Tough, Antifreezing, and Piezoelectric Organohydrogel as a Flexible Wearable Sensor for Human-Machine Interaction.

Piezoelectric hydrogel sensors are becoming increasingly popular for wearable sensing applications due to their high sensitivity, self-powered performance, and simple preparation process. However, conventional piezoelectric hydrogels lack antifreezing properties and are thus confronted with the liability of rupture in low temperatures owing to the use of water as the dispersion medium. Herein, a kind of piezoelectric organohydrogel that integrates piezoelectricity, low-temperature tolerance, mechanical robustness, and stable electrical performance is reported by using poly(vinylidene fluoride) (PVDF), acrylonitrile (AN), acrylamide (AAm), p-styrenesulfonate (NaSS), glycerol, and zinc chloride. In detail, the dipolar interaction of the PVDF chain with the PAN chain facilitates the crystal phase transition of PVDF from the α to ß phase, which endows the organohydrogels with a high piezoelectric constant d33 of 35 pC/N. In addition, the organohydrogels are highly ductile and can withstand significant tensile and compressive forces through the synergy of the dipolar interaction and amide hydrogen bonding. Besides, by incorporating glycerol and zinc chloride, the growth of ice crystals is inhibited, allowing the organohydrogels to maintain stable flexibility and sensitivity even at -20 °C. The real-time monitoring of the pulse signal for up to 2 min indicates that the gel sensor has stable sensitivity. It is believed that our organohydrogels will have good prospects in future wearable electronics.

All-Polymer Piezoelectric Elastomer with High Stretchability, Low Hysteresis, Self-Adhesion, and UV-Blocking as Flexible Sensor.

All-polymer piezoelectric elastomers that integrate self-powered, soft, and elastic performance are attractive in the fields of flexible wearable electronics and human-machine interfaces. However, a lack of adhesion and UV-blocking performances greatly hinders the potential applications of elastomers in these emerging fields. Here, a high-performance piezoelectric elastomer with piezoelectricity, mechanical robustness, self-adhesion, and UV-resistance was developed by using poly(vinylidene fluoride) (PVDF), acrylonitrile (AN), acrylamide (AAm), and oxidized tannic acid (OTA) (named PPO). In this design, the dipole-dipole interactions between the PVDF and PAN chains promoted the content of ß-PVDF, endowing high piezoelectric coefficient (d33, 58 pC/N). Besides, high stretchability (∼500%), supercompressibility (∼98%), low Young's modulus (∼0.02 MPa), and remarkable elasticity (∼13.8% hysteresis ratio) were achieved simultaneously for the elastomers. Inspired by the mussel adhesion chemistry, the OTA containing abundant catechol and quinone groups provided high adhesion (93.26 kPa to wood) and an exceptional UV-blocking property (∼99.9%). In addition, the elastomers can produce a reliable electric signal output (Vocmax = 237 mV) and show a fast response (24 ms) when subjected to external force. Furthermore, the elastomer can be easily assembled as a wearable sensor for human physiological (body pulse and speech identification) monitoring and communication.

Injectable Intrinsic Photothermal Hydrogel Bioadhesive with On-Demand Removability for Wound Closure and MRSA-Infected Wound Healing.

Photothermal hydrogel adhesives have yielded promising results for wound closure and infected wound treatment in recent years. However, photothermal hydrogel bioadhesives with on-demand removability without additional nanomaterials-based photothermal agents have rarely been reported in the literature. In this work, an injectable intrinsic photothermal hydrogel bioadhesive with an on-demand removal trait is developed through dynamic cross-linking of gelatin (Gel), tannic acid (TA) quinone, and borax for closing skin incisions and accelerating methicillin-resistant Staphylococcus aureus (MRSA) infected wound healing. The TA quinone containing polyphenol and quinone groups with multifunctional adhesiveness and intrinsic photothermal performance confer the hydrogel adhesive with near-infrared (NIR) responsive antibacterial activity. The cross-linking of pH-sensitive boronic ester (polyphenol-B) and Schiff base bonds endow the hydrogel with great self-healing capacity and on-demand removability. Moreover, the hydrogel possesses good biocompatibility, injectability, and hemostasis. The in vivo experiment in a rat cutaneous incision model and full-thickness MRSA-infected wound model indicate that the smart hydrogel can close wounds efficiently and treat infected ones, demonstrating its superiority in noninvasive treatment of cutaneous incisions and enhancing infected full-thickness wound healing.

Soft, Super-Elastic, All-Polymer Piezoelectric Elastomer for Artificial Electronic Skin.

Piezoelectric sensors are widely used in wearable devices to mimic the functions of human skin. However, it is considerably challenging to develop soft piezoelectric materials that can exhibit high sensitivity, stretchability, super elasticity, and suitable modulus. In this study, a soft skin-like piezoelectric polymer elastomer composed of poly(vinylidene fluoride) (PVDF) and a novel elastic substrate polyacrylonitrile is prepared by combining the radical polymerization and freeze-drying processes. Dipole-dipole interaction results in the phase transition of PVDF (α phase to ß phase), which enhances the electrical and mechanical performances. Thus, we achieve a high piezoelectric coefficient (d33max = 63 pC/N), good stretchability (211.3-259.3%), super compressibility (subjected to 99% compression strain without cracking), and super elasticity (100% recovery after extreme compression) simultaneously for the elastomer. The soft composite elastomer produces excellent electrical signal output (Vocmax = 253 mV) and responds rapidly (15 ms) to stress-induced polarization effects. In addition, the elastomer-based sensor accurately detects various physiological signals such as gestures, throat vibrations, and pulse waves. The developed elastomers exhibit excellent mechanical properties and high sensitivity, which helps facilitate their application as artificial electronic skin to sense subtle external pressure in real time.

Strong Biopolymer-Based Nanocomposite Hydrogel Adhesives with Removability and Reusability for Damaged Tissue Closure and Healing.

Bioadhesives are widely used in a variety of medical settings due to their ease of use and efficient wound closure and repair. However, achieving both strong adhesion and removability/reusability is highly needed but challenging. Here, we reported an injectable mesoporous bioactive glass nanoparticle (MBGN)-incorporated biopolymer hydrogel bioadhesive that demonstrates a strong adhesion strength (up to 107.55 kPa) at physiological temperatures that is also removable and reusable. The incorporation of MBGNs in the biopolymer hydrogel significantly enhances the tissue adhesive strength due to an increased cohesive and adhesive property compared to the hydrogel adhesive alone. The detachment of bioadhesive results from temperature-induced weakening of interfacial adhesive strength. Moreover, the bioadhesive displays injectability, self-healing, and excellent biocompatibility. We demonstrate potential applications of the bioadhesive in vitro, ex vivo, and in vivo for hemostasis and intestinal leakage closure and accelerated skin wound healing compared to surgical wound closures. This work provides a novel design of strong and removable bioadhesives.

Piezoelectric Hydrogel for Prophylaxis and Early Treatment of Pressure Injuries/Pressure Ulcers.

Pressure injuries/pressure ulcers (PIs/PUs) are a critical global healthcare issue and represent a considerable burden on healthcare resources. Prevention of PIs/PUs is the least costly approach and minimizes the patient suffering compared with treatment. Besides, sustained tissue load alleviation and microenvironment management are the most crucial properties for dressings in PI/PU prevention. Hydrogel dressings have attracted a lot of attention to prevent PIs/PUs because of their unique mechanical properties and ability to manage the microenvironment of skin. However, auxiliary prophylaxis and early treatment of PIs/PUs remain a challenge and an acute clinical demand. Here, we report on an electroactive hydrogel with large stretchability (∼380%) and skinlike ductility, and Young's modulus (0.48 ± 0.03 MPa) matches that of human skin (0.5-1.95 MPa). The hydrogel displayed piezoelectric properties and mechanical-electric response stability and sensitivity. Our results indicated that the hydrogel was able to promote in vitro angiogenesis under piezoelectric stimulation and exhibited biocompatibility, which has the potential for forming fine vessels at the damaged sites of PIs/PUs. Furthermore, finite element analysis and pressure dispersion experiments demonstrated that the hydrogel was suitable for preventing PIs/PUs by redistributing force, reducing tissue distortion, and maintaining the microenvironment for skin. This work offers a new strategy for designing and evaluating the dressing for prophylaxis and the early treatment of PIs/PUs.

Tough and Highly Efficient Underwater Self-Repairing Hydrogels for Soft Electronics.

The vulnerability of hydrogel electronic materials to mechanical damage due to their soft nature has necessitated the development of self-repairing hydrogel electronics. However, the development of such material with underwater self-repairing capability as well as excellent mechanical properties for application in aquatic environments is highly challenging and has not yet been fully realized. This study designs a tough and highly efficient underwater self-repairing supramolecular hydrogel by synergistically combining weak hydrogen bonds (H-bonds) and strong dipole-dipole interactions. The resultant hydrogel has high stretchability (up to 700%) and toughness (4.45 MJ m-3 ), and an almost 100% fast strain self-recovery (10 min). The underwater healing process is rapid and autonomous (98% self-repair efficiency after 1 h of healing). Supramolecular hydrogels can be developed as soft electronic sensors for physiological signal detection (gestures, breathing, microexpression, and vocalization) and real-time underwater communication (Morse code). Importantly, the hydrogel sensor can function underwater after mechanical damage because of its highly efficient underwater self-repairing capability.