Department of Materials Science & Engineering

A review on Dental Ceramic:

An analytical discussion about

Hydroxyapatite, Chemistry and Processing

By Younes Sina

MSE560: Principles of Ceramic Processing

Presented by Dr. Narendra B. Dahotre

1

Introduction

Hydroxyapatite [Ca10 (PO4)6(OH) 2; abbreviated as HAp] is an inorganic compound

whose chemical composition is similar to the composition of the bone. It is a very

attractive material for biomedical applications such as a bone substitute material in

orthopedics and dentistry due to its excellent biocompatibility, bioactivity and

osteoconduction properties. HAp has been used extensively in medicine and dentistry

for implant fabrication owing to its biocompatibility with human bone and teeth .However

due to its poor mechanical properties, HAp ceramics cannot be used for heavy load

bearing applications, but common uses include bone graft substitution and coatings on

metallic implants.

Biomaterials are a class of engineering materials which can be used in animal body

tissue replacements, reconstructions, and regenerations, without any long term adverse

effect. The development of biomaterials and manufacturing techniques broadened the

diversity of applications of various biocompatible materials. Among the different classes

of biomaterials, bioceramic is one of the promising classes of available biomaterials

used as human body-implants. Few of the bioceramics have similarity with the mineral

part of our bone; however do not match with the intricate structure of the bone. There

are several calcium phosphate ceramics that are considered biocompatible. Of these,

most are resorbable and will dissolve when exposed to physiological environments.

Hydroxyapatite is the most important bioceramic materials for its unique bioactivity and

stability. Unlike the other calcium phosphates, hydroxyapatite does not break down

under physiological conditions.In fact; it is thermodynamically stable at physiological pH

and actively takes part in bonebonding, forming strong chemical bonds with surrounding

2

bone. This property has been exploited for rapid bone repair after major trauma or

surgery. While its mechanical properties have been found to be unsuitable for load-

bearing applications such as orthopedics, it is used as a coating on load bearing implant

materials such as titanium and titanium alloys or composites with other materials.[15]

There are several methods to produce hydroxyapatite powder. The most popular and

widely researched route is solution precipitation. HAp nanoparticles can be prepared

using microwave irradiation. Solgel and hydrothermal routes are the two other important

routes for HAp synthesis. Even HAp can be produced by mechanosynthesis route, in

which case no heat treatment is required to produce crystalline nano HAp. Some other

routes for synthesis of HAp are:

Solid state reaction, plasma technique, hydrothermal hot pressing, ultrasonic spray

pyrolysis, and emulsion system.

Porous HAp is envisaged to have better biocompatibility, as tissues can grow much

faster into the available pores. The pore size can be controlled and also complex

shaped materials can be fabricated. Several efforts (specially processing routes) have

been made to improve the mechanical properties of HAp. Thermal treatment is

necessary to improve the mechanical properties. Even sometimes some amount of

additives can be added to improve the sinterability and mechanical properties without

affecting the bioactivity. Using Ca (OH) 2 additives the sintering temperature can be

increased without any dissociation. [15]

3

Routes for synthesis HAp and modifying of its properties

In this section of review it has been tried to introduce the most important routes for

synthesis of HAp and methods for modifying of HAp’s properties. The objective is

discussion about each method and comparison of their different parameters. Before that

it is necessary to discuss about chemistry of HAp.

Why chemistry of HAp is important?

The composition, physicochemical properties, crystal size and morphology of synthetic

apatites are extremely sensitive to preparative conditions. Furthermore the success and

quality of orthopaedic coatings is also largely dependent upon the HA powder

characteristics. For example spherical powders of narrow size distribution are favoured

in order to enhance excellent heat transfer characteristics to increase deposition

efficiency and decrease coating porosity. As we know synthetic HAp occurs in two

structural forms, hexagonal and monoclinic, which have minor structural differences.

The hexagonal HA form is usually formed by precipitation from supersaturated solutions

at 25 °C to 100 °C and the monoclinic form of HA is primarily formed by heating the

hexagonal form at 850 °C in air and then cooling to room temperature [22]. Although

hydroxyapatite is considered as one of the potential materials for the replacement and

reconstruction of human bone and teeth, and the biocompatibility of this material is well

established, the main problem with this material is its reliability due its very poor

mechanical properties. [15]

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Hydroxyapatite is a thermally unstable compound, decomposing at temperatures from

about 800-1200°C depending on its stoichiometry. The stoichiometry of hydroxyapatite

is highly significant if thermal processing of the material is required.Calcium phosphate

phases of alpha and beta-tricalcium phosphate, tetracalcium phosphate occur with

slight imbalances in the stoichiometric ratio of calcium and phosphorus in HA from the

molar ratio of 1.67. It is also important to know the close relation between the

stoichiometry, acidity and solubility. Thus, it is known that the lower the Ca/P ratio and

the larger the acidity of the environment, the higher will be the solubility of the HA. For

Ca:P < 1, both acidity and solubility are extremely high, and both parameters decrease

substantially for Ca/P ratios close to 1.67, which is the value of stoichiometric

hydroxyapatite. The prevention of the formation of calcium phosphate phases with

relatively higher solubility is significant when stability of hydroxyapatite is an important

issue in the application. It is possible to sinter phase pure hydroxyapatite using

stoichiometric composition at temperatures up to 1300 c. [4]

One of the most important properties of HAp is prosity. The simplest way to generate

porous scaffolds from ceramics such as HA is to sinter particles, preferably spheres of

equal size. With the increase in temperature pore diameter decreases and mechanical

properties increase as the packing of the spheres increases. Hot isostatic pressing can

also be used to further decrease the pore diameter.

During sintering porosity can be increased by adding fillers such as sucrose, gelatin,

and PMMA microbeads to the powder and the wetting solution. One of the most reliable

formulations is the use of an HA powder slurry with gelatin solution. Surface tension

forces cause the formation of soft and spherical porous particles of HA and gelatin. It is

5

possible to produce porous bulk material with an interconnected pore structure with an

average pore size of 100 microns after sintering. One other method for producing

porous ceramics is freeze drying. Freeze drying process can be used to introduce

aligned pores in the final ceramic structure but the generated pore diameters do not

exceed 10 microns.

The most important ways for synthesis of HAp is explained at follow:

Wet chemical process (precipitation route)

This route includes two major reactions: acid-base method and chemical precipitation.

Acid-base method is known as one of the most favorable method in industry because

the only it’s by- product is water. Temperature and PH in this route are very important

factors for having a stable HAp. Increasing of temperature and maintaining PH at 6

affect on Ca/P ratio. Acid-base reaction can be written as:

10 Ca (OH) 2+6 H3 PO 4→Ca10 (PO 4)6 (OH) 2+18 H 2O

The common precipitation reaction can be written as:

10Ca (NO3)2 + 6(NH 3) HPO +2 H2 O → Ca10 (PO4)6 (OH) 2 + 12NH4 NO3 +8 HNO3

In this reaction PH>10 is a necessary condition for a stable and stochiometric form of

HAp.

Precipitation reaction for synthesis nanostructure HAp can be written as:

6

10CaCl2 + 6Na2PO4 + NaOH → Caq+(x/2) (PO4)6(OH)2 + (18+x) NaCl + (1-(x/2)) CaCl2

(x=0,1,2)

Flowchart for the synthesis of the hydroxyapatite powder

Reaction is completed during 24 hours at air. After a complete washing it is dried at 80

C for 24hours and then it is heated for 10 hours with rate of 5 min/C until 1000 C. PH

should be maintained over 7. In these conditions, HAp powder size is less than 10 nm

and with increasing of NaOH, the powder will be stable until 1000C at air.

Wet chemical process, which is based on precipitation route, is the most convenient and

commonly used process. This process is very simple and easy to use. The preparative

reaction and the character of reaction products can be regulated easily. [12]

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Hydrothermal route

Hydrothermal process, which works at high temperature and high pressure, is also one

of the widely used and earliest developed methods for the synthesis of hydroxyapatite.

The process is not only an environmentally benign but also chemical composition and

stoichiometry of the material can be controlled [12]. Hydrothermal synthesis has been

used to transform slurries, solutions, or gels into the desired crystalline phase under

mild reaction conditions typically below 350 oC. Typical powders synthesized by this

method have been shown to consist of needle-like particles between 20 - 40 nm in

diameter and 100-160 nm in length. The motivation for synthesizing HAp by

hydrothermal means is to obtain nanosized particles for infiltration of dentinal tubules for

the alleviation of hypersensitivity, a common problem for millions of children and adults

worldwide.

Single phase hydroxyapatite crystallites with a rod-like morphology were synthesized by

a hydrothermal method at 200 oC under saturated water vapor pressure for 24 hrs from

a precipitate formed by mixing Ca(NO3)2&4H2O, (NH4)2HPO4 and distilled water.

Longer treatment times led to the production of a secondary phase, monetite,

(CaHPO4). However the treatment time had no effect on the particle morphology or size

within the reaction time range of 24-72 hrs. The crystallites were measured to be within

the size range 100-600 nm in length and 10-60 nm in diameter. Early results indicate

smaller, more or spherical particles may be desirable for dentine tubule infiltration. [7]

Hydrothermal techniques give hydroxyapatite powders with a high degree of crystallinity

and better stoichiometry having a wide distribution of crystal sizes. [4]

8

Microwave irradiation route

Use of microwaves as an alternative energy source, due to its environment-friendly,

non-polluting, clean and safe approach, is also one of the most promising and excellent

approaches. The great potential offered by microwave irradiation process, is the

acceleration of chemical reaction. [12]

In this technique usually spray dried hydroxyapatite powder, synthesized through

Solution-precipitation route,is used . The aim is to sinter hydroxyapatite at much lower

temperature using microwave and also synthesizeing the material in such a heating

schedule which can have better properties than conventional dense HAp. The most

challenge part is to sinter HAp powder in a single stage (two segments) heating

schedule without resort to calcinations procedure at higher (800C) temperature.

Based on the sintering studies on solution precipitation spray dried HAp powders, the

following conclusions can be drawn:

a) In one segment sintering, a maximum density of ~97% can be obtained after sintering

at 1100C for 3hrs in conventional sintering and density of ~99% obtained after

microwave sintering at 1000C and 1100C for 0.5 hrs. But in this case samples can be

cracked.

b) However, lesser densification (~95%) is obtained in two segments sintering

processes accomplished by intermediate isothermal holding at 800C followed by

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sintering at 1200C. The samples cannot be cracked after sintered in conventional as

well as microwave sintering. This 5% porosity may leads to more bioactivity.

c) For conventional sintering, holding for 2hrs at 800C and subsequent sintering for 3hrs

at 1200C result almost fully dense microstructure is characterized by equiaxed grains of

1-2μm size.

d) The microwave sintering, can be performed by holding at 800C for 0.5 hrs and

sintering at 1200C for 0.5 hrs also produce dense HAp of faceted grains of 1-2μm size.

The microwave sintering is found to be a time and energy efficient densification

technique in dendifying HAp. [15]

Ultrasonic irradiation route

Ultrasonic irradiation is a novel precipitation method for nanocrystalline HAp

preparation. The chemical effects of ultrasound derive primarily from acoustic

cavitations (the formation, growth and collapse of bubbles). Synthesis of HAp

nanoparticles in ultrasonic precipitation and influence of temperature, [Ca2+], Ca/P ratio

and ultrasonic power on its morphology and crystalline has been recently reported.

In this method there is ability for HAp synthesizing using homogeneous precipitation

method in the field of ultrasonic irradiation. Urea can act as an agent for precipitation.

Basic parameters in this technique are: temperature, concentration, power of ultrasound

field, time and dynamics of ultrasound field effect. [11]

Application of ultrasound for the preparation of nano-sized PLGA/HAp (poly d,l lactide –

co-glycolide) composite particles of spherical morphology has been reported.

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Microscopy analysis results reveal that using the ratio 90 wt% of PLGA in relation to

10% HAp in the steps of synthesis in the field ultrasound highly uniform and spherical

particles with diameter of 250-300 nm can be obtained. The presence of both PLGA and

HAp in these particles can be confirmed by IR spectroscopy. [10]

Sol-Gel technique

Sol Gel technique has been developed and employed to prepare various materials

because it has main advantage of easy control of chemical composition and low

temperature synthesis that are very important for thin film formation.[21]

The use of sol gel routes to form a bioactive hydroxyapatite layer on metal substrates

has recently attracted in the biomedical field. The sol gel method represents the low

temperature way of the production of glasses, ceramic and composite materials with

better purity and homogeneity than high temperature conventional processes. This

process has been used to produce a wide range of compositions (mostly oxides) in

various forms, including powders, org/inorg hybrids, fibers, coating, thin films, monoliths

and porous membranes. One of the most attractive features of the sol gel process is

that it can produce compositions that cannot be created by the conventional methods.

The mixing level of the solution is retained in the final product. In sol gel chemistry, the

metal alkoxides convert to amorphous gels of metal oxides through hydrolysis and

condensation reactions.

Hydroxyapatite can be synthesized using the sol-gel route with proper heat and acid

treatment. There will be no significant differences observed for the powder with and

without alcohol medium excluding the pH and gelation time. Nowadays the sol-gel

route is becoming a unique low-temperature technique to produce ultra fine and pure

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ceramic powders. Recently, hydroxyapatite powders and coatings have been

successfully synthesized by the sol gel method. The process parameters have been

optimized to produce high purity hydroxyapatite.

Fluorinated hydroxyapatite

Fluorinated hydroxyapatite(FHAp) [Ca10(PO4)6(OH)2-2xF2x (0 ≤ x ≤ 1)], where F

partially replaces OH in hydroxyapatite, is potentially a very interesting biomaterial. It

was suggested that fluoride-substituted hydroxyapatite has a better thermal and

chemical stabilities than hydroxyapatite. [6]

FHA exhibits a very attractive combination of stability and biocompatibility. However, it

has been reported that if all of the OH groups in HA are replaced by F to form

fluorapatite (FA), the resulting material is not osteo-conductive. Moreover, the high F

content might lead to severe adverse effects such as osteomalacia . As a result, various

methods have been developed in an attempt to tailor the fluorine content of FHA to

achieve the best biological properties. FHA can be either prepared using a solid-state

reaction or a wet-chemical process, but the later is used more commonly. There are

several methods of synthesizing fluoridated hydroxyapatite with varied fluorine contents,

such as, by sol-gel, a solid state reaction, and pyrolysis methods. The pH-cycling

method as the modified wet chemical process was first introduced to avoid a high

temperature operation and the use of volatilized alcohol (fluorine containing reagent).

Fluorhydroxyapatite is synthesized through a pH-cycling method by varying sodium

fluoride (NaF) concentration in hydroxyapatite suspension as a modified wet-chemical

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process. Synthesized fluorhydroxyapatite powder has been characterized on a

macroscopic level by XRD, FTIR and chemical analysis (AAS, EDTA titration technique

and F-selective electrode), while SEM has provided detailed information at the

microscopic (individual grain) level. The XRD analysis has showed that the

fluorhydroxyapatite sample prepared is nearly pure fluorhydroxyapatite. Only low levels

of specific impurities (such as CaO) have detected and it is also demonstrated that the

crystallites of FHA were nanosize. FTIR investigations also have showed all the typical

absorption characteristics of fluorhydroxyapatite. Chemical analyses (for example AAS

and EDTA titration and F-selective electrode analysis) are used for the determination of

Ca/P molar ratio and calculation of the replaced fluorine content in the crystalline

network of hydroxyapatite. The bulk Ca/P ratio has determined as 1.71 which showes

the measured Ca/P ratio for the synthesized powder is higher than stoichiometric ratio

(1.667) which is expected for a pure HA (or FHA) phase. Also According to the F-

selective electrode analysis result and calculations performed, the achieved formula of

the synthesized fluorhydroxyapatite is Ca10 (PO4)6(OH) 0.7F1.3. Finally, the SEM

technique ascertained that the particles of prepared powder are rod-like. [6]

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Flowchart for the synthesis of the fluorinated hydroxyapatite powder [6]

Discussion

Now after being familiar with different routes of synthesizing of HAp and its derivatives,

we can discuss about the advantages and disadvantages of each process. Some

parameters that are determiner are: Time, economic, porosity, density, environmental

consideration, particle size, morphology, homogeneity, ability of sintering, purity, Ca/P

ratio, and ease of control.

Precipitation method: HA may be synthesized by various methods, including the sol-

gel technique, solid-state reactions at elevated temperatures, chemical precipitation and

biosynthesis routes. The precipitation method appears more favorable due to its

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simplicity, cost effectiveness and its non-polluting nature (that is, its only by -product is

water). The success and quality of orthopedic coatings is largely dependent upon the

HA powder characteristics. These include phase composition, crystallinity, particle size

and powder morphology .Spherical powders of narrow size distribution are favored in

order to enhance excellent heat transfer characteristics to increase deposition efficiency

and decrease coating porosity.

Microwave and hydrothermal: Microwave and hydrothermal routes yield uniform grain

growth along with highly porous crystalline HAp material. The microwave irradiation

process requires less time for the synthesis of hydroxyapatite compared to other

processes. The grain size is found to be in the range 31–54 nm. The dielectric constant

is in the range 9–13. Hydroxyapatite seems to be a potential candidate to act as CO

sensor at an optimum temperature near 125°C. The Ca/P ratio is in the range of 1.6–

1.7, a property which is important in biomedical applications. The dielectric constant for

all the samples is found to be at 400 Hz, in the range 9–13. It is reported in the literature

that the dielectric constant of HAp at 291.5 K was 15.4 at 100 Hz. The difference in the

values can be attributed to the difference in the processing and structure of the end

product [15]. The dielectric behavior of monoclinic HAp as a function of temperature,

showing the phase change from monoclinic to hexagonal at a temperature of 483 K is

already reported. [12]

Wet chemical route: The HAp grown at room temperature, via wet chemical route,

shows non-uniform agglomerates wherein there is large variation in particle sizes. [12]

Microwave sintering: Densification studies of normal pressureless sintered and

Microwave sintered has been carried out. In all cases, Microwave sintered samples

15

showed higher density than normal pressureless sintered samples, when sintered at the

same temperature. It can be noted that the soaking time in case of conventional

sintering is six times longer than microwave sintering. It shows that using microwave the

sintering temperature and time can be reduced a greater extent. Sintering of crack free

body was a big challenge for this material. Calcined at 300 C and 350 C powder always

contained some amount of moisture. At higher temperature this moisture creates stress

and the sample get fractured, although the densities of the fractured parts were good

enough. It has reported that calcining at 800 C gives good results, but still problem

persists during pressing of calcined powders. In this case, the green pellets of spray

dried powders were soaked at 800 C for 2 hrs in case of normal pressureless sintering

and for 30 min in case of microwave sintering. At 800 C moisture goes out creating

some channels in the grain boundary regions. These channels create 5% porosity of

final products.[15]

Conventional methods (wet, dry and hydrothermal routes): the conventional

methods (wet, dry and hydrothermal routes) of preparation of this important bioceramic

material are tedious and time consuming. For example, in one process HAp was

precipitated from aqueous solutions using appropriate amounts of calcium nitrate and

di-ammonium hydrogen phosphate using NH4OH to maintain high pH value and the

mixture was kept stirred for about 2 h, later centrifuged and the product was allowed to

ripen. In another method solid state mixture of tri- and tetracalcium phosphates had to

be heated for several hours at 1283 K in a current of moist air to produce HAp. In yet

another process described a hydrothermal route for the synthesis of HAp in which

dicalcium phosphate was heated with water at 573 K for 10 days in a platinum lined

16

hydrothermal bomb. There is thus great need to develop an efficient route to synthesize

HAp.

Microwave irradiation routes: Recently novel microwave irradiation routes have been

developed for the synthesis of inorganic materials. Products with good structural

uniformity and crystallinity have been obtained in these microwave methods. In this

communication a simple and fast precipitation method is described for the preparation

of hydroxyapatite in which microwave irradiation has been used. [2]

Sol Gel technique: Sol Gel technique has been developed and employed to prepare

various materials because it has main advantage of easy control of chemical

composition and low temperature synthesis that are very important for thin film

formation [21]. Traditionally, this bioceramic, Ca10 (PO4)6(OH) 2, can be

synthesized by solid state reactions, plasma techniques, hydrothermal

hotpressing, and many wet chemical precipitation and mechano-chemical

methods. In wet precipitation method, the chemical reactions take place

between calcium and phosphorus ions under a controlled pH and

temperature of the solution. The precipitated powder is typically calcined at

high temperature in order to obtain a stoichiometric apatitic structure. Slow

titration and diluted solutions must be used to improve chemical

homogeneity and stoichiometry within the system. Careful control of the

solution condition is also required in the wet precipitation methods. In early

reports, the decrease of solution pH below about 9 could lead to the

formation of Ca-deficient apatite structure. In some cases, a well-crystallized

HA phase was only developed while approaching a calcination temperature

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of 1200oC. The sol-gel approach provides significantly easier conditions for

the synthesis of HA. Sol-gel synthesis of HA ceramics has recently attracted

much attention. Sol-gel process refers to a low-temperature method using

chemical precursors that can produce ceramics and glasses with better

purity and homogeneity. This process is becoming a common technique to

produce ultra fine and pure ceramic powders, fibers, coatings, thin films, and

porous membranes. Compare to the conventional methods, the most

attractive features and advantages of sol-gel process include (a) molecular-

level homogeneity can be easily achieved through the mixing of two liquids;

(b) the homogeneous mixture containing all the components in the correct

stoichiometry ensures a much higher purity; and, (c) much lower heat

treatment temperature to form glass or polycrystalline ceramics is usually

achieved without resorting to a high temperature. More recently, the sol-gel

method has been extensively developed and used in biotechnology

applications. [21]

Important parameters in synthesizing of HAp

In previous part we compared the different routes of HAp synthesizing and we

discussed about some parameters that differ depend on the route. In fact only few

parameters between them are more important. For example ease of reaction, controlling

of PH and temperature, time, even though economical and environmental consideration

are not basic than porosity, particle size, and sintering ability. Therefore here we pay an

especial and detailed view to the most vital parameters in production of HAp.

Porosity

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While porous materials have many important applications in, for example, acoustic and

thermal insulation, transportation, filtration, purification, biomaterials, building

constructions, a new generation of porous biomaterials has recently emerged enabling

one to better reproduce the structure of natural bone. Several groups have been

successful in controlling the size, volume, and interconnectivity of pores in their

materials. Nevertheless, there have been only a few reports on the use of porous

materials for dental fillings: a possible reason for this is that porous materials tend to

have poor mechanical properties, while the mastication process produces high

compression and shear stresses that must be supported by the obturation material.

Consequently, nonporous materials, usually hard polymer resins, are more commonly

used to support these stresses. However, by the selection of an appropriate

agglutinating polymer (for a ceramic filler), it is possible to create porous materials with

suitable morphology and mechanical strength. Beyond achieving the correct

morphology, a successful dental obturation material must be chemically compatible with

and adhere to the substrate. Many obturation materials are designed essentially by

controlling only mechanical properties because adding a ceramic, or generally a filler, is

known to improve the mechanical properties of polymers. Here, we should take into

account morphology, chemical structure, mechanical behavior, and surface properties,

considering the combined effect of all of these on tooth ingrowth, as well as the role of

viscoelasticity for implant compliance and performance. Because teeth constitute an

organic–inorganic hybrid, a reliable hybrid for dental applications should contain an

agglutinating polymer that possesses: (i) high shear strength (around 70 MPa)

resistance interfacial stresses during mastication; (ii) appropriate tensile/ compressive

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strength and toughness, because very rigid and tough materials may lead to premature

wear of the real teeth from contact during chewing; (iii) high scratch resistance, to avoid

the occurrence of fissures, cracks, or canals that invite bacterial growth; (iv) good

adhesion with the hydroxyapatite (HAp) powder particles and with the substrate (dentin)

to avoid microfiltration, which produces bacterial growth; (v) high hydrolytic stability for

durability within the environmental conditions of the mouth; and (vi) appropriate

chemistry, to favor nonaggravating molecular recognition by the immune system. For all

composites of the general type polymer plus ceramic, the problem of adhesion between

differing components is an important challenge in product development. In turn,

adhesion depends on surface and interfacial tension values.[5]

Particle size

Materials based on calcium hydroxyapatite (HAp) are finding wide application in

medicine for creating bone implants and carriers of medicines, for filling

chromatographic columns, as adsorbents, and so forth. Various chemical methods are

used to obtain high-quality powders of calcium phosphates, including HAp. The most

popular methods are chemical co-precipitation from water solutions containing the ions

Ca2+, (PO4)3- , and (OH)–, which, interacting with pH > 7, form primary crystallites of

insoluble HAp. The process of obtaining powder for ceramics includes chemical

interaction between the initial components, separating and drying the precipitate, and

disaggregating the dried product. A great deal of attention is now being devoted to

obtaining nanopowders, i.e., powders with particle sizes not exceeding 100 nm.

However, the use of such powders for obtaining ceramic remains problematic.

Nanopowders have a high specific surface area and therefore excess surface energy —

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the driving force of the sintering process. Obtaining ceramic with uniform structure from

nanopowders is a quite difficult problem. Nanoparticles aggregate, and the average

particle size (aggregates) in the powder is 1 – 3μ m. It is such aggregates that play the

determining role in the formation of the microstructure of ceramic. The use of chemical

synthesis to obtain HAp powder with individual particle sizes less than 100 nm results in

the formation of 1 – 15 μm grains, depending on the sintering regime and the method

used to prepare the power. An obvious way out of the technological situation which has

developed is to use a number of techniques that make it possible to decrease the

aggregation of the powder material at different stages. One such technique is to use

surfactants, which modify the surface of particles, and to eliminate milling of the powder

material, assuming formation from highly concentrated suspensions. A number of other

techniques can also be used. The use of various surfactants is well known for obtaining

oxide nanoparticles or particles with intricate shapes. However, in most cases, the

influence of the surfactants or other soluble high-molecular compounds (HMC) used in

synthesis on the behavior of powder material during formation of a ceramic is not

studied. The use of gelatin or polyvinyl alcohol (PVA) in the synthesis of oxide powders

is well known. However, in these cases HMC can be used in substantial quantities to

perform synthesis in a viscous medium, where the mobility of the components is

decreased. Polyvinyl alcohol is a widely used substance in the technology of technical

ceramics based on pure oxides (containing no other components) that give plasticity to

the forming paste and ensure consolidation of material at the formation stage. Polyvinyl

alcohol meets all requirements for an ideal temporary technological binder: chemically

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inert, nontoxic, including at the decomposition stage, completely removed during

calcination before the sintering starts for most oxide materials. [18]

Homogeneity

The increase in density is considered to be due to increase in the homogeneity in the

matrix phase and the lower sintering temperature of hydroxyapatite. With increase in

temperature other phases contribute to the increase in density by enhanced sintering.

[4]

Goal

1) For tooth and bone implants, a primary requirement is that the material be

bioaccepted, because vascularization requires the material to support cellular activity

without eliciting an inappropriate host response on recognition of the foreign molecules

(i.e., molecular recognition). Second, the morphology must be suitable to allow

vascularization and attachment to the existing bone or tooth substrates. Morphological

specifications for bone-implant materials require a pore size in the range of 100–400

μm, with the pores being interconnected and comprising a volume fraction between

50% and 70%. For tooth implants, on the other hand, the mean optimal pore size is 2.90

± 0.22 μm (standard deviation), which is considerably smaller. A precise densitometry

study showed that enamel has a density distribution that is narrower with respect to that

of dentin: namely, the density of enamel is reportedly between 2.49 and 3.00 g/mL

(mean density 2.94 ± 0.03 g/mL), while the density of dentin is between 2.06 and 2.24

g/mL (mean density 2.14 ± 0.01 g/mL). Using these values, it is possible to demonstrate

that the pore volume fraction of dentin is around 27%, a value in good agreement with

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that of 22%, which was obtained using results that have been reported elsewhere.8 One

observes then that pore size and pore volume fraction are smaller for dentin than for

bone.[5]

2) Metallic biomaterials, such as titanium and its alloys, have enjoyed clinical successes

because of their superior strength, durability, corrosion resistance in physiological

environment, biocompatibility and bioinductibility. The high mechanical strength and

toughness of these biometals are the most important advantages over bioactive

hydroxyapatite (HAp) ceramics. Therefore, a system that combines both materials has

the mechanical advantages of the underlying (metallic) substrate and biological affinity

of the HAp surface to natural tissue.

3) In the development of new engineering materials, apart from other required

properties, strong and stiff materials coupled with reasonable ductility are always

targeted. In developing new biomaterials for tissue replacement, the structure and

properties of the tissue which is to be replaced must be taken into consideration,

because, if properties of the new material are significantly different from those of the

host tissue, the material under development will cause dynamic changes in the host

tissue after implantation, as has been discussed in terms of Wolff’s Law, and thus will

not achieve the goals considered in the original conceptual design. [4]

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Wolff’s Low: If a stiff metal or ceramic implant is placed in the bone, the bone will be subjected to lower mechanical stresses, and consequently bone will resorb. [14]

What should be done?

Major drawbacks of all biocompatible ceramics restrict their use in biomedical

applications to some degree. Low strength and fracture toughness of hydroxyapatite

limits its bulk use in many implants. As ZrO2 ceramic is a bioinert material, it does not

directly bond with natural bone in hip-joint replacement. Alumina’s high elastic modulus

causes stress shielding those results in loosening of implants in patients with

osteoporosis. The preparation of a microscale composite material is a promising idea

for improving the mechanical properties of hydroxyapatite. HA may be reinforced with

other ceramics or metals in the form of powders, platelets, or fibers. This approach has

been the subject of extensive study since the successful development of ceramic-matrix

composite materials. In order to synthesize an effective ceramic-matrix composite

material, three conditions should be satisfied. The strength and the stiffness of the

reinforcement must exceed those of the matrix. Along with this requirement the strength

of the interfacial layer between the matrix and the reinforcement should be appropriate

with limited reaction between the matrix and the reinforcement producing bond strength

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neither too weak nor too strong. Also, the difference between the coefficients of thermal

expansion of the phases should be low enough to prevent formation of microcracks

during cooling. The absence of these conditions provides microstructural defects

resulting in deterioration of the mechanical properties of the composite. The

biocompatibility of the reinforcement phase should also be considered when the

ceramic matrix composite is designed to be involved in biomaterials applications. Most

metals react with the HA to form metal oxides and tricalcium phosphate (TCP, Ca3

(PO4)2) or tetracalcium phosphate (TeCP, Ca4 (PO4)2O), leading to a serious

reduction in the biocompatibility of HA. Partially stabilized zirconia has been commonly

used as reinforcement for many ceramics because of its high strength and fracture

toughness. Bioinertness is another merit of the ZrO2. However, extensive reaction

between the HA and the ZrO2 to form TCP and fully stabilized ZrO2 is a big

disadvantage of this approach. Alumina, which is also classified as a bioinert material,

has been widely investigated as a reinforcing agent for HA. When large alumina

platelets were added, the fracture toughness of the HA increased without excessive

reaction between the HA and the Al2O3. However, the improvement in strength was

minimal because of the formation of microcracks around the platelets due to the large

difference in coefficient of thermal expansion between Al2O3 and HA. On the other

hand, when fine Al2O3 powder was used, the formation of the microcracks was

circumvented; however, the improvement in mechanical properties was limited due to

relatively low mechanical properties of the Al2O3 itself . Therefore, it is desirable to

combine the advantages of both materials as reinforcements for the HA: the excellent

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mechanical properties of ZrO2 and the chemical inertness of Al2O3 with respect to the

HA. [4]

Physical properties of HAp, Alumina, and Zirconia have been showed in the following

tables: [4]

a) Typical Mechanical Properties of Dense Hydroxyapatite Ceramics

Theoretical density 3.156 g/cm3

Hardness 500-800 Vickers, 2000-3500 KnoopTensile strength 40-100 MPaBend strength 20-80 MPaCompressive strength 100-900 MPaFracture toughness approx. 1 MPa/m1/2

Young’s modulus 70-120 GPab) Mechanical properties of biomedical grade alumina

Density 3.97 g/cm3 (99.9% Al2O3)

Hardness 2200 VickersBend strength 500 MPaCompressive strength 4100 MPaFracture toughness 4 MPa/m1/2

Young’s modulus 380 GPaThermal expansion coefficient 8x10-6 1/K

c) Mechanical properties of zirconia TZ-3Y

Density 6.05 g/cm3

Hardness 1200 HVBend strength 900-1200 MPaCompressive strength 2000 MPaFracture toughness 7-10 MPa/m1/2

Young’s modulus 210 GPaThermal expansion coefficient 11x10-6 1/K

d) Mechanical properties of a compact human boneTest direction related to bone axis

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Parallel NormalTensile strength (MPa) 124-174 49Compressive strength (MPa) 170-193 133Bending strength (MPa) 160Shear strength (MPa) 54Young’s modulus (GP)a) 17.0-18.9 11.5Work of fracture (J/m2) 6000KIc (MPa*m1/2) 2-12Ultimate tensile strain 0.014-0.031 0.007Ultimate compressive strain 0.0185-0.026 0.028Yield tensile strain 0.007 0.004Yield compressive strain 0.010 0.011

The ideal artificial bone demands good biocompatibility without the possibility of

inflammation or foreign body/toxic reactions. Strong bonding with the host bone, active

bone ingrowth into the graft, and bioabsorbability are also required. Sufficient strength

to resist the mechanical load in the implanted bone is also needed. None of the

biomaterials that have been developed unill now meet all of these criteria. HA has good

biocompatibility and osteoconductivity, however its fragility is a drawback like most

ceramic materials. Therefore, it can be used alone in areas that do not require good

mechanical strength. It can also be used with supplementary metal fixation in areas

which bear large amounts of the mechanical load. The structure of the dense sintered

body is stronger and more able to bond rapidly with host bone, but its use is limited due

to its high level of brittleness and low osteoconductivity and absorbability. Porous HA is

considered a good substitute, because it shows good osteoconduction and is replaced

by the host bone although it is mechanically weak. Patterns of osteoconduction for

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porous HAs vary with pore configuration. In HA, the 50 micron sized pore is enough,

and the 300 micron sized pore is optimum for osteoconduction. Porous HA can be a

useful graft material due to its osteoconductivity and the ease with which its pore

geometry can be controlled. The simplest way to generate porous scaffolds from

ceramics such as HA is to sinter particles, preferably spheres of equal size. With the

increase in temperature, pore diameter decreases and mechanical properties increase

as the packing of the spheres increases. Hot isostatic pressing can also be used to

further decrease the pore diameter. During sintering porosity can be increased by

adding fillers such as sucrose, gelatine, and PMMA microbeads to the powder and the

wetting solution. One of the most reliable formulations is the use of an HA powder slurry

with gelatine solution. Surface tension forces cause the formation of soft and spherical

porous particles of HA and gelatine. It is possible to produce porous bulk material with

an interconnected pore structure with an average pore size of 100 microns after

sintering. It is possible to produce interconnected pores with diameters up to 300

microns by using the polymer foam replication method. Open celled polyurethane foams

can be immersed in ceramic slurries under vacuum to allow the slurry to penetrate into

the pores of the foam. Burn out of foam at 250 °C produces the ceramic replica of the

foam. Using a similar method hydroxyapatite coated zirconia scaffolds with

interconnected pore diameters up to 500 microns have been produced. Zirconia’s

enhanced strength allows a high percentage of porosity in the composite.[4]

However some scientists believe because of poor thermal stability in HAp as indicated

by the decomposition into other phases such as tricalcium phosphate (TCP; Ca3

(PO4)2) at sintering temperatures higher than 900 oC, this phase impurity often results

28

in undesirable fast dissolution rates in vivo. The lack of commercially efficient

techniques in processing pure HA ceramics to full densification without decomposition

has somewhat restricted the wider applications of HA ceramics. In contrast, it is

expected that fluorapatite (Ca5 (PO4)3F) or fluorhydro- xyapatite might have superior

mechanical properties when sintered at high temperatures because of their higher

thermal stability than HA. [19]

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