Polymeric Biomaterials - Biomaterials - page 70

TABLE 3.6 Effect of Gamma Irradiation on Polymers
Which Could Be Cross-Linked or Degraded

Cross-Linking Polymers Degradable Polymers

Polyethylene Polyisobutylene

Polypropylene Poly-α-methylstyrene

Polystyrene Polymethylmethacrylate

Polyarylates Polymethacrylamide

Polyacrylamide Polyvinylidenechloride

Polyvinylchloride Cellulose and derivatives

Polyamides Polytetrafluoroethylene

Polyesters Polytrifluorochloroethylene

Polyvinylpyrrolidone
Polymethacrylamide
Rubbers
Polysiloxanes
Polyvinylalcohol
Polyacroleine

severe problem is the embrittlement resulting in flange breakage, luer cracking, and tip breakage. The
physical properties continue to deteriorate with time, following irradiation. These problems of color-
ation and changing physical properties are best resolved by avoiding the use of any additives which
discolor at the sterilizing dose of radiation (Khang et al., 1996c).

3.5 Surface Modifications for Improving Biocompatibility

Prevention of thrombus formation is important in clinical applications, where blood is in contact, such
as hemodialysis membranes and tubes, artificial heart and heart-lung machines, prosthetic valves,
and artificial vascular grafts. In spite of the use of anticoagulants, considerable platelet deposition and
thrombus formation take place on the artificial surfaces (Branger et al., 1990).
Heparin , one of the complex carbohydrates known as mucopolysaccharides or glycosaminoglycan,
is currently used to prevent formation of clots. In general, heparin is well tolerated and devoid of seri-
ous consequences. However, it allows platelet adhesion to foreign surfaces and may cause hemorrhagic
complications such as subdural hematoma, retroperitoneal hematoma, gastrointestinal bleeding, hem-
orrhage into joints, ocular, and retinal bleeding, and bleeding at surgical sites (Lazarus, 1980). These
difficulties give rise to an interest in developing new methods of hemocompatible materials.
Many different groups have studied immobilization of heparin (Kim and Feijen, 1985; Park et al.,
1988) on the polymeric surfaces, heparin analogs, and heparin-prostaglandin or heparin-fibrinolytic
enzyme conjugates (Jozefowicz and Jozefowicz, 1985). The major drawback of these surfaces is that they
are not stable in the blood environment. It has not been firmly established that a slow leakage of heparin
is needed for it to be effective as an immobilized antithrombogenic agent, if not its effectiveness could be
hindered by being “coated over” with an adsorbed layer of more common proteins such as albumin and
fibrinogen. . Fibrinolytic enzymes, urokinase, and various prostaglandins have also been immobilized
by themselves in order to take advantage of their unique fibrin dissolution or antiplatelet aggregation
actions (Oshiro, 1983).
Albumin-coated surfaces have been studied because surfaces that resisted platelet adhesion in vitro
were noted to adsorb albumin preferentially (Keogh et al., 1992). Fibronectin coatings have been used
in in vitro endothelial cell (EC) seeding to prepare a surface similar to the natural blood vessel lumen
(Lee et al., 1989). Also, algin-coated surfaces have been studied due to their good biocompatibility and