Multifunctional Wearable Device Based on Flexible and Conductive

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Multifunctional Wearable Device Based on Flexible and Conductive Carbon Sponge/Polydimethylsiloxane Composite Yuanqing Li, Wei-Bin Zhu, Xiao-Guang Yu, Pei Huang, Shao-Yun Fu, Ning Hu, and Kin Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11196 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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ACS Applied Materials & Interfaces

Multifunctional Wearable Device Based on Flexible and Conductive Carbon Sponge/Polydimethylsiloxane Composite Yuan-Qing Li,† Wei-Bin Zhu,† Xiao-Guang Yu,† Pei Huang,† Shao-Yun Fu,*,† Ning Hu,†,‡ Kin Liao*,§ †

College of Aerospace Engineering, Chongqing University, Chongqing 400044, P. R. China.

‡

The State Key Laboratory of Mechanical Transmissions, Chongqing University, Chongqing 400044, P. R.

China §

Department of Mechanical Engineering, Khalifa University of Science, Technology, & Research, Abu Dhabi

127788, United Arab Emirates.

ABSTRACT: Wearable devices that can be used to monitor personal health, track human motions, and provide thermotherapy etc. are highly desired in personalized healthcare. In this work, a multifunctional wearable “wrist band” which works as both heater for thermotherapy and sensor for personal health and motion monitoring is fabricated from a flexible and conductive carbon sponge/polydimethylsiloxane (CS/PDMS) composite. The key functional material of the “wrist band”, namely the conductive CS, is synthesized from waste paper by a freeze-drying and high-temperature pyrolysis process. When the “wrist band” works as a heater under 15 V, a stable temperature difference of 20 oC is achieved between the “wrist band” and the ambient. When the “wrist band” serves as a wearable strain sensor, the “wrist band” exhibits fast and repeatable response, and excellent durability within the strain range of 0-20% and the working frequency of 0.01-10 Hz. Finally, the typical applications of the multifunctional wearable “wrist band”, as a heater for thermotherapy, and a sensor for blood pulse, breathe, and walk monitoring, are demonstrated. Due to its low cost, high flexibility, moderate conductivity, and excellent strain sensibility, the as-prepared wearable device based on the CS/PDMS composite is promising to be applied for the provision of personal healthcare. KEYWORDS: composite, wearable device, strain sensor, heater, multifunctionality

1 Environment ACS Paragon Plus


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INTRODUCTION

Recently, the demand for wearable electronics has risen exponentially, as demonstrated by the growth of the wearable fitness market to $ 5 billion in 2015, a 25% increase from 2014 - this growth rate is expected to be sustained over in the next 5 years.1 For example, wearable strain sensors, which detect the electrical signal shift upon mechanical deformation, have attracted tremendous attention due to their wide applications in fitness tracking, personal health monitoring and diagnose.2-4 Although conventional strain sensors based on thin metal-wires and semiconductors are well developed, their forms and rigid nature restrict seamless integration with skin, giving rise to wearability challenges.2, 5 Thermotherapy, also called heat therapy, is one of the most popular physiotherapies useful for treating joint pain, swelling, muscle weakness, and numbness. The therapeutic effects of heat include increasing blood flow and the extensibility of collagen tissues, decreasing joint stiffness, reducing pain, relieving muscle spasms, reducing inflammation, etc.6 The conventional devices adapted for thermotherapy including heat packs and wraps have often caused discomfort or even scalding for their wearers because of rigidity, heavy weight with bulky volume, non-uniformity and uncontrollable heating temperature.7 The technical drawbacks of currently available thermotherapy devices limit their uses mainly in hospitals,8 thus a new generation of thermotheraputic devices is needed. Flexible and stretchable composites that incorporate conductive nanomaterials, such as gold and silver nanowire, carbon nanotube, and graphene, into soft elastomeric materials are particularly powerful alternative to rigid and bulky healthcare monitoring and therapy devices, providing improvements in comfort and reduced social stigma.7-22 However, cost of rawmaterials, complicated procedures, and/or complex equipment involved in the preparation of


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those nanomaterials drastically hamper their large-scale production for mass market penetration.23 On the other hand, a desirable feature missing in emerging wearable healthcare devices is the ability to deliver advanced therapy at the same time of signal collection.24 Therefore, it is highly desirable to develop low-cost multifunctional wearable devices that can achieve personal health monitoring and medical treatment simultaneously. Herein, to integrate multi-functionality to one single device, we report the successful fabrication of a wearable “wrist band”, which not only can achieve personal health and motion monitoring but also can be used as a heater for thermotherapy. The main part of the “wrist band” is a flexible and conductive composite based on conductive filler - carbon sponge (CS) and flexible matrix material - polydimethylsiloxane (PDMS) elastomer. Importantly, the key functional material in the composite, carbon sponge, is prepared from waste paper by a freezedrying and a high-temperature pyrolysis process. It is well known that the main component of waste paper is cellulose, which is the most abundant natural polymer on earth, widely available at low cost.25, 26 The temperature of the “wrist band” is well controlled by adjusting the working voltage. Under compression, the “wrist band” shows fast and repeatable piezoresistive response, and excellent durability within the strain range of 0-20% and the working frequency of 0.0110Hz. Moreover, the flexibility and softness of the CS/PDMS composite provides maximum comfort, system robustness for the “wrist band” as wearable device. Finally, blood pulse, breathe, and walk monitoring, and temperature controllable heating are simultaneously achieved by the multifunctional “wrist band” fabricated.

EXPERIMENTAL SECTION Preparation of carbon sponge. Waste tissue papers were used to prepare the conductive

carbon sponge. First, 5.0 g of waste paper scraps were dispersed in 500 mL of distilled water,



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and then the solution were ultrasonicated for 1h. The uniform mixture obtained is called paper pulp. The paper pulp was poured into a plastic dish and then subjected to freeze-drying to form paper sponge. The thickness of the paper sponge can be controlled by the volume of pulp in the dish. Carbon sponges were prepared by pyrolysis of the paper sponge at 1000oC for 2 h in N2 atmosphere. Preparation of CS/PDMS composites and wrist band. PDMS resin and curing agent (Sylgard 170, Dow Corning) in the mass ratio of 1:1 were mixed homogenously via magnetic stirring. The PDMS mixture prepared was poured into a plastic mold where the CS had been placed. The mold with CS and PDMS was then placed in a vacuum chamber for 5 min for the CS to be infused with PDMS resin and air bubbles to be removed. Finally, the mold with CS and PDMS resin was heated at 80oC for 1h to cure the resin. The thickness of the CS/PDMS samples were controlled by the thickness of CS and the volume of PDMS added. To fabricate the multifunctional “wrist band” based on CS/PDMS composite, the CS obtained was cut into 40×10 mm2 sheets, and two pieces of alumina foil acting as electrodes were soldered with silver paste at the two sides of the CS sheet. Similar to the procedure of preparation of CS/PDMS composite, the CS-with-electrodes were encapsulated with PDMS elastomer. After curing of the PDMS resin, extra parts of the PDMS were cut off, and the dimension of the “wrist band” fabricated is around 80×10×2 mm3. In addition, to monitor the sports performance and breathing, the “wrist bands” were integrated with a sport shoe and waist belt, respectively, their schematics are provided in our previous work.23 Characterizations. All optical pictures presented were taken by a digital camera. The morphology of CS was imaged by a scanning electron microscope (SEM, JEOL7610F). Electrical conductivity of the CS and the CS/PDMS composite was measured with a two-probe


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method using a digital multimeter (ADM-930, 0.1Ω~40MΩ). To minimize the contact resistance between the specimens and the electrodes, the contact points of specimens used for conductivity measurement were coated with silver-paste. For CS/PDMS composite, the CS-with-electrodes were encapsulated with PDMS elastomer before measurement, and its electrical resistance was calculated based on the whole electrical resistance of CS/PDMS with electrodes. Thermal images were taken by an infrared camera (FLIR E6). The tensile and compressive behavior of the CP/PDMS composite were investigated using a Microforce Tester (Instron 5948) at a loading rate of 5 and 0.2 mm/min, respectively. To investigate the strain-sensing performance, the “wrist band” fabricated was fixed between the fixtures of the Microforce Tester, while each electrode of the “wrist band” was connected with the electrode of an electrochemical workstation (Autolab 302N). When the strain was applied to the “wrist band”, the current changing from the “wrist band” was recorded by the electrochemical workstation, the working voltage of the “wrist band” was set as 1V. The relative change of the resistance (RCR) is calculated on the basis of the current monitored: ∆R/R0= (Rs-R0)/R0, where R0 and Rs are the resistance without and with applied strain, respectively. Furthermore, the gauge factors defined as δ(∆R/R0 )/δS, where S denotes the applied strain, are calculated based on the RCR-strain curves plotted.

RESULTS AND DISCUSSION

Due to the growth of population, increasing urbanization, rising standards of living, large quantities of paper is manufactured and consumed each year. The increase of paper consumption has been creating a huge amount of paper waste which contributes 25−40% of global municipal solid waste.27 Recycling paper waste will help to preserve forests as well as solve the environmental problem. In this work, lightweight and porous carbon sponge are first fabricated using waste paper as the raw material. As shown in Figure 1, a few pieces of waste tissue paper



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are being broken down into cellulose microfiber via strong ultrasonication treatment in water solution, where a translucent paper pulp is being produced. Then the dry sponge-like paper is obtained by removing the water in the paper pulp via freeze-drying. The shape and size of the paper sponge are controlled by the shape and volume of the container. The density and porosity of the sponge are also adjustable through controlling the concentration of the paper pulp.

Figure 1. Schematic showing of the fabrication of the multifunctional “wrist band” made of CS/PDMS composite: ① waste paper, ② paper pulp, ③paper sponge, ④ carbon sponge, and ⑤ wrist band.

The black and porous CS was generated by pyrolysis of the paper sponge under N2 atmosphere. It was found that the high temperature pyrolysis process did not change the shape of the sponge, but the volume of the carbon sponge shrinks to only around 20% of that of the initial paper sponge. The shrinkage of CS is induced by the evaporation of volatile organic species and conversion of the organic species to carbon.28, 29 As shown in Figure 2, the CS prepared has a porous and three dimensional (3D) network structure formed by the carbonized cellulose fibers. High magnification SEM image (Figure 2B) indicates that these fibers are belt-like with a width around 10 µm, and most of them have a length up to millimeters. Due to the high porosity, the


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apparent density of the CS is around 0.01 g/cm3. Although it has a low density, the CS shows a moderate electrical conductivity of 13.6 S/m attributed to the 3D network structure.

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Figure 2. SEM images of carbon sponge prepared from waste paper under: (A) low and (B) high magnification with a scale bar of 100 µm and 10 µm, respectively.

The low density, 3D network structure, and moderate electrical conductivity make the carbon sponge an ideal candidate to fabricate flexible conductive composite for wearable devices. PDMS, a silicon based elastomer, is one of the most widely used polymers in fabricating wearable devices because of its high chemical stability and excellent flexibility.23 In this work, the CS/PDMS composites were fabricated by vacuum infusion of PDMS resin into the CS scaffold. The tensile tests were performed to evaluate the mechanical performance of the CS/PDMS composite, and the typical tensile strain-stress curve of CS/PDMS composite is presented in Figure 3. The average elongation at break and the elastic modulus of the CS/PDMS composite samples are 228% and 1.1 MPa, respectively. At the same time, as indicated by the inset of Figure 3, the CS/PDMS composite can be repeatedly bended by 180o without any visible damage.



Figure 3. Typical tensile strain-stress curve of the CS/PDMS composite and the inset photographs show the flexibility of CS/PDMS composite.

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Figure 4. Heating behavior of the CS/PDMS composite under various working voltages: (A) temperature-time curves and (B) infrared camera images.

After infusion of PDMS resin into the CS scaffold, the conductive CS is encapsulated by the PDMS. Because PDMS is an insulator, the surface PDMS layer of the CS/PDMS composites is non-conductive. The bulk electrical conductivity of the CS/PDMS composite measured is 10.5


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S/m and close to that of CS, indicating that the conductive path of CS is well preserved within the CS/PDMS composite. Meanwhile, the electrical conductivity of the CS/PDMS composites is relatively stable in the temperature range of 20-50 oC (Figure S1). Due to its excellent flexibility and moderate electrical conductivity, the CS/PDMS composite is suitable as a wearable heater for thermotherapy or body warming. The electric heating behavior of CS/PDMS composite was investigated by applying various voltages on the “wrist band” fabricated (Figure 1), and the surface temperature evolution of the “wrist band” was monitored by an infrared camera. As shown in Figure 4A, under applied voltage, the surface temperature of the “wrist band” increases within the first several minutes, and then reaches a relatively stable state. The rate of such temperature change depends on the voltage applied, as clearly shown in the graph. At the initial stage, most of the Joule heat converted from the electricity is used to heat up the “wrist band”, and only a small portion of it is dissipated into the surroundings. Following the temperature rise, the heat dissipated into the ambient also increases, until a temperature equilibrium is reached.29 Because the power of a device is proportional with the square of the working voltage, the heating power of the “wrist band” can be controlled by adjusting the working voltage. As indicated by Figure 4B, the stable temperature of the “wrist band” under 5, 10, 15, and 17.5 V is around 29.5, 36, 46, and 61oC, respectively. When the “wrist band” is working under 15 V, a maximum temperature difference of 20 oC with ambient is achieved. This temperature difference is adequate for applications since the typical temperature of human skin is around 32°C and the target temperature for thermotherapy is in the range of 38°C to 50°C.30 Besides the low working voltage, there is no safety threat to the wearer because the conductive CS is encapsulated by the insulating PDMS resin. At high temperature, a temperature distribution non-uniformity as indicated by the hot point is observed in the infrared images of the CS/PDMS composite. The



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main reason is that the CS/PDMS composite is directly heated in air, and the edge part of the CS/PDMS composite dissipates more heat than the center part. In addition, the heat nonuniformity may also result from the structural non-uniformity of the CS/PDMS composite. A further work is needed to improve the temperature uniformity for practical applications. More than simply converting the electricity to thermal energy, the CS/PDMS composites can also function as a wearable strain sensor. In order to investigate the response of the composite to applied strain, the current of the “wrist band” under 1 V is monitored by an electrochemical workstation coupled with a Microforce Tester. The relative change of the resistance (RCR) is calculated on the basis of the current monitored. Figure 5 shows the RCR response of the "wrist band” to the tensile and compressive strain applied. It is obvious that the RCR under compression and tension both increase monotonically with the strain applied. Under tension, an approximately linear relation between the RCR and the tensile strain is seen. At the same time, the gauge factor (GF) calculated is 1.78, which reflects the sensitivity of the “wrist band” is comparable to the conventional metallic strain gauge (GF around 2). Under compression, the sensitivity of the “wrist band” increases with an increase in the strain, and the average GFs in the strain range of 0-5% and 15-25% are 0.47 and 3.19, respectively. It is well known that the sensitivity of the resistive-type sensor is related to the change of the conducting paths and contact resistance in the pressure/strain sensitive materials. To reveal the structural change of the CS in the PDMS resin after the applied strain, the fracture surfaces of the CS/PDMS composite with and without pre-applied strain were observed with SEM. As revealed in Figure 6A, without pre-applied strain, the carbon fibers in PDMS resin are well-connected, and very few cracks or discontinuities can be seen. After a tensile strain is applied on the CS/PDMS composite, some cracks and holes in the CS/PDMS composite appear (Figure 6B), which brings about the


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breakage of the conductive path, and thus leads to the rise of resistance of the CS/PDMS composite. After releasing the applied strain, as indicated in Figure S2, the cracks and holes formed in the CS/PDMS composite disappear, and most of the disconnected conductive paths recover to its initial states due to the reversing deformation of the PDMS.

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Figure 6. SEM images of the fracture surface of CS/PDMS composites without pre-applied strain (A) and with a pre-applied tensile strain around 50% (B), the red circles indicate the formed cracks and holes after a tensile strain is applied.



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The response of the “wrist band” to the compressive loading-unloading cycles is investigated. As shown in Figure 6, the “wrist band” exhibits excellent sensitivity to various cyclic loading modes. When compressed at different strain level, as shown in Figure 7A, the RCR response curves are almost identical except the peak value. At the same time, a slight drop in the RCR peak value is seen in the first several cycles, but stabilized afterwards. This phenomenon is normal for resistive-type strain sensor, which attribute to the internal stress relaxation of the PDMS elastomer and partial cracking or discontinuities of the carbon fiber networks in the PS/PDMS composites.17, 31 To reveal the behavior of the “wrist band” at long-term loading, the RCR response of the “wrist band” under compressive cycles with a static-hold period is studied. As seen in Figure 7B, within one cycle, the compressive strain of the “wrist band” is held for 4.5 s at 20% and 0% strain, respectively. The shape of the RCR-time curve is similar with that of the strain applied. However, when the applied strain is held at 0%, the RCR shows a slow recovery characteristic due to the internal stress of the elastomer. Because the relaxation time of PDMS elastomer is in the scale of 10 s,32 after the strain is returned to 0%, the discontinuities of the carbon fiber conductive path in the PDMS resin are gradually recovered following the relaxation of the internal stress. As a strain sensor, the working frequency of the “wrist band” is also significant to the application in personal health or motion monitoring.31 The effect of loading frequency on the RCR response of the “wrist band” in the range of 0.01 to 10 Hz is studied. The RCR response of the “wrist band” agrees well with the strain applied in the frequency range investigated (Figure 7C). However, the RCR peak value of the “wrist band” under high frequency is apparently larger than that under low frequency. When fixing maximum strain, the strain rate applied for 10 Hz is 1000 times higher than that of 0.01 Hz, which results in the stress applied on the “wrist band”


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under high frequency is considerably higher than that under low frequency. The change in the mechanical response upon increasing the strain rate is attributed to the reduction in the molecular mobility of polymer chains, which increases its inherent stiffness.23,34,35 Furthermore, to demonstrate the reliability of the “wrist band”, 1000 compression cycles under 20% strain were performed. After 1000 cycles, the RCR responses kept stable and reversible, indicating excellent durability of the “wrist band” fabricated (Figure 7D).

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To demonstrate the potential of the “wrist band” as wearable multifunctional device, as presented in Figure 8A, the “wrist band” was put on the wrist of a volunteer to act both as a heater and a strain sensor. As shown in Figure 8B, the infrared image of the “wrist band” on “off mode” shows a greenish yellow color, indicating that the temperature of the “wrist band” is close to the ambient and lower than the body temperature. After setting the “wrist band” to “on” mode at a working voltage of 15 V, within 10 minutes, the infrared image of the “wrist band” turns from homogenous greenish yellow to dark red with a white center. The average surface temperature of the “wrist band” measured close to 45oC. This temperature can be sustained as long as the circuit is connected. Heat transfer from the “wrist band” to the wrist would induces vasodilation and increases blood flow around the joints, which reduces pain and joint stiffness.8 Thus, the “wrist band” can work as a wearable heater for thermotherapy.

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Figure 8. (A) Photograph of the multifunctional “wrist band” worn by a volunteer; (B) infrared images of the “wrist band” as a wearable heater at “on” and “off” modes; (C) the RCR response of the “wrist band” to blood pulse of an adult volunteer; monitoring of the breathing (D) and (E) walking of an adult volunteer using the “wrist band” integrated with a waist belt and a sport shoe respectively.

In addition, the RCR response of the “wrist band” to the blood pulse of a testee within 30 seconds is shown in Figure 8C, it is clear that the RCR output signals exhibit good reproducibility. The average blood pulse calculated is 73 beats per minute, which coincides with the typical heartbeat of a healthy adult. Furthermore, the real-time breathing and walking status of an adult are successfully monitored by integrating the “wrist band” with a waist belt and a sports shoe, respectively. As shown in Figure 8D, the breathing modes, such as regular breathing, deep breathing, and breathing hold, are distinguishable from the RCR response. Moreover, more detailed information such as the breathing frequency and breathing hold time can also be calculated from the RCR response data. Similarly, as shown in Figure 8E, three walking modes including regular walking, fast walking, and holding are identified from the RCR data tracked. Other useful information about the walking space, stress applied on the shoe and shoes/ground contact time can also be analyzed based on the RCR data obtained.



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CONCLUSIONS

In summary, a multifunctional wearable “wrist band” made of a flexible and conductive CS/PDMS composite has been depicted. Notably, the conductive CS is prepared from waste paper by a freeze-drying and high-temperature pyrolysis process. The CS/PDMS composite is successfully prepared by vacuum assisted infusion of the PDMS resin into the CS scaffold. Due to its moderate electrical conductivity (10.5 S/m), the CS/PDMS composite exhibits an excellent electric heating behavior and the heating power of the “wrist band” can be easily adjusted by changing the working voltage. As a result, the maximum temperature difference between the “wrist band” and the ambient (20oC) is achieved under a working voltage of 15 V. As a wearable strain sensor, the “wrist band” shows fast and repeatable response with excellent durability within a strain range of 0-20% and a working frequency of 0.01 to 10Hz. Finally, the typical applications of the “wrist band” as wearable multifunctional devices, including as a heater for thermotherapy, and a senor for blood pulse monitoring, breathing, and walking have been demonstrated. Consequently, the low-cost and conductive network structure of CS combined with the flexibility and softness of the PDMS elastomer make the CS/PDMS composite based device a perfect candidate in multifunctional wearable devices. It is believed that the device based on the CS/PDMS composite will have promising applications in personal healthcare.

ASSOCIATED CONTENT Supporting Information The supporting information includes the effect of the temperature on the relative change of the resistance. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are grateful to the financial support of National Natural Science Foundation of China (Grant No. 11372104, 51373187, 51573200, and 11572321), and Khalifa University Internal Research Funds (No. 210038).

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For Table of Contents Use Only

Multifunctional Wearable Device Based on Flexible and Conductive Carbon Sponge/Polydimethylsiloxane Composite Yuan-Qing Li, Wei-Bin Zhu, Xiao-Guang Yu, Pei Huang, Shao-Yun Fu, Ning Hu, Kin Liao

A multifunctional wearable “wrist band” made of a flexible and conductive carbon sponge/polydimethylsiloxane composite is fabricated, which works as both heater for thermotherapy and sensor for personal health and motion monitoring.


Multifunctional Wearable Device Based on Flexible and Conductive

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