Bioprocessing of Nacreous Coatings on Orthopedic Implant Materials

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Abstract

Nacre from mollusc shells has a complex hierarchical structure composed of an organicinorganic composite, and exhibits remarkable mechanical properties. In addition, nacre is biocompatible and bioactive making it an excellent candidate for biological coatings for orthopaedic applications.

The bioprocessing of nacreous coatings on conventional orthopedic materials via biomineralization of abalone shells was examined in this thesis. The animal reaction to the materials was evaluated by the coating surface morphology, thickness and coating-implant interface, which were characterized using SEM, EDS, XRD and Raman spectroscopy.

In the first test, poly(methyl methacrylate) (PMMA), high density polyethylene (HDPE), and titanium (Ti) substrates were implanted separately on the growth surface of abalone shells to examine the effect of different materials on mineral growth. The abalones were under restricted diet. PMMA and HDPE implants resulted in thicker coatings and were able to achieve the desired nacre structure (thickness of 38.1 ± 28.8 μm and 38.7 ± 22.2 μm, respectively). The titanium implants showed thin and sparse coating and were not able to achieve nacre (thickness of 5.3 ± 3.4μm).

In the second test, the effect of Ti surface modification (micro-porous, nano-porous and smooth surface) was examined. The substrates were implant together on one location of the shell and were under normal feeding conditions. Thick nacreous coatings, 50 to 280 µm, were formed on the Ti surfaces. There was no apparent trend between the type of Ti surface and the coating formed; however, it appeared that coatings on the implants were similar within the same animal. Thus, this indicates that feeding conditions and location of implantation may play a role in coating mineralization. In addition, two new unique features were found in the implants that have not been reported in literature before: vaterite and alternating bands of nacre towers and aragonite grains across the coating surface.

The findings in this thesis therefore suggest that nacreous coatings can be processed on both polymeric and metallic implant materials as long as proper abalone culturing conditions are maintained. The biofabrication techniques developed in this project can be applied to the development of new classes of surface coatings for biomedical implants.

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