On-skin devices have shown great potential for applications in healthcare, behavior monitoring, individual protection, self-powered electronics, and human–machine interaction. With the overwhelming evolution of these fields, on-skin devices are further required to achieve comfortable long-term use with diminished sensory interference. They are even expected to detect action and touch in natural states with minimal loss of touch sensation for more sophisticated applications, such as machine learning of a craftsman’s skills and restoration of limb function. To render these on-skin devices with imperceptibility, various ultrathin, ultralight, gas-permeable functional films are fabricated by electrospinning. However, limited by the random spinning-deposition manner of electrospinning, the functional surfaces of reported imperceptible films are flat planes with inferior optical, thermal, mechanical, and electrical properties, resulting in low performances and restricted application scenarios of imperceptible on-skin devices. The challenge, consequently, is to develop new microstructured material to provide various imperceptible on-skin devices with superior performances.
Recently, researchers led by Profs. L. Pan and Y. Shi (J.-H. Zhang, Y. Ke, Prof. Q. Zhang et al. at Nanjing University, Z. Li at Hohai University, Prof. J. Xu at Shanxi Provincial People’s Hospital, and Prof. J. Du at Inner Mongolia University of Science and Technology) constructed unique, ultrathin, ultralight, gas-permeable electrospun micropyramid arrays (EMPAs) by electrospinning self-assembly to endow imperceptible on-skin devices with excellent performance in various applications (Figures 1, 2). A series of wet heterostructured electrified jets can be assembled into structurally designable EMPAs made from various materials. Benefiting from flexible designability, the optimal optical, thermal, mechanical, and electrical properties of EMPAs are exploited to achieve outstanding performance for imperceptible on-skin devices applied in daytime radiative cooling (temperature drop ~4 °C under one sun), pressure sensing (sensitivity ~19 kPa–1), and biomechanical energy harvesting (conversion efficiency ~42%).
Architecture, growth process, and designability of EMPAs.
J.-H. Zhang et al. found that the formation of EMPAs goes through three stages (Figure 3a): (i) inhomogeneously charged microdomains stemming from the deposition of wet heterostructured electrified jets, (ii) fibrous microdomes carrying negative charges on their tops and connected by means of suspended fibers, and (iii) fibrous micropyramids with negative charges on their tops that can continue to grow. The self-assembly technology is flexible for the structural and material design of EMPAs. Firstly, various parameters, such as voltage, humidity, and temperature in electrospinning self-assembly, can be adjusted to obtain EMPAs with desired sizes (Figure 3b). Moreover, the self-assembly technology is adaptable in that various available materials can be processed into EMPAs, such as PVDF, thermoplastic polyurethane (TPU), and poly(vinyl alcohol) (PVA) (Figure 3c).
Various applications of EMPA on-skin devices with high performance.
Since gradient micropyramid geometry can bring about advanced optical, thermal, mechanical, and electrical properties, EMPA-based on-skin devices exhibit high performance in various application fields, such as daytime radiative cooling, pressure sensing, and bioenergy harvesting. A temperature drop of ~4 °C is obtained via an EMPA-based radiative cooling fabric under a solar intensity of 1 kW m–2 (Figure 4a, b). Additionally, EMPA nanogenerators with high triboelectric and piezoelectric outputs (transfer charge density 105.1 µC m–2, energy conversion efficiency 42%) achieve reliable biomechanical energy harvesting (Figure 4c). More importunately, the high sensitivity (19 kPa–1), ultralow detection limit (0.05 Pa), ultrafast response (≤ 0.8 ms) and good imperceptibility make the EMPA pressure sensor suitable for long-duration bio-health and motion monitoring for particular types of workers, such as drivers and eSports players, without affecting their normal manipulation (Figure 4d–g). Even ultraweak reflected systolic peak of the fingertip pulse and natural finger manipulation over a wide frequency range can be successfully detected by means of this sensor. The findings open an upgradable way for developing next-generation on-skin devices with both superior performance and imperceptibility to meet the high-level requirements in multiple application scenarios.
For more details, please see “Jia-Han Zhang, Zhengtong Li, Juan Xu, Jiean Li, Ke Yan, Wen Cheng, Ming Xin, Tangsong Zhu, Jinhua Du, Sixuan Chen, Xiaoming An, Zhou Zhou, Luyao Cheng, Shu Ying, Jing Zhang, Xingxun Gao, Qiuhong Zhang, Xudong Jia, Yi Shi & Lijia Pan, Nature Communications, 2022, 13, 5839. https://doi.org/10.1038/s41467-022-33454-y”.