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Viscoelastic Characterization
of Agarose Gel Scaffolds

The Challenge:

To Determine Viscoelastic Behavior of

Low Concentration Agarose Type VII Gel

Background

Agarose hydrogels are rapidly becoming a popular option for

an implantable scaffold material due to their biocompatibility,

native tissue-like viscoelastic mechanical properties and ease

of casting into complex shapes and sizes. Hydrogel implants

have been used in applications where structural integrity is

necessary, such as cartilage and skin. With a key functional

role of cartilage being force dampening, any potential

substitute must be capable of sustaining high impact loading.

This series of experiments uses the ElectroForce

3200 test

instrument to characterize the dynamic mechanical properties

of agarose gels at high frequencies.

These mean amplitudes, as measured by stretch ratio,

ranged from 0.99 to 0.70, where the stretch ratio, λ, is

defined as λ=

/ L

, where

is the loaded height and L

is the unloaded height. The displacement amplitude was

±0.5% of the unloaded height.

Figure 1 - 1% Agarose Modulus at λ = 0.9

The contribution to the complex modulus at lower frequencies

is approximately equal, while the storage modulus dominates

at higher frequencies. This is an indication that less energy

is dissipated at higher frequencies.

While this figure provides insight into the material behavior

at λ = 0.9, there is a wide range of other loading scenarios

this material can experience. Figures 2 and 3 quantify the

material responses at different mean stretch ratios. Tan

delta, an indication of the phase shift between displacement

and load, is shown for the 1% agarose gel. There is a much

larger phase shift at lower frequencies. This effect can be

understood in Figure 1 as the loss modulus does not greatly

contribute to complex modulus at higher frequencies.

Dr. Murakami and Dr. Sawae, lead researchers at the

Kyushu Institute of Technology in Japan, were interested

in investigating the mechanical behavior (dynamic modulus

and tan delta) of low concentration agarose gels at high

frequencies.To accomplish this, the ElectroForce 3200 test

instrument, in combination with WinTest

software, was used

to perform DMA (Dynamic Mechanical Analysis) experiments

at frequencies that ranged from 0.1 Hz to 100 Hz at different

mean displacements.

Meeting the Challenge

1% and 2% (wt.) agarose gels were produced from agarose

type VII powder (Sigma-Aldrich) using a standard gelling

protocol supplied by Dr. Murakami. Test specimens, cut

from the bulk gels, were approximately 18 mm in diameter

and 2.5 mm in height. 50 mm

diameter solid platens were

used to secure the sample.

An ElectroForce 3200 test

instrument equipped with a

1000 g load cell was used in

these experiments. DMA was

set up as a frequency sweep

from 0.1 Hz to 100 Hz at a

range of mean displacements.

Materials and Methods

Results

The DMA software immediately produces usable results

at the end of the experiments. One example of this is the

complex modulus shown in Figure 1. As the frequency is

increased, complex modulus asymptotically increases. In

addition, the software generates the two components:

storage and loss modulus.

Bose Corporation – ElectroForce Systems Group

10250 Valley View Road, Suite 113, Eden Prairie, Minnesota 55344 USA

Email: electroforce@bose.com – Website: www.bose-electroforce.com

Phone: 952-278-3070 – Fax: 952-278-3071

Bose, the Bose logo, ElectroForce and WinTest are registered trademarks of Bose Corporation. 032410

Figure 2 - 1% Agarose, Tan Delta at Different Mean λ

Figure 3 - 1% Agarose, Modulus at Different Mean λ

Figure 4 - 2% Agarose, Tan Delta at Different Mean λ

Figure 5 - 2% Agarose, Modulus at a Different Mean λ

Figure 3, complex modulus at different λ, provides more

insight into the material behavior. It can also be seen that

as the mean stretch ratio is decreased (i.e., the material is

more compressed), the modulus of that material is significantly

increased. Analyzing this graph along with tan delta

information from Figure 2 shows that the material behaves

significantly differently depending upon pre-stretch. As

the stretch ratio is decreased, the contribution by the loss

modulus to the complex modulus is substantially reduced.

Therefore, as the material is initially more compressed, the

behavior resembles a more purely elastic material.

Similar results can be seen for the 2% agarose gel with

different magnitudes. Tan delta for the 2% agarose gel

is less than 1% gels at lower frequencies (Figure 4).

Additionally, the complex modulus of the 2% agarose gel,

shown in Figure 5, is greater than the modulus for the 1%

agarose gel.

Plotting this data in three dimensional space leads to further

understanding of experimental results, as shown in Figure 6.

This figure displays the complex modulus of a 1% agarose

gel as a function of both the frequency and mean stretch

ratio. A surface can be fit to this data using mathematical

models which would lead to predictive capabilities. Similar

graphs can be developed displaying tan delta as a function

of both mean stretch ratio and frequency.

Incorporating tan delta into the model would further

strengthen the predictive capabilities of the model. With

this knowledge of material behavior, scaffold design

parameters and clinical application of agarose gels can be

understood prior to development.

This series of experiments demonstrates that the

ElectroForce

3200 test instrument is capable of operating

at the desired high frequencies while measuring small

magnitude loads with accuracy and precision. With a

variety of options for load cells and platens, the

ElectroForce 3200 is well suited for use in testing weak

viscoelastic materials.

Figure 6 - 1% Agarose, Complex Modulus

Summary