Thiol-norbornene Hydrogels with Tunable Mechanical Properties for Engineered Extracellular Matrices

Book excerpt: The extracellular matrix (ECM) governs many cellular processes through biochemical and mechanical cues. Particularly, the effect ECM mechanical properties on cells fate has been well established over the years. Many hydrogel systems have been used to mimic the dynamic stiffening processes occurring in ECM. However, changes in ECM stiffness does not fully recapitulate the mechanics of native ECM, as viscoelasticity is also a major factor contributing to ECM dynamic property. This thesis describes the design and characterization of an enzyme-crosslinked hydrogel system that is not only capable of being stiffened on demand, but also can be tuned to obtain viscoelasticity. The first objective of this thesis was to utilize horseradish peroxidase (HRP) to crosslink thiol-norbornene hydrogel and use mushroom tyrosinase (MT) to create secondary DOPA-dimer crosslinks that stiffened the hydrogel. The cytocompatibility of HRP-mediated thiol-norbornene gelation and the effect of stiffening on cell fate was evaluated. The second objective of this thesis represented the first step towards developing a hydrogel system whose viscoelasticity could be dynamically tuned. Thiol-norbornene hydrogel was designed to yield dynamically adaptable boronic ester bonds via partial enzymatic reaction. Thiol-norborne hydrogel was made to contain hydroxyl phenol as well as boronic acid residues within its network. MT, in this case was used to oxidize the hydroxy phenol moieties into DOPA, which then complexed with boronic acid, created dynamic bonds, introducing viscoelasticity to an initial elastic hydrogel.