Large stroke radially oriented MXene composite fiber tensile artificial muscles

Abstract

Actuation is normally dramatically enhanced by introducing so much yarn fiber twist that the fiber becomes fully coiled. In contrast, we found that usefully high muscle strokes and contractile work capacities can be obtained for non-twisted MXene (Ti₃C₂T_x) fibers comprising MXene nanosheets that are stacked in the fiber direction. The MXene fiber artificial muscles are called MFAMs. We obtained MFAMs that have high modulus in both the radial and axial directions by spinning a solution containing MXene nanosheets dispersed in an aqueous cellulose solution. We observed a highly reversible muscle contraction of 21.0% for a temperature increase from 25° to 125°C. The tensile actuation of MFAMs mainly results from reversible hydrogen bond orientation change during heating, which decreases intra-sheet spacing. The MFAMs exhibited fast, stable actuation to multiple temperature-generating stimuli, which increases their applications in smart textiles, robotic arms, and robotic grippers.

Fig. 1.. Fabrication and characterization of the MFAMs.

(A) Schematic illustration of the fabrication of the MFAMs. Under the notable extensional stresses, the hybrid MXene/CNFs nanosheets were aligned perpendicular to the MFAM fiber. (B) Photograph of a hundred-meter-long MFAM wound around a reel. (C) SEM images of the surface of an MFAM. (D) HR-TEM images of the longitudinal section for an MFAM. (E) Digital photographs and infrared thermal images of the reversible contractile actuation of an MFAM under heating and cooling. (F) Tensile stroke and work capacity versus the applied tensile stress on an MFAM. Scale bars, 1 cm (B), 100 μm (top) and 1 μm (bottom) (C), 50 nm (left), and 10 nm (right) (D), and 5 mm (E).

Fig. 2.. In situ temperature–dependent characterization and MD simulations of the MFAMs.

(A) The Herman’s orientation factor and Poisson’s ratio of the MFAMs at various temperatures. (B) On the basis of in situ X-ray patterns, the d-spacing and the calculated porosity of the MFAMs decrease with increasing temperature from 25° to 125°C. (C) In situ FTIR spectra of the MFAMs when heated from 25° to 125°C. (D) Numerical density simulation results for water molecules at the interface of the hybrid MXene/CNFs nanosheets at different temperatures. (E) Simulation results for the degree of parallel distribution of water molecules within hydrogen-bond networks at the interface of hybrid MXene/CNFs nanosheets when heated from 25° to 125°C. (F) Schematic illustration of the mechanism for the thermally driven contractile actuation of the MFAMs. The decrease in intra-sheet spacing, induced by the orientational rearrangement of nanoconfined hydrogen bond networks, leads to the flattening of hybrid MXene/CNFs nanosheets. This zipping effect reduces the void volume as the temperature increases, resulting in the large observed tensile contraction of the MFAMs.

Fig. 3.. Mechanical and actuation performance of the MFAMs.

(A) Typical stress-strain curves of the MFAMs at different temperatures. (B) Tensile strength and Young’s modulus of the MFAMs at different temperatures. (C) Comparison of tensile strength versus Young’s modulus of the MFAMs and reported polymer fiber–based muscles including LCE fiber, poly(ethylene oxide) (PEO) fiber, proteins fiber, polyvinyl alcohol (PVA) fiber, and natural fiber. (D) The generated temperature and contractile actuation stress of an MFAM as a function of the applied square-wave dc voltage at 0.05 Hz. (E) Contractile actuation stress versus time curves for an MFAM when a square-wave dc voltage of 4 V cm⁻¹ was applied at frequencies from 0.02 to 5 Hz. (F) The tensile strokes and work capacities of the MFAMs under a 0.64-MPa load when a 4–V cm⁻¹ square-wave dc voltage at different frequencies was applied. The insets are optical photographs of tensile contraction when the applied dc voltage was increased from 0 to 4 V cm⁻¹. (G) Contractile actuation stress versus cycle number for an MFAM when a 1-Hz square-wave dc voltage of 4 V cm⁻¹ was applied. (H) Optical photographs (left) and infrared thermal images (right) of an MFAM before actuation and after photothermal actuation by using an 808-nm NIR laser to deliver a power density of 4.59 W cm⁻¹ to the fiber surface. (I) The generated temperature and contractile actuation stress of an MFAM when irradiated using different power intensities from a NIR laser. (J) Comparison of the work capacity versus maximum stroke of the MFAMs and reported non-twisted, thermally driven fiber-based muscles, including GO fiber, LCE fiber, SMP fiber, CNTC fiber, and natural muscles. Scale bars, 5 mm (F), 5 mm (H).

Fig. 4.. Demonstration of contractile actuation of the MFAMs.

(A) Schematic of a textile woven with MFAMs to provide heat-generated programmable shape deformation. (B) Infrared thermal images of thermal actuation for the textile at 25° and 50°C. (C) Schematic illustration and optical photographs of a weight-lifting artificial arm powered by four parallel MFAMs, which were heated by an NIR laser. When the MFAMs were exposed to an 808-nm NIR laser, they contracted like the bicep muscles of an arm. (D) A plot showing the angle of elbow bending during 200 repeated cycles of NIR laser–powered actuation. (E) Eight parallel MFAMs were combined and applied in a robotic gripper for grasping, transferring, delivering, and recovering a toy ball. Scale bars, 1 cm (B), 5 mm (C), and 5 mm (E).