Flexible anisotropic magneto-sensitive elastomer films with out-of-plane particle chain for bionic actuator

Abstract

Owing to the flexible and controllable characteristics, the magnetic composite film becomes an ideal option for future portable, efficient and multifunctional magnetic soft actuators. Here, an anisotropic magneto-sensitive elastomer film (MSEF) composite based on polydimethylsiloxane (PDMS) and high concentration of carbonyl iron particles (CIPs) was developed. The off-plane design of CIP structures endows MSEF excellent field-dependent deformability and actuation ability. The off-plane bending angle of the MSEF-30° in 87mT uniform magnetic field could reach 70.78°. Moreover, the MSEF-20° could lift a heavy object 66.4 times its own weight. The influences of particle chain orientation, CIP contents, thickness and applied magnetic field on the magneto-induced deformation behavior were fully discussed, and finite element calculations were performed to clarify the experimental results. An improved balance equation was proposed and qualitatively explained the experimental results under different magnetic fields. Based on the MSEF composite, biomimetic soft actuators with bidirectional deformation and self-sensing functions were produced respectively. The two easy preparation and programmable soft actuators could achieve the expected actuation effect, indicating that the anisotropic MSEFs have great potential as soft actuators in the fields of biomedicine, microfluidics, intelligent robots and bionics applications.

Introduction

Compared with traditional rigid actuators, the soft actuators, with the superior flexibility, adaptability and deformability, have great potential in the field of biomedicine [1], health monitoring [2], [3], MEMS device [4] and soft robot [5], [6]. Various soft actuators have been designed and fabricated based on different stimuli-responsive mechanism [7], [8], [9], [10], [11]. Among them, the magnetically controlled soft actuators based on magneto-sensitive elastomer materials have attracted growing attentions [12], [13], [14], [15]. Magneto-sensitive elastomer materials are usually fabricated by incorporating micron or nanometer magnetic particles into continuous polymer matrix [16], [17]. By varying external magnetic field, the magnetic polymer composite materials behave elongation, contraction, bending and twisting [7], [18], [19], [20], [21]. The deformation can be achieved with fast response time, small size of device, high power efficiency and low drive voltages, etc [22]. As a result, magneto-sensitive soft actuators have been found in mechatronic and micro-robotic systems [13], [23].

As one kind of magneto-sensitive elastomer, anisotropic magneto-sensitive elastomer film (MSEF) has extraordinary deformability under external magnetic field due to its high saturated magnetization, low stiffness and high flexibility [12], [24], [25], [26]. Therefore, the anisotropic MSEF composite is very suitable for developing the intelligent actuators. Zhang et al. [27] fabricated magnetically actuated three-dimensional mobile microgripper based on the programmable magnetic-sensitive elastomer film. By directly controlling the magnetic force and torque applied to the microgripper, the micro objects could be grasped and transported. Mishra et al. [28] utilized nanoparticle chains oriented in plane and prepared the magnetic elastomer films with anisotropic mechanical response, which enabled the direction-controlled actuation under magnetic field. Schmauch et al. [29] concluded that the chained magnetic microparticles (MMPs) could provide more complex deformation behaviors for soft actuators. The magnetically actuated lifters, valves, and peristaltic pumps were fabricated to demonstrate their capabilities in different work status based on the elastomer films with chained MMPs.

In previous studies, the flexible MSEFs with the magnetic particle chains arranging within the plane were connected to obtain micro actuators. During the pre-structured process of the magnetic sensitive elastomer film, the attractive dipolar interactions pull the magnetic particles together to form chains. One-dimensional arrangement of magnetic particle chains will bring the composite the magnetic anisotropy. The magnetic particle chains are magnetized much more when the chain orientation is parallel to the external magnetic field direction. Consequently, the alignment of the chains along the external magnetic field direction is energetically favorable and the magnetically induced mechanical response is orientation-dependent [28]. The bending, curling and other complex motions under uniform magnetic fields are achieved by magnetic anisotropy [30]. However, there are some unavoidable problems in MSEFs composed of in-plane particle chains. The changes of particle chain orientation will cause composite membrane the combined deformation of bending and torsion [28]. Some special measurement techniques are required to quantitatively investigate the magneto-deformation. In addition, excessive magnetic inclusions in-plane can eliminate the magnetic anisotropy caused by the chain arrangement of particles. Only composite films with magnetic particle mass fraction less than 10.2% have been studied [29].

Therefore, the anisotropic magnetic sensitive elastomer films with particle chain orientation along the thickness direction were investigated in this paper. The distribution of micron magnetic particles inside the MSEF was observed by the X-ray microtomography. Under different magnetic fields, the influence of contents, chain orientation and magnetic field strength on the deformation of the MSEF was explored experimentally. And then, a modified model for magnetically induced deformation was proposed based on magneto-elastic theory. Moreover, the actuation force of the MSEF composite as soft actuator was also evaluated by lifting different loads. Finally, a bidirectional actuator and a self-sensing actuator composed of MSEFs were fabricated, indicating the great potential of MSEFs in the soft actuators field.