Laser-Processed Nature-Inspired Deformable Structures forBreathable and Reusable Electrophysiological Sensors towardControllable Home Electronic Appliances and PsychophysiologicalStress MonitoringHyeokju Chae,† Hyuk-Jun Kwon,‡ Yu-Kang Kim,§ YooChan Won,† Donghan Kim,∥ Hi-Joon Park,*,§

Sunkook Kim,*,† and Srinivas Gandla*,†

†Multifunctional Nano Bio Electronics Lab, Department of Advanced Materials Science and Engineering, Sungkyunkwan University,Suwon 16419, South Korea‡Department of Information and Communication Engineering (ICE), Daegu Gyeongbuk Institute of Science and Technology(DGIST), Daegu 42988, South Korea§Acupuncture & Meridian Science Ressearch Center, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemoon-gu, Seoul 02447,Republic of Korea∥Department of Electronic Engineering, Kyung Hee University, 1732, Deogyoung Road, Giheung, Yongin, Gyeonggi 17104, SouthKorea

*S Supporting Information

ABSTRACT: Physiological monitoring through skin patch stretchable deviceshas received extensive attention because of their significant findings in manyhuman−machine interaction applications. In this paper, we present novelnature-inspired, kiri-spider, serpentine structural designs to sustain mechanicaldeformations under complex stress environments. Strain-free mechanicalstructures involving stable high areal coverage (spiderweb), three-dimensionalout-of-plane deformations (kirigami), and two-dimensional (2D) stretchable(2D spring) electrodes demonstrated high levels of mechanical loading undervarious strains, which were verified through theoretical and experimentalstudies. Alternative to conventional microfabrication procedures, sensorsfabricated by a facile and rapid benchtop programmable laser machine enabledthe realization of low-cost, high-throughput manufacture, followed bytransferring procedures with a nearly 100% yield. For the first time, wedemonstrated laser-processed thin (∼10 μm) flexible filamentary patternsembedded within the solution-processed polyimide to make it compatible with current flexible printed circuit board electronics.A patch-based sensor with thin, breathable, and sticky nature exhibited remarkable water permeability >20 g h−1 m−2 at athickness of 250 μm. Moreover, the reusability of the sensor patch demonstrated the significance of our patch-basedelectrophysiological sensor. Furthermore, this wearable sensor was successfully implemented to control human−machineinterfaces to operate home electronic appliances and monitor mental stress in a pilot study. These advances in novel mechanicalarchitectures with good sensing performances provide new opportunities in wearable smart sensors.

KEYWORDS: laser processed, nature-inspired, kirigami, electrophysiological, serpentine

1. INTRODUCTIONNoninvasive wearable electronic devices acquire and deliverdiscrete meaningful information about the body and externalmovements, which may provide crucial inputs for activecontrol in prosthetics, for control over stress, fatigue, andovertraining in athletics, and for continuous monitoring ofdiagnoses, electric generators, and human−machine interac-tions in biomedicine. However, the most common and criticalissue that hinders the performance of these electronics is themaintenance of device performance under external deforma-tions. To overcome this hindrance, two-dimensional (2D)spring-shaped designs motivated by a three-dimensional (3D)

helical spring have been extensively studied in buildingnumerous stretchable electronic devices.1−5 Alternatively,inherently stretchable conductive materials have gained muchattention in comparison to the current state-of-the-artstretchable sensors.6−10 Similarly, kirigami presents anotherway to produce 2D structural designs into 3D out-of-planedeformable architectures.11−16 Based on these, electronicdevices subjected to complex stress environments, such as

Received: April 16, 2019Accepted: July 11, 2019Published: July 11, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acsami.9b06363ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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knees and elbows, have been used to accommodate largemechanical deformations that stretch in 3D.17 Regardless, thepatterns are only stretchable in uniaxial direction with largedeformations normal to the structure.18 Thus, innovativebreakthroughs in advanced materials through unique, mechan-ically robust geometrical designs have drawn extensiveattention in the field of flexible and stretchable deviceengineering. Currently, the most common approach forfabricating these sensor systems relies on photolithographyand transferring techniques, which are quite costly, timeconsuming, and impractical with roll-to-roll processes.Recently, as an alternative to photolithography, an inexpensivefabrication process, “cut and paste” has been adopted.19,20 Inthis procedure, serpentine-based sensors are carved out ofmetallized polymer sheets with the aid of a benchtopprogrammable cutting machine. Instead, laser ablation offersother benefits such as direct patterning of metallic thin filmswithout damaging the host substrate, resolutions down to 50μm, and layer-by-layer deposition with embedded structures.However, the patterning of metallic thin films followed by thecutting of metallized polymer substrates with embeddedstructures is quite challenging but essential for protectingactive materials from multiple usages that lead to damage.Furthermore, the patch-based sensors have drawn greatattention toward wearable electronics because of theirbreathable and long-term wearable forms.5,21,22 However, theassociated sensor fabrication processes are highly expensive.Therefore, a sensory system with inexpensive fabricationprocesses, a patch with breathable forms for long-term wear,and a sensor with reusable forms for multiple usage greatlybenefit the foremost low-cost wearable electronics.Herein, we present novel nature-inspired mechanical

isolated designs that are robust and stable under significantphysical deformations. Nature-inspired geometric sensordesigns, which include kirigami structures to enable mechanicaldeformability, spider-web patterns for stable and high arealcoverage, and filamentary serpentine patterns for 2Dstretchability, have been adopted to follow-up the dynamicmechanical movements of the target skin during compression,stretching, pressing, and peeling-off. The sensor was fabricatedon a solution-based polyimide (SBPI) flexible substrate thathas exceptional thermal stability and smoothness afterprocessing. Moreover, the self-releasing capabilities of theSBPI layer make the fabrication process more facile. Thedesign architecture, along with the breathable and sticky natureof the patch, demonstrates high levels of spatial sensitivity,mechanical reliability, and repeatability, with excellentreusability. In addition, the fabrication process of the sensorsthat were fabricated through the laser ablation technique isfacile, rapid, and high-throughput. To demonstrate thefeasibility that the proposed sensors are useful in realapplications, electromyography (EMG) sensors were employedto control home electronic appliances and to monitor mentalstress as part of a pilot study. These mechanically isolatedstructural designs produced via facile fabrication method-ologies are not only beneficial for epidermal sensors but canalso be extended to promising futuristic wearable electronicapplications.

2. EXPERIMENTAL SECTIONSBPI (CAS # 872-50-4) was purchased from HD-MicroSystems andused as received.

2.1. Sensor Fabrication. Initially, SBPI was spin-coated onto aprecleaned rigid silicon/glass substrate at 2000 rpm for 30 s and thencured in an oven at 350 °C for 1 h. Next, titanium (Ti), as anadhesion layer of 20 nm, followed by gold (Au) 100 nm wasdeposited by electron beam (E-beam) evaporation on top of thepolyimide (PI) layer. We chose E-beam evaporation to deposit Auinstead of commercially available Au-coated PIs, which were quiteexpensive and thick. Next, we used a facile and rapid programmablelaser pulse cutting machine (λ = 1064 nm, INYA-20, In Lasers, Inc.)to pattern thin metallic films and carve metallized polymer films.Later, the desired AutoCAD design was imported into the XinMarkersoftware, and then the Au/Ti patterns (width 300 μm) were obtainedby removing the excess parts by a laser ablation hatch tool (40%power, 20 KHz frequency, 4000 mm/s marking speed, 1 μm ploydelay, and 4 nm pulse width) within minutes. Once the patterns wereformed, another layer of PI was formed as mentioned above, withspinning at 9000 rpm, which leads to a much thinner layer thanbefore. Thereafter, similar design patterns (widths 400 μm) were fedinto the software with a center position to fix to that of the previousdesign with the substrates placed at their initial positions with the helpof alignment markers; thus, the Au/Ti patterns were exactly alignedwith the top PI patterns (see Figure S4a, Supporting Information).Next, the samples were subjected to laser scribe (80% power, 20 KHzfrequency, 2 mm/s marking speed, 1 μm ploy delay, and 4 nm pulsewidth) completely along the stacking layer of the encapsulated PIstructure (see Movie S2, Supporting Information), followed byimmersion in deionized water maintained at 90 °C for approximately15 min, thereby leaving the undercut PI/Au/Ti/PI flexible films(thickness of 11 μm) to float on water (see Figures S3f−h and S4b,Supporting Information). The films were transferred onto a flexible/glass substrate, with the top PI facing toward the down-side. Then, theelectrodes were bonded to the long thin flexible wire with the aid ofanisotropic conductive film bonding, with the other end soldered to aconnector pin (see Figure S3i, Supporting Information). Finally, thefree-standing serpentine patterns were transferred onto a breathablepatch (medi band, genewel, Korea) that served as a sensor for all ofthe experiments and applications.

2.2. Electrical, Mechanical, and FEM Analysis. All of themeasurements related to mechanical behaviors were carried out by anultimate tensile machine (UTM) Mark 10 model ESM303, whereaselectrical measurements (I−V) and simulations were performed by aKeithley model 4200 SCS system and the finite element method(FEM) by COMSOL Multiphysics.

2.3. Controlling Home Appliances. Initially, signals wereobtained by using the BIOPAC system and were further amplifiedand monitored via a PC by using the Arduino (ATMEGA328 MCU),which were later fed to a microprocessor to generate inputs to relayfor converting raw EMG signals to programmable desired AC voltagesfor turning the electronic appliances ON/OFF.

2.4. Stress Monitoring. The Trier social stress test (TSST)standard protocol was performed to induce acute stress and tomeasure the changes in the electrocardiography (ECG) signals. TheECG signals were collected from the subjects by using MP36 (Biopacsystem, Inc., USA). The R−R intervals that were acquired by a BiopacStudent Lab (ver. 4.1.1, Biopac system, Inc., USA) from the ECGsignals were divided into 5 min periods: baseline, preparation,interview, arithmetic, after 1, after 2, and after 3. The data wereevaluated by using Kubios heart rate variability (HRV) (ver. 3.1., Biosignal Analysis and Medical Imaging Group, Finland) with a samplingrate maintained at a frequency of 2 kHz. The HRV data were analyzedby using GraphPad Prism 5.0.2 software (GraphPad Software, Inc.,CA, USA). Two-way analysis of variance (ANOVA) followed by aBonferroni post-hoc test was examined to know the time effects onthe changes in the heart rate (HR).

3. RESULTS AND DISCUSSION

3.1. Laser-Processed Nature-Inspired Kiri-SpiderSerpentine Structures. Figures 1a and Supporting Informa-tion S1a schematically illustrate the nature-inspired sensor

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electrode design to achieve both deformability and stretch-ability simultaneously. In this design, spiderweb patterns thatpossess high areal coverage with a structurally stable structureare adopted to acquire a high signal performance of the sensor,and then kirigami structures having unique functional proper-ties, such as high 3D out-of-plane deformability and the abilityto produce complex 3D geometries, in combination with 2Dspring stretchable patterns that stretch along the plane toenable robust strain-free mechanical structures that are able tostretch both in 2D and 3D are adopted. To understand moreclearly, the serpentine designs having without and withkirigami structures are compared, as shown in Figure S2Supporting Information. Moreover, the single electrode designpossessing serpentine structures with kirigami interlinkingnetwork and structurally stable spiderweb-like patterns greatlybenefit while handling/transferring the free-standing sensorelectrode design. Figure 1b represents the sensors produced bya user-friendly benchtop programmable laser with thefilamentary patterns exhibiting high levels of deformabilitywhen placed over a spherical surface. The sensor design with a

patch substrate possessing stretchable, sticky, and breathable(porous) forms is shown in Figure 1d,e. The fabricatedinspired structures attached to the stretchable PU foamdemonstrated uniaxial and biaxial stretchability, flexibility,and deformability (see Figure S1d, Supporting Information).The strain-free mechanical structures were fabricated bysequential spin coating of the PI layer, deposition of metallicthin films, and laser patterning and cutting techniques,followed by transferring methods.

3.2. Electrophysiological Sensor Fabrication Process.Figures 2a−j and Supporting Information Figure S3a−irepresents the schematic and experimental process to fabricatethe proposed sensor, with the corresponding multilayereddesign layout depicted in Figure 2k. In our approach, PI is usedinstead of other polymers because of its high thermal stabilityafter processing, smoothness after curing, and a self-releasecapability upon immersion in water from the mother substrates(glass and silicon). Most importantly, no adhesives, such asthermal release tape, or wax adhesives are employed in ourapproach. However, sacrificial layers (SL) are not preferred asthe solution-based PI is processed at 350 °C, which maydamage/modify the SL layer. Moreover, we use a facile andrapid programmable laser pulse cutting machine to pattern thinmetallic films and carve metallized polymer films selectivelyalong the stacking structure. Recently, the “cut and paste”approach has received a large amount of attention because themetallized polymer sensory layouts were able to be directlycarved by a facile, rapid, and inexpensive programmable cuttingmachine. Alternatively, laser patterning offers additionalbenefits such as direct patterning of metallic thin films,resolutions down to 50 μm, and layer-by-layer deposition withembedded structures.For the above-mentioned advantages and to further extend

the scope of these high-throughput manufacturing techniques,the pulse laser ablation technique is executed to fabricate thesensor. To manufacture the proposed device, metallizedpolymer films are fed into the programmable pulse laser forpatterning thin metallic films, followed by deposition of secondlayer PI and laser scribing. It should be noted that the Aupatterns that were obtained with and without line ablation led

Figure 1. (a) Schematic illustration of the nature-inspired kiri-spiderserpentine structural design for EP sensing. (b,c) Electrode patternsproduced by the benchtop programmable pulse laser machineexhibiting high levels of deformability placed over a spherical surface.(d,e) Sensor attached to the sticky, breathable, and stretchable patch.

Figure 2. (a−j) Schematic illustration of the sequential fabrication process and (k) layout of layer-by-layer design of the proposed sensor.

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to smooth and rough edges across the Au patterns (see FigureS4c,f, Supporting Information). Moreover, the sensor that wasfabricated without the top PI layer led to delamination issuesfrom the bottom PI layer (see Figure S4g, SupportingInformation). Therefore, it is necessary to take these issuesinto account to further enhance the mechanical robustness ofthe sensor. This method is similar to the principle of positivephotolithography processing, that is, the excess parts arecompletely ablated, leaving only traces of the Au/Ti patterns ofelectrophysiological (EP) electrodes. As mentioned, our sensorelectrodes were designed to be embedded within the PI layers;therefore, the laser parameters and design widths are subjectedto change pertaining to first the patterning of the Au/Ti/PIlayers and second the carving of the PI/Au/Ti/PI layers.Additionally, the sensor size can be significantly scaled downwith the pattern width as small as 100 and 50 μm, representedin Figure S5, Supporting Information. The sticky andstretchable patch presented here exhibited unprecedentedwater permeability (>20 g h−1 m−2 at a thickness of 250 μm)because of its porous structure, measured according to ASTMF1249 (standard test method for water vapor transmission rateusing a modulated infrared sensor), which is generallysufficient and more than the normal skin permeability of 8−20 g h−1 m−2.23−25 Moreover, the films with water vaporpermeability (WVP) of 20−40 g h−1 m−2 are also reported butare not adherent to skins.21,26−30 Recently, in comparison topolydimethylsiloxane, Ecoflex, and Solaris, the stretchableelastomeric material Silbione (RT Gel 4717 A/B, BluestarSilicones, USA) showed promising characteristics of thematerial like stretchability and sticky nature with WVP as∼10 g h−1 m−2 at a thickness of 100 μm, but it is veryexpensive.5 In comparison to the above-mentioned materials,the patch exhibited high WVP values with stretchability andsticky nature.

Moreover, the sensor reusability was demonstrated just bydipping the patch-based sensor into acetone (see Figure S6,Supporting Information). The filamentary patterns aresuccessfully retrieved from the patch without any structuraldamage, subsequently dried, and transferred to a new patchsubstrate. The epidermal sensor systems produced by thedirect cutting of metallized polymer films with metallic filmsfacing the top showed convincing results compared to that ofconventional Ag/AgCl electrodes; however, the performanceof the sensor after reuse is critical. To overcome this issue, PI-encapsulated EP sensors are reported, despite the sensor beingprocessed by conventional lithography techniques that arequite expensive and time consuming. To balance theseparadoxes, we produced laser-processed EP sensors that arefacile, rapid, and inexpensive with the additional feature ofpatternable Au, followed by PI as a protective layer forreusability.

3.3. Finite Element Analysis and Mechanical Stability.To evaluate the mechanical stability of the sensor structures,we studied its mechanical and electrical characteristics byperforming multiple experimental procedures and simulations(see Figures 3a−f and S7a−f, Supporting Information). All ofthe measurements related to mechanical behaviors were carriedout by an UTM, Mark 10 model ESM303, whereas electricalmeasurements (I−V) and simulations were performed by aKeithley model 4200 SCS system and the FEM by COMSOLMultiphysics. Figure 3 shows the 2D (edge side and plane sidestretching) stretchability and 3D (out-of-plane direction)deformability of our designed kiri-spiderweb serpentineinspired structure. Note that our PI/Au/Ti/PI layers had 3.5× 108 N/m2 ultimate tensile strength (UTS) at strain levels of>60% (max. elongation). To reach this tensile strength, thespecimens were designed to be identical, with the same shapedcontact pad attached to the other end of the structure to

Figure 3. Mechanical behavior and FEA of the proposed sensor. (a) Initial position without any load, (b) deformations along the z-axis, (c,d)uniaxial direction stretching along the diagonal and plane, and (e,f) dynamic uniaxial strain response along the edge and plane.

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achieve better accuracy (see Figure 3a). When the globalstructural strain reached a strain of 95% with a generatedmechanical tensile stress of ∼4.0 × 107 N/m2 (the initial yieldpoint), the free-standing filamentary sensor electrodessubjected to uniaxial strain along the edge started to transitionfrom elastic deformation to plastic deformation (>0.5% strain)near the electrical contact pad connection area (see Figure 3c).Then, the electrical resistance of the proposed sensor electrodedrastically increased when the structural strain exceeded 110%(ultimate global strain, failure point). Similarly, as shown inFigure 3d, the sensor electrodes that were applied on the planeside with uniaxial stretching were able to withstand a structuralstrain of 78% until the initial plastic deformation (>0.5%) ofthe Ti/Au (electrode materials) occurred; the failure strain was97%. We also considered the deformation to be in a normaldirection to the plane of the sensor, reflecting the varioushuman motions (e.g., squeezing, compressing, pushing, andpulling) of daily life. In Figure 3b, the displacement (thenormal direction of the plane of the sensor) was able to besustained as much as 11.5 mm within the elastic deformablerange of the electrode. Here, we would like to note that adetailed stretching procedure is provided in Movie S3,Supporting Information. To achieve the stable flexibility andstretchability in the subject-to-sensor operating performance,the confidential working range should be set within the elasticdeformable range (i.e., the upper tolerance limit: 78% forplanar stretching and 11.5 mm for the direction perpendicularto the plane). Fortunately, a structural strain of 30% is goodenough for the working environment of the sensor because ofthe elastic limit of the human skin.31 Furthermore, ourelectrodes are patterned at approximately 50 μm away from theedge of the PI patterns (see Figure S4a, SupportingInformation), where the stress and strain are highlyconcentrated (Figure 3b−d); this design makes it possiblefor the metallic thin films (which are vulnerable to mechanicaldeformation) to create relatively large global structural strainsbecause the fragile electrodes avoid the mechanical force andthe strain-maximized area. Moreover, according to our FEMresults, the generated stress could be well distributed in ourproposed design when the structure is simultaneously

deformed in various directions. The results obtained by finiteelement analysis (FEA) are in compliance with theexperimental data; the structural designs are favorable forrobust mechanical-free device constructions.The mechanically robust designed electrode network that is

firmly attached to the bio-compatible patch plays a veryimportant role in applying the sensor to the human body: itenhances the adhesion force to the skin, is transparent tomoisture loss, increases durability for long-term use with thecomplex and dynamic movements of the skin, and enables areplacement of the sensor system. It should be noted that thepatch is made from a hyperelastic material that exhibits stresssoftening or progressive damage (i.e., hysteresis) depending onthe previous maximum load, called the Mullins effect.32

Therefore, to minimize this effect and obtain more preciseproperties, a loading/unloading cycling test (up to 30% strain)was performed until hysteresis was not observed (see FigureS8b,c, Supporting Information). Then, we obtained a stress−strain (SS) curve, exhibiting the shape of the graph similar tothat of the typical SS curve of the incompressible solid rubber(see Figure S8a, Supporting Information). Here, we found theUTS of the patch with a thickness of 250 μm as ∼1.8 N/m2 ata maximum elongation of 240%, Young’s modulus as 0.149 N/m2, and Poisson’s ratio as ∼0.49 (see Figure S8a, SupportingInformation). The low modulus nature and the rubberlikebehavior of the sticky patch can enforce a minimal constrainton the sensor structure subjected to motion artifacts andnaturally accommodate skin slippage. On the basis of theproperties of the patch, we conducted a stretching and cyclingtest to understand of the mechanical behavior of the proposedsensor network integrated with the patch. It is observed andunderstood from Figure 4a−c that as the patch is sticky andrubberlike, behavior deformation pertaining to the sensoryelectrodes is relatively constrained in comparison to electrodeswithout the patch, so that the maximum structural strain rangeis reduced. However, the sensor on the patch can confinewithin the target range (the structural strain of <30%) duringstretching. Most importantly, after over 2000 cycles, thesensors maintained mechanical stability with a uniaxial strain of30%, which is within the strain limit of Au; the overall structure

Figure 4. Mechanical behavior and FEA of the proposed sensor with the patch. (a) Initial position without any load and (b) uniaxial strain alongthe edge. (c) Simultaneous measurement of resistance and load along the stretching direction and (d) mechanical stability over 2000 cycles with amaximum strain of 30%.

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was able to return to its normal position without anypermanent structural deformations (see Figures 4d, S8d, andMovie S4, Supporting Information). Therefore, from theresults of the mechanical tests and analyses, we note that thepatch offers great benefits and has sufficient mechanicalstability for application on the human skin.3.4. EP Monitoring. The fabricated sensor is designed to

acquire various EP signals, with the overall sensor design sizehaving a high areal coverage equivalent to that of conventionalelectrodes. Next, the EP signals are continuously acquired by

means of a physiological data acquisition system (BSLBSC-W,Biopac) when the sensor is placed at the desired positioncorresponding to the type of measurement performed, such asEMG, ECG, and electrooculography (EOG), as shown inFigure 5 (see also Figure S9a, Supporting Information).Simultaneously, the EP signals are measured by conventionalelectrodes (Ag/AgCl) to quantify the results from nonconven-tional electrodes. In the case of EMG, three proposed andconventional electrodes were attached to the forearm,especially to the flexor muscles whose location determines

Figure 5. Schematic and graphical representation of both proposed and conventional electrodes for measuring EMG, ECG, and EOG.

Figure 6. (a−c) Conceptual illustration of controlling home electronic applications with the proposed EMG sensors wrapped over the arm. (b,c)represent the wrist position and finger movements with the EMG signals measured simultaneously.

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the quality of the imposed operations, followed by grippingand release of hand clenches. To extract the desired signals andfilter out the unwanted frequencies, the signals were filtered ata high-pass of 0.1 Hz and a low-pass of 40 Hz with a samplerate of 2 kHz. It is noted that in both cases, significant featuresare observed with similar amplitudes; however, the strain-freemechanical structures produced from the rapid laser ablationprocessing benefit the sensor’s robustness to physicaldeformations as well as conformable, reusable, breathableand customizable patterns. Next, the ECG signals were filteredthrough low-pass 0.5 Hz and high-pass 35 Hz, with the ECGsignals recorded at regular intervals by placing the electrodeson the left and right sides of the chest, with the referenceground located on the lower right abdomen. Finally, the EOGmeasurements were performed by moving the eye left (down)and right (up), with the noise and unwanted signals filteredthrough a band-stop filter (0.05−35 Hz) while placing thepositive and negative electrodes to the left of the left eye andright of the right eye, with the reference ground located at thecenter of the forehead. It should be noted that during allmeasurements, the adhesion of the patch is sufficient to peelthe device from the skin without damaging the sensor system.3.5. EMG Sensor for Control over Home Appliances.

EP sensors are essential for delivering various useful pieces ofinformation that pertain not only to biomedicine but also tofind major applications in human−machine interactions.33−37

Among the various physiological signals, EMG signals can beobtained from discrete locations on the body: arm forprosthetic control and hand mimicking robot, leg for

exoskeleton and artificial legs, and face for emotionalrecognition. Herein, the proposed EMG sensor was appliedto control the home electronic appliances individually. Thesequential arrangement of EMG sensors (nine electrodes) waswrapped over the arm to cover the different muscles that areutilized in delivering individual signals depending on the pose.To acquire these signals, home electronic appliances areselectively controlled ON/OFF by two-channel EMG sensorsattached to the forearm. The conceptual illustration anddemonstration of the proposed sensor for controlling homeelectronic appliances, such as a fan, light, and humidifier isshown in Figure 6a−c.Initially, the signals are monitored by using the BIOPAC

system and are further studied to obtain specific signals fromdifferent sensors located on the forearm (see Figure S9b,Supporting Information). Once the location (flexor, extensormuscles) of the two-channel EMG sensors, EMG1 and EMG2(positive, negative, and reference electrodes), was finalized, thesensors were amplified and monitored via a PC by using themicrocontroller and were later fed to a microprocessor togenerate inputs to the relays for converting the raw EMGsignals to programmable desired AC voltages of ON/OFF forthe electronic appliances (see Figure S10a and Movie S5,Supporting Information). The simplified configuration of thesystem and the flowchart showing the algorithm for controllinghome appliances are shown in Figure S10b,c SupportingInformation. As shown in Figure 6b, moving the third finger orarm to the left (radial deviation) or moving it down (flexion)turns the lamp or humidifier or fan, on or off independently.

Figure 7. (a,b) Sequential pictorial and graphical representation of the tasks performed by the participant with simultaneous measurements ofECG. (c) Comparisons of HRV variables obtained from both proposed and conventional electrodes among a series of eight HRV variablescontaining mean RR (ms) (i), mean HR (bpm) (ii), SDNN (ms) (iii), RMSSD (ms) (iv), NN50 (beats) (v), pNN50 (%) (vi), RR triangular index(vii), and LF/HF ratio (viii), where r and p are the Pearson correlation coefficient and value.

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Finally, the sensors delivered better performances with lessexternal noise.3.6. Psychophysiological Stress Monitoring−TSST.

Mental stress reduces performance in daily routine life causedby risky situations and difficult environments that mainly occurat the work place, which, in turn, cause depression,hypertension, cardiovascular disorders, and most saviorcognitive dysfunctions.38−41 The early detection of stress-related issues can prevent diabetes, asthma, and saviorsurgeries. Currently, many techniques have been developedto monitor stress-related concerns, among which HRV dealswith the ECG signal of both instantaneous HR and serialconsecutive time domain intervals of the R-wave, considered tobe reliable for analyzing the autonomous nervous system,which reacts to and controls the external stimuli.40,42−45

However, as it is difficult for physicians to continuously assessthe stress, real-time monitoring is highly desirable.The goal of this study is to acquire the physiological signals

and features of the proposed sensor that showed the mostdistinct reaction to mental stress in accordance with ourprotocol. For this, we employed the TSST standard protocol toinduce acute stress and measure the changes in the ECGsignals obtained from our proposed sensor. The TSST iscarried out with the proposed sensor and conventionalelectrodes attached side by side. We compared each HRVacquired from two electrodes to assess the validity of theproposed sensor. The psychophysiological effects of the TSSTare not limited to an escalating HR but have broad impacts onthe hypothalamic-pituitary-adrenal axis, central nervous sys-tem, immune system, and so on.46 To start the TSST, werecruited six healthy subjects (participants) in their twenties(20−25 years) who did not have any endocrine disorders,cardiovascular diseases, or psychiatric problems. In addition,we used the Center for Epidemiologic Studies-DepressionScale (CES-D) and State-Trait Anxiety Inventory to estimatethe levels of participants’ depressiveness and anxiety,respectively (Table S1, Supporting Information).The TSST comprises of a 5 min baseline, 5 min preparation,

5 min speech, and 5 min computer arithmetic test. Initially, theparticipants are requested to introduce themselves as if in a jobinterview, followed by 5 min of self-introduction preparation.Then, they performed a self-introduction in front of threeresearchers who played the role of an interviewer for 5 min.Next, they took a mental arithmetic test [such as x (four digit)minus y (two digit), with the value minus y done repeatedlyuntil the values were correct, with 8 s given for each problem]on a computer for 5 min, while one researcher stood behindthem to watch their performance.The ECG signals based on the stress assessment method-

ology are shown in Figure 7a,b. To avoid and reduceenvironmental changes that affect the stress task, an ideallaboratory setup is designed to perform this task. During theTSST, it became evident that an increase in the HR occurred.During the course of obtaining the ECG signals imposed bydifferent tasks, each section had a different rate of stress as theHR increased (Figure 7b), which indicate that acute stress waswell induced. The data are communicated as the mean ±standard error of the mean. Two-way ANOVA followed by aBonferroni post-hoc test showed that the time effects on thechanges of the HR were extremely significant [F(6,36) = 34.72,p < 0.0001] and that the changes of the mean HR from boththe electrodes had no significant difference [F(1, 6) = 0, p =1.000] (see Figure S11, Supporting Information). In the HRV

analysis, it is very usual to analyze several variables calculatedfrom the R−R intervals, such as the standard deviation of all ofthe R−R intervals (SDNN), the root mean square ofsuccessive differences (RMSSD), the beat count of successivenormal minus RR intervals more than 50 ms (NN50), thepercentage of NN50 (pNN50), and a low-frequency/high-frequency ratio (LF/HF ratio). Pearson’s correlation analysesshowed that the HRV data of proposed and conventionalelectrodes matched well with each other on a one-to-one basisaccording to the mean RR (r = 1.000, p < 0.0001), mean HR(r = 1.000, p < 0.0001), SDNN (r = 0.9747, p < 0.0001),RMSSD (r = 0.8845, p < 0.0001), NN50 (r = 0.9988, p <0.0001), pNN50 (r = 0.9988, p < 0.0001), RR triangular index(r = 0.9836, p < 0.0001), and LF/HF ratio (r = 0.9993, p <0.0001). We investigated these variables within a 5 min periodand compared them among the proposed and conventionalelectrodes, as shown in Figure 7ci−viii. Although the acquiredHRV data were successful, some subjects were excluded fromthe analysis because of less signal quality due to lots of hairs onthe electrode-attached sites of male participants. Nonetheless,these results would be sufficient for demonstrating the validityof our newly developed sensor electrode.

4. CONCLUSIONSA patch-based sensor with nature-inspired deformablestructures designed to acquire EP sensing exhibited significantmechanical stability without producing any adverse effects onthe skin. The patch having stretchability, sticky, and breathableforms enabled the long-term accessibility of the sensor formultiple usages over time. The sensors produced by facilerapid laser ablation processing greatly surpassed the need forconventional microfabrication techniques with an almost 100%yield, which is essential for low-cost and customizableelectronics. This technique can be applied to futuristicminiaturized electronics devices in which resolutions downto 50 μm and multiple layer-by-layer spincoating followed bypatterning are essential. The advances in novel mechanicalarchitectures with the laser patterning technique are success-fully implemented to control/monitor home electronicappliances/mental stress that can enable new opportunitiesin wearable smart sensors.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b06363.

Illustration of the sequential approach in buildingnature-inspired kiri-spider serpentine structural design;fabrication process by benchtop programmable lasermachine; sensor electrode design parameters, cross-sectional view, thickness, and optical microscopicimages; demonstration of sensor reusability by justdipping in acetone; cyclic response; stress versus strainbehavior of the patch; sensor electrode positions withrespective to EP sensing (EMG, ECG, and EOG);control input signals to the microcontroller andflowchart showing the algorithm process flow incontrolling the home electronic appliances; comparisonof HR variation obtained by TSST (PDF)Laser hatch Au patterning (AVI)Samples subjected to laser scribe completely along thestacking layer of the encapsulated PI structure (AVI)

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Detailed stretching procedure (AVI)Mechanical stability of the sensors after over 2000 cycles(AVI)Demonstration of the proposed sensor for controllinghome electronic appliances, such as a fan, a light, and ahumidifier (AVI)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: acufind@khu.ac.kr (H.-J.P.).*E-mail: seonkuk@skku.edu (S.K.).*E-mail: srinivasiitb24@gmail.com (S.G.).ORCIDSrinivas Gandla: 0000-0002-4586-1483Author ContributionsH.J.C., H.-J.K and Y.-K.K. contributed equally. All authors readand contributed to the manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported in part by the National ResearchFoundat ion of Korea (2018R1D1A1B07048232 ,2014M3A9D7070732, 2017R1A2B4009963). This work wassupported by the Ministry of Trade, Industry, and Energy(MOTIE) and Korea Evaluation Industrial Technology(KEIT) through the Industrial Strategic Technology Develop-ment Program (10080348). This work was partly supported bythe Gyeonggi-do Regional Research Center program ofGyeonggi province (GRRC Sungkyunkwan 2017-B06, Nano-biosensor based on flexible material)

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