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Article Cite This: ACS Omega 2018, 3, 17914−17921
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Synergistic pH and Temperature-Driven Actuation of Poly(NIPAMco-DMAPMA)/Clay Nanocomposite Hydrogel Bilayers Guorong Gao,† Liufang Wang,† Yang Cong,‡ Zhenwu Wang,† Yang Zhou,† Rong Wang,† Jing Chen,† and Jun Fu*,† †
Polymers and Composites Division & Cixi Institute of Biomedical Engineering, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Zhongguan West Road 1219, Zhenhai District, Ningbo 315201, P. R. China ‡ School of Materials and Chemical Engineering, Ningbo University of Technology, Fenghua Road 201, Jiangbei District, Ningbo 315211, P. R. China
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S Supporting Information *
ABSTRACT: Hydrogel actuators that deform in response to external stimuli have promising applications in diverse fields. It is desirable to develop robust hydrogels with multiple and synergistic stimuli responsiveness. This work reports on robust poly(NIPAM-co-DMAPMA)/clay hydrogel bilayers that undergo reversible and repeatable curling/uncurling in response to pH and/or temperature changes. The bilayers are fabricated by sequential synthesis of thermoresponsive Nisopropylacrylamide (NIPAM) and N-[3-(dimethylamino)propyl] methacrylamide (DMAPMA) monomers, which produces robust interface bonding between the layers. Different NIPAM/DMAPMA ratios were used between two gel layers to manipulate the asymmetric responsiveness, thus leading to curling/uncurling of bilayers upon exposure to stimuli. The hydrogel bilayers are fabricated into soft manipulators to grasp and release target objects by pH and temperature changes, and as artificial muscle to lift and move a hanging weight by synergistic pH and temperature stimuli. This work provides a new type of asymmetric responsive hydrogel bilayer with potential applications in biomimetic actuators.
1. INTRODUCTION Hydrogels, composed of cross-linked macromolecules and water, are soft and wet materials.1 This unique nature enables a spatiotemporal control over their swelling or stiffness through stimuli, such as pH,2,3 ionic strength (IS),4,5 temperature,6−11 light,12,13 and electric fields,14,15 which thus generates internal stresses to drive deformation or actuation. Hydrogel actuators have gained increasing research interest in recent years due to their potential applications in soft robotics,16 generators,17 smart lenses, 3 valves, 18 manipulators, 8 and artificial muscles,19,20 etc. The controllable actuation or deformation of responsive hydrogels usually stems from their asymmetric network structures. Hydrogel bilayers, pioneered by Hu et al.,9 have been one of the classic soft actuators that undergo bending under an external stimulus. The difference in thermo-responsiveness of each layer drives the asymmetric swelling/shrinking, which motivates reversible bending/unbending of the bilayer. This basic strategy has been broadly advanced in the past few decades to fabricate biomimetic theragrippers,21,22 varifocal lenses2 and soft switches,23 with a combination with welldesigned structures. The success of bilayer hydrogels also benefits from the recent progress in tough and responsive hydrogels.24−26 For practical applications, excellent strength, © 2018 American Chemical Society
toughness, and fatigue resistance of hydrogels are needed. Nonetheless, a robust interface is critical to avoid delamination during actuations.27,28 Moreover, it is highly desirable to combine two or more, and cooperative responsive mechanisms to achieve multiple functions. Here, we present tough hydrogel bilayers with pH-, temperature-, and synergistic pH/temperature-responsive curling/uncurling properties. The bilayers are composed of hydrogels copolymerized from N-isopropylacrylamide (NIPAM) and N-[3-(dimethylamino)propyl] methacrylamide (DMAPMA) with different NIPMA and DMAPMA feed ratios (Scheme 1a). The hydrogel networks are chemically crosslinked with N,N′-methylene-bisacrylamide (MBAA) and physically crosslinked with montmorillonite (MMT) nanoclays to obtain sufficient mechanical robustness (Scheme 1a).29,30 The hydrogel bilayers are denoted as Nm1Dn1−Nm2Dn2 (m1 < m2, m1 + n1 = 100, m2 + n2 = 100), where N represents NIPAM, D stands or DMAPMA, m1 and m2 are the mole percentages of NIPAM, and n1 and n2 are the mole percentages of DMAPMA in the layers. Received: October 28, 2018 Accepted: December 11, 2018 Published: December 20, 2018 17914
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Scheme 1. Schematic Illustration of (a) the Network Structures and (b) the pH-, Temperature-, and Synergistic pH/ Temperature-Responsive Curling/Uncurling of the Hydrogel Bilayers
Figure 1. Typical tensile stress−strain curves of (a) N95D5, N90D10, N85D15, and N80D20 monolayer hydrogels, and (b) N90D10−N95D5 and N80D20−N95D5 bilayers. (c) The initial modulus (Ei), (d) fracture strain (εf), (e) fracture stress (σf), and (f) tensile work at fracture (Uf) of hydrogels. Error bars indicate the standard deviation for n = 3 measurements for each material.
PNIPAM is widely known to be responsive to temperature changes, with a lower critical solution temperature (LCST) at ∼32 °C. 31 PDMAPMA is pH-responsive due to the protonation−deprotonation of −N(CH3)2 groups with a pKa of ∼9.8.32 Thus, such hydrogel bilayers undergo different swelling/shrinking in the layers upon pH and temperature changes, which results in reversible actuation (Scheme 1b). Upon heating, the Nm2Dn2 layer with more PNIPAM components shrinks first, and thus, the bilayer curls toward the Nm2Dn2 side (Scheme 1b). In acidic solutions, the Nm1Dn1 layer with more DMAPMA components absorbs more water than the Nm2Dn2 layer, and thus, the bilayer curls toward the Nm2Dn2 side (Scheme 1b). Further heating results
in a synergistic pH/temperature-responsive curling of the bilayer (Scheme 1b). The proposed hydrogel bilayers are further fabricated into responsive manipulators to grasp, transport, and release objects.
2. RESULTS AND DISCUSSION 2.1. Preparation, Interfacial Bonding and Mechanical Properties of Hydrogel Bilayers. The bilayers were prepared using a sequential polymerization of NIPAM and DMAPMA monomers with MBAA and MMT clay nanosheets as crosslinkers, where the two layers contained different NIPAM and DMAPMA molar ratios. To establish a robust interface, it is critical to cast the second layer precursor 17915
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Figure 2. Swelling ratios (Qt = Wt/W0) of as-prepared NmDn hydrogels in (a) pH 12 and (b) pH 2 solutions. (c) Equilibrium swelling ratios (Qe = We/Wdry) of various NmDn hydrogels in pH 12 and 2 solutions. (d) Illustration of the protonation−deprotonation of −N(CH3)2 groups in hydrogels. (e) Illustration of the pH-responsive curling/uncurling of hydrogel bilayers. Snapshots of (f) N90D10−N95D5 and (g) N80D20− N95D5 bilayer curling in the pH 2 solution, and subsequent uncurling in the pH 12 solution and (h) corresponding evolution of R. Scale bars, 10 mm.
solution onto the surface of the first layer before the latter is completely polymerized. The precursor solution diffuses into the surface, and thus, the subsequent polymerization leads to the formation of an interlocked interface. Tensile tests of the bilayers demonstrate that the mechanical properties of Nm1Dn1−Nm2Dn2 hydrogels are dominated by the corresponding Nm1Dn1 and Nm2Dn2 hydrogels (Figure 1). Figure 1 shows the typical tensile stress−strain curves of N95D5, N90D10, N85D15, and N80D20 monolayer hydrogels and N90D10−N95D5 and N80D20−N95D5 hydrogel bilayers. The initial modulus (Ei), fracture strain (εf), fracture stress (σf), and tensile work at fracture (Uf) of these hydrogels are calculated and presented in Figure 1c−f. Ei (Figure 1c), εf (Figure 1d), σf (Figure 1e), and Uf (Figure 1f) of the N80D20−N95D5 bilayers fall between the values of the N80D20 and N95D5 hydrogels. Ei, εf, σf, and Uf are 20.1 ± 0.9 kPa, 197.4 ± 19.7%, 47 ± 3.1 kPa, and 39.9 ± 3.2 kJ m−3, respectively, for the N80D20−N95D5 bilayers. In contrast, Ei, εf, σf, and Uf are 19.4 ± 1.9 kPa, 277.2 ± 25.4%, 51.4 ± 2.3 kPa, and 70.3 ± 6.6 kJ m−3, respectively, for the N80D20 hydrogels, and 28.7 ± 2.7 kPa, 142.3 ± 18.7%, 41.3 ± 4.6 kPa, and 27.7 ± 3.7 kJ m−3, respectively, for the N95D5 hydrogels (Figure 1c−f). Similarly, Ei, εf, σf, and Uf of the N90D10− N95D5 bilayers fall between those of the N90D10 and N95D5
hydrogels. Figure S1 (Supporting Information) shows the snapshots of the tensile process of three N80D20−N95D5 samples. None of the hydrogels are fractured at the interface. Instead, all of them are fractured at the relatively weak N95D5 side. Furthermore, the scanning electron microscopy (SEM) image of the interface shows an interlocking structure between two layers (Figure S2, Supporting Information). With the strong interfacial bonding, the bilayers inherit the robust mechanical properties of the parent hydrogels. The N90D10−N95D5 bilayers show εf of 166.3 ± 14.2% and σf of 43.1 ± 3.7 kPa (Figure 1b,d,e). The N80D20−N95D5 bilayers show εf of 197.4 ± 19.7% and σf of 47 ± 3.1 kPa (Figure 1b,d,e). The robustness and, in particular, the flexibility of the bilayers ensured stability upon cyclic deformations without breaking upon external stimuli. According to the results shown in Figure 1a,c,d, Ei of the monolayer NmDn hydrogels decreased with increasing m, but εf increased with increasing m. These results indicate that a high PNIPAM content in the hydrogel is beneficial to the stiffness of hydrogels, whereas more PDMAPMA components in the hydrogel is beneficial to their flexibility. 2.2. pH-Responsive Curling/Uncurling of Hydrogel Bilayers. The NmDn hydrogels are pH-responsive due to the protonation−deprotonation of the −N(CH3)2 groups of the 17916
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Figure 3. (a) Deswelling of NmDn hydrogels at elevated temperatures in pH 2 solutions. (b) Calculated apparent LCST of various NmDn hydrogels. (c) Illustration of the temperature-responsive curling/uncurling of hydrogel bilayers. (d) Snapshots of a N80D20−N95D5 bilayer sequentially curling in the pH 2 solution at 25 and 60 °C, and then uncurling in the pH 2 solution at 25 °C and pH 12 at 25 °C, and (e) corresponding evolution of R. Scale bar, 10 mm.
each cycle (Figure 2h), indicating the suitability for repetitive use. Herein, the N90D10−N95D5 and N80D20−N95D5 bilayers have the same length and width, but the N80D20− N95D5 bilayer showed larger curling degrees than N90D10− N95D5 in the overall curling/uncurling process (Figure 2f,g). The big curling degree of the N80D20−N95D5 strip is attributed to the bigger Qe differences between N80D20 and N95D5 than that between N90D10 and N95D5, both in solutions of pH 12 and 2 (Figure 2c). The big curling degree is beneficial for hydrogel bilayers to be used as manipulators. Hereafter, the N80D20−N95D5 bilayers are used to investigate the temperature responsiveness and biomimetic actuators. 2.3. Synergistic pH/Temperature-Responsive Curling/ Uncurling of Hydrogel Bilayers. The NmDn hydrogels are responsive to temperature changes due to the presence of PNIPAM components in networks. Figure 3a show the deswelling of NmDn hydrogels with increasing temperature from 25 to 83 °C in pH 2 solutions, as featured by the decrease of QT (temperature-dependent equilibrium swelling ratio, =WT/W25 °C) at elevated temperatures. The apparent LCST of the NmDn hydrogels is increased by n, from 32.7 °C of N100D0 to 62.2 °C of N80D20 hydrogels (Figure 3b). The presence of PDMAPMA components promotes the water retention capacity of hydrogels and inhibits the hydrophobic collapse of networks.33 Inspired by this behavior, bilayers with different NIPAM contents and thus different LCST values are fabricated. The difference in LCST of various NmDn hydrogels results in reversible curling of hydrogel bilayers toward the low-LCST sides upon temperature increase (Figure 3c), since the low-LCST side shrinks first at elevated temperatures.
PDMAPMA chains in the network. As shown in Figure 2a,b, in solutions of pH = 12 (Figure 2a) and pH = 2 (Figure 2b), the as-prepared NmDn hydrogels become swelled over time. The equilibrium swelling ratio (Qe) of various NmDn hydrogels increased with n, whereas the same NmDn hydrogels showed larger Qe values in the pH 2 solution than those in the pH 12 solution (Figure 2c). In the order of N95D5, N90D10, N85D15, and N80D20, the Qe value increases from 15.2 ± 0.6 to 16.1 ± 0.5, 17.4 ± 0.4, and 18.3 ± 0.6 in pH 12 solutions, and from 15.4 ± 0.4 to 19.6 ± 0.6, 24.6 ± 0.5, and 31 ± 0.5 in pH 2 solutions. These results indicate that the swelling/ deswelling degree could be conveniently manipulated by changing the PDMAPMA content in the hydrogels. Therefore, bilayers with different PDMAPMA contents show asymmetric swelling/deswelling in response to pH changes, leading to curling/uncurling behaviors. As the bilayer is moved from the pH 12 solution to the pH 2 solution, the Qe increment of N90D10 (=3.5) is much bigger than that of the N90D5 hydrogels (=0.2, Figure 2c). As a result, the N90D10− N95D5 bilayer curls to the N95D5 side (Figure 2e). On the other hand, the N90D10−N95D5 bilayer curls to the N95D5 side, with the radius of curvature (R, Figure 2h) decreasing from 30.7 to 15.6 mm in 10 min. The subsequent transfer of the bilayer to the pH 12 solution leads to the uncurling to a R of 32 mm in 24 min. Similar pH change-induced curling/uncurling was observed in the N80D20−N95D5 bilayer. As shown in Figure 2g, a N80D20−N95D5 strip curled in the pH 2 solution, with R decreasing from 13.9 to 8.1 mm in 10 min. Subsequent immersing in the pH 12 solution leads to an increase in R to 15.9 mm in 24 min. The pH-responsive curling/uncurling was repeated for four cycles for both N90D10−N95D5 and N80D20−N95D5 bilayers, with a steady variation of R for 17917
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Figure 4. Snapshots of the N80D20−N95D5 hydrogel bilayers grasping and releasing objects upon (a) pH and (b) temperature changes. Scale bars, 10 mm.
Figure 5. Snapshots of a N80D20−N95D5 bilayer lifting and letting down the hanging copper ring by synergistic pH and temperature changes. At (a) 0 and (b) 33.3 min, in a solution of pH 2 and 22 °C. (c) At 58.3 min, in a solution of pH 2 and 52 °C. (d) At 181.7 min, in a solution of pH 12 and 22 °C. Scale bar, 10 mm.
The hydrogel bilayers show reversible curling/uncurling under the synergistic pH and temperature stimuli. As shown in Figure 3d, a N80D20−N95D5 strip first curled to the N95D5 side in the solution of pH 2 and 25 °C, with R reduced from 17.4 to 8.6 mm in 275.1 min (Figure 3e); then, it quickly curled to the N95D5 side when transferred to a solution of pH = 2 and 60 °C, with R reducing to 4.8 mm from 275.1 to 279.1 min (Figure 3e). Next, it uncurled when transferred to a pH 2 solution at 25 °C, with R increasing to 7.2 mm from 279.1 to 394.2 min (Figure 3e). Finally, it was uncurled when transferred to a pH 12 solution at 25 °C, with R increasing to 18.3 mm from 394.2 to 600.4 min (Figure 3e). 2.4. pH- and Temperature-Responsive Bilayers as Soft Manipulators. The reversible curling/uncurling of the hydrogel bilayers has been used to perform grasp-and-release activities upon changes in pH and temperature. Figure 4a shows a series of snapshots of the N80D20−N95D5 bilayer that grasps and releases the target object under pH stimulus. In a pH 2 solution, the bilayer curled toward the N95D5 side, and took 23.5 min to bend into a circle and thus captured a ring. Subsequently, the bilayer was transferred into a pH 12 solution, and it uncurled to release the ring in 64 min. This procedure could be found in Movie S1, Supporting Information. Similarly, as the bilayer strip was immersed in water at 60 °C, it was driven by the unbalanced deswelling of the N80D20
and N95D5 sides to bend into a loop in 1.5 min, and captured another N80D20−N95D5 bilayer loop (Figure 4b and Movie S2, Supporting Information). The subsequent transfer of such an assembly into water at 22 °C led to the gradual uncurling of both loops. It took about 52 min to release both bilayers (Figure 4b). 2.5. Synergistic pH/Temperature-Responsive Actuation of the Bilayers. The reversible curling/uncurling of the hydrogel bilayer has been used to lift and release an object upon synergistic pH and temperature changes, mimicking the functions of muscles (Figure 5a and Movie S3, Supporting Information). A weight (270 mg) was mounted onto a N80D20−N95D5 bilayer. The strip curled in a solution of pH 2 and 22 °C to lift the weight up by 6.1 mm in 33 min (Figure 5a,b). Subsequent heating of the solution to 52 °C from 33.3 to 58.3 min resulted in further lifting up of the weight to an additional height of 6.2 mm (Figure 5b,c). Next, when transferred to a solution of pH 12 and 22 °C, the strip uncurled and let down the weight from 58.3 to 181.7 min (Figure 5c,d). During the curling of the N80D20−N95D5 bilayer, the weight received mechanical work P, which was determined by P = (m − ρV)gx (m: mass of the weight, ρ: density of the solution, V: volume of the weight, g: acceleration due to gravity, x: lifting height).20 The P value was estimated to be 15.96 μJ during the pH-responsive lifting and 16.22 μJ during 17918
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Q700). Then, NIPAM, DMAPMA, MBAA, KPS, and TEMED were added and magnetically stirred for 1 h at ice-water temperature. Finally, the precursor solution was injected into glass molds to polymerize at room temperature for 24 h. 4.3. Preparation of Bilayer Nm1Dn1−Nm2Dn2. The Nm1Dn1−Nm2Dn2 bilayers were fabricated through a sequential two-step synthesis. First, the precursor solutions of Nm1Dn1 and Nm2Dn2 layers were prepared as described above, but the dyestuff methylene blue was added to the Nm2Dn2 solution with a concentration of 0.16 g L−1. Next, the Nm1Dn1 solution was injected into a glass mold spaced by a thin (e.g., 0.5 mm) silicone frame, which was then kept at room temperature for 20 min for pre-polymerization. Then, the frame was replaced with a thicker one (e.g., 1 mm), and the Nm2Dn2 solution was injected into the new space in the mold. The Nm1Dn1−Nm2Dn2 hydrogel bilayers were obtained after further polymerization for 1 day. The Nm1Dn1−Nm2Dn2 tensile samples were prepared in glass tubes with 5.5 mm inner diameter. The Nm1Dn1 solution was first pre-polymerized in the tube for 20 min. Then, the Nm2Dn2 solution was added and further polymerized for 1 day. 4.4. Tensile Tests. The tensile tests were conducted on an Instron 5567 testing machine (Instron, MA) equipped with a 500 N load cell at room temperature. Samples with initial size of 45 mm in length and 5.5 mm in diameter with 10 mm gauge length were stretched at 100 mm min−1. The tensile stress (σ) is calculated from σ = F/πr2, where F is the load and r is the radius of the original specimen. The tensile strain (ε) is calculated from ε = (l − l0)/l0 × 100%, where l is the varied length and l0 is the initial gauge length. The initial modulus (Ei) is defined as the slope of the stress−strain curve within the ε range of 5−10%. The tensile work at fracture (Uf) is calculated by integrating the area under the stress−strain curve, as follows
the subsequent synergistic pH/temperature-responsive lifting of the weight. These results indicate that, in contrast to the actuation by a single stimulus, the synergistic actuation by two or more stimuli could result in higher energy input and output. This effect may inspire novel design of soft actuators or devices with synergistic multiple stimuli responsiveness. In this work, however, the sequential synthesis method to fabricate bilayers is not optimal for the fabrication of multiresponsive devices. Recently, many other methods, including macroscopic assembling,25,26 inject printing patterning,34 photolithography,35 and evaporation-programmed heterogeneous structures,6 have been demonstrated to create inhomogeneous structures in responsive hydrogels in a wellcontrolled manner. Thus, numerous smart hydrogel devices have been developed and actuated or deformed in a programmable manner. On the other hand, the actuation rate is still slow, since the deswelling and swelling are slow. It is desirable to further develop novel architecture or structures of hydrogel devices to achieve rapid actuations for practical applications. Furthermore, it is also desirable to develop hydrogel actuators to achieve intelligent bionic actions, such as controllable jump36 and earthworm-like crawling.37 Finally, there are mobile counterions in most of the polyelectrolyte hydrogels, which enable electrical conductivity, and as a result, more intelligent and multifunctional hydrogel soft devices may be developed through integrating the actuation and sense38,39 capabilities of hydrogels.
3. CONCLUSIONS In summary, synergistic pH and temperature-responsive poly(NIPAM-co-DMAPMA)/clay hydrogel bilayers were developed by using a sequential two-step synthesis to generate a robust interfacial bonding. The bilayers show asymmetric pH and temperature responsiveness, which originate from the different NIPAM/DMAPMA ratios used in the two layers. The bilayers exhibit reversible and repeatable curling/uncurling upon pH, temperature and synergistic pH/temperature changes. Taking advantage of the curling/uncurling deformations, the bilayers could be used to grasp, release, lift, and let down target objects. Such biomimetic actuations of the hydrogel bilayers may have the potential for applications as soft robots, manipulators, sensors, and drug-delivery vehicles.
Uf =
∫0
ε
σ dε
4.5. pH-Responsive Swelling of Monolayer NmDn. The as-prepared NmDn hydrogels were successively immersed in solutions of pH = 2 and 12 with a constant IS of 0.1 mol L−1 until an equilibrium was reached at room temperature. At the determined time points, the samples were picked out and weighed. Upon equilibrium, the samples were lyophilized using a freeze dryer. The time-dependent swelling ratio (Qt) was calculated as Qt = Wt/Wdry, where Wt is the weight of the swollen gel at a specific time t, and Wdry is the weight of the corresponding dried gel. The Qt value of the equilibrated samples was defined as the equilibrium swelling ratio (Qe). The pH of the solutions was adjusted by adding appropriate amounts of HCl or NaOH, whereas the IS was adjusted by adding NaCl. 4.6. Temperature-Responsive Deswelling of Monolayer NmDn. The NmDn hydrogels were first swelled to equilibrium at room temperature, and subsequently by transferring to solutions at elevated temperatures (T = 30− 83 °C) until equilibrium again. The samples were frequently picked out and weighed. The temperature-dependent equilibrium swelling ratio (QT) was calculated as QT = WT/W25 °C, where WT and W25 °C are the weight of the gel equilibrated at T and 25 °C, respectively. All of the solutions have the same pH = 2 and IS = 0.1 mol L−1. The LCST was calculated by
4. EXPERIMENTAL SECTION 4.1. Materials. NIPAM, DMAPMA, MBAA, potassium persulfate (KPS), N,N,N′,N′-tetramethylethylenediamine (TEMED), and methylene blue were purchased from Aladdin Chemistry Co., Ltd. Hydrochloric acid (HCl, 37 wt % aq sol.), sodium hydroxide (NaOH), and sodium chloride (NaCl) were bought from Sinopharm Chemical Reagent Co., Ltd. MMT clays (trade mark, “SMJ-N”) were supplied by Zhejiang Fenghong New Material Co., Ltd. All chemicals were used as received. Deionized water was used for all experiments. 4.2. Preparation of Monolayer NmDn. NmDn hydrogels were synthesized by free-radical copolymerization of NIPAM (m% mol L−1), DMAPMA (n% mol L−1), and MBAA (1% mol L−1) in an aqueous suspension of MMT (40 g L−1) by using KPS (0.5% mol L−1) as the initiator and TEMED (1% mol L−1) as the catalyst. First, MMT was added into water and magnetically stirred until the formation of a homogeneous slurry, and the mixture was subsequently subjected to ultrasonication for 30 min with a probe sonicator (Qsonica 17919
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derivation from the QT versus T curve, and selecting the minimum dQT/dT as the LCST. 4.7. pH-Responsive Curling/Uncurling of Bilayers. N90D10−N95D5 (size: 55 × 6 × 1.66 mm3, with 0.86 mm thickness belonging to the N90D10 side) and N80D20− N95D5 (size: 55 × 6 × 1.9 mm3, with 1.1 mm thickness belonging to the N80D20 side) bilayers were first equilibrated in a solution of pH = 12. Then, they were immersed in solutions of pH = 2 (300 mL) for curling and subsequently transferred to a solution of pH = 12 (300 mL) for uncurling. The deformation processes were recorded using a digital camera (Sony Alpha 5000, Tokyo, Japan). The R value of the curled strips was measured by the Image-Pro Plus 6.0 software. All of the solutions have the same IS = 0.1 mol L−1 and were at room temperature. 4.8. Synergistic pH/Temperature-Responsive Curling/ Uncurling of a Bilayer. The N80D20−N95D5 hydrogel bilayer (size: 50 × 8 × 3.06 mm3) was first equilibrated in a solution of pH = 12 at room temperature. Then, in sequence, it was immersed in solutions of pH = 2 and 25 °C, pH = 2 and 60 °C, pH = 2 and 25 °C, and pH = 12 and 25 °C. In each step, it was kept for sufficient time until shape stability was achieved. All of the solutions have the same IS = 0.1 mol L−1. The overall process was video recorded. 4.9. pH-Responsive Grasp and Release. A N80D20− N95D5 bilayer was mounted with a copper handle, and then immersed in a pH = 2 solution to capture the target object, which is a copper ring attached with a piece of silicone rubber. After that, it was transferred to a pH = 12 solution to release the object. The overall process was video recorded. All of the solutions have the same IS = 0.1 mol L−1 and were kept at room temperature. 4.10. Temperature-Responsive Grasp and Release. A N80D20−N95D5 bilayer was mounted with a copper handle, and then immersed in water of 60 °C to capture another curled bilayer loop. After that, it was transferred to water of 22 °C to release the loop. The overall process was video recorded. 4.11. Lift and Let Down of a Copper Ring by Synergistic pH and Temperature Changes. A N80D20− N95D5 bilayer with a copper ring hanging at the bottom was hung to another ring at the top. The system was immersed in a solution of pH = 2 and 22 °C. Once the bilayer curled to equilibrium, the solution was heated to 52 °C with 0.02 °C s−1. After that, the system was transferred to a solution of pH = 12 and 22 °C. The overall process was video recorded.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rong Wang: 0000-0003-1971-0865 Jun Fu: 0000-0002-8723-1439 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51603220 and 21574145), China Postdoctoral Science Foundation (2017T100439 and 2016M600476), and Ningbo Natural Science Foundation (2016A610255).
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02979. Experimental method for SEM characterization; SEM image of a freeze-dried N80D20−N95D5 hydrogel bilayer; snapshots of the tensile process of three N80D20−N95D5 bilayer rods (PDF) pH-responsive grasp and release of a N80D20−N95D5 bilayer (AVI) Temperature-responsive grasp and release of a N80D20−N95D5 bilayer (AVI) Synergistic pH/temperature-responsive lift and let down of a N80D20−N95D5 bilayer (AVI) 17920
DOI: 10.1021/acsomega.8b02979 ACS Omega 2018, 3, 17914−17921
ACS Omega
Article
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