Characterization and Implementation of Low Intensity Pulsed Ultrasound as a Tool to Apply Physical Load to Scaffolds and Bone Cells for Fracture Repair

One current challenge in treating bone fractures is the effective treatment of non-unions and delayed unions. Low Intensity Pulsed Ultrasound (LIPUS) has been approved by the FDA to treat fresh fractures since 1994 and non-unions since 2000 and is an attractive treatment option because it is non-invasive. The mechanism by which it works, however, is not well understood; what is known is largely confined to the resultant changes in chemical output of cells. In this thesis several concepts and techniques were brought together to investigate the following hypothesis: LIPUS produces a measurable physical load that results in measurable deformation and displacement of cells and the extracellular matrix (ECM). It is further hypothesized that changing the LIPUS output parameters will result in measureable changes to loading and deformation. This hypothesis was investigated by building and characterizing a variable output LIPUS system capable of applying a load to a GFP-actin labeled macrophage and a self-assembling peptide hydrogel to model the ECM. Loading and deformation were visualized using 3D fluorescent deconvolution microscopy. Analysis of the deformation showed the matrix to have viscoelastic or elastic behavior when compressed, with higher beam intensities showing higher deformations. Cell studies showed physical displacement and increased macrophage fluorescence suggesting heightened actin polymerization in response to loading. These studies describe potentially powerful techniques for investigating the role of other LIPUS parameters in controlling cell and scaffold displacement and deformation.