Combating surface thrombus formation

Many attempts to create a more biocompatible surface have been based on establishing a new biologic lining on the luminal surface that would "passivate" this initial clotting reaction. These have ranged from non-specifically binding albumin to the surface followed by heat denaturation [96] to non-specifically cross-linking albumin [97], gelatin [98], and collagen [99]. Covalent or ionic binding of the anti-coagulant heparin alone [100], in conjunction with other biologic compounds [101] or with spacer moieties [102], and covalent linkage of thrombomodulin [103] have also been performed. Other studies have focused on modifying the composition of the biomaterial by either increasing hydrophilicity via incorporation of polyethylene oxide groups [104] or creating an ionically charged surface [105].

The success of these approaches has been limited: (1) thrombin is not directly inhibited, therefore fibrinogen amounts remain constant on the material surface permitting platelet adhesion; (2) heparin-coated biomaterials may be subject to heparitinases limiting long-term use of these materials; (3) non-specifically bound compounds are stripped from the surface under the shear stresses of blood flow, thereby re-exposing the thrombogenic biomaterial surface; (4) rapid release of non-specifically bound compounds may create an undesired systemic effect; and (5) charge-based polymers may be covered by other blood proteins such that anti-coagulant effects are masked.

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