It's all in the delivery [electronic resource] : engineering injectable hydrogel carriers for cell and drug therapy
Keywords: Bioengineering.

The repair of damaged tissues through cell and growth factor administration is a promising strategy in regenerative medicine. However, clinical success has been limited due to the lack of effective delivery methods. While deemed minimally invasive, the conventional approach of direct injection in saline compromises the efficacy of the biological payloads. Post-injection cell survival is often dismally low, owing to the combination of shear forces during injection and hostile environments at the injury site. Growth factors delivered by bolus injections face rapid clearance and distribution to off-target sites, leading to suboptimal concentrations at the sites of therapy and potential side effects in normal tissues. These are major therapeutic hurdles because symptomatic relief following cell therapy is often correlated with the number of surviving donor cells. In the case of growth factor therapy, spatiotemporal control over growth factor distribution is critical for normal recovery, as tissue regeneration is a tightly coordinated process. This thesis presents a biomaterials approach to overcome these hurdles by harnessing polymer and protein engineering strategies to design injectable hydrogel carries for optimal cell and drug delivery. A broad survey of protein-engineered biomaterials is first given in Chapter 1 to provide a foundation for the design principles, synthesis, and characterization methods employed in subsequent chapters. Chapter 2 describes a mechanistic investigation into the physical forces imposed on cells during injection, using alginate polysaccharides as model carriers. This study reveals extensional stresses to be the dominant cause of cell death, and that cell viability can be restored by pre-encapsulation in hydrogel carriers that exhibit thixotropy, or the ability to shear-thin and self-heal. Subsequent chapters describe the design and characterization of a thixotropic protein-engineered hydrogel system called MITCH, or Mixing-Induced Two-Component Hydrogels, and their applications as cell and drug delivery vehicles in regenerative medicine. The crosslinking of the MITCH network relies on the reversible binding between complementary peptide domains, enabling gelation and cell encapsulation by simple mixing at physiological conditions. The design rationale and demonstration of thixotropy, cyto-compatibility, and three-dimensional cell encapsulation are discussed in Chapter 3. In Chapter 4, the specificity and stoichiometric precision of the peptide-peptide crosslinking interactions are highlighted as a distinguishing feature of MITCH. Polymer physics considerations are combined with protein science methodologies to enable the predictable tuning of macroscopic-level gel mechanics through molecular-level variations of component concentration and stoichiometric ratio. The remainder of the thesis focuses on the utility of the MITCH material as a delivery carrier for cell and drug regenerative therapy. Cell protection is demonstrated in Chapter 5, where adipose-derived stem cells injected subcutaneously in mice exhibit improved retention when encapsulated and delivered in MITCH, relative to saline and control biomatrices. Moreover, histological analyses of explants show endogenous cell invasion and signs of native extracellular matrix remodeling at day 3. The simple mixing protocol allows the encapsulation and release of peptide drugs and growth factors in their bioactive state. Chapter 6 describes the engineering of an affinity- and avidity-based peptide drug delivery system, developed by using the molecular recognition domains in MITCH. Fusion of angiogenic peptides to MITCH-specific affinity tags enables drug immobilization, sustained release, and prolonged local drug availability. This controlled release strategy induces higher levels of endothelial cell migration and matrix invasion compared to delivery from saline alone. Chapter 7 presents a therapeutic angiogenesis strategy by dual delivery of human induced stem cell-derived endothelial cells (hiPSC-ECs) and vascular endothelial growth factor (VEGF). This chapter also introduces MITCH 2.0, a new class of protein polymer/synthetic polymer hybrid hydrogel system created to improve tunability and ease of synthesis. Similar to the original version, MITCH 2.0 protects cells during injection and delivers drugs with tunable kinetics. In a murine hindlimb ischemia model, hiPSC-ECs co-delivered with VEGF in MITCH show improved post-transplantation viability and restore blood perfusion to the ischemic limb. All in all, by improving the delivery of cells and biochemical factors, the biomaterials work completed here provides enabling tools to advance other biological research endeavors, to ultimately realize the clinical translation and success of regenerative medicine.