# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 3 Device Applications
### Applications for Moisture-Induced Electricity  
For moisture-induced electricity, due to its instantaneous pulse nature, researchers initially focused on its application in sensing technologies. For example, a 2017 study presents a self-powered, wearable graphene fiber power generator based on moisture-induced electricity [65]. The device is made of graphene oxide fibers, which are highly oriented to facilitate ion transport based on the ion concentration gradient mechanism, enabling graphene oxide functional group hydrolyzed from environmental moisture (Figure 10a). Key performance parameters include an open-circuit voltage output of 355 mV and a short-circuit current density of 1.06 mA/cm\({}^{2}\) under 65% relative humidity. This output can be further enhanced by connecting multiple fiber units. Four units can generate a voltage of 1.32 V. Furthermore, the device can be integrated into textiles, allowing for information storage and expression based on moisture variation, highlighting its potential for wearable electronics.  
In 2018, another study demonstrated a self-powered wearable electronic device based on moisture-induced electricity. This device leverages the strong hydrophilic properties of titanium dioxide (TiO\({}_{2}\)) nanowire networks, which are able to capture moisture from the surrounding environment or even human breath [66]. Based on the streaming potential, this captured moisture is then converted into electricity, producing an open-circuit voltage of up to 0.5 V and a power density of approximately 4 \(\upmu\)W/cm\({}^{2}\) (Figure 10b). The TiO\({}_{2}\) nanowire networks not only provide flexibility but also maintain robust performance after  
Figure 10: Applications for moisture-induced electricity. **(a)** Wearable device. Reproduced from ref. [65] with permission from Elsevier, copyright 2017. **(b)** Moisture-induced electricity based on TiO\({}_{2}\) nanowire network. **(c)** Working as a breath sensor. **(d)** Working as a touch sensor. Reproduced from ref. [66] with permission from John Wiley and Sons, copyright 2018. **(e)** PSSA is used to obtain moisture-induced electricity. **(f)** Continuous voltage output characteristics. Reproduced from ref. [44] with permission from Royal Society of Chemistry, copyright 2019.  
numerous bending cycles, making the device suitable for wearable applications. It can serve as the core component for a self-powered breathing monitor or touch sensor (Figure 10c,d), eliminating the need for external power sources and minimizing maintenance.  
Because of the instantaneous pulse nature of moisture-induced electricity, its application as a power source is rare. However, in 2019, a study introduced a new way to utilize moisture-induced electricity as a power source by providing sustained output [44]. This work introduces a polymer moist-electric generator (PMEG) using a polyelectrolyte membrane of poly(4-styrenesulfonic acid) (PSSA) to generate electricity from ambient environmental moisture (Figure 10e). The device produces an open-circuit voltage of approximately 0.8 V and a short-circuit current density of around 0.1 mA/cm\({}^{2}\) with a 1 cm\({}^{2}\) PSSA membrane. Remarkably, this output can be sustained for over 4 h without significant decay (Figure 10f). Hence, the PMEG can be employed to harvest energy from environmental moisture sources, such as human breath, and serve as a power source for flexible electronics. The study also highlights the movement of protons following the ion concentration gradient within the PSSA membrane for electricity generation, providing a promising future for moisture as a microscale power source.  
While moisture-induced electricity has traditionally been limited to sensing technologies due to its instantaneous pulse nature, recent advancements have greatly enhanced its potential as a power source. With sustained electricity output, new opportunities are expected in this field for flexible and wearable electronics.