Biomedical Engineering Reference
In-Depth Information
and continuous layer of biomimetic apatite throughout the thickness, without compromis-
ing porosity (Segvich et al. 2008).
Metals
Metal implants are most commonly used for their load-bearing and tribological properties.
The most prevalent metal implant materials include titanium and its alloys, cobalt-chromium
alloys, and stainless steel. Metal implants biomimetically coated with BLM enhance
osteoconduction and osseointegration. Heat treating titanium with NaOH results in the
formation of a sodium titanate layer at the surface of the implant. When this pretreated
titanium is immersed in SBF, sodium ions at the titanate surface are rapidly exchanged
with hydronium ions in the fluid. This exchange functionalizes the surface with Ti-OH
groups. Titanium's isoelectric point is at pH 5.8; therefore, a negative charge on the surface
arises when immersed in SBF that has a pH of 7.4. As sodium ion exchange continues, a
thin calcium titanate layer forms on top of the sodium titanate layer in about 30 min as a
result of electrostatic interaction between negatively charged units of repolymerized TiOH
groups and calcium in the SBF. Amorphous calcium phosphate precipitates on the surface
within 36 h due to electrostatic interaction between the increasingly positive charge of the
calcium titanate layer and negatively charged phosphate ions in SBF. This interaction leads
to crystalline apatite formation within 48 h after immersion (Takadama et al. 2001).
BLM coatings on metal implants enhance osteointegrative properties in vitro and in vivo
(Stigter et al. 2004; Stigter, de Groot, and Layrolle 2002; Schliephake et al. 2006; Bernhardt
et al. 2005; Fujibayashi et al. 2004). BLM coatings deposited on implants from solution at
ambient temperature and pressure are more favorable for osteoconduction than coatings
deposited by high-temperature processing methods since the calcium/phosphate ratio
and crystal structure of the mineral coating are more conducive to osteoconduction (Wang
et al. 2004). For instance, in a goat orthotopic model, BLM-coated Ti 6 Al 4 V femoral implants
exhibited significantly greater implant/bone contact compared to noncoated implants
(Barrere et al. 2003a, 2003b; Habibovic et al. 2005).
Ceramics
Ceramics are another class of biomaterials that have been successfully used in a variety
of orthopedic and dental implants, such as acetabular cups, extracochlear implants, ilial
crest replacement, alveolar ridge maintenance, dental crowns, inlays, onlays, and veneers
(Hench 1991). Although ceramics do not exhibit the same mechanical strength as metals,
they are ideal for some bone microenvironments since they are more susceptible to incor-
poration into the surrounding tissue while their degradative byproducts are biologically
tolerable.
Bioactive glasses improve integration and fixation with host bone tissue. When immersed
in SBF, dissolution of calcium and silicate from these bioactive glasses plays an important
role in mineralization. Ceramic surfaces, such as metal surfaces, exchange hydronium
ions for cations, thereby functionalizing the surface to present nucleation sites for miner-
alization (Abe, Kokubo, and Yamamuro 1990; Takadama et al. 2001). These functionalized
Si-OH surface groups drive the deposition of calcium and phosphate ions, resulting in
BLM formation and a subsequent depletion of phosphate ions from the media. Bioglass
45S5, Ceravital-type glass ceramic, glass ceramic A-W, sintered HA, apatite/β-tricalcium
phosphate, and calcium sulfate exchange Ca + ions with the solution resulting in BLM for-
mation at the surface (Kokubo and Takadama 2006). Nonbioactive glass ceramics do not
Search WWH ::




Custom Search