Electrophoretic deposition of polymer-carbon nanotube–hydroxyapatite composites

Surface & Coatings Technology 203 (2009) 1481–1487

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Electrophoretic deposition of polymer-carbon nanotube–hydroxyapatite composites K. Grandfield, F. Sun, M. FitzPatrick, M. Cheong, I. Zhitomirsky ⁎ Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7

a r t i c l e

i n f o

Article history: Received 5 September 2008 Accepted in revised form 24 November 2008 Available online 3 December 2008 Keywords: Electrophoretic deposition Film Hydroxyapatite Carbon nanotube Composite Chitosan Alginic acid Hyaluronic acid

a b s t r a c t Electrophoretic deposition (EPD) has been utilized for the fabrication of composites containing multiwalled carbon nanotubes (MWCNT) in a biopolymer matrix. Chitosan – MWCNT films were obtained by cathodic EPD. Alginic acid – MWCNT and hyaluronic acid – MWCNT films were deposited by anodic EPD. Biopolymer – MWCNT – hydroxyapatite (HA) films were prepared as monolayer nanocomposites containing MWCNT and HA in a biopolymer matrix or laminates, containing biopolymer – MWCNT layers separated by biopolymer – HA layers. The thickness of individual layers can be varied in the range of 0.2–5 µm by variation in deposition time. Obtained films provided corrosion protection of NiTi shape memory alloys in Ringer's physiological solution. The deposition mechanisms, properties and applications of the obtained composite films are discussed. © 2008 Elsevier B.V. All rights reserved.

1. Introduction EPD has attracted substantial attention for the fabrication of thin films and coatings of ceramics [1–4], glasses [5], polymers [6], carbon nanotubes (CNT) [7] and composite materials [8,9] for the surface modification of biomedical implants. EPD is achieved via the motion of charged particles towards an electrode under the influence of an electric field [9–13]. Various mechanisms of particle coagulation at the electrode surface and the deposit formation were described in the literature [9,10,14]. The interest in EPD for biomedical applications stems from the high purity of deposited materials, the possibility of uniform deposition on substrates of complex shape, relatively high deposition rate and good control of deposit microstructure. Significant interest has been generated in the EPD of cationic and anionic polysaccharides, such as chitosan [15,16], alginic acid [17] and hyaluronic acid [18]. This interest is attributed to the biocompatibility, good film forming properties, antimicrobial and other functional properties of the natural polysaccharides. Chitosan, alginic acid and hyaluronic acid have been utilized in thin film biosensors [19], coatings for drug carriers [20], stents [21], prosthetic cartilages [22], vascular grafts [23], bone substitute implants [24] and extrasynovial tendons [25]. There is an increasing interest in the use of the polysaccharides for the fabrication of drug-eluting, hemocompatible and antimicrobial coatings [26–28]. Recently it was discovered that

⁎ Corresponding author. Tel.: +1 905 525 9140; fax: +1 905 528 9295. E-mail address: [email protected] (I. Zhitomirsky). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.11.022

polysaccharides form interfaces between organic and inorganic components in bones and govern the crystallization of HA nanoparticles [29]. This discovery has generated a new wave of interest in the fabrication of organic–inorganic bone substitute composites containing natural polysaccharides. The combination of polymeric and inorganic phases is a common feature of various natural materials, including bone, which can be considered as a multilayer composite, containing layers of collagen fibers and HA crystals [30]. There are two important factors for the fabrication of implant materials: the desired surface chemistry to support or stimulate an appropriate host response and the appropriate microstructure to achieve desired mechanical and other functional properties. HA is a commonly used biomaterial due to its structural and chemical similarities to the mineral component of natural bones [30]. CNT with their small dimensions, high aspect ratio, bioactive properties, high strength and stiffness have excellent potential to stand in for the reinforcing collagen fibrils in the bone-substitute materials [31]. The applications of HA-CNT composites in bone-substitute biomedical implants have been described in a recent review paper [31]. However, the sintering of such composites presents difficulties, attributed to oxidation of CNT at elevated temperatures. Moreover, the microstructure of sintered HA is different from that of nanostructured HA in bones. The unique mechanical and other functional properties of bones result from unique multilayer microstructure, reinforced by collagen fiber layers, which are separated by HA layers, containing oriented HA nanoparticles with c-axes parallel to the collagen fibers. The interaction of the collagen fibers and HA is mediated by the polysaccharide interface [29].

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Fig. 1. Structures of (a) chitosan, (b) alginic acid and (c) hyaluronic acid.

alginic acid – HA [17] and hyaluronic acid – HA [18] composite coatings were prepared by EPD. This method offers the advantages of room temperature processing and enables co-deposition of nanostructured HA and other functional materials, such as drugs and antimicrobial agents [34–36]. It is known that polysaccharides can be used for the dispersion of CNT. The wrapping mechanism of the dispersion has been described in the literature [37,38]. Therefore, it is expected that polysaccharides can be used for the co-deposition of HA and CNT. The goal of this investigation was EPD of composite films containing MWCNT and polysaccharides. The results presented below indicate that chitosan, alginate and hyaluronate can be used for the dispersion, charging of MWCNT and EPD of composite polysaccharide –MWCNT films. EPD has been utilized for the fabrication of polysaccharide – MWCNT – HA nanocomposite films, which were obtained as monolayers containing MWCNT and HA in a polysaccharide matrix or laminates, containing polysaccharide – MWCNT layers separated by polysaccharide – HA layers. We discuss the deposition mechanisms, properties and applications of the obtained films. 2. Experimental procedures

EPD is ideally suited for the deposition of laminates of different materials and can be investigated for the fabrication of CNTpolysaccharide-HA composite coatings with microstructures and compositions similar to natural bones. The fabrication of CNTpolysaccharide-HA nanocomposites requires the dispersion of HA nanoparticles and CNT in the polysaccharide solutions. In the previous investigations [17,18,32–36], various polysaccharides have been utilized for the electrosteric stabilization of HA nanoparticles in suspensions and EPD of composite coatings. Chitosan – HA [32,33],

Chitosan (degree of deacetylation of 85%), sodium alginate, acetic acid, Ca(NO3)2·4H2O, (NH4)2HPO4, NH4OH (Aldrich) and sodium hyaluronate (Alfa Aesar) were used as starting materials. Chitosan was dissolved at pH = 3 in an acetic acid solution. Stoichiometric HA nanoparticles were prepared by a wet chemical technique described in previous works [32,33,39]. Precipitation was performed at 70 °C by the slow addition of a 0.6 M ammonium phosphate solution into a 1.0 M calcium nitrate solution. The pH of the solutions was adjusted to 11 by NH4OH. Stirring was performed during 8 h at 70 °C and then 24 h at

Fig. 2. TGA and DTA data for (a) as-received MWCNT and films prepared at deposition voltage of 20 V from polymer solutions without MWCNT (P) and containing MWCNT: (b) 0.5 gL− 1 chitosan and 0.5 gL− 1 chitosan containing 0.1 gL− 1 MWCNT, (c) 0.5 gL− 1 alginate and 0.5 gL− 1 alginate containing 0.1 gL− 1 MWCNT, (d) 0.5 gL− 1 hyaluronate and 0.5 gL− 1 hyaluronate containing 0.1 gL− 1 MWCNT.

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room temperature. The precipitate was washed with water and finally with ethanol. It has been previously reported [32,33] that the average length of the needle-shaped HA crystals, prepared by this method, is about 150 nm and the average aspect ratio is 8. The long axis of the needles corresponded to the c-axis of the hexagonal HA structure. MWCNT were provided by Arkema, the average diameter of the MWCNT was ∼15 nm and length ∼0.5 µm. Deposits were obtained on graphite (30×20×1 mm) and Nitinol (40×30×0.2 mm) (Alfa Aesar) substrates. Nitinol is a shape memory alloy containing approximately 50 at.% nickel and 50 at.% titanium. The EPD cell included a substrate centered between two Pt counterelectrodes. The distance between the substrate and counterelectrodes was 15 mm. EPD was performed at deposition voltages of 10–50 V from the 0.5–1 gL− 1 polysaccharide solutions in a mixed ethanol–water solvent (60–87 vol.% ethanol) containing 0–4 gL− 1 HA and 0–0.5 gL− 1 MWCNT at 20 °C. The deposits were studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) using thermoanalyzer (Netzsch STA-409). TGA and DTA studies were carried out in air at a heating rate of 5 °C min− 1. The X-ray diffraction (XRD) studies were performed using a diffractometer (Nicolet I2) with monochromatized CuKα radiation at a scanning speed of 1°min− 1. The microstructures of the deposited films were investigated using a JEOL JSM-7000F scanning electron microscope (SEM). The protective properties of films were studied using a potentiostat (PARSTAT 2273, Princeton Applied Research). Testing was carried out in Ringer's physiological solution (NaCl 8.6 gL− 1, CaCl2·2H2O 0.33 gL− 1, KCl 0.30 gL− 1) [32,33]. A conventional three-electrode cell was utilized, with a platinum mesh being the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The potentiodynamic polarization curves were obtained at a scan rate of 1 m Vs− 1.

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The electric field provided electrophoretic motion of anionic alginate macromolecules towards anode, where pH decreased owing to the electrochemical decomposition of water: 2H2 O→O2 þ 4Hþ þ 4e−

ð5Þ

The neutralization of COO− groups of alginate in the low pH region at the anode surface resulted in the deposition of alginic acid (Fig. 1): Alg− þ Hþ →H–Alg

ð6Þ

It is suggested that the adsorption of anionic alginate on the surface of MWCNT provided a negative charge required for the anodic EPD. Hyaluronic acid – MWCNT films were obtained by anodic EPD. However, the proposed mechanism of hyaluronic acid deposition [18] is

3. Results and discussion Chitosan, alginate and hyaluronate were used for the electrosteric stabilization of MWCNT in suspensions and EPD of composite films. Cathodic deposits were obtained from chitosan solutions containing MWCNT. The mechanism of chitosan deposition has been described in the literature [15,16]. Chitosan (Fig. 1) can be protonated and dissolved in acidic solutions. At low pH, protonated chitosan becomes a cationic polyelectrolyte: CHIT–NH2 þ H3 Oþ →CHIT–NHþ 3 þ H2 O

ð1Þ

However, the increase in solution pH results in a decreasing charge and at pH=6.5 chitosan's amino groups (Fig.1) become deprotonated. High pH can be generated at the cathode surface using the cathodic reaction: 2H2 O þ 2e− →H2 þ 2OH−

ð2Þ

In the EPD process, electric field provides electrophoretic motion of charged chitosan macromolecules to the cathode, where chitosan forms an insoluble deposit: − CHIT–NHþ 3 þ OH →CHIT–NH2 þ H2 O

ð3Þ

It is suggested that the adsorption of chitosan on the MWCNT surface provided a positive charge of MWCNT for cathodic EPD of composite films. In contrast, alginate – MWCNT films were prepared by anodic deposition. The mechanism of deposition of pure alginic acid films was described in a previous investigation [17]. The dissociation of sodium alginate Na–Alg in aqueous solution resulted in the formation of anionic Alg− species: þ

−

Na–Alg→Na þ Alg

ð4Þ

Fig. 3. SEM images of surfaces of composite films prepared at deposition voltage of 20 V from different solutions: (a) 0.5 gL− 1 chitosan containing 0.05 gL− 1 MWCNT, (b) 0.5 gL− 1 chitosan containing 0.1 gL− 1 MWCNT, (c) 0.5 gL− 1 alginate containing 0.2 gL− 1 MWCNT.

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different from the mechanism of alginic acid deposition [17]. The pH reduction at the electrode surface resulted in charge compensation of negatively charged COO− groups of hyaluronate (Fig. 1). However, at pHb 2.5 hyaluronate was positively charged owing to the protonation of the NH groups [40]. It is known that hyaluronic acid exhibits an isoelectric point located at pH = 2.5 [40]. The positive charge of hyaluronic acid at the anode surface prevented anodic deposition in aqueous solutions [18]. However, EPD from mixed ethanol–water solutions enabled the formation of anodic deposits [18]. The deposit formation was attributed to cross-linking of hyaluronic acid in ethanol– water solutions in acidic conditions at the electrode surface. The crosslinking in acidic ethanol–water solutions [41] occurs without change in chemical structure and is related to the formation of hydrogen-bonding network among the chains. According to the literature [41], the crosslinking of hyaluronic acid results in water insoluble gels. The composite polysaccharide-MWCNT films prepared by cathodic and anodic EPD were studied by TGA, DTA and SEM. Fig. 2 compares TGA and DTA data for as-received MWCNT, pure polymer films and composite polymer-MWCNT films. The data for as-received MWCNT showed that burning out of MWCNT was achieved in the range of 550– 670 °C. The TGA and DTA results revealed a difference in thermal behavior of films prepared from pure polymer solutions and the solutions containing MWCNT. The TGA data (Fig. 2b,c,d) showed that burning out of films prepared from pure polymer solutions was achieved at lower temperatures. The DTA data for the films prepared from the polymer solutions containing MWCNT showed broader exothermic peaks or additional peaks at higher temperatures, compared to the DTA peaks for pure polymers. The difference can be

Fig. 5. High magnification SEM images of fractures of composite films prepared on graphite substrates at deposition voltage of 20 V and deposition time of 5 min. from different solutions: (a) 0.5 gL− 1 chitosan containing 0.1 gL− 1 MWCNT, (b) 0.5 gL− 1 alginate containing 0.1 gL− 1 MWCNT and (c) 0.5 gL− 1 hyaluronate containing 0.1 gL− 1 MWCNT.

Fig. 4. SEM images of fractures of composite films (F) prepared on graphite substrates (S) at deposition voltage of 20 V and deposition time of 5 min. from different solutions: (a) 0.5 gL− 1 chitosan containing 0.1 gL− 1 MWCNT and (b) 0.5 gL− 1 hyaluronate containing 0.1 gL− 1 MWCNT.

attributed to higher burning out temperature of MWCNT compared to pure polymers. The results indicated the formation of composite films. The TGA data are in a good agreement with the results of SEM investigations. SEM images of the film surfaces showed MWCNT in the polymer matrix (Fig. 3a,b,c). The increase in MWCNT concentration in the solutions resulted in an increasing amount of MWCNT in the deposits (Fig. 3a,b). SEM observations of the cross sections of the composite films showed that film thickness was varied in the range of 0.5–10 µm by variation of the deposition voltage in the range of 20–50 V and deposition time in the range of 1–10 min (Fig. 4). The high magnification images of the cross sections showed MWCNT in chitosan (Fig. 5a), alginic acid (Fig. 5b) and hyaluronic acid (Fig. 5c) matrix. Some MWCNT are aligned perpendicular to the film surface (Fig. 5a). The amount of MWCNT aligned perpendicular to the film surface increased

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with increasing voltage in the range of 20–50 V. The alignment of CNT perpendicular to the substrates during EPD was observed in other investigations [42,43]. The electrophoresis of MWCNT, which have a high aspect ratio, can be described by the equations derived for infinitely long cylindrical colloidal particles [44]: μ 8 = er e0 f=2η

ð7Þ

μ jj = er e0 f=η

ð8Þ

where μ? and μO are electrophoretic mobilities in transverse and tangential fields, respectively, εr is relative permittivity of the medium, ε0 is the permittivity of vacuum, ζ is zeta potential and η is viscosity. Eqs. (7) and (8) indicate that μO N μ?. Therefore, higher deposition rate of MWCNT aligned parallel to electric field can be expected. Moreover, it is suggested that electric field can provide rotation and preferred alignment of the MWCNT in the suspensions. The EPD method has been further developed for the fabrication of polysaccharide – MWCNT – HA films. The composite films were obtained as monolayers, containing MWCNT and HA nanoparticles in a polysaccharide matrix or laminate structures, containing composite polysaccharide – MWCNT layers separated by polysaccharide – HA layers. Fig. 6 shows SEM images of composite films containing MWCNT and HA in a chitosan matrix. The increase in HA concentration in the suspensions resulted in increasing amount of HA particles in the deposits (Fig. 6a,c). The addition of MWCNT to the chitosan solutions containing HA resulted in the formation of monolayer chitosan – MWCNT – HA films (Fig. 6b,d). The results of SEM observations are in good agreement with TGA and DTA data shown in Fig. 7. The addition of HA to chitosan solutions resulted in co-

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deposition of chitosan and HA, as indicated by additional weight loss in the TGA data for the composite film compared to the weight loss of as-prepared HA. The total weight loss of 6.6 wt.% for pure HA at 800 °C can be attributed to dehydration. In contrast, the composite film showed a total weight loss of 41.9 wt.%, related to HA dehydration and burning out of chitosan. The observed DTA exothermic peaks in the range of 230–500 °C for the composite HA-chitosan films can be attributed to burning out of chitosan. Such DTA peaks were not observed for as-prepared HA powder (Fig. 7). However, similar DTA peaks were observed for chitosan (Fig. 2b). The TGA data presented in Fig. 7 showed additional weight loss for the films prepared from the 0.5 gL− 1 chitosan solutions, containing 1 gL− 1 HA and 0.1 gL− 1 MWCNT, compared to the films prepared from the 0.5 gL− 1 chitosan solution, containing 1 gL− 1 HA without MWCNT. The total weight loss at 800 °C for the films containing MWCNT was found to be 45.7 wt.%. The additional weight loss can be attributed to burning out of MWCNT. The TGA data for the films prepared from suspensions containing MWCNT showed weight loss at higher temperatures, compared to the films without MWCNT. Corresponding DTA data (Fig. 7f) showed additional exotherms at higher temperatures in agreement with the data shown in Fig. 2b. Therefore, the TGA and DTA data indicate the formation of composite chitosan – HA – MWCNT films. MWCNT with their high aspect ratio and excellent mechanical properties have a potential to strengthen and toughen HA. The use of MWCNT for the reinforcement of HA has generated significant interest for biomedical applications [31]. However, the sintering of such composites presents difficulties due to the thermal degradation of MWCNT at temperatures exceeding 600 °C (Fig. 2a). It is known that HA can be sintered at temperatures of 1150–1200 °C [39]. The high sintering temperature introduces the problems related to the oxidation and degradation of MWCNT and metallic substrates. The

Fig. 6. SEM images of composite films prepared from 0.5 gL− 1 chitosan solutions containing (a) 0.5 gL− 1 HA, (b) 0.5 gL− 1 HA and 0.1 gL− 1 MWCNT, (c) 4 gL− 1 HA, (d) 4 gL− 1 HA and 0.1 gL− 1 MWCNT, arrows show MWCNT.

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Fig. 7. (a,b,c) TGA and (d,e,f) DTA data for (a,d) as-prepared HA, and films deposited at deposition voltage of 20 V from 0.5 gL− 1 chitosan solutions, containing (b,e) 1 gL− 1 HA and (c,f) 1 gL− 1 HA and 0.1 gL− 1 MWCNT.

sintering of HA coatings on the metallic substrates results in the metal-catalyzed decomposition of HA and the formation of other calcium phosphate phases with a higher densification temperature [39]. Moreover, the products of HA decomposition have lower chemical stability and higher solubility compared to HA. The sintering of HA at high temperatures results in grain growth. Therefore, the properties of the sintered HA could be different from the properties of natural bone nanocomposite materials, which contain HA nanoparticles. The use of chitosan enabled room temperature processing of composite materials containing HA and MWCNT and avoided the sintering problems. In our investigation, needle-shaped HA nanoparticles were used for EPD. The long axis of the needles corresponded to the c-axis of the hexagonal HA structure. Eqs. (7) and (8) predicted higher deposition rate for the HA particles with c-axis perpendicular to the substrate from the suspensions containing randomly oriented HA particles. However, the XRD pattern shown in Fig. 8 indicated that HA needle-shaped nanoparticles were oriented in a chitosan matrix parallel to the substrate surface. The intensity of (100) and (300) peaks increased and the intensity of (002) decreased drastically for HA in a chitosan matrix of the composite films compared to the intensities of corresponding HA peaks in as-prepared powders. Therefore, c-axes of the particles are oriented parallel to the surface of the Nitinol

Fig. 8. X-ray diffraction patterns of (a) as-prepared HA powder and (b) film deposited at a deposition voltage of 20 V from 0.5 gL− 1 chitosan solution, containing 1 gL− 1 HA (● — peaks of HA, corresponding to JCPDS file 09-0432, ■ — nitinol substrate).

substrate. Similar orientation was observed in the chitosan – HA films deposited on graphite substrate. Such orientation was not observed in alginic acid – HA and hyaluronic acid – HA layers. Therefore, the orientation can be attributed to the interaction of amine groups of chitosan and Ca ions in the HA structure [45]. The chitosan-HA layers containing HA needle-shaped particles oriented parallel to the substrate were separated by chitosan layers reinforced by MWCNT in composite films shown in Fig. 9. The EPD of chitosan-MWCNT was performed at voltage of 20 V in order to reduce MWCNT alignment perpendicular to the substrate. It was shown that the thickness of layers can be varied in the range of 0.2–5 µm by variation in deposition time. Fig. 9 compares typical SEM images of laminates, containing chitosan-MWCNT layers of different thickness. It is important to note that the use of chitosan as a matrix for all layers enabled integration of the individual layers into the multilayer structure. Moreover, the use of chitosan as a common dispersant, charging additive and binder for HA and MWCNT enabled co-deposition of both materials and homogeneous distribution of HA and MWCNT in the polymer matrix. It is in this regard that previous investigations highlighted the importance of the use of common solvent–dispersant–binder systems for the deposition of multilayer coatings and films, containing layers of different materials [46]. It is expected that the incorporation of MWCNT into the composite coatings, containing HA and chitosan, will enable the reinforcement of the composite structures. Similar to natural bones, polysaccharides can be used to create an interface between fibrous reinforcing components and HA nanoparticles. Moreover, the composite films offer other benefits for implant applications. The results of potentiodynamic investigations shown in Fig. 10 indicated that composite films provided corrosion protection of Nitinol substrates in Ringer's

Fig. 9. (a, b) SEM images of laminates of chitosan-HA layers, prepared from 0.5 gL− 1 chitosan solutions containing 1 gL− 1 HA, and chitosan – MWCNT layers prepared from 0.5 gL − 1 chitosan solutions containing 0.1 gL− 1 MWCNT, arrows show chitosan – MWCNT layers, deposition voltage 20 V.

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Acknowledgement The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. References [1] [2] [3] [4] [5] [6] [7]

Fig. 10. Tafel plots for (a) bare Nitinol substrate and (b) coated Nitinol substrate with a film prepared from 0.5 gL− 1 chitosan solutions containing 1 gL− 1 HA and 0.1 gL− 1 MWCNT at deposition voltage of 20 V and deposition time 5 min.

physiological solutions. Fig. 10 compares the Tafel curves for the uncoated and coated Nitinol. It can be seen that the corrosion current and therefore the rate of corrosion of the coated sample was reduced by the film. These results indicated that the composite films acted as protective layers and improved the corrosion resistance of the Nitinol substrates. Nitinol is widely used in numerous biomedical applications, such as stents, orthopaedic implants, orthodontic arch wires, guide wires and jaw surgical implants. However, the biocompatibility of Nitinol is still a subject of controversy [47]. Surface modification techniques are used to improve biocompatibility and reduce the dissolution of Ni from Nitinol in physiological environments [47]. 4. Conclusions Composite chitosan – MWCNT films were prepared by cathodic EPD. Anodic EPD has been utilized for the deposition of alginic acid – MWCNT and hyaluronic acid – MWCNT films. Film thickness was varied in the range of 0.5–10 µm by variation of the deposition voltage in the range of 20–50 V and deposition time in the range of 1–10 min. It was shown that biopolymer – MWCNT – HA films can be prepared as monolayer nanocomposites containing MWCNT and HA in a biopolymer matrix or laminates, containing biopolymer – MWCNT layers separated by biopolymer – HA layers. The thickness of individual layers can be varied in the range of 0.2–5 µm by variation in deposition time. The composition of the films can be varied by the variation of HA and MWCNT concentration in the suspensions. Obtained films provided corrosion protection of NiTi shape memory alloys in Ringer's physiological solution. The proposed method offers the advantage of room temperature processing of composite materials.

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