Electrophoretic deposition of double-layer HA/Al composite coating on NiTi

Characterization and tribological behaviour of Indian clam seashell-derived hydroxyapatite coating applied on titanium alloy by plasma spray technique

Various hydroxyapatite (HA) powders synthesized at different temperatures are deposited on titanium alloy by using an atmospheric plasma spray process. These different HA powders were synthesized from Indian clam seashells through the hydrothermal technique at varying temperatures from 700 to 1000 °C for a 2 h time duration in our previous study. The synthesized HA powders are spray-dried to obtain agglomerated powders suitable for spraying during the coating application. Crystallite size, Ca/P ratio, and crystallinity of agglomerated HA powders and their respective coatings are estimated by standard methods. The microstructure and phases of the feedstock and coating materials are investigated by using a field-emission scanning electron microscope (FESEM) and X-ray diffractometer (XRD), respectively. Further, the HA coatings are characterized in terms of surface roughness, microhardness, porosity, adhesion strength, and wear resistance through the stylus profilometer, Vickers micro-hardness tester, image analysis technique, scratch tester, and ball-on-disc tribometer, respectively. The average surface roughness (R_a) and porosity of the coating are decreased with an increase in the synthesis temperature. The minimum R_a and porosity obtained for the 1000 °C coating sample suggest a high degree of melting of such powder particles. However, the highest adhesion strength noticed in the case of the 900 °C coating sample is due to the high compatibility of such coating material with Ti-alloy substrate in terms of thermal properties. The 900 °C coating sample has also shown the highest microhardness and wear-resistance properties due to its maximum crystallinity among all the HA coatings.

Structural characterization, mechanical, and electrochemical studies of hydroxyapatite-titanium composite coating fabricated using electrophoretic deposition and reaction bonding process

In the present work, hydroxyapatite (HA)-titanium (Ti, 20 wt%) composite coating was coated on NiTi alloy substrate by EPD (electrophoretic deposition) process. Before applying the coating, the HA powder was composed with Ti powder using a ball milling process. Influence of the ball milling time on morphology and phase structure of HA-Ti powder was evaluated using TEM and XRD analysis. After composing the HA particles with Ti, the HA-Ti composite powders were coated on the NiTi substrate by the EPD process in an n-butanol medium for 2 min, with the applied voltage of 60 V. XRD and SEM analysis were utilized to evaluate the phase analysis and morphology of the coatings. Mechanical and electrochemical characteristic of the coatings were also assessed using the micro-indentation, micro-scratch, and polarization tests, respectively. The results revealed that the milling process time had a significant influence on reaction bonds and optimum mixing time was 4 hr. Micro-hardness of the HA-Ti composite coating (304 HV) was substantially higher than the HA coating (72 HV). Also, as the HA coating was composed with Ti particles, the amount of force (in the micro-scratch test) required for detaching the coating from the NiTi substrate increased from 7.1 to 17.8 N. The polarization results showed that the HA-Ti composite coating had a higher electrochemical resistance compared with the HA coating. Corrosion resistance of the NiTi alloy coated with HA increased from 133 kΩ.cm² to 2,720 kΩ.cm² after composed with the Ti particles.

Effect of Ta2O5 content on the osseointegration and cytotoxicity behaviors in hydroxyapatite-Ta2O5 coatings applied by EPD on superelastic NiTi alloys

In the present study, the different contents of tantalum pentoxide (Ta₂O₅: 10, 15, 20 and 30 wt%) nanoparticles were introduced into the natural hydroxyapatite (nHA) coating structure on NiTi substrate through electrophoretic deposition (EPD) method. The phase compositions of coatings were perused before and after the sintering at 800 °C for 1 h by XRD. The incorporation of 30wt%Ta₂O₅ into nHA matrix induced the formation of undesirable soluble Ca₃(PO₄)₂ phase in composite coating. The FESEM images showed that the density of continuous nHA coating increased by compositing with Ta₂O₅. The maximum adhesion strength of 28.3 ± 0.7 MPa accomplished from the nHA-20 wt%Ta₂O₅ composite coating. The Ni ions concentration measurement results from the passivated-NiTi with nHA and nHA-(10, 15 and 20)wt%Ta₂O₅ coatings during 30 days of immersion in PBS clarified the positive role of Ta₂O₅ in decreasing the Ni leaching due to the lowering the open porosities of nHA structure. The biological response of the coating surfaces was assessed in vitro by cell culturing and MTS assay. By considering the morphology and density of adsorbed cells on each coating, the improved biocompatibility of nHA coating in the presence of Ta₂O₅ was justified by scrutinizing the surface roughness, wettability and charge. The highest cell attachment and proliferation on nHA-20 wt%Ta₂O₅ coating was related to owning the lowest roughness, wetting angle of 34^o ± 0.5 and the highest negative surface charge density. Also, the concentration of the highest negative charge density on nHA-20 wt%Ta₂O₅ coating surface in the SBF solution caused to the enhancement of the amount of the apatite nuclei through providing more sites to calcium absorption.

Synthesis of hydroxyapatite for biomedical applications

The current need for long lasting implants and bone substitutes characterized by biocompatibility, bioactivity and mechanical properties, without the immune rejection is a great challenge for scientists. These bone substitute structures should be prepared for individual patients with all details controlled on the micrometer level. Similarly, nontoxic, biocompatible targeted drug delivery systems which allow controlling the rate and time period of the drug delivery and simultaneously eliminating toxic and side effects on the healthy tissues, are of great interest. Extensive attempts have been made to develop a simple, efficient, and green method to form biofunctional scaffolds and implant coatings possessing the above mentioned significant biocompatibility, bioactivity and mechanical strength. Moreover, that could also serve as drug delivery systems. Hydroxyapatite (HA) which is a major mineral component of vertebrate bones and teeth is an excellent material for these purposes. In this literature review the biologically inspired scaffolds, bone substitutes, implants characterized by mechanical strength and biocompatibility, as well the drug delivery systems, based on hydroxyapatite are discussed.