This document discusses hydroxyapatite (HAp), a calcium phosphate ceramic used in biomedical applications. It describes the chemistry and structure of HAp and reviews several common synthesis methods, including wet precipitation, hydrothermal, microwave irradiation, ultrasonic irradiation, and sol-gel techniques. It also discusses modifying HAp properties through doping, such as creating fluorinated hydroxyapatite, and controlling porosity for tissue growth. The document provides details on optimizing various synthesis parameters and processing routes to produce HAp powders and coatings with desired compositions, sizes, morphologies, and mechanical properties for biomedical and dental applications.

1. 1
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

2. 2
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

3. 3
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]

4. 4
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]

5. 5
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

6. 6
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:

7. 7
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]

8. 8
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]

9. 9
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

10. 10
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.

11. 11
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

12. 12
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
process. Synthesized fluorhydroxyapatite powder has been characterized on a

13. 13
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]

14. 14
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

15. 15
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

16. 16
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

17. 17
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 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

18. 18
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
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

19. 19
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
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)

20. 20
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 —
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

21. 21
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
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

22. 22
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
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

23. 23
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]
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]

24. 24
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
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

25. 25
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
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]

26. 26
a) Typical Mechanical Properties of Dense Hydroxyapatite Ceramics
Theoretical density 3.156 g/cm3
Hardness 500-800 Vickers, 2000-3500 Knoop
Tensile strength 40-100 MPa
Bend strength 20-80 MPa
Compressive strength 100-900 MPa
Fracture toughness approx. 1 MPa/m1/2
Young’s modulus 70-120 GPa
b) Mechanical properties of biomedical grade alumina
Density 3.97 g/cm3 (99.9% Al2O3)
Hardness 2200 Vickers
Bend strength 500 MPa
Compressive strength 4100 MPa
Fracture toughness 4 MPa/m1/2
Young’s modulus 380 GPa
Thermal expansion coefficient 8x10-6 1/K
c) Mechanical properties of zirconia TZ-3Y
Density 6.05 g/cm3
Hardness 1200 HV
Bend strength 900-1200 MPa
Compressive strength 2000 MPa
Fracture toughness 7-10 MPa/m1/2
Young’s modulus 210 GPa
Thermal expansion coefficient 11x10-6 1/K
d) Mechanical properties of a compact human bone
Test direction related to bone axis
Parallel Normal
Tensile strength (MPa) 124-174 49
Compressive strength (MPa) 170-193 133
Bending strength (MPa) 160
Shear strength (MPa) 54
Young’s modulus (GP)a) 17.0-18.9 11.5
Work of fracture (J/m2) 6000
KIc (MPa*m1/2) 2-12
Ultimate tensile strain 0.014-0.031 0.007
Ultimate compressive strain 0.0185-0.026 0.028
Yield tensile strain 0.007 0.004
Yield compressive strain 0.010 0.011

27. 27
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
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

28. 28
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
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]

29. 29
References
1. A.Nakahira, S.Konishi, F.Nishimura, T.Murakami, Y.Honda, C.Karatan, M. Tamai, H.
Aritani, Local structure of bioactive hydroxyapatite with solid solution of cation by
XAFS, http://www.uvsor.ims.ac.jp/activity/AR2002/research%EF%BC%95/nakahira.pdf
2. B. Vaidhyanathan, K. Jrao, Rapid microwave assisted synthesis of hydroxyapatite,
Bull. Mater. Sci., Vol. 19, No. 6, December 1996, pp. 1163-I 165
.3. E. B.Kenney, Overview of Periodontal Regenerative Surgery,
www.dent.ucla.edu/pic/ppt/Regenerative-Surgery.ppt
4. E. Sahin, Synthesis and characterization of hydroxyapatite-alumina-zirconia
biocomposites,september 2006,
http://library.iyte.edu.tr/tezler/master/malzemebilimivemuh/T000548.pdf
5. H. E. Hagg Lobland, L. Hoang, J. R.Rodriguez, S.Vargar,
Porous hydroxyapatite-based obturation materials for dentistry, J. Mater, Res., Vol. 23,
No. 6, Jun 2008
6. H.Eslami, M.Solati-Hashjin,M.Tahriri, Synthesis and characterization of
nanocrystalline fluorinated ydroxyapatite powder by a modified wet-chemical process,
Journal of Ceramic Processing Research, Vol. 9, No. 3, pp. 224~229 (2008)
7. J. S. Earl, D. J. Wood, S. J. Milne, Hydrothermal synthesis of hydroxyapatite,
Institute of Physics Publishing Journal of Physics: Conference Series 26 (2006) 268–
271
8. K.Prabakaran, S.Kannan,S. Rajeswari, Development and Characterisation of Zirconia
and Hydroxyapatite Composites for Orthopaedic Applications,Trends Biomater. Artif.
Organs, Vol 18 (2), January 2005 http://www.sbaoi.org

30. 30
9. M.B.Nair, H. K. Varma, T.V. Kumary, S.Babu S,A.John, Cell Interaction Studies with
Novel Bioglass Coated Hydroxyapatite Porous Blocks, Trend Biomater . Artif.Organs,
vol 19(2), pp108-114 (2006), http://www.sbaoi.org
10. M. Jevtić, M. Mitrić, N. Ignjatović, D. Uskoković, Sonochemical preparation of
Hydroxyapatite/poly (d I Lactide co Glycolide) composite,
http://www.itn.sanu.ac.yu/images/Posteri/2008/Amsterdam.pdf
11. M. Jevtić, M. Mitrić, N. Ignjatović, D. Uskoković, Sonochemical Synthesis of
hydroxyapatite using homogenious precipitation agent-urea,
http://www.itn.sanu.ac.yu/images/Posteri/2006/Yucomat06-Marija.pdf
12. M. P. Mahabole, R. C. Aiyer, C .V. Rramakrishna, B. Sreedhar, R. S. Khairnar,
Synthesis, characterization and gas sensing property of
hydroxyapatite ceramic, Bull. Mater. Sci., Vol. 28, No. 6, October 2005, pp. 535–545
13. N. Monturapoj, Nano-size Hydroxyapatite Powders Preparation by Wet-Chemical,
Journal of Metals, Materials and Minerals, Vol.18 No.1 pp.15-20, 2008
14. P.J.Prendergast, R.Hulskes, The Biomechanics of Wolff’s Law: Recent Advances,
http://www.springerlink.com/content/k077t02638p61634/
15. S. Nath, B. Basu, A.Sinha,A Comparative Study of conventional sintering with
Microwave Sintering of Hydroxyapatite Synthesized by Chemical Route, Trends
Biomater. Artif. Organs, Vol. 19(2), pp 93-98 (2006) http://www.sbaoi.org
16. T.Scharnweber ,C. Santos , R.P.Franke, M.M.Almeida, Influence of Spray-dried
Hydroxyapatite-5-Fluorouracil Granules on Cell Lines Derived from Tissues of

31. 31
Mesenchymal Origin, Molecules 2008, 13, 2729-2739; DOI: .3390/molecules13112729,
www.mdpi.org/molecules
17. T.Levingstone,L. Looney , Plasma Spraying of Hydroxyapatite,
www.mecheng.dcu.ie/MPRC/MPRCPosters/TanyaLevingsto...
18. T. V. Safronova, M. A. Shekhirev,V. I. Putlyaev, Ceramics based on calcium
hydroxyapatite synthesized in the presence of PVA,Glass and Ceramics Vol. 64, Nos.
11 – 12, 2007
19. S.C. Bayne,J.Thompson ,Dental porcelain ,
http://www.dent.unc.edu/portfolios/bayne/dental-materials/ ,
20. S. Kehoe, M. Ardhaoui, J. Stokes, Critical analysis of the production of
hydroxyapatite (HA) powder applied onto orthopedic implants,
http://www.nuigalway.ie/ncbes/news/pdf/paper96.pdf
21. U. Vijayalakshmi,S. Rajeswari , Preparation and Characterization of Microcrystalline
Hydroxyapatite Using Sol Gel Method , Trends Biomater. Artif. Organs, Vol. 19(2), pp
57-62 (2006) http://www.sbaoi.org
23 .Z. M. Markovic, B. O. Fowler, M.S. Tung, Preparation and Comprehensive
Characterization of a Calcium Hydroxyapatite, Reference Material, Journal of Research
of the National Institute of Standards and Technology, Volume 109, Number 6,
November-December 2004