Nanomaterials for bioelectronics and integrated medical systems

Soft Bioelectronics for Neuroengineering: New Horizons in the Treatment of Brain Tumor and Epilepsy

Soft bioelectronic technologies for neuroengineering have shown remarkable progress, which include novel soft material technologies and device design strategies. Such technological advances that are initiated from fundamental brain science are applied to clinical neuroscience and provided meaningful promises for significant improvement in the diagnosis efficiency and therapeutic efficacy of various brain diseases recently. System-level integration strategies in consideration of specific disease circumstances can enhance treatment effects further. Here, recent advances in soft implantable bioelectronics for neuroengineering, focusing on materials and device designs optimized for the treatment of intracranial disease environments, are reviewed. Various types of soft bioelectronics for neuroengineering are categorized and exemplified first, and then details for the sensing and stimulating device components are explained. Next, application examples of soft implantable bioelectronics to clinical neuroscience, particularly focusing on the treatment of brain tumor and epilepsy are reviewed. Finally, an ideal system of soft intracranial bioelectronics such as closed-loop-type fully-integrated systems is presented, and the remaining challenges for their clinical translation are discussed.

3D printed electronics with nanomaterials

A large variety of printing, deposition and writing techniques have been incorporated to fabricate electronic devices in the last decades. This approach, printed electronics, has gained great interest in research and practical applications and is successfully fuelling the growth in materials science and technology. On the other hand, a new player is emerging, additive manufacturing, called 3D printing, introducing a new capability to create geometrically complex constructs with low cost and minimal material waste. Having such tremendous technology in our hands, it was just a matter of time to combine advances of printed electronics technology for the fabrication of unique 3D structural electronics. Nanomaterial patterning with additive manufacturing techniques can enable harnessing their nanoscale properties and the fabrication of active structures with unique electrical, mechanical, optical, thermal, magnetic and biological properties. In this paper, we will briefly review the properties of selected nanomaterials suitable for electronic applications and look closer at the current achievements in the synergistic integration of nanomaterials with additive manufacturing technologies to fabricate 3D printed structural electronics. The focus is fixed strictly on techniques allowing as much as possible fabrication of spatial 3D objects, or at least conformal ones on 3D printed substrates, while only selected techniques are adaptable for 3D printing of electronics. Advances in the fabrication of conductive paths and circuits, passive components, antennas, active and photonic components, energy devices, microelectromechanical systems and sensors are presented. Finally, perspectives for development with new nanomaterials, multimaterial and hybrid techniques, bioelectronics, integration with discrete components and 4D-printing are briefly discussed.

Advanced Stretchable Photodetectors: Strategies, Materials and Devices

Past decades have witnessed the generation of new stretchable photodetectors in electronic eyes, health sensing, wearable devices, intelligent monitoring and other fields. Stretchable devices require not only outstanding performance but also excellent flexibility, adaptability and stability. Innovative strategies have been proposed to realize the stretchability of devices. In addition, novel functional materials including zero-dimensional nanomaterials, one-dimensional inorganic nanomaterials, two-dimensional layered materials, organic materials, and organic-inorganic composite materials with excellent properties are emerging to continuously improve the performance of devices. Here, the recent research progress of stretchable photodetectors in terms of both various design methods and functional materials is outlined. The optical performance and stretchable properties are also comprehensively reviewed. Finally, a summary and the challenges associated with the application of stretchable photodetectors are presented.

Nanomaterial-based biohybrid hydrogel in bioelectronics

Despite the broadly applicable potential in the bioelectronics, organic/inorganic material-based bioelectronics have some limitations such as hard stiffness and low biocompatibility. To overcome these limitations, hydrogels capable of bridging the interface and connecting biological materials and electronics have been investigated for development of hydrogel bioelectronics. Although hydrogel bioelectronics have shown unique properties including flexibility and biocompatibility, there are still limitations in developing novel hydrogel bioelectronics using only hydrogels such as their low electrical conductivity and structural stability. As an alternative solution to address these issues, studies on the development of biohybrid hydrogels that incorporating nanomaterials into the hydrogels have been conducted for bioelectronic applications. Nanomaterials complement the shortcomings of hydrogels for bioelectronic applications, and provide new functionality in biohybrid hydrogel bioelectronics. In this review, we provide the recent studies on biohybrid hydrogels and their bioelectronic applications. Firstly, representative nanomaterials and hydrogels constituting biohybrid hydrogels are provided, and next, applications of biohybrid hydrogels in bioelectronics categorized in flexible/wearable bioelectronic devices, tissue engineering, and biorobotics are discussed with recent studies. In conclusion, we strongly believe that this review provides the latest knowledge and strategies on hydrogel bioelectronics through the combination of nanomaterials and hydrogels, and direction of future hydrogel bioelectronics.

Electrical Stimulation of Human Adipose-Derived Mesenchymal Stem Cells on O2 Plasma-Treated ITO Glass Promotes Osteogenic Differentiation

Electrical signals represent an essential form of cellular communication. For decades, electrical stimulation has been used effectively in clinical practice to enhance bone healing. However, the detailed mechanisms between electrical stimulation and bone healing are not well understood. In addition, there have been many difficulties in setting up a stable and efficient electrical stimulation system within the in vitro environment. Therefore, various conductive materials and electrical stimulation methods have been tested to establish an effective electrical stimulation system. Through these systems, many studies have been conducted on the effects of electrical stimulation on bone healing and osteogenic differentiation. However, previous studies were limited by the use of opaque conductive materials that obscure the cells; fluorescent observations and staining are known to be two of the critical methods to confirm the states of the cells. Indium tin oxide (ITO) glass is known to have excellent transparency and conductivity, but it is challenging to cultivate cells due to low cell adhesion characteristics. Therefore, we used O2 plasma treatment to increase the hydrophilicity and wettability of ITO glass. This enhanced cell affinity to the glass, providing a stable surface for the cells to attach. Then, electrical stimulation was applied with an amplitude range of 10 to 200 µA at a frequency of 10 Hz. Our results demonstrated that the osteogenic differentiation efficiency was maximized under the amplitude conditions of 10 µA and 50 µA. Accordingly, the results of our study suggest the development of an excellent platform in the field of biological research as a good tool to elucidate various mechanisms of cell bioactivity under electrical conditions.

Recent Advances in High-Mobility and High-Stretchability Organic Field-Effect Transistors: From Materials, Devices to Applications

Stretchable organic field-effect transistors (OFETs) are one of the essential building blocks for next-generation wearable electronics due to the high stretchability of OFET well matching with the large deformation of human skin. In recent years, some significant progress of stretchable OFETs have already been made via the strategies of stretchable molecular design and geometry engineering. However, the main opportunity and challenge of stretchable OFETs is still to simultaneously improve their stretchability and mobility. This review covers the recent advances in the research of stretchable OFETs with high mobility. First, the core stretchable materials are summarized, including organic semiconductors, electrodes, dielectrics, and substrates. Second, the materials and healing mechanism of self-healing OFET are summarized in detail. Subsequently, their different configurations and the potential applications are summarized. Finally, an outlook of future research directions and challenges in this area is presented.

Adsorption of Cr(VI) onto cross-linked chitosan-almond shell biochars: equilibrium, kinetic, and thermodynamic studies

In this study, to remove Cr(VI) from the solution environment by adsorption, the almond shell was pyrolyzed at 400 and 500 °C and turned into biochar (ASC400 and ASC500) and composite adsorbents were obtained by coating these biochars with chitosan (Ch-ASC400 and Ch-ASC500). The resulting biochars and composite adsorbents were characterized using Fourier transform infrared (FTIR) spectroscopy; Brunauer, Emmett, and Teller (BET) surface area; scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX); and the point of zero charge pH (pHpzc) analyses. The parameters affecting the adsorption were examined with batch adsorption experiments and the optimum parameters for the efficient adsorption of Cr(VI) in 55 mg L−1 solution were determined as follows; adsorbent dosages: 5 g L−1 for biochars, 1.5 g L−1 for composite adsorbents, contact time: 120 min, pH: 1.5. It was seen that the temperature did not affect the adsorption much. Under optimum conditions, Cr(VI) adsorption capacities of ASC400, ASC500, Ch-ASC400, and Ch-ASC500 adsorbents are 11.33, 11.58, 37.48, and 36.65 mg g−1, respectively, and their adsorption percentages are 95.2%, 97.5%, 94.3%, and 94.0%, respectively. Adsorption data were applied to Langmuir, Freundlich, Scatchard, Dubinin-Radushkevic, and Temkin isotherms and pseudo-first-order kinetic model, pseudo-second-order kinetic model, intra-particle diffusion model, and film diffusion model. The adsorption data fitted well to the Langmuir isotherm and pseudo-second-order kinetic models. From these results, it was determined that chemical adsorption is the dominant mechanism. Also, both intra-particle diffusion and film diffusion is effective in the adsorption rate. For all adsorbents, the Langmuir isotherm proved to be the most appropriate model for adsorption. The maximum monolayer adsorption capacities calculated from this model are 24.15 mg g−1, 27.38 mg g−1, 54.95 mg g−1, and 87.86 mg g−1 for ASC400, ASC500, Ch-ASC400, and Ch-ASC500, respectively. The enthalpy change, entropy change, and free energy changes during the adsorption process were calculated and the adsorption was also examined thermodynamically. As a result, adsorption occurs spontaneously for all adsorbents.

Employing defined bioconjugates to generate chemically functionalised gold nanoparticles for in vitro diagnostic applications

Novel methods for introducing chemical and biological functionality to the surface of gold nanoparticles serve to increase the utility of this class of nanomaterials across a range of applications. To date, methods for functionalising gold surfaces have relied upon uncontrollable non-specific adsorption, bespoke chemical linkers, or non-generalisable protein-protein interactions. Herein we report a versatile method for introducing functionality to gold nanoparticles by exploiting the strong interaction between chemically functionalised bovine serum albumin (f-BSA) and citrate-capped gold nanoparticles (AuNPs). We establish the generalisability of the method by introducing a variety of functionalities to gold nanoparticles using cheap, commercially available chemical linkers. The utility of this approach is further demonstrated through the conjugation of the monoclonal antibody Ontruzant to f-BSA-AuNPs using inverse electron-demand Diels-Alder (iEDDA) click chemistry, a hitherto unexplored chemistry for AuNP-IgG conjugation. Finally, we show that the AuNP-Ontruzant particles generated via f-BSA-AuNPs have a greater affinity for their target in a lateral flow format when compared to conventional physisorption, highlighting the potential of this technology for producing sensitive diagnostic tests.

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.

Enzyme-functionalised, core/shell magnetic nanoparticles for selective pH-triggered sucrose capture

Diabetes is a chronic metabolic disease which leads to high glucose levels in the blood, with severe consequences for human health. Due to the worldwide appeal for the reduction in calorie intake, this study presents the development of a nanomaterial able to capture sucrose selectively, thus providing a tool to remove naturally occurring sucrose from food, such as fruit juices, producing low-calorie juices for consumption. Magnetite nanoparticles (Fe3O4 NPs) coated with an inert material (SiO2) and functionalised with the enzyme invertase were designed to remove sucrose from solutions. Fe3O4 NPs were synthesised using the co-precipitation method, whereas the coating with a silica shell was done by the Stöber method. Its physicochemical characteristics were determined, with excellent stability over time. On the other hand, the invertase enzyme was extracted from dry Baker's yeast, purified and immobilised on the surface of the silica-coated Fe3O4 NPs. pH-triggered sucrose capture occurred at pH 3.0 once invertase with protonated catalytic residues was able just to bind with sucrose in a highly selective way. After a short, 1 min interaction, approximately 13.5 mmol L-1 of sucrose was captured per gram of nanomaterial and removed with the use of an external permanent magnet. The complex sucrose/nanomaterial was washed, and the released sucrose was put into buffered solution (pH = 4.8), where it underwent hydrolysis to yield inverted sugar. On the other side, sucrose-free nanomaterial was reused with no loss of enzymatic capability to capture sucrose at pH = 3.0 and maintained the invertase activity at pH 4.8 in ten consecutive rounds of re-use. As sucrose was recovered in the form of inverted sugar, not just low sugar beverage could be obtained, but also a high valued market product. Thus, the developed technology allows for the commercialisation of low-calorie food, offering healthier options to consumers and helping to fight diabetes and obesity.

Design of Stretchable Holey Gold Biosensing Electrode for Real-Time Cell Monitoring

In bioelectronics, gold thin films have been widely used as sensing electrodes for probing biological events due to their high conductivity, chemical inertness, biocompatibility, wide electrochemical window, and facile surface modification. However, they are intrinsically not stretchable, which limits their applications in detecting biological reactions when a soft biological system is mechanically deformed. Here, we report on a nanosphere lithography-based strategy to generate ordered microhole gold thin-film electrodes supported by elastomeric substrates. Both experimental and theoretical studies show that the presence of microholes substantially suppresses the catastrophic crack propagation-the main reason for electrical failure for a continuous gold film. As a result, the holey gold film achieves a ∼94% stretchable limit, after which the conductivity is lost, in contrast to ∼4% for the nonstructured counterpart. Furthermore, the pinhole gold electrode is successfully used to monitor the H₂O₂ released from living cells under dynamic stretching conditions.

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.

3D printed nanomaterial-based electronic, biomedical, and bioelectronic devices

The ability to seamlessly integrate functional materials into three-dimensional (3D) constructs has been of significant interest, as it can enable the creation of multifunctional devices. Such integration can be achieved with a multiscale, multi-material 3D printing strategy. This technology has enabled the creation of unique devices such as personalized tissue regenerative scaffolds, biomedical implants, 3D electronic devices, and bionic constructs which are challenging to realize with conventional manufacturing processes. In particular, the incorporation of nanomaterials into 3D printed devices can endow a wide range of constructs with tailorable mechanical, chemical, and electrical functionalities. This review highlights the advances and unique possibilities in the fabrication of novel electronic, biomedical, and bioelectronic devices that are realized by the synergistic integration of nanomaterials with 3D printing technologies.

Surface Ligands as Permeation Barrier in the Growth and Assembly of Anisotropic Semiconductor Nanocrystals

Because of the large surface-to-volume ratio of colloidal nanocrystals (NCs), surfactant molecules grafted at the NC surface play an important role in NC growth, interparticle interaction, processing, and application. For this reason, much progress has been made in understanding the surface chemistry of NCs along with the organic ligand shell, particularly in terms of grafted polar groups. However, most explanations of aliphatic counterparts are based on spherical NCs that usually have a dilute ligand layer. In anisotropic NCs such as nanorods and nanoplatelets, the linearly extended dimension results in a high-density aliphatic layer on the NC surface. Unlike spherical NCs, the compact organic shell could serve as a permeation membrane, effectively impeding a penetration of foreign molecules toward the NC surface. In this Perspective, we highlight the effects of ligand configuration on the properties of anisotropic NCs by exploring morphologies, assembled superstructures, and surface reaction of anisotropic NCs.

Ligands as a universal molecular toolkit in synthesis and assembly of semiconductor nanocrystals

The multiple ligands with different functionalities enable atomic-precision control of NCs morphology and subtle inter-NC interactions, which paves the way for novel optoelectronic applications.

Successful exploitation of semiconductor nanocrystals (NCs) in commercial products is due to the remarkable progress in the wet-chemical synthesis and controlled assembly of NCs. Central to the cadence of this progress is the ability to understand how NC growth and assembly can be controlled kinetically and thermodynamically. The arrested precipitation strategy offers a wide opportunity for materials selection, size uniformity, and morphology control. In this colloidal approach, capping ligands play an instrumental role in determining growth parameters and inter-NC interactions. The impetus for exquisite control over the size and shape of NCs and orientation of NCs in an ensemble has called for the use of two or more types of ligands in the system. In multiple ligand approaches, ligands with different functionalities confer extended tunability, hinting at the possibility of atomic-precision growth and long-range ordering of desired superlattices. Here, we highlight the progress in understanding the roles of ligands in size and shape control and assembly of NCs. We discuss the implication of the advances in the context of optoelectronic applications.

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.

Direct cation exchange of CdSe nanocrystals into ZnSe enabled by controlled binding between guest cations and organic ligands

Zn chalcogenides are suitable candidates for blue-emitting fluorophores in light-emitting devices. In particular, the efforts to grow ZnSe nanocrystals (NCs) with fine control over size and shape via bottom-up approaches have faced challenges because of the slow decomposition of Zn precursors. In this study, we report direct cation exchange from CdSe NCs to ZnSe. Absorption spectroscopy and density functional theory (DFT) analysis reveal that the reactivity of cation exchange depends on the degree of complexation between organic ligands and Zn halides. We controlled the binding strength of Zn complexes by changing the organic ligands and halogen species that bind with Zn. Appropriate binding strength allows for the release of Zn ions and their facile incorporation into CdSe seed NCs. Under our experimental conditions, trioctylphosphine oxide (TOPO)-ZnI2 drives the efficient cation exchange reaction whereas TOPO-ZnCl2 induces no cation exchange of CdSe NCs. In addition, functional groups vary the binding strength between Zn and ligands. Oleylamine (OLAm)-ZnI2, which has a weaker ligand-ZnI2 binding than TOPO-ZnI2, breaks down the original morphologies of host CdSe NCs due to the very fast exchange rate. On the other hand, the TOPO-ZnI2 complex induces a mild exchange rate, leading to transformation into various morphologies such as CdSe nanorods (NRs) and nanoplatelets (NPLs) into CdSe/ZnSe heterostructures inaccessible via other synthesis methods. The incorporation of Zn into various morphologies of CdSe results in tunable optical transitions in blue-UV regions. The synthesis of heterostructured NCs in an elongated morphology is possible, opening opportunities in photocatalysis, light emitting diodes, and luminescent solar concentrators.

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.

Solution-processed thin films of semiconducting carbon nanotubes and their application to soft electronics

Semiconducting single-walled carbon nanotube (SWNT) networks are promising for use as channel materials in field-effect transistors (FETs) in next-generation soft electronics, owing to their high intrinsic carrier mobility, mechanical flexibility, potential for low-cost production, and good processability. In this article, we review the recent progress related to carbon nanotube (CNT) devices in soft electronics by describing the materials and devices, processing methods, and example applications in soft electronic systems. First, solution-processed semiconducting SWNT deposition methods along with doping techniques used to achieve stable complementary metal-oxide-semiconductor devices are discussed. Various strategies for developing high-performance SWNT-based FETs, such as the proper material choices for the gates, dielectrics, and sources/drains of FETs, and methods of improving FET performance, such as hysteresis repression in SWNT-based FETs, are described next. These SWNT-based FETs have been used in flexible, stretchable, and wearable electronic devices to realize functionalities that could not be achieved using conventional silicon-based devices. We conclude this review by discussing the challenges faced by and outlook for CNT-based soft electronics.

High-performance stretchable conductive nanocomposites: materials, processes, and device applications

Highly conductive and intrinsically stretchable electrodes are vital components of soft electronics such as stretchable transistors and circuits, sensors and actuators, light-emitting diode arrays, and energy harvesting devices. Many kinds of conducting nanomaterials with outstanding electrical and mechanical properties have been integrated with elastomers to produce stretchable conductive nanocomposites. Understanding the characteristics of these nanocomposites and assessing the feasibility of their fabrication are therefore critical for the development of high-performance stretchable conductors and electronic devices. We herein summarise the recent advances in stretchable conductors based on the percolation networks of nanoscale conductive fillers in elastomeric media. After discussing the material-, dimension-, and size-dependent properties of conductive fillers and their implications, we highlight various techniques that are used to reduce the contact resistance between the conductive filler materials. Furthermore, we categorize elastomer matrices with different stretchabilities and mechanical properties based on their polymeric chain structures. Then, we discuss the fabrication techniques of stretchable conductive nanocomposites toward their use in soft electronics. Finally, we provide representative examples of stretchable device applications and conclude the review with a brief outlook for future research.

Wearable and Implantable Soft Bioelectronics Using Two-Dimensional Materials

Soft bioelectronics intended for application to wearable and implantable biomedical devices have attracted great attention from material scientists, device engineers, and clinicians because of their extremely soft mechanical properties that match with a variety of human organs and tissues, including the brain, heart, skin, eye, muscles, and neurons, as well as their wide diversity in device designs and biomedical functions that can be finely tuned for each specific case of applications. These unique features of the soft bioelectronics have allowed minimal mechanical and biological damage to organs and tissues integrated with bioelectronic devices and reduced side effects including inflammation, skin irritation, and immune responses even after long-term biointegration. These favorable properties for biointegration have enabled long-term monitoring of key biomedical indicators with high signal-to-noise ratio, reliable diagnosis of the patient's health status, and in situ feedback therapy with high treatment efficacy optimized for the requirements of each specific disease model. These advantageous device functions and performances could be maximized by adopting novel high-quality soft nanomaterials, particularly ultrathin two-dimensional (2D) materials, for soft bioelectronics. Two-dimensional materials are emerging material candidates for the channels and electrodes in electronic devices (semiconductors and conductors, respectively). They can also be applied to various biosensors and therapeutic actuators in soft bioelectronics. The ultrathin vertically layered nanostructure, whose layer number can be controlled in the synthesis step, and the horizontally continuous planar molecular structure, which can be found over a large area, have conferred unique mechanical, electrical, and optical properties upon the 2D materials. The atomically thin nanostructure allows mechanical softness and flexibility and high optical transparency of the device, while the large-area continuous thin film structure allows efficient carrier transport within the 2D plane. In addition, the quantum confinement effect in the atomically thin 2D layers introduces interesting optoelectronic properties and superb photodetecting capabilities. When fabricated as soft bioelectronic devices, these interesting and useful material features of the 2D materials enable unconventional device functions in biological and optical sensing, as well as superb performance in electrical and biochemical therapeutic actuations. In this Account, we first summarize the distinctive characteristics of the 2D materials in terms of the mechanical, optical, chemical, electrical, and biomedical aspects and then present application examples of the 2D materials to soft bioelectronic devices based on each aforementioned unique material properties. Among various kinds of 2D materials, we particularly focus on graphene and MoS₂. The advantageous material features of graphene and MoS₂ include ultrathin thickness, facile functionalization, large surface-to-volume ratio, biocompatibility, superior photoabsorption, and high transparency, which allow the development of high-performance multifunctional soft bioelectronics, such as a wearable glucose patch, a highly sensitive humidity sensor, an ultrathin tactile sensor, a soft neural probe, a soft retinal prosthesis, a smart endoscope, and a cell culture platform. A brief comparison of their characteristics and performances is also provided. Finally, this Account concludes with a future outlook on next-generation soft bioelectronics based on 2D materials.

Synthesis of indacenodithienothiophene-based conjugated polymers containing electron-donating/accepting comonomers and their phototransistor characteristics

IDTT-based conjugated polymers with electron-accepting comonomers exhibit higher hole mobility (10-fold) and photoresponsivity (2-fold) than those with electron-donating comonomers.

A semi-permanent and durable nanoscale-crack-based sensor by on-demand healing

Although sensitivity and durability are desirable in a sensor, both of them cannot be easily achieved. Site-specific and effective signal acquisition on the limited area of a sensor inevitably allows fatigue accumulation and contamination. For example, an ultrasensitive nanoscale-crack-based sensor for detecting a mechanical stimulus with tremendous sensitivity (a gauge factor greater than 2000 under 2% strain), yet limited durability (up to a few thousand stretching cycles in tensile tests) has been presented previously. Herein, we suggest a simple yet robust nanoscale-crack-based sensor that achieves remarkable durability through the use of a self-healable polymer. The self-healable polymer helps the crack gap recover and maintain high stability for 1 million cycles under 2% strain. Moreover, site-specific recovery with infrared light irradiation was demonstrated with monolithic arrayed sensors. The proposed strategy provides a unique solution to achieving highly enhanced durability and high mechanosensitivity, which are typically incompatible.