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  to non-specifically cross-linking albumin , gelatin , and collagen . Covalent or ionic binding of the anti-coagulant heparin alone , in conjunction with other biologic compounds  or with spacer moieties , and covalent linkage of thrombomodulin  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  or creating an ionically charged surface .
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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