Bio-Inspired Magnetically Controlled Reversibly Actuating Multimaterial Fibers

Abstract

Movements in plants, such as the coiling of tendrils in climbing plants, have been studied as inspiration for coiling actuators in robotics. A promising approach to mimic this behavior is the use of multimaterial systems that show different elastic moduli. Here, we report on the development of magnetically controllable/triggerable multimaterial fibers (MMFs) as artificial tendrils, which can reversibly coil and uncoil on stimulation from an alternating magnetic field. These MMFs are based on deformed shape-memory fibers with poly[ethylene-co-(vinyl acetate)] (PEVA) as their core and a silicone-based soft elastomeric magnetic nanocomposite shell. The core fiber provides a temperature-dependent expansion/contraction that propagates the coiling of the MMF, while the shell enables inductive heating to actuate the movements in these MMFs. Composites with mNP weight content ≥ 15 wt% were required to achieve heating suitable to initiate movement. The MMFs coil upon application of the magnetic field, in which a degree of coiling N = 0.8 ± 0.2 was achieved. Cooling upon switching OFF the magnetic field reversed some of the coiling, giving a reversible change in coiling ∆n = 2 ± 0.5. These MMFs allow magnetically controlled remote and reversible actuation in artificial (soft) plant-like tendrils, and are envisioned as fiber actuators in future robotics applications.

Keywords: inductive heating; magnetic nanocomposite; multimaterial fibers; plant inspired movements; remote actuation; shape-memory polymers; soft actuators and robotics; tendrils.

Figure 1

Schematic illustration of the fabrication and actuation of multimaterial fibers via inductive heating. (A) The polymer fibers are extruded as a monofilament from a mixture of polymer and crosslinker, i.e., triallyl isocyanurate (TAIC). (B) The fibers are crosslinked by gamma irradiation at ambient temperature. The molecular arrangement in the fiber after extrusion and crosslinking is presented with polyethylene crystallites (), amorphous polymer segments () and covalent netpoints (). (C) The crosslinked fibers are programmed by stretching and twisting. (D) MMFs are prepared from a programmed core fiber (by stretching and twisting) and a composite shell comprising mNPs () and silicon elastomer presented as amorphous segments (). (E) The MMF coils upon heating by inductive heating in an AMF and changes reversibly in the number of coils, coil diameter and length on cyclic heating and cooling by switching the field ON and OFF. In the course of inductive heating with magnetic field ON, the heat is generated in mNPs () in the shell and transferred to the core, which leads to the melting of PE crystals as indicated by the resulting amorphous system (), while recrystallization occurs on cooling with the magnetic field OFF. The temperature reached in the composites can be controlled by using a variable current generator as well as by changing the type and amount of mNPs.

Figure 2

Transmission electron microscopic investigation of the spherical SiO₂-coated Fe₃O₄ mNP (i–iii) and non-coated spherical Fe₃O₄ mNP (iv–vi).

Figure 3

Heating profile of PDMS-mNP composite upon stimulation in alternating magnetic field; PDMS-SP20-15 (red), PDMS-Sp20-20 (green), PDMS-Sp30-15 (blue), and PDMS-Sp30-20 (light grey). (B) Heating and cooling profiles of MMFs containing SMCF and a PDMS-mNP as shell, with the same color code as in (A).

Figure 4

(A) The POM images of MMFs: (i) MMF-Sp20-15, (ii) MMF-Sp20-20, (iii) MMF-Sp30-15 and (iv) MMF-Sp30-20. (B) Stress–strain curves of PDMS-mNP composite fibers (without SMCF): PDMS-Sp20-15 (red), PDMS-Sp20-20 (green), PDMS-Sp30-15 (blue) and PDMS-Sp30-20 (light grey) in comparison to PDMS (black).

Figure 5

(A) Image of the MMF after synthesis and before actuation. (B) MMF held in the magnetic coil before its actuation and with magnetic field turned OFF. (C) MMF in the magnetic coil after heating by turning the magnetic field ON, and consequent coiling. (D) Cross-section of coiled MMF. (E) Macroscopic images of coiled MMFs after their actuation by magnetically triggered heating. All images in this figure are from MMF-Sp30-20. Brightness of images was increased for better readability.