A Paper‐Based Wearable Moist–Electric Generator for Sustained High‐Efficiency Power Output and Enhanced Moisture Capture

Abstract

Disposable wearable electronics are valuable for diagnostic and healthcare purposes, reducing maintenance needs and enabling broad accessibility. However, integrating a reliable power supply is crucial for their advancement, but conventional power sources present significant challenges. To address that issue, a novel paper‐based moist–electric generator is developed that harnesses ambient moisture for power generation. The device features gradients for functional groups and moisture adsorption and architecture of nanostructures within a disposable paper substrate. The nanoporous, gradient‐formed spore‐based biofilm and asymmetric electrode deposition enable sustained high‐efficiency power output. A Janus hydrophobic–hydrophilic paper layer enhances moisture harvesting, ensuring effective operation even in low‐humidity environments. This research reveals that the water adsorption gradient is crucial for performance under high humidity, whereas the functional group gradient is dominant under low humidity. The device delivers consistent performance across diverse conditions and flexibly conforms to various surfaces, making it ideal for wearable applications. Its eco‐friendly, cost‐effective, and disposable nature makes it a viable solution for widespread use with minimal environmental effects. This innovative approach overcomes the limitations of traditional power sources for wearable electronics, offering a sustainable solution for future disposable wearables. It significantly enhances personalized medicine through improved health monitoring and diagnostics.

An innovative paper‐based moist–electric generator is designed to power disposable wearable electronics by capturing ambient moisture. This eco‐friendly, cost‐effective device, featuring a nanoporous biofilm and Janus paper layer, delivers sustained high‐efficiency output across various humidity levels. Their breakthrough offers a sustainable power solution, driving advancements in personalized health monitoring and diagnostics within wearable technology.

1. Introduction

The emergence of wearable electronics has revolutionized personalized medicine by enabling continuous, real‐time health monitoring and diagnostics.^{[ 1} , ^{2 ]} Those devices, which can be long‐term, persistent wearables or short‐term, disposable wearables, offer innovative solutions for painless and non‐invasive health management.^{[ 2} , ³ , ⁴ , ^{5 ]} Disposable applications have gained significant attention because of their unique advantages, such as simplicity, ease of use, and cost‐effectiveness.^{[ 6 ]} Short‐term wearables are particularly valuable for diagnostic and therapeutic purposes where frequent updates and quick obsolescence are expected. They reduce the need for maintenance and enable broad accessibility. A vital consideration in the development of wearable electronics, especially disposable ones, is the integration of a power supply that supports eco‐friendly, cost‐effective, and sustainable operation. For wearables to function independently, they must incorporate efficient and reliable power sources.^{[ 7 ]} Traditional power sources, including primary and secondary batteries and supercapacitors, have been used in wearables.^{[ 8} , ^{9 ]} However, they present several limitations: they are often bulky, environmentally unfriendly, costly, and require frequent recharging or replacement. Most importantly, using those devices for one‐time or short‐term missions can be wasteful. Emerging energy harvesting techniques, including mechanical, solar, thermal, and biological methods, have been explored.^{[ 10 ]} However, those methods are often restricted to specific body locations and constrained by complex device configurations and non‐eco‐friendly materials, all of which hinder their practicality and sustainability.

Recently, electricity generation from ambient moisture has emerged as a promising alternative for wearable power sources.^{[ 11} , ^{12 ]} This approach leverages the ubiquity of moisture, providing a sustainable and straightforward mechanism for energy generation. The simple device configuration and the potential for eco‐friendly materials make moisture–electric generators particularly suitable for disposable wearable applications. The basic mechanism of electricity generation in moist–electric generators is based on the facile formation and disruption of hydrogen bonds in water molecules when they interact with surface functional groups.^{[ 13} , ¹⁴ , ^{15 ]} This interaction generates numerous positive and negative ions. Surface functional groups become stationary and negatively charged, while positive proton ions are released into the liquid water, becoming mobile. When a concentration gradient of these positive ions is established across the reaction zone, a potential difference is induced between two electrodes, allowing a current to flow as ions move from areas of higher concentration to lower concentration. Therefore, the key requirements for developing efficient moist–electric generators include: i) effectively capturing ambient moisture, ii) generating sufficient ions, and iii) creating and maintaining ionic gradients. Previous efforts have increased the surface area to enhance interaction with gaseous water and provide more active functional groups to generate more ions.^{[ 16} , ¹⁷ , ^{18 ]} However, creating a gradient of these dissociated ions remains a significant challenge. Establishing a gradient of functional groups through various surface modification techniques, such as electrochemical polarization, laser irradiation, and plasma treatment, can capture moisture differentially and lead to gradient ion diffusion, thereby inducing a built‐in potential gradient (Figure 1a).^{[ 19} , ^{20 ]} However, these techniques require precise control and involve complex fabrication processes. Alternatively, a gradient in moisture uptake can be established with homogeneous functional groups by using an asymmetric device configuration, where the moisture contact is intentionally directed from one side (Figure 1b).^{[ 13} , ¹⁴ , ^{21 ]} While this method can simply form a moisture gradient, electricity generation ceases when the material becomes saturated with moisture, which requires the moisture to evaporate before the cycle restarts. Recent research has discovered that materials with functional groups and nanochannels can form an electric double layer at the interface between the material surface and a liquid (Figure 1c).^{[ 21} , ^{22 ]} This interface generates electrostatic attraction, allowing many oppositely charged ions to accumulate within the Debye length.^{[ 21 ]} When the Debye lengths overlap, ions become very mobile, creating a streaming potential driven by capillary forces within the uniformly structured 3D nanochannels. Alternatively, nanoporous materials combined with a moisture gradient configuration enable dynamic adsorption and desorption at the porous interface, leading to surface charging, which can be analyzed using a leaky capacitor model.^{[ 22 ]} These nanomaterial‐based approaches simplify the formation of ionic gradients and offer a more efficient mechanism for sustained electricity generation. However, forming nanosized structures presents a technical challenge in device fabrication, requiring advanced techniques and precise control to achieve the desired structure.

Figure 1.

Mechanisms and device configuration of moist–electric generators. There are three types of conventional moist–electric generators. They a) use a gradient of functional groups, b) employ a gradient of moisture uptake, and c) exploit the solid‐liquid interaction in nanoporous structures. d) Our proposed moist–electric generator features an innovative gradient of functional groups and moisture uptake, driven by an in situ formed nanoporous spore biofilm and integrates a hydrophobic–hydrophilic Janus paper for effective water harvesting. The conceptual illustration showcases the generator's practical application as a wearable power.

Despite significant recent advancements in moist–electric generators, existing approaches have not been tailored for disposable wearable applications. Additionally, these generators typically require high relative humidity to function effectively, with performance dropping significantly in low humidity, limiting their practical use in wearable applications. Our study addresses these gaps by introducing a revolutionary simple, eco‐friendly, low‐cost, and disposable design that achieves high‐performance moisture capture and power efficiency independent of humidity conditions. We present an innovative moist–electric generator created on naturally porous, biodegradable paper material rich in functional groups (Figure 1d). The device was fabricated using a well‐established wax‐printing method developed in our previous research.^{[ 7 ]} Performance is innovatively enhanced by incorporating bacterial endospores through the natural capillary action of the paper. These endospores possess abundant biological functional groups and exhibit excellent water uptake capability, increasing their diameter by up to 12%.^{[ 23 ]} This process results in a vertical gradient of hygroscopic and functional groups within the paper. Furthermore, in situ biofilm formation of endospores within the paper substrate allows simple, low‐cost fabrication of the nanoporous structure. Additionally, the moisture gradient is established by sealing the bottom of the paper while leaving the top open. Overall, our device enables the formation of gradients in functional groups and moisture uptake, while the nanoporous structure within the biofilm generates additional streaming potential or surface charging. Finally, a hydrophobic–hydrophilic directional‐wicking Janus paper layer is added on top of the generator to enhance air moisture harvesting capability, ensuring efficient operation even under low humidity conditions. Water effectively condenses on the hydrophobic side of the paper, and the collected droplets are unidirectionally transported to the moist–electric generator below. The synergy between the biofilm and the Janus paper layer represents a novel approach to maintaining power generation across varying humidity conditions, particularly in wearable electronics that demand reliable and sustainable power sources.

2. Results and Discussion

In recent research, paper has emerged as a very promising substrate for disposable wearable applications because of its inherent flexibility, disposability, biocompatibility, eco‐friendliness, and breathability.^{[ 24} , ^{25 ]} Despite its lack of stretchability, which may cause fatigue and discomfort during prolonged use, paper is well‐suited for short‐term applications and can be effectively miniaturized and integrated into devices, addressing those concerns. Moreover, the efficient liquid handling capabilities of paper and the straightforward fabrication of devices using printing techniques present significant opportunities for wearable devices focused on sweat collection and analysis, offering simplicity and cost‐effectiveness.^{[ 26 ]}

Recent advancements have used paper's rich surface chemistry with various functional groups and its microporous structure to develop simple and low‐cost moist–electric generators. Gao et al. achieved a voltage of 0.25 V from untreated printed paper,^{[ 27 ]} while Zhang et al. demonstrated 0.33 V generation from a paper‐based device through water evaporation.^{[ 28 ]} Despite their advantages of being cost‐effective, disposable, and easy to fabricate and operate, paper‐based moist–electric generators suffer from low electrical performance, necessitating substantial surface treatment or functionalization. Challenges persist in establishing functional group gradients within a paper, limiting its potential practical use. Current power generation mechanisms primarily rely on asymmetric device configurations, achieved by sealing one electrode to create a gradient in water adsorption.

Here, we present an innovative paper‐based moist–electric generator that integrates a gradient of functional groups and a nanoporous structure within the paper substrate. This innovation leverages in situ spore biofilm formation within the paper (Figure 2a). By sealing the bottom electrode while leaving the top electrode open (Figure 2b), we combine all three mechanisms of moist–electric power generation to enhance performance (Figure 1). These processes can be easily conducted in situ within the paper, ensuring cost‐effectiveness, scalability, and mass‐production feasibility.

Figure 2.

In situ biofilm formation and sporulation within paper. a) Process of gradient formation of spore biofilm within paper. b) Asymmetric electrode deposition to enable a gradient of moisture adsorption. c) Bacterial growth curve according to initial bacterial concentration and nutrient supply. d) Quantification of bacterial biofilm formation over time using bicinchoninic acid (BCA) assay kit. e) SEM images of bacteria after 0, 2, and 4 h of inoculation within paper. f) Fluorescent microscopic images of harvested cells after 4, 12, 18, and 24 h of sporulation. Vegetative cells appear bright green, while spores fluoresce deep red.

2.1. Formation of Functional Groups, Moisture Adsorption, and Nanoporous Structures Within Paper

Recently, bacterial biofilms and bacterial nanowires have emerged as innovative hygroscopic materials for moist–electric generators due to their rich functional groups and nano‐sized structures.^{[ 29} , ³⁰ , ^{31 ]} The narrow nanopathways formed by these biological entities enable continuous moisture adsorption and electricity production based on streaming potential and surface charging.^{[ 22} , ^{29 ]} However, working with live materials is labor‐intensive, expensive, and time‐consuming, while controlling their critical parameters remains a challenge. Furthermore, no study has demonstrated these materials within paper or other disposable substrates suitable for specific short‐term wearable applications. The power‐generating mechanisms have been limited to nanostructure‐based approaches due to technical difficulties in gradient deposition of biological materials.

Here, we propose a method to create a gradient of spore‐forming bacterial cells using paper's capillary force. By converting live bacterial cultures into metabolically dormant spores, we innovatively create a device for practical use and long‐term storage. The detailed fabrication process is as follows: i) Substrate Preparation: We selected Grade 1 Whatman filter paper as our substrate. Hydrophobic and hydrophilic regions were defined using a commercial wax printer, followed by heat treatment.^{[ 32 ]} ii) Introduction of Bacteria: An aquatic culture of spore‐forming Bacillus subtilis was introduced into the patterned hydrophilic region (Figure 2a). iii) Biofilm Formation: Capillary action drove the culture to permeate the paper, creating a vertical gradient of bacterial concentration from the top, where the culture was introduced. The average pore size of the filter paper (≈10 µm) allowed bacterial cells to penetrate the structure, where they interacted within the 3D matrix, adhering to the paper fibers and forming a biofilm.^{[ 33 ]} As expected, the biofilm exhibited a gradient in cell density across different depths of the paper. iv) Sporulation: Bacterial sporulation was initiated as nutrients were depleted.^{[ 34 ]} The spores of B. subtilis are robust, enhancing the device's stability, storability, reliability, and operational capability without requiring continuous viability for sustained power generation.^{[ 34} , ^{35 ]} v) Electrode Deposition: Once the spore‐based biofilm was fully formed and dried, graphite electrodes were asymmetrically deposited on the top and bottom of the biofilm (Figure 2b). The bottom electrode was sealed, while the top remained open to facilitate gradient moisture adsorption.

Initially, we investigated bacterial growth, proliferation, and biofilm formation within paper, comparing their performance to conventional agar culturing platforms. We first observed that the introduced nutrient solution evaporated within 2 h (though slightly more than 1 h). Based on this observation, we selected 1 and 2 h nutrient supply intervals as variables in the subsequent experiments. Figure 2c illustrates how bacterial growth and proliferation varied based on initial concentration and nutrient supply frequency. Higher initial concentrations with hourly nutrient supply demonstrated rapid growth, reaching maximum levels within 4 h. Notably, the culturing capabilities on paper were comparable to those on conventional agar, consistent with our previous findings. This similarity is largely attributed to the paper‐based platform's ability to mimic the 3D microbial environment in vivo.^{[ 36} , ^{37 ]} However, when nutrient supply intervals were extended to 2 h, bacterial growth on paper was significantly reduced, requiring an additional hour to achieve the same cell count as when nutrients were supplied hourly. We quantitatively characterized the biofilm using the established bicinchoninic acid (BCA) assay kit and compared biofilm formation on our paper substrate with conventional agar models (Figure 2d).^{[ 37 ]} Biofilms are typically characterized and quantified based on the proteins that make up a significant portion of the biofilm, embedded within the extracellular polymeric substance (EPS).^{[ 38 ]} The BCA assay relies on the reduction of copper ions by EPS proteins, with their concentration correlating to the degree of reduction, which is detected as strong UV absorbance at 562 nm. Biofilm formation was confirmed across all bacterial concentrations after 4 h of incubation on the paper‐based platform with hourly nutrient supply, comparable to results observed on agar under the same conditions. However, when nutrient supply intervals were extended to every 2 h, insufficient biofilm formation was observed within 4 h on paper. This indicates that while initial bacterial concentration did not significantly influence biofilm formation, nutrient supply frequency played a critical role. Biofilm formation was further examined using scanning electron microscopy (SEM) with a bacterial culture concentration equivalent to 0.2 optical density (OD) at 562 nm (Figure 2e). Initially, at t = 0 h, bacterial aggregates adhered to the paper fibers, followed by the development of a thin yet continuous biofilm embedded in the EPS by t = 2 h. By t = 4 h, SEM imaging revealed a thick, uniform, and densely packed biofilm covering the entire substrate surface. Following full biofilm formation, sporulation in B. subtilis was induced by nutrient depletion. Every six hours, bacterial cells were harvested from the paper substrate, and sporulation progress was monitored using fluorescent microscopy (Figure 2f). Harvested cells were stained with two fluorescent dyes: 5(6)‐carboxyfluorescein diacetate (cFDA) and propidium iodide (PI). cFDA highlighted vegetative bacterial cells in bright green, while the PI caused bacterial spores to fluoresce in deep red.^{[ 34 ]} Fixed samples on glass slides were observed under fluorescence microscopy, revealing an increasing proportion of red‐stained spores over time. The transition from predominantly green to red fluorescence over 24 h confirms the ability of vegetative cells to undergo sporulation on the paper substrate, with the majority reaching this dormant state within ≈24 h. Phase‐contrast imaging (Figure S1, Supporting Information) further corroborated this transition, depicting round‐shaped spores as phase‐bright caused by dehydration and increased refractive index.^{[ 39 ]}

Fourier transform infrared (FTIR) spectroscopy revealed abundant functional groups present in both B. subtilis vegetative cells and spores (Figure 3a‐i).^{[ 40 ]} The paper substrate also exhibited additional oxygen‐containing groups, supporting its suitability as a substrate for moist–electric generators (Figure 3a‐ii). Upon biofilm formation, the EPS showed a distinct increase in functional groups (Figure 3a‐iii). Particularly during sporulation within the biofilm, there was an enhanced intensity of hydroxyl groups compared to biofilms containing vegetative cells, indicating improved hydrophilicity (Figure S2, Supporting Information). This enhancement facilitates moisture adsorption and water flow within the paper substrate. While further investigation is warranted, these findings suggest that functional groups are maximized when spores are developed within the biofilm structure. The natural introduction of an aquatic bacterial culture through capillary action is anticipated to establish a gradient of bacterial cells enriched with functional groups. Subsequent biofilm formation and sporulation in situ further contribute to the formation of this functional group gradient. To validate the gradient formation, we observed cell permeation first in the horizontal direction (Figure 3b).

Figure 3.

Characterization of functional groups and their gradient within paper. a) FTIR spectra comparing: i) vegetative cells versus spores, ii) paper only versus spores in paper, and iii) vegetative cells in biofilm versus spores in biofilm. b) Horizontal observation of cell penetration within the paper matrix. c) Vertical observation of cell penetration within the paper matrix.

If the horizontal spreading of a droplet across the paper is equivalent to its vertical spread, that shows that a horizontal flow evaluation reflects the vertical distribution, given the negligible gravitational forces compared to capillary action and surface tension within the microstructure of the paper matrix. Using a filter paper thickness of 180 µm, we monitored cell permeation horizontally up to 180 µm, with observations taken at 60 µm intervals. As anticipated, horizontal cell penetration through the paper matrix decreased significantly with increasing distance, resulting in a noticeable gradient of cell concentration (Figure 3b). This gradient formation was influenced by various external environmental factors such as temperature and initial sample volume (Figure S3, Supporting Information). Elevated temperatures accentuated gradient differences between the initial drop point and the 180 µm distance, while higher initial culture volumes led to greater concentration disparities over distance. Our findings were further validated by observing vertical penetration, which showed higher cell numbers at the top compared to the bottom (Figure 3c). These findings underscore the controllability and sensitivity of gradient formation within the paper substrate, highlighting its potential for tailored applications in moisture‐sensitive devices and biosensing technologies. Further exploration of these environmental influences could enhance the precision and versatility of gradient formation techniques in paper‐based moist–electric generators.

The in situ formation of bacterial biofilms within the microstructured paper matrix facilitates the straightforward fabrication of nanoporous structures capable of rapid establishment of streaming potential or surface charging. Our spore biofilm created random nanochannels among spores, EPS, and paper fibers (Figure S4, Supporting Information), promoting moisture gradient through a three‐dimensionally distributed nanoporous network. Similar to our control over cell penetration and gradient formation, we demonstrated the ability to manipulate porosity, pore size, and potential liquid transport capabilities (Figure 4a,b). Higher temperatures resulted in denser biofilm structures with smaller pores, enhancing liquid handling efficiency. Similarly, increased culture volume significantly augmented biofilm density, further reducing pore sizes. These findings highlight the versatility and precision in forming nanoporous structures within paper substrates, showcasing promising advancements for moist–electric generators. Further investigation into temperature, culture volume, and other relevant factors could optimize nanoporous design, thereby enhancing the functionality and overall performance of such devices.

Figure 4.

Morphological characterization of nanoporous biofilm structures. a) Biofilm formation with varying initial culture volumes at 37 °C. b) Biofilm formation with varying initial culture volumes at 20 °C.

2.2 Power Performance and Generation Mechanism

Our moist–electric generator features a gradient of functional groups, a gradient of water adsorption, and a nanostructured architecture. As shown in the previous section, environmental conditions determine whether a gradient of spores is established and the structure of biofilms. A quantitative evaluation of the moisture‐induced potential was performed at 20 °C and air relative humidity (RH) levels of 88.6% and 18.9% (Figure 5a). When the spore‐biofilm was prepared at 37 °C, the device sustainably and reliably produced 0.45 V for more than 2000 min. As the formation temperature decreased to 20 and 0 °C, the voltage significantly reduced to 0.37 and 0.15 V, respectively. Even the control, bare paper without spore‐based biofilms, generated a gradually increasing voltage output, reaching a maximum of 0.15 V after 1500 min. That is because of the natural functional groups in the paper,^{[ 27 ]} which take considerable time to generate power through water adsorption, ion generation, and ion migration. Under 20% RH, the overall voltage outputs were lower than at 90% RH, but they followed the same trend, with respect to the temperature of biofilm formation.

Figure 5.

Power performance and generation mechanism. a) Power generation of our innovative moist–electric generator using spore‐based biofilm under various biofilm conditions and humidity levels. By sealing the bottom electrode and leaving the top electrode open, we integrate all three mechanisms of moist–electric power generation to enhance overall performance. b) Power generation of our moist–electric generator with the water uptake gradient removed. c) Power generation when the functionalized biofilm is removed, and spores, in varying concentrations, are dispersed throughout the paper as a gradient functional group (RH 90%).

When the water adsorption gradient was removed by opening the bottom electrode, no power was induced under high RH, which was unexpected (Figure 5b). This phenomenon indicates that the synergistic cooperation of the water adsorption gradient and nanostructure‐based ionic behaviors is the dominant power mechanism, rather than the gradient of functional groups. However, under low RH, very small voltages were induced from the spore‐based biofilms formed at higher temperatures. This small voltage induction is mainly because of the gradient of functional groups, as slow water wicking under low RH creates an ionic gradient throughout the paper. From this experiment, we understand that the functional group gradient is critical under low RH levels, while other mechanisms are important under high RH.

The bacterial endospores used in our device offer a dual advantage: they not only act as highly effective moisture capture agents due to their hygroscopic nature but also contribute to the formation of nanochannels that significantly enhance the device's power generation capabilities. This dual functionality is especially valuable in our system, where the seamless integration of moisture capture and power generation into a single, cohesive process is essential for maintaining efficiency, particularly in wearable electronics. To further prove the significance of the nanochannel‐based power mechanism, we prepared control samples without biofilms (Figure 5c). Instead, spores with functional groups were dispersed through the capillary action of the paper with the sealed bottom electrode to guarantee the gradient of functional groups and water adsorption. Even with significantly increasing spore concentrations up to OD 2.0, the power generation was not comparable to that with biofilm. Its maximum power density normalized to the surface area of the moist–electric generator was limited to 0.1 µW cm^{− 2} (Figure S5, Supporting Information), while our device with functionalized nanochannel‐based spore biofilm generated 0.5 µW cm^{− 2}, achieving five times better performance. This experimental result demonstrates that the gradient of functional groups is not a critical factor in generating power under high RH.

The experimental results highlight the importance of the functionalized nanochannel architecture in our moist–electric generator. The biofilm's formation temperature plays a crucial role in determining the voltage output, with hotter temperatures leading to better performance. The removal of the water adsorption gradient severely affects power generation under high RH, underscoring its dominant effect on the nanostructure‐based mechanism. In contrast, under low RH, the gradient of functional groups becomes more critical because of the slow water‐wicking. The control experiments further confirm that the gradient of functional groups alone is insufficient to achieve high power generation, especially under high RH. The significantly higher power density achieved with the nanostructured spore biofilm emphasizes the importance of integrating all three mechanisms—functional group gradient, water adsorption gradient, and nanostructured architecture—to optimize the performance of moist–electric generators under varying humidity levels for practical environmental use.

Furthermore, evidence suggests that endospores can withstand harsh environments for hundreds of years,^{[ 35 ]} indicating that our fabricated devices can be stored long‐term while maintaining a similar level of voltage output (Figure S6, Supporting Information). These findings provide valuable insights for the design and development of future wearable electronics, particularly those requiring eco‐friendly, cost‐effective, and sustainable power sources. By optimizing environmental conditions for biofilm formation and structural components, we can enhance the efficiency and reliability of these devices, paving the way for their broader application across various fields.

The scalable integration of moist–electric generators is essential for practical power generation. Thanks to their compact design and straightforward paper printing fabrication (Figure S5, Supporting Information), the devices can be easily arranged in simple serial and parallel configurations to enhance power output (Figure 6a). For instance, connecting two devices in a series generates an open circuit voltage of 1.2 V at 90% RH, while six units in a series achieve 2.7 V (Figure 6b). This shows a strong linear relationship between voltage output and the number of devices, demonstrating excellent scalability. When 20 units were connected in a series, the system could charge an external supercapacitor (470 nF, charging curve in Figure S7, Supporting Information) and generate sufficient power to illuminate a liquid–crystal display (LCD) (Figure 6c), highlighting its potential for powering small electronic devices. As shown in Figure S8 (Supporting Information), connecting the units in parallel can significantly enhance the current output. More importantly, the moisture–electric generator can produce a short‐circuit current close to 2.5 µA at 90% RH by directionally adsorbing water molecules from the atmosphere (Figure S9, Supporting Information). Even under low humidity conditions (e.g., 20% RH), the generator can still deliver a short‐circuit current of ≈1.6 µA. Although the power output of moisture–electric generators is inherently low,^{[ 41} , ⁴² , ^{43 ]} they can still provide sufficient electric power for external loads. According to Ohm's law, the output voltage (V) and current (I) across a load can be calculated as:^{[ 44 ]}

$$V=\frac{V_{\mathrm{oc}}{R}_{\mathrm{load}}}{R_0+R_{\mathrm{load}}}$$ (1)

$$I=\frac{V}{R_{\mathrm{load}}}$$ (2)

where V _oc, R _load, and R ₀ represent the open‐circuit voltage, load resistance, and internal resistance of moist–electric generator, respectively. When R _load equals R ₀, the output will reach its maximum value, which is considered the optimal working condition for the generator.^{[ 45 ]} Additionally, the paper‐based moist–electric generator displayed notable flexibility, maintaining stable performance at various bending angles (Figure 6d). Notably, the endospore biofilm exhibits strong self‐healing properties.^{[ 46 ]} Even after repeated bending, it is expected to return to its original state upon exposure to ambient moisture. This flexibility enabled the device array to conform to the skin using 3 M Tegaderm medical transparent tape (Figure 6e). In one demonstration, sixteen devices connected in series formed the initial “B” for Binghamton University, with the open‐electrode sides exposed to ambient moisture. Under 90% RH, the integrated on‐chip light‐emitting diode (LED) lit up, powered by the paper‐based system. Moreover, the device's environmental effect is minimal, as it can be completely incinerated in just 10 seconds without leaving any hazardous residues (Figure 6f). This characteristic ensures that the moist–electric generators do not contribute to electronic waste, promoting sustainable disposal practices. Our moist–electric generator excels in scalability, flexibility, wearability, and environmental disposability. Its capability to generate power efficiently, maintain performance under various conditions, conform to flexible surfaces, and be safely disposed of underscores its potential for a wide range of practical applications. These features position it as a promising solution for sustainable, portable power generation.

Figure 6.

Scalability, flexibility, wearability, and disposability of our moist electric generator. a) Scaled‐up integration of devices using serial and parallel arrangements, b) Open circuit voltages from a series of device units, each with a unique serial number, c) Demonstration of application capability by lighting a segment liquid–crystal display (LCD), d) Performance stability under different bending angles, e) Wearable power generation.

2.2. Enhanced Water Harvesting with Hydrophobic–Hydrophilic Janus Paper

Moist–electric generators initially capture ambient gaseous water, transforming it into liquid water, which facilitates ion dissociation of functional groups on the device's surface. That efficiency of the capture of water directly influences the overall performance of the moist generators, leading to greater and more sustained power output.^{[ 14 ]} The inclusion of more functional hydrophilic groups on a 3D porous structure has been a unique solution, yet the limited device dimensions pose a challenge to significantly improving moisture capture capability. Consequently, most existing moist–electric generators exhibit low performance under low humidity conditions. Previously, atmospheric water harvesting has been extensively explored to alleviate clean water scarcity.^{[ 47} , ^{48 ]} Inspired by nature, researchers have mimicked desert beetles and horned lizards, which collect dew on their body surfaces using alternating hydrophilic and hydrophobic regions.^{[ 49} , ^{50 ]} Innovative Janus hydrophobic–hydrophilic membranes have been proposed for efficient fog and moisture harvesting.^{[ 51 ]} Our research group has even developed Janus membrane paper, demonstrating efficient sweat collection while keeping the skin dry through unidirectional sweat transport.^{[ 52 ]} However, these water harvesting techniques have never been applied to moist–electric generators until now.

Herein, we report a novel use of Janus hydrophobic–hydrophilic paper for efficient moisture harvesting, which maximizes power performance. When the hydrophobic side of the Janus paper faces ambient moisture and the hydrophilic side contacts the moist–electric generator, enhanced water harvesting improves performance and reduces dependency on humidity levels. It is well‐known that condensation rates are significantly greater on hydrophobic surfaces than on hydrophilic surfaces because a hydrophobic surface induces the air to shed droplets.^{[ 53 ]} These droplets are effectively delivered to the generator via unidirectional water transport. The collected water is self‐pumped from the hydrophobic side to the hydrophilic side, continuously removing water from the hydrophobic side and keeping it dry.^{[ 51} , ^{52 ]} Fabricating single‐layered Janus paper involves two simple steps: automatic printing of hydrophobic waxes on paper and heating the printed paper for wax penetration. The depth of wax penetration is controlled by the temperature and time in the oven. At 150 °C for 40 s, unidirectional water transport is achieved. The working mechanism is explained in detail in our previous reports (Figure 7a).^{[ 52} , ^{54 ]} In summary, water droplets penetrate the hydrophobic layer when the hydrostatic pressure (F _H; downward) and Laplace pressure (F _L; downward) exceed the hydrophobic pressure (F _P; upward) on the hydrophobic side. Once the droplets pass through, the capillary force (F _C) of the hydrophilic layer and the surface tension (F _A) of the accumulated droplets facilitate unidirectional transport from the hydrophobic to the hydrophilic layers. In contrast, droplets on the hydrophilic side spread out without penetrating the hydrophobic layer, as the horizontal capillary force and surface tension are insufficient to overcome the vertical hydrophobic pressure. This unidirectional transport capability of Janus paper is demonstrated even when placed vertically or under mechanical bending (Figure 7b).

Figure 7.

Advanced moisture capture through hydrophobic–hydrophilic Janus Paper. a) Contact angle measurement showing a water droplet placed on the hydrophobic and hydrophilic sides of the Janus paper, along with a schematic illustration of the water transport mechanism. b) Dynamic water transport study through the Janus papers under various conditions – i) a droplet is placed on the hydrophobic side facing upward, ii) the Janus paper is flipped, and water is applied to the hydrophobic side facing downward, iii) the Janus paper is curved, and water is dropped on the hydrophobic side facing upward, and iv) the Janus paper is placed vertically, and water is applied to the hydrophobic side. c) Water harvesting performance: Under different relative humidity (RH) conditions, the water harvesting efficiency is assessed by measuring the amount of water transported and collected in a tube through the backside of different papers – 1) Janus paper with the hydrophobic side facing ambient moisture, 2) Janus paper with the hydrophilic side facing ambient moisture, and 3) Hydrophilic filter paper. i) Schematic illustration, ii) continuous recording of open circuit voltage, and iii) polarization and power curves under different relative humidity (RH) levels: d) without and e) with Janus paper.

The water harvesting performance was characterized by weighing water transported and collected through the back of different papers under varying RH conditions. Three samples were prepared: 1) Janus paper with the hydrophobic side facing ambient moisture, 2) Janus paper with the hydrophilic side facing ambient moisture, and 3) hydrophilic filter paper. All samples were cut into 2 cm × 2 cm pieces, placed on a tube, and tested in a humidity‐controlled chamber. When the hydrophobic side of Janus paper faced moisture, droplets coalesced and unidirectional transport occurred, collecting water in a tube. Water harvesting increased with humidity and did not saturate, enabling continuous collection. At ≈90% RH, the Janus paper transported about 2.5 mL hr⁻¹ cm^{− 2} of liquid, demonstrating its efficiency in continuous moisture collection. Even under lower humidity conditions, such as 20% RH, the paper continued to effectively harvest water, with a slightly reduced but still significant transport rate of ≈0.4 mL hr⁻¹ cm^{− 2}. Conversely, with the hydrophilic side facing the atmosphere, droplets were absorbed and spread on the surface without penetrating, resulting in no collected water. Hydrophilic filter paper collected water at higher humidity but saturated quickly, ceasing further collection and failing under low humidity. The excellent water harvesting capability of Janus paper enhances moist–electric generator performance even at low RH levels. Without Janus paper, generator outputs (open circuit voltage, power, and current) significantly decrease with lower RH (Figure 7d). However, with Janus paper, the device remains sensitive to humidity (Figure 7e). At 18.9% RH, the open circuit voltage exceeded 0.4 V, with maximum power and current densities of 0.4 and 2.5 µA cm^{− 2}, respectively, comparable to values at higher RH levels. The integration of Janus hydrophobic–hydrophilic paper significantly improves the efficiency and performance of moist–electric generators, particularly under low humidity conditions. This innovative approach leverages natural principles of water collection and advanced material design, offering a promising solution for enhanced power generation from ambient moisture.

3. Future Direction

Future research will focus on several key areas to build on the promising results of this study. Optimizing the conditions for biofilm formation, including nutrient supply frequency, initial bacterial concentration, and environmental factors, could enhance the efficiency and consistency of the spore biofilm, improving overall device performance. Exploring the integration of other eco‐friendly and biocompatible materials with the paper substrate may enhance the functional group density and moisture adsorption capacity, further improving the power output and sustainability of the moist–electric generators. Developing miniaturized versions of the device and integrating them with other electronic components will be essential for creating fully functional, compact wearable devices. Research should focus on seamless integration with sensors, displays, and communication modules to enable comprehensive health monitoring and diagnostics. Conducting a thorough environmental impact assessment, including lifecycle analysis and disposal practices, will ensure the sustainable development and deployment of the paper‐based moist–electric generators. Investigating the biodegradability and recyclability of all components will be crucial for minimizing electronic waste.^{[ 55 ]} Extensive field testing under various environmental conditions and real‐world scenarios will validate the practical applicability and reliability of the paper‐based moist–electric generator. Collaborations with healthcare providers and industry partners could facilitate the development of specific applications, such as disposable medical sensors and diagnostic tools. Finally, investigating the combination of moist–electric generators with other energy harvesting techniques, such as solar or thermal, could provide hybrid solutions for wearable electronics, enhancing their versatility and energy efficiency. Addressing these areas will enhance the performance, sustainability, and applicability of paper‐based moist–electric generators, paving the way for their widespread adoption in wearable technology and beyond.

4. Conclusion

This study presents the development of an innovative paper‐based moist–electric generator designed to overcome the limitations of traditional power sources for disposable wearable electronics. The device integrates a gradient of functional groups, moisture adsorption gradient, and functionalized nanochannel architecture within a biodegradable paper substrate. Leveraging the natural capillary action of the paper, the in situ formation of bacterial endospore biofilms enhances the device's performance and sustainability. The key findings demonstrate that the in situ formation of spore biofilms within the paper substrate creates a nanoporous structure that significantly improves moisture adsorption and ion generation. The spore biofilm exhibited a gradient in cell density, enhancing the functional group distribution and moisture uptake capacity. The device showed high voltage and power output, with performance influenced by the biofilm formation temperature and environmental conditions. The removal of the water adsorption gradient severely affected power generation under high RH, highlighting its crucial role in the nanostructure‐based power‐generating mechanism. The device's compact design and straightforward fabrication process allow for easy integration in serial and parallel configurations to enhance power output. The device maintained stable performance under various bending angles, demonstrating its flexibility and potential for wearable applications. Additionally, incorporating a Janus hydrophobic–hydrophilic paper layer significantly improved moisture harvesting, enabling efficient operation even under low humidity conditions. This enhancement ensures continuous power generation by effectively capturing ambient moisture and transporting it to the generator. Overall, our paper‐based moist–electric generator offers a cost‐effective, eco‐friendly, and sustainable solution for powering disposable wearable electronics. Its ability to generate power efficiently, maintain performance under varying conditions, conform to flexible surfaces, and be safely disposed of underscores its potential for a wide range of practical applications. These features position the device as a promising solution for sustainable, portable power generation in future wearable technologies.

5. Experimental Section

Materials

Whatman Filter Paper (3 MM) was sourced from Sigma–Aldrich. Graphite ink (E3449) was obtained from Fisher Scientific Company, LLC, and silver/silver‐chloride (Ag/AgCl, E2414) ink was acquired from Ercon Inc. The Bicinchoninic Acid (BCA) assay kit was acquired from Thermo Scientific Pierce. Additionally, sodium chloride (NaCl), agar, tryptone, yeast extract, carboxyfluorescein diacetate (cFDA), propidium iodide (PI), and phosphate‐buffered saline (PBS) were all purchased from Sigma–Aldrich.

B. subtilis Inoculum

The Bacillus subtilis strain 168, obtained from the American Type Culture Collection (ATCC), was grown in a Luria Broth (LB) medium with gentle shaking at 37 °C for 24 h. Centrifugation is used to harvest the B. subtilis when the culture reached an optical density (OD) of 2.0 at 562 nm. The collected cells were then serially diluted to adjust their concentration for use in subsequent experimental procedures. That meticulous preparation ensures the reproducibility and reliability of the bacterial samples, providing a standardized foundation for exploring the bacterium's potential applications.

Biofilm Formation and Sporulation of the B. subtilis

The B. subtilis culture, harvested at the 2.0 OD, was used to build diluted concentrations of 0.2, 0.5, and 1.0 OD. The four concentrations were sequentially applied to the front side of the paper‐based moist–electric generator. The device, featuring an open window, acted as a hydrophilic and porous microbial culture reservoir. Nutrients were supplied by adding 20 µL of fresh LB to each reservoir every hour or every 2 h over 6 h to form a biofilm. After this initial phase, the nutrient supply was stopped overnight to encourage the formation of spores.

Fabrication of Paper‐Based Moist–Electric Generator

A sheet of Whatman 3 MM filter paper was initially prepared with hydrophobic wax boundaries using a commercial wax printer (Xerox Phaser, ColorQube 8570). Wax patterns were applied to both sides of the paper, and subsequent thermal treatment at 150 °C for 30 s defined the front side and the back side of the device. Graphite was screen‐printed as the current collector on both compartments using a sacrificial mask.

SEM Analysis

The biofilm and endospore biofilm samples were treated with a 2.5% glutaraldehyde solution at 4 °C overnight, followed by thorough washing with a 0.1 m PBS solution. Subsequently, the samples were dehydrated using an ethanol series (20%, 40%, 60%, 80%, and 99%). They were then coated with carbon using a 208HR Turbo Sputter Coater (Cressington Scientific Instruments, UK). The samples were examined using a field‐emission scanning electron microscope (FE‐SEM, Supra 55 VP, Carl Zeiss AG, Germany) to image their microstructure and morphology.

BCA Protein Measurement

The Thermo Scientific Pierce BCA protein assay kit is based on the reduction of copper ions (Cu²⁺) to cuprous ions (Cu¹⁺) by the EPS proteins in an alkaline medium. The Cu¹⁺ ions are detected through a colorimetric reaction with the very selective and sensitive bicinchoninic acid (BCA), which exhibits strong UV absorbance at 562 nm. This absorbance is nearly linear with increasing concentrations of EPS proteins, allowing for accurate quantification.

Electrical Measurement Setup

The open circuit voltage of the moist–electric generator was evaluated using a data acquisition system (DI‐4108U, DataQ, USA). The polarization curve and power output were determined by measuring the voltage drops across a series of selected external resistors (no resistor, 470, 250, 162, 100, 71, 47.5, 32, 22, 15, 10, 2, 1.5, 0.45, and 0.35 kΩ) and calculating the corresponding current and power values. The power and current densities were normalized to the surface area of the device.

Fabrication of Janus paper

The Janus paper layer was prepared following the method detailed in our previously published papers.^{[ 52} , ^{54 ]} Briefly, wax patterns were printed on the paper using the commercial printer and then partially penetrated the paper through heat treatment at 150 °C for 40 s. The finished Janus paper was placed on top of the moist–electric generator with the hydrophobic side facing the air to enhance moisture harvesting and the hydrophilic side in direct contact with the device.

Serial Connection of the Devices

Two methods for connecting the devices were employed. First, 20 units of the moist–electric generators were printed back‐to‐back, demonstrating a series connection using carbon paste, with a small monitor screen added at the end of the circuit (Figure 6c). Second, a 15‐device stack was fabricated following the same steps as for a single device, with additional processes for packaging and conductive traces to enhance wearable applications (Figure 6e). For the serial connection of individual devices, carbon paste was used for the front connection, with a cut in the middle to allow space for the via path. The via paths were prepared using Ag/AgCl ink. For the on‐chip power demonstration in papertronics, an LED was mounted and connected to the device stack.

Statistical Analysis

The experimental data presented in this study were obtained from at least three identical experiments. The results are expressed as the mean ± standard error of these replicates.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Gao Y., Elhadad A., Choi S., A Paper‐Based Wearable Moist–Electric Generator for Sustained High‐Efficiency Power Output and Enhanced Moisture Capture. Small 2024, 20, 2408182. 10.1002/smll.202408182

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.