Abstract Body: Despite continued advancements to treatment options, 25% of post-myocardial infarction (MI) patients develop heart failure (HF) within one year of discharge. Large vessel reperfusion is effective in the clinic but fails to address downstream microcirculation damage, with persistent obstruction in ~35% of the initial area at risk. Because up to 50% of the affected cardiomyocyte population enters a stunned or hibernating state, we hypothesize that early microvascular regeneration will rescue contraction for functional recovery. Our previous work identified VEGF, IGF-1, and PDGF (VIP) as a potent proangiogenic protein cocktail to stimulate revascularization in vivo using a fully defined collagen and heparin-modified alginate biomaterial. In this study, we optimize release order to recapitulate the native vascular development cascade by embedding IGF-1 in alginate microspheres to prolong co-release of VIP up to 21 days in vitro. Assessment of in vivo release demonstrates sustained release for up to 10 days, with release order conserved. Assessment in a rat ischemia/reperfusion MI model with implant of unloaded biomaterial or VIP-loaded biomaterial versus sham follows functional remodeling over 3 months (n=9-10/group). Longitudinal echocardiography demonstrates decreased wall thinning and conserved ejection fraction in the loaded and unloaded biomaterial groups versus sham. 4D ultrasound enables regional assessments of 3D strain, with co-registered scans following Dobutamine infusion identifying rescuable hibernating myocardium. At 3 months, optical mapping of ex vivo Langendorf perfused hearts with a voltage-sensitive dye shows increased border zone conduction velocity and decreased effective refractory period in both the loaded and unloaded groups compared to sham, with ongoing analyses evaluating regional action potential duration and arrhythmia incidence. Quantification of regional vascularization is ongoing and expected to elucidate interactions between therapeutic microvascular regeneration by VIP-loaded biomaterial and structural remodeling, infarct size, regional contractility, and arrhythmia risk in the post-MI heart, supporting this new therapeutic approach.