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The influence of microporous structures in the walls of small-diameter arterial prostheses was investigated with the aim of minimizing thrombosis and enhancing endothelialization of blood-contacting surfaces. Six types of spongy polyurethane-polydimethylsiloxane grafts (PUG), 1.5-mm in an internal diameter and 1.5–2 cm in length, were implanted end-to-end in the infrarenal aorta of 66 adult rats. Some had a continuous inner skin and a hydraulic permeability (HP) of Oml/min/cm2at the standard transmural pressure of 120mmHg (PUG-S-0). Some had a discontinuous inner skin with some isolated windows connecting penetrating micropores though the graft wall and a mean HP ranging from 11 (PUG-S-11) to 37 (PUG-S-37) or 58 (PUG-S-58) ml/ min/cm2. The rest had a microporous inner surface with penetrating micropores through the graft wall and a mean HP of 2.7 (PUG-2.7) or 39 (PUG-39) ml/min/cm2. PUG which had a HP of less than 2.7 ml/min/cm2showed poor patency. PUG with a HP of more than 11 ml/min/cm2had acceptable patency, but endothelialization was limited to their anastomoses. In contrast, the patent PUG-S-37 and PUG-S-58 were largely endothelialized and all but one of the patent PUG-39 implants were completely endothelialized. In conclusion, penetrating micropores through the graft wall appear to inhibit critical mural thrombosis. A microporous inner surface seems to be superior to a skinned inner surface in achieving a high degree of endothelialization.

Worldwide, 30–50 million men and women benefit from implanted devices. Therefore, the clinical experiences with synthetic materials in intimate contact with human cells and tissues exceeds by three orders of magnitude the number of laboratory animals sacrificed in the elusive quest for “biocompatibility.” Furthermore, the much longer duration of observation characteristic of human implants, compared to animal studies, adds two orders of magnitude to the superiority of clinical and anatomopathological experience over laboratory animal studies. This glaring disparity must be kept in mind in the assessment of materials through in vitro and small animal studies, and the regulatory agencies’ pronouncements based on such evidence. Another sobering fact is that over 95% of the materials utilized for implants are standard commercial substances originally developed for industrial purposes. Those products which have been found to be appropriate for a specific medical device are labeled “biomaterials” (or more modestly, materials for medical use) on the grounds of established specifications and quality control, supported by continuing feedback from the clinical experience. The major obstacle to the advent of custom-designed biomaterials is that the medical device market is so small (not in numbers of implants and impact on health care budgets, but in the quantity of material used per implant) that large-scale production and amortization of industrial production expenditures is well nigh impossible for truly novel substances. In litigious countries such as the USA, suppliers are pulling out of the market because of the excessive cost of defending against legal action when materials allegedly fail in the body environment. Against this background, biomaterials science is bravely searching for new solutions to old problems. The ill-defined property of biocompatibility is slowly making way for the notion of bio acceptance as we expand our knowledge of the cellular aspects of tissue-material interactions. Bioacceptance can seek two diametrically opposed end points: biointegration, meaning that the material and the surrounding living structures form a continuum with stable, low-grade interactions; and biopassivation, meaning that the material is hardly recognized by surrounding body fluids or tissues and that its stealth characteristics can persist for clinically meaningful periods of use.

This study addresses a long-lasting controversy, ‘Which is a better blood-contacing surface morphology, smooth or rough?’ and indicates the importance of micropores penetrating through the graft wall in minimizing thrombosis. Four types of spongy polyurethanepolydimethylsiloxane (Cardiothane 51; Kontron Instruments, Everett, MA, USA) vascular grafts (PUG), 1.5 mm ID and 1.5–2cm in length, fabricated by a spray, phase-inversion technique, were implanted end-to-end in the infrarenal aorta of 58 adult rats. Some grafts had a continuous inner skin and, consequently, a hydraulic permeability of 0 ml/min per cm2 (PUG-S-0). Some had an inner skin with isolated pores and a mean hydraulic permeability of 11 ml/min per cm2 (PUG-S-11). Some had a microporous luminal surface with a mean hydraulic permeability of either 2.7 ml/min per cm2 (PUG-2.7) or 39 ml/min per cm2 (PUG-39). Twelve PUG-S-0, 6 PUG-S-11, 23 PUG-2.7, and 17 PUG-39 were evaluated between 2 hours and 3 months post-implantation. Almost all PUS-S-0 occluded with thrombus soon after implantation. PUG-2.7 had poor patency. Both PUG-S-11 and PUG-39 showed acceptable patency. Endothelialization, however, was limited to 1–2 mm from proximal and distal anastomoses in any patent PUG-S-11. In contrast, all but one of the patent PUG-39 were completely endothelialized. The extent of mural thrombosis decreased in the order from PUG-39 to PUG-S-11, PUG-2.7, and PUG-S-0. In conclusion, micropores penetrating through the graft wall, as reflected by hydraulic permeability values, appear to inhibit critical mural thrombosis and to promote a high degree of endothelialization.