Stimuli-Responsive Polymer Actuator for Soft Robotics

Abstract

Polymer actuators are promising, as they are widely used in various fields, such as sensors and soft robotics, for their unique properties, such as their ability to form high-quality films, sensitivity, and flexibility. In recent years, advances in structural and fabrication processes have significantly improved the reliability of polymer sensing-based actuators. Polymer actuators have attracted considerable attention for use in artificial or biohybrid systems, as they have the potential to operate under diverse conditions with high durability. This review briefly describes different types of polymer actuators and provides an understanding of their working mechanisms. It focuses on actuation modes controlled by diverse or multiple stimuli. Furthermore, it discusses the fabrication processes of polymer actuators; the fabrication process is an important consideration in the development of high-quality actuators with sensing properties for a wide range of applications in soft robotics. Additionally, the high potential of polymer actuators for use in sensing technology is examined, and the latest developments in the field of polymer actuators, such as the development of biohybrid polymers and the use of polymer actuators in 4D printing, are briefly described.

Keywords: actuator; external stimuli; polymers; sensor; soft biohybrid robotics.

Figure 16

(a) CP formation by reacting metal ions with bridging ligands and also CP formation via polymerization of MCMs. Reproduced with permission from reference [97]. (b) The recycling process of shape-memory polymers in repeated programming and photoinduced 4D printing. (c) The recovery angle versus time of LMPCs in a total of 25 cycles while irradiating with 808 nm laser (0.3 W/cm²) for 60 s. Reproduced with permission from reference [98].

Figure 1

Schematic classification of stimuli-responsive polymer actuators by types of external stimuli, fabrication methods, and types.

Figure 2

(a) Operation model of custom-developed microelectronics devices for device interfacing and experimental controlling. (b) Setpoint, feedback, and output curves of a typical feedback-driven positioning experiment. Position feedback was provided by integrated magnetic sensors. The proportional gain constant was set to K_P = 200. Reprinted with permission from reference [29]. (c) A photograph of the c-PEDOT:PSS-PET substrate immersed in water. (d) Plots of the normalized thickness (upper) and optical transmittance of the c-PEDOT:PSS films (lower) concerning the duration of immersion in DI water and phosphate-buffered saline (PBS). Reprinted with permission from reference [30]. (e) Schematic illustration of the actuation mechanism in the case of a trilayer conjugated polymer. Reprinted with permission from reference [26]. (f) Prospective 3D printed soft robot, where both the EP actuator and robotic body are printed using a single printing process. Cut-away view to show the internal components. Reprinted with permission from reference [28].

Figure 3

(a) Schematic illustration of the preparation process. Reprinted with permission from reference [35]. (b) Photograph and schematic illustration of the ultrahigh-molecular-weight gel [36]. (c) Photograph of a flat individual polyelectrolyte-based GPE (PGPE) and of an LED powered by a 2.4 V device; here, two 1.2 V PGPE supercapacitor devices are connected in series. Reprinted with permission from reference [34].

Figure 4

(a) The biohybrid composite fabrication steps are conducted using bovine pericardium (BP). Step I: intact BP is divided into three portions for processing, and the first portion is left untreated. Step II: the second portion of the sac is decellularized using sodium deoxycholate. Step III: coating of polycaprolactone: chitosan polymer layer on the decellularized BP via electrospinning. Reprinted with permission from reference [43]. (b) Electronic functionalization of plant roots. ETE-S polymerizes on the roots of intact bean plants catalyzed by endogenous plant cell wall peroxidases and H₂O₂. Reprinted with permission from reference [48]. (c) Ejection of guest–host granular hydrogel from the syringe through a 27 G needle onto a surface, and two-component granular hydrogels injected into rat hearts either with myocardial infarction (MI) or no MI. Reprinted with permission from reference [47].

Figure 5

(a) The molecular mechanism of the dual-SME. Black dots: net points; blue lines: molecular chains of low mobility below T_trans; red lines: molecular chains of high mobility above T_trans. Reprinted with permission from reference [53]. (b) The alignment of polymer chains, i.e., entangled in their permanent form and aligned when stretched in their temporary form. Reprinted with permission from reference [52]. (c) Schematic demonstration of this smart shape-memory textile used in a smart cloth. Reprinted with permission from reference [55].

Figure 6

(a) Various actuator constructions using conducting polymers differ in bulk expansion, bending bilayer, buckling trilayer, and bending trilayer in air. Reprinted with permission from reference [56]. (b) Photograph and schematic illustration depicting the structure and assembly procedure of the actuator. Reprinted with permission from reference [57].

Figure 7

Thermal sensing actuator design principle. (a) Schematics showing the hand withdrawal reflex consists of a thermoreceptor, sensory neurons, spinal cord, motor neurons, and muscle. (b) Working mechanism of the TSA simulating the function of hand withdrawal reflex, with thermal sensing potential (V_thermal), action potential (V_action) and a smart control system. Reprinted with permission from reference [60]. (c) Image of the fiber temperature sensor sewn onto the tip of a hand glove. (d) Temperature response of the fiber sensor to repetitive touch of a hot (45 °C) or cold (5 °C) object. Reprinted with permission from reference [61].

Figure 8

(a) Photoactuation of the polymer ribbons incorporating H1 [62]. Thickness 25 μm. (b,c) Linear dichroism of azobenzene-doped ultra-drawn ultrahigh-molecular-weight polyethylene films. Insets are photographs taken of the corresponding ultra-drawn films, with the transmission axis of the polarizer at 0° indicated in white. Reprinted with permission from reference [63].

Figure 9

(a) Schematic representation of a magnetically active strain sensor on a PET film with resistance variation correlation with the magnetic actuator displacement after correction of the low-frequency offset. Inset resistance variation correlation with the magnetic actuator displacement after correction of the low-frequency offset. Reprinted with permission from reference [64]. Novel dual-alignment processing method for HPMSA to align magnetic particles and PVDF crystals. (b) Random Fe₃O₄, f-CNT, and amorphous and crystal phases of PVDF. (c) Magnetic alignment: movement of f-CNT and PVDF crystals due to Fe₃O₄ alignment with an external magnetic field. (d) Mechanical alignment: further alignment of Fe₃O₄, f-CNT, and PVDF crystals (transformation from amorphous phase) with mechanical uniaxial stretching of HPMSA [65].

Figure 10

(a) Initial emf response (squares) and response after 6 months (circles) to pH for an ISE with a PDMA membrane (doped with ionophore and ionic sites) photographed onto a polypropylene-based electrode body and nano-graphite solid contact, relative to a free-flowing double-junction reference electrode. The pH was adjusted by the addition of 1.0 M HCl or 1.0 M NaOH to 10 mM sodium phosphate buffer solution (pH 7.1). The pH shown on the x-axis was measured using a pH glass electrode.Reprinted with permission from reference [68]. (b) schematic representation of our pH sensor consisting of a PANI membrane on interdigital electrodes supported by a PI substrate. The transformation of PANI protonated in acid solution and deprotonated in basic solution [69].

Figure 11

(a) The schematics of the fabricated p(D-co-M) sensor and the copolymer’s predominant protonation state at various pH values. Reprinted with permission from reference [72]. (b) A schematic diagram representing the deposition of gas molecules on the surface of a conducting polymer composite film consisting of inorganic nanoparticles. Reprinted with permission from reference [73].

Figure 12

Proposes structures of (a) SC/PEA/KB(10) and (b) PSCD⊃PEA/KB(10). Reprinted with permission from reference [74]. Schematic illustration of the wearable FSS for real-time stress management. (b) Schematic illustration of the wearable FSS that enables cortisol monitoring through a CNT-based sensor with an MIP. (c) Schematic illustration of a single fabric sensor and magnified image of the fabric sensor. (d) Schematic illustration and current response in cortisol recognition. The red solid line indicates the current response after cortisol recognition. Reprinted with permission from reference [75].

Figure 13

Electrosynthesis strategies are based on the direct formation of quinones. Schematic representations of the electro-crosslinking, through direct electro-oxidation of catechol/gallon moieties of (a) PAH/bis catechol film [77]. (b) Representation of Al³⁺/TA@AgNP film assembly. Reproduced with permission from reference [78].

Figure 14

(a) Schematic illustration of constructing a mesoporous CP layer on 2D functionalized surfaces. (a) Simplified schematic diagram of PS-b-PEO. (b) PS-b-PEO dissolved in the mixed solution to form spherical micelles. (c) The micelles are tightly arranged on the GO surface. (d) Micelles attract Py monomers to form complex micelles. (e) Monomer polymerizes in situ to form a polymer network. (f) Removal of the template to obtain sandwich-structured mesoporous PPy nanosheets [80]. (g) Schematic illustration of the synthesis route employed for the preparation via RAFT-mediated aqueous polymerization-induced disassembly (PIDA). Reproduced with permission from reference [81].

Figure 15

(a) Dispersion polymerization of tulip-derived α-methylene γ-butylrolactone (MBL). Reproduced with permission from reference [89]. (b) Schematic illustrating the Au assembly process of polymer substrates. Reproduced with permission from reference [90].