Bioinspired, Highly Stretchable, and Conductive Dry Adhesives Based on 1D−2D Hybrid Carbon Nanocomposites for All-in-One ECG Electrodes Downloaded via UNIV OF EDINBURGH on January 7, 2019 at 07:20:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Taehoon Kim,† Junyong Park,†,§ Jongmoo Sohn,‡ Donghwi Cho,† and Seokwoo Jeon*,† †
Department of Materials Science and Engineering, KAIST Institute for the Nanocentury, KAIST, Daejeon 34141, Republic of Korea Wearable Computing Research Team, ETRI, Daejeon 34129, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Here we propose a concept of conductive dry adhesives (CDA) combining a gecko-inspired hierarchical structure and an elastomeric carbon nanocomposite. To complement the poor electrical percolation of 1D carbon nanotube (CNT) networks in an elastomeric matrix at a low filler content (∼1 wt %), a higher dimensional carbon material (i.e., carbon black, nanographite, and graphene nanopowder) is added into the mixture as an aid filler. The co-doped graphene and CNT in the composite show the lowest volume resistance (∼100 ohm·cm) at an optimized filler ratio (1:9, total filler content: 1 wt %) through a synergetic effect in electrical percolation. With an optimized conductive elastomer, gecko-inspired high-aspect-ratio (>3) microstructures over a large area (∼4 in.2) are successfully replicated from intagliopatterned molds without collapse. The resultant CDA pad shows a high normal adhesion force (∼1.3 N/cm2) even on rough human skin and an excellent cycling property for repeatable use over 30 times without degradation of adhesion force, which cannot be achieved by commercial wet adhesives. The body-attachable CDA can be used as a metal-free, all-inone component for measuring biosignals under daily activity conditions (i.e., underwater, movements) because of its superior conformality and water-repellent characteristic. KEYWORDS: biomimetics, stretchable conductors, nanocomposites, conductive dry adhesives, ECG electrodes ue to the great advances in flexible and stretchable materials, they have been extensively explored to realize smart, skin-like electronics for various ubiquitous applications, such as wearable energy modules,1−3 artificial tactile sensors, 4−10 and epidermal medical patches.11−13 Such body-attachable electronic systems, in general, are made of two major components: a durable skin adhesive and a stretchable circuit. For the latter part, researchers have focused on the development of highly stretchable and conductive electrodes composed of emerging architectures (serpentine or mesh) or elastomeric nanocomposites.11−14 However, the challenges for developing highly durable and nonirritating adhesives for human skin and integrating those adhesives into skin-patch devices have limited the development due to the difficulty in the modification of wet
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© 2016 American Chemical Society
adhesive chemistry. Recently, gecko-inspired structures consisting of hierarchically structured, high-aspect-ratio micro- and nanopillars have been proven to repeatedly adhere and detach from the rough surface of human skin by collective van der Waals interaction.15,16 With optimization of the material and design, the adhesion force of gecko-inspired dry adhesives on human skin reaches ∼1.8 N/cm2, comparable to that of commercial wet adhesives. A remaining issue for dry adhesivebased technologies is the relatively poor bonding at the interface between an elastomeric adhesive part and a metallic conductive part, which is essentially required for electrical Received: February 23, 2016 Accepted: March 17, 2016 Published: March 17, 2016 4770
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Figure 1. Concept and strategy for realizing conductive dry adhesives. (a) Schematic illustration of the fabrication procedure for conductive dry adhesives and their application to ECG electrodes. (b) Digital images of a large-area silicon mold with gecko-inspired micropatterns and a replicated conductive dry adhesive (∼4 in.2). (c) Digital images of conductive dry adhesives conformally attached on human skin (i: wrist, ii: stomach, iii: ankle). (d) Digital images of the stretched and banded conductive dry adhesives. (e) Digital image of LED operation with a conductive dry adhesive as an interconnection.
dimensional (2D) graphene nanopowder, are simultaneously incorporated into an elastomeric matrix for a synergetic effect in electric percolation. Through the optimization of the ratio between the major and aid fillers, a relatively high conductivity of the elastomer nanocomposites (∼0.01 S/cm), which is enough to obtain biosignals from human skin,22 is achieved, even at a low filler content (∼1.0 wt %). A large-area CDA pad (∼4 in.2), which is attached to various conductive substrates with surface roughness ranging from 0.45 to 39.6 nm as electrical interconnection, successfully operates a light-emitting diode (LED) while bearing the heavy weight (∼1 kg). The mechanical compliance of the CDA pad enables its utilization as a metal-free, all-in-one electrocardiogram (ECG) electrode with the integrated function of adhesion on human skin. Highly accurate ECG signals providing sufficient information to predict cardiovascular problems can be stably collected through the CDA pad, even in extreme moving states and underwater environments because of its superior conformality and waterrepellent characteristic, which cannot be achieved by conventional wet adhesive and metal-electrode-based ECG systems.
interconnection with the upper circuit in device architectures. The metallic part in the integrated systems also strongly restricts the usability of dry adhesives for flexible and stretchable electronics, which should satisfy mechanical compliance: generally at least 30% deformation for human motion in daily life.17 Despite the significance of developing a united, multifunctional component endowing both adhesivity and conductivity, even in the bending and stretching states for practical applications, the current research and development trend for dry adhesives have focused only on a mimicking strategy for gecko-like structures and their adhesion properties.18−21 Here, we first propose a single stretchable and conductive dry adhesive (CDA) based on gecko-inspired architectures and nanocomposites. Unlike the existing dry adhesives that are generally made of insulating elastomer,18−21 the CDA developed here employs a conductive elastomer composed of carbon nanofillers and poly(dimethylsiloxane) (PDMS). Two different forms of carbon nanomaterials, consisting of a onedimensional (1D) carbon nanotube (CNT) and a two4771
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Figure 2. Relationship between the structural integrity of high-aspect-ratio, gecko-inspired micropatterns and adhesivity. (a) Calculated optimal elastic modulus for preventing lateral collapse of micropatterns as a function of the aspect ratio (AR). (b) Cross-sectional SEM images of the replicated micropatterns as a function of AR and the elastic modulus of the replicating materials. (c) Nondestructive structural imaging via confocal microscopy (lower) and confirmation of its validity by SEM (upper). (d) Images indicating different types of defects, such as lateral collapse and missing. (e) Measured normal adhesion force on a skin replica as a function of pillar height (inset: a top-view SEM image of the skin replica). (f) Cyclic adhesion property of the conductive dry adhesives compared to a commercial 3M wet adhesive. (g) SEM images of the micropatterns according to the cyclic use to demonstrate the degradation of structural integrity.
RESULTS AND DISCUSSION
the design and preparation of gecko-inspired Si microstructures. The geometric parameter of gecko-inspired structures is one of the key factors determining the bulk adhesion of the resultant CDA pad. The shape and area of a spatula tip and the aspect
Concept and Strategy for Realizing Conductive Dry Adhesives. Figure 1 describes the overall concept of this study. The fabrication procedure of the CDA pad begins with 4772
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Figure 3. Electrical properties of the conductive dry adhesives. (a) Measured volume resistivity of the conductive dry adhesives consisting of 1D−2D hybrid carbon fillers with different composition ratios (main filler: CNT, total amount of filler: 1 wt %). (b) Resistance change of the conductive dry adhesives under tensile stretching and bending. (c) SEM images of the fractured surfaces of conductive dry adhesives with various filler composition. (d) Schematic illustrations of the distribution of the fillers and electrical percolation in conductive dry adhesives matched with (c). (e) Digital images of the LED operation with conductive dry adhesives as interconnection attached on various conductive substrates (weight: 1 kg). (f) Surface profiles of various conductive substrates. (g) Relationship between the contact resistance and LED output.
human skin (rms ∼0.47 μm), a higher pillar with a wider spatula tip is required.15,16 Intaglio Si platforms containing square arrays of mushroom-shaped holes with aspect ratios
ratio of the supporting micropillars simultaneously influence the contact area with a target substrate.23 To enhance the adhesion between the CDA pad and the rough surface of 4773
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linking agent to validate the theoretical prediction (Figure 2b). The diameter of the pillars is fixed at 5 μm, and the height of the pillars is varied from 10 to 20 μm. When the pillar height is 10 μm (i.e., the aspect ratio is 2), the softer PDMS of ∼1.5 MPa can support the structure without any lateral collapse. When the pillar height increases to ∼15 μm, lateral collapse occurs in the case of softer PDMS, whereas defect-free replication is achieved using harder PDMS (∼3.5 MPa). For taller pillars of ∼20 μm, the PDMS cannot maintain the structural integrity, as expected by calculation. However, the harder PDMS shows a lower degree of lateral collapse per unit area compared to softer PDMS (Figure S3). The structural integrity of the CDA pads can also be visualized using 3D confocal microscopy because PDMS emits green photoluminescence (∼560 nm) under 488 nm excitation (Figure 2c).29 The defect structures (i.e., lateral collapse and missing pillars) are sufficiently observable through the high-resolution confocal imaging (Figure 2d). This method is more advantageous for nondestructive inspection of structures compared to SEM imaging, which sometimes causes unwanted distortion of high-aspect-ratio micropillars by an electron charging effect30 and essentially requires a thin metal coating on polymeric surfaces. Through microstructure inspection, the relationship between the structural integrity and the bulk adhesion properties of the CDA pad is systematically analyzed as a function of the aspect ratio of the micropillars. The average adhesion force of the CDA pads is measured by a normal pulling test with a replicated skin surface as a target substrate (Figure 2e). The RMS value of the skin surface and the elastic modulus of the conductive PDMS used here are ∼0.47 μm and 2.3 MPa, respectively. CDA pads with aspect ratios ranging from 2 to 4 are repeatedly attached to and detached from the skin replica over 30 times with a preload of ∼0.5 N/cm2. During the cyclic measurement, cleaning of adhesives is performed by gentle tapping with 3M tape or DI water every six cycles (Figure 2f). As the aspect ratio increases to 3 (identical to a pillar height of 15 μm), the adhesion force gradually improves to ∼1.3 N/cm2, which is slightly higher than the average adhesion force (∼1.1 N/cm2) of commercial wet adhesives (3M) used 30 times. However, when the aspect ratio of the pillars reaches ∼4, the average adhesion force decreases due to lateral collapse between neighboring micropillars. Although the initial adhesion of the CDA pad with an aspect ratio of ∼4 is slightly stronger than that of the CDA pad with an aspect ratio of ∼3, the degradation of structural integrity is severe for the former case during repetitive compressive contact (Figure 2g). The cyclic performance proves that the CDA pads perfectly recover their adhesive capability after surface cleaning, indicating that the pads are semipermanently reusable, whereas the commercial wet adhesives rapidly lose their adhesion properties and cannot be restored even after cleaning. Electrical Properties of Conductive Dry Adhesives. Figure 3 shows the electrical characteristics of the CDA composed of 1D−2D hybrid carbon nanofillers and PDMS. First, the high-aspect-ratio (∼1000), multiwalled CNTs (M95, Carbon Nanomaterial Tech. Ltd.) are physically dispersed in PDMS prepolymer without chemical planetary mixing.31 After the cross-linking of the prepolymer by heat treatment, the solid film has a volume resistivity of ∼12 000 ohm·cm at a CNT content of 0.4 wt %. Although the resistivity can be reduced by ∼150 ohm·cm at a CNT content of 1.0 wt % (Figure S4), this value is still not sufficient for use in electronic devices; at least ∼100 ohm·cm is required to obtain meaningful biosignals (P, QRS, and T curves) from ECG monitoring.22 PDMS
from 2 to 4 are prepared by photolithography and deep reactive ion etching (DRIE) processes using silicon-on-insulator (SOI) wafers (Figure S1).24 A wider spatula tip (∼7 μm) at the bottom end of the holes compared to the diameter (∼5 μm) of the holes can be achieved by excessive etching of SOI wafers during the DRIE process, which we refer to as a footing effect.25 The viscous PDMS prepolymer embedding 1D−2D hybrid carbon nanofillers with a low loading rate (∼1 wt %) is dispensed on the Si platform and filled into high-aspect-ratio holes by vacuum-assisted capillary filling (Figure 1a). After optimal heat treatment to cure the prepolymer (∼120 °C for 2 h), which is a relatively harsh condition compared to that for curing normal PDMS because fillers generally act as crosslinking inhibitors,26 the CDA pad with mushroom-shaped micropillar arrays is inversely replicated from the Si platform. The resultant product can be directly applied to skin-attachable electronic systems, especially for ECG monitoring, as a metalfree single component because of its integrated functions of conductivity and adhesivity. The diffraction colors from the Si platform and the CDA pad and microstructures prove the successful fabrication with large-area uniformity (∼4 in.2) (Figure 1b). The large-area CDA pad can be wrapped on various curved surfaces of the human body due to its superior flexibility and conformality (Figure 1c). The CDA pad almost preserves the intrinsic stretchability and bendability of the elastomer because minimized loading (∼1.0 wt %) of the conductive fillers in the composite is achieved through the synergetic percolation of 1D−2D hybrid carbon nanomaterials (Figure 1d and Figure S2). The successful LED operation using the CDA pad as an electrical interconnection with the bearing weight (∼1 kg) experimentally supports that it can be potentially used for simplified skin-patch electronic systems as an integrated component, which plays two different roles, as an electrode and as an adhesive, simultaneously (Figure 1e). Effect of Structural Factors of Conductive Dry Adhesives on Bulk Adhesion. Figure 2 addresses the relationship between the structural integrity of the highaspect-ratio structures in the CDA pad and the bulk adhesivity. The mechanical compliance of micropillars on the CDA pad against the target surface is determined by functions of the elastic modulus of the materials and the aspect ratio of the pillars. In general, high-aspect-ratio micropillars are desired to maximize the contact area and adhesion force between the CDA pad and the rough skin surface. However, the achievable aspect ratio of the micropillars is limited by the intrinsic elastic modulus of constructing soft materials, which should overcome the critical elastic modulus at given geometrical conditions (i.e., height, radius, density, and spacing) to prevent the lateral collapse of the micropillars.27,28 The theoretical maximum height of the micropillars without lateral collapse can be expressed by ⎛ π 4E R ⎞1/12 ⎛ 12E R3(W /2)2 ⎞1/4 eff ⎟⎟ hmax = ⎜⎜ 11 eff 2 ⎟⎟ ⎜⎜ γs γ − ν 2 (1 ) ⎠ ⎝ s ⎠ ⎝
(1)
where Eeff is the effective elastic modulus, R is the radius of the pillars, W is the period between neighboring pillars, and γs and ν are the surface energy and Poisson’s ratio of the material, respectively. From eq 1, the required elastic modulus of the construction materials for defect-free replication of micropillars can be extracted (Figure 2a). Two types of conductive elastomer with different moduli (∼1.5 and ∼3.5 MPa) are prepared by optimizing the amounts of the filler and the cross4774
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Figure 4. Surface characteristics and washability of gecko-inspired, conductive dry adhesive (CDA) pads. (a) Cross-sectional images of liquid droplets (i.e., diethylene glycol, propylene carbonate, and DI water) on surfaces of gecko-inspired CDA and flat conductive pads. (b) Digital image of liquid droplets on the gecko-inspired CDA pad. (c) Comparison of contact angles between gecko-inspired CDA and flat conductive pads for various liquid droplets. (d) Demonstration of washability and reusability of the CDA pad (yellow arrows: dust).
motion in daily living, is not significantly deteriorated (∼14% decreased conductivity). The CDA pad has enormous merit in bending circumstances because a bending angle up to ∼70 degrees does not affect the electrical properties of the CDA (Figure 3b). From the morphology of the fractured surfaces, we can analyze the distribution of carbon nanofillers in the matrix and electrical paths (Figure 3c and d). A small amount of wide and flat 2D carbon fillers, such as graphene and graphite, plays a key role as an activator to revive the partially “dead” percolation region where the conductive path is disconnected.35,36 However, as the ratio of secondary 2D carbon fillers to CNTs increases in the composite film, the major electrical path is confused due to the exponentially increased areal coverage of 2D carbon fillers compared to CNTs, and the electrical synergetic effect between 1D and 2D carbon fillers disappears (Figures S6 and S7). The CB (∼15 nm) is not large enough to connect the uncoupled percolation branches of CNTs but forms clusters around the already formed CNT networks. The LED arrays, electrically supplied by the attached CDA pad to various conductive substrates as an interconnecting layer withstanding the weight (∼1 kg), demonstrate the useful conductivity of the 1D−2D carbon hybrid nanocomposite (Figure 3e). Au, ITO, FTO, and single-layer graphene are used as the top layer for conductive glass substrates with different surface roughness (Figure 3f). Although the light-emitting power of an LED is varied by the intrinsic resistance of the conductive substrates, the CDA pad successfully operates the LED arrays by strongly attaching to the conductive substrates, ignoring surface roughness (Figure 3g).
prepolymer incorporated with CNTs over 1.0 wt % is difficult to use for capillary filling into micrometer-sized, deep-hole structures because the viscosity of prepolymer is too high due to the nondispersed, entangled CNT clusters in the elastomeric matrix.32 A higher level of electrical properties while maintaining the low loading rate of the fillers in an insulating matrix requires the use of a co-doping system composed of 1D and 2D carbon nanofillers. With the CNTs as the main filler, which can form a major electrical path in the composite, wide and flat 2D carbon materials, including carbon black (CB), nanostructured graphite (NanoG), and graphene nanopowder (GNP) (Figure S5), are tested as a secondary aid filler to enhance the electrical percolation. Except for the case of CB, which is apt to form aggregated clusters due to an extremely small average size (∼15 nm) of particles, a small amount of GNP and NanoG in CNT-based composites with the optimized mixing ratio of 9:1 (CNTs: aid fillers) helps to reduce the volume resistivity below ∼100 ohm·cm, which represents ∼33% enhanced conductivity compared to the composite containing only CNTs at the identical loading rate (Figure 3a).33 Among tested aid fillers, GNP shows the best material compatibility with CNT from the perspective of conductivity enhancement. Fixing the total filler content of GNP and CNTs at 1 wt % in the CDA pad, the resistance change (ΔR/R0) is measured to evaluate the dynamic electrical behavior of CDA under stretching and bending. Although the resistance of the CDA pad is linearly increased before fracture (∼100%), which is similar to the electrical behavior from other heterogeneous composite systems,34 the electrical performance at 30% elongation, represented as a stretching rate for human 4775
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Figure 5. ECG measurement under various conditions. (a) Comparison of the ECG signals using conductive dry adhesives (CDA) and commercial electrodes with 3M adhesives under normal conditions (inset in the red box: magnified view of the measured ECG wave indicating P, QRS, and T waves). (b) Comparison of ECG signals using CDA and commercial electrodes with 3M adhesives under an immersed condition (inset in the lower graph: magnified view of the detached commercial electrode due to water intrusion). (c) Series of ECG signals using conductive dry adhesives under various movement conditions (i.e., wrist curl, squat, and writing).
Multifunctional Single Component for ECG Measurement. Figures 4 and 5 demonstrates the practical application of the CDA pad to ECG monitoring systems. One of the valuable properties of the CDA pad that distinguishes it from conventional ECG electrodes is the self-cleaning capability. The high-aspect-ratio micropillars with spatula tips greatly increase the contact angle between various types of liquid droplets (diethylene glycol, propylene carbonate, and DI water) and the CDA pad’s surface (Figure 4a and b).37 The contact angle between DI water and the CDA pad with a pillar aspect ratio of ∼3 reaches ∼151°, and other liquid droplets (i.e., diethylene glycol and propylene carbonate) also show a similar large contact angle of ∼140° (Figure 4c). This superhydrophobicity makes the CDA pad semipermanently reusable by simply washing out the dust on its surface without pattern damage (Figure 4d and Movie S1) and enables the CDA pad to operate under immersed conditions by avoiding water permeation. The multifunctionality (i.e., adhesivity, conductivity, and superhydrophobicity) of the CDA pad is practically demonstrated through the ECG application. The CDA pad is directly connected to a wave simulator with the aid of an electric wire and is attached to three different positions (i.e., chest and left and right wrists) to obtain biosignals from a heart rate. All measured ECG signals are filtered by an eighth-order low-pass filter with a cutoff frequency of 40 Hz, followed by 60 Hz notch filtering. The heartbeat wave obtained from the CDA pad in a normal environment provides meaningful results with distinctly separated P, QRS, and T curves, which provide medical information about cardiovascular problems, with negligible fluctuation (Figure 5a). The recorded waveform is similar to that obtained from commercial ECG electrode systems (3M). The CDA pad collects the long-term, intact biosignals while maintaining conformal contact between the
pad and human skin, even in an underwater environment, whereas wet adhesive-based commercial electrodes are immediately detached from the body and lose biosignals within 5 s due to water infusion at the edge of the adhesive (Figure 5b and Movie S2). Although the amplitude of signals measured under immersed conditions using the CDA pad is reduced by 25.6% compared to that measured under dry conditions, the recorded waveform accurately reflects the P, QRS, and T curves due to its superior conformality and water-repellent characteristic.38 The conformality of the CDA pad is also wellmaintained during light and extreme human motions (e.g., wrist curl, squat, and writing), facilitating real-time tracing of high-risk vascular diseases (e.g., arrhythmia and myocardial infarction) in daily life (Figure 5c).
CONCLUSIONS In this work, we demonstrate a new type of dry adhesives based on gecko-inspired microstructures and conductive elastomeric nanocomposites. The defect-free, replicated CDA pad with mushroom-like micropillars shows good cyclic adhesion (∼30 times) with a normal adhesion force of ∼1.3 N/cm2 on human skin, which is comparable to that of commercial wet adhesives. The 1D−2D carbon hybrid filler system is introduced to enhance the electrical percolation and to maximize the conductivity in the elastomer matrix at low concentration (∼1 wt %) of fillers with an optimized mixing ratio of 9:1 (CNTs:graphene). The composite film with optimized conditions possesses superior stretchability of over ∼100% and electrical conductivity (∼100 ohm·cm) that is sufficient to operate LEDs. The CDA developed here has superhydrophobic surfaces that have a self-cleaning capability, and it can be potentially applicable to ECG monitoring systems as a costeffective and reusable all-in-one type electrode. 4776
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dry adhesives for the medical skin patches were attached onto the volunteer’s forearm and left leg and the ECG module (ADS1x9x ECGFE, Texas Instruments). Characterization. The micropillar structures and fractured surfaces of the co-doping composite were visualized via field-emission scanning electron microscopy (S-4800, Hitachi) at an accelerating voltage of 5−10 kV. The nondestructive surface profiles were directly captured using a confocal microscope (LV100POL, Nikon). The surface properties were analyzed using a contact angle goniometer (Phoenix 300, SEO).
MATERIALS AND METHODS Preparation of Mushroom-Shaped Hole Patterns. A mold containing mushroom-shaped hole arrays was fabricated by conventional photolithography and etching techniques. SOI wafers (Winwin Tech. Ltd.) composed of three layers (top bare silicon layer, BOX insulating layer (acting as an etching stop layer), and handle layer) were prepared. A top, bare silicon layer with three different thicknesses was used to fabricate three different pillar heights from 10 to 20 μm thickness with a step of 5 μm. In the first step, a negative photoresist (SU-8, MicroChem) was coated on the top bare silicon layer by spincoating. Hole patterns were made on the photoresist by conventional photolithography with a prepatterned chrome mask with a square array of holes of 5 μm in diameter, followed by a developing process to remove uncured photoresist. Deep reactive-ion etching was successively performed to create vertical holes in the top layer until the BOX layer was exposed. Under equal etching conditions, the top bare silicon layer was further etched in the lateral direction to fabricate mushroom-shaped pillars with tips. The tip width and thickness were controlled by the etching time. After the process, the microholepatterned SOI mold was treated with a self-assembled-monolayer solution. Preparation of Conductive Elastomers. The PDMS base (Sylgard 184, Dow Corning) and carbon nanofillers were first dispersed by hand using a spatula and were then mixed in a planetary mixer (ARE-310, Thinky) for 5 min at 2000 rpm and degassed for 1 min at 2200 rpm. Then, a PDMS curing agent was added (10:1 ratio of the base and curing agent) and incorporated into the mixture by stirring manually before mixing again for 5 min at 2000 rpm and degassing for 1 s at 2200 rpm. Fabrication of Conductive Dry Adhesives. The prepared mixture composed of CNT, second filler (CB, NanoG, or GNP), and PDMS was poured onto the microhole patterned SOI mold. Subsequently, a thin, flat backing layer of conductive dry adhesive was achieved through the spin-coating process (1000 rpm, 60 s). To degas bubbles in the composite mixture, the SOI mold with the flat conductive elastomer was placed in a vacuum chamber for 1 h, followed by curing at 120 °C for 2 h. The cured conductive dry adhesives were carefully peeled off of the SOI mold. Bulk Adhesion Test of the Conductive Dry Adhesives. Adhesion measurements were conducted with a pulling force sensor (FGJN-5B, Shimpo). To quantify the adhesion properties of the adhesives with skin, a skin replica was fabricated as follows. First, thermal-curable soft silicone resin (Ecoflex 0050, Smooth-on) was cast onto a volunteer’s arm and was cured for 30 min. The cured inverseskin surface was peeled from the arm, and PUA (MINS-ERM, Minuta Tech.) was poured on the inverse-skin surface. After full curing for 1 h under a UV lamp (150 mW/cm2), the PUA skin replica was peeled off from the Ecoflex mold. Sequentially, conductive dry adhesives were applied to the skin replica under a 0.5 N/cm2 preload. The adhesion force was measured by raising the sensor until adhesion failure. The adhesion test was conducted 30 times for each sample under the same conditions. The durability test of the conductive dry adhesives was also conducted for 30 repeating cycles, and each sample was cleaned every six cycles by Scotch tape and a DI water wash. Electrical Conductivity Measurement of Conductive Dry Adhesives. The top−bottom electrical conductivity of the micropillar arrays was calculated as σ=
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01355. Additional figures for schematic illustration of preparation steps for deep-etched Si mold and the loss of the electrical synergetic effect in 1D−2D hybrid carbon nanocomposites on increasing the aid-filler ratio, mechanical properties (tensile elastic modulus) of 1D− 2D hybrid carbon nanocomposites, SEM images for structural integrity of the mushroom-shaped micropillar arrays and fractured surface of hybrid carbon nanocomposites, and TEM images for carbon nanomaterials used here in detail (PDF) Movie 1 (AVI) Movie 2 (AVI)
AUTHOR INFORMATION Corresponding Author
*E-mail (S. Jeon):
[email protected]. Present Address
§ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the ICT R&D program of MSIP/ IITP [B0101-15-0239, Human Friendly Devices (Skin Patch, Multimodal Surface) and Device Social Framework Technology] and the Global Frontier Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014 M3A6B3063708). REFERENCES (1) Zhang, K.; Wang, X.; Yang, Y.; Wang, Z. L. Hybridized Electromagnetic−Triboelectric Nanogenerator for Scavenging Biomechanical Energy for Sustainably Powering Wearable Electronics. ACS Nano 2015, 9, 3521−3529. (2) Kim, S. L.; Choi, K.; Tazebay, A.; Yu, C. Flexible Power Fabrics Made of Carbon Nanotubes for Harvesting Thermoelectricity. ACS Nano 2014, 8, 2377−2386. (3) Kim, S. J.; We, J. H.; Cho, B. J. A Wearable Thermoelectric Generator Fabricated on a Glass Fabric. Energy Environ. Sci. 2014, 7, 1959−1965. (4) Park, J.; Lee, Y.; Hong, J.; Ha, M.; Jung, Y.-D.; Lim, H.; Kim, S. Y.; Ko, H. Giant Tunneling Piezoresistance of Composite Elastomers with Interlocked Microdome Arrays for Ultrasensitive and Multimodal Electronic Skins. ACS Nano 2014, 8, 4689−4697. (5) Park, H.; Jeong, Y. R.; Yun, J.; Hong, S. Y.; Jin, S.; Lee, S.-J.; Zi, G.; Ha, J. S. Stretchable Array of Highly Sensitive Pressure Sensors
I t V A
where I is the measured current, V is the applied voltage, t is the sample thickness measured by SEM, and A is the sample/electrode contact area. The applied voltage and measured current were obtained using linear scan voltammetry in an electrochemical scanning system. The electrical robustness test was conducted using the same methods under stretching up to ∼100% and bending up to a bending angle of 70 degrees. ECG Signal Measurement. Three identical conductive dry adhesives were prepared to acquire an ECG signal. The conductive 4777
DOI: 10.1021/acsnano.6b01355 ACS Nano 2016, 10, 4770−4778
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DOI: 10.1021/acsnano.6b01355 ACS Nano 2016, 10, 4770−4778
