Soft media like hydrogels are used as substrates for cellular engineering of different cell types and the stiffness of these media plays an important role in the cellular behavior like adhesion, proliferation, matrix deposition and differentiation. The global mechanical properties of such substrates were previously calculated using spherical beads embedded in micro-liter samples and then deflecting them by a magnetic manipulator . Knowing the force and the displacement the elastic modulus was evaluated using closed form solutions. However in the context of our studies for biological applications there are circumstances where the local elastic properties of the hydrogel play an important role. For example in the vicinity of a cell, the cell will distort the gel and the modulus of elasticity of the surface may get altered, particularly if the gel is non-linear and if the distortions are substantial. In order to measure these local properties a four magnet manipulator based on Amblard, et al  was built, which is capable of applying large forces on micron sized particles embedded in the media.
Force/Torque Field Generation to Obtain Local Properties within Soft Hydrogels
Chippada, U, Yurke, B, Shreiber, DI, Schloss, R, Jiang, X, Li, L, & Langrana, NA. "Force/Torque Field Generation to Obtain Local Properties within Soft Hydrogels." Proceedings of the ASME 2007 Summer Bioengineering Conference. ASME 2007 Summer Bioengineering Conference. Keystone, Colorado, USA. June 20–24, 2007. pp. 257-258. ASME. https://doi.org/10.1115/SBC2007-176081
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