Macroporous-Enabled Highly Deformable Layered Hydrogels with

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Macroporous-Enabled Highly Deformable Layered Hydrogels with Designed pH Response Minggan Li, Dehi Joung, and Dae Kun Hwang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00653 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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Macroporous-Enabled Highly Deformable Layered Hydrogels with Designed pH Response

Minggan Li1,2,3, Dehi Joung1,2,3, and Dae Kun Hwang1,2,3* 1

2

3

Department of Chemical Engineering, Ryerson University 350 Victoria Street, Toronto, Ontario, M5B 2K3, Canada

Keenan Research Centre for Biomedical Science, St. Michael’s Hospital, 30 Bond Street, Toronto, Ontario, M5B 1W8, Canada

Institute for Biomedical Engineering, Science and Technology (iBEST), a partnership between Ryerson University and St. Michael’s Hospital, 30 Bond Street, Toronto, Ontario, M5B 1W8, Canada

Correspondence to Prof. D. Hwang* E-mail: [email protected] Keywords: Stimuli-responsive hydrogels, sensor, swelling, flow lithography

1 ACS Paragon Plus Environment

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Abstract Environment-responsive hydrogel structures are of great interests in materials research and have a wide range of applications. By using a flow lithography technique, we report a one-step and high throughput fabrication method for the synthesis of highly pH-responsive hydrogels with designed shape transformations. In this method, heterogeneous hydrogels with porous and nonporous layers are synthesized using a single UV exposure in a microfluidic channel. During the UV polymerization the porous layers, which are formed by using polymerization induced phase separation (PIPS), significantly increase the swelling capability and enhance the swelling rate of the hydrogels. Since the flow-lithography approach allows various patterns of porous/non-porous layers with great control and enables the simple integration of PIPS, resultant layered hydrogels show extraordinary deformations with desired pH response. More importantly, our fabrication approach can not only make 2D deformation of hydrogel structures such as bending, but also can achieve 3D structural deformation such as helical and buckling structures, enabled by non-uniform UV polymerization we develop.


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Introduction Stimuli-responsive hydrogels have been widely explored in many areas owing to their extraordinary environmental responsiveness, diverse chemical functionality and tunable physical properties.1-6 Layered hydrogel structures are of particular interest because of their structural simplicity and designable shape response to the environmental conditions. Layered hydrogel structures are composed of stimuli-responsive and stimuli-nonresponsive stripes, and they are bond together to form an inhomogeneous hydrogel structure. The structural inhomogeneity converts the hydrogel physical response into desired shape transformation, which has been enabled to achieve customized functions in various applications, such as artificial muscles,7,8 autonomous flow control,9 programmable sheet transformation,10,11 shape memory materials,12-14 environmental sensors,15,16 soft machines,17-19 and bio-inspired actuators.20 Theoretical analysis of the hydrogel structural mechanics have further promoted the understanding of the actuating mechanism11,21-24 and improved the hydrogel structure design for precise shape response and complex structure formation.25,26 Given the structural inhomogeneity of the layered hydrogels, their fabrication normally involves multi-step processes to form the stimuli-responsive and nonresponsive layers. Conventional techniques for patterned microstructures, such as photopatterning,9,11 material deposition,16 and molding19-21 have been commonly adapted to form the structures by sequential polymerization of the responsive and nonresponsive layers; however, these methods require cumbersome multistep alignments and precise positioning procedures. Furthermore, these methods are low in efficiency because of their batch production nature, which makes it difficult to achieve a high throughput for the massive production of sensors and actuators.


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Recently, stop flow lithography (SFL) was exploited for the one-step fabrication of bihydrogel particles by simultaneously polymerizing the parallel streams of pH-responsive and pHnonresponsive hydrogel precursors in a microfluidic channel.27 This route provides a simple and high throughput method for the fabrication of layered hydrogel structures. To obtain large and designable shape transformation triggered by environmental stimuli, materials selected for layers must exhibit a significantly different but easily tunable responsiveness. However, the resulting bihydrogels from this route display a limited deformation arising from the internal mechanical constrain of nonporous polymers, which allows only a small bending of their bihydrogels even at the strongest stimuli. This limitation restricts its shape design and the sensing resolution. In addition, the nonporous polymer also suffers from low swelling-and-deswelling rates because of its low molecular diffusion rate - reaching its equilibrium state, for each measurement takes a minimum of 40 min for the bihydrogels.27 Fabrication methods that can create layered hydrogel structures with rapid and designable shape response in a simple and high throughput fashion are still not available, essentially limiting their functionalities and applications. Here we propose a fabrication route for layered-hydrogel structures based on UV polymerization induced phase separation (PIPS) in combination with SFL, which synthesizes macroporous hydrogel structures that display large and rapid pH-responsive deformation that are not attainable to date. As schematically shown in Figure 1a, in this fabrication process, two or more photocurable prepolymer solutions with or without acrylic acid (AA) are co-flowed through a microfluidic channel. As the flow is stopped UV light is projected to the channel through a photomask to simultaneously polymerize the multiple streams, forming a layered hydrogel structure with designed geometry and dimensions (Figure 1b). Owing to the lubrication layers at the channel walls caused by oxygen inhibition28, synthesized hydrogels can be flushed out of the


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channel by resuming the flow. This process is repeated, and layered hydrogel structures are synthesized in a one-step and continuous mode. More importantly, with no additional fabrication steps, by just introducing proper porogens into the prepolymer solutions, one can simultaneously make hydrogels with costumed shape and tunable micropores and regulate their pore structures by varying the concentration and type of porogen, as studied in our previous work.29,30 The controllable micropores are developed while UV PIPS occurs during the UV polymerization process. These porous hydrogels have a much larger swelling ratio than the nonporous ones owing to the available free volume of the porous matrix,31 as experimentally demonstrated in the Supporting Information (Table S1 and Figure S1-S4). This swelling ratio can be fifteen times larger than nonporous hydrogels.32 In addition, the increased surface area and free chain mobility of macroporous hydrogel can enable equilibrium state establishment in a few seconds,32 much quicker than the responsive time of 40 min by using nonporous hydrogel. Results and Discussion We first made a simple bilayered structure as shown in Figure 1a, in a two-stream microfluidic channel. One stream for the nonporous part contains a UV curable solution of polyethylene glycol diacrylate (PEG-DA) and photoinitiator (PI) to create the pH-nonresponsive layer (nonporous); the other stream for the porous part contains PEG 250 as the porogen and AA as the pH-responsive compound to synthesize the pH-responsive layer (Figure 1b). As exposed to a high pH-buffer solution, a large swelling ratio of the pH-stimuli layer allows this two-stripe hydrogel to rapidly deform into a highly curved structure (complete circle) in as quick as 10 seconds (Figure 1c, d).


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We then performed a serial of pH-responsive deformation experiments on bilayered hydrogels of initial thickness 50 µm, length 1 mm and width 70 µm (50 µm for the porous and 20 µm for the nonporous respectively, as shown in the inset of Figure 2) from pH 2 to 10. As pH increases, the pH-responsive side in the presence of AA swells. This dimension expansion, as a result, exerts compressive stress on the nonresponsive side. The final deformed structures depend on the pH values applied. Owing to the macroporous structures in the hydrogel layer, a small change in pH can lead to a rapid and large deformation of the bilayer. As pH is changed from 2 to 7, a straight bilayer stripe can transform into a half circle, complete circle and even spiral formation. Our bilayered strip shows a large pH response from pH 2 to 7. This linear pH-response of the freestanding hydrogels, not attainable in earlier studies, can be a useful candidate for a pH-sensor in the microfluidics-based system. Further increase of pH to 10 leads to a smaller bending angle change while the hydrogel reaches to their maximum swelling state, as shown in Figure 2 inset. The deformation of the bilayer structure is mechanically stable and only bending presents if the length ratio of hydrogel width (w) to thickness (h) is lower than a critical value about 2,33 which is the case of the bilayer deformation in Figure 2. However, when the length ratio is greater than the critical value so that the bilayer stripe becomes a thin film as shown in Figure 3 inset, out of plane deformation may occur, leading to the buckling structure.32 To study the buckling of the film, we made thin and long bilayer hydrogel films with pre-swelling thickness 20 µm and length 1 mm. The width of the non-responsive part is 20 µm while the responsive part has various width 180 µm, 160 µm and 140 µm. When exposed to pH 3 buffer solutions, they slightly deform as shown in Figure 3. With an increase of the pH value to 4 and 7, these bilayer hydrogel films show different buckling modes depending on their width. The buckling is induced by the instability of the swelling hydrogel film constrained by the stiff non-responsive part. The


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wavelength of the buckles is determined by the width of the hydrogel part and can be approximated by λ=3.2w.32 For the bilayer films we made, the theoretical wavelengths are 576 µm, 512 µm, and 448 µm respectively, which do not strictly apply to the actual wavelength 700 µm, 400 µm and 380 µm of the hydrogel films in this study. This discrepancy may be caused by the assumption of the theory that the hydrogel film is constrained by a stiff and non-deformable part; while the non-responsive parts of our films are not strong enough to withstand the compressive stress of the swelling hydrogel without elastic deformation. Nevertheless, the theory provides a useful reference for our bilayer hydrogel structure design. Interestingly, as the buckling matures along with a pH increase from 4 to 7, the number of the buckles remains same, although their amplitudes grow, as shown in Figure 3 a, b and c respectively. This is because that the number of the buckles is determined by the geometry of the hydrogel film, not the swelling ratio between the two layers, according to the buckling theory. 33 This may be useful in sensor design to achieve reliable measurements of the environmental conditions. By changing the dimensions of the photomask and microfluidic channel, and the configuration of the co-flow streams, this method enables us to facilely make different layered hydrogel structures with great control. Figure 4 shows two-layer and three-layer hydrogel structures with different geometrical configurations of the pH-responsive and non-responsive streams. Figure a-e are thick structures with the height 50 µm, thus there are only bending deformations; while Figure f is thin hydrogel film with height 20 µm and buckling presents. When the two-layer structure is diagonally configured, a gradual change of the hydrogel swelling makes the bending curvature that varies along the longitudinal direction under pH environment.


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The curvature reaches its maximum point at the middle of the structure, forming a ribbon-like shape, as shown in Figure 4 a. If a small non-responsive part is integrated into a hydrogel strip, the bending will only present at the non-responsive section and a Christmas candy shapes can be formed (Figure 4 b). We can also fabricate a two-layer structure and let it bend into a 3dimensional spiral shape under pH environment as seen in Figure 4 c. We use the same twostream configuration as those in Figure 2, with one stream is pH-responsive and the other nonresponsive. However, when we polymerize the structures in the channel, we lifted the focus of the objective for 20 µm to form a non-uniform polymerization of the structure. As the focus is lifted, the polymerization is stronger at the lower part of the structure and is slightly weaker on the upper part due to the non-uniform UV intensity near the focal plane,34,35 as shown in Figure 4 inset and Supplementary Information Figure S5. In addition to the in-plane bending caused by the pH-responsive stream, this structure will also deflect in the direction perpendicular to the bending plane due to gradients of AA concentration in the pH-responsive stream, leading to a 3D spiral shape. This method can be readily extended to the fabrication of three or multi-layer structures by incorporating more streams in the microfluidic channel. By changing the aligning position and orientation of the pH-responsive and non-responsive streams, different structural inhomogeneity can be obtained to achieve designed shape response as shown in Figure d and e. As the thickness of the hydrogels is decreased to 20 µm, the structure becomes multi-layer film and buckling may occur. Figure 4 f is a long and thin three-layer hydrogel strip with two side layers pH-responsive and the middle one non-responsive. As the side layers swell under a pH environment, the structure forms a wavy ribbon. These layered hydrogel structures may allow for many more shape-response formations through proper structure design.


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Conclusion We have developed a one-step method for the fabrication of hydrogel structures highly responsive and sensitive to pH by simple integration of SFL and UV-polymerization induced phase separation. Porous hydrogel structures developed in our resulting hydrogels enable large, rapid and controllable shape-deformations triggered by environment conditions. We demonstrated that the method is able to make different structures for desired shape transformation in a short time 10 s, such as two-layer strip bending, thin film buckling and versatile geometrical configuration for desired shapes. With increasing applications of stimuliresponsive hydrogels, the method we proposed will help for the quick design and easy production of layered hydrogel structures widely used in sensing, actuating, tissue engineering and drug delivery.

Materials and Methods The fabrication system for layered hydrogel structures is based on a stop flow lithography setup.34 A metal arc lamp (Lumen 200, Prior Scientific, Rockland, MA, USA) was connected to the Axio Observer inverted microscope to provide the UV exposure source. UV exposure time is controlled by a UV shutter (Lambda SC, Sutter Instruments, Novato, CA, USA). The prepolymer solutions were supplied through a pneumatic tubing system, which consisted of a pressure regulator (Type 100LR, ControlAir, Amherst, NH USA), serially connected to a three-way solenoid valve (Model 6014, Burkert, Germany) and a PDMS channel. The UV shutter and the solenoid valve were controlled by a program in Labview (National Instruments, Austin, TX, USA) through a digital controller (NI 9472, National Instruments, Austin, TX, USA) to control the UV exposure and prepolymer flow cycle. The microscope equipped with 10×/0.3 objective


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(N-Achroplan, Ec plan-Neofluar and korr LD Plan-Neofluar, Carl Zeiss, Jena, Germany) was used as the synthesis platform. The desired UV excitation (350 nm) required by polymerization was attained by filtering the UV light source through a UV filter set (11000v3, Chroma, VT, USA). The UV intensity is 280 mw/cm2 after passing the filter set. AUTOCAD 2011 was used to design the transparency photomasks. Photomasks were printed at a resolution of 25 000dpi (CAD/Art Services, OR, USA). For 50 µm thickness hydrogel structures, a two-inlet PDMS channel with 60 um channel depth and 300 um width were used for the polymerization. A rectangular mask with length 4 mm and width 300 um was placed in the UV path to form the mask-defined shape of layered structures. The chemical used are all purchased from Sigma-Aldrich unless otherwise stated. The pHresponsive stream contains 20% Acrylate Acid, 50% PEG-250, 26% PEG-DA 250 and 4% Darocur 1173 and the non-responsive stream contains 98% PEG-DA 250 and 2% Darocur 1173. The two prepolymer solutions flowing through the PDMS channel were stopped and subsequently exposed to UV light through the mask. A 10X objective was used and 300 ms UV exposure time was applied. During UV polymerization, the applied UV light polymerized the solution into the photomask defined shapes; while macropores were formed through UVpolymerization induced phase separation. The synthesized hydrogel structures were then flushed out by resuming the flow. This process can be repeated to continuously produce hydrogel structures without sticking to the walls of the channels owing to the oxygen inhibition layers near the channel walls. For 20 µm hydrogel film fabrication, a PDMS channel with 30 um channel depth and 300 um width was used and the composition of the two streams are the same as thick strips. Three-layer hydrogel structures were made through a three-inlet channel by co-flowing


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three streams of prepolymer solutions, each containing a pH-responsive or non-responsive compound. After fabrication, hydrogel structures were washed three times using ethanol to remove the unreacted monomers. Then the hydrogel structures were transferred to small dished to observe their shape transformation under the microscope in different pH conditions. We changed pH values of the solution by adding 0.5 mol/L of HCI or NaOH to an alkaline buffer (pH = 10.3, Sigma-Aldrich) in a dropwise manner. The pH of the solutions were measured by a benchtop pH meter (Eutech CyberScan 1500 pH/mV Benchtop Meter). Images were taken by a charge coupled device (CCD) camera (QImaing, Canada).


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Supporting Information Table S1 Figure S1-S5 Acknowledgment The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (Discovery grant no. 386092) for supporting this research.


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Figure 1. One-step fabrication of layered hydrogel structures with a large and rapid pH response. a) In a two-stream microfluidic channel, one stream contains acrylate acid, porogen, UV curable prepolymer, and photoinitiator (PI) to form the porous pH-responsive part and the other stream contains only UV curable prepolymer and PI for the synthesis of the nonporous pHnonresponsive part. The two solutions are co-flowed in a PDMS channel and are polymerized by UV light through a photomask as the flow is paused. After UV polymerization, the synthesized structure is flushed out of the channel by resuming the flow; b) Schematic of the synthesized two-stripe structure with the porous pH-responsive part and nonporous nonresponsive part; c) Schematic of the large deformation of the structure when exposed to a pH solution; d) Microscope image of a deformed structure. Scale bar is 100 µm.


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Figure 2. Bending angles of a two-stream structure when exposed to different pH values. Inset in the up-left corner shows the definition of the bending angle. The bending angle shows a linear increase with the increase of pH from 2 to 7. The inset in the bottom-right corner shows the bending angle change from pH 2 to 10. We measure the bending angles of the bilayered hydrogel by changing pH values of the solution in the range of 2 to 10 with five cycles. The bending angles are the average value of five measurements at each pH. Scale bar is 100 µm.


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Figure 3. Buckling of bilayer hydrogel films. The swollen structures buckle due to the instability of the films. The wavelength of the buckle is determined by λ=3.2w, where w is the width of the pH-responsive part. Scale bar is 100 µm.


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Figure 4. Various configurations of layered hydrogel structures with designed shape-formation responses to pH 8 buffer solutions. a) Bilayer structure with diagonal interface; b) Bilayer structure with partial diagonal interface; c) Bilayer structure with gradient of AA in the hydrogel stream; d) and e) are three-layered structures with different geometrical configurations; and f) the parallel three-layer structure becomes a ribbon-like stripe as the two hydrogel streams buckles under a pH 8 buffer solution. Scale bar is 100 µm.


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TOC entry. Keywords: Stimuli-responsive hydrogels, sensor, swelling, flow lithography Minggan Li, Dehi Joung, and Dae Kun Hwang Title:

Macroporous-Enabled Highly Deformable Layered Hydrogels with Designed pH Response ToC figure


Macroporous-Enabled Highly Deformable Layered Hydrogels with

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