Thermally driven carbon nanotube@polycaprolactone coaxial artificial muscle fibers working in subzero environments

ABSTRACT Artificial muscle fibers driven electrothermally with excellent properties of response, stroke, and work capacity are expected to serve in some intelligent structures and systems. However, muscle fibers that operate in subzero environments are highly needed in industrial production and aerospace applications but remain challenging. Herein, we reported a coaxial artificial muscle fiber by electrospinning a sheath of polycaprolactone (PCL) nanofibers on the surface of a carbon nanotube (CNT) fiber core, achieving the actuation in response to thermal at subzero temperatures. The CNT@PCL coaxial muscle fiber under 0.3 MPa achieved a maximum contractile stroke of ~18% as the temperature changed from −130°C to 45°C. The actuation mechanism at subzero temperatures of this muscle fiber was analyzed in combination with the temperature-deformation schematic curve of different polymers. Furthermore, a temperature sensor based on this muscle fiber was developed, due to the excellent linear relationship between the contraction and temperature. A 3D-printed prosthetic arm was designed to further exhibit the application demonstrations of this muscle fiber in subzero environments. This work provides new insights into artificial muscle fibers for serving in extreme environments with ultralow temperatures. GRAPHICAL ABSTRACT

Electrothermally driven artificial muscle fibers can rely on the Joule heating to complete the actuation process without the assistance of additional external substances [22][23][24], and thus they have high flexibility for broad application scenarios. The most typical example is the electrothermally driven artificial muscles from fishing lines and sewing threads reported by Haines et al., which achieved a great breakthrough in actuation and work capacity [9]. The polyamide 6,6 (PA66) muscle fiber produced a maximum contractile stroke of 49% by mandrel-coiled structure and a maximum specific work of 2.48 kJ/kg by the self-coiled structure, respectively. Recently, a sheath-run polyimide (PI)-polydimethylsiloxane (PDMS)@carbon nanotube (CNT) muscle fiber reported by Hu et al. had a new advancement in fast response, which generated a contractile stroke of 7.3% at 8 Hz and 14.3% at 1 Hz when applied a square-wave voltage [25]. Additionally, Wang et al. endowed the multifunctionality to electrothermally driven muscle fibers by using the twisted elastomer fiber coated with a buckled CNT sheet, which integrated the functions of actuation and self-sensing in a single fiber [26].
Despite the fact that the electrothermally driven muscle fibers have been rapidly developed, it should be noted that, these muscle fibers operating in extreme temperature environments (e.g. space, Antarctica, Arctic, mountain, and so on) are highly in demand yet rarely achieved [27,28]. Very recently, our group started to focus on the development of yarn muscles for serving in high-temperature environments. The CNT/poly (p-phenylene benzobisoxazole) (PBO) composite yarn muscles with high-temperature tolerance up to 300°C had a high contractile stroke of 18.9% and long-term stability according to the high-temperature resistance of PBO [27]. Artificial muscle fibers operating in low-temperature environments also started to attract attention, and polymer materials for these muscle fibers should be concerned about the brittle failure. Zhang et al. reported a PI/Cu/PDMS composite yarn muscle that achieved a tensile actuation of 20.7% under 1.2 MPa load with the temperature from −50°C to 160°C [28]. The working temperature conditions of these polyethylene and polyamide yarn muscles cannot exceed below −50°C. Once the critical temperature is exceeded, the polymer materials will undergo irreversible brittle damage. Therefore, it is still a great challenge to develop artificial muscles that tolerate lower subzero environments to serve the more extreme application requirements.
In this paper, a sheath-run artificial muscle fiber by electrospinning a sheath of polycaprolactone (PCL) nanofibers on the surface of a CNT fiber core was presented, which achieved the actuation in response to thermal at a subzero temperature of −130°C. To the best of our knowledge, this polymer-based muscle fiber breaks through the current temperature limitations on artificial muscle fibers for subzero environments. Combined with the temperature-deformation schematic curve of different polymers (such as PA66 and polyacrylonitrile), the actuation mechanism at subzero temperatures of this muscle fiber was analyzed in detail. According to the excellent linear relationship between the contraction and temperature, a temperature sensor using this muscle fiber was developed. Furthermore, a 3D-printed prosthetic arm was designed to further exhibit the application demonstrations of this muscle fiber at subzero temperatures. We believe that this CNT@PCL muscle fiber can serve in extreme environments with ultralow temperatures, thereby facilitating the development of next-generation intelligent structures and systems.

Preparation of CNT fibers
The CNT ribbons were prepared by floating catalytic chemical vapor deposition as reported in our previous work [29][30][31]. In brief, a mixture of water, ethanol, thiophene, and ferrocene with precise proportions was injected into a tube furnace at 1300°C with a feeding rate of 30 mL h −1 with hydrogen (2400 sccm) as the reducing agent and argon (2400 sccm) as a carrier gas. The generated CNTs formed continuous aerogel sheets, which were further passed through the water bath to form CNT ribbons. The CNT fibers were obtained by twisting the CNT ribbons with a stepper motor. The surface morphology of the CNT ribbon and CNT fiber is shown in Figure S1.

Fabrication of the coaxial muscle fibers
First, PCL particles were dissolved in a mixed solution of DCM and DMF with a weight ratio of 3:1 and prepared as a precursor solution with a mass fraction of 14%. The solution was stirred at room temperature for 12 h and then loaded into the syringe. For electrospinning, the ends of the CNT fiber were fixed on a metal fixture and rotated around the axis at 300 rpm to receive nanofibers. The needle with an inner diameter of 0.41 mm was 7 cm away from the CNT fiber, and the solution was flowed at a rate of 1 mL h −1 with an applied voltage of 22 kV. After electrospinning, the CNT@PCL fibers were placed in a vacuum oven and kept at 40°C for 6 h to remove the residual organic solvents. Subsequently, the CNT@PCL fiber was inserted into a twist of 1200 turns/m to form a coiled muscle fiber.
For the fabrication of the CNT@PAN muscle fiber and CNT@PA66, the parameters of electrospinning are roughly the same. Differently, PAN powder was dissolved in DMF solution with a mass fraction of 14% and then stirred in a water bath at 80°C until completely dissolved. PA66 particles were dissolved in a mixed solution of anhydrous formic acid and glacial acetic acid with a weight ratio of 1:2 and prepared as a precursor solution with a mass fraction of 14%. The PA66 solution was stirred in a water bath at 80°C for 12 h.

Characterization and measurement
Scanning electron microscope (SEM, S-4800, Hitachi) was used to characterize the surface morphology of the CNT@PCL muscle fibers. The glass transition temperature of polymeric materials was characterized by the differential scanning calorimeter (DSC, 200 F3, Netzsch). The contraction of CNT@PCL muscle fibers at different temperatures was measured by the thermomechanical analyzer (TMA, 402 F3, Netzsch).
For the actuation measurement, a 3D-printed double-layer device filled with liquid nitrogen was used to provide a subzero environment ( Figure S2). One end of the CNT@PCL coaxial muscle fiber was fixed, and the other end of this fiber was tethered to avoid the rotation but allow the lengthwise change. The actuation properties and environment temperatures were measured by a contactless electromagnetic displacement sensor (M18, Shanghai Muxi Electronic Technology Co., Ltd.) and a contactless infrared temperature sensor (T10S-B-HW, Miaoguan Technology Co., Ltd.; HX-RS-LZ2206, Beijing Huaxia Risheng Technology Co., Ltd.), respectively ( Figure S3). A programmable DC power supply (2200-60-2, Keithley) was used to provide DC voltage to the CNT@PCL muscle fibers.

Preparation and characterization of the CNT@PCL muscle fiber
The schematic diagram of the preparation of CNT@PCL muscle fibers is shown in Figure 1a (see the experimental section for details). In brief, a CNT@PCL coaxial fiber was prepared by uniformly wrapping the PCL nanofibers on the surface of the CNT fiber core by electrospinning. Then, this coaxial fiber was inserted into the twist density to obtain the CNT@PCL muscle fiber. Figure 1b schematically shows the coaxial structure of the CNT@PCL muscle fiber. Figure 1c-f show the surface and cross-sectional morphology of the CNT@PCL fibers. The PCL nanofibers are uniformly wrapped on the surface of the CNT fiber (Figure 1c). After further inserting twist density to form the coiled structure, the PCL nanofiber sheath layer remains intact (Figure 1d,e). As shown in the cross-sectional SEM image of the CNT@PCL fiber (Figure 1f), the diameter of the fiber core is ~154 μm and the thickness of the sheath layer is ~48 μm. Figure 1g,h show the element mapping for oxygen and carbon that is selected in Figure 1f, indicating the coaxial structure of the CNT@PCL muscle fibers.
The FTIR spectrum of the CNT@PCL muscle fiber is shown in Figure S4. The peak at 3440 cm −1 is the stretching vibration absorption peak of the hydroxyl group, and the two peaks near 2960 cm −1 are the stretching vibration of the methylene group. The peaks at 1747 cm −1 and 733 cm −1 are the carbonyl and the bending vibration absorption peak of hydroxyl, respectively. These results prove that the PCL nanofibers were successfully wrapped on the surface of the CNT fiber. TG analysis under the air atmosphere and DSC analysis under the nitrogen atmosphere of the polymer materials are shown in Figure S5 and S6, respectively. The decomposition temperature of PCL is ~300°C and the melting temperature of PCL is 40-60°C, which determines that the upper limit of the working temperature of the CNT@PCL muscle fiber is 60°C.

Thermally actuation of the CNT@PCL muscle fiber working in subzero environments
The contraction properties of the CNT@PCL muscle fiber working in subzero environments were first characterized. The CNT@PCL muscle fiber driven electrothermally under 5 V voltage provided a reversible maximum contraction of 15% in a −60°C environment (Figure 2a). The surface temperature of the muscle fibers achieving the maximum contraction is ~46.5°C ( Figure S7). As a sheath-run muscle fiber, the thickness of the sheath layer affects the actuation performance of the CNT@PCL muscle fibers [24]. Figure 2b shows the contraction of five types of muscle fibers with different diameters (different thicknesses of the sheath) under different loads. To ensure that the CNT fiber core generated the same heat per unit length, the maximum contraction of different muscle fibers under the same current of 0.1 A was tested. The muscle fiber with a diameter of 497 μm under 0.06 N load has a maximum contractile stroke of ~12%. The contraction of the muscle fibers did not increase monotonically with the increase of the diameter, while the contraction showed an increasing trend with the increase of the spring index. As the spring index of the muscle fibers increased from 1.15 to 1.59, the maximum contraction increased from 7.5% to 11.6% ( Figure S8). In addition, different ply numbers of the muscle fibers also have an effect on the contraction properties [32]. Four types of muscle fibers with different ply numbers (1-ply, 2-ply, 4-ply, and 6-ply) were prepared for investigating the effect on maximum reversible contraction, which were tested in the same temperature range from −130°C to 45°C. As shown in Figure S9, the maximum contraction increased as the ply numbers increased from 1-ply to 4-ply and then started to decay. The maximum contractile stroke of 4-ply muscle fibers is approximately twice that of 1-ply fibers. This is attributed to the fact that the 4-ply muscle fiber provided more space for contraction [32].
As shown in Figure 2c, the maximum work capacity of the CNT@PCL muscle fiber under 0.4 MPa was 53 J kg −1 , which is ~7 times that of typical mammalian skeletal muscle (7.7 J kg −1 ) [33]. In addition, the contractile stroke of the CNT@PCL muscle fiber at different operating voltages with the dependence of temperature was measured (Figure 2d). The contractile stroke and temperature show a consistent increase with the increase of the operating voltages, which provides the possibility for these muscle fibers to be used as temperature sensors, and it will be discussed in the following section.

Actuation mechanism
The working mechanism of the CNT@PCL muscle fiber is the same as the most thermally driven artificial muscle fibers with the coiled structure previously reported [34][35][36]. When thermal stimulation is applied, the muscle fibers expand significantly in the radial direction that causes fiber untwisting, thereby resulting in the axial shrinkage ( Figure S10). In this process, the coiled structure has the effect of amplifying the contraction. Different thermal expansion coefficients of different polymers cause discrepancies on actuation properties of different muscle fibers [37,38]. Thermal expansion of polymers is attributed to the increase in the vibration amplitude of the molecules within the material with the increase of the temperature, which is reflected macroscopically in the expansion of the volume [39]. For the thermal expansion of amorphous polymers, thermal stimulation usually causes the change of the internal molecular chains [9]. Figure 3a schematically shows the temperature-deformation curve of an amorphous polymer, which includes three states and two regions. When the temperature is very low, this system is in a glass state where neither the molecular chains nor the segments can move, and only the atoms or groups vibrate near the equilibrium position. Once the temperature is higher than the glass transition temperature, this system is in a highly elastic state where the molecular chain cannot move, but the segments start to move. In other words, the conformation of the molecular chain is changed in this state, which results in a large deformation of the polymer. As the temperature continues to increase until exceeding the viscous flow temperature (melting temperature), this system is in a viscous flow state where the entire molecular chain can move and the deformation is irreversible. The vibration amplitude of the molecules in these three states is different, which leads to their different contributions to the deformation of the macroscopic system. The two transition regions between these three states were the glass transition region and the viscoelastic transition region, respectively. The deformation degree of the system changes abruptly in the two transition regions. The melting temperature of the polymer determines the extreme working environment, while the glass transition temperature affects the preliminary working temperature of artificial muscle fibers. The actuation polymers in previously reported artificial muscles driven thermally have a high glass transition temperature (above zero) [40][41][42], and these preliminary temperatures of deformation are usually room temperature, whose reaction process occurs in the transition zone from the highly elastic state to the viscous flow state. If the temperature of the working environment is lowered, the polymer will be in a glassy state, which requires a large heat input to produce the deformation that is required for the artificial muscle fibers to operate. On the contrary, polymers with low glass transition temperatures require less heat input to achieve the actuation process in a subzero environment. Here, a polymer with a low glass transition temperature, such as PCL (T g = −60°C) [43,44], is chosen to enable the artificial muscle fibers to achieve actuation in the subzero environment.
To verify the above theoretical analysis, three polymers (PA66, PAN, and PCL) with different glass transition temperatures were prepared for coaxial muscle fibers (CNT@PA66 muscle fiber, CNT@PAN muscle fiber, and CNT@PCL muscle fiber) by the same method and their actuation performance at low temperatures was measured by the same test conditions. As shown in Figure 3c, the glass transition temperatures obtained by DSC of three polymers were T g PAN : 105°C, T g PA66 : 60°C, and T g PCL : −60°C, respectively. Then, the contractile strokes tested by TMA of three coaxial muscle fibers were characterized in the low-temperature environment (Figure 3b). The CNT@PCL muscle fiber provided a contractile stroke of ~10% in a fixed temperature range from −130°C to −35°C, while the contractile strokes of the CNT@PA66 muscle fiber and the CNT@PAN muscle fiber were only ~1%. The coefficient of linear thermal expansion (CLTE) of these three coaxial muscle fibers was further calculated by the following equation: α t = (L 0 -L 0 )/[L 0* (T t -T 0 )], where α t , T 0 , T t , L 0 , and L 0 are CLTE, the original temperature, the working temperature, the length of the fiber at original temperature, and the length of the fiber at working temperature, respectively. As shown in Figure 3d, the CLTE of the three muscle fibers all changed abruptly near the glass transition region. The CLTE of the CNT@PCL muscle fiber was significantly larger than that of the CNT@PAN muscle fiber and the CNT@PCL muscle fiber at the temperature from −130°C to 35°C ( Figure S11). This is attributed that PCL underwent the transition from a glassy to a highly elastic state, while PAN and PA66 were only in the glassy state as the temperature changed from −130 C to 35°C. Therefore, these results demonstrate the excellent actuation performance of PCL at subzero temperatures.

Application demonstrations
PCL has an excellent sensitivity of deformation to temperature, especially at subzero temperatures [43,44]. As schematically shown in Figure 4a, a temperature sensor based on the linear relationship between contraction and temperature of the CNT@PCL muscle fiber was designed. In this temperature sensor, the length of the fiber at different temperatures corresponds to the scale bar on the dial one by one. For the stability of the fast thermal response, the relationship of all curves between contraction and temperature of the muscle fiber was similar with the increase of the temperature from −130°C to 35°C at different heating rates (1 K min −1 , 5 K min −1 , 10 K min −1 , and 20 K min −1 ), which increased monotonically (Figure 4b). After three repetitive tests, the relationship between contraction and temperature had not significantly changed (Figure 4c), which demonstrates that the temperature sensor has excellent repeatability. As shown in Figure 4d, the muscle fiber accurately measured the temperature change with an interval of 1°C and the relationship between contraction and temperature was almost hysteresis-free in the temperature range from 24°C to 36°C, which proves the accuracy of the temperature sensor. The temperature sensor had rapid responsiveness, when the temperature was changed, the response time was ~1 s (Figure 4e). To verify the reproducibility, the muscle fiber continuously worked for 120 h in a transitional environment switching back and forth between 40°C and −130°C. After working for 120 h, the curve of the contraction versus the temperature was well in agreement with the first measured curve (Figure 4f). This result indicates that the temperature sensor can serve for a long time.
Due to the excellent deformation driven thermally of PCL at subzero temperatures, the CNT@PCL muscle fibers show valuable application scenarios in extreme environments. For example, this muscle fiber can replace the motors that cannot work in extremely cold environments to actuate some intelligent structures and systems (Figure 5a). To exhibit the application demonstrations of the CNT@PCL muscle fiber at subzero temperatures, a 3D-printed prosthetic arm based on this muscle fiber was designed (Figure 5b). To simulate extreme working conditions at low temperatures, the CNT@PCL muscle fiber was placed in an industrial refrigerator with a temperature of −40°C. When applied 12 V voltage, this muscle fiber provided a contractile stroke of ~15% within 6 s to actuate the prosthetic arm to produce a large angle change from 125° to 88°. After switching off the power, the muscle fiber returned to its original state within 20 s.

Conclusion
In summary, a coaxial artificial muscle fiber by electrospinning a sheath of PCL nanofibers on the surface of a CNT fiber core was successfully reported, achieving the actuation in response to thermal at subzero temperatures. According to the ultralow temperature tolerance of PCL, this CNT@PCL muscle fiber can serve in very large subzero temperature ranges (over −130°C) with a maximum contractile stroke of ~18%, which breaks through the current temperature limitations on artificial muscle fibers for subzero environments. This muscle fiber can also be efficiently driven in the above-zero temperature range (below the melting temperature of 45°C for PCL). Due to the excellent linear relationship between the contraction and temperature from −130°C to 45°C, a temperature sensor using this muscle fiber was designed, providing an excellent self-sensing function that can obtain the working state of the muscle fibers based on the ambient temperature without the displacement sensors. Furthermore, a 3D-printed prosthetic arm was designed to further exhibit the application demonstrations of this muscle fiber at subzero temperatures. This muscle fiber is expected to replace the traditional motor drive units in intelligent structures and systems working at subzero temperatures, thereby contributing advance efforts toward the fields of space exploration, polar research, and mountain surveying.

Disclosure statement
No potential conflict of interest was reported by the authors.