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Duan et al. Soft Sci. 2025, 5, 4 https://dx.doi.org/10.20517/ss.2024.46 Page 5 of 15

including antibacterial, antiviral, fully biocompatible and biodegradable characteristics [22,34,35] . The green
solvent glycerol was selected as the plasticizer for the flexible substrate, which contains a multitude of
hydroxyl groups in its molecular structure and is capable of reacting with the numerous carboxyl groups
present in the chitosan-acetic acid mixed solution. The glycerol generates ester bonds and forms strong
hydrogen bonding interactions with the polymer molecules in the solution, thus improving the softness,
ductility and toughness of the base film. Furthermore, the moisturizing properties of the chitosan substrate
are exploited to prolong its service life . Chitosan films offer several advantages over gels traditionally
[36]
based on polyacrylic acid and polyvinyl alcohol. These include a lower cost, a lower risk of allergic
inflammation such as itching and redness due to chitosan’s good biocompatibility, and a higher
[37]
breathability .

The fabrication process of the biocompatible electrode patch film for bladder monitoring is illustrated in
Figure 2A and the Supplementary Materials. The structural properties of chitosan, which comprises free
+
amino groups in its molecules, are exploited to convert its glucose amino groups to R-NH in dilute acid,
3
thereby forming a polycationic gel solution [Supplementary Figure 1] [38,39] . These positively charged chitosan
particles are susceptible to electrostatic adsorption on negatively charged biological skin, while the amino
groups inhibit bacteria by binding to the negative electrons of human skin [Figure 2B]. The chitosan gel was
electrochemically modified using glycerol, and the stress-strain curve (σ-ε curve) and open-circuit potential-
electrochemical impedance spectroscopy (OCP-EIS) were employed to further characterize the electrode
flexible substrate following modification with different volumes of glycerol [Figure 2C and D]. As the
concentration of glycerol increased, the peak stress in the σ-ε curve exhibited an upward trend, while the
resistance in the Nyquist plot exhibited a downward shape. This indicates that the tensile strength and
conductivity of the film were increasing. It is noteworthy that, in the σ-ε curve, a decrease in flexibility was
observed when the volume of glycerol exceeded 6%. This can be attributed to glycerol’s plasticizing effect,
whereby glycerol esters exhibit high resistance to fracture and can form a protective film to enhance the
tensile strength and flexibility of the material. When the glycerol concentration exceeds the optimal level, it
exerts a plasticizing effect, leading to a reduction in the toughness of the film. It is also important to control
the amount of glycerol added in the experiment, as too much or too little will affect the performance of the
membrane. Given that the biocompatible chitosan membrane in this paper is a flexible substrate that needs
to be worn for a long period of time and come into direct contact with the human body to conduct signals,
we chose to increase the volume of glycerol to 8%, in order to provide maximum tensile strength and
conductive ability for bladder status monitoring and analysis. To evaluate the compatibility of the chitosan-
acetic acid conductive film with human skin, we measured the surface energy of the film using a surface
contact angle goniometer. The contact angle of water on the film was found to be 87.1°, while that of
diiodomethane was 42.7° [Supplementary Figure 2A]. Using the Owens-Wendt theory, we calculated the
surface energy of the chitosan film to be 39.95 mJ/m². This surface energy value is close to the range of
surface energy for human skin (35-45 mJ/m²), indicating that the material possesses good biocompatibility,
making it suitable for applications in skin-contacting wearable devices [40,41] . Furthermore, we conducted skin
[42]
irritation tests to assess the changes in skin condition over time after wearing the conductive film
[Supplementary Figure 2B]. No significant redness or itching was observed on the skin following the
removal of the patch, providing clear evidence of the material’s excellent biocompatibility. The patches were
characterized for tensile strength, mechanical bending and adhesion [Supplementary Figure 3A-C].

Figure 2E presents a voltammetry characteristic curve of the chitosan-acetate conductive film at different
rates (5-30 mV/s) within the voltage range of -1.5-1.5 V. The results demonstrate that the peak value of the
voltammetry characteristic curve increases with the rate, indicating that the charge transfer within the film
is more active at higher rates [Supplementary Figure 4A]. This implies that the film has a superior fast