Recently, flexible and wearable electronics have gained considerable research interest due to their potential applications in wearable devices, energy storage materials, electronic skins, sensors, etc. Compared to elastomers, hydrogels demonstrate more potential for flexible electronics because of their biomimetic structures, suitable mechanical properties and excellent biocompatibility. Among all the designs, nanocomposite hydrogel-based strain and pressure sensors which can transmit external stimulus to electrical signals have been intensely investigated due to their high mechanical strength, considerable conductivity and outstanding sensitivity. Numerous reports have been dedicated to the designs, preparations and applications of nanocomposite hydrogels. This review provides an up-to-date and comprehensive summary of research progresses of nanocomposite hydrogel-based strain and pressure sensors including designing strategies, preparing methods and applications of the five nanofiller based hydrogel sensors including carbon nanotube based, graphene oxide based, MXene based, polymer nanofiller based and other nanofiller based sensors. Representative cases are carefully selected and discussed regarding the fabrication, merits and demerits, respectively. Finally, perspectives and challenges are presented for the designs of future nanocomposite hydrogel-based strain and pressure sensors.

Zwitterionic hydrogels exhibit eminent nonfouling and hemocompatibility. Several key challenges hinder their application as coating materials for blood-contacting biomedical devices, including weak mechanical strength and low adhesion to the substrate. Here, we report a poly(carboxybetaine) microgel reinforced poly(sulfobetaine) (pCBM/pSB) pure zwitterionic hydrogel with excellent mechanical robustness and anti-swelling properties. The pCBM/pSB hydrogel coating was bonded to the PVC substrate via the entanglement network between the pSB and PVC chain. Moreover, the pCBM/pSB hydrogel coating can maintain favorable stability even after 21 d PBS shearing, 0.5 h strong water flushing, 1000 underwater bends, and 100 sandpaper abrasions. Notably, the pCBM/pSB hydrogel coated PVC tubing can not only mitigate the foreign body response but also prevent thrombus formation ex vivo in rats and rabbits blood circulation without anticoagulants. This work provides new insights to guide the design of pure zwitterionic hydrogel coatings for biomedical devices.

Flexible and stretchable electronics have received tremendous attention for next-generation human-friendly electronic applications. However, fabrication of transparent, fully recyclable and stretchable electronic sensors with low-temperature stability using biocompatible natural polymer-based hydrogels still remains a great challenge. In this study, a green and fully recyclable stretchable electronic sensor with high transparency and ultra-low operating temperature is constructed using ionic conductive gelatin organohydrogels. These gelatin organohydrogels are prepared by a simple strategy of immersing gelatin pre-hydrogels in citrate (Na₃Cit) water/glycerol solutions. The existence of Na₃Cit in the organohydrogel not only induces the formation of multiple non-covalent cross-linking points, endowing the organohydrogel with high mechanical performances, but also makes the organohydrogel have excellent ionic conductivity. The organohydrogel is also highly transparent and exhibits outstanding antifreezing properties. The mechanical robustness, conductivity and transparency of the organohydrogel can be well maintained even at −60 °C. As a result, a stretchable and transparent electronic sensor based on this organohydrogel is fabricated, which is strain-sensitive with a large linear sensing window and excellent stability. More importantly, the organohydrogel-based electronic sensor can be fully recycled due to the reversible non-covalently crosslinked structure, and the recycled organohydrogel regains its mechanical and sensing properties. The obtained sensors could precisely detect various human activities even below −30 °C, indicating the potential applications of the organohydrogel-based electronic sensor in flexible and stretchable electronics in a broad range of temperature.