Effect of Surface Chemistry on Hemolysis, Thrombogenicity, and Toxicity of Carbon Nanotube Doped Thermally Sprayed Hydroxyapatite Implants.

Assessing blood compatibility is crucial before in vivo procedures and is considered more reliable than many in vitro tests. This study examines the physiochemical properties and blood compatibility of bioactive powders ((0.5-2 wt % carbon nanotube (CNT)/alumina)-20 wt %)) produced through a heterocoagulation colloidal technique followed by ball milling with hydroxyapatite (HAp). The 1 wt % CNT composite demonstrated a surface charge ∼5 times higher than HAp at pH 7.4, with a value of -11 mV compared to -2 mV. This increase in electrostatic charge is desirable for achieving hemocompatibility, as evidenced by a range of blood compatibility assessments, including hemolysis, blood clotting, platelet adhesion, platelet activation, and coagulation assays (prothrombin time (PT) and activated partial thrombin time (aPTT)). The 1 wt % CNT composite exhibited hemolysis ranging from 2 to 7%, indicating its hemocompatibility. In the blood clot investigation, the absorbance values for 1-2 wt % CNT samples were 0.927 ± 0.038 and 1.184 ± 0.128, respectively, indicating their nonthrombogenicity. Additionally, the percentage of platelet adhered on the 1 wt % CNT sample (∼5.67%) showed a ∼2.5-fold decrement compared to the clinically used negative control, polypropylene (∼13.73%). The PT and aPTT experiments showed no difference in the coagulation time for CNT samples even at higher concentrations, unlike HAC2 (80 mg). In conclusion, the 1 wt % CNT sample was nontoxic to human blood, making it more hemocompatible, nonhemolytic, and nonthrombogenic than other samples. This reliable study reduces the need for additional in vitro and in vivo studies before clinical trials, saving time and cost.

Thermal spray processes influencing surface chemistry and in-vitro hemocompatibility of hydroxyapatite-based orthopedic implants.

Orthopedic implants made from titanium are a popular choice in the medical field because of their remarkable strength-to-weight ratio. Nevertheless, they may not interact well with human blood, resulting in thrombosis and hemolysis. In fact, non-hemocompatibility is believed to be responsible for about 31 % of medical device failures in the US alone, requiring painful and expensive revision surgery. To address this issue, bioactive hydroxyapatite coatings are applied to Ti-6Al-4V implants using thermal spray techniques. However, the temperature used during thermal processing impacts the coating's surface properties, affecting the mechanical and biological properties. Furthermore, the effectiveness of HA coatings on titanium for orthopedic applications has not been validated by biocompatibility tests, particularly hemocompatibility. In this study, we aimed to investigate the relative efficacy of three thermal spray processes of different temperature ranges: Atmospheric plasma spray (APS) (high temperature), Flame spray (FS) (moderate temperature), and High-Velocity Oxy-Fuel spray (HVOF) (low temperature), and study their impact on coating's surface properties, affecting blood components and implant's strength. The crystallinity of the HA coating increased by 32 % with a decrease in the operating temperature (APS < FS < HVOF). HVOF coating exhibited a ~ 34 % and ~ 120 % improvement in adhesion strength and ~ 31 % and 59 % increment in hardness compared to APS and FS coating, respectively, attributed to its low porosity, low coating thickness (~55 µm), and high degree of crystallinity. The HVOF coating showcased a significant increase in non-hemolytic behavior, with hemolysis rates ~8 and ~ 11 times lower than APS and FS coatings, respectively, owing to its smooth texture and high degree of crystallinity (p < 0.05). Furthermore, the HVOF coating exhibited minimal blood clotting based on the whole blood clotting assay, again confirmed by PT and aPTT assays showing delayed clotting time, indicating its non-thrombogenic behavior. The number of platelets adhered to the three coatings showed no significant difference compared to Ti-6Al-4V. APS and FS coatings showed low platelet activation, unlike HVOF coating and titanium, which revealed round platelets, similar to the negative control. Neither titanium nor HA coatings exhibited antibacterial properties, which may be due to their high affinity for organic substances, which promotes bacterial adhesion and replication. Among the three thermal processes, HVOF coating displayed good apatite growth, non-hemolytic, and non-thrombogenicity with no platelet activation owing to its low processing temperature, high degree of crystallinity (89.7 %), hydrophilicity, smooth (~4 µm) and dense (~97 %) microstructural properties. The results demonstrated that the HVOF-HA coating presented in this work meets the hemocompatible requirements and shows promise for prospective application as an orthopedic implant. Furthermore, this study has the potential to significantly reduce the use of animals in in-vivo research and improve their welfare while also cutting costs.

An Investigation into the Relative Efficacy of High-Velocity Air-Fuel-Sprayed Hydroxyapatite Implants Based on the Crystallinity Index, Residual Stress, Wear, and In-Flight Powder Particle Behavior.

Due to its resemblance to the bone, hydroxyapatite (HA) has been widely used for bioactive surface modification of orthopedic implants. However, it undergoes significant thermal decomposition and phase transformations at a high operating temperature, leading to premature implant failure. This investigation uses high-velocity air-fuel (HVAF) spray, an emerging low-temperature thermal spray technique, to deposit HA over the Ti-6Al-4V substrate. Coating characteristics, such as the crystallinity index and phase analysis, were measured using X-ray diffraction, Raman analysis, and Fourier transform infrared spectroscopy, residual stress using the sin2ψ method, and tribological performance by a fretting wear test. The coating retained an over 90% crystallinity index, a crystallite size of 41.04 nm, a compressive residual stress of -229 ± 34.5 MPa, and a wear rate of 1.532 × 10-3 mm3 N-1 m-1. Computational in-flight particle traits of HA particles (5 to 60 µm) were analyzed using computational fluid dynamics; it showed that 90% of particles were deposited at a 700 to 1000 m/s velocity and a 900 to 1450 K temperature with a 2.1 ms mean residence time. In-flight particle oxidation was minimized, and particle impact deformation was maximized, which caused severe plastic deformation, forming crystalline, compressive residual stressed coatings. The thermal decomposition model of low-temperature HVAF-sprayed HA particles helped to understand the implants' crystallinity index, residual stress, and tribological characteristics. Hence, this experimental and computational analysis shows that the HVAF process can be a promising candidate for biomedical applications for having strong and durable implants.

Improvement in Slurry Erosion and Corrosion Resistance of Plasma-Sprayed Fly Ash Coatings for Marine Applications.

Fly ash (FA), a multicompound mineral, is an industrial waste produced during coal burning in thermal power stations. It has been regarded as the most environmentally hazardous material. Furthermore, handling FA has been a significant challenge for many developing countries. Therefore, researchers have been exhorted to enhance its usage to counter its handling issues. FA is enriched with mullite, silica, and alumina. Having such mineralogy, FA can be envisaged as a promising candidate for combating erosion and corrosion in marine environments. With this motivation, the research aims to deposit as-received FA using the plasma-spraying technique onto a marine-grade steel substrate without additives and assess the performance of such coatings for erosion and corrosion properties. The coating has exhibited more than 100% improvement in microhardness. The erosion resistance was improved by ∼11% compared to that of the uncoated sample, which is attributed to the hardness to elastic modulus ratio (H/E) and its unique mineralogy. The minor improvement in erosion resistance was attributed to the coating's poor fracture toughness. The erosion study shows that slurry concentration and rotational speeds were the most influential parameters. The scar depth was significantly shallower for FA-coated samples. The corrosion resistance has improved only by ∼13.49%, owing to the porous nature of the coating. Therefore, such coatings with appropriate improvements in their properties are expected to assuage both environmental and industrial challenges.

Bioactive surface modifications through thermally sprayed hydroxyapatite composite coatings: a review of selective reinforcements.

Hydroxyapatite (HA) has been an excellent replacement for the natural bone in orthopedic applications owing to its close resemblance to the bone properties; however, it is brittle and has low strength. Surface modification techniques have been able to allay such mineral issues by depositing on substrate. These methods, being economical, impart mechanical strength without compromising biocompatibility. In this review article, the discussion is confined to plasma spray (high temperature) and other low temperature surface modification techniques: high-velocity oxy-fuel (HVOF) and cold spray. The processing temperature seems to significantly affect the performance of implants deposited with HA. Monolithic HA may not add enough strength to the bioimplants. Hence, this review discusses selective reinforcements to HA and their roles in enhancing the properties. Herein, a variety of selective reinforcements are discussed, such as carbon allotropes: graphene, carbon nanotubes, and nano diamond; metallic materials: Ag, Sr, Mg, and Ti; ceramic materials: Al2O3, SiO2, ZrO2, and TiO2; multi-materials: Al2O3-CNT/HA, Al2O3-TiO2/HA and others; and functionally graded composites: HA, 20 and 50 wt% Ti-6Al-4V/HA layered coating. Most of these reinforcements could not trade-off between biocompatibility and strength. The detailed in vitro and in vivo studies are still lacking. The literature on the relative effectiveness of these reinforcements is scanty, while the interface between HA coating and reinforcements is seldom explored. This review presents the suitability of thermal spray techniques based on the microstructure, mechanical, and biological properties. Therefore, it is envisaged that the present review can intrigue future researchers to understand the scope of surface coatings in achieving the better performance of implants at clinical trials.