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. 2021 Dec 22;15(1):47.
doi: 10.3390/ma15010047.

Flexible Ni/NiOx-Based Sensor for Human Breath Detection

Le Duc-Anh Ho  1 , Vu Binh Nam  1 , Daeho Lee  1
Affiliations

Flexible Ni/NiOx-Based Sensor for Human Breath Detection

Le Duc-Anh Ho et al. Materials (Basel). .

Abstract

We developed a simple methodology to fabricate an Ni/NiOx-based flexible breath sensor by a single-step laser digital patterning process of solution-processed NiOx thin-film deposited using NiOx nanoparticle ink. Laser-induced reductive sintering phenomenon enables for the generation of three parts of Ni electrodes and two narrow NiOx-sensing channels in between, defined on a single layer on a thin flexible polymer substrate. The Ni/NiOx-based breath sensor efficiently detects human breath at a relatively low operating temperature (50 °C) with fast response/recovery times (1.4 s/1.7 s) and excellent repeatability. The mechanism of the gas-sensing ability enhancement of the sensor was investigated by X-ray photoelectron spectroscopy analysis. Furthermore, by decoupling of the temperature effect from the breathing gas, the response of the sensor due to the temperature alone and due to the chemical components in the breathing gas could be separately evaluated. Finally, bending and cyclic bending tests (10,000 cycles) demonstrated the superior mechanical stability of the flexible breath sensor.

Keywords: Ni; NiOx; breath sensor; flexible sensor; laser digital patterning; laser-induced reductive sintering process; nanoparticles; nickel; nickel oxide.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript or in the decision to publish the results.

Figures

Figure 1
Figure 1
Transmittance electron microscopy (TEM) image (a), high-resolution TEM (HR-TEM) image (b), and selective-area electron diffraction (SAED) pattern (c) of the NiOx nanoparticles.
Figure 2
Figure 2
(a) Schematic of the fabrication procedures of the flexible Ni/NiOx-based breath sensor on polyimide (PI). (b) (i) Schematic drawing, (ii) optical microscopy image, and (iii) scanning electron microscopy (SEM) image of the Ni/NiOx-based breath sensor. (c) Chemical compositions of the as-synthesized NiOx thin film, reductive sintering Ni electrode, and the NiOx-sensing channel, analyzed by energy-dispersive X-ray spectrometry.
Figure 3
Figure 3
X-ray photoelectron spectroscopy (XPS) results from the as-synthesized NiOx thin film and the NiOx-sensing channel of the breath sensor for (a) Ni 2p3/2 and (b) O 1s.
Figure 4
Figure 4
(a) Schematic diagram of the experimental setup to test the breath sensing performance of the senor. (b) Electrical resistance variation of the Ni/NiOx-based breath sensor to the normal breathing rate at the operating temperature of 50 °C. (c) Response and recovery times of the breath sensor to the normal breath rate at 50 °C. (d) Response curves of the breath sensor to the different breathing rates at 50 °C. (e) Maximum responses of the breath sensor to the normal breath rate at different temperatures.
Figure 5
Figure 5
(a) Electrical resistance variation of the breath sensor corresponding to temperature change only. (b) Data fitting to determine the thermal sensitivity index (B-value) of the breath sensor. (c) Response curve of the breath sensor covered by a medical tape to the normal breath rate at 50 °C.
Figure 6
Figure 6
(a) Schematic illustration of the horizontal bending and vertical bending states of the sensor. (b) Electrical resistance change (R/Ro) of the breath sensor under various bending radii under each bending state. (c) Response curve of the Ni/NiOx-based breath sensor to the normal breathing rate at the operating temperature of 50 °C under a bending condition. (d) Relative resistance changes of the breath sensor during a cyclic bending test (10,000 cycles).
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