Enhancing Actuation Response in Haptic Displays Utilizing Nanoparticle-Based Magnetorheological Elastomers

Despite the importance of touch for humans, interactive tactile (haptic) displays can still be considered in their infancy. While several actuation techniques have been developed, all these methods still present major drawbacks either in terms of the power demanded for actuation or their large size, thereby discouraging their use in high-resolution haptic displays. <i>Magnetorheological elastomers</i> (MREs) constitute an ideal candidate for a low-power and compact solution to be integrated into a programmable haptic interface. MREs are smart composite materials made of a soft elastomeric matrix and magnetic micro-/nanoparticles. Few studies have focused on the actuation of MREs, and those that do, mostly report the use of relatively large magnetic microparticles inside of elastomeric matrices. In contrast, much smaller nanoparticles would be required to fabricate films on the order of hundreds of nanometers in thickness, able to actuate into sharp textures at the microscopic scale. This research project focuses on building <i>high-resolution </i>haptic interfaces by developing flexible magneto-responsive thin-films based on magnetic <i>nanoparticles</i>. In the future these materials will be integrated into a nonvolatile magnetic control system that can produce local magnetic fields to actuate the film and generate the desired resolution of three-dimensional shapes and textures. <br/><br/>In this study, we fabricated MREs composed of soft, low-remanence ferromagnetic iron nanoparticles with isotropic and anisotropic arrangement in an elastomeric matrix (Ecoflex 00-30) and optimized them by characterizing their magnetic and mechanical behavior. Films with nanoparticle concentrations varying between 2% and 8% by volume were fabricated. All films showed large visible deformations at the millimeter scale at small applied magnetic fields of only 100-300 mT. However, the magnetically-induced deformations increased with the nanoparticle concentration in the films up to 6 vol. % for both isotropic and anisotropic films. Furthermore, isotropic 6 vol. % films exhibited 1.6x the deformation of anisotropic films at the highest applied field (300 mT). These isotropic films were able to achieve deflections of 0.73 mm and 1.6 mm at 100 mT and 300 mT, respectively. Moreover, we characterized the morphology of magnetic particles from SEM cross section images of the optimal 6 vol.% isotropic and anisotropic films. While the isotropic films showed random dispersion of the nanoparticles in the matrix, the anisotropic ones confirmed preferential alignment of the nanoparticles along the magnetization direction. The magnetic hysteresis behavior of isotropic and anisotropic films was measured to visualize the difference in remanent magnetization between the two cases and a small but evident increase in magnetic anisotropy matched the particle arrangement of the anisotropic films.<br/><br/>To compare with the literature, identical MRE films were also fabricated utilizing soft iron microparticles. In comparison with the nanoparticle-based anisotropic MREs, at a field of 100 mT, the analogous microparticle-based films showed 25-30% lower deflection. The latter were only able to match the nanoparticle films’ deflection at higher fields of almost 300 mT. This performance difference between nanoparticle and microparticle films was attributed to the increased anisotropic film stiffness resulting from the larger micrometer-size particles. Finally, the optimal nanoparticle-based isotropic MRE was utilized to create an enlarged programmable braille display which can be refreshed real time by actuating the deflection of specific buttons to construct different letters and words. In conclusion, this research showed for the first time the benefits of utilizing nanoparticles to engineer MREs with enhanced magnetomechanical response, enabling the realization of a reconfigurable haptic interface prototype with the potential for scalability and the promise of enabling numerous future haptic innovations.