Anna M. Vilardell

Cold spray as an emerging technology for biocompatible and antibacterial coatings: state of art

The use of coatings in biomaterials has been fundamental on the applicability of many medical devices and has helped improve mechanical properties such as wear and fatigue and biological properties such as biocompatibility and bioactivity of implant prosthesis, thus, in essence, ameliorating human quality life. The aim of the present paper is to give a review on cold spray (CS) coating systems that are emerging in orthopedics industry (internal fixation systems and prosthesis) as well as those for antibacterial purposes (in body and touch external surfaces). These studies are very new, the oldest dating from the half of last decade and most deal with the improvement of biocompatibility and bioactivity of hard tissue replacement; therefore, research on biocoatings is in constant development with the aim to produce implant surfaces that provide a balance between cell adhesion and low cytotoxicity, mechanical properties, and functionalization. CS offers many advantages over conventional high-temperature processes and seems to be able to become competitive in front of the low-temperature techniques. It is mainly cost effective, appropriate for oxygen-sensitive materials, and environmentally green. It basically involves the use of feedstock material in powder form, which is supersonically sprayed onto the appropriate substrate but without any melting as it occurs in conventional thermal spray processes. Biocompatible metallic materials and polymers have been successfully deposited by this method because it is based on the plasticity of the coating material; pure ceramic deposits, for example of hydroxyapatite, are still a challenge.

Hierarchical structures of anodised cold gas sprayed titanium coatings

ABSTRACT Cold gas spray (CGS) titanium coatings have been produced to obtain porous and rough coatings with enhanced mechanical performance. The coatings from optimal spraying conditions reached tensile strength values up to 40 MPa, shear strength up to 39 MPa and a loss mass of 37 mg/100 cycles in abrasive testing, values in accordance with the ASTM standards to be applied for orthopaedic joint prostheses. An innovative hierarchical structure (micro-nano) consisted of a TiO2 nanotubes top layer obtained by anodisation onto a CGS Ti coating. The present paper focuses on the characterisation of both surfaces, as-sprayed CGS Ti layer and double Ti-TiO2 layer, in terms of mechanical properties, surface topography and wettability (contact angle). There were not significant changes in micro-roughness, Ra∼40 µm and Ra∼30 µm, but a significant decrease in contact angle, from ≈26° up to 0°, was observed between these two structures. This behaviour indicates that the combination of the CGS + anodising results in promising high roughness superhydrophilic surfaces, ideal for biomedical applications.

Osteoblastic cell response on high-rough titanium coatings by cold spray

AbstractHighly rough and porous commercially pure titanium coatings have been directly produced for first time by the cold spray technology, which is a promising technology in front of the vacuum plasma spray for oxygen sensitive materials. The wettability properties as well as the biocompatibility evaluation have been compared to a simply sand blasted Ti6Al4V alloy substrate. Surface topographies were analysed using confocal microscopy. Next, osteoblast morphology (Phalloidin staining), proliferation (MTS assay), and differentiation (alkaline phosphatase activity) were examined along 1, 7 and 14 days of cell culture on the different surfaces. Finally, mineralization by alizarin red staining was quantified at 28 days of cell culture. The contact angle values showed an increased hydrophilic behaviour on the as-sprayed surface with a good correlation to the biological response. A higher cell viability, proliferation and differentiation were obtained for highly rough commercial pure titanium coatings in comparison with sand blasted substrates. Cell morphology was similar in all coatings tested; at 14 days both samples showed extended filopodia. A higher amount of calcium-rich deposits was detected on highly rough surfaces. In summary, in-vitro results showed an increase of biological properties when surface roughness increases.

Functionalized coatings by cold spray: An in vitro study of micro- and nanocrystalline hydroxyapatite compared to porous titanium

Three different surface treatments on a Ti6Al4V alloy have been in vitro tested for possible application in cementless joint prosthesis. All of them involve the novelty of using the Cold Spray technology for their deposition: (i) an as-sprayed highly rough titanium and, followed by the deposition of a thin hydroxyapatite layer with (ii) microcrystalline or (iii) nanocrystalline structure. Primary human osteoblasts were extracted from knee and seeded onto the three different surfaces. Cell viability was tested by MTS and LIVE/DEAD assays, cell differentiation by alkaline phosphatase (ALP) quantification and cell morphology by Phalloidin staining. All tests were carried out at 1, 7 and 14 days of cell culture. Different cell morphologies between titanium and hydroxyapatite surfaces were exhibited. At 1 day of cell culture, cells on the titanium coating were spread and flattened, expanding the filopodia actin filaments in all directions, while cells on the hydroxyapatite coatings showed round like-shape morphology due to slower attachment. Higher cell viability was detected at all times of cell culture on titanium coating due to a better attachment at 1 day. However, from 7 days of cell culture, cells on hydroxyapatite showed good attachment onto surfaces and highly increased their proliferation, mostly on nanocrystalline, achieving similar cell viability levels than titanium coatings. ALP levels were significantly higher in titanium, in part, because of greatest cell number. Overall, the best cell functional results were obtained on titanium coatings whereas microcrystalline hydroxyapatite presented the worst cellular parameters. However, results indicate that nanocrystalline hydroxyapatite coatings may achieve promising results for the faster cell proliferation once cells are attached on the surface.

Cold Spray Coatings for Biomedical Applications

Cold spraying (CS) offers many advantages in front of the conventional thermal spraying processes and is becoming competitive in several industrial sectors. The biomedical industry is a quite well-established field, but still there are many challenges to solve where improvements in surface engineering can play a great role. The use of coatings in biomaterials has been fundamental on the improvement of mechanical as well as biological properties, thus, ameliorating human quality life. Studies about cold-sprayed coatings are emerging in orthopedics industry (internal fixation systems and prosthesis) as well as for antibacterial purposes (in body and touch external surfaces). These works are very new, and most deal with the improvement of biocompatibility and bioactivity of hard tissue replacement. Several combinations of substrate and coating materials are attempted, even trying to overcome any limitation on the spraying of ductile materials; biocompatible metallic materials, bioactive ceramics and polymers, and combinations have been successfully deposited by this method. Therefore, research on biocoatings is in constant development with the aim to produce implant surfaces that provide a balance between cell adhesion and low cytotoxicity, mechanical properties, and functionalization.

In-vitro study of hierarchical structures: Anodic oxidation and alkaline treatments onto highly rough titanium cold gas spray coatings for biomedical applications

Hierarchical structures were obtained applying two different nanotexturing surface treatments onto highly rough commercial pure titanium coatings by cold spray: (i) anodic oxidation and (ii) alkaline treatments. An extended surface characterization in terms of topography, composition, and wettability has been performed to understand how those parameters affect to cell response. Primary human osteoblasts extracted from knee were seeded onto the as-sprayed titanium surface before and after the nanotexturing treatments. Cell viability was tested by using MTS and LIVE/DEAD assays, as well as osteoblasts differentiation by alkaline phosphatase (ALP) quantification at 3 and 10 days of cell culture. The combination of micro-/nano-roughness results in a significantly increase of cell proliferation, as well as cell differentiation after 10 days of cell culture in comparison with the non-treated coatings.