2019: “Depletion effect” of polymers helps assembling micro mechanical objects!

“Polymer-Induced Self-Assembly of a Three-Dimensional Microfabricated Structure”
Published in R. Kawai, Y. Mori, H. Suzuki, J. MicroElectroMechanical Syst., 28(4), 678-684, 2019.

Three-dimensional (3D) self-assembly of mesoscale parts with tens to hundreds of micrometer scales, independent of the material properties, is demonstrated by simply adding an inert polymer, i.e., polyethylene glycol, with large molecular weight (4MDa) at a concentration of micrograms per milliliters. It is most likely that the depletion volume effect, a thermodynamical effect to maximize the entropy of the system, induces face-to-face bonds with microfabricated parts having flat faces. The optimum polymer concentration range yielding high assembling efficiency is found, where the bonding energy is large enough to stabilize the bonding and viscosity is small enough to allow frequent collisions. Using this strategy, a site-specific assembly of 60 μm cubic parts in cavities engraved on large receptor parts, distributed on either 2D or 3D faces, is successfully demonstrated.