Materials have come a long way in recent years. Novel properties and functionalities are increasingly embedded in them to facilitate their use in specific applications. Not only can traditional properties like rigidity, elasticity or conductivity be tweaked to a fine degree, but materials can be designed to be adaptable or undergo self-repair if damaged. Magnetic properties can also be embedded in them providing precise remote control functionalities.

However, when attempting to embed multiple properties or functions in a single material, compromises must usually be made. Magnetic control may be achieved at the cost of flexibility, or durability, for example.

Conventional methods for creating soft magnetic materials generally involve dispersing magnetic particles such as iron oxide (Fe₂O₃), neodymium iron boron (NdFeB), or rare-earth compounds into a polymer matrix. While this approach provides a strong magnetic response, it often stiffens the polymer, making them unsuitable for wearable devices or soft robots, where they will often be deformed.

To overcome this limitation, Assistant Professor Tan Yu Jun from NUS Mechanical Engineering developed a versatile and multifunctional soft magnetic material called magneto-iono-elastomers (MINEs) in which a magnetic ionic liquid (MIL) containing 1-Ethyl-3-methylimidazolium chloride (Emim) and tetrachloroferrate (FeCl₄) ions is embedded within a soft urethane group-based polymer. The resulting material is clear, magneto-responsive, flexible, self-healing and conductive, making it suitable for many applications, from wearable electronics to flexible sensors, and smart touchscreens.

"By leveraging the intrinsic magnetic properties of the magnetic metal ion, we have developed a material that is particle-free, soft, and self-healing. This represents a significant departure from traditional approaches and enables the creation of multifunctional materials with unprecedented combinations of properties, Tan explained.

Properties

For MINEs to work effectively in real-world applications, they must exhibit strong responsiveness to magnets. This requires a high concentration of MIL to be integrated into the polymer. To achieve this, the team used a highly cross-linked polymer, which permitted 80% MIL content while at the same time retaining transparency and stretch . The material could be expanded up to 6 times its original size and return to its original shape, demonstrating both elasticity and durability. The key to this multifunctionality is the confinement of magnetic anions (FeCl₄) through strong yet reversible intermolecular interactions.

“Soft robots made from MINEs can shift shape or move just by applying a magnetic force. No wires, no motors, just pure control using magnets. This is effective for tasks like picking up delicate objects, or navigating tight spaces, making them perfect for exploring hazardous environments,” Tan explained.

The MINEs also become more conductive as the amount of MIL increases. With 80% MIL, the material’s electrical conductivity improves 1,300 times compared to a version with only 20% MIL. The MINEs were able to light up an LED while still responding to a magnetic field, something standard magnetic materials cannot do. Because MINEs can separate their electrical and magnetic functions, they show promise as highly reliable materials for advanced applications like wireless sensors, flexible electronics, and smart wearable devices. In wearables for example, their flexibility allows for user comfort, while their conductivity contributes to reliable signal transmission.

Adding to their potential for advanced applications is the fact that the MINEs can also repair themselves when damaged. A version with 20% MIL can heal 88.7% of its original mechanical performance within three days at room temperature, while higher MIL content improves healing even more. Notably, mild heat sped up the self-healing process of these materials.

“Wearables and soft robotics are bound to bend, stretch, get scratched, and eventually wear out. MINEs can self-repair, so if they get punctured, torn or cut, they fix themselves without any manual intervention. This extends the life of devices. Smartphones and tablets may one day repair their own scratches, or windows may one day double as interactive screens with augmented reality functions,” Tan explained.

MINEs offer a unique combination of properties once thought incompatible. This will not only redefine the advanced materials landscape, but also promises to advance sustainability in material management with the lifespan of products extended so as to reduce waste, lower resource consumption and allow for improvements in the environmental impact of the electronics and robotics industries.