Sarkar, S.; (2011) Development of a synthetic small calibre vascular bypass graft. Doctoral thesis , UCL (University College London).

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Abstract

Polyurethanes are an attractive class of material for bioprosthesis development due to the ability to manipulate their elasticity and strength. However, their use as long term biological implants is hampered by biodegradation. A novel polyurethane has been developed which incorporates nano-engineered polyhedral oligomeric silsesquioxane within poly(carbonate-urea) urethane to improve the biostability of the latter. Previous investigators have found this material to be cytocompatible and to have low thrombogenicity. The medium and long term clinical results of currently available prosthetic small calibre vascular bypass grafts are poor, due to neo-intimal hyperplasia associated with their non-compliant properties. The investigation reported here commences with the benchtop manufacture of compliant small calibre grafts using an original extrusion- phase inversion technique. The reproducibility of the technique as well as the effect on the pore structure of different coagulation conditions is demonstrated. Fundamental mechanical characterisation of the grafts produced is then presented, by way of tensillometry to demonstrate the viscous and elastic properties of the material. These are made more relevant to the clinical setting with functional mechanical characterisation of the grafts, showing graft compliance in a biomimetic flow circuit along with viscoelastic hysteresis, along with burst pressure testing. An examination of burst pressure testing methodology is also shown, in the light of the various non-standardised strategies reported in the graft-testing literature. Mechanical characterisation shows the short-term safety for use, but durability studies in the biological haemodynamic environment serve to assess longer term fatigability as well as confirming biostability. This has been reported using a stringent ovine carotid interposition model which remained patent over the full investigation period representing at least 45 million pulsatile cycles. Physico-chemical analysis; integrity of the structure, microstructure and ultrastructure; preservation of mechanical properties and immunohistological analysis were used to examine the grafts after implantation to show their healing properties and biostability.

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