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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 7

composed of smooth muscle, such as

the bladder, will exhibit a high level of

stress relaxation.

Fatigue testing,

also known as

dynamic testing, helps to character-

ize the lifetime properties of materials

that experience constant mechanical

loading. For example, a human heart

beats approximately 35 million times

per year. In order to ensure that an

engineered tissue material used in the

heart will last a minimum of 10 years,

the tissue needs to be subjected to at

least 350 million cycles of mechanical

loading. In order to complete this test-

ing in a reasonable amount of time, the

testing machine needs to run at a high

frequency. Using a dynamic testing

machine at 100 Hz, a researcher could

simulate 10 years of wear on a mate-

rial in just a few weeks. Fatigue testing

of biomaterials helps ensure that the

material will not yield, become dam-

aged, or fail within the required lifetime.

TESTING PARAMETERS

While mechanical testing of tis-

sues can range from a quick pull or

failure test to a months-long fatigue

test, all testing methods are critical to

understanding the bulk mechanical

properties of biomaterials and predict-

ing how those materials will behave

in the human body. In addition to the

variability inherent to biological mate-

rials, experimental setup for biomate-

rials testing also presents a challenge.

Testing biological materials can be

challenging due to the need for highly

sensitive force measurement, delicate

specimens that are difficult to grip, and

strainmeasurement that is often impos-

sible with traditional extensometers.

Highly sensitive force measure-

ment is needed for mechanical testing

of soft biomaterials. Soft tissue spec-

imens are usually small and the loads

required to pull these materials apart

are generally in the gram-force range.

With biological materials that require

pre-cycling before pull to failure, it

is important to ensure that the force

transducer is not only verified for accu-

racy in the load range for pre-cycling,

but also has the capacity to measure

force at specimen failure.

Determining the proper gripping

solution for testing biomaterials is

crucial to achieving consistent results.

The delicate nature of soft biomaterials

in combination with testing in physi-

ological conditions makes gripping a

challenge. Testing tissues in physio-

logical conditions to properly mimic

the environment inside the human

body is relatively easy to achieve using

a heated bath. However, consistently

inserting a specimen in a set of grips

while submerged in a bath can be diffi-

cult. To save time and make sure spec-

imen alignment is repeatable, using a

bath with a lifting mechanism is rec-

ommended so that the specimen can

be inserted into the grips and then sub-

mersed as the bath raises into place.

When choosing a gripping mechanism

for soft tissues, side-acting grips that

are manual, spring actuated, or pneu-

matically closed generally work well.

In addition, the gripping face should

feature a material with high friction to

avoid specimen slippage while mini-

mizing the necessary clamping force.

When tensile testing soft tissues

to failure to determine maximum force

or maximum strain at break, calculat-

ing strain from the machine’s extension

is widely accepted as suitable. This

method of measuring strain is calcu-

lated by taking the machine’s cross-

head extension data and dividing it by

the initial grip separation. However,

Flexural testing of rat bone.

Tensile testing of an insect wing.