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Bio-inspired Programmable Polymer Gel Controlled by Swellable Guest Medium Heng Deng, Yuan Dong, Jheng-Wun Su, Cheng Zhang, Yunchao Xie, Chi Zhang, Matthew R. Maschmann, Yuyi Lin, and JIAN LIN ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07837 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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ACS Applied Materials & Interfaces

Bio-inspired Programmable Polymer Gel Controlled by Swellable Guest Medium Heng Deng [+], Yuan Dong [+], Jheng-Wun Su, Cheng Zhang, Yunchao Xie, Chi Zhang, Matthew R. Maschmann, Yuyi Lin, and Jian Lin* Department of Mechanical & Aerospace Engineering, University of Missouri-Columbia, Columbia, Missouri 65211, USA. [+] Heng Deng and Yuan Dong contributed equally to this work

Keywords: programmable polymer, responsive, bio-inspired, laser direct writing, origami.

ABSTRACT Responsive materials with functions of forming three-dimensional (3D) origami and/or kirigami structures have a broad range of applications in bioelectronics, metamaterials, microrobotics, and microelectromechanical (MEMS) systems. To realize such functions, building blocks of actuating components usually possess localized inhomogeneous so that they respond differently to external stimuli. Previous fabrication strategies lie in localizing non-swellable or less-swellable guest components in their swellable host polymers to reduce swelling ability. Herein, inspired by ice plant seed capsules, we report an opposite strategy of implanting swellable guest medium inside non-swellable host polymers to locally enhance the swelling inhomogeneity. Specifically, we adopted a skinning effect induced surface polymerization combined with direct laser writing to control gradient of swellable cyclopentanone (CP) in both vertical and lateral directions of the non-swellable SU-8. For the first time, the laser direct writing was used as a novel strategy for patterning programmable polymer gel films. Upon stimulation of organic solvents, the dual-gradient gel films designed by origami or kirigami principles exhibit reversible 3D shape transformation. Molecular dynamic (MD) simulation illustrates that CP greatly enhances diffusion rates of stimulus solvent molecules in the SU-8 matrix, which offers the driving force for the programmable response. Furthermore, this bio-inspired strategy offers unique capabilities in fabricating 1

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responsive devices such as a soft gripper and a locomotive robot, paving new routes to many other responsive polymers.

1. INTRODUCTION 2


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Shape transformation phenomena are ubiquitous on every scale of organization in livings: from a single molecule to individual organs1-2. Such phenomena have triggered tremendous enthusiasm in mimicking them in man-made materials, which breeds a class of materials—programmable and responsive materials3-4. These materials undergo spontaneous shape reconfiguration from planar films into three-dimensional (3D) structures, attracting burgeoning interests in fields of energy harvesting, metamaterials, soft robotic, sensors, and multifunctional bio-scaffolds3-8. Programmable and responsive polymer gel is a major branch in this field, which is of broad interest and has attracted considerable attention9-10. In general, shape transformation of polymer gel is rooted from inhomogeneous swelling behaviors in the material9. In order to create such an inhomogeneity, the most common used philosophy is to locally weaken the swelling ability of polymeric gel. In this case, swellable polymeric gel is normally used as host matrix, then non-swellable or less-swellable guest components are introduced to the host matrix. These guest components, composed by different chemical composition or highly cross-linked polymer, exhibit less volumetric expansion than the host polymeric gel, which locally restrict volumetric expansions of the host matrix to change the shape of the polymer films6, 11-14. Generally, there are two major ways of achieving the transformation. The first one is to make these guest components be integrated on the surface of host polymer gels, forming a bilayer or multilayer structures14-15. For example, metal film16 or non-swellable polymer layer17 are bonded to the host polymer gels. The second strategy is to embed

these guest

components into the matrix of swellable host gels, forming monolithic films18-19. This could be achieved either by introducing external components such as chemically distinct polymers, non-swellable fiber and microplatelets, or by locally cross-linking some domains of polymer to form less swellable domains6, 11-13, 20. By a marriage with traditional photolithography6, 11, 20-23

, or printing technique such as 4D printing18, 24, electrical ionoprinting19, and magnetic 3



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patterning25-26, the non-swellable or less-swellable guest components were patterned on or in the host polymeric films to control the local swelling behaviors of the films. However, preparation of programmable polymer gels through these strategy often involves complicated chemical modification and patterning process such as numerous photolithographic and deposition steps. They render either fabrication complicated or limitation in the freedom of controlling the local swelling inhomogeneity19, thus lessening their wide-spread applications. To move the field forward, a new design philosophy is desired.

Figure 1. (a) A scheme showing anisotropic structure of ice plant seed capsules and its movement by uptaking water (Modified from Reference 14). (b) Schemes of spatially controlling swellable guest medium in polymer matrix to fabricate dual-gradient programmable gels. (c) Schemes of fabrication process for a dual-gradient SU-8 gel film.

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Herein, inspired by a bio-prototype of ice plant seed capsules (IPSC)27, we report a totally new philosophy of introducing swellable guest mediums into their non-swellable host polymer films to locally enhance their swelling ability. The IPSC exhibit hydration-triggered morphology change driven by anisotropic swelling behaviors upon uptaking water within plant cells. It results in origami-like responsive movement. Studies show that this hydration-triggered out-of-plane motion is resulted from the swellable medium, non-lignified cellulose fillers, which is embedded in non-swellable lignified cell walls. The IPSC possess a primary structure composed of active keels and passive backings, in which the swellable medium is distributed in a dual-gradient way (Figure 1a). The swellable mediums absorbed a large amount of water resulting volume expansion, while the non-swellable matrix restrain the volume expansion in the vertical and lateral direction, which lead to complex shape transformation. Inspired by this, we present a monolithic SU-8 gel film with controlled spatial concentrations of the swellable guest medium, cyclopentanone (CP), inside the non-swellable host matrix of SU-8 in both vertical and lateral directions (Figure 1b). To generate a vertical gradient of CP in the SU-8 gel film, a skinning effect of surface polymerization induced by oven heating was used (Figure 1c)28. Laser direct writing (LDW)29-32—a mask-free technique with low cost, high efficiency, and flexible designability—controls a lateral gradient of CP (Figure 1c). These dual gradients designed by origami and kirigami principles enable to form a controllable swelling field, which endows reversible shape transformation of the SU-8 gel films to complex 3D structures when stimulated by organic solvents or vapors (Figure 1b). Moreover, this bio-inspired strategy enables fabrication of a soft gripper and a locomotive soft robot. Up to our knowledge, this design and fabrication strategy is not yet reported in monolithic polymer gel films. Especillay, the powerful LDW technique, which has been applied for advanced fabrication of flexible supercapcitor, electrochemical catalyst and photodetector29-32, was used to pattern programmable polymer gels for the first time. This transformative way of fabricating 3D responsive structures shows a substantial progress in 5



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making multifunctional devices for widespread applications, which would be readily applied to other responsive polymer systems with different material structures and responsive characteristics.

2. EXPERIMENTAL SECTION 2.1. Fabrication of programmable SU-8 films. A standard processing procedure of SU-8 (SU-8 2000 series photoresist bought from MicroChem) films includes following steps: spin coating, pre-exposure soft-baking, UV exposure, post-exposure baking (PEB). After the spin coating, a wet SU-8 film goes through a soft bake (hotplate baking at 95 oC for 3 min) to evaporate all the CP. As a result, it eliminates the possibility of creating a concentration gradient of the imbedded CP. Crosslinking proceeds upon a UV exposure. PEB further drives a polymerization catalyzed by the acid generated in the UV exposure step. With this standard procedure, the obtained SU-8 films are almost CP-free and not responsive to acetone. In a modified procedure of fabricating vertical-gradients SU-8 films, all other steps were the same except that the step of soft-baking by hotplate heating was replaced by oven heating. In details, the SU-8/CP solution (the content of solvent in solution is ~ 36%) was first spin coated on an aluminum plate at 3000 rpm for 30 s and then soft-baked in an oven at 50 ℃ for different durations. Then the film was irradiated by a UV light (UVP high intensity B-100 series lamp, 21700 µw/cm2) for 3 min. Finally, the obtained film was baked on a hot plate at 130 oC for 3 min to further polymerize the film. Due to the skinning effect created in the oven heating step, the CP was already trapped inside the film. Almost no further evaporation was induced on this step of heating on the hotplate. To prepare control samples, which contain homogenous CP, the SU-8/CP solution was first spin coated on an aluminum plate at 3000 rpm for 30 s. Then the solution was covered by a piece of quartz followed by the UV exposure and PEB. The coverage prevents solvent evaporation during UV irradiation and PEB steps. Thus, it leads to homogeneous distribution of CP inside the SU-8 matrix. 6


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To fabricate the dual-gradient SU-8 films, the SU-8/CP solution was first spin coated on an aluminum plate at 3000 rpm for 30 s. Laser direct writing (LDW) was then conducted on the surface of the film using a desktop CO2 laser in a scanning mode. The pulse width of the laser is ~14 µs. The laser power was fixed at 8 W during processing. This low power was used to evaporate the CP solvent in the SU-8 film. All experiments were performed under ambient conditions. Origami patterns were first designed by AutoCAD, and then DXF files were loaded onto the software that controls the movement of the laser beam. After the laser induction, the area irradiated by laser became dry due to solvent evaporation. Then the film was kept in an oven for a certain time to generate vertical CP gradient in the areas where the film was not irradiated by the laser. After that, the film was polymerized by the UV exposure and PEB steps on a hot plate at 130 oC for 3 min. Kirigami cutting was performed by the CO2 Laser in the cutting mode. In this mode, the laser power is increased to 40 W to cut off the areas which are not desired in the film. The cutting pathways were first designed by AutoCAD, and then DXF files were loaded onto the software to control movement of the laser beams for cutting. 2.2. Characterization. FTIR was performed by Thermo Nicolet Avatar 360 FT-IR spectrometer (Thermo Electron Inc. USA). In the FITR spectrum, carbonyl peak at 1750 cm-1 and aromatic C-C stretching peak at 1500 cm-1 are related to CP and SU-8 molecule, respectively. The relative ratio between CP and SU-8 can be calculated by integration of these two peaks. In the calculation, we set the prepared SU-8 sample with homogeneous CP as a reference (termed as R), because we assume that no CP is evaporated due to a protection by the quartz cover. In this reference sample the concentration of CP is 36.8 %. Assuming the IR absorption is followed by the Lambert-Beer law33, the CP concentration in samples (S), which is termed as Cs, could be is written as the following formula: ×

Cs =

× × 36.8%

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where AS1750 and AS1500 are the integration of the peaks at 1750 cm-1 and 1500 cm-1 in the tested sample “S”; AR1750 and AR1500 are the integration of the peaks at 1750 cm-1 and 1500 cm-1 in the reference samples. The cross-section images of SU-8 gels are taken by microscope (MU130, AmScope). Graphical analysis was done by the software of AmScope. The swelling strains were calculated by measuring sample dimensions before and after being swelled in acetone. In detail, the swelling strain is expressed as the ratio of changed length ∆L to the original length L of the investigated sample. The videos and photographs of self-folding behaviors of SU-8 gel films were taken by a Sony digital camera. 2.3. MD simulation of acetone absorption and diffusion in SU-8 films. MD simulation was carried out by the LAMMPS package which has been used by us in a previous study on a conversion of polymers to graphene34. Small organic molecules (such as CP, acetone) and polymer molecules were simulated by the OPLS potential35, which ascribes the intermolecular energies including Van der Waals and electrostatic energies, as well as intramolecular energies such as bonding energy, angle energy and torsion energy. The OPLS potential gives the Lennard-Jones distance and energy parameters of each site of the organic molecules. The Lorentz−Berthelot rule is applied to these inter-site interactions. The long-range electrostatic interaction in the sites of the organic molecules is accounted by means of the PPPM method36. The non-bonded repulsive interaction and the real-space part of the Coulomb and dispersion terms are shifted to zero at a truncated radius of 1 nm. To initialize systems, equilibrium structures of the molecules were constructed and geometrically optimized in Material Studio 7.0. Partial charges of each atom were calculated by the first principle calculation using the VAMP package integrated in Material Studio 7.0. Then locations, bonds, and charges of each molecule were output to molecule template files, which were then used by Moltemplate package (http://www.moltemplate.org/) to build input files for LAMMPS. Periodic boundary conditions were implemented in all three Cartesian coordinate directions. The temperature 8


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and pressure were maintained using the Nose-Hoover thermostat and barostat. The detailed method is presented in Supplementary Note 1. 2.4. FEA modeling of programmable SU-8 films. All FEA modeling was carried out using ABAQUS. The traditional bilayer structure is adopted in our FEA modeling, in which the SU-8 gels were composed by components made of all hyperelastic materials with different swelling properties. The details of modeling process are given in Supplementary Note 2.

3. RESULT AND DISCUSSION Firstly, we fabricated the self-folding SU-8 gel films driven by active CP with a vertical gradient. The starting material is commercial SU-8 solution in which the CP is the solvent and SU-8 monomer is the solute. The acid-initiated cationic polymerization of the SU-8 is initiated by an ultraviolet (UV) exposure and subsequent post-exposure baking (PEB)37-38. In a traditional fabrication process suggested by the manufacturer’s protocol, a hotplate is used in the pre-exposure baking step to maximize the CP removal (Figure S1a). If an oven is used for in this step (Figure S1b), a skinning effect happens, resulting in a highly dense surface which inhibits evaporation of solvent from the bottom of the film28. We found out that the oven heating induced a mass loss of < 16% percent after 50 min while hotplate heating resulted in ~28% in just ~3 min (Figure S2). The resulted SU-8 gel film exhibited reversible self-folding behaviors upon exposure to acetone. The folding direction is always inwards, curving from the bottom to the top surface of the film. To investigate response mechanism, a responsive SU-8 gel film of 10 mm × 1 mm x 25 µm was prepared. Response time was recorded according to evolution of end-to-end distances of the film (two ends are labeled as “a” to “b” in Figure 2a). When immersed in acetone, the flat SU-8 film folds to a closed circle in 6 s (Figure 2a-b and Movie S1). When the folded film is placed in water it recovers to flat state in 5 s. This reversible self-folding can be cycled > 50 times, indicating its good stability.

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Figure 2. (a) Photograph of a vertical-gradient SU-8 film immersed in acetone. (b) Distance between two ends of a SU-8 strip immersed in acetone/water and its corresponding response and recovery time. (c) The FT-IR spectra of a SU-8 film without CP (I), bottom surface of a vertical-gradient SU-8 gel film (II), top surface of a vertical-gradient SU-8 gel film (III), and pure CP (IV). (d) Cross-section optical images of a vertical-gradient SU-8 gel film before and after being immersed in acetone. (e) Schemes of a vertical-gradient SU-8 gel film and its corresponding self-folding behaviors. (f) Cross-section optical images of a dry SU-8 film 10


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before and after being immersed in acetone. (g) Cross-section optical images of a SU-8 film with homogenous CP before and after being immersed in acetone. (h) Scheme and optical images of a vertical-gradient SU-8 gel film in lifting a Cu film upon stimulation of acetone.

We hypothesize that these interesting self-folding behaviors are rooted from the vertical-gradient distribution of CP in the SU-8 matrix. To test this hypothesis, we performed Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) on the top and bottom surfaces of the SU-8 gel film (Figure 2c). In a mixture of SU-8 and CP, only CP possesses a carbonyl group, whose peak appears at 1750 cm-1 in the FTIR spectrum37. By integrating the carbonyl peak of CP at 1750 cm-1 and aromatic C-C stretching peak of SU-8 molecule at 1500 cm-1, the concertation of CP in bottom and top layers are quantified to be 25.76% and 13.61 % (Details are described in Method), indicating a possible gradient of CP in the vertical direction. We also measured strains of the top and bottom surfaces of the gel films folded in acetone. They are termed as αt and αb. The film with a CP gradient shows αt and αb of 4.5 % and 11.8 % (Figure 2d), respectively. From a geometric perspective39, the curving of SU-8 is driven by different αb and αt as shown in Figure 2e. The curvature (C = 1/r)

of a strip is defined as ~ (α − α ), where h is thickness of the gel film. A positive value

of (αb-αt) results in the folding direction from the bottom to the top surface. To further confirm this hypothesis, we fabricated two kinds of control samples. One is a SU-8 film with almost no CP. The other is a SU-8 film with homogenously distributed CP. The former one was prepared by using a hotplate to possibly evaporate all CP (Figure S1a). FTIR spectrum shows almost no carbonyl peak, indicating undetectable CP (the concertation of CP is ~ 0.5%) in the film (Figure S3). The latter one was made by covering a quartz slide on the top surface of the SU-8 film before baking so that the evaporation of CP was prevented through the interface of the SU-8 and air (Figure S1c). Almost identical FTIR spectra on the bottom and top surfaces of the SU-8 film indicate that CP is homogenously distributed in the 11



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SU-8 matrix (Figure S3). Unlike the SU-8 films with vertical-gradient CP (termed as vertical-gradient SU-8), these two kinds of SU-8 films don’t fold upon the acetone stimulus (Figure S4). The cross-section optical image of the SU-8 film without CP shows that αt and αb are close to 0% (Figure 2f), indicating that dry SU-8 does not swell in acetone. The SU-8 film with homogenous CP shows almost the same αt and αb (18.6 % and 19.9%, respectively) (Figure 2g). The diffusion coefficients of acetone in these two systems were also measured, in which the one in the SU-8 film with homogeneous CP exhibited is much larger than the one in SU-8 system without CP (Figure S5). These results suggest that the CP serves as active swellable guest medium in the SU-8 gel to enhance the absorption of acetone. The bottom region with a higher concentration of CP absorbs more acetone than the top region with a lower concentration of CP, resulting in significant different swelling strains in different regions. Moreover, although the thickness of the SU-8 gel film is ~25 µm, the folding generates a considerable mechanical force. As depicted in Figure 2h, a SU-8 film actuator can lift a copper foil which is > 30 times of its body weight. Moreover, cyclability and folding properties of the gel films can be modulated by fabrication parameters, such as oven heating time and PEB temperature (Figure S6). Generally, a longer oven heating and a higher PEB temperature lead to slower response but resulted gel films usually show a higher cyclability and generate larger mechanical forces. Besides acetone, the vertical-gradient SU-8 gel films are also responsive to other organic solvents. We totally tested nine organic solvents. The films showed different response to these solvents (Figure S7). The films show noticeable respond to dichloromethane (DCM), acetone, tetrahydrofuran (THF), and dimethylformamide (DMF) with response time less than 20 s. It’s worth mentioning that the film has a fastest respond time of about 1 s in DCM (Movie S2). In addition, the films also respond to the solvent vapors and recover in air (Figure S8), indicating a possibility of using them as sensing elements for chemical-mechanical sensors. 12


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Figure 3. Snapshots of a crosslinked SU-8 system (a) and a SU-8 system containing 35.8 wt% CP (b). These systems are partially sliced from the whole systems between 0.4z and 0.6z, where z is the total thickness of the systems. The magnified parts of (a) and (b) show cross-linked epoxy groups. The atoms are colored with black (carbon), red and green (oxygen) and white (hydrogen). The C and O atoms in CP are colored with blue. (c) Snapshots of 500 acetone molecules into a dry SU-8 system before and after simulation of 2 ns. (d) Snapshots of 500 acetone molecules into a SU-8 system containing 35.8 wt% CP before and after simulation of 2 ns. The atoms are colored with black (carbon), red (oxygen) white (hydrogen) and light blue (Ar). CP molecules are colored with blue and acetone molecules are colored with orange. (e)-(f) S (A2) vs time for a dry SU-8 system (e) and a SU-8 gel system with 35.8 wt% CP (f). Black dot line is plotted with original data and the red dotted lines are fitted curves.

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Experimental results show that CP in the SU-8 polymer matrix serves as active swellable guest medium that absorbs external solvents like acetone. The CP concentration has anisotropic distribution in the vertical direction of the gel film. To further understand the mechanism of asymmetrical acetone absorption by these gradual CP, molecular dynamics (MD) simulation was conducted at an atomistic level to verify the following two main hypotheses: (1) affinity of acetone toward CP molecules is higher than that of SU-8 matrix; (2) acetone exhibits a larger diffusivity in the SU-8 gel containing CP than that in dry SU-8. Firstly, we investigated the absorption affinity of acetone molecules toward SU-8 and CP by calculating their radial distribution functions (RDFs)40. The system construction and RDF calculation are described in Supplementary Note 1.1 and Figure S9. RDFs show that the acetone is more easily absorbed by CP than by SU-8 (Supplementary Note 1.1 and Figure S10). These results verify that CP has a higher affinity to acetone than that of the SU-8, thus it is reasonable to hypothesize that a region with a higher concentration of CP in the SU-8 matrix would exhibit an enhanced absorption of acetone. In other words, acetone diffusivity in this region would be higher than that with a lower concentration of CP. To verify this hypothesis, SU-8 systems with varied concentrations of CP (0%, 15.6%, 26.9%, and 35.8%) were polymerized by our developed algorithm as described in Supplementary Note 1.2 and Figure S11-12. Representative snapshots of the developed SU-8 polymer systems are shown in (Figure 3a-b). The results in Table S1 show that the density of a dry SU-8 with 80% degree of polymerization is around 1.10 g/cm3, which is in the range of 1.07–1.20 g/cm3 for commercial products41, indicating an accuracy of the developed algorithm. Then acetone molecules were introduced into these polymer systems for an investigation of the diffusion process (details are shown in Supplementary Note 1.3 and Figure S13). Two representative systems are shown in Figure 3c-d. In the dry SU-8 polymer system, almost no acetone molecules diffuse into its matrix (Figure 3c). In contrast, in SU-8 gel with 35.8 wt% of CP acetone diffuse much faster (Figure 3d). The quantitative diffusivity of acetone in these two 14


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systems were calculated by solving the partial differential equation of Fick's law (Supplementary Note 1.3). Figure 3e-3f and Figure S14 show the volume related function S (defined by Equation S7) as time t in different SU-8 systems. By calculating the slopes of these curves, the diffusivity of acetone in these systems can be derived (Table S2). The diffusivity increases as the concentration of CP increases. The diffusivity in the SU-8 gel with 35.8 wt% CP is ~28 times of that in the dry one. All of these MD simulation results reveal that CP serves as swellable guest medium in the SU-8 gel to enhance the diffusion of acetone. And the enhanced diffusion rate guarantte stonger swelling behivour within a given time frame. Therefore, it is reasonable to infer that different CP concentrations result in different swelling ratios, causing the film to fold from a region with a higher concentration of CP to the region with a lower one, which is in a good agreement with the experimental results.

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Figure 4. (a) Scheme of DLW on a vertical-gradient SU-8 film for patterning active creases. (b) English letters (the logo of University of Missouri) evolved from dual-gradient SU-8 films by the origami principle. (c) A series of helix structures evolved from dual-gradient SU-8 gel films. The widths of active crease and passive domain are both 1 mm. (d) A series of 3D origami structures formed by dual-gradient SU-8 gel films. (e) FEA modeling of 2D planar structure with an origami pattern evolved to a 3D closed box. (f) Photographs of self-folding process of a box evolved from SU-8 gel films with origami patterns.

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The SU-8 gel with a sole vertical gradient only exhibits unidirectional bending movement (Figure 2a). To achieve a complex shape transformation, patterning of active and passive creases on the gel film is needed. Inspired by the origami art the films are often patterned by lithography followed by deposition of multiple-layer materials15, 42. However, these processes add unavoidable complexity and cost to the systems. Herein, we employed a new technique of direct laser writing (DLW) (Figure 4a). In the process, the low power CO2 laser (8 W) generates transient heat, which evaporates CP in the SU-8 gel films but does not change the chemical property of the SU-8 and does not affect the following polymerization after UV exposure and PEB (Figure S15 and S16). It should be noted that, partial SU-8

is

ablated by the laser, resulting in thickness reduction (Figure S17).After laser exposure, the CP concentrations of the bottom and top layers in the exposed areas are significantly decreased to 2.0 wt% and 1.7 wt% from 25.76 wt% and 13.6 wt%, respectively. Thus, a lateral gradient of CP is created. The exposed areas serve as passive domains because they are deprived of swelling ability caused by removal of CP. The non-exposed areas serve as active creases due to vertical-gradient CP in the polymer matrix. Therefore, active and passive creases were formed on the SU-8 gel films by controlling the vertical and lateral gradients of CP. We used the DLW to generate a series of origami patterns on the SU-8 gel film. The dual-gradient film exhibits a complex 3D shape transformation. For example, as shown in Figure 4b, SU-8 gel films with the patterned creases were reversibly evolved into alphabet letters (Movie S3). Moreover, by changing the widths and angles of the active creases, the SU-8 gel films with similar patterns are evolved into helix structures with precisely tuned screw pitches (Figure 4c and Figure S18). With different patterns on the SU-8 films, more 3D structures (such as tetrahedron and pyramid) were reversibly formed upon the acetone stimuli (Figure 4d, Figure S19, and Movie S4). The shape-transformation process of the dual-gradient SU-8 gel film was modeled by finite element analyses (FEA). We adopted a 17



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modified bilayer model to construct 3D computational models for the shape change of the planar SU-8 films in ABAQUS (Supplementary Note 2). As shown in Figure S20 the bilayer film folds to a certain curvature. With an origami pattern of the active and passive creases the 2D model is evolved to a 3D closed box (Figure 4e). Moreover, the stress was most concentrated in the area of active creases (Figure 4f). These modeling results agree well with the experiment. They suggest that the developed methodology enables to provide an assistance in designing programmable 3D objectives from 2D origami films although more sophisticated and complex models are needed. As the pre-patterned creases play important roles on the shape transformation of the films, in some cases like the cross-shaped crease (Figure S21), out-of-plane forces generated by these creases are counteracted by one another. As a result, it is very difficult for theses creases to fold a film into a targeted 3D configuration. In these cases, kirigami principle is helpful to realize it43. By strategically configuring arrays of cuts in the SU-8 gel films by the increasing power of the CO2 laser, the counteracted forces are released and the creases enable to direct a film into a targeted 3D configuration. By a combined strategy of kirigami and origami principles we realized a 3D table which was evolved from a planar film (Figure 5a-c).

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Figure 5. (a)-(c) Photographs of a 3D table generated by a combination of origami and kirigami principles using a dual-gradient SU-8 gel film. (a) A SU-8 gel film before being immersed in acetone; (b) and (c): the 3D table from top-view (b) and side-view (c). (d) Photographs of a soft gripper made by a dual-gradient SU-8 gel film. (e) Photographs showing the process of a SU-8 soft gripper grasping a cargo from acetone. (f) A scheme showing DLW in fabricating soft moving robots. (g) Photographs of the moving robot crawling upon cycling between the acetone vapor and air.

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Moreover, we employed this strategy to fabricate more responsive 3D devices. For instance, we fabricated a SU-8 gripper which exhibited a grasping-like motion in acetone (Figure 5d). This SU-8 gripper grasped an object with > 40 times of its body weight when exposed to acetone and then released the object to water when recovered (Figure 5e, Figure S22, and Movie S5). In addition, we have also shown that our dual-gradient design strategy can create a device that transfers chemical affinity energy into mechanical energy. Here we fabricated a SU-8 moving robots with active creases having different widths (0.5 mm, 1mm and 2 mm, respectively). Because a wider active crease generates a higher output stress than a narrower one44, it results in a faster response in the cycled acetone vapor as shown in Figure S23 and S24. Therefore, asymmetric active creases result in asymmetric stresses, which makes the film fold into an arch structure in the acetone vapor. After the acetone stimulus is removed, the stress generated by swelling is released and the arch structure recovers to a flatten plane, making the film stretch forward along the direction of the wider active crease. By switching on and off the acetone vapor (Figure S23), the moving SU-8 robot repeated cycling movement of arching and stretching forward just like a creeping inchworm (Figure 5g and Movie S6).

4. CONCLUSION In summary, inspired by the swellable structure of IPSC, we developed a new strategy of fabricating programmable and responsive gel films by controlling spatial concentrations of active swellable guest medium in dual directions of a SU-8 matrix. Upon organic solvent stimuli, the dual-gradient films designed with origami and kirigami principles exhibited complex 3D shape transformation from 2D counterparts. Moreover, they can also serve as functional materials for responsive soft gripper and soft moving robot. These interesting structures and devices show potentials of being adapted to a wide range of applications such as sensing, artificial muscles, and robotics. The present study also provides a new route to 20


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other responsive polymer systems with different material structures and responsive characteristics.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detail descritptions of MD simulaiton and FEA simulation, mass loss curve of SU-8 in oven and hotplate, response properties of vertical-gradient SU-8 films fabricated with different processing parameters, additional FI-IR spectra of SU-8 samples, additional optical images of SU-8 samples.

AUTHOR INFORMATION Corresponding Author *Telephone: 573-882-8427. E-mail: [email protected] (J. L.) ORCID Jian Lin: 0000-0002-4675-2529 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work was supported by University of Missouri-Columbia start-up fund, University of Missouri System Research Board, University of Missouri Research Council, Oak Ridge Associated Universities (ORAU) Ralph E. Powe Junior Faculty Award, NSF IGERT program (Award number: 1069091). The computations were performed on the HPC resources at the

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University of Missouri Bioinformatics Consortium (UMBC) supported in part by NSF (award number: 1429294).

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