Tissue responses to implanted materials - Biomaterials and Devices for the Circulatory System - page 12

haematopoietic stem cells in the creation of a biologically nonthrombogenic
neointima (Rafi i et al. , 1995).
The fi rst successful prosthetic valve replacement was reported in early
1960 (Harken et al. , 1960). Unfortunately to date prostheses atraumatic to
blood elements with no thromboembolic potential have not been devel-
oped. Valve thrombosis has been defi ned as any thrombus, in the absence
of infection, attached to or near an operated valve that occludes part of the
blood fl ow path or that interferes with function of the valve (Edmunds et
al. , 1996). The incidence of major embolism in the absence of antithrom-
botic therapy has been reported at four per hundred patient-years
(Cannegieter et al. , 1994). This risk may be minimised with the regular use
of anticoagulants that, apart from fi nancial implications, impose inherent
haemorrhagic risks to patients.

1.6 Biofi lm

One of the most disastrous complications and subsequent reason for failure
of surgically implanted devices remains infection. The ability of microor-
ganisms to form an irregular fi lm was fi rst demonstrated by Zobell in 1943
(Zobell, 1943). The defi nition of a microbial biofi lm entails accumulation of
microorganisms and connecting extracellular products on a surface or even
at some distance away from a surface (Costerton et al. , 1987; Gregor et al. ,
1995). Biofi lm development proceeds in a step-wise fashion and certain
discrete stages have been identifi ed. These include the deposition of a con-
ditioning fi lm, initial microbial approach and attachment followed by micro-
bial growth and colonisation, and fi nally biofi lm formation (Habash and
Reid, 1999). Following implantation medical devices come into direct
contact with host interstitial fl uids and the bloodstream in the case of car-
diovascular implants. A variety of circulating extracellular proteins includ-
ing serum albumin, fi brinogen, collagen and fi bronectin participate in
forming an initial conditioning fi lm (Reid, 1999). Biomaterial surface char-
acteristics (chemistry, charge and hydrophobicity) may infl uence the extent
of this phenomenon (Uyen et al. , 1990; Zeng et al. , 1999) as may other local
mechanical parameters such as wall shear rate (Pitt and Cooper, 1986).
Recent data also suggest that different proteins undergo adsorption via
different mechanisms. It has been shown that while albumin undergoes
adsorption via a single step, fi brinogen adsorption is a more complex, mul-
tistage process (Roach et al. , 2005). It has already been discussed that the
profi le of adsorbed serum proteins plays a pivotal role in human macro-
phage behaviour (Jenney and Anderson, 2000). The fi nal two-dimensional
spatial distribution of adsorbed proteins is not that of a homogeneous fi lm
but may be visualised by transmission electron microscopy as island-like
depending on substrata wettability (Uyen et al. , 1990; Busscher et al. , 1991).

￿ ￿ ￿ ￿ ￿