| BackgroundSince Dr Branemark opened the new era of "osseointergration", scientists are trying every methods to get a faster and stable osseointergration. Naturally titanium is considered bioinert and modifications must be taken to get a bioactive surface. Why does the surface show a high bioactivity? The exact mechanism is still unknown.Surface topography and roughness is conventionally focused. Usually under0.5μm is considered a smooth surface. According to the previous studies in vitro, the proliferation of osteoblast is not active and the interfacial bonding is not strong on a smooth surface. Creating a roughen surface by different modifications is widely accepted these years. Through simulating microenvironment like the extracellular matrix, not only the bone-binding protein secretion promoted, but also the combination between bone and implant is stronger.Recently scientists have shifted more attention to the surface characteristics such as surface energy, charge and hydrophility, which may be the direct factor in contacting with the osteoblast and bone-induced protein.Acid-etching is easy controlled and effective, it becomes one of the most common methods of surface modification. By etching the oxide layer, not only surface contaminations are removed but also pits of various dimensions are created on its surface, which leads to a rapid bone growth and good mechanical adherence. Acid-etching associated with other treatments are proved to have a higher bioactivity, it can be either the first or the last step of the previous treatment. No matter which step it is, the surface characteristics will be definitely changed. Especially when etching is the last step, it will directly influence the reaction of cells and proteins for its unique characteristics.From1950s, acid are widely used to remove the oxide layer and contaminations on metals in industry. But soon industrialist realized during etching, hydrogen would diffuse into the matrix and the enrichment of hydrogen will greatly reduce the material’s mechanical properties.In some severe cases, the materials would directly fracture which is called hydrogen embrittlement. Titanium is sensitive in absorbing hydrogen. The existence of hydride will induce the ductility and enhance fragility. Heat treatment is an effective way to reduce hydrogen. As the gas coming out and the stress releasing, the metal may recover the mechanical properties well. The heating atmosphere and temperature varied between different requirements.Some articles have reported hydrogen residual on acid-etched implants but the influence of the mechanical properties and biocompatibility is seldom reported. What’s more, according to the previous studies, heating can effectively reduce hydrogen content but the changes on the surface topography and characteristics is still unknown. Hence, the study in this paper is to find out the principles and behaviors in acid etching and subsequent heat treatment, and focus the impacting factor of hydrogen diffusion and its influence on the mechanical properties and biocompatibility. ObjectiveThe study in this paper is to find out the principles and behaviors in acid etching and subsequent heat treatment, and focus the impacting factor of hydrogen diffusion and its influence on the mechanical properties and bioactivity.Methods and materials1. Preparation of the titanium specimensPart1:Commercial pure Ti metal are abraded and then washed.The specimens were etched at40℃,50℃,60℃,70℃at15min,30min,45min,60min,respectively. Sixteen test groups(four temperatures and four etching times respectively) with three specimens in each group were prepared. After etching, the specimens were rinsed thoroughly with distilled water. Abrading disks are considered control group.Part2:The acid-treated samples (soaked in49%H2SO4and19%HCl, volume ratio1:1,60minutes,60℃)were heated in the range350℃(HT350)、450℃(HT450)、550℃(HT550) at a rate of10℃min-1in the electric furnace in air and were kept at the desired temperature for a period of1h, finally allowed to cool to room temperature at the natural rate of the furnace, finally kept in a dry and seal conditioner.2. Apatite-forming ability of different Ti specimens in an SBFSpecimens of acid-etched and all heat groups were soaked in acellular simulate body fluid(SBF) with ion concentrations nearly equal to those of human blood plasma. The samples were removed from the SBF after3day and7day respectively, then gently washed with distilled water and dried in air.3. Analysis of the surface of the Ti specimens(1)Observation of the surface texture and roughness:surface texture was observed under a scanning electron microscope(SEM); surface roughness of all groups was measured using a surface roughness testing system. (2)Analysis of the surface chemistry:Wettability of the specimens was examined by an optical contact angle (CA) measuring device; Surface crystallographic structure of the specimens was analyzed using thin film X-ray diffraction (XRD); The concentration of hydrogen in the specimens was measured using a H-N-O measurement measurement system. The distribution of oxygen and hydrogen of etched and450℃group was analyzed by Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS). In order to examine the chemical composition of the specimens, X-ray photoelectron spectroscopy (XPS) was utilized.(3)Analysis of the mechanical property:Tensile strength was detected by the tensile test using the electronic tensile testing system. Meanwhile parts of the specimens ahead of crack were taken down to observed the microcracks under scanning electron microscope.4. Statistical analysisAll statistical analysis were carried out with SPSS19.0software.In first part, the experiment was set as a factorial test. The Pearson correlation coefficient(r) was used to analyze the main factor between temperature and time in surface topography, microrough, contact angle, hydrogen concentration and tensile strength observation. The correlation coefficient was among-1to1, which means the closer the correlation coefficient of±1, the stronger influencing factor it was.In second part, significant difference test was determined using one-way analysis of variance(ANOVA) followed by LSD post hoc tests, P<0.05is considered the significant difference between the groups.ResultsTitanium lost its metallic luster after etching and became gray. At lower temperature such as40℃and50℃, the reaction was slowly; Meanwhile the reaction of60℃and70℃was faster, leaving countless and uniform micropits on its surface. Surface roughness was corresponded to that in topography. Contact angle decreased obviously in all groups after etching even the micropits were not formed on the surface. Temperature played a more important factor than time in surface roughness (Temperature r=0.883, Time r=0.446) and contact angle (Temperature r=-0.806, Time r=-0.424). Titanium hydride appeared on all etched surface excepted40℃,15min group detected by XRD. Meanwhile, hydrogen volume increased with time and temperature; time (r=r=0.803)played a dominant factor in hydrogen concentration than temperature(r=0.473). Tensile strength also decreased with increasing time and temperature which was corresponded to the same conclusion of hydrogen concentration (Time r=-0.575, Temperature r=-0.504).After heating, titanium surface became gold (350℃), violet (450℃) and gray purple (550℃). There’s no significant changes in surface morphology in350℃and450℃. In550℃, the micropits turned blunt and fleet. Hydrophility increased with heating temperature. Total hydrogen volume dropped at the same time with titanium hydride decomposed. Rutile was found on550℃group. From TOF-SIMS that the intensity of oxygen is much denser in450℃group than the non-heat specimen meanwhile the intensity of hydrogen was low after heat treatment. XPS illustrated more hydroxyl group appeared with increasing heating temperature.Compared with the acid-etched groups, more hydroxyapatite was deposited on all heat groups in day3and day7observed and detected by SEM and XRD.Conclusion1. Titanium hydride is the main form on etching surface. The condition of acid etching should be strictly controlled since the tensile strength decreased with more hydrogen diffusion, which leads a higher possibility of hydrogen embrittlement. 2.Temperature was relatively important in modifying surface topography, roughness and contact angle, however time played a key role in hydrogen concentration and tensile strength. Higher temperature and less time was suggested through the experiment to get a rough surface and avoid too much hydrogen diffusion.3. Titanium surface became hydrophilic after acid etching.4. Heat treatment decomposed titanium hydride, decreased the total hydrogen volume and changed the distribution of hydrogen.5. Hydride-rich surface didn’t improve the bioactivity in hydroxyapatite deposition. The more hydroxyl groups on titanium surface, the higher bioactivity was performed. |
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