Interpenetrating network hydrogels via low-temperature frontal polymerization and 3D printing

The development of hydrogels with energy-saving and versatile manufacturing, shaping and processing capabilities is highly desirable but challenging. Here, we report a hydrogel precursor system consisting of hydroxypropyl acrylate (HPA), N-vinylpyrrolidone (NVP), and polycaprolactone (PCL), which allows for frontal polymerization (FP) at significantly lower temperatures for the rapid preparation of robust interpenetrating network (IPN) hydrogels, while maintaining compatibility with a variety of manufacturing platforms. This system achieved a record-low frontal temperature of 45.4 °C, significantly lower than the 87–296 °C range for conventional FP systems. The obtained hydrogels demonstrated favorable water swelling behavior, mechanical properties, and biocompatibility. Notably, the hydrogel system is compatible with diverse three-dimensional (3D) printing techniques including direct ink writing (DIW) and digital light processing (DLP). We also explored the integration of 3D printing with self-healing strategies, where hydrogen bond-based self-healing enabled the recovery of 91.04 % of the initial mechanical strength within 10 h, allowing the fabrication of complex 3D architectures. This finding offers an IPN hydrogel protocol compatible with multiple manufacturing methods including energy-saving FP, and DIW, DLP, and self-healing 3D printing, solving the limit in shaping and processing to obtain hydrogel objects with wanted architectures for various applications. This work also greatly widens the implementing temperature for FP systems, which is meaningful for future material fabrications and applications.