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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

2 6


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



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




Static mechanical testing


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.


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




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.