Ultrafast Response Polyelectrolyte Humidity Sensor for Respiration

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Applications of Polymer, Composite, and Coating Materials

Ultrafast Response Polyelectrolyte Humidity Sensor for Respiration Monitoring Jianxun Dai, Hongran Zhao, Xiuzhu Lin, Sen Liu, Yunshi Liu, Xiupeng Liu, Teng Fei, and Tong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18904 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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ACS Applied Materials & Interfaces

Ultrafast Response Polyelectrolyte Humidity Sensor for Respiration Monitoring Jianxun Dai†, Hongran Zhao†, Xiuzhu Lin†, Sen Liu†, Yunshi Liu†, Xiupeng Liu†, Teng Fei*,†,‡ and Tong Zhang*,† †State

Key Laboratory on Integrated Optoelectronics, College of Electronic Science and

Engineering, Jilin University, Changchun 130012, P.R. China ‡State

Key Laboratory of Transducer Technology, Shanghai 200050, P.R. China

KEYWORDS: polyelectrolyte humidity sensors, in situ preparation, click reaction, ultrafast response, breath monitoring

ABSTRACT: Respiration monitoring is important for evaluating human health. Humidity sensing is a promising way to establish a relationship between human respiration and electrical signal. This work describes polymer humidity sensors with ultrafast response for respiration monitoring. The humidity-sensitive polyelectrolyte is in situ crosslinked on the substrate printed with interdigitated electrodes by a thiol-ene click reaction. The polyelectrolyte humidity sensor owns rapid water adsorption/desorption ability, excellent stability and repeatability. The sensor with ultrafast response and recovery (0.29 s/0.47 s) when changing humidity between 33% and 95% shows good application prospects in breath monitoring and touchless sensing. Different respiration patterns can be distinguished and the breath rate/depth of detection subjects can also be determined by the sensor. In addition, the obtained sensor can sense the skin evaporation by a non-contact way.

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1. INTRODUCTION Respiration, the exchange of gases between the body and the environment, is critical for supporting human activities.1,2 Many illnesses and discomforts will lead to the change of respiratory frequency and depth,3 such as heart disease, pneumonia, bronchitis, sleep apnea syndrome (SAS), as well as hyperpyrexia caused by infection. Abnormal respiration is usually a caution of individual’s physical problem. Different methods have been developed to monitor human respiration. Original way of breath rate measurement needs the help of pressure sensors to detect the expansion and constriction of the chest or the airflow from the nose.4-7 The thermal sensors take advantage of the temperature variation between inhale and exhale airflow to monitor breath.8,9 Airborne ultrasound was also used to detect variations in the velocity of sound caused by air flow.10 However, conventional breath monitors are uncomfortable, cumbersome and costly, and sick patients cannot cooperate well. Therefore, convenient and real-time breath monitoring and diagnostic breath analysis are significant. In recent years, humidity sensors have been utilized for breath monitoring. The human exhaled air is always in a high relative humidity (RH) range (>90%), which won’t be influenced by movements, environments or seasons. It is available and convenient to monitor breath via humidity response, based on carbon materials,11-15 metallic oxides,16,17 nanophase Si18,19 and supramolecular materials.20,21 Inorganics or graphene materials suffer from their poor film-forming ability and processability for preparing electronic devices. Polymer humidity sensors own the advantages of controlled structure, facile solution process and batch preparation, which are promising in breath measurement. Importantly, for respiration monitoring, humidity sensors have to work in high humidity environments for a long time, so the stability of the sensors under high humidities must


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be guaranteed. However, humidity sensors based on linear polyelectrolytes usually could not endure high humidity atmospheres, due to the structure limitation of amphiphilic polymers. Another critical point is that a single respiratory period lasts 3-4 s, which presents a great challenge for humidity sensors’ response and recovery time (1 s or even less). In this work, to monitor respiration, polymer humidity sensors were in situ prepared based on poly(mercaptopropyl polyhedral oligomeric silsesquioxane-1,4-divinylbenzene-sodium p-styrene sulfonate

hydrate)

(PMDS)

and

poly(pentaerythritol

tetra(3-mercaptopropionate)-1,4-

divinylbenzene-sodium p-styrene sulfonate hydrate) (PPDS). The crosslinked framework guarantees the stability of the sensor in high humidity atmospheres. Short response/recovery time (0.29 s/0.47 s) was obtained based on PMDS sensor, benefiting from the rigid hydrophobic framework and proper hydrophilic structure of PMDS. PMDS sensor was used for breath monitoring and different respiration patterns could be distinguished. The breath signals during exercise were well recorded and systematically analyzed. What’s more, on the basis of evaporation of sweat, the non-contact sensing of finger motions was also researched. 2. EXPERIMENTAL SECTION Materials. Mercaptopropyl polyhedral oligomeric silsesquioxane (MPOSS) was synthesized according to the reported method.22 Pentaerythritol tetra(3-mercaptopropionate) (PETMP) was obtained from Tokyo Chemical Industry (Shanghai) Development Co., Ltd. 1,4-divinylbenzene (DVB) was obtained from Tianjin Guangfu Technology Development Co., Ltd., China. Sodium p-styrene sulfonate hydrate (SSS) and benzoin dimethyl ether (DMPA) was acquired from Aladdin Industrial Corporation. Tetrahydrofuran (THF) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Methanol was purchased from Beijing Chemical Co., Ltd., China. All chemicals


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were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. In situ preparation of humidity sensors. Take PMDS sensor as an example, MPOSS (M1, 50.0 mg), DVB (M2, 30.0 mg), SSS (M3, 5.6 mg) and DMPA (catalyst, 4.0 mg) were dissolved in the mixed solvents of methanol (0.75 mL) and THF (0.5 mL) (the molar ration of M1: M2: M3 is 1:3.75:0.5). Then 1.5 μL of the obtained mixture was drop-casted onto a ceramic plate (7 mm×5 mm, 0.6 mm in thickness) with four pairs of interdigitated carbon electrodes (electrodes width and distance: 0.3 mm). The substrates covered with precursor solution were irradiated under UV light (365 nm, 0.12 J/cm2) for 30 minutes via the CL-1000L UV crosslinker (UVP, America). In the end, the crosslinked humidity sensors were rinsed with the mixed solvents of methanol and THF to remove unreacted monomers and the catalyst. The thickness of the sensing film is ~30 μm. PPDS sensor was prepared with a similar method with PETMP (M1’) and the molar ratio of M1’: M2: M3 is 2:3.75:0.5. Measurements. FT-IR spectra of the polymers were recorded with a Perkin-Elmer spectrometer, using KBr pellet (KBr Pellet, Supporting Information). Elemental Analysis (EA) of carbon, hydrogen, and sulfur were performed by Flash EA 1112, CHNS-O elemental analysis instrument. The film thickness was measured by Caikon DMM-330C metallographic microscope. The humidity sensing properties of the sensors were measured on a Keysight E4990A impedance analyzer. The voltage used throughout this work was AC 1 V, while the frequency varied from 100 Hz to 100 MHz. All the humidity sensing measurements were carried out at 20 °C under standard atmospheric pressure. The RH atmospheres were produced by different saturated salt solutions in their equilibrium states including LiCl for 11% RH, MgCl2 for 33% RH, Mg(NO3)2 for 54% RH, NaCl for 75% RH, KCl for 85% RH and KNO3 for 95% RH, respectively, at 20 °C.23


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The RH and temperature atmospheres in temperature characteristics measurement was provided by GRZ5702 temperature and humidity generator (Xianghe, China). 3. RESULTS AND DISCUSSION High humidity stability and rapid response are necessary for the real-time respiration monitoring. Based on above requirements, crosslinked polyelectrolytes (PMDS and PPDS) were in situ prepared by the thiol-ene click reaction directly on the ceramic substrates printed with interdigitated electrodes. The schematic diagram for preparing PPDS and PMDS sensors is shown in Figure 1. The crosslinked three-dimensional network structures cannot dissolve by water or organic solvents, which can ensure the stability of the polymers. The frameworks were constructed by different hydrophobic monomers (MPOSS, PETMP and DVB) and moderate hydrophilic monomer (SSS). The molar ratio of the hydrophilic monomer was kept the same in the two polyelectrolytes.

Figure 1. The schematic diagram for preparing PPDS and PMDS sensors.

Materials Characterization. In order to determine the component of PPDS and PMDS, the Fourier Transform Infrared Spectroscopy (FT-IR) measurements were carried out, and the results were presented in Figure 2. The characteristic peak at 1735 cm-1 can be ascribed to the ester group and the peak of C-O appears at 1141 cm-1, which only shows in PPDS. As for PMDS, the


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characteristic peak of Si-O-Si and Si-C appears at 1120 cm-1 and 806 cm-1, respectively. The peaks at 2918, 2853 and 1412 cm-1 are designated to stretching and bending vibration of C-H in -S-CH2-, and the peak at 1036 cm-1 is ascribed to S=O bond coming from SSS. From the EA results shown in Table S1, there is subtle difference between the theoretical value and practical value in the content of carbon, hydrogen and sulfur.

Figure 2. FT-IR spectra of polymers PPDS and PMDS.

Humidity Sensing Properties and Ultrafast Response. The PMDS and PPDS sensors in a batch were tested at different RHs and the impedance signal results are shown in Figure 3a. There is only tiny response for the two sensors at the humidity range of 11% to 54% RH. At high humidity range, PPDS sensors show an incredibly small variation. By contrast, PMDS sensors own an impedance change of nearly two orders of magnitude. It is considered that the rigid cage-like framework structure and better polymerization contribute to the better sensitivity of PMDS sensor. The response and recovery time of PMDS and PPDS sensors was also measured as shown in Figure 3b,c, respectively. The response and recovery time was calculated from the impedance difference after the sensor was put in 33%/95% RH atmosphere with a 90% change, for the adsorption and desorption processes, respectively. PMDS sensor’s response/recovery time between 33% and 95% RH is 0.29 s/0.47 s, and that of PPDS sensor is 0.75 s/0.48 s. Considering the sensitivity and


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response time, the PMDS sensor was selected for the following tests. PMDS sensor’s responses at different frequencies were researched (Figure S1, Supporting Information). The linearities of the curves from 54% to 95% RH were calculated, and the best linear degree occurs at 1 kHz (Adj. Rsquare = 0.9994). Meanwhile, it is available to avoid polarization of polyelectrolyte at low frequency. The humidity hysteresis is 2.6% RH and the adsorption and desorption curves almost coincide with each other at 54% - 95% RH (Figure S2, Supporting Information). Due to the fact that the temperature of exhalation is about 35 ºC, the sensing curves of PMDS sensor at different temperatures were explored, and the temperature compensation coefficient is 0.29 ± 0.05% RH/ºC at high humidity range (54% - 95% RH) (Figure S3, Supporting Information). If the feeding amount of SSS changes, the sensitivity will decrease or the recovery time will become longer (Figure S4 and Figure S5, Supporting Information).

Figure 3. (a) The dependence of impedance on RH for PPDS sensors and PMDS sensors (Error bar curves of 5 sensors in a batch, 1V AC, 1 kHz). Response of (b) PMDS sensor and (c) PPDS sensor to humidity change between 33% RH and 95% RH.

Repeatability and Stability. For a humidity sensor, the repeatability and stability are key properties for practical applications. The continuous response and recovery curve between 33% RH and 95% RH is shown in Figure 4a. The consistent baseline (sensor’s impedance modulus under 33% RH) and four-time fast responses demonstrate perfect repeatability of the PDMS sensor. The response and recovery time of PMDS sensors were monitored for 24 days, and the results are


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presented in Figure 4b. The response-time range is 0.08-0.35 s and recovery-time range is 0.330.56 s, demonstrating the ultrafast sensing characteristic could be well maintained. The stable fast response/recovery PMDS sensor is beneficial for long-time breath monitoring. The impedance responses of PMDS sensor at different RHs were monitored for more than 2 months and shown in Figure 4c. At 75% RH, a junction humidity between insensitive area and sensitive area, the impedance modulus fluctuation is obvious. However, the impedance modulus at high humidity range was stable after 50 days.

Figure 4. (a) The continuous response and recovery curve between 33% RH and 95% RH for 4 cycles. (b) The response-time/recovery-time stability of PMDS sensor. (c) The long-term impedance stability of PMDS sensor under different RHs.

Sensing Mechanism. To explore the origin of humidity sensing properties of PMDS sensor, complex impedance spectroscopy (CIS) and equivalent circuit (EC) are shown in Figure S6 (Supporting Information). At 11% - 54% RH, the CIS curves are straight lines parallel to the Xaxis (Figure S6a-c, Supporting Information), which can be equal to a single capacitor Cf 24 (phase angel is about -90° in Figure S7 (Supporting Information)). Few water molecules are adsorbed on the surface of sensitive film and it is hard for SSS to ionize.25 At 11% - 54% RH, there isn’t any response and the impedance modulus are very large (~80 MΩ) as shown in Fig 3a, mainly coming from Cf. At 54% - 95% RH, the CIS curves are composed of a semicircle and a single line, and the EC can be described as the Rf and Cf in parallel, and the Zw in series.26 With the increase of RH, more water molecules are adsorbed on the sensitive film. It is much easier for Na+ to ionize in


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continuous water layer and Na+ and H3O+ become the main conducting particles. So, the impedance modulus decreases with the increasing RH (Figure 3a). On the basis of the fundamental characteristics, the fast response mechanism was also analyzed. The effect of functional groups and cage-like framework on water molecules can be seen in Figure 5. The rapid adsorption and desorption of water molecules (blue and red arrows) is due to the synergism of hydrophobic and hydrophilic parts. From low to high humidity (Figure 5a), the tiny amount of polar SSS adsorbs a few water molecules onto the surface of sensitive film and it’s easy to reach the dynamic balance condition quickly. From high to low humidity (Figure 5b), the silicon-oxygen bond, benzene ring and alkyl chain can accelerate the desorption of water molecules, which makes the sensor return to the high impedance state easily. Appropriate ratio of hydrophilic and hydrophobic component in the polymer framework is the origin of rapid response to humidity change.

Figure 5. The schematic of water adsorption (a) and desorption (b) in the polymer.

Breath Monitoring. The normal respiration of an adult lasts 3-4 s, and the exhaled air is high humidity flow. The fast arrival of equilibrium in high humidity is a big challenge for humidity sensors. Six breath states were simulated and recorded by the PMDS sensor. The mask with a


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breather value assembled a PMDS sensor (Figure S8, Supporting Information). The sensor could distinguish the variation of breath rate and depth among a normal breath, a fast breath, a slow breath, a deep breath, a random breath and a paused breath. Due to 2-order-of-magnitudes sensitivity, the change of breath depth is obvious in semi-log coordinates, shown in Figure S9 (Supporting Information). As shown in the Figure 6, it is worthy to note that almost every cycle has the inhaled platform and exhaled platform. The signal equilibrium value is very important and beneficial to recognize out the single inspiration and expiration (the data of reference sensor in Figure S10, Supporting Information). The respiration of resting is steady (Figure 6a), and that of doing exercises is fast because of more oxygen demand (Figure 6b). When someone is sleeping, metabolism and breath are both slower (Figure 6c). When doing yoga, taking a deep breath contributes to relax the whole body (Figure 6d). When an emergency happens to somebody, the respiration would be irregular (Figure 6e). If someone has SAS, the breath will display a brief pause (Figure 6f). The human body states and different health conditions will lead to the changes in breath frequency and depth. The PMDS sensor has potential applications in personal state monitoring and diagnostic breath analysis.


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Figure 6. The six respiration patterns recorded by PMDS sensor, (a) normal breath, (b) fast breath, (c) slow breath, (d) deep breath, (e) random breath and (f) paused breath.

Followings are concrete tests to evaluate 5 subjects via respiration during light and vigorous exercise. Figure S11a (Supporting Information) shows one of the signals (Figure S11b-e (Supporting Information) shows the rest of the signals) in which Subject 1 did (1) a normal breath, (2) a shallow and fast breath, (3) a 5-second pause in continuous breath and (4) a deep and slow breath. Different persons have different breath habits and various comprehending about shallow and fast, or deep and slow breath patterns. Though the output signals are not the same, the depth and rate of respiration are shown clearly. The normal breath signals are regarded as the normalized standard in the follow tests. Subjects did Motion state 1 (light exercise) and Motion state 2 (vigorous exercise) in about 4 minutes, which were recorded by PMDS sensor, and shown in Figure S12 (Supporting Information). The rate of respiration (RR) can be easily figured out by


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counting the numbers of peaks. The depth of respiration (DR) is calculated based on the impedance change in every respiration state (Depth of Respiration Calculation Method, Supporting Information). To evaluate the breath condition of each subject, the respiration index (RI) is defined by Equation (1)27 RIN =

RRN × DRN RR0 × DR0

(1)

(N = 1, 2)

Here, RRN is the rate of respiration in Motion state 1 or Motion state 2, RR0 is the rate of resting respiration, DRN is the depth of respiration in Motion state 1 or Motion state 2, DR0 is the depth of resting respiration. The product of RR and DR can semiquantitative represent person’s air flow. An increase in exercise intensity will certainly lead to the increase of air flow (RIN>1). The calculated data of every subject is shown in Figure 7. Comparing with resting respiration, there is an increase in the RR and DR of most subjects generally. However, Subject 3 has a decrease in RR (Figure 7a), but a larger increase in DR (Figure 7b) in the Motion states. Actually, the air flow of Subject 3 still increases (RI1=1.32, RI2=1.68 in Figure 7c). For Subject 1, RIN is much larger than the others (RI1=2.32, RI2=5.14), which indicates Subject 1 gets tired easily and lacks of exercise. On the contrary, subject 5 has a smaller RIN similar to 1 (RI1=1.25, RI2=1.27), which demonstrates a stronger physiological state. To some degree, RIN can represent a person’s physical ability parameter.


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Figure 7. (a) The respiration rate of subjects during rest, Motion state 1 and Motion state 2. (b) The respiration depth of subjects during rest, Motion state 1 and Motion state 2. (c) Respiration index of subjects during Motion state 1 and Motion state 2 (normalized to resting breath). Motion state 1: getting on the exercise bike at 18 km/h. Motion state 2: getting on the exercise bike at 23 km/h.

Based on the PMDS sensor’s rapid response and good stability, the real-time breath monitoring tests were carried out. Via the 6 respiration patterns experiments and sampled 5 subjects’ respirations during exercise, the respiration signals were clearly obtained and well analyzed. The reported humidity sensors developed for breath monitoring were summarized in Table 1. The majority of fast-response humidity sensors for breath monitoring based on 2D materials, metal oxides or nanophase silicon. Comparing with the polymers, inorganic materials have problems in film-forming property, modification processing and batch preparation. However, most polymer sensors have longer response and recovery time in breath monitoring.30-32 To the best of our


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knowledge, only a few researches33,34 on the breath monitoring with fast-response polyelectrolyte humidity sensors has been reported. Table 1. Comparison of humidity sensing performance of PMDS sensor with reported works on breath monitoring Humidity sensitive materials

Sensitivity

Materials type

Response time (s)

Recovery time (s)

Reference

Wrinkled graphene GO Graphene WS2 Columnar TiO2 Columnar TiO2 Nanocrystal Si Nanophase Si PSS@SNs PPPS PMMA/MAPTAC PMDS

0.31 (R/R0/%RH) 2357 103 (R/R0) 105 (R/R0) 105 (R/R0) 104 105 0.0369 (log Z/% RH) 102

2D material 2D material 2D material 2D material Metal oxide Metal oxide Si Si Polymer Polymer Polymer Polymer

0.0125 0.03 0.6 5 0.09 0.05 0.04

Ultrafast Response Polyelectrolyte Humidity Sensor for Respiration

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