Rapid Recovery Hydrogel Actuators in Air with Bionic Large-Ranged

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Applications of Polymer, Composite, and Coating Materials

Rapid Recovery Hydrogel Actuator in Air with Bionic Large-ranged Gradient Structure Yun Tan, Di Wang, Hua-Xiu Xu, Yang Yang, Xiong-Lei Wang, Fei Tian, Pingping Xu, Wenli An, Xu Zhao, and Shimei Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13235 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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ACS Applied Materials & Interfaces

Rapid Recovery Hydrogel Actuator in Air with Bionic Large-ranged Gradient Structure Yun Tan, Di Wang, Huaxiu Xu, Yang Yang, Xiong-Lei Wang, Fei Tian, Pingping Xu, Wenli An, Xu Zhao, Shimei Xu* Y. Tan, D. Wang, Y, Yang, P. Xu, W. An, X. Z, S. Xu College of Chemistry, Sichuan University, Chengdu 610064, China. E-mail: [email protected] H. Xu, F. Tian Polymer Research Institute, Sichuan University, Chengdu 610064, X-L. Wang College of Chemical Engineering, Sichuan University, Chengdu 610064, China. Keywords: hydrogel actuators, temperature-responsive, largeranged, gradient structures, microfluidics

Abstract

Fast

recovery

in

non-aqueous

environment

is

a

big

challenge for hydrogel actuators. In this work, a temperature-

ACS Paragon Plus Environment

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responsive hydrogel actuator with outstandingly rapid recovery in

air

was

gradient

reported.

structure

hydrophilic

was

monomer

isopropylacrylamide utilizing

The

a

hydrogel fabricated

hydroxyethyl (NIPAM)

facile

with

in

by

bionic

copolymerization

acrylate

the

(HEA)

dispersion

electrophoretic

large-ranged

method.

of

The

and

of N-

Laponite

deformation

degree and time can be regulated by varying concentration of HEA to

change

swelling

lower

of

the

critical hydrogel.

solution A

dynamic

temperature

(LCST)

and

equilibrium

between

the

water into and out of the hydrogel was observed and the hydrogel showed no shrink above LCST. The synthesized hydrogels showed fast response in hot water and rapid recovery in air. Such nonshrink

characteristics

possible

for

these

and

excellent

hydrogels

to

reversibility

be

used

as

made

it

temperature-

controlled microfluidic switch. The work provided an approach to design fast recovery hydrogel actuators by the incorporation of hydrophilic monomers and extend the application of the hydrogel actuators into fields such as soft robots, micromanipulation, microfluidics and artificial muscles in various environments.

1. INTRODUCTION As

smart

soft

materials,

hydrogel

actuators

are

able

to

transform their shape, size and position by external stimulus, such as temperature,1-4 light,5-7 pH,8,

9

humidity,10,

11

special ions


2

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or

field.12-14

electric

applications drivers,17,

18

biomimetic hydrogel

in many

They

have

fields, such

also

demonstrated

promising

as

soft robots,15,

16

smart

microfluidic valves,19-21 biosensors,22 as well as

machines

actuators,

and

muscles.23,

artificial

temperature-responsive

24

Among

hydrogel

the

actuators

are predominantly investigated since temperature is regarded as indispensable acrylate

and

based

easy-to-control

thermo-sensitive

stimuli.

polymers

Acrylamide

are

widely

used

and to

prepare the smart hydrogels because of their easy regulation of LCST,25,

26

optical properties,27 hydrophilic and hydrophobicity

properties.28,

29

response/recovery

Recent speed

and

progress deformation

on

improving

degree

of

the

hydrogel

actuators is mainly based on two strategies: one is to fabricate anisotropic

structure,

such

as

structure,16,

bilayer

30-32

programmed heterogeneous structures33-35 or gradient structure;17, 36, 37

the other is to reduce gel thickness in order to increase

the response rate of the material,38 while maintaining a suitable strength and robustness of the thin film9,

32, 39

or to introduce

porous structure in order to achieve rapid transfer of water37. However, the existing temperature-responsive hydrogel actuators still have some stubborn problems: (1) the recovery speed falls far

behind

speed/response different

the

response

speed

deciding

is

speed

in

the

factors:

the

(the range former

ratio of is

of

0.02-0.5) decided

recovery due by

to the


3


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swelling process of the hydrogel in water while the latter by response temperature; (2) water is needed to swell the hydrogel in

the

recovery

process

due

to

the

water

loss

during

the

response process, therefore seriously affecting the application of

hydrogel

actuators;

(3)

the

reversible

response

is

uncontrollable since it is hard to recover to the original size and volume of the hydrogel when swollen in water. There are few attempts to realize the response in a non-aqueous environment since the traditional temperature-responsive behavior is mainly based

on

the

interaction

between

water

and

temperature-

responsive hydrogels. Inspired from the motion of mimosa leaves, Zheng and co-authors have designed a lower critical solution temperature-upper bilayer

hydrogel

critical actuator

solution by

temperature

the

water

(LCST-UCST)

self-circulation

mechanism to realize the actuation in water, oil or open-air environments.31 However, the transfer time of water between the bilayer hydrogels is lagged behind the response time and leads to a long recovery time of tens of minutes above LCST or below. Therefore, obtain

a

universal

hydrogel

strategy

actuators

that

is can

still have

highly fast

desired

to

recovery

in

different kinds of external environments. Herein, inspired by continuous gradient structure of organism with dynamic equilibrium mechanism of water, we developed a novel gradient hydrogel actuator by incorporation of


4

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hydrophilic monomers into temperature-responsive hydrogel matrix. The hydrogel not only met the requirement of rapid recovery and excellent reversibility, but also realized the recovery in air. The mechanism of fast recovery is explored and a temperature-sensitive on/off fluid switch is designed to highlight the advantages of the new system.

2. EXPERIMENT SECTION 2.1 Materials The synthetic “Laponite XLG” ((Mg5.34Li0.66Si8O20(OH)4)Na0.66) was purchased from Rockwood, Co., U.S., and dried at 120oC for 4 h. N-isopropylacrylamide (NIPAM) was provided by J&K and was used after recrystallization from n-hexane. Hydroxyethyl acrylate (HEA), dimethylaminoethyl

Methacrylate

(DMEMA),

N-hydroxymethyl

acrylamide

(HMAM),

potassium peroxydisulfate (KPS), tetramethylethylenediamine (TEMED) and rhodamine B (RB) were purchased from Aladdin Company. Acrylamide (AM) was provided by Chengdu Kelong Chemicals Co.. Sodium pyrophosphate (SPH) was purchased from Shanghai Kechang Fine Chemicals Co.. Deionized water (18.2 MΩ at 25oC) from a water purification system (UPT-110T) was used throughout the experiments. All reagents were of analytical grade. 2.2 Preparation of Bionic Gradient Hydrogel Bionic gradient hydrogels were prepared by the in-situ copolymerization of NIPAM and hydrophilic monomers in a dispersion of Laponite under a DC electric field. Typically, 0.3 g of Laponite was dispersed in 4.5 mL of deionized water containing SPH (mass ratio of Laponite XLG to SPH was 1: 0.076). After stirring for 20 min, 0.6 g of NIPAM and 77 μL of hydroxyethyl acrylate (HEA) were added to the dispersion under stirring for 30 min, followed by


5


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0.5 mL of 20 mg mL-1 KPS and 13 μL of accelerator TEMED under ice-water bath stirring for 5 min. Afterwards, the dispersion was injected into the mold (80 mm × 80 mm × 1 mm) composed of graphite electrodes and silicone and allowed to maintain for 10 min at a voltage of 1 V. Then, the electric field was removed and the hydrogel was aged for 24 hours until bionic gradient hydrogel (GN8E1) was obtained. In this work, bionic gradient hydrogel was referred to as “GNmYx”, where N and Y stood for NIPAM and hydrophilic monomers respectively (for a specific hydrogel, Y was replaced by the abbreviation of a specific monomer, such as E for HEA, D for DMEMA, M for HMAM and A for AM, respectively), and m to x presented the molar concentration ratio of NIPAM to hydrophilic monomers. A total molar concentration of two monomers was 1.2 mol L-1 in all hydrogels. As a control, the PNIPAM/Laponite gradient hydrogel (GN9) was prepared following the same process above, except that no hydrophilic monomers were introduced. 2.3 Characterization Scanning electron microscopy (SEM) morphologies of freezing-dried bionic gradient hydrogels in lyophilizer (LGJ-10C) were examined at scanning electron microscope (PHENOM PRO.V) at an accelerating voltage of 10 kV. Prior to freezing-dried, the samples were freezed by liquid nitrogen. The samples were sputtered with gold before observation. As-prepared gradient hydrogel samples at 25oC and 50oC were rapidly cooled to -20oC by cool stage respectively. Then, the samples were measured in scanning electron microscope (PHENOM PRO.V) at an accelerating voltage of 15 kV. Water state of hydrogel was analyzed by differential scanning calorimetry (DSC) on TA DSC Q200 by fast cooling the samples to 253.3 K, followed by reheating to 293.3 K at scanning rate


6

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of 1oC min-1. The melting point of water in the sample was determined through the temperature at the maximum enthalpy, and the free water content (ωfw) was calculated by the following equation (1): Q

(1)

ωfw = ΔH × 100

where ΔH was the melting enthalpy of the free water with the same as that of the bulk water (ΔH=333.5 J g-1), and Q was the absorption heat during the melting process of water. Q was calculated according the area of the enthalpy peak.40 The hydrogel actuator (mo) was dried for 12 hours in an oven at 80oC, followed by oven-drying at 100oC for 12 h and vacuum-drying at 120oC for 12 h, respectively to obtain a constant weight (me). The solid content (ωs) of the actuators was calculated by the following equation (2): me

ωs = mo × 100%

(2)

The hydrogel actuator was cut into a size of 25 mm × 5 mm × 1 mm, and weighed (M) at 25oC. Then, the actuator was placed in the deionized water at 25oC, and then removed and weighed (Mt) at different time interval. The water content (Wc) of the actuators at 25oC was calculated by the following equation (3):

Wc =

Mt - M × ωs Mt

× 100%

(3)

Where ωs stood for the solid content of the hydrogel.


7


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Similarly, Wc at 50oC was measured by the process above but at 50oC. 2.4 Bending Behavior of Bionic Hydrogel Actuators The as-prepared hydrogel was cut into strips of 25 mm × 5 mm ×1 mm. One end of the hydrogel was fixed while the other end was free to suspend in the air. Then, the hydrogel was put into deionized water at 50oC to record the time and bending angle. The response time was defined as the time to reach the maximum bending angle. Then the hydrogel was placed in water or air at 25oC to record the time and bending angle. The recovery time defined as the time to restore to original shape and volume. The above procedure was repeated five times for each sample. 2.5 Microfluidic Temperature-controlled Switch Temperature-control microfluidic switch was designed as water transfer device. A total of 5 liquid transmission channels (“1”, “2”, “3”, “4” and “5”) were set in the device. Among them, the hydrogel actuator was used as a temperature-controlled microfluidic switch to regulate the closed or open status of the liquid transmission channel through different water temperatures. The temperatures of cold water and hot water were 25oC and 50oC, respectively.

3 RESULTS AND DISCUSSION The gradient hydrogel was prepared using an electrophoresis method (Figure 1a). Negatively charged Laponite platelets were inclined to move towards anode during the copolymerization of monomers and finally a gradient gel network formed due to the crosslinking effect of Laponite. The long-ranged gradient network structure of the hydrogel actuator (GN8E1) was observed along the direction of electric field by scanning electron microscopy (SEM) (Figure 1b and c). The network size increases from 0.35 μm on the anode side to 4.83 μm on the cathode side


8

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(Figure 1d), which is in good agreement with the natural gradient structure of the mimosa leaves or pine cone.41 The gradient span of the network was about 700 μm long showing large-ranged distribution, which was expected to be more conducive to the generation of driving force and the water transfer in the external stimulation.

Figure 1 a) Scheme of synthesis of the bionic gradient hydrogel (a1-a4),

polymerization

dispersion

(a5)

and

gradient

network

structure (a6); b) SEM of freeze-dried GN8E1 with large-ranged gradient structure along the direction of electric field; c) Enlarged SEM images and d) Network size statistics (statistic sample numbers (n>200)) of GN8E1 at three different locations. The scale bars of b and c were 100 μm and 10 μm, respectively. 3.1 Deformation Performance of Hydrogel Actuators


9


Page 10 of 22

The driving performance of the hydrogel actuators was evaluated by measuring the bending angle (θ) in hot water (Figure 2a). The maximum bending angles of GN9 and GN8E1 were reached at 106o and 102o at 50oC in deionized water, respectively (Figure 2b). It was mainly attributed to the long-range gradient structure of actuators which provided a large driving force by asymmetric curling of thermo-sensitive polymers above LCST. The response time of GN8E1 was 24 s longer than the response time of GN9 (24 s). But it is worth noting that the recovery time of GN8E1 was only 60 s which was 36 s shorter than the one of GN9. In addition, the hydrogel actuators exhibited good reversibility after the introduction of additional hydrophilic monomers (Figure S1). However, in contrast with GN8E1, the response and recovery time in the other hydrogels was longer or the bending angles were smaller (Figure S2).


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Figure 2 a) Scheme of measuring the bending angle (θ) of the hydrogel actuators, b) The reversible bending response of the hydrogel actuators at 25oC and 50oC (inset graph corresponding to the optical images of GN8E1 and GN9 with reversible bending and recovering in deionized water at 25oC and 50oC respectively), c) Storage modulus histogram of the hydrogel actuators at 25oC. The scale bars were 10 mm. The main reasons for these results were possibly contributed to the change of the low critical solution temperature (LCST) and water transfer performance in the gradient hydrogels. So we evaluated the LCST of both anode side and cathode side of the gradient hydrogels by measuring their storage modulus dependent of the temperature (Figure S3). The storage modulus of the hydrogel actuators in the anode side in is higher than the one in the cathode side at 25oC (Figure 2c). The introduction of hydrophilic monomers increased the LCST of the hydrogels. Moreover, there exhibited a slight difference in LCST for two sides of the gradient hydrogels: the LCST in the anode side was a little higher than the one in the cathode side. It can be explained as that higher crosslinking density in the anode side limited the motion of PNIAM chains in some degree, and the temperature gap between LCST and external temperature played an important role in the deformation degree and response time. As a result, the hydrogel with more hydrophilic monomers showed smaller bending angle and longer response time instead. Besides, the introduction of hydrophilic monomers into the hydrogels also promotes the transfer of water and enhances the swelling of the hydrogels below LCST (Figure S4). It is well known that a temperature-sensitive hydrogel is inclined to shrink above LCST. The hydrogel needs to swell in the water environment below LCST for reversible recovery when it is used as an actuator.


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However, in our work, it was found that water content of GN8E1 increased by 11.4% above LCST instead when putting into the water at 50oC for 8 min (Figure 3a). Meanwhile, the size of GN8E1 expanded to 110% at 50oC in water for 2 min (Figure 3b). In contrast, the size of GN9 decreased by 15%. Above LCST, PNIPAM in GN8E1 produced an asymmetrical force upon spontaneous curling, so that the hydrogel actuators bended. The curing of PNIPAM can be confirmed by the decrease of the hydrogel transparency. Therefore, the hydrogel can still carry out deformation even if it is swollen. At 50oC, the hydrogel actuators bent into circular arcs, and the degree of bending can be measured by the center angle (α) (Figure 3c). The α values of both GN9 and GN8E1 were more than 180o and their maximum bending angle can be achieved by only 40 s. The smaller α value of GN8E1 is caused by increased gravity and size which enlarged bending resistance due to the swelling at 50oC. This was also one of reasons for the slower response of GN8E1. Besides, the gel modulus had an effect on the recovery process since the recovery of shape needed to overcome internal stress. It explained why GN8D1 with high modulus showed much longer recovery time instead.

Figure 3 a) Water content-time curves of hydrogel actuator at 50oC.

b)

The

size

changes

of

hydrogel

actuators

placed

in

deionized water for 2 min at 50oC. c) The relationship between the α and the time for 2 min at 50oC, Illustration was schematic


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diagram of

the center angle (α) measurement

of the

hydrogel

actuators. The scale bars were 10 mm. 3.2 Rapid Recovery of Hydrogel Actuators in Air

Figure 4 a) The reversible response and recovery of GN24E1 at 50oC in water and at 25oC in air, respectively; b) Comparison of bending velocity and Rs/Rc between the hydrogel actuator and other hydrogel actuators (No.1-10). The detailed information was shown in Table S1.


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Figure 5 a) Optical images of the size changes of the hydrogel actuators placed in deionized water for 2 min at 50oC. b) GN24E1 bending at 25oC and 50oC in the liquid paraffin respectively. c) Water state of GN24E1 by DSC measurement at 25oC (GN24E1-25) and 50oC (GN24E1-50) respectively. d and e) SEM images of GN24E1 at 25oC and 50oC along the direction of the electric field with cool stage. f) Water transfer schematic diagram of the hydrogel actuators with dynamic equilibrium mechanism at 25oC and 50oC. Scale bars were 10 mm (a and b) and 50 μm (d and e), respectively. According to the swelling results of the GN9 and GN8E1, we concluded that the hydrogel actuators can retain constant size without shrinkage above LCST by adjusting the ratio of NIPAM to HEA. As a result, a quick recovery in non-aqueous environment even in air can be achieved. As expected, we got positive results from the hydrogel GN24E1. The reversible


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bending of GN24E1 was measured with response at 50oC in water and recovery at 25oC in air respectively. The excellent and fast reversible property of GN24E1 were displayed by five-cycle experiments with response time of 21 s in water and recovery time of 29 s in air, and the bending angle reached 103o (Figure 4a, Supporting Information Movie S1). GN24E1 showed even shorter response time than GN9. It was mainly because the non-swelling property of GN24E1 maintained the repulsive force between polymer chains during the bending. Compared with the GN8E1 which showed a recovery time of 60 s in water, a faster recovery of GN24E is partly attributed to smaller resistance in air. We summarized the bending velocity and the ratio of response to recovery time (Rs/Rc) of the previously reported hydrogel actuators in the literature (Figure 4b). It is found that our temperature-responsive hydrogel actuator has fast bending velocity (0.05 s-1) and large Rs/Rc value (0.72), superior to those bendable hydrogel actuators. Moreover, those hydrogel actuators were limited in water environment for response and recovery (Supporting Information, Table S1). The temperature-responsive bending/unbending actuator was highly reversible and repeatable since there was no size change and volume contraction in water at 50oC (Figure 5a). When the hydrogel actuator was put into liquid paraffin at 25oC, the bending angle of actuator was 20.5o (Figure 5b). In the liquid paraffin at 50oC, the bending angle of the actuator reached 50.3o after 39 s, and water droplets are observed on the actuator due to the water of hydrogel actuator to be lost above LCST (Supporting Information Movie S2). However, when the actuator was placed in water at 50oC, there was no shrink and water loss observed. It was speculated that there was a dynamic equilibrium mechanism of swelling/deswelling the process of the water transfer in hydrogel and the external environment. The dynamic equilibrium mechanism of water can change the state of water in the actuator before and after bending. The free water content of GN24E1 was 52.1% and 58.8% at 25oC and


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50oC respectively by DSC measurement (Figure 5c). It further indicated that the total water content of the GN24E1 was almost unchanged, but the content of free water and bound water changed due to dynamic transfer of water at 25oC and 50oC (Supporting Information, Table S2). The bound water content of the GN24E1 decreased and the free water content increased when the temperature increased from 25oC to 50oC. Meanwhile, SEM images of GN24E1 showed that the size of the network at 25oC (2.3-8.8 μm, Figure 5d) was slightly smaller than the size of the network 50oC (4.9-10.5 μm, Figure 5e). At 50oC, the larger network size may be related to the increase of free water content which led to expansion of the network and a new balance between the curl of hydrophobic segment and the stretch of hydrophilic chains (Figure 5f). Through the water transfer between the hydrogel actuator and the external environment, the conversion of free water and bound water was promoted, and the rapid recovery of the hydrogel actuator was realized. 3.5 Temperature-controlled Microfluidic Switch

Figure 6 Temperature-controlled microfluidic switch, (a) and (c) were the schematic diagram of closed and open switch. (b) and (d) were the optical images of closed and open switch at


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different time, respectively. The temperature of cold water and hot water were 25oC and 50oC, respectively. “1”, “2”, “3”, “4” and “5” stood for five different channels. The scale bars were 10 mm. Considering the specific non-shrink characteristics above LCST and excellent reversibility, the hydrogel actuator can be designed as a microfluidic control switch besides of soft robot or bionic muscles. At 25oC, the actuator was not bent, and the liquid cannot flow through the channel due to the closed switch below LCST (Figure 6a and b, Supporting Information Movie S3). When the temperature increased to above LCST (50oC), the actuator overcame the water pressure and gradually bent. The switch was opened and the 20 mL of liquids flowed away within 20 s (Figure 6c and d, Supporting Information Movie S4). Once the cold water was injected again at room temperature, the switch was closed again because of good reversibility and unshrinkable size of the hydrogel actuator. The hydrogel actuator showed reliable on/off switch control.

4. CONCLUSION In summary, we designed a temperature-responsive hydrogel actuator with bionic large-ranged gradient structure to achieve distinguished reversible and rapid recovery in diverse environments. Introduction of hydrophilic monomers brought out the dynamic equilibrium of water in the hydrogel. It ensured the stable gel size without shrink or swell, and thus avoided the gel network collapse above LCST. This fact made it possible for these hydrogels to realize fast recovery in non-aqueous environment. In addition, the actuator demonstrated a reliable and easy way to be used as temperature-controlled microfluidic switch. The work provides new insight into the design of soft hydrogel actuators and has great potential in diverse applications.


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ASSOCIATED CONTENT Supporting

Information.

Characterization

methods:

Rheological

measurement of bionic hydrogel actuators, effect of temperature on

deformation

of

bionic

hydrogel

actuators;

as

well

as

supporting data and movies: bending property of other hydrogel actuators, response and recovery time histogram of other hydrogel actuators, rheological curves of the hydrogel

actuators

with

temperature

sweep,

swelling property of the

hydrogel actuators.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The work was supported by NSFC-Xinjiang joint fund for local outstanding youth (No. U1403392), National Natural Science Foundation of China (No. 51773132) and Sichuan Science and Technology Program (2018HH0024).


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ABBREVIATIONS NIPAM, N-isopropylacrylamide; HEA, Hydroxyethyl acrylate; LCST, Lower critical solution temperature; UCST, Upper critical solution temperature; Rs, Response time; Rc, Recovery time.

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Rapid Recovery Hydrogel Actuators in Air with Bionic Large-Ranged

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