There is a need for off-the-shelf, small-diameter vascular grafts that are safe and exhibit high long-term patency. Decellularized tissues can potentially be used as vascular grafts; however, thrombogenic and unpredictable remodeling properties such as intimal hyperplasia and calcification are concerns that hinder their clinical use. The objective of this study was to investigate the long-term function and remodeling of extracellular matrix (ECM)-based vascular grafts composited with antioxidant poly(1, 8-octamethylenecitrate-co-cysteine) (POCC) with or without immobilized heparin. Rat aortas were decellularized to create the following vascular grafts: 1) ECM hybridized with POCC (Poly-ECM), 2) Poly-ECM subsequently functionalized with heparin (Poly-ECM-Hep), and 3) non-modified vascular ECM. Grafts were evaluated as interposition grafts in the abdominal aorta of adult rats at three months. All grafts displayed antioxidant activity, were patent, and exhibited minimal intramural cell infiltration with varying degrees of calcification. Areas of calcification co-localized with osteochondrogenic differentiation of vascular smooth muscle cells, lipid peroxidation, oxidized DNA damage, and cell apoptosis, suggesting an important role for oxidative stress in the calcification of grafts. The extent of calcification within grafts was inversely proportional to their antioxidant activity: Poly-ECM-Hep>ECM>Poly-ECM. The incorporation of antioxidants into vascular grafts may be a viable strategy to inhibit degenerative changes.

Small-diameter vascular grafts developed from vascular extracellular matrix (ECM) can potentially be used for bypass surgeries and other vascular reconstruction and repair procedures. The addition of heparin to the ECM improves graft hemocompatibility but often involves chemical crosslinking, which increases ECM mechanical stiffness compared to native arteries. Herein, we demonstrate the importance of maintaining ECM mechanocompatibility, and describe a mechanocompatible strategy to immobilize heparin onto the ECM via a biodegradable elastomer. Specifically, poly (1,8-octamethylene citrate)-co-cysteine (POC-Cys) was hybridized to the ECM, forming a polymer-ECM composite that allows for heparin immobilization via maleimide-thiol “click” chemistry. Heparinized composites reduced platelet adhesion by >60% in vitro, without altering the elastic modulus of the ECM. In a rat abdominal aortic interposition model, intimal hyperplasia in heparinized mechanocompatible grafts was 65% lower when compared to ECM-only control grafts at 4 weeks. In contrast, grafts that were heparinized with carbodiimide chemistry exhibited increased intimal hyperplasia (4.2 fold) and increased macrophage infiltration (3.5 fold) compared to ECM-only control grafts. All grafts showed similar, partial endothelial cell coverage and little to no ECM remodeling. Overall, we describe a mechanocompatible strategy to improve ECM thromboresistance and highlight the importance of ECM mechanical properties for proper in vivo graft performance.

Thrombosis within small-diameter vascular grafts limits the development of bioartificial, engineered vascular conduits, especially those derived from extracellular matrix (ECM). Here we describe an easy-to-implement strategy to chemically modify vascular ECM by covalently linking a collagen binding peptide (CBP) to heparin to form a heparin derivative (CBP–heparin) that selectively binds a subset of collagens. Modification of ECM with CBP–heparin leads to increased deposition of functional heparin (by ~7.2-fold measured by glycosaminoglycan composition) and a corresponding reduction in platelet binding (>70%) and whole blood clotting (>80%) onto the ECM. Furthermore, addition of CBP–heparin to the ECM stabilizes long-term endothelial cell attachment to the lumen of ECM-derived vascular conduits, potentially through recruitment of heparin-binding growth factors that ultimately improve the durability of endothelialization in vitro. Overall, our findings provide a simple yet effective method to increase deposition of functional heparin on the surface of ECM-based vascular grafts and thereby minimize thrombogenicity of decellularized tissue, overcoming a significant challenge in tissue engineering of bioartificial vessels and vascularized organs.

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Heparinized mechanocompatible polymer-extracellular matrix (ECM) composites to improve vascular graft performance are presented by J. A. Wertheim, G. A. Ameer, and co-workers on page 1594. Polymer-coated ECM is visualized by the purple color. The composite vascular graft reduced intimal hyperplasia in a rodent model, revealed by a-smooth muscle actin staining (green). The method shows promise for tissue engineering where immobilization of bioactive molecules is desirable.