4D printing of biodegradable shape memory double-network hydrogel for highly bionic devices

Hydrogels are attractive for bionic devices due to their sensing ability and flexibility, similar to human skin. However, current hydrogels hardly combine mechanical, water retention, sensing, actuating, and degradation performances, which significantly limits the application of hydrogels in highly bionic devices. In this paper, a biodegradable shape memory 4D printing hydrogel ink was prepared by bio-polyurethane (BPU), carboxymethyl chitosan (CMCS), and carbomer (CBM). The hydrogel ink had a high tensile strength (stress of 0.66 MPa, elongation at break of 643%), outstanding water retention (>85.87%), ionic conductivity (8.59 S m−1), and excellent sensing performance (S = 0.051 kPa−1, GF = 2.9). Fourier transform infrared reflection, X-ray diffractometer, and X-ray photoelectron spectroscopy data showed that the BPU, CMCS, and CBM form a double network structure through chemical, hydrogen, and ionic bonding cross-linking, respectively. After 4D printing, a continuous pore structure could be observed by scanning electron microscopy in the hydrogel model. The continuous pore structure provided channels for the movement of ions in the hydrogel model so that the pressure could be converted into a specific signal. Following the signal, a computer-controlled temperature rapidly heated the hydrogel model to 50 °C, and the hydrogel model could change shape autonomously. The excellent properties and highly bionic functions of biodegradable shape memory double-network hydrogel have broken through the limitations of applications in artificial intelligence robotics, human-machine interfaces, tissue engineering, and other fields.