“Magnetic marshmallows” for soft robotics: magneto-mechanical characterization and application in switchable adhesion structures

Soft magnetic composites exhibit fast and programmable macroscopic deformations in magnetic fields, making them promising for applications in soft untethered robotics or in designing surfaces with switchable adhesion or wetting properties. However, due to the incompressible nature of soft, non-porous actuators, their compression or elongation leads to important shape change due to lateral expansion or compression, respectively. In practice, bending and folding remain preferred actuation modes. Here, we explore the potential of magnetic elastomer "marshmallows" as compressible actuators with low Poisson's ratio. Using a sacrificial salt pellet template method, we fabricate polydimethylsiloxane foams with open porosity filled with carbonyl iron particles. The obtained foams exhibit strong reversible compression under the influence of a magnetic field gradient. We reveal the significance of stress accumulation in the direction of the field gradient due to magnetic body forces and the key role of the foam thickness in magnetic strain. We propose a simple analytical model based on the action of a magnetic body force that explains these effects. Finally, we demonstrate the development of a novel switchable adhesion structure, in which the magnetic foam covered with a pressure-sensitive adhesive serves as a compressible actuator able to switch between adhesive and non-adhesive states.