Biomedical Engineering Reference
In-Depth Information
models include the Poisson's ratio as a determinant of this change. Samples are compressed using
solid platens, with nothing in the radial direction except fluid, and the stress and/or strain response
over time is collected.
5.2.1.2 TensileTesting
The tensile properties of a sample can be determined from both equilibrium stress-strain measure-
ments, as well as time-varying data (i.e., creep and stress relaxation) [
644
]. As with compression data,
the Young's modulus can be determined from the linear region of the equilibrium stress-strain curve.
The sample should be fixed firmly at the grips such that failure occurs within the working length (i.e.,
near the center of the sample). Specimen lengths should be significantly greater than their widths to
ensure uniform strain through the working length. Extensometers or optical techniques are used to
monitor the strain in the region of interest, which is plotted alongside the applied stress for analysis.
5.2.1.3 Shear Testing
Typically, shear tests are conducted on cylindrical samples in a setup similar to unconfined compres-
sion tests. A flat platen is placed on the sample, and a small tare load is applied to assure uniform
contact. Shear tests can use either rotational [
520
,
645
] or translational [
70
,
646
] displacement
strategies. As with the compressive and tensile properties, it is important to characterize both the
equilibrium and dynamic responses of the sample under shear. The equilibrium shear modulus, G,
is calculated from the linear region of the stress-strain curve. The dynamic complex shear modulus,
G*, is calculated using the applied and signal response to a series of oscillatory stimuli.
5.2.1.4 FrictionTesting
While many theories exist describing how cartilage exhibits the frictional properties it does, most
testing approaches focus on quantifying the forces present as two surfaces slide across one another.
Biotribology studies of articular cartilage have focused on lubrication mechanisms at the whole joint
level [
647
] as well as at the cartilage tissue level [
648
]. A variety of experimental configurations
have been used, including pendulums [
30
,
649
], oscillating arthrotripsometers [
79
,
650
,
651
], atomic
force microscopy [
77
,
652
], and plug-on-plate configurations. The latter technique is currently the
most common approach. This method involves moving a sample translationally or rotationally with
respect to a fixed surface or plate. Normal and frictional forces are measured, allowing calculation of
the coefficient of friction. Frictional properties are sensitive to variations in bathing solution, sliding
rate, and fluid pressurization within the tissue [
78
,
81
].
5.2.1.5 FatigueTesting
The durability of cartilage or a tissue engineered construct is perhaps the most important pa-
rameter associated with its overall functionality. Unlike the previous mechanical tests which are
non-destructive, fatigue testing applies repeated loading until the sample fails [
70
,
653
]. Usually a
specific type of loading is focused on, such as compression, tension, or shear, and repeated cycles
are applied until the sample is noticeably affected (cracks, fissures, tears, etc.). Fatigue life is defined
as the number of cycles necessary till failure, which can depend on the applied stress, strain, and
frequency.
