Electrochemical surface modification of titanium in dentistry

Dental Materials Journal 2009; 28(1): 20－36 Review Electrochemical surface modification of titanium in dentistry Kyo-Han KIM1,2,3,* and Narayanan RAMASWAMY1,2 1 Department of Dental Biomaterials, School of Dentistry, Kyungpook National University, Daegu, Korea Brain Korea 21 Project, Kyungpook National University, Daegu, Korea 3 Institute for Biomaterials Research & Development, Kyungpook National University,Daegu, Korea Corresponding author, Kyo-Han KIM; E-mail: kyohan@mail.knu.ac.kr 2 Titanium and its alloys have good biocompatibility with body cells and tissues and are widely used for implant applications. However, clinical procedures place more stringent and tough requirements on the titanium surface necessitating artificial surface treatments. Among the many methods of titanium surface modification, electrochemical techniques are simple and cheap. Anodic oxidation is the anodic electrochemical technique while electrophoretic and cathodic depositions are the cathodic electrochemical techniques. By anodic oxidation it is possible to obtain desired roughness, porosity and chemical composition of the oxide. Anodic oxidation at high voltages can improve the crystallinity of the oxide. The chief advantage of this technique is doping of the coating of the bath constituents and incorporation of these elements improves the properties of the oxide. Electrophoretic deposition uses hydroxyapatite (HA) powders dispersed in a suitable solvent at a particular pH. Under these operating conditions these particles acquire positive charge and coatings are obtained on the cathodic titanium by applying an external electric field. These coatings require a post-sintering treatment to improve the coating properties. Cathodic deposition is another type of electrochemical method where HA is formed in situ from an electrolyte containing calcium and phosphate ions. It is also possible to alter structure and/or chemistry of the obtained deposit. Nano-grained HA has higher surface energy and greater biological activity and therefore emphasis is being laid to produce these coatings by cathodic deposition. Key words: titanium, anodic oxidation, electrophoresis, cathodic deposition Received Sep 29, 2008: Accepted Dec 2, 2008 1. Introduction In the past 20 years, the number of dental implant procedures has increased steadily worldwide, reaching about one million dental implantations per year [1]. The clinical success of oral implants is related to their early osseointegration. Geometry and surface topography are crucial for the short- and long-term success of dental implants. There are two types of response after implantation. The first type involves the formation of a fibrous soft tissue capsule around the implant. This fibrous tissue capsule does not ensure proper biomechanical fixation and leads to clinical failure of the dental implant. The second type of bone response is related to direct bone– implant contact without an intervening connective tissue layer. This is what is known as osseointegration [1]. Surface parameters may play an important role to obtain effective implant–tissue interaction and osseointegration. Modification of the titanium native surface is usually required to meet these requirements. A surface modification based approach can completely exploit the excellent properties of titanium such as mechanical strength and bioinertness. Though many methods of titanium surface modification are known, electrochemical methods are relatively simple and cheap techniques among them. These broadly consist of anodic and cathodic treatments. Anodic oxidation is the chief anodic technique. Electrophoretic and cathodic HA depositions are the cathodic techniques. By anodic oxidation it is possible to engineer the roughness, porosity and chemical composition for improved biocompatibility. The anodic oxide can have interconnected pores (0.5–2 μm in diameter) and intermediate roughness (0.60–1.00 μm). In addition, anodic oxide can be flat layer or tubular and can have amorphous or anatase phase. Heat-treatment or anodic oxidation at high voltages can produce a mixture of anatase and rutile in the oxide. Various elements can be doped from the electrolyte onto the oxide film and incorporation of these elements improves the properties of the oxide for effective bioimplantation. Calcium and phosphorus are deposited on the titanium oxide during anodization from a bath containing calcium acetate and glycerophosphate and are useful for the formation of HA. Electrophoretic deposition of HA uses HA powders dispersed in a suitable solvent and coatings are obtained by applying voltages of the order of 20200V. The coating density is improved by a further sintering at 600°C or above. Dynamic voltage process uses different potentials applied at different rates. Using this method, small particles as well as large particles can be deposited. Cathodic deposition is Dent Mater J 2009; 28(1): 20－36 another electrochemical method where HA is formed from an electrolyte containing calcium and phosphate ions. By adjusting the pH of the bath, current density and bath agitation, it is possible to obtain tailormade calcium phosphate coatings. As the implants have high clinical success rate if they contain coatings with the grain sizes of few nanometers, emphasis is being laid to produce these on titanium substrates. This paper gives a brief summary of anodic oxidation, electrophoretic and cathodic HA deposition on titanium substrates outlining the process details, merits and demerits. 2. Anodic oxidation Thermal treatment, particularly at temperatures above 200°C, significantly changes the microstructural properties of the oxide film. The film thickness increases from a few nanometers (of the natural oxide) to several tens of nanometers [2]. Also the microstructure changes from amorphous or poorlycrystalline to microcrystalline. Thermal treatment of titanium surfaces has been reported to improve the resistance of the surface towards release of soluble species of titanium or other alloying metal cations [3] and also influence the specific protein adsorption pattern in contact with blood [4]. Although, thermal treatment of titanium implant surfaces has been demonstrated to be beneficial for the behavior of titanium in contact with bone tissue, treatments above 500-600°C causes a reduction in strength and/ or fatigue resistance of the titanium alloys; for load-bearing implants, this is not tolerable. In such a case, anodic oxidation is a very useful way of growing the oxide on titanium surface for improved biocompatibility. 2.1 Mechanism of anodic oxidation of titanium The anodic oxidation of titanium is categorized by solid state diffusion in the oxide or by dissolutiondeposition in the electrolyte. Overall reactions leading to oxidation at the anode can be written [5] as: At Ti/Ti oxide interface: Ti → Ti 2+ + 2e － (1) At Ti oxide/electrolyte interface: 2H2O → 2O2 － + 4H+ ( oxygen ions react with titanium to form oxide) ( 2 ) 2H2O → O2(gas) + 4H+ + 4e－ (oxygen gas evolves) At both interfaces: (3) Ti2+ + 2O2 － → TiO2 + 2e－ 21 (4) The titanium and oxygen ions formed in these redox reactions are driven through the oxide by the externally applied electric field, leading to growth of the oxide. Since anodic titanium oxides have a high resistivity relative to the electrolyte and the metallic parts of the electrical circuit, the applied voltage drop will mainly occur over the oxide film of the anode. As long as the electric field is strong enough to drive the ions through the oxide, a current will flow and the oxide will continue to grow. Therefore there is usually a linear relationship between the final oxide thickness and the applied voltage. The oxides usually grow at the rate of 1.5 – 3 nm/V (also called as growth constant) in the various electrolytes. However, this relationship holds good below the dielectric breakdown limit of the oxide, which is around 100V depending on electrolyte and other process conditions [5]. The anodization process can be carried out either at constant current (galvanostatic process) or at constant voltage (potentiostatic process). At galvanostatic operation, voltage will change. If the anodizing is carried out at voltages above the breakdown limit (spark anodizing), the oxide will no longer be resistive enough to prevent further current flow and oxide growth. At such high voltages, the process will lead to increased gas evolution and sparking. During anodic oxidation of titanium metal, oxygen gas evolution is usually observed, which contributes to reduce the current efficiency of the growth process [6-7]. 2.2 Various oxidizing electrolytes Titanium can be anodically oxidized in a) acid and b) non-acid electrolytes. Sulphuric acid is a very common electrolyte for oxidation of titanium and Ti6Al-4V and studied extensively [8-11]. When titanium is immersed in an electrolytic solution as an anode and current is drawn, the oxygen generated at the anodic surface combines with the reactive titanium to form titanium oxide. The thickness of the oxide layers is a function of applied potential [8], anodizing time [9] and electrolyte temperature [9]. The effect of the electrolyte concentration shows that, in general, the anodic forming voltage apparently decreases with increase of the concentration of all the electrolytes employed in this study. This phenomenon can be explained on the basis of the ‘electrical double layer’ mode [12]. It has been proposed that during electrochemical anodizing, the ‘electrical double layer’ forms at the oxide film/ electrolyte interface, which consists of an excess or deficit of electrons on the metal side and of an excess or deficit of ions on the electrolyte side. These couplings of electrons and ions during anodizing normally result in a certain gradient of the concen- 22 Dent Mater J 2009; 28(1): 20－36 Fig. 1 Surface morphology of anodic oxide films of titanium in different compositions of β-glycerophosphate (β-GP) in 0.1 M of calcium acetate: (A) 0.02 M β-GP (B) 0.03 M β-GP (C) 0.04 M β-GP (D) 0.05 M β-GP. tration distribution of the electrolyte at the oxide film/electrolyte interface, i.e., the inner layer of the lower concentration and the outer layer of the higher concentration. In this situation, if an increase of the electrolyte concentration is sufficient to heighten the lowered concentration of the inner layer, the electrochemical reaction at the interface accelerates and then the electrical resistance will be reduced. Eventually the anodic forming voltage decreases with increase of the electrolyte concentration [12]. Besides the acids, many electrolytes like a) sodium phosphate and isopropyl phosphate in ethylene glycol [13], b) ammonium pentaborate [14], and c) calcium acetate and calcium glycerophosphate [15-16] are used for producing anodic oxides on titanium alloys. For an efficient electrolyte of high voltage anodic oxidation using electrolytes containing calcium and phosphorus, requirements of calcium salts are a) good solubility and b) sufficient passivation of the titanium surface at high positive potentials. And these requirements are met with, by only a few calcium salts. Due to their positive charge, calcium ions are repelled by the positive charge of the anodically polarized titanium surface thus reducing the extent of their incorporation in the coating, while anions such as phosphate ions are attracted by the positively charged titanium surface and are incorporated at a higher rate. Ca/P ratio in the coating is very low. A chelating agent namely EDTA increases the content of calcium in the coating and thereby increases the Ca/P ratio [17]. EDTA increases the solubility of calcium through complex formation and at the same time, the positively charged calcium ions are converted to negatively charged complex ions that facilitate incorporation into the surface oxide film at the positively charged titanium surface. 2.3 Properties of the oxide films 2.3.1 Morphology Anodic oxidation of titanium can be designed to produce different oxides containing different pore morphology. Porous anodized films on titanium are good for implant applications. The open porosity opens up possibilities with regard to drug incorporation and release around titanium implants [18]. Porosity can be achieved by increasing the current density, concentration of the electrolyte or bath temperature. Simultaneous dissolution and formation, results in creation of porous, columnar oxide [19]. Pore diameter increases with increasing electrolytic voltage [11]. Size and distribution of the pores on anodic oxide on titanium alloys depend on the substrate structure. This is indicated by faster dissolution on the vanadium-enriched β phase than on the α phase [20] of Ti-6Al-4V alloy. Fig. 1 shows representative scanning electron micrographs of titanium surfaces anodized for 30 minutes at 350V from an electrolyte containing 0.1M calcium acetate and 0.02 to 0.05M β-calcium glycerophosphate. The surfaces indicated the presence of oxide with interconnected pores (0.5–2 μm in Dent Mater J 2009; 28(1): 20－36 Fig. 2 X-ray diffraction patterns of anodic oxide films obtained at 350V from electrolyte containing 0.1M calcium acetate and different concentrations of βglycerophosphate (β-GP): (a) 0.02 M β-GP (b) 0.03 M β-GP (c) 0.04 M β-GP (d) 0.05 M β-GP diameter). The pore diameter increased with an increase in β-GP content. Some microcracks were observed in the oxide film formed with increased electrolyte concentration. The size of pores, which originate from a spark on the interface of the oxide and electrolyte, is related to the nature and concentration of ions in the electrolyte [21]. 2.3.2 Crystallinity By adjusting the operating conditions it is possible to get amorphous or crystalline oxides during anodic oxidation. At lower applied voltages the oxide film is amorphous. With increasing voltages, the structure of the oxide film is changed from an amorphous to a crystalline oxide [22]. TiO2 can exist in three different crystalline forms called anatase, rutile and brookite, all with different physical properties. Anatase is formed at lower potentials and the dual structure of anatase and rutile at higher potentials [23]. Oxide crystallinity is also influenced by the concentration of calcium or phosphorus incorporated into the oxide. The degree of oxide crystallinity increased with an increase in the concentration of calcium incorporated into the oxide [23]. Anodic oxidation of titanium from electrolyte containing calcium glycerophosphate / calcium acetate [15; 21] produced films with different crystallinity. The amount of amorphous structure increased, and the X-ray diffraction peaks (Fig. 2) corresponding to anatase oxide became lower Fig. 3 23 XRD patterns of oxidized (350V from an electrolyte containing 0.15 M calcium acetate and 0.02M βcalcium glycerophosphate ) and hydrothermally treated for 4 hours at a) 200ºC b) 250ºC c) 300ºC. as the β-GP concentration increased [21]. 2.3.3 Composition The anions in the solution like sulphates or phosphates are also co-deposited along with oxides. Incorporation of P increases takes place in the form of PxOy [24] and contributes to the corrosion resistance of the oxide films, particularly. Anodic oxidation of titanium from calcium acetates and glycerophosphates at voltages exceeding 250V are reported [15; 23; 25-26]. No calcium and phosphorus containing phases were detected by XRD up to 350V. At the voltage of 450V, the titanium peak was significantly reduced and new Ca, P, Ti and O containing compounds are formed. And these calcium phosphates become dominant at 500V. The contents of both calcium and phosphorus in the coatings increased from a starting voltage of 250V to 450V. However, the Ca/P ratio increased up to 400V and then decreased at 450V. The Ca/P ratio was 1.3 at 250V and 1.8 at 400V and decreased to 1.5 at 450V [23]. 2.3.4 Oxide type By anodic oxidation it is possible to get amorphous or crystalline oxide depending upon the applied voltage and electrolyte used. Sulphuric acid anodization yields anatase at 90V, dual phase of anatase & rutile at 155V and rutile at 180V [27]. Anodic oxidation is carried out on titanium from electrolytes containing 24 Dent Mater J 2009; 28(1): 20－36 Fig. 4 Morphologies of surfaces obtained after oxidation (350V from an electrolyte containing 0.15 M calcium acetate and 0.02M β-calcium glycerophosphate) and hydrothermal treatment : a) 4 hours at 200ºC b) 4 hours at 300ºC. calcium acetates and glycerophosphates at voltages exceeding 250V [25-26]. At 250V, the oxidized layer was mainly composed of anatase. With increasing voltages, rutile began to appear gradually so that the structure consisted of a mixture of anatase and rutile. At the voltage of 450V, the titanium peak was significantly reduced and new Ca, P, Ti and O containing compounds are formed in addition to anatase and rutile. And these calcium phosphates rather than rutile become dominant at 500V [23]. 2.3.5 Roughness Oxide films on titanium and titanium alloys prepared above the breakdown limit (200V or higher) show increased surface roughness and a three-dimensional oxide structure consisting of numerous open pores. Their topography shows that they are both porous and relatively rough, but at the same time lack sharp edges. At higher applied voltage, the oxide layers were slightly cracked and the surface became irregular and rough [25]. The roughness of the anodic oxide layers of titanium obtained from the electrolyte containing calcium acetate and glycerophosphate is in the range of 0.3-0.9 μm depending on the current density, concentration of the electrolyte and the applied voltage [28]. 2.4 Hydrothermal treatment after anodizing Oxide layer produced by a combination of anodic oxidation and hydrothermal reaction were reported to consist of anodic oxides and HA, and these coatings were reported to adhere to titanium substrates [15]. The use of a mixture of calcium acetate and β-calcium glycerophosphate as an electrolyte was reported to produce porous and adhesive anodic oxide films. The anodic oxide films containing calcium and phosphorus provide precursors for the further formation of HA through hydrothermal treatment. Oxide was produced at 350V from an electrolyte containing 0.02M calcium glycerophosphate and 0.15M calcium acetate. No cracks were observed on the oxide films and the Ca/ P ratio was reported be 1.67 [29]. Hydrothermal treatment was performed on this coating (Ca/P ratio is 1.67) by high-pressure steam in an autoclave for either 2 or 4 hours at 200 or 250 or 300°C to produce HA needles. From the X-ray diffraction analyses of the hydrothermally treated samples (Fig. 3), it is clear that a mixture of apatite-like structures, rutile and anatase are found. Significantly higher concentration of calcium and phosphorus are detected on the crystals of hydrothermally treated surface than on the anodized oxide surface. This is attributed to the diffusion of calcium and phosphorus from the anodic oxide into HA crystals during hydrothermal treatment [30]. As such, the increase of temperature and pressure during the hydrothermal treatment suggests an acceleration of the diffusion and ion exchange process, which included the outward migration of Ca and P ions to the solid-liquid interface and the HA crystallization during hydrothermal treatments. This is probably ascribed to sufficient time for atom arrangement during the formation of the HA needles [29]. The needles were observed to be formed on the surface, while the rest were formed at angles to the surface of the anodic oxide with random orientations originating from the pores (Fig. 4). Increased HA quantity is observed at higher temperature, higher pressure and longer times. Crystallinity of the coatings increased with increase in hydrothermal treatment periods [29]. X-ray analyses also indicated a higher crystallinity as Dent Mater J 2009; 28(1): 20－36 Fig. 5 25 (a) Total protein and (b) alkaline phosphatase activity, during the culture of human embryonic palatal mesenchymal cells on (1) titanium (2) titanium anodized from 0.1M calcium acetate (CA) and 0.02M β- calcium glycerophosphate (β-GP) (3) titanium anodized from 0.1M CA and 0.03M β-GP (4) titanium anodized from 0.1M CA and 0.04M β-GP (5) titanium anodized from 0.1M CA and 0.05M β-GP temperature and pressure during hydrothermal treatment were increased. 2.5 In vitro studies The relations between biomaterials and adjacent tissues are directly related to the surfaces of materials. These events are accompanied by absorption and incorporation of biological molecules and the attachment of surrounding cells [31]. The functional activity of cells in contact with implant surfaces are governed by implant surface. Protein synthesis is an important marker for evaluating cell function. Matrix proteins in bone have been reported to play a crucial role in the calcification and architectural construction of these hard tissues. Alkaline phosphatase (ALP) activity and osteocalcin production are used as biochemical markers for determining osteoblast phenotype and are considered to be important factors in determining bone mineralization [32]. Osteoblast differentiation, as indicated by ALP production, was enhanced on anodized surfaces. It was also concluded that the phenotypic expression of osteoblast was enhanced by the presence of calcium phosphate and higher roughness on anodized titanium surfaces [21; 32]. It can be seen from Fig. 5 that, human mesenchymal cells cultured on anodized surfaces exhibited significantly higher total protein quantity and alkaline phosphatase activity. Fig. 5 shows that, ALP production increased with the β-GP concentration of electrolyte. Osteocalcin production by cells cultured on anodized and hydrothermally treated surfaces was significantly higher as compared with cells cultured on control Ti surfaces. Also anodized and hydrothermally treated surfaces played a role in enhancing bone apposition on the implant surface [21; 32]. 2.6 In vivo studies There is appreciation that surface roughness alters cultured osteoblast differentiation and the ability of the osteoblast to produce bone matrix proteins. The anodic oxide containing porous structure produces a rough surface and hence results in increased bone-toimplant contact and enhances the biomechanical interlocking of implant with bone at early times after placement to provide more bonding strength [27]. Primary fixation is one of the most important factors in establishing adequate osseointegration between bone and fixture. One measure of this factor is the value of the removal torque. A significantly higher removal torque suggests the possibility of bone ingrowth into the porous oxide surface, thereby allowing greater mechanical interlocking between the bone and the implants. Removal torque will also be influenced by the change in surface composition during anodization and the presence of HA needles on the surface-treated implants after hydrothermal treatments [16]. Histomorphologic study in rabbit of titanium surfaces anodized at 350V using 0.02M calcium glycerophosphate and 0.15M calcium acetate and hydrothermally treated at 200°C for 4 hours [16] showed a significantly higher removal torque for the anodized implants (Fig. 6). This suggests the possibility of bone ingrowth into the porous oxide 26 Fig. 6 Dent Mater J 2009; 28(1): 20－36 (a) Mean values and (b) standard deviation of removal torque value of each group after 6 and 12 weeks implantation. p values are computed by one-way ANOVA. Group I represents uncoated titanium. Group II represents titanium implants anodized at 350V from 0.15M calcium acetate (CA) and 0.02M β- calcium glycerophosphate (β-GP). Group III represents titanium implants anodized at 350V from 0.15M calcium acetate (CA) and 0.02M β- calcium glycerophosphate (β-GP) and then hydrothermally treated for 4 hours at 200ºC. Fig. 7 Bone–implant interface for (a) titanium implants at 6 weeks after implantation (b) anodized implants at 6 weeks after implantation (c) anodized & hydrothermally treated implants at 6 weeks after implantation (d) titanium implants at 12 weeks after implantation (e) anodized implants at 12 weeks after implantation (f) anodized & hydrothermally treated implants at 12 weeks after implantation Anodization conditions: 350 V; electrolyte containing 0.02M calcium glycerophosphate and 0.15M calcium acetate; Hydrothermal conditions: 4 hours in steam at 200°C Dent Mater J 2009; 28(1): 20－36 surface, thereby allowing greater mechanical interlocking between the bone and the implants [16]. The cortical bone around the implant was actively remodeled and showed numerous large marrow spaces [16]. With an increase in implantation period to 12 weeks, the bone–implant contact area and the density of cortical bone were improved, with reduction of marrow spaces in cortical bone observed around the inserted implants (Fig. 7). The presence of HA or calcium phosphate results in more rapid osseointegration and the development of increased interfacial strength that results from the early skeletal attachment and increased bone contact with the implant surface [16; 33-35]. 3. Electrophoresis: Electrophoretic deposition (EPD) of HA represents an important technological process because of its simplicity and low cost of the process. Advantages also include ability to coat with uniform thickness, wide range of thicknesses, ability to coat complex shapes, and ease of chemical composition control [3642]. These coatings have strong adhesion to the substrate and are mainly composed of pure phases without any metastable or mixed phases. In this process, HA particles are suspended in suitable solvents like polyvinyl alcohol or N, Ndimethyl formamide. Size of the particles to be deposited by electrophoresis technique is important because the particles must be fine enough to remain in suspension during the coating process. The solution must be maintained at an appropriate pH such that the HA particles acquire positive charge Fig. 8 27 and are deposited on the cathodic titanium under the action of electric field. The liquid medium used to suspend the particles should have a dielectric constant that gives effective coating and the particles must be colloidal stable in that medium [43]. Electrophoresis can produce coatings with a range of thickness. One disadvantage of this process is the requirement of a post-coating sintering at about 800°C. 3.1 Mechanism: There are two steps involved in electrophoretic deposition. The first step involves the migration of particles (which acquire positive charge) under the influence of an electric field applied to a stable colloidal suspension. The second step involves the deposition on the metallic substrate. Driving force of the deposition process is the applied electric field. Depending on the mode and sequence of voltage applied, the electrophoretic deposition can be carried out at i) constant voltage or ii) dynamic voltage. 3.2 Electrophoresis at constant voltage HA suspensions in polyvinyl alcohol and N,Ndimethyl formamide were used and coatings were obtained on titanium by applying voltages in the range of 10-200V [44]. With increasing voltages [44], more HA is deposited (Fig. 8). High voltages give rise to rough HA coatings. With increasing HA contents of the bath, HA levels of the coatings also increased (Fig. 9). For a low HA concentration, the coating was very rough and a great level of agglomeration is deposited. At higher HA concentrations, the coating becomes more uniform and crack-free and there is less agglomeration. At very high concentrations of HA, many cracks can be found [44]. These results Surface morphology by FE-SEM of coatings obtained at HA particle concentration of 0.1% at different applied voltages: (a) 20 V/ 10 min (b) 200 V/ 3 min 28 Dent Mater J 2009; 28(1): 20－36 show that HA powder concentration, applied potential and electrophoresis time have a significant effect on the deposited coating morphology. Suspension presedimentation was found to have a significant effect on the removal of the agglomeration in the coating. By applying low voltages and presedimentation, uniform and smooth hydroxyapatite coating can be prepared [44]. The deposition process fit an empirical model: W = W0(1－e-kt) Fig. 9 Surface morphology of coatings obtained at 20 V for 10 min at different HA particle concentration levels: (a) 0.1% (b) 0.2% (c) 0.5％. (5) where W0 is the weight of the substrate before the deposition, W is the weight of the titanium after deposition for t seconds and k is the kinetic constant. A kinetic constant of 9.99×10-2 provides a reasonably good estimate for the deposition mass at different deposition time [44]. 3.3 Dynamic voltage electrophoresis Under the application of low voltages (<20V), deposition of small HA particles has been reported [40]. These are dense coatings and have high bond strengths but lower biocompatibility. Application of higher voltages (>200V) for periods longer than 10 seconds was reported to produce a coating with larger HA particles, more porous microstructure and poor adhesion to the substrate [45]. Aapplying constant potential during EPD resulted in coatings that either have high bond strength or are porous to allow bony ingrowth. Using dynamically applied voltage a method was developed to produce a gradient structure, in which the part of the layer attached to the substrate is dense, while the outer layer is porous [42]. Electrophoretic deposition of HA powders on titanium was carried out in incremental voltage steps. These definite voltage steps of deposition resulted in a gradient coating, in which, coating layer attached to the substrate was made of fine HA particles and was dense, while the outer layer was made of bigger particles and was porous (Fig. 10). It was observed that the dense HA inner layer exhibited high bond strength, whereas the outer layer showed lower bond strength [42]. Further sintering was done for 2 hours at 800°C. Sintering does not alter the chemical composition of the HA by forming decomposition products or metastable products [42]. From Fig. 11 a, b and c it can be seen that all the coatings produced using the repeated dynamic deposition process were dense, crack-free, and uniform. However, under constant voltage (10 V) and at a high HA concentration (0.5％), cracks in the coatings were observed (Fig. 11 a1, b1, c1). These coatings serve as testimony to the advantage of using dynamic electrophoretic deposition. During electrophoretic deposition, the electrophoretic velocity (ν) of the charged particles can be 29 Dent Mater J 2009; 28(1): 20－36 field [42]. During the dynamic process, the electrophoretic velocity (ν) in Eq. 6 takes the form: dv = Q/4πrη×dE (7) where E is a variable during the dynamic voltage. Since E is time dependent [E = f(t)], the electrophoretic velocity (ν) for the dynamic process can be described by the following equation: ν= Fig. 10 Schematic representation of gradient HA coating by dynamic voltage described by the following equation [45]: ν = QE/4πrη (6) where Q, r, η, and E represent the charge, particle radius, viscosity of the suspension, and the potential difference applied to the suspension, respectively. In suspensions with a low concentration of solids, η is often considered a constant. Under this condition, the electrophoretic velocity is mainly a function of the electric field and the particle size. When E is constant, the suspension has a distribution of particle sizes, and particles with different Q/r ratios have different electrophoretic mobility, thereby resulting in segregation effects observed during the EPD process. In addition, in a suspension of particles with mixed radius, preferential deposition of finer particles is expected due to the mobility of finer particles as compared to bigger particles. Mobility of the particles can also be increased by increasing the applied potential, and thus providing the opportunity for bigger particles to be deposited. These theories help to explain the production of a porous and roughened coating at a higher electric field and a dense coating of finer particle size at a lower electric ∫Q/4πrη× d f(t) (8) As such, the electrophoretic velocity during the dynamic process is a function of time and the particle size. From Eq. 6, it is known that particles with same size are deposited simultaneously under constant voltage, resulting in a layer of predominantly mono-sized particles. Eq. 8 shows that by applying a dynamic voltage, particles with more wide distribution of sizes are deposited, and thus results in a coating consisting of various sized particles [42]. It is also suggested that the uniformly, densely packed mono-sized particle can achieve 64％ of the theoretical density. By the use of this dynamic process, the small particles can fill up the gaps in between the large particles. Further sintering the coatings for 2 hours at 800°C presented a more homogeneous and crack-free structure and caused no deterioration or decomposition of HA [42] and may retain its biocompatibility. 4. Electrochemical cathodic deposition: In this method, calcium phosphate coatings are formed on the titanium cathode from a bath containing dissolved calcium and phosphorus compounds. This process is characterized as a procedure performed using the ambient temperature that results in good conformability to the shape of the component [46-47]. These coatings also exhibit a control over crystallinity under milder conditions and shorter reaction times. Furthermore, a film thickness of less than 1μm can be achieved. Reduction of the film thickness leads to an increased resistance to delamination, which is observed frequently for thicker coatings [48]. 4.1 Process variables: Concentrations of calcium and phosphorus in the electrolyte, pH of the electrolyte, cathodic current density, time, processing temperature and pressure are the parameters influencing the type of calcium phosphate deposit. Following examples illustrate these effects: The electrochemical methods for depositing bioactive apatite coatings with control of phase and composition generally involve reaction conditions comprising elevated temperatures (37°C) or higher 30 Dent Mater J 2009; 28(1): 20－36 Fig. 11 Surface morphologies of HA coatings with similar deposit mass values prepared by repeated dynamic voltage at 0.1％ HA concentration: (a) two depositions, (b) three depositions, and (c) four depositions. Surface morphologies of HA coatings obtained at a constant voltage of 10 V at 0.5％ HA concentration after: (a1) 2 min, (b1) 5 min, and (c1) 7 min. pressure [49]. A pulsed electrochemical deposition technique was used to produce CaP coatings on porous titanium substrates under milder conditions (pH 4.4, 25°C) [50] but post-treatment with sintering under vacuum at high temperatures between 300 and 800°C was required to improve the bonding of the coating to the substrate. A galvanostatic technique was used to produce HA at nearphysiological pH (6.4) and body temperature with no requirements for post-treatment [51]. Homogeneous HA coatings have been obtained by cathodic deposition (－12.5 mA/cm2, 1 h) in a calcium phosphate-containing electrolyte at 80–200°C [52]. However, these large crystallites that grow perpendicularly to the substrate surface along their c-axis have an adverse impact on the mechanical properties of these films. Shirkanzadeh [53] used cathodic polarization to produce HA nanocrystallites on titanium surfaces at －1.4 V (SCE) in (Ca(NO3)2/ NH4H2PO4, pH = 6.0) at 85°C over 2 h. Another method is cathodic deposition of octacalcium phosphate (Ca8H2(PO4)6･5H2O) or brushite Dent Mater J 2009; 28(1): 20－36 (CaHPO4･2H2O) which can then be transformed into HA by a subsequent heat treatment at 425°C or by alkalization [54]. It can be said that, HA can be produced directly as a product of the cathodic polarization or by secondary treatment like heat-treatment. Its properties can also be improved by a secondary annealing treatment. 4.2 Mechanism: The main cathodic reactions involved are hydrogen reduction and oxygen reduction which are dependent on the pH of the bath. If the pH is high then oxygen reduction occurs so as to produce hydroxide ions in the vicinity of the cathode. These hydroxides also help in the supersaturation of PO43－ near the cathode and increase the local pH there. Calcium ions are driven to the cathode under the action of the electric field. Cathodic reactions at various voltage polarization curves on pure titanium in the mixed solution of Ca(NO3)2･4H2O and NH4H2PO4 in the potential range of –0.1 to 3V (vs. Ag/AgCl) are as follows [55]: O2 + 2H2O + 4e－ → 4OH－ (－0.1 to －0.3V)(0.1 mA/cm2 ) ( 9) H2PO4 － + H2O + 2e－ → H2PO3 － +2OH－ ( 10 ) 2H+ + 2e － → H2 (－0.3 to －1.1V)(0.3 mA/cm2) ( 11 ) 2H2PO4 － + 2e－ → 2HPO42－ + H2 ( 12 ) 2HPO42－ + 2e－ → 2PO43－ + H2 (－1.1 to –1.5V) (1 mA/cm2) ( 13 ) 2H2O + 2e － → H2 + 2OH－ (－1.5 to －3V)( 3mA/cm2) ( 14 ) Under these conditions calcium ions react with PO43- (produced from reaction 13) and OH－ (produced from reactions 9, 10 and 14) to produce HA. The pH in the vicinity of the cathode greatly influences the type of calcium phosphates formed. Addition of H2O2 to baths containing calcium and phosphorus ions promotes HA coating [56]. A redox reaction produces supersaturation of OH－ ions near the electrode and raises the local pH. H2PO4 and HPO42- dissociate to give PO43- as shown by: H2PO4 － → H+ +HPO42 － ( 15 ) HPO42 － → H+ +PO43 － ( 16 ) This local effect induces heterogeneous nucleation on the metal surface serving as the electrode. Addition of H2O2 to the solution prevents 31 Fig. 12 XRD of calcium phosphate coating on titanium from electrolyte containing 0.042M Ca(NO3)2 and 0.025M NH4H2PO4 (pH 4.1) at (a) 10mA/cm2 (b) 20mA/cm2 (c) 30mA/cm2 (d) 40mA/cm2 (e) 50mA/cm2. H2 gas generation at the cathode and promotes nucleation and growth of HA coating [56]. 4.3 Properties of cathodic calcium phosphate coatings on titanium 4.3.1 Various phases HA coatings are produced on cathodic titanium [55; 57] and Ti-6Al-4V [58-59] by electrochemical methods. All these works report the deposition of calcium phosphate on the cathodic titanium surface from baths containing Ca(NO3)2 and NH4H2PO4. Most of the electrolytic deposition is done in acidic calcium phosphate solutions and form brushite at lower current densities [55]. Increasing current densities tend to promote the formation of HA. Fig. 12 shows that dicalcium phosphate is the main constituent at current densities less than 5 mA/cm2; HA is the main constituent above 40 mA/cm2 [60]. Other then the electrochemical treatment, the brushite coatings can also be aged to convert them to apatite [54; 57; 61-62]. Electrolytic deposition done in neutral calcium phosphate electrolytes produced HA directly [48; 51] 4.3.2 Morphology: Nucleation of HA crystals during electrodeposition can occur wither as instantaneous nucleation or as progressive nucleation [63]. Nucleation is said to be instantaneous whenever the rate of formation of a nucleus at a given site is expected to be at least 60 times greater than the expected rate of coverage of the site by growth only. And nucleation is said to be progressive when the expected coverage of a site by growth is at least 20 times greater than the coverage 32 Dent Mater J 2009; 28(1): 20－36 Fig. 13 Surface morphology of calcium phosphate coating on titanium from an electrolyte containing 0.042M Ca(NO3)2 and 0.025M NH4H2PO4 (starting pH 4.1 ) produced at (a) 10 mA/cm2 (b) 30 mA/cm2 (c) 50 mA/cm2. of the same site by the act of nucleation. And the growth can be one- or two- or three- dimensional resulting in different shapes of the deposits like needles, discs, hemispheres, etc depending on deposit/substrate binding energy and their crystallographic misfit. In the electrodeposition of HA from aqueous electrolytes, during the first 12 minutes or so, the nucleation is instantaneous and is accompanied by a two-dimensional growth. Subsequently, the nucleation becomes progressive and is accompanied by a three-dimensional growth [63]. Enhanced bath agitation by the use of ultrasonics promotes instantaneous nucleation and reduces the rate of crystal growth during the first 12 minutes of deposition. Therefore the HA crystals have sizes of the order of 18 and 25 nm [64]. Subsequently the nucleation changes to progressive and growth is three-dimensional and therefore the coatings have spherical or near-spherical particles. Calcium phosphate coatings are deposited on titanium plate by an electrochemical method in SBF maintained at 5-62°C. Titanium plates are maintained at -2V in simulated body solution for 5, 10 and 30 minutes and 1 and 2 hours. The deposits are amorphous at 5, 22 and 37°C and are crystalline at 52 and 62°C. In the electrochemical synthesis of calcium phosphate in this temperature range, the diffusion process is the rate-determining step [52]. Low temperature electrochemical method among electrochemical synthesis at various temperatures can deposit defect-free or pore-free HA crystals [65]. Calcium phosphate coatings obtained at different current densities from acidic baths showed different morphology depending upon the applied current. At lower current densities the deposits have needle structure; with increasing currents these needles become blunt (Fig. 13) and show coarse structure [60]. 4.3.3 Bond strength: Coatings deposited at room temperature show stronger adhesion than those produced at elevated temperature [57]. HA coatings on titanium alloy were prepared by an alkaline treatment of electrodeposited precursors [66]. The bond strength of the coatings increased with the decrease of current density in the range of 0.2 – 15 mA/cm2 and reached 14 MPa at 0.2 mA/cm2. These results indicated that the dissolution and bond strength degradation of the electrodeposited coatings were much lower than those of the plasma-sprayed coatings [66]. It was also shown that the HA coatings prepared from neutral electrolytes had bond strengths lower than the prescribed 18 MPa for implant applications [64; 66]. 4.4 In vitro studies: Thin HA coatings were deposited on titanium using periodic pulse plating [48]. The thickness of the 33 Dent Mater J 2009; 28(1): 20－36 current density of 20 mA/cm2 and ultrasonic agitation showed acicular deposit. When the current density is increased to 50 mA/cm2, ultrasonic agitation retained the acicularity of the calcium phosphate deposit. The coating produced at a current density of 50 mA/cm2 from magnetic paddle stirring condition contained globular deposits [71]. Therefore it is clear that, the shape of HA crystals of the coating can be altered by changing the stirring conditions. HA coatings were produced on titanium from neutral electrolytes containing calcium nitrate and ammonium hydrogen phosphates under ultrasonic agitation [64; 73]. The HA crystal sizes were in the range of 15-25 nm and these are showed to have higher osteoblast activity (Fig. 14) compared with the uncoated titanium [64]. Fig. 14 Total protein activity on (a) uncoated titanium (control) (b) HA coating obtained on titanium Ca(NO3)2 and 0.025M NH4H2PO4 7.4 ) at 10 mA/cm2 (CD10) (c) HA coating obtained on titanium Ca(NO3)2 and 0.025M NH4H2PO4 7.4) at 15 mA/cm2 (CD 15) from 0.042M (starting pH from 0.042M (starting pH coatings was about 200 nm. Cell culture experiments showed that these coatings had no cytotoxicity when cultured with MG63 osteoblastic cells in vitro and supported cell growth for 2 days, outperforming the control untreated titanium. Cell culture tests also revealed the cell adherence and cell proliferation leading to infer that the electrochemically deposited HA had crystals in the size lower than 100 nm are beneficial for cell adhesion [64; 67]. 4.5 Nano-grained cathodic calcium phosphate coatings Nanocrystalline HA coatings attract cells more easily than their coarse-grained counterparts as cells and tissues are accustomed to interacting with nano sized structures inside the body [68-69]. Electrochemically deposited nano-grained calcium phosphate coatings were produced on titanium alloy substrates [70-72]. Different coatings were produced by using different cathodic current densities and/or agitation of the electrolytic bath at acidic pH [71]. Ultrasonated bath produced coatings containing dicalcium phosphate dihydrate grain sizes were in the range of 50-100 nm. With increasing current density, HA content in the coatings increased [71]. The coating obtained using a 5. Future research trend Anodic oxidation of titanium attracted much attention in the 1950s and 1960s and still continues to be a fascinating area of research. Self-organized nano titanium oxide tubes offer great scope for research because the tubular structure act as better scaffold for cell reactions than the conventional flat oxide. Coatings containing nano-grained HA act as better colonies for implant-cell interactions than the more coarse-grained HA and recent trend of research in synthesis of HA is oriented towards achieving this objective. 6. Concluding remarks Some aspects of electrochemical anodic oxidation and electrochemical HA deposition related to titanium surfaces are presented in this article. Titanium oxide films find applications in implant applications because of their porous nature and their rough surfaces. 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