BOTTLEBRUSH HYDROGELS WITH TISSUE-MIMETIC MECHANICAL AND SWELLING PROPERTIES

Abstract

The following dissertation is focused on developing polymer gels and elastomers inspired by biological tissues utilizing architecturally diverse bottlebrush polymer networks. We endeavor to address three inherent problems of current bioengineered hydrogels, i.e., mechanical mismatch with the surrounding tissue, invasive implantation of gel-based biomedical devices, and leaching of reagents used during chemical cross-linking. Specifically, we introduce a novel class of injectable hydrogels with linear- bottlebrush-linear (LBL) architecture where the middle brush block is synthesized from hydrophilic poly(ethylene glycol) (PEG) and thermosensitive poly(N-isopropylacrylamide) (PNiPAM) as the linear blocks. The thermosensitive chains trigger prompt gelation at 37°C upon injection into the body using polymer concentrations in the range of 5 to 30%. The low solution viscosity (10^2~10 ??. ?) at room temperature, accompanied by prompt gelation at elevated temperatures facilitates 3D printing. The designed physically crosslinked brush-like macromolecules concurrently provide a platform where seemingly incompatible traits such as softness, firmness, strength, and fluidity merge into one material. The resulting hydrogels mimic the deformation response of supersoft adipose and brain tissues while withstanding deformations of 700%.Next, we endeavor to mimic the iso-compositional swelling behavior of biological tissues, where a diverse family of mechanical phenotypes with Young’s modulus ranging from supersoft brain (~10^2 Pa) to tough skin (~10^6 Pa) contain a narrow range of water concentration (60-80 wt.%). This behavior is impossible in linear polymer networks because the stiffness and swelling of linear networks are coupled through crosslink density such that any variation in modulus results in a corresponding shift in swelling ratio. To resolve this issue, we explored the potential of brush-like networks to tune gel modulus independently of swelling ratio by regulating network strand flexibility, i.e., varying side chain length, grafting, and crosslink density [???,??,??]. A library of mechanical properties of dry and swollen polybutyl acrylate brush elastomers with systematically varied sidechain length, grafting density, and crosslink densities were analyzed to validate the idea.Finally, we showcase the ability of bottlebrushes as re-processable thermosets through dynamic covalent crosslinking of a polymer network. We offer a solution to combine closed-loop recycling and upcycling capabilities using thermoreversible Diels–Alder cycloadditions to enable reversible disassembly into polymer melts and on-demand reconfiguration to an elastomer of either lower or higher stiffness. The crosslink density was tuned by loading the functionalized networks with a controlled fraction of dormant crosslinkers and crosslinker scavengers, such as furan-capped bis-maleimide and anthracene, respectively. The resulting modulus variations precisely followed the stoichiometry of activated furan and maleimide moieties, demonstrating the lack of side reactions during reprocessing. The presented circularity concept is independent of the backbone or side chain chemistry, making it potentially applicable to a wide range of brush-like polymers. While this concept opens the door to new horizons for recycling and upcycling of thermosets, it also mimics tissues’ stiffness variation during exercise, danger prevention, and excitement.