Dynamic Creation of 3D Hydrogel Architectures via Selective Swelling

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Applications of Polymer, Composite, and Coating Materials

Dynamic Creation of 3D Hydrogel Architecture via Selective Swelling Programmed by Interfacial Bonding Riku Takahashi, Hiroki Miyazako, Aya Tanaka, and Yuko Ueno ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05552 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Dynamic Creation of 3D Hydrogel Architecture via Selective Swelling Programmed by Interfacial Bonding Riku Takahashi*, Hiroki Miyazako, Aya Tanaka & Yuko Ueno NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato -Wakamiya, Atsugi, Kanagawa, 243-0198, Japan. E-mail: [email protected] Keywords: Hydrogels, 3D architecture, Swelling, Adhesion, Lithography

Abstract The topological features of material surfaces are crucial to the emergence of functions based on characteristic architectures. Among them, the combination of surface architectures and soft materials, which are highly deformable and flexible, has great potential as regards developing functional materials towards providing/enhancing advanced functions such as switchability and variability. Therefore, a simple yet versatile method for creating 3D architectures based on soft materials is strongly required.

In this study, hydrogels are selected as the soft materials and

hydrogel film/rigid substrate layer composites are fabricated to obtain a 3D hydrogel architecture based on swelling instability. When a hydrogel film weakly attached to a rigid substrate is exposed to water, swelling-driven compressive stress induces buckle-delamination of the film from the substrate. By utilizing the chemical modification of a rigid substrate and a conventional photolithography technique, the delamination location is successfully controlled, resulting in a highaspect-ratio folding architecture at an arbitrary position. In addition, we systematically designed the delamination geometry and chemically tuned the swelling ratio of the hydrogel, leading to the discovery of several new morphology transitions and relationships between the morphologies and the controllable parameters. This work provides a new approach to fabricating highly programmable 3D architectures of soft materials.

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1. Introduction In nature, the topological features of material surfaces are extremely important as regards the emergence of functions such as the low friction of shark skins,1-3 nature of lotus leaves,4-6

the highly water-repellent

and the structural colors of Morpho butterflies.7-9

Although a vast

number of methods have been developed to mimic their functions by utilizing characteristic architectures,10-14

the next goal is to find a way to create such architectures on “soft materials”,

which have highly deformable and flexible properties. This is because the deformable architecture provides/enhances advanced functions such as those found in nature.15-18

One example of a soft

functional architecture is the clusters of suckers on the tentacles of an octopus, which exhibit strong, reversible, highly repeatable adhesion under wet/dry conditions based on suction force generated by the deformation of soft architectures.15,16

Another example is cactus spines, which can collect

water by fog-harvesting, and the water-capture efficiency is enhanced by moving the architecture.17,18

Thus, the combination of specific architectures and soft materials offer great

potential for developing functional materials not only inspired by but also moving beyond nature. From the viewpoint of practical applications, a simple yet programmable method for creating soft 3D architectures is definitely required. With the aim of fabricating soft 3D architectures simply, many researchers have devoted considerable effort to investigating the mechanical instabilities of thin layers compressed on substrates as a convenient and low-cost method.19-22 One of the most investigated systems is a solid substrate covered with polymer gel, which is classified as a soft and wet material.

When a

surface-attached polymer gel is immersed in an appropriate solvent, swelling generates a compressive stress within the polymer gel, resulting in instability patterns including wrinkles, creases, folds, ridges and buckle-delaminations.23-26

Among them, buckle-delamination is very


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attractive because it enables the creation of architectures with a higher aspect ratio (more than 1) with a hollow structure than the wrinkle or crease patterns (less than 1), and it is useful for tuning cell arrangements, fluidic devices and optical transmittance.26-28 Furthermore, extensive approaches were reported for inducing buckle-delamination with an external stimulus.29,30 Remarkably, Velanker et al. reported swelling-induced buckle-delamination that created high-aspect-ratio folding (roughly ~ 5). exposed it to solvent for swelling.

31

They fabricated a PDMS/glass layer composite and

During the swelling process, the biaxial compressive stress

reached a critical value, resulting in the partial delamination of the PDMS film at random positions and the creation of a buckling architecture to relieve compressive stress.

Furthermore, Xu et al.

successfully demonstrated how to control the position at which the architecture formed by utilizing electrochemistry.32 They fabricated a hydrogel/electrode layer composite and induced a chemical reaction on the electrode that enabled the delamination of the hydrogel layer artificially.

By

applying the stimuli-responsive hydrogel, dynamic/reversible architecture was also demonstrated. To expand this concept, it is potentially important to systematically reveal the relationships between the morphology of buckle-delamination and the controllable parameters. Here, we demonstrate a simple yet universal method for tuning the morphology of buckledelamination of a hydrogel at an arbitrary position on hydrogel/solid substrate layer composites. To programmably induce swelling instability, which leads to buckle-delamination, the interfacial bonding of the layer composites is chemically controlled by using silanization and a conventional lithography technique.33-37

The hydrogel on the substrate, where the interfacial adhesiveness has

been eliminated, delaminates spontaneously and transforms from flat 2D layer shape to a folding 3D architecture (Figure. 1) during swelling.

We systematically control the initial conditions (e.g.

hydrogel composition, hydrogel geometry and the patterns of surface bonding) to tune the 3D


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architectures.

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This method not only allows us to design complicated 3D hydrogel architectures

with a high aspect ratio (roughly ~ 1) at a desired position with high reproducibility, but also to create several new morphology transitions. Finally, taking advantage of the biocompatibility of hydrogels,38-41 we further demonstrate the use of the architecture based on buckle-delamination for 3D cell culture. This work substantially expands the parameter-space for creating 3D hydrogel architecture based on buckle-delamination and shows the potential use of such architectures.

Figure 1. Schematic illustration of a 3D hydrogel architecture formed via selective swelling controlled by interfacial bonding. (a,b) A cleaned coverslip is modified with 3-(trimethoxysilyl) propylmethacrylate (TMSPMA) to promote the covalent anchoring of the hydrogel layer. (c,d,e) The desired silanized surface pattern is designed by utilizing a photolithographic technique and oxygen plasma etching. (f,g) Thin hydrogel film is synthesized on the substrate, which provides partial bonding. The reaction cell is assembled with a coverslip and a spacer, and then a pre-gel solution is loaded in the cell and UV polymerization performed for 600 sec. (h) Due to the different swelling behaviors of the hydrogel between the bonded region and the free region, a 3D hydrogel architecture is formed spontaneously during the swelling process.


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2. Experimental Section Materials: Unless otherwise specified, the chemicals employed in this study were used as received without further purification.

Acrylamide (AAm), as a monomer for AAm-based gels, was

purchased from Tokyo Kasei Co., Ltd.

Fluorescein o-acrylate (FL), as a fluorescent monomer

for AAm-based gels was purchased from Sigma-Aldrich.

N,N’-methylenebis [acrylamide]

(MBAA), as a cross-linker for AAm-based gels, and lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate (LAP), as a UV initiator for the polymerization, were purchased from Tokyo Kasei Co., Ltd..

Dimethyl sulfoxide (DMSO) was purchased from Wako Pure Chemical Industries and

used as the solvent for the FL and NIPAM monomer solution.

For covalently anchoring

hydrogels to substrates, 3-(trimethoxysilyl)propyl methacrylate (TMSPMA) was purchased from Sigma-Aldrich.

The sodium chloride used to change the ionic strength of the solvent was

purchased from Kanto Chemical Co., Inc. purchased from Kanto Chemical Co., Inc.

To clean the substrates, sodium hydroxide was

For the surface modification of the hydrogels to attach

cells, sulfosuccinimidyl 6-(4'-azido-2'-nitrophenylamino) hexanoate (sulfo-SANPAH), as a heterobifunctional cross-linker, and type I collagen, as an extracellular matrix (ECM), were purchased from Sigma-Aldrich.

For cell culture, C2C12 cells were purchased from ATCC.

DMEM with high glucose and fetal bovine serum were purchased from Thermo Fisher Scientific. Penicillin-streptomycin and trypsin-EDTA solution were purchased from VWR.

For cell

visualization, paraform aldehyde and TritonX-100 were purchased from Sigma-Aldrich. PBS(-) solution and phalloidin rhodamine were purchased from Thermo Fisher Scientific.

Preparation of pattern silanized substrates: The surface of the substrates (No. 1, 18×18 mm coverslip, purchased from Matsunami Glass Ind., Ltd.) were chemically modified with TMSPMA


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as described previously.41

Briefly, the substrates were immersed in 0.1M NaOH solution for 24

hours to clean their surfaces.

Then, the substrates were thoroughly rinsed with deionized water

and completely dried with nitrogen. 10 min.

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he cleaned substrates were treated with oxygen plasma for

After that, silanization was carried out by immersing the substrates for 2 hours at room

temperature in freshly prepared 1vol% TMSPMA solution (160 ml EtOH, 36 ml deionized water, 4 ml of acetic acid and 2 ml of TMSPMA). dried with nitrogen.

The substrates were then rinsed with the EtOH, and

They were subsequently heated in a vacuum oven (110℃, 30 min) to

complete the dehydration. To fabricate a silanization pattern on the substrate, we adopted conventional photolithography and an etching method. 33-37 A positive resist (S-1813, Shipley) was spin coated on every substrate (4000 rpm, 40 s) and pre-baked on a hotplate for 2 min at 90 ℃.

The

photoresist covered substrates were exposed to a patterned UV light source (40%, 1 s, NEOARK Corporation) and developed (40 s, 351, Shipley). (the data were drawn by Draftsight 2018).

The UV light pattern was designed by 2D CAD

In this study, we focused on a simple stripe pattern

with a fixed length (10 mm) and various widths (200 – 1000 um).

Development was stopped by

immersion in deionized water for 30 seconds, and the substrates were post-baked on a hotplate for 5 min at 90 ℃.

Then, the substrates that were partially covered with photoresist were introduced

into the vacuum chamber of a plasma etcher (CV-e300, Plasmec.).

A 30 s etching process (30

mbar, 30 mL/min in O2 flow rate, 100 W) removed the TMSPMA that was not protected by the photoresist layer.

The photoresist was then removed by soaking in acetone for 1 min and the

substrate was dried completely with nitrogen.


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Synthesis of hydrogels on pattern silanized substrates: To prepare the reaction cell, 4-mm wide PET tape (thickness 30 μm, purchased from NITTO DENKO Corporation) and 4-mm wide Scotch tape (thicknesses: 60 μm, 120 μm, purchased from 3M) were attached on each side of the pattern silanized substrate.

For the PAAm hydrogel, we prepared deionized water containing 4 M AAm

monomer, 0.01-1.0 mol% MBAA as a chemical cross-linker and 0.1 mol% LAP as an initiator (all relative to the monomer).

For the FL-labeled PAAm hydrogel, 12.5 μl of FL monomer solution

(1wt% in DMSO) was added to 250 μl of the AAm pre-gel solution. 20 μl of pre-gel solution was dropped onto the reaction cell, and then a clean coverslip was placed on it as shown in Figure. 1f. Photo-induced free radical polymerization was carried out with a UV lamp for 10 min (UV light intensity was 10 mW/cm2).

After that, the coverslip was

gently removed and the surface with the attached hydrogel was immersed in a large amount of deionized water for 24 hours to allow the gel to reach equilibrium.

At different swelling times,

the swelling-driven 3D hydrogel architecture was photographed from above using a phase-contrast microscope.

Confocal fluorescence microscopy: To visualize the 3D hydrogel architecture under a confocal fluorescence microscope, network backbone.

PAAm based FL copolymer (FL-co-PAAm) was used as the hydrogel

The structure was then imaged using a FLUOVIEW 1200 (Olympus)

confocal laser scanning microscope.

Z-stack images for a slide thickness of ~10 μm were imaged

and a 3D reconstruction image was created using Image J.

Image J was also used to measure the

dimensions of the 3D hydrogel architectures, and quantification was performed based on images collected from three separate experiments.


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Cell culture and fluorescence dying: The C2C12 cells were cultured in DMEM (high glucose), which contained 10% fetal bovine serum and 1% penicillin-streptomycin. incubated at 37℃ in 5% CO2.

The cells were

0.25% trypsin-EDTA solution was used for cell detachment.

The

number of cells in the cell suspension was counted by an automated cell counter (BIO-RAD, TC10), and the concentration of the cell suspension was adjusted to 2x105 cells /mL. Before seeding cells into the gel channels, type I collagen was immobilized on the surface of the gel channels as an extracellular matrix (ECM) by using sulfo-SANPAH.

The sulfo-

SANPAH solution was injected into the gel channel, then 365 nm UV light was irradiated for 10 min to react chemically with the gel surface.

Subsequently, type I collagen solution was supplied

into the gel channel, which was incubated at 4℃ overnight.

After that, the gel channels were

immersed in the culture medium and incubated at 37℃ in 5% CO2 for 1 h.

Then, 20 μL of the

cell suspension was injected using a micropipette, and the injected gel channels were placed on a 35 mm glass bottom dish (MATSUNAMI, D11140H) with the culture medium.

The

proliferation and migration of the cells inside the gel channel were observed at 1 h intervals with an inverted phase contrast microscope (BZ-X710, Keyence), which was equipped with a 20x lens (CFI 60 Series, Nikon), a metal halide lamp, and a chamber with a controller to maintain a temperature of 37℃ and a 5% CO2 concentration (KIW (S/N 160828), TOKAI HIT). The cells in the gel channels were fixed 5 days after seeding.

The cells were incubated

in 4% paraform aldehyde water solution for 30 min at room temperature, and rinsed with PBS(-) solution three times.

Then, the cells were incubated in a 0.1% TritonX-100 water solution for 10

min at room temperature, and then rinsed with PBS(-) solution three times.

Finally, the cells

were incubated in phalloidin rhodamine solution (0.5% in PBS(-)) for 2 h at room temperature under a light-shielded condition, and then rinsed three times with PBS solution.


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3. Results & Discussion Fabrication of 3D hydrogel architectures via selective swelling We fabricated 3D hydrogel architectures in three steps. In the first step, we prepared a pattern silanized substrate possessing selective adhesiveness (Figure. 1a-e).

Cleaned coverslips were

modified with a functional silane, 3-(trimethoxysilyl) propyl methacrylate (TMSPMA) as shown in Figure. S1a.

Subsequently, TMSPMA was partially removed by utilizing the conventional

photolithographic technique and oxygen plasma etching.33-37

In this study, we designed a simple

rectangular pattern (10 mm long with various widths (200 – 1000 um)) for the non-adhesive region. In the second step, polyacrylamide (PAAm) cross-linked by N,N’-methylenebis (acrylamide) (MBAA) was synthesized on the TMSPMA-patterned substrate with a 60 μm thick spacer as a model hydrogel (Figure. 1f and S1b).

The PAAm gel on the TMSPMA modified

surface was covalently anchored to the substrate during synthesis.

The strength of the interfacial

adhesiveness between the hydrogel and the substrate is sufficient to prevent delamination even after the hydrogel has swollen (Figure. S2a).

On the other hand, the PAAm gel on the surface

with the TMSPMA removed did not adhere to the substrate (Figure. S2b), resulting in a freestanding hydrogel layer.

Therefore, the confined and free-standing regions of the surface-

attached hydrogel can be designed by controlling the interfacial adhesiveness (Figure. 1g). In the third step, the surface-attached hydrogel with patterned bonding was immersed in deionized water to allow the gel to reach an equilibrium swelling state.

Due to the difference

between the boundary conditions of the confined region and the free-standing region, the swelling behavior is divided into two types (see Figure. S3 for schematic illustrations of different swelling behaviors); one is confined swelling, which is only able to increase the thickness of a gel when anchored to the substrate,23,24

and the other is side-fixed swelling, which can swell in the


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thickness direction without any restriction from the substrate but cannot expand to the fixed region. As a result, side-fixed swelling induces the buckling deformation of the hydrogel layer, resulting in the dynamic creation of 3D hydrogel architectures on the substrate (Figure. 1h). To investigate the formation of 3D hydrogel architectures, we used phase-contrast microscopy to observe the swelling process of the surface-attached hydrogel with patterned bonding (pattern size: 10 mm length and 600 μm width) (Figure. 2a).

At an early stage of

swelling (~45 s), rapid buckling, which has a straight-sided blister-like shape32, was observed (see Supplementary Movie 1).

As the swelling time increased (~170 s), a telephone-cord like

buckling and architecture growth were observed.

After ~600 s swelling, the hydrogel film

reached an equilibrium state and the wavelength, λ, and amplitude, A, were measured and found to be ~ 700 μm and ~ 300 μm, respectively.

We consider that the telephone-cord buckling

architecture was induced by the coupling of X- and Y-axis buckling. mechanism of architecture forming as shown Figure. 2b.

Herein, we illustrate

The partially surface-attached hydrogel

film has a smooth flat shape before swelling (Figure. 2b-i).

When the sample was immersed in

deionized water, the hydrogel wants to swell, thus inducing a high swelling pressure.

To focus

on the free-standing region of the hydrogel, swelling along the X-axis is particularly restricted due to the existence of the surface-attached region (Figure. 2b-ii).

Therefore, the swelling pressure

on the X-axis quickly reached a critical buckling stress, resulting in rapid buckling deformation to relieve this stress.

At that time, negative pressure was generated under the buckling deformed

gel as previously reported.31,32 To relieve this pressure, water was induced to diffuse through the gel and flow from an open edge.

Subsequently, the buckling part swells further because of the

detachment from the substrate, which reduces the swelling restriction, thus inducing Y-axis buckling and architecture growth (Figure. 2b-iii).

Finally, we could observe a buckling repeated


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architecture (high aspect-ratio fold with telephone-cord buckling) in an equilibrium state, similar to previous reports of swelling-induced delamination and folding.31,32

We should note that the

architecture is highly reproducible even after several dehydration/hydration cycles (see Figure S4 and Supplementary Movie 2). Such telephone-cord buckling has been widely observed in many compressed thin filmsubstrate systems for several decades.42-44 Thin films are normally stressed, with biaxial residual stress residing in unpassivated films. This stress is relieved by partial delamination from the substrate, resulting in 3D architecture formation.


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Figure 2. Observation of 3D hydrogel architecture formation. (a) Phase-contrast image shots of the 3D architecture during swelling in deionized water. After the appearance of rapid buckling (~ 45 s), telephone-cord buckling and architecture growth were observed (~ 170 s). Finally, the architecture reached an equilibrium state (~ 600 s) and the characteristic sizes (wavelength, λ and amplitude, A) were measured. (b) Schematic illustration of the mechanism of 3D hydrogel architecture formation. (i) Partially surface-attached flat hydrogel film was fabricated and immersed in deionized water. The telephone-cord buckling architecture was induced by two coupled buckling: (ii) X-axis buckling and (iii) Y-axis buckling. At the early stage of swelling, the swelling pressure along with X-axis reached a critical buckling stress, resulting in X-axis buckling deformation with water diffusing through the gel and flowing from an open edge. After that, the buckling part swelled further due to detachment from the substrate, showing Y-axis buckling and its growth. Detailed observation of 3D hydrogel architecture through confocal fluoresce microscopy To obtain more detailed information about the 3D hydrogel architecture, we performed confocal fluorescence microscopy, which can visualize 3D architecture via reconstruction from Z-stack images.

To observe a fluorescent image, AAm and a small amount of fluorescent monomer,

Fluorescein o-acrylate (FL), were copolymerized (P(FL-co-AAm)) on a pattern silanized substrate (non-adhesive pattern size: 10 mm long and 600 μm wide) and used instead of pristine PAAm gel. We then immersed the sample in deionized water and the 3D architecture formed.

The samples

were then placed in a water chamber to prevent dehydration during observation (Figure. 3a). As shown in Figure. 3b and Supplementary Movie 3, a reconstructed 3D confocal image was successfully obtained. From the X-Z plane sectional view (Figure. 3c) and the X-Y plane sectional view obtained at the upper surface of surface-attached hydrogel (Figure. 3d), we found that the telephone-cord buckling architecture has a continuous hollow structure.

In addition to this, we

could observe a slight difference in thickness between the hydrogel on the adhesive region (111.6±5.4 μm) and that on the non-adhesive region (76.9±3.7 μm).

This result coincides well

with the swelling behavior differences described above (Figure. S3).


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We should note that the existence of FL in the PAAm polymer backbone (about 0.05wt% for an as-prepared sample) does not affect the swelling ratio, mechanical properties (see Supplementary Figure. S5) or forming 3D hydrogel architectures.

Therefore, we used the P(FL-

co-AAm) hydrogel as a model hydrogel layer for the rest of our experiments, unless otherwise specified.

Figure 3. Observation of 3D hydrogel architecture using confocal laser scanning microscope. (a) To visualize the 3D architecture of the hydrogel, a small amount of fluorescein o-acrylate (FL) and acrylamide (AAm) were co-polymerized with cross-linker. The resultant hydrogel also formed a 3D architecture via selective swelling. To prevent drying during observation, a water chamber was assembled by using a silicone spacer and a coverslip. (b) Reconstructed 3D confocal image of FL-labeled hydrogel. (c) X-Z plane sectional view. (White square in (b)) (d) X-Y plane sectional view at the upper surface of confined gel. (Dashed line in (c)). Even in the absence of FL, we observed exactly the same mechanical properties and swelling behavior.


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Effects of hydrogel cross-linking density and non-adhesive geometry on 3D architectures The equilibrated 3D hydrogel architectures had a highly ordered periodic morphology, considering that the architecture is an instability pattern that is determined by the initial conditions such as gel composition, hydrogel layer thickness and the geometry of the non-adhesive region of the substrate. To control the 3D hydrogel architecture, we tuned two parameters of P(FL-co-AAm) hydrogel namely the cross-linking density from 0.01 mol% to 1.0 mol% and the non-adhesive stripe width (W) from 200 um to 1000 μm.

To compare the architectures, we observed each sample with an

X-Y plane sectional view at the upper surface of the confined gel, which showed the characteristic feature of the architectures (Y-axis buckling), as shown in Figure. 4a. As a result, we found various types of architectures and classified the shapes of these architectures into five categories (Figure. 4b).

(i) period-double channel – there is some space between two side walls, which have a

pattern with twice the wavelength, (ii) fold channel – the side wall pattern changes into selfcontacting sharp tips, (iii) wrinkle channel – the side wall undulates sinusoidally without sharp tips, (iv) fold ridge – there is no space between two walls that have periodic self-contacting sharp tips, and (v) straight ridge – the side wall shape becomes smooth and straight. We should note that the main difference between (i, ii, iii) channel architectures and (iv, v) ridge architectures is whether the X-Z cross-section of the delaminated region forms self-contacting or smooth.

In

addition to this, when the cross-linking density is less than 0.1 mol%, surface creasing can be found on the confined gel as previously reported,23,24 however, the crease pattern is not polygonal but oriented perpendicular to the non-adhesive stripe region.

Although this anisotropic crease

formation is interesting, we focus on 3D hydrogel architecture in this study.

The detailed

relationship between creasing and selective swelling will be reported in a forthcoming paper.


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We measured quantitatively the architecture dimensions including height, H, amplitude, A and wavelength, λ (Figure. 4c). which are defined in Figure. 4b.

The colors of the bars represent the types of architecture,

As regards H, which is related to the X-axis buckling as shown

in Figure. 2b-ii, the values increased monotonically with increases in non-adhesive width or decreases in cross-linking density (Figure. 4c-i).

With A and λ, which are related to the Y-axis

buckling, the values were roughly controlled by controlling the non-adhesive width (Figure. 4cii,iii).

However, the value is highly dependent on the type of instability pattern.

Therefore, we

should consider which parameter determined the various types of architectures.


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Figure 4. Tuning of 3D hydrogel architecture with various parameters. (a) Images of X-Y plane sectional view with various cross-linking densities (x-axis) and free-standing stripe widths (y-axis).


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The appearance of the 3D hydrogel architectures was classified into 5 categories from the images; (i) period-double channel, (ii) fold channel, (iii) wrinkle channel, (iv) fold ridge and (v) straight ridge. Each inset color square in the lower left corner reveals the type of structures; (i) red, (ii) orange, (iii) yellow, (iv) green and (v) blue. (b) Schematic illustration of the 5 categories of 3D hydrogel architecture. (c) Size of (i) height, (ii) amplitude and (iii) wavelength of the 3D hydrogel architectures with various cross-linking densities and non-adhesive widths for the hydrogel layer. The error bars are standard deviations obtained from the results of 3-5 samples, and were smaller than the symbol unless otherwise stated.

We define two parameters when drawing a phase diagram of 3D hydrogel structures. One is the size ratio, which is determined by the non-adhesive width (W) divided by the as-prepared thickness of the gel (T0) (Figure. 5a) as an indicator of the deformability of the hydrogel layer. The height of the architecture, which is related to X-axis buckling, is increased with an increase in the size ratio (Figure. 4c-i).

We consider that the deformability, especially for the Y-axis, is

increased with increases in the height of architecture due to the release from substrate as mentioned in Figure. 2b.

The other parameter is mismatch strain (ε), which is estimated with the following

equation, as an indicator of the compression by swelling (Figure. 5b). ε=1―

𝑙0

(1)

𝑙

where, l0 and l are the as-prepared gel size and free-swollen gel size.

The swelling of the hydrogel

on a non-adhesive region is restricted by the confined gel on the two adjacent sides, thus, the nonadhesive region experiences compressive stress, which can be estimated as the mismatch strain, inducing the buckling deformation.

This mismatch strain is calculated from the swelling ratio

measured in another simple experiment (Figure. S6). We consider that 3D hydrogel architectures are induced by buckling instability determined by the balance between deformability and compression caused by the swelling of the hydrogel. By using these two parameters, we draw a phase diagram as shown in Figure. 5c.

To

improve the reliability of the diagram, we fabricated 3D hydrogel architectures with different as-


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prepared thicknesses (30 and 120 μm) as shown in Figure. S7 to increase the number of plots. The plot size and color in the diagram represent the as-prepared thickness of the gel and the architecture shape as defined in Figure. 4b, respectively.

When the size ratio (W/T0) is low (less

than ~3), the deformability is small, resulting in no Y-axis buckling and therefore a smooth ridge architecture.

However, if the mismatch strain is large enough to induce Y-axis buckling, we can

observe fold ridge architectures.

On the other hand, when the size ratio is high (more than ~20),

the deformability is also increased, resulting in a more complicated pattern that is a double-period channel architecture.

With a middle range size ratio (5~20), the value of the mismatch strain (ε)

is strongly related to the architecture type.

With a low mismatch strain, we observe a wrinkle

channel architecture that is highly periodic and sinusoidal.

With an increase in the mismatch

strain, some parts of the architecture side wall fold into self-contacting sharp tips, forming a fold channel architecture.

With a further increase in mismatch strain, the fold channel architecture

bifurcates into a more complicated period-double channel which is similar to the morphology found in bi-compressed diamond-like-carbon/silicon layer composite.42

However, we should

note that this morphology has never been observed before in a thin gel film-substrate system.

We

think that the high swelling nature of the gel enables the telephone-cord buckling architecture to transform the period-double morphology, which is known to be one of the instability patterns at a high compression state19.

In another case, we can observe a shape transition from fold “channel”

to fold “ridge” due to an increase in wall thickness.

Interestingly, even when the initial gel

thicknesses differ, the resultant architectures with the same size ratio correspond well with each other, indicating that size ratio is important parameter for instability pattern formation.

Overall,

we can qualitatively predict the architecture shape by setting the initial conditions, including gel


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composition (swelling ratio), gel geometry (thickness) and the geometry of non-adhesive region (width). Under our experimental conditions, all the samples exhibit X-axis buckling (Fig.2b-ii), indicating that the critical buckling stress is less than the swelling pressure. A comparison of the theoretically calculated critical buckling stress with the swelling pressure provides good support for this prediction (see Supporting Information, Figure S8 and Table S1).42 Unless otherwise specified, for the rest of the experiments, we chose 0.3 mol% and 60 μm, respectively, for the cross-linking density and thickness of the hydrogel layer.

Figure 5. Schematic illustrations of the definitions of (a) aspect ratio and (b) mismatch strain. (a) Size ratio is determined by the non-adhesive width (W) divided by the as-prepared thickness of the gel (T0), indicating the deformability of the hydrogel layer. (b) Mismatch strain is characterized as the effective degree of compression experienced by a boundary confined gel multiplied by the


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strain required to return a chemically identical, unconfined gel to its initial lateral dimensions. (c) A phase diagram for understanding the tendency of 3D architecture type. Various combinations of (a) size ratio, W/T0, and (b) mismatch strain, ε, were experimentally investigated and classified into 5 architecture types. The error bars are standard deviations obtained from the results of 3-5 samples, and were smaller than the symbol unless otherwise indicated.

Expandability of the method for creating 3D hydrogel architectures Following the above concept, we were able to develop 3D hydrogel architectures with more complex arrays by designing different patterns of non-adhesive geometry on the silanized substrate (Figure. 6).

We used three basic shapes, dot, stripe and branch to build four geometric patterns

with different features.

In addition to the single straight lines that we have already investigated,

we designed (i) periodic circular dots on a square lattice, (ii) periodic straight lines, (iii) honeycomb networks and (iv) a square grid (Figure. 6a).

After the fabrication of the hydrogel

layer on the variously patterned silanized substrates, swollen samples were observed with phasecontrast microscopy (Figure. 6b) and confocal fluorescence microscopy (Figure. 6c-e). circular dot pattern, we find a three-fold symmetric morphology.

For the

Surprisingly, the dot pattern,

which is a closed system between the hydrogel layer and the substrate, also successfully induced a 3D hydrogel architecture via selective swelling.

These results show that the hydrogel layer

allows water penetration from outside into the space surrounded by the hydrogel layer and the substrate.

As shown in Figure S9 in Supplementary Information, this process is relatively slow

(~ 4 hours) compared with the open system that has a stripe pattern (~ 600 sec, Fig. 2a). we can freely design the array pattern without depending on closed/open patterns. pattern, we can successfully fabricate long-range ordered channel-like architectures.

However,

For the stripe Regardless

of the stripe length and the closed/open system, regular sinusoidal architectures are fabricated (Figure S10).

In addition to this, we constructed a honeycomb-like branching channel

architecture with excellent reproducibility.

We should note that when the number of branches is


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increased, the architecture shape is not uniquely determined. well programmed.

However, at least the position is

These results demonstrate that by designing a suitable non-adhesive pattern,

we can obtain complicated 3D hydrogel architecture arrays. Our proposed method for creating a 3D hydrogel architecture is based on swelling, which is a very common property of hydrogels.

Similar to a previous work32, we further functionalized

the P(FL-co-AAm) gels by introducing a stimuli-responsive unit, which can tune the swelling ratio with an external stimulus.

By utilizing sodium acrylate (SA) and N-isopropylacrylamide

(NIPAM) as functional unit45-47, we successfully demonstrated 3D hydrogel architectures with ionic strength and thermo-responsiveness (see Supporting Information, Figure S11, Supporting Movie 4). These results show the extensibility of the method for creating a 3D hydrogel architecture and provide a promising approach for developing dynamic soft substrates.


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Figure 6. Various architecture patterns programmed by non-adhesive geometry on a silanized substrate. (a) Various silanization pattern (black and white parts are silanized and etched parts, respectively); (i) dot, (ii) line, (iii) 3-point junction and (iv) 4-point junction. (b) Phase-contrast images of swollen samples with different silanization patterns. (c) 3D confocal reconstruction image of FL-labeled hydrogel. (d,e) X-Y plane sectional view ((d) high magnification and (e) array pattern) at the upper surface of hydrogel bonded on the substrate.

Cell culture in channel-like hydrogel architectures Finally, we demonstrate cell culture in a channel-like hydrogel architecture to confirm that these 3D hydrogel architectures can be utilized as biocompatible three-dimensional spaces.

To culture

cells on the P(FL-co-AAm) gels (architecture shown in Figure. 3), type I collagen was immobilized as an extracellular matrix (ECM) by using the heterobifunctional cross-linker, sulfosuccinimidyl 6-(4'-azido-2'-nitrophenylamino) hexanoate (sulfo-SANPAH), which can chemically conjugate the gel surface with ECM (Figure. 7a).48,49

We first pipetted the sulfo-SANPAH solution into

the gel channel, then we performed UV irradiation to obtain a chemical reaction with the gel surface. Subsequently, type I collagen solution was supplied into the gel channel and incubated overnight at 4℃.

After that, mouse myoblasts cells, C2C12, were gently injected into the gel

channel and cultured for 5 days until it became confluent.

As shown in time-lapse images and a

movie obtained with phase-contrast microscopy (Figure. 7b and Supplementary Movie 5),

we

successfully enclosed cells in the gel channel at 0h (inset yellow and white arrows represent the external and internal cells, respectively),

and they were cultured to confluence after 106h.

For

a detailed observation, we undertook phalloidin staining for F-actin to visualize cells under confocal fluorescence microscopy.

From the 3D confocal reconstruction of a gel channel

containing C2C12 cells, we found that the cells grew vigorously in the channel, and they were closely packed from the bottom to the top of the channel (Figure. 7c).

In addition, the cells were

oriented along the wall of the channel (Figure. 7c-ii) and we quantitively perform image processing


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to evaluate the cell alignment (Figure S12). This result indicates a possible application of the 3D cell culture system. We should note that some cells were cultured outside the channel due to leakage from the edges during injection.

However, we think that this problem may be solved by

designing a connector as with micro-fluidic devices and utilizing an adhesion technique with soft and wet materials.50,51

In future work, this hydrogel 3D architecture will have great potential to

be used to fabricate a 3D co-culture system for investigating the cellular interaction between cells on the outside and inside of gel channels, which can allow a cell secretion substance to penetrate thanks to the high permeability of hydrogels.52


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Figure 7. Cell culture utilizing 3D hydrogel architecture. (a) Schematic illustration of cell culture process. Surface of channel-like hydrogel is modified by type I collagen, which is an extracellular matrix (ECM), using 　 the heterobifunctional cross-linker, sulfosuccinimidyl 6-(4'-azido-2'nitrophenylamino) hexanoate (sulfo-SAMPAH). (b) Time-lapse microscopy of the cells during cell culture in channel-like hydrogels. Inset white and yellow arrows in phase-contrast image at 0 h represent the external and internal cells of the channel-like hydrogel, respectively. After 106 h, the image in the inset yellow box area was observed with confocal fluorescent microscopy. The inset dashed line represents the X-Z slice position for (c-i). (c) 3D confocal reconstruction of channel-like hydrogel (green, FL) and F-actin in culture C2C12 cells (red, phalloidin). From the X-Z slice image as shown in (i), the X-Y slice images in three positions were defined; (ii) top of channel, (iii) upper surface of confined gel, and (iv) bottom of confined gel.

4. Conclusion In summary, we systematically investigated the morphologies of buckle-delamination in a hydrogel/solid substrate layer composite controlled by parameters such as delamination geometry and the swelling ratio of the hydrogel.

Taking advantage of a conventional photolithographic

technique and oxygen plasma etching, we successfully fabricated a layer composite where the desired interface is bonded. Therefore, we could systematically design the delamination geometry and chemically tune the swelling ratio of the hydrogel, and thus induce buckle-delamination at an arbitrary position with various swelling ratios. From a detailed observation of 3D architectures, we suggested the architecture formation mechanism based on buckle-delamination. In addition, we revealed the relationships between the architecture morphologies and the parameters, leading to the discovery of several new morphology transitions. Using the characteristic feature of the architectures as a basis, we classified the morphologies of these architectures into five categories and drew a phase diagram to predict the architecture morphologies by setting the parameters. Furthermore, we investigated 3D architecture formation on more complicated patterns such as circular dots, 3-point junctions and 4-point junctions. Moreover, utilizing the easy chemical modification on a hydrogel surface, the high permeability of hydrogels and their characteristic “channel” architecture, we further demonstrated cell encapsulation in a hydrogel architecture and


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long-term cell culture.

This work provides a flexible approach for designing 3D soft architectures

that can be applied to a microfluidic device, a controllable adhesion/friction pad under water, and an advanced cell culture substrate.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:XXX



Dynamic creation of hydrogel 3D architecture via selective swelling (MP4)



Reproducibility of 3D hydrogel architecture formation (MP4)



Three-dimensional confocal reconstruction of a 3D hydrogel architecture (MP4)



Dynamic deformation of 3D hydrogel architecture based on thermal responsive gel (MP4)



C2C12 cells were cultured in the 3D channel-like hydrogel architecture (MP4)



Additional information for materials and methods; procedure for chemically anchoring a hydrogel with a substrate; photographs of hydrogel layer on adhesive/non-adhesive substrates; schematic illustration for swelling behavior; dimension reproducibility of the 3D hydrogel architecture; swelling and mechanical properties of hydrogels; images of X-Y plane sectional view with varying hydrogel thickness; calculation of critical buckling stress; time-lapse observation of 3D hydrogel architecture on a circular swellable pattern; 3D hydrogel architectures with varying stripe length; stimuli-responsive 3D hydrogel architectures; evaluation of the cell alignment (PDF)


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Acknowledgements We gratefully acknowledge Dr. H. Miyazako for fruitful discussions regarding the theoretical model of 3D hydrogel architecture formation and experimental setup for cell culture. We are also grateful to Dr. A. Tanaka for the experimental setup for confocal microscopy and the soft photolithography process.

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Topic of Contents figure


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Dynamic Creation of 3D Hydrogel Architectures via Selective Swelling

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