Multiresponsive Reversible Deformation of Patterned Polyacrylamide

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Multi-responsive Reversible Deformation of Patterned Polyacrylamide Hydrogel Constructed by a Computer-Assisted Dispenser Xiao-Qin Zhang, Di Wu, Jun-Bo Hou, Jun-Feng Feng, Duo Ke, Sheng Zhang, and Bang-Jing Li ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00196 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Polymer Materials

Multi-responsive Reversible Deformation of Patterned Polyacrylamide Hydrogel Constructed by a Computer-Assisted Dispenser Xiao-Qin Zhang †, Di Wu †, Jun-Bo Hou †, Jun-Feng Feng ‡, Duo Ke ‡, Sheng Zhang*,†, Bang-Jing Li*,‡ †

State Key Laboratory of Polymer Materials Engineering (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu 610065, Sichuan, China

‡

Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, Sichuan, China

Corresponding authors: [email protected], [email protected] KEYWORDS: patterned hydrogel, stimuli responsive material, reversible transformation, morphing structure, programmed deformation

ABSTRACT

Materials that can undergo shape deformation have potential applications in biomedical field, yet it has always been a challenge to pattern hydrogels and accurately program

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target 3D shapes through a simple method. Here, we develop a facile method that permits fabrication of inhomogeneous hydrogels, which can reversibly deform from planar sheet to 3D shape in response to multi-stimuli, such as pH values, SLDC (solutions with lower dielectric constant), and SBF (simulated body fluid). By printing NaOH solution ink on classic PAAm hydrogel sheet, the amide side groups of PAAm hydrolyzed to acrylate. The swelling mismatch of the printed and unprinted areas generates internal stress to activate hydrogel deformation. Adjusting concentration of NaOH solution or number of times of printing, it is convenient to introduce in-plane and through-thickness gradient distribution to hydrogel sheet. The inkjet printing process is easily controlled by computer, enabling the direct printing of batched and complicated patterns. 3D structures, such as helix, tube and surface with constant Gaussian curvature can be achieved simply.

1. INTRODUCTION

Natural biological tissues are the best inspirations for constructing shape deformation materials, such as Venus flytraps, pods and pine cones that undergo morphological changes to capture preys or release seeds.1 Due to the wide application in various fields, like biological medicine, soft robot, and actuator, smart hydrogels capable of shape deformation have attracted increasing attention.2-13 The key is to obtain an appropriate structure that can deform to an accurate three-dimensional (3D) shape. So, it is significant to construct complex shape deformation by a simple method for adapting to diverse environments that may arise in the future. 2


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Under external stimuli, materials with inhomogeneous or asymmetric distribution cause the swelling mismatch, thereby forming internal stresses and inducing the shape deformation. How to prepare inhomogeneous materials is crucial for programable and deformable hydrogels. Generally, there are two main methods in constructing inhomogeneous materials: 1) multistep deposition; 2) multistep photolithography. Multistep deposition can be used to prepare bilayer or multilayer hydrogels, in which another kind of hydrogel is deposited on the surface of the preformed one, or monomer solutions containing different cross-linking components are sequentially polymerized in different layers. Then, the material with nonuniform distribution in the thickness direction is obtained. Due to the different properties of various layers of gels, the different swelling behavior of different layers results in targeted shape deformations. Early to 1995, Hu et al. prepared a bilayer inhomogeneous structure via multistep deposition polymerization of polyacrylamide (PAAm) and poly(N-isopropyl acrylamide) (PNIPAM), and the hydrogel sheets showed bending and folding deformations.14 Recently, some researchers have fabricated hydrogels with bending, twisting, and biomimetic helical shape deformations through the multistep deposition.15-17 However, to a large extent, the various shape transformations prepared by multistep deposition largely depend on the elaborate mold design and fabrication, every deformation need to reproduce different molds, especially for complex deformations. Noticeably, multistep deposition is a stepwise process and do not allow the continuous and batched production.

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The inhomogeneous materials can also be synthesized through multistep photolithography. Firstly, the surface of a monomer solution is covered by a patterned photomask, and the hydrogel is formed by photoinitiated polymerization. Secondly, the preformed hydrogel, soaked in another monomer solutions, is covered by a different photomask during the next photoinitiated polymerization. Finally, different hydrogel networks are formed in masked areas and unmasked areas. The nonuniform distribution structure in thickness direction and/or in plane of gels can be adjusted by changing the grayscale distribution of the masked patterns and/or by controlling the irradiation dose. Then, under external stimuli, pre-programmed shape deformations can be obtained owing to the uneven swelling/deswelling of patterned and un-patterned parts. By multistep photolithography, some groups have prepared surfaces with constant Gaussian curvature (spherical caps, saddles and cones), adjustable-parameter helical structures, and DNA sequence-directed shape transformations.

18-20

Although many

sophisticated shape changes can be realized in this way, the shape deformations strongly rely on the patterns of the photomask. To obtain various deformations, we have to re-prepare photomasks with different patterns. Moreover, a potential problem of detached layers might be caused by both methods. There are some other methods in preparing non-uniformly distributed materials. The researchers realized stimuli-responsive shape transformations by bonding different preformed hydrogels through hydrogen bonding or the supramolecular interactions, or by patterning hydrogels of polyelectrolyte using metal electrode.21-23 Similarly,

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separated polymerization and re-adhesion is a stepwise process and the electrical patterning depends on the shapes of metal electrode. Recently, Wang et al. prepared programmable complex shape deformation sheets by printing the ink of Fe3+ solution onto the surface of hydrogel composing of sodium polyacrylate, polyacrylamide and polyvinylpyrrolidone through ion inkjet printing.24 The facile inkjet printing process is easily controlled by computer. This technique paves the way for convenient continuous and batched fabrication that was not attainable in earlier works, however, the swelling/deswelling was triggered by solvent (water/ethanol), which was not convenient for application, especially in the biological system. In addition, continuous variation swelling is hard to produce since the polymer matrix composed of three kinds of polymers is not very homogeneous. As a result, it is hard to achieve complex 3D structures with constant Gaussian curvature. Here, we report a simple printing method that allows facile and batched fabrication of multi-stimuli triggered shape-changing hydrogels. The swelling behaviour of patterned hydrogels can be precisely adjusted by this method. Therefore, fine-tuning of hierarchical continuously swelling is easy to implement. Shape deformations can be occurred reversibly from planar sheets to complex 3D shapes with constant Gaussian curvature in response to multi-stimuli, such as pH, SLDC (solutions with lower dielectric constant) or SBF (simulated body fluid). It should be noted that this patterned hydrogel is particularly suitable for biomedical applications since the shape change can be activated by water/simulated body fluid. 5



2.

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Materials and Methods

2.1. Materials Acrylamide

(AAm),

potassium

persulfate

(KPS),

N,

N,

N',

N'-

tetramethylethylenediamine (TEMED), N, N'-methylenebis (acrylamide) and tris(hydroxymethyl)aminomethane were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). NaOH, HCl, MgCl2, K2HPO4·3H2O, NaHCO3, sodium sulfate anhydrous and NaCl supplied by Kelong Reagent Co., Ltd. (Chengdu, China). Buffer of mixed phosphate (pH=4), calcium chloride anhydrous, were supplied by Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). KCl was purchased from Ruijinte Reagent Co., Ltd. (Tianjin, China). All other reagents and solvents were of analytical grade and were used directly without further purification. 2.2. Preparation of polyacrylamide (PAAm) hydrogels

The hydrogels were prepared by copolymerization of a mixture of acrylamide (AAm)， N, N, N', N'-tetramethylethylenediamine (TEMED) with N, N'-methylenebis (acrylamide) (MBAAm) as cross-linker and potassium persulfate (KPS) as initiator in water. General procedure: AAm (1775.0 mg, 25 mmol), MBAAm (1.0 mg, 0.025 mol%) and KPS (6.8 mg, 0.1 mol%) were dissolved in H2O (5 mL). After purging with nitrogen for 5 min, it was cooled for 5 minutes at -5 ℃. Subsequently, TEMED (2.5 μL, 0.1 mol%) was added to the mixing solution and then the mixing solution was added into a mold consisting of two glass substrates separated with a 1 mm-thick silicone 6


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rubber spacer. In the next step, it was sealed at room temperature for 12 h. The asprepared hydrogels were directly used in this work. From Figure S2 and Table S1, it can be seen that the elongation at break and the breaking strength increase as the decrease of concentration of MBAAm. When the MBAAm concentration was 0.025 mol%, the elongation at break of the hydrogel was 2147%, and the break strength was 0.24 MPa, which was enough to be printed and maintained the stable shape after the shape deformation. 2.3. Preparation of SBF.25 The simulated body fluid (SBF, pH 7.2) was prepared by dissolving NaCl (7.996 g), NaHCO3 (0.350 g), KCl (0.224 g), K2HPO4·3H2O (0.228 g), MgCl2·6H2O (0.305 g), 1.0 mol dm−3 HCl (40 cm3), CaCl2 (0.278 g), Na2SO4 (0.071 g), tris(hydroxymethyl)aminomethane (6.057 g) in deionized water (1.0 L). 2.4. Printing on the hydrogel surfaces A dispenser (GJH-3A331) with a movable platform was purchased from Gaojinhao Science and Technology Ltd. (Guangdong, China), in which internal and external diameters of nozzle are 60 μm and 190 μm, respectively. NaOH aqueous solution is used as the ink, and the areal concentration of printing is 5.5 μL/cm2 (details shown in Table S2). An as-prepared hydrogel sheet is fixed on the printing platform to be printed. Then, different patterns, which is designed by a computer, are printed on one or both surfaces of the hydrogel sheets, and the printing process can be repeated for many times. 7



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Although there is a little amount of water loss in hydrogel during printing within tens of seconds, the influence on shape deformation can’t be found. After printing, the hydrogel is sealed by a mold consisting of two glass substrates separated with a 1 mmthick silicone rubber spacer to prevent water loss, and it is moved to a thermostat (30 ℃) for 70 min, because the hydrolysis of patterned hydrogels and the corresponding swelling ratio reach a constant after 70 min, as shown in Figure S5. Finally, the patterned hydrogel is cut and put into water to further deform for 3 hours (swelling equilibrium time, shown in Figure S4), when deformations of patterned hydrogel can keep stable. 2.5. The reversible deformation of patterned hydrogels The deformed patterned hydrogel is immersed into buffer (pH=4), SBF, 1 M NaCl solution, 65% (vol %) ethanol aqueous solution and 65% (vol %) isopropanol aqueous solution for 6 hours, respectively, to diminish the swelling mismatch to recover to its original flat shape. Then, the deformed hydrogel was taken out and put in water for shape deformation again. 2.6. Characterization Fourier transform infrared (FT-IR) spectra (KBr method) were recorded on a Varian Scimitar1000 Fourier transform IR. Tensile strength measurements were carried out on an INSTRON 5567 universal tensile tester at room temperature, using a rectangle sample (thickness of 1 mm) with the gauge length of 20 mm and a strain rate of 40 8


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mm·min−1. To quantitatively analyze the effect of dispenser patterning on the swelling properties of PAAm hydrogel, the thickness of all samples in the experiment is of 1 mm unless otherwise stated. Circular hydrogel sheets with diameter of 12 mm were printed on one surface, and then tested for swelling ratio of these sheets in equilibrated state. Swelling ratio in diameter, S, of the printing and no printing circular hydrogel sheets, S was measured by S = d/d0, in which d and d0 were the diameter of a circular hydrogel sheet in equilibrated state (swelling in water for 3 hours, swelling equilibrium time, as shown in Figure S4) and as-prepared state, respectively. Supporting videos and photos were taken by Nikon, D7100. The measurements of carbon-nitrogen elementary analysis were carried out on a Flash EA 1112 Elemental Analyzer using sulfanilamide as standard. The hydrolysis of printed hydrogels was measured by means of a previous report.26 The treatment of the instrumental data was described in equation (1). The printed hydrogels which were known to contain NH4+ ion was pretreated in order to obtain accurate results. The sample was printed first with NaOH solution at 30 ℃ and the hydrolysis of PAAm was terminated by immersing it in water. The hydrogel sample freeze-dried under vacuum for removing water and NH3. Hydrogels can’t be heated in the presence of strong bases or acids otherwise further hydrolysis of the polymer would have taken place. hydrolysis% = [1 - (Nt / Ct) / (No / Co)] x 100% (1)

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In which, Co and No were the carbon content and nitrogen content (wt %) of as-prepared hydrogels respectively, and Ct and Nt were the carbon content and nitrogen content (wt %) of printed hydrogels respectively. 3. RESULTS AND DISCUSSION 3.1 The computer-assisted dispenser and the hydrolysis of polyacrylamide A commercial flatbed inkjet dispenser, made up with a printer and a movable platform, was used in present work to print patterns on the hydrogel surfaces (Figure S1 and Video S1). PAAm, a well-known biocompatible polymer, was selected as the polymer matrix and prepared by a classic method.27-28 NaOH aqueous solution was used as the ink and the inkjet printing process was controlled by computer. As shown in Figure 1, the amide side groups in PAAm polymer chains hydrolyzed to acrylate groups when the NaOH ink was printed on surfaces of PAAm sheets. The hydrolysis reaction of PAAm related to the nucleophilic addition of a hydroxy ion (OH-) to the carbonyl (C=O) of the acrylamide and elimination of the amine (NH3).29 The FT-IR spectrum (Figure S3) showed that the stretching vibration peak of the carbonyl group (C=O) in the amide group (-CONH2) appeared at about 1670 cm-1, and the stretching vibration of the carbonyl group (C=O) in the anionic carboxyl group (-COO-) appeared at 1560 cm-1, indicating that PAAm partially hydrolyzed to form PAA-Na after printing.30 The PAANa is more hydrophilic than PAAm and also biocompatible, and then during swelling in water the swelling mismatch of the printed and unprinted areas of the hydrogels can generate stresses to activate hydrogel deformations.31 10


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Figure 1. The schematic of inhomogeneous distribution introduced to hydrogel sheets by the hydrolysis of patterned polyacrylamide (PAAm). 3.2 Controllable introduction of in-plane and through-thickness gradient distribution in hydrogel

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Figure. 2 The diffusion measurement of NaOH ink in plane of the patterned hydrogel: a) A schematic of partially printed hydrogel. b) Partially printed hydrogel. c) Partially printed hydrogel after swelling in water. d) The hydrolysis of partially printed hydrogel in different distance to the boundary of printing. All scale bars are 1 cm. Error bars: s. d. of three independent measurements. To quantitatively determine the amount of hydrolysis of patterned hydrogels made by the dispenser printing and give a breakdown of the swelling properties of these hydrogels, all samples in the experiment are of 1 mm in thickness unless otherwise stated. It can be seen from Figure 2 that during the printing, the in-plane diffusion of ink on the hydrogel surface caused its hydrolysis (2.25%) at distance of 1 mm near the printing boundary, and the hydrolysis decreased with the distance and disappeared after 2 mm, forming a flexible transition at the boundary. The results suggest that the hydrolysis of printed hydrogel reacted near the boundary and would not appear in whole hydrogel. As shown in Figure 3a, the results demonstrated that the swelling ratio gradually increased from 252% to 341% with increasing concentration of NaOH aqueous solution from 1 to 5 mol/L, because the corresponding hydrolysis of printed PAAm hydrogels increased from 2.22% to 7.77%. In Figure 3b, when the number of printing times increased from 1 to 3 (using 4 M NaOH aq. as ink), the hydrolysis of PAAm hydrogels increased from 7.07% to 11.76% and the swelling ratio increased from 324% to 387%. There is no significant difference among the 1st, 2nd and 3rd printing times mainly due to the self-retarding effect of PAAm. That is, the rate of

12


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hydrolysis is remarkably retarded as a result of the increased degree of hydrolysis.30 After printing 1 times, the self-retarding effect of PAAm become obvious because the amount of NaOH increase dramatically. When rectangular hydrogel sheets were printed with a stripe for different number of printing times, the bending angle of these hydrogels grew from 397o to 498o (details shown in Figure S6). It reveals that the swelling ratios of hydrogel and the degree of deformation can be adjusted by concentration of NaOH ink and the number of printing times.

Figure 3. a) The hydrolysis and the corresponding swelling ratio of patterned hydrogel using NaOH aqueous solution ink with different concentration. b) The hydrolysis and hydrogel swelling ratio of different number of printing times and the corresponding bending angle of rectangular hydrogel sheets printed with stripe. c) The deformation schematic of circle hydrogel printed on one surface. Swelling ratio in diameter, S, of the unprinted and printed circular hydrogel sheets. S was measured by S = d/d0, in which 13



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d and d0 are the diameter of a circular hydrogel sheet in equilibrated and as-prepared states, respectively. Error bars: s. d. of three independent measurements. The scale bar is 1 cm.

Table 1. The element content of top printing surfaces and bottom noprinting surfaces of hydrogels. All results were obtained from average value of three independent measurements.

Original

Top printing

Bottom noprinting

surfaces

surfaces

surfaces

C(wt%)

41.7

39.3

38.9

N(wt%)

16.0

14.3

14.5

0

5.1

2.8

Element content

The degree of hydrolysis (%)

Comparing to multistep photolithography, this method is very convenient and provides versatile surface printing of batched patterns on a large-sized hydrogel sheet (30 cm x 30 cm). Furthermore, the gradient structure can be incorporated into the thick direction by controlling the inkjet process, which allows the control of deforming direction. It is known that a hydrogel with in-plane gradient distributions has equal access to deform upward or downward since there is no gradient distributions in its thickness direction.3214


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34

To study the hydrolysis of PAAm hydrogels at thickness direction, gel samples

printed with NaOH solution (4 mol/L) was freeze-dried for elemental analysis in its cross-section. The results suggested that the hydrolysis of top printing surface (5.1%) of hydrogels was higher than that of the bottom no printing surface (2.8%) (Table 1). This is due to a gradual decrease in the concentration of NaOH solution as it diffuse from the upper printing surface to the bottom surface. As a result, the top printing surface of hydrogels had a higher swelling ratio than that of the bottom no printing one, and then the nonuniform distribution in the thickness direction led to a dome shape pointed to the direction of printing (Figure 3c). It reveals that in-plane and throughthickness gradient structure can be easily and controllably introduced into patterned hydrogels by this method. Therefore, both the swelling ratio and the degree of deformation can be adjusted, and then designable and programmable complex shape deformations can be allowed. Additionally, printed gels are transparent and colorless, it can be dyed in required colors. 3.3 The patterning of hydrogel and programing of shape deformations Up to now, versatile strategies have been developed to realize programmable deformations. For example, Nie et al. introduced responsive fibrous-like components in planar sheets to obtain various helical shapes by controlling the oblique angle of fibrous-like components.19 Wu group demonstrated that many complex 3D deformations could be realized by patterning site-specific dome-like structures on both sides of hydrogel sheets.35 Santangelo and co-workers obtained very complex shapes 15



with constant Gaussian curvature

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through fabricating continuous gradient swelling

fields in the planar sheet.18 Each of these strategies needed a specific preparation to get the prescribed structure. However, by this method, the fibrous-like components, sitespecific dome-like building blocks, and continuous gradient swelling fields can be easily printed in the hydrogel sheets. Figure 4a and 4b are hydrogel sheets patterned with separated vertical stripes and stripes that are at 45° to the long edge direction, respectively. The dark fibrous-like regions are printed by NaOH ink (4 mol/L) and show high-swelling due to the hydrolysis of PAAm. Figure 4d, e and Figure S7a show the patterned hydrogels printed on both surfaces using series of dome-like or bending structures as building blocks. The dome-like or dark stripe regions contain highswelling PAA-Na caused by printing. Particularly, Figure 4c and 4f are patterned hydrogels with gradient distributions, in which 0~4 means the concentration (0~4 mol/L) of NaOH ink.

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Figure 4. The designs of patterned hydrogel surfaces by dispenser. a) and b) The hydrogel sheets patterned with separated vertical lines and lines at 45° along the long edge direction on only one surface. d) and e) The hydrogels patterned on both surfaces for letter “SCU” and a lotus. c) and f) Patterned hydrogels with gradient distribution, in which 0~4 represent the different concentration of NaOH ink (0~4 mol/L). All scale bars are 1 cm. Different swelling ratios of the unprinted and printed parts lead to the deformation of the hydrogels. As shown in Figure 5, when the patterned hydrogel swelling free in water, the hydrogel sheets printed with patterns in Figure 4a, b, d, e deform into a tube, a righthanded helix (Video S2), letter “SCU” and a lotus (Video S3), respectively. Remarkably, when the NaOH ink of different concentrations is used to constructed the gradient distribution of hydrogel, the swelling ratios of hydrogel gradually decrease or increase along the radius (Figure 4c, f). And then a dome with a positive Gaussian curvature and a saddle with a negative Gaussian curvature are obtained (Figure 5c, f). Besides, there are some other designed patterns and the corresponding 3D shapes in Figure S7. Notice that by controlling number of printing times to fabricate a gradient structure, the dome can also be achieved (Figure S7f).

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Figure 5. The complex 3D shapes deformed from 2D patterned hydrogel sheets. a) a tube. b) a right-handed helix. c) a dome. d) letter “SCU”. e) a lotus. f) a saddle. All scale bars are 1 cm. 3.4 Triple-responsive reversible deformation of patterned hydrogel Many smart hydrogels with inhomogeneous structure reported previously show designable and controllable deformations in response to external stimuli, such as temperature, pH, light, special ions, or solvent.2,3,5,8-11 In present work, the deformation was reversible in response to not only pH, SLDC, but also SBF. Firstly, in pH=4, the swelling ratio of the printing hydrogels dramatically decreased from 327% to 160%, which was very similar to that of no printing hydrogels (163%) (Figure 6a). This great volume phase transition of patterned gels is since that the ionization equilibrium constant (pKa) of poly acrylic acid is approximately 4.2. When pH

Multiresponsive Reversible Deformation of Patterned Polyacrylamide

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