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Page 2 of 9 Ma et al. Soft Sci 2024;4:8 https://dx.doi.org/10.20517/ss.2023.41

[6-8]
diagnostics . Wearable sensors can be classified into physical and biochemical types based on target
physiological signals. Specifically, wearable physical sensors are designed to monitor biophysical signals
such as body motions, blood pressure, heart rate, or body temperature [9-13] , while biochemical sensors focus
on the analysis of biomarkers in body fluids to indicate the health state at the molecule level [14-17] . The
comprehensive evaluation of the body state for precious health management and diagnosis calls for next-
generation hybrid wearables capable of physical and biochemical sensing concurrently [18,19] . A
straightforward way to achieve this goal is integrating various sensors with different sensing mechanisms
into a single device [20-22] . However, this add-on design inevitably results in increased complexity in
fabrication, signal processing, and device integration.

Among various biofluids, sweat may be the best choice for wearable sensing to indicate the dynamic body
[23]
state at the molecular level . This is because sweat contains a wealth of biochemical information that is
composed of various metabolites (glucose, lactate), electrolytes (sodium, chloride, potassium), proteins,
[24]
hormones, and peptides . Moreover, compared to other biofluids such as tear and saliva, sweat can be
easily accessed in a non-invasive and on-demand manner and readily analyzed using skin-interfaced
+
electronic devices. Among various sweat electrolyte ions, the level of sodium ions (Na ) can reflect the
body’s electrolyte balance, osmolality regulation, and acid-base equilibrium, which is also a biomarker for
the diagnosis of cystic fibrosis [25-27] . Among the biophysical signals, the pulse caused by the periodic flow of
blood can reflect the health state of the heart, and pulse monitoring has wide applications in the diagnosis of
cardiovascular diseases, physical fitness, and health management . Simultaneous detection of these two
[28]
basic yet important physiological signals may give a full picture of the health state of an individual.
However, this can only be done using two sensors with existing strategies.

Herein, we show that a single wearable sensing unit can detect sweat Na concentration and heart rate
+
simultaneously. This was achieved by introducing a hydrogel interface to a solid Na selective electrode. In
+
previous reports, the introduction of a hydrogel film between skin and electrochemical sensing electrodes
has proven to be an efficient way to continuously collect and analyze natural sweat [16,29] . Thus, the hydrogel
interface is quite attractive to eliminate the use of chemical, exercise, or thermal stimuli for sweat induction
[30]
and may offer unique insight into body physiology at the rest state . In this work, considering hydrogels
also have the piezoionic response to pressure [31-33] , we show that the wrist pulse-induced potential signal with
+
a periodic pattern can also be monitored during the sensing of sweat Na . Thus, the pulse and sweat Na
+
concentration can be easily analyzed from the single potential response. We believe this dual-mode sensor
has the potential to pave the way for the next generation of multifunctional wearable sensors for
synchronous biophysical and biochemical sensing.

EXPERIMENTAL
Chemicals and materials
Selectophore grade sodium ionophore X, 3,4-ethylene-dioxythiophene (EDOT), poly (sodium 4-styrene
sulfonate) (NaPSS), sodium tetrakis [3,5-bis(trifluoromethyl)phenyl] borate (Na-TFPB), polyvinyl chloride
(PVC), bis(2-ethylhexyl) sebacate (DOS), acrylamide (AM), N, N’-Methylenebisacrylamide (Bis), poly
(ethylene glycol) diacrylate (PEGDA), and (2-hydroxy-2-methylpropiophenone) (HMPP) were obtained
from Sigma-Aldrich. Polyvinyl butyral (PVB) and sodium chloride (NaCl) were obtained from Aladdin
(Shanghai, China). Tetrahydrofuran and methanol were obtained from Macklin (Shanghai, China). A CHI
660D electrochemical analyzer (CH Instruments) was used to carry out electrochemical measurements. A
commercial blood pressure and pulse meter (YE 660D, Yuyue Medical Co., Ltd., China) was used for the
pulse test.