Biocompatibility of cardiovascular devices - Biomaterials and Devices for the Circulatory System - page 60

their own phenotypic behavior as well as the behavior of other cells
(Menconi et al. , 1992).

3.2.6 Shear stresses and mechanics

Shear stresses and mechanical alterations of devices are almost immediate
consequences of fl ow establishment after the implantation of cardiovascu-
lar devices. These external mechanical factors signifi cantly impact cell sig-
naling responses, mesenchymal and EC phenotypic changes, and alterations
in mitotic indices and cytoskeletal reorganizations. In left ventricular assist
devices (LVADs), strategies to effi ciently deliver pump volume with optimal
fl uid dynamics is ideal to minimize hemolysis and thrombosis associated
with fl ow-related mechanical stresses to whole blood (Treichler et al. , 1993).
The modulation of biomaterial and platelet and coagulation cascade activa-
tion interactions by the environmental biomechanical conditions is also an
important consideration when examining the biocompatibility of biomate-
rials. This has been exemplifi ed in a direct comparison of platelet activation
by stainless steel stents with or without gold coating which demonstrated
that the thrombogenic profi les of each of these materials was signifi cantly
dependent on the fl ow patterns to which they were exposed (Kolandaivelu
and Edelman, 2004). Similarly, rates of shear stress can affect platelet
content on devices. Higher rates of shear may increase platelet deposition
on bypass conduits, but may also cause dislodgment of adherent platelets
depending on the amount of time high shear rates are maintained (Badimon
et al. , 1987). Similar results have been demonstrated on various metallic
alloys like titanium (Ti) and diamond-like carbon (DLC) (Schaub et al. ,
2000).
Local hemodynamic conditions also signifi cantly affect EC phenotypic
behavior including alignment and mitotic index (Flaherty et al. , 1972; Reidy
and Langille, 1980; Davies et al. , 1986; Levesque et al. , 1986). Increased EC
pinocytosis and prostacyclin production under both pulsatile and steady
shear conditions, and increased endothelin production under low shear
(Davies et al. , 1984; Frangos et al. , 1985; Sharefkin et al. , 1991) can have
signifi cant effects on the healing, thrombotic, and endothelial regenerative
processes which are thought to modulate device biocompatibility. In addi-
tion, fl ow-related shear stresses impact the effi cacy of cell seeding strategies
utilized in the development of tissue engineered or biohybrid devices. Rota-
tional shear stresses (both high and low rates) have been shown to induce
signifi cant EC desquamation from fi bronectin-coated ePTFE grafts in an
ex vivo fl ow system (Greisler et al. , 1990a). Hydrogel coatings containing
ECs in cell seeding experiments have demonstrated improved EC retention
rates in response to pulsatile ex vivo fl ow compared with fi bronectin coated
grafts (Gosselin et al. , 1996).

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