N. Cinca

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.

Behavior of NiTi Wires for Dampers and Actuators in Extreme Conditions

Shape memory alloys are considered smart materials because of their singular thermo-mechanical properties, due to a thermoelastic martensitic transformation, enabling possible uses as actuators (because of mechanical recovery induced from temperature changes) and as dampers (because of hysteresis). NiTi wires for dampers in Civil Engineering had been characterized and tested in facilities. Guaranteed performance needs to know behavior during fatigue life and knowledge of effects in the event of extreme conditions, as eventual overstraining. In this work, we check the possibilities to absorb mechanical energy on the fatigue life depending on stress level and explore the consequences of overstraining the material during installation, the possibilities of partial healing by moderate heating, and some effects of over-stressing the wires. The mechanical energy absorbed by the unit weight of damper wire might be very high during its lifetime if maximum stresses remain relatively low allowing high fatigue life. We show also some results on NiTi wire working as an actuator. The lifetime mechanical work performed by an actuator wire can be very high if applied stresses are limited. The overstraining produces relevant “residual” deformation, which can be to some extent reversed by moderate heating at zero stress. The reason for the observed characteristics seems to be that when external high stresses are applied to an NiTi wire, it undergoes some plastic deformation, leaving a distribution of internal stresses that alter the shape and position of the macroscopic stress-strain transformation path.

Structure characterization and wear performance of NiTi thermal sprayed coatings

NiTi shape memory alloy (SMA) has been studied for many years for its shape memory and pseudoelastic properties, as well as its biocompatibility, which make it suitable for many biomedical applications. However, SMA NiTi is also interesting for relevant wear resistance near the transition temperature which, along with its high oxidation and corrosion resistance, suggests its use as a coating to increase the lifetime of some components. Also, whereas bulk material properties have been characterized in respect of the nominal composition, manufacturing methods and thermo-mechanical treatments, NiTi overlays have been investigated much less. Most existent works in this field specifically deal with magnetron sputtering technology for thin films and its use in micro-devices (micro-electro-mechanical systems, MEMS), just some works refer to vacuum plasma spraying (VPS) for thicker coatings. The present paper explores and compares the microstructure and wear-related properties of coatings obtained from atomized NiTi powders, by VPS as well as by atmospheric plasma spraying (APS) and high velocity oxygen fuel (HVOF) techniques. In the present case, the wear behaviour of the NiTi deposits has been studied by rubber-wheel equipment and ball-on-disk tests. The results obtained at room temperature show that the APS-quenched coatings exhibit a preferential dry sliding wear mechanism, while the VPS and HVOF coatings show an abrasive mechanism.

Improved bonding strength of bioactive cermet Cold Gas Spray coatings

The fabrication of cermet biocompatible coatings by means Cold Gas Spray (CGS) provides prosthesis with outstanding mechanical properties and the required composition for enhancing the bioactivity of prosthetic materials. In this study, hydroxyapatite/Titanium coatings were deposited by means of CGS technology onto titanium alloy substrates with the aim of building-up well-bonded homogeneous coatings. Powders were blended in different percentages and sprayed; as long as the amount of hydroxyapatite in the feedstock increased, the quality of the coating was reduced. Besides, the relation between the particle size distribution of ceramic and metallic particles is of significant consideration. Plastic deformation of titanium particles at the impact eased the anchoring of hard hydroxyapatite particles present at the top surface of the coating, which assures the looked-for interaction with the cells. Coatings were immersed in Hanks solution for 1, 4 and 7 days; bonding strength value was above 60 MPa even after 7 days, which enhances common results of HAp coatings obtained by conventional thermal spray technologies.

Cold Gas Sprayed Stellite-6 Coatings and their Wear Resistance

Here we report on the successful deposition of stellite-6 coatings by Cold Gas Spray (CGS) technology and their tribological properties in correlation with the as-sprayed structure.The deposits were dense and showed satisfactory interparticle bonding. The deformation mechanisms involved within the process are evaluated with regard to the particle shape and dendritic deformation at the boundaries and also internal structural defects. Despite the lower hardness of CGS coatings compared to HVOF ones, they displayed relatively good wear rates under both abrasive and sliding conditions. The wear mechanisms are discussed mainly in terms of local oxidation and possible martensitic phase transformation of stellite.

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.

Structural Characterization of Intermetallic NiTi Coatings Obtained by Thermal Spray Technologies

NiTi alloy is known to have a potential interest specially for its superelasticity and shape memory effect (SME). Most of the research undergone up to now investigates such properties in bulk materials. The few works related to overlays (coatings) use magnetron sputtering for thin films and Vacuum Plasma Spray (VPS) for thicker coatings. VPS is the mostly used thermal spray technique regarding the Ti oxidation problems which considerably affect the so desired SME. The present work compares the microstructure of VPS NiTi coatings with those of High Velocity Oxygen Fuel HVOF and a liquid cooling device coupled to a Atmosphere Plasma Spraying (APS+Q), with optimized spraying conditions trying to minimize Ti oxidation. Apart from their characterization by Scanning Electron Microscopy, the main purpose is to further study how particles are retained within the coating so that Transmission Electron Microscopy is also employed. This has allowed the observation of amorphous and nanocrystalline phases as well as to the change from cubic B2 NiTi lattice to metastable lattices.

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.