Derby Group

Nanoindentation of Biological Tissues and Hydrogels

Dr Michelle Oyen (Cambridge University) recently visited us at the School of Materials on Friday 13th February 2009, to give a seminar. Her presentation, was entitled ‘Nanoindentation and Microindentation Studies of Biological Tissues and Hydrogels”. We are grateful to her for accepting our invitation and for giving an excellent talk which fits well with the groups interests.

The abstract of her talk is below:

‘Interest in the mechanical behaviour of compliant biological tissues and is growing, and the emphasis on applications necessitating mechanically-robust implant materials motivates further study in this field. Nanoindentation has emerged as a leading technique for mechanical characterization; contact-based techniques are particularly attractive for materials that are difficult to test in tension, including soft biological tissues and hydrogels. However, commercial nanoindenter instrumentation is optimized for stiff, hard materials and therefore testing softer materials can be challenging. In the current study, mechanical testing by nanoindentation and larger scale microindentation were performed on a series of materials including polymeric validation standards, hydrogels of different gel composition and concentration, and hydrated biological tissues including bone and cartilage. Most tests were conducted under displacement control with a spherical indenter tip and using a load-relaxation test method to assess time-dependent deformation; creep characterization was considered when working in load control. Data analysis included a variety of different approaches, including a purely elastic analysis, a linear viscoelastic analysis and a computational analysis based on flow and poroelasticity. Quantitative results compare well for nanoindentation and microindentation, with mean elastic modulus values in the range of 100 kPa to 1-2 MPa for hydrogels and cartilage, respectively. Effects of both air dehydration and ethanol immersion are considered, and while these treatments both change the material elastic modulus by several orders of magnitude, the “dry” materials are found to exhibit profoundly different timedependent behavior depending on the dehydration method. The effects of finite layer thickness were considered in microindentation tests, and the result of faster equilibration for thinner gels was consistent with finite element modelling results for poroelasticity in finite layers. Nanoindentation is found to be a viable technique for mechanical characterization of compliant and hydrated tissues although challenges remain for widespread implementation and routine characterization due to the limitations of commercially available instrumentation.’

Tags: dr michelle oyen, hydrogels, nanoindentation

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