A ring-shaped hydrophilic polyethylene terephthalate (PET) fabric acts a dual role as both the adhesive film and the sweat collector, connecting the VLD and HLD to create an integrated 3D-LD system. To evaluate perspiration performance, a comparison was conducted between the 3D-LD and the PDMS substrate. The simulation of the perspiration process was characterized by a modified sweating model (Fig. 1(g)). Both the 3D-LD and PDMS substrates were positioned over the sweating pores to observe fluid mechanics during ongoing perspiration flow. The results demonstrated that the 3D-LD outperforms the PDMS substrate in terms of permeability. It enables unimpeded gas transport and perspiration within the internal channel, ensuring continuous adheres closely to the artificial skin.

Wearable electronics face a significant challenge related to the limited permeability of electronic materials/devices. This issue results in sweat accumulation across the interface of the device and skin following a specific period of use^[1−3]. Not only does it bring about discomfort for users regarding thermos-physiology, but it also has a detrimental effect on interface adhesion and signal quality, thus hindering exact signal monitoring during prolonged periods^[4−6]. Nevertheless, the advancement of breathable electronics tackles this challenge by enabling the release of gases, either passive or active, vapors, and sweat via micro/nano apertures^[7−10]. Consequently, these devices provide a user-friendly interface within the interface of the skin and the device, enabling improved stability in signal monitoring during long-term healthcare periods, even in sweating conditions^[11−15].

The integrated sweat-permeable electronics system comprises three main components: serpentine metallic interconnects, VLD channel, and HLD channel (Figs. 1(b)−1(d)). These components act as a substrate to facilitate controlled directional sweat discharge and to support versatile electronic circuits using established processing techniques. The developed serpentine metallic interconnects enable conformal contact to the skin, ensuring low skin-electrode impedance and stable electrical conductivity even under deformation, and providing excellent electrode permeability (Figs. 1(c) and 1(d)). The VLD features a spatially distributed design with a hydrophilicity gradient channel. This configuration enables rapid pumping of sweat produced across the interface of the device and skin towards the backside, encouraging sweat to form droplets rather than dispersing on the surface. (Fig. 1(e)). Conversely, the HLD possesses in-plane liquid-transport capability, permitting sweat to disperse within the channel along a predetermined pathway. This ensures that sweat ultimately reaches the collector, where it either evaporates into the ambient surroundings or exits through the outlet, resulting in dripping (Fig. 1(f)). Through strategic pore design for dripping sweat at the 3D-LD substrate edge, flexible devices can seamlessly integrate at the middle of substrates without obstructing the sweat pathway. This arrangement allows for efficient sweat management without compromising the functionality of the integrated electronics.

Recently, a collaborative team led by Professor Yu at City University of Hong Kong developed an integrated permeable wearable electronic system based on the concept of a vertical liquid diode in three dimensions (3D-LD) (Nature, https://doi.org/10.1038/s41586-024-07161-1)^[16], which is illustrated in Fig. 1(a). The technology provides a versatile platform/substrate for seamless integration with established skin-integrated and wearable electronics. The 3D-LD employs nature-inspired microstructures that enable directional and spontaneous fluid transport, eliminating the need for specific materials. The system consists of a horizontal liquid diode (HLD) for in-plane liquid transport and a vertical liquid diode (VLD) for perspiration channels (Fig. 1(b)). The microstructure design allows for rapid self-pumping of sweat that comes from the interface of skin-device to the outlet while preventing any backward flow. This unique feature enables exceptional air and sweat permeability, facilitating the direct integration of high-performance wearable electronics on top of this platform.

In summary, the 3D-LD technology provides a system-level integration with moisture permeability that shows excelled rates of perspiration by ×1000 times. This exceptional capability facilitates seamless, uninterrupted, and thermo-comfort healthcare monitoring. Comprehensive user trials have confirmed its superior performance in the stable and comfortable skin-device interface, even amid perspiration circumstances. The 3D-LD-based ECG monitor exhibits reduced motion artifacts when compared to conventional approaches, ensuring dependable signals for an extended period of one week. These results establish 3D-LD technology as an exceptionally favorable choice for wearable healthcare monitors designed for user-friendly, long-term use. To augment its capabilities and enable mass production, optimizing device architecture, functional materials, and manufacturing techniques is crucial. The inclusion of a washable and reusable adhesive backing is anticipated to enhance the device's reusability and cost-efficiency. These advancements will foster widespread adoption of the technology and its seamless integration into routine healthcare monitoring practices.

To examine the effect of sweat/moisture control on both device performance and wearability, test subjects wore four different types of patches: commercial electrocardiogram (ECG), PDMS, VLD, and 3D-LD. Throughout the 30-min test, it was noted that the conventional patches of commercial ECG and PDMS would detach from the skin, whereas the patches of VLD and 3D-LD exhibited secure affixation to the body (Fig. 1(h)). This showcased the superior wear-comfort capability of the presented patches of the VLD and 3D-LD, attributed to the exceptional vapor and sweat permeability of the 3D-LD (Fig. 1(i)). The 3D-LD also demonstrated superior thermal comfort after exercise compared to the patches of commercial ECG and PDMS, owing to its rapid sweat transport and evaporation capabilities. The impact of the sweat-accumulation process on bio-signal acquisition was assessed by utilizing three categories of electrodes: commercial ECG electrodes, PDMS/PI/Au-meshed dry electrodes, and 3D-LD/PI/Au-meshed electrodes. While the incorporation of the Au mesh slightly elevated the skin-electrode impedance in the 3D-LD-based electrodes, it offered exceptional adherence and insulation from external interference. The signal-to-noise ratios from the commercial ECG or PDMS electrodes steadily declined with prolonged exercise/sweating, posing greater difficulty in reading as perspiration accumulated across the electrode-skin interface. Conversely, the 3D-LD-based electrodes demonstrated outstanding permeability of moisture, ensuring consistent skin-electrode impedance and ECG signals before/after the process of sweating (Fig. 1(j)). The universality and scalability of the 3D-LD technology were further showcased in the portable electronic gadgets: soft ECG monitoring systems. The ECG acquisition employs a detachable design feature utilizing a magnetic coupling approach (Fig. 1(k)). With a sophisticated circuit design, the device maintained stable functionality even under various deformations. The device was attached to the subject’s chest, and the ECG data can be wirelessly transported to a cellphone (Fig. 1(l)).