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Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 27 of 35

Figure 9. Biochemical sensors. (A) Schematic of the PANI-based hierarchically designed nanocomposite (left) and current level
according to the concentration of glucose (right). Reproduced with permission from ref [196] . Copyright 2021, Elsevier Ltd; (B) Schematic
of the Ni-Co MOF nanosheet coated rGO/PU fiber (left) and the glucose sensor integrated into the elastic fabric (middle). Measured
glucose content in sweat by the wearable sensor for one day (right). Reproduced with permission from ref [197] . Copyright 2021, American
Chemical Society; (C) Image and mechanism of the CMP-based biochemical sensor (left). Current level according to the concentration
of lactate (right). Reproduced with permission from ref [154] . Copyright 2022, Wiley-VCH GmbH; (D) Schematic illustration and SEM
image of the Au/SWCNT/AuNPs composite (left). Voltage level according to the concentration of sodium before and after 500 bending
cycles (right). Reproduced with permission from ref [199] . Copyright 2021, Elsevier B.V. AuNPs: Gold nanoparticles; CE: counter electrode;
CMP: Pt-decorated CNT; CNT: carbon nanotube; MOF: metal-organic framework; PANI: polyaniline; PU: polyurethane; PVA: poly(vinyl
alcohol); RE: reference electrode; rGO: reduced graphene oxide; SWCNT: multi-walled carbon nanotube; TEGO: thermally-exfoliated
GO; WE: working electrode.

Sodium ions are relevant to the hydration status and can be utilized as a biomarker for diagnosing stroke
and kidney issues . Lim et al. fabricated a sodium sensor using Au/SWCNT/AuNPs nanocomposites as
[198]
ion-selective electrodes . The Au pad at the bottom aided in material integration without delamination,
[199]
while the AuNPs at the top contributed to a higher capacitance and larger surface area, resulting in highly
sensitive and stable sodium concentration measurements [Figure 9D, left]. By integrating the Au/SWCNT/
AuNPs electrode with a thin-film circuit, a wireless, flexible, and real-time on-skin sodium sensor could be
developed [Figure 9D, right]. The entire sensor demonstrated a sensitivity of 55.5 mV decade and high
-1
mechanical stability, with stable performance after 500 bending cycles.

CONCLUSION AND OUTLOOK
Numerous studies over the past decade have focused on developing soft conductive nanocomposites for
wearable biosignal recording devices. These materials have much lower Young’s moduli than conventional
rigid electronic materials, making them ideal for wearable sensors that conform to the shape of the human
body. By reducing mechanical mismatch, soft nanocomposites help mitigate side effects that frequently
occur with rigid wearable electronics when attached to the soft curvilinear human body.

Material characteristics of nanocomposites vary significantly depending on the types of nanofiller
incorporated, such as carbon-based nanomaterials, CPs, metal-based nanomaterials, and LMs [200-202] . Each
filler has been successfully incorporated into a soft matrix and further optimized for use as soft wearable
sensors, including electrophysiological, strain, pressure, and biochemical sensors. However, it is still
challenging to achieve both high conductivity and softness simultaneously because excessive nanofiller
loading leads to degradation of the soft mechanical property. Moreover, the intrinsic viscoelasticity of
polymeric matrices causes electrical and mechanical hysteresis, making them susceptible to long-term
repeated use.