Colloidal and Electrophoretic Behavior of Chitosan-Bioactive Glass Coating on Stainless Steel 316L

Accepted Article: Colloidal and electrophoretic behavior of Chitosan-Bioactive glass

coating on Stainless Steel 316L

Mehrad Mehdipoura, Abdollah Afsharb, Roozbeh Siavash MoakharC, Milad Mohebalid

(a,b,c) Department of Materials Science and Engineering, Sharif University of Technology, Azadi

Avenue, Tehran, Iran, P.O. BOX 11155-9466

(d)Department of Material Science and Engineering, K. N. Toosi University of Technology,

Pardis St., Vanak Sq., Tehran, Iran

+98-912-2470433

+98-21-88080513

[email protected], [email protected]

Abstract

Submicron sol-gel synthesized bioactive glass (BG) particles were deposited along with chitosan

in composite form onto a stainless steel substrate by an electrophoretic deposition (EPD)

technique. In order to evaluate the optimum Chitosan (CS) solution/ethanol ratio of the

suspension, potential and deposition rate were measured. potential of the suspension with

40% CS solution content- ethanol weight ratio reached an upper limited 30 (mV).

Triethanolamine (TEA) dispersant was added to the suspension to evaluate its influences on EPD

process. Composite coating was characterized by of Scanning Electron Microscopy (SEM),

Thermal Gravimetric Analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). In

presence of TEA in the suspension, Microstructural studies of the coating revealed more

agglomerated particles. FTIR results showed that TEA dispersant reduces amid group adsorption

and impedes structural O-H adsorption, indicating a reduction in overall adsorption of chitosan

on BG particles. The results of thermo gravimetric studies confirmed that the presence of

chitosan in composite deposition is 33.7%. In order to investigate the kinetic behavior of the

EPD process, kinetic constant was estimated by measuring deposition yield at the beginning of

the process and it was then compared with a theoretical model in the literature so the kinetic

constant was found to be 5.52E-04 and 9.87E-04 for suspensions with/without TEA dispersant.

Examination of chronoampermetry diagrams indicated the diffusion controlled mechanism of the

coating process.

Keywords: Electrophoretic deposition, Bioactive glass, Chitosan, Zeta potential,

Triethanolamine (TEA).

1. Introduction

Bioactive glass (BG) is a prominent material in biomedical applications for its biocompatibility,

bioactivity and is therefore highly investigated as a coating for implant applications [1]. 45S5

was the first bioactive glass synthesized by Hench et al. [2]. Many experiments have since been

conducted to coat bioactive glass on metallic implants in order to improve the osseointegration

process via favorable chemical and biological interactions of bioactive glass within body. Sol-gel

and melt processing are the two main synthesis methods of bioactive glass. In order to obtain BG

particles with high specific surface area, improved bioactivity and controlled composition, sol-

gel is preferred over melt processing [3, 4].

Different methods including plasma spray, sol-gel, sputtering techniques and electrophoretic

deposition (EPD) have been employed for deposition of BG on implants [5-7]. The increasing

attention in EPD process as an effective technique for deposition of biomaterials is due to its

features such as low equipment cost, the possibility of deposition on substrates of complex

shape, and high purity and microstructural homogeneity of deposits. In EPD, ceramic charged

particles are dispersed in a suspension and move toward the working electrode in an applied

electric field [8]. EPD yields a homogeneous deposition with a uniform structure. Fig. 1 is shown

the scheme of the EPD process. In this cell positive charged particles move toward cathode

electrode in an applied electric field.

Fig. 1. Electrophoretic deposition (EPD) cell showing positively charged particles in suspension

moving towards the cathode electrode [8].

Successful application of EPD depends on stability of the suspension and parameters of the

deposition.

Suspension in EPD process is a complex system in which each component plays an important

role on deposition efficiency. A stable suspension is a prerequisite to achieve coatings with high

packing density deposition. A suitable means for evaluation of colloidal suspension is zeta

potential measurement [9]. Zeta potential is related to mobility of the particles which is

controlling interaction strength of the colloid particles and provides information about the

mechanism of deposition and agglomeration of the particles in the suspension [10].

There are many investigations in literature concerning stability of the suspension in EPD.

Yamashita et al. [11] showed that particle size is an important factor in mobility of charged

particles in a colloidal suspension. Wang et al. [12] investigated stability of submicron

hydroxyapatite particles in an organic solution of ethanol in various pH by studying conductivity

and zeta potential. Zhang et al. [13] suggested a kinetic model for deposition of ceramic particles

in EPD process based on mass conservation law, defining deposition yield as an exponential

function of time.

Dispersants such as acids, bases and certain electrolytes are added to a suspension in order to

control charge of the particles and improve the coating process. C. Deng et al. [14] compared the

effect of different additives in stability of hydroxyapatite particles in ethanol solvent by studying

viscosity and particle size, and N, N-dimethyl formamide (DMF) was found to be the most

suitable additive. H. Xu et al. [15] showed that adding triethanolamine (TEA) to a suspension

containing Yttria Zirconia particles in acetyl acetone augments pH of the suspension and

maximizes zeta potential. M. F. De Riccardis et al. [16] used TEA as a dispersant in an ethyl-

alcohol solvent for deposition of alumina-zirconia composite coating.

A new approach in EPD involves composite coating of ceramics and polymers. Chitosan based

composites are particularly interesting in biomedical applications for their biocompatibility.

Also, its cationic nature in aqueous solutions and excellent film forming properties make

chitosan a suitable choice for EPD process [17]. Chitosan binds ceramic particles to the substrate

and forms composite coating, eliminating the problems associated with sintering of ceramic

particles.

This work deals with the effects of Chitosan (CS) solution/ethanol ratio and TEA additive on

stability of the suspension. Zeta potentials of the suspensions with and without TEA are

compared and an attempt is made to describe EPD process through a previously developed

kinetic model. Finally the effect of TEA on the coating is studied by FTIR and SEM.

2. Material and Method

Chitosan (CS) with molecular weight of 200 kDa and degree of deacetylation of about (85%),

Ethanol, Tetraethyl orthosilicate (TEOS), Tetraethyl phosphate (TEP), HNO3 (65%),

Ca(NO3)2.4H2O, Mg (NO3)2.6H2O, Triethanolamine (TEA) and citric acid (CH3COOH) were

provided by Merck.

2.1. Synthesize of BG particles

Bioactive glass with SiO2-CaO-P2O5-MgO composition was synthesized by a sol-gel method

described in Ref. [18, 19]. To obtain a submicron powder, the as-synthesized bioactive glass was

grinded by a SPEX mill (Retsch Co., Germany; Model No.: PM 200) for 4 hr.

2.2. Preparation of BG-Chitosan suspension

Chitosan was dissolved in 1% acetic acid and was then used for preparation of the suspension for

EPD process. EPD was performed using a solution of 2 g/l bioactive glass particles in different

ratios of 0.5 g/l chitosan solutions - ethanol solvent. Triethanolamine (TEA) was used in order to

investigate the effect of dispersant in stability of the suspension. The suspensions were

ultrasonicated (Sonicatore 4000) for 30 min to achieve a homogeneous dispersion of bioactive

glass particles.

2.3. Electrophoretic deposition of BG- Chitosan composite coating

316L stainless steel plates, 20*50 mm, were used as cathode and anode. In Order to improve

adhesion of the coating, surface preparation involved sand blast, washing with distilled water,

rinsing and degreasing by ultrasonic cleaning in acetone for 10 min and a final drying step. The

distance between the cathode and the counter electrode was 10mm. Deposition was performed at

a constant voltage of 15 v.cm−2 by a DC rectifier (Model JSD-302D).

2.4. characterization of composite coating

Zeta potential of the particles was measured by zeta sizer (Malvern, Model sizer 3000 HsA,

England). Thickness of the deposited coating was measured by eddy current method (ED10

Eddy, Dual Scope MP40, Germany). Eddy current techniques are used to nondestructively

measure the thickness of nonconductive coatings on nonferrous metal substrates. According to

this method, the most painters and powder coating are measured over any metal. This test was

carried out according to the standard of the ASTM E376-11.

The deposited coating was scratched to prepare a sample for FTIR and thermo gravimetric

analysis (TGA). FTIR was used to investigate the surface bonds of bioactive glass particles.

FTIR was performed using Perkin-Elmer RX1device in the wavenumber range of 400-4000 cm-1

and a resolution of 4 cm-1. TGA was performed using a thermoanalyzer (Mettler SF1) operating

in air at a heating rate of 5˚C/min.

In order to compare the amount of agglomeration in coatings prepared from different

suspensions, a scanning electron microscopy (SEM) (Philips XL30, Holland) was used. A thin

section was taken from the coating and was Au sputtered about 10 nm thick before SEM

analysis.

3. Results and discussion

3.1. Electrophoretic deposition of bioactive glass powder in various CS

solution/ethanol ratios

Successful practice of EPD relies highly on stability of the particles in the CS solution and

ethanol ratio, which is studied here through potential measurement. The reasons of choosing

this solution are that ethanol leads to enhance resident of the suspension and it is more suitable

than distilled water and CS solution content as protonated CS becomes adsorbed on BG particles

[19]. According to Fig.2, potential of BG particles in the suspension with 30% CS solution

content- ethanol weight ratio corresponded to 18.4 mV. By increasing CS solution content of the

suspension to 40% and 50%, zeta potential reached an upper limit of approx. 31±1 (mV), but

decreased to 23.5±1 (mV) afterwards. According to these results, potential was significantly

decreased by increasing CS solution content from 40% to 50%.

Fig. 2. Zeta potential measured as a function of CS solution

Indeed CaO and MgO which are components of bioactive glass coating are not stable in water

[20, 21] and are easily dissolved in the suspension, leading to an increase of pH in different CS

solution/ethanol weight ratios of the suspension, as given in Table 1. Following reactions take

place when these components dissolve in water.

MgO + H2O → Mg2+ + 2(OH)-

CaSiO3 + H2O → Ca2+ + SiO2 + 2(OH)-

CaO + H2O → Ca2+ + 2(OH)-

Surface of the particles saturated by protonated chitosan might be neutralized by OH- ions from

these reactions, and consequently potential is reduced [20]. Increasing of the CS solution leads

to reduce pH solution because CS has dissolved in acidic solution (1% Acidic Acid solution).

Table 1.Variation of pH in different CS solution/ethanol weight ratios before and after adding

BG particles

Fig.3. Effect of CS solution content in the disperse medium on the thickness of BG coating.

Fig. 3 shows the thickness of the coatings prepared by EPD deposition from suspensions with

various CS solution/ethanol weight ratios. Maximum thickness was obtained from the suspension

with 30% CS solution content while maximum potential corresponded to the suspension with

CS solution/ethanol

ratio

pH Before adding

BG particles

pH After adding

BG particles

Zeta potential

(mv)

30 3.98 5.22 18.4

35 3.90 5.1 22.7

40 3.84 4.81 31.5

45 3.72 4.7 25.3

50 3.70 4.73 23.5

40% CS solution content, meaning that the thickness of deposition is not directly proportional to

potential of the particles. This was investigated through conductivity measurements.

Conductivity of the suspensions with 30%, 40% and 50% CS solution content were measured to

be 414, 698 and 956 (µs/cm), respectively, i.e. increasing CS solution content increases

conductivity of the suspensions and reduces mobility of the particles [12, 19]. Therefore, BG

particles with optimum CS solution-ethanol weight ratio (30%) can achieve maximum yield

deposition as a balance is made between stability of the suspension and ready deposition onto the

electrode.

3.2. Study of TEA behavior on the EPD process

3.2.1. FTIR study

Fig. 4 shows FTIR spectra of the coatings prepared using suspensions with 30% CS solution

content with/without TEA. FTIR spectra shows characteristic bands of P-O bonds of PO43- at

463-468 cm-1, Si-O-Si of silicate group at 1060-1080 cm-1, C-O at 1426-1445 cm-1 and C-H at

2810-2885 and 2920 cm-1 (for CH2 and CH3 groups, respectively). The band at 1574 cm-1 is

assigned to N-H bonds of amide group and bands at 3430 cm-1 and 3654 cm-1 correspond to O-H

bonds of adsorbed water and structural hydroxyl group, respectively [22].

Addition of TEA to the suspension leads to disappearance of the band for structural hydroxyl

group of chitosan (3654 cm-1), i.e. TEA interferes with the interaction of chitosan’s hydroxyl

group and the surface of the particles [23]. NH2 bands at 1574 cm-1 are also decreased. It could

be concluded that addition of TEA reduces adsorption of chitosan.

Fig.4. FTIR spectrum of composite coatings deposited from different suspension: a) 1(g/l) TEA,

b) without TEA.

Parameters related to the stability of the suspension are given in Table 2. According to these

results, presence of TEA in suspension decreases potential of BG particles. This, along with the

results from FTIR shows that stability of the suspension reduced in presence of TEA.

Table 2.Parameters of suspensions with/without TEA

Samples Zeta potential (mv) Conductivity (µs/cm2) Mobility

(μm.cm/V.s)

0 g/L TEA 18.4±1.6 414±10 0.487±0.042

1 g/L TEA 14.4±1.1 540±10 0.382±0.029

3.2.2. Evaluation of kinetic behavior

In order to investigate the kinetic behavior of the process, deposition yield was measured at the

beginning of EPD. Fig. 5 shows the effect of TEA on the rate of deposition as a function of the

duration of EPD. Increasing deposition weight is initially linear and then deviates slightly when

the concentration of the particles in the suspension is reduced in the rest of the process. The

deposition thickness grows during EPD process and the effective voltage for BG particles is

dropped. Consequently, the rate of coating’s deposition is reduced.

Fig.5. Deposition yield as a function of deposition time.

Zhang et al.[13] suggested an empirical model for deposition yield, Eq. 1:

W=W0 (1-e -Kt) (1)

Where W0 is the initial weight of the powder in the suspension, t is time and K is the kinetic

constant (S-1). Fig. 6 is obtained by re-writing this equation in logarithmic form (Eq. 2) and

solving for kinetic constant.

Ln W0 – Ln (W0-W) = Kt (2)

The kinetic constant from the slope of diagram was found to be 5.52E-04 and 9.87E-04 S-1 for

suspensions with/without TEA, respectively, i.e. presence of TEA modifies kinetic behavior and

reduces deposition rate.

Fig.6. Logarithmic yield deposition versus time for estimation of kinetic constant

3.2.3. Chronoampermetry of EPD, and electrodeposition mechanism

The diagram of current density during EPD in a constant voltage of 15 V is shown in Fig. 7.

Current density decreases along EPD due to electrode polarization and formation of an insulating

layer. Polarization takes place rapidly and this is the insulating layer growth which is the major

reason of the voltage drop [24]. It was also observed that the current density of the suspension

with TEA was higher compared to the one with no TEA, i.e. TEA increases conductivity of the

suspension as seen in Table 2. As a result, when conductivity is high, not only the rate of

agglomeration will increase and larger agglomerates would be formed, but also the large number

of free ions in the suspension may become main current carrier and hence the speed of the

particles would be reduced [12].

Fig.7. Chronoampermetry of the BG coating in suspension with: a) 1 g/l TEA, b) no TEA

Ma et al. [25] explained EPD mechanism in coating of ceramics through Cottrell equation. They

found out that the EPD of PZT particles in an ethanol suspension is controlled by diffusion.

Cottrell equation defines current density in terms of time as followed (Eq. 3):

(3)

Where i is the current density (A/cm2), F is a Faraday’s constant, D is the diffusion coefficient

(cm2/s), C* is the suspension concentration (Mol/cm3) and t is the deposition time (s).

Fig. 8 shows the linear relationship between i and t -1/2 drawn from the experimental data for

EPD in constant voltage of 15V from the suspensions with/without TEA; this trend obeys

Cottrell equation and EPD mechanism is controlled by diffusion. Furthermore, presence of TEA

in the suspension leads to a decrease in the slope of the diagram which is directly related to the

2/12/1

*2/1

t

CnFDi

diffusion coefficient. Since the concentrations of the suspensions with/without TEA are equal, it

could be concluded that diffusion coefficient is reduced upon TEA addition.

Thickness of the composite coating after 10 min EPD decreased from 12±1 µm to 7±1 µm when

TEA was added to the suspension. This confirms that an increase in conductivity of the

suspension reduces thickness of the coating.

Fig.8. Diagrams of deposition current density against t-1/2for the BG coating from suspensions

with: a) 1 g/l TEA, b) no TEA

3.3. Coating characterization

Fig. 9 shows SEM images of the coatings prepared from suspensions with/without TEA.

As described above, adding TEA dispersant to the suspension reduces zeta potential.

Consequently, possibility of adsorption and agglomeration of BG particles through Van der

Waals forces increases. Therefore, more agglomerated BG particles were observed in the coating

deposited from a suspension with TEA.

Fig. 9. Microstructure of the BG coating from suspensions with: a, c) no TEA (200X, 1000X), b,

d) 1g/l TEA (200X, 1000X).

Thermo gravimetric diagrams for coating prepared from 0.5 g/L chitosan solution with 2 g/L BG

particles are shown in Fig. 10. Three stages in loss weight of chitosan-BG composite coating are

recognizable in this diagram: at 97.5 °C, at 250–300 °C and finally at 500-570 °C (Fig. 8). The

amount of weight loss in below 100 °C was found to be 3.7% which is due to the water loss. The

second decomposition corresponds to dehydration of the polysaccharide rings of chitosan and

also decomposition of acetylated and deacetylated units of the polymer; in this stage deposition

yield decreased 28.4 %. In the final step, pyrolysis corresponds to decomposition (thermal and

oxidation) of residual chitosan. Total weight loss at 850 °C was found to be 37.4%. Therefore,

the results of thermo gravimetric studies confirmed the presence of chitosan in composite

deposition and its amount was measured to be 33.7%, in consistence with investigations of D.

Zhitomirsky [17], Natalia Davidenko [26], and K. Grandfield [27].

Fig. 10. TG and DTG data for the composite coating prepared from 0.5 g/L chitosan solution and

2g/L bioactive glass particles in the suspension with 30 % CS solution/ethanol weigh ratio.

4. Conclusion

Chitosan-bioactive glass composite was coated successfully on a stainless steel substrate.

potential was measured in solvents with various CS solution/ethanol weight ratios and it was

determined that although increasing CS solution content increases potential of the particles

until 40% CS solution-ethanol weight ratio, it reduces the deposition rate. Particle mobility,

potential, and thickness of the coating were measured in order to characterize the colloidal

suspension and investigate the effect of TEA. It was found out that addition of TEA reduces

adsorption of chitosan on BG particles and potential of the particles from 18.4 to 14.4 mV. The

microstructure of composite coating showed that in the present of TEA in the suspension

agglomeration was increased. By comparing experimental data with a theoretical model, kinetic

constant with/without TEA was estimated to be 5.52E-04 and 9.87E-04 S-1, respectively. Finally,

chronoampermetry diagrams show that diffusion controlled mechanism is dominated.

5. Reference

[1] Q.Z. Chen, I.D. Thompson, A.R. Boccaccini, Biomaterials 27 (2006) 2414.

[2] L.L. Hench, Journal of the American Ceramic Society 81 (1998) 1705.

[3] M. Pereira, J. Jones, R. Orefice, L. Hench, Journal of Materials Science: Materials in Medicine 16

(2005) 1045.

[4] A. Doostmohammadi, A. Monshi, R. Salehi, M.H. Fathi, Z. Golniya, A.U. Daniels, Ceramics

International 37 (2011) 2311.

[5] A. Ladwig, S. Babayan, M. Smith, M. Hester, W. Highland, R. Koch, R. Hicks, Surface and

Coatings Technology 201 (2007) 6460.

[6] G.E. Stan, C.O. Morosanu, D.A. Marcov, I. Pasuk, F. Miculescu, G. Reumont, Applied Surface

Science 255 (2009) 9132.

[7] S.-H. Jun, E.-J. Lee, S.-W. Yook, H.-E. Kim, H.-W. Kim, Y.-H. Koh, Acta Biomaterialia 6

(2010) 302.

[8] A.R. Boccaccini, S. Keim, R. Ma, Y. Li, I. Zhitomirsky, Journal of The Royal Society Interface 7

(2010) S581.

[9] L. Besra, M. Liu, Progress in Materials Science 52 (2007) 1.

[10] A. Jada and S. Erlenmeyer, Journal of Colloid Science and Biotechnology 1(2012) 1–8.

[11] K. Yamashita, M. Matsuda, Y. Inda, T. Umegaki, M. Ito, T. Okura, Journal of the American

Ceramic Society 80 (1997) 1907.

[12] C. Wang, J. Ma, W. Cheng, R. Zhang, Materials Letters 57 (2002) 99.

[13] J. Zhang, B.I. Lee, Journal of the American Ceramic Society 83 (2000) 2417.

[14] C. Deng, J. Weng, Q.Y. Cheng, S.B. Zhou, X. Lu, J.X. Wan, S.X. Qu, B. Feng, X.H. Li, Current

Applied Physics 7 (2007) 679.

[15] H. Xu, I.P. Shapiro, P. Xiao, Journal of the European Ceramic Society 30 (2010) 1105.

[16] M.F. De Riccardis, D. Carbone, A. Rizzo, Journal of Colloid and Interface Science 307 (2007)

109.

[17] D. Zhitomirsky, J.A. Roether, A.R. Boccaccini, I. Zhitomirsky, Journal of Materials Processing

Technology 209 (2009) 1853.

[18] A. Saboori, M. Rabiee, F. Moztarzadeh, M. Sheikhi, M. Tahriri, M. Karimi, Materials Science

and Engineering: C 29 (2009) 335.

[19] M. Mehdipour, A. Afshar, Ceramics International 38 (2012) 471.

[20] S. Hayashi, Z.-e. Nakagawa, A. Yasumori, K. Okada, Journal of the European Ceramic Society

19 (1999) 75.

[21] B. Ferrari, R. Moreno, P. Sarkar, P.S. Nicholson, Journal of the European Ceramic Society 20

(2000) 99.

[22] B.D. Mistry, M.B. D., A Handbook Of Spectroscopic Data Chemistry. Oxford Book Company,

2009.

[23] R.H.R. Castro, D. Gouvêa, Journal of the European Ceramic Society 23 (2003) 897.

[24] P.S. Nicholson, P. Sarkar, X. Haung, Journal of Materials Science 28 (1993) 6274.

[25] J. Ma, W. Cheng, Materials Letters 56 (2002) 721.

[26] N. Davidenko, R.G. Carrodeguas, C. Peniche, Y. Solís, R.E. Cameron, Acta Biomaterialia 6

(2010) 466.

[27] K. Grandfield, I. Zhitomirsky, Materials Characterization 59 (2008) 61.