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Ma et al. Soft Sci 2024;4:8 https://dx.doi.org/10.20517/ss.2023.41 Page 7 of 9

Figure 4. On-body test. The red curves indicate the detection of the pulse. The purple cures indicate simultaneous detection of the
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sweat Na and pulse.

On-body real-time monitoring of sweat Na and pulse
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The dual mode sensor was used to monitor sweat Na and pulse simultaneously by attaching it to the wrist
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of a female volunteer aged 25 [Figure 4]. Before the test, a waterproof film was added between the skin and
hydrogel to prevent the hydrogel from absorbing sweat; thus, the potential baseline remained stable, and the
periodic fluctuation was caused by the pulse. Upon removing the waterproof film, the potential baseline
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gradually increased with time as the hydrogel collected the Na from sweat. After five minutes, the
waterproof film was added again. As a result, the potential baseline stabilized to -43 mV while the pulse
potential signal remained. The rising amplitude of the potential baseline from -47 to -43 mV meant that the
Na concentration increased from 1 to 1.10 mM, and the sweat Na concentration was 6.99 mM. In addition,
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the pulse information of 89 beats/min can be read out from the number of peaks. Both the Na
concentration and pulse are in agreement with the results measured separately, demonstrating the
practicability of this wearable sensor for the simultaneous detection of the physical and biochemical signals.
We also demonstrated the on-body tests on the other two volunteers, and the concurrent pulse monitoring
and sweat Na sensing can be achieved for each volunteer [Supplementary Figure 16].
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CONCLUSIONS
In summary, we developed a dual-mode sensor capable of the simultaneous detection of both physical and
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biochemical signals by integrating a hydrogel with a Na selective electrode. The solid-state Na selective
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electrode was endowed with pressure-sensing capability based on the piezoionic response. In addition, the
hydrogel also works as a flexible skin interface for sampling sweat. Thus, we can simultaneously detect the
mechanical pressure and Na concentration using a single sensor. We also demonstrated its application as a
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wearable device for the simultaneous detection of sweat Na and pulse. We believe that this integrated dual-
mode sensor will pave the avenue for the next-generation multifunctional wearable sensors for applications
in healthcare.

DECLARATIONS
Authors’ contributions
Conceptualization, methodology, and writing-original draft: Ma B
Methodology and data curation: Huang K
Writing-original draft: Chen G
Investigation: Tian Y, Jiang N