Biomedical Engineering Reference
In-Depth Information
contact. recently it has been reported that thermal oxidation of niTi alloys
enhances the hydrophilic character and statistically increases both albumin
and fibronectin adsorption.
61
A significant decrease in the contact angle has
been also observed after UV radiation of Ti-6al-4V and Ti-6al-4V oxidized
alloy.
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one further issue is that cleaning and sterilization methods of the implants
produce significant changes in contamination and surface energy.
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6.5 Trends in the development of metallic
biomaterials
Metallic biomaterials have been the most suitable for bone replacement
up to now. In the past, they were basically borrowed from other fields of
applications, mostly the aerospace industry. after the World War II, a number
of high-performance materials originally developed for military purposes
became available for clinical use. In the 20th century, efforts to enhance
their biological response have been focused on their surface modification by
using physical, chemical or biological methods. additionally, the research
and development of titanium alloys without toxic elements lead to the
production of Ti-6Al-4Nb, the first alloy specifically developed for medical
applications.
nowadays there have been many approaches to designing the next generation
of bone implants. These can be grouped as proposals for new compositions,
with non-toxic and non-allergic elements, or for new microstructures obtained
through the introduction of novel processing techniques.
6.5.1 Materials with a lower Young's modulus
Common metallic biomaterials have a modulus that is much higher than
that of bone (see Table 6.4). assuming isoelastic behaviour of the implant/
bone system during service, the implant takes over a considerable part of
the mechanical load, thereby shielding the remaining bone that surrounds
the implant. reduction of physiological loads on the bone induces resorption
mechanisms that lead to a drop in bone density. In addition, tissue resorption
increases micromotions at the bone-implant interface facilitating the formation
and migration of wear debris via biological fluid transport. All these features
yield premature failures of the implant that overall are associated with the
'stress yielding' phenomena.
The elastic modulus of common Ti and Ti-base alloys is lower than that
of Co-base alloys and stainless steel, but it is still much higher than that
of cortical bone (Table 6.4). Thus a reduction in the Young's modulus is
regarded as a high priority in the overall strategy of the design of Ti alloys
for biomedical applications, which is focused on a new generation of b type
