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Enhancement of Interfacial Adhesion Using Micro/Nanoscale Hierarchical Cilia for Randomly Accessible Membrane-Type Electronic Devices Youngkyu Hwang,†,§,¶ Seonggwang Yoo,†,¶ Namsoo Lim,† Sang Myeong Kang,† Hyeryun Yoo,† Jongwoo Kim,‡ Yujun Hyun,† Gun Young Jung,† and Heung Cho Ko*,† †

School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryong-Dong), Buk-Gu, Gwangju 61005, Republic of Korea ‡ Center for Convergent Research of Emerging Virus Infection, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea S Supporting Information *

ABSTRACT: The recent technology of transfer printing using various membrane-type flexible/stretchable electronic devices can provide electronic functions to desirable objects where direct device fabrication is difficult. However, if the target surfaces are rough and complex, the capability of accommodating surface mismatches for reliable interfacial adhesion remains a challenge. Here, we demonstrate that newly designed nanotubular cilia (NTCs), vertically aligned underneath a polyimide substrate, significantly enhance interfacial adhesion. The tubular structure easily undergoes flattening and wrapping motions to provide a large conformal contact area, and the synergetic effect of the assembled cilia strengthens the overall adhesion. Furthermore, the hierarchical structure consisting of radially spread film-type cilia combined with vertically aligned NTCs in specific regions enables successful transfer printing onto very challenging surfaces such as stone, bark, and textiles. Finally, we successfully transferred a temperature sensor onto an eggshell and indium gallium zinc oxide-based transistors onto a stone with no electrical failure. KEYWORDS: transfer printing, flexible electronics, interfacial adhesion, multiscale roughness, nanotubular cilia, hierarchical structure which can be applied for footpads of stickybots,25 biomedical patches,26,27 and transfer-printing heads.28,29 Controlling the geometry in terms of aspect ratio,30−32 end shape,33−38 and pillar slanted angle39,40 further modulates the interfacial adhesion. Regarding the formation of nanostructures, various shapes, including nanorods,41,42 nanofibers,43−47 and nanotubes,42,46,48−50 can be created by a molding process against various templates based on polydimethylsiloxane (PDMS),45 MnO2,48 polyester membranes,46 or anodized aluminum oxide (AAO).41−44,47 In particular, the fabrication of polymer nanostructures based on AAO templates is widely used because the material itself is inert in common organic solvents used in the molding process, and controlling several

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he recent technology of developing membrane-type flexible/stretchable electronic devices that are light, deformable, imperceptible, and eco-friendly1−4 enables “stick-and-play” systems, which provide electronic functions to desired surfaces. The basic strategy when direct fabrication is extremely difficult is to create devices as thin as possible5−7 followed by transfer printing onto diverse target surfaces, such as textiles,8 skin,9,10 and the brain.11,12 The indirect method naturally encounters interfacial adhesion between the device layer and target surfaces, particularly those with multiscale roughness. For example, in the case of chemical aspects, glues,13,14 tapes,15 and surface treatments with UVO16/O2 plasma17 and self-assembled monolayers18,19 are widely used. Regarding physical aspects, decreasing the thickness of a device reduces the critical adhesion energy for reliable conformal contact.20 Introducing micro/nanostructures such as artificial gecko foot hairs21−24 can also enhance interfacial adhesion with anisotropic intensity in attaching and detaching processes, © XXXX American Chemical Society

Received: March 19, 2019 Accepted: August 21, 2019

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Figure 1. Development of a PI substrate with NTCs and enhanced interfacial adhesion. (a−e) Illustration of the fabrication process for a PI substrate with NTCs. Top-view TEM images and cross-sectional SEM images of the selected areas in (c) and (d) are added. An illustration of the backside of the sample in Figure 1e is also added. (f) Photographic image of a PI substrate (lateral dimensions: 1 cm × 2 cm) after NTCassisted transfer printing onto a paper sheet. (g) Peel force of a PI substrate with alternating flat (lateral dimensions = 3 mm × 1 cm) and NTCs (lateral dimensions = 3 mm × 1 cm, average length of NTCs = 6 μm) regions that was transferred onto a paper sheet during the 90° peel test. (h and i) Photographic and SEM images of the surfaces of the paper sheet and the PI substrate with NTCs (NTC side face up) after peeling the sample of (f); the contrast between the NTCs region and flat region is inverted compared to that in the image of (f).

parameters including the polymer infiltration time, molecular weight of the polymer, and pore dimensions of the AAO enables the modulation of the morphology.51 To transfer electronic devices onto desired nonuniform and complex surfaces, the capability of adhering to such randomly rough surfaces is important. In the case of artificial gecko foot hairs, the main efforts toward robust and reversible performance of the dry adhesive would sacrifice the ability to achieve sufficient contact area or the omnidirectional adhesion capability. For example, a cylinder shape37 has no directional preference in bending, but the solid structure requires excessive preload conditions for a sufficient contact area. On the other hand, cuboid38 and disk33 shapes can easily wrap to relatively smooth surfaces, but the directional preference hampers conformal contact with unpredictable complex surfaces. Next, the transfer process should not alter or degrade the characteristic features of desired objects. For example, the use of a toxic solvent or glue is not acceptable for agricultural and stockbreeding products. Preloading conditions, which are generally applied to some extent (i.e., 0.1−0.3 N/cm2).23,24 for the attachment of artificial gecko foot hairs may cause unwanted damage to particularly fragile, tiny, or living things. This study addresses the aforementioned issues by introducing nanoscale tubular structures underneath a thin polyimide (PI) substrate that also serves as a platform for mounting membrane-type electronic devices; the nanoscale tube arrays are called nanotubular cilia (NTCs). Regarding the geometric aspect, the tubular shape easily deforms to accommodate surface mismatches and thereby enlarge the interfacial contact area. For the transfer process, we use just water without mechanical preloading or glue to minimize the physical and chemical damage to various surfaces. A mechanical peel test and numerical simulation of the transferred polymer substrates

with/without NTCs and the examples of transfer printing with diverse sizes and patterns (triangle, square, and circle) of substrates onto various target surfaces (a paper sheet, a leaf, a stone, a piece of bark, an adhesive bandage, and an eggshell) confirm the feasibility of this method. Finally, we successfully transferred a temperature sensor onto an eggshell for an ecofriendly in situ internal monitoring system and indium gallium zinc oxide (IGZO)-based transistors with radially spread hierarchical cilia onto a stone.

RESULTS AND DISCUSSION Fabrication of the PI Substrate with NTCs. Figure 1 shows the fabrication procedure to create a PI substrate with NTCs using an AAO template. A detailed fabrication process is presented in the Experimental Section. Briefly, after the plasma-enhanced chemical vapor deposition (PECVD, Ar/ N2O/SiH4 = 100/30/15 sccm, 100 W, 2 Torr) of a SiO2 layer (thickness = 200 nm) on polished aluminum foil (thickness = 250 μm, Figure 1a), patterning through a commercial photolithography process generates a masking layer (lateral dimensions of an individual open area = 3 mm × 1 cm) (Figure 1b). The anodization of Al using a phosphoric acid (H3PO4) solution (1.6 wt % in deionized (DI) water, 160 V) generates an AAO template with arrays of pores; in this process, an anodizing time of 1 to 10 h generates pores with a depth range of 2 to 15 μm, whereas the average diameter (∼100 nm) and density (7.6 μm−2) of the pores are almost unchanged. Dipping the AAO template in H3PO4 solution (6 wt % in DI water, 35 °C) for 40 min widens the pores; see the transmission electron microscope (TEM) and scanning electron microscope (SEM) images of a representative AAO template (pore depth = 6 μm, pore diameter = 196 nm, and pore density = 7.6 μm−2) in Figure 1c and Figure S1. Next, B

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Figure 2. Origin of the enhanced interfacial adhesion of NTCs and examples of transfer printing. (a) Contact width with the corresponding estimated van der Waals force and fracture force of a nanotube (outer diameter = 196 nm) to a planar surface as a function of the thickness of the tube wall at various downward body loads. (b) Stress distribution and geometric structures, as obtained by FEM simulation, of a PI nanocylinder and a PI nanotube on a planar surface after applying a downward body load of 23 μN. SEM shows a magnified image of a flattened NTC after drying. (c−h) Photographic images of various PI patterns, such as a circle, a triangle, and a square, transferred onto paper sheets and adhesive bandages. The PI patterns were deposited by Cr (thickness = 100 nm) for high optical contrast. The red, green, and yellow boxes correspond to the magnified photographic images of the selected area. (i) Average pressures required for detaching PI substrates (lateral dimensions = 5 mm × 5 mm) from various surfaces (paper, a leaf, cotton, a branch, and an eggshell). The pressure was generated by an air blow gun. Photographic images of the samples before the detaching process are added.

removes the AAO template and floats the PI substrate on the surface of the etchant solution. Next, the PI substrate is placed in fresh DI water, as shown in Figure 1e, to rinse the surface of the NTCs; the rinsing process is carried out two times. Finally, the floated sample is transferred to a paper sheet in wet conditions and completely dried at room temperature to complete the transfer printing (Figure 1f). Enhanced Interfacial Adhesion of PI Substrate with NTCs. To confirm the effect of the NTCs on the interfacial adhesion to complex surfaces, we performed a 90° peel test at a linear motion speed of 3 mm/s using a PI substrate with alternating flat (lateral dimensions = 3 mm × 1 cm) and NTC (lateral dimensions = 3 mm × 1 cm, average length of NTCs = 6 μm) regions that was transferred onto a paper sheet (Figure 1g and Figure S5); see more detailed photographs and a schematic illustration of the experimental setup for the 90° peel test in Figure S6. The average peel force of the NTCs is 0.017 N/cm, which is more than 5 times higher than that of the flat region (0.0033 N/cm). We note that the additional suction of air underneath the sample or exposure to water vapor during the drying process tends to increase the interfacial adhesion by 54% and 55%, respectively, presumably because the additional air-driven force or prolonged drying

spin-coating a PI precursor solution [poly(pyromellitic dianhydride-co-4,4′-oxidianiline), amic acid, Sigma-Aldrich] onto a patterned AAO template and annealing at 150 °C for 3 h forms a PI film with NTCs infiltrating even 15 μm deep pores (Figures S2 and S3); the thickness of the tube walls becomes 20 nm at 1 μm below the top and 17 nm at 5 μm below the top according to TEM images after focused ion beam etching. According to the literature for the case of polystyrene with a sufficiently high molecular weight (e.g., Mn > 18 100 g/mol), a short infiltration time (e.g., < 18 h) and proper template pore size range of diameter (e.g., 150−300 nm) induce hollow nanotubes rather than solid nanorods.52 In the case of using PI precursor solutions (viscosity = 6−10 poise), a nanotubular structure can be achieved under similar conditions (infiltration time = 3 h, pore size = 196 nm). Importantly, according to a detailed SEM image analysis, the NTCs have nanoholes (hole diameter = 50−200 nm) on the tube walls (Figure S4). The reactive ion etching (RIE, O2, 20 sccm, 50 mTorr, 150 W) of the sample through a SiO2 masking layer, which is generated by the same process as that in Figure 1b, generates a PI substrate in a desired shape with patterned NTCs (lateral dimension = 1 cm × 2 cm) (Figure 1d). Dipping the sample in HF solution (49−51% HF, Duksan) C

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directly influence the van der Waals force according to the equation53

time allows the NTCs to adjust themselves to mitigate the surface mismatch more efficiently (see a more detailed study in Figure S7). Interestingly, a surface examination of the paper sheet and the PI substrates after the peel test clearly reveals that many NTCs were fractured, thereby leaving residues on both surfaces; see the SEM images in Figure 1h and i. Moreover, the bundle of fractured NTCs on the paper sheet became flattened, resulting in a large conformal contact with the cellulose fibers. These observations indicate that the peel force is mainly determined by the strength of the NTCs because the interfacial adhesion between the NTCs and the cellulose is strong enough even though no additional glue is used in this test. Dipping the paper with the fractured NTCs in water for 30 min or sonicating in water for 5 min removed most of the NTC residues (Figure S8). Origin of Enhanced Interfacial Adhesion from NTCs. To determine the origin of the enhanced interfacial adhesion arising from the NTCs, it is important to understand the contribution of an individual NTC to the conformal wrapping onto a certain surface and the force required for fracture (the so-called fracture force) (Figure S9 and Supporting Information). First, in terms of conformal wrapping, we performed a numerical simulation based on the finite element method (FEM) to predict the geometric shapes of a nanocylinder cilium (NCC, diameter = 196 nm) and an NTC (outer diameter = 196 nm; thickness of tube wall was varied) after placing them onto a planar surface and applying various downward body loads (Figure 2a). In the wet transfer and drying process, the capillary action occurring outside (for both the cases of the NCC and NTC) and inside (for the case of an NTC only) a cilium during the drying process should provide a downward body load. The theoretical calculation for the outside of an NCC, with several assumptions including (1) a fixed geometry of the cylindrical structure and (2) a water bridge formed between the cylinder and planar hydrophilic substrate, results in an attractive capillary force of 17.5 μN (see the details in Figure S10 and Supporting Information). On the other hand, in the case of an NTC, the capillary force is expected to be larger than that of the NCC due to several reasons. First, an NTC can be easily flattened due to its hollow structure, which is expected to increase the capillary force for the outside of the NTC. Second, there is a slight capillary force arising from the inside of the NTC with a nanohole, which is calculated to be 1.17−4.67 nN for the nanohole diameter range of 50−200 nm (see the details in Figure S11 and the Supporting Information). Although the value of the NCC is expected to be lower than that of an NTC, it is worth comparing the structural effect only of an NCC and an NTC on the geometric structure together with the stress distribution under the same downward body load conditions. We set the value to be 23 μN, corresponding to the conditions matching the actual shape of the flattened NTCs in SEM images. In the case of the NCC, a high stress is concentrated on the local region contacting the substrate, and it deforms little with a cross-sectional contact width of 34 nm (i.e., 17 nm for a halfside cylinder) (left side of Figure 2b). On the other hand, an NTC with a wall thickness of 20 nm is easily flattened, except for the rounded edges, to provide sufficient contact (contact width = 215 nm) under the same downward body load conditions (23 μN), which corresponds to the NTC used in this study (right side of Figure 2b); see the nonlinear curves for the contact width as a function of the thickness of the tube wall in Figure 2a. The contact width after deformation should

F=

Abl 6πz 0 3

where A is the Hamaker constant, z0 is the distance between the two flat regions, and b and l are the contact width and length of the contact area, respectively. We assumed that l is 6 μm for an NTC and that z0 and A are 0.3 nm and 1 × 10−19 J, respectively, which are typical values for two solids.54 Judging from the stress−strain curve of PI (ultimate strength = 400 MPa),55 we can estimate the fracture force of an individual cilium as a function of the thickness of the tube wall (bottom in Figure 2a). As expected, the value increases with the wall thickness to reach a maximum for the cylinder shape; the fracture forces of the NTC and NCC are estimated to be 4.4 and 12.1 μN, respectively. Even though the strength of the individual NTC is smaller than that of the NCC, we believe that the synergetic effect of assembled NTCs with high density, such as the case in Figure 1e, reinforces the overall adhesion force. Transfer Printing of Diverse Patterns of PI Substrates with NTCs. To demonstrate the feasibility of this method, diverse patterns of PI substrates with NTCs (length = 6 μm) were transferred onto various target surfaces. For example, for the sub-millimeter scale, a circle pattern (diameter = 500 μm), a square pattern (side = 500 μm), and a triangle pattern (side = 3 mm) are successfully transferred onto a paper sheet (double A, Figure 2c to f). On a centimeter scale, a square pattern (side = 1 cm) is transferred onto an adhesive bandage (Young Chemical Co, Ltd.) without significant distortion, whereas the same pattern without NTCs is partially delaminated (Figure 2g and h). Adhesion strength can vary depending on the target substrates. To determine the interfacial adhesion of PI substrates (lateral dimension = 5 mm × 5 mm, thickness = 1.2 μm) onto various surfaces with arbitrary shapes (for example, a paper, a leaf, cotton, a branch, and an eggshell are used for demonstration, but water-dissolvable target surfaces were not available in this study), a detachment test was performed by blowing air (Figure 2i), which is more meaningful in a practical manner because air blowing occurs outside naturally, and a nonplanar target surface is not able to maintain 90° of the detached PI in the entire area during the peeling process (see the experimental setup in Figure S12). The average critical pressures required for detaching PI substrates with NTCs (without NTCs) are 6.44 kPa (0.95 kPa) for a paper, 0.23 kPa (0.03 kPa) for a leaf, 0.26 kPa (0.05 kPa) for cotton, 0.81 kPa (undetectable) for a branch, and 9.11 kPa (undetectable) for an eggshell. PI substrates with NTCs still adhered to a paper sheet and an eggshell even after the prolonged dipping of the paper sheet and egg with the transferred PI substrates in water for 30 min. In the case of the eggshell, reusing the PI substrate with NTCs after dissolving the eggshell using diluted HCl(aq) resulted in a decrease in the adhesion (critical air pressure from 9.11 to 3.23 kPa). Randomly Accessible Micro/Nanoscale Hierarchical Cilia. To enhance the accessibility to various complicated surfaces, we designed the main PI substrate with three types of peripheral cilia. The parameters used to describe the geometrical design are the diameter of the main PI substrate (d = 1 or 5 mm), the interval distance between neighboring peripheral cilia (s = 10 or 20 μm), the length of a peripheral D

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Figure 3. Transfer printing of PI substrates with various types of hierarchical cilia. (a) Illustration of the PI substrate with hierarchical cilia consisting of alternating MFCs and NTCs and the parameters used to describe the geometrical design. SEM image (blue box) of NTC region on a peripheral cilium after removing the AAO template. (b−d) SEM images of the PI substrates (d = 5 mm, s = 20 μm) with three types of cilia transferred on a textile: (b) MFCs only, (c) NTCs only, and (d) hierarchical cilia (MFCs-NTCs). (e−j) SEM and photographic images of the PI substrates with hierarchical cilia (d = 1 mm and 5 mm, s = 10 μm) on a stone (e−h) and a piece of bark (i, j). The green, blue, yellow, and red boxes correspond to magnified images, respectively. (k) Photographic images of a PI substrate (lateral dimensions = 2 cm × 2 cm) with nine regions of NTCs (each lateral dimension = 3 mm × 3 mm) as a function of the drying time after dropping a droplet of aqueous rhodamine B solution (Sigma-Aldrich, 10 wt %). (l) Average peel force profile of the PI substrate with alternating flat (lateral dimensions = 3 mm × 1 cm) and NTCs (lateral dimensions = 3 mm × 1 cm, average length of NTCs = 6 μm) regions that was transferred onto a paper sheet using diverse amounts of glue during the 90° peel test; the initial concentration of PVA is noted. Inset shows the profile of the peel force of the PI substrates with no glue and 0.5 and 2 wt % of PVA. (m−o) Photographic and SEM images of a PI substrate with hierarchical cilia (d = 5 mm, s = 10 μm) transferred onto a textile material using PVA glue (initial concentration of PVA solution = 2 wt %). The green and red boxes correspond to the magnified images. E

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Figure 4. Examples of stick-and-play systems based on PI substrates with NTCs. (a) Illustration and photographic images of a temperature sensor (PI/Au/Cr/PI = 1.8 μm/70 nm/6 nm/1.8 μm, dimensions = 9 mm × 7 mm) with NTCs only transferred on an eggshell. (b) Profiles of the temperatures measured by the temperature sensor and a commercial sensor (MR-2040 k-type, Miraetech) after placing the sample inside an oven at 60 °C; the temperature inside the eggshell was simultaneously monitored by the commercial sensor that was inserted through a tiny hole in the eggshell. Separately, the temperature of the oven was monitored by the two sensors without an egg for comparison. (c) Illustration of the PI substrate with radially spread hierarchical cilia consisting of alternating MFCs and NTCs. (d−h) Photographic, SEM, and optical microscope images of IGZO-based TFT arrays developed on a PI substrate with hierarchical cilia (MFCsNTCs) after transfer printing onto a basalt tile. (i) IDS−VG and IDS−VDS curves of the representative IGZO-based TFT in (e). F

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transferred the PI substrate with hierarchical cilia (d = 5 mm, s = 10 μm) onto a textile material (Figure 3m to o); see the PVA residue accumulated around the region of NTCs. Other glues such as PDMS and a PAA-based hydrogel can also promote interfacial adhesion (Figure S16). Stick-and-Play System Based on NTC-Assisted Transfer Printing. To demonstrate the feasibility of the ciliaassisted transfer method for “stick-and-play” systems at the electronic level, we first developed an eco-friendly membranetype temperature sensor. The fabrication begins with the creation of a PI substrate with NTCs on a nonpatterned AAO template. The sequential deposition of Cr and Au layers by sputtering (Ar, 15 sccm, 290/310 V, 5 mTorr, Korea Vacuum Tech., Ltd.) and coating with a top PI layer generates a temperature sensor (PI/Au/Cr/PI-NTC = 1.8 μm/70 nm/6 nm/1.8−6 μm, lateral dimensions of PI substrate, line width and length of Au/Cr in heater region = 15 μm and 110 mm; for a more detailed fabrication process, see Experimental Section). Figure 4a shows the final sensor transferred onto an eggshell using the steps shown in Figure 1d to e; see the calibration curve in Figure S17 and the Supporting Information. For comparison, we inserted a commercial temperature sensor (MR-2040 k-type, Miraetech) inside the eggshell through a tiny hole to simultaneously monitor the temperatures inside and on the surface of the eggshell. After placing the egg with the sensors inside an oven that was already set to 60 °C and equilibrated for 3 h, we monitored both signals from the two sensors (Figure 4b). As a preliminary test, both sensors without an egg show a rapid increase upon equilibration at the oven temperature. The temperature in the oven fluctuated due to the oscillation between the heating and cooling modes. On the other hand, with an egg, the temperature measured by the membrane-type sensor increases slowly and reaches a plateau after ca. 200 min, which indicates that the temperature of the eggshell is almost identical to that inside due to its high heat capacity. We note that the temperature measured by the membrane-type sensor is higher by 1−3 °C than that measured by the commercial sensor, presumably due to the heterogeneous distribution of temperature in the egg (Figure S17 and the Supporting Information). We believe that it is possible to monitor the temperature inside an egg using a membrane-type sensor because of its unique properties, such as its extremely low heat capacity (∼3.5 × 10−4 J/K,59,60 cf. egg ≈ 190 J/K,61 Table S1), high ratio of lateral contact area to volume (2.8 × 105 m−1 for the case of ideal contact), and good conformal contact with the eggshell through the NTCs (thermal conductivity of PI and air = 0.1− 0.35 W/m·K and 0.026 W/m·K, respectively);61 even though the thermal conductivity of PI is much lower than that of metal (e.g., Au = 315 W/m·K),62 the thin structure design and the use of NTCs overcomes this issue. We also created membrane-type IGZO-based thin-film transistor (TFT) arrays and laminated the device with a PI substrate with radially spread hierarchical cilia (Figure 4c); for a more detailed fabrication process, see the Experimental Section and Figures S18 and S19. Membrane-type IGZObased TFT arrays with radially spread hierarchical cilia were successfully transferred onto a basalt tile (Figure 4d−h). The radially spread hierarchical cilia with MFCs and NTCs wrapped well even on the rough and porous surface; see the backside of the NTC region noted by the red arrow in Figure 4f. The transferred device shows the typical electronic performance of an IGZO-based TFT with no significant

cilium (L = 1 mm), the length/width/thickness in the region of the microfilm-type cilium (MFC) (lMFC/wMFC/tMFC = 300/ 10/2 μm), and the length/width/thickness in the region of the NTCs (lNTCs/wNTCs/tNTCs = 20/10/1 μm) (Figure 3a and Figure S13); the values of d and s will be specified hereafter. The PI substrates (d = 5 mm, s = 20 μm) with the three types of peripheral cilia, (1) MFCs only, (2) NTCs only, and (3) hierarchical cilia consisting of alternating MFCs and NTCs, were transferred onto a textile material. In the case of MFCs only, only partial contact to the yarn-scale (diameter ≈ 100 μm) surface roughness is allowed (Figure 3b); reducing the thickness of the MFCs (e.g., thickness 3 μm) does not allow conformal contact. In the case of NTCs only, assembled NTCs excessively increase the stiffness of the peripheral cilia for sufficient conformal wrapping (Figure 3c). In contrast to the MFCs and NTCs alone, the hierarchical cilia with MFCs and NTCs provide more reliable conformal contact to the textile with tighter adhesion in the regions of the NTCs (Figure 3d). The design of the hierarchal cilia is very powerful for conformal wrapping on complex surfaces such as a stone (d = 1 mm, s = 10 μm, Figure 3e to h; we note that the delamination of peripheral cilia with MFCs only occurs after drying; see Figure S14) and a piece of bark (d = 5 mm, s = 10 μm, Figure 3i and j). To further enhance the interfacial adhesion, we can use a small amount of glue without losing characteristic features of the target surfaces. As expected, the cilia structure, the NTCs in particular, effectively confines the glue inside the regions; see the sample images with nine regions of NTCs as a function of the drying time after dropping a droplet of aqueous rhodamine B solution (10 wt %, Sigma-Aldrich, Figure 3k, Figure S15). According to the literature, water with meniscuses in nanostructures evaporates more slowly than water on a planar region.56 In the hierarchical cilia, we believe that the water in the NTC regions evaporates slower than that on the MFC regions and thereby induces the natural migration of the solute in the NTC regions. Moreover, the capillary flow of the solution also occurs from the planar region to the NTC regions, which is another origin of the confining effect of glue in the NTC regions.57,58 To determine the effect of using glue on interfacial adhesion, we carried out a 90° peel test of the PI substrates (lateral dimension = 2 cm × 1 cm) with alternating flat (lateral dimensions = 3 mm × 1 cm) and NTC (lateral dimensions = 3 mm × 1 cm, length of NTCs = 6 μm) regions that were transferred onto a paper sheet with/without the use of poly(vinyl alcohol) (PVA, Sigma-Aldrich). The initial PVA concentration was varied to reduce the amount of residual PVA after complete drying (Figure 3i). As expected, the interfacial adhesion of the regions of NTCs significantly increases with the initial concentration of the PVA solution. For example, the average peel force at 2 wt % PVA(aq) becomes 2.5 and 0.28 N/cm in the NTCs and flat regions, whereas the values without glue are 0.017 and 0.0033 N/cm. It is natural that the use of glue increases the adhesion; the peel force of the flat regions with 2 wt % PVA(aq) is higher by 16 times than that of the NTC regions without PVA. However, it should be stressed that the synergetic effect of the use of cilia and a small amount of glue dramatically enhances the interfacial adhesion in desirable and specific regions by 147 times. Under 2 wt % PVA(aq) conditions, we successfully G

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solution. After rinsing the PI substrate with DI water two times, placing it onto a desired surface and drying completely at room temperature finish the transfer process. Fabrication of the Temperature Sensor (Figure 4a). After the generation of the PI substrate (thickness = 2 μm) on an AAO template with no pattern, sputtering Cr and Au layers (thickness = 6 and 70 nm, Ar, 15 sccm, 310 and 290 V, 5 mTorr, Korea Vacuum Tech., Ltd.) on the PI substrate and wet etching (CR-7 OMG for Cr, Cyantek, Gold Etch for Au, Transene) with a patterned PR layer form the metal line for the temperature sensor. After encapsulating the metal with a PI layer [thickness = 2 μm, one cycle of coating and curing (conditions: 4000 rpm for 60 s, 150 °C for 3 h)], the RIE (O2, 20 sccm, 150 W, 50 mTorr, 16 min) of the PI layer through a patterned SiO2 masking layer (thickness = 200 nm) completes the fabrication of the temperature sensor (lateral dimensions = 9 mm × 7 mm). Dipping the sample into HF solution (49−51% HF, Duksan) removes the AAO template and floats the temperature sensor on the surface of the etchant solution. After rinsing the temperature sensor with DI water two times, placing it onto a desired surface and drying completely at room temperature finish the transfer process. Fabrication of IGZO-Based TFT Arrays (Figure 4c−h). After the generation of a PI substrate (thickness = 1.8 μm) on a Si wafer coated with a SiO2/GeOx (thickness of top SiO2 layer = 200 nm, PECVD, Ar/N2O/SiH4, 100/35/15 sccm, 100 W, 2 Torr, 270 °C, and the thickness of the bottom GeOx layer = 300 nm, Ar/O2, 10/5 sccm, 60 W, 2.5 mTorr), the additional deposition of SiO2 (PECVD, thickness = 200 nm, Ar/N2O/SiH4, 100/35/15 sccm, 100 W, 2 Torr, 300 °C) on the PI substrate forms a barrier layer against the thermal expansion of the PI substrate and thereby allows a broad range of fabrication temperatures. Next, sputtering Mo (thickness = 70 nm, Ar, 15 sccm, 240 V, 5 mTorr) and patterning by photolithography and RIE (SF6/O2, 10/40 sccm, 100 W, 100 mTorr, 1 min) form a gate electrode layer. Depositing SiO2 (PECVD, thickness = 100 nm, Ar/ N2O/SiH4, 45/50/5 sccm, 100 W, 2 Torr, 300 °C) and patterning by photolithography and RIE (CF4/O2, 50/2.5 sccm, 100 W, 100 mTorr, 3 min) form a gate dielectric layer. After the deposition of the IGZO (thickness = 15 nm, sputter, Ar/O2, 15/0.15 sccm, 50 W, 2.5 mTorr), a further annealing process [IR furnace (RTA), Ajeon Heating Industrial Co., Ltd., 300 °C, 99 min, air, 0.3 L min−1], patterning by photolithography, and wet etching with a dilute HCl solution (DI water/HCl = 50:1, HCl: 35%, OCI) form a channel layer. After patterning of a PR layer (AZ-5214), coating with Cr and Au (thickness = 10 and 90 nm) followed by a lift-off process using acetone forms the source/drain electrodes. Next, an SU-8 passivation layer (thickness = 100 nm) is formed by spin-coating a diluted precursor solution (wt % ratio for the SU-8 2010/SU-8 thinner = 8:92, MICRO CHEM, at 3000 rpm for 15 s); exposing a desired area to UV light through a Cr mask using a mask aligner (Ca-6M, Shinu MST, illumination: 8.5 mW/cm2, 6 s); pre- and postbaking at 95 °C for 2 and 15 min, respectively; developing in SU-8 developer (Micro Chem, SU-8 developer) for 45 s; and hard-baking at 140 °C for 30 min. After encapsulation of the devices with a PI film (thickness = 1.8 μm), RIE through the SiO2 masking layer (thickness = 200 nm) completes the fabrication of membrane-type IGZO-based TFT arrays. Dipping the sample into DI water removes the GeOx sacrificial layer and floats the temperature sensor on the surface of the water. Placing it onto a desired surface and drying completely at room temperature finish the transfer process. Lamination of Membrane-Type Electronic Devices on the PI Substrate with Hierarchical Cilia (Figure 4c−h). A PI substrate with hierarchical cilia [MFC region: (lMFC/wMFC/tMFC = 960 μm/1 mm/2 μm), NTCs region: (lNTCs/wNTCs/tNTCs = 40 μm/1 mm/1 μm)] on peripheral cilia (d = 6.4 mm, s = 0, L = 7.34 mm) is fabricated through the aforementioned procedures. Dipping the sample into HF solution (49−51% HF, Duksan) removes the AAO template and floats the PI substrate on the surface of the etchant solution. After rinsing the PI substrate with DI water two times, floating it on the surface of DI water and freezing the DI water generate an ice handling disk to temporarily support the PI substrate. We note that the ice handling disk does not cause excessive stress to

electrical failure; see the electrical characteristics of a representative TFT in Figure 4i and Table S2. A detachment test by blowing air on PI substrates with two types of radially spread hierarchical cilia (the value of both wMFC and wNTCs = 100 μm and 1 mm) and electronic devices reveals critical pressures of 0.38 and 1.16 kPa, respectively. The higher adhesion by using wider peripheral cilia is due to its larger contact area. Furthermore, the critical pressure increased to 2.98 kPa after using 2 wt % PVA glue (Figure S20). In this case, even after exposure to an air blowing pressure sufficient to tear the cilia and thereby detach the central device region, residual torn cilia were observed on the basalt tile (Figure S21). The modification of the peripheral cilia such as by using a structure with a tapered thickness can be one of the solutions to avoid this phenomenon.

CONCLUSION In summary, the use of NTCs underneath the PI substrate enhances interfacial adhesion to a rough surface even without the use of glue due to their efficient flattening and wrapping motions during the drying process. Furthermore, hierarchical cilia (MFCs-NTCs) allow conformal wrapping on various much more complex surfaces by the synergetic effect of the coarse and fine motions of the MFCs and NTCs, respectively. This method can be used for the development of eco-friendly “stick-and-play” systems, such as a temperature sensor with NTCs on an eggshell and IGZO-based TFTs on a stone. We also note that there is room to develop hierarchical ciliaassisted transfer printing further to address technical issues of the reusability, the range of available target surfaces, and the cost efficiency. EXPERIMENTAL SECTION Preparation of the AAO Template (Steps Shown in Figure 1a to c). The fabrication process begins with polishing aluminum foil in a perchloric acid solution (0.5 wt % in EtOH) (70.0% HClO4, Samchun, EtOH, Merck KGaA) by applying 20 V for 240 s using a Keithley 2400/4200 instrument (Keithley Instruments, Inc.). After depositing a SiO2 layer (thickness = 200 nm) on the polished Al foil using PECVD (Ar/N2O/SiH4, 100/35/15 sccm, 100 W, 2 Torr, 150 °C, SNTEK), coating a photoresist layer (PR, GXR-601) at a speed of 4000 rpm for 35 s, exposing to UV light using a mask aligner (CA-6 M, Shinu MST, illumination power = 8.5 mW/cm2, exposure time = 7 s), and developing the PR layer using a developer (AZ 500 MIF, Merck, 15 s), a masking pattern of the PR layer is created. Wet etching of the unprotected area using a buffered oxide etchant (BOE 6:1, J. T. Baker) and removing the PR masking layer by a cleaning process (acetone, isopropyl alcohol, DI water) generate a patterned SiO2 masking layer. Anodization of the unprotected area of Al foil in a phosphoric acid solution (0.2 wt % in DI water) (85.0% H3PO4, OCI Company Ltd.) under 160 V for varied times (from 3600 to 36 000 s) generates the AAO template with nanopores (pore depth = 2−15 μm, pore diameter = 196 nm, density = 7.6 μm−2). Dipping the AAO template into phosphoric acid (6 wt % in DI water) at 35 °C for 40 min widens the pores. Fabrication of the PI Substrate with NTCs (Steps Shown in Figure 1c to e). Spin-coating poly(pyromellitic dianhydride-co-4,4′oxidianiline) amic acid solution (4000 rpm, 60 s, Sigma-Aldrich) on the AAO template and curing at 150 °C for 3 h form a PI film; two coating cycles are used to obtain a total thickness of 4 μm for the peel test. After the formation of a patterned SiO2 masking layer (thickness = 200 nm) using the aforementioned process, the RIE of the unprotected area in the PI layer (O2, 20 sccm, 150 W, 50 mTorr, 16 min) generates a desired shape of the PI substrate. Dipping the sample into HF solution (49−51% HF, Duksan) removes the AAO template and floats the PI substrate on the surface of the etchant H

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ACS Nano Notes

the PI substrate during the dissolving process to release the PI substrate, whereas the use of a PVA handling disk induces crack formation during the dissolving process in water due to the swelling effect. After a photocurable adhesive (Norland optical adhesive 81, Norland Products, Inc.) was coated on the PI/ice disk, a thick PI film (thickness = 25 μm) was transferred on top of the PI/ice disk using a PDMS stamp to support the center area mechanically. Next, the device layer (membrane-type IGZO-based TFT array/PI) was finally transferred onto the PI film using a PDMS stamp, and the Norland optical adhesive was photocured using UV light through a mask aligner (CA-6 M, SHINU MST) to complete the lamination process. Characterization Method. The 90° peel test was performed by using a digital force gauge (DS2-5N, Imada Co, Ltd.) and a motorized linear actuator at 0.3 mm/s (LTS-HS, Newport). SEM, TEM, AFM, and optical microscope images were obtained by using a Hitachi S4700 microscope (Hitachi, Ltd.), a Tecnai G2 F30 S-Twin, a Park Systems XE-100 (Park Systems Corp.), and a BX51 system microscope (Olympus), respectively. The specimens for TEM analysis were prepared with a focused ion beam system (FB-2100, Hitachi). Fluorescence intensity was observed by using a confocal laser scanning microscope (Olympus FV1000). Numerical Simulation of NCC and NTC. The mechanical simulation was carried out by FEM using elastoplastic modeling in COMSOL Multiphysics 5.3. The required parameters, including the Young’s moduli and yield stresses of PI, are obtained from the literature.43 For the efficiency of the calculation, we used the halfshape of the cylinder and tube in two dimensions (diameter of NCC = 196 nm, outer diameter of NTC = 196 nm; thickness of tube wall is varied).

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2018R1A2B2005067) and the KIST Institutional Program (Project No. 2Z05730-19-029) and a GIST Research Institute (GRI) grant funded by the GIST in 2019. REFERENCES (1) Salvatore, G. A.; Münzenrieder, N.; Kinkeldei, T.; Petti, L.; Zysset, C.; Strebel, I.; Büthe, L.; Tröster, G. Wafer-Scale Design of Lightweight and Transparent Electronics that Wraps around Hairs. Nat. Commun. 2014, 5, 2982. (2) Klatenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.; Drack, M.; Schwodiauer, R.; Graz, I.; BauerGogonea, S.; Bauer, S.; Someya, T. An Ultra-Lightweight Design for Imperceptible Plastic Electronics. Nature 2013, 499, 458−463. (3) Araki, H.; Kim, J.; Zhang, S.; Banks, A.; Crawford, K. E.; Sheng, X.; Gutruf, P.; Shi, Y.; Pielak, R. M.; Rogers, J. A. Materials and Device Designs for an Epidermal UV Colorimetric Dosimeter with Near Field Communication Capabilities. Adv. Funct. Mater. 2017, 27, 1604465. (4) Ju, S.; Facchetti, A.; Xuan, Y.; Liu, J.; Ishikawa, F.; Ye, P.; Zhou, C.; Marks, T. J.; Janes, D. B. Fabrication of Fully Transparent Nanowire Transistors for Transparent and Flexible Electronics. Nat. Nanotechnol. 2007, 2, 378−384. (5) Nawrocki, R. A.; Matsuhisa, N.; Yokota, T.; Someya, T. 300-nm Imperceptible, Ultraflexible, and Biocompatible E-Skin Fit with Tactile Sensors and Organic Transistors. Adv. Electron. Mater. 2016, 2, 1500452. (6) Ren, H.; Cui, N.; Tang, Q.; Tong, Y.; Zhao, X.; Liu, Y. HighPerformance, Ultrathin, Ultraflexible Organic Thin-Film Transistor Array via Solution Process. Small 2018, 14, 1801020. (7) Choi, M. K.; Yang, J.; Kim, D. C.; Dai, Z.; Kim, J.; Seung, H.; Kale, V. S.; Sung, S. J.; Park, C. R.; Lu, N.; Hyeon, T.; Kim, D.-H. Extremely Vivid, Highly Transparent, and Ultrathin Quantum Dot Light-Emitting Diodes. Adv. Mater. 2018, 30, 1703279. (8) Yoon, J.; Jeong, Y.; Kim, H.; Yoo, S.; Jung, H. S.; Kim, Y.; Hwang, Y.; Hyun, Y.; Hong, W.; Lee, B. H.; Choa, S.; Ko, H. C. Robust and Stretchable Indium Gallium Zinc Oxide-Based Electronic Textiles Formed by Cilia-Assisted Transfer Printing. Nat. Commun. 2016, 7, 11477. (9) Kim, J.; Lee, M.; Shim, H. J.; Ghaffari, R.; Cho, H. R.; Son, D.; Jung, Y. H.; Soh, M.; Choi, C.; Jung, S.; Chu, K.; Jeon, D.; Lee, S. T.; Kim, J. H.; Choi, S. H.; Hyeon, T.; Kim, D. H. Stretchable Silicon Nanoribbon Electronics for Skin Prosthesis. Nat. Commun. 2014, 5, 5747. (10) Yeo, W.-H.; Kim, Y.-S.; Lee, J.; Ameen, A.; Shi, L.; Li, M.; Wang, S.; Ma, R.; Jin, S. H.; Kang, Z.; Huang, Y.; Rogers, J. A. Multifunctional Epidermal Electronics Printed Directly onto the Skin. Adv. Mater. 2013, 25, 2773−2778. (11) Norton, J. J. S.; Lee, D. S.; Lee, J. W.; Lee, W.; Kwon, O.; Won, P.; Jung, S.-Y.; Cheng, H.; Jeong, J.-W.; Akce, A.; Umunna, S.; Na, I.; Kwon, Y. H.; Wang, X.-Q.; Liu, Z.; Paik, U.; Huang, Y.; Bretl, T.; Yeo, W.-H.; Rogers, J. A. Soft, Curved Electrode Systems Capable of Integration on the Auricle as a Persistent Brain-Computer Interface. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 3920. (12) Jeong, G. S.; Baek, D.-H.; Jung, H. C.; Song, J. H.; Moon, J. H.; Hong, S. W.; Kim, I. Y.; Lee, S.-H. Solderable and Electroplatable Flexible Electronic Circuit on a Porous Stretchable Elastomer. Nat. Commun. 2012, 3, 977. (13) Mu, X.; Xin, X.; Fan, C.; Li, X.; Tian, X.; Xu, K.-F.; Zheng, Z. A Paper-Based Skin Patch for the Diagnostic Screening of Cystic Fibrosis. Chem. Commun. 2015, 51, 6365−6368.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02141. Additional figures and tables, calculation of the minimum radius of a target surface for conformal contact with a PI film, capillary action outside and inside an NTC during the transfer process, and calibration curve of the temperature sensor in Figure 4b (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Youngkyu Hwang: 0000-0002-7997-5649 Seonggwang Yoo: 0000-0002-9126-927X Namsoo Lim: 0000-0001-7497-9440 Jongwoo Kim: 0000-0002-7419-021X Yujun Hyun: 0000-0001-8529-2302 Gun Young Jung: 0000-0003-1163-8651 Heung Cho Ko: 0000-0002-9078-3146 Present Address §

Present address: School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore. Author Contributions ¶

Y.H. and S.Y. contributed equally to this work.

Author Contributions

Y.H., S.Y., and H.C.K. designed the experiments. Y.H., S.Y., N.L., S.M.K., H.Y., Y.H., and G.Y.J. performed the experiments and analysis. J.K. performed the physical calculation of the NTC. Y.H., S.Y., J.K., and H.C.K. wrote the manuscript through the contributions of all authors. All authors have given their approval to the final version of the manuscript. I

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