Hierarchical Cross-Linked Poly(caprolactone-co-urethane) toward Connective Tissue-like Properties and Multifunctional Integration

It is a great challenge to regulate the mechanical properties precisely with both softness and toughness so as to match the requirements of various flexible device applications. Inspired by the three-dimensional network of collagen in connective tissues, a novel biopolymer poly(caprolactone-co-urethane) with hierarchical cross-links was synthesized on the basis of polycaprolactone (PCL) and composite chain extenders of tannic acid (TA) and 1,4-butanediol (BDO). The biopolymer P(CL-TA(x)/BDOy) possesses both physical and chemical cross-links in coordination with phase separation microstructure, sensitive to the molar ratio of TA/BDO. The optimal P(CL-TA(1)/BDO1) shows connective tissue-like properties, possesses an initial modulus less than 0.5 MPa but high toughness (61.2 MJ m(-3)), unique adhesive property, and shape memory behavior. A flexible actuator was prepared by P(CL-TA(1)/BDO1), exhibiting the shape transformation behavior by an electricity stimulation. This work provides a novel facile method to design biopolymers, including tuning the mechanical properties and integrating multifunctions through the hierarchical cross-link strategy.