Resistance to Biodegradative Stress Cracking in Microporous Vascular Access Grafts

Degradative cracking, more commonly known as "environmental stress cracking" (ESC) has been observed in many implanted polyetherurethane elastomers. This phenomenon has been attributed to biochemical and cellular interactions at the surface of the implanted material causing polymer chain cleavage. This may result in surface fissuring followed by the deep cracking associated with considerable biodegradation of the polymer, resulting in loss of mechanical strength and the formation of aneurysms in an in vivo situation.

These cracking effects are believed to be due to mechanical stress combined with the oxidising actions of macrophages and giant cells, as surface cracking has been observed to occur directly under adherent macrophages on a polyetherurethane implant. These cells form part of the body's immune response which uses enzymes and reactive oxygen species (02, 02, HO* and H₂O₂) to degrade foreign material.

We describe a modification of an in vitro test method developed by Zaho et al. [1] using glass wool and a Hydrogen Peroxide/Cobalt (II) Chloride (H₂O₂/CoCl₂) mixture to replicate the oxidising effects of macrophages in vivo. The modifications were made to establish a routine testing system for resistance to biodegradation which could be used to screen a range of polymers designed for use in microporous vascular grafts. The grafts are pre-stressed by a method devised by Stokes et al. [2] where each graft is stretched to a predetermined elongation over a mandrel and the strain is fixed by tying PTFE tape around each end of the mandrel.

Initial comparative testing used two materials fabricated into vascular grafts with identical dimensions and an interior diameter of 5 mm. The first, a polyetherurethane-Estane 5714 F1 (BF Goodrich Specialty Plastics UK Ltd.), showed the in vitro test method to have an acceleration factor of approximately nine times over in vivo trials resulting in catastrophic failure during the sixth week. The second material, a new generation polycarbonate-based polyurethane, Chrono Flex (Cardio Tech International Ltd.), displayed outstanding resistance against environmental stress cracking, maintaining its structure throughout the entire test period of thirty-five weeks. These results will be followed up with long-term in vivo implantation studies to corroborate these findings.