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A Transparent Polymeric Strain Sensor for Monitoring Vital Signs and Beyond Hanguang Wu, Qiang Liu, Wencheng Du, Chun Li, and Gaoquan Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19014 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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ACS Applied Materials & Interfaces

A Transparent Polymeric Strain Sensor for Monitoring Vital Signs and Beyond Hanguang Wu, Qiang Liu, Wencheng Du, Chun Li and Gaoquan Shi* ((Optional Dedication)) Dr H. G. Wu, Q. Liu, Dr W. C. Du, Dr C. Li, Prof. G. Q. Shi Department of Chemistry, MOE Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua University, Beijing 100084, People’s Republic of China E-mail: [email protected] KEYWORDS: strain sensor, vital signs, sensitivity, polymer, subtle human motion

ABSTRACT: Wearable sensors that can precisely detect vital signs are highly desirable for monitoring personal health conditions and medical diagnosis. In this paper, we report an ultrasensitive strain sensor consisting of a 150 nm-thick highly conductive dimethylsulfoxide (DMSO) doped poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) sensing layer and an elastic fluorosilicone rubber substrate. This sensor exhibits a high sensitivity at small strains (e.g., gauge factor at 0.6% strain = 280), low limit of detection (< 0.2% strain), and excellent repeatability and cycling stability. Therefore, it is promising for practical detecting vital signs, tiny human motions, and sounds. Furthermore, the semitransparent shallow blue color and the soft rubbery substrate make the strain sensor beautiful and comfortable to human body.

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1. INTRODUCTION Wearable sensors that can precisely detect vital signs have attracted a great deal of attention because they provide the possibility of monitoring personal health conditions conveniently, continuously and comfortably.1−4 In recent years, a variety of flexible pressure sensitive transistors and piezoresistors have been explored to sensitively detect pulse, respiration and/or blood pressure.5−8 However, the fabrication of these devices usually involves complicated procedures by using expensive starting materials and/or instruments. The flexible strain sensors consisting of flexible substrates and conductive sensing layers have a simple configuration, making them to be convenient and cheap for fabrication and operation.2, 9, 10 Unfortunately, they usually exhibit low sensitivity or unsatisfactory accuracy at extremely small strains around 0.5%; thus they are unable to sensitively respond the small deformations induced by vital signs. The sensitivity of a strain sensor is usually evaluated by its gauge factor (GF). GF is defined as (∆R/R0)/ε, where ∆R = R − R0, R0 and R are the resistances of the strain sensor before and after deformation, and ε is the strain. GF depends on the structures and properties of substrate and sensing layers.11−13 For example, the strain sensor with a nanoscale cracked thin film of platinum particles on polyethylene terephthalate (PET) exhibited a high GF of about 16,000 at 2% strain; thus it can be used to detect high frequency vibrations.14,

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However, the high modulus of PET substrate makes the sensor be unable to conformally attach to human body and the weak forces provided by vital signs cannot induce sufficient strain variations to deliver accurate responses. Therefore, the responses of this strain sensor to human pulses are inaccurate and unstable. To address these problems, silicone-based elastomers such as poly(dimethylsiloxane) (PDMS), Ecoflex, and Dragon Skin were widely used as the substrate layers. A strain sensor with micro-cracked graphene woven fabrics on a PDMS film exhibited a high GF of 500 at 2% strain.16 A similar sensor with a sensing layer of 2


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cracked gold/titanium thin film also showed a large GF of 5,000 at 1% strain.17 However, the irreversible strains upon repeated deformations of the elastomer substrates deteriorated the signal repeatability and stability of the sensors described above, especially at ultrasmall strains. On the other hand, the sensing layers of previously reported strain sensors were usually made of highly conductive metal nanomaterials or carbon nanomaterials.18−25 All-polymer strain sensors have rarely been developed, while they are expected to be light, flexible and biocompatible. In this article, we report an ultrasensitive strain sensor consisting of a sensing layer of dimethylsulfoxide (DMSO) doped PEDOT:PSS and an elastic substrate of fluorosilicone rubber (FSR). Furthermore, the negative influence of irreversible strain of the substrate on the performance of strain sensor at extremely small strains was eliminated by pre-stretching the substrate to its irreversible strain before coating the sensing layer. Thus, this strain sensor can precisely detect vital signs including pulse and respiration, and other tiny human motions such as muscle’s motions and facial expressions. It can also monitor audios sounds because of its fast response and high detecting accuracy. 2. EXPERIMENTAL SECTION 2.1 Fabrication of PEDOT-SS. FSR film was prepared through compression-molding the premix of FSR pre-polymer and a curing agent (for details, see the Supporting Information). First, the substrate was pre-stretched to a defined strain of 4%, and oxidized by exposure to oxygen plasma for 10 min to activate its surface. Then, the DMSO doped PEDOT:PSS was spin-coated on the FSR substrate under prestretching state by using a Spin Coater (KW-4A). Different times of spin coating were applied to modulate the thickness of the micro-cracked PEDOT:PSS sensing layer and the density of micro-cracks. Successively, the DMSO doped PEDOT:PSS/FSR bilayer composite film was stretched to 50% strain, constructing microcracks in the sensing layer. Finally, copper wires were glued to the two ends of the sensing 3



layer with silver conductive paste to form a strain sensor, and a PDMS film was used to cover the sensing layer and the electrodes as a protective layer. 2.2 Characterization. Scanning electron micrographs were taken out by using a Sirion 200 field-emission scanning electron microscope (FEI, USA) at an accelerating voltage of 10.0 kV. UV-vis spectra was taken out by using a U-3010 spectrophotometer (Hitachi, Japan). The electromechanical characterizations of the strain sensor were performed by mounting it onto an Instron 3342 universal testing machine, and the electrical signal was recorded by use of a CHI440A electrochemical workstation. Electrical conductivity was carried out using a fourprobe conductivity tester; the reported electrical conductivity is the average of five measurements. The tensile properties of the FSR substrate was measured by using a tensile apparatus (CMT4104, Shenzhen SANS Testing Machine Co., Ltd., China) at a crosshead speed of 200 mm/min. 3.

RESULTS AND DISCUSSION Figure 1a schematically illustrates the procedure of fabricating the typical DMSO doped

PEDOT:PSS/FSR bilayer strain sensor (PEDOT-SS). The low modulus of FSR substrate is comparable with the modulus of the human skin (4.6~20 MPa),26 providing the strain sensor good comfort with the human body (Figure S1, Supporting Information). In order to remove the influence of FSR’s permanent deformation on the performance of sensor, we pre-stretched FSR to a strain of 4%, a value equals to its permanent irreversible strain after recovering from a maximum strain of 50% (Figure S2a). Then the surface of FSR was treated by air plasma to generate a netlike texture by oxidation (Figure S3), and to increase its surface hydrophilicity via inducing oxygen-containing polar groups. Successively, a 150 nm-thick DMSO doped PEDOT:PSS film was coated on the plasma-treated substrate by spin-coating. This composite film was further stretched to 50% to generate micro-cracks. After releasing, the FSR substrate recovered to the length with additional 4.0% (the permanent irreversible strain), and the 4


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fractured PEDOT:PSS fragments exactly interconnected with each other to form a sensing layer with micro-cracks (Figure 1b and 1c). This PEDOT-SS is transparent with a shallow blue color (see inset in Figure 1d), and it showed the optical transmittance of 67% in the visible range (Figure 1d).

Figure 1. (a) Schematic illustration of fabricating a DMSO doped PEDOT:PSS/FSR bilayer strain sensor (PEDOT-SS). (b and c) SEM images of the PEDOT-SS at different magnifications. (d) Transmittance spectra of the PEDOT-SS in the visible wavelength range from 400 to 800 nm; inset shows the photograph of a PEDOT-SS.

The PEDOT-SS demonstrates linear current-voltage curves at various strains (Figure 2a), reflecting its perfect ohmic behavior.11 The sensitivity of the strain sensor was tested by monitoring the variation of its relative resistance (∆R/R0) upon stretching to different strains (ε). As shown in Figure 2b, ∆R/R0 increases with ε, and the slope of this curve is called GF. The GFs of this strain sensor were measured to be 200~1100 at strains of 0−2%. These high GF values are partly attributed to the high conductivity of DMSO doped PEDOT:PSS film (600 ± 95 S cm−1), providing the sensor with an extremely small R0 (see Figure S4). DMSO doping weakens the Coulomb interaction between positively charged PEDOT and negatively charged PSS, greatly enhancing the electron hopping between PEDOT chains to increase the 5



conductivity of this polymer.27 The plots of ∆R/R0 versus ε of PEDOT-SS recorded during loading and unloading processes with a maximum strain of 2% are nearly identical, reflecting the negligible hysteresis of the sensor. This is mainly due to that FRS can strongly adhere the PEDOT:PSS sensing layer via the electrostatic interactions between PSS and FSR chains (Figure S5).27 In comparison, the sensor with a PDMS substrate showed a much larger hysteresis during the process of loading-unloading to a maximum strain of 1.0% (Figure S6). According to the measured tension-recovery stress-strain curves of PDMS and FSR, the hysteresis of FSR (37.5%) is much higher than PDMS (19%) (see Figure S2). Thus, the high hysteresis of the strain sensor with PDMS as the substrate is attributed to the weak interfacial adhesion and the detachment of the sensing layer from the substrate. Actually, the peeling force of FSR/PEDOT:PSS interface (0.15 N) was tested to be about 15 times that of PDMS/PEDOT:PSS interface (0.01N). The resistance variations of PEDOT-SS at peak strains of 0.2%, 0.5%, 1.0%, and 2.0% were measured to be 0.34, 1.6, 3.5, and 15, respectively (Figure 2c). The large resistance variation (34%) at 0.2% strain indicates that this strain sensor satisfies the requirements of detecting small deformations.

Figure 2. Electromechanical behaviors of the typical PEDOT-SS. (a) Current-potential plots of the strain sensor at different strains. (b) ∆R/R0-ε curves of the strain sensor during the loading-unloading cycle. (c) Multicycle tests of relative resistance variation of the strain sensor upon stretching to different maximum strains. (d) Relative resistance responses of the strain sensor as a function of the 6


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loading frequency at a strain of 1.0%. (e) Relative resistance response of the strain sensor to different strains after a stepped strain change. (f) Durability test of the strain sensor by repeatedly stretching it to a maximum strain of 1.0% for 2,000 cycles.

The resistance responses of PEDOT-SS were recorded during the repeated loadingunloading cycles with a maximum strain of 1.0% at the frequencies ranging from 1/8 to 2 Hz, showing only slight variations (Figure 2d). This result indicates the high stability and fast responses of this sensor, which should be attributed to the consistency of the sensing fragments movement and the deformation of the substrate. Thus, the stability and response speed of the sensor are determined by the strength of the interfacial adhesion between the sensing layer and the FSR substrate. In addition, the resistance response to a given strain keeps identical after a stepped strain change (for example, 0.2% to 2.0% to 0.2%), reflecting the good repeatability of this sensor (Figure 2e, Figure S7). The sharp overshoot at each applied strain is caused by the tensile stress-relaxation of the sensor, attributing to the viscoelasticity of the FSR substrate.23, 28 The resistance variation of this strain sensor at 1% strain kept nearly unchanged after 2,000 cycles of loading-unloading, demonstrating its excellent robustness and stability (Figure 2f). The sensing mechanism of the PEDOT-SS is considered to be the widening of its microcracks and the disconnection of the conducting fragments in its sensing layer during stretching; thus its sensitivity should be significantly affected by the shapes and density of the micro-cracks. We constructed micro-cracks with different shapes and densities via changing the thickness of sensing layer. When the thickness of the sensing layer was as thin as 70 nm, the cracks in PEDOT:PSS film were generated along the surface cracks of FSR substrate, and the sensing film was fractured into micro-sized flakes (see Figure S9). Upon stretching, the resistance variation of the sensing layer was caused by the following two opposite factors: the widening of the cracks perpendicular to the stretching direction and the reconnection and/or overlap of the cracks parallel to the stretching direction induced by the lateral Poisson 7



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compression.29 Therefore, the GF of the sensor was relatively small (45 at strain of 2%), while the sensing range was measured to be as wide as 8% strain (Figure S8). Upon increasing the thickness of the sensing layer to 150 nm, the nearly cut-through cracks within the PEDOT:PSS film propagated in the direction perpendicular to the pre-stretching direction of the sensor (See Figure 1b), and many jagged edges were formed along the cracks. Consequently, the sensing mechanism was changed. In this case, we monitored the morphological evolution of the sensing layer upon stretching by using SEM (Figure 3a-c). The change in the resistance of sensing layer upon stretching is attributed to the disconnection-reconnection of the zip-like cracks (Figure 3d). Therefore, the sensing mechanism of the PEDOT-SS with a 150 nm-thick sensing layer is similar to that of the strain sensor based on nano-cracked Pt film.14, 15 During the uniaxial stretching, the lateral Poisson compression makes the small edge steps remain in contact until the gap distance (δ) overcomes the crack asperity height (hi) (see Figure 3d). According to the previous report, before the complete break of the conductive pathways caused by the disconnection of all the crack edges, the overall effect of the disconnection-reconnection of the crack edges is to reduce the conductivity of the sensing layer as the extension proceeds.14 Consequently, the GF of strain sensor can be deduced by the following equation (Supporting Information):

GF =

1 N hi −ε L0

(1)

where N is the total number of the cracks, hi is the average height of crack asperity, L0 is the initial length of the sensor, and ε is the deformation strain of the sensor. During the stretching process, we reasonably consider hi , L0 are constants. Therefore, it can be seen from the equation that the value of GF increases with the increase in ε during the uniaxial extension, being consistent well with the experimental results (Figure 2b). Furthermore, when the sensor is stretched to a given strain (ε), GF value depends on only the number of the cracks (N). For 8


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PEDOT-SS, N decreases significantly as the thickness of its PEDOT:PSS sensing layer increases from 150 to 300 nm because of the enhanced mechanical strength of PEDOT:PSS film (Figure S10), leading to a sharp increase in GF value (see Figure S8). This result is consistent well with the theoretical prediction of Equation 1. Unfortunately, the detectable strain range decreased from 2% for the sensor with a 150 nm-thick sensing layer to 0.63% for the counterpart with a 300 nm-thick sensing layer (Figure S8). Thus, the thickness of the sensing layer was optimized to 150 nm for fabricating the typical PEDOT-SS for further studies.

Figure 3. (a) SEM images of a PEDOT-SS at 0% strain. (b) SEM images of a PEDOT-SS at 1% strain. (c) SEM images of a PEDOT-SS at 2% strain. (d) Schematic illustration of sensing mechanism of the sensor upon stretching.

As described above, PEDOT-SS has high sensitivity, low hysteresis, fast response, and good stability. Therefore, it is expected to be able to precisely detect vital signs. Furthermore, the elastic soft FSR substrate and the biocompatible PEDOT:PSS sensing layer further improve the comfort and safety of this strain sensor to human body. Among vital signs, pulse 9



pressure is one of the most important indicators of person’s artery condition because the pathological changes (blood pressure, heart rate, etc.) of a person’s body condition are reflected in pulse; thus pulse detection plays an important role in non-invasive medical diagnosis.30, 31 For detecting pulse, PEDOT-SS was placed on the radial artery of wrist (inset of Figure 4a) or neck (Figure S11) of the first author of this paper to detect the subtle pressure and differentiate the pulses before and after doing exercise. The fast response of the sensor provides high resolution signals with detailed clinical information of pulse (Figure 4a). Under the normal condition, the pulse frequency was measured to be 78 beats/min. Each pulse has a regular and repeatable shape, and the typical radial artery pulse waveform has three distinguishable peaks, corresponding to percussion wave (P-wave), tidal wave (T-wave), and diastolic wave (D-wave), respectively. After doing exercise, the pulse frequency increased to 102 beats/min, and the amplitudes became much stronger. The pulse curves recorded from the neck gave signals with similar shapes (Figure S11), but the intensity of each peak was much higher than that of wrist pulse, indicating that the pulse pressure at neck is stronger than that at wrist. In comparison with the previous flexible strain sensors, PEDOT-SS can monitor pulses much more accurately.9-10, 12, 14, 32 The superior performance of PEDOT-SS is mainly attributed to its high sensitivity at small strains (the deformation of the pulse on wrist is around 0.6%, Table S1), the soft FSR substrate and the strong adhesion force between sensing and substrate layers. The small deformations induced by the weak forces of pulse can be accurately and smoothly captured.

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Figure 4. Demonstration of detecting the vital signs by PEDOT-SS. (a) The pulses at wrist. (b) The heartbeats at chest. (c) Respirations under normal and deep breathing conditions.

Heartbeat and respiration are also important vital physiological signals, which are used to diagnose human diseases such as sleep apnea syndrome and sudden infant death syndrome.33 The PEDOT-SS can be integrated on tights (inset in Figure 4b) to detect the human heartbeats and the respiration (Figure 4b and Figure 4c). The close-fitting performance of the tights provides the strain sensor high accuracy and good repeatability among the detection. The heartbeats detected by placing PEDOT-SS on the chest (Figure 4b and Figure 4c) accurately reflect the frequency of pulse. However, the responses are much weaker than those of pulses because of the barrier effect of the sternal. Figure 4c shows the different amplitudes of the signal peaks under normal and deep breathing conditions, explicitly manifesting the discriminable rate and depth of the respiration.32 PEDOT-SS can also be used to detect subtle human-activities, such as facial expression and the tiny motions of epidermis or muscles (Figure 5a, 5b). The PEDOT-SS showed a relative high ∆R/R0 (≈ 30 or 3,000%) when detecting the epidermis motion of the first 11



author′s opisthenar upon clenching her hand (Figure 5a). In addition, the PEDOT-SS attached on the arm exhibited a rapid and dramatic enhancement in ∆R/R0 by stressing the muscle, indicating that the tiny motions of muscles could also be monitored during its tension and relaxation processes (Figure 5b). Furthermore, recognition of human emotions is critical in the human monitoring field.34 This strain sensor can also sensitively detect smiling with a ∆R/R0 responses higher than 200% by adhering it to the skin near the mouth (Figure 5c). Sounds are deformations of air with relatively high frequencies. PEDOT-SS can also recognize different voices generated by a commercial loudspeaker (Figure S12a), including words (Figure 5d), sentences (Figure S12b), and different audio files such as piano (Figure 5e) and helicopter (Figure S12c and S12d). In the case of detecting words or sentences, every syllables were reflected into the resistance variations of the sensor, and the intensity of each signal is related to the loudness of the corresponding pronunciation (Figure 5d, Figure S12b). When sensing the audio sounds (Figure 5e, Figure S12c), the responses of the transducer are well synchronous to the original audio signals (green inset in Figure 5e). Especially, the characteristic peaks are retained with high fidelity.

Figure 5. Demonstration of detecting tiny strains by PEDOT-SS. (a) Extending-clenching of a hand. (b) Muscular tension and relaxation. (c) smile. (d) A word generated by a loudspeaker. (e) Piano audio; Inset: the sound wave profile. 12


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4.

CONCLUSION In summary, we developed a high-performance transparent strain sensor simply by

depositing a DMSO doped PEDOT:PSS film on FSR substrate and further treated by stretching/releasing. This sensor has a highly conductive sensing layer with micro-cracks and a soft rubbery substrate, and these two layers have strong interfacial adhesion force. Furthermore, the influence of the irreversible strain of elastic substrate was removed for the first time by pre-stretching it before coating the sensing layer. As a result, this strain sensor showed superior performances in detecting vital signs, including pulse, heartbeat, and breathing, and it can also be applied to sense tiny motions of human and sounds. This strain sensor is lightweight, beautiful in color and comfortable to human body. Thus, it is attractive and promising for practical applications, especially in human-activity detecting and personalized health monitoring.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: am-2017-154254. Additional experimental details, Mechanism Analysis, Figures S1−S12 and Table S1.

AUTHOR INFORMATION Corresponding Autor *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

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This work was supported by National Key R&D Program of China (2016YFA0200202), the National Natural Science Foundation of China (51433005, 21674056, 51673108), and the China Postdoctoral Science Foundation (2017M610861). The authors thank Dr. Luqi Tao from Institute of Microelectronics and Tsinghua National Laboratory for Information Science and Technology for his help in testing sound signals.

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A Transparent Polymeric Strain Sensor for ... - ACS Publications

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