POTENTIAL FOR NANOSTRUCTURED COATINGS FOR BIOMEDICAL APPLICATIONS
Tuesday, March 16, 2010 at 10:21PM Thermal spray coatings offer a means to help the human body to accept and to attach to orthopaedic and dental implants. In addition, they may serve to provide wear and corrosion protection. Both ceramic and metal-base materials are thermal sprayed as biomedical coatings.
Thermal spray nanostructured ceramic coatings possess superior strength/hardness, bond strength and toughness [[i]]; whereas, nanostructured metal coatings possess superior strength/hardness and resistance to localized corrosion [[ii], [iii]]. These unique characteristics have led to development of thermal spray nanostructured biomedical coatings. The National Research Council of Canada (NRC) has developed and evaluated ceramic-base nanostructured coatings for implant use. An overview of NRC’s findings on these coatings, as well as on some ideas on potential nanostructured coatings for biomedical coating applications are presented below.
The National Research Council of Canada (NRC) has developed and evaluated the application of nanostructured titanium oxide (n-TiO2) [[iv]] and hydroxyapatite (HA) [[v]] coatings for biocompatible implants. NRC’s n-TiO2 coating possessed higher hardness, greater bond strength, equivalent or higher osteoblast cell proliferation, and higher contact surface between bone and coating, as compared to current commercial coating - hydroxyapatite (HA). HVOF applied nanostructured HA coatings resulted in higher crystallinity, bioactivity and bond strength compared to its conventional counterpart.
It is important to point out that when depositing HA and nanostructured ceramic coatings, the selection of the thermal spray process can strongly influence the coatings’ properties and performance. HVOF process has demonstrated the ability to apply HA coatings with the preferred high crystallinity and nanostructured ceramic coatings with superior strength and toughness, compared to APS applied coatings of the same composition [5, [vi]].
One of the key issues when depositing ceramic coatings onto metal substrates relates to the reduced bond strength resulting from differences in coefficient of thermal expansion (CTE). Some research has been carried out with diopside (CaMgSi2O6), a pyroxene mineral that has a CTE similar to that of titanium alloy. When thermal sprayed onto Ti alloy substrate, this coating possessed a bond strength superior to that of HA [[vii]]. A nanostructured diopside coating will likely provide superior wear-resistance, toughness, and adhesion.
Another approach towards improving bond strength between the bioactive/biocompatible ceramic layer and the metal substrate has been studied by Oktar et al. [[viii]]. Their approach was to introduce a bond coat layer with a CTE between that of the HA top coat and the Ti alloy substrate. By introducing a bond coat layer of Al2O3-TiO2 via thermal spray, bond strength was increased by at least 25%. Should a nanostructured Al2O3-TiO2 coating, similar to that used by the US Navy [[ix]], be applied for this application, one may observe a further increase in bond strength and in durability of the coating system.
Metal coatings such as Ti and NiTi have also been applied or studied for biomedical applications. Ti coatings [[x], [xi]] provide a high level of biocompatibility, a low level of toxicity, and high resistance to corrosion by body fluids because of the presence of a very protective oxide layer. NiTi coatings have been studied for biomedical coating application because of its good corrosion resistance and biocompatibility. By using a deposition process that limits exposure of these metal coatings to high temperatures while attaining high density, nanostructured coatings of Ti and NiTi may be attained. These coatings will likely provide superior strength and wear resistance, while maintaining or improving their corrosion resistance [[xii]]. Alternate thermal spray process, spray parameters and/or coating composition can be incorporated to attain a porous nanostructure coating.
With the increase in average age of the population as well as in the average life-expectancy in North America, any extension in the life and enhancement in the performance of implant coatings will be beneficial to the recipients’ quality of life and budget. Thermal spray nanostructured coatings may play a key role in extending and enhancing the life and performance of implants.
REFERENCES
[i] G.E. Kim, Nanostructured Coatings and Their Benefits for Wear Applications, available at: http://blog.fwgts.com/blog/2010/1/13/nanostructured-coatings-and-their-benefits-for-wear-applicat.html, 2010.
[ii] M.R. Zamanzad-Ghavidel, K. Raeissi, A. Saatchi, “ The effect of surface morphology on pitting corrosion resistance of Ni nanocrystalline coatings”, Materials Letters 63 (2009) 1807–1809
[iii] M.M. Sharma and C.W. Ziemian, “Pitting and Stress Corrosion Cracking Susceptibility of Nanostructrued Al-Mg Alloys in Natural and Artificial Environments”, Journal of Materials Engineering and Performance, Volume 17(6) December 2008
[iv] R. S. Lima, B. R. Marple, H. Li and K. A. Khor, “Biocompatible nanostructured high-velocity oxyfuel sprayed titania coating: Deposition, characterization, and mechanical properties”, Journal of Thermal Spray Technology, Volume 15(4) December 2006, 623-627
[v] R.S. Lima, K.A. Khor, H. Li, P. Cheang, B.R. Marple, “HVOF spraying of nanostructured hydroxyapatite for biomedical applications”, Materials Science and Engineering A 396 (2005) 181-187
[vi] J.D. Haman, A.A. Boulware, L.C. Lucas, and D.E. Crawmer, “High-Velocity Oxyfuel Thermal Spray Coating for Biomedical Applications”, Journal of Thermal Spray Technology, Volume 4(2) June 1995
[vii] W. Xue, X. Liu, X. Zheng, C. Ding, “Plasma-sprayed diopside coatings for biomedical applications”, Surface & Coatings Technology 185 (2004) 340-345
[viii] F. N. Oktar, M. Yetmez, S. Agathopoulos, T.M. Lopez Goerne, G. Goller, I. Ipeker, J.M.F. Ferreira, “Bond-coating in plasma-sprayed calcium phosphate coatings”, J Mater Sci: Mater Med (2006) 17:1161–1171
[ix] G.E. Kim, United States Navy and Nanostructured Coatings, available at: http://blog.fwgartner.com/blog/2009/10/7/united-states-navy-and-nanostructured-coatings.html, 2009.
[x] J. Kawakita, S. Kuroda, S. Krebs, H. Katanoda, “In-situ densification of Ti coatings by the warm spray (two-stage HVOF) process”, Materials Transactions, Volume 47, Issue 7, July 2006, Pages 1631-1637
[xi] T.S. Price, P.H. Shipway, and D.G. McCartney, “Effect of Cold Spray Deposition of a Titanium Coating on Fatigue Behavior of a Titanium Alloy”, Journal of Thermal Spray Technology Volume 15(4) December 2006, 507-512
[xii] G.E. Kim, NANOSTRUCTURED COATINGS AND THEIR POTENTIAL BENEFITS FOR CORROSION APPLICATIONS, available at: http://blog.fwgartner.com/blog/2010/2/1/nanostructured-coatings-and-their-potential-benefits-for-cor.html, 2010.
George E. Kim, Ph.D.
F.W. Gartner
Perpetual Technologies, Inc.
email: gkim@perpetualtech.ca
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