  Being able to precisely characterize the mechanical properties of soft
microparticles is essential for numerous situations from the understanding of
the flow of biological fluids to the development of soft micro-robots. Here we
present a simple measurement technique for the Poisson's ratio of soft
micron-sized hydrogels in the presence of a surrounding liquid. This methods
relies on the measurement of the deformation in two orthogonal directions of a
rectangular hydrogel slab compressed uni-axially inside a microfluidic channel.
Due to the in situ character of the method, the sample does not need to be
dried, allowing for the measurement of the mechanical properties of swollen
hydrogels. Using this method we determine the Poisson's ratio of hydrogel
particles composed of polyethylene glycol (PEG) and varying solvents fabricated
using a lithography technique. The results demonstrate with high precision the
dependence of the hydrogel compressibility on the solvent fraction and
character. The method, easy to implement, can be adapted for the measurement of
a variety of soft and biological materials.
