Shape memory and sensing hydrogels: from fundamentals to PAA-SF composites for integrated intelligent systems

Abstract

Advanced materials for soft robotics, biomedical devices and wearable electronics are spearheaded by intelligent hydrogels that are dynamically triggered by environmental stimuli. Within this field, two pivotal yet historically separate domains are represented by shape memory hydrogels (SMHs) and conductive hydrogels used for strain sensing. A comprehensive analysis of recent progress regarding the integration of these functionalities into a single material platform is provided in this review. Poly(acrylic acid) (PAA) and silk fibroin (SF) composites are highlighted as a transformative model system. The fundamental mechanisms and current challenges associated with thermally, chemically and optically responsive SMHs, as well as flexible hydrogel strain sensors driven by ionic or electronic conduction, are outlined initially. Subsequently, the unique advantages of composites comprising PAA and SF are established. The inherent brittleness of PAA is overcome by the inclusion of SF as a mechanical enhancer. Furthermore, novel nonthermal and water triggered shape memory with programmable recovery kinetics is dynamically regulated by the SF framework. Strategies for the design of such multifunctional composites, including network interpenetration, nanomaterial incorporation and dynamic bond integration, are systematically evaluated.Emerging applications in biocompatible soft actuators, implantable devices and advanced sensors are also explored. Finally, prevailing challenges, such as the compromise between strength, sensitivity and extended stability, alongside the complexity of dual sensor and actuator integration, are identified. Future directions are then proposed for the development of robust, environmentally adaptive and fully integrated intelligent hydrogel systems intended for emerging technologies.