Existing ionic artificial muscles still require a technology breakthrough for much faster response speed, higher bending strain, and longer durability. Here, we report an MXene artificial muscle based on ionically cross-linked Ti 3 C 2 T x with poly(3,4 ethylenedioxythiophene)-poly(styrenesulfonate), showing ultrafast rise time of within 1 s in DC responses, extremely large bending strain up to 1.37% in very low input voltage regime (0.1 to 1 V), long-term cyclic stability of 97% up to 18,000 cycles, markedly reduced phase delay, and very broad frequency bandwidth up to 20 Hz with good structural reliability without delamination under continuous electrical stimuli. These artificial muscles were successfully applied to make an origami-inspired narcissus flower robot as a wearable brooch and dancing butterflies and leaves on a tree as a kinetic art piece. These successful demonstrations elucidate the wide potential of MXene-based soft actuators for the next-generation soft robotic devices including wearable electronics and kinetic art pieces.

Here, we report MXene-based AWIS actuators and their soft robotic applications to kinetic art pieces. Simultaneous intercalation and self-assembly of PP chains ionically bonded with Ti 3 C 2 T x provide a synergistically favorable architecture for fast charge transport and ion intercalation/de-intercalation, resulting in exceptionally high actuation performance compared with those characteristics of neat PP and neat Ti 3 C 2 T x . Ionically cross-linked Ti 3 C 2 T x exhibited not only remarkable improvement of high bending strain ~1.37% without any signs of back-relaxation but also excellent cycling stability (more than 18,000 cycles), strong blocking forces, and highly durable stability for several weeks. We successfully applied our actuator for a narcissus-inspired kinetic robot controlled by electric signals as a wearable brooch. In addition, we successfully demonstrated dancing leaves and butterflies on a tree as kinetic art soft robots by using the MXene-based AWIS actuator.

However, because of the lack of stretchability of MXene, it is difficult to fabricate surface-agnostic MXene coating, which could have good adhesion to polymer electrolyte layers and withstand extreme mechanical deformation. Thus far, for flexible energy storage and electrode applications, Ti 3 C 2 T x composites with various polymers—such as sodium alginate ( 27 ), polyvinyl alcohol ( 31 ), polypyrrole ( 32 ), and polydiallyldimethylammonium chloride ( 31 )—have been developed with high tensile strength but with a compromise on conductivity. Therefore, conducting polymers should be polymeric matrices that can maintain the conductivity and act as a binder to enhance the mechanical strength of Ti 3 C 2 T x . A previous study developed Mo 1.33 C–poly(3,4 ethylenedioxythiophene) (PEDOT)–poly(styrenesulfonate) (PSS) (PP) composites based on ultrathin flexible solid-state supercapacitors that showed a higher capacitance (1310 F cm −3 ) ( 24 ) because of a synergistic effect of increased interlayer spacing between Mo 1.33 C MXene layers due to insertion of conductive PEDOT and surface redox processes of PEDOT and MXene. PP has been studied as a conducting electrode material for use in AWIS actuators; however, it exhibits high response time and low bending strain because of its low capacitance ( 33 ). Because of the hydrophilic nature of PP when coupled with active and charged surface groups (such as ─SO 3 H and ─SO 3 − ), PP is a potentially ideal candidate for polymeric matrices for Ti 3 C 2 T x ; also, PEDOT molecules have a positive surface, which can electrostatically interact with the negatively charged surface of Ti 3 C 2 T x . To date, no ionically cross-linked form of MXene with a polymer has been reported as an electrode for AWIS actuators.

Newly discovered materials under the broad title MXene are part of a young family of 2D transition metal carbides and/or nitrides with the formula M n+1 X n T x , where M represents an early transition metal (e.g., Ti, Zr, V, Nb, Ta, or Mo); X represents carbon and/or nitrogen; T x represents the number of surface terminations, such as O, OH, and/or F; and n = 1, 2, or 3. They are very promising candidates for energy storage devices, transparent conductive electrodes, electromagnetic interference shielding, etc. ( 23 – 29 ). Among more than 20 different types of MXene, Ti 3 C 2 T x has been the most studied variety of MXene to date because of its metal-like conductivity and high storage capacity. Interlayer spacing of Ti 3 C 2 T x can be changed via intercalation and de-intercalation of ions (K + , Mg 2+ , Na + , etc.) ( 30 ). Thus, Ti 3 C 2 T x is a good candidate electrode material for AWIS actuators.

However, for AWIS actuators, the essential requirements are a considerable bending strain, fast response time, low driving voltage, and high durability in the air. Conducting electrodes are critical components in AWIS actuators, covering the two opposite faces of the ionic polymer membrane. These electrodes must be compatible and compliant with the ionic polymer membrane during bending and must maintain their electrical conductivity and high capacitance. As far as electrode materials are concerned, carbon-based materials, because of their high theoretical capacity, are attracting extensive attention for AWIS actuators ( 2 , 4 , 8 – 20 ). Recently, several approaches using nanomaterials or designing nano-architectures with enhanced electrochemical properties or charge storage/transport mechanisms have been developed for next-generation high-performance AWIS actuators. For example, millimeter-long single-walled carbon nanotube (CNT)–based actuators have exhibited high strain ( 21 ); three-dimensional (3D) nanocarbon electrodes (reduced graphene oxide–CNTs) with highly stable porous networks showed promising cycling stability (1 million cycles) in electrochemical actuators ( 22 ). 3D graphitic carbon nitride and nitrogen-doped graphene heteronanostructure-based AWIS actuator electrodes were found to exhibit bending strain (0.52%) at low frequency (0.1 Hz) with 93% actuation retention ( 4 ), and a graphdiyne-based actuator showed promising electromechanical transduction efficiency of up to 6.03% with 0.8% bending strain ( 16 ). However, the sluggish kinetics of large-size ionic-liquid cations, poor transport, severe lack of intercalated ions, and high response time during conversion reaction lead to poor rate capacities under high frequency. These limitations mainly arise from sluggish insertion/extraction and diffusion of ionic liquid ions at the interface of the polymer membrane and across the conducting electrode ( 18 ). Therefore, new electrode materials that can exceed the requirements of next-generation AWIS actuators are strongly required.

The past several decades in the fields of bioinspired and biomimetic technology have seen electroactive polymer (EAP) actuators receive enormous interest as artificial muscles for use in soft robotics, wearable touch-feedback systems, stretchable and flexible electronics, and microelectromechanical systems ( 1 – 3 ). Among EAP actuators, ionic polymer-metal composite actuators have emerged as one of the most attractive EAP actuators due to their light weight, superior performance, air-working stability under low driving voltage, and ease of fabrication at low cost ( 2 , 4 – 6 ). The electrochemical strain of air-working ionic soft (AWIS) actuators depends on storing electrical energy in a double interface and converting it to mechanical energy by reversible migration of ion intercalation and de-intercalation at the interface of the electrode and the electrolyte membrane and in the electrode ( 7 ).

RESULTS

Structure and chemical characterizations of ionically cross-linked Ti 3 C 2 T x MXene Ti 3 C 2 T x MXene was prepared using LiF with HCl for selective etching of Al atoms out of MAX phase Ti 3 AlC 2 (see Materials and Methods and fig. S1). The crystal structure of layered Ti 3 C 2 T x was terminated by T x (─OH, ─O, and ─F) on two surfaces. Figure 1A schematically illustrates the procedures for preparing the ionically cross-linked Ti 3 C 2 T x -PP electrodes and fabricating the AWIS actuators (see details in Materials and Methods). Briefly, the delaminated Ti 3 C 2 T x suspension was mixed with PP at two different ratios, 1:4 and 1:2, of Ti 3 C 2 T x :PP. Schematic representation of the layered structure of MXene ionically cross-linked with PP is displayed in Fig. 1A and fig. S2A. The transmission electron microscopy (TEM) image (fig. S1B) shows a few layers of Ti 3 C 2 T x sheets. The highresolution TEM image (fig. S1C) shows lattice fringes that further confirm the high-level crystallinity of MXene; the interlayer spacing corresponding to the (002) plane is 0.91 nm. A scanning electron microscopy (SEM) image shows the several-micrometer nacre-like layered stacking of exfoliated Ti 3 C 2 T x MXene (Fig. 1B). SEM images of the 1:2 ratio Ti 3 C 2 T x -PP are shown in Fig. 1C and indicate that nacre-like layered stacking of Ti 3 C 2 T x remains, with large interlayer spacing, because of the construction of interlayer support of PP due to the high electrostatic/ionic attraction. Further, Raman spectroscopy was used to understand the molecular-level interaction that occurred between Ti 3 C 2 T x and PP. Figure 1D shows representative bands of Ti 3 C 2 T x in the range of 150 to 750 cm−1. The Raman ω 1g (E g ) mode of about 166 cm−1 at room temperature mainly corresponds to the in-plane vibrations of Ti2 and C atoms (34). The Raman modes at 200 and 689 cm−1 correspond to out-of-plane stretching vibrations of Ti, C, and O atoms (34). Another Raman mode was attributed to in-plane vibrations of surface termination ─O─, ─OH, and ─F (34). However, the intensity (position) of the ω 1g (E g ) Raman mode was reduced (blue shifted) with cross-linking of PP because of compressive stress on the first several layers of atoms of Ti 3 C 2 T x ; also, surface atoms closely pack, resulting in a blue shift in vibrational wave number (35). According to the law of energy relation, there is evidence of enlarged interlayer spacing of MXene because the Raman effect is an inelastic phenomenon. Pristine PP exhibited a prominent peak at 1433 cm−1 corresponding to Cα = Cβ symmetrical stretching vibration of the five-membered thiophene rings on PEDOT and PEDOT’s oxyethylene ring mode at ∼988 cm−1 (36). After cross-linking with Ti 3 C 2 T x , this peak was red-shifted (~7.47 cm−1) because of the change of PEDOT chains from benzoid to quinoid structure (Fig. 1D), which was interpreted as a relief of the physical restriction of the PEDOT’s oxyethylene ring, providing an ability to free vibrations (36). PEDOT has two well-known resonant structures: coil phase benzoid and linear or expanded coil phase quinoid in the ground state. The quinoid phase structure causes charge delocalization on PP film and results in enhanced carrier density (37–39). Eventually, the introduction of PP with Ti 3 C 2 T x incited the phase change of the PEDOT chains from benzoid to quinoid structure because of strong π-stacking interactions between PEDOT and the Ti 3 C 2 T x basal plane. This intermolecular interaction was also verified by monitoring Raman spectral changes (highlighted with green and red color rectangle boxes) of Ti 3 C 2 T x before and after mixing with PP; the band of Ti 3 C 2 T x is more sensitive to intermolecular charge transfer. Because of strong noncovalent hydrogen bonding between ─O─ and ─OH termination of Ti 3 C 2 T x with ─SO 3 H and SO 3 − group of PSS, the Raman band (150 to 230 cm−1) corresponding to Ti, C, and O atoms of Ti 3 C 2 T x gradually weakened and shifted to higher Raman wave numbers (220 to 270 cm−1) corresponding to Ti, C, O, and H atoms. Thus, the coulombic interaction between PSS and PEDOT led to phase separation between them. Subsequently, PEDOT not only transformed to quinoid phase structure (fig. S2, B and C), but there was also an alignment of linear PEDOT on the surface of Ti 3 C 2 T x due to π-π interaction (40). The quinoid structure retained PEDOT rings in the same plane (37), facilitating π electron delocalization and resulting in an increased conductivity of Ti 3 C 2 T x -PP. In addition, the attenuated total reflectance Fourier transform infrared (FTIR) spectra (fig. S3; see the Supplementary Materials for details) validated the Raman results. Further, variations in structures, compositions, and surface chemical states of electrode materials of Ti 3 C 2 T x and PP and materials after interaction of Ti 3 C 2 T x with PP were analyzed by x-ray photoelectron spectroscopy and x-ray diffraction (XRD) patterns, as shown in fig. S4 (see the Supplementary Materials for details). These results indicate that PP was intercalated into the MXene layers and ionically cross-linked with the Ti 3 C 2 T x surface. Fig. 1 Synthesis and characterization of ionically cross-linked Ti 3 C 2 T x MXene. (A) Schematic representation of the synthesis of ionically cross-linked Ti 3 C 2 T x MXene. (B) SEM image of Ti 3 C 2 T x (scale bar, 500 nm). (C) SEM image of Ti 3 C 2 T x -PP (scale bar, 2 μm). (D) Raman spectra of Ti 3 C 2 T x , PP, and Ti 3 C 2 T x -PP. a.u., arbitrary units. (E) Representation of the phase change of benzoid PEDOT into quinoid PEDOT.

Fabrication of MXene-based AWIS actuators and assessment of their electrochemical and mechanical properties For better understanding of the effect of MXene on the actuation performance of AWIS actuators, we prepared flexible ionic actuators by drop casting of symmetric electrode material with two different weight ratios of Ti 3 C 2 T x to PP [Ti 3 C 2 T x :PP = 1:4 (T1PP4) and Ti 3 C 2 T x :PP = 1:2 (T1PP2)]; samples were cut into favorable sizes and shapes (see fig. S5A, and details are in Materials and Methods). Figure 2 (A and C) provides the cross-sectional SEM images of the different actuators. T1PP4 and T1PP2 electrodes had more macroscale porosity than the pristine PP electrode; it seems that PP intercalated between the Ti 3 C 2 T x layers. The enhanced porosity will enlarge the accessible surface area of the electrode, which could improve the electrochemical performance with high specific capacitance. The SEM images show the electrode material uniformly coated on the Nafion electrolyte membrane (Nafion membrane containing EMIm+:BF 4 − ionic liquid), with almost identical thickness on both sides. Asymmetric coating of the electrode material on the electrolyte membrane could cause asymmetric actuation due to the high dependency of actuation on the bending stiffness of the actuator. An additional dominant factor in the electrode coating is adhesion with the electrolyte membrane. Weak adhesion leads to flaking of electrode materials, as shown in an image of the pristine MXene electrode (Fig. 2D), and significantly reduces the performance and durability of the actuator. However, strong adhesion between the Ti 3 C 2 T x -PP electrodes and the electrolyte membrane was established, and no flaking or delamination was observed at the boundary of the two layers, as shown in Fig. 2E. T1PP2 and T1PP4 electrodes show a fractal nature near the edge of the electrolyte membrane, acting as distributed electrode capillaries, which leads to good adhesion between them. The bending displacement of the Ti 3 C 2 T x -based AWIS actuator is small at 0.1 Hz under low driving voltage (0.5 V), as shown in fig. S6, due to lack of good adhesion and flexibility in comparison to the Ti 3 C 2 T x -PP–based actuator. It is well known that the capacitance of electrodes directly influences the magnitude of bending deformation and the response time of ionic actuators. Fundamental properties—such as the porosity, hydrophilicity, and chemical affinity of applied electrode materials—are essential, key parameters for the capacitive characteristics of actuators. In this work, to evaluate the energy storage and charge transfer capability, we conducted electrochemical measurements on whole actuator devices and all free electrodes in the ionic liquid electrolyte. For comparison, cyclic voltammogram (CV) of PP, Ti 3 C 2 T x , and Ti 3 C 2 T x -PP actuators was measured at the scan rate of 10 mV s−1 with an electrochemical window of 2 V, as shown in Fig. 2F. The CV curves display regular rectangular shapes with broad humps, demonstrating excellent capacitive behavior with good ion response (29). CV curves of all actuators do not have prominent redox peaks, but pseudocapacitive contribution could not be avoided, indicating that the charge/discharge of actuator electrodes maintains a pseudo-constant rate over the complete voltammetric cycle due to intercalation or electrosorption on the interface surface of the electrode. To elucidate the contribution of the pseudocapacitance, we measured the CV of a free-standing MXene-based electrode using a three-electrode system in ionic liquid electrolyte solution [see fig. S5 (B and C)]. The fractal surface of the electrode at the Nafion interface can be seen in the cross-sectional SEM image of the Ti 3 C 2 T x -PP actuator; this surface assists the intercalation of the ion inside the electrode. The T1PP2 actuator shows the highest energy storage capability, with a volumetric capacitance of 932 F cm−3 at a scan rate of 10 mV s−1, which is nearly four times higher than that of the PP actuator (238 F cm−3); this high value is the result of a sufficient amount of pseudocapacitive Ti 3 C 2 T x material loading (28). Various previous studies on MXene have examined the restacking tendency between the MXene layers, which inhibits reversible ion and charge transfer to some extent (32). Thus, the PP structure can serve as an interactive pillar to prevent restacking of the MXene sheets, to increase the active areas, and to assist in fast reversible electron and ion transport at the interface. Besides this, fig. S5 (E to G) shows CV results at various scan rates from 10 to 500 mV s−1; these were obtained to further realize the capacitance behavior of all actuators due to a direct relation between higher scan rate and higher frequency bandwidth in actuators. Meanwhile, CV curves obtained at a scan rate of 500 mV s−1 deviate slightly from the rectangular shape, with a larger broad hump; this is mainly caused by low diffusion of the electrolyte ions under high scan rate. Further, electrochemical impedance spectroscopy (EIS) results support the above arguments. A Nyquist plot of all actuators (Fig. 2G) had a semicircle (high-frequency region) and a straight line (low-frequency region) corresponding to charge-transfer resistance (R ct ) and the solid-state diffusion of ions, respectively. The R ct value of the T1PP2 electrode (~54 ohms) is smaller than those of the T1PP4 (>73 ohms) and PP (>130 ohms) electrodes, indicating high charge transport in the T1PP2 electrode, which could contribute to the high capacitance. The series resistance at the high frequency of the T1PP2 electrode is low compared with those of the other electrodes because the incorporation of conduction bands facilitates high electron transport as a result of enhanced electrical conductivity. The electric resistance values of T1PP2 and T1PP4 are lower than those of the neat PP and MXene electrodes. It is confirmed that once the PSS and PEDOT molecules are assembled between the MXene nanosheets, a conductive network is formed via the aligned quinoid phase of PEDOT, maintaining increased interlayer spacing via hindering of layer stacking due to hydrogen bonding between PSSH/PSS− and Ti 3 C 2 T x . The resulting fast and high transport of electrons and ions in Ti 3 C 2 T x -PP is associated with a largely reduced R ct value. The bulk electrical conductivity values of the PP and MXene-based electrodes, as measured by the four-probe method, were calculated to be 14,590.56, 16.504, 14.45, and 0.014466 S cm−1 for T1PP2, T1PP4, Ti 3 C 2 T x , and the pristine PP electrodes, respectively (Fig. 2H). The observed notable improvement in electrical conductivity of the T1PP2 electrode might be due to the modification of the electronic structure of Ti 3 C 2 T x via direct ionic cross-linking with PSS and the linearly aligned high conducting quinoid PEDOT phase, which supports the EIS results. In the case of T1PP2, the synergistic effect of Ti 3 C 2 T x and the balanced quinoid PEDOT phase content led to high electrical conductivity in comparison with T1PP4. In addition, according to the strain-stress curve (Fig. 2I), the Young’s modulus of the T1PP2 electrode is 667.92 MPa, which is smaller than that of the neat PP electrode (1781.45 MPa); however, the strain failure percentage is two times higher than that of PP. Although the tensile strength (23.29 MPa) of T1PP2 is lower than that of the PP electrode, it is significantly higher than that of human skeletal muscle (0.3 MPa) (1). Fig. 2 Morphological, electrical, and electrochemical characterization of all AWIS actuators. (A to C) Cross-sectional SEM images of PP, T1PP4, and T1PP2 electrodes, respectively. Scale bar, ~15 μm. Insets: SEM images of electrode surface. Scale bars, ~2 μm. (D) Optical image of ionic soft actuators with the pristine Ti 3 C 2 T x MXene–based electrode, showing flaking of few MXene sheets under bending. (E) Optical image of ionic soft actuators with the Ti 3 C 2 T x -PP–based electrode, indicating good adhesion and flexibility. (F) CV curves of four actuators at a scan rate of 10 mV s−1. (G) EIS curve of all AWIS actuators. Inset: The magnified high-frequency region. (H) The volumetric capacitance value of all AWIS actuators at a scan rate of 10 mV s−1 and electrical conductivity of all electrodes. (I) Stress-strain curves of PP, T1PP2, and T1PP4 electrode materials.