Planar Biaxial testing of soft tissues is often required to fully characterize the inherent anisotropic properties of the tissue or to set-up biaxial stress-strain states to provide more accurate in vivo simulation. With uniaxial testing, fibers may realign along the test axis, altering the mechanical properties of the tissue. In addition, constitutive models cannot be developed based on uniaxial testing alone. The gripping technique must be capable of securely holding soft tissues, without causing damage, and lateral deformations must be unrestricted in order to ensure homogeneous specimen deformation in the gauge area under biaxial loading. In addition to this, strain measurement must not damage the tissue or cause stress concentrations and be able to account for strain in all directions of loading.
To meet these challenges we used a low-force planar biaxial soft tissue system that was developed to perform mechanical testing and property analysis of soft planar biomaterials, native tissues, and tissue-engineered scaffolds. The configurable system for soft tissue mechanical testing consists of four fatigue-rated actuators mounted to an air-suspended isolation table. The biaxial testing system is capable of running both uniaxial and biaxial tests to offer ultimate flexibility for soft tissue testing. This system uses our 8800 digital controller that is capable of providing true planar-biaxial control, giving both translation and deformation control in both axes. This aids in the ability for highly accurate specimen center-point control, allowing the use of optical instruments mounted above the specimen. The exceptional resolution of the controller ensures accuracy when measuring the extremely low loads associated with many soft tissues.
A simple gripping method based on sutures and pulleys was used to distribute the loading forces equally around the specimen, allowing simultaneous testing along the x and y axes. The entire system is normally configured with a temperature-controlled environmental bath for simulation of physiological conditions during biaxial mechanical testing.