An In Vitro Biological And Tribological Evaluation On The Application Of DLC Coating On Orthodontic Bracket And Archwire

ObjectiveStainless steel and Nickel-titanium (NiTi) alloys have been applied in orthodontictreatment for several decades due to their biocompatibility and mechanical properties.Nevertheless, they are still controversial in long-term clinical applications because of thehigh friction coefficient and the decreasing of corrosion resistance in active saliva orfluoride solutions. The long-term biocompatibility and frictional characteristics can beimproved by using various surface modification methods. Diamond-like carbon (DLC)films are very promising candidates for this bio-application owing to their good biocompatibility and tribological properties, and it has been widely expected to be adaptedas new biocompatible coatings. The purpose of this study was to investigate the potentialinterest in DLC coating for applying to orthodontic appliances. Hence DLC films weredeposited on orthodontic archwires and brackets, and an in vitro biological andtribological evaluation was carried out.Methods1. DLC films were prepared on the surfaces of316L stainless steel byMirror-Confinement-type Electron Cyclotron Resonance (MCECR) plasma sputteringtechnique, and the friction behaviour and abrasion performance of DLC films wereinvestigated using a Pin-on-Disk friction and wear tester.2. The bacterial adhesion and antibacterial property of the DLC coating wasevaluated against streptococcus mutans MS95051. Cell behavior of human periodontalligament fibroblasts was evaluated in terms of cell adhesion on the DLC films, cellviability/proliferation (MTT assay) and pattern of cell growth.3. We developed a specific fretting test which permitted the micro-slidings tosimulate tooth movement. The fretting machine is very promising for investigatingbracket-to-wire angulation and rotation, studying environmental conditions, and thefrequency of the oscillation, as well as other parameters that could have tribologicaleffects.4. DLC films were deposited on Ni-Ti orthodontic archwires, and the influences of afluoride-containing environment on the surface topography and the friction coefficientbetween brackets and archwires were subsequently investigated.5. DLC films were deposited on the stainless steel brackets and archwires. Thetribological behavior of archwire-bracket contacts was investigated at a normal load of1Nand a frequency of0.5Hz for displacement amplitude of±150μm. Variousarchwire-bracket combinations were investigated under dry and artificial saliva conditions.After fretting tests the surface morphology of archwires and brackets were analyzed byusing scanning electron microscopy (SEM). Results1. DLC films were deposited on stainless steel, and the results of frictional testsshowed that the samples coated by DLC films exhibited lower and more stable frictioncoefficient than that of uncoated samples. The mean abrasion life of carbon film reached avalue of8000cycle.2. The study examined the biological behavior of human periodontal ligamentfibroblasts on the DLC-coated surface. The results demonstrated that DLC-caoted surfacewell supported the growth, spreading, attachment and proliferation of cell. Meanwhile, theSEM images as well as the counting results of bacterium revealed that the multiplicationof streptococcus mutans was not obvious on the DLC-coated surfaces than on theuncoated surface.3. A specific fretting apparatus consisting of reciprocating tangential displacementswas developed to study the tribological behavior of archwire-bracket contacts.4. The results confirmed the superior nature of the DLC coating, evident as fewersurface roughness variation for DLC-coated Ni-Ti archwires after immersion in a highfluoride ion environment. Moreover, the friction tests shown that the application of DLCcoating significantly decreased the fretting wear and the coefficient of friction both inambient air and artificial saliva.5. The DLC-coated stainless steel wires showed significantly lower frictioncoefficient than the uncoated wires independently of the applied environments.Nevertheless, the DLC-coated and uncoated brackets showed no significant differences inthe friction coefficient. Microscopic analysis showed that low wear took place for theDLC-coated surfaces. In addition, the dominant defect caused by mechanical treatmentduring manufacturing process was especially seen on the external edges of the bracket slotbottom surface. The debris of DLC layer could not remain on such ridged contact surfaceand form a transfer film, which favored the lubrication of the interface of wire-bracketcoupling.Conclusions 1. The MCECR deposition technique allows the growth of dense, homogeneous DLCcoatings on stainless steel substrates. DLC coatings provide a suitable surface for cellattachment, spreading and proliferation, further suggesting the lack of cytotoxicity of thiscoating. In addition, the antibacterial tests show that DLC films exhibit antibacterialproperties.2. The fretting machine appears suitable for evaluating the frictional behavior oforthodontic bracket-wire combinations because of its controllable test parameters, such ascontact configuration and positioning. It permits standardizing the testing methods andachieving reproducible findings.3. DLC coating is recommended in order to reduce the fluoride-induced corrosionand improve the orthodontic friction for Ni-Ti archwires.4. The application of DLC coating on archwires significantly decreases theorthodontic fretting wear and coefficient of friction. Unfortunately it does not affect thefrictional properties for brackets at present, and the design, production techniques andfinal polishing process of the bracket slot bottom should be improved.In a word, the cellular biocompatibility of the DLC coatings, allied with the excellentphysico-chemical performance, anticipates a wide range of applications in the dentalbiomedical field.