Bioceramics And Their Applications

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 apatê, 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. Applications
1.4.1. 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 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 other hand, 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 (2–12 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 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 300ºC) 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 interaction with 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.

3.2.2. Mechanical properties

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 diamond” or “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 1000~1200ºC, and from tetragonal to cubic at 2370ºC. 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.

4.3.2. Mechanical properties

The strength of the partially stabilized zirconia with yttrium oxide (Y–TZP) showed the highest flexural strength and fracture toughness. However, the Weibull modulus was lower than the yttrium magnesium oxide-stabilized zirconia (Y–Mg–PSZ). It is also interesting that the increased fracture toughness is due to a phase transformation caused by cessation of crack propagation.

4.4. Applications

Yttrium-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 alumina–alumina 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.