Electrostatic Interaction-Based High Tissue Adhesive, Stretchable Microelectrode Arrays for the Electrophysiological Interface

Stimuli-Responsive Hydrogel Adhesives for Wound Closure and Tissue Regeneration

Sutures and staplers, as gold standards for clinical wound closure, usually cause secondary tissue injury and require professional technicians and equipment. The noninvasive hydrogel adhesives are used in various biomedical applications, such as wound closure, tissue sealing, and tissue regeneration, due to their remarkable properties. Recently-developed hydrogel adhesives, especially stimuli-responsive hydrogels, have shown great potential owing to their advantages in regulating their performance and functions according to the wound situations or external conditions, thus allowing the wounds to heal gradually. However, comprehensive summary on stimuli-responsive hydrogels as tissue adhesives is rarely reported to date. This review focuses on the advances in the design of various stimuli-responsive hydrogel adhesives over the past decade, including the systems responsive to pH, temperature, photo, and enzymes. Their potential biomedical applications, such as skin closure, cardiovascular and liver hemostasis, and gastrointestinal sealing, are emphasized. Meanwhile, the challenges and future development of stimuli-responsive hydrogel adhesives are discussed. This review aims to provide meaningful insights for the further design of next-generation of hydrogel adhesives for wound closure and tissue regeneration.

Enhancing the mechanical strength of 3D printed GelMA for soft tissue engineering applications

Gelatin methacrylate (GelMA) hydrogels have gained significant traction in diverse tissue engineering applications through the utilization of 3D printing technology. As an artificial hydrogel possessing remarkable processability, GelMA has emerged as a pioneering material in the advancement of tissue engineering due to its exceptional biocompatibility and degradability. The integration of 3D printing technology facilitates the precise arrangement of cells and hydrogel materials, thereby enabling the creation of in vitro models that simulate artificial tissues suitable for transplantation. Consequently, the potential applications of GelMA in tissue engineering are further expanded. In tissue engineering applications, the mechanical properties of GelMA are often modified to overcome the hydrogel material's inherent mechanical strength limitations. This review provides a comprehensive overview of recent advancements in enhancing the mechanical properties of GelMA at the monomer, micron, and nano scales. Additionally, the diverse applications of GelMA in soft tissue engineering via 3D printing are emphasized. Furthermore, the potential opportunities and obstacles that GelMA may encounter in the field of tissue engineering are discussed. It is our contention that through ongoing technological progress, GelMA hydrogels with enhanced mechanical strength can be successfully fabricated, leading to the production of superior biological scaffolds with increased efficacy for tissue engineering purposes.

Step-by-step fabrication of heart-on-chip systems as models for cardiac disease modeling and drug screening

Cardiovascular diseases are caused by hereditary factors, environmental conditions, and medication-related issues. On the other hand, the cardiotoxicity of drugs should be thoroughly examined before entering the market. In this regard, heart-on-chip (HOC) systems have been developed as a more efficient and cost-effective solution than traditional methods, such as 2D cell culture and animal models. HOCs must replicate the biology, physiology, and pathology of human heart tissue to be considered a reliable platform for heart disease modeling and drug testing. Therefore, many efforts have been made to find the best methods to fabricate different parts of HOCs and to improve the bio-mimicry of the systems in the last decade. Beating HOCs with different platforms have been developed and techniques, such as fabricating pumpless HOCs, have been used to make HOCs more user-friendly systems. Recent HOC platforms have the ability to simultaneously induce and record electrophysiological stimuli. Additionally, systems including both heart and cancer tissue have been developed to investigate tissue-tissue interactions' effect on cardiac tissue response to cancer drugs. In this review, all steps needed to be considered to fabricate a HOC were introduced, including the choice of cellular resources, biomaterials, fabrication techniques, biomarkers, and corresponding biosensors. Moreover, the current HOCs used for modeling cardiac diseases and testing the drugs are discussed. We finally introduced some suggestions for fabricating relatively more user-friendly HOCs and facilitating the commercialization process.

Liquid metal-hydrogel composites for flexible electronics

As an emerging functional material, liquid metal-hydrogel composites exhibit excellent biosafety, high electrical conductivity, tunable mechanical properties and good adhesion, thus providing a unique platform for a wide range of flexible electronics applications such as wearable devices, medical devices, actuators, and energy conversion devices. Through different composite methods, liquid metals can be integrated into hydrogel matrices to form multifunctional composite material systems, which further expands the application range of hydrogels. In this paper, we provide a brief overview of the two materials: hydrogels and liquid metals, and discuss the synthesis method of liquid metal-hydrogel composites, focusing on the improvement of the performance of hydrogel materials by liquid metals. In addition, we summarize the research progress of liquid metal-hydrogel composites in the field of flexible electronics, pointing out the current challenges and future prospects of this material.

A multifunctional sensor for real-time monitoring and pro-healing of frostbite wounds

Flexible epidermal sensors based on conductive hydrogels hold great promise for various applications, such as wearable electronics and personal healthcare monitoring. However, the integration of conductive hydrogel epidermal sensors into multiple applications remains challenging. In this study, a multifunctional PAAm/PEG/hydrolyzed keratin (Hereinafter referred to as HK)/MXene conductive hydrogel (PPHM hydrogel) was designed as a high-performance therapeutic all-in-one epidermal sensor. This sensor not only accelerates wound healing but also provides wearable human-computer interaction. The developed sensor possesses highly sensitive sensing properties (Gauge Factor = 4.82 at high strain), strong mechanical tensile properties (capable of achieving a maximum elongation at break of 600 %), rapid self-healing capability, stable self-adhesive capability, biocompatibility, freeze resistance at -20 °C, and adjustable photo-thermal conversion capability. This therapeutic all-in-one sensor can sensitively monitor human movements, enabling the detection of small electrophysiological signals for diagnosing relevant activities and diseases. Furthermore, using a rat frostbite model, we demonstrated that the composite hydrogel sensor can serve as an effective wound dressing to accelerate the healing process. This study serves as a valuable reference for the development of multifunctional flexible epidermal sensors for personal smart health monitoring. STATEMENT OF SIGNIFICANCE: Accelerated wound healing reduces the risk of wound infection, and conductive hydrogel-based sensors can monitor physiological signals. The multifunctional application of conductive hydrogel sensors combined with wound diagnostic and therapeutic capabilities can meet personalized medical requirements for wound healing and sensor monitoring. The aim of this study is to develop a multifunctional hydrogel patch. The multifunctional hydrogel can be assembled into a flexible wearable high-performance diagnostic and therapeutic integrated sensor that can effectively accelerate the healing of frostbite wounds and satisfy the real-time monitoring of multi-application scenarios. We expect that this study will inform efforts to integrate wound therapy and sensor monitoring.

Using Human-Induced Pluripotent Stem Cell Derived Neurons on Microelectrode Arrays to Model Neurological Disease: A Review

In situ physiological signals of in vitro neural disease models are essential for studying pathogenesis and drug screening. Currently, an increasing number of in vitro neural disease models are established using human-induced pluripotent stem cell (hiPSC) derived neurons (hiPSC-DNs) to overcome interspecific gene expression differences. Microelectrode arrays (MEAs) can be readily interfaced with two-dimensional (2D), and more recently, three-dimensional (3D) neural stem cell-derived in vitro models of the human brain to monitor their physiological activity in real time. Therefore, MEAs are emerging and useful tools to model neurological disorders and disease in vitro using human iPSCs. This is enabling a real-time window into neuronal signaling at the network scale from patient derived. This paper provides a comprehensive review of MEA's role in analyzing neural disease models established by hiPSC-DNs. It covers the significance of MEA fabrication, surface structure and modification schemes for hiPSC-DNs culturing and signal detection. Additionally, this review discusses advances in the development and use of MEA technology to study in vitro neural disease models, including epilepsy, autism spectrum developmental disorder (ASD), and others established using hiPSC-DNs. The paper also highlights the application of MEAs combined with hiPSC-DNs in detecting in vitro neurotoxic substances. Finally, the future development and outlook of multifunctional and integrated devices for in vitro medical diagnostics and treatment are discussed.

Mussel-Inspired Wet-Adhesive Multifunctional Organohydrogel with Extreme Environmental Tolerance for Wearable Strain Sensor

As a flexible artificial material, the conductive hydrogel has broad application prospects in flexible wearable electronics, soft robotics, and biomedical monitoring. However, traditional hydrogels still face many challenges, such as long-term stability, availability in extreme environments, and long-lasting adhesion to the skin surface under sweaty or humid conditions. To circumvent the above issues, one kind of ionic conductive hydrogel was prepared by a simple one-pot method that dissolved chitosan (CS), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), tannic acid (TA), and 2-methoxy-ethyl acrylate (MEA) into dimethyl sulfoxide (DMSO)/H2O solvent. The resulting hydrogel showed excellent tensile properties (1440%), extreme environmental tolerance (-40-60 °C), adhesion (72 KPa at porcine skin), ionic conductivity (0.87 S m-1), and high-efficiency antibacterial property. Furthermore, the produced organohydrogel strain sensor exhibited high strain sensitivity (GF = 4.07), excellent signal sensing capabilities (human joint movement, microexpression, and sound signals), and long-term cyclic stability (400 cycles). Looking beyond, this work provides a simple and promising strategy for using hydrogel sensors in extreme environments for e-skin, health monitoring, and wearable electronic devices.

A Nonswelling Hydrogel with Regenerable High Wet Tissue Adhesion for Bioelectronics

Reducing the swelling of tissue-adhesive hydrogels is crucial for maintaining stable tissue adhesion and inhibiting tissue inflammation. However, reported strategies for reducing swelling always result in a simultaneous decrease in the tissue adhesive strength of the hydrogel. Furthermore, once the covalent bonds break in the currently reported hydrogels, they cannot be rebuilt, and the hydrogel loses its tissue adhesive ability. In this work, a nonswelling hydrogel (named as "PAACP") possessing regenerable high tissue adhesion is synthesized by copolymerizing and crosslinking poly(vinyl butyral) with acrylic acid, gelatin, and chitosan-grafted N-acetyl-l-cysteine. The tissue adhesive strength of the obtained PAACP reaches 211.4 kPa, which is approximately ten times higher than that of the reported nonswelling hydrogels, and the hydrogel can be reused for multiple cycles. The as-prepared hydrogel shows great potential in soft bioelectronics, as muscle fatigue is successfully monitored via the electrode array and strain sensor integrated on PAACP substrates. The success of these bioelectronics offers potential applicability in the long-term diagnosis of muscle-related health conditions and prosthetic manipulations.

Determination of Stiffness and the Elastic Modulus of 3D-Printed Micropillars with Atomic Force Microscopy-Force Spectroscopy

Nowadays, many applications in diverse fields are taking advantage of micropillars such as optics, tribology, biology, and biomedical engineering. Among them, one of the most attractive is three-dimensional microelectrode arrays for in vivo and in vitro studies, such as cellular recording, biosensors, and drug delivery. Depending on the application, the micropillar's optimal mechanical response ranges from soft to stiff. For long-term implantable devices, a mechanical mismatch between the micropillars and the biological tissue must be avoided. For drug delivery patches, micropillars must penetrate the skin without breaking or bending. The accurate mechanical characterization of the micropillar is pivotal in the fabrication and optimization of such devices, as it determines whether the device will fail or not. In this work, we demonstrate an experimental method based only on atomic force microscopy-force spectroscopy that allows us to measure the stiffness of a micropillar and the elastic modulus of its constituent material. We test our method with four different types of 3D inkjet-printed micropillars: silver micropillars sintered at 100 and 150 °C and polyacrylate microstructures with and without a metallic coating. The estimated elastic moduli are found to be comparable with the corresponding bulk values. Furthermore, our findings show that neither the sintering temperature nor the presence of a thin metal coating plays a major role in defining the mechanical properties of the micropillar.

Advances in Hemostatic Hydrogels That Can Adhere to Wet Surfaces

Currently, uncontrolled bleeding remains a serious problem in emergency, surgical and battlefield environments. Despite the specific properties of available hemostatic agents, sealants, and adhesives, effective hemostasis under wet and dynamic conditions remains a challenge. In recent years, polymeric hydrogels with excellent hemostatic properties have received much attention because of their adjustable mechanical properties, high porosity, and biocompatibility. In this review, to investigate the role of hydrogels in hemostasis, the mechanisms of hydrogel hemostasis and adhesion are firstly elucidated, the adhesion design strategies of hemostatic hydrogels in wet environments are briefly introduced, and then, based on a comprehensive literature review, the studies and in vivo applications of wet-adhesive hemostatic hydrogels in different environments are summarized, and the improvement directions of such hydrogels in future studies are proposed.