Localizing strain via micro-cage structure for stretchable pressure sensor arrays with ultralow spatial crosstalk

The micro-cage structure is formed with the help of the photo-reticulated strain localization films (prslPDMS), which shows the strain local confinement effect and could be used to prepare ultralow crosstalk sensor arrays.
Published in Materials
Localizing strain via micro-cage structure for stretchable pressure sensor arrays with ultralow spatial crosstalk

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Large-scale and high-density flexible sensing arrays are very important for tactile detection, but there is usually large crosstalk in such devices, which will greatly weaken the detection accuracy. Crosstalk such as leakage, breakdown or external electromagnetic interference can usually be avoided with the help of electronic components or back-end signal processing, but mechanical crosstalk caused by deformation in flexible devices cannot be eliminated using the above methods. This mechanical crosstalk can sometimes be used to enhance local stimulation, but it also requires a large amount of data analysis to achieve precise detection. Therefore, it is essential to design a novel structure to reduce the mechanical crosstalk. Herein, the authors demonstrate the photo-reticulated strain localization films (prslPDMS) to prepare the ultralow crosstalk sensor array, which form a micro-cage structure to reduce the pixel deformation overflow by 90.3% compared to that of conventional flexible electronics.

As shown in Fig. 1b, doping benzophenone into PDMS will inhibit its crosslinking when exposed to UV light, thus forming the photo-reticulated PDMS. The micro-cage structure is formed after encapsulating with another layer of electrodes, and pressure sensor could be prepared within the cage. Besides, the boundary of the micro-cage composed by photo-reticulated PDMS could separate different sensors and prevent unnecessary deformation diffusion, showing the effect of strain local confinement, so it is called photo-reticulated strain localization PDMS (prslPDMS). A simple two-dimensional model and deformation simulation analysis are proposed to analyze the deformation of micro-cage structure under external pressure (Fig. 1c-d).  The results demonstrate that the former realizes the small elongation strain due to the strain confinement of prslPDMS spacer, but PET substrate with high toughness and non-stretchable characteristics shows the large deformation. 

Fig. 1 | Principle of ultralow crosstalk sensor with micro-cage structure based on prslPDMS layer. a Schematic illustration of multiple mechanoreceptors in glabrous skin for accurate tactile perception. b Proposed chemistry of benzophenone inhibiting PDMS crosslinking under UV light to prepare prslPDMS layer. c Simple geometric analysis of micro-cage structure formed by prslPDMS under external pressure. d Two-dimensional deformation simulation analysis of the PDMS & prslPDMS and PET under external pressure, and the micro-cage structure of the former enables the small elongation strain compared with the latter. e Quantitative analysis of displacement variation along the x-axis for different models, and the displacement of adjacent pixel using prslPDMS layer decrease by 90.3% compared with PET. f Crosstalk isolation versus the ratio of spacer length to pixel length, the better isolation effect could be achieved when its value is greater than 1: 2.

Based on the above principles, the authors prepare ultralow crosstalk sensor array, which consists of three parts: the patterned Ag NFs interdigital electrodes, the patterned prslPDMS layer and graphene attached to the PDMS with pyramid microstructures. The prslPDMS layer possesses the similar stretchability (~ 5.08MPa) and transmittance (91.85% in visible light) with PDMS, allowing the preparation of transparent stretchable devices. Since the high precision transparent stretchable prslPDMS film could achieve strain local confinement, it can be used to prepare multilayer devices and improve the mechanical stability, which could provide a solid foundation for more sophisticated electronics (Fig. 2d). Figure 2e and 2f show the 6 × 6 sensor array is well attached on the palm no matter in the flat or curly state. 

Fig. 2 | Structure and design concept of the pressure sensor. a Schematic illustration of the structure of pressure sensor arrays. b SEM images of prslPDMS layer with different resolutions, which could reach 100 μm; The right photographs demonstrate the patterns on various substrates (top: glass, bottom: silicon wafer), indicating its good adaptability. c Tensile properties (left) and UV-visible spectra (right) of prslPDMS exhibit the excellent stretchability and transparency. d Schematic showing the design concept for the ultralow crosstalk sensor, where the prslPDMS layer perform the effect of strain local confinement for mechanical stability as well as multilayer device. Optical photographs of sensor arrays attached on palm no matter in the flat (e) or curly (f) state, which achieve the transparency of 50.36 % and thickness of 60.59 μm.

Additionally, the authors further analyze the mechanical crosstalk isolation of the micro-cage structure formed by prslPDMS in three-dimensional model. Similarly, the large Poisson's ratio of PET will lead to its lack of stretchability, so when three pixels are under external forces at the same time, it will inevitably cause the surrounding pressure. The deformation spillover to adjacent pixel could be alleviated by using the stretchable materials, such as PDMS, but it still cannot be eliminated well. The cage-structured sensor based on prslPDMS layer could effectively avoid the diffusion of strain, and clear multi-pixel stimulation could be obtained through direct measurement, greatly reducing the back-end calibration processing (Fig. 3c). The crosstalk isolation of prslPDMS model is calculated as 32.14 dB, which is significantly better than the devices using PDMS (11.86 dB) or PET (5.38 dB) substrate. Hence, the sensor array with prslPDMS layer could show the clear pressure imaging of the mold letter "B" (Fig. 3d).

Fig. 3 | Measurement and analysis of ultralow crosstalk sensor arrays. a Three-dimensional analysis of micro-cage structure formed by prslPDMS layer to isolate mechanical crosstalk, and the stressed pixels will affect the adjacent pixels from three directions (0°, 45°, 90°). b Displacement of PET and prslPDMS spacer model in different directions, and the model with spacer layer has almost no additional deformation to adjacent pixels. c Crosstalk simulations and corresponding voltage variation of sensors based on PET, PDMS and PDMS & prslPDMS layer, it could be found that the sensor arrays using the prslPDMS layer has less mechanical crosstalk and a clear pressure imaging. d A 6 × 6 sensor array used for pressure detection, and the imaging of the letter "B" could be seen clearly for the device with the prslPDMS layer. e Multi-channel voltage variation of sensor array to detect the letter "B". f Demo of application scenarios. The pixels of the sensor array are divided into four regions, which could be used to control the model movement in the game. 

More details can be found in our paper "Localizing strain via micro-cage structure for stretchable pressure sensor arrays with ultralow spatial crosstalk" published in Nature Communications.

Link to article:  https://www.nature.com/articles/s41467-023-36885-3

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