Page 99 - Biomedical Engineering and Design Handbook Volume 2, Applications

78 MEDICAL DEVICE DESIGN

Current commercial polymeric graft designs can be divided into both textile and nontextile forms
(Brewster, 2000). Textile-based grafts are generally composed of woven or knitted polyethylene tereph-
thalate (Dacron), while the nontextile grafts are usually fashioned from expanded polytetrafluoroethylene
(ePTFE). Other polymeric materials investigated or used in the manufacture of vascular grafts include
polyamides or nylons, nonexpanded PTFE, polyvinyl alcohol (Ivalon), vinyl chloride-vinyl acetate
copolymers (Vinyon-N), polyacrylonitrile (Orlon), and polyurethanes (Greisler, 1991; Eberhart et al.,
1999; Brewster, 2000). Although the majority of these materials have been abandoned, polyurethanes
have enjoyed continued interest as a source material, especially for small diameter vascular grafts,
despite concerns over their long-term degradation performance (Eberhart et al., 1999). The polyuretha-
neurea Vectra graft (Thoratec Corp, Pleasanton, CA) is in clinical use in the United States for vascular
access, and there are a number of improved polyurethane formulations on the horizon that should address
the limitations of early generation polyurethane polymers (Kannan et al., 2005; Kapadia et al., 2008).
The preferred graft material differs for both implantation site and use, and selection is based upon
patency rates, absence of complications, convenience or handling characteristics, and cost (Brewster,
2000). Large-diameter grafts for use in the aorta and major arteries have generally been made of Dacron,
while medium-size grafts are primarily constructed of ePTFE. Smaller vessels located in the infrain-
guinal region (below the area where the legs connect with the torso) represent a significant challenge due
to the traditionally poor patency rates with artificial grafts in these vessels (Veith et at., 1986; 1988;
Londrey et al., 1991). Current evidence suggests that replacement of the damaged vessel with an auto-
genous vein or artery graft is the procedure of choice rather than implantation of an artificial graft (Faries
et al., 2000). Unfortunately, preexisting disease, anatomic or size limitations, and other factors may rule
out an autogenic source for vessel replacement, thereby forcing the use of an artificial (usually ePTFE)
vascular graft or allograft, such as a human umbilical vein (HUV) graft (Harris, 2005). It is in these
smaller vessels that the promise of tissue-engineered prosthesis are expected to have the greatest impact.
The different construction techniques used to manufacture textile grafts have an effect on the final
device properties. Graft porosity is considered to be a prime factor determining a number of han-
dling, performance, and outcome (Wesolowski et al., 1961) characteristics for textile implants.
Knitted textile grafts possess a high porosity and generally require a special yet simple procedure
called preclotting prior to implantation to prevent excessive leakage of blood through the graft wall.
Traditionally, preclotting is performed with a sample of the patient’s blood; coagulation is initiated
to fill the open pores and interstices with fibrin. As of 1997, most (>80 percent) implanted grafts
came presealed with collagen, gelatin, or other biological compounds (e.g., albumin) directly from
the manufacturer in an effort to reduce time spent on graft preparation (Brewster, 2000) and limit
surface thrombogenicity (Greisler, 1991). In contrast, woven textile grafts do not generally require
preclotting but possess less desirable handling properties, such as increased stiffness (Brewster,
2000) and a tendency to fray when cut (Greisler, 1991). Examples of woven and knitted polyester
aortic grafts are shown in Fig. 3.10. Nontextile PTFE grafts can possess differing levels of porosity,
depending on the processing technique employed. This feature may influence the extent of the heal-
ing process (Golden et al., 1990; Kohler et al., 1992; Contreras et al., 2000).

3.5.4 Complications and Management

Graft “healing” or incorporation into the vasculature is a complex process that is the subject of
numerous studies and reviews (Greisler, 1991; Davids et al., 1999; Zilla et al., 2007). Graft healing
is affected by the graft material, the graft microstructure and surface characteristics, hemodynamic
and biomechanical factors, and eventually the interplay between these characteristics and the cellu-
lar and humoral components involved in the ongoing incorporation of the graft (Greisler, 1991).
Incorporation of the graft into the vasculature can occur through ingrowth of tissue from the anasto-
motic ends, from tissue growth through the graft wall, and from deposition of circulating cells onto
the vessel surface (Zilla et al., 2007). Humans, in contrast to many animal models, usually fail to
fully endothelialize the inner surface of a vascular graft, possessing instead some near-anastomic
endothelialization and scattered islands of endothelial cells on a surface rich in fibrin and extracel-
lular matrix (Berger et al., 1972; Sauvage et al., 1974; Sauvage et al., 1975; Pasquinelli et al., 1990).
The lack of full healing has prompted extensive research into the mechanism of endothelialization
and methods to improve it in the clinical setting.