A photothermal nanocomposite hydrogel actuator with electric field

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A photothermal nanocomposite hydrogel actuator with electric field-induced gradient and oriented structure Yang Yang, Yun Tan, Xionglei Wang, Wenli An, Shimei Xu, Wang Liao, and Yu-Zhong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17907 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Applied Materials & Interfaces

A photothermal nanocomposite hydrogel actuator with electric field-induced gradient and oriented structure Yang Yang1, Yun Tan1, Xionglei Wang2, Wenli An1, Shimei Xu*1, Wang Liao1, Yuzhong Wang1 1

Center for Degradable and Flame-Retardant Polymeric Materials, National

Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, College of Chemistry, Sichuan University, Chengdu 610064, China 2

College of Chemical Engineering, Sichuan University, Chengdu 610064, China

KEYWORDS: (graphene oxide; photothermal conversion; gradient hydrogel; near-infrared trigger; actuator)

ABSTRACT: Recent research of hydrogel actuators is still not sophisticated enough to meet the requirement of fast, reversible, complex and robust reconfiguration. Here, we present a new kind of poly(N-isopropylacrylamide)/graphene oxide gradient hydrogel by utilizing direct current electric field to induce gradient and oriented distribution of graphene oxide into poly(N-isopropylacrylamide) hydrogel. Upon near-infrared light irradiation, the hydrogel exhibited excellent comprehensive

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actuation performance as a result of directional bending deformation, promising great potential in the application of soft actuators and optomechanical system.

Near-infrared (NIR) light responsive actuators have attracted great attention for their applications in artificial muscles,1 microvalves,2 biomedical devices3 and soft robots,4 because the NIR light has the advantages of remote triggering, tunable intensity and wavelength, rapid turn on or off, as well as high spatial and temporal precision.5 Among the NIR light responsive actuators, hydrogels have been widely investigated due to their dramatic change in physical/chemical properties, exhibiting great prospects in various applications, such as microfluidic valves,6 intelligent actuators,7 cell scaffolds8 and controlled release systems.9 Generally, the NIR photosensitive hydrogel actuators convert NIR optical signal to locomotion, and show expanding/shrinking, bending/unbending or diverse complex deformation.10-12 However recent research of hydrogel actuators is still not sophisticated enough to meet the requirement of fast, reversible, complex and robust reconfiguration. To address the issue, some strategies have been developed to endow the hydrogels with anisotropic structures, such as stripes,13-14 double-layer structure,15-16 Janus structure17 and gradient pore structure,18 as well as orientation of anisotropic fillers (e.g., carbon nanotubes, liquid crystal and titanate nanosheets).14, 19-20 Among them, the bilayer structure is widely adopted due to relatively simple fabrication. However, in general, poor compatibility between the two layers is one of main problems existing in bilayer hydrogel actuators, which might lead to delamination and poor driving capability.


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Nature provides a rich source of inspiration for the design and manufacture of high performance synthetic materials.21 A gradient structure or an oriented structure is widely adopted in the response of the human body to external stimuli. Luo and co-workers18 have developed a gradient porous hydrogel by hydrothermal process, and the hydrogel strip could bend from 0o to 70o in 5 s after NIR laser irradiation (980 nm, 2.2 W cm-2). Wani et al.22 introduced a liquid crystal into the photoresponsive actuator to form an aligned structure, which acted as a mimicking natural flytrap that was capable of autonomous closure and object recognition. Although a few reports have developed gradient or aligned structure for improving the actuation, the current methods for producing the structures are complex and only limited to a single gradient structure or single aligned structure. Therefore, a combination of both gradient and aligned structure in one material is a big challenge, especially by a facile and simple method. In our previous work, we reported a gradient nanocomposite hydrogel with bionic gradient structure by a facile electrophoresis method.23 The external direct current (DC) induced a gradient distribution of negatively charged Laponite in the hydrogel matrix, which led to gradient stress due to gradient crosslinking of Laponite. Inspired by the work, here we present a simple but versatile fabrication method of fast NIR light responsive gradient hydrogel actuator by using negatively charged graphene oxide

(GO)

to

replace

Laponite.

Meanwhile

thermosensitive

poly(N-isopropylacrylamide (PNIPAm) was used to translate the thermal signal into mechanical motion. Among thermoresponsive polymers, such as non-ionic,24 3 ACS Paragon Plus Environment


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microgel-based25 and acrylate-based thermoresponsive polymers,26-27 PNIPAm exhibits a LCST at around 32oC, which is close to body temperature, and thus has potential application in biomaterials. It is well known that GO are of high photo-thermal conversion efficiency, and could convert NIR light into heat and further trigger an expansion/contraction of the resultant nanocomposites when incorporated into thermosensitive hydrogels forming isotropic actuators.2, 28 In our work, the introduction of GO gradient structure is expected to improve the actuation. Interestingly, GO was also found to be macroscopically oriented in the hydrogel along the direction of DC field. Such a composite material with orientation of nanosheets over a macroscopic size scale is expected to show unique anisotropic features in mechanical toughness and response performance.

The one-pot fabrication strategy of the PNIPAm/GO gradient hydrogel is schematically illustrated in Figure 1. Negatively charged GO moves to the anode during the electrophoresis. By free radical polymerization, the PNIPAm chains are chemically cross-linked by N,N'-methylenen bisacrylamide (BIS) while physically cross-linked by the hydrogen bonding between the amide groups of PNIPAm chains and the oxygen-containing groups on the GO. Finally, the hydrogel with gradient and oriented GO along the direction of the electric field was obtained, and the resultant hydrogel was denoted as GOmN1-En, where m and n represent the mass concentration of GO (mg mL-1) and the intensity of electric field (V mm-1), respectively, and N1 represents concentration of NIPAm i.e., 1 mol L-1.


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Figure 1. Scheme of preparation of PNIPAm/GO gradient hydrogel.

SEM observation revealed increasing pore size from the anode to cathode side of the hydrogel (Figure 2a).

The hydrogel in the anode side was dense and compact

while the one in the cathode side exhibited an interconnected macroporous structure. The gradient of the GO was also confirmed by a gradient fluorescence along the direction of electric field after labeling the GO with rhodamine 6G (R6G) solution through confocal laser scanning microscope (CLSM) observation, while the control hydrogel showed a homogeneous fluorescence (Figure S1 and S2).



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Figure 2. (a) SEM images of freeze-dried GO1N1-E1 gradient hydrogel; (b-e) 2D SAXS images of a GO1N1-E1 hydrogel. The hydrogel was exposed to an X-ray beam from the orthogonal (b) and parallel (c) directions to the applied electric field. (d) Azimuthal angle (φ) plots for the 2D SAXS images in (b) and (c). (e) 2D SAXS images of a GO1N1-E1 gradient hydrogel were measured from the anode(1), middle (2) and cathode (3) side respectively, and Azimuthal angle (φ) plots for the 2D SAXS images in (e).


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Besides of gradient distribution of GO, microscopical orientation of GO was observed by 2D SAXS analysis (Figure 2b and 2c). The hydrogel exhibited an elliptical diffusive pattern (Figure 2b) or isotropic scattering (Figure 2c). It was similar to previous reports on the field alignment of the 2D nanosheets.20 Accordingly, the azimuthal angle (φ) plot of the 2D SAXS pattern measured from the orthogonal direction to the electric field has two peaks, at θ = 180o and 360o (Figure 2d, red), whereas the measurement parallel to the electric field shows a single plateau (Figure 2d, blue). On the basis of the fitting of the azimuthal angle plots, the orientation degree of GO is calculated using the following equation (Eq. 1) Π= (180o-Ho) /180o

(1)

where Π is the degree of orientation, H represents the width of the half peak height of the I-azimuthal angle curve. The orientation degree of GO in the hydrogel was calculated to be 0.77, which was relatively high compared with other electrically oriented 2D nanosheet. Meanwhile 2D SAXS images of GO1N1-E1 hydrogel measured at different positions indicated the different orientation degree of GO (Figure 2e). These results showed that under the induction of electric field, GO not only took a migration toward the anode, but also occurred in the directional arrangement, finally resulted in the formation of gradient-oriented hydrogel.

The GO sheets participated in the formation of the network by acting as a physical crosslinker due to the interaction between GO and PNIPAm (Figure S3 and S4). It was also confirmed by the decreased pore size in the GO2N1-E1 hydrogel due to the



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increasing of GO concentration compared with the GO1N1-E1 hydrogel (Figure S5). Meanwhile GO nanosheets were well dispersed in the hydrogels (Figure S6). The PNIPAm/GO gradient hydrogel began to expel the water after NIR light irradiation (808 nm, 1.2 W cm-2), resulting in bending of the gel to angle θ (Figure 3a). The infrared imaging illustrated the temperature in the cross section of the gradient hydrogel increased from the cathode side to the anode side due to the gradient distribution of GO along the DC direction after irradiated with NIR light (Figure 3b). A higher GO concentration in the anode side led to a faster heating rate which produced a greater deswelling rate compared with that of in the cathode side. So the hydrogel strip bent towards the direction of the anode side. Similarly, with an increase of GO concentration in the hydrogel network, the rise rate of temperature became faster, resulting in more rapid response to the NIR light. However, the bending extent of the gel decreased instead with the GO concentration of 2 mg mL-1 (Figure 3c). The reason can be explained as the gradient hydrogel containing more GO has a greater cross-linking density, which restricts the motion of the polymer chains (Figure S5). As a control sample, the GO0N1-E1 hydrogel exhibited no bending angle change within 240 s (Figure S7). With the increasing of electric field intensity, the PNIPAm/GO hydrogels exhibited more rapid response to the NIR light irradiation (Figure 3d). The hydrogel prepared with the electric field intensity of 2 V/mm reached a bending degree 274o at 45 s under NIR radiation. The larger intensity of the electric field could result in the greater gap of GO concentrations between the two sides (Figure S8). This can be 8 ACS Paragon Plus Environment

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confirmed by the fact that an obvious bilayer was observed when the DC intensity reached 2.5 V/mm (Figure S9). However, the latter showed decreased bending angle due to uncontinuous distribution of GO. This result indicated that the GO concentration gradients within the gels were the driving force for their bending, and the flexion of the gels were easily controlled by adjusting the GO gradient distribution in the gels. The hydrogel exhibited more rapid photoresponses with the increasing of light intensity, where the hydrogel strip could bend from 0 to 300o within 40 s under 2 W cm-2 of NIR light (Figure 3e). To our knowledge, this bending degree in the same time is top-listed among hydrogel actuators in the literature.15-16, 29-30

Figure 3. (a) Scheme illustrating the definition and calculation of bending angles and the photographs are the GO1N1-E1 gradient hydrogel before and after being irradiated for 50 s. (b) Infrared thermal images of PNIPAm/GO gradient hydrogel. (c) Actuation degree for the GOmN1-E1 gradient hydrogels with m = 0, 0.5, 1 and 2. (d) 9 ACS Paragon Plus Environment


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Actuation degree for the GO1N1-En gradient hydrogels with n = 0, 1, 2, and 2.5. (e) Photo-responsive time-bending angle curves of the hydrogel strip after being irradiated by 808 nm NIR light with different intensities. (f) Reversible actuation behaviors of the PNIPAm/GO gradient hydrogel for five cycles of on/off NIR irradiation. Scale bar 1cm for (a and d) and 1mm for (b) respectively.

In addition to rapid photoresponses, the hydrogel also exhibited a precise reversible bending motion in response to NIR light (Figure 3f). Once exposed under NIR light, the GO1N1-E1 hydrogel exhibited a rapid shrinking due to water loss and reached a 360o bending, then the hydrogel slowly reswell to the equilibrium state and restored its original shape within 30 min after being placed into water. After five cycles of the photoresponsive processes, the gradient hydrogels retained actuation similar to that of the original samples, demonstrating the structure stability of the gradient hydrogels. In order to achieve the bionic driving performance, the gradient hydrogel was used as the gripper to grab heavy objects. The gradient hydrogel extruded the water and curved under NIR light (Figure 4a). Simultaneously, the mechanical property of the hydrogel was improved and showed higher extensibility after the NIR light irradiation (Figure 4b). The GO1N1-E1 gel can lift weight by the bending under NIR light irradiation (Figure 4c). Even after loading cargo with a mass of more than 25 times that of the dried hydrogel, the gradient hydrogel could complete lifting after NIR irradiating for 30 s and lift the weight to a height of 8 mm after irradiating for 60 s (Figure 4d). The results show that gradient hydrogels have potential applications as optical converters for artificial muscle and soft robots. 10 ACS Paragon Plus Environment

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Figure 4. (a) The photographs are size changes of the GO1N1-E1 hydrogel before and after being irradiated for 50 s. (b) The GO1N1-E1 hydrogel sample exhibits ultrahigh tensibility against extensive stretching after light irradiated. (c) The weight-lifting process of the GO1N1-E1 hydrogel with 808 nm NIR light irradiation. (d) NIR-driving actuation movement under load of the gradient hydrogel after irradiation for 60 s. The scale bar is 1 cm.

In summary, a novel NIR light-driving PNIPAm/GO hydrogel with gradient and oriented structure has been successfully developed via electrophoresis and in situ polymerization. The work provides not only a new insight on the design of hydrogel actuators with comprehensive actuation performance by introducing the gradient and oriented structure, but also a versatile method to adjust the actuation behaviors by simply changing electophoresis conditions. The PNIPAm/GO gradient hydrogel has 11 ACS Paragon Plus Environment


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NIR photoresponsive property, rapid NIR response kinetics, and programmable locomotion, along with a facile fabrication process, which makes it an outstanding candidate in many applications, such as tissue engineering, soft actuators, and other intelligent biomimetic devices.

ASSOCIATED CONTENT

Supporting information Experimental section, LCSM, FTIR, viscosity, TEM, AFM, 2D SAXS and SEM measurements.

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the NSFC-Xinjiang joint fund for local outstanding youth (No.U1403392).

REFERENCES


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A photothermal nanocomposite hydrogel actuator with electric field

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