Bone Tissue Engineering Using Colloidal Gels and Native Extracellular Matrix Biomaterials

Self-assembling and shear-responsive biomaterials possess favorable rheological and viscoelastic properties to be injectable and facilitate minimally invasive surgery. The current thesis work describes the evaluation of malleable colloidal gel scaffolding technology for the regeneration of bone tissue in non-load bearing critical-sized defects. This represents the first attempt to form colloids exclusively from biomaterials found in the microenvironment of healing bone fractures including hyaluronic acid (HA), hydroxyapatite (HAP), bone and cartilage extracellular matrix (ECM). Work in the current thesis evaluated preliminary in vitro and in vivo efficacy of several injectable colloids. Specifically, HA-HAP colloids exhibiting desirable rheological, viscoelastic, and swelling properties for surgical placement and defect site retention were identified from an array of formulations. Subsequent formulation refinement incorporated micronized decellularized cartilage (DCC) and demineralized bone matrix (DBM) particles into HA-HAP colloids, which did not result in any significant decreases in measured fluid properties. Coupling ECM microparticles within a colloidal fluid carrier was hypothesized to enhance the regenerative capacity of HA-HAP colloidal formulations. In vitro studies demonstrated evidence of temporal chondrogenic, hypertrophic, and osteogenic gene expression in response to changes in colloidal composition. More specifically, the inclusion of DCC led to hypertrophic chondrogenesis while both HAP and demineralized bone matrix DBM colloidal formulations appeared to direct cell lineage down an osteogenic pathway in a temporal manner similar to expression profiles observed in native bone fracture healing. In vivo studies demonstrated the feasibility and efficacy of colloidal scaffolds in critical-sized rat calvarial defects. Although no significant differences in regenerated bone were observed in defects treated with colloidal formulations compared to negative control, the presence of endochondral (EC) derived ossification foci were only observed in HA-HAP and HA-HAP-ECM treated defects. Thus, definite advantages of using colloidal gel technology were observed in these preliminary studies, but future iterations of the implant formulation design may yield enhanced bone tissue regeneration. Ultimately, the idea of colloidal-based tissue implants has been taken from concept to practice, produced promising results for the treatment of non-load bearing bone defects, and has given rise to numerous areas of tangential research to refine the technology.