Magneto-mechanical composites

When micrometer-size magnetic particles are suspended in a fluid, they can give a change mechanical properties of several orders of magnitude when the fluid is subjected to a magnetic field. By encasing the fluid in an elastomeric matrix, we have created a composite that not only exhibits significant modifications in the mechanical properties in a magnetic field but also magnetic shape-memory [1] and switchable adhesion [2].

We have designed a material that displays magnetically addressable shape-memory and switchable adhesion by dispersing droplets of a commercial magneto‐rheological fluid in a crosslinked poly(dimethylsiloxane) (PDMS) matrix, shown schematically in Figure 1a. The composite can undergo substantial stiffening when subjected to a magnetic field, with an exceptional increase in storage modulus of approximatively thirty times at a field of 600 mT (Figure 1b). For magnetically addressable shape‐memory, the PDMS matrix acts as the stable phase and the magnetic fluid as the programmable phase and an applied deformation can be programmed in the composite through the application of a magnetic field (Figure 1c). The composite relaxes back to its original form in less than one second when the magnetic field is released.

Enlarged view: fig1_MMSys — *Fig. 1.* (a) Three-dimensional reconstruction of the internal structure of the material obtained from tomography data, providing a 3D visualization of the two different phases in the composite (cube edge = 390 µm). The effect of the magnetic field on the internal structure of the magnetic phase in a droplet is schematically shown. (b) Experimental demonstration of the influence of the magnetic field on the stiffness of the material. A disk of material is able to carry a weight of 250 g when subjected to a magnetic field from a permanent magnet (left), while it becomes soft and compliant under the same weight when the magnet is removed (right). (c) Embossing a disk of material demonstrating magnetic shape-memory. The material can be programmed to retain a certain shape in a magnetic field, and returns to the original shape after removal of the field. A disk of material (initial Stage 0) is embossed using a stamp with a defined profile (Stage 1) and subsequently an out-of-plane magnetic field is applied with a permanent magnet (Stage 2). The material retains the programmed shape for several days (Stage 3) until the magnetic field is removed, after which it returns to the original shape in less than 1 second (Stage 4). Reproduced from Ref. [1]

The fact that the shape‐memory mechanism shown by this material requires no heat, in conjunction with recent advances in magnetic control systems, opens up new possibilities for applications such as biomedical and wearable devices, which are operated in heat‐sensitive environments. For example, catheters that are pushed through blood vessels to the surgical site in the body during minimally invasive operations could change their stiffness. This has the advantage that they only have to solidify when needed and therefore produce fewer side effects such as thrombosis – for example when sliding through a blood vessel. Based on a straightforward emulsion process, this material composite can not only be extended to different classes of polymer matrices and active fluids, but can also be manufactured with a wide range of methods, including casting, injection molding, and additive manufacturing.

The same material also exhibits magnetically switchable adhesion. When subjected to a magnetic field of 250 mT, the magneto-rheological fluid develops a yield stress, which dramatically enhances the composite's adhesive properties, resulting in a nine-fold increase of the pull-off force. A demonstration of this effect is shown in Figure 2. This surprising effect could extend the use of adhesives to new applications requiring actuation of adhesive forces, such as soft robotic grippers and actuators, climbing robots, and medical or wearable devices.

*Fig. 2*. Demonstration of magnetically-switchable adhesion. A disk with a weight of 20 g, made of non-magnetic acrylic, sticks to the composite when a magnetic field is applied, but detaches under its own weight when the magnetic field is removed.

References

Magnetically Addressable Shape‐Memory and Stiffening in a Composite Elastomer
P. Testa, R. W. Style, J. Cui, C. Donnelly, E. Borisova, P. M. Derlet, E. R. Dufresne and L. J. Heyderman, external pageAdv. Mater. 2019, 31, 1900561.
Additional links: external pageCover Image
Switchable adhesion of soft composites induced by a magnetic field
P. Testa, B. Chappuis, S. Kistler, R. W. Style, L. J. Heyderman and E. R. Dufresne, external pageSoft Matter 2020,16, 5806 - 5811