High-performance poly(acrylic acid) hydrogels formed with a block copolymer crosslinker containing amino-acid derivatives

Two block copolymers containing two amino-acid derivatives, PEO-b-PLAA and PEO-b-PAAC, were fabricated through atom transfer radical polymerization (ATRP) or reversible addition-fragmentation chain transfer polymerization (RAFT). Then, they were employed as a macro-crosslinker to prepare high-performance poly(acrylic acid) (PAA) hydrogels named "PxAy" or "TyAz". There were numerous synergistic noncovalent interactions with hydrogen bonds between the macro-crosslinker and PAA chains, as well as entanglement of polymer chains. Hence, the hydrogels exhibited desirable mechanical properties and self-healing abilities. For PxAy hydrogels, the maximum fracture elongation and fracture strength were 9800% and 120.01 kPa, respectively. Moreover, the enhanced physical interaction enabled the hydrogels to have rapid self-healing abilities without stimulation. The hydrogels showed >80% self-healing efficiency and exhibited ∼10-3 S cm-1 electrical conductivity upon the introduction of KCl. Meanwhile, benefitting from doubling the number of carboxyl groups in the macro-crosslinker of the TyAz hydrogels compared with the PxAy hydrogels, the mechanical properties of TyAz hydrogels could be promoted further and notch-insensitivity could be observed. Tough, adhesive, self-healable, and conductive PAA hydrogels with different structures of amino-acid derivatives could aid the development of macro-crosslinkers.

Doping engineering of conductive polymer hydrogels and their application in advanced sensor technologies

Conductive polymer hydrogels are emerging as an advanced electronic platform for sensors by synergizing the advantageous features of soft materials and organic conductors. Doping provides a simple yet effective methodology for the synthesis and modulation of conductive polymer hydrogels. By utilizing different dopants and levels of doping, conductive polymer hydrogels show a highly flexible tunability for controllable electronic properties, microstructures, and structure-derived mechanical properties. By rationally tailoring these properties, conductive polymer hydrogels are engineered to allow sensitive responses to external stimuli and exhibit the potential for application in various sensor technologies. The doping methods for the controllable structures and tunable properties of conductive polymer hydrogels are beneficial to improving a variety of sensing performances including sensitivity, stability, selectivity, and new functions. With this perspective, we review recent progress in the synthesis and performance of conductive polymer hydrogels with an emphasis on the utilization of doping principles. Several prototype sensor designs based on conductive polymer hydrogels are presented. Furthermore, the main challenges and future research are also discussed.

On the Race for More Stretchable and Tough Hydrogels

Hydrogels are tridimensional networks that are able to retain important amounts of water. These soft materials can be obtained through self-assembling processes involving either hydrophilic molecules or polymers, allowing the formation of the corresponding covalently and physically cross-linked networks. Although the applicability of hydrogels in biomedicine has been exponentially growing due to their biocompatibility and different responses to stimuli, these materials have exhibited the particular feature of poor mechanical strength, and consequently, are brittle materials with low deformation. Due to this reason, a race has started to obtain more stretchable and tough hydrogels through different approaches. Within this context, this review article describes the most representative strategies and examples involving synthetic polymers with potential for biomedical applications.