Important Considerations for Soft Tissue Testing

0 0

Soft tissue test equipment is specialized in several respects. Numerous factors such as environmental conditions, mechanical strength characteristics of the samples, elastic and stress-strain characteristics of the samples, and grip choices should all be considered when planning soft tissue tests.

Compared to testing bone, tissues are generally soft and can undergo large deformations without failing. Non-load bearing soft tissues can benefit from mechanical characterization tests, including lung, kidney, liver, intestines, esophagus and stomach. Tissue test samples need to be kept moist and at 37 degrees Celsius.

Which mechanical properties are important?

Elastic properties are important to determine how the specimen material reacts under pushing, pulling and stretching conditions inside the body. Elastic properties include testing for large but finite deformations, nonlinear stress-strain properties, and anisotropic multiaxial material properties. Anelastic properties include stress relaxation, creep, hysteresis, strain rate dependence, preconditioning behaviors, and viscoelasticity. In fact, stress-strain curves of soft tissues are generally nonlinear. Soft tissues are considered to be complex structures with finite deformation characteristics, exhibiting nonlinear anisotrophic behaviors. Key viscoelastic properties include creep, stress relaxation and hysteresis.

Tissue replacement materials should always be tested in true working environments. Biological test samples can be small and delicate with low strength characteristics, so it is especially important to configure the system using a high resolution test controller outfitted with low force load cells and precision displacement or strain sensors.

Soft tissue test machines

We offer a variety of specialized soft tissue test equipment. The 840 Dynamic Fatigue Biomedical Materials Tester, which is a part of a family of our electrodynamic machines, is a popular choice for our customers. This machine works well for soft tissue as well as bone, performing tests from sub gram range (0.01 grams) to larger orthopedic solutions up to 2.5 kN (500 lb).

Another great choice for testing tissues include planar biaxial test machines. The 574 Family Planar Biaxial Electrodynamic Fatigue Test Machine consists of four individually controlled servoactuators programmed to create synchronized yet independent control load and strain, meaning the machine can independently pull a tissue sample in four different directions in synchronized motions. It generates high precision states of stress along two axis (XY plane) in a biaxial mode. With the use of independent actuators comes high resolution and testing flexibility in evaluating soft tissue materials. The actuator force rating, type and quantity are configured to application requirements. The same system can be also be used to perform single axis tests using two of the four actuators. With our modular approach, it is possible to convert a single channel system into four channels and also to create four single station test machines out of a single 4 channel planar biaxial system. The 574 is perfect for research testing environments when your requirements may transform over time. 

Testing grips and accessories for tissue

Our test machines are designed to fit a variety of testing grips and accessories. Customers testing tissues often require adding a biomedical bath to their test machines. Modular physiological baths produce temperature-controlled saline environments that replicate in-vivo conditions. Baths are commonly used to ensure the biological samples mechanical properties are maintained or that replacement materials are tested in true working environments. Our grips and fixtures can be configured to fit inside a bath. 

For tissue testing, the mechanical grips and test fixture design requirements are wide and varied because they are specially engineered to match each test sample type, size and anticipated mechanical response. Tissue-engineered grips are configured to match sample characteristics by blending different grip design features including line contact jaws, roughened jaw surfaces, interlocking wave jaws, adhesively bonded specimen tabs, staples and even sample stitched ends. 

TestResources offers a unique gripping solution for our planar biaxial test machines. For very small tissue samples we can use hypodermic needles which slide through slots in the grips and into the sample. This technique provides axial compliance to accommodate biaxial strain in a planar biaxial system. This solution also speeds up the testing process which can be essential when testing numerous samples.

Another tissue testing solution is wave jaws, which are better for testing tissues rather than traditional flat faced jaws because they have an increased surface area. Their interlocking radii help hold the tissue tight to reduce high stress concentration points that damage or tear the sample. Pneumatic grips are optimal due their active clamping that reduces the Poisson's ratio of the sample. In other words, as the sample is pulled, it stretches out so that active clamping is necessary to avoid slippage. We often recommend the G229 Pneumatic grips with wave jaws for such cases.

In cases with especially difficult to hold materials we can offer cryogenic wave jaws that utilize either liquid nitrogen or a chiller. These specially designed jaws will freeze the tissue sample in the grip area allowing it to be held in place without freezing the entire sample. The G149 Pneumatic Grips work well with the cryogenic wave jaw option. Most of our grips can be made with stainless steel for use in saline solution to mock physiological conditions.

Our applications engineers have advanced experience in soft tissue testing and are directly available by phone to help configure the best system to meet your testing needs.