Scalable and continuous fabrication of bio-inspired dry adhesives with a thermosetting polymer

Sung Ho Leea, Sung Woo Kima, Bong Su Kanga, Pahn-Shick Changbc and Moon Kyu Kwak*a
aSchool of Mechanical Engineering, Kyungpook National University, Daegu, 41566, Korea
bDepartment of Agricultural Biotechnology, Seoul National University, Seoul 08826, Republic of Korea. E-mail: mkkwak@knu.ac.kr
cResearch Institute of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea

Received 30th November 2017 , Accepted 26th January 2018

First published on 29th January 2018


Many research groups have developed unique micro/nano-structured dry adhesives by mimicking the foot of the gecko with the use of molding methods. Through these previous works, polydimethylsiloxane (PDMS) has been developed and become the most commonly used material for making artificial dry adhesives. The material properties of PDMS are well suited for making dry adhesives, such as conformal contacts with almost zero preload, low elastic moduli for stickiness, and easy cleaning with low surface energy. From a performance point of view, dry adhesives made with PDMS can be highly advantageous but are limited by its low productivity, as production takes an average of approximately two hours. Given the low productivity of PDMS, some research groups have developed dry adhesives using UV-curable materials, which are capable of continuous roll-to-roll production processes. However, UV-curable materials were too rigid to produce good adhesion. Thus, we established a PDMS continuous-production system to achieve good productivity and adhesion performance. We designed a thermal roll-imprinting lithography (TRL) system for the continuous production of PDMS microstructures by shortening the curing time by controlling the curing temperature (the production speed is up to 150 mm min−1). Dry adhesives composed of PDMS were fabricated continuously via the TRL system.


Introduction

Recently, many research groups have made great efforts towards applying bio-inspired functional surfaces in actual industries. One of the most popular commercialization topics in biomimetics is gecko-foot-inspired dry adhesives. The foot of the gecko, which is composed of 30–130 μm long setae and 0.2–0.5 μm spatula-shaped terminating structures, is made up of numerous micro/nano hierarchical structures and can be attached to the surface of an object by van der Waals forces.1 Various micro/nano-structure fabrication methods have been developed in the field of microelectromechanical systems, and many researchers have been studying the application of dry adhesives that mimic the foot of the gecko. As a result, the performance of dry adhesives has gradually improved.2–11 Furthermore, because of the increasing demand for micro/nano-structure-based functional surfaces in many fields in recent years, many studies on large-area and continuous fabrication of micro/nano-structures have been conducted.12–24 The demand for dry adhesives has increased, as in other natural products. Dry adhesives with high adhesion are mostly made of polydimethylsiloxane (PDMS). Unfortunately, PDMS is not a suitable material for a continuous-fabrication system due to its long curing time (approximately one hour). Almost-continuous-fabrication methods of micro/nano-structures are mainly based on roll-type equipment. Large-area fabrication and continuous production are performed with roll-type photolithography or roll-to-roll nanoimprint lithography (R2RNIL), which needs a short patterning time.17,21,23,25–27 In roll-type photolithography and R2RNIL, micro/nanostructures are successfully defined rapidly using UV-curable resins, which need a few seconds for curing. Recently, dry adhesives have been continuously produced by roll-type devices with UV-curable resin.14 Dry adhesives made with UV-curable materials are advantageous for productivity and pull-off force with high preload. However, their application could be limited due to the high preload required. The high preload means that dry adhesives made from UV-curable materials can give a polyethylene terephthalate (PET) film-like feeling in actual use where precise control of preload is difficult. PDMS has a low elastic modulus (approximately 1.8 MPa) and low surface energy (approximately 22.8 mJ m−2) and is widely used for soft lithography as a stamp or resin. PDMS has the advantage of conformal contact without high preload. Given these advantages, PDMS has been utilized in various applications in many fields.28–37 A dry adhesive with PDMS can be the most feasible application but it is difficult to apply continuously because it typically takes approximately one hour or more to cure depending on temperature.33 To solve this problem and utilize PDMS, several research groups have been studying how a PDMS structure can be fabricated rapidly. They have developed photo-definable PDMS with a photoinitiator and cross-linker, and successfully fabricated a micro/nano-structure that consists of UV-curable PDMS.38–40 However, the basic chemical properties of UV-curable PDMS are the same as those of the typical thermosetting PDMS. Unfortunately, UV-curable PDMS is relatively hard to use as a dry adhesive due to its rigidity and lack of conformal contact performance. In this study, we developed a thermal roll-imprinting lithography (TRL) system for a continuous PDMS molding process by controlling the PDMS curing time, which depends on curing temperature and the thickness of the sample. Dry adhesives composed of PDMS were fabricated continuously as a feasible application. A film-type dry adhesive without any supporting substrates was continuously cured at a rate of 150 mm min−1 in roll-type equipment. Compared with the conventional PDMS molding method, PDMS microstructures can be fabricated at a high speed with TRL equipment, and large-area continuous fabrication of PDMS micro/nano-structures is possible via roll-type equipment.

Experimental

Principle of the continuous-fabrication system of film type PDMS

PDMS has a high transparency and low surface energy, and consists of a mixture elastomer base and curing agent (Sylgard 184, Dow Corning Corp.). The elastic modulus of PDMS is approximately 1.8 MPa.32 Generally, PDMS is used in a 10[thin space (1/6-em)]:[thin space (1/6-em)]1 mixture ratio of the elastomer base, such as a platinum-based catalyst, and the curing agent, such as a siloxane oligomer. PDMS is cured as an elastomer by forming organometallic cross linkages (SI–CH2–CH2–SI bonding) through heat energy and platinum-based catalyst in the elastomer base (Fig. 1a).8,35,41
image file: c7sm02354e-f1.tif
Fig. 1 (a) Schematic illustration of the chemical reaction of PDMS. (b) PDMS curing time according to thickness and curing temperature. (c) Curing time according to thickness at specific temperatures (100 °C and 150 °C). (d) SEM image of a dry adhesive cured at 100 °C in 150 seconds. (e) SEM image of a dry adhesive cured at 150 °C in 150 seconds. The scale bars are 30 μm.

The organometallic cross-linking reaction could be accelerated and the required curing time for PDMS can be reduced by increasing the temperature. For instance, on the basis of the data sheet, a curing time of 15 min at 150 °C is enough to fully cure PDMS (Table 1).33 Aside from curing temperature, the coating thickness of the pre-cured PDMS also affects the curing time. To investigate the effect of temperature and coating thickness on the curing time of PDMS, the PDMS curing times of various thickness samples were measured at 100/110/130/150 °C (Fig. 1). As shown in Fig. 1b and c, 1 mm-thick PDMS was cured in only 150 s at 130 °C. With the use of this property, PDMS could be used as a resin in continuous-fabrication roll-type equipment and successfully cured in a film form (approximately 500 μm thickness) at 130 °C in 3 min. In the PDMS curing experiment, fully cured PDMS was shown to have a well defined microstructure, while the uncured PDMS structure was collapsed (Fig. 1d and e). About 2940 microstructures, consisting of 10 μm pillars and 11 μm tips, were arrayed in a square millimetre.

Table 1 PDMS curing time with respect to temperature (from commercial data sheet)
Temperature (°C) Curing time (hours)
25 24
65 4
100 1
150 0.25


Setup for roll-type equipment

The continuous-fabrication system consists of a belt-type mold, a feeder for the thin film coating of PDMS, a 500 mm-long hot plate for supplying heat energy, and a motor (RK 2 series, Oriental Motor) for driving the TRL system (Fig. 2 and Fig. S1, ESI). The entire system was operated by the tension of the belt-type mold via one driven roller, and the fed film was moved by contact friction with the mold. Generally, the PDMS curing process takes approximately one hour in an oven at 70 °C to 80 °C. In this experiment, however, the temperature of the hot plate was set to 130 °C to cure the PDMS more rapidly, and sufficient heat energy was able to shorten the curing time. According to a previous experiment, PDMS curing needed only 2–3 min at 130 °C. In the TRL system, using a typical mold wrapped on the roller is inefficient from a productivity perspective, because the roller would have to slowly roll one cycle in 2–3 min. To satisfy this condition and ensure good productivity, a relatively long heating time was achieved while continuously moving using a 1000 mm-long belt-type flexible mold instead of a typical roll-type mold. The spacing between the belt-shaped mold and the hot plate was maintained at 1 mm or less so that the loss of thermal energy was minimized and the apparatus was smoothly operated. The curing temperature of 130 °C is too high to maintain the shape of a typical PET film or tri-acetyl-cellulose film. Thus, a Kapton film was used in this equipment. The Kapton film was the supporting film of PDMS, and the substrate of the belt-type mold showed thermally stable behavior up to 400 °C.42 Thus, thermal degradation could be prevented despite repeated uses during curing. In a typical continuous process, the resin is applied to the substrate before contact with the mold. However, because of the high viscosity of the PDMS resin, the degassed PDMS was coated onto the flexible mold to fill all patterns on the mold in the TRL equipment.
image file: c7sm02354e-f2.tif
Fig. 2 (a) Schematic description of the TRL system for continuous fabrication with PDMS resin. (b) Schematic illustration of uncured PDMS dispensing and demolding. (c) SEM image of the dry adhesive demolding from the mold. The scale bar is 30 μm.

Fabrication of a dry adhesive master mold

To fabricate a gecko-foot-inspired master mold, a silicon-on-insulator (SOI) wafer that consists of a 400 μm handle layer, a 3 μm buried oxide layer, and a 20 μm device layer was prepared. Photolithography and dry reactive ion etching (DRIE) were implemented twice, because the gecko-foot-inspired dry adhesive has a hierarchical structure composed of two parts, namely, the tip and the pillar. In the first step, the photoresist (PR) layer to be used as a sacrificial layer for etching was patterned using photolithography, and the SOI wafer was etched to a depth of 1 μm using DRIE to form the tip. After removing the sacrificial PR layer, the pillar part (with 20 μm thickness) was defined by second photolithography and DRIE through the same method. In the second step, a bare silicon wafer was bonded to the device layer side of the patterned SOI wafer using fusion bonding. The handle layer was removed by mechanical grinding and chemical wet etching in a solution that contained KOH. The chemical etching was stopped at the buried oxide layer, and the oxide layer was removed by wet etching with a buffered oxide etcher. Finally, the dry adhesive master mold with a hierarchical structure was successfully fabricated after surface treatment with C4F8 gas for easy demolding (Fig. S2, ESI).

Fabrication of a flexible dry adhesive belt-type mold

The mold utilized for the TRL system must be flexible and thermally stable. Polyurethane acrylate (PUA) is a UV-curable and well-known material for rigi-flex lithography. PUA was utilized for imprinting and as a resin for the roll-to-roll system because of its flexibility and rigidity. To confirm the thermal stability of PUA, a line pattern with 21 μm width and 40 μm period was baked for 1 h at 150 °C. As a result, the line pattern had more identical dimensions than it did prior to baking (Fig. S3, ESI). Therefore, PUA was a suitable material for fabricating a flexible mold in the continuous-fabrication system of PDMS. The belt-type mold composed of PUA (MINS311RM, Minuta Tech) was fabricated several times using the tiling method on the Kapton film (DuPont, USA). Typically, PUA patterning is performed with a urethane-coated PET film for handling support. A surface-modified PET film with fine adhesion and resin is utilized in the imprinting lithography process as a flexible substrate. However, a PET film shows low thermal stability. Usually, the glass transition of PET is initiated at approximately 80 °C and is melted at approximately 270 °C. Meanwhile, the Kapton film is reinforced with thermosetting linkable polyimides formed by aromatic dianhydride condensation to operate at a high temperature. The Kapton film shows thermal stability and can be maintained at approximately 230 °C without deformation due to the thermoset polymer matrix (Fig. 1b).42–46 In this case, the Kapton film is a suitable material for the supporting film in the TRL system because the PET film is deformed at the process temperature (approximately 130 °C). To enforce adhesion with PUA, a primer (Glass Primer, Minuta Tech) was applied on the Kapton film. After the imprinting process, the belt-type mold with a dry adhesive structure was treated with L-SAM (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-1-trichlorosilane, a kind of silane, for 40 min at 120 °C for easy demolding. Finally, a 1000 mm-long flexible belt-type mold for the TRL system was prepared (Fig. 3).
image file: c7sm02354e-f3.tif
Fig. 3 (a) Schematic image of the fabrication process of a belt-type PUA mold. (b) Cross-sectional SEM image of the PUA mold. (c) Tilted SEM image of the fabricated gecko-foot-inspired dry adhesive. (d) Photograph of the fabricated gecko-foot-inspired dry adhesive. The scale bars of (b) and (c) are 20 and 50 μm, respectively. (e) Photograph of the 1000 mm-long belt-type mold for wrapping on two rollers.

Pull off-force measurements

Equipment for pull-off force and durability measurements of dry adhesives was built (Fig. S4a, ESI). The jig with a glass substrate to contact with a dry adhesive was moved vertically by a motor-driven crank. The dry adhesive was placed on the stage assembled with an z-axis micro manual stage for controlling the preload and a tiltable micro manual stage for adjusting the horizontal level. To measure the pull-off force between the dry adhesive and glass substrate, two load cells having a gram scale resolution and capable of responding within 100 ms were set below the stage set. The pull-off force was measured with respect to various preloads and a durability test was conducted at a rate of 50 cycles min−1 with 5 N preload at room temperature.

Results and discussion

The PDMS curing process is conducted by mixing a curing agent and a polymer base and the cured layer boundary is propagated from the bottom side of the precured PDMS layer to the upper side on a hot plate. Fig. 1c depicts the PDMS curing time according to thickness at temperatures of 100 °C and 150 °C. In the same-thickness sample, the PDMS curing time differed with respect to temperature. However, the difference in results at the same temperature was smaller than that with the thickness sample result even though the PDMS amount was significantly different. Thus, temperature affects the PDMS curing process more significantly than the PDMS amount does. The belt-type flexible mold was wound on two rolls with moderate tension. In the ordinary PDMS patterning process, the degassing process was performed. In the process of manufacturing a columnar pattern like a dry adhesive, this degassing process is indispensable to fill the hole with precured PDMS. In this process where continuous patterning is performed, PDMS is filled using a blade-type resin coater because the hole cannot be filled with pre-cured PDMS using a vacuum chamber. To ensure enough time for filling resin into holes, degassed PDMS was directly coated onto the mold, as shown in Fig. 2. Considering that the pattern of the dry adhesive was composed of a tip with 12 μm diameter and 1 μm thickness and a pillar with 10 μm diameter and 19 μm height, bubbles from the holes of the mold were raised through the precured PDMS by capillary force. The net (resultant) force (Fnet) for pushing bubbles up can be expressed by the following:
Fnet = Fc + FbFd
where Fc is the capillary force between the mold and precured PDMS, Fb is the buoyant force of air bubbles in PDMS, and Fd is the drag force in PDMS. From a micro perspective, the buoyant force is more negligible than the capillary force. Hence, the equation is simply summarized as follows:
Fnet = FcFd
The capillary force can be expressed by50
Fc = 2γ[thin space (1/6-em)]cos[thin space (1/6-em)]θ/r × A
where γ is the surface energy of PDMS (approximately 22.8 mJ m−2),48 θ is the contact angle (CA) of liquid PDMS on the PUA surface, r is the radius of a pillar, and A is the cross-sectional area of a pillar. The static CA of PDMS on the PUA mold was measured using a CA analyzer (DSA 100, Kruss, Germany) as 13.7°. 5 μm and 25 μm2 were applied in the above equation for r and A, respectively. The capillary force (Fc) was calculated to be 4.4664 × 10−8 N. The drag force of bubbles (Fd) in the PDMS was calculated using the following equation:51
Fd = 3πμVD
where μ is the viscosity of PDMS (3900 cPs),26 V is the velocity in the medium with viscosity μ, and D is the sphere's diameter. We applied the velocity (2 μm s−1) and diameter (14.5 μm) of the bubble in a pillar (Fig. S5, ESI) to the above equation. The drag force (Fd) in the PDMS was calculated to be 6.1237 × 10−9 N. As a result, bubbles in the negative mold could emerge because Fnet is a positive value (3.8540 × 10−8 N). Furthermore, the weight of the bubble in the unit pillar, Wbubble, can be easily calculated to be 1.8879 × 10−8 N (1.9264 × 10−9 kg). These values can explain why the dry adhesive mold was filled by the PDMS in a short time without the degassing process. In demolding, the work of adhesion can theoretically explain why cured PDMS was demolding from the flexible mold. The Kapton film was utilized as a substrate for handling cured film-type PDMS dry adhesives. The work of adhesion between two solid materials can be expressed by the following:
image file: c7sm02354e-t1.tif
where γ is the surface tension, and the subscripts d and p are the dispersion and polar terms, respectively.52 On the basis of the literature, the surface tensions of materials are shown in Table 2. The work of adhesion values calculated in Table 2 are WPUA/PDMS = 47.3 mJ m−2 and WPDMS/Kapton[thin space (1/6-em)]film = 63.7 mJ m−2. These values mean that cured PDMS can adhere to the Kapton film successfully because WPUA/PDMS is larger than WPDMS/Kapton[thin space (1/6-em)]film. To confirm the performance of the dry adhesive fabricated with the TRL, a pull-off force test and a durability test via repeated attachment and detachment on glass were performed (Fig. 4). As shown in Fig. 4, the pull-off force of the dry adhesive against the glass substrate was a maximum of 11.2 N per unit area, and the durability test of the dry adhesive was performed up to 5000 cycles. In the durability test, the pull-off force of the dry adhesive maintained 83% of its initial value (9.3 N per unit area) after 5000 cycles. In addition, to check the PDMS properties, a tensile test was conducted using a PDMS sample cured at 150 °C. The elastic modulus of PDMS which was cured at 150 °C was measured with 2.05 MPa (Fig. S6, ESI), and it was not far apart from the elastic modulus of a PDMS specimen cured at typical temperature. The gecko-foot-inspired dry adhesives developed in this study have good adhesion and do not leave any contaminants on the counter substrate after detachment. In addition, the adhesive can maintain its performance even after 5000 repeated uses. Therefore, the adhesive was applied to a clean glass transportation system for a feasible application. To demonstrate the application, as shown in Fig. 5a, a simple glass transportation system with a robot arm was designed using dry adhesives. The dry adhesion module consisted of four dry adhesives with an area of 3 cm × 3 cm combined with the metal and thick PDMS backing layer (5 mm) because the dry adhesive fabricated in the TRL system is thin. The adhesive was assembled using a robot arm with three degrees of freedom. The robot arm worked at 2.4 m s−2 acceleration in a horizontal direction, and a 470 mm × 370 mm × 2.7 mm (approximately 1.2 kg) glass was utilized in the transportation test. To confirm the practical use and versatile processes of the adhesive, as shown in Fig. 5c, the glass transportation system was represented using time lapse images with various motions (z, r, θ directions). As shown in Fig. 5c, the dry adhesive used in the glass transportation system could grab the glass substrate well in a vertical motion and also showed stable adhesion even during rotation or translational motion.
Table 2 Surface tensions of polymers and a film
Material γd [mJ m−2] γp [mJ m−2] γTot [mJ m−2]
PUA47 21.6 11.2 32.8
PDMS48 21.7 1.1 22.8
Kapton film49 50.0 3.0 53.0



image file: c7sm02354e-f4.tif
Fig. 4 Performance of the dry adhesive. (a) Measurement result of the pull-off force test. The inset shows the tested sample. (b) Durability test up to 5000 cycles of attachment and detachment. Inset: SEM image presents microstructures after 5000 cycles.

image file: c7sm02354e-f5.tif
Fig. 5 (a) Illustration of the robot arm with dry adhesives for a glass transportation system. (b) The dry adhesive module for glass transportation. (c) Time lapse images of glass transportation using the dry adhesive module-attached robot arm.

Conclusions

Although PDMS has many advantages from various points of view, it has not been considered as a resin for a roll-type continuous-fabrication system because of its long curing time. In this study, we investigated the continuous-production method of a thermosetting polymer and its application. We developed a TRL system for continuous fabrication of PDMS using custom-built roll-type equipment. We also demonstrated the possibility of applying PDMS via gecko-foot-inspired dry adhesive fabrication. PDMS was cured at 130 °C in 3 minutes, and developing equipment that realizes continuous patterning of PDMS at a production speed of 150 mm min−1 is possible. To demonstrate its application, a dry adhesive with hierarchical structures was fabricated continuously and rapidly with the TRL system. Furthermore, to check the feasibility of fabricated dry adhesives with the TRL system in real industries, a glass transportation system was successfully implemented with the dry adhesives applied on a robot arm. Its high productivity and product performance show that the TRL system is a promising production technique in the continuous production of thermosetting polymers and is expected to be used in various industries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (2016R1A2B4007858) funded by the Korean Government (MSIP) and the Fire Fighting Safety & 119 Rescue Technology Research and Development Program funded by the Ministry of Public Safety and Security (2015-72).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sm02354e

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