Liquid metal-filled magnetorheological elastomer with positive piezoconductivity

Abstract

Conductive elastic composites have been used widely in soft electronics and soft robotics. These composites are typically a mixture of conductive fillers within elastomeric substrates. They can sense strain via changes in resistance resulting from separation of the fillers during elongation. Thus, most elastic composites exhibit a negative piezoconductive effect, i.e. the conductivity decreases under tensile strain. This property is undesirable for stretchable conductors since such composites may become less conductive during deformation. Here, we report a liquid metal-filled magnetorheological elastomer comprising a hybrid of fillers of liquid metal microdroplets and metallic magnetic microparticles. The composite's resistivity reaches a maximum value in the relaxed state and drops drastically under any deformation, indicating that the composite exhibits an unconventional positive piezoconductive effect. We further investigate the magnetic field-responsive thermal properties of the composite and demonstrate several proof-of-concept applications. This composite has prospective applications in sensors, stretchable conductors, and responsive thermal interfaces.

Fig. 1

Production of the liquid metal-filled magnetorheological elastomer (LMMRE). a Schematic of the procedure for fabricating the LMMRE. b Scanning electron microscopy (SEM) images of the obtained LMMRE. c–e Energy dispersive X-ray spectroscopy (EDS) element mappings of the LMMRE. Scale bars are 10 µm

Fig. 2

Resistivity of the liquid metal-filled magnetorheological elastomer (LMMRE) upon the application of mechanical loadings. a Resistance-strain curve of the LMMRE. b Scanning electron microscopy (SEM) images of the LMMRE under relaxed and tensile conditions (scale bars are 100 µm). Resistivity-strain curves and the resistance changes under cyclic loading under c, d compression, e, f stretching, and g, h bending

Fig. 3

Investigating the properties of the nickel (Ni)-liquid metal-filled magnetorheological elastomer (LMMRE). a Scanning electron microscopy (SEM) images and the energy dispersive X-ray spectroscopy (EDS) mapping of the Ni-LMMRE (LMMRE using Ni microparticles). b Resistivity-strain curve of the Ni-LMMRE under compression and stretching. c, d Piezoconductive coefficient-strain curve of the iron (Fe)- and Ni-LMMRE under compression and stretching. e Resistance changes of the composite under cyclic bending. The strip sample was attached to the index finger

Fig. 4

Response of the liquid metal-filled magnetorheological elastomer (LMMRE) to magnetic field. a The resistance changes of iron (Fe)- and nickel (Ni)-LMMRE (LMMRE using Fe and Ni microparticles) in the magnetic field. The value of the error bar is the standard deviation of the sample resistivity under five measurements. b The resistance change of Fe-LMMRE in periodic magnetic field

Fig. 5

Applications of the liquid metal-filled magnetorheological elastomer (LMMRE) in heating devices. a Operating principle of the pressure-sensitive heating device. b Temperature change on the film at different times after applying magnets. c Temperature change vs pressure plot of the heating device. d Localised heating effect of the device. e Temperature change vs tensile strain plot. f Exploded schematics and thermal images of the hand-held heating column. Scale bars are 1 cm