Wireless Self-Oscillating Systems with Modular Design for Multi-Modal Motion and Versatile Functions

Self-oscillating mechanism requires feedback loops that connect the material properties, deformation, and the external stimuli, to generate a motion constantly switching between multiple metastable states. These self-sustained actuating systems use unmodulated external stimuli, which is advantageous since these energy sources and controls are cheaper, less bulky, and easier to maintain. Utilizing their capabilities of generating continuous motions under unmodulated controls, self-oscillating material systems have been applied in soft robotics, material transport, energy conversion, and sensing.<br/>Currently, self-oscillating systems are mainly demonstrated using the following driving mechanisms: reversible chemical reactions to convert chemical energy into kinetic energy, such as BZ reaction and the <i>cis</i>-<i>trans</i> isomerization of azobenzene; converting material volumetric change to motion such as moisture absorbent membrane swells and dehydrates in a humidity spatial gradient; using the different swelling rates of hydrogels in various solvents to cause buckling and generate wave-pattern self-oscillation movements. In recent years, the self-oscillation motion based on self-shadowing has gained much attention, with exemplary material systems such as liquid crystal networks and responsive hydrogels. These materials undergo photothermal deformation when irradiated by a light source; the deformation blocks the light or moves the prior irradiated area out of illumination, leading to the recovery of the deformed state and re-exposing the material to light, forming a cycle between the irradiated state and the shadowed state. This mechanism is advantageous in producing stable periodic self-oscillation.<br/>A common limitation of these traditional self-oscillating mechanisms is that they require the whole or at least most of the self-oscillating actuators to be made up of specific materials, such as liquid crystal materials, humidity-sensitive materials, and hydrogels. This material requirement limits the mechanical properties and compatible operating environment, thus hindering the universality.<br/>To address this gap, we propose a modular self-oscillating system using unmodulated light and magnetic fields. The self-oscillating system is composed of two components, a driving module and a functional module. The driving module provides active magnetic force to induce deformation in the functional module, and the functional module counteracts with elastic force under deformation. The generated motion is governed by the tug-of-war of the two opposing forces. The underlying mechanism to generate self-sustained oscillation is the negative feedback loop connecting the two forces and the deformation: the two forces are deformation-dependent and the system is out of equilibrium so that the deformation is dynamically adjusted.<br/>The self-sustained oscillation in this work has collective advantages over other self-oscillating mechanisms. Besides having large oscillating amplitude and tunable frequency, the modular design to separate the driving module and the functional module makes the system versatile: the functional module can be changed or functionalized to meet the requirements of various applications, making multi-mode oscillation possible, such as bending, linear translation, twisting, and combination of these modalities to increase the complexities of motions that can be achieved. We demonstrate using a surface-patterned silk fibroin film as a dynamic display device, a reflector as a light scanning and detection device, and a piezoelectric polyvinylidene fluoride (PVDF) film to harvest electricity from the oscillation. We envision the multifaceted versatility of the self-sustained oscillation system—motion modalities, materials selection, and functions—will enable them to be readily used in various applications in a wide range of environments.