Phosphate Chemical Conversion Research Articles

Effect of different sealers on the properties of phosphate conversion coatings on 30CrMnSi alloys

PurposeThis study aims to investigate a chromium-free sealing treatment process to replace the chromate sealing process in response to the environmental hazards caused by chromate in the Phosphate chemical conversion (PCC) coating post-treatment sealing process.Design/methodology/approachIn this paper, chromium-free sealing technology was used to post-treat PCC coatings. Scanning electron microscopy was used to investigate the structure of the surface of the PCC coatings after the sealing treatment, and the corrosion resistance, hydrophobicity and bonding were tested using an electrochemical workstation, a copper sulfate spot-drop test, a lacquer bonding test, a contact angle meter and a neutral salt spray test.FindingsChromium-free closure makes the grain distribution on the surface of the PCC coating more uniform and dense, and forms an organic film on the surface of the coating, which significantly improves the corrosion resistance and hydrophobicity of the PCC coating, does not affect the coating film bonding force and has similar performance with potassium dichromate solution.Originality/valueThe results show that the corrosion resistance of PCC coatings after chromium-free sealing treatment is improved, and chromium-free sealing has the potential to replace chromium sealing.

Influence of Surfactant Types on the Anti-Corrosion Performance of Phosphate Chemical Conversion Coated Mg-8wt.%Li Alloy

In this work, the morphology, anti-corrosion performance and degradation mechanisms of two phosphate chemical conversion coatings containing the AEO (fatty alcohol polyoxyethylene ether) and AES (fatty alcohol polyoxyethylene ether sodium sulfate) on an as-cast Mg-8wt.%Li alloy were explored and compared. Although two coating layers had a petal-shaped structure and were composed of leaf-shaped particles, the coating layer of the AES-coated sample was relatively dense due to the smaller size of the formed petal-shaped structure. Based on the electrochemical data and hydrogen evolution measurements, the corrosion protectability of the coating layer on the AES-coated sample was better than that on the AEO-coated sample. The determined corrosion current densities (icorr) of the AES-coated and AEO-coated samples in the 3.5 wt.% NaCl solution were, respectively, 7.8 mA·cm−2 and 11.7 mA·cm−2, whereas the icorr value of the coated sample without a surfactant was 36.2 mA·cm−2.

Friction, wear, and anti-scuffing behaviors of zinc phosphate film on cylinder liners

Zinc phosphate chemical conversion coating possesses excellent tribological properties, which has attracted increasing attention. In this work, the friction, wear and anti-scuffing performance of zinc phosphate film applied on the cylinder liners were studied using a self-designed friction and wear experimental apparatus, which can sufficiently simulate the working conditions of the piston ring and cylinder in a real engine. The experimental results indicate that comparing with the uncoated cylinder liner, the friction force and total wear losses of friction pairs for the zinc phosphate coating are reduced by 8% and 27.5%, respectively, which is attributed to the formation of the protective film containing metal oxides and iron phosphate. In the scuffing test, uncoated cylinder liner exhibits better anti-scuffing performance because of the spalling of the cast iron graphite. On the contrary, the zinc phosphate coating particles remaining on the worn surface after oil-cut off are peeled off and acted as the abrasive at the contact interface, which accelerates the plastic deformation of friction pairs and the cast iron graphite is covered by the plastic flow of the material.

Construction of Magnesium Phosphate Chemical Conversion Coatings with Different Microstructures on Titanium to Enhance Osteogenesis and Angiogenesis.

Titanium (Ti) and its alloys are widely used as hard tissue substitutes in dentistry and orthopedics, but their low bioactivity leads to undesirable osseointegration defects in the early osteogenic phase. Surface modification is an important approach to overcome these problems. In the present study, novel magnesium phosphate (MgP) coatings with controllable structures were fabricated on the surface of Ti using the phosphate chemical conversion (PCC) method. The effects of the microstructure on the physicochemical and biological properties of the coatings on Ti were researched. The results indicated that accelerators in PCC solution were important factors affecting the microstructure and properties of the MgP coatings. In addition, the coated Ti exhibited excellent hydrophilicity, high bonding strength, and good corrosion resistance. Moreover, the biological results showed that the MgP coatings could improve the spread, proliferation, and osteogenic differentiation of mouse osteoblast cells (MC3T3-E1) and vascular differentiation of human umbilical vein endothelial cells (HUVECs), indicating that the coated Ti samples had a great effect on promoting osteogenesis and angiogenesis. Overall, this study provided a new research idea for the surface modification of conventional Ti to enhance osteogenesis and angiogenesis in different bone types for potential biomedical applications.

Deposition and corrosion performance of phosphate-polylactic acid composite coatings on WE43 magnesium alloy

The degradation profile of Mg alloys in biomedical environments can be controlled by applying various types of surface coatings. Phosphate chemical conversion coatings (CaP) and polylactic acid (PLA) biopolymer coatings are biocompatible and promising for the corrosion protection of Mg alloys. The aim of this work was to investigate the physicochemical properties and corrosion performance of inorganic, organic, and mixed inorganic-organic coatings. Three types of surface modification, CaP, PLA, and a combined CaP-PLA coating were obtained on the surface of the magnesium alloy WE43. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and infrared spectroscopy were employed to characterize the surface and interfacial morphology, along with the composition of the coatings before and after corrosion experiments. The protective performance of obtained coatings was examined by electrochemical impedance spectroscopy and immersion testing in Hank's solution. Coated samples showed significantly improved corrosion resistance. The values of polarization resistance based on impedance measurements were 164; 8688; 69,410; and 228,369 Ω cm2 for WE43 alloy, CaP, PLA, and CaP-PLA coatings, respectively. The results allowed to propose the mechanism of the coatings growth and their further corrosion degradation. A two-component CaP-PLA coating provided reliable corrosion resistance in Hank's solution for 14 days.

Synthesis and characterization of strontium substituted monetite coatings via mild phosphate chemical conversion

Strontium (Sr) substitution into calcium phosphate (CaP) materials has gained acceptance in research into the enhancement of physiochemical and osteogenic potential. Conventional synthesis of metal cationic substitution into CaP coatings on metal substrate usually takes relatively high temperature (﹥100 °C) and hours of processing time. Herein, a mild phosphate chemical conversion method was developed for preparing Sr-substituted CaP coatings on surface of titanium (Ti) in the presence of increasing amount of Sr (40–60 mol%). The results indicated that the complete and even coatings could be produced on the whole surface of Ti only within 50–60 % Sr and the coatings represented a unique lamellar texture in micro/nano-scale. As Sr concentration increased, the phases of coatings were predominantly indexed to monetite and SrHPO4, which presented a highly oriented crystal alignment. With high content of Sr, monetite, instead of brushite, was deposited in the coatings. The possible mechanism of coating formation was proposed that α-SrHPO4 and α-monetite were co-deposited in the acidic phosphating solution and finally a compound of (CaxSr1-x)HPO4 was produced on surface of Ti. Results revealed that coatings formed at high degree of Sr substitution exhibited relative improved mechanical and electrochemical properties. The coatings enhanced the cell focal adhesion and proliferation of BMSCs compared with that of the bare Ti.

Tuning the nucleation kinetics of phosphate chemical conversion coating on Mg-Gd-Y-Zr magnesium alloy: The effect of pretreatment and organic additive

In order to obtain a more protective phosphate conversion coating with a denser architecture, the nucleation kinetics of phosphate chemical conversion coating on Mg-Gd-Y-Zr magnesium alloy was tuned in this work. A pretreatment process was proposed and organic additives were incorporated, which aims at increasing the ionic produce (Jsp) at the interface for increasing σ, and decreasing the critical ionic product (JC,sp), respectively. Results prove that the pretreatment of bare alloys in a phosphate bath could increase the ion products of MgHPO4 / MnHPO4. The addition of benzalkonium chloride could neutralize the charges of crystals, and in turn promote the nucleation kinetics. A denser and more protective conversion coating could consequently be obtained.

Corrosion resistance multi-functional chemical conversion coating on Mg-Gd-Y-Zr alloy with high absorbance and electrical conductivity

In this work, a new design of phosphate chemical conversion coating with low electrical contact resistance and high light absorption (C-E-PCC) on Mg-Gd-Y-Zr (Mg-RE) alloy was proposed. The conversion coating was stained with black dye aiming to blacken PCC, and post-treated with silane loaded nano-Ag wire, achieving low electrical contact resistance (ECR) due to the overlapping network of Ag wires on the surface of PCC. The experimental results indicated that the hydrogen evolition volume of C-E-PCC was 2.54 ± 0.09 ml/cm2 immersion 3.5 % wt NaCl for 120 h. The absorption in the visible light range of C-E-PCC was 30%. Moreover, the ECR of C-E-PCC was 11.00 ± 0.47 Ω. The aforementioned results implied that the PCC not only with high absorbance and low ECR but also with high corrosion protection.

The Influence of pH Value on the Microstructure and Properties of Strontium Phosphate Chemical Conversion Coatings on Titanium.

Strontium (Sr) is a trace element in the human body that can promote bone formation and inhibit bone absorption. A conversion coating of strontium phosphate (Sr-P) on the surface of titanium (Ti) can improve its biological properties and has many potential applications in the fields of dentistry and orthopedics. In the present study, Sr-P coatings with SrHPO4 and Sr3(PO4)2 crystals on Ti are prepared by a phosphate chemical conversion (PCC) treatment and the effect of pH values on the properties of the Sr-P coatings is researched. The results prove that the phase composition, morphology, and corrosion resistance of the coated Ti vary according to the pH values of the PCC solution. The morphology of the conversion deposition on Ti changes from plat-like to cluster-like and then to homogeneous microcrystals as the pH value changes from 2.50 to 3.25. Only discrete SrHPO4 crystals are generated on the substrate at lower pH values, while relatively stable Sr3(PO4)2 and SrHPO4 crystals grow and subsequently form an integrated coating on the Ti as the pH exceeds 2.50. The cross-sectional morphologies and bonding strength of different coatings are also researched. The corrosion resistance of coated Ti improves compared with that of bare Ti because of the Sr-P coatings with a Sr3(PO4)2 phase. In addition, it is indicated that the Sr-P coatings on Ti can improve the adhesion and differentiation of BMSCs.

Effect of Reaction Temperature on the Microstructure and Properties of Magnesium Phosphate Chemical Conversion Coatings on Titanium.

Magnesium phosphate (MgP) has garnered growing interest in hard tissue replacement processes due to having similar biological characteristics to calcium phosphate (CaP). In this study, an MgP coating with the newberyite (MgHPO4·3H2O) was prepared on the surface of pure titanium (Ti) using the phosphate chemical conversion (PCC) method. The influence of reaction temperature on the phase composition, microstructure, and properties of coatings was systematically researched with the use of an X-ray diffractometer (XRD), a scanning electron microscope (SEM), a laser scanning confocal microscope (LSCM), a contact angle goniometer, and a tensile testing machine. The formation mechanism of MgP coating on Ti was also explored. In addition, the corrosion resistance of the coatings on Ti was researched by assessing the electrochemical behavior in 0.9% NaCl solution using an electrochemical workstation. The results showed that temperature did not obviously affect the phase composition of the MgP coatings, but affected the growth and nucleation of newberyite crystals. In addition, an increase in reaction temperature had a great impact on properties including surface roughness, thickness, bonding strength, and corrosion resistance. Higher reaction temperatures resulted in more continuous MgP, larger grain size, higher density, and better corrosion resistance.

Effects of nucleation pretreatment on corrosion resistance of conversion coating on magnesium alloy Mg-10Gd-3Y-0.4Zr

To improve corrosion resistance of phosphate chemical conversion coatings (PCCs) on magnesium alloy Mg-10Gd-3Y-0.4Zr, a new route was proposed. The phosphate conversion treatment was preceded by two nucleation pretreatment processes in the form of physical adhesion and chemical adsorption. The present study focused on the effects of two nucleation pretreatments of silane coupling agent KH-550 + nano-SiO2 sol process and Ti/Zr surface modification treatment on microstructure and corrosion resistance of PCCs on magnesium alloy Mg-10Gd-3Y-0.4Zr. Results demonstrate that nucleation pretreatment can prefabricate dispersed nuclei on substrate's surface, which significantly enhances the nucleation rate during the initial stage of coating formation, and decreases the crystal size of PCCs. Corrosion resistance of the PPCs was increased by a factor of 4. The Ti/Zr pretreatment demonstrates a superior application perspective due to the ease-of-fabrication and simplicity.

Calcium-Zinc Phosphate Chemical Conversion Coating Facilitates the Osteointegration of Biodegradable Zinc Alloy Implants by Orchestrating Macrophage Phenotype.

Zinc (Zn) alloys provide a new generation for orthopedic applications due to their essential physiological effects and promising degradation properties. However, excessive release of Zn ions (Zn2+ ) during degradation and the severe inflammatory microenvironment are not conducive to osseointegration, which is determined by the characteristics of the implant surface. Therefore, it is essential to modulate the release rate of Zn alloys by surface modification technology and endow them with anti-inflammatory and osteogenic effects. In this study, two kinds of phosphate chemical conversion (PCC) coatings with different compositions and morphological structures are prepared, namely Zn-P (with disk-like crystals) and Ca-Zn-P (with lamellar crystals). Although all the PCC-coated Zn implants have low cytotoxicity, Ca-Zn-P show better osteoimmunomodulation effects in several aspects: the induction of the M2-phenotype macrophage polarization and thus promotion of osteogenesis in vitro; the regulation of the bone immune microenvironment which is conducive to tissue regeneration and osseointegration in vivo; and the release of ions (through PI3K/AKT and Wnt signaling pathways) and the morphological structures (through RhoGTPase signaling pathways) act as possible mechanisms of M2 polarization. The Ca-Zn-P coating can be considered to provide new insights into bone immunomodulation and osseointegration.

Effect of Substrates Performance on the Microstructure and Properties of Phosphate Chemical Conversion Coatings on Metal Surfaces.

Phosphate chemical conversion (PCC) technology has attracted extensive attention for its ability to regulate the surface properties of biomedical metals. However, titanium (Ti)-based alloys exhibit inertia because of the native passive layer, whereas zinc (Zn)-based alloys show high activity in acidic PCC solutions. The substrate performance affects the chemical reaction in the phosphating solution, which further leads to diversity in coating properties. In this work, the zinc-phosphate (ZnP) coatings are prepared on Ti alloy (TA) and Zn alloy (ZA) substrates using the PCC method, respectively. The coatings prepared herein are detected by a scanning electron microscope (SEM), X-ray diffractometer (XRD), laser scanning confocal microscope (LSCM), universal testing machine, contact angle goniometer, and electrochemical workstation system. The results show that the substrate performance has little effect on the phase composition but can significantly affect the crystal microstructure, thickness, and bonding strength of the coatings. In addition, the ZnP coatings improve the surface roughness of the substrates and show good hydrophilicity and electrochemical corrosion resistance. The formation mechanism of the ZnP coating is revealed using potential-time curves, indicating that the metal–solution interfacial reaction plays a dominant role in the deposition process.

Effect of Surfactants on the Corrosion Protectability of Calcium Phosphate Conversion Coatings on Duplex Structured Mg-8Li (in Wt.%) Alloy

Calcium phosphate chemical conversion coatings with the additions of sodium lauryl sulfate (SLS) and dodecanesulfonic acid sodium (DSS), respectively, were prepared on the surface of the Mg-8Li alloy. The surface and cross-sectional corrosion morphologies, compositions, and corrosion behavior of the coated surfaces in 3.5 wt.% NaCl solution were respectively investigated by using a scanning electron microscope (SEM), energy dispersive spectrometer (EDS), electrochemical workstation, hydrogen evolution apparatus, and optical microscope (OM). The results demonstrated that Ca-P coatings had a petal-like structure being composed of leaf-like particles. After being respectively performed for 30 min in conversion solutions containing SLS and DSS, the corresponding average film thicknesses of surface coatings were 27 μm and 7 μm. In addition, the corrosion current densities of coated surfaces by using the conversion solutions containing SLS and DSS were 1.438 × 10−5 A·cm−2 and 4.719 × 10−5 A·cm−2, respectively. The effect of surfactants on phosphate chemical conversion coating was discussed in detail.

Effect of reaction time on the microstructure and properties of in-situ hopeite chemical conversion coatings formed by self-corrosion on zinc alloy

Zinc (Zn) and its alloys are considered as promising biodegradable metals due to its suitable degradability and important physiological functions. However, in the process of degradation, excessive release of Zn ions (Zn2+) will significantly affect the cytocompatibility and antibacterial properties. Surface modification technology can not only control the corrosion behavior of biodegradable metals but also improve its biocompatibility and antibacterial properties according to clinical requirements. In this study, an in-situ hopeite (HP, Zn3(PO4)2·4H2O) coating was prepared on Zn alloy by phosphate chemical conversion (PCC) method using Zn2+ provided by self-corrosion of the substrates. And the effect of phosphating time on the microstructure, phase composition, corrosion resistance, and wettability of HP coatings was investigated. The results showed that a complete HP coating was formed on the Zn alloy after reaction for 1 min, even if Zn2+ were supplied by self-corrosion without an additional Zn source. It also revealed that the reaction time had a marked impact on the morphologies and microstructures of the HP coating. The electrochemical test results proved that the prepared HP coatings had obvious protection on the Zn alloy and could regulate the corrosion rate of the substrate. Meanwhile, the Zn alloy modified by the PCC method also possessed excellent hydrophilicity. These primary findings might support new opportunities in the exploration of controllable coatings on biomedical Zn alloys.

Zinc-Doping Induces Evolution of Biocompatible Strontium-Calcium-Phosphate Conversion Coating on Titanium to Improve Antibacterial Property.

Implant-associated infections (IAI) remains a common and devastating complication in orthopedic surgery. To reduce the incidence of IAI, implants with intrinsic antibacterial activity have been proposed. The surface functionalization and structure optimization of metallic implants can be achieved by surface modification using the phosphate chemical conversion (PCC) technique. Zinc (Zn) has strong antibacterial behavior toward a broad-spectrum of bacteria. Herein, Zn was incorporated into strontium-calcium-phosphate (SrCaP) coatings on titanium (Ti) via PCC method, and the influence of its doping amount on the phase, microstructure, antibacterial activity, and biocompatibility of the composite coating was researched. The results indicated that traces of Zn doping produced grain refinement of SrCaP coating with no significant effect on its phase and surface properties, while a higher Zn content induced its phase and microstructure transformed into zinc-strontium-phosphate (SrZn2(PO4)2). SrCaP-Zn1 and SrCaP-Zn4 represented trace and high content Zn-doped coatings, respectively, which exhibited a similar bacterial attachment for a short time but showed inhibition of biofilm formation after continuous incubation up to 24 h. The killing rates of SrCaP-Zn1 coating for Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) reached 61.25% and 55.38%, respectively. While that data increased to 83.01% and 71.28% on SrCaP-Zn4 coating due to the more-releasing Zn2+. Furthermore, in vitro culture of MC3T3-E1 cells proved that the Zn-doped coatings also possessed excellent biocompatibility. This study provides a new perception for the phase and microstructural optimization of phosphate coatings on implant surfaces, as well as fabricating promising coatings with excellent biocompatibility and antimicrobial properties against IAI.

Microstructural evolution of strontium‑zinc-phosphate coating on titanium via changing Zn2+ concentration in phosphate solution for enhanced osteogenic activity

The microstructure of the phosphate coating on implant surface is critical for successful osseointegration. Based on the morphological variability of phosphate crystals, the microstructure of the coating can be regulated by designing the composition of the phosphate solution. In this study, a strontium‑zinc-phosphate (SrZn2(PO4)2, SZP) coating was prepared on titanium (Ti) surface by phosphate chemical conversion (PCC) method. The effect of Zn2+ concentration in phosphate solution, which was regulated by increasing the addition amount of Zn(H2PO4)2·2H2O, on the microstructure and properties of the SZP coating was investigated. The characterization of the coating was carried out by scanning electron microscope (SEM), X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrometer (XPS), laser scanning confocal microscope (LSCM), contact angle goniometer, and electrochemical workstation system. The results revealed that the increase of Zn2+ concentration in phosphate solution could induce the crystal morphology of the SZP coating to evolve from irregular granules (Zn(H2PO4)2·2H2O: 1 g/L) to regular cubic blocks (Zn(H2PO4)2·2H2O: 8 g/L). This morphological evolution process was accompanied by an increase in thickness (from 4.8 ± 1.2 to 20.6 ± 1.9 μm), an improvement in wettability and corrosion resistance, as well as a decrease in the release amount of Sr2+ (from 3.97 ± 0.26 to 0.86 ± 0.12 ppm) and Zn2+ (from 0.31 ± 0.01 to 0.18 ± 0.01 ppm) after 21 days of immersion. In vitro evaluations proved that the adhesion, proliferation, and differentiation of MC3T3-E1 pre-osteoblasts on SZP coatings, especially for the coating with tiny cubic-block crystals, were significantly enhanced compared with pure Ti. This study clarified the rule of morphological evolution of the SZP crystal that is caused by changing the Zn2+ concentration in the phosphate solution, which contributes to the structural optimization of the SZP coating to obtain high osteogenic activity.

Influence of Surface Post-Processing on Crystal Refinement and Characteristics of Hopeite Coating by Phosphating

Hopeite (Zn3(PO4)2·4H2O) conversion coating was prepared on pure titanium using mild phosphate chemical conversion technology. In this investigation the effect of post-processing after phosphating was focused on coating microstructure, chemical composition, adherence and corrosion resistance. Different sequential order of drying and rinsing of the coated samples were carried out. The results demonstrated that the priority shift of rinsing or drying did not change the hopeite phase of the conversion coating, but played a significant role in the crystal size and coating microstructure. Compared with the traditional preparation of the coating, a more compact and uniform coating with finer crystals grains (3–5 μm) was obtained by the unconventional post-treatment of drying before rinsing, along with a higher bonding strength and corrosion resistance.

Comparative investigation of hydroxyapatite coatings formed on titanium via phosphate chemical conversion

Phosphate chemical conversion coatings have attracted much attention on surface modification of metallic implant, due to their in-situ growth pattern and good mechanical performances. Nevertheless, it is difficult to use this method for preparing a pure hydroxyapatite (HA) coating. In this study, a pure HA coating was prepared on titanium (Ti) in an acidic phosphate aqueous via a simple single-step chemical conversion without alkali post-treatment. The microstructure, chemical composition, electrochemical and mechanical performances of the HA coating were characterized in comparison with those of a precursor dicalcium phosphate (CaP) coating which was alkali post-treated (CaP-OH). Results revealed that the one-step chemically converted HA coating was homogeneously nano-structured with higher phase purity and crystallinity. Moreover, the HA coating was thicker (~6.5 μm) than the CaP-OH one (~40 μm) but with better mechanical properties and corrosion resistance. The results encouraged the direct preparation of crystalline HA coating with superior properties on surface of Ti via a single-step phosphate chemical conversion.

Strontium-zinc phosphate chemical conversion coating improves the osseointegration of titanium implants by regulating macrophage polarization

The immune response of peri-titanium (Ti) implants, which is mainly determined by the surface properties of the Ti implant, may lead to a prolonged healing period and even implantation failure. The phosphate chemical conversion (PCC) technique is an effective method for simultaneously regulating the crystallographic morphological and elemental compositions of Ti implant surfaces. However, whether the PCC coating induces an immune response or further affects osteogenesis is unclear. In this study, we prepared three kinds of PCC-coated Ti implants, i.e., Sr-Zn-P A (strontium zinc phosphate with rectangular blocky crystals), Sr-Zn-P B (strontium zinc phosphate with lamellar crystals) and Sr-Ca-P (calcium strontium phosphate), which were composed of different phosphate crystals or had different morphologies. Although all PCC-coated Ti implants showed good biocompatibility with both rat bone marrow mesenchymal stem cells (rBMMSCs) and rat macrophages, different effects on rBMMSC osteogenesis and macrophage polarization were found in this study: 1) Sr-Zn-P B preferentially induced rBMMSC osteogenesis and M2 polarization to a greater extent than the other implants because of its surface morphology and ion release; 2) under three culture conditions (coculture, transwell and conditioned medium), Sr-Zn-P B was found to enhance the production of anti-inflammatory and pro-osteogenic factors via M2 polarization; 3) characterization of the osteogenesis and immune response in rat subcutaneous models and rat femoral defect models revealed that Sr-Zn-P B could perform well in osteoimmunomodulation in vivo; and 4) through RNA sequencing analysis, the up-/downregulation of several signaling pathways was identified as a potential reason for M2 polarization on Sr-Zn-P B. In conclusion, this study demonstrates that Sr-Zn-P B is a promising scaffold for osteoimmunomodulation and provides new insight into bone defect repair.