CHARACTERIZATION OF HYBRID ORGANIC-INORGANIC COATING FOR PERMANENT MAGNETS IN ORTHODONTIC APPLICATIONS

Aim. The Nd-Fe-B type permanent magnets possess high magnetic properties achieved for smaller size compared to others permanent magnet and with lower cost. Contrary to traditional force delivery system, magnetic brackets should be able to provide a controlled force released, no friction and no material fatigue. The main reason limiting long-term clinical application of magnets in orthodontic field is their marked corrosion tendency in middle aggressive media containing chloride such as saliva. This is due both to the high porosity of Nd-Fe-B alloys and to the inhomogeneity of the magnets surfaces that lead to uncoated areas. The imperfections in the coating represent the starting point for preferential pitting corrosion. The corrosion implys the mechanical deterioration and the decrease of the magnetic properties neverthless the release of citotoxic agents. Thereby it becomes necessary to overcome the corrosion resistance lack of Nd-Fe-B magnets with new wear resistant encapsulating materials and surface coatings. Materials and methods. The coating tested consists in a metal substrates (such as Au, Ni, Zn, etc..) and a coupling agent. In first step the mechanical characterisation was carried out acquiring knowledge about how, at different thicknesses of the coupling agent, the coating affects the magnetic force. The measurements of the magnetic force were performed by using an universal testing machine model Tenso Test TT2, 5-GU, Lonos with a 10 N load-cell and sensibility of 0.001 N. The cross-head speed was 0.05 mm/s. In the second step we investigated the corrosion resistance of the new coating for Nd-Fe-B magnets in corrosive environments by immersing them in a system consisting of Fusayama artificial saliva media at pH 5.5. Electrochemical impedance spectroscopy measurements and potentiodynamic polarization tests during 3 days of immersion were performed. Results. Up to 30 coupling agent layers, the magnetic force is constant with a variation from the central value of less than the 3%. The statistical analysis (ANOVA) of the data of forces measured each 0.1mm shows that the data in the samples with 7, 8, 11, 15, and 30 coupling agent layers, measured at the same distance, are normally distributed, as if they are belonging to the same population. Analyzing the acquired data by EIS is possible to evidence as coupling agent coated samples at 0 hours have higher protective performances (109 Ω) then uncoated one (104 Ω), similar behaviour was found to high immersion times (72 hours). These results confirm that the addiction of coupling agent layer on a magnet surface enhance its durability in biological electrolyte solution. Similar results are obtained for samples with higher dipping layer suggesting that this multi-steps procedure stabilize the electrochemical activity of Nd-Fe-B magnets. Conclusions. First results of this research show that the limiting aspect preventing the use of Nd-Fe-B magnets for orthodontic applications, as rapid corrosion in saliva, can be avoided by using coupling agents as surface coating. In fact, it was evidenced that the coupling agents provides a barrier effect with the formation of stable structure of hybrid organic-inorganic coating.