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Bio‐Inspired Untethered Automatic Light Harvesting System Based on Liquid Crystal Elastomers Tubular Actuator

Bio‐Inspired Untethered Automatic Light Harvesting System Based on Liquid Crystal Elastomers... IntroductionWith the rapid growth of human demand for energy, in order to ensure a vibrant, healthy, and friendly future, it is urgent to promote the use of affordable, easy‐to‐obtain, and sustainable energy development. The two pillars of this sustainable energy source are energy efficiency and renewable energy.[1,2] As a kind of energy that can be obtained and supplemented at any time in nature, solar energy is a typical renewable energy source, accompanied by the growth of solar‐related industries. However, in different latitudes, different seasons, and times, the solar energy received by the surface of various harvest solar equipment on the ground dramatically varies with the illumination angle of the solar light.[3–5] For example, the energy density received at the surface of the general device in the sunlight at 30° is only 50% of when it is vertical. The loss of solar capture caused by this natural incident angle is a key factor to limit the high efficiency of solar energy.[4]Most organisms can produce a stress response after receiving external stimulus signals, and light signals are the most common stimulus signal to organisms in nature.[6] The most common example is the phototropism of sunflowers, which controls the bending and rotation of its stems to make the flower disk face the sun, maximizing the use of solar energy. The direction of movement of organisms, especially plants, is strictly determined by the direction of environmental stimuli, that is tropic movement.[7,8] In tropic movement, the direction of the organism can be searched for as the stimulus that follows, and it can spontaneously adjust its movement to the direction of the movement signal. In solar automation and robot systems, adding this adaptive tropic movement is not only a way to solve the low efficiency of solar energy absorption. However, a current technology for artificial phototropism light‐harvesting system with the motor system is capable of accurate direction with high efficiency but still limited to bulky and expensive.[6,9,10]Liquid crystal elastomer (LCE) is a slightly cross‐linked network that combines the entropy elasticity of elastomers and the self‐organization of the liquid crystal mesogens.[11–16] Due to its unique actuation properties, such as large and recoverable deformability and programmability of actuation,[17–21] LCE emerges as an attractive and promising material candidate for autonomous light‐harvesting with features of simple design, small size, and low cost. The driving method of LCE is mainly thermal, such as direct environmental heating,[22,23] photo‐thermal,[24] photo‐chemical effects,[25–28] or electric heating,[17,29–31] which transform the liquid crystal mesogen from a nematic phase to an isotropic phase, causing macroscopic deformation of the elastomer. Several photo‐responsive LCE actuators have been explored in light‐harvesting systems. An auto‐artificial phototropism system using a photo‐responsive LCE actuator with a sheet‐like structure was demonstrated and it could effectively improve the solar collection of solar panels, but the payload of the actuator was less than 0.2 g.[32] A heliotracking device based on sunlight‐driven LCE was reported by using 8 actuators with a “dog bone” design to improve the actual payload up to 62.5 g/actuator, but this system was limited by a complicated fabrication process and a small max‐titling angle of only 15° that would largely influence the efficiency of light‐harvesting.[33] MXene‐reinforced photo‐responsive soft actuators with tubular structures were also demonstrated for the light‐harvesting systems, where the application of tubular structure increased the mechanical payload of the actuator, but the payload was still small for the practical devices due to low photo‐thermal conversion efficiency.[34] Compared to the photo‐responsive method based on photothermal or photochemical effects, the electric‐thermal effect can offer much higher energy conversion efficiency to the LCE actuator, which can bear a heavier payload to carry solar cells for practical application.In this letter, inspired by the phototropism of sunflowers, an untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells has been proposed. This portable system consisting of a light sensor, micro‐controller, and copper wire heater, combines the advantages of an LCE actuator and electrical control system and is capable of adjusting the solar cell facing the sun automatically through controls of bending and rotation of LCE tubular actuator to maximize the absorption of solar energy. Compared with the traditional mini photovoltaic (mini‐PV) panels, the absorption efficiency of solar energy by the light‐harvesting system is increased by 27.68% for the whole day and 230.15% in the morning or dawn for sunlight with a small incident angle. The proposed untethered automatic light‐harvesting system is compact, inexpensive, and can provide a large payload, showing great application potential in solar automation and robot systems, and energy‐saving fields.Results and DiscussionElectrical Driven and Precise Control of the LCE Tubular ActuatorFigure 1a demonstrates the photo image of the bending LCE tubular actuator (θ presents the bending angle) when current is applied to one heating wire. The heating efficiency and bending speed can be increased with the increase of the current. The dynamic relationship of bending angle (θ) and current (from ≈0.3–0.8 A) in a time range of 0 to 200 s during heating is shown in Figure 1b when the current is applied to one heating wire. Due to the influence of the LCE cylindrical structure, the maximum bending angle of our actuator is around 60°. Herein, when the bending angle of the actuator reaches around 60°, the input current is immediately cut off. The results show that when the input current is greater than 0.3 A, the actuator will eventually bend to the maximum angle. When the current is greater than 0.7 A, the bending time of the actuator is around 23 s, and saturates under a larger current without further improvement, which indicates that the highest stable temperature generated by the heating wire is achieved. The bending process under current is also demonstrated in Movie S1, Supporting Information.1Figurea) Image of bending LCE tubular actuator when current is applied to one heating wire, θ presents the bending angle. b) The dynamic relationship of bending angle and current during heating. c) The thermal image of LCE tubular actuator. d) The relationship between the highest stable temperature produced by the heating wire and the input current.The thermal image of the LCE tubular actuator captured by an infrared thermal camera (FLIR ONE pro) is demonstrated in Figure 1c. The relationship of the highest stable temperature of LCE versus current is plotted in Figure 1d. It can be seen that, with the increase of current from 0.3 to 0.8 A, the highest stable temperature increases from 43.4 to 176.8 °C. Due to the barrel structure of the actuator, the bending caused by the heating of the local area is related to the LCE contracted ratio of the heated area, while the temperature affects the contracted ratio of the LCE within a certain period of time. In our experimental curve, it is shown as the difference in bending speed at different heating power. When the heating temperature is 145.3 °C (0.7 A), the bending speed of the actuator is saturated, so continuing to increase the temperature (176.8 °C, 0.8 A) will not increase the bending speed. When the input current is 0.3 A, the heating temperature (43.4 °C) is not enough to make the local LCE contract sufficiently, so the actuator cannot meet the bending requirement of 60°. When the input current increases to 0.4 A, the heating temperature (61.3 °C) is closer to or reaches the nematic‐isotropic transition temperature of LCE, so the local LCE can be fully contracted, and the tubular actuator can be bent to 60°. In subsequent experiments, 0.7 A current can meet the requirements of the maximum bending speed, and 0.4 A is the minimum current we need to maintain the bending angle.System Integration and TestingA fabricated LCE tubular actuator was first glued to an acrylic plate, and then a spring (50 mm × 10 mm × 0.6 mm) was put inside the cylinder and fixed on the acrylic plate in order to ensure that the tubular actuator can be restored upright after bending. The electrical wires of the light sensor module and the mini‐PV will pass through the spring. The mini photovoltaic panel (100 mm × 100 mm × 3 mm, 50 g) was used to absorb light energy, with a maximum working voltage of 7.2 V and a minimum current of 0.33 A. The light sensor module consists of four photoresistor probes, which were glued to the lower surface of the mini‐PV panel. The photo image of the LCE tubular actuator loaded with photovoltaics is shown in Figure 2a.2Figurea) Photo image of LCE tubular actuator loaded with photovoltaic. b)The photo images of the tubular actuator loaded with photovoltaic with different input current ratios of I) 4:0, II) 0:4, III) 4:1, and IV) 4:4 in range of operation current from the bottom wire and right wire, the red, orange, and blue color represents 4 (big current), 1 (small current), and 0 (no current) input; c) the relationship of the input current ratio of two adjacent heating wires (bottom and right) in the range from 0:4 to 4:0 and the corresponding bending angle of the actuator along the right direction.In the automatic light‐harvesting system, the LCE tubular actuator is required to maintain its bending angle and direction under a load of mini‐PV and light sensors. The prepared mini‐PV and four photoresistors (total weight 16.5 g) are connected to one end of the LCE actuator through the insulating tape, and another end was fixed on the substrate. Then, apply an electric current to a copper heating wire, and the LCE tubular actuator will bend toward the direction of the heating wire with the mini‐PV and light sensors. An alternating current (AC) square wave current was used to maintain a certain bending angle of the actuator by adjusting the duty cycle. In the beginning, the duty cycle of the input current was 50%, and after reaching a designed angle, the duty cycle was adjusted to 25%, allowing the actuator to maintain this angle.When the current is applied to two adjacent heating wires at the same time, the actuator can bend in the direction between them. The actuator can obtain different bending directions when controlling the input current ratio of adjacent heating wires, as shown in Figure 2b. When the input current is applied to one heating wire, for example, the bottom one or the right one, the resulting bending angle can reach up to 95° along the corresponding direction, where the current ratio of the bottom wire and right wire is 4:0 and 0:4, as shown in Figure 2b‐I,II. As the ratio of two input currents changes, the bending angle, as well as the direction of the LCE tubular actuator, changes accordingly. Figure 2b‐III,IV demonstrates the photo images of the actuator with a current ratio of bottom and right wires of 4:1 and 4:4. It is worth noticing that the input current is operated in a range of 0.4–0.7 A and the red, orange, and blue color represents big (4), small (1), and 0 (no) current input, respectively, It can be seen that the bending direction of the actuator can precisely be controlled according to environmental light exposure by adjusting the value and ratio of current of adjacent heating wires, thus the mini‐PV device can maintain the maximum light absorption efficiency dynamically. Figure 2c shows the relationship of the input current ratio of two adjacent heating wires (bottom and right) in the range from 0:4 to 4:0 and the corresponding bending angle of the actuator along the right direction (0°–95°). The rotation and bending process of the LCE tubular actuator loaded with photovoltaic is demonstrated in Movies S2–S4, Supporting Information.Stimulated by the phototropism of sunflower (Figure 3a), an untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells has been proposed, which makes the solar cell facing the sun automatically through controls of bending and rotation of LCE tubular actuator to maximize the absorption of solar energy. The circuit diagram of the untethered automatic light‐harvesting system is depicted in Figure 3b. The photoresistors in the four directions of the photoelectric module read the light intensity of the four points respectively, then the microcontroller DFRobot UNO 3 (5 V) (Figure 3c) analysis the direction of the light source through the four‐point positioning. According to the light source direction data, the microcontroller controls the current and duty cycle of the four copper heating wires of the LCE tubular actuator, respectively, through the Quad motor driver shield for Arduino (TB6612FNG) (Figure 3d). The LCE tubular actuator can bend toward the light source, helping the mini‐PV mounted on top to trace the light, absorbing the light energy with higher efficiency. A lithium battery (NICJOY 7.4 V, 20 000 mAh) equipped with a battery fuel gauge module (LSP1005P) was used as the power source for the system to store light energy from photovoltaics (Figure 3e).3Figurea) The phototropism of sunflowers and the schematic of light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells. b) Circuit diagram of the untethered automatic light‐harvesting system. c) Microcontroller (DFRobot UNO 3); d) Quad motor driver shield (arduino TB6612FNG); e) Battery fuel gauge module (LSP1005P).It is worth noticing that if driven by direct current (DC), the LCE tubular actuator would exhibit unexpected morphological changes when the operating current was applied for a long time. It is due to the thermal diffusion that affects a larger area of the actuator, resulting in a decrease in the degree of bending of the tubular actuator and then unpredictable geometric deformation. In order to solve this problem, the AC was used to replace the DC current. By controlling its duty cycle of AC square wave, the actuator can be stabilized at a certain bending angle (shown in Movie S5, Supporting Information).The tropic movement of plants to light ensures that they can receive maximum sunlight. The proposed untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators is demonstrated in Figure 3a. A mini‐PV panel integrated with photoresistors was loaded on the head of the LCE tubular actuator, where the light energy was collected through the mini‐PV panel and stored on the prepared lithium battery, which also supplied power to the actuator and the control system. Four photoresistors were installed under the mini‐PV to detect light intensities in four directions, and the detection signals from four photoresistors were collected and analyzed by the controller to determine the direction of the light source. The controller was preprogrammed to allow individual control of the heating time and output current of individual copper heating wires through the quad motor driver shield. The copper heating wire corresponding to the photoresistor with the highest light intensity would be heated, and the controller would further adjust the output power according to the maximum difference between the four light intensities. The LCE tubular actuator would bend toward the light source, thereby lowering the angle of the mini‐PV mounted on the head to the light. When the magnitudes of signals from the four lights tended to be the same, the system would reduce the output power to the copper heating wire. At this time, the restoring force generated by the spring that was installed in the LCE tubular actuator would balance the force of the LCE bent by heating and the gravity of the mini‐PV. As a result, the system can maintain the angle between the mini‐PV with the light, leading to the high efficiency of light energy absorption. Once the difference between the four light intensities exceeds the set threshold, the controller will re‐adjust the output power according to the difference to maintain the stability of the system.An Untethered SystemFigure 4 demonstrates the phototropism of our light‐harvesting system, where the LCE tubular actuator (Figure 5a‐I) bends toward the light source to the left (Figure 4a‐II), and then follows the light source back to its original position (Figure 4a‐III). First, the light source was moved to about a 45° angle position to the upper left, then the tubular actuator took 40 s to direct the photovoltaic panel toward the light source. Then, the actuator can maintain this angle and direction to maintain the maximum absorbed light energy of the mini‐PV. After maintaining for 50 s, the light source was moved directly above, and the system took 55 s to return to its original state. Due to long‐term heating, it takes a certain time for the LCE on the left to cool and recover. Therefore, it takes a longer time to guide the system to the right compared to directing it to the left. During the movement, the controller will adjust the output power according to the change in the position of the light source. The relationship of bending angle and average input power versus time of LCE tubular actuator during bending and recovering with a light source is shown in Figure 4b. It can be seen that the actuator bends with high input power, maintains with low input power, and recovers with high power on another heating wiring. The controller can bend the mini‐PV in all directions and maintain the angle by controlling the current input of the four copper heating wires, as shown in Figure 4c.4Figurea) LCE tubular actuator loaded mini‐PV bends and recovers with a light source. Step I: original position; Step II: bending towards left; Step III: recovering to the original position. b) The relationship of bending angle and average input power versus time of LCE tubular actuator during bending and recovering with a light source. c) The tubular actuator loaded with mini‐PV bends light source in all directions.5Figurea) Photograph of the untethered light‐harvesting system under sunlight at the time of I) 10:00 am, II) 12:00 am, III) 2:00 pm, and IV) 5:00 pm, respectively. b) The light energy absorbed by the photovoltaic was used to charge a mobile phone. c) Capacity in charging mobile phones through automatic light‐harvesting system (blue curve) and mini‐PV (green curve) directly, respectively. The bar chart shows the increase of capacity for the automatically untethered system (blue bar chart) and mini‐PV (green bar chart) over every single time period from 9:00 to 17:00.To test the ability of our system to work under 1 day's sunlight, the untethered automatic light‐harvesting system was exposed outdoors for a whole day (Movie S6, Supporting Information). The photograph of our untethered light‐harvesting system is demonstrated in Figure 5a at times 10:00 am (Figure 5a‐I), 12:00 am (Figure 5a‐II), 14:00 pm (Figure 5a‐III), and 17:00 pm (Figure 5a‐IV), respectively. It can be seen that our untethered light‐harvesting system can dynamically adjust the direction of the solar cells toward the direction of sunlight, enabling the maximal absorption efficiency of sunlight through the bending of the tubular actuator. In Figure 5b, after adding a suitable constant voltage step‐down module to the system, the light energy absorbed by the photovoltaic can be restored and used to charge a mobile phone. The whole system is compact with a size of 15 cm × 15 cm × 8 cm, and a weight of 700 g, which can provide a large payload of more than 110 g. The system is portable and can be easily carried by a private vehicle or even bicycle for handphone or electronic device charge in a wild environment that is short of power supply.The change in power in each time period was recorded by the battery capacity indicator module. As a comparison, the light energy absorbed by an equivalent mini‐PV placed on the ground without an automatic light‐harvesting system was also recorded, as shown in Figure 5c. It can be seen that the capacity in charging mobile phones through our untethered automatic light‐harvesting system and mini‐PV directly is represented by blue and green curves, respectively. The bar graph shows the increase of capacity for automatic light harvesting untethered system (blue bar chart) and mini‐PV (green bar chart) over every single time period from 9:00 to 17:00. The total electricity capacity absorption curve of the two methods is a typical S‐curve, showing the trend of increased acceleration and then deceleration. Compared with the single mini‐PV (green curve), the speed of the untethered automatic light‐harvesting system (blue curve) for solar energy collection is significantly improved (means a large difference in bar chart) in the morning time period (9:00–11:00) and the afternoon time (14:00–17:00).The angle between the sunlight illumination and the normal direction of the PV‐plane play an important role in the low energy absorption efficiency of mini‐PV. From Figure 5c, it can be seen that the strategy of enabling mini‐PV tracking the sun to improve the efficiency of light absorption is successful especially when the sunshine is obliquely illuminated on the ground. The stored electricity from light energy of the untethered automatic light‐harvesting system (blue bar chart) in recorded eight time periods is 110, 230, 377, 390, 382, 377, 251, and 208 mAh, respectively, with an increase of 129.17%, 38.55%, −4.07%, −0.51%, 25.25%, 41.73%, 33.51%, and 230.15%, respectively, comparing to the single mini‐PV without this system. It can be seen that the smaller the angle between sunlight and ground, the more obvious the improvement of the light absorption efficiency of mini‐PV with an automatic light‐harvesting system. During the noon time (11:00 to 13:00), the single mini‐PV does not show a decrease in absorption efficiency caused by the sunlight angle (near 90°). Since the light‐harvesting system requires a certain amount of power to maintain the operation of the control system, thus the efficiency is slightly lower (−4.07%, −0.51%). Once the angle of sunlight on the ground decreases (in other time periods except 11:00–13:00), the light energy absorption efficiency of mini‐PV with an automatic light‐harvesting system improves significantly (25.25–230.15%).Totally, from 9:00 am to 17:00 pm, the amount of electricity stored by the untethered automatic light‐harvesting system is 2325 mAh, which is 27.68% higher than 1821 mAh stored by the mini‐PV alone. The bar chart showing the increase of electricity per unit time can directly reflect the improvement of the untethered automatic light‐harvesting system's light energy efficiency absorption during the solar oblique illumination period. The highest increase of electricity per unit time of mini‐PV without an untethered automatic light‐harvesting system (green bar chart) was 393 mAh, and the lowest was only 48 mAh. The light absorption efficiency of the unit time is reduced by up to 87.79%.ConclusionIn summary, we demonstrate an untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells, inspired by the phototropism of sunflowers. This system, consisting of a light sensor, microcontroller, and copper wire heater, is capable of adjusting the solar cell facing the sun automatically through controls of bending and rotation of the LCE tubular actuator to maximize the absorption of solar energy. Compared with the traditional mini photovoltaic panels, the absorption efficiency of solar energy by the light‐harvesting system is increased by 27.68% for the whole day, and 230.15% in the morning or dawn for sunlight with a small incident angle. This compact portable system with a single tubular design can provide a large payload and can be used in handphone or electronic device charge in a wild environment that is short of power supply. Regarding the problems of incident angle modifiers and high commercial automatic costs encountered in the current solar energy field, the proposed untethered automatic light‐harvesting system with the capability of artificial tropic movement shows great application potential in solar automation and robot systems, and energy‐saving fields.Experimental SectionMaterialsLiquid crystal mesogen 1,4‐Bis‐[4‐(3‐aryloyloxypropyloxy)benzoyloxy]‐2‐methylbenzene (RM257, 95%) was provided by Nanjing Leyao Technology Co. Ltd. (China). Triglycol dimercaptan (EDDET, 95%) and pentaerythritol tetrakis (3‐mercaptopropionate) (PETMP, 95%) were purchased from J&K Scientific (China). 2‐Hydroxy‐1‐[4‐(2‐hydroxyethoxy)phenyl]‐2‐methyl‐1‐propanone (HHMP, 95%) and iron(III) chloride solution (45% FeCl3 basis) were purchased from Aladdin Bio‐Chem Technology Co. Ltd. (China). Dipropylamine (DPA, 98%) was purchased from Mackin (China), which was mixed into a 2 wt% toluene solution before use. Toluene was purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. (China), which was dried with a 5A molecular sieve before use. Polydimethylsiloxane (PDMS, 184 Silicone elastomer base) was purchased from DOWSIL (US). Polyimide (PI) precursor solution (10–12% solids) was provided by Jingai Microelectronics (China).Fabrication of Liquid Crystal Elastomer FilmThe preparation method of the LCE film was based on previous reports with small changes.[35] First, 2.7 mL of toluene was mixed with 7.2 g (12.2 mmol) of RM257, then the mixture was heated at 85 °C to completely dissolve into a transparent solution. After cooled down to room temperature, the mixture was added to 2.021 g (11.3 mmol) of spacer EDDET, 0.160 g (0.3 mmol) of cross‐linker PETMP, and 0.05 g (0.22 mmol) photoinitiator HHMP, with a shaker to mix the mixture till completely dissolved. Then, 1 mL of prepared 2% catalyst DPA toluene solution was added into the mixture and then the mixture was stirred, cast into a rectangular Teflon mold (100 mm ×100 mm), degassed for 5 min, and then solidified at room temperature for 24 h. Finally, the resultant composite film was dried in an oven at 85 °C for solvent evaporation, leading to a loosely cross‐linked LCE film. The chemical structures of RM257, EDDET, and PETMP are shown in Figure 6a. Figure 6b shows the shapes of the LCE tubular actuator during the heating and cooling process, where the liquid crystal network changes between the nematic phase (without current) and the isotropic phase (with a current of 0.3 A).6Figurea) Chemical structures of RM257, EDDET, and PETMP. b) LCE tubular actuator during the heating and cooling process, where the liquid crystal network changes between the nematic phase (without current) and isotropic phase (with a current of 0.3 A). c) The preparation process of copper heating wire. Step I: a PI precursor layer with a thickness of 3 µm was spin‐coated on the copper surface of an FPC film. Step II: the PI/Cu/PI film (3 µm/18 µm/12 µm) was totally inverted and placed on the glass substrate spin‐coated with 1 mm PDMS, and then solidified in a 50 °C oven for 1 h. Step III: An UV laser marking machine was used to etch the required wire shape on the surface PI layer (12 µm) and remove the unnecessary PI around the wires. Then, the iron(III) chloride solution was applied to etch the corresponding copper layer that was uncovered by the surface PI layer. Step IV: the PI layer with a thickness of 3 µm as encapsulation was formed and patterned by ICP under O2 and CHF3 plasma. d) A copper heating wire with a width of about 120 µm. e) Fabrication of LCE tubular actuator. Step I: four copper heating wires were first transferred onto the prepared LCE film (50 mm × 50 mm × 0.7 mm) using water‐soluble tape. Step II: the copper heating wires were sandwiched with another film of the same size. Step III: the sample was rolled into a tubular shape and stretched along the longitudinal direction (up‐down direction) to align the liquid crystal mesogens.Preparation of Copper Heating WireThe preparation of copper heating wire is shown in Figure 6c. First (step I), a PI precursor layer with a thickness of 3 µm was spin‐coated on the copper surface of a flexible printed circuit (FPC, PI/Cu; 12 µm/18 µm), and then cured at a 300 °C oven with nitrogen protection for 12 h. Second (step II), the PI/Cu/PI film (3 µm/18 µm/12 µm) was totally inverted and placed on the glass substrate spin‐coated with 1 mm PDMS, and then solidified in a 50 °C oven for 1 h. Third (step III), an UV laser (355 nm, 5 W) marking machine was used to etch the required wire shape on the surface PI layer (12 µm) and removed the unnecessary PI around the wires. Then, the iron(III) chloride solution was applied to etch the corresponding copper layer that was uncovered by the surface PI layer. Finally (step IV), the PI layer with a thickness of 3 µm as encapsulation was formed and patterned by inductively coupled plasma (ICP) under O2 and CHF3 plasma. A copper heating wire with a width of about 120 µm was obtained, as shown in Figure 6d. The copper wire was removed from the PDMS surface on the substrate with a water‐soluble tape before use.Fabrication of LCE Tubular ActuatorThe preparation method of the LCE tubular actuator follows the method reported in ref. [17]. Four copper heating wires were first transferred onto the prepared LCE film (50 mm × 50 mm × 0.7 mm) using water‐soluble tape, where the tape was removed by soaking in water. The PI layer with a thickness of 3 µm can help the copper heating wire adsorption on the LCE film (Figure 6e‐I). After drying, the copper heating wires were sandwiched with another film of the same size (Figure 6e‐II). The whole sample was then compressed by an object with 500 g weight into a sandwich‐like structure at 85 °C. A certain margin of around 2 mm was reserved beyond the adhesive parts on both sides of the upper and lower layers of LCE to facilitate subsequent bonding, and the excess LCE film was trimmed. Then, the sample was rolled into a tubular shape (Figure 6e‐III) and then stretched along the longitudinal direction (up‐down direction) to align the liquid crystal mesogens, where the LCE tube was kept in the stretched state and exposed under a UV lamp (355 nm, 5 W) for 30 min. Finally, the input electrical wire was soldered to the copper heating wire to form the LCE tubular actuator.AcknowledgementsThis work was supported by the National Natural Science Foundation of China (NSFC) (62175098), and Guangdong Basic and Applied Basic Research Foundation (2021B1515020097).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.I. Vera, L. 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Reed, C. N. Bowman, RSC Adv. 2015, 5, 18997. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advanced Materials Interfaces Wiley

Bio‐Inspired Untethered Automatic Light Harvesting System Based on Liquid Crystal Elastomers Tubular Actuator

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Wiley
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© 2023 Wiley‐VCH GmbH
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2196-7350
DOI
10.1002/admi.202202513
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Abstract

IntroductionWith the rapid growth of human demand for energy, in order to ensure a vibrant, healthy, and friendly future, it is urgent to promote the use of affordable, easy‐to‐obtain, and sustainable energy development. The two pillars of this sustainable energy source are energy efficiency and renewable energy.[1,2] As a kind of energy that can be obtained and supplemented at any time in nature, solar energy is a typical renewable energy source, accompanied by the growth of solar‐related industries. However, in different latitudes, different seasons, and times, the solar energy received by the surface of various harvest solar equipment on the ground dramatically varies with the illumination angle of the solar light.[3–5] For example, the energy density received at the surface of the general device in the sunlight at 30° is only 50% of when it is vertical. The loss of solar capture caused by this natural incident angle is a key factor to limit the high efficiency of solar energy.[4]Most organisms can produce a stress response after receiving external stimulus signals, and light signals are the most common stimulus signal to organisms in nature.[6] The most common example is the phototropism of sunflowers, which controls the bending and rotation of its stems to make the flower disk face the sun, maximizing the use of solar energy. The direction of movement of organisms, especially plants, is strictly determined by the direction of environmental stimuli, that is tropic movement.[7,8] In tropic movement, the direction of the organism can be searched for as the stimulus that follows, and it can spontaneously adjust its movement to the direction of the movement signal. In solar automation and robot systems, adding this adaptive tropic movement is not only a way to solve the low efficiency of solar energy absorption. However, a current technology for artificial phototropism light‐harvesting system with the motor system is capable of accurate direction with high efficiency but still limited to bulky and expensive.[6,9,10]Liquid crystal elastomer (LCE) is a slightly cross‐linked network that combines the entropy elasticity of elastomers and the self‐organization of the liquid crystal mesogens.[11–16] Due to its unique actuation properties, such as large and recoverable deformability and programmability of actuation,[17–21] LCE emerges as an attractive and promising material candidate for autonomous light‐harvesting with features of simple design, small size, and low cost. The driving method of LCE is mainly thermal, such as direct environmental heating,[22,23] photo‐thermal,[24] photo‐chemical effects,[25–28] or electric heating,[17,29–31] which transform the liquid crystal mesogen from a nematic phase to an isotropic phase, causing macroscopic deformation of the elastomer. Several photo‐responsive LCE actuators have been explored in light‐harvesting systems. An auto‐artificial phototropism system using a photo‐responsive LCE actuator with a sheet‐like structure was demonstrated and it could effectively improve the solar collection of solar panels, but the payload of the actuator was less than 0.2 g.[32] A heliotracking device based on sunlight‐driven LCE was reported by using 8 actuators with a “dog bone” design to improve the actual payload up to 62.5 g/actuator, but this system was limited by a complicated fabrication process and a small max‐titling angle of only 15° that would largely influence the efficiency of light‐harvesting.[33] MXene‐reinforced photo‐responsive soft actuators with tubular structures were also demonstrated for the light‐harvesting systems, where the application of tubular structure increased the mechanical payload of the actuator, but the payload was still small for the practical devices due to low photo‐thermal conversion efficiency.[34] Compared to the photo‐responsive method based on photothermal or photochemical effects, the electric‐thermal effect can offer much higher energy conversion efficiency to the LCE actuator, which can bear a heavier payload to carry solar cells for practical application.In this letter, inspired by the phototropism of sunflowers, an untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells has been proposed. This portable system consisting of a light sensor, micro‐controller, and copper wire heater, combines the advantages of an LCE actuator and electrical control system and is capable of adjusting the solar cell facing the sun automatically through controls of bending and rotation of LCE tubular actuator to maximize the absorption of solar energy. Compared with the traditional mini photovoltaic (mini‐PV) panels, the absorption efficiency of solar energy by the light‐harvesting system is increased by 27.68% for the whole day and 230.15% in the morning or dawn for sunlight with a small incident angle. The proposed untethered automatic light‐harvesting system is compact, inexpensive, and can provide a large payload, showing great application potential in solar automation and robot systems, and energy‐saving fields.Results and DiscussionElectrical Driven and Precise Control of the LCE Tubular ActuatorFigure 1a demonstrates the photo image of the bending LCE tubular actuator (θ presents the bending angle) when current is applied to one heating wire. The heating efficiency and bending speed can be increased with the increase of the current. The dynamic relationship of bending angle (θ) and current (from ≈0.3–0.8 A) in a time range of 0 to 200 s during heating is shown in Figure 1b when the current is applied to one heating wire. Due to the influence of the LCE cylindrical structure, the maximum bending angle of our actuator is around 60°. Herein, when the bending angle of the actuator reaches around 60°, the input current is immediately cut off. The results show that when the input current is greater than 0.3 A, the actuator will eventually bend to the maximum angle. When the current is greater than 0.7 A, the bending time of the actuator is around 23 s, and saturates under a larger current without further improvement, which indicates that the highest stable temperature generated by the heating wire is achieved. The bending process under current is also demonstrated in Movie S1, Supporting Information.1Figurea) Image of bending LCE tubular actuator when current is applied to one heating wire, θ presents the bending angle. b) The dynamic relationship of bending angle and current during heating. c) The thermal image of LCE tubular actuator. d) The relationship between the highest stable temperature produced by the heating wire and the input current.The thermal image of the LCE tubular actuator captured by an infrared thermal camera (FLIR ONE pro) is demonstrated in Figure 1c. The relationship of the highest stable temperature of LCE versus current is plotted in Figure 1d. It can be seen that, with the increase of current from 0.3 to 0.8 A, the highest stable temperature increases from 43.4 to 176.8 °C. Due to the barrel structure of the actuator, the bending caused by the heating of the local area is related to the LCE contracted ratio of the heated area, while the temperature affects the contracted ratio of the LCE within a certain period of time. In our experimental curve, it is shown as the difference in bending speed at different heating power. When the heating temperature is 145.3 °C (0.7 A), the bending speed of the actuator is saturated, so continuing to increase the temperature (176.8 °C, 0.8 A) will not increase the bending speed. When the input current is 0.3 A, the heating temperature (43.4 °C) is not enough to make the local LCE contract sufficiently, so the actuator cannot meet the bending requirement of 60°. When the input current increases to 0.4 A, the heating temperature (61.3 °C) is closer to or reaches the nematic‐isotropic transition temperature of LCE, so the local LCE can be fully contracted, and the tubular actuator can be bent to 60°. In subsequent experiments, 0.7 A current can meet the requirements of the maximum bending speed, and 0.4 A is the minimum current we need to maintain the bending angle.System Integration and TestingA fabricated LCE tubular actuator was first glued to an acrylic plate, and then a spring (50 mm × 10 mm × 0.6 mm) was put inside the cylinder and fixed on the acrylic plate in order to ensure that the tubular actuator can be restored upright after bending. The electrical wires of the light sensor module and the mini‐PV will pass through the spring. The mini photovoltaic panel (100 mm × 100 mm × 3 mm, 50 g) was used to absorb light energy, with a maximum working voltage of 7.2 V and a minimum current of 0.33 A. The light sensor module consists of four photoresistor probes, which were glued to the lower surface of the mini‐PV panel. The photo image of the LCE tubular actuator loaded with photovoltaics is shown in Figure 2a.2Figurea) Photo image of LCE tubular actuator loaded with photovoltaic. b)The photo images of the tubular actuator loaded with photovoltaic with different input current ratios of I) 4:0, II) 0:4, III) 4:1, and IV) 4:4 in range of operation current from the bottom wire and right wire, the red, orange, and blue color represents 4 (big current), 1 (small current), and 0 (no current) input; c) the relationship of the input current ratio of two adjacent heating wires (bottom and right) in the range from 0:4 to 4:0 and the corresponding bending angle of the actuator along the right direction.In the automatic light‐harvesting system, the LCE tubular actuator is required to maintain its bending angle and direction under a load of mini‐PV and light sensors. The prepared mini‐PV and four photoresistors (total weight 16.5 g) are connected to one end of the LCE actuator through the insulating tape, and another end was fixed on the substrate. Then, apply an electric current to a copper heating wire, and the LCE tubular actuator will bend toward the direction of the heating wire with the mini‐PV and light sensors. An alternating current (AC) square wave current was used to maintain a certain bending angle of the actuator by adjusting the duty cycle. In the beginning, the duty cycle of the input current was 50%, and after reaching a designed angle, the duty cycle was adjusted to 25%, allowing the actuator to maintain this angle.When the current is applied to two adjacent heating wires at the same time, the actuator can bend in the direction between them. The actuator can obtain different bending directions when controlling the input current ratio of adjacent heating wires, as shown in Figure 2b. When the input current is applied to one heating wire, for example, the bottom one or the right one, the resulting bending angle can reach up to 95° along the corresponding direction, where the current ratio of the bottom wire and right wire is 4:0 and 0:4, as shown in Figure 2b‐I,II. As the ratio of two input currents changes, the bending angle, as well as the direction of the LCE tubular actuator, changes accordingly. Figure 2b‐III,IV demonstrates the photo images of the actuator with a current ratio of bottom and right wires of 4:1 and 4:4. It is worth noticing that the input current is operated in a range of 0.4–0.7 A and the red, orange, and blue color represents big (4), small (1), and 0 (no) current input, respectively, It can be seen that the bending direction of the actuator can precisely be controlled according to environmental light exposure by adjusting the value and ratio of current of adjacent heating wires, thus the mini‐PV device can maintain the maximum light absorption efficiency dynamically. Figure 2c shows the relationship of the input current ratio of two adjacent heating wires (bottom and right) in the range from 0:4 to 4:0 and the corresponding bending angle of the actuator along the right direction (0°–95°). The rotation and bending process of the LCE tubular actuator loaded with photovoltaic is demonstrated in Movies S2–S4, Supporting Information.Stimulated by the phototropism of sunflower (Figure 3a), an untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells has been proposed, which makes the solar cell facing the sun automatically through controls of bending and rotation of LCE tubular actuator to maximize the absorption of solar energy. The circuit diagram of the untethered automatic light‐harvesting system is depicted in Figure 3b. The photoresistors in the four directions of the photoelectric module read the light intensity of the four points respectively, then the microcontroller DFRobot UNO 3 (5 V) (Figure 3c) analysis the direction of the light source through the four‐point positioning. According to the light source direction data, the microcontroller controls the current and duty cycle of the four copper heating wires of the LCE tubular actuator, respectively, through the Quad motor driver shield for Arduino (TB6612FNG) (Figure 3d). The LCE tubular actuator can bend toward the light source, helping the mini‐PV mounted on top to trace the light, absorbing the light energy with higher efficiency. A lithium battery (NICJOY 7.4 V, 20 000 mAh) equipped with a battery fuel gauge module (LSP1005P) was used as the power source for the system to store light energy from photovoltaics (Figure 3e).3Figurea) The phototropism of sunflowers and the schematic of light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells. b) Circuit diagram of the untethered automatic light‐harvesting system. c) Microcontroller (DFRobot UNO 3); d) Quad motor driver shield (arduino TB6612FNG); e) Battery fuel gauge module (LSP1005P).It is worth noticing that if driven by direct current (DC), the LCE tubular actuator would exhibit unexpected morphological changes when the operating current was applied for a long time. It is due to the thermal diffusion that affects a larger area of the actuator, resulting in a decrease in the degree of bending of the tubular actuator and then unpredictable geometric deformation. In order to solve this problem, the AC was used to replace the DC current. By controlling its duty cycle of AC square wave, the actuator can be stabilized at a certain bending angle (shown in Movie S5, Supporting Information).The tropic movement of plants to light ensures that they can receive maximum sunlight. The proposed untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators is demonstrated in Figure 3a. A mini‐PV panel integrated with photoresistors was loaded on the head of the LCE tubular actuator, where the light energy was collected through the mini‐PV panel and stored on the prepared lithium battery, which also supplied power to the actuator and the control system. Four photoresistors were installed under the mini‐PV to detect light intensities in four directions, and the detection signals from four photoresistors were collected and analyzed by the controller to determine the direction of the light source. The controller was preprogrammed to allow individual control of the heating time and output current of individual copper heating wires through the quad motor driver shield. The copper heating wire corresponding to the photoresistor with the highest light intensity would be heated, and the controller would further adjust the output power according to the maximum difference between the four light intensities. The LCE tubular actuator would bend toward the light source, thereby lowering the angle of the mini‐PV mounted on the head to the light. When the magnitudes of signals from the four lights tended to be the same, the system would reduce the output power to the copper heating wire. At this time, the restoring force generated by the spring that was installed in the LCE tubular actuator would balance the force of the LCE bent by heating and the gravity of the mini‐PV. As a result, the system can maintain the angle between the mini‐PV with the light, leading to the high efficiency of light energy absorption. Once the difference between the four light intensities exceeds the set threshold, the controller will re‐adjust the output power according to the difference to maintain the stability of the system.An Untethered SystemFigure 4 demonstrates the phototropism of our light‐harvesting system, where the LCE tubular actuator (Figure 5a‐I) bends toward the light source to the left (Figure 4a‐II), and then follows the light source back to its original position (Figure 4a‐III). First, the light source was moved to about a 45° angle position to the upper left, then the tubular actuator took 40 s to direct the photovoltaic panel toward the light source. Then, the actuator can maintain this angle and direction to maintain the maximum absorbed light energy of the mini‐PV. After maintaining for 50 s, the light source was moved directly above, and the system took 55 s to return to its original state. Due to long‐term heating, it takes a certain time for the LCE on the left to cool and recover. Therefore, it takes a longer time to guide the system to the right compared to directing it to the left. During the movement, the controller will adjust the output power according to the change in the position of the light source. The relationship of bending angle and average input power versus time of LCE tubular actuator during bending and recovering with a light source is shown in Figure 4b. It can be seen that the actuator bends with high input power, maintains with low input power, and recovers with high power on another heating wiring. The controller can bend the mini‐PV in all directions and maintain the angle by controlling the current input of the four copper heating wires, as shown in Figure 4c.4Figurea) LCE tubular actuator loaded mini‐PV bends and recovers with a light source. Step I: original position; Step II: bending towards left; Step III: recovering to the original position. b) The relationship of bending angle and average input power versus time of LCE tubular actuator during bending and recovering with a light source. c) The tubular actuator loaded with mini‐PV bends light source in all directions.5Figurea) Photograph of the untethered light‐harvesting system under sunlight at the time of I) 10:00 am, II) 12:00 am, III) 2:00 pm, and IV) 5:00 pm, respectively. b) The light energy absorbed by the photovoltaic was used to charge a mobile phone. c) Capacity in charging mobile phones through automatic light‐harvesting system (blue curve) and mini‐PV (green curve) directly, respectively. The bar chart shows the increase of capacity for the automatically untethered system (blue bar chart) and mini‐PV (green bar chart) over every single time period from 9:00 to 17:00.To test the ability of our system to work under 1 day's sunlight, the untethered automatic light‐harvesting system was exposed outdoors for a whole day (Movie S6, Supporting Information). The photograph of our untethered light‐harvesting system is demonstrated in Figure 5a at times 10:00 am (Figure 5a‐I), 12:00 am (Figure 5a‐II), 14:00 pm (Figure 5a‐III), and 17:00 pm (Figure 5a‐IV), respectively. It can be seen that our untethered light‐harvesting system can dynamically adjust the direction of the solar cells toward the direction of sunlight, enabling the maximal absorption efficiency of sunlight through the bending of the tubular actuator. In Figure 5b, after adding a suitable constant voltage step‐down module to the system, the light energy absorbed by the photovoltaic can be restored and used to charge a mobile phone. The whole system is compact with a size of 15 cm × 15 cm × 8 cm, and a weight of 700 g, which can provide a large payload of more than 110 g. The system is portable and can be easily carried by a private vehicle or even bicycle for handphone or electronic device charge in a wild environment that is short of power supply.The change in power in each time period was recorded by the battery capacity indicator module. As a comparison, the light energy absorbed by an equivalent mini‐PV placed on the ground without an automatic light‐harvesting system was also recorded, as shown in Figure 5c. It can be seen that the capacity in charging mobile phones through our untethered automatic light‐harvesting system and mini‐PV directly is represented by blue and green curves, respectively. The bar graph shows the increase of capacity for automatic light harvesting untethered system (blue bar chart) and mini‐PV (green bar chart) over every single time period from 9:00 to 17:00. The total electricity capacity absorption curve of the two methods is a typical S‐curve, showing the trend of increased acceleration and then deceleration. Compared with the single mini‐PV (green curve), the speed of the untethered automatic light‐harvesting system (blue curve) for solar energy collection is significantly improved (means a large difference in bar chart) in the morning time period (9:00–11:00) and the afternoon time (14:00–17:00).The angle between the sunlight illumination and the normal direction of the PV‐plane play an important role in the low energy absorption efficiency of mini‐PV. From Figure 5c, it can be seen that the strategy of enabling mini‐PV tracking the sun to improve the efficiency of light absorption is successful especially when the sunshine is obliquely illuminated on the ground. The stored electricity from light energy of the untethered automatic light‐harvesting system (blue bar chart) in recorded eight time periods is 110, 230, 377, 390, 382, 377, 251, and 208 mAh, respectively, with an increase of 129.17%, 38.55%, −4.07%, −0.51%, 25.25%, 41.73%, 33.51%, and 230.15%, respectively, comparing to the single mini‐PV without this system. It can be seen that the smaller the angle between sunlight and ground, the more obvious the improvement of the light absorption efficiency of mini‐PV with an automatic light‐harvesting system. During the noon time (11:00 to 13:00), the single mini‐PV does not show a decrease in absorption efficiency caused by the sunlight angle (near 90°). Since the light‐harvesting system requires a certain amount of power to maintain the operation of the control system, thus the efficiency is slightly lower (−4.07%, −0.51%). Once the angle of sunlight on the ground decreases (in other time periods except 11:00–13:00), the light energy absorption efficiency of mini‐PV with an automatic light‐harvesting system improves significantly (25.25–230.15%).Totally, from 9:00 am to 17:00 pm, the amount of electricity stored by the untethered automatic light‐harvesting system is 2325 mAh, which is 27.68% higher than 1821 mAh stored by the mini‐PV alone. The bar chart showing the increase of electricity per unit time can directly reflect the improvement of the untethered automatic light‐harvesting system's light energy efficiency absorption during the solar oblique illumination period. The highest increase of electricity per unit time of mini‐PV without an untethered automatic light‐harvesting system (green bar chart) was 393 mAh, and the lowest was only 48 mAh. The light absorption efficiency of the unit time is reduced by up to 87.79%.ConclusionIn summary, we demonstrate an untethered automatic light‐harvesting system with artificial phototropism based on LCE tubular actuators and solar cells, inspired by the phototropism of sunflowers. This system, consisting of a light sensor, microcontroller, and copper wire heater, is capable of adjusting the solar cell facing the sun automatically through controls of bending and rotation of the LCE tubular actuator to maximize the absorption of solar energy. Compared with the traditional mini photovoltaic panels, the absorption efficiency of solar energy by the light‐harvesting system is increased by 27.68% for the whole day, and 230.15% in the morning or dawn for sunlight with a small incident angle. This compact portable system with a single tubular design can provide a large payload and can be used in handphone or electronic device charge in a wild environment that is short of power supply. Regarding the problems of incident angle modifiers and high commercial automatic costs encountered in the current solar energy field, the proposed untethered automatic light‐harvesting system with the capability of artificial tropic movement shows great application potential in solar automation and robot systems, and energy‐saving fields.Experimental SectionMaterialsLiquid crystal mesogen 1,4‐Bis‐[4‐(3‐aryloyloxypropyloxy)benzoyloxy]‐2‐methylbenzene (RM257, 95%) was provided by Nanjing Leyao Technology Co. Ltd. (China). Triglycol dimercaptan (EDDET, 95%) and pentaerythritol tetrakis (3‐mercaptopropionate) (PETMP, 95%) were purchased from J&K Scientific (China). 2‐Hydroxy‐1‐[4‐(2‐hydroxyethoxy)phenyl]‐2‐methyl‐1‐propanone (HHMP, 95%) and iron(III) chloride solution (45% FeCl3 basis) were purchased from Aladdin Bio‐Chem Technology Co. Ltd. (China). Dipropylamine (DPA, 98%) was purchased from Mackin (China), which was mixed into a 2 wt% toluene solution before use. Toluene was purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. (China), which was dried with a 5A molecular sieve before use. Polydimethylsiloxane (PDMS, 184 Silicone elastomer base) was purchased from DOWSIL (US). Polyimide (PI) precursor solution (10–12% solids) was provided by Jingai Microelectronics (China).Fabrication of Liquid Crystal Elastomer FilmThe preparation method of the LCE film was based on previous reports with small changes.[35] First, 2.7 mL of toluene was mixed with 7.2 g (12.2 mmol) of RM257, then the mixture was heated at 85 °C to completely dissolve into a transparent solution. After cooled down to room temperature, the mixture was added to 2.021 g (11.3 mmol) of spacer EDDET, 0.160 g (0.3 mmol) of cross‐linker PETMP, and 0.05 g (0.22 mmol) photoinitiator HHMP, with a shaker to mix the mixture till completely dissolved. Then, 1 mL of prepared 2% catalyst DPA toluene solution was added into the mixture and then the mixture was stirred, cast into a rectangular Teflon mold (100 mm ×100 mm), degassed for 5 min, and then solidified at room temperature for 24 h. Finally, the resultant composite film was dried in an oven at 85 °C for solvent evaporation, leading to a loosely cross‐linked LCE film. The chemical structures of RM257, EDDET, and PETMP are shown in Figure 6a. Figure 6b shows the shapes of the LCE tubular actuator during the heating and cooling process, where the liquid crystal network changes between the nematic phase (without current) and the isotropic phase (with a current of 0.3 A).6Figurea) Chemical structures of RM257, EDDET, and PETMP. b) LCE tubular actuator during the heating and cooling process, where the liquid crystal network changes between the nematic phase (without current) and isotropic phase (with a current of 0.3 A). c) The preparation process of copper heating wire. Step I: a PI precursor layer with a thickness of 3 µm was spin‐coated on the copper surface of an FPC film. Step II: the PI/Cu/PI film (3 µm/18 µm/12 µm) was totally inverted and placed on the glass substrate spin‐coated with 1 mm PDMS, and then solidified in a 50 °C oven for 1 h. Step III: An UV laser marking machine was used to etch the required wire shape on the surface PI layer (12 µm) and remove the unnecessary PI around the wires. Then, the iron(III) chloride solution was applied to etch the corresponding copper layer that was uncovered by the surface PI layer. Step IV: the PI layer with a thickness of 3 µm as encapsulation was formed and patterned by ICP under O2 and CHF3 plasma. d) A copper heating wire with a width of about 120 µm. e) Fabrication of LCE tubular actuator. Step I: four copper heating wires were first transferred onto the prepared LCE film (50 mm × 50 mm × 0.7 mm) using water‐soluble tape. Step II: the copper heating wires were sandwiched with another film of the same size. Step III: the sample was rolled into a tubular shape and stretched along the longitudinal direction (up‐down direction) to align the liquid crystal mesogens.Preparation of Copper Heating WireThe preparation of copper heating wire is shown in Figure 6c. First (step I), a PI precursor layer with a thickness of 3 µm was spin‐coated on the copper surface of a flexible printed circuit (FPC, PI/Cu; 12 µm/18 µm), and then cured at a 300 °C oven with nitrogen protection for 12 h. Second (step II), the PI/Cu/PI film (3 µm/18 µm/12 µm) was totally inverted and placed on the glass substrate spin‐coated with 1 mm PDMS, and then solidified in a 50 °C oven for 1 h. Third (step III), an UV laser (355 nm, 5 W) marking machine was used to etch the required wire shape on the surface PI layer (12 µm) and removed the unnecessary PI around the wires. Then, the iron(III) chloride solution was applied to etch the corresponding copper layer that was uncovered by the surface PI layer. Finally (step IV), the PI layer with a thickness of 3 µm as encapsulation was formed and patterned by inductively coupled plasma (ICP) under O2 and CHF3 plasma. A copper heating wire with a width of about 120 µm was obtained, as shown in Figure 6d. The copper wire was removed from the PDMS surface on the substrate with a water‐soluble tape before use.Fabrication of LCE Tubular ActuatorThe preparation method of the LCE tubular actuator follows the method reported in ref. [17]. Four copper heating wires were first transferred onto the prepared LCE film (50 mm × 50 mm × 0.7 mm) using water‐soluble tape, where the tape was removed by soaking in water. The PI layer with a thickness of 3 µm can help the copper heating wire adsorption on the LCE film (Figure 6e‐I). After drying, the copper heating wires were sandwiched with another film of the same size (Figure 6e‐II). The whole sample was then compressed by an object with 500 g weight into a sandwich‐like structure at 85 °C. A certain margin of around 2 mm was reserved beyond the adhesive parts on both sides of the upper and lower layers of LCE to facilitate subsequent bonding, and the excess LCE film was trimmed. Then, the sample was rolled into a tubular shape (Figure 6e‐III) and then stretched along the longitudinal direction (up‐down direction) to align the liquid crystal mesogens, where the LCE tube was kept in the stretched state and exposed under a UV lamp (355 nm, 5 W) for 30 min. Finally, the input electrical wire was soldered to the copper heating wire to form the LCE tubular actuator.AcknowledgementsThis work was supported by the National Natural Science Foundation of China (NSFC) (62175098), and Guangdong Basic and Applied Basic Research Foundation (2021B1515020097).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.I. Vera, L. 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Journal

Advanced Materials Interfaces – Wiley

Published: Apr 1, 2023

Keywords: automatic; bio‐inspired; light harvesting; liquid crystal elastomers; untethered

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