Self-Healable, Self-Adhesive and Degradable MXene-Based Multifunctional Hydrogel for Flexible Epidermal Sensors.

Conductive hydrogels have garnered significant interest in the realm of wearable flexible sensors due to their close resemblance to human tissue, wearability, and precise signal acquisition capabilities. However, the concurrent attainment of an epidermal hydrogel sensor incorporating reliable self-healing capabilities, biodegradability, robust adhesiveness, and the ability to precisely capture subtle electrophysiological signals poses a daunting and intricate challenge. Herein, an innovative MXene-based composite hydrogel (PBM hydrogel) with exceptional self-healing, self-adhesive, and versatile functionality is engineered through the integration of conductive MXene nanosheets into a well-structured poly(vinyl alcohol) (PVA) and bacterial cellulose (BC) hydrogel three-dimensional (3D) network, utilizing multiple dynamic cross-linking synergistic repeated freeze-thaw strategy. The hydrogel harnesses the presence of dynamically reversible borax ester bonds and multiple hydrogen bonds between its constituents, endowing it with rapid self-healing efficiency (97.8%) and formidable self-adhesive capability. The assembled PBM hydrogel epidermal sensor possesses a rapid response time (10 ms) and exhibits versatility in detecting diverse external stimuli and human movements such as vocalization, handwriting, joint motion, Morse code signals, and even monitoring infusion status. Additionally, the PBM hydrogel sensor offers the added advantage of swift degradation in phosphate-buffered saline solution (within a span of 56 days) and H2O2 solution (in just 53 min), maintaining an eco-friendly profile devoid of any environmental pollution. This work lays the groundwork for possible uses in electronic skins, interactions between humans and machines, and the monitoring of individualized healthcare.

Highly Sensitive Biosensors Based on All-PEDOT:PSS Organic Electrochemical Transistors with Laser-Induced Micropatterning.

Recent advances in numerous biological applications have increased the accuracy of monitoring the level of biologically significant analytes in the human body to manage personal nutrition and physiological conditions. However, despite promising reports about costly wearable devices with high sensing performance, there has been a growing demand for inexpensive sensors that can quickly detect biological molecules. Herein, we present highly sensitive biosensors based on organic electrochemical transistors (OECTs), which are types of organic semiconductor-based sensors that operate consistently at low operating voltages in aqueous solutions. Instead of the gold or platinum electrode used in current electrochemical devices, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) was used as both the channel and gate electrodes in the OECT. Additionally, to overcome the patterning resolution limitations of conventional solution processing, we confirmed that the irradiation of a high-power IR laser (λ = 1064 nm) onto the coated PEDOT:PSS film was able to produce spatially resolvable micropatterns in a digital-printing manner. The proposed patterning technique exhibits high suitability for the fabrication of all-PEDOT:PSS OECT devices. The device geometry was optimized by fine-tuning the gate area and the channel-to-gate distance. Consequently, the sensor for detecting ascorbic acid (vitamin C) concentrations in an electrolyte exhibited the best sensitivity of 125 µA dec-1 with a limit of detection of 1.3 µM, which is nearly 2 orders of magnitude higher than previous findings. Subsequently, an all-plastic flexible epidermal biosensor was established by transferring the patterned all-PEDOT:PSS OECT from a glass substrate to a PET substrate, taking full advantage of the flexibility of PEDOT:PSS. The prepared all-plastic sensor device is highly cost-effective and suitable for single-use applications because of its acceptable sensing performance and reliable signal for detecting vitamin C. Additionally, the epidermal sensor successfully obtained the temporal profile of vitamin C in the sweat of a human volunteer after the consumption of vitamin C drinks. We believe that the highly sensitive all-PEDOT:PSS OECT device fabricated using the accurate patterning process exhibits versatile potential as a low-cost and single-use biosensor for emerging bioelectronic applications.

Injectable, Intrinsically Antibacterial Conductive Hydrogels with Self-Healing and pH Stimulus Responsiveness for Epidermal Sensors and Wound Healing.

Torealize intelligent and personalized medicine, it is a huge challenge to develop a hydrogel dressing that can be used as a sensor to monitor human health in real-time while promoting wound healing. Herein, an injectable, self-healing, and conductive chitosan-based (CPT) hydrogel with pH responsiveness and intrinsic antibacterial properties was fabricated via a Schiff base linkage and a hydrogen bond. Due to the introduction of Schiff base bonds, the injectable CPT hydrogel exhibits various excellent properties, such as pH responsiveness to sol-gel transition, self-healing properties, and broad-spectrum antibacterial properties even without additional antibacterial agents. In vitro experiments verify the excellent biocompatibility of the as-prepared hydrogel. An in vivo experiment in a mouse full-thickness skin-wound model was performed to confirm the outstanding effect on wound healing. Meanwhile, as epidermal sensors, the conductive hydrogel that perceives various human activities in real-time could provide the real-time analysis of the patient's healthcare information. Based on these excellent properties, the CPT hydrogel, as a biological dressing with a sensing function, lays a solid foundation for the further realization of personalized medicine.

The design, fabrication, and applications of flexible biosensing devices.

Flexible biosensors form part of a rapidly growing research field that take advantage of a multidisciplinary approach involving materials, fabrication and design strategies to be able to function at biological interfaces that may be soft, intrinsically curvy, irregular, or elastic. Numerous exciting advancements are being proposed and developed each year towards applications in healthcare, fundamental biomedical research, food safety and environmental monitoring. In order to place these developments in perspective, this review is intended to present an overview on field of flexible biosensor development. We endeavor to show how this subset of the broader field of flexible and wearable devices presents unique characteristics inherent in their design. Initially, a discussion on the structure of flexible biosensors is presented to address the critical issues specific to their design. We then summarize the different materials as substrates that can resist mechanical deformation while retaining their function of the bioreceptors and active elements. Several examples of flexible biosensors are presented based on the different environments in which they may be deployed or on the basis of targeted biological analytes. Challenges and future perspectives pertinent to the current and future stages of development are presented. Through these summaries and discussion, this review is expected to provide insights towards a systematic and fundamental understanding for the fabrication and utilization of flexible biosensors, as well as inspire and improve designs for smart and effective devices in the future.

Fabrication of Low-Cost and Highly Sensitive Graphene-Based Pressure Sensors by Direct Laser Scribing Polydimethylsiloxane.

The cost-effective production of flexible interconnects is a challenge in epidermal electronics. Here we report a low-cost approach for producing and patterning graphene films from polydimethylsiloxane films by direct laser scribing in ambient air. The produced graphene films exhibit high electrical conductivity and excellent mechanical properties and can thus be used directly as a flexible conductive layer without the need for metals. The skinlike pressure sensor with these layers exhibits ultrahigh sensitivity (∼480 kPa-1) while maintaining the fast response/relaxation time (2 µs/3 µs) and excellent cycle stability (>4000 repetitive cycles). Moreover, it can naturally attach to the skin to monitor the wrist pulse. In addition, a 7 × 7 sensor array has been fabricated, which possesses the capability to detect the spatial distribution of pressure. This device has great potential for application in epidermal electronics because of its low cost and high performance.

Connexins: sensors of epidermal integrity that are therapeutic targets.

Gap junction proteins (connexins) are differentially expressed throughout the multiple layers of the epidermis. A variety of skin conditions arise with aberrant connexin expression or function and suggest that maintaining the epidermal gap junction network has many important roles in preserving epidermal integrity and homeostasis. Mutations in a number of connexins lead to epidermal dysplasias giving rise to a range of dermatological disorders of differing severity. 'Gain of function' mutations reveal connexin-mediated roles in calcium signalling within the epidermis. Connexins are involved in epidermal innate immunity, inflammation control and in wound repair. The therapeutic potential of targeting connexins to improve wound healing responses is now clear. This review discusses the role of connexins in epidermal integrity, and examines the emerging evidence that connexins act as epidermal sensors to a variety of mechanical, temperature, pathogen-induced and chemical stimuli. Connexins thus act as an integral component of the skin's protective barrier.