Ze-Kun
Chen
a,
Wei-Wei
Bai
a,
Ying-Qian
Huo
*b and
Jing-Hui
He
*a
aSuzhou Key Laboratory of Novel Semiconductor-Optoelectronics Materials and Devices, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China. E-mail: jinghhe@suda.edu.cn
bDepartment of Neurology, Binzhou Medical University Hospital, Binzhou, Shandong Province 256603, P.R. China. E-mail: yqhuo_BMU@126.com
First published on 10th April 2023
Snoring monitoring is a valid method to assess human health. However, it is challenging to sense the humidity of breath flows that have huge oscillations, requiring highly robust sensors. Herein, a humidity sensor based on ion-in-conjugation croconate polymers was fabricated for snoring monitoring for the first time. The sensor showed rapid response/recovery to monitor human breathing. It has a wide working range from 11% to 95% RH (relative humidity) with a time stability of up to 28800 seconds (8 hours). Normal breath, snoring, and apnea are distinguished and recorded timely. The fabricated noninvasive and point-of-care snoring detection device exhibits comparable accuracy to commercial devices, manifesting the potential of ion-in-conjugation polymers in future breath monitoring systems.
The performance of humidity sensors relies on the working principle and sensing materials being applied, such as two-dimensional materials,11 polymers,12 and ceramics.13 Among them, conjugated polymers have attracted increasing attention due to their structural diversity, tunability, controllable performance, and low cost.14 However, many conjugated polymers suffer from long-time instability and low sensitivity to humidity.15 Recently, ion-in-conjugation polymers, containing ionic states in the conjugated backbones, were investigated owing to their unparalleled physical and chemical properties.16a Our previous work has shown that ion-in-conjugation polymers have strong affinity to polar molecules, including water, because of highly polar ionic fragments, rendering breath monitoring viable.16b In addition, the strong inter-molecular interactions and good crystallinity of ion-in-conjugation polymers can resist the corrosion of water vapor and provide good robustness for humidity sensing. Humidity sensors were prepared from two polycroconate materials: poly(1,5-diaminoanthraquinone-croconate) (1,5-PDAC) and poly(2,6-diaminoanthraquinone-croconate) (2,6-PDAC).
In this study, we demonstrate for the first time that ion-in-conjugation polymers can be utilized for snoring monitoring and breath apnea, comparable to commercial apparatuses (Fig. 1). As a proof of concept of ion-in-conjugation polymers, two polycroconates were prepared, which showed impedance ranging from 104 to 108 Ω, the time of response and recovery within seconds and the humidity hysteresis less than 6% toward the atmosphere of relative humidity from 11% to 95%. Normal breath and snoring were successfully distinguished and recorded remotely using ion-conjugated-polymer-based resistive sensors to monitor the humidity of exhaled gas from nostril outlets with a time stability of up to 28800 s. The noninvasive and point-of-care snoring detection device is comparable to the accuracy of commercial devices, indicating the potential commercial application of ion-in-conjugation polymers in future screening snoring and apnea systems based on humidity.
Furthermore, it can be seen from X-ray diffraction (XRD) data that 1,5-PDAC has better crystallinity than 2,6-PDAC in Fig. S3a,† possibly due to the strong double hydrogen bond interaction provided by ion-conjugated polymers.19 In terms of electronic properties, the Tauc plots show that the energy gaps (Eg) of 1,5-PDAC and 2,6-PDAC, shown in Fig. S3b,† are 1.44 eV and 1.57 eV, respectively, which means that their properties of semiconducting are appropriate for humidity sensing. In terms of thermal stability, we compared the weight loss of the polymers at 200 °C in a nitrogen environment to exclude the influence of material hygroscopicity. The above-mentioned results indicate that 1,5-PDAC has better thermostability than 2,6-PDAC (Fig. S4a and b†). 1,5-PDAC shows a smaller specific surface area than 2,6-PDAC (Fig. S5a and b†) calculated using the Brunauer–Emmett–Teller (BET). These properties prove that the two polymers are suitable for humidity sensing.
As one of the most important indexes to evaluate the reliability of humidity sensors, humidity hysteresis is defined as the maximum RH difference at the identical impedance modulus on the curve when the sensor experiences humidity moisture adsorption and desorption.20 Humidity hysteresis is caused by the higher impedance of the adsorption process due to the higher exothermic adsorption rate than the endothermic desorption rate16a (calculation details are shown in Fig. S7†). In Fig. 3b and d, the humidity hysteresis of 1,5-PDAC and 2,6-PDAC is 6.4% and 9%, respectively.
In terms of humidity sensor sensitivity, the response time (tres, time when the response change reaches 90% of the maximum signal) and recovery time (trec, time when the response drop change reaches 90% of the steady response) are other significant characteristics for breath monitoring. We made several pulses at 11% RH and 95% RH at 20 Hz, as shown in Fig. S8a and d† to obtain the tres and trec of 1,5-PDAC, and 2,6-PDAC-based humidity sensors, respectively. The tres of the 2,6-PDAC sensor is 120 s when switching from 11% to 95% RH in Fig. S8b,† and the trec is approximately 10 s, as shown in Fig. S8c.† In contrast, the tres of the 1,5-PDAC sensor is short enough (10 s) to have the potential to track human breathing (Fig. S8e and f†). Compared with the reported polymer materials, the tres of 1,5-PDAC is short enough to successfully monitor human breathing, which is superior to many other sensory materials.21–23 In summary, the humidity sensor based on 1,5-PDAC can operate over a broad range of RH, and the time stability of 1,5-PDAC up to 28800 seconds (8 hours) also lays the foundation for its application in the field of screening snoring and apnea (Fig. S9†).
The humidity sensing properties of 1,5-PDAC are the strongest among these two polymers. The complex impedance spectra of 1,5-PDAC, as shown in Fig. 4a, indicate that the arc radius is larger at low humidity (11–33% RH), while the arc gradually becomes a semicircle with increasing humidity. The equivalent circuit of the complex impedance spectra of 1,5-PDAC is formed by a resistor (Rf) and a condenser (Cf) in parallel24,25 (Fig. 4c). As the humidity increases to 95%, the radius of the semicircle decreases, the straight line followed becomes longer, and the equivalent circuit becomes a Warburg impedance (Zw)26 series connection with resistors and capacitors in parallel (Fig. 4d). The Warburg impedance results from the diffusion of charge carriers between the hygrosensitive material and the electrode interface. At low humidity, a small number of water molecules are physically adsorbed on the hygrosensitive material surface in the form of hydrogen bonds. Therefore, physically adsorbed water molecules are unable to move freely, resulting in difficulties in ionic conduction. The equivalent circuit shown in Fig. 4c confirms the above logic. With the gradual increase in humidity, the number of physically adsorbed water molecules increases to form multiple layers of water. The movement of ions in the continuous water layer is mainly from H+ and H3O+, which are formed by the ionization of H+ and transferred through the Grotthuss chain reaction (H2O + H3O+ → H3O+ + H2O),27 resulting in the decrease of impedance with the equivalent circuit in Fig. 4d. Similarly, the complex impedance plot of 2,6-PDAC indicates the same characteristics as 1,5-PDAC in Fig. 4b. Compared with 2,6-PDAC, the spectral radius of 1,5-PDAC shrinks faster, and the straight line becomes longer, indicating that 1,5-PDAC has the strongest adsorption capacity among these two polymers.
Fig. 4 Complex impedance plots of a humidity sensor based (a) 1,5-PDAC and (b) 2,6-PDAC. (c and d) The electric equivalent circuits. |
Fig. 5 (a) Sketch of breath snore monitoring; (b) schematic diagram of breath monitoring based on the 1,5-PDAC humidity sensor; (c) human breath test in two different modes; (d) sensor cycle stability test; (e) real-time monitoring of normal breathing (the blue line is the commercial detector, the green line is the point-of-care sensor and the magnified illustration in Fig. S10a†); (f) real-time monitoring of snoring (the blue line is the commercial detector, the red line is the point-of-care sensor and the magnified illustration in Fig. S10b†); (g) real-time monitoring of apnea in the process of snoring. |
A volunteer wore a headphone and breathed in two different models (snoring and normal). The interdigital electrode covered by the polymers is placed 1 cm away from her nostril outlet without the need for an invasion into her nostril. In the breath monitoring test, the 1,5-PDAC-based humidity sensor was integrated into the headphone under ambient conditions, with the RH of exhaled and inhaled gas ranging from 11% to 95%. The results showed that under such an RH range, the humidity sensor can synchronously respond and recover with breath regardless of different breath modes, indicating that the sensor has the ability to capture the rate of human breath and ensure the utility of the device (Fig. 5c). After converting the impedance to analogue voltage (0–4 V), the signal is transmitted to a computer or mobile phone for facile remote monitoring. Compared to normal breathing, the further increase in the output signal during snoring is due to the higher humidity during deep breathing, resulting in a lower impedance of the humidity sensor. Therefore, normal breathing and snoring can be distinguished (Fig. 5c). To explore its potential application, we further tested the circulatory stability of the above-mentioned breath sensors (Fig. 5d). The device can accurately distinguish snoring and normal breath modes, indicating the potential for respiratory health detection.
We compared the data with a commercial snoring detector (EEG-1200C from Nihon Kohden Corp) to demonstrate the sensitivity and commercial potential of our noninvasive and point-of-care sensor. To realize synchronous monitoring using the commercial detector and our sensors, we first reduced the sampling frequency to 1 Hz to allow long-term operation. The commercial detector was tested using invasive methods that fix the detector inside the nasal cavity. In contrast, our sensors used a noninvasive method for point-of-care snoring detection. A volunteer wore the commercial monitor and our noninvasive sensor at the same time (Fig. S11†) to synchronize the output of sensitivity and voltage signal of real-time breathing data. Initially, during the normal breathing period, our noninvasive sensor had the same sensitivity as the commercial detectors, and the signal fluctuated in line with breathing (Fig. 5e). As snoring occurs, slower breathing (Fig. S9a and b†) presents a plateau in the data of our sensor, comparable and synchronal to the plateau of the commercial detector (Fig. 5f). Furthermore, the disappearance of data fluctuations for more than 10 seconds, belonging to apnea, will cause an alarm remotely through 5G or Bluetooth transmission (Fig. 5g). With these functions, the sensor has great commercial potential for point-of-care detection of the snoring process and apnea.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sd00042g |
This journal is © The Royal Society of Chemistry 2023 |