Abstract Body: Intimal hyperplasia (IH) is a maladaptive fibroproliferative response following arterial bypass grafting (BPG) responsible for late BPG failure and thrombosis. Our previous work demonstrated a promising role for periadventitial hydrogel-based small interfering RNA (siRNA) delivery at the time of arterial injury in reducing downstream IH. However, prior hydrogel formulations were limited by poor adhesion to the vascular adventitia, off-target effects, and divergent viscoelastic properties in comparison to a native BPG. We describe a novel gelatin-based hydrogel utilizing polyacrylic acid (PAA) functionalized with benzaldehyde (BLA) to facilitate improved adherence to the vascular adventitia through dynamic covalent Schiff base crosslinking without compromising transfection efficiency. Viscoelastic testing utilizing oscillatory shear rheometry and stress relaxation testing of the PAA-BLA hydrogel demonstrated tunable viscoelastic properties dependent on PAA-BLA concentration which were similar to a greater saphenous vein and femoral artery (Figure 1A), along with improved viscous dissipation and stress relaxation in comparison to our prior hydrogel, CLICKGel, and a commercially available vascular sealant. These findings suggest our PAA-BLA hydrogel closely mimics the mechanical environment of native vasculature. Further, a 1.67% PAA-BLA hydrogel loaded with siGAPDH and jetPEI complexes demonstrated 49% GAPDH mRNA reduction compared to no treatment using in vitro human aortic endothelial cells (HuAoEC) after 24 hours of transfection (Figure 1B). To test in vivo transfection, we performed bilateral common carotid artery (CCA) dissection in rats and perivascular application of the PAA-BLA hydrogel (n = 5). PAA-BLA hydrogel loaded with siGAPDH and jetPEI was applied to the right CCA with PAA-BLA hydrogel alone applied to the left CCA as an internal control. After 24h, qPCR demonstrated 56% GAPDH mRNA reduction compared with control (Figure 1C). In conclusion, the PAA-BLA gelatin-based hydrogel represents a promising strategy for periadventitial targeted siRNA-based gene therapy delivery directly to the arterial-BPG anastomosis with mechanical properties similar to native vessels, improved adhesion through dynamic covalent Schiff base cross-linking and efficient siRNA delivery and transfection.