1. INTRODUCTION

Over the past few decades, flexible electrothermal heaters have attracted considerable attention owing to their broad range of potential applications, including thermochromic displays^{[1, 2]}, thin-film defrosters^{[3- 8]}, wearable thermotherapy systems^{[8- 11]}, biomedical and prosthetic devices^{[12, 13]}, and flexible gas sensors^{[14, 15]}. In particular, with the growing demand for efficient point-of-care solutions, body-attachable heaters have emerged as a promising platform for smart healthcare applications. For such applications, heaters must be sufficiently breathable to permit efficient moisture transport from the skin and highly deformable to conform to human body movements during wear. Therefore, conductive fabrics have recently emerged as promising candidates for skin-mountable heating platforms owing to their unique mesh geometry featuring uniformly distributed conductive fibers and pores, lightweight nature, and conformal integration with the human body^{[16, 17, 19- 22]}.

Insulating fabrics have previously been conformally coated with carbon nanotubes (CNTs) via a facile dip-coating process to fabricate wearable heaters^{[16, 17]}. Despite the simple, rapid fabrication, CNT-coated fabric heaters still require relatively high operating voltages. For example, an input voltage as high as 15–20 V is required to achieve a temperature of ~50 °C, which substantially limits their practical applicability in mobile wearable systems. This limitation is presumably attributed to the high contact resistance at CNT–CNT junctions^[18].

Silver nanowires (AgNWs) have emerged as attractive alternatives to CNTs for fabricating low-voltage fabric heaters owing to their superior electrical and thermal conductivities^{[17, 19, 20]}. Notably, AgNW-based heaters require input voltages as low as 3 V to reach temperatures of ~40 °C. Carbonization of fabrics through facile thermal treatment has also been proposed as an effective approach for fabricating low-voltage fabric heaters, enabling operating temperatures exceeding 100 °C at an input voltage of 3 V^[21].

In another approach, polymeric microfibers were sequentially coated with copper nanowires and a protective rubber layer, followed by weaving into a mesh structure^[22]. The resulting fabric heaters reached temperatures as high as 57 °C under an input voltage of 3 V while maintaining excellent breathability and mechanical stability. Electroless deposition techniques have also been employed to fabricate conductive fabric heaters^{[23, 24]}. Following a subsequent hydrophobic treatment, the conductive fabric exhibited a high static contact angle (SCA) of 158° while preserving its low-voltage heating performance, reaching a temperature of ~73.4 °C at an input voltage of only 0.5 V.

However, most previously reported fabric heaters inevitably rely on cumbersome and time-consuming fabrication processes, presenting a major obstacle to their practical implementation in wearable systems. In this study, we present a simple yet highly efficient strategy for fabricating conductive superhydrophobic fabric mesh heaters by combining facile dip-coating of highly conductive AgNWs with conformal vapor deposition of a thin polydimethylsiloxane (PDMS) overlayer.

The resulting AgNW/PDMS-coated fabric mesh (APFM) heater combines several desirable features, including low-voltage operation (~70.1 °C at 1.2 V), robust superhydrophobicity (SCA > 150°, contact angle hysteresis (CAH) < 10°, excellent water repellency, and a sliding angle < 10°), high breathability, and a cost-effective, straightforward fabrication process, establishing it as a promising platform for highly efficient electrothermal heating applications.

2. EXPERIMENTAL

3. RESULTS AND DISCUSSION

The superhydrophobic APFM was fabricated via a facile two-step coating route, as schematically illustrated in Fig. 1(a). Figs. 1(b)–(d) display top-view SEM images of the fabric mesh in its pristine state, following AgNW dip-coating, and after subsequent PDMS vapor deposition, respectively. The pristine mesh consists of periodically woven fibers forming regularly distributed open pores (Fig. 1(b)). Following five dip-coating cycles, the woven framework was uniformly and densely covered with an interconnected AgNW percolation network (Fig. 1(c)), indicating high electrical conductivity. The AgNW network remained intact without noticeable morphological degradation after the PDMS vapor coating process (Fig. 1(d)). Crucially, the two-step coating process preserved the original open-pore architecture of the woven mesh, verifying the conformal nature of the deposition.

Fig. 1. Fabrication of the AgNW/PDMS-coated fabric mesh (APFM). (a) schematic illustration of the fabrication sequence, and top-view SEM images of the fabric mesh (b) in its pristine state, (c) after the AgNW dip-coating, and (d) after the PDMS coating, scale bars: 500 µm (inset: magnified SEM images for each case, scale bars: 50 µm).

The electrical properties of the APFM were modulated by controlling the number of AgNW dip-coating cycles. Fig. 2(a) presents the sheet resistance ($R_s$) of the APFM as a function of coating cycles. As shown in Fig. 2(a), the $R_s$ decreased monotonically with increasing coating cycles, which is attributed to the formation of an increasingly dense AgNW percolation network. After five dip-coating cycles, a minimum $R_s$ of 1.4 ± 0.04 Ω sq^-1 was achieved. Furthermore, increasing the AgNW density effectively reduced sample-to-sample variations in $R_s$, with the standard deviation decreasing from ~2.3 Ω sq^-1 after a single cycle to ~0.04 Ω sq^-1 after five cycles. These results confirm that the dense AgNW network provides both superior electrical conductivity and high process reproducibility.

Fig. 2. Sheet resistance (R_s) measurement. (a) R_s of the fabric mesh as a function of the number of AgNW dip-coating cycles and (b) comparison of R_s values of the five-cycle coated fabric mesh before and after the PDMS coating.

Fig. 2(b) compares the $R_s$ of the five-cycle coated sample before and after PDMS vapor deposition. The $R_s$ further decreased to 1.2 ± 0.07 Ω sq^-1 (from 1.4 ± 0.04 Ω sq^-1) after PDMS coating. This reduction is attributed to the conformal PDMS overlayer, which mechanically constrains the AgNW percolation network, thereby expanding the effective contact area between adjacent AgNWs and reducing junction contact resistance.

The surface wetting properties of the APFM were characterized by measuring the static contact angle (SCA) and contact angle hysteresis (CAH). Superhydrophobicity is conventionally defined by an SCA exceeding 150° and a CAH below 10°^[25]. Fig. 3(a) compares the SCA values of three different surfaces: a PDMS-coated flat substrate (flat PDMS), a PDMS-coated fabric mesh (PFM), and the APFM. Photographs of water droplets on each surface are shown in the inset of Fig. 3(a).

Fig. 3. Static contact angle (SCA) of the APFM. (a) SCAs measured on different surface models: flat PDMS, PFM, and APFM (inset: digital images of water droplets sitting on the respective surface models) and (b) uniformity of SCA for each surface model.

The flat PDMS exhibited an SCA of 105.7 ± 0.3°, consistent with the intrinsically low surface energy of PDMS^[26]. This indicates that PDMS vapor deposition is an effective method for hydrophobizing various substrates. When PDMS was coated onto the pristine fabric mesh, the resulting PFM exhibited an increased SCA of 141.7 ± 0.7°. This enhancement occurs because the microscale roughness of the woven mesh promotes a stable Cassie–Baxter wetting state by trapping air within the open pores, thereby minimizing the solid–liquid contact area^{[27, 28]}.

The incorporation of the AgNW network between the mesh substrate and the PDMS overlayer introduced nanoscale roughness, generating hierarchical micro/nanostructures that trap additional air at the solid–liquid interface. Consequently, the APFM exhibited an exceptional SCA of 154 ± 2.2°. Furthermore, all surface models displayed uniform wetting characteristics across their entire surfaces (Fig. 3(b)). The variation among average SCA values measured at five distinct locations (top, bottom, center, left, and right) on the APFM was within ~2.6°, confirming structural and chemical uniformity across the entire mesh.

Dynamic wetting behavior was investigated by evaluating the CAH, defined as the difference between advancing and receding contact angles. As shown in Fig. 4(a), the flat PDMS exhibited a CAH of 31.5 ± 0.5°, in agreement with literature values^{[29, 30]}. In contrast, the APFM exhibited a markedly lower CAH of 6.1 ± 0.1°, representing an ~80.6% reduction compared to flat PDMS. This low CAH is attributed to the hierarchical surface roughness, which stably traps air beneath the droplet, drastically lowering both the solid–liquid contact area and the droplet adhesion force.

Fig. 4. Contact angle hysteresis (CAH) of the APFM. (a) CAHs measured on the flat PDMS and APFM and (b) sequential snapshots of a water droplet released onto the ~10°-inclined APFM from a height of ~30 mm, scale bar: 10 mm.

Because the CAH of the APFM was well below 10°, droplets rolled and bounced freely off the surface. To examine this dynamic behavior, a 10 µL water droplet was dropped from a height of ~30 mm onto an APFM inclined at ~10°. As shown in the sequential snapshots in Fig. 4(b), the droplet rebounded completely upon impact without droplet breakup or surface wetting. Following initial impact, the droplet underwent a secondary bounce and transitioned smoothly into rolling at 162 ms, completely rolling off the surface within 216 ms. This confirms the superior dynamic superhydrophobicity and low roll-off resistance of the APFM.

The APFM was subsequently evaluated as a low-voltage electrothermal heater. Fig. 5(a) displays the time-dependent temperature profiles of the APFM heater at applied voltages from 0.3 to 1.2 V in 0.3 V intervals. Upon voltage application, the heater temperature rose rapidly, reaching a steady-state value within seconds. When the voltage was turned off, the temperature rapidly returned to ambient levels, demonstrating rapid thermal response and operational stability over repeated cycles.

Fig. 5. Joule heating performance of the APFM heater. (a) time-dependent temperature profiles of the device at applied voltages ranging from 0.3 to 1.2 V and (b) steady-state temperature (T_ss) as a function of applied voltage.

Fig. 5(b) plots the steady-state temperature ($T_{ss}$) as a function of applied voltage. A maximum $T_{ss}$ of 70.1 ± 11.5 °C was attained at an operating voltage of only 1.2 V, highlighting the remarkable low-voltage heating capability of the APFM. Although minor device-to-device temperature variations were observed due to localized AgNW density variations, $T_{ss}$ demonstrated strong linear dependence on the applied voltage ($R^2 \approx 0.993$). This linearity facilitates predictable and precise temperature control by adjusting the input voltage.

These findings demonstrate that the APFM heater is well-suited for portable and wearable thermotherapy applications. In particular, combining low-voltage operation with robust superhydrophobicity enables reliable, safe performance in humid or wet environments.

4. CONCLUSIONS

In summary, a low-voltage-driven superhydrophobic heater was successfully developed using a highly conductive, hierarchically roughened fabric mesh (APFM). The APFM was fabricated via sequential AgNW dip-coating and PDMS vapor deposition. The resulting mesh demonstrated outstanding electrical conductivity ($R_s = 1.2 \pm 0.07\ \Omega\ \text{sq}^{-1}$) and robust superhydrophobicity (SCA = 154 ± 2.2°, CAH = 6.1 ± 0.1°, and sliding angle < 10°). Operated as an electrothermal heater, the device reached a steady-state temperature of 70.1 ± 11.5 °C at an input voltage of only 1.2 V with excellent linearity ($R^2 \approx 0.993$). These results establish the APFM as an efficient, waterproof platform for next-generation wearable and skin-attachable thermotherapy devices.