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Cui SY, Lu YY, Kong DP, Luo HY, Peng L et al. Laser direct writing of Ga2O3/liquid metal-based flexible humidity sensors. Opto-Electron Adv 6, 220172 (2023). doi: 10.29026/oea.2023.220172
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Article Open Access
Laser direct writing of Ga2O3/liquid metal-based flexible humidity sensors
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Abstract
Flexible and wearable humidity sensors play a vital role in daily point-of-care diagnosis and noncontact human-machine interactions. However, achieving a facile and high-speed fabrication approach to realizing flexible humidity sensors remains a challenge. In this work, a wearable capacitive-type Ga2O3/liquid metal-based humidity sensor is demonstrated by a one-step laser direct writing technique. Owing to the photothermal effect of laser, the Ga2O3-wrapped liquid metal particles can be selectively sintered and converted from insulative to conductive traces with a resistivity of 0.19 Ω·cm, while the untreated regions serve as active sensing layers in response to moisture changes. Under 95% relative humidity, the humidity sensor displays a highly stable performance along with rapid response and recover time. Utilizing these superior properties, the Ga2O3/liquid metal-based humidity sensor is able to monitor human respiration rate, as well as skin moisture of the palm under different physiological states for healthcare monitoring.-
Keywords:
- laser direct writing /
- liquid metal /
- humidity sensors /
- flexible electronics /
- wearable sensors
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References
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Laser direct writing of Ga2O3/liquid metal-based flexible humidity sensors 
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Cui SY, Lu YY, Kong DP, Luo HY, Peng L et al. Laser direct writing of Ga2O3/liquid metal-based flexible humidity sensors. Opto-Electron Adv 6, 220172 (2023). doi: 10.29026/oea.2023.220172
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Figure 1.
Design and fabrication of flexible capacitive humidity sensors. (a) Fabrication processes of flexible Ga2O3/LM humidity sensors, including ultrasonication, spraying coating and laser sintering. (b) Schematic of the mechanism to form GWLM films by laser sintering and the sensing mechanism of Ga2O3/LM-based humidity sensors.
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Figure 2.
Characterizations of flexible humidity sensors. SEM images of GWLM (a–d) with and (a, e) without laser sintering. (f) EDX images of the Ga, In, and O distributions. (g) Histogram of diameter size distribution for the unsintered GWLM particles on the PI film. (h) Resistivity of the laser induced conductive GWLM paths at different laser fluences. (i) The minimum resolution of sintered LM path at a laser fluence of 9.4 J/cm2. (j–l) Schematics of Ga2O3/LM-based humidity sensors with various fabrication parameters (i.e. widths and lengths of electrodes, UV laser fluence) (top). Cycle measurements of Ga2O3/LM-based humidity sensors by periodically varying the humidity from 30% RH to 95% RH (bottom).
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Figure 3.
(a) Photo of a flexible Ga2O3/LM-based humidity sensor. (b) Capacitance change of Ga2O3/LM-based humidity sensor at different RHs. The inset shows the image of this sensor. (c) A temperature dependent test of Ga2O3/LM-based humidity sensor via varying temperatures from 25 °C to 45 °C. The temperature in the oven was recorded by a commercial thermal sensor. (d, e) Long-term stability measurement (50 cycles) under 95% RH. (f) Cycle measurements of four different batches of humidity sensors by periodically varying the humidity from 30% RH to 95% RH.
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Figure 4.
(a) Photos of a humidity sensor on a commercial mask worn on the subject’s face. (b) Human respiration test of a subject by mouth at a rest state. (c) Response and recovery time of the sensor. (d–f) Real-time monitoring of respiratory rate by nose of a subject at a rest state. Real-time monitoring of palm moisture while (g) drinking hot water and (h) exercising.
