Shape memory nanocomposite fibers for untethered high-energy microengines

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Science  12 Jul 2019:
Vol. 365, Issue 6449, pp. 155-158
DOI: 10.1126/science.aaw3722

Getting the most out of muscles

Materials that convert electrical, chemical, or thermal energy into a shape change can be used to form artificial muscles. Such materials include bimetallic strips or host-guest materials or coiled fibers or yarns (see the Perspective by Tawfick and Tang). Kanik et al. developed a polymer bimorph structure from an elastomer and a semicrystalline polymer where the difference in thermal expansion enabled thermally actuated artificial muscles. Iterative cold stretching of clad fibers could be used to tailor the dimensions and mechanical response, making it simple to produce hundreds of meters of coiled fibers. Mu et al. describe carbon nanotube yarns in which the volume-changing material is placed as a sheath outside the twisted or coiled fiber. This configuration can double the work capacity of tensile muscles. Yuan et al. produced polymer fiber torsional actuators with the ability to store energy that could be recovered on heating. Twisting mechanical deformation was applied to the fibers above the glass transition temperature and then stored via rapid quenching.

Science, this issue p. 145, p. 150, p. 155; see also p. 125


Classic rotating engines are powerful and broadly used but are of complex design and difficult to miniaturize. It has long remained challenging to make large-stroke, high-speed, high-energy microengines that are simple and robust. We show that torsionally stiffened shape memory nanocomposite fibers can be transformed upon insertion of twist to store and provide fast and high-energy rotations. The twisted shape memory nanocomposite fibers combine high torque with large angles of rotation, delivering a gravimetric work capacity that is 60 times higher than that of natural skeletal muscles. The temperature that triggers fiber rotation can be tuned. This temperature memory effect provides an additional advantage over conventional engines by allowing for the tunability of the operation temperature and a stepwise release of stored energy.

Miniature engines or motors are of practical interest for emerging applications ranging from microrobotics, lab-on-a-chip technology, and smart textiles, to microelectromechanical systems and miniaturized medical devices (1). Making large-stroke, high-speed, high-energy rotating microengines showing simplicity and robustness has long remained challenging. Different mechanisms and materials have been sought to provide torsional rotations, such as shape memory alloys (2), piezoelectric ceramics (3), and electroactive polymers (4). The most promising rotating performances have been achieved using the concept of twisted fibers (5), as the insertion of the twist amplifies the strokes and work capacities compared with those of nontwisted or noncoiled fibers.

Following the demonstration of electrochemically driven motors based on twist-spun carbon nanotube (CNT) yarns (6), highly coiled CNT yarns have emerged. The helical topology enables a conversion of the yarn volumetric expansion into tensile contraction and torsional untwist, delivering high gravimetric work capacities (710). The volume change is driven by the expansion of infiltrated guest materials in response to heat (7), liquid adsorption (8), or by the surface tension of liquids diffused through gaps of hierarchically arranged helical CNT fibers (10). Nevertheless, the difficulty and high cost of fabricating CNT muscles has restricted their applications. Alternatively, simple and low-cost engines can be made by twisting polymer fibers. For example, the twisted rubber band used in airplane toys untwists because of the entropic elasticity of polymer chains (11). However, this type of engine suffers from a low energy density owing to the low Young’s and shear moduli of elastomers. Twisted nylon-6,6 fibers contract and untwist in response to thermal expansion with a high energy density but necessitate large temperature changes (12). Buckled sheath-core rubber muscles that operate electrically have also been proposed as rotary motors (13). Here, we show that the twist insertion into shape memory nanocomposite fibers is efficient to create hook-free and high-energy microengines. Because of the energy storage capability (14) and the triggering of the untwist by a small increase in temperature above the switching temperature Tsw (which is in the range of an involved thermal transition) by environmental heating, these multifunctional micromachines can work untethered.

We start with polyvinyl alcohol (PVA)–based shape memory polymer (SMP) fibers with high strength and toughness (15), in which crystallites form the permanent netpoints. A 2-cm-long, 40-μm-diameter PVA fiber (Fig. 1A) was first heated to the programming temperature Td ~ 100°C (which is above the glass transition temperature Tg ~ 80°C). Isobaric twist [under conditions of constant tensile force provided by a weight of 1.2 g attached to the fiber end (unless otherwise noted, the weight is fixed at 1.2 g)] was then inserted into this fiber at 7500 turns per meter of fiber length, with a rotation speed of 60 revolutions per minute (rpm). Afterwards, the twisted SMP fiber was quenched to room temperature Tr to fix the coiled structure (Fig. 1B). The coiled structure can be retained without being hooked because of the glassy nonequilibrium conformation of the helically configured polymer chains. Upon reheating the twisted fiber to a temperature above Tsw (14), the one end-tethered fiber rotated its free end to revert to its original, equilibrium shape via untwist (Fig. 1C). A practical example of the programming and untwist of the twisted fiber in response to heat (shape memory effect) is shown in movie S1.

Fig. 1 SMP fibers for rotating microengines.

(A) Scanning electron microscopy (SEM) images of a 40-μm-diameter PVA fiber. Its cross-sectional morphology is shown in fig. S11. (B) SEM image of the coiled PVA fiber. (C) Evolution of the morphology of the twisted fiber as it untwists in response to heating. (D) Tensile stress versus strain curves at Tr for pure PVA fibers and PVA nanocomposite fibers. (E) The torque needed to twist the pure PVA, PVA-SWNT, and PVA-GO fibers until their rupture at Tr. The quantitative measurements of the torque and applied twist angle were obtained using homemade instruments (fig. S6).

The energy that a rotating engine provides via shape recovery is a function of the energy absorbed during the twist programming at Td. The polymer fibers can be stiffened by the inclusion of reinforcing nanofillers and can be made more efficient for high-energy microengines. We prepared single-walled carbon nanotube (SWNT)– and graphene oxide (GO)–doped PVA fibers by using a wet-spinning method upon injection of a PVA-SWNT dispersion or PVA-GO solution in an aqueous solution of Na2SO4 as a coagulating bath (16). The wet-spun composite fibers have 5 weight % (wt %) SWNT and 5 wt % GO nanoparticles (15).

Quantitative characterizations were achieved by measuring the stress needed to stretch or the torque needed to twist the fibers at Tr. The pure PVA fiber shows high toughness and a tensile Young’s modulus of 4.9 GPa at Tr (Fig. 1D). The incorporation of SWNTs or GO decreases the strain to failure but increases the Young’s modulus up to 13.5 and 12.5 GPa, respectively. Both nanofillers have nearly the same reinforcement efficiency on the tensile properties. By contrast, a greater torque is needed to twist the PVA-GO fiber to a given angle compared with that needed to twist pure PVA and PVA-SWNT fibers (Fig. 1E). By considering the fiber to be a noncoiled cylinder, the shear stress and shear strain can be calculated. The elastic shear moduli are deduced from the ratio of shear stress and shear strain at small twist angles (shear strain < 0.02) (fig. S1). Note that at higher strains, plastic deformation takes place, followed by coiling at extreme twist levels. Nevertheless, the exact onset of coiling could not be directly measured from the shear stress–shear strain curves. The SWNT and GO nanoparticles boost the shear modulus (~1.3 GPa) of PVA fiber up to 2.3 and 6.8 GPa, respectively. Relative to SWNTs aligned along the fiber axis (17), GO has a more pronounced effect on the improvement of torsional properties because of its two-dimensional (2D) structure. The shear is applied perpendicular to the nanotube orientation direction but remains within the plane of 2D GO platelets. The nanosheets allow the fiber to sustain a high torque under twist (fig. S2). We therefore chose the strong PVA-GO fibers to investigate in more depth the torsional rotation of shape memory microengines.

We investigated the effect of programming temperature Td on the torsional untwist for PVA-GO fibers. A higher torque is needed to twist the fibers at a lower Td, indicating that more torsional mechanical energy is stored during programming (Fig. 2A). When reheating the fiber in free load conditions at a rate of 5°C/min (unless otherwise noted, the heating rate is fixed at 5°C/min), higher full rotations are generated for fibers that have been initially programmed at lower Td (Fig. 2B). A maximum value of 6123 turns/m was recorded by varying the temperature by ~160°C. Fibers that have been twisted at greater Td recover their shape at higher temperatures but show decreased full rotations. The recovery rate Rr (the ratio of recovery number of rotations to the applied number of turns) decreases as the programming temperature increases. The best Rr (82%) is obtained for a programming temperature near 80°C, at which a maximum mechanical energy was stored.

Fig. 2 Temperature memory of twisted fibers.

(A) 3-cm-long, 40-μm-diameter PVA-GO fibers were programmed by inserting a twist of 7500 turns/m with a rotation rate of 60 rpm at different temperatures Td ranging from 80° to 140°C. The specific torque versus twisted angle was recorded at each temperature. (B) Recovery rotation with free load for the twisted fibers that are programmed at different Td. (C) Evolution of rotation speed with temperature obtained by the first derivative of the curve of rotation versus time shown in fig. S3. (D) Recovery-specific torque as twisted fibers, two ends of which are tethered to prevent fiber untwisting, are heated.

On the basis of the first derivative of the curve of rotation versus time (fig. S3), the rotation rate can be calculated and is plotted as a function of temperature in Fig. 2C. The twisted fiber untwists with a peak rate at a well-defined temperature, which is equal to Td, showing a temperature memory effect (TME) closely reminiscent of the TME previously observed in tension or contraction (1821). The rotation rate largely depends on the heating rate. Upon immediate heating from Tr to 100°C, the fiber rotates at its free end a 5000-times-heavier syringe needle and a paper paddle to a peak rotation rate of 600 rpm in 2 s. It maintains rotation for ~5 s and ~50 full turns (fig. S4 and movie S2). The cycling shape memory behavior of PVA-GO fibers is characterized (fig. S5) and stable cyclic performances are demonstrated (15). Additionally, as thermally powered shape memory nanocomposite microengines, the twisted fibers can be principally triggered to untwist by any kind of heating method. An example of heating the fiber in viscous silicon oil is demonstrated in movie S3.

We directly measure the generated torque upon reheating the programmed fiber at fixed deformation from Tr to 220°C using homemade instruments (15) (fig. S6). The fiber programmed at 100°C generated a maximum gravimetric torque of ~21 N·m/kg (Fig. 2D), which is lower than that of bacterial flagella (200 N·m/kg) (22, 23) but higher than that of previously developed artificial torsional motors (table S1). Even by varying Td far from 100°C, this value decreases but still remains on the order of 10 N·m/kg. Moreover, the recovery torque peak occurs at the temperature at which the fibers were programmed. This is a consequence of the TME. The distinctive feature of temperature memory with Tsw from 80° to 140°C, compared with other rotating engines, enables the shape memory nanocomposite microengines to be customized for releasing the stroke or torque in the vicinity of a certain temperature needed for a particular application.

The generated mechanical energy is expressed as U=Γdθ when the fiber rotates by an angle θ against a given torque Γ. Twist programming was applied onto small-diameter pure PVA, PVA-SWNT, and PVA-GO fibers, which were obtained by thermally drawing the as-prepared fibers by 200%. The reduction of the fiber diameter (from 40 to 23 μm) retards the onset of coiling, thus largely increasing the amount of twist that can be inserted before the direct contact of neighboring coils (12). A greater torque is needed to twist PVA-GO fiber at Td ~ 100°C as compared with that needed for pure PVA and PVA-SWNT fibers because of the torsional reinforcement effect of GO additives (Fig. 3A), which was also observed at Tr (Fig. 1E).

Fig. 3 High energy density of shape memory microengines.

(A) Torque needed to twist 3-cm-long, 23-μm-diameter pure PVA, PVA-SWNT, and PVA-GO fibers by 1885 rads (10,000 turns/m) at a rate of 60 rpm at Td ~ 100°C. After twisting, the fibers are quenched in air to form coils with and without being torque balanced for further characterizations. (B) Recovery torque generated by the coiled fibers that have been quenched without being hooked when they are reheated. (C and D) Recovery angle upon reheating in conditions of variable applied torques τapplied for the programmed fibers. (E and F) The gravimetric work capacities as a function of the ratio of applied torque τapplied to the blocking torque τmaximum. The coiled fibers used for (C) and (E) are formed by quenching in air without being hooked, therefore losing some twist; whereas for (D) and (F), the coiled fibers are quenched by balancing the twist inserted into the fibers. The hook is removed before the fiber is stimulated to rotate.

Upon reheating the two ends-tethered twisted fibers from Tr to 210°C, the recovery torque reaches a maximum blocking torque τmaximum at 100°C (Fig. 3B). It reaches ~0.27 μN·m for PVA-GO fiber, which is higher than 0.12 and 0.11 μN·m, respectively, achieved for PVA-SWNT and pure PVA fiber. We measured the recovery angle against an applied torque τapplied below τmaximum. As the temperature increases to an operation window (ΔT, in the vicinity of Td ~ 100°C), in which the recovery torque remains superior to τapplied, the fiber starts to untwist and provide mechanical output. For PVA-GO fiber, as the applied torque increases from 20 to 80% of the blocking torque, the recovery rotation decreases and the needed operation window ΔT narrows from 120° to 70°C (Fig. 3C). An optimum work capacity is achieved at an intermediate load. The gravimetric work capacity is defined here as the amount of work done by the fibers, divided by the mass of the fibers. Figure 3E shows the gravimetric mechanical output as a function of the ratio of τappliedmaximum. In response to a temperature variation of 100°C in the vicinity of Td, the twisted PVA-GO fiber can rotate against a load of 50% of τmaximum to provide a maximum gravimetric work capacity as high as 1800 J/kg (fig. S7). This energy density is far higher than that of mammalian skeletal muscles (39 J/kg) (24) and higher than that of most of the previously reported artificial rotary engines (table S1).

The wet-spun PVA fibers do not 100% fix the twisted deformation because of polymer chain relaxation and thermal expansion. These effects result in the loss of some turns of twist after being quenched in air. A torque-balanced structure can be added to prevent such untwist and further enhance the mechanical output. The hook is immediately removed before the fiber is stimulated to rotate. Figure 3D shows the recovery rotations against different loads for the twisted PVA-GO fibers that have been quenched and hooked. At the same load, the fibers rotate by a larger number of turns compared with twisted fibers quenched without being torque balanced (Fig. 3C) and thus provide a greater gravimetric energy density of 2766 J/kg at an optimum τapplied (50% of τmaximum) (fig. S8 and Fig. 3F).

For comparison, PVA-CNT fibers and pure PVA fibers were used to perform exactly the same protocols to characterize their optimum energy density. Energy densities of 628 and 1115 J/kg were observed for twisted PVA-CNT fibers that had been quenched without and with being torque balanced, respectively (Fig. 3, E and F, and fig. S9); whereas pure PVA fibers showed similar energy generation capabilities, 632 and 925 J/kg for the fibers quenched without and with being hooked, respectively (Fig. 3, E and F; fig. S10; and table S1).

With this work, we have created rotating microengines based on multifunctional SMP nanocomposite fibers that can store mechanical energy and rotate in response to heat. This in-demand shape recovery can be triggered by environmental heating at a temperature that depends on the programming temperature (temperature memory). PVA fibers that are torsionally stiffened by the inclusion of 2D GO lead to untethered microengines with high gravimetric work capacity. Additionally, the distinctive temperature memory feature enables a large tunability of the storage and release of mechanical energy.

Supplementary Materials

Materials and Methods

Supplementary Text

Figs. S1 to S19

Table S1

References (2549)

Movies S1 to S4

References and Notes

  1. Materials and methods are available as supplementary materials.
Acknowledgments: We thank I. Ly for SEM images of the fibers. Funding: A.L. and K.K. were financially supported by the Helmholtz Association through program-oriented funding. Author contributions: P.P. and J.Y. conceived and designed the research project. W.N. prepared the wet-spun fibers. J.Y. and C.Z. performed the shape memory characterizations. P.M. designed instruments for characterizations of torsional properties of fibers. J.Y., C.Z., K.K., A.L., and P.P. analyzed the data. J.Y. and P.P. wrote the paper. All authors discussed the results and commented on the manuscript. Competing interests: A.L. and K.K are co-inventors on patents in the field of polymer-based shape memory materials and fibers. Data and materials availability: All data are available in the main text or the supplementary materials.
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