Highly crystalline HA coating reduces HA dissolution in body fluids. ACKNOWLEDGMENTS. The authors would like to thank NIH SBIR program for financial ...
Mater. Res. Soc. Symp. Proc. Vol. 1140 © 2009 Materials Research Society
1140-HH03-03
Improved Mechanical Properties of Nanocrystalline Hydroxyapatite Coating for Dental and Orthopedic Implants Wenping Jiang1, Jiping Cheng2, Dinesh K. Agrawal2, Ajay P. Malshe1, Huinan Liu1 1 NanoMech LLC, 535 W. Research Blvd, Suite 135, Fayetteville, AR 72701, USA. 2 Materials Research Institute, Penn State University, University Park, PA 16802, USA. ABSTRACT Hydroxyapatite (HA) has been widely used as a coating material for orthopedic/dental applications due to its similar chemical composition to natural bone mineral and its capability to promote bone regeneration. It has been reported that HA with nano-scale crystalline features and controlled porosity and pore size could promote osseointegration (that is, direct bonding to natural bone). So far, a number of methods have been developed or used commercially to deposit HA on metal implants, such as electrophoretic deposition, sputter, dip coating, spin coating and plasma spray. It is, however, very challenging to produce a nanocrystalline HA coating with desirable nano-features and surface roughness as well as controlled pore size and porosity for dental/orthopedic implants. It is also necessary for nano-HA coating to have good adhesion strength to metallic substrates and sufficient mechanical properties for load-bearing conditions. Therefore, a novel hybrid coating process, combing electrostatic spray coating (ESC) technique with a novel non-conventional sintering, was developed to meet requirements for dental implants in this study. Specifically, HA nanoparticles were deposited on titanium substrates using ESC technique and the green HA coating was then sintered in a controlled condition. The produced HA coating were characterized for grain size and pore size using an environmental scanning electron microscope (ESEM), the composition and Ca/P ratio using Energy Dispersive X-ray (EDX) analysis, and crystalline phases using X-ray diffraction (XRD). The results demonstrated that a nanocrystalline HA coating with a grain size from 50 to 300 nm and a gradient of nano-tomicron pore sizes were fabricated successfully using this novel coating process. The controlled nano-scale grain size and a gradient of pore sizes are expected to promote bone cell functions and facilitate bone healing. Besides biological properties, such HA coating was also characterized for its mechanical properties, such as adhesion strength, hardness and toughness. Microscratch test results showed that the critical load of coating de-lamination reached as high as 10 N. In conclusion, this study demonstrated that it is very promising to scale up this novel hybrid coating process (ESC followed by a novel sintering process) for dental/orthopedic implant applications. INTRODUCTION Hydroxyapatite (HA) has been widely used as a coating material for dental/orthopedic applications due to its excellent biocompatibility and bioactivity to promote natural bone-implant integration. It has been reported that HA with nano-scale crystalline features and controlled porosity and pore size could promote osseointegration (that is, direct bonding to natural bone). So far, a number of methods have been developed or used commercially to deposit HA on metal implants, such as electrophoretic deposition, sol-gel, sputter, dip coating, spin coating and plasma spray. It is, however, very challenging to produce a high quality nanocrystalline HA coating with desirable nano-features and surface roughness as well as controlled pore size and
porosity for biomedical implants. It is also necessary for nano-HA coating to have excellent adhesion strength on metallic substrates to prevent coating delamination. Therefore, in this study, a novel hybrid coating process, combining NanoMech’s patented NanoSpray® coating technology with a microwave sintering process, was developed to meet mechanical and biological requirements for dental/orthopedic implants. MATERIALS AND METHODS HA nanoparticles were deposited on titanium (Ti) substrates using NanoMech’s patented Nanospray® coating system which was developed based on the electrostatic spray coating technology (Figure 1). The HA nanoparticles were generally electrically insulating in nature and can carry the static charge over a distance of a few tens of centimeters. The HA particles were charged when they exit the powder spray gun and exposed to an electrostatic field generated by a pointed electrode, which is typically of a few tens of kilovolts. The charged particles followed the electric field lines toward the grounded objects (titanium substrates in this study) and form a uniform coating perform. The produced HA coating was then sintered in a microwave furnace for maximal consolidation without significant grain growth. The sintering of HA-coated Ti implants need to be done in an air environment in order to achieve desirable nano-HA chemistry (Ca/P ratio of 1.60 ± 0.06 to mimic natural bone mineral). The parameters for the sintering were set at a temperature of 1000-1300 ºC for 5-20 minutes.
Figure 1: The schematic sketch showing the mechanism of NanoSpray® of HA nanoparticles. The produced HA coating was characterized for grain size and pore size using an environmental scanning electron microscope (ESEM), the composition and Ca/P ratio using Energy Dispersive X-ray (EDX) analysis, and crystalline phases using X-ray diffraction (XRD). Such HA coating was further characterized for its mechanical properties, such as adhesion strength (scratch resistance), hardness and toughness. The microscratch test method is commonly used to measure the critical load and then correlate it to the coating adhesion. Thus, microscratch testing according to ASTM C1624 was carried out for the HA-coated samples. The diamond stylus was drawn on top of each sample by using an increasing load, between 10 to 120N, at
constant velocity of 10 mm/min, until some well-defined failure occurred. The normal load under which the de-lamination of the coating from the Ti implants occurred was defined as critical load. RESULTS Figure 2 showed the results of HA nanocoatings deposited by NanoSpray® system before the microwave sintering. The HA coating surface morphology was characterized using SEM (Figure 2a-c). The chemical composition of HA coating before microwave sintering was characterized using EDX and the results are shown in Figure 2e in comparison with original HA particles (Figure 2d). The coating thickness variation was thoroughly characterized using cross sections and statistical results showed the thickness of 60 ± 2.1 µm. A representative cross sections of the deposited HA nanocoatings is shown in Figure 2f. (a)
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Figure 2: (a-c) SEM images of HA coating deposited by NanoSpray® system (before microwave sintering) on titanium substrates from low to high magnifications. (d) EDX results of HA particles before the coating process. (e) EDX results of HA nanocoatings on titanium substrates. (f) SEM micrograph of the cross-section of the HA nanocoating on titanium substrates to show the coating thickness (59.4 µm) and uniformity. The scale bars are 100 µm for (a) and (f), 20 µm for (b), 10 µm for (c). After the microwave sintering, the results demonstrated that a nanocrystalline HA coating with a grain size from 50 to 300 nm and a gradient of nano-to-micron pore sizes were fabricated successfully using this novel coating process (Figure 3). The controlled nano-scale grain size and a gradient of pore size are expected to promote bone cell functions and facilitate bone healing. EDX results showed that the nano-HA coating had a Ca/P ratio of 1.6 (Figure 3d), very close to natural bone. XRD results confirmed that the nano-HA coating was highly crystalline after sintering (Figure 3e).
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Figure 3: (a-c) SEM micrographs of HA nanocoating after microwave sintering. (a) Low magnification to show uniformity. (b) High magnification to show nano-scale features and nanoporosity. (c) Cross-section of HA coating to show the thickness of coating (~ 60 μm) and its uniformity. (d) EDX results of HA nanocoatings after microwave sintering showed the Ca/P ratio of 1.60 ± 0.06 (mimic natural bone mineral). (e) XRD results of HA nanocoatings after microwave sintering confirmed presence of highly crystalline HA. Optical examination at the end of the microscratch test coupled with both acoustic emission response and frictional properties variation during the test provided complete insight into the coating adhesion. Microscratch test results showed that the critical load of coating delamination reached as high as 10 N. CONCLUSIONS This study demonstrated that it is very promising to scale up this novel hybrid coating process (NanoSpray® coating followed by a microwave sintering) for commercial dental/orthopedic implants. This coating process offers high deposition rate, suitability for various composite coatings, compatibility with simple and complex geometries, flexibility, low energy consumption and low cost. The results demonstrated that the application of this coating process could reduce or even eliminate the formation of amorphous phase HA, which is soluble in body fluids and results in subsequent dissolution of the material before natural bone tissue integrates. Technically, the HA nanocoatings fabricated by this coating process have the following benefits. • Improved adhesion strength prevents delamination; • Biomimetic chemistry to natural bone tissues (Ca/P ratio very close to natural bone); • Large effective surface areas enhance cell attachment and growth; • Nano-scale roughness promotes implant-tissue integration; • Nano-to-micron pores provide more anchor sites for inducing cell activities;
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Highly crystalline HA coating reduces HA dissolution in body fluids.
ACKNOWLEDGMENTS The authors would like to thank NIH SBIR program for financial support.
