Design and Characterization of Multifunctional and Responsive Polymeric Scaffolds for Tissue Engineering
An ideal biomaterial scaffold for tissue engineering must satisfy multiple design criteria to effectively support tissue regeneration. In addition to providing physical support, scaffolds must mimic key features of the native extracellular matrix, degrade in concert with tissue remodeling, and present biological signals in a controlled manner to direct cellular behavior. Achieving these functions remains a significant challenge due to the dynamic nature of the tissue environment during healing. This dissertation details three distinct polymeric platforms to address these requirements by exploiting dynamic covalent crosslinking, polymer end-functionalization, controlled radical polymerization, and biorthogonal click chemistries.
A bioinspired, low-energy process to produce mechanically tunable biopolymer fibers was developed by leveraging dynamic covalent chemistry. Extensible alginate fibers were successfully drawn from aqueous precursors containing phenylboronic acid-functionalized polyethylene glycol and alginate. Tensile testing of the fibers revealed that elastic modulus, tensile strength, and ultimate strain could be tuned by changing polymer structure, molecular weight, and concentration. Modifying alginate with methacrylate groups enabled post-drawing photopolymerization to stabilize the drawn fibers in physiological media. The fibers demonstrated humidity-responsive contractile behavior akin to spider silks. This work provided a versatile strategy for controlling the formation and mechanical properties of pultrusion-derived fibrous networks, highlighting new opportunities for biomimetic and stimulus-responsive materials.
Biologically responsive functionality in mechanically robust polymeric biomaterials was introduced by leveraging protease activity as a cell-mediated trigger to regulate scaffold remodeling in polycaprolactone (PCL)-based scaffolds. A mass spectrometry-based screening method was used to identify peptide sequences susceptible to cell-mediated proteolysis, which were subsequently integrated into the PCL backbone. Samples containing the degradable peptide motif degraded significantly faster in the presence of collagenase and human mesenchymal stromal cells (hMSCs) compared to controls, confirming that material remodeling was governed by sequence-specific proteolytic cleavage. These findings demonstrated that integrating protease-sensitive peptides into the polymer backbone can be used to program cell-mediated degradation of mechanically robust polyester biomaterials.
The same protease-responsive design principle was applied to control the delivery of pro-angiogenic signals from PCL-based scaffolds. Excessive or prolonged exposure to angiogenic factors such as vascular endothelial growth factor (VEGF) can result in the formation of immature and leaky vasculature and impair regenerative outcomes by promoting bone resorption. Localized and cell-responsive delivery strategies are therefore needed to better mimic the dynamic signaling environment of healing tissues. Protease-sensitive peptide linkers were used to tether a VEGF-mimetic peptide to the surface of solvent-cast 3D-printed PCL scaffolds functionalized with azide-bearing polymethacrylate bottlebrushes. The combination of surface-initiated polymerization and strain-promoted azide-alkyne cycloaddition chemistry enabled grafting of polymer bottlebrushes with tunable chain lengths and high density peptide immobilization. Scaffolds cultured with hMSCs or human vascular endothelial cells (hUVECs) showed that VEGF-mimetic peptides were selectively released in the presence of hMSCs but not hUVECs. These findings established a strategy for regulating both the dosage and timing of therapeutics delivery through tunable bottlebrush architectures and cell-responsive peptide linkers.
The platforms developed in this dissertation provide new strategies for designing multifunctional and responsive polymeric scaffolds that more closely mimic and respond to the dynamic processes that occur during tissue healing and regeneration.