Polyacrylonitrile‐carbon Nanotube‐polyacrylonitrile: A ... · turing a polyacrylonitrile-carbon...

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Polyacrylonitrile-carbon Nanotube-polyacrylonitrile: A Versatile Robust Platform for Flexible Multifunctional Electronic Devices in Medical Applications

Toan Dinh,* Van Dau, Canh-Dung Tran, Tuan-Khoa Nguyen, Hoang-Phuong Phan, Nam-Trung Nguyen, and Dzung Viet Dao

Dr. T. Dinh, Dr. T.-K. Nguyen, Dr. H.-P. Phan, Prof. N.-T. Nguyen, Dr. D. V. DaoQueensland Micro- and Nanotechnology CentreGriffith UniversityBrisbane, Queensland 4111, AustraliaE-mail: [email protected], [email protected]. V. Dau, Dr. D. V. DaoGriffith School of Engineering and Built EnvironmentGriffith UniversityQueensland, AustraliaDr. C.-D. TranUniversity of Southern QueenslandQueensland, Australia

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.201900014.

DOI: 10.1002/mame.201900014

approaches for health-care monitoring and medical treatment.[1–4] These elec-tronic devices offer unique characteristics such as flexibility, stretchability, and wear-ability, which can comfortably be affixed on human skin without any adverse effect.[5,6] Therefore, wearable devices can be used for real-time monitoring of physi-ological measurements such as tempera-ture, pressure, heart pulse rate, as well as for clinical treatment including on-skin thermal therapy.[7–9]

Wearable thermal management is of great interest for the treatment of numerous arthritis and rheumatism symptoms, as well as joint pains, sciatic, and bruising.[10–12] For thermal therapy, heat pad and wearable heater can be used to soothe such symptoms. The heater generates and maintains a desired temperature to dilate blood vessels and stimulate blood circulation. A number of heaters have been successfully demon-strated employing conventional metals (e.g., silver) as well as advanced nanoma-terials (e.g., graphene and carbon nano-tube).[13–17] However, such heaters either

lack flexibility and wearability or are constructed on high den-sity substrates with slow thermal response and high power consumption.[16,18–20]

In addition to wearable therapeutic heaters, flexible skin-like physical sensors with low mass density, low young modulus, and porosity, such as temperature and flow sensors, will pro-vide a great opportunity for monitoring motion and health con-dition of human.[1,3,21,22] Sensing functionalities embedded in the same medical treatment platform will lead to a new class of multifunctional electronic devices.[11,23,24] However, very little progress on multifunctional sensors and electronic devices has been recently achieved because such systems require complex and advanced fabrication processes. Moreover, conventional electronic systems with sensitivity and accuracy used to employ functional elements (e.g., heaters and strain sensors) have been fabricated on rigid planar substrates such as silicon wafer or glass slices. The electric heaters and strain sensors with flex-ible substrates have attracted considerable interest for wear-able applications.[25–27] For example, lightweight, bendable, and

Flexible Electronics

Flexible multifunctional electronic devices are of high interest for a wide range of applications including thermal therapy and respiratory devices in medical treatment, safety equipment, and structural health monitoring systems. This paper reports a scalable and efficient strategy of manufac-turing a polyacrylonitrile-carbon nanotube-polyacrylonitrile (PAN-CNT-PAN) robust flexible platform for multifunctional electronic devices including flexible heaters, temperature sensors, and flexible thermal flow sensors. The key advantages of this platform include low cost, porosity, mechanical robustness, and electrical stability under mechanical bending, enabling the development of fast-response flexible heaters with a response time of ≈1.5 s and relaxation time of ≈1.7 s. The temperature-sensing functionality is also investigated with a range of temperature coefficient of resistances from −650 to −900 ppm K−1. A flexible hot-film sensing concept is successfully demon-strated using PAN-CNT-PAN with a high sensitivity of 340 mV (m s−1)−1. The sensitivity enhancement of 50% W−1 is also observed with increasing supply power. The low cost, porosity, versatile, and robust properties of the proposed platform will enable the development of multifunctional electronic devices for numerous applications such as flexible thermal management, tempera-ture stabilization in industrial processing, temperature sensing, and flexible/wearable devices for human healthcare applications.

Advances in medical technologies promise the tremendous enhancement of our life quality. To date, wearable electronics have demonstrated themselves as one of the most effective

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flexible multifunctional electronic platforms for flexible and wearable applications are desired for developing equipments for personal healthcare.[1,28] Therefore, it is necessary to develop low-cost, robust heaters with integrated functionality (e.g., tem-perature and flow sensing) fast response and high efficiency in power consumption for flexible and wearable applications. Electrospun polyacrylonitrile-based (PAN) carbon nanofibers have been demonstrated for flexible lithium-ion batteries.[29] In addition, carbon nanotube (CNT) fibers showed high mechan-ical robustness, structural flexibility, thermal stability, and elec-trical conductivities for wearable applications.[30] Consequently, oriented CNT and PAN fibers could be formed as a platform for developing flexible multifunctional electronic devices in medical applications.

In this work, we report an efficient approach to develop a low-cost, robust, lightweight, and flexible platform for tempera-ture control and thermal management. The platform is based on a carbon nanotube (CNT) membrane arranged between two layers of electro-spun polyacrylonitrile (PAN). The flexible heater yields a relatively fast thermal response of 1.5 s, and a relaxation time of 1.7 s while maintaining the targeted tem-perature under bending conditions. The platform is versatile in terms of temperature-sensing functionality owing to the large temperature coefficient of resistance (TCR) from −650 to −900 ppm K−1. Furthermore, the novel PAN-CNT-PAN can function as a sensitive and flexible flow sensing for monitoring human respiration. The PAN-CNT-PAN platform has high elec-trical stability under bending (e.g., temperature stability of the heater under bending disturbances). The platform can be also integrated using both temperature-sensing and flow-sensing functionalities with high sensitivity (e.g., 340 mV (m s−1)−1.

We employed an electro-spinning process to synthesize polyacrylonitrile (PAN) fibers and to form the first PAN pro-tective porous layer. Figure 1 shows the spinning process to form CNT films on the as-synthesized PAN layer. A highly oriented CNT membrane was pulled out from a CNT wafer and was then laminated on the lightweight protective layer of PAN fibers. Figure 1a (top-left) illustrates the CNT fibers well-oriented in the pulling direction. This CNT film is a highly conductive layer while the porous PAN covering layer acts as an insulation substrate. Therefore, the CNT-conductive layer is electrically isolated without electric shock effects on human body. If the CNT is too resistive, it will require a high applied voltage to the device and lead to unsafe use for wearable appli-cations. Figure 1a (top-right) shows the as-fabricated PAN fibers showing its high porosity. Subsequently, the electrical interconnects can be simply formed using conductive paste owing to this porosity nature of PAN fibers. The porosity of PAN fibers results in a low mass density and a fast thermal response time. In addition, the porosity of PAN will allow the diffusion of silver paste, enabling the simplicity in fabrication. While the PAN layer is porous, its insulation properties were confirmed to be excellent for long-term services. The manufac-turing process of the PAN-CNT-PAN platform are detailed in the experimental section.

The Raman spectrum of the CNT membrane (Figure 1b) shows three dominant peaks at the wavenumbers of 1350, 1577, and 2685 cm−1 that correspond to the D, G, and 2D bands of carbon materials, respectively.[31,32] While the D band

indicates the disorders in CNT structures and the G band quantifies the vibration of carbon atoms, the 2D band associ-ates with the stress in CNT. The CNT layer was located in the central plane of the platform (e.g., between two PAN layers), creating a PAN-CNT-PAN sandwich structure to limit the impact of mechanical deformation under bending conditions. Figure 1c reveals the structure of the PAN-CNT-PAN platform. The top and bottom layers of PAN were designed with the same thickness. Therefore, both upward bending and down-ward bending have negligible impact on performance of the thermal devices. The thickness of each PAN layer was meas-ured to be approximately 10 μm. Since both CNT and PAN are lightweight, flexible, the PAN-CNT-PAN platform possesses unique characteristics including flexibility and high porosity. Thus, the present platform is bendable and flexible as shown in Figure 1d.

The typical requirements of a heater for thermal therapy are included but not limited to the flexibility, the capability of reaching to a sufficiently high temperature at a low applied voltage/power, quick thermal response and relaxation with time, and stability with cycling test. Here, we investigated the heating properties of our present platform. The maximum temperatures generated on the heater with different applied voltages are shown in Figure 2a. The average temperature was measured to be 35, 44, and 57 °C, corresponding to the sup-plied voltages of 12, 15, and 18 V, respectively. The results indi-cate the capability of using PAN-CNT-PAN platform for heating purposes, as thermal therapy applications typically require a temperature below 50 °C.

The power consumption P of the heater is determined as: P = V2/R, where R and V are the electrical resistance of the heater and applied voltage, respectively. According to the heat transfer equilibrium, the supplied power is equal to the lost power including the heat conduction, radiation, and convection where the latter is the dominant factor. The energy loss due to heat convection can be described as ΔP = h × A × ΔT,[11] where h, A, and ΔT are the heat transfer coefficient, surface area of the heater (e.g., 2 × 1.5 cm2), and temperature difference between the heater and the surrounding environment, respec-tively. Figure 2b shows the relationship between the power dis-sipated on the heater and the steady state temperature of the heater. A relatively low power consumption of 0.45 W was used to rise the temperature of the heater up to 57 °C. This power consumption is comparable with that of other heaters reported in literature.[18,33,34] The applications of PAN-CNT-PAN heaters typically require a temperature lower than 80 °C that is much lower than working temperature limit of thermally stable PAN (300 °C).[35]

The fast response and relaxation times of the heater are important parameters for thermal therapy applications. Figure 2c shows a relatively fast response time of 1.5 s of the heater with a supply voltage of 12 V, which is one of the fastest response times compared with other flexible heaters reported to date.[18,36,37] In addition, the relaxation time of 1.7 s was found, Figure 2d. We attribute these fast response and relaxation of the heater with different applied voltages to the lightweight and porosity of the PAN-CNT-PAN platform. For example, the thermal constant τ (i.e., response and relaxation time) can be estimated as τ = Rth × Cth, where Rth = 1/(hA) is the thermal




resistance,[38] where h and A are the convection coefficient and surface area, respectively. Cth = m × C is the thermal capaci-tance; m and C are the mass and heat capacitance, respec-tively. The detailed model of the thermal time constant of the platform is presented in the Supporting Information. Since our heater is lightweight, thin, well aligned, and porous, both thermal resistance and thermal capacitance are small, leading to a fast response/relaxation time of the heater.

For practical applications, the longevity of a heater can degrade its performance due to a high temperature required at the steady state as well as an additional power compen-sated for the heat loss. Therefore, Figure 3a shows the repeat-ability of the thermal response versus a cyclic applied voltage,

confirming the stability of the heater. Furthermore, the gener-ated temperature at a constant applied voltage of 15 V under different bending conditions is almost constant, indicating the excellent temperature stability which is a desired characteristic for wearable thermal therapy applications. The heating perfor-mance was also confirmed by testing in different days (e.g., long-term services, not shown here) with good temperature stability as well as electrical stability. It can be seen that the sta-bility of the heater is attributed to the design of a CNT layer at the neutral/middle plane of the platform, where the strain induced is minimised under bending conditions. The results indicate the potential of using PAN-CNT-PAN platform for flex-ible thermal management.


Figure 1. Fabrication of the PAN-CNT-PAN platform. a) A image of CNT spinning process of the CNT membrane from a CNT wafer. Figure 1a(top-left) shows the oriented CNT film formed by the spinning process. Figure 1a(top-right) shows the polyacrylonitrile (PAN) fibers synthesized by the electro-spinning method. b) Raman spectrum of CNT showing three dominant peaks of carbon materials. c) Schematic sketch of the PAN-CNT-PAN platform with a CNT layer arranged in the middle of the structure (e.g., neutral plane) to avoid the effect of bending on the electrical properties of the CNT layer. d) The final PAN-CNT-PAN platform showing its outstanding flexibility.



Low mass density, flexibility, and biocompatibility are required for thermal therapy, since the heater is typically attached directly to human skin. Therefore, we next inves-tigated the stable performance of the flexible heater under bending condition. Figure 4 shows that in the steady state, the temperature distribution on the heater are similar under both upward bending (Figure 4a) and downward bending conditions (Figure 4b). The heater can be wrapped on a cur-vature surface while maintaining the constant temperature (Supporting Information). In addition, we used PAN-CNT-PAN heaters attached on human body without any adverse impacts on human skin and the immune systems. This agrees with the biocompatibility of PAN reported in the lit-erature.[39] The stability of the temperature response and

the flexibility of the present heater with its biocompatibility enable the development of high-performance heaters in thermal therapy applications.

The lightweight, flexible, and biocompatible PAN-CNT-PAN platform can be also attached to human skin for monitoring the body temperature, where the temperature is typically below 50 °C. The temperature sensitivity was characterized by per-forming current-voltage (I–V) measurements under different temperatures (Figure 5a). The linear I–V characteristics of the platform at elevated temperatures indicate the good Ohmic contact of the CNT film to the electrodes. In addition, all the I–V curves pass through the origin of the coordinate system, indicating that the thermoelectric effect does not affect the measurement results. Figure 5b shows the increasing current


Figure 3. a) Stability of the heater with ON and OFF response at 15 V and 0 V, respectively after several cycles; and b) Stability of the heater under bending conditions.

Figure 2. PAN-CNT-PAN platform based heaters. a) Temperature profile of the CNT heater with different applied DC voltages. b) Heating temperature plotted versus applied power. c) Thermal response (∼1.5 s) of the present platform with a DC voltage of 12 V. d) Relaxation time of approximately 1.7 s.



with increasing temperature, indicating the improvement of the electrical conductivity by the thermal energy. At 1 V, the measured current I increased approximately 4% from 1.201 to 1.248 mA when the temperature increased from 21.8 to 80.6 °C. The electrical resistance of the temperature sensor is calculated using Ohm’s law R = V/I, and the normalized resistance change of the sensor is expressed as ΔR/R = (R − Ro)/Ro,[38] where Ro is the resistance at the reference temperature. Figure 5c shows the decrease of the electrical resistance with increasing temper-ature. The decrease of the electrical resistance with increasing temperature indicates a negative temperature coefficient of resistance (TCR) of the sensor as TCR = ΔR/R × 1/ΔT where ΔT is the temperature difference.[38] Figure 5d shows a TCR value ranging from −650 to −900 ppm/K, which is comparable to that of sensitive temperature sensors.[40–42] To explain the

temperature-sensing mechanism of the PAN-CNT-PAN sensor, we hypothesize that the carriers are trapped in the CNT defects and disorders (see Raman spectrum in Figure 1b). When the temperature increases, these trapped carriers are excited by the thermal energy, leading to an increase in the conductivity of the CNT and a negative TCR of the platform. The elec-trical resistance of the PAN-CNT-PAN film can be described

as 1

812 2R R

t

h

eexp

E

kTc

gap

� �= + +

,[40] where Rc is the contact

resistance between CNT film and electrode, which is neglected; h and e are Planck’s constant and primary charge; ∣t∣2 is the probability of transferring an electron over the band gap of CNT with an energy of Egap. This equation suggests a decrease of the electrical resistance with increasing temperature as observed in Figure 5.


Figure 4. Demonstration of flexible heaters. Infrared images of the present platform based heater with different stress conditions a) upward bending and b) downward bending.

Figure 5. Temperature sensing demonstration. a) Current–voltage (I–V) characteristics of the PAN-CNT-PAN temperature sensors at elevated tem-peratures. b) The zoom-in I–V characteristics at a voltage range of 0.97 to 1 V. c) Electrical resistance of the present temperature sensor plotted versus temperature. The error bar shows the resistance variation under bending conditions (e.g. bending radius of 2 cm). d) Temperature coefficient of resist-ance (TCR) of the platform based temperature sensor ranging −650 to −900 ppm K−1 plotted versus temperature.




Furthermore, real-time detection of human respiration has been proven to be an effective treatment method for respiratory diseases.[43] However, conventional sensing technologies have employed nasal cannulas to be invasively placed in nostrils, resulting in uncomfortable breathing of respiratory patients. As PAN-CNT-PAN has a large negative TCR and excellent electrical conductivity, it would be a suitable platform for the development of flexible thermal flow sensors for monitoring human respiration. Figure 6a shows the flow-sensing concept of our PAN-CNT-PAN platform. Under a constant applied current, the temperature increases, resulting in a decrease of the electrical resistance owing to the negative TCR of the plat-form. When flow is applied, the temperature of the hot-film decreases owing to more effective heat transfer to the sur-rounding environment. Consequently, the resistance of the sensor increases, corresponding to an increase of the meas-ured voltage (Figure 6b). When the flow is ON, the measured voltage immediately increases with a high signal-to-noise ratio (Figure 6b). The sensitivity was measured to be approximately 340 mV (m s−1)−1, which is comparable to or even much better than conventional MEMS (Miro ElectroMechanical Systems thermal flow sensors.[44,45] The response is repeatable with three cycles of ON and OFF. In addition, the measured voltage increases with increasing applied currents, corresponding to an increase of the relative resistance change (50%/W, Figure 6c). This high sensitivity indicates the suitability of PAN-CNT-PAN platform for flow sensing applications. In addition, a thermal response/recover time of less than 100 ms was measured for the sensor (Figure S5, Supporting Information), corresponding

to a bandwidth of >10 Hz. This bandwidth satisfies the require-ment of human breathing measurements of 12 and 20 breaths per minute. We attached the sensor on a curvature surface (e.g., a tube with a diameter of 2 cm) and measured the real-time response of the sensor to the human blowing. The imme-diate response with a high signal-to-noise ratio was observed in Figure 6d. Under a constant applied current of 5 mA, the output voltage change was approximately 100 mV and the repeatability was also recorded with four cycles of blowing. In practical applications, the sensor can be comfortably affixed to the human upper lip to measure human respiration in real-time, due to the lightweight and flexibility of PAN-CNT-PAN. The real-time response with high signal-to-noise ratio indicates the high potential of using PAN-CNT-PAN thermal flow sensors for wearable applications such as monitoring of human respiration.

We successfully demonstrated a facile fabrication strategy to develop multifunctional flexible electronic devices using the sandwich platform of PAN and CNT layers. The heating capa-bility was characterized with a relatively fast response time of 1.5 s and relaxation time of 1.7 s, low power consump-tion, outstanding flexibility, and stability under bending distur-bances. The platform also can serve as a flexible temperature sensor with a relatively large negative temperature coefficient of resistance from −650 to −900 ppm K−1. The efficient flow sensing properties of PAN-CNT-PAN were also demonstrated with a high sensitivity of 340 mV (m s−1)−1 and linear increase of flow sensing performance with increasing supply power. Com-bined with these superior properties, the low cost, porosity, and

Figure 6. Flow sensing functionality. a) Hot-film flow sensing concept. b) Response of the flow sensor with different applied currents at an air velocity of 1.2 m s−1. c) Performance of the hot-film flow sensor with different applied powers. The error bar shows the variation of resistance under bending conditions (e.g. bending radius of 2 cm). d) Sensor response to human respiration.




robust PAN-CNT-PAN platform make it capable of a wide range of flexible and wearable applications including flexible thermal management, temperature sensing, detection of human respi-ration, and other multifunctional healthcare devices.

Experimental SectionElectron-spun PAN and CNT membrane: Electrospinning was

used to fabricate a thin and porous polyacrylonitrile (PAN) layer (Supporting Information). Polyacrylonitrile (powder copolymer 99.5%AN/0.5%MA, Sigma Aldrich) was dissolved in dimethylformamide (N,N-dimethylformamide HCON(CH3)2, 99.0%, Sigma Aldrich) with a concentration of 10% (polymer/solvent). The solution was stirred at room condition for 30 min and then introduced into a 1.0 mL glass syringe with a 30 gauge stainless needle. A 30 mm plastic cylindrical collector was covered by a cooper foil or a stainless steel mesh and located 60 mm from the needle. The collector rotated at a speed of 400–500 rpm to fabricate a layer of PAN fibers. A high voltage of 8 kV was applied between the needle (positive electrode) and the collector (negative electrode/ground) to generate a strong electrical field. Under the electrical field, the solution was injected through the needle with a flow rate of 0.01 mL minute−1 by a syringe pump (KDScience) into nanofibers then landed on the moving face of the collector to create the PAN layer.

A spinnable carbon nanotube (CNT) matrix was grown on a silica wafer via chemical vapor deposition (CVD) approach using cyclohexane as a carbon source. The growth parameters of CNT wafers were controlled by growth time and the thickness of the catalyst. The wafer was then diced into smaller fragments (Figure 1). The oriented CNT membrane was pulled out from the fragment and laid on top of the electrospun PAN layer. Since CNT was pulled out from the wafer/fragment, the pulling force enabled the highly oriented CNT fibers. The collector was manually rotated to form a CNT layer on the as-fabricated PAN layer. Another PAN layer was then deposited by electrospinning on the CNT membrane to form a sandwich of PAN-CNT-PAN platform (Supporting Information).

Fabrication of PAN-CNT-PAN based devices: The all-spun PAN-CNT-PAN platform was cut into small strips with a dimension of 2 × 1.5 cm2. A typical A4 printing paper was also cut and employed as a substrate for the devices. Highly conductive silver paste (1863616, RS Components) was used to establish electrical contacts and interconnections with copper tapes. The device was cured in an oven at 100 °C for 30 min.

Measurement and characterisation of multifunctional devices: The Raman spectrometry was performed using Raman Spectrometer Renishaw Invia (514 nm). The temperature response and infrared images were characterized using an infrared camera (GOBI-1513, Xenics Infrared Solutions). Temperature sensors were characterized in a precisely controlled temperature oven. Electrical measurements were performed using a source-meter (2450, Keithley). Flow-sensing characterisation was performed using a channel (length of 1.5 m, diameter of 5 cm).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsT.D. and V.D. contributed equally to this work. This work was performed in part at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and micro-fabrication

facilities for Australian researchers. This work has been partially supported by Australian Research Council grants LP150100153 and LP160101553.

Conflict of InterestThe authors declare no conflict of interest.

Keywordscarbon nanotubes, electrospinning, flexible flow sensing, flexible heating devices, flexible temperature sensing

Received: January 9, 2019Revised: February 20, 2019

Published online:

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