The viability, and hence clinical utility of an engineered tissue implant relies on rapid integration with the blood supply of the recipient. Angiogenic growth factors including basic fibroblast growth factor (bFGF) and platelet- derived growth factor-BB (PDGF-BB) are critical regulators of vascular invasion. During wound healing, these growth factors exhibit spatially- and temporally-restricted patterns of expression. The administration of angiogenic growth factors by conventional methods has yielded limited success in promoting clinically-relevant vascular responses to implants - in part because these methods provide minimal control over the timing and localization of growth factor activity. The long-term goal is to use ultrasound to actively control cellular behavior - such as migration, survival, proliferation, and differentiatio - involved in tissue regeneration thus enhancing construct viability and functionality. Ultrasound is ideally suited to modulate cellular activities within implanted scaffolds since it can be applied non-invasively, focused with sub-millimeter precision, delivered in a spatio-temporally controlled manner to sites deep within the body, and translated clinically. The objective of this application is to use ultrasound - specifically a mechanism termed acoustic droplet vaporization (ADV) - to generate tightly controlled vascularization within an implanted tissue scaffold via the controlled release of angiogenic growth factors. ADV is a phenomenon whereby emulsified liquid droplets are converted into gas bubbles upon exposure to ultrasound above a certain acoustic amplitude. Therapeutic agents, encapsulated within the emulsion, are released upon ADV. The central hypothesis is that ADV can be used to spatio- temporally control the release of encapsulated bFGF and PDGF-BB within an implanted tissue scaffold. The rationale for the proposed research is that ADV enables the sequential release of bFGF and PDGF-BB, which exhibit mutual antagonism when presented simultaneously but synergistically enhance vascular growth when presented sequentially. Guided by strong preliminary data, the hypothesis will be tested via three specific aims: 1) Develop acoustically-sensitive emulsion formulations with distinct release thresholds;2) Demonstrate vascular in-growth in a subcutaneously implanted composite scaffold following the ultrasound-triggered release of bFGF;and 3) Demonstrate vascular in-growth in an ischemic hind limb model following the sequential release of bFGF and PDGF-BB at the site of vascular injury. Overall, the two growth factor delivery system optimized in the first aim will be evaluated in vivo using ectopic and orthotopic models in the second and third aims, respectively. Successful completion of these studies is significant since controlled release of angiogenic growth factors and subsequent vascularization within scaffolds is expected to facilitate the clinical translation of engineered tissue. Ultimately, this ultrasound-responsive scaffold system is a platform technology that could enable spatial and temporal control over the release of other regenerative growth factors critical for tissue regeneration and integration.
The proposed research is relevant to public health since an ultrasound-responsive scaffold system is expected to further the clinical translation and viability of engineered tissue scaffolds by providing spatial and temporal control over the release of regenerative growth factors. Additionally, the system will minimize the cost and risk associated with conventional growth factor-based therapies. Thus, the proposed research - which would ultimately reduce patient morbidity, mortality, and pain associated with conventional surgical reconstruction and organ transplantation - is relevant to the mission of the NIH.