The quest for miniaturized soft bioelectronic devices

Photoresist Development for 3D Printing of Conductive Microstructures via Two-Photon Polymerization

The advancement of electronic devices necessitates the development of three-dimensional (3D) high-precision conductive microstructures, which have extensive applications in bio-electronic interfaces, soft robots, and electronic skins. Two-photon polymerization (TPP) based 3D printing is a critical technique that offers unparalleled fabrication resolution in 3D space for intricate conductive structures. While substantial progress has been made in this field, this review summarizes recent advances in the 3D printing of conductive microstructures via TPP, mainly focusing on the essential criteria of photoresist resins suitable for TPP. Further preparation strategies of these photoresists and methods for constructing 3D conductive microstructures via TPP are discussed. The application prospects of 3D conductive microstructures in various fields are discussed, highlighting the imperative to advance their additive manufacturing technology. Finally, strategic recommendations are offered to enhance the construction of 3D conductive microstructures using TPP, addressing prevailing challenges and fostering significant advancements in manufacturing technology.

PolyPyrrole based-impedimetric aptasensor for selective determination of beta-HCG from urine sample

Herein, a conjugated conducting polymer-based impedimetric aptasensor has been developed to detect beta-human chorionic gonadotropin (bHCG), the one of the important biomarkers in gynecology, from synthetic human urine samples. In this context, gold electrodes were, firstly coated with pyrrole and pyrrole-3-carboxylic acid to obtain the poly(pyrrole-pyrrole-3-carboxylic acid) [poly(Py-PyCOOH)] conductive copolymer by cyclic voltammetry (CV). Then, bHCG-specific peptide aptamer was covalently linked onto the surface via applying a well-known carbodiimide-succinimide chemistry. The sensor developed was characterized to confirm modification steps via both electrochemical methods including CV, electrochemical impedance spectroscopy (EIS), and chronoamperometry and physico-chemically via attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), atomic force microscope (AFM), and contact angle measurements (CA). The analytical performance of the sensor was evaluated in the concentration range from 1 μg/mL to 100 μg/mL for successful detection of bHCG even in the presence of interference agents. The results have also revealed that the sensor could be classified as a promising alternative to its benchmark commercial clinical methods due its superior properties such as cost-friendliness, easy-to-prepare, stable, robust, and selectivity / sensitivity.

Shape-persistent ladder molecules exhibit nanogap-independent conductance in single-molecule junctions

Molecular electronic devices require precise control over the flow of current in single molecules. However, the electron transport properties of single molecules critically depend on dynamic molecular conformations in nanoscale junctions. Here we report a unique strategy for controlling molecular conductance using shape-persistent molecules. Chemically diverse, charged ladder molecules, synthesized via a one-pot multicomponent ladderization strategy, show a molecular conductance (d[log(G/G0)]/dx ≈ -0.1 nm-1) that is nearly independent of junction displacement, in stark contrast to the nanogap-dependent conductance (d[log(G/G0)]/dx ≈ -7 nm-1) observed for non-ladder analogues. Ladder molecules show an unusually narrow distribution of molecular conductance during dynamic junction displacement, which is attributed to the shape-persistent backbone and restricted rotation of terminal anchor groups. These principles are further extended to a butterfly-like molecule, thereby demonstrating the strategy's generality for achieving gap-independent conductance. Overall, our work provides important avenues for controlling molecular conductance using shape-persistent molecules.

Wireless Battery-free and Fully Implantable Organ Interfaces

Advances in soft materials, miniaturized electronics, sensors, stimulators, radios, and battery-free power supplies are resulting in a new generation of fully implantable organ interfaces that leverage volumetric reduction and soft mechanics by eliminating electrochemical power storage. This device class offers the ability to provide high-fidelity readouts of physiological processes, enables stimulation, and allows control over organs to realize new therapeutic and diagnostic paradigms. Driven by seamless integration with connected infrastructure, these devices enable personalized digital medicine. Key to advances are carefully designed material, electrophysical, electrochemical, and electromagnetic systems that form implantables with mechanical properties closely matched to the target organ to deliver functionality that supports high-fidelity sensors and stimulators. The elimination of electrochemical power supplies enables control over device operation, anywhere from acute, to lifetimes matching the target subject with physical dimensions that supports imperceptible operation. This review provides a comprehensive overview of the basic building blocks of battery-free organ interfaces and related topics such as implantation, delivery, sterilization, and user acceptance. State of the art examples categorized by organ system and an outlook of interconnection and advanced strategies for computation leveraging the consistent power influx to elevate functionality of this device class over current battery-powered strategies is highlighted.

Alternating Current Electroluminescence for Human-Interactive Sensing Displays

The development of stimuli-interactive displays based on alternating current (AC)-driven electroluminescence (EL) is of great interest, owing to their simple device architectures suitable for wearable applications requiring resilient mechanical flexibility and stretchability. AC-EL displays can serve as emerging platforms for various human-interactive sensing displays (HISDs) where human information is electrically detected and directly visualized using EL, promoting the development of the interaction of human-machine technologies. This review provides a holistic overview of the latest developments in AC-EL displays with an emphasis on their applications for HISDs. AC-EL displays based on exciton recombination or impact excitations of hot electrons are classified into four representative groups depending upon their device architecture: 1) displays without insulating layers, 2) displays with single insulating layers, 3) displays with double insulating layers, and 4) displays with EL materials embedded in an insulating matrix. State-of-the-art AC HISDs are discussed. Furthermore, emerging stimuli-interactive AC-EL displays are described, followed by a discussion of scientific and engineering challenges and perspectives for future stimuli-interactive AC-EL displays serving as photo-electronic human-machine interfaces.

Real-Time Monitoring of Cell Adhesion onto a Soft Substrate by a Graphene Impedance Biosensor

Soft substrates are interesting for many applications, ranging from mimicking the cellular microenvironment to implants. Conductive electrodes on such substrates allow the realization of flexible, elastic, and transparent sensors. Single-layer graphene as a candidate for such electrodes brings the advantage that the active area of the sensor is transparent and conformal to the underlying substrate. Here, we overcome several challenges facing the routine realization of graphene cell sensors on a canonical soft substrate, namely, poly(dimethylsiloxane) (PDMS). We have systematically studied the effect of surface energy before, during, and after the transfer of graphene. Thus, we have identified a suitable support polymer, optimal substrate (pre)treatment, and an appropriate solvent for the removal of the support. Using this procedure, we can reproducibly obtain stable and intact graphene sensors on a millimeter scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements, we infer that the softness of the substrate is slightly affected after the graphene transfer. However, we can modulate the stiffness using PDMS with differing compositions. Finally, we show that graphene sensors on PDMS can be successfully used as soft electrodes for real-time monitoring of the cell adhesion kinetics. The routine availability of single-layer graphene electrodes on a soft substrate with tunable stiffness will open a new avenue for studies, where the PDMS-liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity, and transparency.

A self-assembled implantable microtubular pacemaker for wireless cardiac electrotherapy

The current cardiac pacemakers are battery dependent, and the pacing leads are prone to introduce valve damage and infection, plus a complete pacemaker retrieval is needed for battery replacement. Despite the reported wireless bioelectronics to pace the epicardium, open-chest surgery (thoracotomy) is required to implant the device, and the procedure is invasive, requiring prolonged wound healing and health care burden. We hereby demonstrate a fully biocompatible wireless microelectronics with a self-assembled design that can be rolled into a lightweight microtubular pacemaker for intravascular implantation and pacing. The radio frequency was used to transfer energy to the microtubular pacemaker for electrical stimulation. We show that this pacemaker provides effective pacing to restore cardiac contraction from a nonbeating heart and have the capacity to perform overdrive pacing to augment blood circulation in an anesthetized pig model. Thus, this microtubular pacemaker paves the way for the minimally invasive implantation of leadless and battery-free microelectronics.

Soft Human-Machine Interface Sensing Displays: Materials and Devices

The development of human-interactive sensing displays (HISDs) that simultaneously detect and visualize stimuli is important for numerous cutting-edge human-machine interface technologies. Therefore, innovative device platforms with optimized architectures of HISDs combined with novel high-performance sensing and display materials are demonstrated. This study comprehensively reviews the recent advances in HISDs, particularly the device architectures that enable scaling-down and simplifying the HISD, as well as material designs capable of directly visualizing input information received by various sensors. Various HISD platforms for integrating sensors and displays are described. HISDs consist of a sensor and display connected through a microprocessor, and attempts to assemble the two devices by eliminating the microprocessor are detailed. Single-device HISD technologies are highlighted in which input stimuli acquired by sensory components are directly visualized with various optical components, such as electroluminescence, mechanoluminescence and structural color. The review forecasts future HISD technologies that demand the development of materials with molecular-level synthetic precision that enables simultaneous sensing and visualization. Furthermore, emerging HISDs combined with artificial intelligence technologies and those enabling simultaneous detection and visualization of extrasensory information are discussed.

Handheld device quantifies breath acetone for real-life metabolic health monitoring

Non-invasive breath analysis with mobile health devices bears tremendous potential to guide therapeutic treatment and personalize lifestyle changes. Of particular interest is the breath volatile acetone, a biomarker for fat burning, that could help in understanding and treating metabolic diseases. Here, we report a hand-held (6 × 10 × 19.5 cm³), light-weight (490 g), and simple device for rapid acetone detection in breath. It comprises a tailor-made end-tidal breath sampling unit, connected to a sensor and a pump for on-demand breath sampling, all operated using a Raspberry Pi microcontroller connected with a HDMI touchscreen. Accurate acetone detection is enabled by introducing a catalytic filter and a separation column, which remove and separate undesired interferents from acetone upstream of the sensor. This way, acetone is detected selectively even in complex gas mixtures containing highly concentrated interferents. This device accurately tracks breath acetone concentrations in the exhaled breath of five volunteers during a ketogenic diet, being as high as 26.3 ppm. Most importantly, it can differentiate small acetone changes during a baseline visit as well as before and after an exercise stimulus, being as low as 0.5 ppm. It is stable for at least four months (122 days), and features excellent bias and precision of 0.03 and 0.6 ppm at concentrations below 5 ppm, as validated by proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS). Hence, this detector is highly promising for simple-in-use, non-invasive, and routine monitoring of acetone to guide therapeutic treatment and track lifestyle changes.

Scalable-produced 3D elastic thermoelectric network for body heat harvesting

Flexible thermoelectric generators can power wearable electronics by harvesting body heat. However, existing thermoelectric materials rarely realize high flexibility and output properties simultaneously. Here we present a facile, cost-effective, and scalable two-step impregnation method for fabricating a three-dimensional thermoelectric network with excellent elasticity and superior thermoelectric performance. The reticular construction endows this material with ultra-light weight (0.28 g cm^-3), ultra-low thermal conductivity (0.04 W m^-1 K^-1), moderate softness (0.03 MPa), and high elongation (>100%). The obtained network-based flexible thermoelectric generator achieves a pretty high output power of 4 μW cm^-2, even comparable to state-of-the-art bulk-based flexible thermoelectric generators.

Ultra-soft and highly stretchable tissue-adhesive hydrogel based multifunctional implantable sensor for monitoring of overactive bladder

A highly stretchable and tissue-adhesive multifunctional sensor based on structurally engineered islets embedded in ultra-soft hydrogel is reported for monitoring of bladder activity in overactive bladder (OAB) induced rat and anesthetized pig. The use of hydrogel yielded a much lower sensor modulus (1 kPa) compared to that of the bladder (300 kPa), while the strong adhesiveness of the hydrogel (adhesive strength: 260.86 N/m) allowed firm attachment onto the bladder. The change in resistance of printed liquid metal particle thin-film lines under strain were used to detect bladder inflation and deflation; due to the high stretchability and reliability of the lines, surface strains of 200% could be measured repeatedly. Au electrodes coated with Platinum black were used to detect electromyography (EMG). These electrodes were placed on structurally engineered rigid islets so that no interfacial fracture occurs under high strains associated with bladder expansion. On the OAB induced rat, stronger signals (change in resistance and EMG root-mean-square) were detected near intra-bladder pressure maxima, thus showing correlation to bladder activity. Moreover, using robot-assisted laparoscopic surgery, the sensor was placed onto the bladder of an anesthetized pig. Under voiding and filling, bladder strain and EMG were once again monitored. These results confirm that our proposed sensor is a highly feasible, clinically relevant implantable device for continuous monitoring OAB for diagnosis and treatment.

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human-machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.

Wireless Non-Invasive Monitoring of Cholesterol Using a Smart Contact Lens

Herein, a wireless and soft smart contact lens that enables real-time quantitative recording of cholesterol in tear fluids for the monitoring of patients with hyperlipidemia using a smartphone is reported. This contact lens incorporates an electrochemical biosensor for the continuous detection of cholesterol concentrations, stretchable antenna, and integrated circuits for wireless communication, which makes a smartphone the only device required to operate this lens remotely without obstructing the wearer's vision. The hyperlipidemia rabbit model is utilized to confirm the correlation between cholesterol levels in tear fluid and blood and to confirm the feasibility of this smart contact lens for diagnostic application of cholesterol-related diseases. Further in vivo tests with human subjects demonstrated its good biocompatibility, wearability, and reliability as a non-invasive healthcare device.

Stretchable and Self-Adhesive PEDOT:PSS Blend with High Sweat Tolerance as Conformal Biopotential Dry Electrodes

Dry epidermal electrodes that can always form conformal contact with skin can be used for continuous long-term biopotential monitoring, which can provide vital information for disease diagnosis and rehabilitation. But, this application has been limited by the poor contact of dry electrodes on wet skin. Herein, we report a biocompatible fully organic dry electrode that can form conformal contact with both dry and wet skin even during physical movement. The dry electrodes are prepared by drop casting an aqueous solution consisting of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), poly(vinyl alcohol) (PVA), tannic acid (TA), and ethylene glycol (EG). The electrodes can exhibit a conductivity of 122 S cm^-1 and a mechanical stretchability of 54%. Moreover, they are self-adhesive to not only dry skin but also wet skin. As a result, they can exhibit a lower contact impedance to skin than commercial Ag/AgCl gel electrodes on both dry and sweat skins. They can be used as dry epidermal electrodes to accurately detect biopotential signals including electrocardiogram (ECG) and electromyogram (EMG) on both dry and wet skins for the users at rest or during physical movement. This is the first time to demonstrate dry epidermal electrodes self-adhesive to wet skin for accurate biopotential detection.

Intelligent Nanomaterials for Wearable and Stretchable Strain Sensor Applications: The Science behind Diverse Mechanisms, Fabrication Methods, and Real-Time Healthcare

It has become a scientific obligation to unveil the underlying mechanisms and the fabrication methods behind wearable/stretchable strain sensors based on intelligent nanomaterials in order to explore their possible potential in the field of biomedical and healthcare applications. This report is based on an extensive literature survey of fabrication of stretchable strain sensors (SSS) based on nanomaterials in the fields of healthcare, sports, and entertainment. Although the evolution of wearable strain sensors (WSS) is rapidly progressing, it is still at a prototype phase and various challenges need to be addressed in the future in special regard to their fabrication protocols. The biocalamity of COVID-19 has brought a drastic change in humans' lifestyles and has negatively affected nations in all capacities. Social distancing has become a mandatory rule to practice in common places where humans interact with each other as a basic need. As social distancing cannot be ruled out as a measure to stop the spread of COVID-19 virus, wearable sensors could play a significant role in technologically impacting people's consciousness. This review article meticulously describes the role of wearable and strain sensors in achieving such objectives.

Recent Progress in Materials Chemistry to Advance Flexible Bioelectronics in Medicine

Designing bioelectronic devices that seamlessly integrate with the human body is a technological pursuit of great importance. Bioelectronic medical devices that reliably and chronically interface with the body can advance neuroscience, health monitoring, diagnostics, and therapeutics. Recent major efforts focus on investigating strategies to fabricate flexible, stretchable, and soft electronic devices, and advances in materials chemistry have emerged as fundamental to the creation of the next generation of bioelectronics. This review summarizes contemporary advances and forthcoming technical challenges related to three principal components of bioelectronic devices: i) substrates and structural materials, ii) barrier and encapsulation materials, and iii) conductive materials. Through notable illustrations from the literature, integration and device fabrication strategies and associated challenges for each material class are highlighted.

A Microclip Peripheral Nerve Interface (μcPNI) for Bioelectronic Interfacing with Small Nerves

Peripheral nerves carry sensory (afferent) and motor (efferent) signals between the central nervous system and other parts of the body. The peripheral nervous system (PNS) is therefore rich in targets for therapeutic neuromodulation, bioelectronic medicine, and neuroprosthetics. Peripheral nerve interfaces (PNIs) generally suffer from a tradeoff between selectivity and invasiveness. This work describes the fabrication, evaluation, and chronic implantation in zebra finches of a novel PNI that breaks this tradeoff by interfacing with small nerves. This PNI integrates a soft, stretchable microelectrode array with a 2-photon 3D printed microclip (μcPNI). The advantages of this μcPNI compared to other designs are: a) increased spatial resolution due to bi-layer wiring of the electrode leads, b) reduced mismatch in biomechanical properties with the nerve, c) reduced disturbance to the host tissue due to the small size, d) elimination of sutures or adhesives, e) high circumferential contact with small nerves, f) functionality under considerable strain, and g) graded neuromodulation in a low-threshold stimulation regime. Results demonstrate that the μcPNIs are electromechanically robust, and are capable of reliably recording and stimulating neural activity in vivo in small nerves. The μcPNI may also inform the development of new optical, thermal, ultrasonic, or chemical PNIs as well.

A Tissue-Like Soft All-Hydrogel Battery

To develop wearable and implantable bioelectronics accommodating the dynamic and uneven biological tissues and reducing undesired immune responses, it is critical to adopt batteries with matched mechanical properties with tissues as power sources. However, the batteries available cannot reach the softness of tissues due to the high Young's moduli of components (e.g., metals, carbon materials, conductive polymers, or composite materials). The fabrication of tissue-like soft batteries thus remains a challenge. Here, the first ultrasoft batteries totally based on hydrogels are reported. The ultrasoft batteries exhibit Young's moduli of 80 kPa, perfectly matching skin and organs (e.g., heart). The high specific capacities of 82 mAh g^-1 in all-hydrogel lithium-ion batteries and 370 mAh g^-1 in all-hydrogel zinc-ion batteries at a current density of 0.5 A g^-1 are achieved. Both high stability and biocompatibility of the all-hydrogel batteries have been demonstrated upon the applications of wearable and implantable. This work illuminates a pathway for designing power sources for wearable and implantable electronics with matched mechanical properties.

Soft Bioelectronics Based on Nanomaterials

Recent advances in nanostructured materials and unconventional device designs have transformed the bioelectronics from a rigid and bulky form into a soft and ultrathin form and brought enormous advantages to the bioelectronics. For example, mechanical deformability of the soft bioelectronics and thus its conformal contact onto soft curved organs such as brain, heart, and skin have allowed researchers to measure high-quality biosignals, deliver real-time feedback treatments, and lower long-term side-effects in vivo. Here, we review various materials, fabrication methods, and device strategies for flexible and stretchable electronics, especially focusing on soft biointegrated electronics using nanomaterials and their composites. First, we summarize top-down material processing and bottom-up synthesis methods of various nanomaterials. Next, we discuss state-of-the-art technologies for intrinsically stretchable nanocomposites composed of nanostructured materials incorporated in elastomers or hydrogels. We also briefly discuss unconventional device design strategies for soft bioelectronics. Then individual device components for soft bioelectronics, such as biosensing, data storage, display, therapeutic stimulation, and power supply devices, are introduced. Afterward, representative application examples of the soft bioelectronics are described. A brief summary with a discussion on remaining challenges concludes the review.

Beyond homogeneous dispersion: oriented conductive fillers for high κ nanocomposites

Rational design of structures for regulating the thermal conductivities (κ) of materials is critical to many components and products employed in electrical, electronic, energy, construction, aerospace, and medical applications. As such, considerable efforts have been devoted to developing polymer composites with tailored conducting filler architectures and thermal conduits for highly improved κ. This paper is dedicated to overviewing recent advances in this area to offer perspectives for the next level of future development. The limitations of conventional particulate-filled composites and the issue of percolation are discussed. In view of different directions of heat dissipation in polymer composites for different end applications, various approaches for designing the micro- and macroscopic structures of thermally conductive networks in the polymer matrix are highlighted. Methodological approaches devised to significantly ameliorate thermal conduction are categorized with respect to the pathways of heat dissipation. Future prospects for the development of thermally conductive polymer composites with modulated thermal conduction pathways are highlighted.

Electronic Drugs: Spatial and Temporal Medical Treatment of Human Diseases

Recent advances in diagnostics and medicines emphasize the spatial and temporal aspects of monitoring and treating diseases. However, conventional therapeutics, including oral administration and injection, have difficulties meeting these aspects due to physiological and technological limitations, such as long-term implantation and a narrow therapeutic window. As an innovative approach to overcome these limitations, electronic devices known as electronic drugs (e-drugs) have been developed to monitor real-time body signals and deliver specific treatments to targeted tissues or organs. For example, ingestible and patch-type e-drugs could detect changes in biomarkers at the target sites, including the gastrointestinal (GI) tract and the skin, and deliver therapeutics to enhance healing in a spatiotemporal manner. However, medical treatments often require invasive surgical procedures and implantation of medical equipment for either short or long-term use. Therefore, approaches that could minimize implantation-associated side effects, such as inflammation and scar tissue formation, while maintaining high functionality of e-drugs, are highly needed. Herein, the importance of the spatial and temporal aspects of medical treatment is thoroughly reviewed along with how e-drugs use cutting-edge technological innovations to deal with unresolved medical challenges. Furthermore, diverse uses of e-drugs in clinical applications and the future perspectives of e-drugs are discussed.

Functionalized Elastomers for Intrinsically Soft and Biointegrated Electronics

Elastomers are suitable materials for constructing a conformal interface with soft and curvilinear biological tissue due to their intrinsically deformable mechanical properties. Intrinsically soft electronic devices whose mechanical properties are comparable to human tissue can be fabricated using suitably functionalized elastomers. This article reviews recent progress in functionalized elastomers and their application to intrinsically soft and biointegrated electronics. Elastomers can be functionalized by adding appropriate fillers, either nanoscale materials or polymers. Conducting or semiconducting elastomers synthesized and/or processed with these materials can be applied to the fabrication of soft biointegrated electronic devices. For facile integration of soft electronics with the human body, additional functionalization strategies can be employed to improve adhesive or autonomous healing properties. Recently, device components for intrinsically soft and biointegrated electronics, including sensors, stimulators, power supply devices, displays, and transistors, have been developed. Herein, representative examples of these fully elastomeric device components are discussed. Finally, the remaining challenges and future outlooks for the field are presented.

Wireless Power Transfer and Telemetry for Implantable Bioelectronics

Implantable bioelectronic devices are becoming useful and prospective solutions for various diseases owing to their ability to monitor or manipulate body functions. However, conventional implantable devices (e.g., pacemaker and neurostimulator) are still bulky and rigid, which is mostly due to the energy storage component. In addition to mechanical mismatch between the bulky and rigid implantable device and the soft human tissue, another significant drawback is that the entire device should be surgically replaced once the initially stored energy is exhausted. Besides, retrieving physiological information across a closed epidermis is a tricky procedure. However, wireless interfaces for power and data transfer utilizing radio frequency (RF) microwave offer a promising solution for resolving such issues. While the RF interfacing devices for power and data transfer are extensively investigated and developed using conventional electronics, their application to implantable bioelectronics is still a challenge owing to the constraints and requirements of in vivo environments, such as mechanical softness, small module size, tissue attenuation, and biocompatibility. This work elucidates the recent advances in RF-based power transfer and telemetry for implantable bioelectronics to tackle such challenges.

Nano-biosupercapacitors enable autarkic sensor operation in blood

Today's smallest energy storage devices for in-vivo applications are larger than 3 mm³ and lack the ability to continuously drive the complex functions of smart dust electronic and microrobotic systems. Here, we create a tubular biosupercapacitor occupying a mere volume of 1/1000 mm³ (=1 nanoliter), yet delivering up to 1.6 V in blood. The tubular geometry of this nano-biosupercapacitor provides efficient self-protection against external forces from pulsating blood or muscle contraction. Redox enzymes and living cells, naturally present in blood boost the performance of the device by 40% and help to solve the self-discharging problem persistently encountered by miniaturized supercapacitors. At full capacity, the nano-biosupercapacitors drive a complex integrated sensor system to measure the pH-value in blood. This demonstration opens up opportunities for next generation intravascular implants and microrobotic systems operating in hard-to-reach small spaces deep inside the human body.

Flexible and biodegradable electronic implants for diagnosis and treatment of brain diseases

In the diagnosis and treatment of brain diseases, implantable devices have immense potential for intracranial sensing of brain activity and application of controlled therapy for providing feedback to the sensing. Flexible materials are preferred for implantable devices, as they can minimise implanted device-brain tissue mechanical mismatch. Moreover, biodegradable implantable devices can reduce potential immunological side-effects. Biodegradability also helps avoid the burdensome secondary surgery for retrieving the implanted device. In this study, we reviewed recent advancements related to the use of flexible and biodegradable type of implantable devices for the diagnosis and treatment of brain diseases. Representative cases of intracranial sensing and feedback therapy are introduced, and then a brief discussion concludes the review.

Wearable and Implantable Soft Bioelectronics: Device Designs and Material Strategies

High-performance wearable and implantable devices capable of recording physiological signals and delivering appropriate therapeutics in real time are playing a pivotal role in revolutionizing personalized healthcare. However, the mechanical and biochemical mismatches between rigid, inorganic devices and soft, organic human tissues cause significant trouble, including skin irritation, tissue damage, compromised signal-to-noise ratios, and limited service time. As a result, profuse research efforts have been devoted to overcoming these issues by using flexible and stretchable device designs and soft materials. Here, we summarize recent representative research and technological advances for soft bioelectronics, including conformable and stretchable device designs, various types of soft electronic materials, and surface coating and treatment methods. We also highlight applications of these strategies to emerging soft wearable and implantable devices. We conclude with some current limitations and offer future prospects of this booming field.

Emerging Functional Biomaterials as Medical Patches

Medical patches have been widely explored and applied in various medical fields, especially in wound healing, tissue engineering, and other biomedical areas. Benefiting from emerging biomaterials and advanced manufacturing technologies, great achievements have been made on medical patches to evolve them into a multifunctional medical device for diverse health-care purposes, thus attracting extensive attention and research interest. Here, we provide up-to-date research concerning emerging functional biomaterials as medical patches. An overview of the various approaches to construct patches with micro- and nanoarchitecture is presented and summarized. We then focus on the applications, especially the biomedical applications, of the medical patches, including wound healing, drug delivery, and real-time health monitoring. The challenges and prospects for the future development of the medical patches are also discussed.

Soft implantable drug delivery device integrated wirelessly with wearable devices to treat fatal seizures

Personalized biomedical devices have enormous potential to solve clinical challenges in urgent medical situations. Despite this potential, a device for in situ treatment of fatal seizures using pharmaceutical methods has not been developed yet. Here, we present a novel treatment system for neurological medical emergencies, such as status epilepticus, a fatal epileptic condition that requires immediate treatment, using a soft implantable drug delivery device (SID). The SID is integrated wirelessly with wearable devices for monitoring electroencephalography signals and triggering subcutaneous drug release through wireless voltage induction. Because of the wireless integration, bulky rigid components such as sensors, batteries, and electronic circuits can be moved from the SID to wearables, and thus, the mechanical softness and miniaturization of the SID are achieved. The efficacy of the prompt treatment could be demonstrated with animal experiments in vivo, in which brain damages were reduced and survival rates were increased.

Advances in drug delivery technology for the treatment of glioblastoma multiforme

Glioblastoma multiforme (GBM) is a particularly aggressive and malignant type of brain tumor, notorious for its high recurrence rate and low survival rate. The treatment of GBM is challenging mainly because several issues associated with the GBM microenvironment have not yet been resolved. These obstacles originate from a variety of factors such as genetics, anatomy, and cytology, all of which collectively hinder the treatment of GBM. Recent advances in materials and device engineering have presented new perspectives with regard to unconventional drug administration methods for GBM treatment. Such novel drug delivery approaches, based on the clear understanding of the intrinsic properties of GBM, have shown promise in overcoming some of the obstacles. In this review, we first recapitulate the first-line therapy and clinical challenges in the current treatment of GBM. Afterwards, we introduce the latest technological advances in drug delivery strategies to improve the efficiency for GBM treatment, mainly focusing on materials and devices. We describe such efforts by classifying them into two categories, systemic and local drug delivery. Finally, we discuss unmet challenges and prospects for the clinical translation of these drug delivery technologies.

Enhancement of skin permeability with thermal ablation techniques: concept to commercial products

Traditionally, the skin is considered as a protective barrier which acts as a highly impermeable region of the human body. But in recent times, it is recognized as a specialized organ that aids in the delivery of a wide range of drug molecules into the skin (intradermal drug delivery) and across the skin into systemic circulation (transdermal drug delivery, TDD). The bioavailability of a drug administered transdermally can be improved by several penetration enhancement techniques, which are broadly classified into chemical and physical techniques. Application of mentioned techniques together with efforts of various scientific and innovative companies had made TDD a multibillion dollar market and an average of 2.6 new transdermal drugs are being approved each year. Out of various techniques, the thermal ablation techniques involving chemicals, heating elements, lasers, and radiofrequency (RF) are proved to be more effective in terms of delivering the drug across the skin by disrupting the stratum corneum (SC). The reason behind it is that the thermal ablation technique resulted in improved bioavailability, quick treatment and fast recovery of the SC, and more importantly it does not cause any damage to underlying dermis layer. This review article mainly discussed about various thermal ablation techniques with commercial products and patents in each classes, and their safety aspects. This review also briefly presented anatomy of the skin, penetration pathways across the skin, and different generations of TDD.

Skin-interfaced sensors in digital medicine: from materials to applications

The recent advances in skin-interfaced wearable sensors have enabled tremendous potential towards personalized medicine and digital health. Compared with traditional healthcare, wearable sensors could perform continuous and non-invasive data collection from the human body and provide an insight into both fitness monitoring and medical diagnostics. In this review, we summarize the latest progress of skin-interfaced wearable sensors along with their integrated systems. We first introduce the strategies of materials selection and structure design that can be accommodated for intimate contact with human skin. Current development of physical and biochemical sensors is then classified and discussed with an emphasis on their sensing mechanisms. System-level integration including power supply, wireless communication and data analysis are also briefly discussed. We conclude with an outlook of this field and identify the key challenges and opportunities for future wearable devices and systems.

Materials engineering, processing, and device application of hydrogel nanocomposites

Hydrogels are widely implemented as key materials in various biomedical applications owing to their soft, flexible, hydrophilic, and quasi-solid nature. Recently, however, new material properties over those of bare hydrogels have been sought for novel applications. Accordingly, hydrogel nanocomposites, i.e., hydrogels converged with nanomaterials, have been proposed for the functional transformation of conventional hydrogels. The incorporation of suitable nanomaterials into the hydrogel matrix allows the hydrogel nanocomposite to exhibit multi-functionality in addition to the biocompatible feature of the original hydrogel. Therefore, various hydrogel composites with nanomaterials, including nanoparticles, nanowires, and nanosheets, have been developed for diverse purposes, such as catalysis, environmental purification, bio-imaging, sensing, and controlled drug delivery. Furthermore, novel technologies for the patterning of such hydrogel nanocomposites into desired shapes have been developed. The combination of such material engineering and processing technologies has enabled the hydrogel nanocomposite to become a key soft component of electronic, electrochemical, and biomedical devices. We herein review the recent research trend in the field of hydrogel nanocomposites, particularly focusing on materials engineering, processing, and device applications. Furthermore, the conclusions are presented with the scope of future research outlook, which also includes the current technical limitations.

Flexible Electrochemical Bioelectronics: The Rise of In Situ Bioanalysis

The amalgamation of flexible electronics in biological systems has shaped the way health and medicine are administered. The growing field of flexible electrochemical bioelectronics enables the in situ quantification of a variety of chemical constituents present in the human body and holds great promise for personalized health monitoring owing to its unique advantages such as inherent wearability, high sensitivity, high selectivity, and low cost. It represents a promising alternative to probe biomarkers in the human body in a simpler method compared to conventional instrumental analytical techniques. Various bioanalytical technologies are employed in flexible electrochemical bioelectronics, including ion-selective potentiometry, enzymatic amperometry, potential sweep voltammetry, field-effect transistors, affinity-based biosensing, as well as biofuel cells. Recent key innovations in flexible electrochemical bioelectronics from electrochemical sensing modalities, materials, systems, fabrication, to applications are summarized and highlighted. The challenges and opportunities in this field moving forward toward future preventive and personalized medicine devices are also discussed.

Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability

Wearable electronics have revolutionized the way physiological parameters are sensed, detected, and monitored. In recent years, advances in flexible and stretchable hybrid electronics have created emergent properties that enhance the compliance of devices to our skin. With their unobtrusive attributes, skin conformable sensors enable applications toward real-time disease diagnosis and continuous healthcare monitoring. Herein, critical perspectives of flexible hybrid electronics toward the future of digital health monitoring are provided, emphasizing its role in physiological sensing. In particular, the strategies within the sensor composition to render flexibility and stretchability while maintaining excellent sensing performance are considered. Next, novel approaches to the functionalization of the sensor for physical or biochemical stimuli are extensively covered. Subsequently, wearable sensors measuring physical parameters such as strain, pressure, temperature, as well as biological changes in metabolites and electrolytes are reported. Finally, their implications toward early disease detection and monitoring are discussed, concluding with a future perspective into the challenges and opportunities in emerging wearable sensor designs for the next few years.

A Multifunctional Electronic Skin Empowered with Damage Mapping and Autonomic Acceleration of Self-Healing in Designated Locations

Integrating self-healing capabilities into soft electronic devices and sensors is important for increasing their reliability, longevity, and sustainability. Although some advances in self-healing soft electronics have been made, many challenges have been hindering their integration in digital electronics and their use in real-world conditions. Herein, an electronic skin (e-skin) with high sensing performance toward temperature, pressure, and pH levels-both at ambient and/or in underwater conditions is reported. The e-skin is empowered with a novel self-repair capability that consists of an intrinsic mechanism for efficient self-healing of small-scale damages as well as an extrinsic mechanism for damage mapping and on-demand self-healing of big-scale damages in designated locations. The overall design is based on a multilayered structure that integrates a neuron-like nanostructured network for self-monitoring and damage detection and an array of electrical heaters for selective self-repair. This system has significantly enhanced self-healing capabilities; for example, it can decrease the healing time of microscratches from 24 h to 30 s. The electronic platform lays down the foundation for the development of a new subcategory of self-healing devices in which electronic circuit design is used for self-monitoring, healing, and restoring proper device function.

Material Design and Fabrication Strategies for Stretchable Metallic Nanocomposites

Stretchable conductive nanocomposites fabricated by integrating metallic nanomaterials with elastomers have become a vital component of human-friendly electronics, such as wearable and implantable devices, due to their unconventional electrical and mechanical characteristics. Understanding the detailed material design and fabrication strategies to improve the conductivity and stretchability of the nanocomposites is therefore important. This Review discusses the recent technological advances toward high performance stretchable metallic nanocomposites. First, the effect of the filler material design on the conductivity is briefly discussed, followed by various nanocomposite fabrication techniques to achieve high conductivity. Methods for maintaining the initial conductivity over a long period of time are also summarized. Then, strategies on controlled percolation of nanomaterials are highlighted, followed by a discussion regarding the effects of the morphology of the nanocomposite and postfabricated 3D structures on achieving high stretchability. Finally, representative examples of applications of such nanocomposites in biointegrated electronics are provided. A brief outlook concludes this Review.

Multi-Access Networking with Wireless Ultrasound-Powered Implants

Multi-access networking with miniaturized wireless implantable devices can enable and advance closed-loop medical applications to deliver precise diagnosis and treatment. Using ultrasound (US) for wireless implant systems is an advantageous approach as US can beamform with high spatial resolution to efficiently power and address multiple implants in the network. To demonstrate these capabilities, we use wirelessly powered mm-sized implants with bidirectional communication links; uplink data communication measurements are performed using time, spatial, and frequency-division multiplexing schemes in tissue phantom. A 32-channel linear transmitter array and an external receiver are used as a base station to network with two implants that are placed 6.5 cm deep and spaced less than 1 cm apart. Successful wireless powering and uplink data communication around 100 kbps with a measured bit error rate below 10^-4 are demonstrated for all three networking schemes, validating the multi-access networking feasibility of US wireless implant systems.

Fundamental Limits to the Electrochemical Impedance Stability of Dielectric Elastomers in Bioelectronics

Incorporation of elastomers into bioelectronics that reduces the mechanical mismatch between electronics and biological systems could potentially improve the long-term electronics-tissue interface. However, the chronic stability of elastomers in physiological conditions has not been systematically studied. Here, using electrochemical impedance spectrum we find that the electrochemical impedance of dielectric elastomers degrades over time in physiological environments. Both experimental and computational results reveal that this phenomenon is due to the diffusion of ions from the physiological solution into elastomers over time. Their conductivity increases by 6 orders of magnitude up to 10^-8 S/m. When the passivated conductors are also composed of intrinsically stretchable materials, higher leakage currents can be detected. Scaling analyses suggest fundamental limitations to the electrical performances of interconnects made of stretchable materials.

Emerging combination strategies with phototherapy in cancer nanomedicine

Based on the challenges in single-mode phototherapy, this review summarizes the significant research progress in combinatorial strategies with phototherapy.

Implantable Neural Interfaces and Wearable Tactile Systems for Bidirectional Neuroprosthetics Systems

Neuroprosthetics and neuromodulation represent a promising field for several related applications in the central and peripheral nervous system, such as the treatment of neurological disorders, the control of external robotic devices, and the restoration of lost tactile functions. These actions are allowed by the neural interface, a miniaturized implantable device that most commonly exploits electrical energy to fulfill these operations. A neural interface must be biocompatible, stable over time, low invasive, and highly selective; the challenge is to develop a safe, compact, and reliable tool for clinical applications. In case of anatomical impairments, neuroprosthetics is bound to the need of exploring the surrounding environment by fast-responsive and highly sensitive artificial tactile sensors that mimic the natural sense of touch. Tactile sensors and neural interfaces are closely interconnected since the readouts from the first are required to convey information to the neural implantable apparatus. The role of these devices is pivotal hence technical improvements are essential to ensure a secure system to be eventually adopted in daily life. This review highlights the fundamental criteria for the design and microfabrication of neural interfaces and artificial tactile sensors, their use in clinical applications, and future enhancements for the release of a second generation of devices.

Battery-free, wireless sensors for full-body pressure and temperature mapping

Thin, soft, skin-like sensors capable of precise, continuous measurements of physiological health have broad potential relevance to clinical health care. Use of sensors distributed over a wide area for full-body, spatiotemporal mapping of physiological processes would be a considerable advance for this field. We introduce materials, device designs, wireless power delivery and communication strategies, and overall system architectures for skin-like, battery-free sensors of temperature and pressure that can be used across the entire body. Combined experimental and theoretical investigations of the sensor operation and the modes for wireless addressing define the key features of these systems. Studies with human subjects in clinical sleep laboratories and in adjustable hospital beds demonstrate functionality of the sensors, with potential implications for monitoring of circadian cycles and mitigating risks for pressure-induced skin ulcers.

Near-Field Communication Sensors

Near-field communication is a new kind of low-cost wireless communication technology developed in recent years, which brings great convenience to daily life activities such as medical care, food quality detection, and commerce. The integration of near-field communication devices and sensors exhibits great potential for these real-world applications by endowing sensors with new features of powerless and wireless signal transferring and conferring near field communication device with sensing function. In this review, we summarize recent progress in near field communication sensors, including the development of materials and device design and their applications in wearable personal healthcare devices. The opportunities and challenges in near-field communication sensors are discussed in the end.

Bioresorbable Electronic Implants: History, Materials, Fabrication, Devices, and Clinical Applications

Medical implants, either passive implants for structural support or implantable devices with active electronics, have been widely used for the diagnosis and treatment of various diseases and clinical issues. These implants offer various functions, including mechanical support of biological structures in orthopedic and dental applications, continuous electrophysiological monitoring and feedback of electrical stimulation in neuronal and cardiac applications, and controlled drug delivery while maintaining arterial structure in drug-eluting stents. Although these implants exhibit long-term biocompatibility, surgery for their retrieval is often required, which imposes physical, biological, and economical burdens on the patients. Therefore, as an alternative to such secondary surgeries, bioresorbable implants that disappear after a certain period of time inside the body, including bioresorbable active electronics, have been highlighted recently. This review first discusses the historical background of medical implants and briefly define related terminology. Representative examples of non-degradable medical implants for passive structural support and/or for diagnosis and therapy with active electronics are also provided. Then, recent progress in bioresorbable active implants composed of biosignal sensors, actuators for therapeutics, wireless power supply components, and their integrated systems are reviewed. Finally, clinical applications of these bioresorbable electronic implants are exemplified with brief conclusion and future outlook.

Polymer-based flexible bioelectronics

Due to the mechanical mismatch between conventional rigid electronic devices and soft tissues at nature, a lot of interests have been attracted to develop flexible bioelectronics that work well both in vitro and in vivo. To this end, polymers that can be used for both key components and substrates are indispensable to achieve high performances such as high sensitivity and long-term stability for sensing applications. Here we will summarize the recent advances on the synthesis of a variety of polymers, the design of typical architectures and the integration of different functions for the flexible bioelectronic devices. The remaining challenges and promising directions are highlighted to provide inspirations for the future study on the emerging flexible bioelectronics at end.