The Cell–Surface Interaction - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 7

one can increase bony ingrowth proportionally [ 30 ], whereas others acknowledge
the beneficial effects of the oxide layer but do not report major differences in
fibroblast cell response based on oxide thickness alterations [ 31 ]. Clearly, surface
chemistry is an important factor in terms of the cell-material interaction. Here we
briefly discuss the oxide layer of commercial metal implants and explore the effect
of chemistry.

3.1.1 The Oxide Layer

Metal implants such as titanium and stainless steel are termed 'biocompatible' on
the basis of the presence of a surface oxide layer. It is this oxide layer that allows
the implants to have a high degree of corrosion resistance and separates the del-
icate biological environment from the highly reactive and incompatible bulk
material of an implant. In implant-quality electropolished stainless steel (EPSS),
the passive film consists mainly of iron, nickel and chromium in addition to
smaller quantities of elements such as molybdenum and manganese. The distin-
guishing passive film of EPSS is formed through the reaction of chromium within
the steel surface with oxygen. The naturally occurring oxide layer of EPSS is
generally in the region of a few nanometres (2-3-nm) thick [ 32 ].
In contrast, titanium and its alloys form much thicker (5-6-nm) naturally
occurring oxide layers, the composition of which is dominated by titanium, oxygen
and carbon, with the most stable stoichiometry of the oxide layer being TiO 2 . For
dual-phase alloys such as titanium-6% aluminium-7% niobium (TAN), the oxide
layer consists of Al 2 O 3 and Nb 2 O 5 , with aluminium being enriched within the
alpha phase of the oxide and niobium being enriched within the beta phase [ 32 ].
Anodizing is a commercially available process used for increasing the oxide
thickness of clinical implants. With this method it is possible to increase the oxide
thickness by approximately 2-3 nm per volt to produce an oxide layer of

approximately 200 nm.
When the oxide film is mechanically abraded, this allows the release of metal
ions from the highly reactive and incompatible bulk material. The release of
potentially toxic metal ions persists until the oxide layer is regenerated, which for
EPSS devices tested in 0.9% saline solution takes approximately 35 min, com-
pared with approximately 8 min for titanium and titanium alloys [ 33 , 34 ]. Recent
in vitro and in vivo studies have produced a convincing case identifying many of
the components of implant materials, such as chromium, cobalt, iron and nickel, as
the main culprits of toxicity. In particular, it has been shown that the potential
negative effects of released ionic and particulate implant components can impact a
variety of systems, such as excretory, reproduction, vascular, immune and integ-
umentary and nervous systems (see [ 35 ] for a review).