Enhanced biological responses of a hydroxyapatite/TiO2 hybrid structure when surface electric charge is controlled using radiofrequency sputtering

Currently available clinical oral and orthopedic implants have microtopography-altered rough surfaces and chemically added bioactive materials1-4). Numerous reports have shown that bone responses are related to different surface properties associated with microtopography and chemistry of these surfaces5-9). Surfaces of these microstructures can increase the differentiation rate of osteoblastic cells10,11). In addition to the microstructure, chemical modification of surfaces also focuses on metal coatings of potentially bioactive materials, such as hydroxyapatite (HA). It has been suggested that bone formation reactions are enhanced by such bioactive materials12). Several HA-coated implants are already being used for dental and orthopedic treatments13,14). HA is classified as a bioactive material, and bioactive materials have increased surface electric charges. However, it is difficult to control the surface electric charge intensity. Moreover, there has been little research regarding the relationship between surface electric charge intensity and biological responses. Coating implants with bioactive materials have been made possible by several technologies such as solgel-derived coatings, plasma spraying, radiofrequency magnetron sputtering, and ion beam dynamic mixing deposition15). However, a problem with these coating techniques is the adhesive strength between a base material and HA. For example, surface modification by the plasma spray technique is convenient. However, the associative strength of this technique provides a separation strength of about 20 MPa. In general, a desirable adhesive strength between a base material and HA is >50 MPa12). To resolve these problems, radiofrequency magnetron sputtering is useful for depositing bioactive materials. Its main advantage is the excellent adhesiveness that can be achieved and its ability to coat onto various types of substrates, including rough surfaces, either by sandblasting or by acid etching. Using this technology, the adhesive strength between a base material and HA has been shown to be >60 MPa16,17). In addition, the sputtering technique controls chemical deposition and surface electric charge by adjusting the output voltage and provides the self-assembly of a hybrid surface4,18). Sputter-deposited titanium oxide (TiO2) and HA hybrid surfaces are expected to be useful for remodeling surfaces of various biomedical devices and implants17). A recent study demonstrated that hybrid surfaces supported initial protein adsorption as well as the growth and proliferation of osteoblastic cells more readily than conventional HAor TiO2-coated surfaces19,20). These findings suggest that HA and TiO2 exhibit synergy during bone formation. We have been using a modified sputtering technique with dual targets of TiO2 and calcium phosphate ceramics to produce hybridized HA/TiO2 on a titanium surface. This technique confirms that the coated surface is both smooth and homogeneous, and has a high tensile strength and low frictional resistance21,22). However, the association between a HA/TiO2 hybrid structure and biological responses with regard to the surface electric charge used in implant therapies remains unclear. We hypothesized that a HA/TiO2 hybrid surface, for which the surface electric charge was controlled using a dual sputtering deposition technique on a microsurface, Enhanced biological responses of a hydroxyapatite/TiO2 hybrid structure when surface electric charge is controlled using radiofrequency sputtering