Exploring the mechanism behind improved osteointegration of phosphorylated titanium implants with hierarchically structured topography

Near-infrared smart responsive orthopedic implants with synergistic antimicrobial and bone integration-promoting properties

Background

The decline in antibiotic use has made the treatment of post-implant infections increasingly challenging, especially the problem of bacterial invasion caused by inadequate tissue fusion with the implant in the early stages of the implant. Developing multiple methods to reduce bacterial infections through synergies will be superior to a single model of antimicrobial means.

Methods

The composite coating composed of titanium phosphate (TiP)/copper oxide nanoparticles (CuO)/nano-hydroxyapatite (n-HA) named TiP-ua was used to kill Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) under near infrared (NIR) irradiation by means of photothermal therapy (PTT) and photodynamic therapy (PDT) synergism.

Results

The TiP-ua composite coating can reach about 60 °C and produce a certain amount of reactive oxygen species after 15 min irradiation with 980 nm near infrared light with 0.9 W/cm2 power. Under the NIR irradiation of 0.9 W/cm2 power for 10 min, the composite coating can achieve about 90% killing effect on S. aureus and more than 90% killing effect on E. coli. In terms of mouse pre-osteoblasts (MC3T3-E1), TiP-ua showed more superiority in promoting osteogenic differentiation ability. In the mouse infection model, it also showed good antibacterial effect, and could significantly reduce the expression of inflammatory factors and accelerate wound healing. In the bone defect model, the intervention significantly accelerated the regeneration of neobone tissue and enhanced osseointegration capacity.

Conclusions

The experimental results show that TiP-ua coating not only has good photothermal conversion ability, but also has good biosafety, which can accelerate the regeneration and repair of bone tissue around the implant, including accelerating the osteogenic differentiation of cells, and reduce the activity of bacteria to effectively reduce the inflammatory response.

The translational potential of this article

The collaborative antibacterial and bone repair coating in this study has a simple preparation process, high repeatability, high biosafety and positive effect on bone tissue repair, and has great clinical application potential in orthopedics and dental implants.

Investigation on the Osteogenic and Antibacterial Properties of Silicon Nitride-Coated Titanium Dental Implants

The silicon nitride (Si3N4) coating exhibits promising potential in oral applications due to its excellent osteogenic and antibacterial properties. However, a comprehensive investigation of Si3N4 coatings in the context of dental implants is still lacking, especially regarding their corrosion resistance and in vivo performance. In this study, Si3N4 coatings were prepared on a titanium surface using the nonequilibrium magnetron sputtering method. A systematic comparison among the titanium group (Ti), Si3N4 coating group (Si3N4-Ti), and sandblasted and acid-etched-treated titanium group (SLA-Ti) has been conducted in vitro and in vivo. The results showed that the Si3N4-Ti group had the best corrosion resistance and antibacterial properties, which were mainly attributed to the dense structure and chemical activity of Si-O and Si-N bonds on the surface. Furthermore, the Si3N4-Ti group exhibited superior cellular responses in vitro and new bone regeneration and osseointegration in vivo, respectively. In this sense, silicon nitride coating shows promising prospects in the field of dental implantology.

Profiling to Probing: Atomic force microscopy to characterize nano-engineered implants

Surface modification of implants in the nanoscale or implant nano-engineering has been recognized as a strategy for augmenting implant bioactivity and achieving long-term implant success. Characterizing and optimizing implant characteristics is crucial to achieving desirable effects post-implantation. Modified implant enables tailored, guided and accelerated tissue integration; however, our understanding is limited to multicellular (bulk) interactions. Finding the nanoscale forces experienced by a single cell on nano-engineered implants will aid in predicting implants' bioactivity and engineering the next generation of bioactive implants. Atomic force microscope (AFM) is a unique tool that enables surface characterization and understanding of the interactions between implant surface and biological tissues. The characterization of surface topography using AFM to gauge nano-engineered implants' characteristics (topographical, mechanical, chemical, electrical and magnetic) and bioactivity (adhesion of cells) is presented. A special focus of the review is to discuss the use of single-cell force spectroscopy (SCFS) employing AFM to investigate the minute forces involved with the adhesion of a single cell (resident tissue cell or bacterium) to the surface of nano-engineered implants. Finally, the research gaps and future perspectives relating to AFM-characterized current and emerging nano-engineered implants are discussed towards achieving desirable bioactivity performances. This review highlights the use of advanced AFM-based characterization of nano-engineered implant surfaces via profiling (investigating implant topography) or probing (using a single cell as a probe to study precise adhesive forces with the implant surface). STATEMENT OF SIGNIFICANCE: Nano-engineering is emerging as a surface modification platform for implants to augment their bioactivity and achieve favourable treatment outcomes. In this extensive review, we closely examine the use of Atomic Force Microscopy (AFM) to characterize the properties of nano-engineered implant surfaces (topography, mechanical, chemical, electrical and magnetic). Next, we discuss Single-Cell Force Spectroscopy (SCFS) via AFM towards precise force quantification encompassing a single cell's interaction with the implant surface. This interdisciplinary review will appeal to researchers from the broader scientific community interested in implants and cell adhesion to implants and provide an improved understanding of the surface characterization of nano-engineered implants.

A Bioinspired Gelatin-Amorphous Calcium Phosphate Coating on Titanium Implant for Bone Regeneration

Biocompatible and bio-active coatings can enhance and accelerate osseointegration via chemical binding onto substrates. Amorphous calcium phosphate (ACP) has been shown as a precursor to achieve mineralization in vertebrates and invertebrates under the control of biological macromolecules. This work presents a simple bioinspired Gelatin-CaPO4 (Gel-CaP) composite coating on titanium surfaces to improve osseointegration. The covalently bound Gel-CaP composite is characterized as an ACP-Gel compound via SEM, FT-IR, XRD, and HR-TEM. The amorphous compound coating exhibits a nanometer range thickness and improved elastic modulus, good wettability, and nanometric roughness. The amount of grafted carboxyl groups and theoretical thickness of the coatings are also investigated. More importantly, MC3T3 cells, an osteoblast cell line, show excellent cell proliferation and adhesion on the Gel-CaP coating. The level of osteogenic genes is considerably upregulated on Ti with Gel-CaP coatings compared to uncoated Ti, demonstrating that Gel-CaP coatings possess a unique osteogenic ability. To conclude, this work offers a new perspective on functional, bioactive titanium coatings, and Gel-CaP composites can be a low-cost and promising candidate in bone regeneration.

In situ titanium phosphate formation on a titanium implant as ultrahigh bonding with nano-hydroxyapatite coating for rapid osseointegration

Titanium (Ti) has been widely used as a dental implant material due to its excellent mechanical property and good biocompatibility. However, its poor biological activity severely limits its ability to bond with bony tissues. To ameliorate this situation, a preparation method of ultra-high bonding nano-hydroxyapatite (n-HA) coating on the Ti surface is urgently needed. Here, Ti phosphate/n-HA (TiP-Ca) composite coatings with ultra-high bonding were prepared by a two-step hydrothermal treatment. The TiP coating was first formed in situ on the pure Ti substrate and then n-HA crystals further grew on the TiP surface. The formation mechanism of composite coating and reasons for increased bonding strength were systematically investigated. The results show that the TiP-Ca coating remains stable and exhibits an ultra-high bonding strength with the Ti implant (up to 783.30 ± 207.46 N). An effective solution was designed to address the problems of easy peel off. Cell experiments showed that TiP-Ca could promote the adhesion of MC3T3-E1 and expression of OCN, Runx2, and ALP. In vivo evaluation further confirmed that the TiP-Ca composite coating significantly enhanced osseointegration. The designed coating shows great potential in clinical application of implants.

Micron/Submicron Scaled Hierarchical Ti Phosphate/Ti Oxide Hybrid Coating on 3D Printed Scaffolds for Improved Osteointegration

Three-dimensional (3D) printed implants have attracted substantial attention in the field of personalized medicine, but negative impacts on mechanical properties or initial osteointegration have limited their application. To address these problems, we prepared hierarchical Ti phosphate/Ti oxide (TiP-Ti) hybrid coatings on 3D printed Ti scaffolds. The surface morphology, chemical composition, and bonding strength of the scaffolds were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle measurement, X-ray diffraction (XRD), and scratch test. In vitro performance was analyzed by colonization and proliferation of rat bone marrow mesenchymal stem cells (BMSCs). In vivo osteointegration of the scaffolds in rat femurs was assessed by micro-CT and histological analyses. The results demonstrated improved cell colonization and proliferation as well as excellent osteointegration obtained by incorporation of our scaffolds with the novel TiP-Ti coating. In conclusion, micron/submicron scaled Ti phosphate/Ti oxide hybrid coatings on 3D printed scaffolds have promising potential in future biomedical applications.

Design of Cefotaxime Sodium-Loaded Polydopamine Coatings with Controlled Surface Roughness for Titanium Implants

The success rate of dental implants is limited by peri-implant infection and insufficient osseointegration. Therefore, reducing the occurrence of peri-implantitis and promoting osseointegration are in demand. A roughened surface has commonly been applied to improve the osseointegration of implants, but it will accelerate the attachment of bacteria. We have developed novel antibiotic-decorated titanium (Ti) surfaces by the immobilization of dopamine and cefotaxime sodium (CS) simultaneously. Moreover, the surface roughness of the polydopamine (PDA)/CS coating was controlled by the changes in polymerization times as determined by atomic force microscopy. Then, all antibiotic-grafted Ti surfaces could effectively prevent the adhesion and proliferation of both Escherichia coli and Streptococcus mutans in comparison to the pristine control. For the culture and osteogenic differentiation of human umbilical mesenchymal stem cells (hUMSCs) on the substrate surface, PDA/CS coating with polymerization times less than 30 min showed acceptable biocompatibility, but the upregulation of marker genes and proteins was detected when the polymerization time was more than 30 min. Moreover, the best calcium deposition results were found in the 30 min PDA/CS group with or without the addition of osteogenic factors. Therefore, our PDA/CS coating with a polymerization time of 30 min holds great potential to design dental implants with dual bacteriostatic and osteogenic properties.

Evaluation of Bone Response to a Nano HA Implant Surface on Sinus Lifting Procedures: Study in Rabbits

The aim of this study was to evaluate the bone response to two different implant surfaces on sinus lift procedures in rabbits. Bilateral sinus lifting with inorganic bovine bone associated with collagen membrane and immediate implantation were performed in 16 rabbits. Custom mini-implants were randomly installed in the prepared sites: one side received a double acid-etched (DAE) surface and the other a nano-hydroxyapatite (NHA) surface. The animals were euthanized 30 and 60 days after surgery, and biopsies were collected for microtomographic and histomorphometric analysis. After 30 days, no intra- and inter-group statistical differences were observed in microtomographic analysis, while at 60 days, bone analysis showed statistically significant differences between groups (p < 0.05) for all the evaluated parameters. Histomorphometric analysis showed, after 30 days, mean % of Bone-to-Implant Contact (BIC) for DAE and NHA of 31.70 ± 10.42% vs. 40.60 ± 10.22% (p > 0.05), respectively; for % of Bone Area Fraction Occupancy (BAFO), mean values were 45.43 ± 3.597% for DAE and 57.04 ± 5.537% for NHA (p < 0.05). After 60 days, mean %BIC and %BAFO for DAE and NHA implants were statistically significant (p < 0.05). The NHA surface showed superior biological features compared to the DAE treatment, promoting higher bone formation around the implants in an experimental model of bone repair in a grafted area.

Microporous surface containing flower-like molybdenum disulfide submicro-spheres of sulfonated polyimide with antibacterial effect and promoting bone regeneration and osteointegration

Implanted materials with both osteogenic and antibacterial functions are promising for facilitating osteointegration and preventing infection for orthopedic applications. In this work, we synthesized flower-like molybdenum disulfide (fMD) submicro-spheres containing nanosheets, which were incorporated onto the microporous surface of polyimide (PI) via concentrated sulfuric acid, suspending fMD contents of 5 wt% (SPM1) and 10 wt% (SPM2). Compared with sulfonated polyimide (SPM0), both SPM1 and SPM2 with microporous surfaces containing fMD exhibited nano-submicro-microporous surfaces, which improved the surface roughness, wettability, and surface energy. Due to there being more fMD submicro-spheres on the microporous surface, SPM2 revealed a better antibacterial effect than SPM1. In addition, compared with SPM1 and SPM0, SPM2 with more fMD significantly promoted rat bone marrow-derived stromal cell response in vitro. Moreover, SPM2 remarkably enhanced new bone formation and osteointegration in vivo. In summary, the combination of fMD with the microporous surface of SPM2 resulted in a nano-submicro-microporous surface with optimized surface performance, which possessed not only osteogenic bioactivity but also an antibacterial effect. As a bone implant, SPM2 with osteogenic and antibacterial functions may have enormous potential as a bone tissue substitute.

Enhanced Antibacterial Ability and Bioactivity of Polyetherketoneketone Modified with LL-37

Polyetherketoneketone (PEKK) is considered to be a potential substitute material for metal bone implants because of its advantageous biocompatibility, chemical stability, and mechanical properties, but clinical application has been severely restricted due to PEKK's lack of antibacterial ability and biological activity. In this study, LL-37, a natural human antimicrobial peptide, was successfully modified on the PEKK surface with polydopamine as the intermediate layer and released continuously for more than 6 days. The results of the MTT assay, colony counts, and Live/Dead staining demonstrated that compared to unmodified PEKK, the LL-37-modified PEKK significantly inhibited the adhesion, vitality, and bacterial biofilm growth of Staphylococcus aureus and Escherichia coli in a concentration-dependent way. Furthermore, the LL-37-modified PEKK enhanced biocompatibility (cell adhesion and viability) and promoted osteogenic differentiation of human umbilical cord Wharton's jelly-derived mesenchymal stem cells. Our data suggested that LL-37-modified PEKK might be a promising material for use in orthopedic implants.

Mussel-inspired self-assembly engineered implant coatings for synergistic anti-infection and osteogenesis acceleration

Implant associated infections (IAI) and poor osseointegration are the two major causes for titanium implant failure, leading to subsequent financial burden and physical sufferings. Therefore, advanced implants with excellent anti-infection and osseointegration performance are needed. In this work, mussel-inspired tannic acid (TA) mediated layer-by-layer (LbL) self-assembly was used for fabricating bonded polyethylene glycol (PEG) and 8DSS (8 repeating units of aspartate-serine-serine) coatings (Ti/8DSS/PEG) on the surface of titanium implants. The coating is designed to simultaneously reduce bacterial adhesion through the super-hydrophilic effect of PEG and promote osseointegration through the effective biomineralization of 8DSS. The obtained Ti/8DSS/PEG implant exhibits superior anti-biofouling capabilities (anti-protein adhesion and anti-bacterial adhesion against S. aureus and E. coli) and excellent biocompatibility. Meanwhile, the Ti/8DSS/PEG implant accelerates osteoblast differentiation and presents significantly better osteogenic ability than bare titanium implants in vivo. This mussel-inspired TA mediated LbL self-assembly method is expected to provide a multifunctional and robust platform for surface engineering in bone repair.

Hydroxyapatite-Coated Titanium by Micro-Arc Oxidation and Steam-Hydrothermal Treatment Promotes Osseointegration

Titanium (Ti)-based alloys are widely used in tissue regeneration with advantages of improved biocompatibility, high mechanical strength, corrosion resistance, and cell attachment. To obtain bioactive bone–implant interfaces with enhanced osteogenic capacity, various methods have been developed to modify the surface physicochemical properties of bio-inert Ti and Ti alloys. Nano-structured hydroxyapatite (HA) formed by micro-arc oxidation (MAO) is a synthetic material, which could facilitate osteoconductivity, osteoinductivity, and angiogenesis on the Ti surface. In this paper, we applied MAO and steam–hydrothermal treatment (SHT) to produce HA-coated Ti, hereafter called Ti–M–H. The surface morphology of Ti–M–H1 was observed by scanning electron microscopy (SEM), and the element composition and the roughness of Ti–M–H1 were analyzed by energy-dispersive X-ray analysis, an X-ray diffractometer (XRD), and Bruker stylus profiler, demonstrating the deposition of nano-HA particles on Ti surfaces that were composed of Ca, P, Ti, and O. Then, the role of Ti–M–H in osteogenesis and angiogenesis in vitro was evaluated. The data illustrated that Ti–M–H1 showed a good compatibility with osteoblasts (OBs), which promoted adhesion, spreading, and proliferation. Additionally, the secretion of ALP, Col-1, and extracellular matrix mineralization was increased by OBs treated with Ti–M–H1. Ti–M–H1 could stimulate endothelial cells to secrete vascular endothelial growth factor and promote the formation of capillary-like networks. Next, it was revealed that Ti–M–H1 also suppressed inflammation by activating macrophages, while releasing multiple active factors to mediate osteogenesis and angiogenesis. Finally, in vivo results uncovered that Ti–M–H1 facilitated a higher bone-to-implant interface and was more attractive for the dendrites, which promoted osseointegration. In summary, MAO and SHT-treated Ti–M–H1 not only promotes in vitro osteogenesis and angiogenesis but also induces M2 macrophages to regulate the immune environment, which enhances the crosstalk between osteogenesis and angiogenesis and ultimately accelerates the process of osseointegration in vivo.

mTORC2 regulates hierarchical micro/nano topography-induced osteogenic differentiation via promoting cell adhesion and cytoskeletal polymerization

Surface topography acts as an irreplaceable role in the long‐term success of intraosseous implants. In this study, we prepared the hierarchical micro/nano topography using selective laser melting combined with alkali heat treatment (SLM‐AHT) and explored the underlying mechanism of SLM‐AHT surface‐elicited osteogenesis. Our results show that cells cultured on SLM‐AHT surface possess the largest number of mature FAs and exhibit a cytoskeleton reorganization compared with control groups. SLM‐AHT surface could also significantly upregulate the expression of the cell adhesion‐related molecule p‐FAK, the osteogenic differentiation‐related molecules RUNX2 and OCN as well as the mTORC2 signalling pathway key molecule Rictor. Notably, after the knocked‐down of Rictor, there were no longer significant differences in the gene expression levels of the cell adhesion‐related molecules and osteogenic differentiation‐related molecules among the three titanium surfaces, and the cells on SLM‐AHT surface failed to trigger cytoskeleton reorganization. In conclusion, the results suggest that mTORC2 can regulate the hierarchical micro/nano topography‐mediated osteogenesis via cell adhesion and cytoskeletal reorganization.