Biomedical Engineering Reference
In-Depth Information
topographical properties of a biomaterial by inducing various levels of surface
roughness or porosity, which has been shown to directly influence protein
adsorption and cellular behavior. These surfaces can be further modified by the
covalent attachment of adhesion proteins and peptides. Hydroxyapatite
(Ca
10
(PO
4
)
6
(OH)
2
) has been successfully used as coating on metallic hips and
dental implants for a number of years [179]. These metallic implant coatings
encourage ingrowth of bone to implant device, enhance fixation of the implants,
and improve the lifetime of the prostheses [180]. Commercially pure titanium
(cpTi) or titanium alloy compounds (ie: Ti6Al4V) are frequently used as the
metal of choice for HA coatings due to their high mechanical properties and
corrosion resistance [181].
Technically, several procedures have been developed to coat metallic
substrates, such as high temperature plasma-spraying, magnetron sputtering,
electrospraying, pulsed laser deposition, and hot isostatic pressing [94,182].
Coatings carried out at high temperatures usually receive post-immersion in SBF
to induce the formation of an amorphous CaP layer [178]. The pre-treatment of
Ti substrates also is important in the resulting properties of the HA coating.
Researchers have shown how Ti substrates modified prior to the electrochemical
deposition of HA result in inorganic layers with different surface textures and
contact angles [183]. Furthermore, as discussed in greater detail in later sections,
pre- and post-treatments also affected the precipitation of CaP from aqueous
solutions.
6.1.1.
Biological apatite
The formation of a CaP, or biological apatite, layer at the bone-bioceramic
interface is critical to the biomimetic nature of HA coated metallic implants.
During bone remodeling old bone is resorbed by the local acidification of bone
mineral prompted by integrin-mediated adhesion of osteoclasts. New bone is
formed by CaP precipitation from aqueous solution, protein adsorption, and
osteoblast adherence, maturation into osteocytes, and mineralization. Biomimetic
CaP compounds similarly sustain dissolution-reprecipitation cascades in
physiological solution due to the surface hydration of calcium and phosphate
ions present in the biomaterial, along with ionic transfers from surrounding fluids
onto the CaP substrate. Such exchanges at the solid-liquid interface lead to the
formation of biological apatite on the material surface [180]. As reviewed by
Kokubo [184], there is a correlation between apatite formation on surfaces pre-
implantation and
in vivo
bone bioactivity: “a material able to have apatite form
on its surface in SBF can bond to living bone through the apatite layer formed on
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