Flexible sensitive hydrogel sensor with self-powered capability

Recent Progress of Anti-Freezing, Anti-Drying, and Anti-Swelling Conductive Hydrogels and Their Applications

Hydrogels are soft-wet materials with a hydrophilic three-dimensional network structure offering controllable stretchability, conductivity, and biocompatibility. However, traditional conductive hydrogels only operate in mild environments and exhibit poor environmental tolerance due to their high water content and hydrophilic network, which result in undesirable swelling, susceptibility to freezing at sub-zero temperatures, and structural dehydration through evaporation. The application range of conductive hydrogels is significantly restricted by these limitations. Therefore, developing environmentally tolerant conductive hydrogels (ETCHs) is crucial to increasing the application scope of these materials. In this review, we summarize recent strategies for designing multifunctional conductive hydrogels that possess anti-freezing, anti-drying, and anti-swelling properties. Furthermore, we briefly introduce some of the applications of ETCHs, including wearable sensors, bioelectrodes, soft robots, and wound dressings. The current development status of different types of ETCHs and their limitations are analyzed to further discuss future research directions and development prospects.

Construction methods and biomedical applications of PVA-based hydrogels

Polyvinyl alcohol (PVA) hydrogel is favored by researchers due to its good biocompatibility, high mechanical strength, low friction coefficient, and suitable water content. The widely distributed hydroxyl side chains on the PVA molecule allow the hydrogels to be branched with various functional groups. By improving the synthesis method and changing the hydrogel structure, PVA-based hydrogels can obtain excellent cytocompatibility, flexibility, electrical conductivity, viscoelasticity, and antimicrobial properties, representing a good candidate for articular cartilage restoration, electronic skin, wound dressing, and other fields. This review introduces various preparation methods of PVA-based hydrogels and their wide applications in the biomedical field.

Hydroxyapatite cross-linked in situ polyvinyl alcohol hydrogel for bionic calcified cartilage layer

Many tissue engineering constructs have been investigated for repairing the calcified cartilage layer in the recent years, but engineering a consistent and stable interface to facilitate a graft-to-bone fixation remains a concern. In the work, hydroxyapatite (HA) is modified by diisocyanate (HDI) and integrated with polyvinyl alcohol (PVA) to prepare the hydrogel. The IR shows that HDI is grafted onto HA and helps HA to cross-link in situ in the PVA gel network. When the mass ratio of HA/PVA is 3.5 wt%, the swelling rate in the PBS of different pH reduced with time prolong, and the hydrogel takes on good swelling resistance. The tensile strength and toughness are 1890 KPa and 264 KJ/m-3, respectively, while the compression strength reaches 1125 KPa at compressive strain of 60%. The hydrogel not only is well durable via continuous 100 rounds of compression-recovery, but also has excellent bioactivity. The work will provide a platform for developing multifunctional soft tissue scaffold.

Self-Powered Multifunctional Organic Hydrogel Based on Poly(acrylic acid-N-isopropylacrylamide) for Flexible Sensing Devices

Human-machine interactions, medical monitoring, and flexible robots stimulate interest in hydrogel sensing devices. However, developing hydrogel sensors with multifunctions such as good mechanics, electroconductivity, resistance to solvent volatility as well as freezing, self-adhesion, and independence on external power supply remains a challenge. In the work, a poly(acrylic acid-N-isopropylacrylamide) P(AA-NIPAm) organic hydrogel loading LiCl is prepared by ultraviolet cross-linking in ethylene glycol/H₂O. The organic hydrogel exhibits favorable mechanical properties such as an elongation of break at 700% and a breaking strength of 20 KPa, can adhere to various substrates, and resists frost and solvent volatility. Especially, it possesses an excellent conductivity of 8.51 S/m. The organic hydrogel shows wide strain sensitivity based on resistance change, and the gauge factor reaches 5.84 in the range of 300-700%. It has short responsive and recuperative time and is still stable within 1000 rounds. Moreover, the organic hydrogel is also assembled into a self-powered device in which the open-circuit voltage is 0.74 V. The device can transform external stimuli such as stretching or compressing into the output current change, so it detects human motion effectively in real time. The work provides a perspective for electrical sensing engineering.

Recent Development of Self-Powered Tactile Sensors Based on Ionic Hydrogels

Hydrogels are three-dimensional polymer networks with excellent flexibility. In recent years, ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications. In this paper, we introduce the basic properties of ionic hydrogels and highlight their application in self-powered sensors working in triboelectric, piezoionic, ionic diode, battery, and thermoelectric modes. We also summarize the current difficulty and prospect the future development of ionic hydrogel self-powered sensors.