Humidity Sensing of Stretchable and Transparent Hydrogel Films for Wireless Respiration Monitoring

Abstract

Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors. However, traditional humidity sensors suffer from a trade-off between deformability, sensitivity, and transparency, and thus the development of high-performance, stretchable, and low-cost humidity sensors is urgently needed as wearable electronics. Here, ultrasensitive, highly deformable, and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels. Concomitantly, a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area, which can be applied to different polymer networks and facilitate the development of flexible integrated electronics. In addition, sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network, exhibiting excellent water adsorption capacity. Through the synergistic optimization of structure and composition, the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH, which is unprecedented. Moreover, the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity. As a proof of concept, we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission, enabling real-time monitoring of human health status. This work provides a general strategy to construct high-performance, stretchable, and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

Keywords: Hydrogel film; Respiration monitoring; Stretchable and transparent humidity sensors; Ultrasensitive; Wireless and wearable sensor.

Fig. 1

a Schematic and photographs show the excellent performance of the stretchable hydrogel film humidity sensor and its practical wearable application for wireless respiration monitoring. b Scheme illustrating the preparation process of hydrogel film with the polydimethylsiloxane (PDMS) film substrate. c Time-dependent responses of the bulk and film hydrogel sensors to different RH. The green and white areas indicate that the sensor is in high and low (11%) RH environments, respectively. d The relationship between RH and the responses of bulk and film sensors

Fig. 2

a Photographs of the morphology evolutions of the PAM/Carrageenan DN hydrogels that were not soaked and soaked in 25, 50, and 100% glycerol solutions, respectively, after being stored in a dry environment of 25 °C and 43% RH for 48 h. b Time evolution of the weight loss percentage of the hydrogels in a when stored at 25 °C and 43%RH. c DSC curves of hydrogels that were not soaked in glycerol and soaked in 25, 50, 75, and 100% glycerol solutions, respectively. d Contents of freezing and nonfreezing water in hydrogels soaked in glycerol solutions with different concentrations

Fig. 3

a-c The complex impedance spectra of the humidity sensor under 11, 33, and 85% RH environment. d Piecewise fitting of response versus RH within 11–98% RH

Fig. 4

a Cross-sectional Optical microscope images showing the PDMS substrates and hydrogel films with different thicknesses obtained using different spin-coating speeds. b The correlation between spin coating speed and the thickness of hydrogel films. c Transmittance of film hydrogels with different thickness in the visible wavelength range. d Time-dependent responses of the humidity sensors with different film thickness upon exposure to different RH. e Piecewise fitting of the response of sensors with different film thicknesses versus RH. f The dependences of sensitivities (S₁ under 11–37% RH and S₂ under 37–98% RH) on the thickness of the film. g Time-dependent and repeated response of the sensor to 33, 59, and 98% RH in four experimental cycles, respectively. h Quantitative responses extracted from g versus experimental cycle for different RH. i Hysteresis curve of the sensor

Fig. 5

a Photographs show that the hydrogel film withstands 97% tensile strain. b Dynamic response curves to different RH at 0, 10, 15, 20, and 25% tensile strains. c Piecewise fitting of the response of sensors versus RH at different strains. d The relationship between sensitivity and tensile strain. e Dynamic response curves to different RH at 15, 25, 32, and 41 °C. f The relationship between sensitivity and temperature. g Compound plots of responses of the sensor toward strain and RH. h Compound plots of responses of the sensor toward temperature and RH

Fig. 6

a Time-dependent responses (capacitance) and b piecewise fitting of the response of PAM/Tapioca DN hydrogel bulk and film sensors. c Sensitivity comparison of the two hydrogel sensors. d Dynamic response (capacitance) curves, e piecewise fitting of the response, and f sensitivity comparison of hydrogel film sensors soaked in 1, 2, and 3 mol L⁻¹ LiBr solutions. g Dynamic response (conductance) curves and h sensitivity comparison of hydrogel film sensor at 0, 10, 15, 20, and 25% tensile strains. i The radar chart comparing the performance of this humidity sensor with that of others

Fig. 7

a Photograph showing the hydrogel film humidity sensor is integrated in the mask for real-time human respiration monitoring. b Dynamic response curves of the smart mask to fast, normal and deep breathing. c Schematic illustrating a wireless transmission system used to real-time monitor human breathing status and display the data on a mobile phone APP. d Schematic diagram of the principle of wireless circuit with Bluetooth transmission module and alarm. e₁-e₂ The APP displays "NORMAL" and "ALARM," respectively, during normal breathing and apnea