MXene incorporated polymeric hybrids for stiffness modulation in printed adaptive surfaces

Polymeric materials systems developed for actuators and human-machine interfaces suffer from limitations associated with effective force output due to their low mechanical modulus. New material solutions which can provide intrinsic multi-modal responses are needed to reversibly modulate rigidity; to be flexible, stretchable and bendable one moment, and to be rigid, able to bear load and resist deformation at another moment. Thermally modulated phase transition materials are promising for modulation of mechanical properties; however, they have not been explored for electrically driven shape morphing and responsive surfaces which require favourable electrical properties too. Polymers like polyethylene glycol (PEG) allow for low melting point (56 ℃) and high dielectric constant (10), however they are limited by slow crystallization kinetics and large temperature window. We architect an MXene incorporated PEG-water hybrid which allows for both reduction in melting point and rapid heterogeneous nucleation, which in turn increases the crystallization point. Multimodal response is demonstrated via thermal and electrical input, resulting in modulation of 700 times in Young's modulus, 100 times in flexural modulus and 10 times in hardness as well as large actuation strains (~28%) at low electric fields (~0.7 V/µm). They can be printed to create hardness domains, allowing for local and programmable modulation. An all-printed haptic device with an array of 3 × 3 pixels has been demonstrated, capable of independently varying the hardness values for each pixel.