Paolo Testa, first author of the study, holds a model of the structure of the shape-memory material.
Photo courtesy of ETH Zurich

Researchers at the Paul Scherrer Institute (PSI) and ETH Zurich have developed a new material which, due to its magnetism- activated shape memory, retains a given shape when it is in a magnetic field. The material consists of two components: a silicone-based polymer and droplets of a magnetorheological fluid.

The droplets provide the magnetic properties of the material and its shape memory. If the composite material is forced into a certain shape with tweezers and then exposed to a magnetic field, it stiffens and retains this shape – without the support of the tweezers – and does not return to its original shape until the magnetic field is removed.

While comparable materials have consisted of a polymer and embedded metal particles, researchers at PSI and ETH Zurich instead used droplets of water and glycerine to insert the magnetic particles into the polymer. This produces a dispersion similar to that found in milk. As fat droplets are finely dispersed in milk, the droplets of the magnetorheological liquid are finely distributed in the new material.

“Since the magnetically sensitive phase dispersed in the polymer is a liquid, the forces generated when a magnetic field is applied are much larger than previously reported,” explains Laura Heyderman, head of the Mesoscopic Systems Group at PSI and professor at ETH Zurich.

The researchers studied the new material with the Swiss Light Source (SLS) at PSI. The X-ray tomographic images produced with SLS showed that the length of the droplets in the polymer increases under the influence of a magnetic field, and that the carbonyl iron particles in the liquid partially align along magnetic field lines. These factors make the material up to 30x stiffer.

The magnetic shape memory of the new material offers advantages in addition to higher force. Most shape-memory materials react to temperature changes, creating two problems in medical applications: excessive heat causes cell damage, and it is not always possible to guarantee uniform warming of an object that remembers its shape. Both disadvantages can be avoided by controlling shape memory with a magnetic field.

This new material could have numerous applications in different areas:

Medicine

– Catheters that are pushed through blood vessels to the surgical site during minimally invasive operations could change their stiffness. Using shape-memory materials, catheters could solidify only when needed and therefore produce fewer side effects, such as thromboses, when sliding through a blood vessel. Space exploration – The new material could serve as tires that inflate or fold up on their own for rover vehicles. Robotics – Shape-memory materials can perform mechanical movements without a motor, creating new possibilities for automation.

“With our new composite material, we have taken another important step toward simplifying components in a wide range of applications,” says ETH Zurich and PSI materials scientist Paolo Testa, first author of the study. “Our work therefore serves as the starting point for a new class of mechanically active materials.”

The researchers are now publishing their results in the scientific journal Advanced Materials.

Paul Scherrer Institute

ETH Zurich