The rapid evolution of bionic flexible tactile sensors is driven by the overarching goal of emulating human tactile perception to augment robots' perceptual acuity. Conventional electric sensing paradigms grapple with a myriad of challenges, including elevated manufacturing costs and susceptibility to signal interference. Meanwhile, due to the small size, strong flexibility, and high sensitivity, optical sensing modalities are pushing micro/nano fibers (MNFs) into the spotlight. Domestically, the Zhejiang Lab is at the forefront of developing various MNF-based sensors, enabling single/dual-modal detection for applications in human-machine interaction and physiological parameter monitoring. Nevertheless, the challenge of balancing sensitivity and operational range remains unresolved in current methods, compounded by susceptibility to wear-related issues. Thus, we introduce a micro/nano fiber-based flexible tactile sensor unit inspired by fingertip skin microstructures (FIMF). By simulating the biological microstructures and tactile conduction mechanisms of fingertip skin, FIMF achieves the detection of mechanical stimuli and object feature recognition. The advanced sensor structure and functional attributes are significant for applications in flexible bionic devices and advanced robotics technology.

Firstly, the proposed flexible tactile sensing unit FIMF is inspired by the microstructure of fingertip skin and is achieved by embedding an MNF between two layers of polydimethylsiloxane (PDMS) films. The structure is further enhanced by introducing two layers of elastic resin annular ridges on the surface, each with varying stiffness. This design aims to replicate the intricate microstructure of biological fingertip skin and its underlying tactile conduction mechanism. Subsequently, we delve into the influence of PDMS film thickness and the dimensions of the annular ridges on the tactile pressure response of the FIMF sensor. Based on meticulous simulation results, the optimal sensor parameters are identified with a PDMS film thickness of 50 µm, an upper annular ridge thickness of 0.2 mm, and a lower annular ridge thickness of 0.4 mm. Furthermore, we extensively examine the FIMF sensor's response to diverse tactile stimuli, including static and dynamic pressure, and vibrations. Finally, the FIMF's ability to discern object hardness and surface textures is investigated by employing a synergistic approach integrating the mechanical finger's travel distance and the FIMF force feedback to discern object hardness characteristics. Meanwhile, we conduct waveform analysis of transmitted intensity changes over time to perceive and compute object texture. The pursuit of further insight into different textures is accomplished by the application of short-time Fourier transform (STFT) to extract frequency domain features.

The experimental findings underscore that the devised FIMF inspired by the microstructures of fingertip skin presents an amalgamation of wide-ranging dynamic detection capabilities and high sensitivity. Remarkably, it boasts response and recovery times of less than 100 ms, providing the sensor with the capacity to swiftly discern mechanical stimuli (Fig. 7). Furthermore, the sensor exhibits exceptional robustness and elevated static/dynamic stability, which is a testament to the robust encapsulation of its diverse structural layers (Fig. 8). Expanding its sensing range is proven instrumental in significantly enhancing the sensitivity for minute pressure ranges (0-2 N), thereby achieving an enhancement of approximately fourfold compared to recently reported MNF tactile sensors. A pivotal facet arises from the microstructure integration to amplify tactile mechanical stimuli and translate them into MNF deformations. This innovative approach does not need to employ tapering processes that would require reducing the MNF diameter to below 2 µm, which not only streamlines manufacturing but also augments the overall structural resilience (Table 1). In object hardness/texture perception, the FIMF divulges a pertinent trait that the transmitted intensity diminishes with the escalating hardness. This phenomenon arises because stiffer objects induce greater forces and stresses during contact, thus culminating in a more conspicuous attenuation of optical intensity (Fig. 9). The FIMF employs a spatial frequency-based characterization for discerning object texture, and the texture wavelength is derived by dividing the sliding speed by the dominant frequency. Additionally, the STFT of the transmitted light intensity signal provides a richer depiction of intensity fluctuations over time. During scans across regular surface patterns, the light intensity signal engenders periodic motifs at frequencies below 10 Hz. Notably, the number and positioning of these motifs amplify in tandem with increased scanning speeds in the temporal domain (Fig. 11).

We propose a novel micro-nano optical fiber flexible tactile pressure sensor inspired by the fingertip skin microstructure. This sensor combines force sensing with object hardness/texture detection capabilities. The sensor's force conduction performance is enhanced by bionic design to offer a wide detection range (0-16 N), high sensitivity (20.58% N^-1), short response time (86 ms), extended lifespan, and low cost. By demonstrating its functionality, we directly connect this soft sensor to a robotic manipulator, enabling it to differentiate between soft and hard objects, perceive object textures, and measure gripping forces. Consequently, this sensor is suitable for robotic gripping operations. Thus, the proposed sensor possesses structural and functional features reminiscent of human fingertip skin and has promising potential for applications in bionic artificial skin and advanced robotics technology.