BIO

The Alemnis Standard Assembly (ASA) is perfectly adapted to the testing of biomaterials in a range of environments, both in-situ and ex-situ. Some examples include bone micropillars (made from FIB milling) tested inside the SEM, submerged biomaterials tested ex-situ in a liquid cell and analysis of biocompatible coatings for dental applications.

The Liquid Cell (LIC) can be used for a wide range of experiments where the sample needs to be submerged in liquid. It is therefore typically used ex-situ with the Alemnis Standard Assembly (ASA) mounted vertically with the LIC mounted on the load cell. Some examples of how the LIC can be used are:

  • Submerging biomaterials in saline solution or Body-Mimicking Fluid (BMF) to evaluate their mechanical properties when hydrated
  • Electro-chemical tests where the indenter may be used as a current probe and the electrolyte (liquid) may be activated using an external potentiostat for corrosion or passivation studies
  • Studies of hydrogel meso-porous effects, i.e. how the mechanical properties change as a function of the level of hydration.
  • Tribological studies of interaction between the indenter and the sample when submerged in liquid. Often used for simulating prosthetic joints.

Vertically mounted ASA with liquid cell (LIC) for making tests in liquid

The Relative Humidity Module (RHM) is an environmental sample chamber to allow mechanical tests to be performed under conditions of controlled humidity, temperature, or in liquid (sample fully submerged). The typical configuration of the ASA is vertically mounted with the indenter above the sample, but other configurations are also possible depending on options installed.

The relative humidity (RH) can be controlled over the range 5 – 95% with an accuracy of 1.5%. The temperature range is from room temperature to 70°C with an accuracy 0.1°C.

The environmental sample chamber consists of:

– the environmental chamber with all mounts;

– all cables, connectors, pipes;

– controller for relative humidity and temperature closed loop control.

For fully submerged testing without RH and temperature control, the Liquid Cell (LIC) can be used.

The Bio-indenter is an ex-situ configuration of the Alemnis Standard Assembly (ASA) which is specifically designed for characterizing the mechanical properties of tissues and soft materials. It can be configured with the Relative Humidity Module (RHM) and the Miniaturized Load Cell (MLC) to provide a complete testing platform for a wide variety of sample configurations (micropillars, particles, gels, tensile testing, etc..) in a range of environments (controlled relative humidity and temperature, submerged in liquid, etc..). The basic specifications are as follows:

The Bio-indenter configuration can also be coupled with an optical microscope via a piezo-actuated XY displacement stage. The optical microscope has the following specifications:

  • Objective with zoom (0.77 – 9.31x)
  • Working distance 86mm
  • Color camera 1280 x 1024 pixel
  • Coaxial and annular illumination

The Bio-indenter has proved itself for many ground-breaking scientific studies. Here are some examples:

 

Tensile Testing at the Microscale

Focused ion beam (FIB) was used to produce microtensile specimens to mimic ASTM 638 tensile specimen stress conditions. Single crystal Si and GaAs specimens were tested to validate the tensile setup.

Casari et al., European FIB Network, Grenoble, 2018:

https://www.dora.lib4ri.ch/empa/islandora/object/empa%3A17607/datastream/PDF/Casari-2018-Design_and_validation_of_a-%28published_version%29.pdf

Tensile properties of bone extracellular matrix were evaluated in both axial and transverse orientations on the length scale of a single lamellae. Tensile test specimens were fabricated by focused ion beam (FIB) milling of ovine osteonal bone and two distinct failure modes were identified from in-situ observation and post-failure high resolution scanning electron micrographs.

Casari et al, World Congress of Biomechanics, Dublin, 2018:

https://www.dora.lib4ri.ch/empa/islandora/object/empa%3A17605/datastream/PDF/Casari-2018-Tensile_properties_of_bone_extracellular-%28published_version%29.pdf

 

Low temperature (cryo) testing of nanocrystalline PdAu:

Development of the Low Temperature Module (LTM-CRYO) add link to LTM-CRYO page! to investigate deformation mechanisms by strain rate jump micropillar compression experiments at temperatures down to 125 K with hardness mapped as a function of temperature.

Schwiedrzik et al., ECI Nanomechanical Testing, Dubrovnik, 2017:

https://www.dora.lib4ri.ch/empa/islandora/object/empa%3A14860/datastream/PDF/Schwiedrzik-2017-In_situ_micromechanical_testing_inside-%28published_version%29.pdf

A good example of how the Bio-indenter can be used is the example of Ref. 4 which describes micropillar compression of wood (Norway Spruce) samples made by FIB milling. The hierarchical architecture of wood makes it an interesting candidate for structure-property investigation over many length scales. The stem of softwoods features a characteristic pattern of early and latewood forming the so-called growth rings which consist mainly of tracheid cells which are rectangular or circular hollow tubes aligned with the length axis of the stem. In the tracheid cell wall (cw), semi-crystalline cellulose (amorphous cellulose (ac) and crystalline cellulose (cc)) microfibrils are embedded in a polymeric network of hemicellulose (hc) and lignin (li) and aligned parallel to each other similar to the situation in a unidirectionally fibre-reinforced composite. The fibrils are inclined at an angle to the cell axis called the microfibril angle (MFA) leading to a helical structure.

Hierarchical structure of wood from the macro to the nanoscale consisting of amorphous cellulose and crystalline cellulose, hemicellulose and lignin (from Ref. 4)

Although nanoindentation has been used to study wood properties, the interpretation of the data remains problematic owing to the non-homogeneous structure and the local inelastic deformation under the indenter. Therefore, micropillar compression experiments with well-defined boundary conditions and a homogeneous stress state provide a more straightforward interpretation of elastic behavior in wood at the cell wall level and below. In combination with X-ray diffraction and wet chemical analysis, the micropillar compression technique has allowed differences in mechanical behavior between two types of wood tissue (normal and compression) to be explained and the shear yield stress of the polymer matrix and lignin to be measured.

Typical example of micropillar produced by FIB-milling of Norway spruce wood with corresponding stress-strain curve. Scale bar represents 1 µm (From Ref. 4)

Selected References

  1. Schwiedrzik J, Raghavan R, Bürki A, LeNader V, Wolfram U, Michler J, et al. In situ micropillar compression reveals superior strength and ductility but an absence of damage in lamellar bone, Nature Materials 13 (2014) 740-747
  2. Lunt AJ, Mohanty G, Ying S, Dluhoš J, Sui T, Neo TK, et al. A comparative transmission electron microscopy, energy dispersive x-ray spectroscopy and spatially resolved micropillar compression study of the yttria partially stabilised zirconia-porcelain interface in dental prosthesis, Thin Solid Films 596 (2015) 222-232
  3. Lunt AJ, Mohanty G, Neo TK, Michler J, Korsunsky AM. Microscale resolution fracture toughness profiling at the zirconia-porcelain interface in dental prostheses, SPIE Micro+ Nano Materials, Devices, and Applications: International Society for Optics and Photonics; 2015. p. 96685S-S-11.
  4. Schwiedrzik J, Raghavan R, Rüggeberg M, Hansen S, Wehrs J, Adusumalli RB, et al. Identification of polymer matrix yield stress in the wood cell wall based on micropillar compression and micromechanical modelling, Philosophical Magazine 96 (2016) 3461-3478
  5. Groetsch, A. Gourrier, J. Schwiedrzik, M. Sztucki, J. Shephard, J. Michler, P. K. Zysset, U. Wolfram, Simultaneous micropillar compression and X-ray scattering or diffraction to investigate scale effects of strains in mineralised collagen fibres, Paper presented at 12th International Conference on Advances in Experimental Mechanics, Sheffield, United Kingdom (2017)
  6. Ferrand, H.L., F. Bouville, and A.R. Studart, Processing of dense bio-inspired ceramics with deliberate microtexture, arXiv preprint arXiv:1807.04378 (2018)