Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review

Abstract

Respiratory monitoring is a fundamental method to understand the physiological and psychological relationships between respiration and the human body. In this review, we overview recent developments on ultrafast humidity sensors with functional nanomaterials for monitoring human respiration. Key advances in design and materials have resulted in humidity sensors with response and recovery times reaching 8 ms. In addition, these sensors are particularly beneficial for respiratory monitoring by being portable and noninvasive. We systematically classify the reported sensors according to four types of output signals: impedance, light, frequency, and voltage. Design strategies for preparing ultrafast humidity sensors using nanomaterials are discussed with regard to physical parameters such as the nanomaterial film thickness, porosity, and hydrophilicity. We also summarize other applications that require ultrafast humidity sensors for physiological studies. This review provides key guidelines and directions for preparing and applying such sensors in practical applications.

Keywords: fast response; humidity sensor; nanomaterial; physiological monitoring; respiratory monitoring.

Figure 1

(a) Indicators of respiratory system. (b) General features in typical respiratory signals from sensors. t_i: time of inspiration, t_e: time of expiration, t_p: pause time, Pⁱ_O2,CO2: partial pressure of O₂/CO₂ in inspiratory air (ambient air), P^e_O2,CO2: partial pressure of O₂/CO₂ in respiratory air. (c) Example of correlations between humidity and irregular breathing patterns.

Figure 2

(a) Schematic illustration of respiratory monitoring by humidity-sensitive nanomaterials. Nanomaterials on the substrate are directly exposed to breath air with water molecules. (b) Microscopic structure of adsorbed water molecules on the nanomaterial surface. (c) Definition of response and recovery time as t₉₀ in the ideal response of humidity sensors.

Figure 3

Recent trends of the number of publications reporting humidity sensors for respiratory monitoring (filled circles). The number of publications relating to humidity sensors is also shown as open circles. The data were surveyed on 7 January 2022 in Web of Science using the filter: (“humidity sensor”) AND (“breath” OR “respiration”).

Figure 4

Four working principles of fast-response humidity sensors classified by physical origins of sensing.

Figure 5

(a) Typical structure of an impedance-type humidity sensor. A humidity-sensitive nanomaterial film is coated over interdigitated electrodes. (b) Photo of a humidity sensor using a coated nanoparticle film and 20 μm line/spacing interdigitated electrodes on a thermally oxidized silicon substrate. (c) Equivalent electrical circuit of an impedance-type humidity sensor. v: voltage, i: current, Z: whole impedance (v/i), R_n: resistance of nanomaterial film, C_n: capacitance of nanomaterial film, and R_w: surface resistance of adsorbed water layers on nanomaterial surface. Z corresponds to a parallel connection of Z_n and R_w.

Figure 6

Physical origins of impedance modulation. (a) Proton ion conduction via water molecular chains. (b) Carrier modulation of semiconductor nanomaterials by water molecules. (c) Porous media filled by water molecules changing whole permittivity.

Figure 7

(a–f) Pioneering works to monitor breath air in real time by using fast-response impedance-type humidity sensors. (a) Photo of humidity sensor using a spray-coated graphene oxide (GO) film and silver electrodes on a polyethylene naphthalate substrate. (b) Normalized response of humidity sensors dependent on the thickness of graphene oxides. The response of a commercial humidity sensor is shown as a reference. (c) Detection of breath air from speaking and breathing by using an ultrathin graphene oxide humidity sensor. Courtesy: Adapted with permission from ref. [54]. Copyright 2003 American Chemical Society. (d) Atomic force microscope image of a few nanofibers over electrodes. (e) Response of humidity sensor using a few nanofibers. (f) Respiratory monitoring by using a nanofiber humidity sensor in real time. The notations, e and i, refer to the exhaling and inhaling of the subject, respectively. Courtesy: Adapted with permission from ref. [64]. Copyright 2014 Springer Nature.

Figure 8

(a) Schematic of insulating nanoparticle film-based humidity sensor. (b) Respiratory monitoring by using the insulating nanoparticle-based sensor. Courtesy: Adapted with permission from ref. [70]. Copyright 2017 American Chemical Society. (c) A headset-type device using a nanoparticle film-based sensor. (d) Respiratory patterns observed by the monitor. The respiratory rate is intentionally controlled in this case. Courtesy: Adapted with permission from ref. [72] Copyright 2018 IEEE. (e) A nanoparticle film-based sensor attached on a gas mask of conventional respiratory monitoring system. (f) Change in respiratory rate during physical activity. The subject is at rest and running at 3, 6, and 8 km/h on a speed-controlled treadmill successively. Courtesy: Adapted with permission from ref. [73]. Copyright 2019 IEEE.

Figure 9

Demonstration of respiratory monitoring using optical fiber humidity sensors coated with nanomaterials. (a) Schematic of experimental setup for humidity measurement with an enlarged illustration of an MoS₂-coated etched single-mode fiber (ESMF). The humidity responses from human breathing of the MoS₂-coated ESMF sensor and a comparison to a bare ESMF sensor are shown in the middle and right panels, respectively [86]. Courtesy: Adapted with permission from ref. [86]. Copyright 2017 Elsevier. (b) Schematic of a large-angle tilted fiber grating (TFG) with graphene oxide deposited on the fiber surface. The bottom panel shows the humidity responses from normal (Test 1) and rapid (Test 2) breathing measurements [87]. Courtesy: Adapted with permission from ref. [87]. Copyright 2019 Elsevier.

Figure 10

Schematic illustrating humidity-responsive structural colors from coherent (a–c) and incoherent (d) light interference. (a) A 3D photonic crystal composed of an inverse opal photonic gel in hydrophilic ionic liquid that swells and shrinks in response to humidity [92]. Courtesy: Adapted with permission from ref. [92]. Copyright 2018 MDPI. (b) Mesoporous 1D photonic crystal or Bragg stack, composed of alternating layers of nanogels and TiO₂ nanoparticles [93]. Courtesy: Adapted with permission from ref. [93]. Copyright 2018 American Chemical Society. (c) A thin-film assembly of konjac glucomannan (KGM)–metal–substrate humidity sensor and KGM thickness changes as a result of humidity-induced swelling and shrinking [100]. Courtesy: Adapted with permission from ref. [100]. Copyright 2020 American Chemical Society. (d) Disordered assembly of mesoporous titania microspheres showing changes in scattered colors in response to humidity. The insets show models of the porous network inside the titania microsphere under dry (top) and humid (bottom) conditions [110]. Courtesy: Adapted with permission from ref. [110]. Copyright 2021 Royal Society of Chemistry.

Figure 11

Colorimetric respiratory monitoring using structural color-based humidity sensors. (a) Reflectance spectra (left) and wavelength peak positions (right) of a P(AM-MBA)/TiO₂ sensor under exposure to human exhalation. The inset on top shows photographs of the sensor taken at 0.5 s intervals [93]. Courtesy: Adapted with permission from ref. [93]. Copyright 2018 American Chemical Society. (b) Photographs showing colorimetric responses of the KGM humidity sensor at 65–100%RH [100]. Courtesy: Adapted with permission from ref. [100]. Copyright 2020 American Chemical Society.

Figure 12

Physical origin of frequency-based humidity sensor using QCM. (a) Before and (b) after the adsorption of water molecules on QCM. The resonant frequency of the mechanical oscillating substrate is shifted by the adsorption. The advantage of this method is to evaluate the mass of adsorbed water molecules directly.

Figure 13

(a) Scanning electron microscope images of a surface-acoustic wave resonator. (b) Humidity dependence of resonant frequency shift. Inset: Electrospun polyaniline and polyvinyl butyral composite nanofibers. (c) Response and recovery time of the SAW sensor between 11 and 98%RH. Courtesy: Adapted with permission from ref. [116]. Copyright 2012 Elsevier.

Figure 14

Typical mechanism of voltage generation from ambient humidity. A porous nanowire network charged in negative separate charged ions. Thus, a charge imbalance is formed and a potential difference is generated by humidity.

Figure 15

Benefits in each sensing mechanism.

Figure 16

(a) Response (τ_res) and recovery time (τ_rec) of reported fast-response humidity sensors used for respiratory monitoring. A list of the literature is shown in Table S1 of Supplementary Materials. Rn, Cn: resistance and capacitance of nanomaterial film, respectively. Ri: resistance of adsorbed water layer. (b) Design strategy of a structure of nanomaterial films for fast-response humidity sensors.

Figure 17

Conventional methods to monitor respiration activity.

Figure 18

(a) Recognition of verbal commands by moisture patterns of exhaled breath air. Courtesy: Adapted with permission from ref. [111]. Copyright 2021 American Chemical Society. (b) Response with a bare hand vertically approaching and retracting away from the surface. A hand with a glove does not affect the current intensity. Courtesy: Adapted with permission from ref. [70] Copyright 2017 American Chemical Society. (c) A flexible humidity sensor applied to human skin to test skin moisture. (d) Variation in facial water contents under different conditions of the subject: waking up, drinking, washing face, and moisturizing face by moisturizer. Courtesy: Adapted with permission from ref. [170]. Copyright 2017 Wiley.