 ). For example, paper-based supercapacitors have been fabricated by a facile graphite pencil drawing ( Guo et al., 2016 ; Yao et al., 2013 ; Zheng et al., 2011 ), vacuum filtration ( Li et al., 2014 ), or a scalable inkjet printing ( Choi et al., 2016 ), where conductive electrodes (carbon nanotubes, graphene, silver nanowires, polypyrroles, and polyanilines) are typically employed ( Zhang et al., 2015 ) ( Figures 4 A and 4B). Considerable research efforts have been made due to their outstanding reversibility, high-power density, long-life cycles, and safe operation. Of note, the kirigami and origami designs can expand the stretchability of paper-based wearable devices ( Figure 4 A). Li-ion batteries are another powerful and versatile platform that has already shown promise in portable electronic devices owing to their outstanding energy density, high operation voltage, low self-discharging rate, lack of memory effect, long life cycles, and environmentally friendly features ( Zhao et al., 2020 ). Li-ion batteries generally comprise a LiCoO 2 cathode, a graphite anode, and a separator saturated with a liquid organic electrolyte. An example using copper nanoparticles-plated cellulose paper as the anode is provided in Figure 4 C, which illustrates the mechanical flexibility of the resulting battery ( Wang et al., 2018 ). Proper packaging and encapsulations are also essential to construct an integrated wearable self-powered system. To achieve a sustainable and clean energy supply, biofuel cells that leverage the oxidation and reduction processes of renewable fuels can generate energy from biological matters ( Song et al., 2015 ). Representative examples include alcohols, sugar, and amine substances, such as glucose, ethanol, pyruvate, and lactate ( Escalona-Villalpando et al., 2019 ; Li et al., 2020 ), which can potentially enable future sweat-powered skin-interfaced electronics or blood-powered implantable devices ( Bandodkar et al., 2017 , 2020 ). Integrating biofuel cells with cellulose paper substrates, although still in their infancy, can provide a promising alternative for the fabrication of completely sustainable and disposable self-powered wearable electronics. An enzymatic biofuel cell that employs laccase as biocatalyst can generate power (power density: 1.897 W m −3 ; open circuit voltage: 0.14 V) from waste water ( Figure 4 D) ( Li et al., 2020 ). The highest output power density (∼200 μW cm −2 ) is achieved on a cellulose filter paper using glucose as the biofuel ( Desmet et al., 2016 ; Wang et al., 2014 ).

Paper-based energy storage devices

(A) Paper-based supercapacitors with kirigami designs. Adapted with permission from Guo et al. (2016) . Copyright 2016, American Chemical Society.

(B) A solid-state supercapacitor inkjet printed on paper. Adapted with permission from Choi et al. (2016) . Copyright 2016, Royal Society of Chemistry.

(C) Copper-plated paper for foldable lithium ion batteries. Adapted with permission from Wang et al. (2018) . Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(D) Schematic illustrations and performance characterizations of paper-based enzymatic cellulose biofuel cell with bisphenol A as fuel. Adapted with permission from Li et al. (2020) . Copyright 2020, Elsevier.

Other applications

Display is one basic building block of next-generation wearable electronics. Also, wearable display can bridge human-device interactions. Transforming conventional rigid displays into flexible and wearable forms provides fascinating opportunities for wearable technologies ( Shi et al., 2021 ). Regular paper is inherently not suitable for organic light-emitting displays owing to its surface roughness and opaque feature. Highly transparent cellulose nanopaper with low haze and smooth surface has been explored for flexible displays ( Figure 5 A) ( Okahisa et al., 2009 ). The nanopaper comprises nanofibrillated celluloses that are assembled into densely packed thin films, thereby substantially improving optical transmittance and reducing surface roughness ( Fang et al., 2014 ; Zhu et al., 2016 ). The dimensional reduction of microscale fibers in traditional copy paper also is effective for minimizing surface roughness. Besides, filling hierarchically porous cavities with polymers provides an alternative approach to reduce surface roughness for optoelectronic applications ( Ha et al., 2018 ). Light-emitting electrochemical cells can overcome these issues as a result of in situ formation of a light-emitting p–n junction ( Figure 5 B) ( Asadpoordarvish et al., 2015 ). Commonly used cellulose paper with microscale fibers serving as a supporting substrate for flexible organic light-emitting displays has also been reported ( Jeong et al., 2019 ). Specifically, a facile pen-drawing approach is employed. Here, conducting and light-emitting functional materials are loaded into various drawing pens for anode, hole injection layer, hole transfer layer, emission layer, electron transfer layer, electron injection layer, and cathode, respectively ( Figure 5 C). Despite these progresses, paper-based flexible displays still face challenges in terms of thermal stability and mechanical durability.

Other applications of paper-based electronics

(A) An optical image of an organic light-emitting diode on transparent, resin-infiltrated cellulose paper. Adapted with permission from Okahisa et al. (2009) . Copyright 2009, Elsevier.

(B) Photographs of surface-emitting light-emitting electrochemical cells on flexible cellulose paper substrates. Adapted with permission from Asadpoordarvish et al. (2015) . Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(C) Schematic illustrations and photographs of the pen-drawing display on paper. Adapted with permission from Jeong et al. (2019) . Copyright 2019, Nature Publishing Group.

(D) Schematic illustrations of paper-based hygroexpansive electrothermal actuators. Adapted with permission from Hamedi et al. (2016) . Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(E) Optical images of paper-based flexible electric circuits, antenna, transistors, and photodetectors. Adapted with permission from Lin et al. (2017) ; Russo et al. (2011) ; Zschieschang and Klauk (2019) . Copyright 2017, American Chemical Society; copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; copyright 2019, Royal Society of Chemistry.

Given the hygroscopic nature from surface hydroxyl groups, cellulose paper is also exploited for soft actuators. Upon moisture exposure, the cellulose matrix expands or contracts in response to humidity variations ( Amjadi and Sitti, 2016 ). The moisture content in the paper can be modulated by electrothermal heating, which results in dimensional deformations when a strain-constraining layer is integrated ( Figure 5 D) ( Hamedi et al., 2016 ). Cellulose paper-based soft actuators can potentially offer exciting opportunities for liquid transport control in microfluidic devices in terms of micromachine assembly and microactuation ( Hamedi et al., 2016 ). Actuation mechanisms that utilize magnetic and electrostatic fields were also demonstrated. However, these methods usually suffer from the requirement of magnetic additives, high voltage supply, and insufficient movement.

Also, paper's appealing properties make it suitable for a number of other applications. These can be exemplified by transistors ( Zschieschang and Klauk, 2019 ), electric circuits ( Hyun et al., 2013 ; Russo et al., 2011 ), radio frequency identification (RFID) tags ( Wang et al., 2019 ), antennas ( Inui et al., 2015 ; Xu et al., 2020b ), photodetectors ( Gomathi et al., 2017 ; Lin et al., 2017 ; Pataniya and Sumesh, 2020 ), and artificial nerve ( Liao et al., 2020 ) ( Figure 5 E).

Toward highly integrated wearable systems

During the past several years, we have witnessed the significant advancements of paper-based electronics in a variety of research realms, including sensing, energy generation and storage, optoelectronics, and data transmission. In the future, a paper-based integrated wearable electronic system is highly desirable for skin-interfaced biomedical applications. Ideally, the system should comprise biodegradable cellulose supporting substrates and minimal amount of nondegradable functional materials to minimize environmental impact. An all-paper supported wearable system, consisting of vital signal sensing, data processing, power supply, and signal transmission, was recently demonstrated ( Figure 6 ) ( Nassar et al., 2017 ). This highly integrated system uses Post-it Note paper as the supporting substrate, integrated with RFID tag for radio communication, power management circuitry, silicon-based microprocessor (μP), and various sensors. This paper-based wearable system is capable of monitoring a broad spectrum of vital biosignals (e.g., temperature, respiration, heart rates, blood pressure, and sweating) and can be interfaced with a smartphone for wireless data collection, interpretation, and visualization.

A paper-based highly integrated wearable healthcare monitoring system

Adapted with permission from Nassar et al. (2017) . Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Challenges and conclusion

Given the promising properties of cellulose paper in terms of inexpensive and scalable manufacturing, biodegradability, disposability, biocompatibility, breathability, and rich surface chemistry, a variety of paper-based devices have been developed, ranging from sensing, energy storage and generation, optoelectronics, and soft actuators to wireless data communication. These research advancements are driven by the attempt to seek for a more sustainable, disposable, and environmentally friendly alternative of currently polymer-based flexible electronics. Although most current studies use pristine cellulose paper as the supporting substrate, its abundant surface hydroxyl groups and intra- and inter-molecular hydrogen bonds offer many exciting opportunities for hybridization with other functional moieties to meet requirements of customized applications. These modifications generally contribute to the enhanced mechanical strength, decreased surface roughness, and improved thermal and chemical stability against inorganic and organic solvents.

Although a broad range of proof-of-concept paper-based wearable devices have been demonstrated, many challenges still remain, especially for the transition from laboratory-based demonstrations to practical uses. These obstacles involve material and chemistry aspects, which are critical for cellulose paper as a sustainable alternative of polymer substrates. For example, although the hierarchically porous structure formed by the entangled fibrous cellulose network offers excellent breathability for skin-interfaced electronics and large surface area for energy storage-related applications, the porosity inevitably results in the incompatibility with some solution-printing techniques (e.g., inkjet printing) owing to uncontrolled ink spreading. This will further cause conductivity deterioration as a result of ink penetration into a deeper depth of the paper. Moreover, as compared with conventional miniaturized electronic devices, the printing resolution on cellulose paper has yet met the demand for future highly integrated electronics. In addition, the microscale surface roughness and intrinsic opaque nature of commercial cellulose paper limit its application in optoelectronics that typically requires high optical transmittance and low surface roughness. Unlike conventional polymers, pristine cellulose paper is not mechanically or thermally stable. Dimensional deformations caused by moisture absorption will compromise device performance and the reliability of the recorded data.

Although there are multiple options available in the market, adhesiveness is of superior importance for skin-interfaced wearable electronics since conformal contact with human skin is a critical factor of high-fidelity biosignal recording. Motion-induced artifacts during regular human activities will cause increased noise level and signal degradation. Examples of adhesive papers used for wearable electronics include Post-it Note paper and water-soluble adhesive paper. The stickiness is often associated with a thin layer of adhesive polymer, which comes at the sacrifice of breathability to some extent. This issue can probably be addressed by innovative material design that can provide the required adhesiveness while simultaneously preserving the porous structure. While adhesive paper is commercially available, the deposition of conductive electrodes that are typically non-sticky will compromise the overall interface adhesion. Adopting an open-mesh serpentine layout is an effective strategy since this can maximize the proportion of nonconductive adhesive area and minimize deformation-induced local strains. Alternatively, it is desirable to develop adhesive conductive composites, comprising conductive fillers and adhesive polymers, as sensing electrodes.

In the light of an eco-friendly society, paper-based wearable electronics will be completely disposable when biodegradable electronic materials are used for device fabrication. Although various emerging carbonaceous nanomaterials bring promising routes toward paper-based all-biodegradable device, the performance is yet comparable with conventional nonbiodegradable materials. Using some nonbiodegradable components is still indispensable for high-performance tasks, such as data processing and management. Besides, the research of paper-based, highly integrated, wearable electronic system is still in the infant stage, which requires more exploration of fundamental science and technologies at the molecular level and all-paper based system assembly.

These obstacles inevitably impede the advancement and commercialization of cellulose paper-based wearable electronics. Nevertheless, they can be addressed through judicious material innovation, structural engineering, and advancement of other related realms (e.g., fabrication techniques) to satisfy specific demands for various arising applications. For example, stability issues in wet environments can be alleviated through proper chemical functionalization (e.g., silanization); substantial surface roughness and low transmittance can be resolved by downscaling constituent fibrous diameters; inadequate adhesion to skin can be enhanced by hybridizing cellulose paper with adhesive chemicals or proper bioinspired structure engineering (e.g., bioinspired dry adhesive structure [ Chun et al., 2018 ; Wang, 2018 ]); low stretchability can be improved by introducing kirigami design. In addition to conventional copy paper, an expanded array of nanopapers comprising nanofibrillated cellulose is emerging thanks to extensive research efforts on fundamental research studies in terms of materials and chemistry innovations. These nanopapers have raised substantial interests as they bypass existing issues of microscale cellulose counterparts, such as considerably improved surface roughness and transparency. However, microscale and nanoscale cellulose papers are only negligibly stretchable, which might limit their wide adoption in wearable electronics. Nevertheless, we anticipate fertile opportunities of cellulose papers in sustainable wearable electronics considering their mechanical, economic, and environmental benefits.

Acknowledgments

Z.Y. acknowledges the financial support from the NSF grant (ECCS-1917630) and the University of Missouri-Columbia start-up fund. S.B.S. acknowledges the financial support from the NASA-Missouri Space Grant Consortium.

Author contributions

Conceptualization, Y.X., and Z.Y.; investigation, Y.X., Q.F., M.P., G.Z., Y.L., and S.B.S.; writing – original draft, Y.X.; writing – review & editing, Y.X., Q.F., M.P., G.Z., Y.L., S.B.S., and Z.Y.; funding acquisition, Z.Y.

Declaration of interests

The authors declare no competing interests.

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Electronics Research Paper Topics

This list of electronics research paper topics provides the list of 30 potential topics for research papers and an overview article on the history of electronics.

1. Applications of Superconductivity

The 1986 Applied Superconductivity Conference proclaimed, ‘‘Applied superconductivity has come of age.’’ The claim reflected only 25 years of development, but was justifiable due to significant worldwide interest and investment. For example, the 1976 annual budget for superconducting systems exceeded $30 million in the U.S., with similar efforts in Europe and Japan. By 1986 the technology had matured impressively into applications for the energy industry, the military, transportation, high-energy physics, electronics, and medicine. The announcement of high-temperature superconductivity just two months later brought about a new round of dramatic developments.

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Get 10% off with 24start discount code, 2. discovery of superconductivity.

As the twenty-first century began, an array of superconducting applications in high-speed electronics, medical imaging, levitated transportation, and electric power systems are either having, or will soon have, an impact on the daily life of millions. Surprisingly, at the beginning of the twentieth century, the discovery of superconductivity was completely unanticipated and unimagined.

In 1911, three years after liquefying helium, H. Kammerlingh Onnes of the University of Leiden discovered superconductivity while investigating the temperature-dependent resistance of metals below 4.2Kelvin. Later reporting on experiments conducted in 1911, he described the disappearance of the resistance of mercury, stating, ‘‘Within some hundredths of a degree came a sudden fall, not foreseen [by existing theories of resistance]. Mercury has passed into a new state, which . . . may be called the superconductive state.’’

3. Electric Motors

The main types of electric motors that drove twentieth century technology were developed toward the end of the nineteenth century, with direct current (DC) motors being introduced before alternating current (AC) ones. Most important initially was the ‘‘series’’ DC motor, used in electric trolleys and trains from the 1880s onward. The series motor exerts maximum torque on starting and then accelerates to its full running speed, the ideal characteristic for traction work. Where speed control independent of the load is required in such applications as crane and lift drives, the ‘‘shunt’’ DC motor is more suitable.

4. Electronic Calculators

The electronic calculator is usually inexpensive and pocket-sized, using solar cells for its power and having a gray liquid crystal display (LCD) to show the numbers. Depending on the sophistication, the calculator might simply perform the basic mathematical functions (addition, subtraction, multiplication, division) or might include scientific functions (square, log, trig). For a slightly higher cost, the calculator will probably include programmable scientific and business functions. At the end of the twentieth century, the electronic calculator was as commonplace as a screwdriver and helped people deal with all types of mathematics on an everyday basis. Its birth and growth were early steps on the road to today’s world of computing.

5. Electronic Communications

The broad use of digital electronic message communications in most societies by the end of the 20th century can be attributed to a myriad of reasons. Diffusion was incremental and evolutionary. Digital communication technology was seeded by large-scale funding for military projects that broke technological ground, however social needs and use drove systems in unexpected ways and made it popular because these needs were embraced. Key technological developments happened long before diffusion into society, and it was only after popularity of the personal computer that global and widespread use became commonplace. The Internet was an important medium in this regard, however the popular uses of it were well established long before its success. Collaborative developments with open, mutually agreed standards were key factors in broader diffusion of the low-level transmission of digital data, and provided resistance to technological lock-in by any commercial player. By the twenty-first century, the concept of interpersonal electronic messaging was accepted as normal and taken for granted by millions around the world, where infrastructural and political freedoms permitted. As a result, traditional lines of information control and mass broadcasting were challenged, although it remains to be seen what, if any, long-term impact this will have on society.

6. Electronic Control Technology

The advancement of electrical engineering in the twentieth century made a fundamental change in control technology. New electronic devices including vacuum tubes (valves) and transistors were used to replace electromechanical elements in conventional controllers and to develop new types of controllers. In these practices, engineers discovered basic principles of control theory that could be further applied to design electronic control systems.

7. Fax Machine

Fax technology was especially useful for international commercial communication, which was traditionally the realm of the Telex machine, which only relayed Western alpha-numeric content. A fax machine could transmit a page of information regardless of what information it contained, and this led to rapid and widespread adoption in developing Asian countries during the 1980s. With the proliferation of the Internet and electronic e-mail in the last decade of the twentieth century, fax technology became less used for correspondence. At the close of the 20th century, the fax machine was still widely used internationally for the transmission of documents of all forms, with the ‘‘hard copy’’ aspect giving many a sense of permanence that other electronic communication lacked.

8. Hall Effect Devices

The ‘‘Hall effect,’’ discovered in 1879 by American physicist Edwin H. Hall, is the electrical potential produced when a magnetic field is perpendicular to a conductor or semiconductor that is carrying current. This potential is a product of the buildup of charges in that conductor. The magnetic field makes a transverse force on the charge carriers, resulting in the charge being moved to one of the sides of the conductor. Between the sides of the conductor, measurable voltage is yielded from the interaction and balancing of the polarized charge and the magnetic influence.

Hall effect devices are commonly used as magnetic field sensors, or alternatively if a known magnetic field is applied, the sensor can be used to measure the current in a conductor, without actually plugging into it (‘‘contactless potentiometers’’). Hall sensors can also be used as magnetically controlled switches, and as a contactless method of detecting rotation and position, sensing ferrous objects.

9. Infrared Detectors

Infrared detectors rely on the change of a physical characteristic to sense illumination by infrared radiation (i.e., radiation having a wavelength longer than that of visible light). The origins of such detectors lie in the nineteenth century, although their development, variety and applications exploded during the twentieth century. William Herschel (c. 1800) employed a thermometer to detect this ‘‘radiant heat’’; Macedonio Melloni, (c. 1850) invented the ‘‘thermochrose’’ to display spatial differences of irradiation as color patterns on a temperature-sensitive surface; and in 1882 William Abney found that photographic film could be sensitized to respond to wavelengths beyond the red end of the spectrum. Most infrared detectors, however, convert infrared radiation into an electrical signal via a variety of physical effects. Here, too, 19th century innovations continued in use well into the 21st century.

10. Integrated Circuits Design and Use

Integrated circuits (ICs) are electronic devices designed to integrate a large number of microscopic electronic components, normally connected by wires in circuits, within the same substrate material. According to the American engineer Jack S. Kilby, they are the realization of the so-called ‘‘monolithic idea’’: building an entire circuit out of silicon or germanium. ICs are made out of these materials because of their properties as semiconductors— materials that have a degree of electrical conductivity between that of a conductor such as metal and that of an insulator (having almost no conductivity at low temperatures). A piece of silicon containing one circuit is called a die or chip. Thus, ICs are known also as microchips. Advances in semiconductor technology in the 1960s (the miniaturization revolution) meant that the number of transistors on a single chip doubled every two years, and led to lowered microprocessor costs and the introduction of consumer products such as handheld calculators.

11. Integrated Circuits Fabrication

The fabrication of integrated circuits (ICs) is a complicated process that consists primarily of the transfer of a circuit design onto a piece of silicon (the silicon wafer). Using a photolithographic technique, the areas of the silicon wafer to be imprinted with electric circuitry are covered with glass plates (photomasks), irradiated with ultraviolet light, and treated with chemicals in order to shape a circuit’s pattern. On the whole, IC manufacture consists of four main stages:

• Preparation of a design
• Preparation of photomasks and silicon wafers
• Testing and packaging

Preparing an IC design consists of drafting the circuit’s electronic functions within the silicon board. This process has radically changed over the years due to the increasing complexity of design and the number of electronic components contained within the same IC. For example, in 1971, the Intel 4004 microprocessor was designed by just three engineers, while in the 1990s the Intel Pentium was designed by a team of 100 engineers. Moreover, the early designs were produced with traditional drafting techniques, while from the late 1970s onward the introduction of computer-aided design (CAD) techniques completely changed the design stage. Computers are used to check the design and simulate the operations of perspective ICs in order to optimize their performance. Thus, the IC drafted design can be modified up to 400 times before going into production.

12. Josephson Junction Devices

One of the most important implications of quantum physics is the existence of so-called tunneling phenomena in which elementary particles are able to cross an energy barrier on subatomic scales that it would not be possible for them to traverse were they subject to the laws of classical mechanics. In 1973 the Nobel Prize in Physics was awarded to Brian Josephson, Ivan Giaever and Leo Esaki for their work in this field. Josephson’s contribution consisted of a number of important theoretical predictions made while a doctoral student at Cambridge University. His work was confirmed experimentally within a year of its publication in 1961, and practical applications were commercialized within ten years.

13. Laser Applications

Lasers are employed in virtually every sector of the modern world including industry, commerce, transportation, medicine, education, science, and in many consumer devices such as CD players and laser printers. The intensity of lasers makes them ideal cutting tools since their highly focused beam cuts more accurately than machined instruments and leaves surrounding materials unaffected. Surgeons, for example, have employed carbon dioxide or argon lasers in soft tissue surgery since the early 1970s. These lasers produce infrared wavelengths of energy that are absorbed by water. Water in tissues is rapidly heated and vaporized, resulting in disintegration of the tissue. Visible wavelengths (argon ion laser) coagulate tissue. Far-ultraviolet wavelengths (higher photon energy, as produced by excimer lasers) break down molecular bonds in target tissue and ‘‘ablate’’ tissue without heating. Excimer lasers have been used in corneal surgery since 1984. Short pulses only affect the surface area of interest and not deeper tissues. The extremely small size of the beam, coupled with optical fibers, enables today’s surgeons to conduct surgery deep inside the human body often without a single cut on the exterior. Blue lasers, developed in 1994 by Shuji Nakamura of Nichia Chemical Industries of Japan, promise even more precision than the dominant red lasers currently used and will further revolutionize surgical cutting techniques.

14. Laser Theory and Operation

Lasers (an acronym for light amplification by stimulated emission of radiation) provide intense, focused beams of light whose unique properties enable them to be employed in a wide range of applications in the modern world. The key idea underlying lasers originated with Albert Einstein who published a paper in 1916 on Planck’s distribution law, within which he described what happens when additional energy is introduced into an atom. Atoms have a heavy and positively charged nucleus surrounded by groups of extremely light and negatively charged electrons. Electrons orbit the atom in a series of ‘‘fixed’’ levels based upon the degree of electromagnetic attraction between each single electron and the nucleus. Various orbital levels also represent different energy levels. Normally electrons remain as close to the nucleus as their energy level permits, with the consequence that an atom’s overall energy level is minimized. Einstein realized that when energy is introduced to an atom; for example, through an atomic collision or through electrical stimulation, one or more electrons become excited and move to a higher energy level. This condition exists temporarily before the electron returns to its former energy level. When this decay phenomenon occurs, a photon of light is emitted. Einstein understood that since the energy transitions within the atom are always identical, the energy and the wavelength of the stimulated photon of light are also predictable; that is, a specific type of transition within an atom will yield a photon of light of a specific wavelength. Hendrick Kramers and Werner Heisenberg obtained a series of more extensive calculations of the effects of these stimulated emissions over the next decade. The first empirical evidence supporting these theoretical calculations occurred between 1926 and 1930 in a series of experiments involving electrical discharges in neon.

15. Lasers in Optoelectronics

Optoelectronics, the field combining optics and electronics, is dependent on semiconductor (diode) lasers for its existence. Mass use of semiconductor lasers has emerged with the advent of CD and DVD technologies, but it is the telecommunications sector that has primarily driven the development of lasers for optoelectronic systems. Lasers are used to transmit voice, data, or video signals down fiber-optic cables.

While the success of lasers within telecommunication systems seems unquestioned thanks to their utility in long-distance large-capacity, point-to-point links, these lasers also find use in many other applications and are ubiquitous in the developed world. Their small physical size, low power operation, ease of modulation (via simple input current variation) and small beam size mean that these lasers are now part of our everyday world, from CDs and DVDs, to supermarket checkouts and cosmetic medicine.

16. Light Emitting Diodes

Light emitting diodes, or LEDs, are semiconductor devices that emit monochromatic light once an electric current passes through it. The color of light emitted from LEDs depends not on the color of the bulb, but on the emission’s wavelength. Typically made of inorganic materials like gallium or silicon, LEDs have found frequent use as ‘‘pilot,’’ or indicator, lights for electronic devices. Unlike incandescent light bulbs, which generate light from ‘‘heat glow,’’ LEDs create light more efficiently and are generally more durable than traditional light sources.

17. Lighting Techniques

In 1900 electric lighting in the home was a rarity. Carbon filament incandescent lamps had been around for 20 years, but few households had electricity. Arc lamps were used in streets and large buildings such as railway stations. Domestic lighting was by candle, oil and gas.

The stages of the lightning techniques evolution are the following:

• Non-Electric Lighting
• Electric Lighting: Filament Lamps
• Electric Lighting: Discharge Lamps
• Electric Lighting: Fluorescent Lamps
• Electric Lighting: LED Lamps

18. Mechanical and Electromechanical Calculators

The widespread use of calculating devices in the twentieth century is intimately linked to the rise of large corporations and to the increasing role of mathematical calculation in science and engineering. In the business setting, calculators were used to efficiently process financial information. In science and engineering, calculators speeded up routine calculations. The manufacture and sale of calculators was a widespread industry, with major firms in most industrialized nations. However, the manufacture of mechanical calculators declined very rapidly in the 1970s with the introduction of electronic calculators, and firms either diversified into other product lines or went out of business. By the end of the twentieth century, slide rules, adding machines, and other mechanical calculators were no longer being manufactured.

19. Mobile (Cell) Telephones

In the last two decades of the twentieth century, mobile or cell phones developed from a minority communication tool, characterized by its prevalence in the 1980s among young professionals, to a pervasive cultural object. In many developed countries, more than three quarters of the population owned a cell phone by the end of the 20th century.

Cell phone technology is a highly evolved form of the personal radio systems used by truck drivers (citizens band, or CB, radio) and police forces in which receiver/transmitter units communicate with one another or a base antenna. Such systems work adequately over short distances with a low volume of traffic but cannot be expanded to cope with mass communication due to the limited space (bandwidth) available in the electromagnetic spectrum. Transmitting and receiving on one frequency, they allow for talking or listening but not both simultaneously.

For mobile radio systems to make the step up to effective telephony, a large number of two-way conversations needed to be accommodated, requiring a duplex channel (two separate frequencies, taking up double the bandwidth). In order to establish national mobile phone networks without limiting capacity or the range of travel of handsets, a number of technological improvements had to occur.

20. Photocopiers

The photocopier, copier, or copying machine, as it is variously known, is a staple of modern life. Copies by the billions are produced not only in the office but also on machines available to the public in libraries, copy shops, stationery stores, supermarkets, and a wide variety of other commercial facilities. Modern xerographic copiers, produced by a number of manufacturers, are available as desktop models suitable for the home as well as the small office. Many modern copiers reproduce in color as well as black and white, and office models can rival printing presses in speed of operation.

21. Photosensitive Detectors

Sensing radiation from ultraviolet to optical wavelengths and beyond is an important part of many devices. Whether analyzing the emission of radiation, chemical solutions, detecting lidar signals, fiber-optic communication systems, or imaging of medical ionizing radiation, detectors are the final link in any optoelectronic experiment or process.

Detectors fall into two groups: thermal detectors (where radiation is absorbed and the resulting temperature change is used to generate an electrical output) and photon (quantum) detectors. The operation of photon detectors is based on the photoelectric effect, in which the radiation is absorbed within a metal or semiconductor by direct interaction with electrons, which are excited to a higher energy level. Under the effect of an electric field these carriers move and produce a measurable electric current. The photon detectors show a selective wavelength-dependent response per unit incident radiation power.

22. Public and Private Lighting

At the turn of the 20th century, lighting was in a state of flux. In technical terms, a number of emerging lighting technologies jostled for economic dominance. In social terms, changing standards of illumination began to transform cities, the workplace, and the home. In design terms, the study of illumination as a science, as an engineering profession, and as an applied art was becoming firmly established. In the last decades of the 20th century, the technological and social choices in lighting attained considerable stability both technically and socially. Newer forms of compact fluorescent lighting, despite their greater efficiency, have not significantly replaced incandescent bulbs in homes owing to higher initial cost. Low-pressure sodium lamps, on the other hand, have been adopted increasingly for street and architectural lighting owing to lower replacement and maintenance costs. As with fluorescent lighting in the 1950s, recent lighting technologies have found niche markets rather than displacing incandescents, which have now been the dominant lighting system for well over a century.

23. Quantum Electronic Devices

Quantum theory, developed during the 1920s to explain the behavior of atoms and the absorption and emission of light, is thought to apply to every kind of physical system, from individual elementary particles to macroscopic systems such as lasers. In lasers, stimulated transitions between discrete or quantized energy levels is a quantum electronic phenomena (discussed in the entry Lasers, Theory and Operation). Stimulated transitions are also the central phenomena in atomic clocks. Semiconductor devices such as the transistor also rely on the arrangement of quantum energy levels into a valence band and a conduction band separated by an energy gap, but advanced quantum semiconductor devices were not possible until advances in fabrication techniques such as molecular beam epitaxy (MBE) developed in the 1960s made it possible to grow extremely pure single crystal semiconductor structures one atomic layer at a time.

In most electronic devices and integrated circuits, quantum phenomena such as quantum tunneling and electron diffraction—where electrons behave not as particles but as waves—are of no significance, since the device is much larger than the wavelength of the electron (around 100 nanometers, where one nanometer is 109 meters or about 4 atoms wide). Since the early 1980s however, researchers have been aware that as the overall device size of field effect transistors decreased, small-scale quantum mechanical effects between components, plus the limitations of materials and fabrication techniques, would sooner or later inhibit further reduction in the size of conventional semiconductor transistors. Thus to produce devices on ever-smaller integrated circuits (down to 25 nanometers in length), conventional microelectronic devices would have to be replaced with new device concepts that take advantage of the quantum mechanical effects that dominate on the nanometer scale, rather than function in despite of them. Such solid state ‘‘nanoelectronics’’ offers the potential for increased speed and density of information processing, but mass fabrication on this small scale presented formidable challenges at the end of the 20th century.

24. Quartz Clocks and Watches

The wristwatch and the domestic clock were completely reinvented with all-new electronic components beginning about 1960. In the new electronic timepieces, a tiny sliver of vibrating quartz in an electrical circuit provides the time base and replaces the traditional mechanical oscillator, the swinging pendulum in the clock or the balance wheel in the watch. Instead of an unwinding spring or a falling weight, batteries power these quartz clocks and watches, and integrated circuits substitute for intricate mechanical gear trains.

25. Radio-Frequency Electronics

Radio was originally conceived as a means for interpersonal communications, either person-toperson, or person-to-people, using analog waveforms containing either Morse code or actual sound. The use of radio frequencies (RF) designed to carry digital data in the form of binary code rather than voice and to replace physical wired connections between devices began in the 1970s, but the technology was not commercialized until the 1990s through digital cellular phone networks known as personal communications services (PCS) and an emerging group of wireless data network technologies just reaching commercial viability. The first of these is a so-called wireless personal area network (WPAN) technology known as Bluetooth. There are also two wireless local area networks (WLANs), generally grouped under the name Wi-Fi (wireless fidelity): (1) Wi-Fi, also known by its Institute of Electrical and Electronic Engineers (IEEE) designation 802.11b, and (2) Wi-Fi5 (802.11a).

26. Rectifiers

Rectifiers are electronic devices that are used to control the flow of current. They do this by having conducting and nonconducting states that depend on the polarity of the applied voltage. A major function in electronics is the conversion from alternating current (AC) to direct current (DC) where the output is only one-half (either positive or negative) of the input. Rectifiers that are currently, or have been, in use include: point-contact diodes, plate rectifiers, thermionic diodes, and semiconductor diodes. There are various ways in which rectifiers may be classified in terms of the signals they encounter; this contribution will consider two extremes—high frequency and heavy current—that make significantly different demands on device design.

27. Strobe Flashes

Scarcely a dozen years after photography was announced to the world in 1839, William Henry Fox Talbot produced the first known flash photograph. Talbot, the new art’s co-inventor, fastened a printed paper onto a disk, set it spinning as fast as possible, and then discharged a spark to expose a glass plate negative. The words on the paper could be read on the photograph. Talbot believed that the potential for combining electric sparks and photography was unlimited. In 1852, he pronounced, ‘‘It is in our power to obtain the pictures of all moving objects, no matter in how rapid motion they may be, provided we have the means of sufficiently illuminating them with a sudden electric flash.’’

The electronic stroboscope fulfills Talbot’s prediction. It is a repeating, short-duration light source used primarily for visual observation and photography of high-speed phenomena. The intensity of the light emitted from strobes also makes them useful as signal lights on communication towers, airport runways, emergency vehicles, and more. Though ‘‘stroboscope’’ actually refers to a repeating flash and ‘‘electronic flash’’ denotes a single burst, both types are commonly called ‘‘strobes.’’

28. Transistors

Early experiments in transistor technology were based on the analogy between the semiconductor and the vacuum tube: the ability to both amplify and effectively switch an electrical signal on or off (rectification). By 1940, Russell Ohl at Bell Telephone Laboratories, among others, had found that impure silicon had both positive (ptype material with holes) and negative (n-type) regions. When a junction is created between n-type material and p-type material, electrons on the ntype side are attracted across the junction to fill holes in the other layer. In this way, the n-type semiconductor becomes positively charged and the p-type becomes negatively charged. Holes move in the opposite direction, thus reinforcing the voltage built up at the junction. The key point is that current flows from one side to the other when a positive voltage is applied to the layers (‘‘forward biased’’).

29. Travelling Wave Tubes

One of the most important devices for the amplification of radio-frequency (RF) signals— which range in frequency from 3 kilohertz to 300 gigahertz—is the traveling wave tube (TWT). When matched with its power supply unit, or electronic power conditioner (EPC), the combination is known as a traveling wave tube amplifier (TWTA). The amplification of RF signals is important in many aspects of science and technology, since the ability to increase the strength of a very low-power input signal is fundamental to all types of long-range communications, radar and electronic warfare.

30. Vacuum Tubes/Valves

The vacuum tube has its roots in the late nineteenth century when Thomas A. Edison conducted experiments with electric bulbs in 1883. Edison’s light bulbs consisted of a conducting filament mounted in a glass bulb. Passing electricity through the filament caused it to heat up and radiate light. A vacuum in the tube prevented the filament from burning up. Edison noted that electric current would flow from the bulb filament to a positively charged metal plate inside the tube. This phenomenon, the one-way flow of current, was called the Edison Effect. Edison himself could not explain the filament’s behavior. He felt this effect was interesting but unimportant and patented it as a matter of course. It was only fifteen years later that Joseph John Thomson, a physics professor at the Cavendish Laboratory at the University of Cambridge in the U.K., discovered the electron and understood the significance of what was occurring in the tube. He identified the filament rays as a stream of particles, now called electrons. In a range of papers from 1901 to 1916, O.W. Richardson explained the electron behavior. Today the Edison Effect is known as thermionic emission.

History of Electronics

Few of the basic tasks that electronic technologies perform, such as communication, computation, amplification, or automatic control, are unique to electronics. Most were anticipated by the designers of mechanical or electromechanical technologies in earlier years. What distinguishes electronic communication, computation, and control is often linked to the instantaneous action of the devices, the delicacy of their actions compared to mechanical systems, their high reliability, or their tiny size.

The electronics systems introduced between the late nineteenth century and the end of the twentieth century can be roughly divided into the applications related to communications (including telegraphy, telephony, broadcasting, and remote detection) and the more recently developed fields involving digital information and computation. In recent years these two fields have tended to converge, but it is still useful to consider them separately for a discussion of their history.

The origins of electronics as distinguished from other electrical technologies can be traced to 1880 and the work of Thomas Edison. While investigating the phenomenon of the blackening of the inside surface of electric light bulbs, Edison built an experimental bulb that included a third, unused wire in addition to the two wires supporting the filament. When the lamp was operating, Edison detected a flow of electricity from the filament to the third wire, through the evacuated space in the bulb. He was unable to explain the phenomenon, and although he thought it would be useful in telegraphy, he failed to commercialize it. It went unexplained for about 20 years, until the advent of wireless telegraphic transmission by radio waves. John Ambrose Fleming, an experimenter in radio, not only explained the Edison effect but used it to detect radio waves. Fleming’s ‘‘valve’’ as he called it, acted like a one-way valve for electric waves, and could be used in a circuit to convert radio waves to electric pulses so that that incoming Morse code signals could be heard through a sounder or earphone.

As in the case of the Fleming valve, many early electronic devices were used first in the field of communications, mainly to enhance existing forms of technology. Initially, for example, telephony (1870s) and radio (1890s) were accomplished using ordinary electrical and electromechanical circuits, but eventually both were transformed through the use of electronic devices. Many inventors in the late nineteenth century sought a functional telephone ‘‘relay’’; that is, something to refresh a degraded telephone signal to allow long distance telephony. Several people simultaneously recognized the possibility of developing a relay based on the Fleming valve. The American inventor Lee de Forest was one of the first to announce an electronic amplifier using a modified Fleming valve, which he called the Audion. While he initially saw it as a detector and amplifier of radio waves, its successful commercialization occurred first in the telephone industry. The sound quality and long-distance capability of telephony was enhanced and extended after the introduction of the first electronic amplifier circuits in 1907. In the U.S., where vast geographic distances separated the population, the American Telephone and Telegraph Company (AT&T) introduced improved vacuum tube amplifiers in 1913, which were later used to establish the first coast-to-coast telephone service in 1915 (an overland distance of nearly 5000 kilometers).

These vacuum tubes soon saw many other uses, such as a public-address systems constructed as early as 1920, and radio transmitters and receivers. The convergence of telephony and radio in the form of voice broadcasting was technically possible before the advent of electronics, but its application was greatly enhanced through the use of electronics both in the radio transmitter and in the receiver.

World War I saw the applications of electronics diversify somewhat to include military applications. Mostly, these were modifications of existing telegraph, telephone, and radio systems, but applications such as ground-to-air radio telephony were novel. The pressing need for large numbers of electronic components, especially vacuum tubes suitable for military use, stimulated changes in their design and manufacture and contributed to improving quality and falling prices. After the war, the expanded capacity of the vacuum tube industry contributed to a boom in low-cost consumer radio receivers. Yet because of the withdrawal of the military stimulus and the onset of the Great Depression, the pace of change slowed in the 1930s. One notable exception was in the field of television. Radio broadcasting became such a phenomenal commercial success that engineers and businessmen were envisioning how ‘‘pictures with sound’’ would replace ordinary broadcasting, even in the early 1930s. Germany, Great Britain, and the U.S. all had rudimentary television systems in place by 1939, although World War II would bring nearly a complete halt to these early TV broadcasts.

World War II saw another period of rapid change, this one much more dramatic than that of World War I. Not only were radio communications systems again greatly improved, but for the first time the field of electronics engineering came to encompass much more than communication. While it was the atomic bomb that is most commonly cited as the major technological outcome of World War II, radar should probably be called the weapon that won the war. To describe radar as a weapon is somewhat inaccurate, but there is no doubt that it had profound effects upon the way that naval, aerial, and ground combat was conducted. Using radio waves as a sort of searchlight, radar could act as an artificial eye capable of seeing through clouds or fog, over the horizon, or in the dark. Furthermore, it substituted for existing methods of calculating the distance and speed of targets. Radar’s success hinged on the development of new electronic components, particularly new kinds of vacuum tubes such as the klystron and magnetron, which were oriented toward the generation of microwaves. Subsidized by military agencies on both sides of the Atlantic (as well as Japan) during World War II, radar sets were eventually installed in aircraft and ships, used in ground stations, and even built into artillery shells. The remarkable engineering effort that was launched to make radar systems smaller, more energy efficient, and more reliable would mark the beginning of an international research program in electronics miniaturization that continues today. Radar technology also had many unexpected applications elsewhere, such as the use of microwave beams as a substitute for long-distance telephone cables. Microwave communication is also used extensively today for satellite-to-earth communication.

The second major outcome of electronics research during World War II was the effort to build an electronic computer. Mechanical adders and calculators were widely used in science, business, and government by the early twentieth century, and had reached an advanced state of design. Yet the problems peculiar to wartime, especially the rapid calculation of mountains of ballistics data, drove engineers to look for ways to speed up the machines. At the same time, some sought a calculator that could be reprogrammed as computational needs changed. While computers played a role in the war, it was not until the postwar period that they came into their own. In addition, computer research during World War II contributed little to the development of vacuum tubes, although in later years computer research would drive certain areas of semiconductor electron device research.

While the forces of the free market are not to be discounted, the role of the military in electronics development during World War II was of paramount importance. More-or-less continuous military support for research in electronic devices and systems persisted during the second half of the twentieth century too, and many more new technologies emerged from this effort. The sustained effort to develop more compact, rugged devices such as those demanded by military systems would converge with computer development during the 1950s, especially after the invention of the transistor in late 1947.

The transistor was not a product of the war, and in fact its development started in the 1930s and was delayed by the war effort. A transistor is simply a very small substitute for a vacuum tube, but beyond that it is an almost entirely new sort of device. At the time of its invention, its energy efficiency, reliability, and diminutive size suggested new possibilities for electronic systems. The most famous of these possibilities was related to computers and systems derived from or related to computers, such as robotics or industrial automation. The impetus for the transistor was a desire within the telephone industry to create an energy-efficient, reliable substitute for the vacuum tube. Once introduced, the military pressed hard to accelerate its development, as the need emerged for improved electronic navigational devices for aircraft and missiles.

There were many unanticipated results of the substitution of transistors for vacuum tubes. Because they were so energy efficient, transistors made it much more practical to design battery powered systems. The small transistor radio (known in some countries simply as ‘‘the transistor’’), introduced in the 1950s, is credited with helping to popularize rock and roll music. It is also worth noting that many developing countries could not easily provide broadcasting services until the diffusion of battery operated transistor receivers because of the lack of central station electric power. The use of the transistor also allowed designers to enhance existing automotive radios and tape players, contributing eventually to a greatly expanded culture of in-car listening. There were other important outcomes as well; transistor manufacture provided access to the global electronics market for Asian radio manufacturers, who improved manufacturing methods to undercut their U.S. competitors during the 1950s and 1960s. Further, the transistor’s high reliability nearly eliminated the profession of television and radio repair, which had supported tens of thousands of technicians in the U.S. alone before about 1980.

However, for all its remarkable features, the transistor also had its limitations; while it was an essential part of nearly every cutting-edge technology of the postwar period, it was easily outperformed by the older technology of vacuum tubes in some areas. The high-power microwave transmitting devices in communications satellites and spacecraft, for example, nearly all relied on special vacuum tubes through the end of the twentieth century, because of the physical limitations of semiconductor devices. For the most part, however, the transistor made the vacuum tube obsolete by about 1960.

The attention paid to the transistor in the 1950s and 1960s made the phrase ‘‘solid-state’’ familiar to the general public, and the new device spawned many new companies. However, its overall impact pales in comparison to its successor—the integrated circuit. Integrated circuits emerged in the late 1950s, were immediately adopted by the military for small computer and communications systems, and were then used in civilian computers and related applications from the 1960s. Integrated circuits consist of multiple transistors fabricated simultaneously from layers of semiconductor and other materials. The transistors, interconnecting ‘‘wires,’’ and many of the necessary circuit elements such as capacitors and resistors are fabricated on the ‘‘chip.’’ Such a circuit eliminates much of the laborious process of assembling an electronic system such as a computer by hand, and results in a much smaller product. The ability to miniaturize components through integrated circuit fabrication techniques would lead to circuits so vanishingly small that it became difficult to connect them to the systems of which they were a part. The plastic housings or ‘‘packages’’ containing today’s microprocessor chips measure just a few centimeters on a side, and yet the actual circuits inside are much smaller. Some of the most complex chips made today contain many millions of transistors, plus millions more solid-state resistors and other passive components.

While used extensively in military and aerospace applications, the integrated circuit became famous as a component in computer systems. The logic and memory circuits of digital computers, which have been the focus of much research, consist mainly of switching devices. Computers were first constructed in the 1930s with electromechanical relays as switching devices, then with vacuum tubes, transistors, and finally integrated circuits. Most early computers used off-the-shelf tubes and transistors, but with the advent of the integrated circuit, designers began to call for components designed especially for computers. It was clear to engineers at the time that all the circuits necessary to build a computer could be placed on one chip (or a small set of chips), and in fact, the desire to create a ‘‘computer on a chip’’ led to the microprocessor, introduced around 1970. The commercial impetus underlying later generations of computer chip design was not simply miniaturization (although there are important exceptions) or energy efficiency, but also the speed of operation, reliability, and lower cost. However, the inherent energy efficiency and small size of the resulting systems did enable the construction of smaller computers, and the incorporation of programmable controllers (special purpose computers) into a wide variety of other technologies. The recent merging of the computer (or computer-like systems) with so many other technologies makes it difficult to summarize the current status of digital electronic systems. As the twentieth century drew to a close, computer chips were widely in use in communications and entertainment devices, in industrial robots, in automobiles, in household appliances, in telephone calling cards, in traffic signals, and in a myriad other places. The rapid evolution of the computer during the last 50 years of the twentieth century was reflected by the near-meaninglessness of its name, which no longer adequately described its functions.

From an engineering perspective, not only did electronics begin to inhabit, in an almost symbiotic fashion, other technological systems after about 1950, but these electronics systems were increasingly dominated by the use of semiconductor technology. After virtually supplanting the vacuum tube in the 1950s, the semiconductor-based transistor became the technology of choice for most subsequent electronics development projects. Yet semiconducting alloys and compounds proved remarkably versatile in applications at first unrelated to transistors and chips. The laser, for example, was originally operated in a large vacuum chamber and depended on ionized gas for its operation. By the 1960s, laser research was focused on the remarkable ability of certain semiconducting materials to accomplish the same task as the ion chamber version. Today semiconductor devices are used not only as the basis of amplifiers and switches, but also for sensing light, heat, and pressure, for emitting light (as in lasers or video displays), for generating electricity (as in solar cells), and even for mechanical motion (as in micromechanical systems or MEMS).

However, semiconductor devices in ‘‘discrete’’ forms such as transistors, would probably not have had the remarkable impact of the integrated circuit. By the 1970s, when the manufacturing techniques for integrated circuits allowed high volume production, low cost, tiny size, relatively small energy needs, and enormous complexity; electronics entered a new phase of its history, having a chief characteristic of allowing electronic systems to be retrofitted into existing technologies. Low-cost microprocessors, for example, which were available from the late 1970s onward, were used to sense data from their environment, measure it, and use it to control various technological systems from coffee machines to video tape recorders. Even the human body is increasingly invaded by electronics; at the end of the twentieth century, several researchers announced the first microchips for implantation directly in the body. They were to be used to store information for retrieval by external sensors or to help deliver subcutaneous drugs. The integrated circuit has thus become part of innumerable technological and biological systems.

It is this remarkable flexibility of application that enabled designers of electronic systems to make electronics the defining technology of the late twentieth century, eclipsing both the mechanical technologies associated with the industrial revolution and the electrical and information technologies of the so-called second industrial revolution. While many in the post-World War II era once referred to an ‘‘atomic age,’’ it was in fact an era in which daily life was increasingly dominated by electronics.

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Five lessons to level up conservation successfully

Conservation needs to scale successfully to protect nature. A new paper takes lessons from around the world to show how that might be done.

To reverse biodiversity loss and meet ambitious global targets, conservation programmes designed to preserve everything from forests to fish need to work 'at scale'.

Scaling can mean three things. Scaling 'out' means expanding a programme to new people and places, while scaling 'up' means bringing in higher-level institutions, such as governments introducing policies or incentives that make it easier for individuals and private companies to engage.

Scaling 'deep' means changing hearts and minds -- what is socially acceptable. A particularly good example of scaling deep is the 'Don't Mess with Texas' campaign in the 1980s, which successfully made littering a social no-no.

But not every attempt to expand pilot programmes in one or more of these directions works. Now, the Catalysing Conservation team led by Dr Morena Mills at Imperial College London researchers have reviewed conservation initiatives around the world with global experts and come up with five lessons to avoid the pitfalls of ineffective expansion.

The study is published today in Nature Ecology & Evolution , and we spoke to two of the authors on the paper, Dr Thomas Pienkowski and Dr Matthew Clark, both from the Centre for Environmental Policy at Imperial.

Before we dive in, Dr Clark says: "There's no magic bullet -- it's not a case of 'do these five things and you will succeed' -- but we hope these lessons will allow reflection on what hasn't worked, and where we need to go from here."

Lesson 1: There must be a balance between what is effective and what is scalable

Say you have a pilot programme that works with coastal communities to protect fish and other marine resources, aiming to improve the local ecology and economy. Then lots of neighbouring communities take up the programme. Great! This is scaling out, but has it actually been effective? Did it meet the stated goals of protecting marine life and improving local livelihoods? If the answer is no, it has scaled but it is not effective.

Conversely, something can be effective but not scalable. Dr Clark works with communities to support mangrove conservation, which can involve planting programmes. However, many of the seedlings die young. It's possible to use specialised tools and know-how to increase survival rates, which makes the planting more effective, but it is an intensive process, and so not very scalable across rural communities.

The team say these trade-offs between what's scalable and what's effective must be balanced.

Good example: Community-based forestry management in Nepal has been adopted for more than 20,000 forests since the 1980s and appears to have reduced both poverty and deforestation, showing that some initiatives can be both highly scalable and effective.

Lesson 2: Effectiveness can depend on scale

A pilot project that is successful in one area may not work when moved out to a new area. This is common, say the researchers, and can be for a number of reasons: pilots may be in optimal locations and have lots of oversight and investment that expanded programmes won't have, for example.

But it can also work the other way. For example, says Dr Clark: "Where the goal is to protect land for wildlife, larger animals that move over larger areas will only benefit once enough land is conserved, and enough patrols are in place to enforce the protection."

Good example: Cacao agroforestry in Belize became much more effective at scale when a clear market for sustainable cacao emerged and more international companies wanted to promote their use of these products.

Lesson 3: The effects of conservation can change the conditions for further conservation

Sometimes, conservation expansion can backfire even when it's effective. For example, a 10-year project in Mozambique introduced 'no-take' zones for fish and mangrove timber, which increased food security. However, once these areas had regained their value as sources of food and income, conservation support declined, leading to the abandonment of the zones in some areas.

These kinds of feedback loops between environmental change and human behaviour can be negative, as in Mozambique, or positive, where the impact of conservation schemes in one area can lead to neighbouring areas taking them up spontaneously, or where grassroots actions become national policy.

Good example: on the island of Pemba, Zanzibar, protected forest areas initially led to more harvesting on the edges of these zones; but this in turn led to neighbouring communities applying for their own forest protection, spontaneously expanding conversation.

Lesson 4: Pressures to scale can lead to bad practices that undermine long-term outcomes

Ambition is needed to meet ambitious goals, but ambition without care can be harmful. Dr Pienkowski explains one way this can happen: "NGOs [non-governmental organisations] play a really important role in scaling out, providing technical and financial support to local communities. But there can also be blurred boundaries between assistance and coercion.

"This can take the form, for example, of NGOs misleading communities of the benefits they might get from engaging in conservation programmes, or only engaging with people in the community who are most likely to benefit, leaving more vulnerable members behind and widening inequalities."

For example, the REDD+ scheme is designed to help developing countries manage their forests and improve carbon stocks, but its implementation in parts of Tanzania was marred by promised payments not materialising, leading people to abandon conservation efforts and be suspicious of other schemes.

Larger NGOs are often needed to scale programmes, but this can be at the expense of local knowledge and grassroots organisations. For example, 'slash and burn' agriculture is considered bad practice in Europe, so European NGOs may lobby against it, but in communities in Africa it can be well used and an integral part of local ecosystem management.

Good example: Eco-tourism in Costa Rica started locally with support from NGOs, but has now become self-sustaining, meaning it no longer relies on direct aid or other structures that may undermine its long-term success.

Lesson 5: More evidence is needed

Dr Pienkowski explains: "This one is really an appeal from us researchers, who are struggling to develop the evidence base we need to inform more effective scaling strategies. It's very difficult to know which initiatives have gone to scale or not -- this information isn't collected in a systematic or rigorous way."

This is particularly true after programmes have 'ended' -- few NGOs routinely review whether a scheme is still working years after their intervention has ended, or whether it has been abandoned.

Dr Pienkowski concludes: "For those calling for conservation scaling, this is a valuable moment to pause and reflect: with these examples and these lessons, what do we need to change? If we do this, we're more likely to be able to deliver impact at scale and finally bend the curve on biodiversity loss."

• Endangered Plants
• Sustainability
• Environmental Policy
• Land Management
• Legal Issues
• Funding Policy
• Conservation ethic
• Marine conservation
• Biodiversity Action Plan
• Conservation status
• Giant Sequoia tree
• Fujita scale
• Richter magnitude scale

Story Source:

Materials provided by Imperial College London . Original written by Hayley Dunning. Note: Content may be edited for style and length.

Journal Reference :

• Thomas Pienkowski, Arundhati Jagadish, Willow Battista, Gloria Christelle Blaise, Alec Philip Christie, Matt Clark, Antony Philip Emenyu, Abha Joglekar, Kristian Steensen Nielsen, Tom Powell, Thomas White, Morena Mills. Five lessons for avoiding failure when scaling in conservation . Nature Ecology & Evolution , 2024; DOI: 10.1038/s41559-024-02507-4

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