Implantable Electronics Research Articles

Acoustic Tunable Battery‐Free Implants Based on Sustainable Triboelectric Nanogenerators With Metal‐Polymer Intermixing Layers

AbstractUltrasound‐driven triboelectric nanogenerators (US‐TENGs) offer an innovative solution for transcutaneous power transfer, with the potential to enable battery‐free, permanently implantable electronics. However, research to date has primarily demonstrated only fragmentary functionalities for these applications. This work presents the simultaneous transmission of acoustic power and precise acoustic information using a double‐electrode US‐TENG, enabling a battery‐free implant controlled via ultrasound. High and sustained output from a US‐TENG is crucial for operating the versatile system; therefore, a novel triboelectric membrane with a top electrode incorporating a gold‐polymer intermixing layer has been designed. Reversible micro‐cracks form in the intermixing layer, ensuring electrical connectivity under high‐frequency strain. In vivo experiments confirm that the system is biocompatible and can be reliably operated inside living rats. These achievements represent a significant step toward realizing multifunctional implantable electronics that can be reliably powered and controlled by ultrasound.

Improvement of Thermal Stability of Charges in Polylactic Acid Electret Films for Biodegradable Electromechanical Sensors.

Eco-friendly sensors fabricated from biocompatible and biodegradable materials are promising candidates for wearable and implantable electronics due to their environmental sustainability and biosafety. This article reports a fully biodegradable electromechanical sensor (FBES) utilizing a sandwich structure with macro ripple structured polylactic acid (PLA) electret films acting as sensitive layers and molybdenum (Mo) sheets serving as electrodes for a wearable device application. The stability of the space charge stored within the PLA film has been enhanced by introducing an internal cellular structure and improving the polarization process. A macro ripple structure of the PLA layer with higher deformation is a great guarantee for boosting the pressure sensitivity. The results indicate that inserting cell microstructures and optimizing the polarization process significantly improve the charge storage stability of PLA films by nearly 55%. This enhancement is attributed to several factors, including the extended charge drift path of the charges in cellular films, a synergy effect of surface charges, and "macroscopic" dipole charges distributed in the cells. The fabricated sensor achieves a high sensitivity of 1000 pC/kPa, a wide pressure detection range of 0.03-62.4 kPa, and satisfactory stability. Such sensors are not only sensitive to body movements but also to subtle physiological signals, satisfying the diverse needs of wearable healthcare. Importantly, all the composition materials of the sensor can be completely degraded after their service, aligning with the environmentally friendly principles of green development.

Robust and Reversible Thermal/Electro‐Responsive Supramolecular Polymeric Adhesives via Synergistic Hydrogen‐Bonds and Ionic Junctions

Adhesive conducting elastomers are rising materials towards cutting‐edge applications in wearable and implantable soft electronics. Yet, engineering the conductive adhesives with robust and tunable interfacial bonding strength is still in its infancy stage. We herein identify a structurally novel supramolecular polymer scaffold, characterized by synergistic coexistence of hydrogen‐bonding (H‐bonding) interactions and electrostatic ionic junctions, endowing the robust and tunable elastic conducting adhesives with remarkable thermal/electro‐responsive performance. H‐bonding association and electrostatic interaction play orthogonal yet synergistic roles in the strong supramolecular adhesive formation, serving as the leveraging forces for opposing both cohesion and adhesion energy. To do so, six‐arm star‐shaped random copolymers P(DAP‐co‐Thy‐co‐MBT)6 are strategically designed, bearing H‐bonding PDAP (poly(diaminopyridine acrylamide)) and PThy (poly(thymine)) segments, which can form hetero‐complementary DAP/Thy H‐bonding association, along with conductive poly(ionic liquid)s segment: PMBT, (poly(1‐[2‐methacryloylethyl]‐3‐methylimidazolium bis(trifluoromethane)‐sulfonamide)). DAP/Thy H‐bonding association, along with electrostatic ionic interaction, can yield dual supramolecular forces crosslinked polymeric networks with robust cohesion energy. Moreover, coexistence of poly(ionic liquid)s can impact and interfere the configuration of H‐bonding association, liberate more free DAP and Thy motifs to form H‐bonds towards substrate, affording strong surface adhesion in a synergistic manner. This work demonstrates a significant forward step towards potential adhesives devoted to hybrid electronic devices.

Robust and Reversible Thermal/Electro-Responsive Supramolecular Polymeric Adhesives via Synergistic Hydrogen-Bonds and Ionic Junctions.

Adhesive conducting elastomers are rising materials towards cutting-edge applications in wearable and implantable soft electronics. Yet, engineering the conductive adhesives with robust and tunable interfacial bonding strength is still in its infancy stage. We herein identify a structurally novel supramolecular polymer scaffold, characterized by synergistic coexistence of hydrogen-bonding (H-bonding) interactions and electrostatic ionic junctions, endowing the robust and tunable elastic conducting adhesives with remarkable thermal/electro-responsive performance. H-bonding association and electrostatic interaction play orthogonal yet synergistic roles in the strong supramolecular adhesive formation, serving as the leveraging forces for opposing both cohesion and adhesion energy. To do so, six-arm star-shaped random copolymers P(DAP-co-Thy-co-MBT)6 are strategically designed, bearing H-bonding PDAP (poly(diaminopyridine acrylamide)) and PThy (poly(thymine)) segments, which can form hetero-complementary DAP/Thy H-bonding association, along with conductive poly(ionic liquid)s segment: PMBT, (poly(1-[2-methacryloylethyl]-3-methylimidazolium bis(trifluoromethane)-sulfonamide)). DAP/Thy H-bonding association, along with electrostatic ionic interaction, can yield dual supramolecular forces crosslinked polymeric networks with robust cohesion energy. Moreover, coexistence of poly(ionic liquid)s can impact and interfere the configuration of H-bonding association, liberate more free DAP and Thy motifs to form H-bonds towards substrate, affording strong surface adhesion in a synergistic manner. This work demonstrates a significant forward step towards potential adhesives devoted to hybrid electronic devices.

Introductory Review of Soft Implantable Bioelectronics Using Conductive and Functional Hydrogels and Hydrogel Nanocomposites.

Interfaces between implantable bioelectrodes and tissues provide critical insights into the biological and pathological conditions of targeted organs, aiding diagnosis and treatment. While conventional bioelectronics, made from rigid materials like metals and silicon, have been essential for recording signals and delivering electric stimulation, they face limitations due to the mechanical mismatch between rigid devices and soft tissues. Recently, focus has shifted toward soft conductive materials, such as conductive hydrogels and hydrogel nanocomposites, known for their tissue-like softness, biocompatibility, and potential for functionalization. This review introduces these materials and provides an overview of recent advances in soft hydrogel nanocomposites for implantable electronics. It covers material strategies for conductive hydrogels, including both intrinsically conductive hydrogels and hydrogel nanocomposites, and explores key functionalization techniques like biodegradation, bioadhesiveness, injectability, and self-healing. Practical applications of these materials in implantable electronics are also highlighted, showcasing their effectiveness in real-world scenarios. Finally, we discuss emerging technologies and future needs for chronically implantable bioelectronics, offering insights into the evolving landscape of this field.

(Invited) Battery-Free Wearable and Implantable Electronics for Chronic Disease Management

60% of Americans live with at least one chronic disease. These diseases and their associated comorbidities are now the leading causes of death in the United States. The effective management of complex chronic diseases requires body-wide, long-term, accurate, and continuous monitoring of multiple physiological signals from wearable and implantable devices to precisely determine the pathological state. Wearable physiological signal monitoring can dramatically reduce the demand for physician visits and increase patients' engagement and treatment adherence rates. Specifically, battery-free wearables and implantable electronics reduce device volume and mechanical stiffness, significantly improving wear comfort, which is highly desirable for next-generation wearable and implantable electronics. However, battery-free wearables and implantable electronics still face many challenges, mainly wireless energy and data transfer. To address these challenges, my research has been involved in the exploration of rational system design concepts, material and device fabrication innovation, and tailored algorithms to enable smart battery-free wearables and implantable electronics targeting next-generation chronic disease management.Here, I would like to discuss two of my developed technology platforms to elaborate on the concept of battery-free wearable and implantable systems. First, I will describe an RFID-based body area sensor network technology platform. This technology uses electromagnetic waves and RF antennas as energy and data transmission media and has broad applications in sleep tracking, workout monitoring, chronic wound healing, and inflammation management. Second, I will describe a triboelectric transducer-based implantable battery-free device. This technology platform uses ultrasound waves and triboelectric transducers as energy and data transmission media and has broad applications in implantable sensing. Overall, the developed technology platforms can assess multiple health outcomes and treatment responses to various chronic diseases. Ultimately, this technology will help reduce the burden of chronic diseases, lower medical costs, and provide a better quality of life for patients. (1,2,3,4)

High-Performance Printed Ag2Se/PI Flexible Thermoelectric Film for Powering Wearable Electronics.

We report the magnificent thermoelectric properties of the n-type Ag2Se film printed onto a flexible polyimide (PI) substrate. The orthorhombic β-Ag2Se phase of the processed Ag2Se film is confirmed from the X-ray diffractogram. Remarkably, the resulting Ag2Se/PI film exhibits outstanding thermoelectric properties, boasting maximum power factors of 1.4 and 2.1 mW/mK2 at 300 and 405 K, respectively. Furthermore, the flexibility of the Ag2Se/PI film remains intact even after undergoing 1500 bending cycles with no degradation observed in its thermoelectric performance. To demonstrate the practical application of our findings, a flexible thermoelectric prototype is constructed using the fabricated Ag2Se/PI films, which can harvest an impressive output voltage of 52 mV across a temperature difference of 53 K. Additionally, the prototype generates a maximum power output of 7.2 μW with a 40 K temperature difference and can produce 13 mV output voltage when subjected to around a 10 K temperature gradient when the cold side temperature is maintained at 308 K. Moreover, leveraging body heat with just a 1 K temperature variance between the body and the surrounding environment, the prototype could yield an impressive voltage output of 1.6 mV, marking the highest reported voltage output from human body heat to date. Our study not only introduces a cost-effective method for producing high-performance flexible thermoelectric films but also highlights their potential applications in wearable and implantable electronics.

Advancements in atomic-scale interface engineering for flexible electronics: Enhancing flexibility and durability.

Flexible electronics, such as wearable displays, implantable electronics, soft robots, and smart skin, have garnered increasing attention. Despite notable advancements in research, a bottleneck remains at the product level due to the prevalent use of polymer-based materials, requiring encapsulation films for lifespan extension and reliable performance. Multilayer composites, incorporating thin inorganic layers to maintain low permeability towards moisture, oxygen, ions, etc., exhibit potential in achieving highly flexible barriers but encounter challenges stemming from interface instability between layers. This perspective offers a succinct review of strategies and provides atomic-scale interface modulation strategy utilizing atomic layer integration technology focused on enhancing the flexibility of high-barrier films. It delves into bendable multilayers with atomic-scale interface modulation strategies, encompassing internal stress and applied stress modulation, as well as stretchable composite structural designs such as gradient/hybrid, wavy, and island. These strategies showcase significant improvements in flexibility from bendable to stretchable while maintaining high barrier properties. Besides, optimized manufacturing methods, materials, and complex structure design based on atomic-scale interface engineering are provided, better aligning with the future development of flexible electronics. By laying the groundwork for these atomic-scale strategies, this perspective contributes to the evolution of flexible electronics, enhancing their flexibility, durability, and functionality.

Universal hydrogel adhesives with robust chain entanglement for bridging soft electronic materials

Ensuring stable integration of diverse soft electronic components for reliable operation under dynamic conditions is crucial. However, integrating soft electronics, comprising various materials like polymers, metals, and hydrogels, poses challenges due to their different mechanical and chemical properties. This study introduces a dried-hydrogel adhesive made of poly(vinyl alcohol) and tannic acid multilayers (d-HAPT), which integrates soft electronic materials through moisture-derived chain entanglement. d-HAPT is a thin (~1 µm) and highly transparent (over 85% transmittance in the visible light region) adhesive, showing robust bonding (up to 3.6 MPa) within a short time (<1 min). d-HAPT demonstrates practical application in wearable devices, including a hydrogel touch panel and strain sensors. Additionally, the potential of d-HAPT for use in implantable electronics is demonstrated through in vivo neuromodulation and electrocardiographic recording experiments while confirming its biocompatibility both in vitro and in vivo. It is expected that d-HAPT will provide a reliable platform for integrating soft electronic applications.

Multifunctional Conductive Hydrogel Interface for Bioelectronic Recording and Stimulation.

The past few decades have witnessed the rapid advancement and broad applications of flexible bioelectronics, in wearable and implantable electronics, brain-computer interfaces, neural science and technology, clinical diagnosis, treatment, etc. It is noteworthy that soft and elastic conductive hydrogels, owing to their multiple similarities with biological tissues in terms of mechanics, electronics, water-rich, and biological functions, have successfully bridged the gap between rigid electronics and soft biology. Multifunctional hydrogel bioelectronics, emerging as a new generation of promising material candidates, have authentically established highly compatible and reliable, high-quality bioelectronic interfaces, particularly in bioelectronic recording and stimulation. This review summarizes the material basis and design principles involved in constructing hydrogel bioelectronic interfaces, and systematically discusses the fundamental mechanism and unique advantages in bioelectrical interfacing with the biological surface. Furthermore, an overview of the state-of-the-art manufacturing strategies for hydrogel bioelectronic interfaces with enhanced biocompatibility and integration with the biological system is presented. This review finally exemplifies the unprecedented advancement and impetus toward bioelectronic recording and stimulation, especially in implantable and integrated hydrogel bioelectronic systems, and concludes with a perspective expectation for hydrogel bioelectronics in clinical and biomedical applications.

A reversible implantable memristor for health monitoring applications

Conventional implantable electronics based on von Neumann architectures encounter significant limitations in computing and processing vast biological information due to computational bottlenecks. The memristor with integrated memory-computing and low power consumption offer a promising solution to overcome the computational bottleneck and Moore's law limitations of traditional silicon-based implantable devices, making them the most promising candidates for next-generation implantable devices. In this work, a highly stable memristor with an Ag/BaTiO3/MnO2/FTO structure was fabricated, demonstrating retention characteristics exceeding 1200 cycles and endurance above 1000 s. The device successfully exhibited three-stage responses to biological signals after implantation in SD (Sprague-Dawley) rats. Importantly, the memristor perform remarkable reversibility, maintaining over 100 cycles of stable repetition even after extraction from the rat. This study provides a new perspective on the biomedical application of memristors, expanding the potential of implantable memristive devices in intelligent medical fields such as health monitoring and auxiliary diagnostics.

Lead-free dual-frequency ultrasound implants for wireless, biphasic deep brain stimulation

Ultrasound-driven bioelectronics could offer a wireless scheme with sustainable power supply; however, current ultrasound implantable systems present critical challenges in biocompatibility and harvesting performance related to lead/lead-free piezoelectric materials and devices. Here, we report a lead-free dual-frequency ultrasound implants for wireless, biphasic deep brain stimulation, which integrates two developed lead-free sandwich porous 1-3-type piezoelectric composite elements with enhanced harvesting performance in a flexible printed circuit board. The implant is ultrasonically powered through a portable external dual-frequency transducer and generates programmable biphasic stimulus pulses in clinically relevant frequencies. Furthermore, we demonstrate ultrasound-driven implants for long-term biosafety therapy in deep brain stimulation through an epileptic rodent model. With biocompatibility and improved electrical performance, the lead-free materials and devices presented here could provide a promising platform for developing implantable ultrasonic electronics in the future.

Implantable Wet‐Adhesive Flexible Electronics with Ultrathin Gelatin Film

AbstractImplantable flexible electronic has attracted significant research interest in various fields. However, it still faces the challenge of simultaneously achieving tight adhesion to tissues in a mildly wet environment and possessing excellent biocompatibility to reduce immune rejection reactions after implantation. Here, a degradable wet‐adhesive flexible electronic device based on liquid metal and ultrathin gelatin film is developed. The ultrathin gelatin film forms numerous hydrogen bonds with tissue in a slightly humid environment, rapidly constructing a wet‐adhesive interface without damaging tissue structure. Inkjet printing is utilized to pattern the mixture of liquid metal and PVP on the surface of the ultrathin gelatin to create flexible patch. With the excellent conductivity of liquid metal, low toxicity, and similarity to natural tissue components of gelatin, flexible patch exhibits outstanding biocompatibility and fatigue resistance. It can be implanted in the body for up to 6 weeks, retaining monitoring capabilities and resisting 1 000 000 cycles of bending fatigue. This study provides a novel strategy for the future development of implantable flexible electronics.

Dynamic healing-assembly for biocompatible, biodegradable, stretchable and self-healing triboelectric nanogenerators

Triboelectric nanogenerators (TENGs) have great potential as power sources for on-skin and implantable electronics. However, most existing TENGs are not suitable for the above applications due to limited material choices, often characterized by rigidity, susceptibility to damage, biological incompatibility, and non-biodegradability. Herein, we developed a simple self-healing assembly strategy based on introducing dynamic Cu(II)-dimethylglyoxime-urethane groups into a biocompatible and biodegradable polyurethane elastomer, to simultaneously address the above obstacles. The coordination of Cu2+ with dynamic bonds significantly enhanced the mechanical properties of elastomer, resulting in a modulus of 9.4 MPa and toughness of 40.2 MJ/m3. These bonds also gave the elastomer skin-like self-healing capabilities, achieving a healing efficiency of 77.5 % at 80 °C. Subsequently, a highly stretchable and integrated triboelectric nanogenerator (Cu-POU TENG) with self-healing capabilities was developed. The Cu-POU TENG with an effective area of 1.5 × 1.5 cm2 produced an open-circuit voltage of 38 V in single-electrode mode. The Cu-POU TENG could restore its original electrical properties through self-healing after mechanical damage. Furthermore, the Cu-POU TENG exhibited biocompatibility and biodegradability, making it suitable for on-skin and implantable electronics. Notably, the Cu-POU TENG maintained a stable voltage output throughout the degradation process. The integration of self-healing feature enabled a sophisticated structure, allowing the Cu-POU TENG to function effectively as a motion sensor for tracking body movement. This Cu-POU TENG holds great potential for applications in on-skin and implantable electronics.

Bioprinting of Stable Bionic Interfaces Using Piezoresistive Hydrogel Organoelectronics.

Bionic tissues offer an exciting frontier in biomedical research by integrating biological cells with artificial electronics, such as sensors. One critical hurdle is the development of artificial electronics that can mechanically harmonize with biological tissues, ensuring a robust interface for effective strain transfer and local deformation sensing. In this study, a highly tissue-integrative, soft mechanical sensor fabricated from a composite piezoresistive hydrogel. The composite not only exhibits exceptional mechanical properties, with elongation at the point of fracture reaching up to 680%, but also maintains excellent biocompatibility across multiple cell types. Furthermore, the material exhibits bioadhesive qualities, facilitating stable cell adhesion to its surface. A unique advantage of the formulation is the compatibility with 3D bioprinting, an essential technique for fabricating stable interfaces. A multimaterial sensorized 3D bionic construct is successfully bioprinted, and it is compared to structures produced via hydrogel casting. In contrast to cast constructs, the bioprinted ones display a high (87%) cell viability, preserve differentiation ability, and structural integrity of the sensor-tissue interface throughout the tissue development duration of 10 d. With easy fabrication and effective soft tissue integration, this composite holds significant promise for various biomedical applications, including implantable electronics and organ-on-a-chip technologies.

Copolymers of Gelatin-graft-poly(3-hexylthiophene) for Transient Electronics.

As transient electronics continue to advance, the demand for new materials has given rise to the exploration of conducting polymer (CP)-based electronic materials. The big challenge lies in balancing conductivity while introducing controlled degradable properties into CP-based transient materials. In response to this, we present in this work a concept of using conducting polymers attached to an enzymatically biodegradable biopolymer to create transient polymer electronics materials. Specifically, poly(3-hexyl thiophene) (P3HT) is covalently grafted onto biopolymer gelatin, affording graft copolymer gelatin-graft-poly(3-hexyl thiophene) (termed Gel-g-P3HT). The thin films of Gel-g-P3HT that were produced by optimized processing solvent (THF/H2O cosolvent) showed enhanced π-π stacking domains of P3HT, resulting in semiconducting thin films with good electroactivity. Due to the presence of amide bonds in the gelatin backbone, Gel-g-P3HT underwent degradation over a period of 5 days, resulting in the formation of amphiphilic micellar nanoparticles that are biocompatible and nontoxic. The potential of these conductive and degradable graft copolymers was demonstrated in a pressure sensor. This research paves the way for developing biocompatible and enzymatically degradable polymer materials based on P3HT, enabling the next generation of transient polymer electronics for diverse applications, such as skin, implantable, and environmental electronics.

Recent advances in implantable sensors and electronics using printable materials for advanced healthcare

This review article focuses on the recent printing technological progress in healthcare, underscoring the significant potential of implantable devices across diverse applications. Printing technologies have widespread use in developing health monitoring devices, diagnostic systems, and surgical devices. Recent years have witnessed remarkable progress in fabricating low-profile implantable devices, driven by advancements in printing technologies and nanomaterials. The importance of implantable biosensors and bioelectronics is highlighted, specifically exploring printing tools using bio-printable inks for practical applications, including a detailed examination of fabrication processes and essential parameters. This review also justifies the need for mechanical and electrical compatibility between bioelectronics and biological tissues. In addition to technological aspects, this article delves into the importance of appropriate packaging methods to enhance implantable devices' performance, compatibility, and longevity, which are made possible by integrating cutting-edge printing technology. Collectively, we aim to shed light on the holistic landscape of implantable biosensors and bioelectronics, showcasing their evolving role in advancing healthcare through innovative printing technologies.

Oblong-shaped piezoelectric ultrasound energy harvester for high-performance wireless power charging

Wireless power charging for implantable biomedical electronics (IBEs) can potentially eliminate the need for frequent battery-replacement surgeries, ultimately improving patients’ quality of life. Ultrasound wireless power transfer (US-WPT) based on piezoelectric receivers has demonstrated significant potential, particularly for deep-seated IBEs. However, most studies on US-WPT have overlooked the optimization of piezoelectric receiver dimensions relative to transmitted ultrasound beam profiles for maximizing battery-charging efficiency for IBEs. This study revealed that a piezoelectric receiver should have dimensions corresponding to the main lobe width of the transmit beam profile within a focal area. Moreover, its output power should be proportional to the cube of the receiver area. Considering these findings, an oblong-shaped ultrasound transmitter and receiver (OsUTR) was developed for efficient WPT, capable of fully charging commercial batteries. In water, the OsUTR produced an output power per unit area of 246.93 mW/cm2, which is 6.588 times higher than that achieved by previously reported methods. This resulted in an average charging rate of 1.64 mC/s, enabling the fully charging of a 30 mAh commercial battery in 1.33 h. Experiments with a 50 mm thick porcine tissue demonstrated that the OsUTR provided an output voltage and current of 38.4 Vp-p and 103.4 mAp-p, respectively. Consequently, full charging of the battery was successfully achieved in 1.80 h. This high-performance OsUTR can enable long-term use of IBEs. This innovation reduces the burden of battery replacement and expands the applicability of implantable devices, driving significant advancements in the IBEs industry.

Wireless powering solution for implantable electronics based on ultra-low frequency magnetic energy focusing.

Wireless powering solution for implantable electronics based on ultra-low frequency magnetic energy focusing.

Integration of Supercapacitors with Sensors and Energy‐Harvesting Devices: A Review

AbstractThe vigorous development of wearable and implantable electronics provides a series of new solutions for human health monitoring and disease diagnosis and treatment. Considering the particularity of the operating environment, this also puts forward higher requirements for energy storage devices (ESDs). Supercapacitors stand out from many ESDs due to their unique advantages, such as high power density, long life, and ease of fabrication. For different application requirements, supercapacitors are developing toward flexibility, multifunction, and integration. This review highlights the recent progress in developing supercapacitor‐integrated systems. Smart supercapacitors with unique properties, their applications, and integrations with various sensors and/or energy‐harvesting devices are discussed and summarized thoroughly. Furthermore, the all‐in‐one device enabled by compatible materials and ingenious structure design is also described. From practical perspectives, the key challenges facing the development of supercapacitor‐integrated systems are presented, and the suggestions for future research directions are given.