Preparation Of Strontium-containing Phosphate Chemical Conversion Coatings On Medical Titanium And Their Antibacterial And Osteoimmunomodulatory Behaviors

Due to the lack of bioactivity and antibacterial ability,titanium(Ti)implants have exposed some urgent problems in their clinic applications,such as long bone healing periods,insufficient osseointegration,and bacterial infection.Macrophages play a crucial role in the immune microenvironment,which affects the osseointegration of Ti implants as they can trigger the host immune response.To address these issues,modification techniques can be used to give Ti bio-functional surfaces which promote bone generation,inhibit bacterial infection,and regulate immune behavior,ultimately improve the clinical efficacy of Ti implants.One effective method for modifying the surface of Ti implants is the phosphate chemical conversion(PCC)treatment,which creates coatings with excellent biocompatibility and bioactivity.By designing the involved metallic ions,the chemical composition and microstructure of PCC coatings can be modified to achieve dual functional regulation,aligning with the requirements of surface modification for metallic implants.Strontium(Sr),which promotes osteogenesis and inhibits osteolytic,has been widely used to enhance the osteogenic properties of biomaterials.Therefore,the construction of Sr-containing PCC coatings on the surface of Ti implants can effectively improve its osteogenic ability and long-term stability.This study focused on developing bioactive phosphate coatings on Ti using the biofunctional Sr element with the PCC method.The effects of process parameters and substrate surface pretreatment on the formation of the coatings were investigated to optimize the coating quality.The phase composition and crystal morphological evolution of the coatings induced by doping calcium(Ca)and zinc(Zn)ions were analyzed.Based on phase and microstructure modulation,different types of Sr-containing PCC coatings on Ti were constructed.Their physical and chemical properties,including phase composition,crystal morphology,bonding strength,surface roughness,wettability,corrosion resistance,and interfacial bonding characteristics were systematically characterized by X-ray diffractometer(XRD),X-ray photoelectron spectrometer(XPS),Fourier transform infrared spectrometer(FTIR),field emission scanning electron microscope(FE-SEM),laser scanning confocal microscope(LSCM),focused ion beam and transmission electron microscopy system(FIB-TEM),and electrochemical workstation system.Finally,the biological properties of the Sr-containing PCC coatings,including antibacterial and osteoimmunomodulatory activities,were systematically characterized to evaluate the synergistic ability of the coatings to inhibit bacterial infection and promote osseointegration.The results showed that ultrasonic impact,which could promote the growth of strontiumzinc-phosphate(SrZnP)coatings and refine crystals,was a necessary condition for preparing Sr-containing PCC coatings on Ti substrates.However,an excessively high ultrasonic power(300 W)reduced the structural integrity of the coating surface.The optimal pH range for preparing the SrZnP coating was found to be 2.75-3.75,with the best forming quality achieved at pH=3.25.At 50℃ and 60℃,the SrZnP coatings were composed by slab-like Zn3(PO4)2·4H2O crystals and ellipsoidal SrZn2(PO4)2 crystals.As the temperature elevated to 70℃ and 80 ℃,the SrZnP coating transformed into a homogeneous SrZn2(PO4)2 phase,with a change in crystal morphology to cubic.The SrZnP coatings formed on Ti substrates with different surface pretreatment states,including sanding and acid-etching(SA),sandblasting and acid-etching(SLA),alkali washed after sandblasting and acid-etching(SLAA),and ion preloading(IPL),could maintain the SrZn2(PO4)2 phase composition and cubic crystal morphology.Meanwhile,SLA,SLAA,and IPL treatments reduced the coating thickness and improved the bonding strength of the coatings to the Ti substrate.The Sr2+ and Zn2+preloading significantly reduced the crystal size by promoting nucleation.Based on the similar ionic radius,Ca2+ was able to dope into the strontium-phosphate(SrP)coating by replacing Sr2+ in the crystal,and the Ca content in the coating possessed a positive linear correlation with the Ca2+concentration in the solution.In addition,doping Ca2+caused crystal refinement,resulting in a change in crystal morphology from rhombic to lamellar,and a gradual decrease in crystal size and coating thickness with increased Ca2+doping amount.The effect of Zn2+doping on strontium-calcium-phosphate(SrCaP)coating was dependent on the concentration of Zn2+in the solution.When the amount of Zn(H2PO4)2·2H2O added was less than 1.0 g/L,the main phase and surface morphology of the coatings remained Ca2+-doped SrHPO4 with an interlaced structure composed of lamellar crystals.With increasing Zn(H2PO4)2·2H2O addition,the main phase of the coatings evolved to SrZn2(PO4)2,accompanied by an increase in crystal size.Meantime,the morphology evolved to a mixed structure of lamellar and fusiform crystals(2.0 g/L,4.0 g/L),and then to cubic crystals(8.0 g/L).To avoid interference from Ca2+,the effect of Zn2+ concentration in the solution on the crystal morphology of SrZnP coatings was investigated.With increasing Zn(H2PO4)2·2H2O addition(1.0-8.0 g/L),the microstructure of the coatings evolved from irregular hemispherical and slab-like to a stable cubic morphology while maintaining the SrZn2(PO4)2 phase,along with an increase in coating thickness and an improvement in corrosion resistance.According to the phase composition,the Sr-containing phosphate coatings on Ti could be divided into five types,i.e.,SrP,SrCaP,SrCaP-Zn,SrZnP-Ca,and SrZnP.The main constituent phase of SrP coating was SrHPO4,while Ca2+-doped SrHPO4 was the main constituent phase in SrCaP and SrCaP-Zn(with trace contents of Zn)coatings.The main constituent phase of SrZnP-Ca(with trace contents of Ca)and SrZnP coatings was SrZn2(PO4)2.An evolution law existed between the five coatings based on the doping of Ca2+ and Zn2+,which induced the evolution of their phase types and microstructures.Based on the SrP coating,SrCaP was obtained by doping Ca2+,while SrCaP-Zn,SrZnP-Ca,and SrZnP were obtained by doping Zn2+and increasing its content.The thickness of the five coatings ranged from 13-42 μm,and the bonding strength was about 14-26 MPa.The thinnest coating,SrZnP-Ca,had the highest bonding strength.The elastic modulus(E)of the coatings was 20-90 GPa,and the hardness(H)was 2.5-4.0 GPa,with the SrZnP-Ca coating possessing the highest H/E ratio.The coatings exhibited rough surfaces(Sa=270-930 nm)and good wettability(water contact angle θ was 30-85°),with the SrCaP-Zn coating showing low roughness and the best wettability.The electrochemical tests indicated that the five coatings,especially SrZnP-Ca and SrZnP,exhibited favorable corrosion resistance in simulated body fluid.In addition,the interfacial analysis suggested that the coatings with dense internal structures were tightly bonded to the Ti substrates with an amorphous oxide passivation layer.The formation of the coatings depended on the combined promotion of ultrasonic,electrical coupling,and Fe2+.The growth processes of the coatings were characterized by multi-point nucleation at the micro level,accompanied by a synergistic transverse spreading and longitudinal thickening at the macro level.In vitro antibacterial assays showed that the coatings had no obvious difference in the initial adhesion(within 2 h)of S.aureus and E.coli.However,after 24 h,the SrCaP-Zn,SrZnPCa,and SrZnP coatings could inhibit the formation of bacterial biofilm,indicating that Zn2+was a key factor to endow the coating with antibacterial activity.The osteoimmunomodulatory properties of the three antimicrobial coatings were evaluated through in vivo and in vitro tests.The coatings demonstrated good biocompatibility and bioactivity,as evidenced by their ability to enhance the adhesion,growth,and proliferation of BMSCs and macrophages,as well as increase the expression of osteogenic differentiation genes and proteins in BMSCs.Although the SrCaP-Zn coatings induced macrophages to express M1 polarization markers and proinflammatory cytokines,the SrZnP-Ca coatings induced macrophages to express M2 polarization markers,osteogenic genes,and anti-inflammatory cytokines.In vitro cell coculture assays confirmed that the SrZnP-Ca coatings could promote macrophage polarization towards the M2 phenotype,resulting in the expression of more anti-inflammatory and osteogenic factors,thus synergistically promoting the osteogenic differentiation of BMSCs.Conversely,the SrCaP-Zn coatings induced M1 polarization.Finally,rat subcutaneous models and femoral defect models indicated that the SrZnP-Ca coating had a good osteoimmunomodulatory effect in vivo,resulting in the promotion of new bone growth and improved osseointegration around Ti implants.