Human-muscle-inspired single fibre actuator with reversible percolation

Artificial muscles are indispensable components for next-generation robotics capable of mimicking sophisticated movements of living systems. However, an optimal combination of actuation parameters, including strain, stress, energy density and high mechanical strength, is required for their practical applications. Here we report mammalian-skeletal-muscle-inspired single fibres and bundles with large and strong contractive actuation. The use of exfoliated graphene fillers within a uniaxial liquid crystalline matrix enables photothermal actuation with large work capacity and rapid response. Moreover, the reversible percolation of graphene fillers induced by the thermodynamic conformational transition of mesoscale structures can be in situ monitored by electrical switching. Such a dynamic percolation behaviour effectively strengthens the mechanical properties of the actuator fibres, particularly in the contracted actuation state, enabling mammalian-muscle-like reliable reversible actuation. Taking advantage of a mechanically compliant fibre structure, smart actuators are readily integrated into strong bundles as well as high-power soft robotics with light-driven remote control.


Optimization of highly aligned LCFs
LCFs were designed by two-step process via direct melt-spinning, as schematically described in Supplementary Fig. 34a (see Supplementary Fig. 35 for the home-made equipment). First, LCO dope was extruded through the nozzle onto glass plate collector at 15 °C below from Tnp (above 90 °C for all dopes) using heating coil, (Supplementary Video 1) as higher temperature (in isotropic phase) degraded the alignment of LC, while a lower temperature led to the solidification without fluidity. The mesogenic units with random direction in LCO dope were aligned into monodomain along the fiber axis under the external shear. Subsequently, cross-linked network of uniaxial alignment within LCFs was fixed via UV photopolymerization of diacrylate. This continuous melt-spinning process with home-made equipment renders continuous scalable fabrication of highly aligned elastomeric fiber as shown in Supplementary Fig. 34b. Supplementary Fig. 36 exhibits the evolution of alignment state for the as-spun fibers summarized with respect to the fiber draw ratio under various spinning parameters. The draw ratio was controllable by the speed of collector stage movement against the fiber extrusion rate. The gap distance between spinneret and collector plate (d) was kept constantly at 1 mm for the desirable alignment of mesogenic units. Draw ratio below 2 led to the accumulated fiber morphology at substrate surface. Fibers with the draw ratio over 2 showed optical birefringence under the rotatable crossed polarizers perpendicular to the direction of the light propagation verifying the preferential alignment of LC oligomers along the fiber axis ( Supplementary Fig. 37a). While the birefringence becomes stronger along with draw ratio, discontinuous feature appeared above the draw ratio of 30. In the draw ratio of 10 to 30 significant thinning of fiber diameter was observed, which is undesired for the practical treatment and actuating applications. Taken together, an optimal condition was established at the draw ratio of 8 to 9 for the monodomain LC alignment state of LCFs with sufficiently large fiber diameter of ~203 μm ( Supplementary Fig. 37b).

Determination of molecular weight of LC oligomer
Mixture of nematic diacrylate monomer with chemical structure as shown in Supplementary Fig.   38a. The three peaks in the 1 H NMR spectrum (peak b, c, d in Supplementary Fig. 38b) appearing at 5.85-6.45 ppm correspond to six protons in the diacrylate end groups of the LCO. The integration value of these peaks was used to calibrate other peaks and set as six. The peak at 8.15 ppm (peak a) correspond to the four aromatic protons in the LCO with integration value of 71.31.

Chemical analysis of EG
Electrically exfoliated graphene was prepared from graphite foil two electrode systems as shown in Supplementary Fig. 39a. SEM and AFM characterizations identified the average flake size of a 3.3 μm and thickness of 2.01 nm, respectively (Supplementary Fig. 39b to d). High-resolution of C 1s spectra of EG by XPS detected the C=C bonding as the major peak at 284.5 eV that corresponds to sp 2 hybridized carbons from the basal graphitic plane together with the additional peaks at 285.5 eV, 286.5 eV, 287.9 eV, and 288.9 eV assigned to C-C, C-O, C=O, and O-C=O bonding, respectively ( Supplementary Fig. 40a) 57 . Carbon to oxygen (C/O) ratio within the C 1s peak from XPS data was calculated to be ~7, considerably higher than those of graphene oxide (GO) and reduced graphene oxide (rGO) 58 . Further chemical analysis was investigated by Raman spectrum displaying an intense D peak at 1350 cm -1 and G peak at 1590 cm -1 , respectively (Supplementary Fig. 40b) 59 . EG shows the D/G ratio of 0.96 verifying the low degree of defects.
A clear 2D peak at 2710 cm -1 associated with the presence of disorders was also detected 60 . The intensity ratio of 2D/G, which is relevant to the graphitization degree in carbon structure was obtained to be 0.26 for our EG, further suggesting the high quality compared to typical rGO.
Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of EG dispersions proves the highly exfoliated state with the presence of peaks for C-O-C stretching at 1050 cm -1 , C-OH stretching at 1365 cm -1 , and C=O stretching derived from carbonyl and carboxylic groups at 1720 cm -1 with skeletal vibration of graphitic domains at 1610 cm -1 as shown in Supplementary   Fig. 40c 61 .

Detailed mechanical properties characterization of LCF and G-LCFs
Stress-strain curves obtained from tensile tests exhibit semi-soft elastic plateaus originated from the innate ductility of LCE matrix, accompanied by the noticeable strengthening by EG loading.
The tensile strength is dramatically enhanced with EG fillers (up to 15.1-folds for G0.3-LCF), while preserving the maximum elongation below 200% for a well-aligned LC configuration. The increase of Young's modulus from 12.8 (pure LCF) to 80.6 MPa (G0.3-LCF) shows a good agreement with the theoretically predicted values but significant deviation starts to occur above 0.75 vol% (G0.5-LCF) due to the segregation of EG flakes. Toughness also proportionally increases with the EG content from 2.4 (pure LCF) to 22.2 MJ m -3 (G0.3-LCF).

Comparative specification and performances of LCF and G-LCFs
Systematically optimized single LCE fibers show a regular diameter of 203 μm with or without graphene fillers and revealed light density below 1070 kg m -3 . Light driven actuation behaviors estimated by DMA (Q850) reveal ultrafast response time compared to the thermal-driven ones. In addition, these artificial muscles fibers with high contraction rate (45%) can be readily assembled into bundled structures. Correlated performances of the as-prepared artificial muscles such as work capacities and power densities calculated by considering above characteristics were examined in the single strand or fiber bundle level, as summarized in Supplementary Table 1. Overall, substantial enhancements of actuation performances were realized by homogeneous intercalation of graphene fillers in LCE network. Our genuine light-weight artificial muscle fibers showed outstanding actuation stress and work capacity arising from the enhancement of stiffness, as summarized in Supplementary Table 2. It is also noteworthy that our artificial muscle fiber system exceeded the most of previous LCE based actuators, specifically exhibiting the work capacity and power density approximately 3.5 and 17 times higher than natural human muscles. In addition, ultralight all-carbon artificial worm consisting of our high-power composite fibers also showed a superior crawling speed compared to the natural inchworms and attained one of the fastest values among the previous artificial soft crawlers based on diverse strategies of actuation as shown in Supplementary Table 3. Supplementary Fig. 1│a, Photograph of meter-long continuous single strand G-LCF. b, Single level G-LCFs deposited on the fiber collector surface and gathered bundle level of G-LCFs with different number of strands. Scale bar, 5 mm. Fig. 2 │ a, Dispersion test of EG flakes in DMF with additional polydimethylsiloxane (PDMS) oligomer, polystyrene (PS), and LC oligomer in similar molecular weights for 3 weeks. b, Homogeneous EG-LCO hybrid dope with 0.3 wt% EG.