Small caliber vascular grafts. Part I: state of the art

Vascular grafts, devices designed to augment inefficiently functioning vascular systems, represent a significant part of implantable medical devices, with major participation in over a million vascular surgeries performed worldwide. By definition accepted in the art, a small caliber graft is a conduit with internal diameter (ID) of 6 mm or less; large caliber grafts start at ID of 7 mm. While the autologous grafts utilizing saphenous veins (SVG) and internal iliac, or mammary arteries are used exclusively in cardiac artery bypass grafts (CABG) procedures and preferentially in many peripheral indications, and while the use of grafts with biological origin did not proliferate, polymer-based artificial grafts of controlled patterns and porosity are prostheses of choice for the large caliber. The polyester (PET) yarn is knitted or woven into various porous patterns. The PTFE tubes are expanded into porous conduits (ePTFE). Although these technologies are used to produce the grafts with ID larger than 6 mm, the dominant principles are being applied to the development of small caliber graft. Polyurethanes are also evaluated for small caliber application. The grafts (regardless of the ID) produced by the above technologies are porous. This porosity, considered to be critical for proper healing and overall graft patency, causes the blood to leak through the graft wall or at anastomosis through the suture holes. Both the wall leakage and suture hole bleeding remain rather serious drawbacks. Currently, collagen, gelatin, albumin and their derivatives are used as sealants. Various modes of application and degrees of crosslinking are utilized to control in vivo degradation and graft healing. Other hydrogels, both natural and synthetic, could play significant roles as sealants and modifiers of the graft performance. Enhancement of graft patency via improvement of initial hemocompatibility could be achieved by application of bioactive coatings. Heparinized systems seem to dominate in this field, but many new concepts are being investigated. Intraluminal endothelialization via mediating biologicals could open significant potential for synthetic small caliber grafts. Furthermore, porous biodegradable tubes could be used as temporary scaffold to attract and promote cell propagation and ingrowth, the true angiogenesis. Part I of this series discusses the "S.O.T.A" of the small caliber graft. The following parts will discuss concepts needed for development of truly patent small caliber grafts and will report on our progress in the development of biodurable and pulsatile grafts for vascular access, peripheral, and potentially for CABG indications.