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Nanoindentation of Soft materials and Biological Tissues

Marc Farine

office: CLA G 34
phone: +41 44 632 3977
email: farine@imes.mavt.ethz.ch

I. Introduction
Nanoindentation, also known as instrumented indentation, is established as a useful technique for highly localized mechanical characterization of hard materials. It consists of poking samples of choice with tips of known shape made of very stiff materials, such as diamond, monitoring in real time both the tip penetration depth, typically in the range of a few nanometers, and sample reaction forces, typically up to few millinewton. Mechanical
characteristics related to samples are then extracted from force-displacement (P-h) curves, e.g. using Finite Element (FE) modeling.
Over the last decade, a growing interest has formed around the characterization of softer materials such as polymers and biological samples at the microscale with the help of nanoindentation machine.


II. Setup for nanoindentation of soft materials
In the course of the current project, a commercially available Nanoindenter tester is used to address challenges related to the characterization of very soft materials. The improved measurement system providing small force sensing possesses long indenters and a liquid cell allowing indentation of samples fully immersed in water.
The measurement procedure consists of 2 steps. It starts by a pre-approach phase involving movement of the indenter tip towards sample surface and landing of the reference on the sample holder. As a second step, the indenter is driven down towards the specimen surface at a constant speed, recording measurements data (both force and displacement) until reaching a specific contact force. Then, a linear ramp load is applied to the specimen until reaching a specific deformation.

III. Challenges related to Roughness & Capillarity effects
In the case of very soft materials nanoindentation, the forces sensed by the nanoindenter machine are very small such that surface interaction forces become prominent, having a strong impact on experimental data analysis. The importance of the contact interface can be demonstrated with measurements at different humidity conditions and surface topography.

IV. Hyperelastic, Viscoelastic, and Poroelastic material characterization through FE modeling
Non-linear inverse FE analysis has been used to extract mechanical properties from nanoindentation measurements on soft materials. The mechanical models (e.g. hyperelastic-viscoelastic) obtained can be validated when used to predict the behavior of materials undergoing mechanical testing in different configurations (e.g. uniaxial tensile, creep, relaxation tests, inflation tests).


Video of CSEM Bioindenter (external youtube link)

CSM Instruments, Peseux, Switzerland
CSEM, Neuchâtel, Switzerland
Dr. Michelle Oyen, Cambridge University, UK


05/19/16 | Kevin Bircher | ZfM | ETH