Reversible Self-Assembly of 3D Architectures Actuated by Responsive

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Reversible Self-assembly of 3D Architectures Actuated by Responsive Polymers Cheng Zhang, Jheng-Wun Su, Heng Deng, Yunchao Xie, Zheng Yan, and Jian Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14887 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 12, 2017

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ACS Applied Materials & Interfaces

Reversible Self-assembly of 3D Architectures Actuated by Responsive Polymers Cheng Zhang,† Jheng-Wun Su,† Heng Deng,† Yunchao Xie,† Zheng Yan,*,†,‡ and Jian Lin*,† †

Department of Mechanical & Aerospace Engineering and Engineering, University of Missouri, Columbia, Missouri 65211, United States

‡

Department of Chemical

Keywords: reversible, self-assembly, 3D architectures, responsive polymer, origami. ABSTRACT Assembly of three-dimensional (3D) architectures with defined configurations has important applications in broad areas. Among various approaches of constructing 3D structures, stressdriven assembly provides the capabilities of creating 3D architectures in a broad range of functional materials with unique merits. However, 3D architectures built via previous methods are either simple, irreversible or not free-standing. Furthermore, the substrates employed for the assembly remain flat, thus not involved as parts of the final 3D architectures. Herein, we report a reversible self-assembly of various free-standing 3D architectures actuated by the self-folding of smart polymer substrates with programmed geometries. The strategically-designed polymer substrates can respond to external stimuli, like organic solvents, to initiate 3D assembly process and subsequently become the parts of final 3D architectures. The self-assembly process is highly controllable via origami and kirigami design patterned by direct laser writing. Self-assembled geometries include 3D architectures such as “flower”, “rainbow”, “sunglasses”, “box”, “ptramid”, “grating”, and “armchair”. The reported self-assembly also shows wide applicability to various materials including epoxy, polyimide, laser-induced graphene, and metal films. The device examples include 3D architectures integrated with a micro light-emitting diode (µ-LED) 1 ACS Paragon Plus Environment


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and a flex sensor, indictaing the potential applications in soft robotics, bioelectronics, microelectromechanical systems (MEMS) and others.

1. INTRODUCTION Three-dimensional (3D) devices with defined structures have been widely applied in biomedical

devices,1-4

optoelectronics,5-6

microelectromechanical

systems,7-9

near-field

communication,10 etc., due to their unique features compared to conventional two-dimensional (2D) counterparts. For example, 3D scaffolds integrated with sensory systems have the capability of monitoring activity of cardiomyocyte inside the constructs while 2D platforms can only collect signals from the surface.2 Among a great diversity of fabrication methods that have been applied for the controlled formation of 3D structures, the dominant ones, which enable to achieve crossed length scales, usually include 3D printing and stress-driven assembly.11 3D printing provides the greatest versatility in geometric design.12-15 However, its applicability only extends to the structures whose components can only be prepared as inks or patterned by exposure to light.16 A stress-driven assembly method is based on a strategy of assembling predefined 2D precursors to 3D structures driven by mechanical stresses.11,

17

The method has several

advantages. First, fabrication of predefined 2D precursors would avoid direct fabrication of 3D structures, which is always a big challenge. By this means one can take advantage of the exceptional planar devices fabrication processes that are ubiquitous in the state-of-the-art semiconductor technologies.18 Second, the 3D structures can be packed in a temporary state during a transfer process for a better protection and an easy operation, and then assembled to working states by releasing stresses. For instance, large-size antennas, made by shape memory

2 ACS Paragon Plus Environment

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alloys, can be packed into relatively small launchers and then are transformed to 3D structures in space upon stimulation of heat.19 Last but not the least, the stress-driven 3D structures can easily adapt to the working environment. For example, in bioengineering, responsive implantable electronics are transformed into 3D geometries from initial planar device architectures when exposed to physiological conditions. The 3D structures enable to conform to the biological environment by diminishing mechanical and geometrical mismatches, realizing electrically biointerfacing.20 Traditionally, stresses employed in the stress-driven assembly methods arise from residual stresses in thin films,8, 21 capillarity,22-23 or mechanically active materials (such as shape memory polymers and metal alloys).20, 24-25 However, the techniques based on above stresses apply most readily to plates or simple curved structures (for example, scrolls and tubes), which cannot provide complex 3D geometries.11, 26 Moreover, a large portion of these techniques can only assemble irreversible 3D architectures.8, 21 Recently, another source of the stress resulting from compressive buckling of elastomeric substrates has been reported to produce sophisticated and reversible 2D−3D transformations.10-11, 16, 27 Despite the fact that this method can drive the shape transformation of various materials ranging from flexible polymers to brittle inorganic semiconductors and crossing length scales from nanometers to centimeters, the assembled 3D structures are not free-standing because they are tethered on the planar elastomeric substrates. Moreover, they are not self-responsive, restricting the application of this mechanical buckling approach in shape-reconfigurable electronics.11, 28 Herein, we demonstrate reversible self-assembly of 3D architectures actuated by mechanical buckling of responsive polymeric substrates. Specifically, 2D passive films with defined origami and kirigami patterns fabricated by direct laser writing (DLW)29-30 are selectively bonded onto



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the responsive polymeric substrates with programmed geometries which can respond to external stimuli, such as heat,31 light,32 or organic solvents.33-34 Once the substrates are exposed to the stimuli, their self-folding responses impart stresses to 2D origami and/or kirigami patterns through the bonding sites. Therefore, the 2D passive films are mechanically buckled and assembled into 3D architectures. After the stimuli are removed, the 3D architectures return to the 2D forms as the responsive polymeric substrates recover to their planar structures. This proposed strategy not only inherits a wide selection of materials and a broad range of geometric designs, but also is grafted with self-assembly ability from the self-response of the substrates. Moreover, the responsive substrates can be designed as parts of the final 3D architectures, improving the structural complexity for various applications. To conceptually verify this idea, an responsive polymer gel developed by our group,35 was engineered to act as the responsive substrate that enables to impart the mechanically buckling forces to the passive thin films. We demonstrate the effectivness of various passive thin films such as epoxy (SU-8), polyimide (PI), laser-induced graphene (LIG),36 and metal films in realizing the 3D transformation processes. Accordingly, mechanical behaviors were investigated and governing parameters were identified. By utilizing origami and kirigami design principles, we patterned a variety of passive films by the DLW technique, which were then mechanically buckled into the 3D structures such as “rainbow”, “sunglasses”, “box”, “ptramid”, “grating”, and “armchair” when the responsive substrates were stimulated. Moreover, we demonstrated integration of a micro light-emitting diode (µ-LED) and a flex sensor to the reversibly assembled 3D structures. The integrated 3D electronic scaffolds showed reversible transformation between the 2D and the 3D configurations. The proposed strategy paves a route to reversible selfassembly of multifunctional 3D scaffolds being responsive to external stimuli, moving forward


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to shape-reconfigurable 3D elelctronics for application in soft robotics, tissue engineering, and microelectromechanical systems (MEMS).

2. EXPERIMENTAL SECTION Fabrication of responsive SU-8 substrates. The SU-8 solution (SU-8 2000 series photoresist bought from MicroChem) was first spin coated on an aluminum plate for 30 s with a speed of 3000 rpm. Patterns for passive domains were designed in AutoCAD and loaded to the CO2 laser control software. The spin-coated SU-8 gel film was illuminated by a low power laser (8 W) to evaporate the solvent, cyclopentanone (CP), at passive domains and a high power laser (40 W) to cut the desired film out of the whole film. Then it was soft-baked in an oven at 50 ℃ for 3 min. After that, the film was exposed to a UV light (UVP high intensity B-100 series lamp, 21700 µW/cm2) for 3 min. Finally, the film was baked on a hotplate at 95 ℃ for 3 min for further polymerization. Fabrication of passive SU-8 2D precursors. The SU-8 solution was first spin coated on an aluminum plate for 30 s at various speeds (from 1000 – 3500 rpm). After spin-coating, a low power laser (8 W) was used to reduce the thickness of the creases in the passive film and a high power laser (40 W) was used to cut the film with a desired shape. Then, the film was soft-baked on a hotplate at 95 ℃ for 3 min. Subsequently, the film was exposed to the UV light for 3 min. At last, the film was heated at 95 °C for 3 min on a hotplate. Fabrication of other 2D precursors. The PI film (thickness of 25.4 µm, bought from Kapton) was partially carbonized by a laser with a power of 10.4 W at the areas of the creases. Then a higher power of 40 W was employed to totally cut off films with desired shapes. Ti/Au films (8/200 nm) were deposited on the PI film by magnetron sputtering (Kurt Lesker Co-deposition



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sputtering system). Fabrication of LIG-based flex sensor. The surface of the PI film was carbonized by a laser at a power of 10.4 W. To make electrical connection two ends of the LIG-based flex sensor were connected to aluminum wires by silver paint. Assembly of 2D precursors and responsive substrates. One droplet of SU-8 solution was dropped at each bonding site on the responsive substrate, and then the 2D precursor was transferred to the substrate by connecting their desired bonding sites. Then the device was heated at 95 °C for 3 min on a hotplate, exposed to UV for 3 min, and post-baking for 3 min on the hotplate. These processes rendered the polymerization of the SU-8 droplets and bonded the 2D precursor to the responsive substrate. For the assembly of multiple responsive substrates, the central passive regions of substrates were firstly aligned and then SU-8 droplets were pasted to these regions to bond each layer. Assembly of µLED bulb. Firstly, Au electrodes were deposited on the passive SU-8 membrane through a shadow mask. Then the µLED bulb was welded to the electrodes. The electrodes were then connected to a power supply (1.5 V) with a protection resistor (2 kΩ). Measurement. A optical profilometer (Vecco NT 9109) was used to measure the thickness. The I-V curve of the flex sensor was measured by a HP 4156A semiconductor parameter analyzer. The dynamic resistance change was recorded by a Keithley 2400. FEA simulation. FEA was performed by using ABAQUS. A 24.5 mm × 1 mm SU-8 ribbon was created with a thickness of 45 µm. The length and the thickness of the crease was set as 1 mm and 20 µm, respectively. The elastic modulus and Poisson’s ratio of the SU-8 ribbon were set as 4.02 GPa and 0.22.16 In the analysis, we added a bending moment (0.017 N·m) at both ends of the ribbon. A four-node 3D deformable shell unit was set for the analysis. Refined meshes were


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adopted to ensure the accuracy. Structured quadratic elements were used for the geometry. Simpson’s rule with 5 integral points was applied to the thickness.

3. RESULT AND DISCUSSION According to our previous demonstration, SU-8 polymer can be either active or passive depending on fabrication processes.35 In this study, active SU-8 polymer films were employed as the responsive substrates. Passive SU-8 films were employed as the 2D precurser films to illustrate the process. A processing flow chart of the 2D passive SU-8 origami films is illustrated in Figure 1a. First, we prepared a liquid thin film on an aluminum plate by spin-coating of commercial SU-8 solution. Then, we utilized the DLW, which is a mask-free technique with low cost, high efficiency, and ﬂexible design,29-30 to partially ablate the thin film at assigned locations to form an origami pattern (Figure 1a.i). Major deformation of the passive SU-8 thin film occurs at these locations because their bending stiffness were significantly lowered after the partial ablation, as a result of the cubic downscaling of the bending stiffness with the thickness.27 After that, the SU-8 film was completely cut off by the laser (Figure 1a.ii), followed by being treated according to the manufacturer’s protocol (Figure 1a.iii) to obtain the passive film.37-38 Finally, the fabricated film was released by etching the aluminum plate in 20 wt% HCl solution (Figure 1a.iv).



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Figure 1. Schematic of fabrication and self-assembly processes of 3D structures consisting of passive SU-8 films and responsive SU-8 substrates. (a) Fabrication process of passive SU-8 films. (b) Fabrication process of responsive SU-8 substrates. (c) Assembly of passive films and responsive substrates. (d) Self-assembled 3D structures. Scale bars: 5 mm.

The above passive SU-8 origami film was subsequently bonded onto a responsive SU-8 substrate which was prepared by our reported method (Figure 1b).35 A responsive substrate consists of active hinges and passive regions which the 2D precursor films are bonded to. Upon external stimuli (such as acetone), the swellable guest medium, cyclopentanone (CP), which is hosted in the active hinges, absorbs stimuli and makes the active hinges swell. Then the swelling hinges direct the folding of the passive regions. The folding imparts buckling forces to the 2D precursor films for assemble of them into the 3D structures. As illustrated in Figure 1b.i, first, laser was used to remove all the CP in exposed areas which turn into the passive regions. Then, the responsive substrate was shaped by laser cutting (Figure 1b.ii). To fabricate the active hinges, distinguished from the fabrication of the passive SU-8 origami film (95℃ on a hotplate ), a preexposure baking was executed in an oven at a temperature of 50 ℃ to produce a gradient concentration of CP due to a skinning effect (Figure 1b.iii).35 After UV exposure and post8 ACS Paragon Plus Environment

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exposure baking, the responsive substrate was polymerized so that the swellable CP was retained inside the active hinges that fold the passive regions of the substrates. Multilayer responsive SU-8 films can be stacked and bonded as a substrate in order to increase the driving force and complexity of the final 3D structures (Figure 1b.iv). After the 2D passive SU-8 film and the responsive SU-8 substrate were both fabricated, they were bonded via a transfer printing technique. In this case, SU-8 droplets were pasted onto bonding sites of the substrate as adhesive material. After the passive SU-8 film was transferred onto the responsive SU-8 substrate through the assigned bonding sites, the SU-8 droplets were polymerized by UV exposure, resulting in tight bonding of the passive and the responsive SU-8 films. When the resulting sample was immersed into an acetone solution, the active hinges absorbed acetone and rendered the responsive SU-8 substrate to fold, compressing the bonded passive SU-8 origami film into various 3D structures. For instance, as shown in Figure 1d, a “Six-petal flower” shape was evolved from a 2D passive SU-8 origami film. In the process, the hinges of “petals” on the responsive substrates absorbed acetone and drove petals to fold, imparting forces to the top “stamen/pistil” of the passive film, making them be buckled up and form a “Six-petal flower” structure. When the 3D structure was transferred into air or water it recovered to a 2D planar film as a consequence of desorption of acetone in the responsive substrate.



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Figure 2. (a) Schematic of a passive SU-8 ribbon bonded on a responsive SU-8 substrate. The passive ribbon has a crease with length of Lc and thickness of Tc. (b) Photographs of a representative group of self-assembled structures, in which the passive ribbons have creases with various lengths. (c) Photographs of a representative group of self-assembled structures, in which the passive ribbons have creases with various. (d) Relationship of bending degrees and lengths of the crease. (e) Relationship of bending degrees and thicknesses of creases. FEA modeling results of after-assembled SU-8 ribbons with a crease (f) and without a crease (g). Scale bars: 6 mm.

To control the transformation between the 2D and the 3D structures, the origami design principle was selected. Over the past decade, the origami principle has been widely applied to design folded structures with arbitrary geometries, proving its power for the fabrication of 3D objects from 2D sheets.39-42 An origami pattern is made of creases assigned with defined locations and bending angles, which greatly determine the shape transformation. The locations can be designed to control the main backbone fo the final structure. To predict the bending angles, we first investigated their governing parameters. In this case, we used a passive SU-8 ribbon as an example. The ribbon with a crease (length of Lc and thickness of Tc) was bonded to the responsive substrate with an active hinge that generates a bending moment (M) (Figure 2a). 10 ACS Paragon Plus Environment

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Qualitatively, the bending angle of the passive ribbon is determined by the bending moment and the geometry of the crease. The bending moment increased with the length of the active hinge in the substrate (Figure S1). The detailed method for measuring the bending moment is illustrated in Supporting Information. For instance, a length of 1.27 mm generated a bending moment of 0.017 N·m. It was found that this moment was sufficient for driving all self-assembly processes in the following experiments. Since the bending stiffness of the creases was lowered after the laser irradiation, the major deformation occurred at the crease of the ribbon. After a serial of study on the passive ribbons with different Lc and different Tc we further found out that the length and the thickness of the crease are the major geometrical factors that determine the bending angles when the same bending moment is applied.27 Two representative groups of the passive SU-8 ribbons with creases having different Lc and different Tc are bent to various bending angles (θ) (Figure 2b and Figure 2c). It is found that the bending angles exhibit a quite linear relationship with Lc (Figure 2d). For instance, the one with a crease having a length of 0.25 mm is bent to 65.5º. As the length of the crease increases to 2.54 mm θ increases to ~180º. Figure 2e shows the relation of θ versus the thickness of the creases. The fitted curve shows that θ decreases with Tc in a function of Tc-3. The bending angle of a ribbon with a crease having a thickness of 11.7 µm is 105º. When the thickness increases to 16.8 µm, θ significantly decreases to 46º. As further increased to 34.5 µm, θ decreases slowly to 5.2 µm. The relationship of the bending angles with the lengths and the thicknesses of the creases can be explained by the bending beam model (Figure S2). In the model, the bending angle is calculated by θ = ρLc

(1)

where ρ is the curvature of the beam. The expression of the curvature is



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ρ = MS-1

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(2)

where M is the bending moment and the S is the bending stiffness calculated by S= EI

(3)

where E is the Young’s modulus and I is the second moment of area. For a rectangular section, 1 I = 12 WcTc3

(4)

where Wc and Tc are the width and thickness, respectively. Combining Eqs. 1-4, we obtain the bending angle that can be expressed as θ = (12ME-1Wc-1) LcTc-3

(5)

From Eq. 5, we can see that θ linearly increases with Lc and decreases with Tc in a function of Tc3

, agreeing well with the results shown in Figure 2d and Figure 2e. Meanwhile, we also

performed finite element analysis (FEA) to study the the stress distribution in the assembled structures to estimate the possible fracture of the structures. The stress distribution of a folded 3D structues is illustrated in Figure 2f. As a comparison, a ribbon without a crease was also analyzed (Figure 2g). We can observe that stress in the area of the crease in the ribbon is reduced from 4.3 N/m2 to 3.9 N/m2, indicating improvement of structral stability with the design of creases.


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Figure 3. Schematic and photographs of 3D architectures assembled from passive SU-8 ribbons (a-d) and membranes (e-h). Scale bars: 6 mm.

The investigation on the governing parameters that control the self-assembly of the passive films provides a guidance for a sophisticated design of more complex 3D structures. Figure 3 presents examples that are assembled from the 2D passive SU-8 films. Figure 3a-d are 3D structures assembled from the 2D ribbons. Schemes of the 2D ribbons are shown in the left column. Figure 3a is a “rainbow” assembled from three ribbons with various lengths, but the dimensions of their creases are the same. Thus these three ribbons folded to the same bending angels. Movie S1 and S2 show the reversible self-assembly processes of this “rainbow”. Figure 3b is a “jade belt” assembled from a ribbon with two creases. One advantage of the demonstrated self-assembly method is that the responsive SU-8 substrate can be a part of the final structure, increasing complexity of the final 3D structures. This is shown by a pair of “sunglasses” and a 13 ACS Paragon Plus Environment


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“human pyramid” that were formed from the assembled the passive ribbons together with the responsive substrates (Figure 3c-d). Generally, kinks induce stress concentrations, which may result in mechanical fracture when bending and folding movement happen in materials. By kirigami cutting in some areas such stresses can be released.43 Thus, we also combined the kirigami principle44 with the origami one for self-assembly of more geometries in the final 3D structures. Figure 3e-h show the assembled 3D structures (right column) from 2D membranes (left column). Figure 3e is a “box” assembled from a passive SU-8 membrane cut into five continuous pieces of rectangles by the kirigami principle. Figure 3f is a “pyramid” assembled from a membrane in which four triangles are connected at the center with creases. The 2D precursor in Figure 3g is a passive SU-8 membrane with a cut of a strip in the middle (not cut through) and a crease at the midpoint of the strip. During transformation, folding occurs at the crease of the strip while the main part of the membrane does not change. Finally, a “grating” is formed. The similar structure was demonstrated to tune optical transmission.43 Figure 3h shows a row of “armchairs” assembled from a 2D precursor cut with a “clip” shape. More 3D structures such as a “butterfly” are illustrated in Figure S3.


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Figure 4. Schematic and photographs of multi-layer 3D architectures assembled from multimaterials. (a) A bilayer structure of Au coated PI. (b) A bilayer structure of PI and passive SU-8. (c) A three-layer structure of PI, Al, and passive SU-8. Scale bars: 3 mm.

The proposed self-assembly approach is valid for other materials besides the SU-8. Figure 4 shows 3D structures assembled from multi-layer films made of different types of materials on the responsive SU-8 substrates with a “six-petal flower” shape. Figure 4a shows self-assembly of passive PI films coated with 80 nm Au. A layer with a small “three-claw” geometry is bonded onto three central-symmetry petals of the responsive substrate. Another layer with a larger “three-claw” geometry is bonded onto the rest of the petals. The creases (red areas in schemes) are located at the intersection of three claws in each layer (covered by Au coating). These creases were produced by laser induction. The induced areas in the PI sheet are made of porous carbon with a much smaller Young’s modulus than the PI, leading to a much smaller bending stiffness 15 ACS Paragon Plus Environment


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(Eq. 3). Herein, we only consider the bending stiffness of the residual PI film at the creases. Because nanometer thick Au can be also neglected when compared to µm thick PI. When they were exposed to the acetone stimulation, the 2D films were assembled into a “palace lantern”. Figure 4b shows another bilayer structure resembling a “palace lantern” made of different layers with various materials. The bottom layer is the “three-claw” consisting of the passive SU-8 directly bonded onto three separate petals of the responsive substrate. The other layer with a larger “three-claw” made of PI/Au is bonded onto the other three petals. A three-layer “interchange” 3D structure is illustrated in Figure 4c. It is assembled from three membranes made of polyimide, Al, and passive SU-8. Each ribbon has a crease in the middle. Finally, a 3D structure constructed by PI and passive SU-8 is shown in Figure S4. The proposed strategy has demonstrated its power for reversible self-assembly of 3D structures from diverse materials. These structures can serve as scaffolds for 3D electronics. After a 2D passive SU-8 film was prepared, a microscale GaN-based LED with gold electrodes was applied onto the surface of the film (Figure 5a). The bulb was lighted by electricity conducted through external aluminum wires. After exposed to stimilus, the 2D film with the lighting LED was mechanically buckled to a 3D scaffold (Figure 5b). Moreover, we also demonstrated a flex sensor that can be assembled to a 3D structure. The scheme and the photograph of the device are shown in Figure 5c-d, respectively. The flex sensor was made by laser-induced graphene (LIG)

36

attached to a PI film. In this case, the polyimide acts as a

supporting layer while the LIG is the electrical sensing element for the sensor. This flex sensor was bonded onto the responsive SU-8 substrate. When it was mechanically buckled by the responsive substrate, there was a strian happening at the creases. The strain increases the spacing of the conductive network in the LIG, thus lowering the number of contact points and resulting


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in an increase of the electrical resistance.45 As the bending angles increase, the resistane change increases more (Figure S5). The relationship of the resistance change versus the bending angle was measured. I-V curves under different angles were shown in Figure S6. Then normalized resistances (R/R0) were calculated and plotted in Figure 5e. R/R0 shows quite linear relationship with the bending angles. Moreover, R/R0 at 0 degree and 53 degree, show almost no change after 50 cycling (Figure S7), indicating a quite reliable sensing behavior.

Figure 5. 3D scaffolds integrated with electronics. The photographs of the 3D scaffold integrated with a LED bulb and gold electrodes before (a) and after (b) self-assembly. The schematic (c) 17 ACS Paragon Plus Environment


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and photograph (d) of 3D scaffold integrated with a LIG-based flex sensor. (e) The characteristic curve of a flex sensor and measured data during self-assembly. Scale bars: 3 mm.

4. CONCLUSION In summary, we have demonstrated reversible self-assembly of 3D architectures which can be applied to shape-reconfigurable electronics. This assembly not only inherits advantages of the wide material applicability and broad geometric diversity from the mechanical buckling driven self-assembly, but also is grafted with responsive and free-standing nature from the responsive substrate. As a consequence, complex 3D structures were assembled from a set of 2D films. Moreover, 3D geometries were integrated with electronics, including µLED bulbs and LIG flex sensors. Such integrated responsive devices can be used in various fields such as soft robotics, tissue regeneration and monitoring, MEMS, and wearable sensors. The proposed strategy would shed a new light on the development of 3D architectures with complex geometries that are not easily assessible by other assembly methods.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detail descriptions of bending moment measurement, bending beam model, additional optical images of assembled 3D architectures, and measurement of flex sensor.

AUTHOR INFORMATION Corresponding Author


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*

E-mail: [email protected] (J. L.).

*

E-mail: [email protected] (Z. Y.).

ORCID Jian Lin: 0000-0002-4675-2529 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS J.L. acknowledge the financial support from University of Missouri-Columbia start-up fund, Oak Ridge Associated Universities (ORAU) Ralph E. Powe Junior Faculty Award, NASA Missouri Space Consortium (Project: 00049784), NSF IGERT program (Award number: 1069091), and NSF SBIR program (Award number: 1648003).

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Figure 1. Schematic of fabrication and self-assembly processes of 3D structures consisting of passive SU-8 films and responsive SU-8 substrates. (a) Fabrication process of passive SU-8 films. (b) Fabrication process of responsive SU-8 substrates. (c) Assembly of passive films and responsive substrates. (d) Self-assembled 3D structures. Scale bars: 5 mm. 333x157mm (300 x 300 DPI)

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Figure 2. (a) Scheme of a passive SU-8 ribbon bonded on a responsive SU-8 substrate. The passive ribbon has a crease with length of Lc and thickness of Tc. (b) Photographs of a representative group of selfassembled structures, in which the passive ribbons have creases with various lengths. (c) Photographs of a representative group of self-assembled structures, in which the passive ribbons have creases with various. (d) Relationship of bending degrees and lengths of the crease. (e) Relationship of bending degrees and thicknesses of creases. FEA modeling results of after-assembled SU-8 ribbons with a crease (f) and without a crease (g). Scale bars are 6 mm. 337x190mm (300 x 300 DPI)


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Figure 3. Schemes and photographs of 3D architectures assembled from passive SU-8 ribbons (a-d) and membranes (e-h). Scale bars are 6 mm. 287x190mm (300 x 300 DPI)



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Figure 4. Schemes and photographs of multi-layer 3D architectures self-assembled from multi-materials. (a) Bilayer structure of Au coated PI. (b) Bilayer structure of PI and passive SU-8. (c) Three-layer structure of PI, Al, and passive SU-8. Scale bars are 3 mm. 269x189mm (300 x 300 DPI)


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Figure 5. 3D scaffolds integrated with electronics. The photographs of the 3D scaffold integrated with a LED bulb and gold electrodes before (a) and after (b) self-assembly. The scheme (c) and photograph (d) of 3D scaffold integrated with a LIG-based flex sensor. (e) The characteristic curve of the flex sensor and the measured data during self-assembly. Scale bars are 3 mm. 160x141mm (300 x 300 DPI)


Reversible Self-Assembly of 3D Architectures Actuated by Responsive

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