Functional Paper for Magnetically Controlled Soft Actuators and Deformable Sensors

Since its invention from more than 2200 years ago, paper has been indispensable in many parts of our everyday life such as writing, printing, cleaning, and packaging. In recent years, growing interest in the creation of functionalized paper has expanded the application scope of paper for batteries, energy harvesters, actuators, sensors, conductors, semiconductors, and biomedical uses, which exploit the inherent characteristics of paper, namely flexibility, foldability, bendability, recyclability, and lightness in weight. In this study, reprogrammable, magnetically controlled actuators, and highly deformable electrodes and sensors are fabricated by a simple and low-cost process. For the fabrication, commercially available water-soluble cellulose paper, consisting of wooden pulp and sodium carboxymethyl cellulose (Na-CMC) is used, and either Neodymium-Ferro-Boron (NdFeB) microparticles or ethanol-treated carbon black (CB) nanoparticles are uniformly dispersed in an aqueous solution of cellulose paper by sonication. The compound is heated for solvent evaporation where the coating of wooden pulp by CMC and filler particles composite takes place. The CMC-containing paper allows the final product to be glued together or recycled in the easiest way that needs no shredding or chemicals owing to its water-solubility at any temperature. Therefore, conductive paper strips can be joined to form an extended wiring, and recycling can be achieved by dissolving the paper in water and drying the solution to produce a new sheet of paper.<br/>Magnetic cellulose paper for shape-morphing soft actuators is designed to perform dynamic locomotion such as crawling, rolling, sliding, and folding in magnetic fields. To form magnetization profiles on the paper that initially has random domain orientation, magnetic torque is applied to the paper under a programming magnetic field and reorient the NdFeB particles to align the local magnetization, either in-plane or out-of-plane, which determines the locomotion mode. The paper is then folded in origami or selectively integrated onto pristine paper for desired actuation that is driven by an electromagnet with 2-axis Helmholtz coils at a very low magnetic field down to 0.3 mT. Increasing the magnetic field to 2 mT leads to a large-degree deformations up to ±90°. The magnetization profile can be reprogrammed by repeating the same procedure above, but with different alignment under a programming magnetic field. In such manner, the initially torque-driven magnetization changes its orientation and thus the modes of locomotion straightforwardly.<br/>Electrically favorable properties and durability of conductive cellulose paper are illustrated through mechanical deformation tests and sensor demonstrations. The electrical conductivity of the paper is saturated to 118 S/m when the mass ratio of CB to pristine cellulose paper is over 0.4 as the binding sites for CMC/CB in wooden pulp are restricted in amount. Bending up to a bending radius of 60 μm or repeated 180° folding cause no mechanical or electrical damage to the paper-based electrodes (resistance change below 0.1 % and 0.3 %, respectively). While the electrode dissolves completely in DI water, its partial dissolution can be recovered merely by drying the dissolved area without damaging the electrical characteristics. A paper-based parallel plate capacitor manifested negligible difference between the experimental and modeling data, and a capacitive touch sensor with paper electrodes evinces stable capacitance values during repetitive touching. The results unlock the implementation limit of the conductive paper for a variety of conditions that require bending, folding, instant dissolution, recovery after dissolution, lightweight capacitor, etc. Furthermore, the combination of the two types of functional paper provides magnetically controllable electric circuits whose wire connection can be steered and recuperated by maneuverable locomotion under magnetic fields.