Bioceramic

Bioceramics and bioglasses are ceramic materials that are biocompatible.^[1] Bioceramics are an important subset of biomaterials.^[2][3] Bioceramics range in biocompatibility from the ceramic oxides, which are inert in the body, to the other extreme of resorbable materials, which are eventually replaced by the body after they have assisted repair. Bioceramics are used in many types of medical procedures. A primary medical procedure where they are used is implants.^[2] This article is primarily concerned with rigid materials commonly used as surgical implants, though some bioceramics are flexible. The ceramic materials used are not the same as porcelain type ceramic materials. Rather, bioceramics are closely related to either the body's own materials, or are extremely durable metal oxides.

History

Prior to 1925 the materials used in implant surgery were primarily relatively pure metals. However, these are not considered to be ceramics and are therefore outside the scope of this article. The success of these materials was surprising considering the relatively primitive surgical techniques. The 1930s marked the beginning of the era of better surgical techniques and also the first use of alloys such as Vitallium.

In 1969 L. L. Hench and others discovered that various kinds of glasses and ceramics could bond to living bone^[4][5] Hench was inspired with the idea on his way to a conference on materials. He was seated next to a colonel who had just returned from the Vietnam War. The colonel shared that after an injury the bodies of soldiers would often reject the implant. Hench was intrigued and began to investigate materials that would be biocompatible. The final product was a new material which he called Bioglass. This work inspired a new field called bioceramics.^[6] With the discovery of bioglass interest in bioceramics grew rapidly.

On April 26, 1988, the first international symposium on bioceramics was held in Kyoto, Japan.

Applications

A titanium hip prosthesis, with a ceramic head and polyethylene acetabular cup

Ceramics are now commonly used in the medical fields as dental, and bone implants.^[7][8] Artificial teeth, and bones are relatively commonplace. Surgical cermets are used regularly. Joint replacements are commonly coated with bioceramic materials to reduce wear and inflammatory response. Other examples of medical uses for bioceramics are in pacemakers, kidney dialysis machines, and respirators.^[6] The global demand on medical ceramics and ceramic components was about US$9.8 billion in 2010. It is forecast to have an annual growth of 6–7% in the following years, and the world market value will increase to US$15.3 billion by 2015 and reach US$18.5 billion by 2018.^[9]

Mechanical Properties and Composition

Bioceramics are meant to be used in extracorporeal circulation systems (dialysis for example) or engineered bioreactors. But is mostly common as implants.^[10] The Ceramics show numerous applications as biomaterials due to their physicochemical properties. They are also inert in the human body, the hardness and resistance to abrasion also makes them useful for bones and teeth replacement. Some ceramics also have excellent resistance to friction making them useful to replace malfunctioning joints. More properties such as appearance and electrical insulation are also a concern for specific biomedical applications.
Some bioceramics, such as Alumina (Al₂O₃) are used with high pure phase and the lifespan is longer that the patient’s. They can be also used as inner ear ossicles, ocular prostheses, electrical insulation for pacemakers, catheter orifices and in numerous prototypes of implantable systems (cardiac pumps for example). Alumino-silicates are commonly used in dental prostheses, pure or in ceramic-polymer composites. The ceramic-polymer composites are a potential way to filling of cavities replacing amalgams suspected to have toxic effects. The aluminosilicates also have a glassy structure. Contrary to artificial teeth in resin, the colour of tooth ceramic remains stable^[10][11] Zirconia doped with yttrium oxide has been proposed as a substitute for alumina for osteoarticular prostheses. The main advantages are a greater failure strength, and a good resistance to fatigue. Vitreous carbon has interesting properties, is light, resistant to wear and compatible to the blood. It is mostly used in cardiac valve replacement. Diamond can be used for the same application, but in coating form.
Bioactive ceramics are used for orthopaedic, maxillofacial and plastic surgeons. They are used in to prevent a loss of bone substance. Biodegradable ceramics will be resorbed by the organism and replaced by reconstructed tissue, whereas non-biodegradable ceramics are intended for a permanent implantation.^[10][11] Calcium phosphate-based ceramics constitute, at present, the preferred bone substitute in orthopaedic and maxillofacial surgery.^[10] They are very similar to the mineral phase of the bone, in their structure and/or their chemical composition.
Calcium phosphates usually found in ceramics are:
– hydroxyapatite (HAP): Ca₁₀(PO₄)₆(OH)₂;
– tricalcium phosphate β (β TCP): Ca₃ (PO₄)₂;
– mixtures of HAP and β TCP.
This type of material has pores which provide a good bone-implant interface, due to the increase of surface area that encourages cell colonisation and revascularisation. Besides this excellent property, it has lower mechanical strength compared to bone, making highly porous implants very delicate. Since Young's modulus of ceramics is generally much higher than that of the bone tissue, the implant can cause mechanical stresses at the bone interface.^[10]
Table 1: Bioceramics Applications ^[12]

Table 2: Mechanical Properties of Ceramic Biomaterials ^[12]

Multipurpose

A number of implanted ceramics have not actually been designed for specific biomedical applications. And are used in different implantable systems because of their properties and their good biocompatibility. Alumina is one of the most widely used multipurpose ceramics. There is also Alumino-silicates, Alumino-silicate glasses, Zirconia, Vitreous carbon and diamond carbon.
A number of other ceramics have been subjected to biomedical tests for implantation, without currently being developed industrially. Among these ceramics, we can cite silicon carbide, titanium nitrides and carbides, and boron nitride. TiN has been suggested as the friction surface in hip prostheses. While cell culture tests show a good biocompatibility, the analysis of implants shows significant wear, related to a delaminating of the TiN layer. Silicon carbide is another modern day ceramic which seems to provide good biocompatibility and can be used as bone implant.^[10]

Specific Use

Ceramics for specific uses, in addition to their traditional properties, have biological activity. It is also referred as bioactive ceramics. Calcium phosphates, Oxides and hydroxides are an example. There are also natural materials - generally from animal origin; bioglasses and composites, which are a combination of mineral-organic composite materials such as HAP, alumina or titanium dioxide with the biocompatible polymers (polymethylmethacrylate): PMMA, poly(L-lactic) acid: PLLA, poly(ethylene). Composites can be differentiated as bioresorbable or non-bioresorbable. The nonbioresorbable composites are the result of the combination of a non-bioresorbable calcium phosphate (HAP) with a non-bioresorbable polymer (PMMA, PE). These materials may become more widespread in the future, on account of the many combination possibilities and their aptitude at combining a biological activity with mechanical properties similar to those of the bone.

Biocompatibility

Bioceramics are materials that are well used and are known all over the world, specifically in the biomedical area. Their properties are anticorrosive, biocompatibility and aesthetic such as a matching shade. This makes bioceramics very suitable for medical usage. Zirconia ceramic has bioinertness and noncytotoxicity. Carbon with similar mechanical properties of bone is an alternative, for it elicits blood compatibility, no tissue reaction and nontoxicity to cells. None of the three-bioinert ceramics exhibited bonding with the bone. However, the bioactivity of the bioinert ceramics can be achieved by forming composites with bioactive ceramics. Bioglass and glass ceramics are nontoxic and chemically bond to bone. Glass ceramics elicit osteoinductive property. Calcium phosphate ceramics exhibit nontoxicity to tissues, bioresorption and osteoinductive properties. The ceramic particulate reinforcement has led to the choice of more materials for implant applications that include ceramic/ceramic, ceramic/polymer, ceramic/metal composites. Among these composites ceramic/polymer composites have been found to release toxic elements into the surrounding tissues. Metals face corrosion related problems and ceramic coatings on metallic implants degrade over time during lengthy applications. Ceramic/ceramic composites enjoy superiority due to similarity with bone minerals, exhibiting biocompatibility and are able to be shaped into a particular size. The biological activity of bioceramics has to be considered under various in vitro and in vivo studies. The knowledge of performance needs to be considered for the choice of the bioceramic in accordance with the particular site of implantation.^[12]

Processing

Technical ceramics are composed of raw materials generally as powder and of natural or synthetic chemical additives, favoring either compaction (hot, cold orisostatic), or setting (hydraulic or chemical) or accelerating sintering processes. According to the formulation of the bioceramic and the shaping process used, it can obtain ceramics, dense or with variable porosity, cements, ceramic depositions or ceramic composites.^[10] Another way of material processing is developing; it is based on the biomimetic processes aiming at imitating natural and biological processes and offers the possibility of making these bioceramics at ambient temperature than that of conventional or hydrothermal processes [GRO 96]. The prospect of using these very low forming temperatures opens up possibilities for mineral organic combinations with improved biological properties by the addition of proteins and biologically active molecules (growth factors, antibiotics, anti-tumor agents, etc.). However, these materials have poor mechanical properties which can be improved, partially, by combinations with bonding proteins.^[10]

Commercial Usage

The bioactive materials available commercially for clinical use are 45S5 bioactive glass, A/W bioactive glass ceramic, dense synthetic HA, or bioactive composites, such as a polyethylene–HA mixture. All the above bioactive materials form an interfacial bond with adjacent tissue.^[11]
High-purity alumina bioceramics are currently commercially available from various producers. Morgan Advanced Ceramics (MAC) (Worcestershire, UK) began manufacturing orthopaedic devices in 1985 and quickly became a recognised supplier of ceramic femoral heads for hip replacements. MAC Bioceramics has the longest clinical history for alumina ceramic materials, HIP Vitox® alumina since 1985.^[13] Some calcium-deficient phosphates with an apatite structure were thus commercialised as ‘tricalcium phosphate’ even though they did not exhibit the expected crystalline structure of tricalcium phosphate.^[13]
At the present time, numerous commercial products described as HA are available, in various physical forms (e.g. granules, specially designed blocks for specific applications) HA/polymer composite (HA/polyethyelene, HAPEXTM) is also commercially available as ear implants. HA is also now commonly used in abrasives and as a plasma-sprayed coating for orthopedic and dental implants.^[13]

Future trends

One proposed use for bioceramics is the treatment of cancer. Two methods of treatment have been proposed; treatment through hyperthermia, and radiotherapy. Hyperthermia treatment involves implanting a bioceramic material that contains a ferrite or other magnetic material. The area is then exposed to alternating magnetic field, which causes the implant and surrounding area to heat up. Alternatively the bioceramic materials can be doped with β-emitting materials and implanted into the cancerous area.^[2]

Other trends include engineering the materials for specific tasks. Ongoing research involves the chemistry, composition, and micro and nanostructures of the materials to improve their biocompatibility.^[14][15][16]

See also

• Ceramic-impregnated fabrics

References