Titanium and its alloys have been widely used in the manufacture of endosseous implants due to excellent biocompatibility, low elastic modulus, and good chemical stability. However, the titanium based metals are essentially bioinert materials. In order to improve their bioactivity, biological coatings are usually applied. Recent studies found that, compared with hydroxyapatite coating, dicalcium phosphate anhydrous (DCPA, Monetite) coating maybe more bioactive due to higher solubility and release of Ca and P ions. In this work, DCPA coating was prepared through a novel and simple method. Briefly, high concentration DCPA solution was sprayed onto superhydrophilic titanium and the specimens were dried fast in an air-circulating oven. After repeating the process over 100 times, a compact coating was fabricated. The microstructure, chemical composition, wettability, and
Titanium (Ti) and its alloys are important metal materials for the manufacture of load bearing bone implants due to excellent corrosion resistance, biocompatibility, and lower elasticity modulus. However, the Ti based metals are essentially bioinert materials and are insufficient in osseointegration ability. Instead of biological fixation, their combination with human bones mainly depends on mechanical locking. Moreover, a fibrous capsule intervening at the interface of implant and bone is always formed after the implantation, especially when a relatively smooth implant was used [
Bioactive coatings play an important role in improving the osseointegration of metal implant with bone and, among them, the hydroxyapatite (HA) coating is the most widely used [
Several techniques have been developed for preparation of DCPA coating. Kuroda et al. heated the Ti substrate up to 150°C in a solution with calcium and phosphorus ions by large alternating current (up to 25 A). Water gasified vigorously on Ti surface and DCPA was formed. However, the coating grew in the form of lamellar and brick-like clusters and failed to completely cover the substrate [
In this work, a novel DCPA coating preparation method was proposed based on the dissolution characteristic of dicalcium phosphate. Briefly, high concentration DCPA aqueous solution was prepared and then a DCPA coating was prepared on superhydrophilic Ti substrate by repeated spray-drying treatment. The microstructure, chemical composition, wettability, and
Annealed commercially pure Ti pieces (Grade 2, Baoji Titanium Industry Company Ltd., China), cut into 10 mm × 10 mm × 2.5 mm, were used as substrates. The specimens were wet-polished with 280#, 400#, and 800# SiC abrasive paper successively and then ultrasonically washed in acetone and distilled water for 10 min, respectively. After being dried in air, these specimens were pretreated by acid etching to get superhydrophilic surface. Briefly, the Ti specimens were submerged in a mixture of 98% (wt) H2SO4, 37% (wt) HCl, and distilled water with the volume ratio of 5 : 2 : 3 and etched for 30 min. After that, the specimens were sufficiently washed with tap water, neutralized by diluted NaHCO3 solution, and then ultrasonically washed for 10 min in distilled water. The pretreated specimens were placed in distilled water before use.
High concentration DCPA aqueous solutions, 11.5 g/L (unsaturated), were prepared by dissolving analytically pure DCPA powder (Sinopharm Chemical Reagent Co. Ltd., China) into diluted HCl solution of 0.15 mol/L with the assistance of ultrasonicator. Pretreated Ti specimens were taken out of distilled water, placed into an air-circulating oven set at 60°C. Immediately after drying, the specimens were repeatedly sprayed with DCPA solution for certain times by a programmable automatic electric sprayer (TS-T, Shanghai Kezhe Biochemical Technology Co., Ltd., China). The spraying distance was 3 cm, the spray radius was 4 cm, and the spray volume was 0.1 ml per time. A interval of 2 min was set between two successive sprays to ensure complete drying of the specimen surface. The experiment setup is illustrated in Figure
Illustration of coating preparation progress.
Product phases and crystallinity were investigated by X-ray diffraction (XRD, D/MAX-3B, Rigaku Co., USA). The surface morphologies of the specimens were observed with scanning electronic microscope (SEM, S-3000, Hitachi Co., Japan). Surface roughness of specimens was analyzed by laser scanning confocal microscopy (VKX100, Keyence Co., Japan). The contact angle of distilled water on DCPA coating was measured by the static contact angle measuring instrument (JC2000A, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd., China). Bioactivity of DCPA coating was evaluated through the simulated body fluid (SBF) immersion test. The main ion concentrations of the SBF were nearly equal to that of human plasma and the pH was adjusted to 7.4 by trihydroxymethyl aminomethane (Tris) and 1.0 mol/L HCl solution [
Surface morphologies of specimens before and after pretreatment were shown in Figure
Surface morphology of specimens before and after pretreatment: (a) sandpaper polished; (b) acid etched specimens.
Contact angles of specimens before and after pretreatment: (a) sandpaper polished; (b) acid etched specimens.
The changes of contact angles with increasing storage duration.
Figure
Surface morphology changes with increasing spray-drying repetition number: (a) 20 times; (b) 60 times; (c) 100 times; (d) 140 times.
Figure
XRD patterns of specimens after 100 times of spray-drying treatment.
It could be inferred that, after each spraying, the former precipitated granules on substrate would be dissolved and precipitated again during the following drying stage. With the increase of repeating times and the accumulation of solute, a compact DCPA coating would be formed gradually. Compared with that by Kuroda et al., the DCPA coating prepared by this spray-drying method could cover the acid etched surface completely and the granules in the coating were uniform in size, contacted with each other more closely according to SEM observation [
The wettability of DCPA coating was evaluated by measuring the contact angle, as shown in Figure
Contact angles of DCPA coatings: (a) freshly prepared; (b) after 4 weeks storage in air.
Currently, surface of Ti implants designed for osseointegration are usually roughened by sandblast and/or acid etching. Submicron scaled pits generated on the Ti surface mimic the microstructure of natural bone and facilitate osteogenic differentiation of stem cells. Besides, such roughening could impose superhydrophilicity to surface of implant which greatly improves the bioactivity of Ti. For instance, the protein in the body fluid could be rapidly adsorbed to the implant surface at the implantation moment and show conformation favorable for cell attachment, proliferation, and osteogenic differentiation [
The surface morphology observation and XRD results of DCPA coated specimens (spray-drying 100 times) after immersion in SBF for 2 weeks were shown in Figures
Surface morphology of coated specimens after being immersed in SBF for 2 weeks.
Phase analysis of coated specimens after being immersed in SBF for 2 weeks.
DCPA coating was fabricated on titanium substrate through a novel and simple spray-drying method. The coating was compact, fully covered substrate and was consist of uniform sized DCPA granules that packed together closely. The coating showed good wettability and could keep the property for a long time. After immersion in simulated body fluid for 2 weeks, a large amount of bone-like apatite with low crystallinity was induced on the coating surface, which implied good bioactivity. This method is feasible and efficient, and it is expected to be used for other substrates as long as the surface showed good hydrophilicity.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (51275514), Natural Science Foundation of Jiangsu Province (BK20160566), China Postdoctoral Science Foundation (2016M601754), and Jiangsu Planned Projects for Postdoctoral Research Funds (1421601085B).