3D Printing of Biocompatible Shape-Memory Double Network Hydrogels.

Shape-memory hydrogels can be fixed to an arbitrary temporary shape and recover their permanent shape under appropriate stimulus conditions. Their shape-memory behavior and biocompatible mechanical and chemical properties impart them with many biomedical applications. However, like most hydrogels, traditional shape-memory hydrogels suffer from intrinsic brittleness due to the network inhomogeneity and high water content. In the past, the double network (DN) scheme has been proved a robust method to improve the mechanical performance of hydrogels. Although 3D printing of DN hydrogels has been realized before, 3D printable shape-memory DN hydrogels have not been achieved so far. In this work, we propose a one-pot method for printing a biocompatible shape-memory DN hydrogel via fused deposition method. The two networks incorporated to the hydrogel ink are polyacrylamide (PAAm) and gelatin. The PAAm network is covalently cross-linked and responsible for the permanent shape, while the gelatin network has thermoreversible cross-links and responsible for fixing the temporary shape. The DN hydrogel shows 3 to 7 times higher fracture toughness than a single network gelatin or PAAm hydrogel and can be fixed to 300% of its original length under tension and 10% of its original thickness under compression. The ink compositions are tuned for optimal printing quality and shape-memory performance. The robust mechanical integrity and dramatic shape transformation capability of the 3D-printed shape-memory DN hydrogel will open-up new potential applications in transformative medical robots and self-deployable devices.