Research ArticleSOFT ROBOTS

Ionic spiderwebs

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Science Robotics  15 Jul 2020:
Vol. 5, Issue 44, eaaz5405
DOI: 10.1126/scirobotics.aaz5405

Abstract

Spiders use adhesive, stretchable, and translucent webs to capture their prey. However, sustaining the capturing capability of these webs can be challenging because the webs inevitably invite contamination, thus reducing its adhesion force. To overcome these challenges, spiders have developed strategies of using webs to sense prey and clean contaminants. Here, we emulate the capturing strategies of a spider with a single pair of ionic threads based on electrostatics. Our ionic spiderwebs completed consecutive missions of cleaning contamination on itself, sensing approaching targets, capturing those targets, and releasing them. The ionic spiderwebs demonstrate the importance of learning from nature and push the boundaries of soft robotics in an attempt to combine mutually complementary functions into a single unit with a simple structure.

INTRODUCTION

Soft robots made of compliant materials are expected to flexibly handle unpredictable and irregular tasks based on their deformable bodies (1). With the advent of soft robots, individual components, such as soft sensors and actuators, have been actively researched (25). However, for soft robots to fully achieve their potential, each complementary component is required to be combined into a single robotic system (1, 6). Thus far, attempts have been made to combine diverse components, but achieving complementary interactions in a single system still remains a challenge (79). In contrast, biological systems successfully combine mutually complementary functions even in simple structures. For instance, spiders spin adhesive, stretchable, and translucent capture threads on strong structural threads used as a framework (1013). The adhesive coating on the capture thread enables spiders to capture their prey (1214). The translucent feature of capture thread allows for passive camouflage in all backgrounds (12, 13, 15). The stretchability of capture thread helps it to be compliant in various situations (1214). However, the adhesive coating on capture thread inevitably invites contamination of the web (12). Because contamination deteriorates the capturing capability of webs, spiders (i.e., Cyclosa japonica) use unique strategies to minimize and eliminate such contamination (16, 17). Spiders build minimal webs and then wait for prey to come into contact with these webs (12, 13). After sensing vibrations of the web caused by the contact, spiders wrap the weakly caught prey with additional thread to supplement the adhesion force of the existing web (12, 13, 1821). Thus, their sensing strategy not only prevents prey from escaping but also helps to minimize contamination by allowing the spiders to build minimal webs. Furthermore, spiders pull and rapidly release their webs in a manner similar to a slingshot, causing the contaminants on webs to bounce off by inertia force, thus recovering the capturing capability of the webs (Fig. 1B and movie S1). In this regard, spiders can sense and clean using their webs alone, and these functions act synergistically to capture prey.

Fig. 1 Working principle of ISWs.

(A) Spiders can sense and clean using their webs alone, and these functions act synergistically to capture their prey. (B) A spider removing dew drops from its web with slingshot cleaning. Scale bar, 3 mm. (C) ISWs emulating the capturing strategies of a spider. (D) Schematic illustration describing the structural design of a fabricated ISW. (E) Chemical structures of an ionically conductive organogel consisting of EG dissolving LiCl and PAAm chains cross-linked by MBAAm. (F to H) Simplified working mechanisms of electrostatic adhesion capturing, electrostatic induction sensing, and electrostatic vibration cleaning of ISWs.

For the purpose of securing synergistic activity in a simple structure, the components should have similar operation mechanisms. One option that could meet this challenge is electrostatics, where capturing, sensing, and cleaning can be realized using only dielectric elastomers and soft electrodes. They can also be controlled quickly and accurately using an electrical circuit. Thus, adopting electrostatics might allow a simple structure to perform complementary functions and generate synergy (Fig. 1, F to H).

In this article, we emulate the capturing strategies of spiders based on electrostatics in ionic spiderwebs (ISWs) (Fig. 1, A and C; fig. S1; and Movie 1). An ISW was fabricated with ionically conductive and stretchable organogel encapsulated with silicone rubber to form a strand shape (Fig. 1D and figs. S2 and S3) (2224). The organogel consists of covalently cross-linked polyacrylamide (PAAm) chains and ethylene glycol (EG) dissolving lithium chloride (LiCl) (Fig. 1E). Plasma etching and (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trichlorosilane (HDFS) treatment of the silicone rubber markedly boosted cleaning capability and prevented evaporation of the organogel, which enhanced the durability of ISWs (fig. S4) (2, 25). In addition, similar to the structural thread of spiderwebs, nylon thread was used as the framework for ISWs to reinforce their mechanical properties (figs. S5 and S6 and table S1) (1013).

Movie 1. Summary of ISWs.

RESULTS

Capturing by electrostatic adhesion

Spiders firmly capture various types of prey using their adhesive and stretchable webs. To emulate the capturing capability of these webs, electrostatic adhesion capturing was realized through the mechanism shown in Fig. 2 (A to D) (26, 27). In the OFF-state (Vapplied = 0), the two threads remain separated (Fig. 2A). When a high voltage was applied, the ions were aligned along the organogel/silicone rubber interface of each thread, resulting in Maxwell stress between the threads (Fig. 2B) (3). As the threads moved closer, generation of a stronger electric field led to polarization in a target. The charge induced by polarization was attracted by the external electric field, and electrostatic adhesion occurred between the ISW and the target (Fig. 2C). Electrochemical reactions at the interface between an organogel and the metal electrode can be an issue (3). However, the electrostatic adhesion is based on a capacitive system. This lowers concerns about possible electrochemical reactions except for those caused by minimal leakage current, which was less than 2.3 nA/cm (see figs. S7 and S8 and the Supplementary Materials for details) (3). Although unwanted electrochemical reactions may be negligible, the ISWs can undergo breakdown if the electric field across dielectrics exceeds the dielectric strength of silicone rubber (50 to 100 kV/mm) (9). To address this safety issue, the leakage current was continuously measured to allow for immediate shut down of the applied voltage when a sudden increase in leakage current was sensed (see figs. S9 and S10 and the Supplementary Materials for details). To consolidate the working mechanism, the method of image charges was used to derive a mathematical expression for determining the electrostatic adhesion force exerted on a target, and the potential distribution was acquired, as shown in Fig. 2D (see fig. S11 and the Supplementary Materials for details).

Fig. 2 Capturing by electrostatic adhesion.

(A to C) Schematic diagrams illustrating the working mechanism of electrostatic adhesion capturing in the OFF-state (Vapplied = 0) (A) and ON-state (Vapplied > 0) (B) with a cross-sectional view (C). The electric field generated by the potential difference between a pair of ISWs induces polarization in a target, leading to electrostatic adhesion capture. (D) Calculated potential distribution in ISWs and a target. (E to I) Adhesion force was investigated as a function of applied voltages (E), target materials (F), ion concentrations in the organogel (G), tilt angle (H), and applied strain (I). (J) Four pairs of ISWs capturing a 6.8-g aluminum plate. When 50% strain was applied, the ISWs captured a 13.6-g aluminum plate. Error bars in the figures represent SD (n = 5).

To obtain a reproducible measurement of the electrostatic adhesion force, a universal testing machine with a custom acrylic holder was used (see Materials and Methods and movie S2 for details). As the applied voltage increased from 2 to 9 kV, the adhesion force per unit area improved from 0.1 to 0.3 kPa and 0.2 to 1.1 kPa when the target was poly(methyl methacrylate) (PMMA) and aluminum, respectively (Fig. 2E). The adhesion force was investigated with a variety of target materials—such as metals, ceramics, polymers, and natural materials—under a constant applied voltage of 5 kV (Fig. 2F). The highest adhesion force of 0.9 kPa was measured for metals, which have the highest dielectric constant, followed by ceramics (0.7 kPa) and polymers (0.25 kPa). This was because the higher the dielectric constant of the targets, the larger the polarization in the target under the same electric field. The conductivity of the organogel can be controlled by ion concentration. Figure 2G shows the effects of LiCl concentrations ranging from 0.15 mM to 1.5 M on the adhesion force. Ion concentration in the gel did not influence the adhesion force. The forces generated by electrostatic adhesion were investigated at varying tilt angles (Fig. 2H). The adhesion force increased by up to 6.5-fold when the ISW was tilted 80° from the horizontal, suggesting that ISWs can achieve substantial strength when tilted at a large angle (28, 29). All components of the ISW are made of soft materials and can therefore be stretched. When a 50% strain was applied, adhesion force increased by 37%, as shown in Fig. 2I. A decrease in thickness of the silicone rubbers caused by stretching led the gels in a pair of ISWs to be closer to each other. This contributed to the generation of a stronger electric field, thus enhancing the electrostatic adhesion force. To demonstrate capturing capability, four pairs of ISWs were fixed at 1-cm intervals in a zigzag pattern and tilted approximately 80° relative to the horizontal (Fig. 2J and movie S3). Because of the simple structure, 0.2 g of ISWs was able to capture 6.8 g of an aluminum plate. The ISWs readily expanded to 50% of strain in the longitudinal direction and were able to capture a heavier target (13.6 g). Because the stretched ISWs were under tension, they showed less deflection when capturing a target.

Sensing by electrostatic induction

One of the major concerns for spiders capturing prey is contamination of webs due to their strong adhesion force. Similar to spiderwebs, the adhesion force of an ISW can attract undesirable contaminants. For example, an air circulation system that scatters dense particles of dust can accelerate contamination (see Fig. 3A and movie S4 and Materials and Methods for details), and the electrostatic adhesion force of an ISW in the ON-state was substantially reduced to 1.8% after 50 s. Meanwhile, an ISW in the OFF-state still maintained 58.5% of its initial adhesion force because an ISW in the OFF-state was less contaminated compared with an ISW in the ON-state as shown in magnified images (Fig. 3, B and C).

Fig. 3 Sensing by an electrostatic induction.

(A) Air circulation system that scatters dense dust to accelerate contamination. (B) Normalized electrostatic adhesion forces of ISWs after exposure to contaminants in the ON-state and OFF-state. (C) Contaminated surfaces of ISWs. Scale bar, 150 μm. (D and E) Schematic diagrams showing the working mechanism of electrostatic induction sensing. (F) A pushing tester providing consistent control of the moving speed and gap distance for electrostatic induction sensing. (G) Induced voltage in ISWs is investigated as a function of the gap distance between an ISW and a target. (H and I) Peak-to-peak voltages induced by a glass target were measured using an external load with resistance ranging from 1 megohm to 20 gigohms (H) and various charging materials (I). (J) A block diagram showing how the ISW senses the approach of a target and then interacts with a preprogrammed computer. (K and L) A green LED was turned on when the induced voltage exceeded a threshold voltage (0.1 V). Scale bar, 5 mm. Error bars represent SD (n = 5).

To minimize contamination, spiders use a sensing strategy that allows them to build minimal webs. Inspired by this strategy, ISWs were equipped with a sensing capability that allowed them to turn on electrostatic adhesion only when a target is nearby. In this way, ISWs can minimize contamination by reducing the operating time of electrostatic adhesion. As described in Fig. 3 (D and E), making contact with the surroundings can charge the target through contact electrification (3032). Approach of a charged target induces a voltage in the ISWs. Thus, measuring the voltage across an external load allows the relative distance between the ISW and the target to be measured. A target with no surface charge cannot be sensed. However, approaching targets can be sensed in most cases because they will be charged not only when they come into contact with other types of materials—including solids, liquids, and vapors—but also when they come into contact with the same type of material they are made of (3336). Meanwhile, static objects around an ISW do not affect its sensing capability, because voltage is induced only when objects move (see figs. S12 and S13 and the Supplementary Materials for details). Because electrostatic induction sensing converts the mechanical energy of a target to an electrical signal, it has the advantage of negligible power consumption compared with existing proximity sensing technologies (37).

A pushing tester was used to reproducibly control the distance and approaching speed of the targets (Fig. 3F). Static charges were created on the surface of a target by rubbing a glass with various charging materials (see Materials and Methods for details). The sensing capability was tested with various gap distances (d), ranging from 0.5 to 1.5 cm, under a sinusoidal wave of 1 Hz (Fig. 3G). These measured voltages closely traced the position of the target. In addition, contact of a target with an ISW could also be sensed (fig. S14). However, because switching from sensing mode to capturing mode takes time, ISWs are required to sense the approach of a target before contact occurs and then turn on electrostatic adhesion in advance. To obtain the highest sensitivity, the induced voltages were measured with various external loads. The induced voltage was enhanced with increasing resistance of the external load under various target approach speeds of a target (Fig. 3H and fig. S15). The induced voltage was measured when the target was charged by various materials, such as alumina, aluminum, PMMA, leaf, copper, polyimide, and perfluoroalkoxy alkane (PFA) (Fig. 3I). Perceptible values of 0.11 Vpp were measured even when the target was charged with alumina, which is known to have an electron affinity similar to that of the target (38). A circuit was fabricated to demonstrate how an ISW could sense an approaching target and then turn on a power source using a preprogrammed computer (Fig. 3J). When the induced voltage exceeded a threshold voltage of 0.1 V for sensing, the computer allowed the power source to operate. As shown in Fig. 3 (K and L) (as well as in movie S5), a green light-emitting diode (LED) was turned on when an ISW sensed the approach of a target.

Cleaning by electrostatic vibration

A spider can pull and release its web in a manner similar to a slingshot to clean contamination. An ISW emulates a spider’s cleaning strategy to recover its capturing capability by eliminating contaminants. As shown in Fig. 4 (A to C), vibration cleaning based on the electrostatic attraction between ISWs was proposed. In the initial state (Vapplied = 0), the two threads are separated (Fig. 4A). When a high voltage is applied, the threads attract each other by Maxwell stress (Fig. 4B) (3, 39). As the applied voltage approaches zero again, the threads are released (Fig. 4C). Thus, alternating attraction and release caused a vibration in the ISW. When the vibration frequency matched the resonant frequency of the ISW, the vibration amplitude was maximized and contaminants could bounce off by their inertia.

Fig. 4 Cleaning by electrostatic vibration.

(A to C) Schematic diagrams showing the working mechanism of electrostatic vibration cleaning in the initial (A), attracted (Vapplied > 0) (B), and released (Vapplied → 0) (C) states. (D) To determine a resonant frequency of the ISW, the frequency of the applied voltage was swept from 20 to 116 Hz. (E) Resonant frequencies were measured using different length threads. (F) The adhesion force of a contaminated ISW was recovered as vibration cleaning removed contaminants. Scale bar, 300 μm. (G) An x-ray photoelectron spectrometer (XPS) was used to characterize the HDFS on surfaces of ISWs. (H) Wrinkles were formed on the surface of ISWs to enhance their hydrophobicity. Scale bar, 1 μm. (I) Static contact angles depending on surface treatment. (J) Cleaning capabilities of pristine and surface-treated ISWs. A droplet hanging on surface-treated ISWs was removed by vibration, whereas a droplet remained on the pristine ISWs. Scale bar, 5 mm. Error bars represent SDs [in (F), n = 5; in (I), n = 3].

The amplitude of electrostatic vibration was investigated using different frequencies of applied voltage ranging from 20 to 116 Hz, as shown in Fig. 4D (see movie S6 and Materials and Methods for details). When the length of the threads was 6 cm, resonance occurred at 46 Hz (mode number, n = 1) and 92 Hz (n = 2). When the length of the threads decreased from 6 to 3 cm, the resonant frequencies of the first and second mode increased from 46 to 97 Hz and from 92 to 195 Hz, respectively, as shown in Fig. 4E. Therefore, to clean ISWs of various lengths, it is necessary to sweep the entire frequency range containing all resonant frequencies. To verify the cleaning effect by vibration, recovery of the adhesion force was investigated after the surface of an ISW was sufficiently contaminated (Fig. 4F). During 60 s of vibration cleaning including three cycles of frequency sweeps, the adhesion force was restored to 98.7% of the initial force. Insets show the surface of the ISW before and after vibration cleaning.

Compared with other contaminants, a droplet of water is more difficult to remove due to its high surface tension. To clean a droplet of water by electrostatic vibration, the surface of the ISW was physically and chemically treated (see Materials and Methods for details). Perfluorination of ISWs using HDFS can lower surface energy (2, 25). X-ray photoelectron spectroscopy (XPS) was implemented to characterize the functionalized ISWs (Fig. 4G). The spectral peak of fluorine 1s orbital was observed, confirming the presence of fluorine on the ISW after HDFS treatment. To improve the hydrophobicity of ISWs, microwrinkles were formed by air plasma treatment of prestretched ISWs (Fig. 4H) (40, 41). Through HDFS treatment, the static contact angle of water was improved from 122° to 141.3°. Microscale wrinkles with a HDFS treatment maximized the static contact angle from 141.3° to 153.3° (Fig. 4I). Figure 4J shows the cleaning capabilities of pristine and surface-treated ISWs using a frequency sweep of the applied voltage. Droplets of water were initially suspended from each ISW. A droplet on a surface-treated ISW was cleaned during the vibration, whereas a droplet on a pristine ISW was not cleaned off (movie S7).

Ionic spiderwebs

An ISW was fabricated in the form of an orb web, as shown in Fig. 5A. A pair of threads was fixed on nylon threads as a framework (Fig. 5B). The distance between the pairs was 1 cm (do), and the distance between threads in a pair was 3 mm (di). The ISW exhibits passive camouflage in diverse environments (such as a forest, fallen leaves, a brick wall, and the floor of a building) because it is transparent and thin, as shown in Fig. 5C. In an attempt to enhance the adhesion force, the ISW was tilted to 80° from the horizontal. In the initial state, the ISW was contaminated with dust, such as polyurethane particles (Fig. 5D, i). During cleaning, a frequency of the applied voltage was swept from 30 to 60 Hz for 200 s, and the dust that covered the ISW was notably cleaned (Fig. 5D, ii). After the cleaning, the ISW was turned to sensing mode with no applied voltage. When an approaching target caused the induced voltage to exceed the threshold voltage of 1 V, the ISW was rapidly converted to capturing mode (Fig. 5D, iii). Various types of materials—such as a leaf (0.4 g), PMMA (3.1 g), glass (8.5 g), and aluminum (11 g)—were tightly captured by the electrostatic adhesion of the ISW (Fig. 5D, iv). After performing the mission, the targets were released by application of 30 Hz of AC voltage to the ISW (Fig. 5D, v). The entire continuous process is shown in movie S8.

Fig. 5 Ionic spiderwebs.

(A and B) ISW woven in the form of an orb web (A) and exploded view (B). Scale bar, 3 cm. (C) ISWs are passively camouflaged in a variety of environment such as a forest, fallen leaves, a brick wall, and the floor of a building. Scale bar, 4 cm. (D) An ISW is initially covered with dust (i). The dust is removed (ii). The ISW senses the approach of a target and then converted to capturing mode (iii). Leaf, PMMA, glass, and aluminum were sequentially captured by the ISW (iv). The targets were released from the ISW (v).

DISCUSSION

Inspired by how spiders capture their prey in spiderwebs, we emulate the abilities to capture, sense, and clean in our ISWs. Through electrostatics, all of these features were realized with a single pair of threads consisting of ionically conductive organogel encapsulated by a silicone rubber. The ISWs developed in this study robustly captured various types of targets, including an aluminum with a 68-fold greater mass. An ISW’s stretchability enhanced its capturing capability by reducing the thickness of the silicone rubber. The sensing capability of the ISW allowed it to turn on electrostatic adhesion only when a target was nearby to avoid undesirable contamination. This contributed to the maintenance of the adhesion force, which was 32.5 times higher than that of an ISW without sensing capability. The cleaning capability of the ISW, which was reinforced by surface modifications, ensured the removal of inevitable contaminants. With electrostatic vibration cleaning, an ISW was able to recover its adhesion force up to 98.7%. Our approach is potentially applicable to other existing robotic components based on electrostatics, such as electrostatic adhesion grippers, dielectric elastomer actuators, and capacitive tactile sensors. They are expected to equip additional functionalities and broaden the scope of applications potentially without major structural changes. The proposed ISWs offer a design principle for soft robots that can achieve complementary interactions between each robotic component by emulating synergic functions of nature.

MATERIALS AND METHODS

Materials and specimen preparation

Unless otherwise specified, ISWs were fabricated using a PAAm organogel containing LiCl. Acrylamide (AAm; Sigma, A8887) and N,N-methylenebisacrylamide (MBAAm; Sigma, M7279) were used as monomers and cross-linkers for the organogel, respectively. EG (Dae-Jung, 4026-4404) was used as a liquid constituent. LiCl (Sigma, L4408) was used as an ionic charge carrier. Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP; Sigma, 900889) was used as a photo initiator. Silicone tubes with inner diameters of 500 μm and outer diameters of 600 μm (Axel, 1-8194-05, https://axel-search-e.as-1.co.jp/asone/global/g/NC2006025965/) were used as dielectrics in ISWs. HDFS (JSI Silicone Co, H5060.1) was used as surface fluorination agent. Ethyl cyanoacrylate (Henkel, Loctite 406) and a primer (Henkel, Loctite SF770) were used to bond ISWs to nylon threads with a diameter of 0.52 mm (Ourix, Shock Leader) (42). Platinum electrodes were used to connect the organogel with electrical circuits.

Preparation of organogel precursor solution

Unless otherwise specified, the ionic organogel was synthesized by dissolving AAm and LiCl in EG. The molar concentrations of the AAm and LiCl solutions were 3.5 and 1.5 M, respectively. MBAAm [0.124 weight % (wt %)] and LAP (0.012 wt %) with respect to the weight of AAm were added.

Fabrication of ISWs

Unless otherwise specified, surface etching of prestrained (150%) silicone tubes was conducted using an air plasma processing system (FEMTO SCIENCE, COVANCE-1MPR) under conditions of 200 W, 1 hour, and 15 sccm (standard cubic centimeters per minute). The silicone tubes were immersed in a solution of HDFS and hexane mixed at a ratio of 1:500. Self-assembled monolayer formation took place for 20 min. Treated ISWs were then washed with hexane for 10 min. Organogel precursor solution was injected into surface-treated silicone tubes, followed by 365-nm ultraviolet irradiation (CL-1000L, UVP) under inert conditions for 5 min. Pretreatment using a primer was carried out on ISWs at the intersection with nylon thread to promote wetting of the adhesive (42). A pair of ISWs was bonded to nylon threads using ethyl cyanoacrylate. A stereomicroscope (Stereo Discovery V12, Zeiss) was used to observe the fabricated ISW.

Characterization and measurement of electrostatic adhesion capturing

For all measurements of electrostatic adhesion force, a universal testing machine (Instron, 3343) with a static load cell (Instron, 2530-50N) was used at a fixed receding rate of 3 mm/min. Targets and ISWs were fixed in place with a custom acrylic holder. Unless otherwise specified, the adhesion force of ISWs was measured using aluminum as a target under 5 kV of applied voltage. ISWs were connected to a power source via Pt electrodes. A function generator (Agilent, 33612A) with a high-voltage amplifier (Trek, 30/20A) was used as a power source for electrostatic adhesion capturing. The leakage current was measured using a multimeter (Agilent, 34461A). Adhesion force per unit area was calculated in a projection area in which the sample and target met.

Demonstration of stretchable electrostatic adhesion net

Four pairs of ISWs were fixed parallel to the holders. The gaps between each pair were 1 cm, whereas the gap between threads in a pair was 3 mm. The ISW was manually stretched from 8 to 12 cm under the applied voltage. To release the target, 3 kV of AC voltage with an offset of 1.5 kV and a frequency of 30 Hz were applied to the ISW. The targets consisted of aluminum at sizes of 5 cm by 5 cm by 0.1 cm (6.8 g) and 10 cm by 5 cm by 0.1 cm (13.6 g).

Test of contamination

Contamination was tested using a cubic-shaped acrylic system (1.73L). ISWs were set at the center of the system. Each ISW was kept under adhesion ON-state and OFF-state, respectively. Ten grams of silica powder with an average particle diameter of 50 μm was then scattered using a fan for 50 s, and adhesion force was measured every 10 s during this time. An inverted microscope (Olympus Korea, CKX41-A32PH standard) was used to observe contamination on the surface of the ISWs.

Characterization and measurement of electrostatic induction sensing

Unless otherwise indicated, ISWs were connected to an external load of 5 gigohms. An electrometer (Keithley, 6517A) was used to measure induced voltage across the external load. A pushing tester (Labworks, LW139.138-40) with a linear power amplifier (Labworks, PA-138) was used to provide vertical linear motion of the targets. A glass target was charged by rubbing with PFA. The targets moved in a sinusoidal pattern with a frequency of 1 Hz. The reference distance and amplitude were 1 and 0.75 cm, respectively. All measurements of electrostatic induction sensing were conducted in a Faraday cage to prevent ambient electrical noise (see fig. S16 and the Supplementary Materials for details). To show interactions with a computer, LabView (National Instruments) was used for programming.

Characterization and measurement of electrostatic vibration cleaning

A function generator (Agilent, 33612A) with a high-voltage amplifier (Trek, 30/20A) was used as a power source to provide sinusoidal high voltages. To measure the vibration amplitude of ISWs, a high-speed camera (Phantom, V611-32G-MAG) that could run up to 6242 frames per second was used. Contact angles of deionized water on ISW were measured with a contact angle analyzer (FEMTOFAB, Smart Drop) at room temperature. An x-ray photoelectron spectrometer (Kratos, AXIS-HSi) was used to characterize functionalized ISWs. A field-emission scanning electron microscope (Carl Zeiss, Sigma) was used to observe surface morphology of ISWs. To investigate cleaning capability of ISW, 10 g of silica powder and a 5-μl water droplet were used as contaminants. The frequency of the applied voltage was swept from 45 to 100 Hz for 20 s.

Recording of slingshot cleaning by a spider

The slingshot cleaning behavior of a C. japonica specimen was recorded in Galsan-dong, Seosan-si, Chungnam, South Korea.

SUPPLEMENTARY MATERIALS

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Supplementary Text

Fig. S1. ISWs emulating the optimized capturing strategies of spiders.

Fig. S2. Fabrication process of an ISW.

Fig. S3. Magnified image of an ISW.

Fig. S4. Anti-dehydration capability of ISWs under harsh conditions.

Fig. S5. Exploded view of orb spiderwebs.

Fig. S6. Stress-strain curve of tensile test.

Fig. S7. Equivalent circuit of electrostatic adhesion capturing.

Fig. S8. Leakage current of ISW.

Fig. S9. Breakdown voltages of ISW.

Fig. S10. Dielectric breakdown behavior of ISW.

Fig. S11. Problem definition for deriving mathematical expression of electrostatic adhesion force.

Fig. S12. Schematic describing the effect of static objects around the ISW.

Fig. S13. Experimental results describing the effect of static objects around the ISW.

Fig. S14. Induced voltage when a target contacts ISW.

Fig. S15. Induced voltages measured with various resistances of external loads and approaching speeds of a target.

Fig. S16. Demonstration of electrostatic induction sensing under practical conditions.

Table S1. Tensile mechanical properties of spiderwebs and ISWs.

Movie S1. Slingshot cleaning of spiders.

Movie S2. Measurement of electrostatic adhesion force.

Movie S3. Stretchable electrostatic adhesion net.

Movie S4. Test of contamination.

Movie S5. Sensing by electrostatic induction.

Movie S6. Measurement of vibration amplitude.

Movie S7. Cleaning by electrostatic vibration.

Movie S8. Ionic spiderwebs.

References (4345)

REFERENCES AND NOTES

Funding: This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korean Government (nos. 2018M3A7B4089670, 2018R1A3B1052541, and 2015R1A5A1037668). H.-Y.K. acknowledges administrative support from SNU-IAMD. Author contributions: Y.L. and W.J.S. conceived the idea, carried out the experiments, analyzed the data, and wrote the main manuscript text. Y.J., H.Y., and H.-Y.K. developed the mathematical model and wrote the manuscript. M.-Y.K. provided an advice on behavioral characteristic of spiders and videos about them. J.-Y.S. supervised this study and provided intellectual and technical guidance. All authors discussed the results and commented on the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper or the Supplementary Materials.

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