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Photo-directed Morphing Structures of Nanocomposite Shape Memory Hydrogel with High Stiffness and Toughness.

Shape memory hydrogels have drawn increasing attentions in recent years. Practical applications require these hydrogels to have good mechanical properties, as well as contactless stimulations to trigger the shape deformations. Here we report a stiff and tough shape memory hydrogel that can transform to various configurations sequentially by photo-triggered site-specific deformations. Response of the shape memory hydrogel to near infrared (NIR) light irradiation was achieved by incorporating gold nanorods (AuNRs) into the glassy gel matrix of poly(methacrylic acid-co-methacrylamide) without compromising the excellent mechanical properties. Owing to the photo-thermal effect of the AuNRs, the localized temperature rise led to dramatic decrease in Young's modulus (from 200 MPa to 2 MPa) of the pre-stretched hydrogel and bending deformation with programmable direction and amplitude. More complex three-dimensional configurations can be obtained by multi-directional pre-stretching and shape memorizing the individual parts of the nanocomposite hydrogel. Furthermore, the AuNRs embedded in the gel were aligned along the pre-stretching direction, leading to anisotropic plasmon resonance. These photo-mediated programmable deformations of tough shape memory hydrogels should find applications in biomedical and engineering fields.