Tissue responses to implanted materials - Biomaterials and Devices for the Circulatory System - page 6

and of cells of a permanent nature that are unable to replicate themselves,
such as cardiac myocytes. Fibrous capsule formation has been demon-
strated in the epicardium of rats implanted with expanded polytetrafl uoro-
ethylene, a material commonly used in vascular graft manufacturing (Kellar
et al. , 2002). Identifi cation of the processes involved in the infl ammatory
response to biomaterials has allowed research to be targeted towards devel-
oping appropriate anti-infl ammatory strategies to improve the biocompat-
ibility of implanted devices (Bridges and Garcia, 2008).
The development of coatings for the surface of cardiovascular implants
attempts to combat the inevitability of the host's response to device implan-
tation. Surface-coating materials in vascular stents that release a variety of
molecules have been demonstrated directly or shown to have the potential
to reduce infl ammation, thrombosis and in-stent restenosis at the site of
implantation. An in vitro study using biodegradable poly(ester amide)
(PEA) co-polymers showed a reduction in the infl ammatory response and
promotion of healing by reducing levels of pro-infl ammatory IL-6 and IL-1
beta while increasing levels of the anti-infl ammatory IL-1 receptor antago-
nist released by monocytes that adhere to PEA (DeFife et al. , 2009). A
further in vitro study demonstrated the application of a compound derived
from green tea, epigallocatechin-3- O -gallate, to signifi cantly reduce vascu-
lar smooth muscle cell proliferation and platelet adhesion and activation at
the site of implantation (Cho et al. , 2008). The use of cucurmin-coated stents
in rabbit iliac arteries showed a signifi cant reduction in neointimal area
compared with non-coated bare metal stents, indicating a reduction in in-
stent restenosis by using the coated stents (Jang et al. , 2009). Sirolimus-
coated stents have also demonstrated signifi cant reduction in neointimal
formation compared with non-coated stents in porcine carotid arteries
(Tepe et al. , 2006). In addition, surface coatings are being developed to
facilitate local gene delivery (Jewell and Lynn, 2008), which may be applied
to vascular stents and prosthetic valves to modify immune and infl amma-
tory responses (Fishbein et al. , 2005).

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1.3 Immune response

The immune response consists of innate and adaptive immune systems,
which have primarily evolved to recognise and destroy infective organisms.
However, they also respond to tissue injury, as this can be due to or may
result in subsequent infection, and are therefore relevant in the host
response to device implantation. The innate immune system is the initial
generalised response that occurs over the fi rst 12 to 24 hours. It refers to
defence components that exist before the injury. Innate immunity consists
of physical barriers such as skin and mucosal epithelium, blood proteins
such as complement and coagulation factors, and cells. Infl ammation is an