Non-Thrombogenic Polymer Surfaces

Activation of the intrinsic clotting system, and platelet adsorption and activation by a foreign surface are preceeded by adsorption of plasma proteins, often accompanied by change of conformation (denaturing). Protein molecules are modelled as particles having heterogeneous surface characteristics, i.e., separate “patches” of lipophilic (hydrophobic) groups, of H-bonding groups,of fixed aniomic groups, and of fixed cationic groups. Any of these “patches” can anchor the protein to the “foreign” surface if it finds a receptive “patch” on the surface of sufficient area. When thus stabilized on the surface, and in sufficiently close proximity, proteins like fibrinogen can then bind platelets which can become activated. We postulate that protein adsorption is minimized, if the molecular segments of polymer molecules which create the synthetic surface (1) are disordered (amorphous) rather than ordered (crystalline) (2) undergo Brownian fluctuations rather than being fixed in the glassy state (3) are mixed in chemical composition rather than being all of the same type (4) are such as to minimize the bonding energy per segment to any of the receptive sites on the protein (anionic, cationic, H-bonding, hydrophobic) (5) are “diluted” by swelling in water, to reduce the surface density. Using the above model and postulates, we review currently used polymers (vinyls, silicones, polyurethanes etc). Surfaces containing polyethers such as polyethylene oxide (PEO) that are water absorbing and non-crystalline, rank highest in the list of non-thrombogenic polymers. We suggest that this arises from the alternation of hydrophobic CH₂-segments with hydrophilic ether oxygen, and the total absence of ionic groups and bonding hydrogens. Our observations on the blandness of hydrophilic cross-linked polyethers is supported by evidence from the literature concerning nearly total phase exclusion of protein from homogeneous PEO solutions, and the non-adsorption of proteins on surfaces coated by PEO.