For flexible electronic skin, the structure typically consists of three parts: stretchable substrate, sensitive functional component, and stable packaging layer. In practical applications, polydimethylsiloxane (PDMS) (Nan et al., 2024), ecoflex (Li N. et al., 2024), dragon skin (Liu et al., 2024) and styrene-ethylene/butylene-styrene (SEBS) (Shao et al., 2023) films are widely used as the stretchable substrates. To enhance the sensitivity of the electronic skin, a prevalent approach is to integrate the sensitive functional component. In this regard, flexible electronic skin based on highly sensitive piezoelectric polymers and piezoelectric ceramics have been widely employed, facilitating the accurate detection of diverse stimuli (Ha et al., 2019; Han M. et al., 2019; Mahapatra et al., 2021). In addition to piezoelectric materials, most of the functional materials are used as conductors, either by penetrating within the elastic matrix or being dispersed on its surface to form piezoresistive sensors, such as carbon-framed materials (especially carbon nanotubes (CNTs) (Lin et al., 2022), graphene (Zheng et al., 2020), carbon black (CB) (Shao et al., 2023), and carbides (Mxene) (Guo et al., 2021)), metal-based nanomaterials (Raman and Arunagirinathan, 2022), inorganic semiconductor materials (Hu et al., 2024) and hydrogels (Tang et al., 2024).

Among these materials, graphene exhibits exceptional electrical conductivity and is widely employed. For instance, Figure 2A illustrates the graphite nanoplates penetrate with polyurethane (PU) to form nanocomposite film, which demonstrates superior electrical conductivity and extremely high flexibility (Wu et al., 2017). However, the percolation-based sensor rarely exhibits good linear performance on a larger scale due to the uneven mixing and the uncontrollable disruption of conductive filler network. Therefore, surface modification of the mentioned material is required to gain desirable mechanical properties (Lin et al., 2022). In addition, laser-induced graphene combined with an elastic ecoflex polymer forms a stretchable electronic skin that demonstrates high sensitivity and stability (Li Y. et al., 2024). Han Z. et al. (2019) drop casted the carbon black (CB) solutions onto airlaid paper to obtain an ultrahigh sensitivity and flexible pressure sensor. Song et al. (2023) presented a 3D-printed pressure sensor consisting of an interdigital MXene electrode and a porous CNT-PDMS active layer, which yields the highest sensitivity due to the increased contact area, as shown in Figure 2B.

Although promising, ongoing challenges for these carbon-framed functional materials-based devices typically suffer from device-to-device variation. In contrast, well-established inorganic semiconductor-based strain sensor exhibits uniformity and consistency in the device performance, demonstrating the omnidirectional capability (Figure 2C) (Hu et al., 2024).

Flexible electronic skin using metal nanowires have attracted notable attention, attributed to their high electrical conductivity and mechanical flexibility. Figure 2D illustrates a micromolding-based method for printing silver nanowires (AgNWs), which demonstrates potential applications in soft electronics (Liu et al., 2022).

As a promising alternative, hydrogels have gained widespread attention due to their exceptional high stretchability, good conductivity, biocompatibility, and mechanical properties. Figure 2E shows a programmable microfluidic-assisted hydrogel patch that features high stretchability and impressive conductivity (Liao et al., 2024). In addition, Wang et al. developed a stretchable hydrogel-based multimodal electronic skins that can self-calibrate the sensing of any two of three stimuli: strain, temperature, and humidity (Wang W. et al., 2024).

Stable packaging materials play a pivotal role in the performance of electronic skin, providing long-term durability under various conditions (Zhou et al., 2024). The materials such as parylene (Mariello et al., 2021), polyimide (Kim et al., 2018), PDMS (Li J. et al., 2024) and SEBS (Yi et al., 2023) are often employed as the packaging materials for 2D electronic device. For instance, Wang X. et al. (2024) employed PDMS silicone gel to package the flexible pressure sensor array, which serves to isolate the air. This isolation effectively safeguards oxidative behavior of the electrode and sensitive layers within the sensor, thereby ensuring the long-term usability of the sensor. For three-dimensionally (3D) architected electronic skin, Liu et al. (2024) proposed a heterogeneous encapsulation strategy that encapsulates functional components with different layers and materials, which ensures that the surrounding soft materials have similar mechanical properties to the human skin.

In addition to the material point of view, the property of flexible electronic skin can be improved from the engineering strategies, including ultra-thin films, serpentine design, micro-nano structures. The ultra-thin film ensures low modulus mechanics, which endows the sensor with good conformability to curved surfaces, avoiding any significant constraint on natural motions of the skin, when undergoing pressing or stretching (Wu et al., 2020). A dual-sacrificial-layer method has been employed to produce a CNT-based thinnest pressure sensor featuring a thickness of ≈850 nm, as shown in Figure 2F (Hu X. et al., 2023). The sensor achieves superior sensitivity and perfect conformability simultaneously. Most of the existing hydrogels are relatively thick and have poor air permeability. Zhang et al. (2024) presents a ∼10-μm-thick hydrogel sensor, which exhibits great skin compliance.

The electronic skins demonstrate certain stretchability on highly elastic substrates; however, the conductive path is susceptible to fracture when subjected to high strain. To enhance the stretchability of the electronic skin, designs incorporating serpentine patterns and island-bridge structures are proposed. These designs possess low stiffness and good stretchability, avoiding the generation of large or irreversible cracks in a certain strain region and ensure good electrical interconnection between the sensing units (Jo et al., 2024). By analyzing various stretchable 2D and 3D sensing units, including zigzag, rhomb, serpentine, net, wave and spring, the results show that 2D structures have the superiority in thickness and attachable comfort as wearable devices attached on tendons (Shu et al., 2021). Taking advantage of the serpentine structure, Jang et al. (2022) proposed heterogeneous serpentine ribbons, which enable ambulatory electrodermal activity monitoring on the palm in free-living conditions, as shown in Figure 2G. Cai et al. (2021) reported a multifunctional electronic skin based on a patterned serpentine metal film for tactile sensing of pressure and temperature, which exhibited excellent flexibility and wearability. A stretchable ultrasound probe that exploits an island-bridge layout with multilayer electrodes, showing excellent stretchability (Zhu et al., 2018) (Figure 2H). In addition to serpentine patterns and island-bridge structures, spiral form, kirigami-inspired structures and watch-chain shapes (Li et al., 2020; Meng et al., 2022; Che et al., 2024) have also been used.

Micro-nano structures located on the surface or inside the sensing layers have demonstrated their effectiveness in enhancing the sensitivity, stretchability, and stability of the flexible electronic skin. Various forms of micro-nano structures have been employed, such as porous (Xiao et al., 2024), micro-pyramids (Hu X. et al., 2023), nanofibers (Bai et al., 2023), cracks (Zhang et al., 2023), nanowires (Won et al., 2019), interlocking structure (Ilami et al., 2021), interface non-uniform structure (yarn structure) (Liang et al., 2020) and so on (Niu et al., 2021).

Compared with ordinary planar films, the porous structure can achieve higher tensile and perceptual properties (Park et al., 2012). Unlike the salt-templating method (Wang H. et al., 2021), the laser ablation can produce thinner porous structures. The porous structure of the laser-scribed graphene and PU nanomesh-based electronic skin can be attached on finger with clear fingerprint morphology (Qiao et al., 2022). The flexible electronic skin, which utilizes pressure-induced changes in the contact area of microstructures, typically demonstrates superior sensitivity and faster response times compared to alterations in the conductive network of porous structures (Liu et al., 2021). Figure 2I (Yang et al., 2023) presents a flexible electronic skin with gradient pyramidal microstructures fabricated by a multiple laser ablations method. The sensor allows the applications in subtle pulse detection, interactive robotic hand, and ultrahigh-resolution smart weight scale/chair. Besides, the all-fibrous based sensitive mechanoacoustic sensor demonstrates excellent heart signal detection ability (Zhi et al., 2023).

The spider-inspired cracked multifunctional electronic skin has been developed by employing a location-programmable uniaxial stretching method (Zhang et al., 2023). Owing to the strain sensitive cracked straight electrode, the device exhibits a high sensitivity. By employing a laser-induced nanoscale cracking method, Kim et al. (2020) combined the aforementioned engineering strategies and developed a novel flexible electronic skin featuring an ultra-thin crack-based layer and serpentine patterned structure with a thickness of ∼50 μm, as shown in Figure 2J. The sensor allows conformal contact with the epidermis, enabling a more direct measurement of skin deformation.