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Development of calcium phosphate coatings on type 316L SS and their in vitro response.

A simple method was attempted for coating calcium phosphates on type 316L SS through electrochemical deposition. An aqueous solution comprising Calcium chloride dihydrate (Ca[Cl.sub.2] x 2[H.sub.2]O) and Ammonium dihydrogen phosphate (N[H.sub.4][H.sub.2]P[O.sub.4]) adjusted to Ca/P molar ratio of 1.67, was taken as electrolyte for deposition at constant current and temperature at different time duration. The resultant coatings were estimated for its Ca/P ratio, FT-IR spectra to identify the functional groups and further the in vitro behaviour of coatings were studied through cyclic polarisation experiments in simulated body fluid (SBF) conditions. The results from the studies showed that coating weight varied with time duration and applied current. FT-IR spectra indicated the presence of C[O.sub.3] containing apatite and the coatings showed improved resistance to corrosion towards pristine 316L SS.

INTRODUCTION

Metals have a long history of use in restoration of anatomical structures and their mechanical properties. Among commonly available materials like Austenitic stainless steels, Cobalt-chromium alloys and Titanium and its alloys, 316L SS are used as an implant material due to its availability at low cost, easy fabrication and superior mechanical properties (1). However being a bio-inert material, they do not form a chemical bond with bony tissue after implantation. Failures of 316L SS as implants have also been reported widely and this is mainly due to fatigue and corrosion (2). Studies over the past decade indicated that coating of a bioceramic material like Hydroxyapatite [[Ca.sub.10][(P[O.sub.4]).sub.6][(OH).sub.2]] (HAP) on metal reduces the failure of a metal implant and promotes direct bone bonding between osseous tissue and implant surface. The potent of using HAP coatings on metal implants is due to the similarities of HAP with hard tissue and has the advantage of metals providing necessary mechanical features, while HAP supplies needful biological properties.

Various methods of ceramic coating developed during recent years have been considered to improve the wear and corrosion resistance of metallic implants (3). Some of the methods are: plasma spray, physical vapor deposition (PVD), chemical vapor deposition (CVD), ion-plating, dipping, sol-gel coating, blast coating and ion sputtering etc,. Electrochemical deposition is one such method to develop ceramic coating on metal surface. Electrochemical deposition using chemical aqueous solutions can produce ceramic coatings over metal substrates. This method offers a number of advantages over other conventional coating methods (CVD, sputtering , plasma spray etc.,) namely: low temperature process, low cost equipment, easy to control microstructure of coatings and can be used in any shape of substrates (4,5). These advantages make this method more attractive to metallic implants.

In this present study electrodeposition was attempted to develop calcium phosphate coatings similar to HAP phase on type 316L SS. The formed coatings were analysed for Ca/P ratio and a FT-IR spectrum was employed for identification of functional groups. Since corrosion is one of the main parameters to determine the biocompatibility of metals, the corrosion performance of the coatings were analysed through cyclic polarisation experiments in simulated body fluid conditions.

EXPERIMENTAL

Sample preparation

Surgical grade of type 316L SS was used as a substrate for electrolytic coating. The chemical composition of the specimen was (wt %): Cr (18.00), Ni (12.00), Mo (2.50), Mn (1.70), Cu (0.026), Si (0.15), C (0.02) and Fe (balance). The specimens cut into 1[cm.sup.2] surface with 0.5 mm thickness were taken for all the experiments. All metal specimens are finely polished to mirror like finish with 0.5[micro]m diamond paste. Each specimen was cleaned with acetone (organic debris removal) followed by a treatment for 5 minutes in ultrasonic bath filled with deionized water and then dried at room temperature.

Electrochemical deposition

The electrolyte used for the deposition of calcium phosphate (CP) on 316L SS was prepared by adding optimum concentrations of Ca[Cl.sub.2] x 2[H.sub.2]O) and N[H.sub.4][H.sub.2]P[O.sub.4] to attain Ca/P ratio of 1.67. The pH of the mixed solution was measured as 4.4. The deposition was carried out in a three electrode assembly comprising platinum foil as counter electrode, saturated calomel electrode (SCE) as the reference and 316L SS as working electrode. A Elico CL-95 model, potentiostat/galvanostat operating in galvanostatic mode was used to maintain constant current at a predetermined value. CP coatings were carried out at a constant current of 3.0 mA and 12.0 mA at 50[degrees]C at various time intervals (20min, 40min, 60min and 3hrs). The electrochemical yield was determined for each set of applied current at different time duration and analysed for Ca/P ratio.

In vitro studies

The coatings were dried at 300[degrees]C in vacuum for 1 hr for densification and subjected to FT-IR spectra for identification of functional groups. The changes occurring in the coated metal surface in the presence of Ringer's solution that simulate body fluid conditions was studied electrochemically at pH 7.4 by maintaining the temperature at 37 [+ or -] 1[degrees]C. The cyclic polarisation studies were carried out in a three-cell electrode configuration (SCE-reference electrode, Platinum-counter electrode, 316L SS- working electrode). This technique measures the pitting tendencies of a coated metal surface by maintaining the electrode at a constant open circuit potential ([E.sub.c]). The potential was increased from [E.sub.c] in the positive noble direction at a sweep rate of 1mV/sec until [E.sub.b] (break down potential), where the specimens entered the transpassive region. The direction of the potential step was reversed until the repassivation potential ([E.sub.p]) was reached.

RESULTS AND DISCUSSION

Variation of coating density

Reports indicate that coating thickness plays a major role in the implant manufacture. An increase in coating thickness increases the problem related to mechanical competence of the coating. Since the coating weight has direct relationship with the thickness, our present study was concerned with the dependence of coating weight with applied current and time. The changes in weight of the deposits at constant applied current of 3.0 mA (low current) and 12.0 mA (high current) at different deposition times are given in the fig-1.

[FIGURE 1 OMITTED]

It is clear from the figure that the weight of deposits increased with deposition time. Applied current also had a greater impact on the coating weight. High-applied current (12.0 mA) showed increased weight compared to low-applied current (3.0 mA). Prolonged deposition for more than one hour were found to be non-uniform and caused loosening of the deposits. Meroli and Tranquilli showed that a thicker coating will give a brittle material prone to cracking under bending or shearing force accompanied by a decrease in bond strength due to residual stress in the coating (6). The electrochemical deposition of calcium phosphates on 316L SS for one hour produced uniform deposits.

Chemical analysis

The resultant deposits were scraped and analysed for the presence of elements. Calcium and phosphate were detected as the predominant elements in the deposits. Calcium was estimated through standard EDTA titration and Phosphorous was determined gravimetrically as [Mg.sub.2][P.sub.2][O.sub.7]. The Ca/P molar ratio was found to be 1.63 [+ or -] 0.01 for all the deposits. It is interesting to note that Ca/P molar ratio was comparable with Ca/P ratio of normal bone. It has been reported that bone mineral is micro crystalline, non-stoichiometric, structurally imperfect analogue of HAP with a Ca/P ratio slightly less than 1.67 (7).

FT-IR spectra

The FT-IR spectra for one hour deposition for both 3.0 mA and 12.0 mA applied current are given in fig-2. The infra red spectrum for both the deposits was almost similar to those reported for the non-stoichiometric, carbonate containing HAP and bone apatite (8). The peaks at 631 and 3571 [cm.sup.-1] characteristic of HAP and the bands at 1062,601 and 550[cm.sup.-1] derived from P[O.sub.4] modes are apparent in both the spectrums. There is also a sharp peak at 1648[cm.sup.-1] and a broad band at the range of 3000-3600 [cm.sup.-1] indicative of OH group. The infra red spectrum also contains bands at 1400-1450 [cm.sup.-1] and 872 [cm.sup.-1], assigned to C[O.sub.3.sup.2-] ions. These bands are similar to those reported by Blitz and Pellegrino for an inorganic bone (9).

[FIGURE 2 OMITTED]

Cyclic polarisation studies

The electrochemical parameters obtained from the cyclic polarisation curves of the pristine 316L SS and CP deposited 316L SS with applied current at different time duration are given in Fig-3&4. Higher [E.sub.b] and [E.sub.p] values were obtained for all the CP deposited 316L SS compared to pristine 316L SS. The break down potential ([E.sub.b]) value of (+460 mV) with a repassivation potential ([E.sub.p]) of +160 mV for one hr deposition at 12.0 mA current was found to be high compared to other coated specimens. The [E.sub.b] values for the specimens deposited at 12.0 mA applied current for 20 and 40 minutes were +425 mV and +410 mV respectively. Similarly the [E.sub.b] values for deposited samples at 3.0 mA applied current for 20, 40 and 60 minutes deposition were found to be +390, +400 and + 415 mV respectively.

[FIGURE 3 OMITTED]

A large hysteresis loop indicating high corrosion rate was observed for pristine 316 L SS ([E.sub.b]=+310 mV) and CP deposited at 3.0 mA applied current. This could be due to the growth of pits caused by the presence of chloride ion in Ringers solution. Chloride being a relatively small anion with high diffusing power enters the passive film and penetrates to cause damage. The low current deposition though indicates better [E.sub.b] value compared to pristine 316L SS does not act as a strong barrier to prevent the entry of chloride ions and hence results in enhanced corrosion. The depositions at 12.0 mA current with considerable thickness restrict the entry of chloride ion and thus prevent the base metal to corrosion.

CONCLUSION

The electrochemical approach for the fabrication of bioactive calcium phosphate coatings seems to be attractive, since the process parameters are precisely controlled. The deposition produces calcium phosphates, composed of non-stoichiometric, carbonate containing apatite. Prolonged deposition caused loosening of the deposits and may cause decohesion of coatings. The deposition conditions were optimised as 12.0 mA current for 1 hour. The results from cyclic polarisation experiments showed that the CP coatings act as a barrier for corrosion of metal implants. Hence this type of coatings with novel physicochemcial characteristics may be prepared for specific applications.

Acknowledgement

The authors are grateful to Indian Council of Medical Research (ICMR) for providing financial assistance.

REFERENCES

(1.) ASTM standard specification F 55 (1982) and F 138 (1988), Philadelphia

(2.) Williams D.F., J. Mater. Sci., 22, 342 (1987).

(3.) Galon L., Silberman I. and Chain R., J. Electrochem. Soc., 138, 1939 (1991).

(4.) Yen S.K. and Huang T.Y., Mater. Chem. Phy., 56, 214 (1998).

(5.) Izumi K., Marakani M., Deguchi T., Morita A., Tohge N. and Minami T., J. Am. Ceram. Soc. 72, 1465 (1989).

(6.) Merolli A. and Tranquilli Leali P., "Crystallinity of the coating directs in vivo response to hydroxyapatite in the rabbit" In Bioceramics, Anderson O.H. and Yli-urpo A.. (Eds). Butterworth. Heinemann Ltd., London, Vol-7,1994,223.

(7.) Mayer J.L. and Fowler B.O., Inorg. Chem, 21, 3029 (1982).

(8.) Osaka A., Miura Y., Takeuchi K., Asada M. and Takahashi K., J. Mater. Sci. Mate. Med, 2, 51 (1991).

(9.) Blitz R.M. and Pellegrino E.D., Calcif. Tissue. Res.7, 259 (1971).

S. Kannan, A. Balamurugan and S. Rajeswari

Department of Analytical Chemistry

University of Madras

Guindy campus

Chennai-600 025
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Author:Kannan, S.; Balamurugan, A.; Rajeswari, S.
Publication:Trends in Biomaterials and Artificial Organs
Geographic Code:9INDI
Date:Jul 1, 2002
Words:2000
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