Simple and Reliable Fabrication of Bioinspired Mushroom-Shaped

Aug 22, 2016 - Although this method enables reliable and reproducible fabrication of Si master with negative patterns of mushroom-shaped microstructur...
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Simple and Reliable Fabrication of Bioinspired Mushroom-Shaped Micropillars with Precisely Controlled Tip Geometries Hoon Yi,† Minsu Kang,† Moon Kyu Kwak,‡ and Hoon Eui Jeong*,† †

Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea Department of Mechanical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea

‡

S Supporting Information *

ABSTRACT: We present a simple yet scalable method with detailed process protocols for fabricating dry adhesives with mushroom-shaped micropillars of controlled tip geometries. The method involves using photo-lithography with a bilayer stack combining SU-8 and lift-off resist, and subsequent replica molding process. This approach utilizes widely used and commercially available materials and can thus be used to generate mushroom-shaped micropillars with precisely controlled tip diameters and thicknesses in a simple, reproducible, and cost-effective manner. The fabricated mushroom-shaped micropillar arrays exhibited highly different tendencies in adhesion strength and repeatability depending on tip geometries, such as tip diameter and thickness, thereby demonstrating the importance of precise tunability of tip geometry of micropillars. The fabricated dry adhesives with optimized tip geometries not only exhibited strong pull-off strength of up to ∼34.8 N cm−2 on the Si surface but also showed high durability. By contrast, dry adhesives with nonoptimized tips displayed low pull-off strength of ∼3.6 N cm−2 and poor durability. KEYWORDS: dry adhesive, gecko, biomimetics, mushroom structure, micropillar, SU-8, LOR

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pillar structures.1,6,7,16−27 Microstructures with such unique tips exhibit superior pull-off force (maximum reported value of ∼42 N cm−2), whereas nanostructures with simple flat or spatulate tips generate low normal adhesion force.16 These tips provide microstructures with enhanced structural stability, thereby enabling million cycles of repeatable attachments and detachments while maintaining reasonable levels of pull-off and shear forces. These microstructures are scalable with current manufacturing techniques as compared with the hierarchical nanoscale structures.17,18 The unique and superior advantages of mushroom-shaped microstructures enable its practical application in various aspects of daily life as well as in precision industries that require clean, residue-free, and repeatable adhesion.17,19−21 Various methods have been developed to obtain mushroom-shaped micropillars.18,22,23 The most widely used method is deep reactive-ion etching (DRIE) of silicon-on-

INTRODUCTION Bioinspired micro- or nanostructures with protruding tips have been explored extensively in the past decades because of their capability to maximize van der Waals interactions between the tips and contact surfaces, thereby enabling repeatable and reversible dry adhesion to surfaces of varying roughness and orientation.1−9 For example, various gecko-inspired nanostructures with controlled diameter, height, leaning angle, tip shape, and hierarchy have been developed by using different polymeric materials and carbon nanotubes.8,10 These gecko-inspired nanostructures with simple flat or spatulate tips have demonstrated high levels of shear forces of up to ∼40 N cm−2.11 However, these nanostructures have low normal adhesions of typically less than ∼3 N cm−2 and collapse easily after repeated use; these disadvantages significantly limit the practical applications of these nanostructures.11−15 Mushroom-shaped microstructures have been proposed as an efficient form for a bioinspired dry adhesive. As compared with gecko-inspired nanostructures, mushroom-shaped microstructures have thin, circular, wide tips sticking out of the circular © XXXX American Chemical Society

Received: June 17, 2016 Accepted: August 10, 2016

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DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of the fabrication of mushroom-shaped micropillars through photo-lithography with a bilayer combining SU-8 and LOR and a subsequent replica molding with PDMS.

insulator (SOI) wafer with embedded SiO2 layer.17,24,25 In this method, negative patterns of mushroom-shaped micropillars are generated on the SOI wafer by using the notching effect of the embedded oxide layer between the top and bottom Si substrates during DRIE process. Although this method enables reliable and reproducible fabrication of Si master with negative patterns of mushroom-shaped microstructures, the high cost of the SOI wafer and the DRIE process significantly hinders the practical use and commercialization of dry adhesives with mushroom-shaped microstructures. A modified soft-lithographic technique has also been proposed to generate mushroom-shaped structures. In this method, micropillars with protruding tips are generated by inking micropillars with flat or hemispherical tips in a spin-coated precured polymer followed by pressing against a flat substrate and thermal curing.26−28 This approach is relatively simple and cost-effective but has difficulty obtaining uniform mushroom-shaped micropillars over a large area because of the fluidic and viscous nature of uncured prepolymer. As an alternative to this approach, photo-lithographic methods using a bilayer stack of two different photosensitive polymers or double-step UV exposures have been proposed.20−22,29,30 For example, Sameoto and Menon proposed a thick-film lift-off process to define molds using AZ 9260 photoresist and polymethylglutarimide (PMGI). In this method, the AZ9260 top layer is used to define microhole patterns by photo-lithography, and the underlying PMGI layer is used as a sacrificial layer to define protruding, mushroom-shaped tip patterns. Wang et al. reported a doublestep UV exposure technique using AZ P4620 photoresist. In this method, AZ P4620 is exposed to UV light from the back side of the substrate to form undercut microholes during a subsequent development step. Compared with the DRIE of SOI wafer and the inking methods, these photo-lithographic approaches have potential application in scalable fabrication of bioinspired dry adhesives, because these methods enable lowcost and large-area fabrication of mushroom-shaped microstructures in a reproducible manner. Despite these advantages, a very limited number of processes based on photo-lithographic methods have been reported. Detailed fabrication techniques and protocols utilizing widely used photoresist materials, such as SU-8, have been rarely reported. Moreover, adhesion performances (i.e., pull-off strength or repeatability) of mushroom-shaped micropillars with different tip thicknesses have not been reported in detail despite the importance of these tip geometries for high levels of adhesion strength and repeatability. Simple and cost-effective techniques for fabricating mushroom-shaped micropillar arrays that utilize widely used

photosensitive polymers can accelerate the practical use and commercialization of bioinspired synthetic dry adhesives. In this study, we present a simple and scalable fabrication process with detailed protocols for generating bioinspired dry adhesives with mushroom-shaped micropillar arrays utilizing widely used photoresists. In our approach, we utilized a bilayer stack of SU-8 and lift-off resist (LOR, supplied by Microchem Corp) to fabricate a master with negative mushroom-shaped micropillar arrays through conventional photo-lithography. Simple photo-lithography and replica molding process with widely used polymers can be utilized to generate mushroomshaped micropillars with precisely controlled tip diameters and thicknesses. The adhesion performance of the fabricated mushroom-shaped micropillars with different tip geometries was also investigated in detail. A slight modulation of the tip geometry resulted in significant differences in pull-off strength and repeatability.

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EXPERIMENTAL SECTION

Fabrication of SU-8 Master. A silicon wafer was first baked at 70 °C for 10 min in a convection oven for dehydration. A LOR of 10B or 30B (Microchem Corp, USA) was then spin-coated on the Si wafer, followed by baking on a hot plate at 200 °C for 10 min and relaxation for 5 min. The thickness of the LOR layer was controlled by modulating the spin-coating speed. Subsequently, an SU-8 3010 (Microchem Corp, USA) was spin-coated on the LOR layer (2000 rpm, 30 s), followed by a 5 min soft bake at 100 °C and 5 min of relaxation. The wafer was then exposed to UV (λ = 365 nm, dose = 250 mJ cm−2) with a photomask having a microhole array followed by 5 min of relaxation. After UV exposure, the wafer was baked further at 100 °C for 3 min and relaxed for another 5 min. Subsequently, the UV-exposed SU-8 layer was developed using an SU-8 developer (Microchem Corp, USA) for 5 min, followed by rinsing with isopropyl alcohol and blow-drying with nitrogen. The LOR layer was removed selectively with AZ 400 K (AZ Electronics Materials Corp, USA) for 30−300 s to form an undercut for a protruding tip of the mushroomshaped micropillars. Finally, the substrate was rinsed with deionized water and blow-dried with nitrogen. For surface hydrophobization, the fabricated Si master was passivated with C4F8 gas. Fabrication of Mushroom-Shaped Micropillar Array. Mushroom-shaped micropillar arrays with varying geometries were prepared by molding the fabricated Si master with a polydimethylsiloxane (PDMS) prepolymer. A 10:1 mixture of the PDMS prepolymer (Sylgard 184A) and curing agent (Sylgard 184B) was poured onto the master. The master covered with PDMS prepolymer was then placed in a vacuum chamber and degassed for 10 min in several decades of pressure. Subsequently, the Si master was placed in a convection oven and cured at 70 °C for 2 h. Finally, the cured PDMS replica was carefully removed from the master. Adhesion Measurements. Macroscopic pull-off forces of the mushroom-shaped micropillars were evaluated using custom-built B

DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Photographs of (a) the fabricated Si master and (b) dry adhesive with mushroom-shaped micropillar arrays. (c) Optical microscopy images of the fabricated mushroom-shaped micropillars at five different regions: locations i−iv in (b). Each diameter depicted in the enlarged optical images is an averaged value of 10 micropillars in locations i−iv.

Figure 3. Optical microscopy images of the Si masters with negative mushroom-shaped micropillars fabricated through different LOR development times. (a) No development, (b) 60 s, (c) 180 s, and (d) 300 s. The tip diameter increased monotonically with the increase in development time. (e− h) Cross-sectional SEM images of the fabricated Si masters, corresponding to each array shown in a−d. The thicknesses of SU-8 and LOR layers are ∼8.3 and ∼1.15 μm. The post diameter and the pitch of array are ∼19.4 and ∼20.6 μm, respectively. equipment at a relative humidity of 40% and ambient temperature of 25 °C. A circular adhesive patch (diameter = 12 mm, thickness = 2 mm) was attached to a flat Si wafer under controlled preload ranging from 2 to 30 N, and lifted using a motorized stage at a speed of 3 mm s−1 until separation occurred. For statistical significance, adhesion measurement was conducted 20 times for each sample under identical conditions.

LOR is inert to most organic solvents and acids after baking at its glass transition temperature but dissolves in alkaline chemicals such as AZ 400 K without affecting other layers such as SU-8.31 The thickness of the LOR layer can be controlled simply by using different models of LOR and spincoating conditions, which determine the tip thickness of the resulting micropillars. For example, an ∼1.15 μm thick LOR layer was obtained with 2000 rpm spin-coating (30 s) using LOR 10B, and ∼2.75 μm thick LOR layer was obtained with 3000 rpm spin-coating (30 s) using LOR 30B. We found that the baking conditions of the coated LOR layer are critical to obtain a uniform undercut of the LOR layer and prevent delamination of the layer during the LOR development process.

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RESULTS AND DISCUSSION Figure 1 shows the schematic of fabricating mushroom-shaped micropillar arrays. An LOR was spin-coated on a Si wafer, which is used as a sacrificial layer to form an undercut for protruding tips of the mushroom-shaped micropillars. The C

DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a−d) Optical microscopy images of the mushroom-shaped micropillars with different tip diameters replicated from the Si masters shown in Figure 3a−d. (e−h) Tilted SEM images of the fabricated micropillars, corresponding to each array shown in a−d. (insets) Magnified views of each image.

The optimal baking conditions in our study were 200 °C for 10 min with 5 min of relaxation for 1.15 μm thick LOR layer. Subsequently, SU-8 layer was spin-coated on the LOR layer, followed by patterning of the SU-8 layer with conventional photo-lithographic process. When using SU-8 3010, an ∼8.3 μm thick SU-8 layer was obtained with 2000 rpm spin-coating for 30 s. After SU-8 patterning, a uniform undercut of the bottom LOR layer was generated by dipping the substrate in the LOR developer (AZ 400 K). The degree of LOR layer undercut determines the tip diameter of mushroom-shaped micropillars. This parameter was controlled simply by modulating the LOR development time. Finally, mushroomshaped micropillar arrays were obtained through replica molding of the patterned Si master with PDMS. Figure 2a,b shows photographs of the fabricated Si master and the PDMS dry adhesive pad with mushroom-shaped micropillar arrays replicated from the master. The Si master shown in Figure 2a was obtained by spin-coating LOR 10B at 2000 rpm for 30 s on the Si wafer, followed by SU-8 3010 spincoating at 2000 rpm for 30 s on the LOR layer. After SU-8 patterning, the LOR 10B was removed selectively by dipping the substrate in the developer for 90 s. Figure 2a,b clearly shows that a uniform pattern array could be generated over a large area (maximum size of ∼176.7 cm2) with our suggested process (Figure S1). We investigated the uniformity of the fabricated adhesive pads with micropillar arrays by exploring pillars at five regions of the pad using an optical microscope. Figure 2c shows that micropillars at different zones exhibited uniform tip diameters (∼23.31 ± 0.15 μm) and neck diameters (∼19.52 ± 0.03 μm) without defects. The protruding tips of the mushroom-shaped micropillars are critical to obtain high levels of pull-off strength and durability of the pillar array. Therefore, precise tuning of the tip geometry of mushroom-shaped micropillars is critical to develop bioinspired dry adhesive with superior adhesion performance. In our approach, the tip diameter can be modulated simply by controlling the development time of the sacrificial LOR layer. Figure 3a−d shows the optical microscope images of Si masters fabricated with four LOR development times of 0, 60, 180, and 300 s. As shown in the images, the outer diameter of the fabricated microholes was increased monotonically with the increase of development time, whereas the inner diameter of the holes remained nearly the same. This finding demonstrates that the tip diameter of the array can be

controlled precisely by modulating the development time of the LOR layer. Figure 3e−h shows the cross-sectional scanning electron microscope (SEM) images corresponding to the optical images of Figure 3a−d. The SEM images confirm that micropillars with varying tip diameters can be easily generated by controlling the degree of undercut of the sacrificial LOR layer. Figure 4a−d shows the optical microscope images of PDMS micropillar arrays replicated from the Si master shown in Figure 3. The tip diameter of micropillars replicated from a master without LOR removal was ∼19.4 μm. Tip diameters of the pillars were increased by using a master with long LOR development times of ∼22.7, ∼25.5, and ∼28.0 μm for masters with development times of 60, 180, and 300 s. Figure 4e−h shows the tilted SEM images of the replicated mushroomshaped micropillars that correspond to optical images shown in Figure 4a−d. The SEM images also show that mushroomshaped micropillars with controlled tip diameters can be generated by our suggested process. The resulting pillar array has ∼1.15 μm in tip thickness, ∼9.4 μm total height, and ∼40 μm center-to-center pitch. Figure 5 shows the tip diameters of the fabricated mushroom-shaped micropillars as a function of LOR development time for samples with four tip thicknesses of ∼1.15, ∼2.10, ∼2.75, and ∼4.05 μm (Figures S2 and S3). The

Figure 5. Tip diameters of fabricated negative mushroom-shaped micropillars depending on LOR development time for LOR layers of different thicknesses. D

DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Measurement of the pull-off strength of mushroom-shaped micropillars with different tip diameters for four tip thicknesses of (a) ∼1.15, (b) ∼2.10, (c) ∼2.75, and (d) ∼4.05 μm. The tip diameters of the micropillars were controlled by modulating the LOR development time during the Si master fabrication process.

identical conditions. As shown in the figure, micropillars without tips exhibited very low adhesion strength (less than ∼1 N cm−2). On the contrary, micropillars with protruding tips displayed drastically enhanced adhesion strength. For example, dry adhesive pads with a relatively thin tip (∼1.15 μm thickness) and a tip diameter of ∼20.2 μm (LOR development time: 30 s) exhibited maximum pull-off strength of ∼14 N cm−2 under a preload of 20 N (Figure 6a). The pull-off strength decreased after passing the maximum point because of the buckling of the pillars or the later deformation of the back film under an excessive preload.17,24,33 The maximum pull-off strength was increased further to ∼21 and ∼25 N cm−2 with the increase of tip diameter to ∼22.7 μm (LOR development time: 60 s) and ∼25.5 μm (LOR development time: 180 s; Figure 6a). These results indicate that the pull-off strength of dry adhesives with mushroom-shaped microstructures can be modulated by controlling the tip diameters of micropillars without changing the pillar diameter and pitch. The pull-off force of for the flat tip (Pflat) is theoretically given by34

maximum tip diameter that can be obtained from our master without damage was ∼33.6 μm when the tip thickness was ∼4.05 μm (length of the tip protruding out of the pillar post: ∼7.67 μm). Tip thickness was modulated by controlling the speed of spin-coating using different kinds of LOR. Although the undercut rate of the LOR layer was not perfectly linear, the tip diameters of the resulting pillar arrays were increased monotonically for samples having different tip thicknesses with good reproducibility. Although samples with thicker tips require long development time to obtain the same tip diameters, mushroom-shaped micropillars with large variations in tip diameters of up to ∼12 μm can be simply generated with our approach. Controlling the tip diameter and thickness of mushroom-shaped micropillars is not easily achievable with the aforementioned DRIE of SOI wafer and the inking of micropillars with precured polymer. The difficulty arises from the notch phenomenon being affected not only by DRIE etching conditions but also by hole geometries, which in turn makes individually controlling the width and depth of the notch difficult.24,32 Furthermore, controlling the tip thickness requires the use of several SOI wafers with SiO2 layers of different thicknesses, thereby incurring very high costs. In the case of the inking method, controlling the tip diameter and thickness is also highly difficult because of the fluidic nature of the uncured polymer. Although not demonstrated in this paper, the height of the micropillars can also be controlled easily by changing the spin-coated thickness of SU-8 layer. Figure 6 shows the pull-off strength of dry adhesives with different tip diameters and four tip thicknesses of ∼1.15, ∼2.10, ∼2.75, and ∼4.05 μm. For statistical significance, adhesion measurement was conducted 20 times for each sample under

Pflat(PP) = N (PP) 8πKa3w12

(1)

where N(PP) is the number of pillars in contact at preload PP, K is the effective Young’s modulus of the system, and a is the radius of the flat tip. In this paper, w12 is the work of adhesion of the interface. Although eq 1 is for the micropillar arrays with simple flat tips, our experimental results are in a good agreement with the theoretical prediction (Figure S4). More accurate theoretical models can be used to better predict the pull-off force for the mushroom-shaped micropillars.35,36 Notably, samples prepared with excessively long development time (>300 s) exhibited diminished pull-off strengths as E

DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 7. (a−d) Durability test of the fabricated mushroom-shaped micropillars with different tip geometries of (a) ∼1.15 μm tip thickness and ∼22.7 μm tip diameter, (b) ∼2.75 μm thickness and ∼22.5 μm diameter, (c) ∼1.15 μm thickness and ∼28.0 μm diameter, and (d) ∼2.75 μm thickness and ∼29.5 μm diameter. (insets a−d) Detailed geometries of each micropillar used for the corresponding durability tests. (inset c) A magnified view of the pull-off strength at 1−10 test cycles. (c, d) SEM and optical microscopy images of (e) micropillars with ∼1.15 μm tip thickness and ∼28.0 μm tip diameter and (f) micropillars with ∼2.75 μm thickness and ∼29.5 μm diameter before (left image) and after (right image) the measurement tests.

the critical aspect ratio (AR) is given by eq 2; AR is defined as the ratio of the length of the tip (L) protruding out of the pillar post to tip thickness (t) for preventing the wrinkling or folding of the tips.37

compared with samples prepared with proper development time (30−180 s). For example, samples prepared with 300 s of LOR development time demonstrated the maximum pull-off strength of ∼7 N cm−2, which is merely ∼25% of the maximum strength. These samples also showed fairly high standard deviation (∼2−8 N cm−2). This result is due to the micropillars with thin and large tips that enable conformal and large contact against a substrate, but the mechanical stability and durability of the pillars decreased significantly. For example, mushroomshaped micropillars with ∼1.15 μm tip thickness and ∼28.0 μm tip diameter exhibited maximum adhesion strength of ∼30 N cm−2 during the first adhesion measurement test (Figure 6a). However, the averaged pull-off values decreased to ∼7 N cm−2 after 20 cycles of tests because of the low durability of the excessive tip size. This averaged value is expected to decrease further with the increase in measurement test cycles. Apart from the low averaged pull-off strength, these samples also displayed high levels of variations in pull-off strength even at the first adhesion test (Figure 6a). This result is due to the significantly thin and large tips of the pillar arrays that impeded conformal contact of the tips against a substrate because of the wrinkling or folding of the tips during contact of the pad on a substrate. The collapse of tips of mushroom-shaped micropillars has been rarely theoretically studied, whereas the collapse of simple micropillar structures has been frequently explored.30 On the basis of a quantitative model of ground collapse of micropillars,

⎛ E ⎞1/2 1/2 ⎛L⎞ −1/2 ⎜ ⎟ = 18 ·π ·⎜ ⎟ · (t ) ⎝t⎠ ⎝ w12 ⎠

(2)

where E is the Young’s modulus of the material. According to eq 2, the critical AR (L/t) of the protruding tips can be increased by increasing the tip thickness. Therefore, the problems associated with the weak structural durability of the micropillars with large tips can be solved through a slight increase in tip thickness. As shown in Figure 6c, micropillars with ∼2.75 μm tip thickness exhibited a high level of averaged pull-off strength. For example, the averaged pull-off strength of micropillars with ∼22.4 μm tip diameter (LOR-30B development time: 300 s) reached ∼17 N cm−2 and increased further to ∼27 N cm−2 with micropillars with ∼29.5 μm tip diameter (LOR-30B development time: 600 s) with small standard deviations in the pull-off strength. The mushroom-shaped micropillars with thin and large tips (∼1.15 μm tip thickness and ∼28 μm tip diameter) exhibited low averaged adhesion because of low structural durability. However, increasing the tip thickness slightly to ∼2.75 μm resulted in structural durability and significantly stable pull-off strength of micropillars with similarly large tips (∼29.5 μm tip diameter). F

DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Repeatability of the mushroom-shaped micropillars with different tip geometries was investigated by conducting durability tests, where the cycles of attachment and detachment were repeated using dry adhesives with different tip sizes (Figures 7 and S5). As shown in Figure 7a,b, when the tip diameter was relatively small (∼22−23 μm), the micropillar arrays maintained their pull-off strength even after 500 cycles of testing. The reason is that the tip AR (L/t) is relatively small (0.64−1.38). However, when the tip diameter was increased to ∼28−30 μm, pull-off of micropillars with thinner tips (AR: ∼3.70) decreased immediately after the first adhesion measurement (Figure 7c). By contrast, micropillars with slightly thicker tips (AR: ∼1.93) exhibited better durability as compared with micropillars with thinner tips, although a slight decrease in pulloff was observed (Figures 7d and S5). These results can be explained by the large and thin tips of micropillars, which are mechanically unstable and are therefore easily folded and collapsed upon the attachment and detachment against a contact surface (Figure 7e). Micropillars with large but thick tips maintained their structural integrity and adhesion performance even after repeated cycles of attachment and detachment (Figure 7f). Thus, fabrication of mushroom-shaped micropillars with optimized tip geometry is critical to develop bioinspired dry adhesives with superior and stable adhesion performance.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fire Fighting Safety & 119 Rescue Technology Research and Development Program funded by the Ministry of Public Safety and Security (Grant No. MPSS-FFS-2015-73). This work was also supported by the National Research Foundation of Korea grant funded by the Korea government (MSIP) (2016R1A2B2014044, 2013R1A1A1061219)

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REFERENCES

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CONCLUSIONS We have presented a simple yet scalable fabrication technique with detailed protocols for fabricating dry adhesives with mushroom-shaped micropillars of controlled tip diameters and thicknesses. A master with negative mushroom-shaped micropillars can be generated through conventional photo-lithography by using a bilayer stack of top SU-8 layer and bottom sacrificial LOR layer. The thickness and diameter of the mushroom-shaped tips can be controlled by modulating the spin-coating thickness and development time of the LOR layer. This method enables simple, reproducible, and scalable fabrication of mushroom-shaped micropillars with precisely controlled tip geometry. The fabricated mushroom-shaped micropillar arrays exhibit different tendencies in adhesion strength and durability depending on the tip diameter and thickness of micropillars. Macroscopic adhesion measurements of the fabricated dry adhesives demonstrate that tip diameter and thickness of the micropillars are critical to obtain dry adhesives with superior and stable adhesion performance. Our dry adhesives with optimized tip geometry not only exhibit high durability but also show high level of pull-off strength of up to ∼34.8 N cm−2 on the Si surface. This fabrication approach with detailed protocols offers a cost-effective, reproducible, and scalable means for the industrialization and commercialization of bioinspired smart dry adhesives in a variety of applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07337. Photograph; optical microscopy images; SEM images; pull-off strength; and durability performance of the fabricated adhesive samples; table showing comparisons of adhesion performances of mushroom-shaped microstructures with different geometries. (PDF) G

DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b07337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX