Page 15 - Read Online

Ma et al. Soft Sci 2024;4:8 https://dx.doi.org/10.20517/ss.2023.41 Page 3 of 9

Preparation of the hydrogel
The p(AM-co-PEGDA) hydrogel was prepared by dissolving 10 wt% AM, 0.2 wt% Bis, 1 wt% PEGDA, and
1 wt% HMPP in deionized water. AM and HMPP are the monomer and photoinitiator, respectively. Bis and
PEGDA are the cross-linking agents. Then, the solution was injected into the glass cell with a spacer
thickness of 0.2 mm, followed by UV polymerization for 30 s [Supplementary Figure 1]. Under the UV
light, the photoinitiator (HMPP) absorbs the UV energy and forms free radicals to initiate the
polymerization reaction. After polymerization, the hydrogel was immersed in distilled water for seven days,
with water replaced every 12 h to remove unreacted monomers and photoinitiators. The hydrogel was cut
into the films (0.5 cm × 1 cm) for on-skin applications, and each hydrogel film initially contained ~20 μL
water or NaCl solution, which was obtained by weighting the hydrogel films before and after dehydration.

+
Preparation of the Na selective electrode
Graphite ink (LOCTITE EDAG423SS E&C) was screen-printed on the poly (ethylene glycol) terephthalate
(PET) film, followed by drying at 90 °C for 30 min. The reference electrodes were printed with silver/silver
chloride ink (LOCTITEEDAG 6037SS E&C) followed by drying at 80 °C for 30 min [Supplementary Figure
2] . Then, the poly(3,4-ethylene-dioxythiophene) PEDOT:PSS was deposited on the working electrodes as
[34]
an ion-electron transducer through the galvanostatic electrochemical polymerization in a solution
containing 100 mM NaPSS and 10 mM EDOT. A constant current of 35 μA was applied, and the depositing
time was 285 s. We prepared the Na selective membrane solution by dissolving 1.0 wt% Na ionophore X,
+
0.55 wt% Na-TFPB, 33 wt% PVC, and 65.45 wt% DOS in tetrahydrofuran and a PVB-based coating solution
by dissolving 79 mg PVB and 50 mg NaCl in 1.0 mL methanol. Finally, 2.0 μL of Na selective membrane
+
solution and 2.0 μL of PVB solution were dropped on the working electrodes and reference electrodes,
respectively, followed by overnight drying at room temperature [Supplementary Figure 3].

On-body test
To perform the sweat rate test, a piece of filter paper (Cytiva, 0.5 cm × 1 cm) was attached to the wrist to
absorb sweat for an hour. During this process, the paper was weighed every ten minutes. Before each test,
the sweat collection area at the wrist was cleaned with 75% alcohol and deionized water, followed by
removing the water using absorbent cotton. Then, the sensor was positioned on the wrist and covered by a
waterproof adhesive layer to prevent water evaporation. Before the on-body test, the hydrogel was
immersed in 1 mM NaCl solution. All experiments were conducted in strict compliance with the relevant
laws and with the approval of the Scientific Ethical Committee of Southeast University. Three healthy
volunteers were recruited to participate in the on-body tests.

RESULTS AND DISCUSSION
Overview of the dual-mode sensor
Existing strategies for wearable sweat Na detection include optical techniques, e.g., fluorescence or
+
colorimetric analysis [35,36] , and electrochemical methods using potentiometric ion selective electrodes (ISE)
[37]
[38]
+
or organic electrochemical transistors . Among these sensing methods, ISE-based Na sensors are
particularly attractive due to their simple electrode and circuit design, low cost, and ability for continuous
sensing .
[34]
Here, we designed the wearable dual-mode sensor with ion and pressure sensing functions by interfacing a
hydrogel film to a solid Na selective electrode [Supplementary Figure 4], which was attached to the wrist to
+
monitor the sweat Na and pulse simultaneously [Figure 1A]. In our design, the hydrogel film can be used
+
not only for sweat collection but also for pressure sensing by utilizing the piezoionic effect of the hydrogel.
The piezoionic electromechanical response of hydrogel is due to the difference in mobilities of cations and
anions in the hydrogel that generates a streaming potential when pressure is applied [Figure 1B].
[32]