Importance for Dental Health and Digestion

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Applications of Nanocomposite Materials in Dentistry. https://doi.org/10.1016/B978-0-12-813742-0.00008-0

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Hydroxyapatite and its coatings in dental implants

Abu Nasar

Department of Applied Chemistry, Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh, India

8.1 Introduction

The teeth are the hardest part of the human body. They are essential for chewing food, and also play an important role in speech, facial beauty, pronunciation, laughter, smiling, and more. The proper digestion of food is essential for good health, and teeth are part of the first stage of this process. Food should be crushed and chewed before entering the diges- tive system so that the body can absorb nutrients from the food. In the absence of teeth, or if there are defects in the teeth, proper digestion is held up, leading to considerable effects on health. The most common dental problems are tooth decay (also known as dental caries or cavities), enamel degradation, chipped teeth (caused by dental injury/accident), impacted teeth, cracked teeth, missing teeth, and so forth. In many circumstances, dental problems occur due to frequent exposure to certain foods, and bad human habits. Bacteria present in the mouth forms plaque, where bacteria are further reproduced. The plaque interacts with food deposits left on teeth and from acids. The food acid dissolves the enamel and creates cavities, or holes, in teeth. These cavities, or holes, are generally pain- less, until they become large and decimate the nerves and blood vessels inside the teeth.

Therefore, it is essential that any cavity or hole that forms in teeth should be filled as early as possible. Fortunately, for most dental problems, the solution is available in the form of a dental implant, dental filling, bonding, braces, bridges, crowns, dentures, extractions, gum surgery, root canals, veneers, and so forth. However, the solution depends on many factors, such as historical conditions, severity of the problem, dental status, financial con- dition of the patient, and so forth. For example, the dental cavities, or damaged, cracked, or missing teeth are filled, repaired, or implanted as needed by using biocompatible materials. For a material to be used in dental applications, it should fulfill certain requirements. However, there are different performance necessities for specific applications, but excellent biocompatibility and biological safety are primary requirements, and common in each case. The materials to be used for dental applications should fulfill a number of general property requirements, which are outlined as follows:

(i) Physical requirements: The dental materials should have a matching, natural appearance;

high glass transition temperature; low specific gravity; good dimensional stability; high thermal conductivity; and so forth.

(ii) Chemical requirements: They should be chemically inert, insoluble in oral fluids, and should not absorb water or saliva. Further, if a material is to be used as a monomer for making polymer or a composite, it should be polymerized completely without leaching any residual monomer.

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(iii) Biological requirements: The materials should not be toxic or irritating to the patient, and should not sustain the growth of bacteria and fungi. Further, in the unmixed or uncured form, they should not be harmful to the technician.

(iv) Mechanical requirements: The materials should have a high modulus of elasticity, elastic limit, flexural strength, adequate fatigue life, and impact strength. They should also be abrasion resistant.

(v) Thermal requirements: Teeth face greater temperature changes than other parts of the human body, and can tolerate ice-cold temperatures, and the very hot temperatures of coffee and tea. Thus, dental materials should be able to withstand temperature variations without any deterioration. If they are used in making a denture base, their thermal expansion coefficient should match with that of the artificial teeth, and vice versa.

(vi) Miscellaneous: Preferably, the materials should be inexpensive, have a long shelf life, and be easily manipulated. If any damage, cracks, or fractures occur, they can be easily repaired.

Only materials with the preceding properties can be used for dental applications, along with some other specific properties, depending upon their area of application and the nature of their use.

8.2 Human teeth and the biocompatibility of hydroxyapatite

An adult human has a total of 32 teeth, including 8 incisors (4 upper, 4 lower), 4 canines (2 upper, 2 lower), 8 premolars (4 upper, 4 lower), and 12 molars (6 upper, 6 lower). However, in many cases, due to genetic syndromes, natural defects, accidental causalities, or eruption difficulties, more or fewer teeth are present. Each human tooth has a crown, and a root covered by gum tissue, and is made of three primary layers, namely, enamel (the outer layer covering the tooth crown), dentin (the tissue imme- diately underneath the enamel), and pulp (the innermost part). This inorganic component in human teeth is largely made of a calcium phosphate related to hydroxyapatite (HAP). In fact, the HAP is a mineral that belongs to the family of apatite, having the general formula M5(ZO4)3X, where M may be Ca²⁺, Cd²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Zn²⁺ or Mg²⁺, ZO4 may be PO43−, PO43− or SO42− and X may be OH⁻, F⁻ or Cl⁻ and so forth [1]. The chemical formula of HAP is Ca5(PO4)3(OH), but is usually written in the unit cell formula of Ca10(PO4)6(OH)2. It is a mineral, and occurs in nature, yet can be prepared as a synthetic biomaterial with comparable characteristics (chemical composition and other properties) to those of natural teeth. There has been increasing interest in using HAP as a component of making composites for different dental applications.

This is primarily due to three reasons: (i) it is a natural component of enamel and den- tine, (ii) it has many properties comparable with those of natural teeth, and (iii) it can be easily synthesized as a component for dental composites as required. The chemical composition and other properties of HAP are compared with those of adult human calcified tissues in Table 8.1 [2–4].

Table 8.1 indicates that the chemical composition and relevant properties of HAP are comparable with the natural component of teeth and bone. HAP has been uti- lized for an assortment of biomedical applications, such as bone tissue designing, and

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controlled drug release systems [5, 6]. Because of the similarity in chemical composition and characteristic properties of the inorganic constituents of the bone matrix, synthetic HAP shows strong affinity to host hard tissues. HAP has the potential for the formation of chemical bonding with the host tissue, making it advantageous in clinical uses, compared with other bone substitutes [7]. The chief advantages of synthetic HAP are its biocompatibility, slow biodegradability in situ, high moisture resistivity, and good osteoconductive and osteoinductive capabilities [8–10]. A further investigation reflects that sintered HAP has outstanding biocompatibility with soft tissues such as gums, muscle, and skin [9]. These properties have made HAP a potential material for use in dental and orthopedic implants. The additional advantage of using a HAP coating is that it can perform as a reservoir of calcium and phosphate ions, which in turn can stimulate the growth of bone tissue on and toward the implant, ensuring its strong interfacial bond with living tissues [11]. For these reasons, HAP has gained popularity for its use as a coating material on dental and orthopedic implants. Synthetic HAP is also commonly used for bone repair, bone augmentation, coating of implants, and as a component of fillers in bones and teeth [9, 12]. Furthermore, HAP has also proved its active role in tooth polishing and whitening agents in toothpaste [13–15]. Although it has several interesting applications, only HAP coatings on dental implants will be discussed in the following sections.

8.3 Use of hydroxyapatite as a coating in dental implants

The dental implant is a preferred treatment option in the case of missing teeth as a means to protect surrounding tissues and provide proper retention. The implants are commonly used in the form of a staple, screw, plate, and root-formed type. These implants are generally prepared from non-corrosive metals or alloys such as titanium, stainless steel, or cobalt-chrome alloys. However, titanium and its alloys are the most

Component/properties Enamel Dentin Bone HAP

Calcium (Ca), wt% 36.5 35.1 24.8 39.6

Phosphorus (P), wt% 17.7 16.9 15.2 18.5

Ca/P (molar ratio) 1.63 1.61 1.71 1.67

Total inorganic, wt% 97 70 65 100

Total organic, wt% 1.5 20 25 0

Water, wt% 1.5 10 10 0

Elastic modulus (GPa) 80 15 0.34–13.8 10

Tensile strength (MPa) 10 100 150 100

Lattice parameter: a-axis (nm), (±0.0003) 0.6880 0.6887 0.6890 0.6891 Lattice parameter: c-axis (nm), (±0.0003) 0.9441 0.9421 0.9410 0.9430

Crystallinity index (HAP = 100) 70–75 33–37 33–37 100

Table 8.1

Chemical composition and characteristic properties

of enamel, dentin, bone, and HAP [2–4]

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widely used due to their outstanding mechanical properties and adaptability within the physiological system. But these metallic materials have the drawback of failure to ad- just to the local tissue environment. This issue has been solved by coating the metallic substrates with biocompatible materials such as HAP. HAP is a ceramic biomaterial that has been found to exhibit good bioactivity, biocompatibility, osteoconductivity, nontoxicity, and a non-inflammatory nature under both in vitro and in vivo conditions. But due to its highly brittle nature, HAP cannot be used alone as an implant in load-bearing applications. Thus, load-bearing implants can be coated with HAP. The application of HAP coatings is an interesting surface amendment on dental implants.

Because of high biocompatibility with bone, connective tissues, and epithelium, the HAP-coated implants have attracted attention for dental implant applications [16–24].

It has been commonly observed that a thin layer of hydroxyapatite used for coating the surface of dental implants can be beneficial to the osseointegration process, bone- implant contact, healing time, load stress distribution to the surrounding bone, mainte- nance of the bone crest, and so forth. Further, the HAP coating on the metallic surface offers advantages in relation to mechanical properties; in particular, the load-bearing capacity is unaffected, and the surface becomes biocompatible. In the study of interface mechanical characteristics and the histology of CP titanium and HAP coated titanium, it has been observed that the HAP coated system developed five to eight times the mean interface strength of the uncoated, bead-blasted CP titanium system [25].

8.4 HAP coating methods

It has been observed that all hydroxyapatite coatings are not identical, and their nature and quality depends on the deposition methods. It has been established that the characteristics of HAP coating, as obtained by different techniques, are not the same. Even with the same coating method, different properties can be achieved depending on the process parameters. For example, the biochemical composition of plasma-sprayed, HAP-coated implants changes with the change in the coating parameters [26]. Thus, the choice of coating depends on its use. A number of methods of depositing HAP are available. Some commonly used depositing techniques are discussed in the following sections.

8.4.1 Plasma-spraying technique

The plasma spraying method is a widely used technique for the coating of HAP on metallic substrates. It is the only method that has been accepted by the Food and Drug Administration (USA) for biomedical coatings due to its outstanding functioning as related to other deposition techniques [27]. Investigation on plasma-sprayed HAP deposition on metallic surfaces for their applications in dentistry and orthopedics started in the mid-1980s [28]. This method involves the melting of coating powders by employing the heat of ionized inert gas (plasma). This is followed by spraying the molten powders onto the surface. This leads to the formation of a protective coating that provides an obstacle against corrosion, wear, and temperature variation. Plasma spraying of HAP is generally performed under normal atmospheric conditions, contrary to the

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plasma spraying of some metallic powders, during which a vacuum or an inert environment is maintained to diminish oxidation [3]. This coating method has many benefits, such as low cost, fast deposition, and speedy bone healing. Additionally, the threat of thermal deterioration of the coating obtained by this method is much lower than that generated by high-temperature methods [27]. However, a plasma- sprayed HAP coating is associated with the disadvantages of porosity, variable bond strength, and poor adhesion between the coating and the metallic substrate. The bonding of the plasma-sprayed HAP coatings is mechanical in nature, which is supported by the fact that a highly roughed substrate surface has a better bond strength than that of a smooth surface [29]. Studies on bonding strength and process parameters for HAP coatings obtained by the plasma-spraying technique have been successfully performed [30]. Balani et al. [31] have conducted studies on the interaction of plasma- sprayed carbon nanotube- reinforced hydroxyapatite coatings with human osteoblasts in vitro.

They pointed out that the plasma-sprayed HAP/carbon nanotube coating has a uniform distribution of undamaged carbon nanotubes, improvement in fracture toughness by 56%, and an increase in crystallinity from 53.7% to 80.4%. By their findings, they suggested that plasma spraying is a definitive tool for synthesizing HAP/carbon nanotube coatings onto Ti-6Al-4V implants, which brought the benefits of nanotech- nology out of the laboratory for application in real improvements in human health.

The in-vitro study of the biological response of HAP/Ti-6Al-4V composite coatings under simulated body fluids (SBF) was studied, and coatings were found to undergo two biointegration processes—dissolution during the initial 4 weeks soaking in SBF, and the subsequent bone-like apatite crystal precipitation [32]. In a notable work, titanium implants were coated with HAP, employing the improved biomimetic method and plasma spray technique [33]. These implants were characterized by Fourier- transformed infrared spectroscopy, scanning electron microscopy, and X-ray photo- electron spectroscopy, and it was observed that the plasma spray process generates a typical rough topography, mainly consisting of HAP.

The spraying process is influenced by a number of parameters, and the effects of five plasma spray process parameters, including current, carrier gas flow rate, powder feed rate, gas flow rate, spray distance and on the roughness, crystallinity, and purity of HAP coatings was studied by Levingstone et al. [34] using a fractional factorial design. They reported that higher coating roughness was obtained when the current was high, the gas flow rate was low, and the powder feed rate was high. Further, the highest coating crystallinity was observed at high current, low spray distance, and low carrier gas flow rate.

8.4.2 Hot isostatic pressing technique

Hot isostatic pressing (HIP) is a metallurgical process employed to decrease the porosity and increase the density of ceramic materials. The method is used to improve mechanical properties and workability of the materials. As pointed out in the previous section, HAP coatings produced by plasma spraying are porous, and therefore likely to have a greater tendency toward dissolution than the dense HAP coatings. HIP offers a method to produce dense HAP coating on a Ti substrate. In this technique, the pressure

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is exerted using gas at a high temperature. A dense coating with HAP structure with little porosity was obtained using hot isostatic pressing for 35 min at 700–850°C, and a maximum pressure of 1000 bar [35]. This was achieved by air spraying the HAP powder mixed with titanium substrate. The coatings obtained were denser than those produced by plasma spraying with a bonding strength of >62 MPa. In this process, the sintering temperature and pressure can be controlled to produce interconnected porosity, good mechanical properties, and high permeability. It is worth mentioning that studies have also been conducted by combining hot isostatic pressing and plasma spraying to reduce the number of micropores, and improve the mechanical properties [36, 37]. The influence of HIP on phase transformation, compressive strength, Young’s modulus, and density of dental ceramics has been assessed by employing different experimental conditions [38].

8.4.3 Pulsed laser deposition coating technique

In this technique, a high-power pulsed laser beam is focused inside a vacuum cham- ber to strike a target of the material that is to be deposited. This material is vaporized from the target (in a plasma plume), which deposits it as a coating on a substrate. It is a simple, but versatile, coating method, and has been used successfully for making good quality coatings of HAP on different substrates. This method has the advantages of forming a coating with a uniform thickness on flat substrates, the ability to control deposition parameters, and the ability to produce high crystalline coatings.

Khandelwal et al. [39] have successfully deposited HAP coatings on stainless steel 316 L substrate by pulse laser deposition at different energy levels of 300 and 500 mJ.

They reported that variation in the laser energy affects the surface characteristics such particle size, surface roughness, uniformity, and Ca/P ratio of the HAP coating. An amorphous thin film on Ti-6Al-4V was obtained by laser ablation of HAP targets with a KrF excimer laser at the ambient temperature under different working pressures that range from 10⁻⁴ to 10⁻¹ Torr of oxygen [40]. By combining the pulsed laser deposition and post-deposition annealing at 300°C, this amorphous coating of HAP was modified to be pure, adherent, and crystalline, with an additional advantage of not being dissolved in simulated body fluids. By a Rutherford backscattering spectroscopic analysis of HAP film, it has been suggested that PID is a suitable technique to obtain crystalline and adherent hydroxyapatite films on Ti or Ti-6Al-4V substrates [41]. The quality of the HAP deposit indicated that the PLD process could be an interesting option for coating dental implants. HAP thin film on a titanium substrate was also deposited by PLD, with the controlled chemical composition of Ca/P ration of 1.72 ± 0.13, which is close to the stoichiometric ratio of 1.67 [42]. Carbonated hydroxyapatite films were prepared on titanium substrates by pulsed laser deposition at different substrate temperatures ranging from 30 to 750°C [43]. It was reported that the carbonated HAP films were nearly stoichiometric with a Ca/P atomic ratio of 2.0–2.2, and the thickness of the film was strongly dependent on temperature. They [43] reported that the intrinsic hardness of the films was also highly reliant on temperature variation, and increases with a rise in substrate temperature, being as low as 5 GPa at 30°C and as high as 28 GPa at 700°C.

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8.4.4 Electrophoretic deposition coating technique

This is a coating process based on the electrode deposition in which colloidal particles suspended in a liquid migrate under the influence of an electric field (electropho- resis), and are deposited onto an electrode. The method can be applied for ceramic coatings, and has the advantages of cost-effectiveness (as it requires simple and cheap equipment), and flexibility, as it can be used to coat objects with a complex shape and morphology. Electrophoretic deposition coating is particularly helpful in the deposition of HAP coatings because this technique permits the control of the composition, thickness, and microstructure of coatings. The morphology of the coating deposit can be controlled by changing the deposition conditions and size and shape of the ceramic powder, while the thickness can be increased by increasing the deposition time and voltage [44]. An EPD technique has been used to deposit HAP coatings onto different substrates of metal biomaterials, such as Ti, Ti-6Al-4V, and 316L stainless steel [45]. As the HAP coating on this substrate resulted in significant cracking during densification, the dual coating (coat, sinter, coat, sinter) applied as the second coating filled in the valleys in the cracks of the first coating. Ma et al. [46] have used a co-precipitation method to prepare nanosized HAP powder using Ca(NO3)2.4H2O and H3PO4 as starting materials, and the HAP powder obtained was electrophoretically deposited onto a titanium tubular substrate. The coating was reported to be uniform, having no cracks, and with good interfacial bonding and adhesion to the substrate. By employing this technique, uniform HAP-Cu nanocomposite coatings were deposited on Ti6Al4V, and no change in the phase behavior was reported until a heat treatment temperature reached 900°C [47]. It has been suggested that a nanocomposite coating of HAP-3 wt% Cu has higher cytocompatibility and efficient antibacterial activity compared with other specimens, and therefore might be a better candidate for bone tissue engineering applications. The coatings deposited by EPD methods have the advantages of rapid deposition, suitability for the complex shaped substrate, uniformity of thickness, low cost, and control of coating morphology and coating thickness.

However, the requirement of a high sintering temperature can become problematic.

8.4.5 Thermal spraying technique

This is a coating method in which melted or heated materials are sprayed onto a surface. This technique is a collection of different coating methods, which provides functional surfaces to defend or increase the function, and it can be categorized into three major classes, namely, electrical arc spray, plasma arc spray, and flame spray [27]. These energy sources are employed to heat the solid form to a molten and semi- molten form of the coating substance. The resulting heated samples are actuated to the surface. The coatings based on nanoscale HAP suspension and microscale HAP powder were successfully prepared on Ti plates by the thermal spraying technique by using high-velocity suspension flame spraying, atmospheric plasma spraying, and high- velocity oxy-fuel spraying methods [48]. The performance of coatings deposited in this way was evaluated in relation to different aspects, such as surface roughness, microhardness, the bond strength of the layer composites, phase content, and

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crystallinity. However, films deposited by thermal spraying have the disadvantage of low coating adherence, non-uniform crystallinity, and the possibility of crack development on the surface due to high sintering temperatures [27]. The High-Velocity Oxy-Fuel (HVOF) spray process is a relatively new method of thermal spraying, and has been successfully used for deposition of HAP and other bioceramics on metallic surfaces. Haman et al. [49] have used this method to develop the coatings of HAP and fluorapatite powders onto titanium, and characterized the composition, microstructure, morphology, and apatitic structure using different analytical techniques, such as diffuse reflectance Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. They observed some loss in crystallinity of the coatings due to the spraying process. The effect of nano-hydroxyapatite powder (obtained from a novel peroxide- based route using calcium nitrate, phosphoric acid, and hydrogen peroxide as peroxo-precursor) on thermally sprayed HAP coatings onto stainless steel substrates involving the HVOF method was studied [50]. They reported that the hardness and bioactivity of the coatings was achieved after soaking in a simulated body fluid for 14 days. The HAP coatings were a mixture of amorphous and crystalline phases with a hardness value of 2.15 ± 0.08 GPa.

8.4.6 Dip coating technique

In the dip coating method, the substrate is dipped and withdrawn from the solution.

This coating consists of three major steps, namely, dipping, withdrawing and drying, and curing or sintering [27, 51]. After withdrawing the substrate from the medium, a consistent liquid film is carried on the substrate, and after evaporation, a thin coat of deposits is made. In most cases, after drying, an extra post-treatment stage, such as sintering or curing, is desired to acquire the final coating [27]. This method has many advantages, such as being inexpensive, fast, simple, and it offers uniformity of coating and the ability to coat irregular shapes and patterns [51]. This technique has successfully been used to coat HAP on Ti-6Al-4V substrates [52]. The sols, prepared by dispersing HAP crystals of <100 nm length in distilled water or physiological salt solution using an ultrasonic homogenizer, were used in HAP coatings on titanium rods [53]. However, this method has a drawback; the requirement of high sintering temperatures, and thermal expansion mismatch.

8.4.7 Sol-gel technique

This method is widely used in the synthesis of many inorganic materials because of the easy development of crystalline homogeneous films at relatively low temperatures. Further, this method can also be conveniently used for making complex shape coatings. In this method, generally, two solvents are mixed with a calcium phosphate (CaP) precursor. The most commonly used solvents for the sol preparation are water and ethanol. The sol-gel method is a low cost and simple method, which permits the molecular level mixing of the HAP precursors. Kaygili et al. [54] have synthesized nano-crystalline aluminum-containing hydroxyapatite ceramics by using the sol-gel method. They synthesized various undoped and Al-doped (10, 20, 25, 30, 40% Al)

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HAP samples. They prepared the gel by dissolving and stirring Ca(NO3)2.4H2O, and P2O5 precursors having Ca/P ratio of 1.67 in absolute ethanol at the ambient temperature for 30 min followed by vigorous stirring in a vortex for about 10 min. After the gelating step, they heated the sample in a water bath for 2 h at 60°C, dried at 120°C for 15 h, and then sintered at 900°C for 1 h. Ultimately, a white powder sample was obtained. Similarly, they prepared Al-doped (10, 20, 25, 30, and 40% Al) HAP by adding the solution of Al(NO3)2.9H2O in the ethanol to the mixture of the Ca(NO3)2.4H2O and P2O5 by maintaining 1.67 M ratio of (Ca + Al)/P for all the Al-containing samples. The sample was characterized by different techniques such as X-ray diffraction, Fourier transforms infrared spectroscopy, and scanning electron microscopy. It was reported that with the rise of Al percentage, the crystallinity was drastically reduced, and the phase belonging to HAP was changed into the new phases, including aluminum calcium phosphate and/or aluminum phosphate. Combined sol-gel and dip coating methods were used to make HAP coatings on titanium substrates [55]. The better quality coating was achieved by modifying the titanium substrate by adding a calcium titanate (CaTiO3) sublayer, or through additional preheating at 650°C. Azem and Cakir [56] used calcium nitrate and triethyl phosphite as Ca and P precursors (in the stoichiometric proportion of Ca/P = 1.67) with ethanol and water as solvents. The sol was prepared by the dropwise addition of a 4 M ethanol solution of calcium nitrate to hydrolyzed triethyl phosphite (obtained by treating it with distilled water for 24 h), followed by continuous agitation for 30 min. The sol was deposited on unmodified and modified 316L stainless steel by a dipping process, which was performed by dipping the substrate into the solution and pulling it up at a constant rate of 5 cm/s. Finally, the deposited film on the surface was dried at 80°C for 1 h, and then heated at 500°C for 1 h. It has been reported that the coating on the modified surface prepared in the 24 h aged sol solution had an enhancement in the adhesion strength from 29 to 38 mN [57]. Ramires et al. [58] carried out the vitro and in vivo biocompatibility test for dental implants coated with titania/HAP and titania/bioactive glass (BG) composites prepared by a sol-gel method. They reported that HAP and BG sol-gel coated dental implants showed better performance than the uncoated titanium. They reported that in vitro, the HAP coating stimulates osteoblastic cells in producing a higher level of alkaline phosphate and collagen; whereas, in vivo, this surface modification resulted in a higher removal torque and a larger bone-implant contact area.

8.4.8 Sputter coating technique

This method is a vapor deposition technique for making a thin coating by sputtering, and it consists of ejecting a substance from the target to a substrate using energetic particle bombardment. In this method, a gas plasma such as neon, argon, krypton, or xenon is used to eliminate substances from a negatively charged target, which are then deposited as a thin film on the substrate [27]. The HAP powder was successfully deposited on titanium substrate by radio frequency (RF) magnetron sputtering, and the coating was observed to be homogeneous, having low crystallinity and a high (3.0) Ca/P ratio [59]. The radio frequency (RF) magnetron sputtering method was used for making the effective coatings of HAP on different substrates [60–64]. The technique

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has been reported to obtain coatings of thickness ranging from 0.5 to 3 μm, and has the advantages of the formation of a dense and uniform coating of high adhesion strength; but it suffers from the disadvantages of being costly, time-consuming, and the production of a coating of low crystallinity. Further, a higher Ca/P ratio than that of a synthetic HAP coating can be achieved if RF magnetron sputtering is used [28].

On the basis of the preceding discussion, it is clear that each technique has different merits and demerits, and therefore its application in depositing HAP coating in dental implants is based on different parameters, including the nature of use. The relative advantages and disadvantages of different coating techniques have been compared in Table 8.2 [11, 27, 28, 51].

Method

Coating

thickness Advantages Disadvantages Plasma

spraying <20 μm Low cost, high deposition rate, less possibility of coating degradation, fast bone healing

Change of HAP structure during coating, phase change and grain growth of the material because of high temperature, nonuniformity in coating density, relatively poor adhesion, unable to form complete crystalline coating Hot isostatic

pressing

0.2–2.0 mm Dense coating, no shape or dimensional limitation, good temperature control

Costly, unable to coat complex shaped substrate, requirement of high temperature, incompatibility of thermal expansion coefficient, reaction of encapsulation material with HAP coating

Pulsed laser deposition

0.05–5 μm Dense coating, suitable for porous, amorphous and crystalline coatings, control of deposition factors

Low deposition rate, expensive, lack of uniformity, requirement of pretreatment of surface, line of sight technique

Electrophoretic deposition

0.1–2 mm Rapid deposition, suitable for complex shaped substrate, uniform coating thickness, simple setup, low cost, control of coating morphology/

thickness,

Requirement of high sintering temperature, appearance of crack in coating, decomposition of HAP during sintering stage

Table 8.2

Comparison of different techniques for depositing

HAP coatings [11, 27, 28, 51]

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8.5 Conclusion

The coating of HAP on different metallic biomaterials was discussed in detail. Eight common methods of coatings, including plasma spraying, hot isostatic pressing, pulsed laser deposition, electrophoretic deposition, thermal spraying, dip coating, sol- gel coating, and sputter coating were thoroughly evaluated. The different deposition factors influencing the adhesive power of the coating were discussed. The advantages and disadvantages of each method were compiled, and it was concluded that each technique has some advantages and disadvantages over the other. Thus, the choice of a suitable method is a challenging task, because performance and the quality of the HAP coating depend on many factors, such as the nature of the substrate, requirements of the coating quality, the nature/quantity of dopant, and so forth. Thus, all these parameters, along with the nature of the application, and financial aspects, must also be considered while selecting a method for HAP coating on a substrate.

Table 8.2

Comparison of different techniques for depositing HAP coatings [11, 27, 28, 51]—cont'd

Method

Coating

thickness Advantages Disadvantages Thermal

spraying

30–200 μm Rapid deposition, low cost, biocorrosion resistance, suitable for complex shaped substrate

Line of sight technique, fast cooling creates amorphous coatings, appearance of cracks in coatings, lack of uniformity, low porosity

Dip coating <1 μm Low cost, rapid deposition, suitable for complex substrate, uniform coating

Requirement of high sintering temperature, mismatching of thermal expansion

Sol-gel coating 50–400 nm Uniformity in coating, suitable for complex shaped substrate, low processing temperature, high corrosion resistance, very thin and high purity coatings

Appearance of edge cracking, requirement of posttreatment (curing), difficult to control porosity, high permeability, expensive raw materials

Sputter coating 0.5–3 μm Dense and uniform coating on flat surface, high adhesion

Line of sight technique, time- consuming, costly, low deposition rate, produces amorphous coating

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Acknowledgment

The author is grateful to the chairman, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India, for providing the required facilities.

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