Indentation tests of biological materials: Theoretical aspects

The term ‘biological material’ includes many meanings, and here it means materials that constitute living organisms. The variety of material parts of living organisms is huge. They may be hard and soft, elastic and viscoelastic, quite often sizes of constitutive parts are within micro or nano scales and they can be considered as structured biocomposite materials. The traditional methods of materials testing are not applicable to evaluating mechanical properties of materials of very small volumes. It is also very difficult to apply traditional approaches for characterization of very soft materials. Therefore, indentation techniques are widely used to estimate mechanical properties of biological materials. In this review paper, we briefly discuss some results related to mechanics of contact between an indenter and a deformable sample. Then we critically examine the common approaches to interpretation of indentation experimental data. Finally we discuss the results of indentation tests of biomaterials having rather different properties: bones, snake skins, and cartilages, along with resilin and elastin-based materials. We argue that also depth-sensing indentation is a valuable tool for studying mechanical properties of biomaterials, one should be aware that the theoretical models used for justification of modern nanoindentation tests are based on non-adhesive contact, while the influence of adhesive interactions increases as the scale of samples goes down to micro and nanoscales