Fabricating hierarchical micro and nano structures on implantable Co-Cr-Mo alloy for tissue engineering by one-step laser ablation

Femtosecond laser-induced periodic grooves and nanopore clusters make a synergistic effect on osteogenic differentiation

Polyether-ether-ketone (PEEK) materials have good biocompatibility, excellent corrosion resistance, chemical stability and an elastic modulus close to that of natural bone. However, due to its biological inertness, PEEK may affect osteogenic differentiation and leads to osseointegration failure, though PEEK is expected to improve osseointegration. In this work, by changing the power of femtosecond laser, micro-grooves are made on the PEEK surface. As observed by scanning electron microscopy, the trench has a periodic structure, the micro shape is neat, and the trench is also covered with nanometer-level pore clusters. In the in vitro culture experiments, through the proliferation experiment of mouse bone marrow mesenchymalstem cells (mBMSCs), cell viability analysis and alkaline phosphatase activity analysis, it is proven that after femtosecond laser treatment of the PEEK surface, the micro-grooves on the surface and the nanopore clusters due to laser energy ablation can produce a synergistic effect, enhancing the osteogenic differentiation ability of cells, and improving the bone integration ability of PEEK materials.

Laser Direct Writing via Two-Photon Polymerization of 3D Hierarchical Structures with Cells-Antiadhesive Properties

We report the design and fabrication by laser direct writing via two photons polymerization of innovative hierarchical structures with cell-repellency capability. The structures were designed in the shape of “mushrooms”, consisting of an underside (mushroom’s leg) acting as a support structure and a top side (mushroom’s hat) decorated with micro- and nanostructures. A ripple-like pattern was created on top of the mushrooms, over length scales ranging from several µm (microstructured mushroom-like pillars, MMP) to tens of nm (nanostructured mushroom-like pillars, NMP). The MMP and NMP structures were hydrophobic, with contact angles of (127 ± 2)° and (128 ± 4)°, respectively, whereas flat polymer surfaces were hydrophilic, with a contact angle of (43 ± 1)°. The cell attachment on NMP structures was reduced by 55% as compared to the controls, whereas for the MMP, a reduction of only 21% was observed. Moreover, the MMP structures preserved the native spindle-like with phyllopodia cellular shape, whereas the cells from NMP structures showed a round shape and absence of phyllopodia. Overall, the NMP structures were more effective in impeding the cellular attachment and affected the cell shape to a greater extent than the MMP structures. The influence of the wettability on cell adhesion and shape was less important, the cellular behavior being mainly governed by structures’ topography.

Gradient nanostructured titanium stimulates cell responses in vitro and enhances osseointegration in vivo

Background

Though titanium (Ti) is widely used as dental materials in the clinic, effective methods to treat Ti for higher surface biological activity still lack. Through Surface mechanical attrition treatment (SMAT) technology we could endow Ti with gradient nanostructured surface (GNS Ti). To investigate the biocompatibility of GNS Ti for its further application in dental implant field, we study the effects of GNS Ti on cell responses in vitro and osseointegration of the implant with surrounding bone tissues in vivo.

Methods

In this study, GNS Ti was fabricated by SMAT. In vitro experiment, we co-cultured GNS Ti with bone mesenchymal stem cells (BMSCs), surface characterization was detected by transmission electron microscope (TEM). Adhesion, proliferation and differentiation of BMSCs were evaluated by scanning electron microscope (SEM), MTT, flow cytometry (FCM), alkaline phosphatase (ALP) and osteocalcin (OCN) tests. In vivo experiment, the GNS Ti was implanted into the rabbit mandible. Osteogenesis and osseointegration were evaluated by Micro CT, toluidine blue staining, and immunohistochemical staining at 4, 8, and 12 weeks postoperatively.

Results

Both results showed that compared with the coarse grained (CG) Ti, the GNS Ti stimulated the adhesion, proliferation, and differentiation of BMSCs and improved osteogenesis and osseointegration.

Conclusions

This study indicates that gradient nanostructured Ti is a promising material for dental implant application.

Surface Analysis of 3D (SLM) Co–Cr–W Dental Metallic Materials

The surface condition of the materials involved in dentistry is significant for the subsequent operations that are applied in oral cavity. Samples of Co–Cr–W alloy, obtained through selective laser melting (SLM) 3D printing, with different surface states were analyzed. Surface analysis after the 3D printing process and sandblasting was realized from microstructural, chemical composition, profilometry, droplet adhesion, scratch test, and microhardness perspectives. The results presented a hardening process and a roughness modification following the sandblasting procedure, a better adhesion of the liquid droplets, the appearance of micro-cracks during the scratch test, and the oxidation of the sample after the 3D printing process and surface processing.

Cell Type-Specific Adhesion and Migration on Laser-Structured Opaque Surfaces

Cytocompatibility is essential for implant approval. However, initial in vitro screenings mainly include the quantity of adherent immortalized cells and cytotoxicity. Other vital parameters, such as cell migration and an in-depth understanding of the interaction between native tissue cells and implant surfaces, are rarely considered. We investigated different laser-fabricated spike structures using primary and immortalized cell lines of fibroblasts and osteoblasts and included quantification of the cell area, aspect ratio, and focal adhesions. Furthermore, we examined the three-dimensional cell interactions with spike topographies and developed a tailored migration assay for long-term monitoring on opaque materials. While fibroblasts and osteoblasts on small spikes retained their normal morphology, cells on medium and large spikes sank into the structures, affecting the composition of the cytoskeleton and thereby changing cell shape. Up to 14 days, migration appeared stronger on small spikes, probably as a consequence of adequate focal adhesion formation and an intact cytoskeleton, whereas human primary cells revealed differences in comparison to immortalized cell lines. The use of primary cells, analysis of the cell-implant structure interaction as well as cell migration might strengthen the evaluation of cytocompatibility and thereby improve the validity regarding the putative in vivo performance of implant material.

Hierarchical Micro-/Nano-Structures on Polycarbonate via UV Pulsed Laser Processing

Hierarchical micro/-nanostructures were produced on polycarbonate polymer surfaces by employing a two-step UV-laser processing strategy based on the combination of Direct Laser Interference Patterning (DLIP) of gratings and pillars on the microscale (3 ns, 266 nm, 2 kHz) and subsequently superimposing Laser-induced Periodic Surface Structures (LIPSS; 7-10 ps, 350 nm, 100 kHz) which adds nanoscale surface features. Particular emphasis was laid on the influence of the direction of the laser beam polarization on the morphology of resulting hierarchical surfaces. Scanning electron and atomic force microscopy methods were used for the characterization of the hybrid surface structures. Finite-difference time-domain (FDTD) calculations of the laser intensity distribution on the DLIP structures allowed to address the specific polarization dependence of the LIPSS formation observed in the second processing step. Complementary chemical analyzes by micro-Raman spectroscopy and attenuated total reflection Fourier-transform infrared spectroscopy provided in-depth information on the chemical and structural material modifications and material degradation imposed by the laser processing. It was found that when the linear laser polarization was set perpendicular to the DLIP ridges, LIPSS could be formed on top of various DLIP structures. FDTD calculations showed enhanced optical intensity at the topographic maxima, which can explain the dependency of the morphology of LIPSS on the polarization with respect to the orientation of the DLIP structures. It was also found that the degradation of the polymer was enhanced for increasing accumulated fluence levels.

Friction stability and cellular behaviors on laser textured Ti-6Al-4V alloy implants with bioinspired micro-overlapping structures

The grain structure and surface morphology of bio-implants act as a pivotal part in altering cell behavior. Titanium alloy bone screws, as common implants, are prone to screws loosening and complications threat in the physiological environment due to their inferior anti-wear and surface inertia. Manufacturing bone screws with high wear resistance and ideal biocompatibility has always been a challenge. In this study, a series of overlapping morphologies inspired by the hierarchical structure of fish scales and micro bulges of shrimp were structured on Ti-6Al-4V implant by laser texturing. The results indicate that the textured patterns could improve cell attachment, proliferation, and osteogenic differentiation. The short-term response of human bone marrow-derived mesenchymal stem cells (hBMSCs) on the textured surface are more sensitive to the microstructure than the surface roughness, wettability, grain size and surface chemical elements of the textured surfaces. More importantly, the friction-increasing and friction-reducing type overlapping structures exhibit excellent friction stability at different stages of modified simulated body fluid (m-SBF) soaking. The overlapping structure (Micro-smooth stacked ring: MSSR) is more beneficial to promote the formation of apatite. Deposited spherical-like apatite particles can act as a "lubricant" on the MSSR surface during the friction process to alleviate the adhesion wear of the surface. Meanwhile, apatite particles participate in the formation of friction film, which plays an effective role in reducing friction and antiwear in corrosion solution (m-SBF) for a long time. These features show that the combination of soaking treatment in m-SBF solution with laser-textured MSSR structure is expected to be an efficient and environmentally friendly strategy to prolong the service life of bone screws and reducing the complications of mildly osteoporotic implants.

Laser Polishing of Ti6Al4V Fabricated by Selective Laser Melting

Selective laser melting (SLM) is emerging as a promising 3D printing method for orthopedic and dental applications. However, SLM-based Ti6Al4V components frequently exhibit high roughness values and partial surface defects. Laser polishing (LP) is a newly developed technology to improve the surface quality of metals. In this research, LP is applied to improve the surface finish of components. The results show that the laser beam can neatly ablate the aggregates of metallic globules and repair cracks and pores on the surface, resulting in a smooth surface with nanocomposites. Overall, the results indicate that using LP optimizes surface morphology to favor fatigue behavior and osteoblastic differentiation. These findings provide foundational data to improve the surface roughness of a laser-polished implant and pave the way for optimized mechanical behavior and biocompatibility via the laser process.

Polydopamine-Assisted Immobilization of Chitosan Brushes on a Textured CoCrMo Alloy to Improve its Tribology and Biocompatibility

Due to their bioinert and reliable tribological performance, cobalt chromium molybdenum (CoCrMo) alloys have been widely used for articular joint implant applications. However, friction and wear issues are still the main reasons for the failure of implants. As a result, the improvement of the tribological properties and biocompatibility of these alloys is still needed. Thus, surface modification is of great interest for implant manufacturers and for clinical applications. In this study, a strategy combining laser surface texturing and chitosan grafting (mussel inspired) was used to improve the tribological and biocompatible behaviors of CoCrMo. The microstructure and chemical composition were investigated by atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. The tribological properties were discussed to determine their synergistic effects. To evaluate their biocompatibility, osteoblast cells were cocultured with the modified surface. The results show that there is a distinct synergistic effect between laser surface texturing and polymer brushes for improving tribological behaviors and biocompatibility. The prepared chitosan brushes on a textured surface are a strong mechanism for reducing friction force. The dimples took part in the hydrodynamic lubrication and acted as the container for replenishing the consumed lubricants. These brushes also promote the formation of a local lubricating film. The wear resistance of the chitosan brushes was immensely improved. Further, the worn process was observed, and the mechanism of destruction was demonstrated. Co-culturing with osteoblast cells showed that the texture and grafting have potential applications in enhancing the differentiation and orientation of osteoblast cells.

Fabricating Laser-Induced Periodic Surface Structures on Medical Grade Cobalt–Chrome–Molybdenum: Tribological, Wetting and Leaching Properties

Hip-implants structured with anti-bacterial textures should show a low-friction coefficient and should not leach hazardous substances into the human body. The surface of a typical material used for hip-implants, namely Cobalt–Chrome–Molybdenum (CoCrMo) was textured with different types of laser-induced periodic surface structures (LIPSS)—i.e., low spatial frequency LIPSS (LSFL), hierarchical structures consisting of grooves superimposed with high spatial frequency LIPSS (HSFL) and Triangular shaped Nanopillars (TNP)—using a picosecond pulsed laser source. The effect of LIPSS on the wettability, friction, as well as wear of the structures, when slid against a polyethylene (PE) counter surface and biocompatibility was analyzed. Surfaces covered with LSFL show superhydrophobicity and grooves with superimposed HSFL, as well as TNP, show hydrophobic behavior. The coefficient of friction (CoF) of LIPSS against a polyethylene (PE) counter surface was found to be higher (ranging from 0.40 to 0.66) than the CoF of (polished) CoCrMo, which was found to equal 0.22. It was found that the samples release cobalt within biocompatible limits. Compared to polished reference surfaces, LIPSS cause higher friction of CoCrMo against PE contact. However, the wear of the PE counter surface only increased significantly for the LSFL textures. For these reasons, it is concluded that LIPSS are not suitable for a heavily loaded metal-on-plastic bearing contact.