Transdermal Mechanical Loading for Cell Therapy-Based Bone Repair

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Cell therapy for bone repair combined with hydrogels, networks of crosslinked polymer chains with very high water content, is gaining in acceptance as a potential alternative to scaffold-based tissue engineering, especially for smaller scale defects that may be treatable through minimally invasive methods. Injecting cells into a bony defect with a small incision may be preferable to more invasive surgical procedures when clinically indicated. Once at the defect site the cells are left largely unperturbed within the hydrogel as the defect itself would require stabilization to permit healing, a requirement that goes against the therapeutic benefit of physically loading bone forming cells. It is this contradiction that has driven the work outlined in this proposal. Non-invasive, low-intensity pulsed ultrasound has been shown to be effective for transdermal treatment of fresh fractures (38% reduction in clinical and radiographic healing time) and fracture nonunions. While the mechanism through which LIPUS acts is poorly understood we have developed a highly tunable ultrasound system that demonstrates a measurable acoustic radiation force at clinically relevant ultrasound intensities and have shown this force to be capable of physically deflecting both cells and hydrogels. However, to date LIPUS-generated acoustic radiation force has not been paired with cell-loaded hydrogels for bone repair. The goal of this proposal is to combine LIPUS-generated acoustic radiation force and hydrogel-based cell therapy with the belief that both approaches together will enhance repair over either one alone. Using LIPUS- generated loading capable of imparting physical forces on cells, it is our intention to design hydrogel scaffolds that 1) are able to deliver encapsulated viable cells in vivo, 2) can be physically loaded by LIPUS generated acoustic radiation force after implantation and during the healing process and 3) can be modified to transfer varied physical forces from the hydrogel to cells such that healing would be optimized. The objectives of the present research are 1) to evaluate the effect of LIPUS-generated acoustic radiation force on cells embedded in hydrogels with increasing crosslinking densities, 2) to evaluate the effect of radiation force on cells encapsulated in collagen hydrogels of varying mechanical properties to determine the relationship between applied force and hydrogel stiffness on cell behavior, and 3) to use radiation force applied to hydrogels that have been loaded with cells and implanted in bone defect models. Implanted hydrogels containing cells will be loaded transdermally using acoustic radiation force. It is anticipated that the parameters defined in the in vitro studies will result in enhanced in vivo defect healing in hydrogels under acoustic radiation force when compared to either parameter alone.