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Nano Hydroxyapatite Coatings For Next Generation Prostheses
Inframat Corporation (“IMC”) is developing the next generation of nanostructured hydroxyapatite, (“n-HA”), coatings for various prostheses which utilize a room temperature electrophoretic deposition process, (EPD). Research to date has been in an NIH Phase I grant, where Ti 6 Al 4 V coupons were electrophoretically coated, and exposed over several months to simulated body fluids. Animal studies have been proposed in a phase II submittal to NIH. Fig. 1. shows the schematics of the electrophoretic HA nanocoating deposition process, and its possible applications in hips, knees and dentals.
Schematic illustration of electrophoretic process and its applications
Fig.1. Schematic illustration of electrophoretic process and its applications

Benefits derived from this innovative process include:Increased bond strength: Sixteen adhesion tests, (per ASTM F 1147-99), revealed that bond strength of the electrophoretically deposited nanocoating on Ti 6 Al 4 V substrate was > 60 MPa, thereby exceeding 50 MPa FDA requirement. This tensile bond strength is much higher than that for conventional thermally sprayed (30 MPa) and chemical deposited HAp coatings (14 MPa) before in-vitro corrosion exposures in simulated human body fluid. After in-vitro testing, the bond strength of the n-HA coatings remained constant, (> 60 MPa), while the plasma sprayed and chemical deposited HAp coatings exhibited significant bond strength reductions. Improved Corrosion Resistance: EPD HA nanocoatings are 100% dense and exhibit 100% crystallinity, which eliminates the amorphous HA dissolution problem in conventionally applied HAp coatings. Polarization current from corrosion of electrophoretically deposited nano HA in simulated body fluid was 300 fold less than that of a thermally sprayed micron sized HA coating. Feasibility of Graded Nanocoatings: The HAp/titanium interface presents two problems; (1) thermal expansion coefficient mismatch leading to internal stresses in a plasma sprayed coating system, (2) reaction of the HA with the titanium substrate forming non protective oxides thereby resulting in corrosion of the titanium alloy prosthesis. IMC has overcome these problems introducing medical glass, (inert, and biocompatible with human tissue), at the HAp/substrate interface to match the titanium alloy thermal expansion coefficient, thus precluding any thermally induced internal stresses in the coating system. Furthermore, IMC has introduced a proprietary intermediate thin layer between the titanium substrate and the medical glass. This layer is used to completely eliminate any possible physical contact between the body fluid and the titanium substrate for the lifetime of the prosthesis. Extended Prosthesis Lifetime: Our initial experimental results indicate that with IMC’s HAp nanocoating, (viz. higher bond strength, increased corrosion resistance, and no thermally induced stresses), prostheses lifetimes should be significantly lengthened.

Broader Market Applications: Knees and dental implants require high bond strength coatings to accommodate the greater impact loads and related higher stresses than hip prostheses. The high bond strength of EPD deposited HA nanocoatings to the prosthesis substrate is projected to broaden its applications to include knee and dental implants. Fig. 2. shows the market potential of the electrophoretically deposited HA nanocoating.

Potential applications of electrophoretically deposited HA nanocoatings
Fig. 2. Potential applications of electrophoretically deposited Ha nanocoatings, note nano HA will expand the current HA applications to knees and dentals.

Projected Accelerated Healing Rates: The HA nanocoating grain size is ~50 nm, similar to that of biological carbonated apatite in bone (<60 nm in length) and much smaller than conventional HAp coating grain sizes, typically > 1.0 mm. Consequently, higher bioactivity of the HA nanocoating is projected to accelerate osteointegration, and promote reduced time for full unencumbered use of the new prosthesis.

Projected Price: HA coatings in the current commercial market are mainly plasma sprayed, with some being chemical precipitated. With plasma spray operations, booth operation, (thermal spray gun, gases, robot), is costly. Conversely, electrophoretic deposition involves low maintenance and is energy efficient. Consequently, we project that electrophoretically deposited HA nanocoatings will be economically competitive.

 


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