Stimuli-responsive Bottlebrush Elastomers with Tissue-mimetic Mechanical Properties

Abstract

The mechanical property of soft tissues is a unique combination of softness and firmness, where they are initially soft to touch but rapidly stiffens as strain increases, resulting in good resistance to rupture and injury. In addition, many soft tissues have stimuli-responsive functionalities, which means they can quickly and significantly change their mechanical properties upon receiving a neural or biochemical stimulus. The combination of tissue mechanics and stimuli-responsiveness is not only the key in the survival of organisms, but also highly desirable for artificial materials in biomedical or robotic applications.Bottlebrush elastomers are known to have a tissue-mimetic combination of softness and firmness. Their tissue-like softness originates from the suppression of entanglements by the side chains, while their firmness or strain-stiffening from their stretched backbones. More importantly, mechanical properties of bottlebrush elastomers are largely controlled by chain architecture and weakly by chemical composition. As a result, stimuli-responsive chemical components can be incorporated while preserving the tissue-mimetic softness and firmness.In the dissertation, we seek to design novel bottlebrush elastomers that mimic both the mechanical properties and stimuli-responsiveness of tissue for biomedical and soft robotics applications. This is achieved with two distinct approaches, each utilizing different aspects of bottlebrush chemistry: side chain monomers and side chain ends. In the first approach, we introduced crystallizable side chains to bottlebrush elastomers. This results in a tissue-adaptive material that transitions from plastic-hard (10^8~10^9Pa) to tissue-soft (10^3~10^5Pa) at around body temperature. We then investigated how the transition temperature and soft-state modulus can be independently controlled by tuning side chain length and crosslink density. This transition also enables thermal-triggered release of embedded drugs for anti-inflammatory treatment. Second, we incorporated Diels-Alder dynamic crosslinking by functionalizing side chain ends. The versatility of Diels-Alder reactions allows the design of two reconfiguration additives: dormant crosslinkers and crosslinker scavengers, which alters the number of stress-supporting junctions in bottlebrush elastomers upon thermal activation. By varying the type and concentration of additives, we realized two-way network reconfiguration (both hardening and softening) with accurately controlled modulus change, as well as temperature-responsive recycling. Overall, the effect of bottlebrush molecular architecture on the response of bottlebrush elastomers to external stimuli is investigated in this dissertation work, which serves as a foundation for practical applications such as tissue-adaptive implants, drug delivery, and sustainable elastomers.