Mechanical characterization of agarose micro-particles with a narrow size distribution

A micromanipulation technique was used for the mechanical characterisation of two types of agarose microspheres with different material properties. Narrow-size distributions having a mean diameter in the range of 15–22 µm were prepared using (a) conventional emulsification followed by filtration and (b) membrane emulsification. Single microspheres were compressed to a range of deformations at different speeds up to a maximum of ~ 70 µm/s, and then held at constant deformation to permit relaxation to occur. It was found that the loading data could be satisfactorily described by the Hertz equation up to 30% deformation. The Young's moduli calculated on this basis were found to correlate with the gel strength of the agarose which was used to prepare the microspheres. However, the values of the moduli increased with the compression speed and significant stress relaxation occurred. Consequently, a modified Hertz analysis was employed that accounts for the viscoelastic behaviour. Two relaxation times were sufficient to describe the stress relaxation function. The Young's moduli from the Hertz analysis corresponded to the long-time values of the stress relaxation function, which is reasonable given the relatively slow compression speeds used. The predominant process occurring at short times was ascribed to water transport from the interior of the microspheres and the process occurring at longer times was interpreted as that arising from the residual viscoelasticity of the polymer network. As a result of the stress relaxation during loading, the Tatara model could not be used to describe loading data at large deformations.