A physically transient form of silicon electronics

Highly flexible and degradable memory electronics comprised of all-biocompatible materials

Biocompatible materials have received increasing attention as one of the most important building blocks for flexible and transient memories. Herein, a fully biocompatible resistive switching (RS) memory electronic composed of a carbon dot (CD)-polyvinyl pyrrolidone (PVP) nanocomposite and a silver nanowire (Ag NW) network buried in a flexible gelatin film is introduced with promising nonvolatile RS characteristics for flexible and transient memory applications. The fabricated device exhibited a rewritable flash-type memory behavior, such as low operation voltage (≈-1.12 V), high ON/OFF ratio (>10²), long retention time (over 10⁴ s), and small bending radius (15 mm). As a proof of degradability, this transient memory can dissolve completely within 90 s after being immersed into deionized water at 55 °C; it can decompose naturally in soil within 6 days. This fully biocompatible memory electronic paves a novel way for flexible and wearable green electronics.

Phototriggerable Transient Electronics via Fullerene-Mediated Degradation of Polymer:Fullerene Encapsulation Layer

Transient electronics is an emerging class of electronics that has attracted a lot of attention because of its potential as an environmental-friendly alternative to the existing end-of-life product disposal or treatments. However, the controlled degradation of transient electronics under environmentally benign conditions remains a challenge. In this work, the tunable degradation of transient electronics including passive resistor devices and active memory devices was realized by photodegradable thin polymer films comprising fullerene derivatives, [6,6]-phenyl-C61-butyric acid methyl esters (PCBM). The photodegradation of polymer:PCBM under an aqueous environment is triggered by ultraviolet (UV) light. Experimental results demonstrate that the addition of PCBM in commodity polymers, including but not limited to polystyrene, results in a catalytic effect on polymer photodegradation when triggered by UV light. The degradation mechanism of transient electronics is ascribed to the photodegradation of polymer:PCBM encapsulation layers caused by the synergistic effect between UV and water exposure. The polymer:PCBM encapsulation system presented herein offers a simple way to achieve the realization of light-triggered device degradation for bioapplication and expands the material options for tailorable degradation of transient electronics.

A thin, deformable, high-performance supercapacitor implant that can be biodegraded and bioabsorbed within an animal body

It has been an outstanding challenge to achieve implantable energy modules that are mechanically soft (compatible with soft organs and tissues), have compact form factors, and are biodegradable (present for a desired time frame to power biodegradable, implantable medical electronics). Here, we present a fully biodegradable and bioabsorbable high-performance supercapacitor implant, which is lightweight and has a thin structure, mechanical flexibility, tunable degradation duration, and biocompatibility. The supercapacitor with a high areal capacitance (112.5 mF cm^-2 at 1 mA cm^-2) and energy density (15.64 μWh cm^-2) uses two-dimensional, amorphous molybdenum oxide (MoO _x ) flakes as electrodes, which are grown in situ on water-soluble Mo foil using a green electrochemical strategy. Biodegradation behaviors and biocompatibility of the associated materials and the supercapacitor implant are systematically studied. Demonstrations of a supercapacitor implant that powers several electronic devices and that is completely degraded after implantation and absorbed in rat body shed light on its potential uses.

Wearable sensors: At the frontier of personalised health monitoring, smart prosthetics and assistive technologies

Wearable sensors have evolved from body-worn fitness tracking devices to multifunctional, highly integrated, compact, and versatile sensors, which can be mounted onto the desired locations of our clothes or body to continuously monitor our body signals, and better interact and communicate with our surrounding environment or equipment. Here, we discuss the latest advances in textile-based and skin-like wearable sensors with a focus on three areas, including (i) personalised health monitoring to facilitate recording physiological signals, body motions, and analysis of body fluids, (ii) smart gloves and prosthetics to realise the sensation of touch and pain, and (iii) assistive technologies to enable disabled people to operate the surrounding motorised equipment using their active organs. We also discuss areas for future research in this emerging field.

Flexible Cyclic-Poly(phthalaldehyde)/Poly(ε-caprolactone) Blend Fibers with Fast Daylight-Triggered Transience

Cyclic-poly(phthalaldehyde) (cPPHA) exhibits photo-triggerable depolymerization on-demand for applications like the photolithography of microfabricated electronics. However, cPPHA is inherently brittle and thermally sensitive; both of these properties limit its usefulness as an engineering plastic. Prior to this report, small molecule plasticizers are added to cPPHA-based films to make the polymer more flexible. But plasticizers can eventually leach out of cPPHA, then leaving it increasingly more brittle throughout product lifetime. In this research, a new approach to fabricating flexible cPPHA blends for use as spun fibers is achieved through the incorporation of poly (ε-caprolactone) (PCL) by a modified wet spinning method. Among blend compositions, the 50/50 cPPHA/PCL fiber shows fast transience (<50 s) in response to daylight while retaining the flexibility of PCL and mechanical properties of an elastomer (i.e., tensile strength of ≈8 MPa, Young's modulus of ≈118 MPa, and elongation at break of ≈190%). Embedding 2 wt% gold nanoparticles to cPPHA can further improve the transience rate of fibers comprising less than 50% cPPHA. These flexible, daylight-triggerable cPPHA/PCL fibers can be applied to an extensive range of applications, such as wearable electronics, intelligent textiles, and zero waste packaging for which modest mechanical performance and fast transience are desired.

Stretchable, Healable, and Degradable Soft Ionic Microdevices Based on Multifunctional Soaking-Toughened Dual-Dynamic-Network Organohydrogel Electrolytes

Electronic materials and devices that can mimic biological systems featured with elasticity, toughness, self-healing, degradability, and environmental friendliness drive the technological developments in fields spanning from bioelectronics, biomedical diagnosis and therapy, electronic skin, and soft robotics to Internet-of-Things with "green" electronics. Among them, ionic devices based on gel electrolytes have emerged as attractive candidates for biomimetic systems. Herein, we presented a straightforward approach to demonstrate soft ionic microdevices based on a versatile organohydrogel platform acting as both a free-standing, stretchable, adhesive, healable, and entirely degradable support and a highly conductive, dehydration- and freezing-tolerant electrolyte. This is achieved by forming a gelatin/ferric-ion-cross-linked polyacrylic acid (GEL/PAA) dual dynamic supramolecular network followed by soaking into a NaCl glycerol/water solution to further toughen the gelatin network via solvent displacement, thus obtaining a high toughness of 1.34 MJ·cm^-3 and a high ionic conductivity (>7 mS·cm^-1). Highly stretchable and multifunctional ionic microdevices are then fabricated based on the organohydrogel electrolytes by simple transfer printing of carbon-based microelectrodes onto the prestretched gel surface. Proof-of-concept microdevices including resistive strain sensors and microsupercapacitors are demonstrated, which displayed outstanding stretchability to 300% strain, resistance to dehydration for >6 months, autonomous self-healing, and rapid room-temperature degradation within hours. The present material design and fabrication approach for the organohydrogel-based ionic microdevices will provide promising scope for life-like and sustainable electronic systems.

How is flexible electronics advancing neuroscience research?

Innovative neurotechnology must be leveraged to experimentally answer the multitude of pressing questions in modern neuroscience. Driven by the desire to address the existing neuroscience problems with newly engineered tools, we discuss in this review the benefits of flexible electronics for neuroscience studies. We first introduce the concept and define the properties of flexible and stretchable electronics. We then categorize the four dimensions where flexible electronics meets the demands of modern neuroscience: chronic stability, interfacing multiple structures, multi-modal compatibility, and neuron-type-specific recording. Specifically, with the bending stiffness now approaching that of neural tissue, implanted flexible electronic devices produce little shear motion, minimizing chronic immune responses and enabling recording and stimulation for months, and even years. The unique mechanical properties of flexible electronics also allow for intimate conformation to the brain, the spinal cord, peripheral nerves, and the retina. Moreover, flexible electronics enables optogenetic stimulation, microfluidic drug delivery, and neural activity imaging during electrical stimulation and recording. Finally, flexible electronics can enable neuron-type identification through analysis of high-fidelity recorded action potentials facilitated by its seamless integration with the neural circuitry. We argue that flexible electronics will play an increasingly important role in neuroscience studies and neurological therapies via the fabrication of neuromorphic devices on flexible substrates and the development of enhanced methods of neuronal interpenetration.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.

Advanced Materials and Systems for Biodegradable, Transient Electronics

Transient electronics refers to an emerging class of advanced technology, defined by an ability to chemically or physically dissolve, disintegrate, and degrade in actively or passively controlled fashions to leave environmentally and physiologically harmless by-products in environments, particularly in bio-fluids or aqueous solutions. The unusual properties that are opposite to operational modes in conventional electronics for a nearly infinite time frame offer unprecedented opportunities in research areas of eco-friendly electronics, temporary biomedical implants, data-secure hardware systems, and others. This review highlights the developments of transient electronics, including materials, manufacturing strategies, electronic components, and transient kinetics, along with various potential applications.

Biodegradable Optical Fiber in a Soft Optoelectronic Device for Wireless Optogenetic Applications

Optogenetics is a new neuroscience technology that uses light-responsive proteins to stimulate neurons with light and control the emotions and/or behavior of animals. There are a few approaches to deliver light to neurons in vivo, including a using an optical fiber that can send light from an external source to a target neuron, directly inserting a light-emitting device, and shooting light to penetrate tissue from the outside. Among these methods, inserting a wireless light-emitting device that is capable of being used for an experiment while leaving an animal completely free is a method that has been studied in recent years. At the same time, the possibility of causing mechanical and thermal damage to neural tissues has been highlighted as an issue due to the stiffness of robust injection tools and the photoelectric efficiency of light-emitting diodes (LEDs). In this study, we developed a device that can send light from a wireless light-emitting device to a target neuron without mechanical and thermal effects and analyzed the optical and thermal characteristics of the device to be used for optogenetic studies.

Stretchable, dynamic covalent polymers for soft, long-lived bioresorbable electronic stimulators designed to facilitate neuromuscular regeneration

Bioresorbable electronic stimulators are of rapidly growing interest as unusual therapeutic platforms, i.e., bioelectronic medicines, for treating disease states, accelerating wound healing processes and eliminating infections. Here, we present advanced materials that support operation in these systems over clinically relevant timeframes, ultimately bioresorbing harmlessly to benign products without residues, to eliminate the need for surgical extraction. Our findings overcome key challenges of bioresorbable electronic devices by realizing lifetimes that match clinical needs. The devices exploit a bioresorbable dynamic covalent polymer that facilitates tight bonding to itself and other surfaces, as a soft, elastic substrate and encapsulation coating for wireless electronic components. We describe the underlying features and chemical design considerations for this polymer, and the biocompatibility of its constituent materials. In devices with optimized, wireless designs, these polymers enable stable, long-lived operation as distal stimulators in a rat model of peripheral nerve injuries, thereby demonstrating the potential of programmable long-term electrical stimulation for maintaining muscle receptivity and enhancing functional recovery.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

High precision epidermal radio frequency antenna via nanofiber network for wireless stretchable multifunction electronics

Recently, stretchable electronics combined with wireless technology have been crucial for realizing efficient human-machine interaction. Here, we demonstrate highly stretchable transparent wireless electronics composed of Ag nanofibers coils and functional electronic components for power transfer and information communication. Inspired by natural systems, various patterned Ag nanofibers electrodes with a net structure are fabricated via using lithography and wet etching. The device design is optimized by analyzing the quality factor and radio frequency properties of the coil, considering the effects of strain. Particularly, the wireless transmission efficiency of a five-turn coil drops by approximately only 50% at 10 MHz with the strain of 100%. Moreover, various complex functional wireless electronics are developed using near-field communication and frequency modulation technology for applications in content recognition and long-distance transmission (>1 m), respectively. In summary, the proposed device has considerable potential for applications in artificial electronic skins, human healthcare monitoring and soft robotics.

A rewritable optical storage medium of silk proteins using near-field nano-optics

Nanoscale lithography and information storage in biocompatible materials offer possibilities for applications such as bioelectronics and degradable electronics for which traditional semiconductor fabrication techniques cannot be used. Silk fibroin, a natural protein renowned for its strength and biocompatibility, has been widely studied in this context. Here, we present the use of silk film as a biofunctional medium for nanolithography and data storage. Using tip-enhanced near-field infrared nanolithography, we demonstrate versatile manipulation and characterize the topography and conformation of the silk in situ. In particular, we fabricate greyscale and dual-tone nanopatterns with full-width at half-maximum resolutions of ~35 nm, creating an erasable 'silk drive' that digital data can be written to or read from. As an optical storage medium, the silk drive can store digital and biological information with a capacity of ~64 GB inch^-2 and exhibits long-term stability under various harsh conditions. As a proof-of-principle demonstration, we show that this silk drive can be biofunctionalized to exhibit chromogenic reactions, resistance to bacterial infection and heat-triggered, enzyme-assisted decomposition.

Hydroxypropyl Cellulose Adhesives for Transfer Printing of Carbon Nanotubes and Metallic Nanostructures

Transfer printing is one of the key nanofabrication techniques for the large-scale manufacturing of complex device architectures. It provides a cost-effective and high-throughput route for the integration of independently processed materials into spatially tailored architectures. Furthermore, this method enables the fabrication of flexible and curvilinear devices, paving the way for the fabrication of a new generation of technologies for optics, electronics, and biomedicine. In this work, hydroxypropyl cellulose (HPC) membranes are used as water soluble adhesives for transfer printing processes with improved performance and versatility compared to conventional silicone alternatives. The high-water solubility and excellent mechanical properties of HPC facilitate transfer printing with high yield for both metal and carbon nanotubes (CNTs) inks. In the case of metal inks, crack-free stripping of silver films and the simple fabrication of Moiré Plasmonic architectures of different geometries are demonstrated. Furthermore, HPC membranes are used to transfer print carbon nanotube films with different thicknesses and up to 77% transparency in the visible and near infrared region with potential applications as transparent conductive substrates. Finally, the use of prepatterned HPC membranes enables nanoscale patterning of CNT with feature resolution down to 1 µm.

Expandable and implantable bioelectronic complex for analyzing and regulating real-time activity of the urinary bladder

Underactive bladder or detrusor underactivity (DUA), that is, not being able to micturate, has received less attention with little research and remains unknown or limited on pathological causes and treatments as opposed to overactive bladder, although the syndrome may pose a risk of urinary infections or life-threatening kidney damage. Here, we present an integrated expandable electronic and optoelectronic complex that behaves as a single body with the elastic, time-dynamic urinary bladder with substantial volume changes up to ~300%. The system configuration of the electronics validated by the theoretical model allows conformal, seamless integration onto the urinary bladder without a glue or suture, enabling precise monitoring with various electrical components for real-time status and efficient optogenetic manipulation for urination at the desired time. In vivo experiments using diabetic DUA models demonstrate the possibility for practical uses of high-fidelity electronics in clinical trials associated with the bladder and other elastic organs.

Heterogeneous integration of rigid, soft, and liquid materials for self-healable, recyclable, and reconfigurable wearable electronics

Wearable electronics can be integrated with the human body for monitoring physical activities and health conditions, for human-computer interfaces, and for virtual/augmented reality. We here report a multifunctional wearable electronic system that combines advances in materials, chemistry, and mechanics to enable superior stretchability, self-healability, recyclability, and reconfigurability. This electronic system heterogeneously integrates rigid, soft, and liquid materials through a low-cost fabrication method. The properties reported in this wearable electronic system can find applications in many areas, including health care, robotics, and prosthetics, and can benefit the well-being, economy, and sustainability of our society.

Sensors Made of Natural Renewable Materials: Efficiency, Recyclability or Biodegradability-The Green Electronics

Nowadays, sensor devices are developing fast. It is therefore critical, at a time when the availability and recyclability of materials are, along with acceptability from the consumers, among the most important criteria used by industrials before pushing a device to market, to review the most recent advances related to functional electronic materials, substrates or packaging materials with natural origins and/or presenting good recyclability. This review proposes, in the first section, passive materials used as substrates, supporting matrixes or packaging, whether organic or inorganic, then active materials such as conductors or semiconductors. The last section is dedicated to the review of pertinent sensors and devices integrated in sensors, along with their fabrication methods.

Resilient yet entirely degradable gelatin-based biogels for soft robots and electronics

Biodegradable and biocompatible elastic materials for soft robotics, tissue engineering or stretchable electronics with good mechanical properties, tunability, modifiability or healing properties drive technological advance, and yet they are not durable under ambient conditions and do not combine all the attributes in a single platform. We have developed a versatile gelatin-based biogel, which is highly resilient with outstanding elastic characteristics, yet degrades fully when disposed. It self-adheres, is rapidly healable and derived entirely from natural and food-safe constituents. We merge all the favourable attributes in one material that is easy to reproduce and scalable, and has a low-cost production under ambient conditions. This biogel is a step towards durable, life-like soft robotic and electronic systems that are sustainable and closely mimic their natural antetypes.

Naturally Degradable Photonic Devices with Transient Function by Heterostructured Waxy-Sublimating and Water-Soluble Materials

Combined dry-wet transient materials and devices are introduced, which are based on water-dissolvable dye-doped polymers layered onto nonpolar cyclic hydrocarbon sublimating substrates. Light-emitting heterostructures showing amplified spontaneous emission are obtained on transient elements and used as illumination sources for speckle-free, full-field imaging, and transient optical labels are realized that incorporate QR-codes with stably encoded information. The transient behavior is also studied at the microscopic scale, highlighting the real-time evolution of material domains in the sublimating compound. Finally, the exhausted components are fully soluble in water thus being naturally degradable. This technology opens new and versatile routes for environmental sensing, storage conditions monitoring, and organic photonics.

Highly Conductive Collagen by Low-Temperature Atomic Layer Deposition of Platinum

In modern biomaterial-based electronics, conductive and flexible biomaterials are gaining increasing attention for their wide range of applications in biomedical and wearable electronics industries. The ecofriendly, biodegradable, and self-resorbable nature of these materials makes them an excellent choice in fabricating green and transient electronics. Surface functionalization of these biomaterials is required to cater to the need of designing electronics based on these substrate materials. In this work, a low-temperature atomic layer deposition (ALD) process of platinum (Pt) is presented to deposit a conductive thin film on collagen biomaterials, for the first time. Surface characterization revealed that a very thin ALD-deposited seed layer of TiO₂ on the collagen surface prior to Pt deposition is an alternative for achieving a better nucleation and 100% surface coverage of ultrathin Pt on collagen surfaces. The presence of a pure metallic Pt thin film was confirmed from surface chemical characterization. Electrical characterization proved the existence of a continuous and conductive Pt thin film (∼27.8 ± 1.4 nm) on collagen with a resistivity of 295 ± 30 μΩ cm, which occurred because of the virtue of TiO₂. Analysis of its electronic structures showed that the presence of metastable state due to the presence of TiO₂ enables electrons to easily flow from valence into conductive bands. As a result, this turned collagen into a flexible conductive biomaterial.

Hot electrons in a nanowire hard X-ray detector

Nanowire chip-based electrical and optical devices such as biochemical sensors, physical detectors, or light emitters combine outstanding functionality with a small footprint, reducing expensive material and energy consumption. The core functionality of many nanowire-based devices is embedded in their p-n junctions. To fully unleash their potential, such nanowire-based devices require - besides a high performance - stability and reliability. Here, we report on an axial p-n junction GaAs nanowire X-ray detector that enables ultra-high spatial resolution (~200 nm) compared to micron scale conventional ones. In-operando X-ray analytical techniques based on a focused synchrotron X-ray nanobeam allow probing the internal electrical field and observing hot electron effects at the nanoscale. Finally, we study device stability and find a selective hot electron induced oxidization in the n-doped segment of the p-n junction. Our findings demonstrate capabilities and limitations of p-n junction nanowires, providing insight for further improvement and eventual integration into on-chip devices.

Controlling the dissolution of iron through the development of nanostructured Fe-Mg for biomedical applications

In the field of biodegradable metallic materials, rapid and non-uniform biodegradation, caused by uncontrolled corrosion rates, is a potential shortcoming. Among the prominent biodegradable materials, magnesium is an attractive choice, however, it is prone to rapid dissolution. In contrast, iron possesses a slow dissolution rate. To approach the middle ground, instead of making magnesium more corrosion-resistant, the less-explored approach of making iron less corrosion-resistant is employed here. In this study, iron, and magnesium, having contrasting corrosion rates, are combined via magnetron co-sputtering. The idea of combinatorial synthesis is employed to fabricate two model nanostructured Fe-Mg samples, i.e. CSFM-1 (Fe₈₅Mg₁₅), and CSFM-2 (Fe₆₅Mg₃₅), exhibiting a controlled and uniform degradation in phosphate-buffer saline solution. The structural characterization of the two samples demonstrates a substitutional solid solution of bcc-Fe-Mg in CSFM-1 and an amorphous short-range-ordered structure in the CSFM-2 sample. Electrochemical investigation shows increased corrosion rates for the two Fe-Mg samples in comparison to pure Fe, validated by relatively active corrosion potentials, higher corrosion current densities, faster anodic dissolution, and lower charge transfer resistances, governed by chemical composition and non-equilibrium nanostructures. Finally, nano-indentation testing of the two samples reveals relatively higher hardness and lower elastic moduli, a suitable combination for bio-implants. STATEMENT OF SIGNIFICANCE: The use of Mg as a biodegradable in-vivo  implant material is problematic because of its high dissolution rate and potential for hydrogen gas generation. This is the first time that the idea of combinatorial synthesis is employed to fabricate two model nanostructured Fe-Mg systems, i.e. CSFM-1 (Fe₈₅Mg₁₅), and CSFM-2 (Fe₆₅Mg₃₅), exhibiting a controlled and uniform degradation. The structural characterization of the two systems demonstrates a substitutional solid solution of bcc-Fe-Mg in CSFM-1 and an amorphous short-range-ordered structure in the CSFM-2 system. Electrochemical investigation shows increased biodegradation rates for the two Fe-Mg systems in comparison to pure Fe, validated by relatively active corrosion potentials, higher corrosion current densities, faster anodic dissolution, and lower charge transfer resistances, governed by chemical composition and non-equilibrium nanostructures.

A Review of Bioresorbable Implantable Medical Devices: Materials, Fabrication, and Implementation

Implantable medical devices (IMDs) are designed to sense specific parameters or stimulate organs and have been actively used for treatment and diagnosis of various diseases. IMDs are used for long-term disease screening or treatments and cannot be considered for short-term applications since patients need to go through a surgery for retrieval of the IMD. Advances in bioresorbable materials has led to the development of transient IMDs that can be resorbed by bodily fluids and disappear after a certain period. These devices are designed to be implanted in the adjacent of the targeted tissue for predetermined times with the aim of measurement of pressure, strain, or temperature, while the bioelectronic devices stimulate certain tissues. They enable opportunities for monitoring and treatment of acute diseases. To realize such transient and miniaturized devices, researchers utilize a variety of materials, novel fabrication methods, and device design strategies. This review discusses potential bioresorbable materials for each component in an IMD followed by programmable degradation and safety standards. Then, common fabrication methods for bioresorbable materials are introduced, along with challenges. The final section provides representative examples of bioresorbable IMDs for various applications with an emphasis on materials, device functionality, and fabrication methods.

Devising Materials Manufacturing Toward Lab-to-Fab Translation of Flexible Electronics

Flexible electronics have witnessed exciting progress in academia over the past decade, but most of the research outcomes have yet to be translated into products or gain much market share. For mass production and commercialization, industrial adoption of newly developed functional materials and fabrication techniques is a prerequisite. However, due to the disparate features of academic laboratories and industrial plants, translating materials and manufacturing technologies from labs to fabs is notoriously difficult. Therefore, herein, key challenges in the materials manufacturing of flexible electronics are identified and discussed for its lab-to-fab translation, along the four stages in product manufacturing: design, materials supply, processing, and integration. Perspectives on industry-oriented strategies to overcome some of these obstacles are also proposed. Priorities for action are outlined, including standardization, iteration between basic and applied research, and adoption of smart manufacturing. With concerted efforts from academia and industry, flexible electronics will bring a bigger impact to society as promised.

Highly Thermally Stable, Green Solvent Disintegrable, and Recyclable Polymer Substrates for Flexible Electronics

Flexible electronics require its substrate to have adequate thermal stability, but current thermally stable polymer substrates are difficult to be disintegrated and recycled; hence, generate enormous electronic solid waste. Here, a thermally stable and green solvent-disintegrable polymer substrate is developed for flexible electronics to promote their recyclability and reduce solid waste generation. Thanks to the proper design of rigid backbones and rational adjustments of polar and bulky side groups, the polymer substrate exhibits excellent thermal and mechanical properties with thermal decomposition temperature (T_d,5% ) of 430 °C, upper operating temperature of over 300 °C, coefficient of thermal expansion of 48 ppm K^-1 , tensile strength of 103 MPa, and elastic modulus of 2.49 GPa. Furthermore, the substrate illustrates outstanding optical and dielectric properties with high transmittance of 91% and a low dielectric constant of 2.30. Additionally, it demonstrates remarkable chemical and flame resistance. A proof-of-concept flexible printed circuit device is fabricated with this substrate, which demonstrates outstanding mechanical-electrical stability. Most importantly, the substrate can be quickly disintegrated and recycled with alcohol. With outstanding thermally stable properties, accompanied by excellent recyclability, the substrate is particularly attractive for a wide range of electronics to reduce solid waste generation, and head toward flexible and "green" electronics.

Multimodal spectroscopic investigation of the conformation and local environment of biomolecules at an electrified interface

The complex and dynamic interfacial regions between biological samples and electronic components pose many challenges for characterization, including their evolution over multiple temporal and spatial scales. Spectroscopic probes of buried interfaces employing mid-infrared plasmon resonances and time-resolved fluorescence detection in the visible range are used to study the properties of polypeptides adsorbed at the surface of a working electrode. Information from these complementary spectroscopic probes reveals the interplay of solvation, electric fields, and ion concentration on their resulting macromolecular conformations.

Temperature Sensor with a Water-Dissolvable Ionic Gel for Ionic Skin

In the era of a trillion sensors, a tremendous number of sensors will be consumed to collect information for big data analysis. Once they are installed in a harsh environment or implanted in a human/animal body, we cannot easily retrieve the sensors; the sensors for these applications are left unattended but expected to decay after use. In this paper, a disposable temperature sensor that disappears with contact with water is reported. The gel electrolyte based on an ionic liquid and a water-soluble polymer, so-called ionic gel, exhibits a Young's modulus of 96 kPa, which is compatible with human muscle, skin, and organs, and can be a wearable device or in soft robotics. A study on electrical characteristics of the sensor with various temperatures reveals that the ionic conductivity and capacitance increased by 12 times and 4.8 times, respectively, when the temperature varies from 30 to 80 °C. The temperature sensor exhibits a short response time of 1.4 s, allowing real-time monitoring of temperature change. Furthermore, sensors in an array format can obtain the spatial distribution of temperature. The developed sensor was found to fully dissolve in water in 16 h. The water-dissolvability enables practical applications including healthcare, artificial intelligence, and environmental sensing.

From Silk Spinning to 3D Printing: Polymer Manufacturing using Directed Hierarchical Molecular Assembly

Silk spinning offers an evolution-based manufacturing strategy for industrial polymer manufacturing, yet remains largely inaccessible as the manufacturing mechanisms in biological and synthetic systems, especially at the molecular level, are fundamentally different. The appealing characteristics of silk spinning include the sustainable sourcing of the protein material, the all-aqueous processing into fibers, and the unique material properties of silks in various formats. Substantial progress has been made to mimic silk spinning in artificial manufacturing processes, despite the gap between natural and artificial systems. This report emphasizes the universal spinning conditions utilized by both spiders and silkworms to generate silk fibers in nature, as a scientific and technical framework for directing molecular assembly into high-performance structures. The preparation of regenerated silk feedstocks and mimicking native spinning conditions in artificial manufacturing are discussed, as is progress and challenges in fiber spinning and 3D printing of silk-composites. Silk spinning is a biomimetic model for advanced and sustainable artificial polymer manufacturing, offering benefits in biomedical applications for tissue scaffolds and implantable devices.

Biodegradable Materials and Green Processing for Green Electronics

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods. In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest. Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents. This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices.

Skin-Integrated Wearable Systems and Implantable Biosensors: A Comprehensive Review

Biosensors devices have attracted the attention of many researchers across the world. They have the capability to solve a large number of analytical problems and challenges. They are future ubiquitous devices for disease diagnosis, monitoring, treatment and health management. This review presents an overview of the biosensors field, highlighting the current research and development of bio-integrated and implanted biosensors. These devices are micro- and nano-fabricated, according to numerous techniques that are adapted in order to offer a suitable mechanical match of the biosensor to the surrounding tissue, and therefore decrease the body’s biological response. For this, most of the skin-integrated and implanted biosensors use a polymer layer as a versatile and flexible structural support, combined with a functional/active material, to generate, transmit and process the obtained signal. A few challenging issues of implantable biosensor devices, as well as strategies to overcome them, are also discussed in this review, including biological response, power supply, and data communication.

Low-Cost PVD Shadow Masks with Submillimeter Resolution from Laser-Cut Paper

We characterize an affordable method of producing stencils for submillimeter physical vapor deposition (PVD) by using paper and a benchtop laser cutter. Patterning electrodes or similar features on top of organic or biological substrates is generally not possible using standard photolithography. Shadow masks, traditionally made of silicon-based membranes, circumvent the need for aggressive solvents but suffer from high costs. Here, we evaluate shadow masks fabricated by CO₂ laser processing from quantitative filter papers. Such papers are stiff and dimensionally stable, resilient in handling, and cut without melting or redeposition. Using two exemplary interdigitated electrode designs, we quantify the line resolution achievable with both high-quality and standard lenses, as well as the positional accuracy across multiple length scales. Additionally, we assess the gap between such laser-cut paper masks and a substrate, and quantify feature reproduction onto polycarbonate membranes. We find that ~100 µm line widths are achievable independent of lens type and that average positional accuracy is better than ±100 µm at 4”-wafer scale. Although this falls well short of the micron-size features achievable with typical shadow masks, resolution in the tenths to tens of millimeters is entirely sufficient for applications from contact pads to electrochemical cells, allowing new functionalities on fragile materials.

Untethered Single Cell Grippers for Active Biopsy

Single cell manipulation is important in biosensing, biorobotics, and quantitative cell analysis. Although microbeads, droplets, and microrobots have been developed previously, it is still challenging to simultaneously excise, capture, and manipulate single cells in a biocompatible manner. Here, we describe untethered single cell grippers, that can be remotely guided and actuated on-demand to actively capture or excise individual or few cells. We describe a novel molding method to micropattern a thermally responsive wax layer for biocompatible motion actuation. The multifingered grippers derive their energy from the triggered release of residual differential stress in bilayer hinges composed of silicon oxides. A magnetic layer enables remote guidance through narrow conduits and fixed tissue sections ex vivo. Our results provide an important advance in high-throughput single cell scale biopsy tools important to lab-on-a-chip devices, microrobotics, and minimally invasive surgery.

Body-Integrated, Enzyme-Triggered Degradable, Silk-Based Mechanical Sensors for Customized Health/Fitness Monitoring and In Situ Treatment

Mechanical signals such as pressure and strain reflect important psychological and physiological states of the human body. Body-integrated sensors, including skin-mounted and surgically implanted ones, allow personalized health monitoring for the general population as well as patients. However, the development of such measuring devices has been hindered by the strict requirements for human-biocompatible materials and the need for high performance sensors; most existing devices or sensors do not meet all the desired specifications. Here, a set of flexible, stretchable, wearable, implantable, and degradable mechanical sensors is reported with excellent mechanical robustness and compliance, outstanding biocompatibility, remotely-triggered degradation, and excellent sensing performance, using a conductive silk fibroin hydrogel (CSFH). They can detect multiple mechanical signals such as pressure, strain, and bending angles. Moreover, combined with a drug-loaded silk-based microneedle array, sensor-equipped devices are shown to be effective for real-time monitoring and in situ treatment of epilepsy in a rodent model. These sensors offer potential applications in custom health monitoring wearables, and in situ treatment of chronic clinical disorders.

A flexible and physically transient electrochemical sensor for real-time wireless nitric oxide monitoring

Real-time sensing of nitric oxide (NO) in physiological environments is critically important in monitoring neurotransmission, inflammatory responses, cardiovascular systems, etc. Conventional approaches for NO detection relying on indirect colorimetric measurement or built with rigid and permanent materials cannot provide continuous monitoring and/or require additional surgical retrieval of the implants, which comes with increased risks and hospital cost. Herein, we report a flexible, biologically degradable and wirelessly operated electrochemical sensor for real-time NO detection with a low detection limit (3.97 nmol), a wide sensing range (0.01-100 μM), and desirable anti-interference characteristics. The device successfully captures NO evolution in cultured cells and organs, with results comparable to those obtained from the standard Griess assay. Incorporated with a wireless circuit, the sensor platform achieves continuous sensing of NO levels in living mammals for several days. The work may provide essential diagnostic and therapeutic information for health assessment, treatment optimization and postsurgical monitoring.

Intrinsically Stretchable, Transient Conductors from a Composite Material of Ag Flakes and Gelatin Hydrogel

Transient conductors are one of the most important components in transient electronics, which attract great attention because of their environment-friendly and biocompatible characters. To meet the requirement for wearable electronics, good stretchability and mechanical durability are needed for the transient conductors. However, it remains challenging to achieve stretchability and transient behavior simultaneously because of a lack of the proper elastomer. Herein, we demonstrate the first highly stretchable and transient conductor from a composite material of Ag flakes and gelatin hydrogel. It shows a maximum stretchability of more than 100% with minimal resistance increase and a good cyclic durability of 1000 cycles of deformation at 20%. The above mentioned good mechanical properties come from the rational design of the conductor with a seamless interface between the hydrogel and Ag flakes. When the conductor is immersed in water at 60 °C, it can be quickly dissolved within 90 s, and the transient behavior can be controlled by tuning the content of the hydrogel in the conductors and dissolving temperature. These properties make the conductor a good wiring candidate for stretchable and transient electronics.

Wafer-Scale Two-Dimensional MoS2 Layers Integrated on Cellulose Substrates Toward Environmentally Friendly Transient Electronic Devices

We explored the feasibility of wafer-scale two-dimensional (2D) molybdenum disulfide (MoS₂) layers toward futuristic environmentally friendly electronics that adopt biodegradable substrates. Large-area (> a few cm²) 2D MoS₂ layers grown on silicon dioxide/silicon (SiO₂/Si) wafers were delaminated and integrated onto a variety of cellulose-based substrates of various components and shapes in a controlled manner; examples of the substrates include planar papers, cylindrical natural rubbers, and 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibers. The integrated 2D layers were confirmed to well preserve their intrinsic structural and chemical integrity even on such exotic substrates. Proof-of-concept devices employing large-area 2D MoS₂ layers/cellulose substrates were demonstrated for a variety of applications, including photodetectors, pressure sensors, and field-effect transistors. Furthermore, we demonstrated the complete "dissolution" of the integrated 2D MoS₂ layers in a buffer solution composed of baking soda and deionized water, confirming their environmentally friendly transient characteristics. Moreover, the approaches to delaminate and integrate them do not demand any chemicals except for water, and their original substrates can be recycled for subsequent growths, ensuring excellent chemical benignity and process sustainability.

Glymphatic clearance of simulated silicon dispersion in mouse brain analyzed by laser induced breakdown spectroscopy

Silicon-based devices, such as neural probes, are increasingly used as electrodes for receiving electrical signals from neural tissue. Neural probes used chronically have been known to induce inflammation and elicit an immune response. The current study detects and evaluates silicon dispersion from a concentrated source in the mouse brain using laser induced breakdown spectroscopy. Element lines for Si (I) were found at the injection site at approximately 288 nm at 3hr post-implantation, even with tissue perfusion, indicating possible infusion into neural tissue. At 24hr and 1-week post-implantation, no silicon lines were found, indicating clearance. An isolated immune response was found by CD68 macrophage response at 24hr post injection. Future studies should measure chronic silicon exposure to determine if the inflammatory response is proportional to silicon administration. The present type of protocol, coupling laser induced breakdown spectroscopy, neuroimaging, histology, immunohistochemistry, and determination of clearance could be used to investigate the glymphatic system and different tissue states such as in disease (e.g. Alzheimer's).

Wireless implantable and biodegradable sensors for postsurgery monitoring: current status and future perspectives

In in vivo postsurgery monitoring, the use of wireless biodegradable implantable sensors has gained and is gaining a lot of interest, particularly in cases of monitoring for a short period of time. The employment of biodegradable materials allows the circumvention of secondary surgery for device removal. Additionally, the use of wireless communication for data elaboration avoids the need for transcutaneous wires. As such, it is possible to prevent possible inflammation and infections associated with long-term implants which are not wireless. It is expected that microfabricated biodegradable sensors will have a strong impact in acute or transient biomedical applications. However, the design of such high-performing electronic systems, both fully biodegradable and wireless, is very complex, particularly at small scales. The associated technologies are still in their infancy and should be more deeply and extensively investigated in animal models and, successively, in humans, before being clinically implemented. In this context, the present review aims to provide a complete overview of wireless biodegradable implantable sensors, covering the vital signs to be monitored, the wireless technologies involved, and the biodegradable materials used for the production of the devices, as well as designed devices and their applications. In particular, both their advantages and drawbacks are highlighted, and the key challenges faced, mainly associated with fabrication techniques, and control over degradation kinetics and biocompatibility of the device, are reported and discussed.

Flexible and fully biodegradable resistance random access memory based on a gelatin dielectric

The increased public concerns on healthcare, the environment and sustainable development inspired the development of biodegradable and biocompatible electronics that could be used as degradable electronics in implants. In this work, a fully biodegradable and flexible resistance random access memory (RRAM) was developed with low-cost biomaterial gelatin as the dielectric layer and the biodegradable polymer poly(lactide-coglycolide) acid (PLGA) as the substrate. PLGA can be synthesized by a simple solution process, and the PLGA substrate can be peeled off the handling substrate for operation once the devices are fabricated. The fabricated memory devices exhibited reliable nonvolatile resistive switching characteristics with a long retention time over 10⁴ s and a near-constant on/off resistance ratio of 10² even after 200 bending cycles, showing the promising potential for application in flexible electronics. Degradation of the devices in deionized water and in phosphate buffered saline (PBS) solution showed that the whole devices can be completely degraded in water. The dissolution time of the metals and the gelatin layer was a few days, while that for PLGA is about 6 months, and can be modified by changing the synthesis conditions of the film, thus allowing the development of biodegradable electronics with designed dissolution time.

Injectable Biomedical Devices for Sensing and Stimulating Internal Body Organs

The rapid pace of progress in implantable electronics driven by novel technology has created devices with unconventional designs and features to reduce invasiveness and establish new sensing and stimulating techniques. Among the designs, injectable forms of biomedical electronics are explored for accurate and safe targeting of deep-seated body organs. Here, the classes of biomedical electronics and tools that have high aspect ratio structures designed to be injected or inserted into internal organs for minimally invasive monitoring and therapy are reviewed. Compared with devices in bulky or planar formats, the long shaft-like forms of implantable devices are easily placed in the organs with minimized outward protrusions via injection or insertion processes. Adding flexibility to the devices also enables effortless insertions through complex biological cavities, such as the cochlea, and enhances chronic reliability by complying with natural body movements, such as the heartbeat. Diverse types of such injectable implants developed for different organs are reviewed and the electronic, optoelectronic, piezoelectric, and microfluidic devices that enable stimulations and measurements of site-specific regions in the body are discussed. Noninvasive penetration strategies to deliver the miniscule devices are also considered. Finally, the challenges and future directions associated with deep body biomedical electronics are explained.

Materials Strategies and Device Architectures of Emerging Power Supply Devices for Implantable Bioelectronics

Implantable bioelectronics represent an emerging technology that can be integrated into the human body for diagnostic and therapeutic functions. Power supply devices are an essential component of bioelectronics to ensure their robust performance. However, conventional power sources are usually bulky, rigid, and potentially contain hazardous constituent materials. The fact that biological organisms are soft, curvilinear, and have limited accommodation space poses new challenges for power supply systems to minimize the interface mismatch and still offer sufficient power to meet clinical-grade applications. Here, recent advances in state-of-the-art nonconventional power options for implantable electronics, specifically, miniaturized, flexible, or biodegradable power systems are reviewed. Material strategies and architectural design of a broad array of power devices are discussed, including energy storage systems (batteries and supercapacitors), power devices which harvest sources from the human body (biofuel cells, devices utilizing biopotentials, piezoelectric harvesters, triboelectric devices, and thermoelectric devices), and energy transfer devices which utilize sources in the surrounding environment (ultrasonic energy harvesters, inductive coupling/radiofrequency energy harvesters, and photovoltaic devices). Finally, future challenges and perspectives are given.

Flexible Neuromorphic Electronics for Computing, Soft Robotics, and Neuroprosthetics

Flexible neuromorphic electronics that emulate biological neuronal systems constitute a promising candidate for next-generation wearable computing, soft robotics, and neuroprosthetics. For realization, with the achievement of simple synaptic behaviors in a single device, the construction of artificial synapses with various functions of sensing and responding and integrated systems to mimic complicated computing, sensing, and responding in biological systems is a prerequisite. Artificial synapses that have learning ability can perceive and react to events in the real world; these abilities expand the neuromorphic applications toward health monitoring and cybernetic devices in the future Internet of Things. To demonstrate the flexible neuromorphic systems successfully, it is essential to develop artificial synapses and nerves replicating the functionalities of the biological counterparts and satisfying the requirements for constructing the elements and the integrated systems such as flexibility, low power consumption, high-density integration, and biocompatibility. Here, the progress of flexible neuromorphic electronics is addressed, from basic backgrounds including synaptic characteristics, device structures, and mechanisms of artificial synapses and nerves, to applications for computing, soft robotics, and neuroprosthetics. Finally, future research directions toward wearable artificial neuromorphic systems are suggested for this emerging area.

Implantable, Degradable, Therapeutic Terahertz Metamaterial Devices

Metamaterial (MM) sensors and devices, usually consisting of artificially structured composite materials with engineered responses that are mainly determined by the unit structure rather than the bulk properties or composition, offer new functionalities not readily available in nature. A set of implantable and resorbable therapeutic MM devices at terahertz (THz) frequencies are designed and fabricated by patterning magnesium split ring resonators on drug-loaded silk protein substrates with controllable device degradation and drug release rates. To demonstrate proof-of-concept, a set of silk-based, antibiotics-loaded MM devices, which can serve as degradable antibacterial skin patches with capabilities to monitor drug-release in real time are fabricated. The extent of drug release, which correlates with the degradation of the MM skin patch, can be monitored by analyzing the resonant responses in reflection during degradation using a portable THz camera. Animal experiments are performed to demonstrate the in vivo degradation process and the efficacy of the devices for antibacterial treatment. Thus, the implantable and resorbable therapeutic MM devices do not need to be retrieved once implanted, providing an appealing alternative for in-vivo sensing and in situ treatment applications.

Multiscale Soft-Hard Interface Design for Flexible Hybrid Electronics

Emerging next-generation soft electronics will require versatile properties functioning under mechanical compliance, which will involve the use of different types of materials. As a result, control over material interfaces (particularly soft/hard interfaces) has become crucial and is now attracting intensive worldwide research efforts. A series of material and structural interface designs has been devised to improve interfacial adhesion, preventing failure of electromechanical properties under mechanical deformation. Herein, different soft/hard interface design strategies at multiple length scales in the context of flexible hybrid electronics are reviewed. The crucial role of soft ligands and/or polymers in controlling the morphologies of active nanomaterials and stabilizing them is discussed, with a focus on understanding the soft/hard interface at the atomic/molecular scale. Larger nanoscopic and microscopic levels are also discussed, to scrutinize viable intrinsic and extrinsic interfacial designs with the purpose of promoting adhesion, stretchability, and durability. Furthermore, the macroscopic device/human interface as it relates to real-world applications is analyzed. Finally, a perspective on the current challenges and future opportunities in the development of truly seamlessly integrated soft wearable electronic systems is presented.