In vitro investigation of NbTa alloy coating deposited on CoCr alloy for biomedical implants

Surface & Coatings Technology 377 (2019) 124932

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In vitro investigation of NbeTa alloy coating deposited on CoCr alloy for biomedical implants

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Balraj Singha, Gurpreet Singha, , Buta Singh Sidhub a b

Department of Mechanical Engineering, Punjabi University, Patiala, Punjab 147002, India MRS Punjab Technical University, Bathinda, Punjab 151001, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Alloy coating Biomaterials Cobalt-chromium Niobium Plasma spray Tantalum

Cobalt-chromium (CoCr) alloys are widely used as implant materials but their corrosion in physiological conditions can trigger severe immunological response leading to inflammatory and allergic reactions. The present study was aimed at the development, characterization and in vitro evaluation of niobium-tantalum (NbeTa) alloy coating on CoCr alloy. Alloy coating with three different compositions was prepared by varying weight proportion of Nb and Ta (70:30, 50:50 and 30:70) through the plasma spray technique. Pure Nb and Ta coatings were also deposited for comparative analysis. Nb and Ta were recognized as the primary phases in respective coatings. Microcracks were observed in case of pure coatings, while alloy coatings displayed crack-free morphology. The coatings demonstrated microroughned surfaces and hydrophobic properties. Microhardness of the CoCr alloy was notably enhanced with the deposition of coatings. Corrosion investigation revealed that the coated surfaces had higher protection ability than the uncoated CoCr and alloy coatings imparted better corrosion resistance than pure coatings. The alloy coatings were more conducive to cell proliferation and showed superior hemocompatibility. The findings of this study indicate that the surface modification with NbeTa alloy coatings is a promising approach to improve the performance of CoCr alloys for biomedical applications.

1. Introduction Medical implants are commonly manufactured using metallic biomaterials, such as CoCr alloys, stainless steel and titanium and its alloys [1]. Amongst these materials, CoCr alloys are increasingly used for orthopedic implants, primarily in total joint replacements (TJR), due to their high stiffness and excellent wear-resistance in vivo [2]. However, concerns have been raised about the doubtful biologic effects of the corrosion products that emerge from the degradation of CoCr alloys in a physiological environment. Reportedly, the ions (Co2+, Cr3+ and Cr6+) can trigger adverse health issues like cell apoptosis, hypersensitivity, allergic dermatitis, necrosis and genotoxicity [3,4]. In spite of the active research in the field of biomedical implantology, the osteomyelitis (infection) rate is still high (0.5–6%) after TJR as about 12,000 such cases are reported annually [5,6]. The long-term (> 15 years) survival rates of TJR are < 50% [7]. In the next 20 years, the procedures of total knee replacement and total hip replacement in the USA will increase by 401% and 284%, respectively [8]. Therefore, there is a constant need for research and development in the improvement of longevity and biocompatibility of implants. The performance of implantable CoCr alloy can be enhanced with

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various methods; one of which involves coating deposition onto the surface [9,10]. In this regards, a bioactive metallic coating is a potentially important strategy to improve the functional properties of biomedical implants that has been gaining more attention. Amongst metallic biomaterials, Nb and Ta are two bioactive transition metals which have very high corrosion protection ability against aggressive environments [11]. However, their widespread use in biomedical implantology to fabricate a whole-Nb/Ta implant is limited due to certain shortcomings: firstly, the mechanical weakness of Nb and Ta in pure form is the major obstruction to the fabrication of implants in their pure/bulk form [12,13]; secondly, the expensive raw material for these metals contributes to relatively higher manufacturing cost [14,15]; thirdly, the processing of pure/bulk-Ta for the fabrication of a modular implant is very difficult due to its extremely high melting temperature [16]. Fathi et al. [12] suggested that Nb and Ta can be used as a coating on a substrate with suitable mechanical properties to utilize their interesting properties for biomedical implant applications. Separate studies have evaluated the performance of Nb- and Ta-based coatings for implant applications [17–22]. Various researchers have also investigated the combination of Nb with other constituents such as zirconium, strontium, copper, zinc, bioglass, fluorapatite and

Corresponding author. E-mail address: [email protected] (G. Singh).

https://doi.org/10.1016/j.surfcoat.2019.124932 Received 20 June 2019; Received in revised form 7 August 2019; Accepted 24 August 2019 Available online 26 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

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hydroxyapatite etc. to further improve the corrosion protection ability and the biologic response of the coating [23–28]. Similarly, Ta has been combined as a coating component with zirconium, magnesium, titanium, silver and hydroxyapatite etc. for biomedical applications [13,29–32]. In most of the studies related to Nb- and Ta-based coatings, the coatings were prepared via electrodeposition, sol-gel method and sputtering techniques. It is worth mentioning that plasma spraying is the only Food and Drug Administration (FDA) approved method for biomedical coating [33]. Moreover, the plasma spray technique is commercially well-proven and has several advantages over other coating methods such as ease of operation, higher deposition efficiency, strong adhesion and significantly higher coating thickness [34,35]. Although various researchers have studied pure coatings of Nb and Ta for biomedical applications, the combination of Nb and Ta together as a coating is seldom reported. The raw form of Nb (ore) is 8 times cheaper than Ta and thus the combination of Nb with Ta can decrease the manufacturing cost [36,37]. The melting temperature of Nb (2477 °C) is also lower than that of Ta (2996 °C), so combining Nb with Ta can also alleviate the problem related to high-temperature processing of Ta in pure/bulk-form [38]. Moreover, owing to their highly appealing properties as biomaterials, Nb and Ta together as a coating can produce interesting results in terms of better protection ability and biological efficacy. In addition, the comparative in vitro analysis of pure Nb/Ta coating and NbeTa alloy coating remains to be investigated. Therefore, in the present study, pure Nb, pure Ta and NbeTa alloy coatings were developed onto a CoCr alloy via the plasma spray technique. The in vitro analyses of corrosion behavior and biocompatibility along with surface properties evaluation of different coatings has been carried out.

Fig. 1. Morphology of (a) Nb and (b) Ta powder.

Table 1 Spray parameters applied for the deposition of coatings. Spray parameter Arc current (A) Arc voltage (V) Spray distance (mm) Primary gas (Ar) flow rate (SLPM) Secondary gas (H2) flow rate (SLPM)

2. Experimental procedure 2.1. Materials and preparation of coatings

Value 600 63 75 40 5

A medical grade CoCr alloy (ASTM F-1537, Φ15 mm x 5 mm) supplied by Zapp Precision Metals, Schwerte, Germany was used as substrate material. The samples were grit blasted with alumina (particle size 50–70 μm) before coating to obtain a rough surface and subsequently air blasting was performed for the elimination of residual grit. The Nb and Ta powders (BGYST Co. Ltd., Beijing, China) of 75–95 μm particle size were used in this study. The micrograph of powder particles is shown in Fig. 1, which reveals that the particles of both the powders had an irregular morphology. The spray feedstock was prepared by blending Nb and Ta (NbeTa) powders at weight proportion of 70:30, 50:50 and 30:70 (classified as N7T3, N5T5 and N3T7, respectively) in a jar mill. Plasma spraying of pure Nb, pure Ta and NbeTa powders was carried out using MF4 gun (Metallizing Equipment Co. Pvt. Ltd., Jodhpur, India). Table 1 presents the spray parameters applied for the coating process.

microhardness tester (402MVD). To obtain polished cross-section, the samples were firstly cut across the cross-section using a low-speed precision sectioning saw and then mounted in epoxy resin. Subsequently, the polishing of the samples was performed with emery papers upto grade 2000. Finally, alumina slurry was used for buffing of the samples on a napped cloth. A dwell time of 15 s for the 50 gramforce load was used on microhardness tester. The surface properties' measurements were performed in triplicate on five samples from each group. The average ± standard deviation of the measurements is reported in the present study and the statistical comparison was performed using Student's two-tailed t-test with 95% confidence interval (i.e. significant level with p-value < 0.05). OriginPro 8 software (OriginLab Corporation, Washington, USA) was used for statistical analyses.

2.2. Characterization techniques

2.3. Corrosion behavior investigation

Panalytical X'pert Pro X-ray diffractometer (XRD, PN-3040/80) was utilized for identifying the phase constituents of the powders and coatings. The XRD data was acquired using Cu Kα radiation over 20°–80° 2Ɵ range. Crystallite size of the coatings was calculated as per an earlier study [39]. Scanning electron microscopy was performed for the microstructural and compositional examination of the coatings using a JEOL scanning electron microscope (SEM, JSM-6610LV) equipped with an energy-dispersive X-ray spectroscopy (EDX) analyzer. Mitutoyo contact type profilometer (SJ-210) was used for surface roughness analysis. The wettability behavior of the samples was assessed by measuring the static contact angle of Ringer's solution droplet with the surface. For this analysis, the First Ten Angstroms goniometer (FTA 2000) was used. Microhardness (HV) of the coatings was evaluated from the polished cross-section by using Wolpert Wilson

The electrochemical corrosion behavior of the samples was analyzed through the potentiodynamic polarization technique in Ringer's solution (Nice Chemicals Pvt. Ltd., Kochi, India). The electrochemical measurements were obtained with the Gamry potentiostat/galvanostat (G-750) interfaced with a digital computer using a conventional threeelectrode cell in which the uncoated/coated sample, graphite rod and Ag/AgCl (saturated KCl) formed working, counter and reference electrode, respectively. Prior to electrochemical measurements, a relatively steady state of each sample was established by immersing it in Ringer's solution for 24 h. Table 2 presents other parameters related to corrosion testing. The potentiodynamic curves were obtained by conducting the corrosion tests with 5 replicates for each group of samples. The potentiodynamic curves then analyzed with Echem Analyst software (Gamry Instruments, Warminster, USA) using Tafel extrapolation 2

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Table 2 Parameters related to corrosion testing. Parameter

Value 2

Area exposed to Ringer's Solution (cm ) Scan rate (mV/sec) Initial potential (V) Final potential (V)

1 0.5 −0.25 0.25

method. 2.4. In vitro biocompatibility studies Human osteosarcoma MG-63 cell line (National Centre for Cell Science, Pune, India) was employed to investigate the cytotoxic behavior of the samples. For cell culture, Gibco RPMI 1640 media supplemented with 10% (V/V) Foetal Bovine Serum and 1% PenicillinStreptomycin was used. Sterilization of the samples was done by autoclaving prior to the cytotoxicity analysis. Then the cells were grown in a 12-well plate at 6000 cells/well seeding density for 96 h along with the samples. Bare CoCr alloy sample was treated as control. After 96hour incubation, samples and media were extracted from the well plate. Subsequently, the cells were washed with Dulbecco's phosphate-buffered saline (DBPS) solution. Trypsinization of the cells was performed in the next phase with 80 μl of 0.25% Trypsin-EDTA. Afterward, a 96well plate was used for seeding (100 μl/well) of the suspended cells. Finally, cytotoxicity of the samples was assessed using MTS assay (Promega CellTiter 96). MTS assay was performed after 12-hour incubation in 96-well plate. BMG LABTECH microplate reader (CLARIOstar) was employed for determining the spectrophotometrical absorbance at 490 nm. The cell health was analyzed using the Thermo Fisher digital inverted microscope (EVOS XL). Hemolysis test was conducted to assess the hemocompatibility of the samples according to the method previously reported [40].

Fig. 3. XRD diffractogram of as-sprayed (a) Nb, (b) Ta, (c) N7T3, (d) N5T5 and (e) N3T7 coating.

seen from the Fig. 2(a) and (b) that the position of the diffraction peaks of Nb and Ta is almost identical which has also been reported by Seifried et al. [41]. The absence of any amorphous hump/impurity phase and the well-pronounced sharp peaks corresponding to the Nb and Ta phase confirmed the crystalline nature of powders. When both the powders were blended together [Fig. 2(c), (d) and (e)], no position shift was noticed for the diffraction peaks. The XRD diffractograms of as-sprayed Nb, Ta and NbeTa coatings are displayed in Fig. 3. The broadening of diffraction peaks of assprayed coatings was observed as compared to the corresponding feedstock powders [Fig. 2]. Reportedly, the peak broadening is attributed to the reduction in crystallite size and the size of crystallites can even decrease upto 100-folds for plasma-sprayed coating as compared to corresponding feedstock powder [42,43]. In the present study, a decrease in crystallite size was observed for as-sprayed coatings (4.1 ± 1 nm) as compared to the crystallite size of feedstock powders (8.8 ± 1 nm). In the XRD diffractograms of the as-sprayed coatings, low intensity signals corresponding to Nb-oxide [Fig. 3(a), (c), (d) and (e)] and Taoxide [Fig. 3(b), (c), (d) and (e)] were also detected as per JCPDS card #30-0873 and #19-1300, respectively. As thermal spraying involves high-temperature processing, the oxide phases of feedstock powder are usually formed within the coating [44]. The presence of Nb-oxide and Ta-oxide can enhance the performance of coating as the earlier studies have reported that both improve corrosion protectiveness as well as cell viability on the surface [45,46]. In case of alloy coatings [Fig. 3(c), (d) and (e)], the diffraction peaks corresponding to 38.47° and 55.55° 2Ɵ remained unaffected when the composition was varied. A marginal variation in the intensity of the peak corresponding to 69.50° 2Ɵ was noticed with the increment of Ta proportion in the alloy coating. Reportedly, as the proportion of an alloying element possessing higher atomic weight is increased in the coating, it can cause changes in lattice parameters and strain energy leading to variation in the intensities of diffraction peaks [47]. The surface morphology of as-sprayed coatings is shown in Fig. 4. The coatings displayed typical microroughned features of plasma sprayed coatings such as irregular nodular particles, accumulated and well-flattened splats with the subjacent dense background. The absence of partially melted/unmelted particles as well as the presence of wellflattened splats indicates the successful melting and spraying of powders. The surface appearance of the coatings was not notably different.

3. Results and discussion 3.1. Characterization Fig. 2 presents the XRD diffractograms of Nb, Ta and NbeTa powders. The characteristic peaks of Nb [JCPDS card #35-0789] and Ta [JCPDS card #4-0788] were recognized from the patterns. It can be

Fig. 2. XRD diffractogram of (a) Nb, (b) Ta, (c) N7T3, (d) N5T5 and (e) N3T7 feedstock powder. 3

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(caption on next page)

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Fig. 4. Surface morphology and EDX analysis of as-sprayed (a) Nb, (b) Ta, (c) N7T3, (d) N5T5 and (e) N3T7coating.

However, microcracking was noticed in case of pure Nb and Ta coatings [Fig. 4(a) and (b)], while the morphology of alloy coatings [Fig. 4(c), (d) and (e)] was crack-free. Rapid solidification is a major cause of microcrack development in plasma-sprayed coatings [48]. The solidification process of pure Nb and Ta coatings involved cooling of singlephase with homogenous thermal conductivity. But in the case of alloy coatings, the dissimilar thermal conductivities of Nb and Ta can generate thermal gradient across the Nb/Ta boundary which may have led to slower cooling of alloy coatings. Therefore, the crack-free morphology of alloy coatings could be due to this slower cooling rate than the pure Nb and Ta coatings. SEM analysis also revealed the presence of micropores in all the coatings. The micropores present at the implant surface provide greater surface area for the precipitation of natural calcium-phosphate in vivo [49]. It has also been reported that the osseointegration process is enhanced due to the presence of microporosity as it facilitates better mechanical interlocking between the body-tissue and implant surface [50]. The EDX analysis was carried out to detect elemental composition of the coatings and the findings revealed that Nb or/and Ta were the major constituents in the respective coating composition. Furthermore, the EDX analysis also confirmed that the Nb and Ta were consistently distributed within the coating and lump formation was not noticed. 3.2. Analysis of surface properties As can be seen from Fig. 4 that the surface of coatings displayed uneven topography, the surface roughness analysis also revealed the relatively rough nature of the coatings. The average surface roughness (Ra) value for the coatings and uncoated CoCr alloy was 3.49 ± 0.1 μm and 1.16 ± 0.2 μm, respectively. The surface roughness of CoCr alloy was significantly improved with the deposition of pure Nb and Ta as well as NbeTa alloy coatings. However, the no significant difference was observed between the Ra values of the coatings. The final characteristics of the implants are substantially influenced by surface topography. Reportedly, the micrometer surface features are advantageous for the interaction between cells and extracellular matrix and present higher surface area for protein absorption [51,52]. Therefore, better cell adhesion, growth and phenotypic maturation are facilitated by a rough surface [34]. Further, the rough nature of the surface provides better biointegration and sturdiness to an implant in vivo as it promotes osteogenic differentiation of the adjacent cells and stimulates the formation of new bone [19]. The interaction of implant with surrounding cell-tissue is strongly influenced by its surface morphology, topography and chemistry as well [53]. The wettability characteristic of a surface in part indicates its surface chemistry [54]. The contact angle values and droplet profile at different surfaces have been depicted in Fig. 5(a) in which the asterisk (*) represents the statistically significant difference (i.e. p-value < 0.05). The uncoated CoCr alloy displayed hydrophobic properties and with the pure Nb and Ta coatings, the contact angle was significantly increased. The alloy coatings also demonstrated hydrophobicity and N3T7 coating exhibited the highest contact angle amongst them. Subsequent to the implantation, the absorption of proteins (laminin, fibronectin, vitronectin and collagen) starts at the surface from blood or extracellular matrix. The earlier studies have endorsed that the proteins tend to bind in a higher amount and more tightly to hydrophobic surfaces than hydrophilic ones [55–57]. The protein absorption increases the signaling into cells leading to a rise in cell propagation [58]. Reportedly, hydrophobic nature enhances the cellular response to the implant's surface [59]. After implantation, the implants interact in a physiological environment which is delicate but hostile at the same time. Therefore, in addition to favorable biological properties, biomaterials should also

Fig. 5. (a) Contact angle values and droplet profile at different samples, (b) microhardness value for different samples.

possess adequate mechanical strength. Surface hardness, a key mechanical property, can be used for the assessment of functional effectiveness as well as the quality of coating [60,61]. Fig. 5(b) demonstrates the microhardness values for different samples. The microhardness of CoCr alloy was significantly increased with pure Nb and Ta as well as NbeTa alloy coatings. The microhardness of pure Ta coating was significantly higher than the pure Nb coating. In case of alloy coatings, the microhardness of N7T3 and N5T5 coating was not markedly different but N3T7 coating possessed notably higher microhardness than the other two alloy coatings. It is hence inferred from this analysis that pure Ta coating had the highest microhardness and the value of microhardness increased as the proportion of Ta was progressively increased in the alloy coating. The high hardness of the surface enhances the biomechanical compatibility of the metallic implant by facilitating good wear resistance in vivo and improving the long-term survivability [33]. Moreover, coatings with higher hardness impart high load carrying capacity to metallic implants [62].

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electrical contact [67]. Then galvanic corrosion can occur beneath the coating leading to ion leaching which is a possible reason for lower corrosion resistance exhibited by pure Nb and Ta coatings. In the case of alloy coatings, an improvement in protectiveness was noticed with the progressive increase of Ta proportion. This improvement can be attributed to the higher passivation ability possessed by Ta. It has been well documented in the preliminary studies which analyzed the comparative corrosion behavior of Nb and Ta that Ta has better corrosion resistance in chloride as well as acidic environment [68,69]. Fathi et al. [70] and Starikov et al. [71] have also reported that Ta coating demonstrated higher electrode potential i.e. better corrosion protective effect than Nb coating during electrochemical corrosion testing. Therefore, it can be inferred that the corrosion resistance of alloy coatings was enhanced by the higher passivation ability imparted due to the progressive increase of Ta proportion. It has been reported that the higher the corrosion resistance, the stronger the implant-tissue bonding in lesser time which leads to faster recovery of damaged tissue [72]. In addition to better osseointegration, the higher corrosion resistance also enhances the long-term survivability of implants [73].

Fig. 6. Potentiodynamic polarization curves.

3.3. Electrochemical corrosion behavior 3.4. Biocompatibility evaluation

Fig. 6 illustrates the potentiodynamic polarization curves of uncoated and coated CoCr alloy in Ringer's solution. The depicted curves are chosen because their corrosion current density (Icorr) values were closest to the mean Icorr values of each group of samples. The values of corresponding polarization parameters were obtained through Tafel extrapolation and are summarized in Table 3. In the assessment and comparison of corrosion behavior of biomaterials, the corrosion potential (Ecorr) and Icorr are very crucial parameters. The Ecorr value is an indication of the chemical affinity of material towards corrosion and a nobler Ecorr value intimates better passivation ability [24]. The value of Icorr is used for the quantification of corrosion kinetics and a lower Icorr indicates higher degradation resistance of a material [63]. The Ecorr value for uncoated CoCr alloy was the lowest and a significant shift in Ecorr value towards nobler potentials was noticed with the deposition of coatings. This positive shift of Ecorr implies that the coatings reduced the electroactive area which was responsible for corrosion by acting as an effective insulative barrier [64]. Pure Ta coating showed higher Ecorr value than the pure Nb coating; while the Ecorr value further increased for NbeTa alloy coatings as the proportion of Ta incremented in the coating. The uncoated CoCr demonstrated very high Icorr value. A notable decrease in the Icorr value was observed for the coated CoCr samples which can be attributed to the decreased ionic diffusion due to higher protection ability offered by the coatings [65]. Amongst the coated samples, Nb and N3T7 coating revealed the highest and lowest Icorr value, respectively. The results of the electrochemical investigation suggest that the alloy coatings imparted better corrosion protective effect to the CoCr alloy as compared to pure Nb and Ta coatings. The lower corrosion resistance of pure coatings could be arisen by the microcracking observed in both the coatings [Fig. 4(a) and (b)]. The microstructural imperfection (such as a microcrack) provides an active route for the ingress of corrosive electrolyte [66]. In the presence of a conductive medium, galvanic coupling forms when two dissimilar metals establish

3.4.1. Cytotoxicity analysis Cytotoxicity test is the preliminary test in the evaluation of biological safety of biomedical implants. Cytotoxicity of different samples was assessed from the results of cell proliferation (MTS) data which is presented in Fig. 7. The uncoated CoCr alloy exhibited the least cellular proliferation aptitude. Earlier, Plecko et al. [74] conducted a comparative study of the biological behavior of different implant materials and reported that the CoCr alloy demonstrated almost no bioactivity. The pure Nb and Ta coatings revealed marginally improved absorbance values but the cell proliferation was not significantly different as compared to CoCr alloy. However, significantly better cell proliferation was observed in the case of alloy coatings. It has been reported that the biologic response of metallic biomaterials is equated with its corrosion resistance because the corrosion current may adversely affect cell metabolism and behavior [75,76]. Thus the higher cell proliferation in case of alloy coatings is possibly attributed to their higher corrosion resistance. The findings of the present study are consistent with the previous studies which have also shown the positive effect of Nb and Ta on cellular proliferation aptitude [21,77]. Fig. 8 depicts the cell health after incubation with different samples. The microanalysis revealed that the morphology of the cells was not affected when exposed to the coated samples. Reportedly, Nb and Ta are non-cytotoxic metals and they do not trigger any adverse cellular reactions [38]. Therefore, it can be deduced from the results of cytotoxicity analysis that NbeTa alloy

Table 3 Mean values (standard deviation) of potentiodynamic polarization parameters of uncoated and coated CoCr alloy in Ringer's solution. Sample

Ecorr (mV)

Icorr (per cm2)

βa (e−3 V/decade)

Uncoated Nb Ta N7T3 N5T5 N3T7

−392 (12) −331 (9) −306 (4) −301 (3) −297 (5) −286 (7)

3.03 (0.2) μA 456 (6) nA 415 (8) nA 399 (4) nA 391 (3) nA 369 (5) nA

166 (11) 159 (7) 190 (4) 182 (5) 173 (4) 175 (7)

βc (e−3 V/decade) 143 171 176 188 178 169

(9) (8) (8) (4) (6) (3)

Fig. 7. Cell proliferation (MTS) data. 6

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Fig. 8. Cell health after incubation with (a) Uncoated CoCr alloy, (b) Nb, (c) Ta, (d) N7T3, (e) N5T5 and (f) N3T7 coating.

[80]. The alloy coatings demonstrated preeminent hemocompatibility with the lowest HR values (3.11–3.50). As described previously, the ion leaching due to higher corrosion kinetics can increase the HR value. Thus superior corrosion resistance is a possible reason for the minimal hemolysis observed in case of alloy coatings. Additionally, wettability of the surface also plays a vital role in the hemocompatibility behavior of biomaterials. The hydrophobic surface has been ascribed friendlier to RBC and a decrease in HR value with increase in contact angle value has been reported [81,82]. HR value < 5% is considered safe for implantable materials [83]. The HR value for uncoated CoCr was below but very close to the threshold safe value, whereas NbeTa alloy coatings revealed considerably lower HR values than threshold safe value indicating better hemocompatibility. Fig. 10 shows the morphology of erythrocytes after exposure to different samples. Some hemolytic erythrocytes were noticed in the case of uncoated CoCr and pure coatings [Fig. 10(a), (b) and (c)]. Whereas, almost all the erythrocytes which were exposed to NbeTa coatings [Fig. 10(c), (d) and (e)] retained their actual (nearly round) morphology. These results suggest that the NbeTa coatings can successfully prevent the erythrolysis and enhance the hemocompatibility of CoCr implants.

coatings are non-cytotoxic and conducive to cell proliferation. 3.4.2. Hemocompatibility analysis The hemocompatibility testing is indispensable in the evaluation of biomaterials and it has been considered the most common method of biocompatibility investigation for decades [78]. When corrosion of biomaterial takes place, the leaching of ions can increase the pH value of the blood system that causes breakage of red blood cells (RBC)/erythrocyte membrane [79]. This rupturing of RBC leads to intercellular hemoglobin release i.e. hemolysis. Thus hemolysis ratio (HR) is an important index for the assessment of hemocompatibility of a biomaterial. The HR values for different samples are displayed in Fig. 9. The quantitative analysis revealed a significant decrease in HR value for all the coated samples as compared to the uncoated CoCr. Higher hemolytic activity is linked to toxicity and it can even inhibit the cell growth

4. Conclusion In the present study, NbeTa alloy coatings were successfully deposited on CoCr alloy substrates via the plasma spray technique. Microcracking was noticed in case of pure Nb and Ta coatings but alloy coatings demonstrated crack-free morphology. The coatings imparted reasonable roughness to the CoCr alloy surface and no significant difference amongst the surface roughness of the coatings was noticed. Microhardness of the coatings was notably higher than uncoated CoCr and with the progressive increment of Ta proportion, a significant improvement in microhardness was observed. All the coated surfaces had hydrophobic properties. The NbeTa alloy coatings were more corrosion resistant than the uncoated CoCr alloy as well as pure Nb and Ta coatings. The alloy coatings also demonstrated superior

Fig. 9. Value of HR for different samples. 7

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Fig. 10. Morphology of erythrocytes after exposure to (a) Uncoated CoCr alloy, (b) Nb, (c) Ta, (d) N7T3, (e) N5T5 and (f) N3T7 coating.

cytocompatibility with higher cell proliferation of MG-63 cells. Furthermore, the alloy coatings displayed noticeable improvement in the hemocompatibility of CoCr alloy. In summary, the NbeTa alloy coatings could be a promising strategy to enhance the surface properties, corrosion resistance, cytocompatibility as well as hemocompatibility of CoCr implants and thus they deserve further biological evaluation to ascertain their usefulness for clinical applications. The forthcoming stage of research will be focused on the in-depth phase and structural characterization to provide a comprehensive understanding of the alloying process of NbeTa coatings and the microcracking phenomenon in case of pure coatings.

Acknowledgment 1. Zapp Precision Metals, Germany for substrate material. 2. Mechanical Engineering Department, IIT Ropar, India for surface and corrosion analysis. 3. S. Naidu and S. Saproo from Center for Biomedical Engineering, IIT Ropar, India for cytotoxicity analysis.

Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. 8

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