Spatially-Targeted Reinforcement of Hydrogel Microstructure by Cross-Linking of Double Network Hydrogels Using Femtosecond Laser

Controlling elastic modulus as well as breaking stress is crucial for fabricating soft devices. Double network hydrogels (DN gels) have been reported to show high mechanical strength due to their unique network structure formed by two types of polymer networks. However, in the previously reported fabrication methods, DN gels were fabricated by cross-linking two different hydrogel polymers over the entire hydrogel. To the best of our knowledge, spatially selective formation of DN gel structures inside a hydrogel on a micrometer scale and the associated enhancement of local mechanical strength have yet to be demonstrated. In this study, we applied multi-photon polymerization (MPP) using a femtosecond laser to the cross-linking method of hydrogels and attempted spatially-selective formation of DN gels inside a hydrogel block. Hydrogels were prepared by the following two methods for comparison and evaluation. The first method was cross-linking of the 1st gel by UV irradiation followed by cross-linking of the 2nd gel by MPP using a femtosecond laser. By cross-linking the 2nd gel in a limited zone in the 1st gel, DN gel was formed in a spatially-selective manner. The second method was fabricating DN gels using MPP induced by femtosecond laser pulse irradiation for cross-linking of both the 1st and 2nd gels. This method was used to investigate spatially-selective changes in mechanical properties. As result, the stiffness changed in the DN gel where the 2nd gel was spatially-selectively cross-linked. The elastic modulus of the DN gel was higher than that of the single network gel. These results show that the mechanical properties have changed in the zone where the DN gel was locally formed. To demonstrate the potential of the method for soft robotics, we attempted to impart arbitrary mechanical strength in targeted locations and investigated the grasping property of a micro-object. We fabricated U-shaped gel tweezers and a glass bead with a diameter of 100 µm was grasped with the tweezers. The glass bead was slipped through the gel structure due to the significant deformation of the gel when only the single network gel was used because the structure was too flexible to transmit external force to the contact area with the bead. On the other hand, the glass bead was successfully grasped by the U-shaped structure with DN gel microstructures near the contact area with the glass bead, demonstrating the potential of the method for hydrogel-based soft devices.