Vascular implants for peripheral arterial bypass and aortic aneurysm repair - Biomaterials and Devices for the Circulatory System - page 226

genic response and rejection since the materials used are mostly foreign,
and that the behaviour of cells in patients with vascular disease is not
known. An approach which may prove attractive is to use the body as a
bioreactor to engineer a vascular graft by growing tubes of tissue subcuta-
neously. Based on earlier works (Elken, 1960; Schilling et al. , 1964; Sparks,
1969, 1973), Campbell et al. (1999) placed supporting silastic tubings into
the peritoneal cavity of rats and rabbits to form granulation tissue capsule
of multiple layers of myofi broblasts, collagen matrix and single layer of
mesothelial cells which could be everted and used as grafts. They later
showed that the tubes need not be everted and yet have a burst strength of
greater than 2500 mmHg (Chue et al. , 2004). This approach could possibly
ensure no tissue rejection and should limit graft complications if the vessels
can be successfully grown in patients prior to bypass grafting. Theoretically,
one or more vessels of the required diameters and lengths can be grown at
the same or different times. Provided that improved biocompatibility, ade-
quate and stable mechanical properties, vascular reactivity and non-throm-
bogenic surface can be demonstrated in humans, grafts developed this way
could be useful as bypass grafts or arteriovenous fi stula grafts for haemo-
dialysis vascular access. However, as in all approaches, the impending issues
are the length of culture time required to produce a tissue-engineered vas-
cular graft (weeks to months), its vascularisation and vasoactivity. Further-
more, even if a graft with good vascular properties can be produced, how
it will perform in the patients is still not known. The fact that suture/
anastomosis is required, there may still be issues inductive to thrombosis,
atherosclerosis and IH, e.g. compliance matching, confl uent functional
endothelium, injury, etc. as observed in the currently used vascular grafts.
Nevertheless, with more understanding and progress made in tissue engi-
neering, the possibility of a tissue-engineered alternative vascular graft may
not be too far from being realised.

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8.6 Abdominal aortic aneurysm

An aneurysm is a localised dilatation of an artery with an increase in diam-
eter of at least 50% greater than that of the unaffected artery segment. The
majority of aneurysms occur in the infra-renal aorta, generally referred to
as abdominal aortic aneurysms (AAA) and are found mainly in men over
the age of 55 years (O'Kelly and Heather, 1989; Collin et al. , 1988; Singh
et al. , 2001; Wanhainen et al. , 2001). The reported increased prevalence of
AAA may be due to a real rise in incidence as well as improved detection
of asymptomatic aneurysms on imaging performed for other reasons
(Fowkes et al. , 1989). AAAs gradually enlarge at rates that vary between
0.2 and 0.7 cm in diameter per annum (Collin et al. , 1989; Cronenwett et al. ,
1990). If left untreated (and if the patient does not die of an unrelated