Flexible Battery‐Less Bioelectronic Implants: Wireless Powering and Manipulation by Near‐Infrared Light

Bioelectronics, which can provide electrical impulses to precisely modulate the body's neural circuits, spark great interests of industry and academia. Yet the technologies to power and manipulate these devices, such as wireless powering and remote manipulation, remain challenging. Here, by investigating the pyroelectric performances of poly(vinylidene difluoride) (PVDF) and its remote‐manipulation ability under near‐infrared‐ray (nIR) irradiation, a flexible battery‐less implantable device is proposed, constructed by laminated graphene–PVDF–graphene sandwiches, which can be wirelessly powered and supply regulatable electrical pulses for nerve stimulation by nIR irradiation. The flexible and compact device (20 mm × 20 mm in area, 0.2 mm in thickness) can generate electrical pulses with controllable amplitude and width, and shows an excellent ability to stimulate nerves, i.e., sciatic nerve of a frog and a rat heart, by remote control. The flexible and remote‐manipulative battery‐less device should find uses in the design and fabrication of bioelectronics‐related applications.     

Biodegradable Thin Metal Foils and Spin‐On Glass Materials for Transient Electronics

Biodegradable substrates and encapsulating materials play critical roles in the development of an emerging class of semiconductor technology, generally referred as “transient electronics”, whose key characteristic is an ability to dissolve completely, in a controlled manner, upon immersion in ground water or biofluids. The results presented here introduce the use of thin foils of Mo, Fe, W, or Zn as biodegradable substrates and silicate spin‐on‐glass (SOG) materials as insulating and encapsulating layers, with demonstrations of transient active (diode and transistor) and passive (capacitor and inductor) electronic components. Complete measurements of electrical characteristics demonstrate that the device performance can reach levels comparable to those possible with conventional, nontransient materials. Dissolution kinetics of the foils and cytotoxicity tests of the SOG yield information relevant to use in transient electronics for temporary biomedical implants, resorbable environmental monitors, and reduced waste consumer electronics.     

A Bioresorbable Magnetically Coupled System for Low‐Frequency Wireless Power Transfer

Bioresorbable electronic technologies form the basis for classes of biomedical devices that undergo complete physical and chemical dissolution after a predefined operational period, thereby eliminating the costs and risks associated with secondary surgical extraction. A continuing area of opportunity is in the development of strategies for power supply for these systems, where previous studies demonstrate some utility for biodegradable batteries, radio frequency harvesters, solar cells, and others. This paper introduces a type of bioresorbable system for wireless power transfer, in which a rotating magnet serves as the transmitter and a bioresorbable antenna as the remote receiver, with capabilities for operation at low frequencies (<200 Hz). Systematic experimental and numerical studies demonstrate several unique advantages of this system, most significantly the elimination of impedance matching and electromagnetic radiation exposure presented with the types of radio frequency energy harvesters explored previously. These results add to the portfolio of power supply options in bioresorbable electronic implants.     

Materials and Fabrication Processes for Transient and Bioresorbable High‐Performance Electronics

Materials and fabrication procedures are described for bioresorbable transistors and simple integrated circuits, in which the key processing steps occur on silicon wafer substrates, in schemes compatible with methods used in conventional microelectronics. The approach relies on an unusual type of silicon on insulator wafer to yield devices that exploit ultrathin sheets of monocrystalline silicon for the semiconductor, thin films of magnesium for the electrodes and interconnects, silicon dioxide and magnesium oxide for the dielectrics, and silk for the substrates. A range of component examples with detailed measurements of their electrical characteristics and dissolution properties illustrate the capabilities. In vivo toxicity tests demonstrate biocompatibility in sub‐dermal implants. The results have significance for broad classes of water‐soluble, “transient” electronic devices.     

Dissolvable Metals for Transient Electronics

Reactive dissolution and its effects on electrical conduction, morphological change and chemical transformation in thin films of Mg, AZ31B Mg alloy, Zn, Fe, W, and Mo in de‐ionized (DI) water and simulated body fluids (Hanks’ solution pH 5–8) are systematically studied, to assess the potential for use of these metals in water‐soluble, that is, physically "transient", electronics. The results indicate that the electrical dissolution rates in thin films can be much different that traditionally reported corrosion rates in corresponding bulk materials. Silicon metal oxide field effect transistors (MOSFETs) built with these metals demonstrate feasibility for use in transient electronics.     

Biodegradable Polyanhydrides as Encapsulation Layers for Transient Electronics

Bioresorbable electronic systems represent an emerging class of technology of interest due to their ability to dissolve, chemically degrade, disintegrate, and/or otherwise physically disappear harmlessly in biological environments, as the basis for temporary implants that avoid the need for secondary surgical extraction procedures. Polyanhydride‐based polymers can serve as hydrophobic encapsulation layers for such systems, as a subset of the broader field of transient electronics, where biodegradation eventually occurs by chain scission. Systematic experimental studies that involve immersion in phosphate‐buffered saline solution at various pH values and/or temperatures demonstrate that dissolution occurs through a surface erosion mechanism, with little swelling. The mechanical properties of this polymer are well suited for use in soft, flexible devices, where integration can occur through a mold‐based photopolymerization technique. Studies of the dependence of the polymer properties on monomer compositions and the rates of permeation on coating thicknesses reveal some of the underlying effects. Simple demonstrations illustrate the ability to sustain operation of underlying biodegradable electronic systems for durations between a few hours to a week during complete immersion in aqueous solutions that approximate physiological conditions. Systematic chemical, physical, and in vivo biological studies in animal models reveal no signs of toxicity or other adverse biological responses.     

Liquid Metal‐Based Transient Circuits for Flexible and Recyclable Electronics

Transient electronics, arising electronic devices with dissolvable or degradable features on demand, is still at an early stage of development due to the limited choices of materials and strategies. Herein, a facile fabrication method for transient circuits by the combination of room‐temperature liquid metals (RTLMs) as the electronic circuit and water‐soluble poly(vinyl alcohol) (PVA) as the packaging material is reported. The as‐made transient circuits exhibit remarkable durability and stable electric performance upon bending and twisting, while possessing short transience times, owing to the excellent solubility of PVA substrates and the intrinsic flexibility of RTLM patterns. Moreover, the RTLM‐based transient circuit shows an extremely high recycling efficiency, up to 96% of the employed RTLM can be recovered. As such, the economic and environmental viability of transient electronics increases substantially. To validate this concept, the surface patterning of RTLMs with complicated shapes is demonstrated, and a transient antenna is subsequently applied for passive near‐field communication tag and a transient capacitive touch sensor. The application of the RTLM‐based transient circuit for sequentially turning off an array of light‐emitting‐diode lamps is also demonstrated. The present RTLM‐based PVA‐encapsulated circuits substantially expand the scope of transient electronics toward flexible and recyclable transient systems.     

Rational Design of a Printable,Highly Conductive Silicone‐based Electrically Conductive Adhesive for Stretchable Radio‐Frequency Antennas

Stretchable radio‐frequency electronics are gaining popularity as a result of the increased functionality they gain through their flexible nature, impossible within the confines of rigid and planar substrates. One approach to fabricating stretchable antennas is to embed stretchable or flowable conductive materials, such as conductive polymers, conductive polymer composites, and liquid metal alloys as stretchable conduction lines. However, these conductive materials face many challenges, such as low electrical conductivity under mechanical deformation and delamination from substrates. In the present study, a silicone‐based electrically conductive adhesive (silo‐ECA) is developed that have a conductivity of 1.51 × 10⁴ S cm^?1 and can maintain conductivity above 1.11 × 10³ S cm^?1, even at a large stain of 240%. By using the stretchable silo‐ECAs as a conductor pattern and pure silicone elastomers as a base substrate, stretchable antennas can be fabricated by stencil printing or soft‐lithography. The resulting antenna's resonant frequency is tunable over a wide range by mechanical modulation. This fabrication method is low‐cost, can support large‐scale production, has high reliability over a wide temperature range, and eliminates the concerns of leaking or delamination between conductor and substrate experienced in previously reported micro‐fluidic antennas.     

Bioresorbable Wireless Sensors as Temporary Implants for In Vivo Measurements of Pressure

Pressures at targeted locations inside the human body serve as critically important diagnostic parameters for monitoring various types of serious or even potentially fatal medical conditions including intracranial, intra‐abdominal, and pulmonary hypertension, as well as compartment syndromes. Implantable commercial sensors provide satisfactory accuracy and stability in measurements of pressure, yet surgical removal is required after recovery of the patient to avoid infections and other risks associated with long‐term implantation. Sensors that dissolve in biofluids (or, equivalently, bioabsorb or bioresorb) avoid the need for such surgeries, yet current designs involve either hard‐wired connections and/or fail to provide quantitative measurements over clinically relevant lifetimes. Here, a bioresorbable, wireless pressure sensor based on passive inductor‐capacitor resonance circuits in layouts and with sets of materials that overcome these drawbacks is reported. Specifically, optimized designs offer sensitivity as high as ≈200 kHz mmHg^?1 and resolution as low as 1 mmHg. Encapsulation approaches that use membranes of Si₃N₄ and edge seals of natural wax support stable operation in vivo for up to 4 days. The bioresorbable pressure sensing technology reported here may serve as an important solution to temporary, real‐time monitoring of internal pressure for various medical conditions.     

Biocompatible and Biodegradable Materials for Organic Field‐Effect Transistors

Biocompatible‐ingestible electronic circuits and capsules for medical diagnosis and monitoring are currently based on traditional silicon technology. Organic electronics has huge potential for developing biodegradable, biocompatible, bioresorbable, or even metabolizable products. An ideal pathway for such electronic devices involves fabrication with materials from nature, or materials found in common commodity products. Transistors with an operational voltage as low as 4–5 V, a source drain current of up to 0.5 μA and an on‐off ratio of 3–5 orders of magnitude have been fabricated with such materials. This work comprises steps towards environmentally safe devices in low‐cost, large volume, disposable or throwaway electronic applications, such as in food packaging, plastic bags, and disposable dishware. In addition, there is significant potential to use such electronic items in biomedical implants.     

Stretchable Twisted‐Pair Transmission Lines for Microwave Frequency Wearable Electronics

Stretchable electrical interconnects based on serpentines combined with elastic materials are utilized in various classes of wearable electronics. However, such interconnects are primarily for direct current or low‐frequency signals and incompatible with microwave electronics that enable wireless communication. In this paper, design and fabrication procedures are described for stretchable transmission line capable of delivering microwave signals. The stretchable transmission line has twisted‐pair design integrated into thin‐film serpentine microstructure to minimize electromagnetic interference, such that the line's performance is minimally affected by the environment in close proximity, allowing its use in thin‐film bioelectronics, such as the epidermal electronic system. Detailed analysis, simulations, and experimental results show that the stretchable transmission line has negligible changes in performance when stretched and is operable on skin through suppressed radiated emission achieved with the twisted‐pair geometry. Furthermore, stretchable microwave low‐pass filter and band‐stop filter are demonstrated using the twisted‐pair structure to show the feasibility of the transmission lines as stretchable passive components. These concepts form the basic elements used in the design of stretchable microwave components, circuits, and subsystems performing important radio frequency functionalities, which can apply to many types of stretchable bioelectronics for radio transmitters and receivers.     

Microporous Polypyrrole‐Coated Graphene Foam for High‐Performance Multifunctional Sensors and Flexible Supercapacitors

This study reports on the fabrication of pressure/temperature/strain sensors and all‐solid‐state flexible supercapacitors using only polydimethylsiloxane coated microporous polypyrrole/graphene foam composite (PDMS/PPy/GF) as a common material. A dual‐mode sensor is designed with PDMS/PPy/GF, which measures pressure and temperature with the changes of current and voltage, respectively, without interference to each other. The fabricated dual‐mode sensor shows high sensitivity, fast response/recovery, and high durability during 10 000 cycles of pressure loading. The pressure is estimated using the thermoelectric voltage induced by simultaneous increase in temperature caused by a finger touch on the sensor. Additionally, a resistor‐type strain sensor fabricated using the same PDMS/PPy/GF could detect the strain up to 50%. Flexible, high performance supercapacitor used as a power supply is fabricated with electrodes of PPy/GF for its high surface area and pseudocapacitance. Furthermore, an integrated system of such fabricated multifunctional sensors and a supercapacitor on a skin‐attachable flexible substrate using liquid–metal interconnections operates well, whereas sensors are driven by the power of the supercapacitor. This study clearly demonstrates that the appropriate choice of a single functional material enables fabrication of active multifunctional sensors for pressure, temperature, and strain, as well as the supercapacitor, that could be used in wirelessly powered wearable devices.     

Edible Electronics: Biocompatible and Biodegradable Materials for Organic Field‐Effect Transistors (Adv. Funct. Mater. 23/2010)

Biocompatible‐ingestible electronic circuits and capsules for medical diagnosis and monitoring are currently based on traditional silicon technology. Organic electronics has huge potential for developing biodegradable, biocompatible, bioresorbable, or even metabolizable products. An ideal pathway for such electronic devices involves fabrication with materials from nature, or materials found in common commodity products. Transistors with an operational voltage as low as 4–5 V, a source drain current of up to 0.5 μA and an on‐off ratio of 3–5 orders of magnitude have been fabricated with such materials. This work comprises steps towards environmentally safe devices in low‐cost, large volume, disposable or throwaway electronic applications, such as in food packaging, plastic bags, and disposable dishware. In addition, there is significant potential to use such electronic items in biomedical implants.     

Miniaturized Battery‐Free Wireless Systems for Wearable Pulse Oximetry

Development of unconventional technologies for wireless collection and analysis of quantitative, clinically relevant information on physiological status is of growing interest. Soft, biocompatible systems are widely regarded as important because they facilitate mounting on external (e.g., skin) and internal (e.g., heart and brain) surfaces of the body. Ultraminiaturized, lightweight, and battery‐free devices have the potential to establish complementary options in biointegration, where chronic interfaces (i.e., months) are possible on hard surfaces such as the fingernails and the teeth, with negligible risk for irritation or discomfort. Here, the authors report materials and device concepts for flexible platforms that incorporate advanced optoelectronic functionality for applications in wireless capture and transmission of photoplethysmograms, including quantitative information on blood oxygenation, heart rate, and heart rate variability. Specifically, reflectance pulse oximetry in conjunction with near‐field communication capabilities enables operation in thin, miniaturized flexible devices. Studies of the material aspects associated with the body interface, together with investigations of the radio frequency characteristics, the optoelectronic data acquisition approaches, and the analysis methods capture all of the relevant engineering considerations. Demonstrations of operation on various locations of the body and quantitative comparisons to clinical gold standards establish the versatility and the measurement accuracy of these systems, respectively.     

Design for a Dual‐Frequency Antenna‐in‐Package

For an antenna‐in‐package (AiP), via holes are used to connect the antenna ground and system ground. In this letter, a dual‐frequency AiP with a U‐slot embedded in the patch is proposed. By properly arranging three via holes under the non‐radiating edge, an AiP with two resonant frequencies is realized. Then a U‐slot is embedded in the patch to further improve the bandwidth of the AiP. To validate the proposed design, an AiP with the bandwidth of 4.49% at 2.45 GHz and 6.02% at 5.32 GHz is achieved and fabricated. The measured results agree with the simulated results.     

Electrically Enhanced Readout System for a High‐Frequency CMOS‐MEMS Resonator

The design of a CMOS clamped‐clamped beam resonator along with a full custom integrated differential amplifier, monolithically fabricated with a commercial 0.35 μm CMOS technology, is presented. The implemented amplifier, which minimizes the negative effect of the parasitic capacitance, enhances the electrical MEMS characterization, obtaining a 48 × 10⁸ resonant frequency‐quality factor product (Q×f_res) in air conditions, which is quite competitive in comparison with existing CMOS‐MEMS resonators.