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A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 6
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iological materials, both natural
and engineered, are inherently
multifaceted. As such, the bio-
materials that make up one individual
will not have the exact same mechan-
ical properties as those that make up
another. Given the natural variability in
biological tissues—such as bone, ten-
dons, ligaments, and hair—achieving
consistent and repeatable metrics to
effectively characterize the mechani-
cal properties of a biomaterial may be
challenging. In the physical materials
testing industry, biomaterials can gen-
erally be broken down into two catego-
ries: soft and hard tissues. Mechanical
characterization of both natural and
engineered biomaterials is usually
achieved using a combination of both
static and fatigue testing.
Biological materials are
visco-
elastic
because they feature both vis-
cous and elastic properties. Viscosity
is the measure of a fluid’s resistance
to flow, while elasticity is the tendency
of a material to return to its original
state after undergoing deformation. In
mechanical terms, elasticity is modeled
using a spring and viscosity is mod-
eled using a dashpot, which resists
motion via friction. Viscoelastic mate-
rials exhibit time-dependent proper-
ties and thus exhibit both creep and
stress-relaxation.
MECHANICAL TESTING
TECHNIQUES
Static mechanical testing
gener-
ally refers to monotonic compression,
tensile, and flexural testing. However, it
also encompasses simple cyclic testing,
creep, and stress-relaxation testing,
which can help properly characterize
the viscoelastic properties of biologi-
cal materials. When testing biomaterial
strength, simple cyclic testing is often
conducted at the beginning of the test
and is referred to as pre-cycling. Pre-cy-
cling soft tissues before failure helps
align fibers to condition the material.
Creep testing
is a type of static test
that involves holding a specimen in ten-
sion under a constant load. In a purely
elastic material, an applied load in ten-
sion or compression will create some
displacement in a material that will
not change over time. For example, if a
weight were applied to a spring and held
constant over time, the initial exten-
sion of the spring would not increase or
decrease no matter how long the weight
is kept on the spring. This behavior is
expected in purely elastic materials.
However, in viscoelastic materials such
as a tendon containing mostly collagen
fibers, under constant load, a tendon’s
strain or material extension will increase
over time. Materials that exhibit creep
will undergo plastic deformation under
constant load.
Stress-relaxation
is a static test
that involves holding a specimen in
tension at a constant strain or displace-
ment. For example, if a purely elastic
spring is pulled to a displacement, the
resulting force or stress on the spring
would remain constant over time. In
materials that exhibit stress-relaxation,
stress will decrease or relax in response
to the same amount of strain over
time. Biological materials primarily
TECHNICAL SPOTLIGHT
BIOMATERIALS TESTINGAND
CHARACTERIZATION
As scientists and engineers continue to develop and investigate replacement
tissues for patient disease, injury, and aging, proper mechanical
characterization of biological materials is critical.
Tensile testing of biological tissues.