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May 17th, 2013

Bioceramics: Opportunities and challenges

Published on May 17th, 2013 | Edited by: Eileen De Guire

 

Chemical composition of Biolox delta, a zirconia-toughened alumina with small additions of chromium oxide and strontium aluminate. Credit: Pawar, IJACT

Last July’s 4th International Congress on Ceramics was the setting for multiple presentations on the use of advanced ceramics in various industries. Among the application areas covered were biology and medicine. This post is a recap of a paper on the topic from the May/June issue of AcerS’ International Journal of Applied Ceramic Technology.

According to author Vivek Pawar, a materials researcher at Smith and Nephew Inc. (Memphis, Tenn.), seven presentations at the event focused on bioceramics for orthopedic, tissue engineering, and dentistry applications, as well as on innovative manufacturing techniques and novel ceramic materials for use as bearing surfaces.

Pawar writes that the bioceramics used in hard or soft tissue replacement can be classified as bioactive glasses made mainly from calcium oxide, sodium oxide, phosphorus pentoxide, and silica; apatite-based ceramics made from synthetic hydroxyapatite and calcium phosphates; and ceramics that are used as bearing surfaces for orthopedic applications.

Since development of the first bioactive glass by Larry Hench more than 40 years ago, few alterations have been made to the materials’ composition. Pawar reports current research in the area focuses on developing compositions that maintain or increase bioactivity after crystallization during sintering. The goal is to develop low-density, easily machinable materials with fracture toughness greater than 1 MPa m1/2.

A new material aimed at meeting those criteria is a product called ‘Biosilicate’ from Vitrovita (São Carlos, Brazil), which is reported to have antimicrobial properties. In one study assessing the effectiveness of Biosilicate against a variety of microorganisms, the material displayed activity against all the bacteria except one, drastically reducing the number of viable cells in the first 10 minutes of contact.

Hydroxyapatite (HA) coatings are commonly used in orthopedic devices to promote bone in-growth on metallic implants. Pawar writes that current research focuses on increasing the material’s bioactivity by incorporating bioactive ions in the HA crystal structure. Researchers have investigated magnesium, strontium, silver, zinc, titanium, iron, sodium, and potassium cationic substitutions. Anionic substitutions considered include fluorine, chlorine, hydroxide, phosphate, and silicate ions. These ions perturb the HA crystal structure and change solubility. Current work is aimed at understanding how each of these ions affects bioactivity. Substitutions with silver, for example, have increased HA solubility and shown bactericidal effects.

Hip arthroplasty remains the predominant use for ceramic bearing surfaces in orthopedic implants, and materials used in this application have included alumina, yttria-stabilized zirconia, and zirconia-toughened alumina. Newer ceramic materials with higher strength and toughness than alumina and reduced risk of fracture include ‘Biolox delta‘ from CeramTec (Plochingen, Germany). A zirconia-toughened alumina with small additions of chromium oxide and strontium aluminate, the material is “being considered for challenging applications such as hip resurfacing femoral heads and knee femoral components,” Pawar writes.

Another potential bearing material is silicon nitride, which offers high strength and toughness, excellent wear resistance, imaging compatibility, affinity to bone, and an antibacterial surface. Produced by Amedica (Salt Lake City, Utah), Si3N4 is already being used in spinal devices.

In the article Pawar notes, “Although no significant clinical problems have been reported with these two materials, a long-term clinical followup will be required to evaluate the performance of these materials.”

Innovations in ceramic processing techniques are being driven by specific biomedical applications. For example, camphene freeze casting processes are being used to create a 3-D interconnected porous bioceramic scaffold with the aim of producing a bioactive glass scaffold with high strength and bioactivity.

Also proposed is a 3-D printing process for apatite-based ceramics using stereolithography. The method is said to enable production of customized solutions based on clinical needs. A limited clinical study of the technology for repair of large craniofacial bone defects is under way at 3DCeram (Limoges, France).

Finally, nanoceramics with particle sizes of 1 to 100 nm are the focus of considerable research. The unique properties of materials produced using nanoparticles—higher surface-to-volume ratios, no light scattering, and unique mechanical properties in composite form—have led to use in dental fillings and crowns. Bone grafting, bone cement, and bioactive ceramic applications also offer research opportunities. Pawar expects future research in this area will focus on development of methods to produce customized nanoceramics based on patient needs.


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