Biomechanics and Motion Analysis

Agarose Gel Material Properties

Project Coordinator: Qingshan Chen — chen.qingshan@mayo.edu

Figure 65: Complex modulus vs. frequency for different concentration agarose gels

Agarose gel is widely used in various fields of biomedical research, particularly in tissue culture systems because it permits growing cells and tissues in a three-dimensional suspension. This is especially important in the application of tissue engineering concepts to cartilage repair because it supports the cartilage phenotype. Because the tissue pieces are embedded in the agarose gel, understanding the mechanical properties of agarose gels can provide basic background knowledge for tissue engineering investigations that employee dynamic loading to cells or whole tissues in vitro.

Since agarose gel is a porous solid filled with fluid, theoretically the gel and the fluid together are expected to behave as a biphasic substance with characteristic viscoelastic properties. A fractional calculus approach had been proposed as a method of describing the viscoelastic materials and soft tissues. Such models were in sharp contrast to the traditional spring-dashpot models because these models had simple expressions in their constitutive equation and yet represented a fairly complex rheological behavior that was often difficult to mimic with spring-dashpot systems.

Figure 66: Elastic and viscous modulus as a function of frequency for 3% agarose gel

In the present study, the dynamic mechanical properties of agarose gels as a function of frequency were characterized with a simple form of fractional derivative model. The relationship between the model parameters and the agarose concentration was also investigated. Gels with agarose concentration (weight/volume, w/v) of 2%, 3%, 4% and 5% were prepared by dissolving appropriate amount of Difco Bacto agar into the distilled deionized water. Samples 5 mm in thickness were used for testing.

Mechanical properties of the gels were tested in frequency sweep shear mode with a dynamic mechanical analyzer (DMA 2980, TA Instruments, New Castle, DE) over a frequency range of 1-20 Hz at a constant amplitude of 20mm. The individual complex modulus, elastic modulus and viscous modulus were recorded. A fractional derivative model was used to model the stress-strain relationship of the gel.

The model indicated that the elastic modulus of the agarose gel significantly prevailed its viscous modulus. The elastic modulus of agarose gels was usually 1-2 orders of magnitude larger than its viscous modulus (Figure 65. Moreover, elastic modulus and viscous modulus of the agarose gel were only slightly frequency-dependent, verifying its usage in dynamic pressure stimulation in tissue engineering (Figure 66).