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Fast Torsional Artificial Muscles from NiTi Twisted Yarns Seyed M Mirvakili, and Ian W. Hunter ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 29, 2017

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ACS Applied Materials & Interfaces

Fast Torsional Artificial Muscles from NiTi Twisted Yarns Seyed M Mirvakili*, Ian W Hunter BioInstrumentation Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA. E-mail: Seyed M Mirvakili ([email protected], [email protected]) Keywords: Fast Torsional Artificial Muscle, Shape Memory Alloy, Nickel Titanium, Twisted Yarns, Torsional Actuators, NiTi Twisted Yarn. Abstract Torsional artificial muscles made of multi-walled carbon nanotube/niobium nanowire yarns have shown remarkable torsional speed and gravimetric torque. The muscle structure consists of a twisted yarn with half of its length infiltrated with a stimuli-responsive guest material such as paraffin wax. The volumetric expansion of the guest material creates the torsional actuation in the yarn. In the present work, we show that this actuation mechanism is not unique to wax-infiltrated carbon multiwalled nanotube (MWCNT) or niobium nanowire yarns and twisted yarn of NiTi alloy fibers also produces fast torsional actuation. By gold plating half-length of a NiTi twisted yarn and Joule heating it, we achieved a fully reversible torsional actuation of up to 16 °/mm with peak torsional speed of 10,500 rpm and gravimetric torque of 8 N·m/kg. These results favorably compare to those of MWCNTs and niobium nanowire yarns. Introduction The invention of McKibben artificial muscle in 1950s has been an inspiration for number of new artificial muscle designs. For example, pneumatic torsional artificial muscles are one of the derivatives of the McKibben artificial muscle where instead of having a double family of fibers braided around the bladder, a single family of fibers are used.1 This asymmetric braiding

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translates the volumetric expansion in the bladder into a torsional actuation and unwinding of the inextensible fibers. Similarly, twisted yarns of MWCNT2 or niobium nanowires3 can produce a torsional actuation in response to heat when half of the length of the yarn is infiltrated with a stimuli-responsive guest material such as wax.2–5 The MWCNT twisted yarns have shown rotational speeds of up to 11,500 revolutions per minute and a remarkable gravimetric torque of 8 N·m/kg which is higher than that of ungeared commercial direct drive electric motors (2 N·m/kg to 6 N·m/kg).2 In the present work, we show that in addition to nanofiber yarns, twisted yarns of fine NiTi fibers can also produce similar results but with a different working mechanism (video S1). The working mechanism of guest infiltrate twisted nanofiber yarns is based on volumetric expansion of the infiltrated section of the yarn. When the guest material expands in volume in response to a stimulus, since the fibers are inextensible, the only way for the yarn to accommodate this expansion is to untwist. While the infiltrated section is untwisting, the neat section twists and act as a bias spring which returns the paddle to its initial angular position when the voltage is turned off. In contrast, the working principle for NiTi twisted fibers is mainly based on shape recovery due to the shape memory effect and no guest material is required to achieve the torsional actuation. This can be an advantage for NiTi over MWCNT or niobium nanowire yarns for applications where the guest material cannot survive the exposed environment (i.e., in space vacuum). In addition, NiTi microwires are commercially available which makes them suitable for immediate applications. Shape memory alloy wires (e.g., NiTi) have been used in different rotary actuator designs.6–8 It has been demonstrated that by twisting a 100 µm diameter NiTi wire and differentially heating it along its length, the wire retains its programmed shape (i.e., straight wire with no twists) and 2 ACS Paragon Plus Environment

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produces reversible torsional actuation.6 However, the torsional stroke is limited to only 0.6 °/mm and requires having a third electrical contact to the middle of the fiber which can possibly reduce its range of applications. In the present work, we show that by twisting pair(s) of NiTi microwires (methods) and applying heat to half of the yarn’s length we can achieve a reversible torsional and linear actuation. The required heat was generated by Joule heating of the NiTi fibers. To trap the Joule heating to only half of the yarn’s length, resistance of the other half was lowered by a factor of 15 – 20 by electroplating a thin layer of gold on the microwires (methods). The NiTi alloy microwire that we have used in this work has an electrical resistivity of 0.8 µΩ·m and 1 µΩ·m in its Martensite and Austenite phases respectively.9 Since the gold coating has a much higher electrical conductivity than the NiTi microwire, it carries almost all the current. Therefore, almost no gold coated NiTi fiber reaches the activation temperature when the voltage is applied. Thermal imaging of a sample illustrates the differential heating concept (supporting figure S3). Working principle of the NiTi torsional actuator in this work is similar to that of the wax infiltrated yarns with only one major difference. In the wax-infiltrated MWCNT or nanowire yarns, upon Joule heating, volume of the paraffin wax expands by almost 30% during the solid to liquid phase transition process. Since the yarn is prevented from rotation at its both ends (similar to our setup in Figure 1A) and each individual fiber is inextensible, the expansion in volume of the wax-infiltrated section leads to an untwist in that section of the yarn and a twist in the bare part. In contrast, when fully activated, each microwire of our NiTi twisted yarn can contract up to 4.5% in length and expand up to 1.5% in diameter with Poisson’s ratio of 0.33. To accommodate the contractile strain in each microwire, the yarn untwists while the gold coated part twists and stores the energy (Figure 1B). This energy will be released to restore the twisted yarn to its initial 3 ACS Paragon Plus Environment


state during the turn off cycle. To better observe this behavior, we attached a paddle made of a piece of aluminum sheet to the middle of the yarn at the boundary of the gold coated part and the bare section of the yarn. Results and discussion Dynamic mechanical behavior of NiTi alloy is a well-studied subject.10,11 For the purpose of completeness and accuracy, we performed a dynamic mechanical analyzer (DMA) measurement on a piece of 25 µm NiTi alloy microwire (figure 1 C, D) (methods). It is very important to know the behavior of the storage modulus (εʹ) as a function of temperature. From figure 1C we can observe that εʹ increases by almost 18% during the phase transition from Martensite to Austenite. In contrast, in oriented nylon fibers (used in nylon artificial muscles), the storage modulus decreases by almost 88% over similar temperature range.12 The increase in the εʹ can positively affect the torsional rotational speed (since it modifies the torsional spring constant) and reversibility of the torsional stroke. To better understand the actuation mechanism in NiTi twisted yarns, first we investigated the behavior of a twisted yarn consisting of only two NiTi fibers (each 25 µm in diameter) and then we increased the number of fibers (i.e. N) to evaluate the performance scalability. We postulate that the torsional actuation mechanism in NiTi twisted yarns is based on two phenomena: 1) shape recovery of the twisted NiTi individual microwires upon excitation and 2) contraction and expansion in the length and the diameter of the individual microwires respectively. Upon twisting of the yarn in the fabrication process which is analogous to two, four, or six-ply homochiral nanofiber twisted yarns,2 aside from the fiber bundle, each individual NiTi microwire twists as well. In the shape recovery stage, each individual strand untwists to recover its original shape which also causes the bundle to untwist. The twisted NiTi microwires can be seen as intertwined 4 ACS Paragon Plus Environment

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helices with coil index (i.e., the ratio of mean coil diameter to the fiber diameter) of 1. Therefore, we can explain the actuation mechanism from the spring mechanics.13–15 From the equations of torsion and curvature we can find the torsional stroke (∆N/N) to be (supporting information):

∆N ∆L ∆h 1 ∆d ∆n =3 − − + . N L h 2 d n

(1)

where N is the number of turns in the coil, L is the length of individual fibers, h is the length of the active section of the twisted yarn, d is the diameter which is measured from the center of the fibers, n is the number of inserted twist, and ∆n is the change in the inserted twist of the microwire during the shape recovery process. For a one-end–tethered yarn the individual microwires can recover their initial shape in the absence of a counterbalancing torque. Therefore, all the twists in each individual microwire will be translated to the paddle (assuming friction is negligible). However, for a two-end–tethered yarn since one half of the yarn is acting as a bias spring, the fibers in the bare part cannot completely recover their initial shape, therefore, only a fraction of the twist in individual microwires rotate the paddle. We observed that by increasing the load the torsional stroke decreases (figure 2A and C). This decrease can be explained by the fact that when we increase the load, the yarn should untwist to accommodate this increase in tension. However, since the both ends of the yarn are prevented from rotation, this increase in tension acts as an opposing torsional spring which reduces the torsional stroke. From energy point of view, we can rationalize this behavior from the fact that as we increase the load, more of the generated mechanical energy will be consumed by the tensile actuation. Therefore, less energy will be available for torsional actuation which results in less number of untwisting of the individual fibers. This effect can be dominant at high loads since the Young’s modulus increases upon excitation (figure 1C) thus the tensile strain in each fiber



increases (supporting information). Similar to wax-infiltrated MWCNT yarns,2 we observed that the rotational speed has a direct correlation with the load size (figure 2B and C). We achieved fully reversible torsional stroke of 16 °/mm which is higher than the 0.6 °/mm for NiTi monofilament torsional actuators6 and comparable to that of wax-infiltrated MWCNT or nanowire torsional actuators.2,3,5 We measured rotational speed of up to 10,500 rpm (figure 3B). This rotational speed is higher than the 7,200 rpm for niobium nanowire yarn3 and close to the 11,500 rpm for MWCNT yarn.2,5 By applying square shape voltage pulses of 500 ms long we achieved a 1.1% tensile actuation for a twisted pair torsional actuator under a load of 64 MPa. The maximum generated torque ( τm = κθm ) by the NiTi twisted yarns can be estimated from the maximum torsional stroke (θm) and the torsional spring constant of the yarn (κ). The torsional spring constant ( κ = Iωn2 ) can be evaluated from the moment of the inertia of the paddle (

I = m(l 2 + w2 ) / 12 ) and the natural resonance frequency of the yarn (ωn) which can be estimated from the damped oscillation of the paddle at maximum torsional stroke (supporting information). We achieved a gravimetric torque of 8 N·m/kg to 10 N·m/kg which is higher than that of a high performance, ungeared, commercial, direct drive electric motor (2 N·m/kg to 6 N·m/kg) and comparable to what is achieved from MWCNT twisted yarn.2 Figure 4 illustrates the torque which is normalized to the mass of the actuating section of the yarn and number of inserted twist (supporting information) as a function of the input power with is normalized over the length of the yarn. The specific work capacity can be estimated from the maximum torque and torsional stroke ( 1 Wm = τ mθ m ).5 Specific work capacity of up to 170 kJ·m−3 was measured for a yarn with two 4 NiTi strands which is higher than the 3 kJ·m−3 measured from twisted MWCNTs.5 This large 6 ACS Paragon Plus Environment

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difference can possibly be explained by the fact that the torsional spring constant of the twisted NiTi yarns is larger than that of the twisted MWCNTs and Nb nanowire yarns (~10−7 N·m·rad−1 vs ~10−10 N·m·rad−1).3 Figure 4 shows the specific work capacity which is normalized to the mass and square of the number of inserted twist (supporting information) as a function of input power per length of the yarn.

Cycle life We measured the cycle life of a NiTi twisted pair torsional artificial muscle with length of 127 mm and 394 turns/m. The yarn was under a load of 35 MPa. We measured 5,200 cycles with some deterioration in the rotational speed (figure 5A). For this experiment, instead of a square wave signal, we used a ramp with a 400 ms rise time, 600 ms peak time, and 100 ms fall time (Figure 5B). With the square waves, the yarn broke in fewer than 3,200 cycles while ramping the voltage during on and off cycles made the cycle life slightly longer (under the similar experimental condition). We hypothesize that with the square wave signals the inertial mass is higher than that of a ramp signal (due to the higher acceleration that the load experiences) and this sudden acceleration can fatigue the yarn and lower the cycle life.

Conclusion In the present work, we have demonstrated that NiTi twisted fibers, when heated differentially along their length, can produce torsional actuation. Peak speeds of 10,500 rpm with gravimetric torque of 8 N·m/kg to 10 N·m/kg is achieved which compares favorably to those of MWCNT and nanowire twisted yarn torsional artificial muscles. One of the advantages of NiTi twisted fibers is that there is no need for guest materials such paraffin wax or rubber2–4 which allows the device to be used in vacuum as well.



There are several performance metrics that artificial muscles are usually evaluated with. For example, cost, energy/power density, efficiency, and cycle life are among the commonly used metrics. Torsional artificial muscles made from MWCNT and nanowire yarns have shown a long cycle life and high energy and power densities; however, they are not yet commercially available. In contrast, twisted NiTi yarn torsional artificial muscles show similar performance results (except the cycle life) to those of the MWCNT and nanowires yarns and the raw material, NiTi alloy wire, are commercially available.

Methods The twisted yarn was prepared by twisting NiTi wires (each 25 µm in diameter) with transition temperature of 90°C (Dynalloy Inc’s Flexinol®) to form the twisted yarn. Gold plating solution (Spa Plating) was used to gold coat the NiTi yarn’s half length. The gold plating was performed at constant current of 3 mA for 2 to 3 minutes. After finishing the plating, the yarn was rinsed with deionized water 3 times to clean the gold plating solution from the yarn. The gold coated yarn was then mounted on the setup (figure 1A) and twisted to achieved the desired number of twists per length. Dynamic modulus of a single NiTi fiber was measured by a dynamic mechanical analyzer (TA Q800). The temperature was ramped up and down at 1.5°C/min while maintaining the sample under a constant load. A constant voltage power supply was used with a function generator to create the voltage pulses needed for excitation. The power supply and the function generator were connected to the actuator via a solid-state relay (Panasonic AQZ202). For the cycle life measurements, we used a programmable power supply (Agilent B2962A) to create the ramp signal and a laser ring to measure the rotational speed of the actuator. The laser ring was made of 3 pairs of laser diodes 8 ACS Paragon Plus Environment

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(VLM-650-03-LPA) and photo-transistors (PT333-3C) facing each other on a ring made of acrylic. When the paddle blocks the path between the laser and the photo-transistor the voltage drops across the series resistance with the photo-transistor. By measuring the time between the events, we can find the rotational speed of the actuator. For more accurate measurements the torsional stroke was measured by a high-speed camera (1000 fps, SAMSUNG TL350) followed by frame-by-frame analysis in Logger Pro.

Supporting Information A video demonstration of NiTi twisted yarn artificial muscle design and performance. Documentation on the calculation of torque and specific energy, and derivation of the torsional stroke governing equation. Figures showing the illustration of the spring and helical model, dependency of torsional stiffness on number of microwires, and thermal image of an excited NiTi torsional artificial muscle. The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.

Acknowledgment S.M.M. was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Alexander Graham Bell Graduate Fellowship.



References (1) Matsikoudi-Iliopoulou, M. Finite Axisymmetric Deformations with Torsion of An Initially Cylindrical Membrane Reinforced with One Family Inextensible Cords. Int. J. Eng. Sci. 1987, 25, 673–680. (2) Lima, M.D.; Li, N.; De Andrade, M.J.; Fang, S.; Oh, J.; Spinks, G.M.; Kozlov, M.E.; Haines, C.S.; Suh, D.; Foroughi, J.; Kim, S.J.; Chen, Y.; Ware, T.; Shin, M. K.; Machado, L. D.; Fonseca, A. F.; Madden, J. D. W.; Voit, W. E.; Galvão, D. S.; Baughman R. H. Electrically, Chemically, and Photonically Powered Torsional and Tensile Actuation of Hybrid Carbon Nanotube Yarn Muscles. Science 2012, 338, 928 – 932. (3) Mirvakili, S. M.; Pazukha, A.; Sikkema, W.; Sinclair, C. W.; Spinks, G. M.; Baughman, R. H.; Madden, J. D. W. Niobium Nanowire Yarns and their Application as Artificial Muscles. Adv. Funct. Mater. 2013, 23, 4311−4316. (4) Lima, M.D.; Hussain, M.W.; Spinks, G.M.; Naficy, S.; Hagenasr, D.; Bykova, J.S.; Tolly, D.; Baughman, R.H. Efficient, Absorption‐Powered Artificial Muscles Based on Carbon Nanotube Hybrid Yarns. Small 2015, 11, 3113-3118. (5) Chun, K.Y.; Kim, S.H.; Shin, M.K.; Kwon, C.H.; Park, J.; Kim, Y.T.; Spinks, G.M.; Lima, M.D.; Haines, C.S.; Baughman, R.H.; Kim, S.J.; Hybrid Carbon Nanotube Yarn Artificial Muscle Inspired by Spider Dragline Silk. Nat. commun. 2014, 5, 3322. (6) Gabriel, K. J.; Trimmer, W. S. N.; Walker, J. A. A Micro Rotary Actuator Using Shape Memory Alloys. Sens. Actuators 1988, 15, 95–102. (7) Hwang, D.; Higuchi, T. A Rotary Actuator Using Shape Memory Alloy (SMA) Wires. IEEEASME Trans. Mechatron. 2014, 19, 1625–1635. (8) Rodrigue, H.; Bhandari, B.; Han, M.-W.; Ahn, S.-H. A Shape Memory Alloy–Based Soft Morphing Actuator Capable of Pure Twisting Motion. J. Intell. Mater. Syst. Struct. 2015, 26, 1071–1078. (9) “DYNALLOY, Inc. Makers of Dynamic Alloys.” [Online]. Available: http://www.dynalloy.com/tech_sheets.php. [Accessed: 13-Feb-2017]. (10) Kusy, R. P.; Wilson, T. W.; Dynamic mechanical properties of straight titanium alloy arch wires. Dent. Mater. 1990, 6, 228–236. (11) Vitiello, A.; Giorleo, G.; Morace, R. E. Analysis of Thermomechanical Behaviour of Nitinol Wires with High Strain Rates. Smart Mater. Struct. 2005, 14, 215. (12) Mirvakili, S. M.; Hunter, I. W. Multidirectional Artificial Muscles from Nylon. Adv. Mater. 2017, 29, 1604734. (13) Foroughi, J.; Spinks, G.M.; Wallace, G.G.; Oh, J.; Kozlov, M.E.; Fang, S.; Mirfakhrai, T.; Madden, J.D.; Shin, M.K.; Kim, S.J. Baughman, R.H. Torsional Carbon Nanotube Artificial Muscles. Science 2011, 334, 494 – 497. (14) Haines, C.S.; Lima, M.D.; Li, N.; Spinks, G.M.; Foroughi, J.; Madden, J.D.; Kim, S.H.; Fang, S.; de Andrade, M.J.; Göktepe, F.; Göktepe, Ö.; Mirvakili, S. M.; Naficy, S.; Lepró, X.; Oh, J.; Kozlov, M. E.; Kim, S. J.; Xu, X.; Swedlove, B. J.; Wallace, G. G.; Baughman, R. H. Artificial Muscles from Fishing Line and Sewing Thread. Science, 2014, 343, 868-872. 10 ACS Paragon Plus Environment

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(15) Mirvakili, S.M.; Ravandi, A.R.; Hunter, I.W.; Haines, C.S.; Li, N.; Foroughi, J.; Naficy, S.; Spinks, G.M.; Baughman, R.H.; Madden, J.D. Simple and Strong: Twisted Silver Painted Nylon Artificial Muscle Actuated by Joule Heating. Proc. SPIE 2014, 9056, 90560I90560I10.



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Figure 1. (A) A photograph of our setup for measuring the actuation response of the NiTi twisted yarns. Insets: Scanning electron microscopy image of the bare and gold coated sections of the twisted yarn (scale bar: 100 µm for the SEM images and 16 mm for the image of the setup). The loop connects one end of the yarn to the load via a piece of string and provides electrical connection for the yarn. To avoid rotation of the loop during actuation a rod is passed though it and fixed. (B) A diagram illustrating the working mechanism of the actuator. The bottom yellow part of the yarn represents the gold coated section. (C) Storage modulus of a NiTi alloy microwire as a function of temperature. A hysteresis of 30°C is observed during the phase transition. (D) Tanδ showing the transition temperatures for the Martensite and Austenite phases.



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Figure 2. (A) Torsional stroke for a twisted pair of NiTi microwires with inserted twist of 375 turns/m and excitation input power of 875 mW. (B) Rotational speed of the yarn in (A) as a function of input power and load. (C) torsional stroke and rotational speed of the yarn as a function of load for the experiment in part (A) and (B). For lower excitation voltages to ensure that the yarn is reaching the steady state actuation 0.5 Hz square wave pulse was used. For higher voltages since the actuation was fast 1 Hz square wave signal was used. Duty cycle was 50% for all of the pulses.


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Figure 3. (A) and (B) Torsional stroke of a twisted pair with 827 turns/m excited with an 83 ms voltage pulse of 16 V. In (A) the pulse width is smaller than the time it takes for the yarn to reach the maximum torsional stroke. In contrast, in (B) the pulse width is longer and therefore, ringing occurs. (C) Linear actuation of a twisted pair torsional actuator excited with a 500 ms voltage pulse.



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Figure 4. Scaling of torque and specific mechanical energy as a function of number of microwires under isotonic condition: (A) Normalized torque as a function of input power per length of the yarn. As the plot suggests the torque scales with mass of the active section of the yarn as well as the number of inserted twist. (B) The specific mechanical energy scales with mass and square of the inserted twist during the fabrication process. (C) The torsional stroke as a function of input power per length of the yarn for different number of strands. All of the samples were under constant load of 28 MPa.


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Figure 5. (A) Rotational speed of a NiTi twisted yarn actuator as a function of cycle number in a nontemperature controlled environment. (B) The input voltage was ramped to 8.5 V to reduce the inertial mass effect. Torsional stroke response of the yarn (measured with a 1,000-fps camera) shows an oscillation at steady state which can be due to the small number of twists per yarn’s length (394 turns/m) or the fact that the voltage is maintained at the peak after the yarn has reached the steady state torsional stroke. The peak rotational speed of the yarn matches with our measured data from the laser ring. The torsional stroke was measured at cycle number 2,000.


Fast Torsional Artificial Muscles from NiTi Twisted ... - ACS Publications

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