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Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316 L stainless steel for hard tissue fixation
Accepted Manuscript
Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation Z.M. Al-Rashidy , M.M. Farag , N.A. Abdel Ghany , A.M. Ibrahim , WafaI. Abdel-Fattah PII: DOI: Reference:
S2468-0230(17)30033-0 10.1016/j.surfin.2017.03.010 SURFIN 83
To appear in:
Surfaces and Interfaces
Received date: Revised date: Accepted date:
16 January 2017 16 March 2017 20 March 2017
Please cite this article as: Z.M. Al-Rashidy , M.M. Farag , N.A. Abdel Ghany , A.M. Ibrahim , WafaI. Abdel-Fattah , Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation, Surfaces and Interfaces (2017), doi: 10.1016/j.surfin.2017.03.010
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Manuscript Title page:
Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation
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Zainab M. Al-Rashidy1, M.M. Farag2, N.A. Abdel Ghany3, A.M. Ibrahim4, and Wafa I. AbdelFattah1
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Highlights
Electrophoretic deposition (EPD) technique in aqueous electrolyte, as a green and simple method, was used. Borate bioactive glass prepared powder was deposited on 316L stainless steel using EPD technique with optimized parameters.
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316L stainless steel coated borate glass coupons were characterized and subjected to SBF and DMEM solutions, to determine its biocompatibility and corrosion
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resistance.
Potentiodynamic polarization technique was used to evaluate corrosion protection
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of coated samples in SBF and DMEM solutions.
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Title Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation Z.M. Al-Rashidy1, M.M. Farag2, N.A. Abdel Ghany3, A.M. Ibrahim4, and WafaI. Abdel-Fattah1
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Address 1-Refractories and Ceramics Dept., National Research Centre, Dokki, Cairo, Egypt 2- Glass Research Dept., National Research Centre, Dokki, Cairo, Egypt 3-Physical Chemistry Dept., National Research Centre, Dokki, Cairo, Egypt 4-University College for Girls, Ain Shams University, Cairo, Egypt
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Corresponding authors: Dr. Wafa I. Abdel-Fattah, Postal address: National Research Centre, Refractories and Ceramics Dept, Tahrir St.,33 El Bohouth St.(former El-Tahrir St.) Dokki, POB 12622, Cairo, Egypt. E-mail: [email protected]
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N.A. Abdel Ghany Physical Chemistry Dept., National Research Centre, Dokki, 12622, Cairo, Egypt [email protected]
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Co-authors: Z.M. Al-Rashidy Refractories and Ceramics Dept., National Research Centre, Dokki, 12622, Cairo, Egypt [email protected]
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M.M. Farag Glass Research Dept., National Research Centre, Dokki, 12622, Cairo, Egypt [email protected]
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A.M. Ibrahim University College for Girls, Ain Shams University, Cairo, Egypt [email protected]
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Abstract The present work was essentially concerned with an improvement of the biocompatibility of 316L stainless steel (316L SS) by coating its surface by bioactive borate glass layer using electrophoretic deposition (EPD) technique. Suspension of glass particles in double distilled water was used in this study as a coating suspension. The
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EPD parameters pH, applied voltage, glass concentration and time were optimized. The obtained coated substrates were investigated using FT-IR, X-ray diffraction, SEM/EDX analyses. The optimum conditions applied to obtain smooth, uniform, cracked-free, dense and adhesive layers with even glass particles distribution were achieved at voltage 35 V, pH 7 and 15 min deposition time. Additionally, the in vitro biodegradation was compared
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applying biochemical and electrochemical corrosion assessment of the developed glass coatings in two biological fluids, SBF (ISO 23317) and DMEM (ISO 13485) at body temperature (37 oC). The resulted coated 316L SS specimens showed good corrosion resistance in both SBF and DMEM solutions at 37 oC using potentiodynamic polarization
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(PDP) techniques.
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Keywords: borate glass, 316L stainless steel, coating, EPD, SBF/DMEM
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Introduction Metallic alloys of titanium, cobalt, magnesium and 316L stainless steel were used as orthopedic implants due to their high tensile strength, fatigue strength, and fracture toughness. Metallic biomaterials have likely been applied as supporting artificial joints, bone plates, screws and intramedullary nails. Additionally, they have been used for spinal
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fixations and spacers, external fixtures, pacemaker cases, as well as, they have been employed in artificial heart valves, wires, stents and dental implants [1].
Importantly, 316L stainless steel alloys were used vastly for human body implants. Although such metal implants are impressively corrosion resistant in many environments, localized corrosion could be faced when subjected to human body fluids. Reducing
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metallic ions and electrons transport could lead to passive oxide films that hinder corrosion rate. Moreover, some corrosive attacks on stainless steel may occur when subjected to body fluid, whereas, it contains various kinds of corrosive constituents, such as water, dissolved oxygen and large amounts of chloride ions (Cl-) beside other electrolytes like bicarbonate and small amounts of potassium, calcium, magnesium,
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phosphate, sulphate and amino acids, proteins, and plasma. Expectedly, the combined presence of these components results in initiation of some degree of corrosion of metal
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implants throughout the long period of implantation. As a result, the corrosion products can be very harmful to the human body [2-4]. The stainless steel common corrosion products, such as nickel, cobalt, chrome and their compounds are known as an
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allergens[5]. Beyond individual concentrations, a disturbance of the proper behavior of the osteoblast-like bone marrow cells could be expected[5].Consequently, it is essential
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to develop techniques to minimize the corrosion products of such implants within the aggressive body environment. Coating metallic implants with bioactive layer has been
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proposed as a promising approach to overcome such problem. In this context, bioactive glass [6-8], glass-ceramics [9-11]free of cracks and phosphate ceramic family as hydroxyapatite have been widely developed as bioactive coatings for metal implants [1214]due to their controlled surface reactivity beside bone bonding ability. Recently, borate glasses have attracted biomedical scientists attention due to their low chemical durability as fast conversion to HA compared to the widely applied silicate 45S5 bioactive glass[15-17]. Boron among other elements such as nickel, cobalt, chrome 5
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and their compounds play the essential roles in many life processes including embryogenesis. The active anti-inflammatory influence of boron containing compounds with little side effects was evidenced by several researchers[18, 19]. Positive effects in bone, brain, inflammation, and hormone function were demonstrated in a previous study [20].The anti-inflammatory functions of boron with minimum side effects were attributed
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to the suppression of serine proteases that are released by inflammation-activated white blood cells. Reduced reactive oxygen species, generated during neutrophil’s respiratory burst along with T-cell activity suppression and antibody concentrations are additionally effective[15, 21]. Consequently, the boron delivery upon biodegradation of borate glass is of particular interest for biomedical applications [22-27].
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However, research on borate bioactive glass is rather limited [8] while borosilicate glass was reported as coating materials[11]. Reports concerned with the surface of the borate-based glass coatings on load bearing metallic implants are seldom found despite its promising bioactivity.
Various coatings techniques including plasma spraying[10, 28, 29], sol–gel[30-33],
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electrophoretic deposition[34-37], electrochemical deposition, biomimetic process[3842]and sputter coating[43], were reported. The bioactive material coatings are considered
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potential methods for improving the orthopedic devices performance to reduce corrosion and achieve better biocompatibility[4]. Mostly implantable devices are required to exhibit
reaction[44].
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stable long-term performance at the host interface with minimum foreign body
Electrophoretic deposition (EPD) gained considerable attention among biomaterials
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coating techniques specifically bioactive ones and biomedical nanostructures. Its advantages include short processing time, simple apparatus setup, and few restrictions on
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substrate shape, requiring no binder burnout as the green surfaces contain negligible organics. It is usually carried out in the two-electrode cell. The mechanism involves two steps of electrophoresis and deposition. Initially, an electric field is applied between two electrodes. The charged particles being suspended in a suitable liquid move towards the oppositely charged electrode constituting the electrophoresis step. The particles accumulation at the deposition electrode creates a relatively compact and homogeneous film and is referred to as the deposition step. Essentially, a stable suspension containing 6
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the charged particles is free to move when an electric field is applied in a suitable electrolyte. Dispersion media of organic solvents are preferably used more than water in EPD to avoid the problems of electrolysis and gas evolution.
Solutions of high
oxidation–reduction potentials like benzene or ketenes could also be used[45]. However, water is preferable avoiding organic media being costly and for environmental
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impact[45].Further, a heat-treatment step is essential to eliminate porosity and enhance the deposits density.
The present work aimed to improve the biocompatibility and in turn functionality of 316L SS alloy through coating its surface with bioactive borate glass layer using EPD technique. Furthermore, the study mainly aimed to perform coating process in the cost-
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effective and environmental impact aqueous solution, rather than organic solvents. For these purposes, different coating conditions were studied to determine the optimum coating conditions.
2 Materials and Experimental Methods
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2.1. Materials
The bioactive borate glass powder inB2O3-CaO-Na2O-MgO system, with the
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composition shown in Table 1, was prepared by melting process. Reagents grade of H3BO3, CaCO3, and Na2CO3andMgO were mixed thoroughly and further melted in a platinum crucible at 1050 °C for two h using the electrical furnace. The melted glass was
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quenched in water to prevent crystallization and achieve the glass frit. Further, it was dried at room temperature and ground using an electrical porcelain ball mill for 2.5 h
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(Fretch, Germany) and the powder was sieved through sieves of 45 µm and 25 µm apertures, and the particles between those were collected and used for EPD. Bulk glass
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sample was prepared for density measurements.
2.2. Treatment of metal plates Surgical 316L stainless steel (316L SS) plates with a nominal composition
represented in Table 2 were used. The metal plates were polished with emery paper to 1200 grit and degreased with acetone, ethanol for 15 min at 45 °C in an ultrasonic bath and washed with double distilled water. Further, they were treated in 5 mMNaOH at 60 7
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°C for 24 h and further washed with double distilled water in an ultrasonic bath. Finally, they were etched in 1:3 (volume ratio) HF:HNO3 for 10 min (the concentration of HF was 5% and that of HNO3 was 15%) at room temperature. Thereafter, they were washed with double distilled (dd) water in an ultrasonic bath and then dried. To determine the weight of deposited glass, the treated plates (electrodes) were washed with ethanol and
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weighed before and after EPD. The suspension electrolytes were composed of 50 ml of glass powder with concentrations 0.5, 1, 3, or 4 wt% in dd water. To support the dispersion process and prevent glass settling, the constituents were gently stirred with a magnetic stirrer at room temperature. The pH was adjusted and measured (3040 Ion
2.3 Electrophoretic deposition (EPD)
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Analyzer meter).
The electrophoretic deposition was performed at individually several voltages of 10, 20, 25, 30, 35 and 40 V for periods of 15 min each. The dimensions of the working area of both electrodes were 10 x 10 x 1 mm and the separation distance between the
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electrodes was adjusted to 1 cm. The electric voltage was applied by DC power supply (GW Model No: GPC-3060). Samples were pulled out from the suspension after the
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coating process and dried for 24 h at ambient conditions.
2.4. Factors influencing EPD
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Four factors that affected the quality of achieved coatings were investigated including the glass concentrations, pH of the suspensions, applied voltages and
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deposition time.
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3. Characterization The particle size of the glass powder was measured by particle size analyzer and
SEM image. On the other hand, the density of bulk glass with irregular shape was measured by the Archimedes method, using distilled water as a liquid. The density (D) was calculated by the equation: D = Wair L / (Wair – W liquid)
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g/cm3
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Where Wair is the weight of the sample in the air, W
liquid
is the weight of the sample in
the liquid and L is the density of the immersion liquid (equal 1 for water). Fourier transform infrared spectroscopy (model FT/IR-6100 type A) was used at room temperature (~20 oC) within the wavenumber range of 4000–400 cm-1 at a resolution of 2 cm-1.The functional groups and chemical structure of the prepared borate
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glass particles and coatings were determined. The thermal behavior of the borate glass was characterized by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) under the nitrogen atmosphere using DSC-TGA instrument model STD Q600 at a heating rate of 10 oC/min. The surfaces morphology and composition were determined using scanning electron microscope coupled with energy dispersive X-ray
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analysis (SEM/EDX) (model JEOL JXA-840A, Electron probe microanalyzer). X-ray diffractometer(model, BRUKER as, D8 ADVANCE) was used to study the crystal structure of as-prepared glass and glass coatings using Ni-filtered Cu radiation with a tube voltage of 40 kV. A current of 25 mA was utilized. The hydrophilicity and wettability of the achieved borate glass coatings were evaluated by measuring the contact
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angle using horizontal plate camera perpendicular to liquid plate droplet plane using Compact Video Microscope (CVM), manufactured by SDL-UK. About 250 µl of
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distilled water was used as a liquid for measuring the wettability according to ASTM D724-99 and ASTM D5946-96 method.
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3.2. In vitro degradation test
3.2.1. Biological fluids (7 days)
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In vitro degradation tests of glass-coated substrate were followed in SBF solution
ISO 23317, as well as, DMEM solution by measuring the ionic concentrations of the
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released species from glass coatings and stainless steel substrates. Ionic boron, calcium, phosphorus, magnesium and iron along with total proteins levels released into the immersion liquids were measured at the predetermined times (3, 6 h, 1, 3 and 7 d) using either ICP (Model) or colorimetric kits (BIODIGNOSTIC, Egypt). Ionic Boron concentration measured by ICP-OES model, Agilent5100 Synchronous Vertical Dual View (SVDV). The pH of the incubated solutions was also measured.
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3.2.2. Inorganic ionic concentrations Calcium ions react with methyl thymol, in an alkaline medium to form colored compound (blue color) that absorbsat λ = 585nm. Inorganic phosphorus reacts with molybdic acid to form phosphomolybdate complex (blue color). It’s measured at λ = 640nm.
thioglycolic
acid.
The
colored
compound
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The iron is dissociated by hydrochloric acid then reduced to ferrous by the
metal
forms
with
bathophenanthroline is measured colorimetrically at 535 nm. The concentrations of calcium, phosphorus and iron were calculated by applying the following equation: Concn. = (Asample / Astandard) x concn. of standard
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Where, the concentrations of Ca and P ions were in mmol/l and the concentration of Fe ions were in µmol/l.
Magnesium ions react with metallochrome dye calm augite to form chromophore which absorbs at 520 nm. The concentrations of magnesium ions were calculated by the following equation:
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Concn. (mmol/ L) = (Asample/ Astandard) x 1.0mmol/l The protein molecule when treated with an alkaline cupric sulfate, forms a violet color
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that absorbs at 550 nm. The concentration of protein is calculated by the following equation:
g/dl
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Concn.(g/dl) = (Asample/ Astandard) x 5
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3.2.3. Potentiodynamic polarization (PDP) The electrochemical behavior of coated substrate compared with bare 316L SS
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alloy were studied in simulated body fluid (SBF) and Dulbecco's Modified Eagle Medium (DMEM) solutions (represented in Table 3) at pH 7.4, stagnant solution and 37 °C using potentiodynamic polarization (PDP) test in open atmosphere. The device model used was Autolab 302N electrochemical workstation (Metrohm). The potential scanning rate applied to record the polarization curves was 1 mV/Sc, and the potential scan started from -200 mV. The measurements were carried out in three electrode cell, the platinum electrode was used as a counter electrode and saturated calomel electrode (SCE) as a 10
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reference electrode, as well as, the substrate with/without coating was used as a working electrode, with exposed area of 1 cm2. Different parameters, such as corrosion potential (Ecorr), polarization resistance, corrosion rate and pitting area, were calculated from the polarization curves. For reproducibility, five sets of the uncoated and coated samples
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obtained under the optimum coating parameters were measured.
4. Results and discussion 4.1. Characterization of glass
The density of the glass as measured by Archimedes method was 2.43 g/cm3. The bioactive glass which was melted for two h at 1050°C and casted; was transparent and
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colorless granules without crystalline inclusions as confirmed by XRD (Figure 1a).
The particle size of as-prepared glass after grinding for 2.5 h in planetary ball mill was measured by particle size analyzer and SEM image. Figure 1b represents the particle size distribution curve of the ground glass which exhibited a bimodal distribution curve with two maxima. The first one at 10.6 μm was sharp and with higher glass particle
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percentage (58.5 %), while the second one at 45.9 μm was broad for lower glass particle percentage (41.5%). These results were confirmed by SEM image (Figure 2). Figure 1c
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shows DSC and TGA profiles of as-prepared glass. The DSC curve demonstrated a long and sharp endothermic peak at 62 °C, which probably corresponded to the removal of adsorbed water on glass particles surfaces. Another weak and broad endothermic peak
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was observed at 275 °C, which is compared to glass transition temperature (Tg). Interestingly, as no exothermic peak was recorded for the glass sample, proved the
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inability of the glass for crystallization at high temperature. Additionally, TGA curve showed that a total weight loss of only 3% up to 850 °C.
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Figure 2 shows SEM micrograph and EDX analysis of the glass particles. Where,
small homogenous particles with few large ones could be recognized. These glass particles have angular shapes with sharp edges. Also, EDX analysis showed the peaks corresponding to elemental glass components. Boron element couldn’t be detected by such analysis.
4.2. Determination of optimal suspensions and EPD parameters 11
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The coating thickness and morphology depend on several parameters, such as the concentration of glass, deposition time, applied voltage and pH of suspension (electrolyte). In order to obtain the optimum deposited coating, these parameters were fully investigated. Nevertheless, the results of change of glass concentration showed that suspensions with either 0.5 or 1 wt% of glass particles produced a thin layer which could
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not completely cover the metal surface. Further increase of glass concentration to 3 wt% resulted in a thicker coating. The optimum coating was obtained at 4 wt% of glass concentration. Based on this result, the mass (weight gain) of the deposited layers were measured at 10, 20, 25, 30, 35 and 40 applied volts, the coatings weight were 0.1, 0.2, 0.42, 0.4, 2.44 and 0.4 g/mm2 x 10-4, respectively (Figure 3a). The optimum result was
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obtained using 35 V with sufficient weight (2.44 mg/mm2). Accordingly, at such previously mentioned conditions (4 wt% glass and 35 V), the pH of suspension was assessed in this study due to its significant effect on the charge of particles inside the solution. The pH was varied from 6.5 to 11. The results showed that the glass particles were mainly deposited on the cathode (cathodic deposition) in neutral suspension (pH 7),
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which meant that it acquired a positive charge at this pH. Increase of the pH gave raise to coating thickness decrease, which can be explained by a suppressing of the surface
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ionization and lowering of suspension conductivity. Nevertheless, pH 7 showed an optimum coupled with highest coating mass (1.23 g/mm2 x 10-4) (Figure 3c and d). At other respective pH values, the obtained mass values of the glass coatings were 0.62, 0.8,
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0.4, 0.2, 0.76, 0.44 and 0.2 g/mm2 x 10-4 at pH values of 6.5, 7.5, 8.5, 9, 9.5, 10 and 11. However, the coating mass did not decrease linearly with the increase of pH, but there
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were a fluctuation of the deposited masses. This can be explained by the dissolution behavior of glass particles during the deposition process. Though, during electrophoretic
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deposition (EPD) of borate glass particles in aqueous medium, some degree of dissolution of glass can occur throughout the deposition process. Where, there are two possible reactions being; hydration of alkali and alkaline earth ions beside hydrolysis of the borate network. Hydration of alkali ions results in release of such ions into the solution and replaced with protons in glass network for charge balance. Hydrolysis of the borate network is generated from breaking of glass network to form three coordinated borate units and eventually release boric acid, B(OH)3, to the solution [46]. However, at 12
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higher pH, B(OH)-4 anions are more dominant. Therefore, the type of borate unit species released from the glass is pH dependent. As a result, there are interfered deposition and dissolution reactions during the electrophoretic deposition process. It might be that the amount of the deposited glass particles depended on the relative amount of B(OH)3 to B(OH)-4 in the solution beside the amount of the released alkali ions. All these issues the aqueous suspension medium.
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probably caused a fluctuation in the thickness of deposited layer with the change of pH of
4.3. FTIR, XRD, SEM-EDX and contact angle measurements
Figure 4a represents FT-IR spectra of 316L SS, as-prepared borate glass and substrate.
The
following
bands
were
detected
from
FTIR
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glass-coated
spectra[47]:Bending vibration bands at 590 and 636 cm-1 corresponded to [BO3] isolated units. The wave bands at 676 and 715 cm-1 were related to vibration of oxygen bridges between two trigonal boron atoms. The B-O stretching vibration bands in the [BO4] units as detected from spectral splitting in the 810 - 1107 cm-1 spectral region into bands at
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876, 910, 949 and 988 cm-1.However, for glass coating the intensity of the glass characteristic vibrational bands decreased, while, some bands shifted to higher and other
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ones shifted to lower frequencies.
Figure 4b represents XRD patterns of as-prepared glass, 316L SS substrate and the surface of the glass coating. Two diffraction peaks with d-spacing 2.07 Å and 1.79 Å
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were characterizing the 316L SS substrate. Additionally, these two peaks were still predominant in of glass coating, but with lower intensity because of the coverage of the
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substrate surface with glass powder. Moreover, a new diffraction peak at 3.04 Å could be attributed to a formation of the new bond at the glass/substrate interface.
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Figure 5 shows SEM micrographs of glass coating surface coupled with EDX
analysis (a) and cross section of the glass-coated substrate (b).The figure represented a homogeneous, compact and crack-free coating. The EDX analyses showed the same elemental components of glass particles. Additionally, the thickness of glass coating was about 60 µm, as measured from SEM photo of glass-substrate cross-section (Figure 5b). Optical photos (Figure 6) of water drop on 316L SS substrate and substrate with glass coating revealed that the contact angle of the uncoated substrate was 104o, and 13
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decreased to 60ofor the coated substrate. These results showed that borate glass coating changed the metal substrate surface from hydrophobic to hydrophilic properties. Therefore, the borate glass increased the substrate wettability due to its hydrophilic properties which is expected to induce a positive mark for the cell attachment and interaction. Therefore, the achieved glass in this study has hydrophilic properties like the
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well-known Bioglass®[48, 49].
4.4.In vitrodegradation test 4.4.1. pH measurements
The pH of incubated media of the studied glass-coated substrates as a function of
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time is represented in Figure 7a. The pH values of samples immersed in SBF decreased slightly and reached to 7.05 at the end of the incubation time. This decrease is attributed to the consumption of Ca and P ions present in SBF in the hydroxyapatite crystals formation as confirmed by the degradation curve of Ca and P ions (Figure 7c, e). On the hand, pH of sample incubated in DMEM showed different behavior. It increased
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continuously to reach 8.32 at the end of the incubation period. The release of alkaline ions from the glass-coatings to the solution led to increase of DMEM pH. Because of the
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non-buffered nature of such fluid[50](unlike SBF) pH values were clearly affected by increasing alkali ions in the media.
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4.4.2. Ionic release concentrations
The levels of B, Ca, P, Mg, Fe and total proteins released from glass coating into
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SBF and DMEM are shown in Figures 7b-g. The behavior of changing profile of concentrations of B ions released into both solutions was almost the same (Figure 7b).
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However, the concentration of B ions released into SBF was higher than that released from DMEM solution. The initial concentration of B ion was relatively high during initial immersion hours, then it decreased and then increased a little bit at around 20 h and after this period its concentration became nearly constant. This initial fluctuation of B concentration in the solution was due to the conversion of borate glass into hydroxyapatite phase. After the complete conversion of the glass to hydroxyapatite crystals, the concentration of B ions in the solution became constant [51-53]. The 14
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dissolution curves of Ca and Mg ions (Figure 7c and d) in both solutions also showed similar behavior. Moreover, the concentration of Ca and Mg ions in SBF was slightly higher than that in DMEM solution. This was because the starting Ca ionic concentration in SBF (2.5 mmol/dm3) and starting Mg ion concentration (1.0 mmol/dm3) are higher than that in DMEM (1.8 mmol/dm3 and 0.8 mmol/dm3 respectively)[50]. Also, the release
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behavior of P ions into SBF and DMEM was similar (Figure 7e). In contrast, the concentrations of P ions released into SBF were higher than that released into DMEM. In both solutions the concentrations of P ions decreased obviously after one day of incubation. This could be attributed to a consumption of P ions in the formation of hydroxyapatite phase. These results agreed with previous works[54]. Moreover, the
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concentration of Fe ion released into SBF was close to that released into DMEM solution. It increased till the third day of incubation (Figure 7f) then it was almost constant up to the end of immersion time due to the precipitation of hydroxyapatite crystals on the glass coatings. Figure 7g shows the change of concentration of total proteins in DMEM solution during the incubation of glass-coated substrate sample. The concentrations of the
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total proteins increased initially up to the first day of incubation then it started to decrease
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slightly till the end of immersion time.
4.5. In vitro electrochemical corrosion study 4.5.1.Potentiodynamic polarization (PDP) studies
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Figure 8and Table 4show the anodic and cathodic polarization curves and parameters, respectively for bioactive glass coated samples with the uncoated 316L SS in
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DMEM and SBF solutions. It could be realized that 316L SS alloy had active behavior in both DMEM and SBF solutions during cyclic polarization. While, this behavior
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decreased in case of glass coated samples in both solutions. Furthermore, corrosion rates (Mils per year, mpy) of the coated substrates in DMEM and SBF (2.963 mpy and 4.533 mpy, respectively) were lower than corrosion rates of uncoated substrates in both solutions (3.468 mpy and 6.838 mpy, respectively). Moreover, the results showed that the corrosion rate in DMEM was less than that in SBF. This could be attributed to the higher chloride ion concentration in SBF (148.8 mmol/dm3) than in DMEM (118.5 mmol/dm3) which is known as the most corrosive ions for metallic alloy, specifically 316L SS. On 15
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the other hand, the corrosion current densities of the coated samples were reduced apparently in comparison to uncoated samples in DMEM and SBF (Table 4). The data reported in (Table 4) (Pitt. Area, mC/cm2) represented the integrated surfaces under the polarization curves for each zone. The obtained results were expressed in mC and were measured directly using the potentiostat program (area under curve).In case of immersion
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of coated metal electrode in SBF, the free potential was measured relative to a reference electrode without an externally imposed potential or current. From these results, the bioactive borate glass acted as a protective layer leading to a reduction of metal corrosion rate. It was noted that SBF solution was more destructive for metal under investigation
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than DMEM solution.
5. Conclusions
The developed borate bioactive glass was successfully prepared and coated onto 316L SS substrates by the electrophoretic deposition technique in aqueous solution, (green electrolyte), which is a cheap and environmentally desirable solution. The EPD
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parameters, namely, suspension particle concentrations, applied voltage, pH, and deposition time which control the deposition yield were thoroughly investigated. The
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optimum parameters were attained at 4wt% glass level and a potential of 35 V at pH 7 for 15 minutes. Before EPD, the stainless steel substrates were treated to create a rough surface which significantly increased the glass layer thickness during EPD coating
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process. According to the electrochemical measurements, coatings proved noticeable improvement for corrosion protection that satisfies the medical requirements of the
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selected materials.
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Acknowledgements The financial support of the present work within National Research Centre,
Biomaterials group, Cairo, Egypt is appreciated. The authors acknowledge and thank National Research Centre, Central Labs, Cairo, Egypt for providing measurement facilities applied in this research.
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Figure captions Graphical abstract: Schematic illustration of borate glass preparation and its coating on SS steps by EPD process. Figure1. (a) XRD pattern of as-prepared glass and (b) Particle size distribution after grinding in planetary ball mill (2.5 h).DSC-TGA thermal analysis of as
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prepared glass (c).
Figure 2.SEM micrograph of milled glass particulates and its EDX analysis.
Figure 3.EPD coating technique (a) Optical photos of 316L SS plates coated as a function of incremental voltages (10 - 40 volt) and b) Effect of voltages on the deposited glass particles weight. (c) Optical photos of 316L plates coated at
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gradual incremental pH values and (d) Effect of solution pH on the deposited glass particles weight.
Figure 4. FT-IR reflection spectra (a) and XRD patterns (b) of as-prepared borate glass and 316L SS substrate compared to glass coating.
of the glass coating (b).
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Figure 5. SEM micrographs of surface coupled with EDX analysis (a) and cross section
Figure 6. Optical photos of water drop on 316l substrate (SS sample) and substrate with
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glass coating (glass coat. sample) and contact angle measurements. Figure 7.pH change (a) concentration of released B (b),Ca (c), Mg (d), P (e), Fe (f) and
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total proteins (g) into SBF and DMEM solutions as a function of time. Figure 8.Potentiodynamic polarization curves of SS and coated SS in DMEM compared
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to SBF media.
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Table 1.Chemical composition in mol% of glass prepared and used in this study B2O3 CaO Na2O MgO Oxides 60
Mol%
10
20
10
Table 2. Nominal composition of surgical 316L stainless steel (wt %) Si 0.01
Mn 0.01
P 0.29
S 0.001
Ni 19.5
Cr 15.08
Mo 2.74
Cu ----
Al 0.024
Fe Bal.
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C 0.015
Table 3.Ion concentration (mmol/dm3) of mod-DMEM and SBF. Mod-DMEM SBF
K+ 5.4 5.0
Ca2+ 1.8 2.5
Mg2+ 0.8 1.0
Cl118.5 148.8
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Na+ 154.5 142.0
HCO344.0 4.2
HPO420.9 1.0
SO420.8 0.0
Table 4. Main parameters obtained from cyclic polarization method Pol. Resist., KOhms 2.845
Corr. Pot., mV -436
Corr. Curr., uA/cm2 22.86
Pitt. Area, mC/cm2 780.3
Coat in DMEM
7.078
3.331
-635.2
15.58
691.7
Bare in SBF
6.106
1.443
-424.4
13.44
505.9
Coat in SBF
4.533
2.177
-555.8
9.974
449.5
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Bare in DMEM
Corr. Rate, mpy 10.390
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Corr. Rate: corrosion rate, Pol. Resist.: polarization resistance, Corr. Pot.: corrosion potential, Corr. Curr.: corrosion current, and Pitt. Area: pitting area.
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Graphical abstract: Schematic illustration of borate glass preparation and its coating on 316L SS by EPD process.
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Figure1. (a) XRD pattern of as-prepared glass and (b) Particle size distribution after grinding in planetary ball mill (2.5 h).DSC-TGA thermal analysis of as prepared glass (c). 24
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Figure 2.SEM micrograph of milled glass particulates and its EDX analysis.
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Figure 3. EPD coating technique (a) Optical photos of 316L SS plates coated as a function of incremental voltages (10 - 40 volt) and b) Effect of voltages on the deposited glass particles weight. (c) Optical photos of 316L plates coated at gradual incremental pH values and (d) Effect of solution pH on the deposited glass particles weight.
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Figure 4. FT-IR reflection spectra (a) and XRD patterns (b) of as-prepared borate glass and 316L SS substrate compared to glass coating.
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Figure 5. SEM micrographs of surface coupled with EDX analysis (a) and cross section of the glass-coated substrate (b). 28
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Figure 6. Optical photos of water drop on 316L SS substrate (SS sample) and substrate with glass coating (glass coat. sample) and contact angle measurements.
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Figure 7.pH change (a), concentration of released B (b), Ca (c), Mg (d), P (e), Fe (f) and total proteins (g) from glass-coated substrate into SBF and DMEM solutions as a function of time. 30
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Figure 8.Potentiodynamic polarization curves of SS and coated SS in DMEM compared to SBF media.
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Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation Z.M. Al-Rashidy , M.M. Farag , N.A. Abdel Ghany , A.M. Ibrahim , WafaI. Abdel-Fattah PII: DOI: Reference:
S2468-0230(17)30033-0 10.1016/j.surfin.2017.03.010 SURFIN 83
To appear in:
Surfaces and Interfaces
Received date: Revised date: Accepted date:
16 January 2017 16 March 2017 20 March 2017
Please cite this article as: Z.M. Al-Rashidy , M.M. Farag , N.A. Abdel Ghany , A.M. Ibrahim , WafaI. Abdel-Fattah , Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation, Surfaces and Interfaces (2017), doi: 10.1016/j.surfin.2017.03.010
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Manuscript Title page:
Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation
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Zainab M. Al-Rashidy1, M.M. Farag2, N.A. Abdel Ghany3, A.M. Ibrahim4, and Wafa I. AbdelFattah1
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Highlights
Electrophoretic deposition (EPD) technique in aqueous electrolyte, as a green and simple method, was used. Borate bioactive glass prepared powder was deposited on 316L stainless steel using EPD technique with optimized parameters.
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316L stainless steel coated borate glass coupons were characterized and subjected to SBF and DMEM solutions, to determine its biocompatibility and corrosion
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resistance.
Potentiodynamic polarization technique was used to evaluate corrosion protection
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of coated samples in SBF and DMEM solutions.
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Title Aqueous electrophoretic deposition and corrosion protection of borate glass coatings on 316L stainless steel for hard tissue fixation Z.M. Al-Rashidy1, M.M. Farag2, N.A. Abdel Ghany3, A.M. Ibrahim4, and WafaI. Abdel-Fattah1
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Address 1-Refractories and Ceramics Dept., National Research Centre, Dokki, Cairo, Egypt 2- Glass Research Dept., National Research Centre, Dokki, Cairo, Egypt 3-Physical Chemistry Dept., National Research Centre, Dokki, Cairo, Egypt 4-University College for Girls, Ain Shams University, Cairo, Egypt
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Corresponding authors: Dr. Wafa I. Abdel-Fattah, Postal address: National Research Centre, Refractories and Ceramics Dept, Tahrir St.,33 El Bohouth St.(former El-Tahrir St.) Dokki, POB 12622, Cairo, Egypt. E-mail: [email protected]
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N.A. Abdel Ghany Physical Chemistry Dept., National Research Centre, Dokki, 12622, Cairo, Egypt [email protected]
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Co-authors: Z.M. Al-Rashidy Refractories and Ceramics Dept., National Research Centre, Dokki, 12622, Cairo, Egypt [email protected]
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M.M. Farag Glass Research Dept., National Research Centre, Dokki, 12622, Cairo, Egypt [email protected]
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A.M. Ibrahim University College for Girls, Ain Shams University, Cairo, Egypt [email protected]
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Abstract The present work was essentially concerned with an improvement of the biocompatibility of 316L stainless steel (316L SS) by coating its surface by bioactive borate glass layer using electrophoretic deposition (EPD) technique. Suspension of glass particles in double distilled water was used in this study as a coating suspension. The
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EPD parameters pH, applied voltage, glass concentration and time were optimized. The obtained coated substrates were investigated using FT-IR, X-ray diffraction, SEM/EDX analyses. The optimum conditions applied to obtain smooth, uniform, cracked-free, dense and adhesive layers with even glass particles distribution were achieved at voltage 35 V, pH 7 and 15 min deposition time. Additionally, the in vitro biodegradation was compared
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applying biochemical and electrochemical corrosion assessment of the developed glass coatings in two biological fluids, SBF (ISO 23317) and DMEM (ISO 13485) at body temperature (37 oC). The resulted coated 316L SS specimens showed good corrosion resistance in both SBF and DMEM solutions at 37 oC using potentiodynamic polarization
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(PDP) techniques.
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Keywords: borate glass, 316L stainless steel, coating, EPD, SBF/DMEM
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Introduction Metallic alloys of titanium, cobalt, magnesium and 316L stainless steel were used as orthopedic implants due to their high tensile strength, fatigue strength, and fracture toughness. Metallic biomaterials have likely been applied as supporting artificial joints, bone plates, screws and intramedullary nails. Additionally, they have been used for spinal
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fixations and spacers, external fixtures, pacemaker cases, as well as, they have been employed in artificial heart valves, wires, stents and dental implants [1].
Importantly, 316L stainless steel alloys were used vastly for human body implants. Although such metal implants are impressively corrosion resistant in many environments, localized corrosion could be faced when subjected to human body fluids. Reducing
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metallic ions and electrons transport could lead to passive oxide films that hinder corrosion rate. Moreover, some corrosive attacks on stainless steel may occur when subjected to body fluid, whereas, it contains various kinds of corrosive constituents, such as water, dissolved oxygen and large amounts of chloride ions (Cl-) beside other electrolytes like bicarbonate and small amounts of potassium, calcium, magnesium,
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phosphate, sulphate and amino acids, proteins, and plasma. Expectedly, the combined presence of these components results in initiation of some degree of corrosion of metal
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implants throughout the long period of implantation. As a result, the corrosion products can be very harmful to the human body [2-4]. The stainless steel common corrosion products, such as nickel, cobalt, chrome and their compounds are known as an
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allergens[5]. Beyond individual concentrations, a disturbance of the proper behavior of the osteoblast-like bone marrow cells could be expected[5].Consequently, it is essential
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to develop techniques to minimize the corrosion products of such implants within the aggressive body environment. Coating metallic implants with bioactive layer has been
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proposed as a promising approach to overcome such problem. In this context, bioactive glass [6-8], glass-ceramics [9-11]free of cracks and phosphate ceramic family as hydroxyapatite have been widely developed as bioactive coatings for metal implants [1214]due to their controlled surface reactivity beside bone bonding ability. Recently, borate glasses have attracted biomedical scientists attention due to their low chemical durability as fast conversion to HA compared to the widely applied silicate 45S5 bioactive glass[15-17]. Boron among other elements such as nickel, cobalt, chrome 5
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and their compounds play the essential roles in many life processes including embryogenesis. The active anti-inflammatory influence of boron containing compounds with little side effects was evidenced by several researchers[18, 19]. Positive effects in bone, brain, inflammation, and hormone function were demonstrated in a previous study [20].The anti-inflammatory functions of boron with minimum side effects were attributed
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to the suppression of serine proteases that are released by inflammation-activated white blood cells. Reduced reactive oxygen species, generated during neutrophil’s respiratory burst along with T-cell activity suppression and antibody concentrations are additionally effective[15, 21]. Consequently, the boron delivery upon biodegradation of borate glass is of particular interest for biomedical applications [22-27].
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However, research on borate bioactive glass is rather limited [8] while borosilicate glass was reported as coating materials[11]. Reports concerned with the surface of the borate-based glass coatings on load bearing metallic implants are seldom found despite its promising bioactivity.
Various coatings techniques including plasma spraying[10, 28, 29], sol–gel[30-33],
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electrophoretic deposition[34-37], electrochemical deposition, biomimetic process[3842]and sputter coating[43], were reported. The bioactive material coatings are considered
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potential methods for improving the orthopedic devices performance to reduce corrosion and achieve better biocompatibility[4]. Mostly implantable devices are required to exhibit
reaction[44].
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stable long-term performance at the host interface with minimum foreign body
Electrophoretic deposition (EPD) gained considerable attention among biomaterials
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coating techniques specifically bioactive ones and biomedical nanostructures. Its advantages include short processing time, simple apparatus setup, and few restrictions on
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substrate shape, requiring no binder burnout as the green surfaces contain negligible organics. It is usually carried out in the two-electrode cell. The mechanism involves two steps of electrophoresis and deposition. Initially, an electric field is applied between two electrodes. The charged particles being suspended in a suitable liquid move towards the oppositely charged electrode constituting the electrophoresis step. The particles accumulation at the deposition electrode creates a relatively compact and homogeneous film and is referred to as the deposition step. Essentially, a stable suspension containing 6
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the charged particles is free to move when an electric field is applied in a suitable electrolyte. Dispersion media of organic solvents are preferably used more than water in EPD to avoid the problems of electrolysis and gas evolution.
Solutions of high
oxidation–reduction potentials like benzene or ketenes could also be used[45]. However, water is preferable avoiding organic media being costly and for environmental
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impact[45].Further, a heat-treatment step is essential to eliminate porosity and enhance the deposits density.
The present work aimed to improve the biocompatibility and in turn functionality of 316L SS alloy through coating its surface with bioactive borate glass layer using EPD technique. Furthermore, the study mainly aimed to perform coating process in the cost-
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effective and environmental impact aqueous solution, rather than organic solvents. For these purposes, different coating conditions were studied to determine the optimum coating conditions.
2 Materials and Experimental Methods
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2.1. Materials
The bioactive borate glass powder inB2O3-CaO-Na2O-MgO system, with the
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composition shown in Table 1, was prepared by melting process. Reagents grade of H3BO3, CaCO3, and Na2CO3andMgO were mixed thoroughly and further melted in a platinum crucible at 1050 °C for two h using the electrical furnace. The melted glass was
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quenched in water to prevent crystallization and achieve the glass frit. Further, it was dried at room temperature and ground using an electrical porcelain ball mill for 2.5 h
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(Fretch, Germany) and the powder was sieved through sieves of 45 µm and 25 µm apertures, and the particles between those were collected and used for EPD. Bulk glass
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sample was prepared for density measurements.
2.2. Treatment of metal plates Surgical 316L stainless steel (316L SS) plates with a nominal composition
represented in Table 2 were used. The metal plates were polished with emery paper to 1200 grit and degreased with acetone, ethanol for 15 min at 45 °C in an ultrasonic bath and washed with double distilled water. Further, they were treated in 5 mMNaOH at 60 7
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°C for 24 h and further washed with double distilled water in an ultrasonic bath. Finally, they were etched in 1:3 (volume ratio) HF:HNO3 for 10 min (the concentration of HF was 5% and that of HNO3 was 15%) at room temperature. Thereafter, they were washed with double distilled (dd) water in an ultrasonic bath and then dried. To determine the weight of deposited glass, the treated plates (electrodes) were washed with ethanol and
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weighed before and after EPD. The suspension electrolytes were composed of 50 ml of glass powder with concentrations 0.5, 1, 3, or 4 wt% in dd water. To support the dispersion process and prevent glass settling, the constituents were gently stirred with a magnetic stirrer at room temperature. The pH was adjusted and measured (3040 Ion
2.3 Electrophoretic deposition (EPD)
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Analyzer meter).
The electrophoretic deposition was performed at individually several voltages of 10, 20, 25, 30, 35 and 40 V for periods of 15 min each. The dimensions of the working area of both electrodes were 10 x 10 x 1 mm and the separation distance between the
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electrodes was adjusted to 1 cm. The electric voltage was applied by DC power supply (GW Model No: GPC-3060). Samples were pulled out from the suspension after the
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coating process and dried for 24 h at ambient conditions.
2.4. Factors influencing EPD
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Four factors that affected the quality of achieved coatings were investigated including the glass concentrations, pH of the suspensions, applied voltages and
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deposition time.
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3. Characterization The particle size of the glass powder was measured by particle size analyzer and
SEM image. On the other hand, the density of bulk glass with irregular shape was measured by the Archimedes method, using distilled water as a liquid. The density (D) was calculated by the equation: D = Wair L / (Wair – W liquid)
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g/cm3
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Where Wair is the weight of the sample in the air, W
liquid
is the weight of the sample in
the liquid and L is the density of the immersion liquid (equal 1 for water). Fourier transform infrared spectroscopy (model FT/IR-6100 type A) was used at room temperature (~20 oC) within the wavenumber range of 4000–400 cm-1 at a resolution of 2 cm-1.The functional groups and chemical structure of the prepared borate
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glass particles and coatings were determined. The thermal behavior of the borate glass was characterized by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) under the nitrogen atmosphere using DSC-TGA instrument model STD Q600 at a heating rate of 10 oC/min. The surfaces morphology and composition were determined using scanning electron microscope coupled with energy dispersive X-ray
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analysis (SEM/EDX) (model JEOL JXA-840A, Electron probe microanalyzer). X-ray diffractometer(model, BRUKER as, D8 ADVANCE) was used to study the crystal structure of as-prepared glass and glass coatings using Ni-filtered Cu radiation with a tube voltage of 40 kV. A current of 25 mA was utilized. The hydrophilicity and wettability of the achieved borate glass coatings were evaluated by measuring the contact
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angle using horizontal plate camera perpendicular to liquid plate droplet plane using Compact Video Microscope (CVM), manufactured by SDL-UK. About 250 µl of
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distilled water was used as a liquid for measuring the wettability according to ASTM D724-99 and ASTM D5946-96 method.
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3.2. In vitro degradation test
3.2.1. Biological fluids (7 days)
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In vitro degradation tests of glass-coated substrate were followed in SBF solution
ISO 23317, as well as, DMEM solution by measuring the ionic concentrations of the
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released species from glass coatings and stainless steel substrates. Ionic boron, calcium, phosphorus, magnesium and iron along with total proteins levels released into the immersion liquids were measured at the predetermined times (3, 6 h, 1, 3 and 7 d) using either ICP (Model) or colorimetric kits (BIODIGNOSTIC, Egypt). Ionic Boron concentration measured by ICP-OES model, Agilent5100 Synchronous Vertical Dual View (SVDV). The pH of the incubated solutions was also measured.
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3.2.2. Inorganic ionic concentrations Calcium ions react with methyl thymol, in an alkaline medium to form colored compound (blue color) that absorbsat λ = 585nm. Inorganic phosphorus reacts with molybdic acid to form phosphomolybdate complex (blue color). It’s measured at λ = 640nm.
thioglycolic
acid.
The
colored
compound
which
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The iron is dissociated by hydrochloric acid then reduced to ferrous by the
metal
forms
with
bathophenanthroline is measured colorimetrically at 535 nm. The concentrations of calcium, phosphorus and iron were calculated by applying the following equation: Concn. = (Asample / Astandard) x concn. of standard
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Where, the concentrations of Ca and P ions were in mmol/l and the concentration of Fe ions were in µmol/l.
Magnesium ions react with metallochrome dye calm augite to form chromophore which absorbs at 520 nm. The concentrations of magnesium ions were calculated by the following equation:
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Concn. (mmol/ L) = (Asample/ Astandard) x 1.0mmol/l The protein molecule when treated with an alkaline cupric sulfate, forms a violet color
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that absorbs at 550 nm. The concentration of protein is calculated by the following equation:
g/dl
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Concn.(g/dl) = (Asample/ Astandard) x 5
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3.2.3. Potentiodynamic polarization (PDP) The electrochemical behavior of coated substrate compared with bare 316L SS
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alloy were studied in simulated body fluid (SBF) and Dulbecco's Modified Eagle Medium (DMEM) solutions (represented in Table 3) at pH 7.4, stagnant solution and 37 °C using potentiodynamic polarization (PDP) test in open atmosphere. The device model used was Autolab 302N electrochemical workstation (Metrohm). The potential scanning rate applied to record the polarization curves was 1 mV/Sc, and the potential scan started from -200 mV. The measurements were carried out in three electrode cell, the platinum electrode was used as a counter electrode and saturated calomel electrode (SCE) as a 10
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reference electrode, as well as, the substrate with/without coating was used as a working electrode, with exposed area of 1 cm2. Different parameters, such as corrosion potential (Ecorr), polarization resistance, corrosion rate and pitting area, were calculated from the polarization curves. For reproducibility, five sets of the uncoated and coated samples
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obtained under the optimum coating parameters were measured.
4. Results and discussion 4.1. Characterization of glass
The density of the glass as measured by Archimedes method was 2.43 g/cm3. The bioactive glass which was melted for two h at 1050°C and casted; was transparent and
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colorless granules without crystalline inclusions as confirmed by XRD (Figure 1a).
The particle size of as-prepared glass after grinding for 2.5 h in planetary ball mill was measured by particle size analyzer and SEM image. Figure 1b represents the particle size distribution curve of the ground glass which exhibited a bimodal distribution curve with two maxima. The first one at 10.6 μm was sharp and with higher glass particle
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percentage (58.5 %), while the second one at 45.9 μm was broad for lower glass particle percentage (41.5%). These results were confirmed by SEM image (Figure 2). Figure 1c
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shows DSC and TGA profiles of as-prepared glass. The DSC curve demonstrated a long and sharp endothermic peak at 62 °C, which probably corresponded to the removal of adsorbed water on glass particles surfaces. Another weak and broad endothermic peak
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was observed at 275 °C, which is compared to glass transition temperature (Tg). Interestingly, as no exothermic peak was recorded for the glass sample, proved the
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inability of the glass for crystallization at high temperature. Additionally, TGA curve showed that a total weight loss of only 3% up to 850 °C.
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Figure 2 shows SEM micrograph and EDX analysis of the glass particles. Where,
small homogenous particles with few large ones could be recognized. These glass particles have angular shapes with sharp edges. Also, EDX analysis showed the peaks corresponding to elemental glass components. Boron element couldn’t be detected by such analysis.
4.2. Determination of optimal suspensions and EPD parameters 11
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The coating thickness and morphology depend on several parameters, such as the concentration of glass, deposition time, applied voltage and pH of suspension (electrolyte). In order to obtain the optimum deposited coating, these parameters were fully investigated. Nevertheless, the results of change of glass concentration showed that suspensions with either 0.5 or 1 wt% of glass particles produced a thin layer which could
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not completely cover the metal surface. Further increase of glass concentration to 3 wt% resulted in a thicker coating. The optimum coating was obtained at 4 wt% of glass concentration. Based on this result, the mass (weight gain) of the deposited layers were measured at 10, 20, 25, 30, 35 and 40 applied volts, the coatings weight were 0.1, 0.2, 0.42, 0.4, 2.44 and 0.4 g/mm2 x 10-4, respectively (Figure 3a). The optimum result was
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obtained using 35 V with sufficient weight (2.44 mg/mm2). Accordingly, at such previously mentioned conditions (4 wt% glass and 35 V), the pH of suspension was assessed in this study due to its significant effect on the charge of particles inside the solution. The pH was varied from 6.5 to 11. The results showed that the glass particles were mainly deposited on the cathode (cathodic deposition) in neutral suspension (pH 7),
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which meant that it acquired a positive charge at this pH. Increase of the pH gave raise to coating thickness decrease, which can be explained by a suppressing of the surface
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ionization and lowering of suspension conductivity. Nevertheless, pH 7 showed an optimum coupled with highest coating mass (1.23 g/mm2 x 10-4) (Figure 3c and d). At other respective pH values, the obtained mass values of the glass coatings were 0.62, 0.8,
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0.4, 0.2, 0.76, 0.44 and 0.2 g/mm2 x 10-4 at pH values of 6.5, 7.5, 8.5, 9, 9.5, 10 and 11. However, the coating mass did not decrease linearly with the increase of pH, but there
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were a fluctuation of the deposited masses. This can be explained by the dissolution behavior of glass particles during the deposition process. Though, during electrophoretic
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deposition (EPD) of borate glass particles in aqueous medium, some degree of dissolution of glass can occur throughout the deposition process. Where, there are two possible reactions being; hydration of alkali and alkaline earth ions beside hydrolysis of the borate network. Hydration of alkali ions results in release of such ions into the solution and replaced with protons in glass network for charge balance. Hydrolysis of the borate network is generated from breaking of glass network to form three coordinated borate units and eventually release boric acid, B(OH)3, to the solution [46]. However, at 12
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higher pH, B(OH)-4 anions are more dominant. Therefore, the type of borate unit species released from the glass is pH dependent. As a result, there are interfered deposition and dissolution reactions during the electrophoretic deposition process. It might be that the amount of the deposited glass particles depended on the relative amount of B(OH)3 to B(OH)-4 in the solution beside the amount of the released alkali ions. All these issues the aqueous suspension medium.
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probably caused a fluctuation in the thickness of deposited layer with the change of pH of
4.3. FTIR, XRD, SEM-EDX and contact angle measurements
Figure 4a represents FT-IR spectra of 316L SS, as-prepared borate glass and substrate.
The
following
bands
were
detected
from
FTIR
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glass-coated
spectra[47]:Bending vibration bands at 590 and 636 cm-1 corresponded to [BO3] isolated units. The wave bands at 676 and 715 cm-1 were related to vibration of oxygen bridges between two trigonal boron atoms. The B-O stretching vibration bands in the [BO4] units as detected from spectral splitting in the 810 - 1107 cm-1 spectral region into bands at
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876, 910, 949 and 988 cm-1.However, for glass coating the intensity of the glass characteristic vibrational bands decreased, while, some bands shifted to higher and other
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ones shifted to lower frequencies.
Figure 4b represents XRD patterns of as-prepared glass, 316L SS substrate and the surface of the glass coating. Two diffraction peaks with d-spacing 2.07 Å and 1.79 Å
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were characterizing the 316L SS substrate. Additionally, these two peaks were still predominant in of glass coating, but with lower intensity because of the coverage of the
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substrate surface with glass powder. Moreover, a new diffraction peak at 3.04 Å could be attributed to a formation of the new bond at the glass/substrate interface.
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Figure 5 shows SEM micrographs of glass coating surface coupled with EDX
analysis (a) and cross section of the glass-coated substrate (b).The figure represented a homogeneous, compact and crack-free coating. The EDX analyses showed the same elemental components of glass particles. Additionally, the thickness of glass coating was about 60 µm, as measured from SEM photo of glass-substrate cross-section (Figure 5b). Optical photos (Figure 6) of water drop on 316L SS substrate and substrate with glass coating revealed that the contact angle of the uncoated substrate was 104o, and 13
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decreased to 60ofor the coated substrate. These results showed that borate glass coating changed the metal substrate surface from hydrophobic to hydrophilic properties. Therefore, the borate glass increased the substrate wettability due to its hydrophilic properties which is expected to induce a positive mark for the cell attachment and interaction. Therefore, the achieved glass in this study has hydrophilic properties like the
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well-known Bioglass®[48, 49].
4.4.In vitrodegradation test 4.4.1. pH measurements
The pH of incubated media of the studied glass-coated substrates as a function of
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time is represented in Figure 7a. The pH values of samples immersed in SBF decreased slightly and reached to 7.05 at the end of the incubation time. This decrease is attributed to the consumption of Ca and P ions present in SBF in the hydroxyapatite crystals formation as confirmed by the degradation curve of Ca and P ions (Figure 7c, e). On the hand, pH of sample incubated in DMEM showed different behavior. It increased
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continuously to reach 8.32 at the end of the incubation period. The release of alkaline ions from the glass-coatings to the solution led to increase of DMEM pH. Because of the
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non-buffered nature of such fluid[50](unlike SBF) pH values were clearly affected by increasing alkali ions in the media.
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4.4.2. Ionic release concentrations
The levels of B, Ca, P, Mg, Fe and total proteins released from glass coating into
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SBF and DMEM are shown in Figures 7b-g. The behavior of changing profile of concentrations of B ions released into both solutions was almost the same (Figure 7b).
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However, the concentration of B ions released into SBF was higher than that released from DMEM solution. The initial concentration of B ion was relatively high during initial immersion hours, then it decreased and then increased a little bit at around 20 h and after this period its concentration became nearly constant. This initial fluctuation of B concentration in the solution was due to the conversion of borate glass into hydroxyapatite phase. After the complete conversion of the glass to hydroxyapatite crystals, the concentration of B ions in the solution became constant [51-53]. The 14
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dissolution curves of Ca and Mg ions (Figure 7c and d) in both solutions also showed similar behavior. Moreover, the concentration of Ca and Mg ions in SBF was slightly higher than that in DMEM solution. This was because the starting Ca ionic concentration in SBF (2.5 mmol/dm3) and starting Mg ion concentration (1.0 mmol/dm3) are higher than that in DMEM (1.8 mmol/dm3 and 0.8 mmol/dm3 respectively)[50]. Also, the release
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behavior of P ions into SBF and DMEM was similar (Figure 7e). In contrast, the concentrations of P ions released into SBF were higher than that released into DMEM. In both solutions the concentrations of P ions decreased obviously after one day of incubation. This could be attributed to a consumption of P ions in the formation of hydroxyapatite phase. These results agreed with previous works[54]. Moreover, the
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concentration of Fe ion released into SBF was close to that released into DMEM solution. It increased till the third day of incubation (Figure 7f) then it was almost constant up to the end of immersion time due to the precipitation of hydroxyapatite crystals on the glass coatings. Figure 7g shows the change of concentration of total proteins in DMEM solution during the incubation of glass-coated substrate sample. The concentrations of the
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total proteins increased initially up to the first day of incubation then it started to decrease
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slightly till the end of immersion time.
4.5. In vitro electrochemical corrosion study 4.5.1.Potentiodynamic polarization (PDP) studies
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Figure 8and Table 4show the anodic and cathodic polarization curves and parameters, respectively for bioactive glass coated samples with the uncoated 316L SS in
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DMEM and SBF solutions. It could be realized that 316L SS alloy had active behavior in both DMEM and SBF solutions during cyclic polarization. While, this behavior
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decreased in case of glass coated samples in both solutions. Furthermore, corrosion rates (Mils per year, mpy) of the coated substrates in DMEM and SBF (2.963 mpy and 4.533 mpy, respectively) were lower than corrosion rates of uncoated substrates in both solutions (3.468 mpy and 6.838 mpy, respectively). Moreover, the results showed that the corrosion rate in DMEM was less than that in SBF. This could be attributed to the higher chloride ion concentration in SBF (148.8 mmol/dm3) than in DMEM (118.5 mmol/dm3) which is known as the most corrosive ions for metallic alloy, specifically 316L SS. On 15
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the other hand, the corrosion current densities of the coated samples were reduced apparently in comparison to uncoated samples in DMEM and SBF (Table 4). The data reported in (Table 4) (Pitt. Area, mC/cm2) represented the integrated surfaces under the polarization curves for each zone. The obtained results were expressed in mC and were measured directly using the potentiostat program (area under curve).In case of immersion
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of coated metal electrode in SBF, the free potential was measured relative to a reference electrode without an externally imposed potential or current. From these results, the bioactive borate glass acted as a protective layer leading to a reduction of metal corrosion rate. It was noted that SBF solution was more destructive for metal under investigation
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than DMEM solution.
5. Conclusions
The developed borate bioactive glass was successfully prepared and coated onto 316L SS substrates by the electrophoretic deposition technique in aqueous solution, (green electrolyte), which is a cheap and environmentally desirable solution. The EPD
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parameters, namely, suspension particle concentrations, applied voltage, pH, and deposition time which control the deposition yield were thoroughly investigated. The
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optimum parameters were attained at 4wt% glass level and a potential of 35 V at pH 7 for 15 minutes. Before EPD, the stainless steel substrates were treated to create a rough surface which significantly increased the glass layer thickness during EPD coating
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process. According to the electrochemical measurements, coatings proved noticeable improvement for corrosion protection that satisfies the medical requirements of the
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selected materials.
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Acknowledgements The financial support of the present work within National Research Centre,
Biomaterials group, Cairo, Egypt is appreciated. The authors acknowledge and thank National Research Centre, Central Labs, Cairo, Egypt for providing measurement facilities applied in this research.
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Figure captions Graphical abstract: Schematic illustration of borate glass preparation and its coating on SS steps by EPD process. Figure1. (a) XRD pattern of as-prepared glass and (b) Particle size distribution after grinding in planetary ball mill (2.5 h).DSC-TGA thermal analysis of as
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prepared glass (c).
Figure 2.SEM micrograph of milled glass particulates and its EDX analysis.
Figure 3.EPD coating technique (a) Optical photos of 316L SS plates coated as a function of incremental voltages (10 - 40 volt) and b) Effect of voltages on the deposited glass particles weight. (c) Optical photos of 316L plates coated at
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gradual incremental pH values and (d) Effect of solution pH on the deposited glass particles weight.
Figure 4. FT-IR reflection spectra (a) and XRD patterns (b) of as-prepared borate glass and 316L SS substrate compared to glass coating.
of the glass coating (b).
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Figure 5. SEM micrographs of surface coupled with EDX analysis (a) and cross section
Figure 6. Optical photos of water drop on 316l substrate (SS sample) and substrate with
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glass coating (glass coat. sample) and contact angle measurements. Figure 7.pH change (a) concentration of released B (b),Ca (c), Mg (d), P (e), Fe (f) and
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total proteins (g) into SBF and DMEM solutions as a function of time. Figure 8.Potentiodynamic polarization curves of SS and coated SS in DMEM compared
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Table 1.Chemical composition in mol% of glass prepared and used in this study B2O3 CaO Na2O MgO Oxides 60
Mol%
10
20
10
Table 2. Nominal composition of surgical 316L stainless steel (wt %) Si 0.01
Mn 0.01
P 0.29
S 0.001
Ni 19.5
Cr 15.08
Mo 2.74
Cu ----
Al 0.024
Fe Bal.
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C 0.015
Table 3.Ion concentration (mmol/dm3) of mod-DMEM and SBF. Mod-DMEM SBF
K+ 5.4 5.0
Ca2+ 1.8 2.5
Mg2+ 0.8 1.0
Cl118.5 148.8
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Na+ 154.5 142.0
HCO344.0 4.2
HPO420.9 1.0
SO420.8 0.0
Table 4. Main parameters obtained from cyclic polarization method Pol. Resist., KOhms 2.845
Corr. Pot., mV -436
Corr. Curr., uA/cm2 22.86
Pitt. Area, mC/cm2 780.3
Coat in DMEM
7.078
3.331
-635.2
15.58
691.7
Bare in SBF
6.106
1.443
-424.4
13.44
505.9
Coat in SBF
4.533
2.177
-555.8
9.974
449.5
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Bare in DMEM
Corr. Rate, mpy 10.390
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Corr. Rate: corrosion rate, Pol. Resist.: polarization resistance, Corr. Pot.: corrosion potential, Corr. Curr.: corrosion current, and Pitt. Area: pitting area.
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Graphical abstract: Schematic illustration of borate glass preparation and its coating on 316L SS by EPD process.
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Figure1. (a) XRD pattern of as-prepared glass and (b) Particle size distribution after grinding in planetary ball mill (2.5 h).DSC-TGA thermal analysis of as prepared glass (c). 24
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Figure 2.SEM micrograph of milled glass particulates and its EDX analysis.
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Figure 3. EPD coating technique (a) Optical photos of 316L SS plates coated as a function of incremental voltages (10 - 40 volt) and b) Effect of voltages on the deposited glass particles weight. (c) Optical photos of 316L plates coated at gradual incremental pH values and (d) Effect of solution pH on the deposited glass particles weight.
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Figure 4. FT-IR reflection spectra (a) and XRD patterns (b) of as-prepared borate glass and 316L SS substrate compared to glass coating.
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Figure 5. SEM micrographs of surface coupled with EDX analysis (a) and cross section of the glass-coated substrate (b). 28
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Figure 6. Optical photos of water drop on 316L SS substrate (SS sample) and substrate with glass coating (glass coat. sample) and contact angle measurements.
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Figure 7.pH change (a), concentration of released B (b), Ca (c), Mg (d), P (e), Fe (f) and total proteins (g) from glass-coated substrate into SBF and DMEM solutions as a function of time. 30
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Figure 8.Potentiodynamic polarization curves of SS and coated SS in DMEM compared to SBF media.
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