Angiogenic growth factor delivery for vascular regeneration in critical limb ischemia using acoustically-responsive scaffolds

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ABSTRACT The development of techniques to generate functional, perfused blood vessel networks is essential for engineering of viable tissues as well as for the treatment of ischemic diseases. Pro-angiogenic growth factors have been used, either as a standalone therapy or in combination with cell-based techniques, to promote vascularization. Unfortunately, conventional approaches for delivery of pro-angiogenic growth factors have yielded disappointing results in clinical, therapeutic trials. This is because optimal growth factor combinations, concentrations, release kinetics, and spatial presentations are unknown. Thus, there is a need to develop translatable technologies for precisely controlling these parameters so optimization can be achieved. Our long- term goal is to develop implantable biomaterials for tissue regeneration that are spatiotemporally manipulated in a non-invasive, on-demand manner using focused ultrasound. Focused ultrasound is a clinically-used technology with sub-millimeter precision that penetrates deeply within the body. During the prior funding period, we developed a paradigm-changing hydrogel, termed an acoustically-responsive scaffold (ARS) that enables the controlled release of multiple, pro-angiogenic growth factors using ultrasound. An ARS consists of an ultrasound-sensitive emulsion embedded within a hydrogel matrix. Growth factors encapsulated within the emulsion are released when an ARS is exposed to focused ultrasound. This non-thermal release mechanism, termed acoustic droplet vaporization (ADV), is driven by the formation of a bubble within each emulsion droplet, thereby releasing the encapsulated payload. We developed techniques to sequentially release basic fibroblast growth factor (bFGF) and platelet derived growth factor-BB (PDGF-BB) from an ARS. We also demonstrated that ADV can release bioactive bFGF with high specificity, thereby leading to the formation of functional vessels in vivo. The kinetics of bFGF release strikingly impacted the formation of perfused blood vessels. ADV-generated bubbles also dramatically altered the permissiveness of the ARS to cell migration. The objective of this proposal is to understand how vascularization is impacted by spatiotemporal variation of release kinetics of bFGF and PDGF-BB from an ARS as well as permissiveness of the ARS. The central hypothesis is that optimal blood vessel formation can be achieved by precisely controlling growth factor release from ARSs using ADV-generated bubble dynamics. Aim 1 will use ADV-generated bubble dynamics to modulate the kinetics of growth factor release from an ARS. Aim 2 will quantify the impact of bFGF release kinetics and ARS permissiveness on the development and inosculation of cell-loaded ARSs. Aim 3 will demonstrate enhanced vascularization in an atherosclerotic model of hind limb ischemia by sequentially delivering bFGF and PDGF-BB from acellular ARSs. Successful completion of these aims will advance the translation of pro-angiogenic growth factors in acellular and cell-based therapies for vascularization as well as propel other biomedical applications of ADV.