Bioceramics and Their Applications

7/30/2019 Bioceramics and Their Applications

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Bioceramics And Their ApplicationsPosted by siwahid | 0 Comment

1. Hydroxyapatite (HA)

1.1. Introduction

Hydroxyapatite (HA) is a member of the apatite group of ceramics. The term apatite is derived from the Greek

apat, which means deceit or deception. It was called such for its diversity of form and color.

1.2. Source

There are two sources of apatite: one biological and the other from mineral deposits, such as phosphate rock or

phosphorite, a sedimentary rock the essential mineral components of which is carbonate fluoroapatite. Bone and

teeth contain a HA-like mineral component that supports the majority of load in vivo. Deorganized bone and some

sea corals (porites) are used to make implants.

1.3. Properties

The (bio)chemical and mechanical properties of HA are similar to those of bone and teeth. Their molecular structures

are also similar, although the exact nature of the composite, the minerals and proteins, and their interactions are not

fully understood.

1.3.1. Mechanical Properties

There is a wide variation in the reported mechanical properties of HA. Jarcho2)reported that fully densified

polycrystalline specimens of HA synthesized by themhad average compressive and tensile strengths of 917 and 196

MPa, respectively.Kato2) noted a compressive strength of 3000 kg/cm2 (294 MPa), a bending strength of1500

kg/cm2 (147 MPa), and a Vickers hardness of 350 kg/mm2 (3.43 GPa).

1.3.2. Chemical Properties

Hydroxyapatite is considered bioactive, indicating that the ceramic may undergo ionization in vivo and that the rate of

dissolution may depend on many factors includeing degree of crystallinity, crystallite size, processing condition

(temperature,pressure, and partial water pressure), and porosity. Hydroxyapatite is soluble in anacidic solution while

insoluble in an alkaline one and slightly soluble in distilled water. The solubility of sintered HA is very low. The rate of

solubility is 0.1 mg/yearin subcutaneous tissue. Hydroxyapatite reacts actively with proteins, lipids, and otherinorganic

and organic species.1.3.3. Biological properties

1.3.3.1. In vitro cell response

Substituted HA or HAP showed the following cell response: (a) carbonate-substituted apatite stimulate greater activity

of osteoclasts compared with carbonate-free HAP or FAP or F-treated dentin (b) stimulation of proliferation and

phenotypic expression of F-containing HAP or Ftreated bovine bone (c) difference in response between odontoblasts

and osteoblasts

1.3.3.2. Tissue response

HA have bioactivity proerties, ability of the material to directly bond to bone through chemical interaction and not

physical or mechanical attachment. HA also have osteoconductive properties an ability to serve as a scaffold or

template to guide the newly forming bone along its surfaces.

1.4. Applications1.4.1. HA as abrasive

HA or apatitic abrasive (biphasic calcium phosphate) has gained popularity as the abrasive of choice for orthopedic

and dental implant. Implant surface gritblasted with HA or apatitic abrasive was shown to be cleaner (free of

inclusions) compared with alumina [31] and appear to promote higher bone contact.

1.4.2. Bone graft materials and scaffolds

Dental applications of HA materials include: implants as immediate tooth root replacement, alveolar ridge

augmentation , pulp capping , periodontal defects, bone regeneration with guided tissue regeneration membrane ;

alveolar distraction osteogenesis, peri-implantitis defects, reconstruction of severely atrophic human maxillae and
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sinus lifts. Medical applications include: repair of bone defects, chin augmentation, ear implant by itself, or as a

composite with high molecular weight polyethylene, spine cage, tibial osteotomy in patients with osteoarthritis, and as

a percutanous device. HA or HAP is also used as component of calcium phosphate cement, CPC .

1.4.3. Implant coatings

In spite of the many good qualities of HA and related calcium phosphates (e.g. B-TCP) such as bioactivity and

osteoconductivity, they cannot be used in load-bearing areas because of their low fracture strength. On the otherhand, metal implants, primarily titanium (Ti) or Ti alloy, are not bioactive and therefore do not bond directly to bone.

1.4.4. Drug delivery and other applications

HA ceramic is used as gene carrier or transfection agents , for drug delivery such as delivery of anticancer drugs or

bisphosphonate, or as scaffolds for bone regeneration by tissue engineering.

2. Tricalcium Phosphate (TCP)

2.1. Introduction

The term biphasic calcium phosphate (BCP) was first used by Ellinger et al. to describe the bioceramic previously

described as tricalcium phosphate but was shown by LeGeros in 1986 using X-ray diffraction (XRD) to consist of a

mixture of 80% HA and 20% B-TCP.

2.2. Fabrication

Biphasic calcium phosphate (BCP), or intimate mixtures of HA and B-TCP, is obtained when synthetic calcium-

deficient apatites (CDAs) are sintered above 900 C [11,26,29] according to the following reaction:

2.3. Properties

2.3.1. Physicochemical properties

Since B-TCP has a higher solubility than HA, the extent of dissolution of BCP ceramic of comparable macroporosity

and particle size will depend on the HA/B-TCP ratio: the higher the ratio, the lower the extent of dissolution. This

phenomenon may be caused by processing variables (sintering time and temperature).

2.3.2. Mechanical properties

BCP ceramic prepared from a single calcium-deficient apatite phase was reported to exhibit higher compressive

strength (212 MPa) compared with BCP ceramic prepared by mixing two unsintered calcium phosphate

preparations (2 MPa): one that after sintering at 1200 C resulted in only HA and the other that resulted in only B-

TCP [48]. The initial mechanical property is not the best criterion for efficacy of bone ingrowth.

2.3.3. Bioactivity and osteogenic properties

Bioceramics (calcium phosphates, bioactive glass) do not usually have osteoinductive property. However, several

reports indicated osteoinductive properties of some calcium phosphate bioceramics such as those reported for

coralline HA (derived from coral) or observed in some studies using BCP.

2.4. Applications

2.4.1. Applications in dentistry

Dental applications of BCP include prevention of bone loss after tooth extraction, repair of periodontal defects and

sinus lift augmentation.

2.4.2. Applications in orthopedics

Micromacroporous biphasic calcium phosphate bioceramics are largely used in orthopedics and effi cacy has been

demonstrated in numerous preclinical and clinical studies, for example using specific shaped blocks (custom-designed) for spine arthrodesis (cage insert) and wedges for tibial valgization osteotomy of valgization.

3. Alumina (Al2O3)

3.1. Introduction

Aluminium oxide (Al2O3), more commonly known as alumina, is the most widely used oxide ceramic material.

Bauxite (hydrated aluminum oxide) and native corundum (aluminum oxide mineral) are the main sources of high-

purity alumina. As a raw material, Al2O3 powder is produced in large quantities from the mineral bauxite, by the


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Bayer process. Bayer process, which yields D-alumina. The Bayer process involves dissolution of crushed bauxite in

sodium hydroxide (NaOH) solution under pressure at high temperatures (up to 300C) to form a supersaturated

sodium aluminate solution.

3.2. Properties

3.2.1. Physical properties

Additives or impurities determine the colour of alumina, in addition to the sintering atmosphere, and by the interactionwith ionising radiation. Alumina is generally white but can sometimes be pink (88% alumina) or brown (96% alumina).

When chromium oxide (Cr2O3) is added, it reacts with Al2O3 to form a solid solution. The amount of chromium oxide

added will determine whether the colour of alumina changes to pink or ruby. When medical-grade alumina is sintered

in air together with the addition of magnesia, it will appear as ivory. Alumina turns white when it is sintered in reducing

atmosphere or if it contains traces of silica.


Because of their strong bonding, alumina ceramics have very high melting or, more appropriately, dissociation

temperatures, hence the production of alumina ceramics can only be achieved with high-temperature sintering.

During the sintering process, powders are heated usually to two-thirds of their melting temperature. As shown earlier,

during this densification particles bond together to form necks between the particles, which subsequently reduce the

surface area and cause the powder to consolidate.

3.3. Applications

High-purity alumina bioceramics have been developed as an alternative to surgical metal alloys for total hip

prosthesis and tooth implants. Their high hardness, low friction coefficient and the excellent corrosion resistance of

alumina offer a very low wear rate at the articulating surfaces in orthopaedic applications. Alumina has the ability to

be polished to a high surface finish. Other applications for alumina in orthopaedic and maxillofacial applications

include porous coatings for femoral stems, alumina spacers employed specifically in revision surgery (Huckstep and

Sherry, 1996), knee prostheses (see Fig. 10.4), and in the past as polycrystalline and single crystal forms in dental

applications as tooth implants.

4. Zirconia (ZrO2)

4.1. Introduction

Zirconium oxides (zirconia) have been used for the purpose of fabricating implants. Some are called fake diamondor cubic zirconia since some zirconia single crystals can be of gem grade and made into jewels. Some of their

mechanical properties are as good or better than those of alumina ceramics. They are highly biocompatible, like other

ceramics, and can be made into such large implants as the femoral head of a hip joint replacement. Some of their

drawbacks include the fact that they exhibit high density, low hardness, and phase transformations under stress in

aqueous conditions, thus degrading their mechanical properties.

4.2. Source

Zircon (ZrSiO4) is the most commercially important zirconium mineral and is found mostly in the mineral baddeleyite.

Zircon is a gold-colored silicate of zirconium, a mineral found in igneous and sedimentary rock and occurring in

tetragonal crystals colored of many colors. The transparent varieties are usually deposited in beach sand, and are

used as gems. Zircon is first chlorinated to form ZrCl4 in a fluidized bed reactor in the presence of petroleum coke. A

second chlorination is required for highquality zirconium. Zirconium is precipitated with either hydroxides or sulfates,and then calcined to its oxide.

4.3. Properties

4.3.1. Physical Properties

Zirconia undergoes an allotropic phase transition from monoclinic to tetragonal at 1000~1200C, and from tetragonal

to cubic at 2370C. The cubic-to-monoclinic and tetragonal

phase transition is diffusionless and accompanies a volume expansion of about 7%. The cubic structure of zirconia

belongs to the group of fluorite (CaF2) structures.


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The strength of the partially stabilized zirconia with yttrium oxide (YTZP) showed the highest flexural strength and

fracture toughness. However, the Weibull modulus was lower than the yttrium magnesium oxide-stabilized zirconia

(YMgPSZ). It is also interesting that the increased fracture toughness is due to a phase transformation caused by

cessation of crack propagation.

4.4. ApplicationsYttrium-stabilized zirconia has been used to fabricate the femoral head of total hip joint prostheses and has two

advantages over alumina. One is the finer grain size and well-controlled microstructure with minimum residual

porosity, resulting in a better tribological material than with alumina. The other is higher fracture strength and

toughness due to the phase transformation toughening process. Approximately 20% of the prosthetic femoral heads

manufactured in the world are made of ceramic, with a strongly growing market (i.e. more than 25% growth for the

aluminaalumina coupling between 2002 and 2004). Up to the year 2000,approximately 40% of ceramic heads were

zirconia and the remaining alumina.

5. Reference:

1) Kokubo, Tadashi. Bioceramics and their clinical applications. England : Woodhead Publishing.

2) Park, Joon. 2008. Bioceramics - Properties, Characterizations, and Applications. USA : Springer.

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Bioceramics and Their Applications