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Technical University of Munich: biaxial strain measurement of the aortal wall

As part of its research activities the Department of Computational Mechanics at the Technical University of Munich (TUM) is carrying out material modelling of biological tissues under high strain.

This includes developing models of the human vascular system which are of core importance for research on vascular diseases such as abdominal aortic aneurysms. The TUM uses a ZwickRoell biaxial testing machine for this work.

The tensile testing machine previously used at the TUM for testing lung specimens was unsuitable for tests on the aortal wall as far as travel range and tensile force were concerned. In addition, only uniaxial measurements on lung specimens were possible. A biaxial testing machine was therefore obtained from Messphysik for tests on the aortal tissues. This testing machine is specially designed for biaxial tensile tests on natural and artificial elastic tissues, enabling anisotropic properties of these tissues to be determined also. This is an important factor in being able to reach a precise conclusion with regard to an operation as the mechanical properties of the aortal wall vary according to the alignment of the fibers of other types of tissue.

Four linear drives, controlled independently of each other, are incorporated in the ZwickRoell bi-axial testing machine. Force measurement is via (waterproof) load cells, two each in the X and Y directions. In addition there are four testControl measurement and control electronics units plus a bath for optimum temperature control of the medium. The maximum test load is 100 N per load cell, with resolution up to 0.6 mN. Travel range for tensile measurement is 50 mm (resolution 0.1 μm) and maximum speed is 2,000 mm/min. The specimen is mounted away from the test device using the fixtures supplied with the machine and is then inserted into the measuring system ready assembled.

An additional advantage offered by the new testing machine is the laser speckle extensometer laserXtens, which enables non-contact two-dimensional strain and deformation measurement without specimen marking. The measuring principle is based on evaluation of speckle patterns formed on the specimen surface by illumination with a laser; these are reflected and recorded with a camera. The functions of laserXtens  are fully integrated into ZwickRoell’s testXpert software system, which features intuitive operation and powerful control functions.

A short-term objective of the tests on the abdominal aortal wall described is the correlation of biological metabolic activity, which can be measured by means of clinical imaging, with the mechanical properties of the arterial wall. This would make it ultimately possible to reach conclusions regarding local mechanical wall properties using the PET/CT scan without previously having to remove tissue samples surgically. This non-invasive method of forecasting wall properties is an important step on the road to optimizing medical treatment methods for individual patients.