Vascular Grafts: Technology Success/Technology Failure

Abstract

Vascular prostheses (grafts) are widely used for hemodialysis blood access, trauma repair, aneurism repair, and cardiovascular reconstruction. However, smaller-diameter (≤4 mm) grafts that would be valuable for many reconstructions have not been achieved to date, although hundreds of papers on small-diameter vascular grafts have been published. This perspective article presents a hypothesis that may open new research avenues for the development of small-diameter vascular grafts. A historical review of the vascular graft literature and specific types of vascular grafts is presented focusing on observations important to the hypothesis to be presented. Considerations in critically reviewing the vascular graft literature are discussed. The hypothesis that perhaps the "biocompatible biomaterials" comprising our vascular grafts-biomaterials that generate dense, nonvascularized collagenous capsules upon implantation-may not be all that biocompatible is presented. Examples of materials that heal with tissue reconstruction and vascularity, in contrast to the fibrotic encapsulation, are offered. Such prohealing materials may lead the way to a new generation of vascular grafts suitable for small-diameter reconstructions.

Fig. 1.

The structure of an artery that should be significantly recapitulated in a healed, integrated, truly biocompatible vascular graft (anatomical features of the illustration are not to scale). (Attribution—Kelvin Song, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons.)

Fig. 2.

Scanning electron microscopy image of ePTFE showing the fibril–node porous structure.

Fig. 3.

An ePTFE graft after implantation in a sheep for 1 month. Trichrome stain—the foreign body capsule (collagen) is stained darker blue. The central gray zone is a reinforcing wrap around the graft as fabricated by the manufacturer. (Data of Le Zhen, Louis Chen, Elina Quiroga, Jonathan Himmelfarb, and Buddy Ratner.)

Fig. 4.

The time sequence of the inflammatory reaction to implanted “biocompatible” biomaterials. Since the biostable biomaterial (a foreign body) can never be digested by the macrophages, macrophages continue sending signals maintaining the encapsulated state. These same signals also negatively impact the functions of the blood vessel cells (hyperplasia, thrombosis).

Fig. 5.

Schematic diagram of the wall structure of a polyurethane vascular prosthesis now under development at the University of Washington (inner and outer components are not to scale). The orange color suggests the thin gelatin layer lining the lumen of the graft.