Ultrasound-Derived Mechanical Loading with Cell Therapy for Bone Repair

Healing of large bone defects remains a challenge. Regenerative engineering promises an alternative to over 2 million bone-grafting procedures performed annually. We have combined regenerative engineering techniques with existing clinical therapies to develop a treatment strategy for large-scale bone defects. Our novel approach combines hydrogel-based cell delivery with low intensity pulsed ultrasound (LIPUS), an FDA approved treatment for fracture repair. In a clinical implementation of our system, implantation of a cell-laden hydrogel in a bone defect in the patient will allow cell delivery and cell retention within the defect. Transdermal application of LIPUS-derived acoustic radiation force (ARF) will physically load the encapsulated cells to stimulate bone formation while they reside within the defects, without mechanically disrupting the unstable defect. In this dissertation, we aim to determine an optimal combination of hydrogel stiffness and ultrasound intensity to maximize bone formation for two hydrogel systems: collagen and alginate. To understand the synergistic effects of matrix stiffness and ultrasound-derived loading, our in vitro studies evaluate essential cellular responses of intracellular calcium ions and short-term markers such as cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2), which are established load sensitive markers of bone formation. We also study the effect of our optimized system on long-term osteogenic markers, matrix maturation, and the formation of mineralized tissue. The efficacy of our approach in the clinical scenario is further evaluated in vivo by implanting optimal cell-hydrogel constructs in mouse calvarial defects and treating with ultrasound. Our observations reveal that encapsulated cells in optimal hydrogel stiffness respond immediately at the onset of ultrasound by upregulating calcium, COX-2, and PGE2 that further modulate long term osteogenic gene expressions and bone mineral formation. Our in vivo study demonstrates that defects implanted with cell-laden hydrogels and treated with ultrasound had enhanced bone formation through increased donor cell viability and matrix formation. Finally, the results obtained are likely to suggest a minimally invasive, effective healing process that is a combination of cell therapy and appropriate ultrasound treatment for healing non-union and bone defects as compared to traditional surgical solutions.