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

Hydrogels composite optimized for low resistance and loading-unloading hysteresis for flexible biosensors

With the advancement of wearable and implantable medical devices, hydrogel flexible bioelectronic devices have attracted significant interest due to exhibiting tissue-like mechanical compliance, biocompatibility, and low electrical resistance. In this study, the development and comprehensive performance evaluation of poly(acrylic acid)/ N,N'-bis(acryloyl) cystamine/ 1-butyl-3-ethenylimidazol-1-ium:bromide (PAA/NB/IL) hydrogels designed for flexible sensor applications are introduced. Engineered through a combination of physical and chemical cross-linking strategies, these hydrogels exhibit strong mechanical properties, high biocompatibility, and effective sensing capabilities. At 95 % strain, the compressive modulus of PAA/NB/IL 100 reach up to 3.66 MPa, with the loading-unloading process showing no significant hysteresis loop, indicating strong mechanical stability and elasticity. An increase in the IL content was observed to enlarge the porosity of the hydrogels, thereby influencing their swelling behavior and sensing functionality. Biocompatibility assessments revealed that the hemolysis rate was below 5 %, ensuring their suitability for biomedical applications. Upon implantation in rats, a minimal acute inflammatory response was observed, comparable to that of the biocompatibility control poly(ethylene glycol) diacrylate (PEGDA). These results suggest that PAA/NB/IL hydrogels hold promise as biomaterials for biosensors, offering a balance of mechanical integrity, physiological compatibility, and sensing sensitivity, thereby facilitating advanced healthcare monitoring solutions.

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

Multiperformance PAM/PVA/CaCO3 Hydrogel for Flexible Sensing and Information Encryption

Currently, the development of hydrogels with excellent mechanical properties (elasticity, fatigue resistance, etc.) and conductive properties can better meet their needs in the field of flexible sensor device applications. Generally, hydrogels with a denser cross-linking density tend to have better mechanical properties, but the improvement in mechanical properties comes at the expense of reduced electrical conductivity. Directly generating CaCO3 in the hydrogel prepolymer can not only increase the cross-linking density of its network but also introduce additional ions to enhance its internal ionic strength, which is beneficial to improving the conductivity of the hydrogel. It is still a big challenge to directly generate CaCO3 in the static prepolymer solution and ensure its uniform dispersion in the hydrogel. Herein, we adopted an improved preparation method to ensure that the directly generated CaCO3 particles can be evenly dispersed in the static prepolymer solution until the polymerization is completed. Finally, a PAM/PVA/CaCO3 hydrogel with supertensile, compressive, toughness, and fatigue resistance properties was prepared. In addition, the presence of free Na+ and Cl- gives the hydrogel excellent conductivity and sensing performance to monitor daily human activities. On the basis of the application of hydrogels in information communication, we have further deepened this application by combining the characteristics of hydrogels themselves. Combined with ASCII code, the hydrogel can also be applied in information exchange and information encryption and decryption, achieving the antitheft function in smart locks. A variety of excellent performance integrated PAM/PVA/CaCO3 hydrogels have broad application prospects for flexible sensors, highlighting great potential in human-computer interaction and intelligent information protection.

Bioinspired Multiscale Mineralization: From Fundamentals to Potential Applications

The wondrous and imaginative designs of nature have always been an inexhaustible treasure trove for material scientists. Throughout the long evolutionary process, biominerals with hierarchical structures possess some specific advantages such as outstanding mechanical properties, biological functions, and sensing performances, the formation of which (biomineralization) is delicately regulated by organic component. Provoked by the subtle structures and profound principles of nature, bioinspired functional minerals can be designed with the participation of organic molecules. Because of the designable morphology and functions, multiscale mineralization has attracted more and more attention in the areas of medicine, chemistry, biology, and material science. This review provides a summary of current advancements in this extending topic. The mechanisms underlying mineralization is first concisely elucidated. Next, several types of minerals are categorized according to their structural characteristic, as well as the different potential applications of these materials. At last, a comprehensive overview of future developments for bioinspired multiscale mineralization is given. Concentrating on the mechanism of fabrication and broad application prospects of multiscale mineralization, the hope is to provide inspirations for the design of other functional materials.

Ultrahigh-Ionic-Conductivity, Antifreezing Poly(amidoxime)-Grafted Polyzwitterion Hydrogel for Facile Integrated into High-Performance Stretchable Flexible Supercapacitor

Developing wearable supercapacitors (SCs) with high stretchability, arbitrary deformability, and antifreezing ability is still a challenge. In the present work, an ultrahigh-ionic-conductivity, antifreezing poly(amidoxime)-graft-polyzwitterion (PAO-g-PSBMA) hydrogel electrolyte is fabricated by grafting PSBMA in PAO. Owing to the abundant hydrophilic and high ionic adsorption capacity of amidoxime groups in PAO and zwitterion groups in PSBMA, the as-prepared PAO-g-PSBMA hydrogel can facilitate the dissociation of lithium salt and exhibit an ultrahigh ionic conductivity of 29.8 S m-1 at 25 °C and 3.4 S m-1 even at -30 °C. Employing mATi3C2Tx and mSTi3C2Tx, which contain small amounts of PAO-AGE and PAO-g-PSBMA dispersions, respectively, coated onto both sides of the PAO-g-PSBMA hydrogel, we followed a thermal treatment to facilely form integrated stretchable flexible SCs. The as-prepared SCs show an outstanding recoverable tensile stain of 80% and an excellent electrochemical stability under many types and times of arbitrary deformation. More importantly, as-prepared mATi3C2Tx- and mSTi3C2Tx-based SCs present fantastic antifreezing ability and excellent stability with 74.6 and 78.3% retention of the initial capacitance, respectively, even after 1000 times of stretching to 60% at -30 °C. This work offers a new strategy of using PAO-grafted polyzwitterion for obtaining an antifreezing stretchable SC, which shows a high potential for application in next-generation integrated stretchable devices in various fields.

Gesture Recognition System Using Reduced Graphene Oxide-Enhanced Hydrogel Strain Sensors for Rehabilitation Training

Gesture recognition systems epitomize a modern and intelligent approach to rehabilitative training, finding utility in assisted driving, sign language comprehension, and machine control. However, wearable devices that can monitor and motivate physically rehabilitated people in real time remain little studied. Here, we present an innovative gesture recognition system that integrates hydrogel strain sensors with machine learning to facilitate finger rehabilitation training. PSTG (PAM/SA/TG) hydrogels are constructed by thermal polymerization of acrylamide (AM), sodium alginate (SA), and tannic acid-reduced graphene oxide (TA-rGO, TG), with AM polymerizing into polyacrylamide (PAM). The surface of TG has abundant functional groups that can establish multiple hydrogen bonds with PAM and SA chains to endow the hydrogel with high stretchability and mechanical stability. Our strain sensor boasts impressive sensitivity (Gauge factor = 6.13), a fast response time (40.5 ms), and high linearity (R2 = 0.999), making it an effective tool for monitoring human joint movements and pronunciation. Leveraging machine learning techniques, our gesture recognition system accurately discerns nine distinct types of gestures with a recognition accuracy of 100%. Our research drives wearable advancements, elevating the landscape of patient rehabilitation and augmenting gesture recognition systems' healthcare applications.

Guar gum reinforced conductive hydrogel for strain sensing and electronic devices

Hydrophobically associated conductive hydrogels got great attention due to their excellent properties like stretchability, energy dissipation mechanism, and strain sensor. But hydrophobically associated hydrogels have poor mechanical properties and time response to external stimuli. To enhance the mechanical properties and response to stimuli, Acrylamide- co-Butyl acrylate/Gum based conductive hydrogels were prepared. SDS works as a cross-linker and micelle-forming agent while NaCl makes hydrogel as conductive. The results show that our % strain sensing reached up to 400 %, and fracture stress and fracture strain reached to 0.5 MPa and 401 % respectively. Besides this, it's having an excellent response to continuous stretching and unstretching multiple cycles without any fracture up to 180 s and an excellent time response of 190 s. The conductivity of the hydrogel was 0.20 Sm-1. The hydrophobic hydrogels showed a clear and quick response to human motions like finger, wresting, writing, speaking, etc. Interestingly, our prepared hydrogels can detect the mood of the human face. Similarly, the hydrogels were found efficient in bridging the surface of electronic devices with human skin. This indicates that our prepared hydrogels can monitor human body motion and will replace the existing materials used in strain sensors in the near future.

Recent Advances in Bioinspired Hydrogels: Materials, Devices, and Biosignal Computing

The remarkable ability of biological systems to sense and adapt to complex environmental conditions has inspired new materials and novel designs for next-generation wearable devices. Hydrogels are being intensively investigated for their versatile functions in wearable devices due to their superior softness, biocompatibility, and rapid stimulus response. This review focuses on recent strategies for developing bioinspired hydrogel wearable devices that can accommodate mechanical strain and integrate seamlessly with biological systems. We will provide an overview of different types of bioinspired hydrogels tailored for wearable devices. Next, we will discuss the recent progress of bioinspired hydrogel wearable devices such as electronic skin and smart contact lenses. Also, we will comprehensively summarize biosignal readout methods for hydrogel wearable devices as well as advances in powering and wireless data transmission technologies. Finally, current challenges facing these wearable devices are discussed, and future directions are proposed.

Supramolecular Chemistry of Carbon-Based Dots Offers Widespread Opportunities

Carbon dots are an emerging class of nanomaterials that has recently attracted considerable attention for applications that span from biomedicine to energy. These photoluminescent carbon nanoparticles are defined by characteristic sizes of <10 nm, a carbon-based core and various functional groups at their surface. Although the surface groups are widely used to establish non-covalent bonds (through electrostatic interactions, coordinative bonds, and hydrogen bonds) with various other (bio)molecules and polymers, the carbonaceous core could also establish non-covalent bonds (ππ stacking or hydrophobic interactions) with π-extended or apolar compounds. The surface functional groups, in addition, can be modified by various post-synthetic chemical procedures to fine-tune the supramolecular interactions. Our contribution categorizes and analyzes the interactions that are commonly used to engineer carbon dots-based materials and discusses how they have allowed preparation of functional assemblies and architectures used for sensing, (bio)imaging, therapeutic applications, catalysis, and devices. Using non-covalent interactions as a bottom-up approach to prepare carbon dots-based assemblies and composites can exploit the unique features of supramolecular chemistry, which include adaptability, tunability, and stimuli-responsiveness due to the dynamic nature of the non-covalent interactions. It is expected that focusing on the various supramolecular possibilities will influence the future development of this class of nanomaterials.

Machine-Learning Assisted Handwriting Recognition Using Graphene Oxide-Based Hydrogel

Machine-learning assisted handwriting recognition is crucial for development of next-generation biometric technologies. However, most of the currently reported handwriting recognition systems are lacking in flexible sensing and machine learning capabilities, both of which are essential for implementation of intelligent systems. Herein, assisted by machine learning, we develop a new handwriting recognition system, which can be applied as both a recognizer for written texts and an encryptor for confidential information. This flexible and intelligent handwriting recognition system combines a printed circuit board with graphene oxide-based hydrogel sensors. It offers fast response and good sensitivity and allows high-precision recognition of handwritten content from a single letter to words and signatures. By analyzing 690 acquired handwritten signatures obtained from seven participants, we successfully demonstrate a fast recognition time (less than 1 s) and a high recognition rate (∼91.30%). Our developed handwriting recognition system has great potential in advanced human-machine interactions, wearable communication devices, soft robotics manipulators, and augmented virtual reality.

A Strand Entangled Supramolecular PANI/PAA Hydrogel Enabled Ultra-Stretchable Strain Sensor

Hydrogel electronics have attracted growing interest for emerging applications in personal healthcare management, human-machine interaction, etc. Herein, a "doping then gelling" strategy to synthesize supramolecular PANI/PAA hydrogel with a specific strand entangled network is proposed, by doping the PANI with acrylic acid (AA) monomers to avoid PANI aggregation. The high-density electrostatic interaction between PAA and PANI chains serves as a dynamic bond to initiate the strand entanglement, enabling PAA/PANI hydrogel with ultra-stretchability (2830%), high breaking strength (120 kPa), and rapid self-healing properties. Moreover, the PAA/PANI hydrogel-based sensor with a high strain sensitivity (gauge factor = 12.63), a rapid responding time (222 ms), and a robust conductivity-based sensing behavior under cyclic stretching is developed. A set of strain sensing applications to precisely monitor human movements is also demonstrated, indicating a promising application prospect as wearable 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.

Wearable CNTs-based humidity sensors with high sensitivity and flexibility for real-time multiple respiratory monitoring

Sensors, such as optical, chemical, and electrical sensors, play an important role in our lives. While these sensors already have widespread applications, such as humidity sensors, most are generally incompatible with flexible/inactive substrates and rely on conventional hard materials and complex manufacturing processes. To overcome this, we develop a CNT-based, low-resistance, and flexible humidity sensor. The core-shell structured CNT@CPM is prepared with Chit and PAMAM to achieve reliability, accuracy, consistency, and durability, resulting in a highly sensitive humidity sensor. The average response/recovery time of optimized sensor is only less than 20 s, with high sensitivity, consistent responsiveness, good linearity according to humidity rates, and low hysteresis (- 0.29 to 0.30 %RH). Moreover, it is highly reliable for long-term (at least 1 month), repeated bending (over 15,000 times), and provides accurate humidity measurement results. We apply the sensor to smart-wear, such as masks, that could conduct multi-respiratory monitoring in real-time through automatic ventilation systems. Several multi-respiratory monitoring results demonstrate its high responsiveness (less than 1.2 s) and consistent performance, indicating highly desirable for healthcare monitoring. Finally, these automatic ventilation systems paired with flexible sensors and applied to smart-wear can not only provide comfort but also enable stable and accurate healthcare in all environments.

Recent Progress on Self-Healable Conducting Polymers

Materials able to regenerate after damage have been the object of investigation since the ancient times. For instance, self-healing concretes, able to resist earthquakes, aging, weather, and seawater have been known since the times of ancient Rome and are still the object of research. During the last decade, there has been an increasing interest in self-healing electronic materials, for applications in electronic skin (E-skin) for health monitoring, wearable and stretchable sensors, actuators, transistors, energy harvesting, and storage devices. Self-healing materials based on conducting polymers are particularly attractive due to their tunable high conductivity, good stability, intrinsic flexibility, excellent processability and biocompatibility. Here recent developments are reviewed in the field of self-healing electronic materials based on conducting polymers, such as poly 3,4-ethylenedioxythiophene (PEDOT), polypyrrole (PPy), and polyaniline (PANI). The different types of healing, the strategies adopted to optimize electrical and mechanical properties, and the various possible healing mechanisms are introduced. Finally, the main challenges and perspectives in the field are discussed.

Bio-Based Hydrogel Transducer for Measuring Human Motion with Stable Adhesion and Ultrahigh Toughness

Adaptability could meet basic technological application requirements. Therefore, a hydrogel-based transducer with durable adhesion, ultrahigh toughness, and super resilience was highly demanded. Here, a skin-like hydrogel transducer was successfully prepared through introducing carboxymethyl chitosan and sodium caseinate into a polyacrylamide hydrogel system. In addition, the polyacrylamide-sodium casein-carboxymethyl chitosan (PAAM-SC-CC) hydrogel has strong mechanical properties and excellent mechanical flexibility, largely due to the adequate energy dissipation mechanism. Surprisingly, the PAAM-SC-CC hydrogel exhibited stable and reproducible adhesion to various solid substrates and the human skin. Due to abundant free ions driven from sodium caseinate, the PAAM-SC-CC hydrogel could maintain stable and sensitive ionic conductivity without adding additional fillers. Experiments have proved that it can be applied to the field of human motion monitoring with complex signals. Therefore, the PAAM-SC-CC hydrogel sensor could monitor human movement in different strain ranges, including throat movement and joint extension. Such a flexible hydrogel-based transducer with various properties is conceivable to broaden the application field of bioelectrodes, human machines, personalized medical health fields, etc.

Reversible Stimuli-Dependent Aggregation-Induced Emission from a "Nonfluorescent" Amphiphilic PVDF Graft Copolymer

A poly(vinylidine fluoride) graft random copolymer of t-butyl aminoethyl methacrylate (tBAEMA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA, Mn = 300) [PVDF-g-P(tBAEMA-ran-OEGMA), PVBO] is synthesized by atom transfer radical polymerization (ATRP), and PVBO is fractionated to get a highly water-soluble fraction (PVBO-1) showing a reversible on/off fluorescence behavior with gradual increase and decrease in pH, respectively, achieving a maximum quantum yield of 0.18 at pH = 12. PVBO-1 dissolved in water shows large multimicellar aggregates (MMcA), but at pH 12, crumbling of larger aggregates to much smaller micelles occurs, forming nonconjugated polymer dots (NCPDs), as supported by transmission electron microscopy and dynamic light scattering study. The reversible fluorescence on/off behavior also occurs with the decrease and increase of temperature. Theoretical study indicates that, at high pH, most of the amino groups become neutral and exhibit a strong tendency to form aggregates from crowding of a large number of carbonyl and amine groups, minimizing the HOMO-LUMO gap, showing an absorption peak at the visible region, and generating aggregation-induced emission.

Topology Reliable LCST-Type Behavior of ABA Triblock Polymer and Influence on Water Condensation and Crystallization

As a kind of smart material, thermoresponsive hydrogels are widely investigated and applied in many fields. Due to the limitation of the freezing temperature of the water, it is a challenge to further broaden their sol-gel transition temperature (T_gel ) range, especially below 0 °C. Herein, the lower critical solution temperature type of amphiphilic ABA triblock copolymers, synthesized via two-step reversible addition-fragmentation chain transfer (RAFT) polymerization is demonstrated. The hydrophilic A-block and the hydrophobic B-block are composed of poly(N,N-dimethylacrylamide) (PDMAA) and poly(diacetone acrylamide) (PDAAM), respectively. The degree of polymerization (DP) of both A-block and B-block shows a significant influence on the T_gel of triblock copolymer dispersion. By changing the length of these two blocks or physically blending these copolymers dispersions, the T_gel can be well adjusted in a temperature range from 45 to -10 °C. Moreover, When the T_gel is higher than 4 °C, the triblock copolymer coatings show a good anti-fogging property. And when the T_gel is around or lower than the freezing temperature of the water, aqueous dispersions of the triblock copolymer have an ice recrystallization inhibition activity, resulting in the decrease of average maximum grain size (MLGS) of ice crystal.

Self-healing and anti-freezing graphene-hydrogel-graphene sandwich strain sensor with ultrahigh sensitivity

Hydrogels with specially designed structures and adjustable properties have been considered as smart materials with multi-purpose application prospects, especially in the field of flexible sensors. However, most hydrogel-based sensors have low sensitivity, which inevitably affects their promotion in the market. Herein, a strain sensor comprising a poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) hybrid hydrogel sandwiched between two graphene layers was successfully constructed in a facile way, and it exhibited many excellent properties including extremely high sensitivity. The incorporation of glycerol ensured the good flexibility and anti-freezing performance of the hydrogel-based sensor even at -15 °C. The dynamic coordination bonds in the hydrogel-based sensor endowed it with excellent self-healing properties. In particular, the sandwich-structured hydrogel sensor showed a very high gauge factor (GF) value of 39 at the strain of 50%, which is much higher than those of most ordinary hydrogel-based strain sensors. A super stable signal value after 5000 strain cycles and a very short response time of 274 ms guaranteed the long-term usability and sensitivity of the hydrogel-based sandwich sensor. More importantly, the hydrogel-based sandwich sensor could detect both large and tiny human motions accurately and instantly in a series of real-time monitoring experiments, showing great potential for intelligent wearable electronic devices.

Tunable Pressure Sensor of f-Carbon Dot-Based Conductive Hydrogel with Electrical, Mechanical, and Shape Recovery for Monitoring Human Motion

The reversible volume memories of the inner structures of soft materials with controllable hydrophilic-hydrophobic balance have been widely recognized, for example, hydrogels used in pressure sensors. Mechanical stimuli, such as pressure, vibration, and tensile, may influence the deformation of the hydrogel while simultaneously changing the electronic signal. Here, we designed a hydrophobic carbon dot nanoparticle (f-CD) mixed with polyvinyl alcohol and catechol-conjugated chitosan to obtain a hydrogel suitable for pressure and vibration sensor applications. The hydrophobicity of loaded f-CD plays an important role in mechanical performance and electronic signal acquisition. It also affects the different rheological reversibility and shape recovery as an impact on the volume transition. These characteristics are influenced by the compactness, dimensional structure, and density of the fabricated hydrogel. As a result, hydrogels with high hydrophobicity have a stiff structure (shear modulus 8123.1 N·m^-2) compared to that of the hydrophilic hydrogel (ranging between 6065.7 and 7739.2 N·m^-2). Moreover, the mechanically dependent volume transition hydrogel affects the electronic resistivity (up to 17.3 ± 1.3%) and capacitance change (up to 145%) when compressed with different forces. The hydrogel with a controlled hydrophobic-hydrophilic inner structure shows a unique sensitivity and great potential for various applications in wearable electronic skins, real-time clinical health-care monitoring, and human-computer interactions.