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Material mechanical testing is used to characterize and compare different materials. Materials can be characterized as rubbery or hard, for example. The tensile test is a common testing procedure used to provide data for such classifications. Data can be used to form a stress-strain curve which is a basic but very useful tool in the analysis of materials. The discussion below focuses on special considerations needed for tensile testing biological soft tissue (e.g., ligament and tendon) compared to traditional engineering materials (e.g., aluminum and steel).
Traditional Engineering Materials
Common assumptions used for testing and analysis of traditional materials are that they are homogenous, exhibit small deformations and are linearly elastic. These assumptions shape the way in which the materials is tested. Because these materials are homogenous, there is no ‘with the grain’ or ‘against the grain’ manner in which a testing sample must be positioned in the testing device. The exhibition of small deformations allows for the use of displacement transducers with relatively small ranges (often inexpensive and readily available). Young’s modulus (E), the slope of the elastic portion of the stress-strain curve, is a quantity often used to asses a material’s stiffness. The linear elastic assumption makes the determination of E relatively straightforward as it can be assessed anywhere along the initial linear portion of the curve (Fig. 1). The American Society for Testing and Materials (ASTM) has guidelines for test methods of the tensile test for metals in standard ASTM E 8 .
Figure 1. Stress-strain curve of traditional engineering materials
Biological Soft Tissue
The assumptions made for the traditional materials are not valid when testing biological soft tissue samples. These materials are, in general, non-homogenous due to the orientation of their collagen and elastin fibers. Hence, care must be taken when collecting samples and positioning them in testing systems to ensure the directions are consistent with the intended analysis. Biological tissues exhibit large deformation before failure, therefore any transducer used to measure strain will need to accommodate the large movement. Due to the un-crimping of collagen fibers and elasticity of elastin, the initial portion of a biological sample stress-strain curve has a high deformation/low force characteristic known as the toe region. In short, unlike traditional materials this region is non-linear. A linear region is typically identified after the toe region and is used for the determination of E (Fig. 2).
Figure 2. Stress-strain curve of biological tissue
Measurement of force and strain in biological tissue can be accomplished in various ways. In vivo, devices such as buckle transducers , implantable force transducer  and pressure transducers can be used to measure force or pressure. Devices such as commercially available differential variable reluctance transducers (DVRTs) have been used to measure strain . In vitro, force can be measured with standard load cells; however, many groups have developed creative techniques to measure strain. The liquid metal strain gage , photoelastic film  and optical techniques [7, 8] are some examples. The reader is referred to reviews by Fleming and Beynnon  as well as Ravary et al.  for more in-depth discussion of this topic.
Aside from transducers, other considerations must be made when testing biological tissue in vitro. First, in order to study the tissue’s response in vitro, the in vivo environment must be recreated. This typically entails bathing the sample in some type of fluid (e.g., phosphate buffered saline (PBS), culture media, synovial fluid, etc.) at the proper temperature (e.g., 37.5°C) and pH (e.g. 7.4). Second, the mechanical properties of biological tissues are strain or loading rate dependent due to these tissues’ viscoelastic nature. For example, biological tissues typically become stiffer with increasing strain rate (Fig. 3A). As such, predetermined and tightly controlled strain or loading rates must be maintained during testing. Further, many biological tissues in their normal state are preconditioned (e.g., the anterior cruciate ligament) while many are not (e.g., the brain). For a tissue that has not been preconditioned, its response to load or displacement from one cycle to the next will not be similar hence results will not be consistent (Fig. 3B). Depending on the type of tissue being tested or the response of interest (e.g., sudden impact or fatigue failure), preconditioning as part of the testing protocol may or may not be necessary. The type of grips used to securely interface the tissue to the tensile testing machine is another consideration. Interdigitating wave profile, liquid nitrogen-cooled, pneumatic, wedge-action and, in the case where attached bone is present, potting in polymethyl methacrylate (PMMA) are some common ways to connect samples to the testing machine.
Figure 3A. Strain rate dependence
Figure 3B. Preconditioning
Use of Data to Assess Material Performance
Mechanical testing can be used to assess the suitability of replacement grafts. Noyes et al.  subjected various grafts commonly used for anterior cruciate ligament (ACL) reconstruction to tensile loads. These tests provided data to develop stress-strain curves for the graft tissues, from which various mechanical comparators could be computed. The comparators served as a basis of judgment, from a mechanical standpoint, of the suitability of graft performance as an ACL replacement. Similarly, Sauren et al.  sought to investigate the mechanical properties of different parts of porcine aortic valves (the leaflets, the sinus wall and the aortic wall) in different directions. Utilizing the uniaxial tensile and stress-relaxation tests, this group characterized both mechanical and viscoelastic properties of the replacement valves. Comparisons to previously published  in vivo human data were then made.
When conducting mechanical testing it is important to be aware of differences between testing of biological materials vs. traditional engineering materials. By conducting the testing carefully, bioengineers can effectively characterize and compare natural biological materials or materials intended for in vivo replacement.