Electrospinning onto Insulating Substrates via Surface Wettability and Local Humidity Control

Background

Electrospinning—an economical method for producing continuous nanofibers in the sub‑micrometer range—relies on an electric field generated between a high‑voltage source, a spinneret, and a collector. Traditionally, the collector is a conductive substrate (e.g., metal) that serves as the ground electrode, ensuring a stable electric field. When a non‑conductive substrate is required, external conductive layers must be added to provide grounding [4].

Many industrial applications demand deposition onto insulating, flexible polymers [6]. Prior work has achieved this only for very thin insulating layers (≤100 µm) or by employing complex AC pulse‑modulated electrohydrodynamic techniques that require specialized equipment [7]. Although electrospinning onto liquid electrolytes has been explored [9–12], a simple, scalable approach for arbitrary insulating substrates remains elusive.

We introduce a method that exploits the conductivity of adsorbed water layers on hydrophilic surfaces in a high‑humidity environment. A thin monolayer of water can conduct electricity at relative humidity (RH) above 50 % [13]. By controlling local humidity around a plasma‑treated polymer, the adsorbed water acts as the ground electrode, enabling electrospinning independent of substrate thickness and compatible with conventional setups.

Methods

Preparation of a Hydrophilic Polymer Substrate

A 500‑µm acrylic sheet with a naturally hydrophobic surface was subjected to 30 s of oxygen plasma (CUTE, Femto Science, Korea). This treatment introduced silanol groups (SiOH) and reduced the water contact angle from 81.3° to 36.7°, confirming successful hydrophilization [14]. Selective hydrophilization was achieved by masking regions prior to plasma exposure.

Electrospinning Setup

Electrospinning was carried out at room temperature (≈22 °C) and moderate RH (40–50 %). A 10 wt % polyurethane (PU) solution (Pellethane 2363‑80AE; Lubrizol, USA) dissolved in an 80/20 (v/v) THF/DMF mixture served as the spinning fluid. For comparative studies, PU fibers were deposited onto an acrylic collector that combined hydrophilic and hydrophobic regions (Fig. 1a).

Schematic of the electrospinning process on a polymer substrate with local humidity control (a) and a close‑up of the boundary region (b).

Local Humidity Control

To raise humidity adjacent to the collector, a wet paper was placed between the acrylic substrate and the ground electrode (Fig. 1b). Because water vapor diffuses slowly, RH near the substrate reached ~70 % while the syringe tip environment remained at ~50 % (Additional file 1: Figure S2). This localized increase ensures rapid water adsorption on hydrophilic surfaces, facilitating electrostatic coupling [15].

Results and Discussion

Electrospinning Modes

We examined two configurations: (1) far‑field electrospinning (tip‑to‑collector distance = 8 cm, 13 kV DC) and (2) near‑field electrospinning (distance = 1 cm, 2 kV DC, moving tip at 100 mm s⁻¹). In far‑field mode, fibers formed only on hydrophilic areas; the hydrophobic sections remained free of deposition (Fig. 2). The adsorbed water layer on the hydrophilic surface provided the necessary grounding, while the hydrophobic surface inhibited electric field formation. Near‑field experiments demonstrated that fibers could be written directly along the tip trajectory on hydrophilic regions, producing continuous lines; fibers on hydrophobic regions appeared unstable and twisted.

Far‑field electrospun films on surfaces with differing wettability. a and c show macroscopic images; b and d show micrographs of the boundary regions.

Four polymers—PCL, PS, CA, and PVDF—were tested to confirm versatility. In each case, fibers were deposited exclusively on hydrophilic regions (Fig. 3). SEM analysis (Fig. 4) revealed that fiber morphology on the polymer collector matched that produced on conventional metal electrodes, indicating that local humidity control does not compromise fiber quality.

Electrospun fibers of (a) PCL, (b) PS, (c) CA, and (d) PVDF on hydrophilic versus hydrophobic surfaces (scale bar: 10 mm).

SEM images comparing PU fibers on a metal electrode (a, d) without humidity control, metal electrode with humidity control (b, e), and hydrophilic polymer substrate with humidity control (c, f) under 12 kV DC.

Electric field strength also influences deposition patterns. As the applied voltage increased from 6 kV to 16 kV (tip‑to‑collector = 8 cm), the bending instability of the jet intensified, leading to larger loops that deposited fibers on the hydrophobic regions (Fig. 5). At lower voltages, fibers predominantly collected on the hydrophilic surface.

PU fiber deposition on hydrophilic (right) and hydrophobic (left) surfaces at applied voltages of 6, 8, 10, 12, 14, and 16 kV (scale bar: 10 mm).

Near‑field experiments further validated the method. When the collector was a hydrophilic polymer, the tip produced straight, continuous lines, whereas on a hydrophobic collector the fibers were unstable and curved (Fig. 6). This demonstrates that direct patterning on insulating substrates is achievable without external electrodes.

Near‑field electrospun films on (a) hydrophobic and (b) hydrophilic surfaces; (c) digital photo of fibers written on a hydrophilic substrate; (d) micrograph of the same. The vertical line marks the boundary between hydrophilic (left) and hydrophobic (right) regions.

Conclusions

We have established a simple, scalable approach to electrospin onto insulating substrates of any thickness by leveraging surface wettability and controlled local humidity. Oxygen plasma renders the polymer surface hydrophilic, allowing adsorbed water at ~70 % RH to act as a ground electrode. Electrospun nanofibers deposited on the hydrophilic collector match the morphology achieved on conventional metal electrodes, and near‑field writing yields precise fiber patterns on hydrophilic regions of otherwise insulating surfaces. This strategy eliminates the need for complex MEMS or electrode fabrication, enabling the integration of electrospun nanofibers into flexible, wearable electronics and other emerging applications.