Ultrasound as a Physical Force for Scaffold-Based Bone Repair

Non-invasive, transcutaneous low intensity pulsed ultrasound (LIPUS) therapy has shown clinical efficacy in bone healing for over the past two decades; however, the exact mechanism of action remains largely unknown. The goal of this work was to find more conclusive evidence as to how LIPUS works in addition to optimizing the currently used clinical therapy. We developed and characterized our own highly tunable ultrasound system as well as a hydrogel scaffold system for tissue mimetics. A series of experiments evaluated the response of our adjustable ultrasound system to: cells alone; hydrogels alone; cell-hydrogel encapsulation; and cell-hydrogel encapsulation implantation into a mouse calvarial model. By characterizing our tunable ultrasound system with a needle hydrophone, we found that the clinical LIPUS parameter produces a measurable acoustic radiation force previously only recognized in higher intensity ultrasound modalities. Given that bone responds positively to physical forces, we have attempted to relate the documented benefit of LIPUS therapy in bone healing to acoustic radiation forces as a likely candidate for LIPUS efficacy. Through our adjustable LIPUS system and the development of collagen hydrogel scaffolds, we demonstrated that varying LIPUS intensity and duty cycle results in the manifestation of varying physical loads. These loads ultimately lead to the quantifiable deformation of collagen hydrogel scaffolds through the displacement of fluorescent beacons encapsulated within the hydrogels. By application of acoustic radiation force, pre-osteoblast cell-encapsulated hydrogels experienced varied osteogenic response from the clinical intensity of LIPUS based on their collagen concentration. Also, the exposure of cell-hydrogel constructs to ultrasound resulted in the varied upregulation of certain markers indicative of mechanical stress, based on LIPUS intensity and hydrogel collagen concentration. As fractures are typically immobilized during the fracture healing process, local bone cells receive limited beneficial physical loading. To this end, we used a mouse calvarial defect model to encapsulate donor bone marrow stromal cells (BMSCs) within a collagen hydrogel at the defect site to physically load cells during the fracture healing process in hopes to enhance the currently used clinical LIPUS therapy