A comfortable and high-density digital healthcare platform by permeable 3D electronic skin
For bio-integrated electronics, a high degree of wearing comfort consisting of adequate stretchability, softness, and permeability for the living body while maintaining normal electronic functions is required. Although significant progress has been made in the development of soft, stretchable, and multifunctional electronic materials and devices1–4, the permeable (breathable) type of bioelectronics with system-level integration is still in its early stages5. Most permeable electronics have no or low-density integration and are usually wired with external printed circuit boards, which limits functionality, deteriorates user experience, and impedes long-term usability.
Why permeability is important for bioelectronics? Although the softness and stretchability of electronic materials can satisfy part of the wearing comfort, the thermophysiological comfort of the living body relies on the thermal microenvironment between the skin and the covering device/system, which is controlled by balancing the loss of water and heat from the skin surface by sensible or insensible perspiration6. Suppose perspiration cannot escape from the skin quickly and sufficiently. In that case, the accumulation of moisture between the skin and the covering will inevitably lead to uncomfortable thermophysiological sensations such as dampness and clamminess.
However, achieving high integration density on such soft, stretchable, and permeable substrates is very challenging. Patterning complex and high-density microcircuits on permeable, supersoft, and stretchable substrates while maintaining outstanding mechanical, electrical, and electromagnetic performance is very challenging because of the large surface roughness and porosity. As such, we adopted the previously developed methodology7 1) photopatterning ---- pattern transfer ---- selective wetting method to fabricate the complex and high-density upper liquid metal (LM) circuit layer. However, currently, we can only achieve a single-layered micropatterning using this method alone. To further enhance the integration density, we learned from conventional electronic system design and adopted the 3D configuration. This multilayered configuration can largely elevate the level of integration and functional complexity8 because the surface density of electronic components in a single layer usually reaches a bottleneck owning to constraints in fabrication and mechanical design. Therefore, we further adopted the stencil printing technique to generate simple patterns such as circuit traces and contacts in the base circuit layer and pads in the paste mask layer. In this way, we can obtain a 3D permeable and stretchable circuit layout taking both the advantages of photopatterning (high resolution) and stencil printing technique (cost-effectiveness).

Figure. 1 Exploded schematics of a typical P3D-eskin. Liquid metal (LM) microelectrodes are adopted as the reliable interface between the soft, rough fiber mat substrate and the rigid components. Vertical interconnect accesses (VIAs) are used for interlayer electrical connections. Key components in each layer include microcontroller unit (MCU), oscillator, multiplexer (MUX), current mirror, digital-analog-convertor (DAC), operational amplifier (OP-AMP), high voltage module (HV, 20V), and low dropout regulator (LDO, 3.3 V). The dashed lines indicate the distribution and positions of the VIAs in the system.

Figure. 2 Schematic illustration showing the detailed processing flow of layer-by-layer fabrication of P3D-eskins.
The next step is to achieve reliable and stretchable electrical interfaces with various electronic components both in plane and out of plane. To address this challenge, we formulated two different kinds of LM inks, namely the pristine LM and oxidized LM (oLM), serving as a stretchable hybrid LM (hLM) solder for the 3D circuit. The stretchable hLM solder was formed by printing oLM on the paste mask layer as contact pads, and the use of additional pristine LM paste at the pin/oLM interface. The pristine LM showed high fluidity but low wettability to the fibrous poly(styrene-block-butadiene-block-styrene) (SBS) substrate and it was used for the fabrication of stretchable circuit antennas, interconnects, and vertical interconnect accesses (VIAs) on the base and upper circuit layers. As such, the base and upper circuits remained outstanding in-plane stretchability and out-of-plane insulation, unless they were connected with stretchable LM VIAs. Finally, the entire 3D hybrid electronic system was encapsulated conformally with a permeable but waterproof fibre mat to ensure stable functions.
Our P3D-eskin replaces impermeable and rigid printed circuit boards (PCBs) with a skin-like stretchable, soft, and breathable design form factor, while maintaining complex system-level and continuous functions such as data acquisition, signal processing and analysis, intervention, and wireless communication with a mobile device.
We then use the platform to create wireless, battery-powered, and battery-free skin-attached bioelectronic systems that offer complex system-level functions, including stable sensing of bio-signals, signal processing and analysis, electrostimulation, and wireless communication. A wireless transcutaneous electrostimulation and electrophysiological sensing system equipped with a Bluetooth Low Energy built-in microcontroller unit and LM antenna, providing stable wireless control and data transmission functions with the mobile device at a distance of up to 15 m. The embedded electrostimulating electrodes could generate high-voltage electrical pulses with precisely controlled current intensity, frequency, and duty cycle for delivering electrical stimulations to the user/animal’s body. In addition, a battery-free type of P3D-eskin was developed using near-field communication (NFC) technology. A temperature-sensing network consisting of wireless P3D-eskin sensing forty nodes to continuously record the temperature distribution of different positions of the human body. P3D-eskins offer a high degree of wearing comfort and biocompatibility to skin health. Wearing the P3D-eskin during intensive exercise did not lead to the accumulation of sweat, thus avoiding skin dampness, allergy, and inflammation, while the skin covered by the PDMS-eskin showed obvious skin erythema.
Overall, this work is the result of years of effort and the hard work of many people, especially Prof. Zijian Zheng, Dr. Qiuna Zhuang, Prof. Xinge Yu, and Dr. Kuanming Yao, and we believe P3D-eskin will potentially serve as a new digital healthcare platform with both high integration density and wearing comfort for the users.
References:
- Rogers, J. A., Someya, T. & Huang, Y. Materials and mechanics for stretchable electronics. Science 327, 1603–1607 (2010).
- Matsuhisa, N., Chen, X., Bao, Z. & Someya, T. Materials and structural designs of stretchable conductors. Chem. Soc. Rev. 48, 2946–2966 (2019).
- Gong, S., Lu, Y., Yin, J., Levin, A. & Cheng, W. Materials-Driven Soft Wearable Bioelectronics for Connected Healthcare. Chem. Rev. 124, 455–553 (2024).
- Ding, Y. et al. Porous Conductive Textiles for Wearable Electronics. Chem. Rev. 124, 1535–1648 (2024).
- Lee, S. et al. Permeable Bioelectronics toward Biointegrated Systems. Chem. Rev. (2024) doi:10.1021/acs.chemrev.3c00823.
- Huang, Q. & Zheng, Z. Pathway to Developing Permeable Electronics. ACS Nano 16, 15537–15544 (2022).
- Zhuang, Q. et al. Wafer-patterned , permeable , and stretchable liquid metal microelectrodes for implantable bioelectronics with chronic biocompatibility. Sci. Adv. 9, eadg860 (2023).
- Huang, Z. et al. Three-dimensional integrated stretchable electronics. Nat. Electron. 1, 473–480 (2018).
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