Strain-programmable fiber-based artificial muscle

Abstract

Artificial muscles may accelerate the development of robotics, haptics, and prosthetics. Although advances in polymer-based actuators have delivered unprecedented strengths, producing these devices at scale with tunable dimensions remains a challenge. We applied a high-throughput iterative fiber-drawing technique to create strain-programmable artificial muscles with dimensions spanning three orders of magnitude. These fiber-based actuators are thermally and optically controllable, can lift more than 650 times their own weight, and withstand strains of >1000%. Integration of conductive nanowire meshes within these fiber-based muscles offers piezoresistive strain feedback and demonstrates long-term resilience across >10⁵ deformation cycles. The scalable dimensions of these fiber-based actuators and their strength and responsiveness may extend their impact from engineering fields to biomedical applications.

Fig. 1.. Fabrication, and morphological characterization

(A) Bimorph fibers produced via two-step thermal drawing. HDPE = high density PE. (B), A photograph of ~60 m of PMMA-encapsulated bimorph fibers. (C) An illustration of the PMMA cladding removal. (D) Cross-sectional scanning electron microscope (SEM) images of a fiber prior to and following cold drawing. Inset: COCe structure after stretching (Scale bar: 20 μm). (E) Cold drawing process to obtain a spring actuator. Steps 1 to 4 show the stretching process. Upon release, shown in steps 5 to 8, the fibers formed springs. (F and G) Micrographs and photographs of the first-step and second-step artificial muscles (Scale bars in F: 200 μm). (H) Fiber-based muscle contracting in response to temperature increase of ΔT=14 ^oC (spring index: k = 4.70, 6 turns/cm).

Fig. 2.. Mechanical characterization of fiber-based artificial muscle.

(A) Stress-strain curves recorded at different extension rates for precursor fibers (without PMMA cladding) with a cross-sectional area of 300×470 μm². (B) Change in the spring diameter with respect to the deformation rate. (C) Spring diameter and number of coils versus applied pre-strain. Error bars and shaded areas represent average and standard deviation, respectively. The number of samples n=5. (D) Change in the spring index with respect to spring diameter and actuation strain. (E) Change in the actuation stress-strain (grey) and work capacity (green). Attributes of the fibers are k=6, 6 turns/cm (continuous line); k=5, 10 turns/cm (dashed line); k=5.5, 12 turns/cm (dot line). (F) Change in the residual stress with respect to the cross-sectional area. (G) The setup used for the force measurement of the fibers with 300×470 μm² cross-section (scale bar is 5 mm). For the fibers with a 8×12.5 μm² cross-section (scale bar is 200 μm), a similar setup was used. The optical heat source was replaced with a micro-Peltier heater and a force gauge had a higher resolution. (H) Temperature and force responses to photothermal pulses collected for a fiber with 300×470 μm² cross-section (k=5, 8 turns/cm). (I) Generated force vs. the temperature difference (number of cycles 300) for fibers with 200×312 μm² cross-section (k=5, 10 turns/m). Navy blue, cerrulean blue, cyan, and green data point clusters represent different temperature ranges of 3.32±0.26 °C, 5.28±0.37 °C, 9.74±0.34 °C, and 12.98±0.74 °C, respectively. (J) Temperature and force responses to thermal pulses for a fiber with an 8×12.5 μm² cross-section (k=4.6, 60 turns/cm). (K) Change in the efficiency with respect to actuation strain (blue line k=6, 6 turns/cm; green line k=5, 10 turns/cm; grey line k=5.5, 12 turns/cm). (L) Force measured across 300 thermal actuation cycles applied over 3 consecutive days for a fiber with 200×312 μm² cross section (k= 5, 10 turns/cm). Inset: A single actuation cycle.

Fig. 3.. Electrical feedback in fiber-based artificial muscle.

(A) A schematic illustration of the setup for the resistance measurements for fibers with 300×470 μm² cross-sectional area (k = 5, 8 turns/cm). Inset: A SEM image of silver nanowire mesh on the surface of a 300×470 μm² fiber-based muscle. (B) Resistance waveforms collected at different numbers of extension and release cycles. (C) The change in the extended and released resistance for ~12000 cycles. (D) Hysteresis curve showing the resistance vs. the applied strain for a single cycle of deformation. The lines represent average and error bars represent the standard deviation.

Fig. 4.. Thermal actuation of fiber-based muscles.

(A) Schematic illustration of the vertical lift experiment, where t is time, m is mass, and Δx is displacement. (B) A photographic time series collected during a displacement experiment in (A). The heat was applied in 2 s pulses separated by 6 s rest epochs. The load mass is 1 g. (C) Waveforms for the heat pulses (top), the corresponding change in the temperature at the fiber surface (middle), and the displacement of the 1g load. (D) The vertical displacement Δx of a 1g load in response to temperature increase ΔT. (E) Strain measurement across 100 cycles of thermal actuation. (F) The maximum displacement for fiber bundles loaded with weights 1, 2, 3, 4, 5, 10, 50, 100, and 200 g. (G) A printed model of a weight-lifting artificial limb, inspired by a human arm. (H) A photographic time series of the artificial limb lifting a 1g load. The heat was applied using a heat gun for 2 s and then followed by 5 s rest epochs. (A-G) Fiber cross-sectional area is 300×470 μm², spring index k = 5, and the number of turns per cm is 8. (I) The change in the fiber length, a piezoresistive strain feedback signal, and the angle of the arm for the experiment in (H).