Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 338

factors are constantly interacting in a reciprocal fashion in vivo . It is prob-
ably impossible to exactly replicate all these factors using a single or even
multiple devices concurrently. The inherent tension within the arterial wall
is subjected to intricate forces, induced by pulsatile blood fl ow, which con-
tinually interact. Pulse pressure generates axial, radial and circumferential
strain in the blood vessel wall. This strain is counterbalanced by the aniso-
tropic generation of vessel wall tone by VSMC contraction, elastin and
collagen (Dobrin, 1978). This interaction occurs because arteries seek to
maintain constant and optimal homeostatic mechanical state. Experiments
on isolated microvessels, cell-seeded collagen gels and adherent cells iso-
lated in culture suggest that vascular cells and subcellular structures such
as stress fi bres and focal adhesions seek to maintain a constant mechanical
state (Humphrey, 2008).
Circumferential residual strain across the walls of coronary arteries is not
uniform, resulting in non-uniform transmural strain. With an artery being
a composite structure, the constituents of its different layers probably com-
pensate for the non-uniformity of transmural strain, inducing uniform
transmural strain (Guo et al. , 2005). Laminar blood fl ow induces shear
stress which directly interacts with intimal ECs. VSMCs are only directly
subjected to shear stress in areas of intimal damage. In healthy vessels with
an intact intima, VSMCs are indirectly exposed to shear stress initiated by
the transmural fl ow pathway or through interstitial fl ow (Tada and Tarbell,
2002; Wang and Tarbell, 1995). The best method of identifying and measur-
ing the various forces that continuously affect the vessel wall is yet to be
decided. The process may be simplifi ed by employing bioreactors that apply
the two principal forces within a vessel, i.e. stretch and shear stress,
independently.
Mechanical effects on cells have received more and more attention in the
studies of tissue engineering, cellular pathogenesis and biomedical device
design. Anisotropic biaxial cyclic stress, reminiscent of the in vivo cellular
mechanical environment, may promise signifi cant implications for biotech-
nology and human health (Tan et al. , 2008). Examining cardiovascular
system forces will certainly open many more avenues requiring research in
the complex and ever-progressing fi eld of vascular tissue engineering.

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12.8 Conclusions

Despite intensive research exceeding 20 years, TEBVs have yet to replace
autologous arteries and veins as the fi rst choice of surgeons performing
bypass procedures. Synthetic grafts, such as ePTFE, are still regularly used
when autologous vessels are unsuitable despite their associated risks of
infection and poor long-term patency. Although there is a defi nite and
urgent requirement for TEBVs in clinical practice, their reliability in