Structural material with designed thermal twist for a simple actuation

: Materials with desired thermal deformation are very important for various engineering applications. Here, a material with the combination of chiral structure and TiNi shape memory alloy ( SMA ) sheets that performs a twist during heating is proposed. The thermo - mechanical properties of these materials are experimentally investigated. Inspired by this, a car - like material performing translational and rotational motion is designed, which illustrates the potential applications for the next - generation soft robotic devices. Based on this method, one can design remotely manipulated arti ﬁ cial muscles, nanorobots, revolute pairs, and thermal sensors or actuators in a noncontact fashion.

Auxetic metamaterials mainly have three categories: reentrant materials, rigid square rotation materials, and chiral structures [23]. In chiral structures, chiral elasticity theory can describe the coupling among local rotation, bending, and bulk deformation [24][25][26][27]. Based on this theory, the deformation mechanism of tension-torsion coupling [29], dilatation-rotation coupling, and shearrotation coupling [27,28] of chiral structures is well investigated. Our heat-driven actuators are motivated by the chiral structures that have a tension-torsion coupling effect. For example, an elastic bar designed with a manmade architecture performed a twist during compression, which was mainly caused by the circumferential freedoms of a unit cell [29]. Similarly, a tetrachiral cylindrical shell based on the natural plant architecture exhibited a reversible bidirectional twisting deformation in the axial compression and tension processes [30]. Then, threedimensional (3D) architectures with the coupling effect of shear-compression also exhibited a twist behavior under uniaxial loading [31], and the tension-torsion coupling effect was found based on a 3D metamaterial with connecting neighbor chiral honeycomb layers [32].
The aforementioned literature has been focused on the twist behavior caused by uniaxial force, but here, we use the chiral structure in a different way: harvesting twist with heating. The chiral structures are actuated by shape memory alloy (SMA) sheets, which can be deformed when cold but return to their predeformed shapes when heated. The deformation recovery effect and hyperelastic property of SMAs are associated with a diffusionless martensitic transformation [33,34]. Hence, they can be trained to remember their shape at both high temperature phase (austenite phase) and low temperature phase (martensite phase), which was used in this research as a two-way memory effect [34,35]. The combination effects of SMA with purposely engineered topologies have been paid more attention. Grummon et al. [36,37] created a regular cellular material by joining thin-walled superelastic SMA tubes via electrical resistance welding and explored the influence of constitutive behavior on the effective mechanical properties. Hassan et al. [37,38] manufactured truss by assembling cells of chiral honeycomb topology and SMA ribbons, which have a large rotational folding rate to use in new types of deployable antenna reflectors. Moreover, SMAs have been used as torsional actuators by converting the linear actuation motion or directly rotational motion in many practical fields, such as biomedical applications, aerospace applications, and automotive applications. The linear actuation motion caused by the contractile behavior of SMA wires, springs, or thin films is used to generate rotation, and the direct rotational motion with slender SMA torque tubes and rods twisted around the long axis is used to achieve direct rotation [34]. These methods use the SMA component to form rotational actuators. In this research, we combine SMA sheets and chiral structures ( Figure 1a) to realize twistable actuators, as the twist comes from the coupling between SMA sheets and chiral structures, which opens a new avenue for torsional SMA-based actuators. Compared to the fiber-based actuators that can only directly produce pulling force [39][40][41] and the hydraulically amplified self-healing electrostatic actuators that can only directly generate propulsive force [42], our chiral structures actuated by SMA sheets can yield both pulling force and propulsive force. Compared to the electrohydraulic transducer-based actuators [43], our design scheme needs simpler devices and can produce a larger strain. Here, the geometries, kinematics, and coupled thermo-mechanical properties of these motors on a macroscale are experimentally investigated. However, the proposed method is also appropriate on a nanoscale, which can be used in nanorobots and actuators. . θ denotes the angle that the upper square is twisted relative to the lower square. There are four geometrical parameters determining the whole structure with the relation

Geometry design of a motor
, which can basically predict the change of angle θ (Figure 2a, inset). The key idea is using SMA sheets to control the change in angle α and obtain a twist (change in angle θ). Here, = L 40 mm, and = r 20 mm.

Properties of TiNi SMA sheets
In this study, two-way memory TiNi SMA sheets (Zhilian Memory Alloy Co., Ltd., Huizhou, China) with the initial angle ≈°α 90 were used. During heating, the SMA sheet straightened with the decrease in angle α. When the heater was turned off, the SMA sheet slowly returned to its initial shape as it was cooled to room temperature. The properties of TiNi SMAs are shown in Table 1, which are consistent with the earlier literature [33].
Here, the SMA sheet sizes of all samples are as follows:

Fabrication of the motors
For fabricating the motors, a U.S. Strathmore 500 Series 3-ply Bristol card stock (U.S. Strathmore) cut by laser forming the structure and TiNi SMA sheets controlling the angle α were used. The photographs in Figure 2b show how a spoke and the upper/lower square are connected by a TiNi SMA sheet. For each spoke, two SMA sheets were used to connect the upper and lower squares. Therefore, four SMA sheets were needed for the motor with two spokes, and six SMA sheets are needed for that with three spokes, and so on.

Motion capture
To measure the twist angle θ of the motor as a function of temperature, the experimental setup, as shown in Figure 1b (scheme and photograph), was built. A platform controlling the temperature from 20 to°120 C was used to heat the motor, which meant that the heater could heat the SMA sheets in a noncontact way. A temperature sensor was placed in the motor to obtain the temperature near the SMA sheets. A camera from the top was used to record the motion of the upper square M M M M 1 2 3 4 , which can be seen in Figure 1c at each temperature.

Investigation of one SMA sheet
An SMA sheet with one side fixed was heated, and another side was free. At°20 C, the angle α of the SMA sheet was about°90 (see photograph in Figure 1d). During heating, the SMA sheet straightened itself, thus α decreased, as shown in the plot of Figure 1d. The angle α decreased rapidly as the temperature increased from 20 to°95 C and remained constant at°95-120 C; the minimum value of α was about°16 , which reveals that the SMA sheet realized the deformation within the temperature range of°20-120 C. Assembling the SMA sheets to a chiral structure (Figure 1a) to control the angle α caused a twist angle θ of the motor; thus, this mechanism enabled the chiral structure to act as a motor.

Kinematic properties of motors
Experimentally, the twist angle θ, as a function of temperature with different geometrical parameters of L and r, was measured. In Figure 2a, angle θ quickly increased before°95 C and then basically remained constant. A large value of length L resulted in a large change in θ (see the orange data vs blue data, and purple data vs gray data).
Additionally, a small value of r also resulted in a large change in θ (see the orange data vs purple data, and blue data vs gray data). In Figure 2b, we experimentally found that the value of θ of the motor with two spokes was larger than that with three spokes and four spokes, by fixing the other geometrical parameters (see the blue data vs cyan data vs red data), which indicates that more constraints may hamper the twist behavior of the motor. Interestingly, if we change the direction of the SMA sheets connecting spokes with squares, the twist can be both anticlockwise ( Figure 2c model and photo) and clockwise ( Figure 2d model and photo), which can yield both propulsive force and pulling force (see the third column of Figure 2c and d).

Kinematic properties of a car assembled with such motors
Interestingly, such actuators can be used to drive a car. In Figure 3a and b, the size of the car body was × × l l l ). Each wheel with radius = r 20 mm 1 was fixed on the car body based on the center points, such as W 1 (action wheel) and W 2 (driven wheel). The wheels were obtained by changing the squares (e.g., M M M M 1 2 3 4 in Figure 1a) to disks. There were two action wheels and two driven wheels on the car body. Each action wheel was connected to the car body with three spokes (with size × = × s s 6 mm 25 mm 1 2 ) and six SMA sheets, and the SMA sheets were evenly arranged along the red circles with radius = r 12.5 mm 2 as shown in Figure 3a and b. The driven wheels were directly glued on the car body. The assembling location of action wheel W 1 was ( ) ( ) = based on center W 2 (Figure 3a). To enhance the friction between the action wheels and the ground, double-side tapes were used on the edge of action wheels as shown in Figure 3b.
The action wheels on the two sides of the car body can be designed to twist in an identical direction (Figure 3c left) or in opposite directions (Figure 3c right). In Figure 3c, the assembling fashions between the wheels, SMA sheets, and spokes are shown in the photographs, and the arrows indicate the motion directions of the top edges of the action wheels. Figure 3d and e show the walking experiments of the car actuated by the heat-driven motors on the heater from 20 to°120 C recorded using a camera. In Figure 3d, the action wheels on the two sides of the car twist in the same direction (see the white arrows), thus the car exhibited a translational motion (see the red trajectories of the corner points A, B, C, and D and the central point O on the top surface of the car in Figure 3d right, where dots denote starting points and arrows denote terminal points. The measurement method was similar to that in Section 2). This is also demonstrated in Figure 3f with the car body angle ≈ γ 0 1 (red curve), which is defined as the angle between line AB and its initial direction (Figure 3d left). When the action wheels on the two sides of the car twisted in opposite directions (see the white arrows in Figure 3e left), the car presented a clockwise rotation (see the blue trajectories  Figure 3f).
The flaw of this design is that the car can only trigger a stroke, and how to design heat-driven actuators with continuous twist is still an open question.

Coupled thermo-mechanical properties
Here, an experiment to obtain the actuating force of the heat-driven motors is designed. As shown in Figure 4a Figure 4b shows the photographs of the setup in two views. When heating, the motor twisted anticlockwise (see the arrow in Figure 4a) and pulled the strip, and then pulled the force sensor to obtain the actuating force data. Figure 4c shows the actuating forces of the motors with two, three, and four spokes from 20 to°1 20 C. As expected, we found that the motor with more spokes yielded a larger actuating force (brown curves). This indicates that increasing the number of spokes is a good way to enlarge the actuating force of the motor. As the paper strip does not allow extension, the paper disk, strip, and spokes all deformed after they were heated, as shown in the lower right photo in Figure 4b, which led to the reductions in force responses after the peak values (brown and blue curves in Figure 4c).
In the future, a finite element model is needed to predict the interaction between the twisting structures and SMA sheets with heating (such as the deformations of spokes and wheels). Furthermore, the load performance of an SMA sheet, the maximum strokes, force and response speed, and efficiency of the motor structure should be investigated. As SMA can be trained to achieve desired deformation within given temperature range [33], SMA sheets used in this paper that transform in narrower temperature range, such as°20-40 C, need to be further improved and studied for convenient application and low energetic cost. Meanwhile, for the perspective of SMA performance development, it is important to understand the microstructural evolution related to the deformation behavior of SMA [44]. The effect of different stages of deformation on the microstructure evolution of SMA has been reported in the available literature [45], which showed that the deformation twinning and stacking fault ribbons were the main deformation mechanisms. Therefore, the effect of microstructure on deformation correlating with the behavior of the proposed actuator needs to be further investigated. Moreover, the design of heat-driven actuator with continuous twist needs to be further considered.

Conclusions
Here, the heat-driven actuators made by twisting structures and TiNi SMA sheets are designed. The kinematic properties of the actuators with different geometrical parameters and number of spokes were experimentally studied. Based on such motors, we designed a car and then demonstrated its translational and rotational motion driven by heating. To investigate the coupled thermo-mechanical properties, we measured the actuating force of the motor with different number of spokes. In given thermal conditions, the proposed actuators can provide desired twist and torque in a noncontact way, which can be potentially used as remotely controlled revolute pairs in nanorobots, artificial muscles, bioimplants, and thermal sensors.