Article Cite This: Chem. Mater. 2019, 31, 4469−4478
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Fabrication of Asymmetric Tubular Hydrogels through Polymerization-Assisted Welding for Thermal Flow Actuated Artificial Muscles Huijuan Lin,†,‡ Shuanhong Ma,*,† Bo Yu,† Meirong Cai,† Zijian Zheng,§ Feng Zhou,*,† and Weimin Liu†,∥
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†
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hong Kong 999077, Hong Kong ∥ College of Materials, Northwestern Polytechnical University, 127 West Youyi Road, Xian 710072, China S Supporting Information *
ABSTRACT: By nature, diversified motions in most muscle systems are accompanied by fluid transportation. Inspired by the fluid-induced actuation behavior of Urechis unicinctus, we introduce a two-step fixedsite selective surface catalytically initiated radical polymerization strategy to prepare asymmetric thermoresponsive hydrogel tubes (as-TRTs) containing the responsive PNIPAAmx-PAAy part and nonresponsive PAMx-PAAy part with perfect interface fusion, including uniform structure (I), Janus structure (II), and block structure (III). In a typical case, we can easily obtain three kinds of multiple-channel as-TRTs with complex geometries by using multiple wires as a growth template and three kinds of TRTs with patterned, thread, and spiral structures. Meanwhile, the as-TRTs can achieve dynamic bending based on the generated interface stress difference from up to 7 times modulus value change for its thermoresponsive part upon immersing the tubes into a hot water bath or injecting the hot fluid into the channels. The prepared as-TRTs can bend or twist depending on their flexible geometric design. Devices made with these asymmetric hydrogel tubes can capture/release or lift objects both underwater and in air, convert thermal stimulation to the transportation capability of the fluid, gas or mix different kinds of liquids, and act as intelligent multichannel fluidic switchers. The current work is highly anticipated to open new frontiers for developing stimuli-responsive “smart” soft robots. for layered and patterned structures.29−32 However, achieving faster actuation for a traditional bulk hydrogel sheet with large thickness or size of the order of several centimeters still faces difficulties due to its time-dependent response associated with slow diffusion of water molecules.26 In addition, hydrogels are commonly actuated in aqueous solutions and face difficulty to work in air because of their limited volume shrinkage and longer response time in air than in water. Hence, the core issue is to achieve faster actuation of bulk hydrogels with huge volume changes, both in air and underwater, in response to a narrow stimulus change, which can be solved by using hollow or tubular hydrogels.33,34 In typical cases, hollow materials inspired by nature can generate considerable actuation advantages including fast responsive speed and large actuation stress. For example,
1. INTRODUCTION Many organisms possess an amazing innate capability of inducing diversified motions through reversible volume shrinkage and expansion,1−3 while a few organisms rely on dynamic fluid drainage and uptake.4,5 The actuation behavior is followed by complex shape changes in multidimensions, such as axial volume shrinkage/expansion, bending, or even twisting. The fundamental motion mechanisms could be beneficial for modeling soft robotic devices and machines.6−8 Biomimicking the actuation mechanism still remains challenging since it depends on suitable smart materials and the availability of their shaping technology.9 For instance, several soft materials can be employed as smart actuators analogous to hydrogels,10−15 whose swelling/deswelling process is driven by osmotic pressure changes in response to external stimuli such as pH,16 solvent,17 temperature,18,19 electric field,20 magnetic field,21 and light,22,23 and would possibly aid the designing of soft actuation machines24−26 and even intelligent flow control systems.27,28 Actuating hydrogel materials are usually designed © 2019 American Chemical Society
Received: March 8, 2019 Revised: May 17, 2019 Published: May 22, 2019 4469
DOI: 10.1021/acs.chemmater.9b00965 Chem. Mater. 2019, 31, 4469−4478
Article
Chemistry of Materials Scheme 1. Diversified Motions of Urechis unicinctus in Water Based on the Typical Fluid Actuation Mechanism.
(a) Schematic illustration of the actuation mechanism of the Urechis unicinctus in water: reversible volume shrinkage/expansion is relying on dynamic process of drainage and liquid absorption. (b) Snapshots of motion states of Urechis unicinctus in water as (I) original state, (II) shrinkage state, (III) vertical block state and (IV) bending state. Co. Ltd., China), and iron chloride hexahydrate (FeCl3·6H2O, Tianjin Chemical Reagents Corp.), methylene blue (Tianjin Chemical Reagents Corp.), and methyl red (Tianjin Chemical Reagents Corp.). The experimental special wires were purchased from China. All chemical reagents were not further purified. 2.2. Preparation of Uniform/Symmetric Thermoresponsive Hydrogel Tube (TRT) or Tube System (TRTS) and Nonuniform/ Asymmetric Thermoresponsive Hydrogel Tube (as-TRT) or Tube System (as-TRTS). Uniform/symmetric thermoresponsive tube (TRT) or tube system (TRTS) were prepared in solution A by the SCIRP method.35 First, the iron wire or iron wire array was immersed into solution A for SCIRP. After polymerization for 4 min, the hydrogel layer-wrapped iron wire or iron wire array was taken out and then immersed into the 0.06 M FeCl3 solution for 1 h. Next, the samples were taken out and immersed into 100 mL ultrapure water for 10 h to remove the unreacted monomers or Fe3+ residues. Finally, TRT or TRTS was obtained by removing the iron wire or iron wire array. Nonuniform/asymmetric thermoresponsive hydrogel tube (asTRT) or tube system (as-TRTS) with an asymmetric structure was prepared by the fixed-site polymerization strategy. In a typical case, the iron wire or iron wire array was fixed on a 3D-printed mold, and then mixed monomer solution A was poured into the mold until the necessary part of the iron wire or iron wire array was completely immersed, while the remaining part of the iron wire or iron wire array was exposed in air or just protected by 3M tape. After polymerization (SCIRP-1st) for 4 min, the iron wire or iron wire array was rotated with certain angles and then immersed into solution B to allow the remaining part of the wire or iron wire array polymerization (SCIRP2nd) for the other 4 min. Then, following the same experimental steps as that of uniform TRT or TRTS, we obtained as-TRT and as-TRTS. The summary of material details for all involved hydrogel tubes can be found in Table S1. 2.3. Characterization. An electrical universal material testing machine with a 500 N load cell (EZ-Test, SHIMADZU) was used to test the mechanical property of the tubes. The tests were carried out at a rate of 5 mm min−1. The elastic modulus of tubes was calculated from the slope of a stress−strain curve at strain values of 5%−15% in low-temperature (20 °C) conditions and 0−5% in high-temperature (50 °C) conditions. A 14-FW tribometer (HEIDON Co., Ltd.) with a programmatic heating platform was used to measure the friction force for both the inner and external surfaces of the tubes in a low-temperature (20 °C) and high-temperature (50 °C) water bath. The reciprocating mode
inspired by the swift swimming capability of leptocephali, Zhao et al. have reported a hydraulic hydrogel/elastic composite actuator with designable hollow structures that can achieve high speed, high force, and optical and sonic camouflage in water.33 The as-prepared hydrogel actuator can perform extraordinary functions including swimming, kicking rubber balls, and even catching a live fish in water. By nature, Urechis unicinctus, as a tubular species of marine spoon worm, possesses amazing responsive capabilities of diversified motions in water based on the typical fluid actuation mechanism (Scheme 1).4 Imitating such wonderful natural mechanisms boosts the development of similar tubular hydrogel actuators, which demonstrate great potential as soft robots and intelligent flow-control devices. At the same time, actuating tubular hydrogel systems are highly dependent on the process of building a complex tubular asymmetric structure, which still remains a strenuous task. In this paper, we report on the facile preparation of asymmetric hydrogel tubes with designed structures and diverse functionality for fluid-driven artificial muscles by employing the surface catalytically initiated radical polymerization (SCIRP) method. The asymmetric thermoresponsive hydrogel tubes (as-TRTs) containing the responsive PNIPAAmx-PAAy part and nonresponsive PAMx-PAAy part are prepared for the first time by the two-step fixed-site polymerization strategy. The fast, reversible, and diverse actuation behavior of the thermoresponsive part is beneficial to the realization of controllable actuation for structural asymmetric hydrogel tubes when flushed with water above and below the phase transition temperature both in air and underwater. Importantly, the customized hydrogel tubes are useful for various applications, such as soft actuators, underwater catchers, and intelligent flow-control devices.
2. EXPERIMENTAL SECTION 2.1. Materials. The following materials were used in this study: Nisopropylacrylamide (NIPAAm, 99%, J&K Chemical Ltd.), acrylic acid (AA, >99%, TCI), acrylamide (AM, J&K Chemical Ltd.), ammonium persulfate (APS, Tianjin Chemical Reagents Corp.), N,N′-methylene diacrylamide (MBAA, Sinopharm Chemical Reagent 4470
DOI: 10.1021/acs.chemmater.9b00965 Chem. Mater. 2019, 31, 4469−4478
Article
Chemistry of Materials
Figure 1. Design and fabrication of symmetric/asymmetric thermoresponsive hydrogel tube (TRT/as-TRT) actuators. (a) Schematic illustration of the preparation of the asymmetric thermoresponsive hydrogel tube (as-TRT) with Janus structure (II). (b) Schematic diagram showing the thermoresponsive actuation behavior of the TRT and as-TRT both underwater (when the tubes were immersed into the hot water bath) and in air (when hot water was injected into the channel of the tubes) and that the shrinkage (T > LCST) and swelling (T < LCST) behavior of the tubes is reversible. The responsive part is the PNIPAAmx-PAAy hydrogel layer, while the nonresponsive part is the PAMx-PAAy hydrogel layer. was selected while a ball-on-disk contact mode was employed. The ball was a PDMS hemisphere with a diameter of 6 mm. The load was 0.1 N, the sliding velocity was 150 mm min−1, and pure water was used as a lubricant. The temperature-dependent volume shrinkage ratio (M) was calculated using
with a typical dual cross-linking network can be conceived well. The as-prepared thermoresponsive hydrogel tube exhibits a larger volume change compared to traditional hydrogel blocks (Figure S2) in response to temperature stimulus at a macroscale level. In this paper, it has been shown that, besides the facile preparation of uniform thermoresponsive tubular structure (I), hydrogel tubes with different components can also be weld together, for example, responsive and nonresponsive parts can be weld together to form desirable geometric structures such as Janus structure (II), block structure (III), and even a twisted structure. Tightly bonding of hydrogels of different components is very tedious. To prepare the asymmetric thermoresponsive hydrogel tube (asTRT), the fixed-site polymerization strategy was developed. As shown in Figure 1a and Figure S3, any site along the iron wire can be masked with 3M adhesive tape, while the exposed part can be utilized for SCIRP. For instance, after obtaining the responsive PNIPAAmx-PAAy part in solution A, the mask can be removed to proceed into a second SCIRP to obtain the nonresponsive PAMx-PAAy part in solution B. So, we can easily obtain the Janus hydrogel tube (II) (Figure 1a) and block hydrogel tube (III) (Figure S3). Meanwhile, the characterization from Fourier transform infrared (FT-IR) spectra and energy-dispersive spectra (EDS) in Figures S4 and S5 showed that PNIPAAm-PAA/Fe3+ and PAM-PAA/Fe3+ hydrogel layers were successfully formed. As shown in Figure 1b, it can be imagined that the as-prepared as-TRT with a horizontal asymmetric structure can realize fast thermoresponsive bending, while the channel size of as-prepared as-TRT with a block structure can be dynamically adjusted upon the temperature change of the fluid in the tube between above its lower critical solution temperature (LCST) and below its LCST. As far as we know, this polymerization strategy has never been reported in the hydrogel system and is almost impossible to realize by other technologies as well. It is exemplified in Figure 1b that the responsive part (PNIPAAmx-
M = 1 − V /V0 where V represented the equilibrium volume of the tubes in a 20 and 50 °C water bath, and V0 represented the initial equilibrium volume of the tube in a 20 °C water bath. Meanwhile, the LCST of the tube was defined as the temperature at which the volume of the gel tube began to shrink. The morphology of the prepared hydrogel tubes was characterized by employing a scanning electron microscope (JSM-5600LV, JEOL, Japan) at 20 kV. The energy-dispersive X-ray spectra were obtained by using a silicon drift detector (X-MaxN 80, Oxford Instruments, UK) at 15 kV. The chemical compositions of the resulting hydrogel tubes were characterized by Fourier transform infrared (FT-IR) spectroscopy (Nicolet iS10, Thermo Scientific, USA) with the attenuated total reflection method.
3. RESULT AND DISCUSSION 3.1. Preparation of Symmetric/Asymmetric Thermoresponsive Hydrogel Tube (TRT/as-TRT) Actuators. Recently, the SCIRP method has been used to prepare tubular hydrogels.35 As shown in Figure S1, the surface of the iron wire can form one layer of uniform poly(N-isopropylacrylamide-coacrylic acid) P(NIPAAm-co-AA) hydrogel film after immersing it into solution A containing monomers of N-isopropylacrylamide (NIPAAm), acrylic acid (AA), initiator of ammonium persulfate (APS), and cross-linker of N,N′-methylene bisacrylamide (MBA). Then, the P(NIPAAm-co-AA) hydrogel film-coated iron wire was immersed into the FeCl3 solution to allow strong ion coordination between Fe3+ and COO−. After removing the iron wire, the uniform or symmetric thermoresponsive hydrogel tube (TRT) of PNIPAAm/PAA-Fe3+ (I) 4471
DOI: 10.1021/acs.chemmater.9b00965 Chem. Mater. 2019, 31, 4469−4478
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Chemistry of Materials
Figure 2. Diverse TRTs and as-TRTS along with their thermoresponsive behavior and SEM characterization. The model diagram shows the photos before (middle) and after (bottom) encountering fluid-induced thermoresponse for the as-prepared. (a) Uniform TRTs composed by the thermoresponsive part of the PNIPAAm10.0-PAA2.0 network, including the single tube, multichannel tubes, and cross tubes. (b) as-TRTS composed of the thermoresponsive part of the PNIPAAm10.0-PAA2.0 network and nonresponsive part of the PAM10.0-PAA1.0 network with horizontal or longitudinal distribution, including the Janus tube, block tube, patterned tube, and complex tubes. All scale bars are 4 mm. (c) SEM images show the boundary between the thermoresponsive part and non-responsive part of the (left) Janus hydrogel tube and (right) block hydrogel tube.
clearly demonstrated by scanning electron microscopy (SEM) characterization (Figure 2c) and mechanical property tests (Figures S10−S12). This interface fusion is extremely important to avoid any phase transition or tear-off under stress conditions. Above the LCST of the tubes, the responsive part of the as-TRTs generated obvious volume shrinkage, while the nonresponsive part remained unchanged. As a result, such prompt stimulation can induce swift bending of as-TRTs because of asymmetric stress distribution. 3.3. Thermoresponsive Bending Behavior of the Diverse as-TRTs Underwater or Fluidic-Actuation in Air. Owing to the controllability/dominance of the as-TRTs’ growth, various as-TRTs were prepared to produce the programmable thermoresponsive bending behavior. By comparing these with traditional hydrogel actuators, the temperature-responsive bending behavior of our as-TRTs can be realized both in water and in air based on the typical fluiddriven mechanism by using the hollow tubular channels of hydrogels. Figure 3a shows the thermoresponsive bending mechanism of our as-TRTs underwater. In a typical case, above the LCST, the responsive PNIPAAm10.0-PAA2.0 part of asTRTs shrank along with modulus increment, while the nonresponsive PAM10.0-PAA1.0 part remained unaltered. As a result, the as-TRT gradually curved toward the PNIPAAm10.0PAA2.0 part, resulting from the generated stress difference at the interface of two parts. At the beginning of the bending process, the gas in the channel of the gel tube was exhausted as the water started flowing into the channel (I). Furthermore, the gas in the channel was drained, while the cavity of hydrogel
PAAy) of hydrogel tubes undergoes thermoresponsive volume shrinkage above its LCST, while the other part (PAMx-PAAy) remains unchanged. This process is reversible and is accompanied by fast water discharge and uptake, which enables deformation of the hydrogel tubes to form the desired device shapes. Moreover, the wall thickness for different kinds of hydrogel tubes also has an important effect on the temperature responsiveness and actuation behavior (Figures S6−S8). The thin thickness of the tube wall is beneficial to improve its responsiveness. As exemplified in this paper, the customized hydrogel tubes have a variety of applications, such as smart actuators, underwater manipulators, and intelligent flow-control devices. 3.2. Thermoresponsive Behavior and SEM Characterization for Diverse TRTs and as-TRTS. First, the LCST of the hydrogel tubes was measured from in situ monitoring their volume changes along with the temperature rise of the water bath. The experimental results show that the LCST of the PNIPAAmx-PAAy hydrogel tubes with different components is very similar, which lies between 31 and 33 °C (Figure S9). Figure 2a shows the TRTs including the single tube, multichannel tubes, and crossed tubes along with their phase transition while heating up above the LCST and turning nontransparent. Figure 2b shows the as-TRTs including the Janus tube, block tube, patterned tube, and complex array tubes. Unless otherwise specified, the thermoresponsive part is PNIPAAm10.0-PAA2.0, while the nonresponsive part is PAM10.0PAA1.0. The responsive part and non-responsive part of the tubes are visually merged and bonded together, which was 4472
DOI: 10.1021/acs.chemmater.9b00965 Chem. Mater. 2019, 31, 4469−4478
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Chemistry of Materials
Figure 3. Thermoresponsive bending behavior of the diverse as-TRTs underwater or fluidic actuation in air. Schematic diagram shows the bending mechanism of the as-TRTs above its LCST (a) underwater and (b) fluidic actuation in air. (c) Bending angle versus responsive time of the threechannel Janus as-TRTs upon immersing into hot water above the LCST of the tube. The inserted schematic diagram shows the definition of the bending angle as a = 360° − 2θ. (d) Real-time images are captured in the bending process of the three-channel Janus as-TRTs. (e) Actuation schematic diagram and real-time images of the eight-channel Janus as-TRTs (one end was chemically linked together while the other end was completely separated) upon immersing into hot water above the LCST of the tube, realizing the octopus-like maneuvering. (f) Actuation schematic diagram and real-time images of the five-channel Janus as-TRTs (one end was chemically linked together while the other end was completely separated) upon selectively injecting 50 °C hot water into each channel, realizing the finger-like maneuvering in air. (g) Actuation schematic diagram and real-time images of the single-channel as-TRTs with cross component distribution upon injecting 50 °C hot water into the channel, realizing the snake-liked fixed-point bending movement in air. (h) Actuation schematic diagram and real-time images of the single-channel as-TRTs with helical component distribution upon injecting 50 °C hot water into the channel, realizing chiral bending in air. The scale bars are 7 mm.
tube was completely occupied by water (II). With the increment in the bending degree of the gel tube, the water in the cavity was slowly discharged until achieving bending limitation (III). Interestingly, the fluid-driven bending mechanism in air was similar to that of underwater (Figure 3b). The bending processes of as-TRTs were accompanied by the discharge of gas in the cavity and the entrance of the liquid both in air and underwater, while the only prominent difference lays in the manner that the liquid enters the cavity. As shown in Figure 3c, the bending angle of the threechannel Janus as-TRTs linearly increased with the responsive time and remained unchanged after 45 s (Movie 1), which means that our system can realize 360° curving above its LCST within 45 s underwater. The images of the bending process for the three-channel Janus as-TRTs are shown in Figure 3d. Besides achieving such simple bending behavior along one direction, we can easily realize the complex 3D bending
behavior along different directions by using complex as-TRTs. For instance, the eight-channel Janus as-TRTs (where one end of the tube was chemically linked together while their other end was completely separated) can be well prepared. When the eight-channel Janus as-TRTs was immersed into a 50 °C hot water bath (above its LCST), each tube curved toward the responsive part, realizing the octopus-like movement (Figure 3e and Movie 2). The as-TRTs can also be actuated by injecting the hot fluid (above its LCST) into its channels when in air. For example, finger-like bending behavior can be figuratively reproduced in air by injecting the hot fluid into the five-channel Janus asTRTs (where one end was chemically linked together while the other end was completely separated) (Figure 3f and Movie 3). In addition, snake-like fixed-site bending behavior can be easily realized by injecting the 50 °C hot fluid into the channels of as-TRTs with cross component distribution 4473
DOI: 10.1021/acs.chemmater.9b00965 Chem. Mater. 2019, 31, 4469−4478
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Chemistry of Materials
Figure 4. Various as-TRTs can be used to capture and release objects underwater and in air. (a) Change of the elastic modulus for uniform TRTs (PNIPAAmx-PAAy) with different NIPAAm/AA molar ratios between swelling (>LCST) and shrinkage (
