HYBRID BIOMATERIALS FOR ENGINEERING VASCULAR TISSUES - Biologically-Responsive Hybrid Biomaterials - page 362

design and development of stent materials, however, including some degree of
clinical success for drug-eluting stents [3], angioplasty techniques are still
hindered by late thrombosis events and in-stent restenosis, and intense research is
ongoing in an effort to develop more biocompatible materials.
In cases of coronary arteries with greater than 70% occlusion, by-pass
grafting is generally performed. Synthetic vascular conduits composed of
polyethylene terephthalate (Dacron™), expanded polytetraflouroethylene
(ePTFE, Teflon™) and polyurethane have been successfully used in the
replacement of large-calibre ( ² 5 mm) vessels [4]. However, small-calibre vessels
(<5 mm) constructed from synthetic materials for coronary artery by-pass have
poor intermediate and long-term patency rates, attributed to thrombogenic
occlusion of the lumen caused by the blood-contacting surface [5, 6]. Autologous
grafts, including the internal thoracic artery and saphenous vein, remain the
standard for by-pass or replacement of small-calibre vessels. However, many
patients do not have an autologous vessel suitable for use due to pre-existing
pathological conditions or prior surgical harvest.
The urgent requirement to develop alternative vascular structures and to
regenerate tissue following ischemia has led to the development of 'living' grafts
through the field of tissue engineering [7]. Tissue engineering design strategies
are typically based on the seeding of a scaffold material (biological, synthetic, or
a composite of both) with a suitable source of living cells in an appropriate three-
dimensional configuration, which is subsequently conditioned using various
external stimuli, including biochemical or mechanical factors. Each of the three
essential components of the technique have important, specific functions: the
cells, or viable component of the construct, will ultimately develop and remodel
the tissue/graft, and in the case of vascular constructs, should form a thrombo-
resistant coating on the luminal surface. The scaffold material is designed to
deliver cells and other molecules to the implantation site, where they are
maintained in a suitable anatomic configuration, while biochemical or
mechanical stimulation acts to guide optimal cellular and physical properties via
controlled gene expression, differentiation, tissue development and remodeling.
Indeed, 'hybrid' biomaterials can be engineered with additional biological
recognition motifs for interaction with cell receptors and the control of cell
signaling cascades and responses, or for local and controlled delivery of cells,
proteins or gene vectors.