Electrochemical stability and bio-mineralization capability of zinc substituted and elemental zinc reinforced calcium phosphate composite coatings synthesized through pulsed electro-deposition

Electrochemical stability and bio-mineralization capability of zinc substituted and elemental zinc reinforced calcium phosphate composite coatings synthesized through pulsed electro-deposition

Ceramics International xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate...

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Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Electrochemical stability and bio-mineralization capability of zinc substituted and elemental zinc reinforced calcium phosphate composite coatings synthesized through pulsed electro-deposition Rajib Chakrabortya,∗, Monalisa Mandalb, Partha Sahaa a b

Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Zinc substituted calcium phosphate Zinc-calcium phosphate composites Bio-mineralization Composite bio-coatings Electrochemical behaviour Pulsed electro-deposition

Recent studies showed that zinc, as one of the essential element in the human body, is able to promote osteoblast proliferation and bone growth when present with calcium phosphate phases. In this study, zinc substituted and zinc reinforced calcium phosphate composites were prepared by electro chemical deposition from aqueous electrolyte bath. Nucleation rate and reaction time were varied to understand the relation between in-situ formation mechanism of various phases with the varying reaction kinematics and kinetics. The thin functional composite coatings were compared in terms of their surface morphology, lattice structure, surface elemental composition and related electrochemical corrosion resistance along with bio-mineralization capability under simulated body fluid (SBF). On a particular parameter of 10 mA/cm2 current density and 20% duty cycle, nano tubes enriched in elemental zinc; hopeite and calcium orthophosphate composite phases are formed. This unique microstructural morphology and composition make the coating surface 10 times more electrochemical corrosion resistant compared to all other coatings. A high amount of zinc substitution in calcium phosphate phases leads to the development of high compressive lattice strain, which in turn results in rapid dissolution of the coating under contact with SBF. During bio-mineralization, the zinc loading ratio is rapidly decreased for the initial 7 days through preferential dissolution and enrichment in calcium ions and subsequently stabilized at 60% from the initial 83% in nano tube structures. The effective adherence of osteoconduction product on the nano tube structure is achieved post 14 days. In summary, electrochemical deposition was found as an effective way to design various tailor-made/functionally graded zinc-calcium phosphate composite coatings based on specific requirements of biodegradability, biocompatibility and surface properties.

1. Introduction During recent studies, Zinc is evolved as one of the most promising and ideal biodegradable material for use in orthopaedics on account of their desirable biocompatibility along with superior biodegradability, respectively, over and above the magnesium, which generally creates hydrogen pockets due to non-uniform corrosion. These also affect the bone healing process greatly along with callus formation and cortical defects [1]. Another major advantage of zinc is that it does not produce voluminous corrosion products during in-vivo degradation, unlike iron, which is sometimes responsible for blocking the body fluid inside living tissues and thus causes inflammation [2]. On the other hand, zinc can actively stimulate the bone formation and growth of bone tissues through mineralization along with immune regulation [3–7]. However, pure zinc exhibits sufficient toxicity to osteoblast cells along with the

*

decrease in cell viability less than 50% over 7 days of culture and thus restricting their use as an individual component in orthopaedic implants [8–10]. On the other side, calcium phosphate group compounds like hydroxyapatite, calcium orthophosphate etc. are widely used as functional coatings over metallic implant surfaces due to their ceramic nature along with their superior biocompatibility similar to natural bone [11–16]. Various methods namely electrochemical deposition, electrophoretic technique, pulsed laser deposition etc. were adopted to synthesis the thin film structure of hydroxyapatite over metallic implant surfaces. Researchers have also found that nano crystallite hydroxyapatite can promote better bio functionality in terms of faster osteoblast cell proliferation [17]. Also, the substitutions of calcium ions in calcium phosphate compound by other similar ions can inhibit the insitu formed crystal growth and development of nano crystalline phases

Corresponding author. E-mail address: [email protected] (R. Chakraborty).

https://doi.org/10.1016/j.ceramint.2019.07.333 Received 24 June 2019; Received in revised form 19 July 2019; Accepted 29 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Rajib Chakraborty, Monalisa Mandal and Partha Saha, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.07.333

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Altogether 1800 integrated pulses were applied for each coating specimens. Coatings were also carried out at various constant cathode current density of 5, 10 and 20 mA/cm2 based on the findings from authors previously published works related to hydroxyapatite electro depositions [24–26]. Thus, altogether five different samples in a group size of five were prepared for relative comparison in this study. Samples were named as cathode current density in mA followed by duty cycle in percentage. The electro-deposition was carried out with the help of BioLogic SP150 electro-deposition set up in the pulsed DC mode. All the depositions were performed at room temperature, which was around 25 °C.

[18,19]. Zinc ions are efficient enough to produce an antimicrobial response, which preferentially prevent the growth of Streptococcus mutans, Staphylococcus aureus, Escherichia coli and Candida albicans [20,21]. Therefore, incorporation of zinc ions in any functional coatings would stand beneficial to prevent conditions like osteomyelitis [20]. A ceramic-metal composite can be one of the potential solutions to overcome the toxicity related problem of pure zinc during usage as orthopaedic implants, without affecting the desired biocompatibility. Major advantages of the ceramic-metal composite are adjustable corrosion and surface properties, which can be tailor-made by altering the relative weight ratio and electrochemical state of the two individual components present in the composite. Various researchers have successfully synthesized hydroxyapatite-zinc based metal matrix composite through spark plasma sintering, pulsed laser depositions and chemical precipitation method [22–25]. They have successfully incorporated hydroxyapatite particle in the zinc metal matrix or substitution of calcium ion in the hydroxyapatite phase during chemical precipitation method. But very few studies, related to in-situ synthesis along with good amalgamation of zinc-calcium phosphates composites, could be found out. Also, it seems that electrochemical deposition is considered superior for the development of very thin, residual stress-free, welladherent and tailor-made functional coating over any complex shape of metallic implant surface as compared to pulsed laser deposition or plasma sintering [26–28]. Sun et al. have reported the development of zinc-substituted hydroxyapatite coatings through electrophoretic methods which exhibit poor bonding of individual deposited phases due to the absence of molecular reaction/interaction during the deposition stage [29]. Keeping in view of the above scenario, this study aimed to the development of ceramic-metal composite coatings consisting of various calcium phosphate phases along with zinc through the pulsed electrodeposition route which considered being a low cost and easily replicable technique for mass manufacturing of any complex shape of implants. The effects of nucleation rate and duty cycle were also taken into consideration in this study since several in-situ formations and co-deposition of phases occur through various stage of instantaneous reaction in the cathode surface and electrolyte bath. Relative comparison in terms of corrosion, osteoconduction and surface properties were carried out amongst a different set of coatings.

2.3. Characterization of coatings The phase purity and crystallinity of the coatings were analyzed by the X-ray diffraction (Pan Analytical Empyrean diffractometer), using CuKα radiation (wavelength = 0.1546 nm), at a step angle of 0.02 degree, scanning rate 0.05 degree per second and two theta range between 10 and 55 deg. The percentage amount of different phases present in the coating was estimated by Rietveld analysis using TOPAS 4.5 software of BRUKER, Germany. The microstructure, compositions and surface morphology of the coatings were characterized by SEM (EVO18 and Merlin FE-SEM Zeiss) combined with EDS (EDAX, USA). Further, in order to confirm the presence of state of phosphate group in the deposition material, which was major constituent of the complex chemical structure of hydroxyapatite, Fourier transform infrared spectroscopy (FTIR) was carried out in a Nicolet 6700 spectrometer using KBr pallet technique in the range of 4000 to 400 cm−1, with a resolution of 8 cm−1. All measurements were performed at room temperature, which was around 25 °C. The electrochemical behaviour under immersion in simulated body fluid was compared with the help of electrochemical impedance spectroscopy (EIS) through BioLogic SP 150 device set up. A three-electrode system, consisting of an Ag/AgCl electrode as the reference electrode, platinized platinum as the counter electrode and coated sample as the working electrode was used. EIS measurement was performed at OCP condition with a surface area of around 1 cm2 for the working electrode over the frequency range from 106 to 10−2 Hz with 10 mV amplitude. The impedance spectra were analyzed with the help of BioLogic EC Lab software, and the results were presented in the form of standard Nyquist and Bode plots. An equivalent circuit was chosen for this analysis based on the review of similar research work in the recent past on this type of coating. Corrosion behaviour of deposited coatings in simulated body fluid solution was evaluated based on potentiodynamic polarization measurements, by using a potentiostat/galvanostat (BioLogic SP150) with a standard three-electrode cell. All potentials were referred to the saturated Ag/AgCl electrode (RE), in which platinum rod was used as the counter electrode. The potentiodynamic polarization curve was obtained by scanning from −1.0 V to +1.0 V at a scanning rate of 1 mV/ s. The polarization resistance was calculated from the software directly by Tafel extrapolation method. Altogether, five samples were tested of each category for the sake of data reproducibility. The stabilization time of around 60 min in SBF was set for each sample. The surface topography and composition of the corrosion samples were also assessed with the help of SEM micrograph and EDS respectively.

2. Materials and methods 2.1. Substrate preparation Commercial grade stainless steel (SS316) sheet of 180 μm thickness was selected as substrates for electro-deposition. The substrate surface was prepared for electro-deposition by polishing with SiC emery papers (600 and 800 grit) followed by ultrasonic cleaning in distilled water and acetone for several times. Epoxy resin was applied to cover the required surface so that it left an exact exposed area of 6 cm2 on one side of the SS316 plate for deposition. 2.2. Synthesis of coatings The deposition was carried out with the help of a conventional twoelectrode system. SS316 substrate was used as the working electrode, and a platinum rod was used as the counter electrode. The electrolyte solution was prepared by mixing 0.1 M of Zn (NO3)2, 6H2O (Merck, > 98% purity), 0.15 M of CaCl2, 2H2O (Merck, > 98% purity) along with 0.1 M of NH4H2PO4 (Merck, > 98% purity) salt in distilled water. Thus both zinc, calcium and di-hydrogen phosphate ions were present in the bath which took part in various in-situ phase nucleation reactions at the cathode surface. A typical pulsed technique was followed with a varying duty cycle like 10%, 20% and 40% to carry out the in-situ formation and co-deposition of various phases during each integrated pulse of around 1 s.

2.4. Bio-mineralization assessment In order to determine the osteoconductive behaviour of these composite coatings, they were immersed into newly modified SBF for 1, 3 and 14 days at 37 °C. The simulated body fluid (SBF) prepared using the following compositions, NaCl- 7.996 g/l, KCl-0.224 g/L, CaCl2, 2H2O-0.278 g/L, MgCl2, 6H2O- 0.305 g/L, NaHCO3–0.350 g/L, K2HPO4, 3H2O-0.228 g/L, Na2SO4–0.071 g/L, as per various literature 2

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with both the perpendicular flakes and scaffold in these coating surfaces. It seems that the 20% duty cycle is the optimum one for the formation of entangled porous nano-tubes structure on the coating surface. Thus all the composite coatings achieved through pulsed electrodepositions with varying nucleation rate and duty cycles exhibit uniformity, intactness, a strong amalgamation between in-situ phases and many variations in surface morphology as compared to the zinc-hydroxyapatite coatings developed by other researchers through electrophoretic or pulsed laser deposition techniques [23,29]. Yang et al. showed that zinc –hydroxyapatite prepared through the plasma sintering process, are separated mostly at grain boundaries, and preferential corrosion started from these interfaces [22]. In this case, more uniform and intact coatings are assumed to deliver better responses towards cell attachment and subsequent uniform growth on the surfaces without providing any preferential corrosion sites.

recommendation [30,31]. The prepared SBF was buffered to a pH of around 7.4, same as of human blood plasma fluid. The SBF was refreshed at every alternate day to maintain its ion concentration and pH value nearly close to living human body plasma. The solution and sample were also kept under the closed container to avoid contamination or reaction with open air. After every subsequent interval of 1, 3 and 14 days, the specimen was removed from the solution, washed with distilled water and then dried for 1 day in the vacuum desiccator. To characterize the effect of surface features on apatite growth under SBF, the surface morphology and elemental analysis of all the specimens were studied in the scanning electron microscope (Zeiss EVO 18 and Zeiss Merlin FE-SEM) equipped with energy-dispersive X-ray spectroscopy (EDAX, USA). Since the coatings were generally nonconductive, they were gold-coated through the sputter coating process before examination under SEM. 3. Results and discussions

3.2. Phase composition and fraction analysis in XRD 3.1. Surface morphology The x-ray diffraction patterns of all the composite coating samples are presented in Fig. 2 and some of the individual peaks are compared in Fig. 3. The diffraction data are in good agreement with the calcium orthophosphate (01-072-0713), hydroxyapatite (00-009-0432), hopeite (00-033-1474), spencerite (00-035-0631) and zinc (00-004-0831) of the standard ICCD data base. Various phases of calcium phosphates, zinc substituted calcium phosphates along with pure zinc are developed under different nucleation rate and duty cycles. In 10 mA-20DC sample, the prominent peaks of CaHPO4 (112), Zn3 (PO4)2 (422) and pure metallic zinc (100 and 002) along with zinc substituted CaHPO4 (004) are evident. Zinc substitution in calcium orthophosphates phase only takes place along (004) surface parallel planes and only during reactions under 20% duty cycle. The amount of zinc substitution in calcium orthophosphate phases are increasing with increasing nucleation rate at 20% duty cycle. Thus the nano tubes formed in 10 mA-20DC samples, are mostly comprised of calcium orthophosphate and hopeite phase along with metallic nano crystallite (30 nm) zinc phase as reinforcement in between. The most stable hydroxyapatite phases (111, 002, 211 and 202) are formed in all other samples except 10 mA-20 DC sample. More or less up to 5 wt percentage of pure zinc are found in each composite coating along with the highest wt.% of 29.7 in calcium phosphate-zinc nano tube structure [Table 1]. On the other side, spencerite formation, which is similar to the zinc substituted hydroxyapatite phase, is formed only in three samples namely 5 mA-20 DC, 20 mA-20 DC and 10 mA-10 DC to a level of 7.1, 10.8 and 13.1% respectively. The highest amount of hydroxyapatite formation of around 34.5 wt % is evident at low nucleation rate and moderate duty cycle. It proves that sufficient relaxation time between the two stages of reactions promotes the formation of more stable hydroxyapatite phase. At higher nucleation rate and higher duty cycle, the hydroxyapatite phase formation is reduced by 50% and 80% respectively. Moreover, an increasing amount of calcium orthophosphate phase, which is formed during the first stage of electrochemical reaction, is found along with the zinc dominated phases. This is on account of lack of sufficient reaction time for hydroxyapatite formation during the second stage of reaction along with more and more engagement of hydroxyl and phosphate ions in the first stage of the reaction. On relative comparison of hydroxyapatite (002) peak, it is observed that with increasing nucleation rate and reaction cycle time, the peak got shifted towards higher 2theta value side. This signifies the larger incorporation of zinc-substituted calcium sites in hydroxyapatite crystal with increasing nucleation rate or duty cycle [27]. The peak broadening also indicates lattice disorder on account of the presence of zinc ion impurities replacing calcium ion sites in hydroxyapatite crystals [34,35]. The reduction in crystallite size of calcium phosphate phases from 87 nm to 15 nm is also supporting the fact of a larger amount of zinc substitution with increasing duty cycle. Some of the

Surface morphology plays an important role in deciding the successful absorption of protein and cells on the surface during interaction with body fluid along with uniform growth and sustenance of live cell on the surfaces. Various types of morphologies were observed under varying nucleation rate along with duty cycles. These are compared with the help of SEM(SE) images and presented in Fig. 1. At low magnification, coatings formed with 10 mA/cm2 cathode current density is appeared more uniform and intact as compared to respective low (5 mA/cm2) and high (20 mA/cm2) current density. It seems that at the 10 mA/cm2 current density, the nucleation rate of all reactions reached to its optimum level, and thus equilibrium is reached between the insitu phase formation and subsequent uniform co-deposition on the cathode surface. At higher magnification, various microstructural formations are evident on the coating surfaces. The unique one, the intact scaffold made of 80–100 nm diameter nanotube was observed in 10 mA_20 DC sample. The nano tubes are so strongly entangled with each other that a porous coating layer formed on the surface with no direct access to the substrate. It seems that the nanotubes are not formed and grown individually during in-situ formation and co-depositions, rather a spider's net-like formation is happened along with strong bonding through amalgamation between the tubes. Thus it gives a better strength of the overall coatings as compared to similar formation made of individual nano tubes. At lower nucleation rate (5 mA/cm2), the mostly broom-like morphology of surface perpendicular flakes are observed. The flakes are around 1 μm in thickness along with 200–400 μm long and originated from each individual point towards all direction in the coating surface. The outer edges of the flakes are sharp and prominent. Formations of few surface parallel planes are also observed in this sample. Randomly deposited flakes are also observed as result of a higher nucleation rate. Both surface perpendicular, surface parallel along with randomly oriented planes are observed. The flakes are 3–4 μm thick along with blunt and rough edges on the surfaces, which indicates growth by deposition. On the variation of duty cycle vis-à-vis reaction stabilization time keeping the nucleation rate constant at (10 mA/cm2), geometrical changes in the deposited flakes are observed. Thus, it can be presumed that the variation of the duty cycle able to govern the formation and development of various surface parallel crystals planes during deposition. Based on increasing stabilization time, various types of crystal planes are dominant on the coating surfaces. At lower duty cycle (10% DC), square-shaped around 10–20 μm in size of surface parallel flake formation is evident. Whereas, at a higher duty cycle, the formation of surface perpendicular flakes of around 1–2 μm wide took place along with the formation of 100–150 nm diameter spherical particle attached to the outermost coating surface. The scaffold made of the attached spherical particle is not so dense that the body fluid can directly interact 3

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Fig. 1. Comparison of coatings surface morphology with the help of SE−SEM images at low and high magnifications (a and f) 10 mA-20 DC, (b and g) 5 mA-20DC, (c and h) 20 mA-20 DC, (d and i) 10 mA-10 DC and (e and j) 10 mA-40 DC.

larger calcium ions. Mostly (110) and (202) surface parallel planes of spencerite phase are evident on the coatings in the form of surface parallel square flakes deposited in the flower-like petal construction. Along with increasing nucleation rate, this peak got shifted towards higher two theta side in addition to the development of more and more compact crystal. The crystallite size of the spencerite phase is in the

researchers have reported the formation of Parascholzite phase due to the incorporation of zinc ion into hydroxyapatite crystal [32,33,36]. Whereas, Spencerite phase formation is evident in few coatings in which zinc ion take part in the reaction instead of calcium ion. This is presumed to be happening on account of better mobility of the zinc ion inside porous coating due to their smaller ionic radius as compared to 4

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on the wall of the nano tubes and at the nano tubes junction, respectively. Thus it indicates that the high amount of in-situ formed elemental zinc helps to develop the nano tubes made of in-situ formed calcium orthophosphate phase and zinc phosphate phases by acting as a binder element. Several studies have indicated that the presence of nano crystalline zinc or zinc oxide helps to promote the faster proliferation of osteoblasts and bone growth over the implant surface. Keeping in view of this, it may be noted that this kind of in-situ formed scaffold made of biocompatible nano fibres can act as a potential candidate to promote better bonding of cells along with faster proliferations [36]. In 5mA-20 DC samples, very high zinc loading was observed in broom-like surface perpendicular flakes originated from one point. CaZn/P ratio was about 1.22, which is consistent with the theoretical value determined considering the relative amount of phase presence as per Table 1. The lower nucleation rate enables replacement of most the calcium ions (zinc loading~ 98%) in all the in-situ formed calcium phosphate phases with the lower ionic radius of zinc ions. The lower nucleation rate helps to provide longer diffusion time vis-à-vis larger replacement of the calcium ions from the in-situ formed calciumphosphate coating surface. The lower nucleation rate also helps to promote the formation of the higher amount of stable hydroxyapatite phase in the coating due to the availability of larger relaxation time for the second phase of the reaction of formation of hydroxyapatite from calcium orthophosphate phase. The high amount of zinc loading in place of the calcium ions was found as around 94% under a higher nucleation rate in 20 mA-20 DC sample. Although, the concentration of hydroxyapatite phase was decreased by around 50% along with the same amount of increment in the calcium orthophosphate phase as compared to that of 5 mA-20DC samples. This indicates that at higher nucleation rate, the faster movement of zinc ions with low ionic radius towards cathode surface effortlessly substitute the calcium ions with bigger ionic radius in the in-situ formed phases by a larger amount. Due to a high concentration of zinc ions on the cathode surface spencerite phase was formed limited to the maximum of 10.8% in weight. CaZn/P ratio in the 10 mA-10 DC samples was in between 1.7 and 1.9. Although as per theoretical stoichiometric ration, it should fall near to 1.4 considering the outcome of phase analysis results in Table 1. Additional 21% zinc overloading is observed in calcium ortho phosphate and hydroxyapatite phases under 10% duty cycle. With an increase of duty cycle to 40% lead to the formation of a layer of spherical particles uniform scaffolds which is made of the almost equal amount of hydroxyapatite and zinc substituted hydroxyapatite phase with a CaZn/P ratio of 1.7. The surface perpendicular flakes beneath of the spherical particle layer are found to be formed with zinc-rich phases.

Fig. 2. Relative comparison of X-ray diffraction pattern of composite coating surfaces in full scanning range.

range of 13–16 nm size. The high concentration of zinc ion in the electrolyte bath favours the formation of various zinc phosphate (Zn3(PO4)2) phase during electro-deposition which is a stable corrosion product formed over pure zinc implant under contact with body fluid as mentioned in various research studies [37–40]. The formations of probable detrimental corrosive by-products like zinc oxide and calcium oxide, which may cause inflammation in live tissues around the implant, are not detected in the present study. The direct formation of this stable zinc phosphate phase is thus can be treated as beneficial to protect the surface from any sort of preferential corrosion under contact with body fluid without making abnormalities to the surrounding body cells. On relative examination of X-ray diffraction patterns along with surface micrograph, it can be presumed that the hopeite phase took an active part in the formation of surface perpendicular broom stick like planes in samples 5 mA-20 DC, 20 mA-20 DC and 10 mA-40 DC. Thus, various amount of hopeite phase in the coatings affects the size and surface characteristics of the perpendicular deposited flakes (211). In case of the 10 mA-40 DC sample, the surface parallel plane of hydroxyapatite along with perpendicular broom-stick flakes ornamented with spherical nano particle is observed. The high reaction time prefers the formation of more stable hydroxyapatite phase, and due to high reaction rates, the formed phases are converted into a spherical structure on account of low surface tension. On relative comparison of the lattice parameters of various formed crystallite phases through Riet-Veld refinement, it is observed that residual lattice strains are increased in formed calcium orthophosphate phase with increased nucleation rate. Whereas, change in duty cycle vis-à-vis reaction time directly affect the second stage of the reaction of hydroxyapatite formation and its residual lattice strain. The lattice structures of other zinc-rich phases along with pure zinc are not greatly affected with changing nucleation rate or duty cycle.

3.4. FTIR analysis of coating surface Fig. 4 compares the FTIR spectrum of different coatings surfaces in terms of their specific bond characteristics and subsequent bond energy. Absorbed water band is observed over a broad area between 2600 and 3600 cm−1 along with an explicit peak attributed to O-H stretching at 3600 cm−1. The broad peaks of absorbed water are on account of the present of hopeite and spencerite phases in the coatings. Several absorption peaks in the 1400-1500 cm−1 region are related to the strong adsorption of CO32− in the formed phases [18]. With increasing nucleation rate and duty cycle, the peaks got shifted towards higher wave numbers side, which indicates the formation of stronger carbonate bonding with the formed phases. Broader peak attributed to O-H vibration in between 1600 and 1700 cm−1 range is observed in the 5mA20DC coating, which is having the highest amount of formed hydroxyapatite phase. O-H vibration peak shifted towards the lower wavenumber side with increasing duty cycle. It signifies that more reaction on time is responsible for producing weaker O-H bond in the formed phases. Peaks attributed to PO4 y3 vibration are identified between 1000 and 1100 cm−1 range. A broader peak region which indicates the formation of a more polarized bond is evident in all the samples except

3.3. Elemental composition analysis The relative quantitative elemental composition of calcium, zinc and phosphorous in various formed morphologies of the coating surfaces was assessed with the help of point and area mapping through energy-dispersive X-ray spectroscopy. The result showed that in 10 mA–20 DC sample, the CaZn/P ratio is in the range of 2.2–2.4, which is on the higher side as compared to both the individual phases of calcium orthophosphate (Ca/P~ 1) and zinc phosphate (Zn/P~ 1.5) present in that coating. This seems to be on account of the presence of elemental zinc in this particular coating in high amount (29.7 wt %). Also, zinc loading amount (Zn/Zn + Ca) is found as 82% and 88–91% 5

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Fig. 3. Relative comparison of X-ray diffraction pattern of composite coating surfaces at various peak positions along.

related to the formation of PO4 y1 symmetric stretching are found at 960 cm−1 regions. The larger presence of the symmetric stretching mode of phosphate bond suppresses the formation of PO4 y4 bending (560- 600 cm−1) or y2 vibration mode (490 cm−1). The prominent

10 mA-20 DC sample. An increment in duty cycle makes this PO4 y3 vibration bond more polarized along with uniform and stronger. Whereas, increment in nucleation rate help to produce a more polarized bond only. Under high nucleation rate and duty cycle, a broader peak

Table 1 Comparison of the lattice parameter of various formed crystallite phases. Sample

10 mA-20 DC 5 mA-20 DC 20 mA-20 DC 10 mA-10 DC 10 mA-40 DC

Calcium ortho phosphate

Zinc

Hopeite

hydroxyapatite

Spencerite

a (Å)

b (Å)

c (Å)

Wt %

a (Å)

c (Å)

Wt %

a (Å)

b (Å)

c (Å)

Wt %

a (Å)

c (Å)

Wt %

a (Å)

b (Å)

c (Å)

Wt %

5.843 5.632 5.937 5.668 5.838

15.313 15.285 15.117 15.263 15.007

6.301 6.358 6.378 6.349 6.319

30.5 22.5 42.2 46.1 50.01

2.666 2.619 2.617 2.643 2.621

4.818 5.095 5.080 4.854 5.009

29.7 4.8 3.5 3.7 2.8

10.728 10.580 10.551 10.578 10.555

18.306 18.254 18.252 18.251 18.210

5.023 5.022 5.009 5.021 5.011

39.8 31.1 24.7 14.3 42.9

9.279 9.226 9.304 9.454

6.925 6.872 6.902 6.769

34.5 18.8 22.8 4.24

10.351 10.063 10.064

5.246 5.434 5.409

11.175 11.760 11.724

7.1 10.8 13.1

6

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Fig. 4. Detailed FTIR spectra of various coating surfaces comparing their bond strength and bond characteristics.

peak at 850 cm−1 related to the presence of HPO4 bond is evident. The highlighted peak positions are in-line with the finding of other researchers [41–46]. FTIR spectra suggest the presence of strong amalgamation between various calcium phosphates and zinc in-situ formed phases, which is not reported to be produced by any other method like electrophoretic deposition or pulsed laser technique tried by other researchers [23,29].

3.5. Electrochemical behaviour assessment 3.5.1. Open circuit potential Open circuit potential trend (Fig. 5) for an initial period gives an idea about the stabilization time and rate under contact with simulated body fluid (SBF). A slow dissolution rate, along with high passivation phenomena makes the coatings more corrosion resistant at post-implementation in the human body. Out of all the deposited coatings, 10 mA-20 DC exhibits the highest level of passivation from its initial contact point with SBF due to the presence of interconnected nano tube mesh structures. Nano tubes wall contain protective zinc phosphate layer which possess the least active bond on the outer wall and thus least sites for electrochemical cell formation, responsible for initialization of corrosion. Although, the interconnected mesh-type structures provide sufficient porosity to absorb micrometre size live-cell effectively on the coatings surfaces. 10mA20DC samples also exhibit 3 fold higher passivation under SBF as compared to all other coating surfaces. In all other samples except 10 mA-40DC, steady-state dilutions were observed with an initial sharp decrease. This seems to be on account of the presence of spencerite

Fig. 5. Comparison of OCP trend of coatings surfaces depicts their initial stabilization characteristics under contact with SBF.

phase in those coatings which slowly discharges zinc ions in the solution due to the high concentration difference. Very fast passivation rate was observed for 10mA-40 DC sample for the initial state of contact with SBF. After 2000 s, the OCP reduced by almost 50% as compared to initial OCP. This is on account of the transformation of metastable calcium orthophosphate phase to saturated hydroxyapatite phases 7

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Fig. 6. Comparison of (a) Nyquist (b) Bode phase and (c) Bode impedance plots of different coating surfaces based on (d) typical equivalent circuit.

formed with spherical shape apatite structure gives its initial protection towards corrosion resistance. Although, the electrolyte penetrates to the bottom layer of flakes through bigger voids and slowly started corrode the base surface which was holding the top spherical apatite layers with the flakes. With this, the spherical apatite layer gets detached from the coatings top surface, and the overall coating surface loses its protection immediately. Thus at low-frequency region, 10 mA-40 DC coatings exhibit poor performance towards corrosion protection due to the dissolution of its spherical apatite layer from the top contact surface. One broad capacitance peak was observed in bode phase plots of all three other coatings. Their performances were similar at the high-frequency region with the same capacitance and impedance values. At low-frequency region, 5 mA-20 DC coatings performance was lower in terms of corrosion protection followed by 10 mA-10 DC and 20 mA-20DC samples. Similarity found out of these three coating samples are that the presence of hydroxyapatite phase with high internal compressive residual lattice strain as compared to the formation the hydroxyapatite spherical particles in 10 mA-40DC sample [Table 1]. The trend of corrosion performance of these three coatings is exactly matching with the trend of hydroxyapatite phase present in this coatings. The zinc incorporation in the hydroxyapatite and calcium orthophosphate lattice may be one of the causes for the distortion of the hydroxyapatite lattice structure and subsequent deterioration of their corrosion protection performance. Another cause may be the presence of elemental zinc in the coatings without having any strong bonding with any of the in-situ formed phases. This can also cause rapid dissolution from the coatings top contact surface during initial contact with SBF, which is observed as a downward hump in the OCP trend for these three coatings. Table 2

under contact with SBF. Due to high cycle on time, the second stages of the reaction of hydroxyapatite formation were unable to take place smoothly during in-situ formation and co-depositions. Pitting phenomena are also observed during stabilization, which is considered on account of the dilution of amorphous phases from the 10 mA-40 DC coating formed with high cycle on-time. 3.5.2. Electrochemical impedance spectroscopy The corrosion resistance capabilities of each composite coating were assessed with the help of electro impedance spectroscopy considering coatings electrical components as represented in Fig. 6d. In Nyquist plot, 10 mA-20 DC samples showed extremely high polarization and charge transfer resistance vis-à-vis high corrosion protection performance as compared to all other coating surfaces due to the presence of nanotube mesh structures [Fig. 6a]. It also demonstrates prominent dual capacitance peak in the Bode phase plot, one at a highfrequency side and another at low-frequency zone along with almost 1.5 times higher charge storage capacity as compared to all other coatings. Bode impedance plots also recognize these facts with an observation that throughout the test frequency region, 10 mA-20DC coatings, the resistance is almost one fold higher compared to all other coatings [Fig. 6c]. This signifies that the zinc-calcium orthophosphatehopeite nanotube mesh structures able to exhibits a high amount of capacitive charge storage along with superior corrosion protection performance at any frequency region. The electrochemical behaviour of 10 mA-40DC coatings are also found as similar to the 10 mA-20 DC coatings mostly at a high-frequency level due to the presence of the spherical apatite layer throughout on outermost surface. Thin scaffold 8

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potential, the one at 150 mV and other at 400 mV. This seems to be on account of the formation of the zinc doped calcium orthophosphate phases with a slightly different lattice parameter, which produces strain along different orientation as compared to the other two samples. On comparison of the Ecorr and Icorr values, it is observed that with increasing nucleation rate vis-à-vis deposition current density corrosion rate decreases along with increasing passivation effect in the coating. This seems to be on account faster reaction completions, which help to reduce the amount of zinc substitution in the formed calcium phosphate phases. With increasing duty cycle vis-à-vis reaction on time, I corr value decreased rapidly till 20% duty cycle and achieved a steady equilibrium state over and above 20% duty cycle. Comparing both the trend, 10 mA-20 DC sample can be declared as optimum coating samples in terms of superior corrosion resistance performance along with low corrosion rate and high cathodic passivation phenomena [Fig. 7 b and c]. In terms of Icorr values, these composite coatings exhibit 25 times high corrosion resistance compared to the similar type of zinchydroxyapatite composite coatings developed by Yang et al. [22].

Table 2 Comparison of equivalent circuit values/sq. cm of coating area. Sample

R1 (Ω)

Q2(F)

R2 (KΩ)

Q3 (F)

R3 (KΩ)

10 mA-20 DC 5 mA-20 DC 20 mA-20 DC 10 mA-10 DC 10 mA-40 DC

15.3 13.5 15.9 12.9 17.3

4.78E-05 1.69E-04 8.82E-05 6.92E-05 5.31E-05

491.0 11.7 22.1 4.4 0.6

7.76E-07 1.28E-04 8.18E-06 3.33E-05 2.26E-06

452.9 2.2 44.3 44.9 43.3

compares the equivalent electrical circuit values of various coatings surfaces. Composites formed with other physical method and without having in-situ formation, experienced two capacitive loops in Nyquist plots, one at high frequency due to the formation of zinc phosphate corrosion products and another at low-frequency level due to charge transfer and electrical double layer formation [22]. Comparing this, here only one capacitive loop is present due to a strong amalgamation between in-situ formed phases in all Nyquist plots. Two loops are evident in only bode phase plot of 10 mA–20 DC sample due to the presence of in-situ formed stable zinc phosphate phases on it [Fig. 6 b].

3.5.4. Surface condition post potentiodynamic polarization Fig. 8 compares SEM (SE) images of the coating surfaces post potentiodynamic polarization test to find out the most significant corrosion sites/phases in each of the individual samples. In 10 mA-20DC samples, the nano tubes are almost found intact with either no prominent corrosion mark on the outer walls or neither the decrease in their size and diameter. Only the junction point humps of the nano tubes which is attributed to the accumulation of additional elemental zinc phases remain as sharp edges post preferential dissolution of zinc. Thus it can be presumed that the extra amount of elemental zinc from the junction points are dissolved without affecting the elemental zinc, which takes part in the nano tube formation along with hopeite and calcium orthophosphate phases. The unique bonding in the form of nanotube keeps them intact in the coating. On the other hand, 10 mA40 DC sample loses its spherical apatite layer structure throughout from the top surface without affecting the bottom layer much. In the other three samples, prominent pitting/dissolution marks are evident on the surfaces of the deposited flakes. These results are coinciding with the EIS and PDP behaviour of these three coating surfaces. In 5 mA-20 DC and 20 mA-20 DC samples, layer by layer dissolution is observed along with surface out of spherical particle scaffold. Whereas, a complete distortion of the contact surface and shape of the deposited flakes are observed in 10 mA-10DC coatings. On further investigation with high magnification image, deep interconnected lamellar type pitting mark is observed over the surface of the square flakes. Thus it can be envisaged that the presence of hopeite phase in the

3.5.3. Potentiodynamic polarization Fig. 7a represents the potentiodynamic polarization trend of all the composite coatings on both the cathodic and anodic regions. 10 mA20DC coating showed passivation throughout the entire cathodic region of the curve past equilibrium region. Although with immediate entering in the anodic zone, steady-state dissolution is observed, this is mostly due to the presence of elemental zinc in the coating. In 10 mA-40 DC sample, dissolution started from the cathodic region itself and continues to the anodic region with an almost same rate of reaction. Thus ceramic phase dissolution can be envisaged from 10 mA to 40 DC coatings surface, which contains a very low amount of elemental zinc amongst all the coatings. The breakaway potential is found in the range of 300, 500 and 150 mV for 5 mA-20 DC, 10 mA-10 DC and 20 mA-20DC coating respectively. The breakaway of the passivation layer can be considered on account of the presence of zinc-substituted hydroxyapatite and calcium orthophosphate phases in these three coatings surfaces. The lowest breakaway potential in 20 mA-20 DC sample is due to the formation of hydroxyapatite phase with more and more zinc ion incorporation, which in terms create more residual compressive stress in the lattice. This is also recognized by comparison of the lattice parameter of hydroxyapatite presented in table no 1. Any residual stress in the lattice always depicts the poor corrosion resistance performance of that particular phase. 20 mA-20 DC sample exhibits dual break away

Fig. 7. Comparison of (a) potentiodynamic polarization curve along with E corr/I corr trend with (b) current density and (c) duty cycle of in-situ reaction. 9

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Fig. 8. Comparison of surface condition of (a) 10 mA-20DC, (b) 5 mA-20DC, (c) 20 mA-20 DC, (d and e) 10 mA-10DC and (f) 10 mA-40DC sample post potentiodynamic polarization test.

reported to exhibit antibacterial properties and thus slow dilution of this phases creates a body fluid layer rich with antibacterial properties around the implant and tissues interaction surfaces [48,49]. At a few sites, initiations of spherical apatite colony formation are evident. Although after 3 days, no more new sites of dissolution are evident, rather the lamellar pitting marks which were observed on day 1 goes deeper in the deposited flakes in 5 mA–20 DC and 10 mA-10 DC samples. The apatite colony expanded horizontally across the other intact areas of the coating surfaces. In 20 mA-20 DC samples, the deposition structure is got effected from its sidewall and not so much on the flat surfaces. Thus, a relatively larger and intact growth of apatite scaffold is observed in 20 mA-20DC samples as compared to the other two samples. After 14 days, the spherical apatite scaffold becomes denser and uniform in nature without having any significant dissolution/pitting mark on the surface. The size of the formed spherical apatite are controlled by the individual surface properties and thus found the diameter of the spherical particle as 30 nm, 70 nm and 350 nm for 5 mA-20DC, 20 mA-20DC and 10 mA-10 DC samples respectively. The porosity and roughness of the formed apatite surface which is governed by the size of the formed apatite, can affect the cell attachment and cell/protein absorption phenomena to a great extent. Thus it may be noted that the specific presence and amalgamation of phases can able to produce a significant effect in the bio-mineralization process and subsequent cell attachment/growth process under contact with SBF. In 10 mA-40 DC sample, the spherical particle layer, which was formed during deposition of coatings, mostly dissolved away on day 1 of interaction. This is presumed on account of the presence of zinc doped calcium phosphate phases with excessive lattice strain due to the high amount of zinc substitution. On day 3, a new spherical apatite layer formation is evident mostly on the edges of the deposited flakes, which in turn got detached from the surface on day 14 due to the subsequent dissolution of flake top layers. Dissolution produces a rough surface on the flake edges as compared to the deposited surface. Fig. 10 compares the zinc loading (Zn/Ca + Zn) and Ca + Zn/P variation trend of the various coating surfaces with immersion time in SBF. Amongst all the coatings, 10 mA-20DC sample exhibit quick stabilization within 7 days in terms of both of zinc content and CaZn/P ratio with close matching to the stoichiometric value of 1.67. The overall zinc loading ratio stabilizes at near to 60%. However, the actual zinc substitutions to calcium phosphate phases are much lower to around 25–30% in this sample due to the presence of a large amount of elemental zinc phase inside the nano tubes wall structure. An increasing trend of relative zinc contains in 10 mA-10 DC and 20 mA-20DC samples which are on account of the dissolution of high strain calcium phosphate phases from the coating surfaces. This also led to an increase in the CaZn/P relative ratio in those samples. 5 mA-20 DC samples

form of spherical particle scaffold acts as reinforcement in the deposited flake and thus able to retain the stability of the deposited flakes during corrosion. A low amount of hopeite phase (50–60% as compared to other samples) or presence in the other form leads to the complete deformation of deposited flakes in 10 mA-10 DC sample during corrosion. In 10 mA-40DC sample, the further increase in hopeite phase able to keep the zinc substituted calcium orthophosphate phase intact. Keeping in view of the post corrosion surface condition of 10 mA–20 DC and 10 mA-40 DC sample, it can be presumed that the hopeite and calcium orthophosphate phase take part in active corrosion resistance bonding when present at a particular ratio and lattice orientation during in-situ formation and co-deposition at the cathodic surface in which hopeite act as reinforcement in the deposited structure. 3.6. Bio-mineralization under contact with SBF Various types of interaction mechanism are observed under contact with the simulated body fluid [Fig. 9]. The degree of interaction is not directly related to the amount and type of individual in-situ formed phase, rather their physical presence/amalgamation to the coatings along with zinc doping in calcium phosphate phases plays a major role. Both the dilution as well as apatite formation phenomena along with the conversion of metastable phases to higher stable phases are observed in these coatings. Although the highest amount of elemental zinc is observed in 10 mA–20 DC sample, the strong bonding along with other calcium phosphate and zinc substituted calcium phosphate phases make them more resistible towards rapid corrosion and a steady state dilution are observed. On day 1, spherical apatite formation is initiated from the cross-sectional junction of the nano tubes structure where a relatively high concentration of elemental zinc is found as compared to the nano tube outer wall surface. The presence of elemental zinc may have diluted from the junction points and develop the necessary site for the growth of spherical apatite. After 3 days, prominent numbers of formed spherical apatite of around 90–120 nm in diameter are visible on the surface of the nanotubes. After 14 days, the embedding of a thin apatite layer in the form of a fish-scale structure is prominent over the nanotubes wall which in turn increases the nano tube diameter to 100–130 nm along with increasing the roughness of the outer wall. The rough outer wall with a nano-textured apatite layer would provide a better platform for stronger cell attachment along with uniform growth [47]. A significant amount of dissolution is observed in 5 mA–20 DC, 20 mA-20 Dc and 10 mA-10DC samples on day 1 from the deposited flakes. It may be noted that these three coatings contain the maximum amount of zinc-substituted hydroxyapatite and calcium orthophosphate phases, which tends to wash away under direct interaction SBF. Till date, zinc loaded calcium phosphate and hydroxyapatite have been 10

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Fig. 9. SE−SEM images of various coating surface morphology under contact with SBF for 1, 3 and 14 days depicting their osteoconduction capability.

Fig. 10. Trend of (a) CaZn/P ratio and (b) relative zinc loading ratio with immersion time in SBF. 11

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exhibit tremendous zinc losing trend from day1 to day 14, and the same is prominent with deep pitting marks on the flakes. 5 mA-20DC, 10 mA10 DC and 10 mA-40 DC samples exhibit deterioration of CaZn/P ratio with immersion time and keeping in view the zinc loading trend it indicates the domination of dilution of zinc phosphate phases in 5 mA–20 DC and 10 mA-40 DC samples. Whereas, the dilution of calcium phosphate phases mostly occurs in 10 mA-10 DC samples, which lead to an increase in the zinc loading ration also.

behavior for bioabsorbable stents, Adv. Mater. 25 (18) (2013) 2577–2582. [3] C.J. Frederickson, K. Jae-Young, A.I. Bush, The neurobiology of zinc in health and disease, Nat. Rev. Neurosci. 6 (6) (2005) 449–462. [4] M. Yamaguchi, Role of zinc in bone formation and bone resorption, J. Trace Elem. Exp. Med. 11 (1998) 119–135. [5] Y.Z. Tang, H.F. Chappell, M.T. Dove, R.J. Reeder, Y.J. Lee, Zinc incorporation into hydroxyapatite, Biomaterials 30 (2009) 2864–2872. [6] A. Ito, H. Kawamura, M. Otsuka, M. Ikeuchi, H. Ohgushi, K. Ishikawa, K. Onuma, N. Kanzaki, Y. Sogo, N. Ichinose, Zinc-releasing calcium phosphate for stimulating bone formation, Mater. Sci. Eng. C 22 (2002) 21–25. [7] E.S. Thian, T. Konishi, Y. Kawanobe, P.N. Lim, C. Choong, B. Ho, M. Aizawa, Zincsubstituted hydroxyapatite: a biomaterial with enhanced bioactivity and antibacterial properties, J. Mater. Sci. Mater. Med. 24 (2013) 437–445. [8] N.S. Murni, M.S. Dambatta, S.K. Yeap, G.R. Froemming, H. Hermawan, Cytotoxicity evaluation of biodegradable Zn-3Mg alloy toward normal human osteoblast cells, Mater. Sci. Eng. C 49 (2015) 560–566. [9] J. Kubásek, D. Vojtěch, E. Jablonská, I. Pospíšilová, J. Lipov, T. Ruml, Structure, mechanical characteristics and in vitro degradation, cytotoxicity, genotoxicity and mutagenicity of novel biodegradable Zn–Mg alloys, Mater. Sci. Eng. C 58 (2016) 24–35. [10] Z. Tang, H. Huang, J. Niu, L. Zhang, H. Zhang, J. Pei, J. Tan, G. Yuan, Design and characterizations of novel biodegradable Zn-Cu-Mg alloys for potential biodegradable implants, Mater. Des. 117 (2017) 84–94. [11] L.T. de Jonge, S.C.G. Leeuwenburgh, J.G.C. Wolke, J.A. Jansen, Organic-inorganic surface modifications for titanium implant surfaces, Pharm. Res. 25 (2008) 2357–2369. [12] M.A. Lopez-Heredia, J. Sohier, C. Gaillard, S. Quillard, M. Dorget, P. Layrolle, Rapid prototyped porous titanium coated with calcium phosphate as a scaffold for bone tissue engineering, Biomaterials 29 (2008) 2608–2615. [13] A.R. Boccaccini, S. Keim, R. Ma, Y. Li, I. Zhitomirsky, Electrophoretic deposition of biomaterials, J. R. Soc. Interface 7 (Suppl. 5) (2010) S581–S613. [14] C.J. Chung, H.Y. Long, Systematic strontium substitution in hydroxyapatite coatings on titanium via micro-arc treatment and their osteoblast/osteoclast responses, Acta Biomater. 7 (2011) 4081–4087. [15] S. Shadanbaz, G.J. Dias, Calcium phosphate coatings on magnesium alloys for biomedical applications: a review, Acta Biomater. 8 (2012) 20–30. [16] T. Roland, H. Pelletier, J. Krier, Scratch resistance and electrochemical corrosion behavior of hydroxyapatite coatings on Ti6Al4V in simulated physiological media, J. Appl. Electrochem. 43 (2013) 53–63. [17] H. Zhou, J. Lee, Nanoscale hydroxyapatite particles for bone tissue engineering, Acta Biomater. 7 (2011) 2769–2781. [18] V. Krishnan, A. Bhatia, H. Varma, Development, characterization and comparison of two strontium doped nano hydroxyapatite molecules for enamel repair/regeneration, Dent. Mater. 32 (2016) 646–659. [19] K.P. Tank, P. Sharma, D.K. Kanchan, M.J. Joshi, FTIR, Powder XRD, TEM and Dielectric Studies of Pure and Zinc Doped Nano-Hydroxyapatite, Crystal Research and Technology, 2011, pp. 1309–1316. [20] J.H. Shepherd, D.V. Shepherd, S.M. Best, Substituted hydroxyapatites for bone repair, J. Mater. Sci. Mater. Med. 23 (2012) 2335–2347. [21] V. Aina, L. Bergandi, G. Lusvardi, G. Malavasi, F.E. Imrie, I.R. Gibson, G. Cerrato, D. Ghigo, Sr-containing hydroxyapatite: morphologies of HA crystals and bioactivity on osteoblast cells, Mater. Sci. Eng. C 33 (2013) 1132–1142. [22] Hongtao Yang, Xinhua Qu, Wenjiao Lin, Cong Wang, Donghui Zhu, Kerong Dai, Yufeng Zheng, In vitro and in vivo studies on zinc-hydroxyapatite composites as novel biodegradable metal matrix composite for orthopedic applications, Acta Biomater. 71 (2018) 200–214. [23] B.M. Hidalgo-Robatto, M. López-Álvarez, A.S. Azevedo, J. Dorado, J. Serra, N.F. Azevedo, P. González, Pulsed laser deposition of copper and zinc doped hydroxyapatite coatings for biomedical applications, Surf. Coat. Technol. 333 (2018) 168–177. [24] Xi Chen, Qi-Li Tang, Ying-Jie Zhu, Cai-Lian Zhu, Xi-Ping Feng, Synthesis and antibacterial property of zinc loaded hydroxyapatite nanorods, Mater. Lett. 89 (2012) 233–235. [25] N. Lowry, M. Brolly, Y. Han, S. McKillop, B.J. Meenan, A.R. Boyd, Synthesis and characterisation of nanophase hydroxyapatite co-substituted with strontium and zinc, Ceram. Int. 44 (2018) 7761–7770. [26] R. Chakraborty, S. Sengupta, P. Saha, K. Das, S. Das, Synthesis of calcium hydrogen phosphate and hydroxyapatite coating on SS316 substrate through pulsed electrodeposition, Mater. Sci. Eng. C 69 (2016) 875–883. [27] R. Chakraborty, V. Seesala, S. Sengupta, S. Dhara, P. Saha, K. Das, S. Das, Comparison of Osteoconduction, cytocompatibility and corrosion protection performance of hydroxyapatite-calcium hydrogen phosphate composite coating synthesized in-situ through pulsed electro deposition with varying amount of phase and crystallinity, Surf. Interfaces 10 (2018) 1–10. [28] R. Chakraborty, P. Saha, A comparative study on surface morphology and electrochemical behaviour of hydroxyapatite-calcium hydrogen phosphate composite coating synthesized in-situ through electro chemical process under various deposition conditions, Surf. Interfaces 12 (2018) 160–167. [29] Guangfei Sun, Jun Ma, Shengmin Zhang, Electrophoretic deposition of zinc-substituted hydroxyapatite coatings, Mater. Sci. Eng. C 39 (2014) 67–72. [30] D. Gopi, A. karthika, Development of lotus like hydroxyapatite coating on HELCDEB treated titanium by pulsed electrodeposition, Mater. Lett. 105 (2013) 216–219. [31] A. Oyane, H.M. Kim, T. Furuya, T. Kokubo, T. Miyazaki, T. Nakamura, Preparation and assessment of revised simulated body fluids, J. Biomed. Mater. Res. A 56 (2003) 188–195. [32] S. Ziani, S. Meski, H. Khireddine, Characterization of magnesium-doped

4. Conclusions Zinc-calcium phosphate composites were successfully developed by electrochemical deposition from the single aqueous electrolyte bath containing zinc, calcium and phosphate ions, through the in-situ formation and co-deposition techniques. Nucleation rate and duty cycle were varied to find out the relation between physical characteristics of formed composite with operational parameters. Various phases namely elemental zinc, calcium orthophosphate, hydroxyapatite, hopeite and spencerite were formed with varying quantity and characteristics under different parameter conditions. Based on this entire study, following conclusion points are stipulated. a. Nucleation rate and duty cycle greatly influence the various reaction stages and thus lead to the formation of altogether different phases with varying morphology and relative weight percentage under the various combination of these two electrochemical deposition parameters. b. At a particular parameter of 10 mA/cm2 current density and 20% duty cycle, the highest amount of nano crystallite elemental zinc presence are there in the composite which aids to produce a unique nano tubes structure through the amalgamation of other nano crystallite calcium phosphate and zinc phosphate phases. c. The high amount of zinc substitution in calcium phosphate phases are evident under both low nucleation rate due to the higher diffusion time as well as high nucleation rate due to the faster movement of the zinc ion compared to calcium ions. d. The high amount of zinc substitution in calcium phosphate phases generates greater compressive lattice strain, which in turn leads to the faster dissolution of these phases under contact with SBF and exhibits poor corrosion resistance performance. e. The higher duty cycle leads to the formation of less hydroxyapatite phase due to unavailability of sufficient reaction time for the second phase of the conversion of calcium orthophosphate to hydroxyapatite f. 10 mA-20 DC coating exhibits the highest amount of electrochemical corrosion resistance due to its intact nano-tube scaffold structure along with the formation of nano-spherical apatite during osteoconduction under SBF. g. During bio-mineralization, high amount of zinc ion dissolution phenomena is observed in most of the coatings along with the incorporation of more and more calcium ions for the first 7 days of immersion. This would help to create a larger number of sites for new bone formation over the coating surfaces. In summary, this systematic study suggests that pulsed electro-deposition can be a promising strategy to develop the functional coatings of zinc-calcium phosphate composite over the metallic implant and the surface properties can be tailor-made in terms of adjustable biodegradability, biocompatibility and corrosion resistance performance. References [1] T. Kraus, S.F. Fischerauer, A.C. Hänzi, P.J. Uggowitzer, J.F. Löffler, A.M. Weinberg, Magnesium alloys for temporary implants in osteosynthesis: in vivo studies of their degradation and interaction with bone, Acta Biomater. 8 (3) (2012) 1230–1238. [2] P.K. Bowen, J. Drelich, J. Goldman, Zinc exhibits ideal physiological corrosion

12

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[33] [34] [35]

[36]

[37]

[38]

[39]

[40]

[41]

hydroxyapatite prepared by sol–gel process, Int. J. Appl. Ceram. Technol. 11 (1) (2014) 83–91. F. Miyaji, Y. Kono, Y. Suyama, Formation and structure of zinc-substituted calcium hydroxyapatite, Mater. Res. Bull. 40 (2005) 209–220. Y. Tang, H.F. Chappell, M.T. Dove, et al., Zinc incorporation into hydroxylapatite, Biomaterials 30 (2009) 2864–2872. A.R. Boyd, L. Rutledge, L.D. Randolph, B.J. Meenan, Strontium-substituted hydroxyapatite coatings deposited via a co-deposition sputter technique, Mater. Sci. Eng. C 46 (2015) 290–300. P.V. Gnaneshwar, et al., Ramification of zinc oxide doped hydroxyapatite biocomposites for the mineralization of osteoblasts, Mater. Sci. Eng. C 96 (2019) 337–346. L. Berzina-Cimdina, N. Borodajenco, Research of calcium phosphates using fourier transform infrared spectroscopy, in: T. Theophile (Ed.), Infrared Spectroscopy – Materials Science, Engineering and Technology, 2012, pp. 123–148. H. Yang, C. Wang, C. Liu, H. Chen, Y. Wu, J. Han, Z. Jia, W. Lin, D. Zhang, W. Li, Evolution of the degradation mechanism of pure zinc stent in the one-year study of rabbit abdominal aorta model, Biomaterials 145 (2017) 92–105. L. Zhao, Z. Zhang, Y. Song, S. Liu, Y. Qi, X. Wang, Q. Wang, C. Cui, Mechanical properties and in vitro biodegradation of newly developed porous Zn scaffolds for biomedical applications, Mater. Des. 108 (2016) 136–144. Y. Chen, W. Zhang, M.F. Maitz, M. Chen, H. Zhang, J. Mao, Y. Zhao, N. Huang, G. Wan, Comparative corrosion behavior of Zn with Fe and Mg in the course of immersion degradation in phosphate buffered saline, Corros. Sci. 111 (2016) 541–555. B. León, Pulsed laser deposition of thin calcium phosphate coatings, in: J.A. Jansen,

[42]

[43]

[44]

[45]

[46]

[47] [48]

[49]

13

B. León (Eds.), Thin Calcium Phosphate Coatings for Medical Implants, Springer, New York, 2009, pp. 101–156. R. Cai, H. Wang, M. Cao, L. Hao, L. Zhai, S. Jiang, X. Li, Synthesis and antimicrobial activity of mesoporous hydroxylapatite/zinc oxide nanofibers, Mater. Des. 87 (2015) 17–24. Z. Geng, Z. Cui, Z. Li, S. Zhu, Y. Liang, Y. Liu, X. Li, X. He, X. Yu, R. Wang, X. Yang, Strontium incorporation to optimize the antibacterial and biological characteristics of silver-substituted hydroxyapatite coating, Mater. Sci. Eng. C 58 (2016) 467–477. B.-D. Hahn, Y.-L. Cho, D.-S. Park, J.-J. Choi, J. Ryu, J.-W. Kim, C.-W. Ahn, C. Park, H.-E. Kim, S.-G. Kim, Effect of fluorine addition on the biological performance of hydroxyapatite coatings on Ti by aerosol deposition, J. Biomater. Appl. 27 (2013) 587–594. Y. Huang, X. Zhang, R. Zhao, H. Mao, Y. Yan, X. Pang, Antibacterial efficacy, corrosion resistance, and cytotoxicity studies of copper-substituted carbonated hydroxyapatite coating on titanium substrate, J. Mater. Sci. 50 (2015) 1688–1700. J. Shen, Y. Qi, B. Jin, X. Wang, Y. Hu, Q. Jiang, Control of hydroxyapatite coating by self-assembled monolayers on titanium and improvement of osteoblast adhesion, J. Biomed. Mater. Res. B Appl. Biomater. 105 (2017) 124–135. S.V. Dorozhkin, Nanosized and nanocrystalline calcium orthophosphates, Acta Biomater. 6 (2010) 715–734. N. Jones, B. Ray, K.T. Ranjit, A.C. Manna, Antibacterial activity of ZnO nanoparticle suspensions on a broad spectrum of microorganisms, FEMS Microbiol. Lett. 279 (2008) 71–76. R.J. Chung, M.F. Hsieh, K.C. Huang, L.H. Perng, F.I. Chou, T.S. Chin, Anti-microbial hydroxyapatite particles synthesized by a sol–gel route, J. Sol. Gel Sci. Technol. 33 (2005) 229–239.