Simple Approach to High-Performance Stretchable Heaters Based on

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Simple Approach to High-Performance Stretchable Heaters Based on Kirigami Patterning of Conductive Paper for Wearable Thermotherapy Applications Nam-Su Jang, Kang-Hyun Kim, Sung-Hun Ha, Soo-Ho Jung, Hye Moon Lee, and Jong-Man Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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ACS Applied Materials & Interfaces

Simple Approach to High-Performance Stretchable Heaters Based on Kirigami Patterning of Conductive Paper for Wearable Thermotherapy Applications Nam-Su Jang,1 Kang-Hyun Kim,1 Sung-Hun Ha,1 Soo-Ho Jung,2 Hye Moon Lee,2,* and JongMan Kim1,3,* 1

Department of Nano Fusion Technology and BK21 Plus Nano Convergence Technology

Division, Pusan National University, Busan 46241, Republic of Korea 2

Powder & Ceramics Division, Korea Institute of Materials Science, Changwon 51508, Republic

of Korea 3

Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of

Korea

ACS Paragon Plus Environment

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ABSTRACT Recent efforts to develop stretchable resistive heaters open up the possibility for their use in wearable thermotherapy applications. Such heaters should have high electrothermal performance and stability to be used practically, and the fabrication must be simple, economic, reproducible, and scalable. Here we present a simple yet highly efficient way of producing high-performance stretchable heaters, which is based on a facile kirigami pattering (the art of cutting and folding paper) of a highly conductive paper for practical wearable thermotherapy. The resulting kirigami heater exhibits high heating performance at low voltage (> 40 °C at 1.2 V) and fast thermal response (< 60 s). The simple kirigami patterning approach enables the heater to be extremely stretchable (> 400%) while stably retaining its excellent performance. Furthermore, the heater shows the uniform spatial distribution of heat over the whole heating area, and is highly durable (1000 cycles at 300% strain). The heater attached to curvilinear body parts shows stable heating performance even under large motions while maintaining intimate conformal contact with the skin thanks to the high stretchability and sufficient restoring force. The usability of the heater as a wearable thermotherapy device is demonstrated by increased blood flow at the wrist during operation.

KEYWORDS: stretchable conductor, skin-mountable heater, kirigami patterning, conductive paper, wearable thermotherapy


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1. INTRODUCTION In recent decade, flexible and stretchable resistive heaters have gained great attention due to their considerable potential for various applications in next-generation devices, such as window defrosters,1–4 thermochromic displays,5 flexible gas sensors,6 prosthetic skin devices,7 and wearable thermotherapy patches.8–10 A number of different conductive materials have been extensively explored for flexible and stretchable devices, including metallic nanowires,1–3,8,11–15 superaligned carbon nanotube sheets,4,5 hybrid of nanowires and carbon nanotubes,16,17 graphene films,6,18 pencil graphite,19 patterned metal thin-films,7,10,20 and metals on structural and sacrificial electrospun fibers.9,21 Based on the advance in conductive nanomaterials technologies, notable attention has been recently devoted to developing skin-mountable resistive heaters based on conductive nanomaterials to replace conventional thermotherapy methods such as heat packs and wraps, which are heavy and bulky and have limited wearability due to their mechanical rigidity.22 For this application, mechanical stretchability of heating devices is important for maintaining intimate conformal contact with the skin for effective heat transfer, comfortable wearability, and stable heating performance, all while enduring large strains during body motion. In this regard, highly stretchable conducting element that can retain the electrical conductivity under various mechanical deformations is a key factor for successful development of skin-mountable heaters. To date, stretchable heaters have mainly been demonstrated using two representative strategies. First, random networks of functional conductive materials such as metallic nanowires,14,15 metalcoated polymeric fibers,21 and metallic glass nanotroughs9 are attached to stretchable elastomeric substrates. In this way, simple structures of stretchable heaters can be achieved through relatively facile fabrication. However, the stretchability of the resulting devices is generally limited to tens


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of percent strains due to severe loss of the electrical conductivity when subjected to large strains. More importantly, the heating performance gradually and unavoidably degrades upon stretching, even at relatively low levels of strains. Therefore, an additional feedback control module is inevitably required to adjust the output temperature for practical applications. The second approach uses stretchable architectures such as fractal-inspired stretchable heaters,7 serpentine-mesh heaters made of metal nanowire percolation networks,8 and heating pads supported by serpentine metallic springs.10 Well-designed stretchable heater architectures make it possible to accommodate mechanical stresses while retaining the electrical properties by predominantly changing their structures under deformations. This results in stable electrothermal performance even under large strains (> 100%). However, the fabrication is quite complex and time-consuming, which is one of the biggest challenges for practical use. This work introduces high-performance stretchable paper heater based on simple kirigami geometry on a highly conductive paper substrate sandwiched between thin elastomers. The synergetic combination of kirigami patterning and conductive paper synthesis techniques can simultaneously address the critical limitations regarding the ‘fabrication’ and ‘performance’ in the conventional wearable resistive heaters: 1) the entirely solution-processed conductive paper is patterned into kirigami geometry by a very simple, fast, and inexpensive plotting approach without any cumbersome or complex fabrication processes, and 2) rational geometrical design of the kirigami pattern allows the paper conductor to be extremely stretchable without almost any loss in performance. The resulting stretchable kirigami heater exhibits superior electrothermal performance at low voltage (> 40 °C at 1.2 V), rapid thermal response (< 60 s), uniform, stable heating upon stretching (up to 400% strain), and highly reproducible performance (1000 cycles with a maximum strain of 300%). High stretchability and sufficient restoring force of the


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kirigami heater enables the device to be attached conformally to various body parts independent of the size and movement, possibly allowing efficient and uniform heat transfer to the skin even at large-area toward practical skin-mountable thermotherapy.

2. EXPERIMENTAL DETAILS Synthesis of Conductive Paper. Aluminum (Al) paper was prepared by a facile chemical solution process at room temperature, as described in our previous works.23–25 The moisture was first fully removed from a pristine paper sheet in a convention oven at 85 °C to prevent the formation of non-conductive components such as aluminum hydroxide (Al(OH)3) and aluminum oxide (Al2O3) during processing. The paper was then coated with a catalyst by immersing it in a chemical solution mixed with 5 vol% of titanium isopropoxide (Ti(O-i-Pr)4) and 95 vol% of dibutyl ether (O(C4H9)2). Afterwards, the paper was kept in the solution for 40 min for uniform coating of the catalyst onto all of the cellulose fibers in the paper sheet. The paper sheet was then immersed in an Al precursor solution of AlH3{O(C4H9)2} for 2 h to decompose the precursor into Al, 1.5H2, and O(C4H9)2 on all fiber surfaces. The nucleated Al sites gradually grew, and highly conductive Al features eventually covered the surfaces of each fiber and tightly filled in the empty spaces between them.

Fabrication of stretchable heaters. The proposed stretchable heaters were simply fabricated by embedding kirigami-patterned Al paper in a highly elastic silicone elastomer. Silicone prepolymer (Ecoflex 00-50, Smooth-On) was mixed with a curing agent at a weight ratio of 1:1 and then first spin-coated onto a cleaned polyethylene terephthalate (PET) supporting film at 700 rpm for 35 s. This was followed by partial curing in a convection oven at 70 °C for 60 s. A piece


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of the Al paper was then stacked firmly onto the partially cured ecoflex layer under slight pressure. The sticky surface of the partically cured ecoflex is greatly helpful for conformal and intimate bonding with the Al paper without any air voids at the interface. A second lasyer of ecoflex (top polymer) was then spin-coated onto the Al paper stacked on the partially cured ecoflex (the bottom polymer) under the same coating conditions as above. Next, the top and bottom ecoflex layers were entirely solidified at the same time by thermal curing at 70 °C for 5 min to make the sandwich structure mechanically stable. Kirigami designs were patterned by cutting through the sandwich structure using a commercially available computer-controlled electronic cutting machine (Cameo, Silhouette). Finally, stretchable kirigami heater was completed by peeling off the patterned sandwich structure from the supporting PET film. A band-type kirigami heater was fabricated by simply connecting both ends of the ecoflex/Al paper/ecoflex sandwich structure using the ecoflex after patterning into simple kirigami geometry.

Characterization. The detailed morphology and chemical composition of the Al paper were investigated with a field-emission scanning electron microscope (FESEM; S7400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDS). A four-point probe system (FPP; CMT-SR 1000N, Advanced Instrument Technology) was used to measure the sheet resistance distribution on the Al paper. The FPP measurements were performed at least three times at each location (top, bottom, center, left, and right) and averaged. The electrical resistance of the kirigami conductors and heaters was measured using a digital multimeter (34465A, Keysight Technologies) connected to a computer through a RS232 data


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cable. To operate the kirigami heaters, an input voltage was applied using a DC power supply (E3649A, Agilent Technologies). Tensile strains were applied to the kirigami conductors and heaters using a computer-controlled stage (JSV-H100, JISC) equipped with a push-pull force gauge (HF-10, JISC). The electrothermal properties of the stretchable kirigami heaters were characterized by measuring the maximum temperature using an infrared (IR) thermal camera (Ti400, FLUKE), which is capable of measuring temperatures of up to 1200 °C. A light-emitting diode (LED) circuit was simply constructed by connecting the fabricated stretchable kirigami interconnect and the power supply to a green LED with a turn-on voltage of ~2.5 V in cascade. The light intensity of the LED was monitored in real-time while gradually deforming the kirigami interconnect by applying various tensile strains. The current-voltage (IV) curves of the LED circuit under various strains were recorded using an impedance analyzer (IviumStat, Ivium Technologies). The kirigami heaters were then attached to the wrist of one of the authors and activated. The blood flow was then characterized by examining the blood perfusion26 using a laser Doppler blood perfusion imager (PeriScan PIM3, Perimed AB).

3. RESULTS AND DISCUSSION Figure 1(a) shows a digital image of the synthesized Al paper. Al is chosen for the synthesis of conductive papers mainly due to its high electrical conductivity and cost-effectiveness in manufacturing. A rectangular paper sheet with a typical size of 14.5 × 12 cm2 was used as a template in the present work, but the conductive paper can be prepared in a reproducible manner with any size or shape via further optimization of the solution-based manufacturing setup. This can also be potentially linked to a roll-to-roll process for mass-production because the fabrication


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method is entirely based on solution-based processes without any complicated high-vacuum systems. Figure 1(b) shows SEM images of the detailed surface morphology of the synthesized Al paper, indicating that the Al features are uniformly formed on the cellulose fibers and the space between them. The presence of the Al features was also quantitatively confirmed by the strong Al peak around 1.5 keV in EDS analysis, as shown in Figure 1(c). The electrical properties of the synthesized Al paper were characterized by measuring the sheet resistance (Rs) distribution using the FPP method. The Rs values measured on the Al paper were quite uniform independent of locations while representing low average Rs of ~75 mΩ/sq, as shown in Figure 1(d). The excellent electrical performance of the Al paper originates from the densely interconnected Al features in the paper structure, which provide stable paths for electron conduction. This suggests that Al paper can be used as a multifunctional conducting material in various potential flexible electronics due to the high conductivity in conjunction with the desirable nature of a paper such as high flexibility and lightweight. Figure 2(a) shows a digital image of the kirigami-patterned paper conductor prepared by a simple cutting process that is the easiest yet efficient way of plotting a paper with various designs. It is important to note that the kirigami patterning process can also be used to greatly enhance the extensibility of elastic conductors such as silver nanowire/polydimethylsiloxane composite films.27 In the present work, we chose a simple kirigami design with straight, linearly arranged cut lines for convenience in the design and fabrication. Prior to the electrical characterizations, the structural stability of the Al paper upon cutting was first investigated by examining the cut surface using SEM and EDS analyses, as shown in Figure 2(b). The morphology of the cut surface was found to differ slightly from that of the top surface (Figure 1(b)) due to local deformations of the Al features induced by the translational motion of the


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cutting tool. However, the cut surface was still covered well with the Al features while stably maintaining the electrical paths. Besides, the EDS analysis also revealed a strong Al peak that was consistent with the SEM investigation. However the intensity of the carbon (C) peak became slightly higher than that for the top surface (Figure 1(c)). This could be due to the possible loss of Al features during the cutting process. This means that the interfacial adhesion between the Al features and paper structure is sufficiently strong to withstand the cutting process without degradation in the electrical performance. It is important to note that the simple kirigami patterning approach makes any flexible, but non-stretchable, substrates highly stretchable. Figure 2(c) shows the normalized resistance of the pristine, single-cut, and kirigami-patterned paper conductors in response to applied strain. As expected, the pristine and single-cut paper conductors were torn at tensile strains less than 9% and permanently lost their electrical conductivity. In contrast, the kirigami-patterned paper conductor could be stretched to ~500% while stably maintaining its mechanical and electrical performance, as shown in Figure 2(c). This clearly suggests that the synergetic combination of the large-area conductive paper synthesis and very simple kirigami patterning techniques can be an efficient way of demonstrating various high-performance stretchable electronic devices. When applying strains higher than 500%, the electrical resistance of the kirigami-patterned paper conductor increased and showed a staircase profile. The resistance eventually became infinite at ~643% strain. This occurs because the junctions connecting the cut lines are gradually broken in response to the increased strain. To examine this behavior more precisely, stress analyses were conducted for the device under various strains using the finite element method (FEM), as shown in Figure 2(d). In this case, two-dimensional (2D) models were analyzed for


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convenience because the simulations were just intended to investigate the spatial stress distribution on the device upon stretching. When stretched, the stress was predominantly concentrated at the junction areas of the device and gradually increased in proportion to the applied strain. The concentrated stress gradually breaks the junctions when it exceeds the critical level that each junction can withstand before fracture, which leads to a gradual degradation in the electrical performance. This also implies that the maximum stretchability (i.e., maximum strain before fracture) of the device can be easily controlled by varying the design parameters of the simple kirigami pattern, such as the horizontal gap (gx) and vertical gap (gy) between the cut lines and the cut length (lc), as shown in Figure 2(a). Figures 2(e)–(g) show the normalized resistances of the kirigami-patterned paper conductors with different designs as a function of the applied strain. When increasing gx with fixed values of gy and lc, the maximum stretchability of the device decreased (Figure 2(e)). This occurs because the bending angle of the cut lines with respect to the junction becomes larger with increasing gx, even under the same tensile strain. This leads to more significant stress concentration at the junctions and results in easier destruction, as shown in Figure S1(a) in the Supporting Information (SI). Increasing lc with fixed values of gx and gy improved the maximum stretchability of the device (Figure 2(f)). This occurs because higher lc allows for the same longitudinal extension of the device while inducing much lower stress at the junctions due to smaller bending angle of the cut lines, even under the same tensile strain. This result is illustrated in Figure S1(b) in the SI. Increasing gy with fixed values of gx and lc degraded the maximum stretchability of the device (Figure 2(g)). In this case, the extension length of the device should increase under the same tensile strain because the initial length of the device is increased with increasing gy. Therefore,


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the bending angle of the cut lines becomes larger, which leads to much easier tearing at the junctions due to higher stress concentration, as shown in Figure S1(c) in the SI. The experimental and theoretical observations give us a clear chance to design stretchable conductors ensuring both high conductivity and high stretchability. To demonstrate the practicability of the kirigami-patterned paper conductor as stretchable interconnects, the fabricated conductor was integrated to a light-emitting diode (LED), as shown in Figure S2 in the SI. Figure S2(a) shows sequential digital images of the stretchable LED circuit with various strains applied to the conductor. The light intensity of the LED was kept stably without any signs of degradation, even when the kirigami conductor was subjected to high strains of up to 400%. The electrical robustness of the LED circuit against tensile strain was more quantitatively conformed by examining the current-voltage (I-V) characteristics of the circuit upon stretching and releasing, as shown in Figure S2(b). The curves under different strains were almost identical to each other, representing great potential for high-performance stretchable system applications. Figure 3(a) shows a schematic illustration of the fabrication sequence for the stretchable kirigami heater. The stretchable heater made of Al paper sandwiched with thin elastomeric polymers was easily prepared by the simple single-step cutting process after spin-coating the elastomer, as shown in Figure 3(b). The width and length of the heating area were ~33 and ~10 mm, respectively. The cross-sectional SEM image of the fabricated heater in Figure 3(c) shows the elastomer/Al paper/elastomer sandwich structure. The elastomeric polymers coated on both sides of the Al paper play a fundamental role in protecting the Al paper from the environment and preventing direct contact with the heater conductor when attached to human skin. In particular, the excess elastomer on both sides of the heater does not contain any Al paper and can


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serve as elastic mechanical springs. In the present work, the coating thickness of the elastomeric polymer was carefully determined to be ~200 µm in consideration of both the mechanical robustness under stretching and efficient heat transfer to the skin. Figure 3(d) shows the force-strain (F-S) relationship of the kirigami-patterned conductors obtained with and without the polymer coating. The F-S curves can be divided into two sections with different slope. In the low strain regime (section I, up to ~17% strain), both conductors showed similar behavior with a relatively steep slope, as shown in the inset in Figure 3(d). This probably resulted from the relatively large force that is required to initiate out-of-plane deformation of the cut lines. However, in the high strain regime (section II, > ~17%), the F-S curve of the polymer-coated kirigami conductor had higher slope compared to that of the pristine one, which was mainly due to the presence of the elastic springs. This implies that the restoring force exerted by the elastic springs allows the stretched heater structure to return almost to its initial state upon releasing the applied strain by pulling the springs back to equilibrium based on Hooke’s law. The effect of the elastic springs on the deformation and restoration of the heater structure is visualized in Figures 3(e) and 3(f). Both devices showed similar deformed shapes upon stretching. However, the kirigami conductor without the elastic springs (without polymer coating) did not fully return to its original shape after removing the applied strain, as shown in Figure 3(e). This is probably because the stress concentrated at the junctions exceeds the elastic deformation capacity of the cut lines when subjected to large strains. In contrast, the initial shape of the device with the elastic springs almost recovered its original shape by sufficient restoring force of the sandwich-structured heater, as shown in Figure 3(f). This is greatly desirable for


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efficiently transferring heat to human skin by enabling to accommodate various motions while maintaining intimate conformal contact to the body parts. Figures 4(a) and 4(b) show the time-dependent temperature profiles and steady-state temperatures of the stretchable heater under various input voltages, respectively. The heater was found to be relatively fast enough to reach the steady-state temperature within ~60 s, regardless of the magnitude of the applied voltage. This also suggests that the time-dependent responses of the kirigami heater are comparable to those of other types of stretchable heaters based on percolation networks of conductive nanomaterials.8,28 The heater also distinctly responded to even small differences in the input voltage (0.3 V), which suggests that precise control of output temperature is possible. In particular, the high electrical conductivity of the kirigami heater enabled temperatures as high as ~43 °C with an input voltage of only 1.2 V. It is well known that the electrical resistance of metals increases with temperature. The resistance change was investigated as a function of temperature to determine the effects on the electrothermal performance, as shown in the inset in Figure 4(b). The resistance of the Al paper sheet was almost linearly proportional to the temperature, indicating the maximum increase of ~10% at 50 °C. The temperature-dependent increase of the resistance may not have a significant effect on the heater performance because of the high conductivity of the Al paper. However, it may provide a potential opportunity to detect the temperature by Joule heating for use as a builtin temperature sensor based on the linear relationship between the temperature and resistance. The kirigami heater was also found to be highly stable and reversible, even under repeated onoff operations, as shown in Figure 4(c). Figure 4(d) shows the sequential IR camera images of the kirigami heater under stepwise stretching from 0% to 400% at 1.2 V. Uniform temperature distribution was achieved throughout the whole heating area, even under high strains. This


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originates from the fact that the kirigami pattern is deformed into a mesh geometry with uniformly spaced heating lines when stretched. In addition to the temperature uniformity, the maximum temperature was kept almost constant in each stretched state. This was due to the constant resistance of the device, which was independent of applied strain (Figure 4(e)). This is one of the most important advantages of the proposed kirigami heater compared to other stretchable heaters made of percolation networks of conductive nanomaterials. Such stretchable heaters inevitably suffer from gradual degradation in the electrothermal performance, mainly due to a considerable loss of current paths in the percolation network upon stretching.9,14,21 This might critically limit their use in practical skin-mountable heater applications because of the difficulty of maintaining constant temperature upon various motions of the body parts. The electrical properties of the kirigami heater were also highly robust under repeated stretching loads of up to 1000 cycles with a maximum strain of 300%. The heater temperature at 1.2 V was also stably maintained without significant fluctuations, as shown in Figure 4(f). The long-term electrical stability of such heaters is also one of the most important requirements in their practical applications. To determine this, the electrical resistance of the fabricated kirigami heater was monitored in real-time without (Figure S3(a) in the SI) and with an input voltage (Figure S3(b) in the SI, 1.5 V; corresponding temperature: ~54 °C). The electrical resistance of the device remained nearly constant during the test, which suggests that the heater is electrically stable without significant oxidation of the Al doped in the paper substrate. Besides, the electrothermal performance of the kirigami heater was also remarkably stable under different types of mechanical deformations, such as bending and twisting, as shown in Figure 4(g). The excellent and stable electrothermal performance of the kirigami heater coupled


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with high stretchability and simple, low-cost and reproducible fabrication makes it ideal for practical skin-mountable heater applications. To demonstrate the kirigami heaters as an efficient wearable heat source, the devices were attached to the wrist and elbow. Prior to this, as shown in Figure S4 in the SI, we fabricated the stretchable kirigami heaters with the same design but different lengths (L = 10, 30, and 50 mm) based on consideration of the wide variation of different movement ranges of the target objects. Figure 5(a) shows a digital image of a kirigami heater (L = 10 mm) attached to the wrist. The corresponding IR camera images of the heater operated at 1.2 V indicate that the device maintains conformal contact with the wrist with stable heating performance during flexion and extension motions, as shown in Figure 5(b). In the similar way, a longer kirigami heater (L = 30 mm) was attached to the elbow and subjected to a large bending motion (Figure 5(c)). Figure 5(d) shows a series of IR camera images of the heater attached to the elbow joint during flexion and extension. With an input voltage of 2 V, heat was generated evenly on the whole heating area of the device and remained stable, even under large motions (i.e., maximum flexion and extension). In particular, the stretched heater nearly perfectly recovered to its original shape upon extension thanks to the sufficient restoring force exerted by the elastic springs, as discussed in Figure 3(f). To achieve enhanced wearability and easy-to-wear of device, we also fabricated a band-type kirigami heater without any post processing to attach it to target objects. Based on the high stretchability of the kirigami conductor in conjunction with the restoring force by the built-in elastic springs, the band-type heaters can be easily worn on the objects with different arc lengths while maintaining intimate conformal contact to the object surfaces, as shown in Figures 5(e) and 5(f). IR camera images of the band-type heater worn on the wrist are shown in Figure 5(g),


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which also demonstrate the practicality of our device as an efficient, wearable heat source. As mentioned above, high stretchability of the kirigami conductor enables the band-type device to be tightly attached to various objects with different arc lengths. However, once the device without the elastic springs is exposed to large strain on a large object, it is difficult to attach again to smaller-sized objects that induce smaller strain. This results from irreversible deformation of the cut lines in the device without a polymer coating, as shown in Figure S5(a) in the SI. This critically hinders use in practical applications. In contrast, the elastic springs on the band-type heater allow it to be tightly attached repeatedly to multiple body parts with different arc lengths (the wrist and arm) regardless of the order (i.e., wrist arm wrist), as shown in Figure S5(b) in the SI. This means that the elastic springs play an important role in stabilizing the overall heater structure when attached to various body parts in a way that the device can reliably function by forming a stable contact with the target surfaces. Note that the band-type kirigami heater can also be easily scaled up to a size large enough to be worn on large body parts such as the knee and thigh, as shown in Figure S6 in the SI. Superficial heat increases the blood flow by inducing vasodilation, which can alleviate pain and joint stiffness.29 We investigated the applicability of our heater as a wearable thermotherapy technique. The heat-dependent change in blood flow was characterized by measuring the total local microcirculatory blood perfusion, which is a relative measure of the blood flow, at the wrist after attaching an open-type kirigami heater (not a band-type). The electrothermal properties (i.e., the temperature and input power with respect to the applied voltage) of the tested heater model were already provided in Figure 4. Figure 6(a) shows a series of colored, two-dimensional (2D) Doppler images obtained from the wrist under a stepwise increase in the voltage applied to the heater. The average perfusion value obtained from each Doppler image is plotted in Figure 6(b).


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As expected, the perfusion signal increased with the applied voltage, indicating a corresponding increase in the blood flow caused by thermal expansion of the microcirculation vascular system upon heating. In particular, a relatively steep increase in the perfusion signal was observed with an input voltage of 1.2 V, which highlights the potential for use in low-voltage wearable thermotherapy applications. In addition, the benefits of heat on the blood flow are also demonstrated using the band-type kirigami heater attached to the wrist, as shown in Figure S7 in the SI. The blood flow at the wrist was dramatically increased throughout the whole heating area within 3 min after turning-on an input voltage, which suggests that the heat generated by our mesh-type heater can be quite rapidly spread to the target skin area. This also indicates that the kirigami heater is still effective in transferring sufficient heat to the skin even at low voltage despite the reduction of the contact area between the heater and skin due to the out-of-plane deformation upon stretching. These results suggest that the proposed fabrication approach can facilitate further demonstration of large-scale thermotherapy devices to cover various body parts, regardless of the size, due to the large-area fabrication capability.

4. CONCLUSION In summary, we have demonstrated a new type of skin-mountable heater based on kirigamipatterned conductive paper, which can overcome the major issues of other methods related to performance degradation upon stretching and complex fabrication. A very simple kirigami patterning approach coupled with a solution-based synthesis technique of conductive paper enabled to fabricate the high-performance stretchable heater in a fast, low-cost, and reproducible manner. The resulting kirigami heater showed a rapid and stable electrothermal response and uniform temperature distribution at low input voltage, even when subjected to high strains. The


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heating performance was also highly robust against various mechanical deformations, such as bending and twisting as well as repeated stretching under high strain. The kirigami heater could be stably attached to various curvilinear body parts, regardless of the size and motion, while still maintaining intimate conformal contact with the skin due to the high stretchability and structural restoration ability enabled by the built-in elastic springs. Stable and uniform heating was also achieved with the kirigami heater attached tightly to the wrist and elbow while performing large motions, leading to efficient heat conduction to the skin. Lowvoltage local heating of the wrist with the integrated kirigami heater was highly effective at increasing blood flow in a controllable manner, which could be applied for efficient pain management. These results demonstrate that the stretchable kirigami heater has great potential for practical applications in wearable thermotherapy.

ASSOCIATED CONTENT Supporting Information. Stress analysis of the stretchable kirigami conductors with various designs through FEM simulation (Figure S1), sequential digital images and corresponding I-V curves of LED circuits with kirigami-patterned stretchable interconnect under various tensile strains (Figure S2), normalized electrical resistance of stretchable kirigami heater as a function of time (day) (Figure S3), stretchable kirigami heaters with the same design but different length (Figure S4), sequential digital images of band-type kirigami heaters without and with polymer coating after being attached to the body parts with different arc lengths (wrist arm wrist) (Figure S5), digital images of band-type kirigami heater worn on large body parts (Figure S6), color Doppler images showing the blood perfusion on the human wrist with a band-type kirigami


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heater attached in the initial (0 V) and activated (2.5 V) states (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H. M. Lee), [email protected] (J. -M. Kim)

ACKNOWLEDGEMENT This

research

was

supported

by

the

Basic

Science

Research

Program

(No.

2015R1A2A2A01004038) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.

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Figure 1. Aluminum-doped conductive paper. (a) digital image of the synthesized Al paper (scale bar: 20 mm), (b) SEM image of Al paper surface morphology (scale bar: 50 µm) (inset: magnified SEM image of Al-coated cellulose fiber (scale bar: 5 µm)), (c) EDS spectrum obtained from the surface of the Al paper, and (d) sheet resistance distribution measured on the Al paper (in (a)) at different locations (top, bottom, center, left, and right).


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Figure 2. Kirigami-patterned stretchable paper conductor. (a) digital image of the as-prepared stretchable kirigami conductor (scale bar: 10 mm), (b) SEM image and EDS spectrum characterized on the cut surface of the Al paper (scale bar: 10 µm), (c) normalized electrical resistance of the pristine, single-cut, and kirigami-patterned Al papers as a function of applied strain, (d) FEM analysis of the stress distribution on the kirigami conductor under various tensile strains, and (e), (f) and (g) normalized resistance of the stretchable kirigami conductors with various designs as a function of applied strain.


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Figure 3. Kirigami-patterned stretchable heater. (a) schematic illustration of the fabrication process, (b) digital image (scale bar: 10 mm) and (c) cross-sectional SEM image of the fabricated stretchable heater (scale bar: 200 µm), (d) force-strain curves of devices with and without polymer coating (elastic springs) (inset: magnified F-S curves in the low strain regime), and digital images of devices (e) without and (f) with polymer coating under various strains (scale bars: 10 mm).


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Figure 4. Electrothermal performance of stretchable kirigami heater. (a) temperature profile and (b) maximum temperature of device as a function of applied voltage (inset: normalized resistance of the Al paper as a function of temperature), (c) temperature profile of device under repetitive voltage on (1.2 V) and off (0 V) cycles, (d) IR camera images of device under various strains, normalized electrical resistance and maximum temperature of device at 1.2 V (e) as a function of applied strain and (f) according to repeated stretching cycles (1000 cycles with a maximum strain of 300%), and (g) maximum temperature of device at 1.2 V under bending and twisting deformations (inset: corresponding digital and IR camera images).


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Figure 5. Demonstration of stretchable kirigami conductors as a skin-mountable heater. (a) digital image of device integrated onto the wrist joint (scale bar: 10 mm), (b) IR camera images of device on the wrist joint under flexion and extension motions (applied voltage: 1.2 V), (c) digital image of device integrated onto the elbow joint (scale bar: 20 mm), (d) IR camera images of device on the elbow joint under flexion and extension motions (applied voltage: 2 V), digital images of the band-type stretchable kirigami heater attached to (e) a glass vial (scale bar: 10 mm) and (f) the wrist (scale bar: 20 mm), and (g) IR camera images of device attached to the wrist joint before (left, 0 V) and after applying voltage (right, 2.5 V).


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Figure 6. Blood flow characterization. (a) sequential color Doppler images showing the blood perfusion on the human wrist under various voltages applied to the kirigami-patterned stretchable heater attached to the wrist (scan area: 30 × 30 mm2, holding time at each step: 3 min), and (b) average blood perfusion value extracted from (a) as a function of applied voltage.


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TABLE OF CONTENTS GRAPHIC


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Simple Approach to High-Performance Stretchable Heaters Based on

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