Double-network hydrogels with adjustable surface morphology and multifunctional integration for flexible strain sensors

An Antibacterial, Highly Sensitive Strain Sensor Based on an Anionic Copolymer Interpenetrating with κ-Carrageenan

Polysaccharide-based hydrogels are suitable for use in the field of flexible bioelectronics due to their benign mechanical properties and biocompatibility. However, the preparation of hydrogel sensors with high performance without affecting their physicochemical properties (e.g., flexibility, toughness, self-healing, and antibacterial activity) remains a challenge and needs to be solved. Herein, a metal ion cross-linking reinforced, double network hydrogel was formed from a 2-acrylamide-2-methylpropanesulfonic acid (AMPS) copolymer interpenetrating κ-carrageenan (CAR), followed by immersing the gel in a Cu2+ ion solution to obtain an antibacterial CAR/P(AM-co-AMPS)-Cu2+ conductive hydrogel. LiCl was added as the electrolyte. The presence of the LiCl electrolyte and sulfonated molecular chain units not only gives the hydrogel good electrical conductivity (conductivity up to 2.68 S/m) but also improves the sensitivity of the hydrogel as a stress-strain sensor, with a hydrogel sensitivity GF of up to 3.76 in the 20%-100% strain range and response time of up to 280 ms. The CAR double-helical structure and sol-gel properties and the interaction of multiple noncovalent bonds between polymers provide the hydrogel with excellent self-healing, with a self-healing efficiency of 68%. In addition, the electrostatic interaction of Cu2+ with Escherichia coli cells can inhibit their growth, exhibiting good antibacterial properties with an inhibition circle diameter of 20.5 mm. This work could provide an effective strategy for antibacterial multifunctional CAR-based bionic sensors.

Mechanically Robust and Highly Conductive Poly(ionic liquid)/Polyacrylamide Double-Network Hydrogel Electrolytes for Flexible Symmetric Supercapacitors with a Wide Operating Voltage Range

Flexible electronic devices, such as supercapacitors (SCs), place high demands on the mechanical properties, ionic conductivity, and electrochemical stability of electrolytes. Hydrogels, which combine flexibility and the advantages of both solid and liquid electrolytes, will meet the demand. Here, we report the synthesis of novel poly(ionic liquid)/polyacrylamide double-network (DN) (PIL/PAM DN) hydrogel electrolytes containing different metal salts via a two-step γ-radiation method. The resultant Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte possesses excellent mechanical properties (tensile strength of 3.64 MPa, elongation at break of 446%) and high ionic conductivity (24.1 mS·cm-1). The corresponding flexible SC based on the Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte (SC-Li2SO4) presents improved ion diffusion, ideal electrochemical double-layer capacitor behavior, good rate capability, and excellent cyclic stability. Moreover, symmetric SC-Li2SO4 achieves a wide operating voltage range of up to 1.5 V, with a maximum energy density of 26.0 W h·kg-1 and a capacitance retention of 94.1% after 10,000 galvanostatic charge-discharge cycles, owing to the deactivation of free water molecules by the synergistic effect of PIL, PAM, and SO42-. Above all, the capacitance of SC-Li2SO4 is well-maintained after overcharge, overdischarge, short circuit, extreme temperature, compression, and bending tests, indicating its high security and flexibility. This work reveals the enormous application potential of PIL-based conductive hydrogel electrolytes for flexible electronic devices.

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

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

High-performance flexible strain sensors based on silver film wrinkles modulated by liquid PDMS substrates

Flexible strain sensors based on controllable surface microstructures in film-substrate systems can be extensively applied in high-tech fields such as human-machine interfaces, electronic skins, and soft robots. However, the rigid functional films are susceptible to structural destruction and interfacial failure under large strains or high loading speeds, limiting the stability and durability of the sensors. Here we report on a facile technique to prepare high-performance flexible strain sensors based on controllable wrinkles by depositing silver films on liquid polydimethylsiloxane (PDMS) substrates. The silver atoms can penetrate into the surface of liquid PDMS to form an interlocking layer during deposition, enhancing the interfacial adhesion greatly. After deposition, the liquid PDMS is spontaneously solidified to stabilize the film microstructures. The surface patterns are well modulated by changing film thickness, prepolymer-to-crosslinker ratio of liquid PDMS, and strain value. The flexible strain sensors based on the silver film/liquid PDMS system show high sensitivity (above 4000), wide sensing range (∼80%), quick response speed (∼80 ms), and good stability (above 6000 cycles), and have a broad application prospect in the fields of health monitoring and motion tracking.

Ultra-robust, high-adhesive, self-healing, and photothermal zwitterionic hydrogels for multi-sensory applications and solar-driven evaporation

Zwitterionic hydrogels have received considerable attention owing to their characteristic structures and integrating multifunctionality. However, the superhydrophilicity-induced poor mechanical properties severely hinder their potential applications. Besides, from the perspective of wide applications, zwitterionic hydrogels with integrated high mechanical properties, conductivity and multifunctionalities including self-adhesive, self-healing, and photothermal properties are highly desirable yet challenging. Herein, a new class of high-performance and multifunctional zwitterionic hydrogels are designed based on the incorporation of polydopamine-coated liquid metal nanoparticles (LM@PDA). Due to the efficient energy dissipation endowed by the isotropically extensible deformation of LM@PDA and the multiple interactions within the hydrogel matrix, the resultant hydrogels exhibited ultrahigh robustness with tensile strength of up to 1.3 MPa, strain of up to 1555% and toughness of up to 7.3 MJ m-3, superior or comparable to those of most zwitterionic hydrogels. The introduced LM@PDA also endows the hydrogels with high conductivity, versatile adhesion, autonomous self-healing, excellent injectability, three-dimensional printability, degradability, and photothermal conversion performance. These preferable properties enable the hydrogels promising as wearable sensors with multiple sensory capabilities for a wide range of strain values (1-500%), pressures (0.5-200 kPa) and temperatures (20-80 °C) with an impressive temperature coefficient of resistance (up to 0.15 °C-1). Moreover, these hydrogels can be also applied as solar evaporators with a high water evaporation rate (up to 2.42 kg m-2 h-1) and solar-thermal conversion efficiency (up to 90.3%) for solar desalination and wastewater purification. The present work can pave the way for the future development of zwitterionic hydrogels and beyond.

All polymeric conductive strain sensors with excellent skin adhesion, recovery, and long-term stability prepared from an anion-zwitterion based hydrogel

Developing a high-performing hydrogel with long-lasting skin adhesion, high ionic conductivity, mechanical stability, and fatigue resistance is a crucial issue in the field of wearable electronic devices. Because of their weak mechanical properties, zwitterion-based hydrogels are not suitable for application in wearable strain sensors despite their excellent adhesion to the skin. In this study, a hydrogel of polymer without additive was prepared by using polymerizable monomers consisting of zwitterionic 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS), anionic 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (AMPSs), and acrylamide (AAm); the hydrogel is abbreviated as P(AMPSs/VIPS-co-AAm). The P(AMPSs/VIPS-co-AAm) hydrogel shows exceptional adhesive strength, reaching up to 26.29 kPa (lap shear to porcine skin) and high stretchability (with a fracture strain of 1282% and stress of 40 kPa). The high polarity of the AMPSs/VIPS pair improves the interfacial adhesion to the skin, the internal cohesion and recovery tendency. Unique structural characteristics of the hydrogel impart excellent fatigue resistance, network toughening, and electrical stability after multiple deformations. Thus, the prepared hydrogel has an ionic conductivity (0.51 S m^-1), strain sensitivity, and long-term skin adhesion, and it demonstrates potential to be applied for wearable strain sensors.

Poly(N,N-dimethyl)acrylamide-based ion-conductive gel with transparency, self-adhesion and rapid self-healing properties for human motion detection

Flexible strain sensors have been extensively studied for their potential value in monitoring human activity and health. However, it is still challenging to develop multifunctional flexible strain sensors with simultaneously high transparency, strong self-adhesion, fast self-healing and excellent tensile properties. In this study, we used N,N-dimethylacrylamide (DMA) in the imidazolium-based ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide ([BMIM][Tf₂N]) for "one-step" UV irradiation. A poly(N,N-dimethyl)acrylamide (PDMA) ion-conductive gel was prepared by site polymerization. Based on the good compatibility between PDMA and ionic liquid, the prepared ion-conductive gel has good transparency (∼90%), excellent stretchability (1080%), strong self-adhesion (67.57 kPa), fast self-healing (2 s at room temperature) and great antibacterial activity (∼99% bacterial killing efficiency). Moreover, the strain sensor based on the PDMA ion-conductive gel has good electromechanical performance and can detect different human motions. Based on the simple and easy-to-operate preparation method and the endowed multifunctionality of the PDMA ion-conductive gel, it has broad application prospects in the field of flexible electronic devices.

Cutting-Edge Progress in Stimuli-Responsive Bioadhesives: From Synthesis to Clinical Applications

With the advent of "intelligent" materials, the design of smart bioadhesives responding to chemical, physical, or biological stimuli has been widely developed in biomedical applications to minimize the risk of wounds reopening, chronic pain, and inflammation. Intelligent bioadhesives are free-flowing liquid solutions passing through a phase shift in the physiological environment due to stimuli such as light, temperature, pH, and electric field. They possess great merits, such as ease to access and the ability to sustained release as well as the spatial transfer of a biomolecule with reduced side effects. Tissue engineering, wound healing, drug delivery, regenerative biomedicine, cancer therapy, and other fields have benefited from smart bioadhesives. Recently, many disciplinary attempts have been performed to promote the functionality of smart bioadhesives and discover innovative compositions. However, according to our knowledge, the development of multifunctional bioadhesives for various biomedical applications has not been adequately explored. This review aims to summarize the most recent cutting-edge strategies (years 2015-2021) developed for stimuli-sensitive bioadhesives responding to external stimuli. We first focus on five primary categories of stimuli-responsive bioadhesive systems (pH, thermal, light, electric field, and biomolecules), their properties, and limitations. Following the introduction of principal criteria for smart bioadhesives, their performances are discussed, and certain smart polymeric materials employed in their creation in 2015 are studied. Finally, advantages, disadvantages, and future directions regarding smart bioadhesives for biomedical applications are surveyed.

Simulation of the peel of hydrogels with stiff backing

In this work, the peel of hydrogels under a stiff backing constraint was studied using a finite element method. The finite element method was first validated by comparing the simulation results to theoretical predictions and experimental measurements. Then, the method was used to investigate the effects of adhesion thickness, adhesion length and backing thickness on the peel behaviors, as well as the stress distribution within the adhesion layer. The results indicated that the peel force-displacement curve has a constant profile when the adhesion thickness and backing thickness are prescribed so long as the adhesion length is sufficiently long. The peak peel force increases with the adhesion length and then plateaus. The larger the intrinsic peak stress or the thicker the backing, the higher the plateau. The steady-state peel force is independent of the backing thickness, while positively correlated with the strain energy storage of the hydrogel adhesion layer. The critical vertical displacement corresponding to the peak peel force increases with the hydrogel thickness and decreases with the backing thickness. However, the critical vertical displacement corresponding to the steady-state peel force increases with the backing thickness. The present work puts forward an effective numerical approach to probe the peel of hydrogels, which is beneficial for the design of relevant structures.

Dually cross-linked single network poly(ionic liquid)/ionic liquid ionogels for a flexible strain-humidity bimodal sensor

Gel electrolytes have aroused extensive interest for diverse flexible electronics due to their high ionic conductivity and inherent stretchability. However, gel electrolytes still face challenges in terms of mechanical properties, fatigue resistance, and environmental adaptation, which severely limit the practical application of gel-based electronics. In this paper, we have synthesized a novel polymerizable ionic liquid [SBMA][AA] by mixing zwitterionic sulfobetaine methacrylate with acrylic acid. Then a dually cross-linked single network poly(ionic liquid)/ionic liquid (DCSN PIL/IL) ionogel was prepared by a simple one-step photopolymerization of the [SBMA][AA] in another IL 1-ethyl-3-methylimidazolium dicyanoamide ([EmIm][DCA]). The synergistic effect between covalent crosslinking and dynamic physical crosslinking points endows the ionogel with good mechanical properties as well as outstanding fatigue resistance. Gratifyingly, the entrapment of [EmIm][DCA] in the ionogel matrix yields excellent environmental adaptability and high ionic conductivity. Meanwhile, the DCSN PIL/IL ionogel also exhibited strong adhesive capacity due to the abundance of carboxyl and sulphonic acid groups. The outstanding electromechanical properties make the DCSN PIL/IL ionogel a perfect candidate for strain sensors to monitor diverse human body activities, such as the movement of the thumb knuckle and handwriting. Interestingly, the DCSN PIL/IL ionogel also displayed high responsiveness to humidity. Therefore, it is believed that this DCSN PIL/IL ionogel offers a broad prospect in flexible strain-humidity bimodal sensors.