Auckland Bioengineering Institute

Biaxial testing

Tissue sample undergoing a biaxial test.

Biological membranes, such as skin, are typically complex heterogeneous materials, exhibiting complicated mechanical behaviour (anisotropic, viscoelastic, and non-linear stress–strain response, varying with age, body location, and sex). In order to characterize the mechanical properties of such materials, the membrane must be subjected to a rich set of deformations in order to tease out the contributions of the various factors that govern its behavior. To achieve this, we have built a unique biaxial testing device that can subject membranes in vivo to a wide set of controlled deformations, while recording boundary forces and surface strain fields.

The rig is equipped with sixteen actuators, arranged in a circular array, capable of incremental motions of 0.2 µm over a range of 50 mm. Each actuator is equipped with a custom-built 2D force transducer, from which the applied force vectors at each of the sixteen attachment points are recorded. The transducers have an operational range of ±3 N and a SNR of 0.07%. An area scan camera is used to record the geometry and the internal deformations of the samples. The deformations of regions within the geometry are traced mathematically as displacements of subimages (typically 64 pixel × 64 pixel) using a phase corrected cross-correlation technique (Malcolm et al. 2002). This technique is able to resolve displacements as small as 0.05 pixel, giving a resolution for the measured displacements of better than 1 μm.

This device has been used to identify the mechanical behavior of living skin at various sites on the body, across many individuals. The equipment and analysis, however, are applicable to characterizing the mechanical behavior of a wide range of membranous materials.

Researchers and graduate students

Funding partners

  • Skin NERF
  • MSI (TechNZ grant) 


  •  Nielsen, P. M. F., Hunter, P. J. & Smaill, B. H. Biaxial testing of membrane biomaterials: testing equipment and procedures. Journal of Biomechanical Engineering - Transactions of the ASME 113, 295-300 (1991).
  •  Nielsen, P. M. F., Malcolm, D. T. K., Hunter, P. J. & Charette, P. G. Instrumentation and procedures for estimating the constitutive parameters of inhomogeneous elastic membranes. Biomech model mechanobiol 1, 211-218, doi:10.1007/s10237-002-0019-7 (2002).
  •  Malcolm, D. T. K., Nielsen, P. M. F., Hunter, P. J. & Charette, P. G. Strain measurement in biaxially loaded inhomogeneous, anisotropic elastic membranes. Biomech model mechanobiol 1, 197-210, doi:10.1007/s10237-002-0018-8 (2002).
  • Kvistedal, Y. A. & Nielsen, P. M. F. Estimating material parameters of human skin in vivo. Biomech model mechanobiol 8, 1-8, doi:10.1007/s10237-007-0112-z (2009).
  • Jor, J. W. Y., Nash, M. P., Nielsen, P. M. F. & Hunter, P. J. Estimating material parameters of a structurally based constitutive relation for skin mechanics. Biomech model mechanobiol 10, 767-778, doi:10.1007/s10237-010-0272-0 (2011).