A highly sensitive, low-cost, wearable pressure sensor based on conductive hydrogel spheres

High-Performance Pressure Sensors Based on Shaped Gel Droplet Arrays

Polymer gel-based pressure sensors offer numerous advantages over traditional sensing technologies, including excellent conformability and integration into wearable devices. However, challenges persist in terms of their performance and manufacturing technology. In this study, a method for fabricating gel pressure sensors using a hydrophobic/hydrophilic patterned surface is introduced. By shaping and fine-tuning the droplets of the polymer gel prepolymerization solution on the patterned surface, remarkable sensitivity improvements compared to unshaped hydrogels have been achieved. This also showcased the potential for tailoring gel pressure sensors to different applications. By optimizing the configuration of the sensor array, an uneven conductive gel array is fabricated, which exhibited a high sensitivity of 0.29 kPa-1 in the pressure range of 0-30 kPa, while maintaining a sensitivity of 0.13 kPa-1 from 30 kPa up to 100 kPa. Furthermore, the feasibility of using these sensors for human motion monitoring is explored and a conductive gel array for 2D force detection is successfully developed. This efficient and scalable fabrication method holds promise for advancing pressure sensor technology and offers exciting prospects for various industries and research fields.

Hydrophobically Associated Hydrogel for High Sensitivity and Resolution of an Interdigital Electrode Pressure Sensor

Hydrogel-based flexible strain sensors have been known for their excellent ability to convert different motions of humans into electrical signals, thus enabling real-time monitoring of various human health parameters. In this work, a composite hydrogel with hydrophobic association and hybrid cross-linking was fabricated by using polyacrylamide (PAm), surfactant sodium dodecyl sulfate (SDS), lauryl methacrylate (LMA), and polypyrrole (PPy). The dynamic dissociation-conjugation among LMA, SDS, and PPy could dissipate energy to improve the toughness of hydrogels. The SDS/PPy/LMPAm composite hydrogel with a toughness of 1.44 MJ/m3, tensile fracture stress of 345 kPa, tensile strain of 1021%, and electrical conductivity of 0.57 S/m was obtained. Furthermore, an interdigital electrode flexible pressure sensor was designed to replace the bipolar electrode flexible pressure sensor, which greatly improved the sensitivity and resolution of the pressure sensor. The SDS/PPy/LMPAm composite hydrogel-based interdigital electrode flexible pressure sensor showed extraordinary stability and identified different hand gestures as well as monitored the pulse signal of humans. Moreover, the characteristic systolic and diastolic peaks were clearly observed. The pulse frequency (65 times/min) and the radial artery augmentation index (0.57) were calculated, which are very important in evaluating the arterial vessel wall and function of human arteries.

Machine Learning-Enhanced Biomass Pressure Sensor with Embedded Wrinkle Structures Created by Surface Buckling

Flexible piezoresistive sensors are core components of many wearable devices to detect deformation and motion. However, it is still a challenge to conveniently prepare high-precision sensors using natural materials and identify similar short vibration signals. In this study, inspired by microstructures of human skins, biomass flexible piezoresistive sensors were prepared by assembling two wrinkled surfaces of konjac glucomannan and k-carrageenan composite hydrogel. The wrinkle structures were conveniently created by hardness gradient-induced surface buckling and coated with MXene sheets to capture weak pressure signals. The sensor was applied to detect various slight body movements, and a machine learning method was used to enhance the identification of similar and short throat vibration signals. The results showed that the sensor exhibited a high sensitivity of 5.1 kPa-1 under low pressure (50 Pa), a fast response time (104 ms), and high stability over 100 cycles. The XGBoost machine learning model accurately distinguished short voice vibrations similar to those of individual English letters. Moreover, experiments and numerical simulations were carried out to reveal the mechanism of the wrinkle structure preparation and the excellent sensing performance. This biomass sensor preparation and the machine learning method will promote the optimization and application of wearable devices.

Pathways toward wearable and high-performance sensors based on hydrogels: toughening networks and conductive networks

Wearable hydrogel sensors provide a user-friendly option for wearable electronics and align well with the existing manufacturing strategy for connecting and communicating with large numbers of Internet of Things devices. This is attributed to their components and structures, which exhibit exceptional adaptability, scalability, bio-compatibility, and self-healing properties, reminiscent of human skin. This review focuses on the recent research on principal structural elements of wearable hydrogels: toughening networks and conductive networks, highlighting the strategies for enhancing mechanical and electrical properties. Wearable hydrogel sensors are categorized for an extensive exploration of their composition, mechanism, and design approach. This review provides a comprehensive understanding of wearable hydrogels and offers guidance for the design of components and structures in order to develop high-performance wearable hydrogel sensors.

High-Performing Conductive Hydrogels for Wearable Applications

Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human-machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity.

Hydrogel fibers for wearable sensors and soft actuators

Owing to superior softness, wetness, responsiveness, and biocompatibility, bulk hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, optics, and coatings. The one-dimensional (1D) hydrogel fibers possess the metrics from both the hydrogel materials and structural topology, endowing them with extraordinary mechanical, sensing, breathable and weavable properties. As no comprehensive review has been reported for this nascent field, this article aims to provide an overview of hydrogel fibers for soft electronics and actuators. We first introduce the basic properties and measurement methods of hydrogel fibers, including mechanical, electrical, adhesive, and biocompatible properties. Then, typical manufacturing methods for 1D hydrogel fibers and fibrous films are discussed. Next, the recent progress of wearable sensors (e.g., strain, temperature, pH, and humidity) and actuators made from hydrogel fibers is discussed. We conclude with future perspectives on next-generation hydrogel fibers and the remaining challenges. The development of hydrogel fibers will not only provide an unparalleled one-dimensional characteristic, but also translate fundamental understanding of hydrogels into new application boundaries.

Gel Electrolytes for Electrochemical Actuators and Sensors Applications

Advanced functional materials, especially gel electrolytes, play a very important role in the preparation of electrochemical actuators and sensors, and have received extensive attention. In this review, a general classification of gel electrolytes is firstly introduced according to the type of medium. Then, the research progress of gel electrolytes with different types used to fabricate electrochemical actuators is summarized. Next, the current research progress of gel electrolytes used in different types of electrochemical sensors, including strain sensors, stress sensors, and gas sensors is introduced. Finally, the future challenges and development prospects of electrochemical actuators and sensors based on gel electrolytes are discussed. The huge application prospects of gel electrolyte are worthy of further focusing by researchers, which will have an indispensable impact on human life and development.

Highly Sensitive Zwitterionic Hydrogel Sensor for Motion and Pulse Detection with Water Retention, Adhesive, Antifreezing, and Self-Healing Properties

The design and synthesis of conductive hydrogels with antifreezing, long-term stable, highly sensitive, self-healing, and reusable is a critical procedure to enable applications in flexible electronics, medical monitoring, soft robotics, etc. Herein, a novel zwitterionic composite hydrogel possessing antifreezing, fast self-healing performance, water retention, and adhesion was synthesized via a simple one-pot method. LiCl, as an electrolyte and antifreeze, was promoted to dissociate under the electrostatic interaction with zwitterions, resulting in the composite hydrogels with high electrical conductivity (7.95 S/m) and excellent antifreeze ability (-45.3 °C). Meanwhile, the composite hydrogels could maintain 97% of the initial water content after exposed to air (25 °C, 55% RH) for 1 week due to the presence of salt ions. Moreover, the active groups of zwitterions could form conformal adhesion between the composite hydrogels and skin, which was particularly crucial for the stable signal output of the sensor. The dynamic borate ester bonds, active group of zwitterions, and the hydrogen bond between different components could achieve rapid self-healing (2 h, self-healing efficiency to 97%) without any external intervention. Notably, the developed P_BAS-Li (poly(vinyl alcohol) _Borax/acrylamide/zwitterionic-LiCl) hydrogel not only succeeded in sensitively detecting human motions but also could precisely captured handwritings signals and subtle pulse waves on the neck and wrist. The above findings demonstrated the great potential of P_BAS-Li hydrogels in the field of flexible electronic devices.

Multitasking smart hydrogels based on the combination of alginate and poly(3,4-ethylenedioxythiophene) properties: A review

Poly(3,4-ethylenedioxythiophene) (PEDOT), a very stable and biocompatible conducting polymer, and alginate (Alg), a natural water-soluble polysaccharide mainly found in the cell wall of various species of brown algae, exhibit very different but at the same complementary properties. In the last few years, the remarkable capacity of Alg to form hydrogels and the electro-responsive properties of PEDOT have been combined to form not only layered composites (PEDOT-Alg) but also interpenetrated multi-responsive PEDOT/Alg hydrogels. These materials have been found to display outstanding properties, such as electrical conductivity, piezoelectricity, biocompatibility, self-healing and re-usability properties, pH and thermoelectric responsiveness, among others. Consequently, a wide number of applications are being proposed for PEDOT-Alg composites and, especially, PEDOT/Alg hydrogels, which should be considered as a new kind of hybrid material because of the very different chemical nature of the two polymeric components. This review summarizes the applications of PEDOT-Alg and PEDOT/Alg in tissue interfaces and regeneration, drug delivery, sensors, microfluidics, energy storage and evaporators for desalination. Special attention has been given to the discussion of multi-tasking applications, while the new challenges to be tackled based on aspects not yet considered in either of the two polymers have also been highlighted.

Biocompatible Conductive Hydrogels: Applications in the Field of Biomedicine

The impact of COVID-19 has rendered medical technology an important factor to maintain social stability and economic increase, where biomedicine has experienced rapid development and played a crucial part in fighting off the pandemic. Conductive hydrogels (CHs) are three-dimensional (3D) structured gels with excellent electrical conductivity and biocompatibility, which are very suitable for biomedical applications. CHs can mimic innate tissue's physical, chemical, and biological properties, which allows them to provide environmental conditions and structural stability for cell growth and serve as efficient delivery substrates for bioactive molecules. The customizability of CHs also allows additional functionality to be designed for different requirements in biomedical applications. This review introduces the basic functional characteristics and materials for preparing CHs and elaborates on their synthetic techniques. The development and applications of CHs in the field of biomedicine are highlighted, including regenerative medicine, artificial organs, biosensors, drug delivery systems, and some other application scenarios. Finally, this review discusses the future applications of CHs in the field of biomedicine. In summary, the current design and development of CHs extend their prospects for functioning as an intelligent and complex system in diverse biomedical applications.

Emerging Fabrication Strategies of Hydrogels and Its Applications

Recently, hydrogels have been investigated for the controlled release of bioactive molecules, such as for living cell encapsulation and matrices. Due to their remote controllability and quick response, hydrogels are widely used for various applications, including drug delivery. The rate and extent to which the drugs reach their targets are highly dependent on the carriers used in drug delivery systems; therefore the demand for biodegradable and intelligent carriers is progressively increasing. The biodegradable nature of hydrogel has created much interest for its use in drug delivery systems. The first part of this review focuses on emerging fabrication strategies of hydrogel, including physical and chemical cross-linking, as well as radiation cross-linking. The second part describes the applications of hydrogels in various fields, including drug delivery systems. In the end, an overview of the application of hydrogels prepared from several natural polymers in drug delivery is presented.

Revolution in Flexible Wearable Electronics for Temperature and Pressure Monitoring—A Review

In the last few decades, technology innovation has had a huge influence on our lives and well-being. Various factors of observing our physiological characteristics are taken into account. Wearable sensing tools are one of the most imperative sectors that are now trending and are expected to grow significantly in the coming days. Externally utilized tools connected to any human to assess physiological characteristics of interest are known as wearable sensors. Wearable sensors range in size from tiny to large tools that are physically affixed to the user and operate on wired or wireless terms. With increasing technological capabilities and a greater grasp of current research procedures, the usage of wearable sensors has a brighter future. In this review paper, the recent developments of two important types of wearable electronics apparatuses have been discussed for temperature and pressure sensing (Psensing) applications. Temperature sensing (Tsensing) is one of the most important physiological factors for determining human body temperature, with a focus on patients with long-term chronic conditions, normally healthy, unconscious, and injured patients receiving surgical treatment, as well as the health of medical personnel. Flexile Psensing devices are classified into three categories established on their transduction mechanisms: piezoresistive, capacitive, and piezoelectric. Many efforts have been made to enhance the characteristics of the flexible Psensing devices established on these mechanisms.

Shaping Macromolecules for Sensing Applications—From Polymer Hydrogels to Foldamers

Sensors are tools for detecting, recognizing, and recording signals from the surrounding environment. They provide measurable information on chemical or physical changes, and thus are widely used in diagnosis, environment monitoring, food quality checks, or process control. Polymers are versatile materials that find a broad range of applications in sensory devices for the biomedical sector and beyond. Sensory materials are expected to exhibit a measurable change of properties in the presence of an analyte or a stimulus, characterized by high sensitivity and selectivity of the signal. Signal parameters can be tuned by material features connected with the restriction of macromolecule shape by crosslinking or folding. Gels are crosslinked, three-dimensional networks that can form cavities of different sizes and forms, which can be adapted to trap particular analytes. A higher level of structural control can be achieved by foldamers, which are macromolecules that can attain well-defined conformation in solution. By increasing control over the three-dimensional structure, we can improve the selectivity of polymer materials, which is one of the crucial requirements for sensors. Here, we discuss various examples of polymer gels and foldamer-based sensor systems. We have classified and described applied polymer materials and used sensing techniques. Finally, we deliberated the necessity and potential of further exploration of the field towards the increased selectivity of sensory devices.

Insole-Based Systems for Health Monitoring: Current Solutions and Research Challenges

Wearable health monitoring devices allow for measuring physiological parameters without restricting individuals' daily activities, providing information that is reflective of an individual's health and well-being. However, these systems need to be accurate, power-efficient, unobtrusive and simple to use to enable a reliable, convenient, automatic and ubiquitous means of long-term health monitoring. One such system can be embedded in an insole to obtain physiological data from the plantar aspect of the foot that can be analyzed to gain insight into an individual's health. This manuscript provides a comprehensive review of insole-based sensor systems that measure a variety of parameters useful for overall health monitoring, with a focus on insole-based PPD measurement systems developed in recent years. Existing solutions are reviewed, and several open issues are presented and discussed. The concept of a fully integrated insole-based health monitoring system and considerations for future work are described. By developing a system that is capable of measuring parameters such as PPD, gait characteristics, foot temperature and heart rate, a holistic understanding of an individual's health and well-being can be obtained without interrupting day-to-day activities. The proposed device can have a multitude of applications, such as for pathology detection, tracking medical conditions and analyzing gait characteristics.

Epidermis-Inspired Wearable Piezoresistive Pressure Sensors Using Reduced Graphene Oxide Self-Wrapped Copper Nanowire Networks

Wearable piezoresistive sensors are being developed as electronic skins (E-skin) for broad applications in human physiological monitoring and soft robotics. Tactile sensors with sufficient sensitivities, durability, and large dynamic ranges are required to replicate this critical component of the somatosensory system. Multiple micro/nanostructures, materials, and sensing modalities have been reported to address this need. However, a trade-off arises between device performance and device complexity. Inspired by the microstructure of the spinosum at the dermo epidermal junction in skin, a low-cost, scalable, and high-performance piezoresistive sensor is developed with high sensitivity (0.144 kPa-1 ), extensive sensing range ( 0.1-15 kPa), fast response time (less than 150 ms), and excellent long-term stability (over 1000 cycles). Furthermore, the piezoresistive functionality of the device is realized via a flexible transparent electrode (FTE) using a highly stable reduced graphene oxide self-wrapped copper nanowire network. The developed nanowire-based spinosum microstructured FTEs are amenable to wearable electronics applications.

Skin-like hydrogel devices for wearable sensing, soft robotics and beyond

Skin-like electronics are developing rapidly to realize a variety of applications such as wearable sensing and soft robotics. Hydrogels, as soft biomaterials, have been studied intensively for skin-like electronic utilities due to their unique features such as softness, wetness, biocompatibility and ionic sensing capability. These features could potentially blur the gap between soft biological systems and hard artificial machines. However, the development of skin-like hydrogel devices is still in its infancy and faces challenges including limited functionality, low ambient stability, poor surface adhesion, and relatively high power consumption (as ionic sensors). This review aims to summarize current development of skin-inspired hydrogel devices to address these challenges. We first conduct an overview of hydrogels and existing strategies to increase their toughness and conductivity. Next, we describe current approaches to leverage hydrogel devices with advanced merits including anti-dehydration, anti-freezing, and adhesion. Thereafter, we highlight state-of-the-art skin-like hydrogel devices for applications including wearable electronics, soft robotics, and energy harvesting. Finally, we conclude and outline the future trends.

Solution-Processable Conductive Composite Hydrogels with Multiple Synergetic Networks toward Wearable Pressure/Strain Sensors

A biocompatible, flexible, yet robust conductive composite hydrogel (CCH) for wearable pressure/strain sensors has been achieved by an all-solution-based approach. The CCH is rationally constructed by in situ polymerization of aniline (An) monomers in the polyvinyl alcohol (PVA) matrix, followed by the cross-linking of PVA with glutaraldehyde (GA) as the cross-linker. The unique multiple synergetic networks in the CCH including strong chemical covalent bonds and abundance of weak physical cross-links, i.e., hydrogen bondings and electrostatic interactions, impart excellent mechanical strength (a fracture tensile strength of 1200 kPa), superior compressibility (ε = 80%@400 kPa), outstanding stretchability (a fracture strain of 670%), high sensitivity (0.62 kPa^-1 at a pressure range of 0-1.0 kPa for pressure sensing and a gauge factor of 3.4 at a strain range of 0-300% for strain sensing, respectively), and prominent fatigue resistance (1500 cycling). As the flexible wearable sensor, the CCH is able to monitor different types of human motion and diagnostically distinguish speaking. As a proof of concept, a sensing device has been designed for the real-time detection of 2D distribution of weight or pressure, suggesting its promising potentials for electronic skin, human-machine interaction, and soft robot applications.

Bio-Inspired Hydrogels via 3D Bioprinting

Many soft tissues of the human body such as cartilages, muscles, and ligaments are mainly composed of biological hydrogels possessing excellent mechanical properties and delicate structures. Nowadays, bio-inspired hydrogels have been intensively explored due to their promising potential applications in tissue engineering. However, the traditional manufacturing technology is challenging to produce the bio-inspired hydrogels, and the typical biological composite topologies of bio-inspired hydrogels are accessible completed using 3D bioprinting at micrometer resolution. In this chapter, the 3D bioprinting techniques used for the fabrication of bio-inspired hydrogels were summarized, and the materials used were outlined. This chapter also focuses on the applications of bio-inspired hydrogels fabricated using available 3D bioprinting technologies. The development of 3D bioprinting techniques in the future would bring us closer to the fabrication capabilities of living organisms, which would be widely used in biomedical applications.

Stretchable and tough conductive hydrogels for flexible pressure and strain sensors

Flexible pressure and strain sensors have great potential for applications in wearable and implantable devices, soft robotics and artificial skin. Compared to flexible sensors based on filler/elastomer composites, conductive hydrogels are advantageous due to their biomimetic structures and properties, as well as biocompatibility. Numerous chemical and structural designs provide unlimited opportunities to tune the properties and performance of conductive hydrogels to match various demands for practical applications. Many electronically and ionically conductive hydrogels have been developed to fabricate pressure and strain sensors with different configurations, including resistance type and capacitance type. The sensitivity, reliability and stability of hydrogel sensors are dependent on their network structures and mechanical properties. This review focuses on tough conductive hydrogels for flexible sensors. Representative strategies to prepare stretchable, strong, tough and self-healing hydrogels are briefly reviewed since these strategies are illuminating for the development of tough conductive hydrogels. Then, a general account on various conductive hydrogels is presented and discussed. Recent advances in tough conductive hydrogels with well designed network structures and their sensory performance are discussed in detail. A series of conductive hydrogel sensors and their application in wearable devices are reviewed. Some perspectives on flexible conductive hydrogel sensors and their applications are presented at the end.

Capacitive Pressure Sensors Containing Reliefs on Solution-Processable Hydrogel Electrodes

Highly sensitive capacitive-type pressure sensor has been achieved by fabricating reliefs on solution-processable hydrogel electrodes. Hybrid PVA/PANI hydrogels (PVA, poly(vinyl alcohol); PANI, polyaniline) with a fully physically cross-linked binary network are selected as the electrodes of the pressure sensors. On the basis of the solution processability, reliefs are fabricated on the surface of PVA/PANI hydrogel electrodes by a template method. The gauge factor (GF) is enhanced by introducing reliefs and regulated by controlling the composition and relief dimension of hydrogel electrodes. The optimized pressure sensor containing reliefs achieves the highest GF of 7.70 kPa^-1 and a sensing range of 0-7.4 kPa. Furthermore, the freezing and drying problems of the hydrogel sensors are overcome by introducing a binary solvent of water/glycerol and the pressure sensing ability at -18 °C has been achieved. Finally, monitoring of various pressures in daily life, such as joint bending, blowing, and brush writing, is demonstrated using such pressure sensors.

Gelatin methacryloyl-based tactile sensors for medical wearables

Gelatin methacryloyl (GelMA) is a widely used hydrogel with skin-derived gelatin acting as the main constituent. However, GelMA has not been used in the development of wearable biosensors, which are emerging devices that enable personalized healthcare monitoring. This work highlights the potential of GelMA for wearable biosensing applications by demonstrating a fully solution-processable and transparent capacitive tactile sensor with microstructured GelMA as the core dielectric layer. A robust chemical bonding and a reliable encapsulation approach are introduced to overcome detachment and water-evaporation issues in hydrogel biosensors. The resultant GelMA tactile sensor shows a high-pressure sensitivity of 0.19 kPa-1 and one order of magnitude lower limit of detection (0.1 Pa) compared to previous hydrogel pressure sensors owing to its excellent mechanical and electrical properties (dielectric constant). Furthermore, it shows durability up to 3000 test cycles because of tough chemical bonding, and long-term stability of 3 days due to the inclusion of an encapsulation layer, which prevents water evaporation (80% water content). Successful monitoring of various human physiological and motion signals demonstrates the potential of these GelMA tactile sensors for wearable biosensing applications.

NIR-responsive multi-healing HMPAM/dextran/AgNWs hydrogel sensor with recoverable mechanics and conductivity for human-machine interaction

Conductive and self-healing hydrogel sensor is perspective in human-machine interaction applications. However, the design of ideal self-healing hydrogels are always challenging. Herein, by introducing disulfide modified Ag nanowires (AgNWs), we show a novel self-healing hydrogel strain sensor with superior mechanics, conductivity, antibacterial property, and firstly realizing of self-healing with both recovery of mechanics and sensing properties. We demonstrate that the covalent and reversible non-covalent hydrophobic blocks in hydrophobic modified polyacrylamide (HMPAM) achieves the basic self-healing network; dextran with plentiful hydroxyl groups synergistic helps the self-healing by hydrogen bonds; disulfide on the AgNWs surface forms a NIR-responsive and dynamic Ag-S coordination bridge between HMPAM and AgNWs. The resulted hydrogel sensor exhibits comprehensive electromechanical properties, and precisely monitors human motion and subtle electromyography (EMG) signals. Importantly, we firstly achieved the recovery of sensing properties on human motion detection and EMG signal detection after self-healing. This work provides a promising exploration to manufacture bionic strain sensors for potential applications in wearable electronics.

Mineralized Soft and Elastic Polymer Dot Hydrogel for a Flexible Self-Powered Electronic Skin Sensor

We propose an integrated, self-powered, flexible electronic skin device containing an alginate-derived polymer dot (A-PD)-incorporated mineralized hydrogel-based energy storage unit and a chitosan-derived n-type carbon dot (N-CD)-based solar cell for an energy-harvesting unit. This study demonstrates a unique architecture of mineralized hydrogel comprising A-PD-incorporated poly(acrylic acid) (PAA)/CaCO₃/laponite containing soft and sensitive layers, deposited with a polyaniline electrode to serve as an energy storage unit. The self-assembly was achieved through the ionic cross-linking between A-PD and PAA driven by the mineralization process, resulting in excellent dimensional stability and improved mechanical properties of the hydrogel. The sp² carbon-rich A-PD enhances the electrochemical performance and the overall photon-to-electrical conversion and storage efficiency for self-powered devices by the formation of the bridge of electrons between the ionized polymer and metal ion. The capacitive sensor developed in this study exhibits high sensitivity in detecting small pressure changes, such as the falling of small water droplets. The self-powered sensing device can detect and monitor various human motions continuously by harvesting light energy from outdoor sunlight. Furthermore, the energy-autonomous device exhibits unique responses for handwriting characters stably and repeatedly. The proposed system may be applicable to human-machine interfaces, biomonitoring systems, secure communication, and wearable devices.

Freezing-Tolerant, Highly Sensitive Strain and Pressure Sensors Assembled from Ionic Conductive Hydrogels with Dynamic Cross-Links

Conductive hydrogels have attracted intensive attention for versatile functions in flexible electronics because of their unique combination of mechanical flexibility and conductivity. However, hydrogels containing plenty of water inevitably freeze at subzero temperature, leading to invalid electronics with failed mechanical advantages and negligible conductivity. Moreover, the inferior elasticity and fatigue resistance of hydrogels result in unstable sensing performance and poor reusability of hydrogel-based electronics. Herein, a freezing-tolerant, high-sensitive, durable strain and pressure sensor was constructed from an ionic conductive chitosan-poly(acrylamide-co-acrylic acid) double-network [CS-P(AM-co-AA) DN] hydrogel with dual-dynamic cross-links (chitosan physical network and ionic coordination [CO₂LFe^III]), which was feasibly fabricated by soaking the CS-P(AM-co-AA) composite hydrogel in FeCl₃ solution. The ions immobilized in dynamic cross-links exerted crucial effects on improving mechanics [prominent tensile performance, supercompressibility, extraordinary elasticity, fast self-recovery capacity, and remarkable fatigue resistance (1000 cycles)]; meanwhile, the free ions in the hydrogel rendered the hydrogel excellent conductivity and strong freezing tolerance concurrently. The sensor assembled from the DN hydrogel exhibited cycling stability and good durability in detecting pressure, various deformations (elongation, compression, and bend), and human motions even at a low temperature (-20 °C). Notably, the sensitivity on detecting strain and pressure at both room and subzero temperature was superior than most of the reported organohydrogel and hydrogel sensors. Thus, we believe that this work will provide a platform for construction and application of high-sensitive strain and pressure hydrogel sensors with cycling stability and good durability in a wide temperature range.

Polyacrylamide/Phytic Acid/Polydopamine Hydrogel as an Efficient Substrate for Electrochemical Enrichment of Circulating Cell-Free DNA from Blood Plasma

A facile method has been developed for the rapid and efficient enrichment of DNAs from different media including synthetic single-strand DNAs (ssDNAs) from buffer solutions and cell-free DNAs (cfDNAs) from blood plasma through electric field-driven adsorption and desorption of DNAs by a polyacrylamide/phytic acid/polydopamine (PAAM/PA/PDA) hydrogel. The as-prepared PAAM/PA/PDA hydrogel possesses regular porosity with a large surface area, strong electric field responsiveness/good conductivity, and a rich aromatic structure, which can be used as an ideal adsorbent for DNA enrichment under a positive electric field. The enriched DNAs can be released efficiently when the positive electric field is converted to a negative electric field. The PAAM/PA/PDA hydrogel-based electrochemical method enables the completion of the process of DNA adsorption and release within 5 min and exhibits reasonable enrichment efficiencies and recovery rates of various DNAs. For instance, the high enrichment sensitivity (0.1 pmol L-1) together with the excellent recovery (>75%) of an ssDNA with 78 nucleotides is obtained. Combined with the PCR amplification technique, the practicability of the as-proposed method is demonstrated by the screening of circulating tumor DNAs (ctDNAs) with a BRAFV600E mutation in cfDNAs from the blood plasma samples of patients with papillary thyroid cancer or thyroid nodule and random patients from a clinical laboratory.

Carbon Nanotubes/Hydrophobically Associated Hydrogels as Ultrastretchable, Highly Sensitive, Stable Strain, and Pressure Sensors

Conductive hydrogels have become one of the most promising materials for skin-like sensors because of their excellent biocompatibility and mechanical flexibility. However, the limited stretchability, low toughness, and fatigue resistance lead to a narrow sensing region and insufficient durability of the hydrogel-based sensors. In this work, an extremely stretchable, highly tough, and anti-fatigue conductive nanocomposite hydrogel is prepared by integrating hydrophobic carbon nanotubes (CNTs) into hydrophobically associated polyacrylamide (HAPAAm) hydrogel. In this conductive hydrogel, amphiphilic sodium dodecyl sulfate was used to ensure uniform dispersion of CNTs in the hydrogel network, and hydrophobic interactions between the hydrogel matrix and the CNT surface formed, greatly improving the mechanical properties of the hydrogel. The obtained CNTs/HAPAAm hydrogel showed excellent stretchability (ca. 3000%), toughness (3.42 MJ m^-3), and great anti-fatigue property. Moreover, it exhibits both high tensile strain sensitivity in the wide strain ranges (gauge factor = 4.32, up to 1000%) and high linear sensitivity (0.127 kPa^-1) in a large-pressure region within 0-50 kPa. The CNTs/HAPAAm hydrogel-based sensors can sensitively and stably detect full-range human activities (e.g., elbow rotation, finger bending, swallowing motion, and pronouncing) and handwriting, demonstrating the CNTs/HAPAAm hydrogel's potential as the wearable strain and pressure sensors for flexible devices.

Self-healing conductive hydrogels: preparation, properties and applications

Conductive hydrogels have generated great interest in biomedical and electrical fields. However, conventional conductive hydrogels usually lack self-healing properties, which might be unfavorable for their application. Conductive self-healing hydrogels with excellent performance for applications in the biomedical and electrical fields are growing in number. In this review paper, the progress related to conductive self-healing hydrogels is summarized. The self-healing mechanism is classified to demonstrate the design and synthesis of conductive self-healing hydrogels and their applications in tissue engineering, wound healing, electronic skin, sensors and self-repaired circuits are presented and discussed. The future development of conductive self-healing hydrogels and problems that need to be solved are also described.

Visual Evoked Potentials Used to Evaluate a Commercially Available Superabsorbent Polymer as a Cheap and Efficient Material for Preparation-Free Electrodes for Recording Electrical Potentials of the Human Visual Cortex

The aim of this study was to investigate the use of inexpensive and easy-to-use hydrogel “marble” electrodes for the recording of electrical potentials of the human visual cortex using visual evoked potentials (VEPs) as example. Top hat-shaped holders for the marble electrodes were developed with an electrode cap to acquire the signals. In 12 healthy volunteers, we compared the VEPs obtained with conventional gold-cup electrodes to those obtained with marble electrodes. Checkerboards of two check sizes—0.8° and 0.25°—were presented. Despite the higher impedance of the marble electrodes, the line noise could be completely removed by averaging 64 single traces, and VEPs could be recorded. Linear mixed-effect models using electrode type, stimulus, and recording duration revealed a statistically significant effect of the electrode type on only VEP N75 peak latency (mean ± SEM: 1.0 ± 1.2 ms) and amplitude (mean ± SEM: 0.8 ± 0.9 µV) The mean amplitudes of the delta, theta, alpha, beta, and gamma frequency bands of marble electrodes were statistically significantly different and, on average, 25% higher than those of gold-cup electrodes. However, the mean amplitudes showed a statistically significant strong correlation (Pearson’s r = 0.8). We therefore demonstrate the potential of the inexpensive and efficient hydrogel electrode to replace conventional gold-cup electrodes for the recording of VEPs and possibly other recordings from the human cortex.

One-Step Preparation of a Highly Stretchable, Conductive, and Transparent Poly(vinyl alcohol)-Phytic Acid Hydrogel for Casual Writing Circuits

Conductive hydrogels have shown great potential applications in a wide variety of fields, including artificial intelligence devices and biomedical engineering. However, it still remains a great challenge to develop a facile and cost-effective approach to achieve a conductive hydrogel with favorable qualities. Herein, we have changed the traditional ingredient of poly(vinyl alcohol) (PVA) hydrogel by the addition of phytic acid (PA), which could yield a conductive hydrogel through one freeze-thaw cycle. The PVA-PA hydrogel holds several virtues including a large stretchability (about 1100% strain), excellent conductivity (1.34 kΩ cm), and high optical transparence (about 95%). By assembling the PVA-PA hydrogel into a wearable strain sensor, the gel-based sensor has shown good performance for the real-time monitoring of human daily activities and health conditions. Moreover, one formula of the PVA-PA sol ink could rapidly convert to the gel state just by being injected on a flexible substrate under an ice-bath, which would satisfy the demand of casual writing circuits. This one-step preparation method of the PVA-PA hydrogel may open an innovative avenue for the fabrication of easy-molding and functional hydrogels with only two components under mild ambient conditions.

Laser micro-structured pressure sensor with modulated sensitivity for electronic skins

Micro-structured pressure sensors with broad pressure sensing range, high sensitivity and rapid response speeds are highly desired for epidermal electronic skin. The widely used methods to fabricate micro-structured pressure sensors are lithography and biomaterial-replicating, which are either complex in preparation procedure or uncontrollable in micro-structure morphology. In this work, laser micro-structured wearable pressure sensors with high-performance are developed for epidermal electronic skin. Laser micro-engineering, with scalability, high-efficiency, and controllability, is employed to prepare a series of micro-structures on elastomers for modulating and enhancing the sensitivity of the sensors. The device with micro-domes owns a sensitivity of -1.82 kPa^-1, which is approximately 17 times better than the one based on long micro-ridges. Due to the reduced viscous properties of the elastomers by laser micro-engineering, the sensor based on micro-domes demonstrates rapid response/relaxation speeds of 0.036 and 0.052 s, respectively, and a detection limit of 0.001 kPa. Additionally, the device has a good durability (6,000 cycles) with a repeatability deviation of 1.44%, confirming its stability. Combined with near field communication technology, the sensor has been investigated as epidermal electronic skin for health monitoring.

Wearable, Ultrawide-Range, and Bending-Insensitive Pressure Sensor Based on Carbon Nanotube Network-Coated Porous Elastomer Sponges for Human Interface and Healthcare Devices

Flexible and wearable pressure sensors have attracted a tremendous amount of attention due to their wider applications in human interfaces and healthcare monitoring. However, achieving accurate pressure detection and stability against external stimuli (in particular, bending deformation) over a wide range of pressures from tactile to body weight levels is a great challenge. Here, we introduce an ultrawide-range, bending-insensitive, and flexible pressure sensor based on a carbon nanotube (CNT) network-coated thin porous elastomer sponge for use in human interface devices. The integration of the CNT networks into three-dimensional microporous elastomers provides high deformability and a large change in contact between the conductive CNT networks due to the presence of micropores, thereby improving the sensitivity compared with that obtained using CNT-embedded solid elastomers. As electrical pathways are continuously generated up to high compressive strain (∼80%), the pressure sensor shows an ultrawide pressure sensing range (10 Pa to 1.2 MPa) while maintaining favorable sensitivity (0.01-0.02 kPa^-1) and linearity ( R² ∼ 0.98). Also, the pressure sensor exhibits excellent electromechanical stability and insensitivity to bending-induced deformations. Finally, we demonstrate that the pressure sensor can be applied in a flexible piano pad as an entertainment human interface device and a flexible foot insole as a wearable healthcare and gait monitoring device.

Rational Design of Microfabricated Electroconductive Hydrogels for Biomedical Applications

Electroconductive hydrogels (ECHs) are highly hydrated 3D networks generated through the incorporation of conductive polymers, nanoparticles, and other conductive materials into polymeric hydrogels. ECHs combine several advantageous properties of inherently conductive materials with the highly tunable physical and biochemical properties of hydrogels. Recently, the development of biocompatible ECHs has been investigated for various biomedical applications, such as tissue engineering, drug delivery, biosensors, flexible electronics, and other implantable medical devices. Several methods for the synthesis of ECHs have been reported, which include the incorporation of electrically conductive materials such as gold and silver nanoparticles, graphene, and carbon nanotubes, as well as various conductive polymers (CPs), such as polyaniline, polypyrrole, and poly(3,4-ethylenedioxyythiophene) into hydrogel networks. Theses electroconductive composite hydrogels can be used as scaffolds with high swellability, tunable mechanical properties, and the capability to support cell growth both in vitro and in vivo. Furthermore, recent advancements in microfabrication techniques such as three dimensional (3D) bioprinting, micropatterning, and electrospinning have led to the development of ECHs with biomimetic microarchitectures that reproduce the characteristics of the native extracellular matrix (ECM). In addition, smart ECHs with controlled structures and healing properties have also been engineered into devices with prolonged half-lives and increased durability. The combination of sophisticated synthesis chemistries and modern microfabrication techniques have led to engineer smart ECHs with advanced architectures, geometries, and functionalities that are being increasingly used in drug delivery systems, biosensors, tissue engineering, and soft electronics. In this review, we will summarize different strategies to synthesize conductive biomaterials. We will also discuss the advanced microfabrication techniques used to fabricate ECHs with complex 3D architectures, as well as various biomedical applications of microfabricated ECHs.

Elastomeric conductive hybrid hydrogels with continuous conductive networks

The DA–PPy–GP ECHs with continuous conductive networks show high force and strain sensitivity.

High-performance transparent pressure sensors based on sea-urchin shaped metal nanoparticles and polyurethane microdome arrays for real-time monitoring

An ultra-sensitive and transparent piezoresistive pressure sensor based on a sea-urchin shaped metal nanoparticle (SSNP)-polyurethane (PU) composite with microdome arrays is successfully fabricated for the first time. The piezoresistive pressure sensor with microdome arrays was prepared using a nanoimprinting process based on an intermediate polymer substrate (IPS) replica mold. It showed a superior sensitivity (71.37 kPa^-1) and a high optical transmittance (77.7% at 550 nm) due to the effective quantum tunneling effect even at small concentrations of conductive SSNP filler (6 mg mL^-1). The high-performance characteristics of the piezoresistive pressure sensor are attributed to the geometric effects of the microdome structure, especially the stress concentration at small contact spots and the deformation of the contact area. The piezoresistive pressure sensor with microdome arrays also exhibited a fast response/relaxation time (30 ms), ultra-low pressure detection (4 Pa), and excellent long-term stability under harsh conditions. In addition, the effectiveness of the piezoresistive pressure sensors in various sensing applications including sensing mapping, human arterial pulse monitoring, and the detection of muscle movement is also successfully demonstrated. It is anticipated that this novel transparent pressure sensor based on a SSNP-PU composite with microdome arrays will be a key component in the development of integrated transparent sensing applications.

High-Performance Resistive Pressure Sensor Based on Elastic Composite Hydrogel of Silver Nanowires and Poly(ethylene glycol)

Improved pressure sensing is of great interest to enable the next-generation of bioelectronics systems. This paper describes the development of a transparent, flexible, highly sensitive pressure sensor, having a composite sandwich structure of elastic silver nanowires (AgNWs) and poly(ethylene glycol) (PEG). A simple PEG photolithography was employed to construct elastic AgNW-PEG composite patterns on flexible polyethylene terephthalate (PET) film. A porous PEG hydrogel structure enabled the use of conductive AgNW patterns while maintaining the elasticity of the composite material, features that are both essential for high-performance pressure sensing. The transparency and electrical properties of AgNW-PEG composite could be precisely controlled by varying the AgNW concentration. An elastic AgNW-PEG composite hydrogel with 0.6 wt % AgNW concentration exhibited high transmittance including T550nm of around 86%, low sheet resistance of 22.69 Ω·sq-1, and excellent bending durability (only 5.8% resistance increase under bending to 10 mm radius). A flexible resistive pressure sensor based on our highly transparent AgNW-PEG composite showed stable and reproducible response, high sensitivity (69.7 kPa-1), low sensing threshold (~2 kPa), and fast response time (20⁻40 ms), demonstrating the effectiveness of the AgNW-PEG composite material as an elastic conductor.

Hydrogel Actuators and Sensors for Biomedical Soft Robots: Brief Overview with Impending Challenges

There are numerous developments taking place in the field of biorobotics, and one such recent breakthrough is the implementation of soft robots-a pathway to mimic nature's organic parts for research purposes and in minimally invasive surgeries as a result of their shape-morphing and adaptable features. Hydrogels (biocompatible, biodegradable materials that are used in designing soft robots and sensor integration), have come into demand because of their beneficial properties, such as high water content, flexibility, and multi-faceted advantages particularly in targeted drug delivery, surgery and biorobotics. We illustrate in this review article the different types of biomedical sensors and actuators for which a hydrogel acts as an active primary material, and we elucidate their limitations and the future scope of this material in the nexus of similar biomedical avenues.

Highly Stable and Flexible Pressure Sensors with Modified Multi-Walled Carbon Nanotube/Polymer Composites for Human Monitoring

A facile method for preparing an easy processing, repeatable and flexible pressure sensor was presented via the synthesis of modified multi-walled carbon nanotubes (m-MWNTs) and polyurethane (PU) films. The surface modification of multi-walled carbon nanotubes (MWNTs) simultaneously used a silane coupling agent (KH550) and sodium dodecyl benzene sulfonate (SDBS) to improve the dispersibility and compatibility of the MWNTs in a polymer matrix. The electrical property and piezoresistive behavior of the m-MWNT/PU composites were compared with raw multi-walled carbon nanotube (raw MWNT)/PU composites. Under linear uniaxial pressure, the m-MWNT/PU composite exhibited 4.282%kPa⁻¹ sensitivity within the pressure of 1 kPa. The nonlinear error, hysteresis error and repeatability error of the piezoresistivity of m-MWNT/PU decreased 9%, 16.72% and 54.95% relative to raw MWNT/PU respectively. Therefore, the piezoresistive response of m-MWNT/PU had better stability than that of raw MWNT/PU composites. The m-MWNT/PU sensors could be utilized in wearable devices for body movement detection, monitoring of respiration and pressure detection in garments.

Rapid thermal responsive conductive hybrid cryogels with shape memory properties, photothermal properties and pressure dependent conductivity

Stimuli responsive cryogels with multi-functionality have potential application for electrical devices, actuators, sensors and biomedical devices. However, conventional thermal sensitive poly(N-isopropylacrylamide) cryogels show slow temperature response speed and lack of multi-functionality, which greatly limit their practical application. Herein we present conductive fast (2 min for both deswelling and reswelling behavior) thermally responsive poly(N-isopropylacrylamide) cryogels with rapid shape memory properties (3 s for shape recovery), near-infrared (NIR) light sensitivity and pressure dependent conductivity, and further demonstrated their applications as temperature sensitive on-off switch, NIR light sensitive on-off switch, water triggered shape memory on-off switch and pressure dependent device. These cryogels were first prepared in dimethyl sulfoxide below its melting temperature in ice bath and subsequently put into aniline or pyrrole solution to in situ deposition of conducting polyaniline or polypyrrole nanoparticles. The continuous macroporous sponge-like structure provides cryogels with rapid responsivity both in deswelling, reswelling kinetics and good elasticity. After incorporating electrically conductive polyaniline or polypyrrole nanoaggregates, the hybrid cryogels exhibit desirable conductivity, photothermal property, pressure dependent conductivity and good cytocompatibility. These multifunctional hybrid cryogels make them great potential as stimuli responsive electrical device, tissue engineering scaffolds, drug delivery vehicle and electronic skin.

Three Dimensional Distribution of Sensitive Field and Stress Field Inversion of Force Sensitive Materials under Constant Current Excitation

Force sensitive conductive composite materials are functional materials which can be used as the sensitive material of force sensors. However, the existing sensors only use one-dimensional electrical properties of force sensitive conductive materials. Even in tactile sensors, the measurement of contact pressure is achieved by large-scale arrays and the units of a large-scale array are also based on the one-dimensional electrical properties of force sensitive materials. The main contribution of this work is to study the three-dimensional electrical properties and the inversion method of three-dimensional stress field of a force sensitive material (conductive rubber), which pushes the application of force sensitive material from one dimensional to three-dimensional. First, the mathematical model of the conductive rubber current field distribution under a constant force is established by the effective medium theory, and the current field distribution model of conductive rubber with different geometry, conductive rubber content and conductive rubber relaxation parameters is deduced. Secondly, the inversion method of the three-dimensional stress field of conductive rubber is established, which provides a theoretical basis for the design of a new tactile sensor, three-dimensional stress field and space force based on force sensitive materials.