Ion Beam and Plasma Technology for Improved Biocompatible Surfaces
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MRS BULLETIN/APRIL 1989
chemical constitutents as HA. The correct proportions of the phosphate and hydroxyl groups must be present, the material should have the proper local chemistry, it should be polycrystalline (i.e., with the correct long-range order) and highly adherent so as not to come off either during surgical insertion or during residence in the body. Bioactivity, or resorbability, is a function of specific material composition and structure. 1 Both amorphous HA (i.e., little or no long-range order) and nonhydrated calcium phosphate (e.g., tricalcium phosphate) are more bioactive (more resorbable) than crystalline HA. This may be used to promote good and newly regenerated bone. An HA film could be grown, for example, on the stem of a hip joint, then inserted, or screwed, into the existing leg bone. In this case, a certain degree of resorbability, or bioactivity on the film surface could be an advantage by encouraging bone intergrowth to firmly fix the stem in place. Once the new bone has grown in, the existence of the HA coating on the implant device may no longer be relevant; thus it could resorb completely with time. Alternatively, an HA coating may be needed to permanently disguise an implant device by providing a biocompatible interface for devices such as electrodes or pacemakers which may contain noncompatible materials. In this case, a nonresorbable, very adherent, and stable HA film is required with little or no bioactivity. Adhesion of synthetic
HA (for cellular attachment) as well as biocompatibility has been shown to depend on the condition of the surface itself.2 This study demonstrated that both surface structure and composition could be altered, with predictable effects on bioreactivity during in vitro tests, through either mechanical polishing or chemical techniques. Both factors may play a role in ion-assisted coating where an ion beam can smooth the surface as well as alter chemical composition.
Previous Methods of Film Deposition Different deposition techniques have been used to produce HA films on metal substrates. Electrophoretic deposition has been used to coat intricately shaped Ti6A14V alloy substrates with 20micron-thick hydroxylapatite films.3 However, electrophoretic deposition does not form a good mechanical bond, and coated substrates must be sintered at 950°C in vacuum. This high temperature heat treatment further induces chemical changes at the interface between coating and substrate so that a Ti-P compound is produced (stoichiometry unknown), depleting the coating of P. High concentrations of tetracalcium phosphate, with a higher Ca/P ratio than normally found in HA, is observed in the coating itself. The effects of these changes in chemistry upon the biological response is as yet unknown. More commonly, plasma spray techniques are used to coat implant devices with thick films of HA (50-200 microns). Good results, in terms of reducing the time required for maximum in vivo interface strength to be achieved, have been found with this type of coating process4 for Ti alloy as
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