For the mechanical characterization of the collagen-mineral composite of bone and other mineralized tissues, a combination of an atomic force microscope (AFM) and an add-on nanoindentation device consisting of a three-plate capacitor is used in cooperation with the Max Planck Institute of Colloids and Interfaces (www.mpikg.mpg.de), Potsdam, Germany.
When voltage is applied to the outer plates of the AFM, the electrostatic force on the center plate drives the indenter into the sample and the changes of the electrostatic capacity due to the displacement of the center plate is a measure for the penetration of the indenter (a three side pyramidal tip) into the sample. The indentations can then be viewed by using the same tip in the AFM allowing high spatial accuracy. The loading-unloading cycles consist of five linear segments and the elastic modulus and hardness are obtained from the first linear region of unloading.
For further information see Tesch et al., Calcif Tissue Int 69;2001, Gupta et al., J Struct Biol 194;2005.