Spatially-selective enhancement of mechanical strength through localized cross-linking of double network hydrogels

To advance the development of flexible materials for soft robotics applications, it is crucial to enhance the elastic modulus and breaking the stress of soft materials, such as hydrogels. Double network hydrogels (DN gels) have displayed remarkable mechanical strength owing to their unique network structure composed of two types of polymer networks. However, current fabrication methods for DN gels entail cross-linking two distinct hydrogel polymers throughout the entire hydrogel matrix. In this study, we focused on employing multi-photon polymerization (MPP) with femtosecond laser pulses as a cross-linking method for hydrogels to achieve spatially selective formation of DN gel structures at the micrometer scale, along with the consequent improvement in local mechanical strength. We assessed the mechanical properties of the fabricated structures and confirmed that the mechanical strength varied within the regions where DN gel was locally formed.