Biomedical Engineering Reference
In-Depth Information
surface of the implant in a proper conformation, osseointegration will be
favoured. In constrast, if the proteins are adsorbed in a state that cannot be
recognized by the cells, the implants may be identified as a foreign material
and trigger an immune response.
50
The cells attach to positively or negatively
charged surfaces but the morphology of the cells can vary depending on the
electrical charge.
51
surface charges are relevant for anchorage-dependent cells such as
osteoblasts. The attachment and spreading of osteoblasts is strong on very
positively charged surfaces, with intimate contact between the ventral cell
membrane and the biomaterials surface.
52
on negatively charged surfaces,
however, osteoblasts tend to show a stand-off morphology with localized
attachment points.
53
as a quantitative measure, the isoelectric point (IeP), or the pH at which
its zeta potential is equal to zero, is often used to investigate the surface
charges. The surface tends to be more negative when IeP is inferior to the
physiological pH (7.4) and more positive when it is superior. evaluation
of the IeP for a solid body, in particular for an implant surface, is not
straightforward. Bearing in mind that all metallic biomaterials used for
bone repair are passive, that is become covered by a thin oxide layer, the
properties of the oxides forming the passive layer become relevant. IePs for
metal oxides and related materials have recently been compiled.
54
Values
for oxides that are present in the passive layers of the metallic biomaterials
(or as coatings) are: 8.8-9.5 for al
2
o
3
, 6.7 for Cr
2
o
3
and 3.5-6.7 for Tio
2
.
Consequently, most of the alloys used for orthopaedic implants exhibit at
their surface a net negative charge at physiological pH.
55
Surface energy
Surface wettability, a measurement of surface energy, influences the degree
of contact with the physiologic environment and is considered to be one
of the most important parameters affecting the biological response to an
implanted material.
56,57
Wettability affects protein adsorption, platelet
adhesion/activation, blood coagulation and cell bacterial adhesion.
58
Generally
the cellular attachment is poor on any hydrophobic surface and high on
moderately hydrophilic surfaces.
surface energy is dictated by composition of the surface but not by
topography (including roughness).
59
It was suggested that wettability,
expressed in terms of contact angle and related time-dependent non-
equilibrium phenomena, should be correlated with crystalline structure and
the chemical composition of the oxide films that form on the surfaces.
60
Titanium surfaces, covered by Ti-rich oxide films, are hydrophilic owing to
the high polarity of the Ti-o bond, although surface contamination, such
as by carbon or hydrocarbon adsorption, produces higher values of water
