Mechanical properties of biomaterials

Materials that are used for biomedical or clinical applications are known as biomaterials. The following article deals with fifth generation biomaterials that are used for bone structure replacement. For any material to be classified for biomedical application three requirements must be met. The first requirement is that the material must be biocompatible; it means that the organism should not treat it as a foreign object. Secondly, the material should be biodegradable (for in-graft only); the material should harmlessly degrade or dissolve in the body of the organism to allow it to resume natural functioning. Thirdly, the material should be mechanically sound; for the replacement of load bearing structures, the material should possess equivalent or greater mechanical stability to ensure high reliability of the graft.

Introduction

The biomaterial term is used for materials that can be used in biomedical and clinical applications. They are bioactive and biocompatible in nature. Currently, many types of metals and alloys (stainless steel, titanium, nickel, magnesium, Co–Cr alloys, Ti alloys),[1] ceramics (zirconia, bioglass, alumina, hydroxyapatite) [1] and polymers (acrylic, nylon, silicone, polyurethane, polycaprolactone, polyanhydrides) [1] are used for load bearing application. This includes dental replacement and bone joining or replacement for medical and clinical application. Therefore their mechanical properties are very important. Mechanical properties of some biomaterials and bone are summarized in table 1.[2] Among them hydroxyapatite is most widely studied bioactive and biocompatible material. However, it has lower young’s modulus and fracture toughness with brittle nature. Hence, it is required to produce a biomaterial with good mechanical properties.

Elastic modulus

Elastic modulus is simply defined as the ratio of stress to strain within the proportional limit. Physically, it represents the stiffness of a material within the elastic range when tensile or compressive load are applied. It is clinically important because it indicates the selected biomaterial has similar deformable properties with the material it is going to replace. These force-bearing materials require high elastic modulus with low deflection. As the elastic modulus of material increases fracture resistance decreases. It is desirable that the biomaterial elastic modulus is similar to bone. This is because if it is more than bone elastic modulus then load is born by material only; while the load is bear by bone only if it is less than bone material. The Elastic modulus of a material is generally calculated by bending test because deflection can be easily measured in this case as compared to very small elongation in compressive or tensile load. However, biomaterials (for bone replacement) are usually porous and the sizes of the samples are small. Therefore, nanoindentation test is used to determine the elastic modulus of these materials. This method has high precision and convenient for micro scale samples. Another method of elastic modulus measurement is non-destructive method. It is also clinically very good method because of its simplicity and repeatability since materials are not destroyed.[3]

Hardness

Hardness is one of the most important parameters for comparing properties of materials. It is used for finding the suitability of the clinical use of biomaterials. Biomaterial hardness is desirable as equal to bone hardness. If higher than the biomaterial, then it penetrates in the bone. As above said, biomaterials sample are very small therefore, micro and nano scale hardness test (Diamond Knoop and Vickers indenters) are used.[3]

Fracture strength

Strength of materials is defined as the maximum stress that can be endured before fracture occurs. Strength of biomaterials (bioceramics) is an important mechanical property because they are brittle. In brittle materials like bioceramics, cracks easily propagate when the material is subject to tensile loading, unlike compressive loading. A number of methods are available for determining the tensile strength of materials, such as the bending flexural test, the biaxial flexural strength test and the weibull approach. In bioceramics, flaws influence the reliability and strength of the material during implantation and fabrication. There are a number of ways that flaws can be produced in bioceramics such as thermal sintering and heating. The importance is for bioceramics to have high reliability, rather than high strength.

Fracture toughness

Fracture toughness is required to alter the crack propagation in ceramics. It is helpful to evaluate the serviceability, performance and long term clinical success of biomaterials. It is reported that the high fracture toughness material improved clinical performance and reliability as compare to low fracture toughness.[4] It can be measured by many methods e.g. indentation fracture, indentation strength, single edge notched beam, single edge pre cracked beam and double cantilever beam.

Fatigue

Fatigue is defined as failure of a material due to repeated/cyclic loading or unloading (tensile or compressive stresses). It is also an important parameter for biomaterial because cyclic load is applied during their serving life. In this cyclic loading condition, micro crack/flaws may be generated at the interface of the matrix and the filler. This micro crack can initiate permanent plastic deformation which results in large crack propagation or failure. During the cyclic load several factor also contribute to microcrack generation such as frictional sliding of the mating surface, progressive wear, residual stresses at grain boundaries, stress due to shear.[3]

Table 1: Summary of mechanical properties of cortical bone and biomaterial

MaterialTensile strength (MPa)Compressive strength (MPa)Elastic modulus (GPa)Fracture toughness (MPa. m-1/2)
Bioglass42[5]500[5]35[6]2[6]
Cortical Bone50-151[5]100-230[7]7-30[6]2-12[6]
Titanium345[8]250-600[9]102.7[8]58-66[8]
Stainless steel465-950[1]1000[9]200[5]55-95[9]
Ti-Alloys596-1100[8]450-1850[9]55-114[8]40-92[8]
Alumina270-500[9]3000-5000[9]380-410[6]5-6[6]
Hydroxyapatites40-300[9]500-1000[7]80-120[6]0.6-1[6]

See also

References
  1. Katti, K. S. (2004). Biomaterials in total joint replacement. Colloids and Surfaces B: Biointerfaces, 39(3), 133-142.
  2. Wang, R. Z., Cui, F. Z., Lu, H. B., Wen, H. B., Ma, C. L., & Li, H. D. (1995). Synthesis of nanophase hydroxyapatite/collagen composite. Journal of materials science letters, 14(7), 490-492.
  3. Kokubo, T. (Ed.). (2008). Bioceramics and their clinical applications. Woodhead Pub. and Maney Pub.
  4. Fischer, H., & Marx, R. (2002). Fracture toughness of dental ceramics: comparison of bending and indentation method. Dental Materials, 18(1), 12-19.
  5. Chen, Q., Zhu, C., & Thouas, G. A. (2012). Progress and challenges in biomaterials used for bone tissue engineering: bioactive glasses and elastomeric composites. Progress in Biomaterials, 1(1), 1-22.
  6. Amaral, M., Lopes, M. A., Silva, R. F., & Santos, J. D. (2002). Densification route and mechanical properties of Si 3 N 4–bioglass biocomposites. Biomaterials, 23(3), 857-862.
  7. Kokubo, T., Kim, H. M., & Kawashita, M. (2003). Novel bioactive materials with different mechanical properties. Biomaterials, 24(13), 2161-2175.
  8. Niinomi, M. (1998). Mechanical properties of biomedical titanium alloys.Materials Science and Engineering: A, 243(1), 231-236.
  9. "Archived copy". Archived from the original on 2013-10-30. Retrieved 2013-05-03.CS1 maint: archived copy as title (link)

Further reading
  • Bhatia, S. K. (2010). Biomaterials for clinical applications. Springer.
  • Hench, L. L. (1993). An introduction to bioceramics (Vol. 1). World Scientific.
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