On-skin devices, functional patches or cloths attached to the skin, hold immense potential in healthcare, behavior monitoring, and human-machine interaction. However, for comfortable long-term use, these devices need to minimize sensory interference and ideally maintain natural touch sensation. Recent research suggests that ultrathin, ultralight, gas-permeable films are ideal for imperceptible on-skin devices. However, existing technologies, particularly electrospinning, often result in flat functional surfaces with limited optical, thermal, mechanical, and electrical properties. Three-dimensional (3D) microstructures, such as micropyramid arrays, offer superior properties due to their gradient space filling, stress distribution, and refractive index. Existing 3D microarray fabrication methods (photolithography, 3D printing) struggle to combine gas permeability with ultralow thickness and gradient geometries. This study addresses this challenge by introducing a novel self-assembly electrospinning technique to create 3D micropyramid arrays with enhanced properties for on-skin applications.

The literature extensively covers on-skin devices and their applications in various fields. However, most existing imperceptible on-skin devices utilize flat surfaces, limiting their performance due to inferior optical, thermal, mechanical, and electrical properties. While 3D microstructures like micropyramid arrays have demonstrated superior performance, current fabrication methods fail to combine this advantage with the requirements of gas permeability, ultrathin and ultralight weight, and gradient geometry. This review highlights the gap in the literature and the need for a new fabrication method that addresses all these challenges. The limitations of photolithography and 3D printing in creating such structures are discussed, setting the stage for the proposed self-assembly electrospinning technique.

This research utilizes a self-assembly electrospinning technique to create electrospun micropyramid arrays (EMPAs). The process involves using a far-field electrospinning apparatus with an electrically grounded aluminum foil collector. Poly(vinylidene fluoride) (PVDF) was initially used as a model material. The formation of EMPAs was systematically investigated using scanning electron microscopy (SEM). The process begins with the deposition of wet, heterostructured electrified jets, forming inhomogeneously charged microdomains. Electrostatic interaction causes positively charged aerial jets to deposit on negatively charged microdomains, leading to the formation of fibrous domes. These domes evolve into EMPAs as the substrate thickness increases. The self-assembly is shown to be highly designable with control over structure achievable through adjustments to voltage, humidity, and horizontal swing distance of the syringe. The methodology also demonstrates the adaptability of the self-assembly technique to various materials such as PVDF, thermoplastic polyurethane (TPU), and poly(vinyl alcohol) (PVA). The imperceptibility of the resulting EMPAs was evaluated through water vapor transmission tests and visual analog score (VAS) surveys in conjunction with object-grasping experiments. Various applications were explored using the EMPAs in radiative cooling, pressure sensing, and bioenergy harvesting. The optical properties of the EMPAs were characterized using spectral reflectance and emittance measurements. Pressure sensing capabilities were evaluated using piezocapacitive sensors, while bioenergy harvesting was assessed using triboelectric and piezoelectric nanogenerators (TENGs and PENGs).

The study successfully demonstrated the creation of ultrathin, ultralight, and gas-permeable EMPAs via a self-assembly electrospinning technique. The resulting EMPAs exhibited high visible to near-infrared (vis-NIR) reflectivity (97.9%) and mid-infrared (MIR) emissivity (76.3%), leading to a -4°C temperature drop in a radiative cooling fabric under 1 kW m⁻² solar intensity. The EMPAs also significantly enhanced the performance of piezocapacitive sensors, achieving high sensitivity (19 kPa⁻¹), ultralow detection limit (0.05 Pa), and ultrafast response (≤0.8 ms). This allowed for the detection of ultraweak fingertip pulses during natural finger manipulation. Moreover, the EMPAs boosted the triboelectric and piezoelectric outputs of nanogenerators, enabling effective biomechanical energy harvesting (105.1 µC m⁻²). Imperceptibility evaluations confirmed minimal sensory interference during prolonged use. The hybrid sensor combining piezocapacitive and triboelectric sensing demonstrated the ability to simultaneously monitor fingertip pulses and complex finger manipulations, suitable for applications requiring interference-free multi-information monitoring, such as in eSports.

The findings successfully address the research question of creating high-performance, imperceptible on-skin devices. The self-assembly electrospinning technique offers a scalable and designable approach to fabricate 3D microstructures with optimal properties for diverse applications. The superior performance in radiative cooling, pressure sensing, and energy harvesting compared to existing technologies demonstrates the potential of EMPAs in various fields. The excellent imperceptibility, verified through both subjective VAS scores and objective grip force experiments, highlights the suitability of EMPAs for long-term, comfortable on-skin applications. The simultaneous monitoring of subtle physiological signals and dynamic movements showcased by the hybrid sensor expands the capabilities of on-skin devices, opening up possibilities for applications requiring simultaneous and high-fidelity information acquisition.

This study successfully developed versatile electrospun micropyramid arrays (EMPAs) using a novel self-assembly technique, enabling the creation of high-performance, imperceptible on-skin devices. The EMPAs demonstrated significant improvements in radiative cooling, pressure sensing, and bioenergy harvesting, with minimal sensory interference. Future research could explore the use of other materials and optimize the EMPAs' structure for further performance enhancements and broader applications. Investigating the integration of more sophisticated functionalities into the hybrid sensor and exploring advanced signal processing algorithms could further improve its capabilities.

While the study demonstrates significant advancements, some limitations exist. The long-term stability of the EMPAs under various environmental conditions needs further investigation. The current study primarily focused on a limited set of materials; exploring a wider range of materials could lead to improved performance. The scope of human subject testing could be broadened to include a more diverse population and longer observation periods. Further optimization of the hybrid sensor's signal processing algorithms could improve accuracy and reduce misidentification rates.