Fig. 1. Characteristic properties and diverse functions and applications of recently developed tactile sensors and soft actuators.

Fig. 2. Fabrication and packaging the optical micro/nanofibers. (a) Evolution of flame-heated taper drawing setup. i) An alcohol burner and its flame; ii) A stage assisted taper drawing setup; iii) An automatic taper drawing setup. Figure reproduced with permission from ref.²⁸, under a Creative Commons Attribution 4.0 International License. (b) Fabrication of MNF with controlled diameter. i) Control of MNF diameter by real time measuring the time interval between two drops; ii) SEM image of an as-fabricated MNF with uniform diameter; iii) Spiral MNF guiding 633-nm-wavelength laser. Figures reproduced with permission from: i-ii) ref.³³, © 2017 Optical Society of America; iii) ref.²⁸, under a Creative Commons Attribution 4.0 International License. (c) Fabrication of MNF array with an electric heater. i) Schematic of a taper drawing setup for fabricating MNF array; ii) Recorded the multimode-induced oscillation for 20 MNFs during the taper drawing process; iii) 20 MNFs in parallel. Figures reproduced with permission from ref.⁵², under a Creative Commons Attribution 4.0 International License. (d) Packaging the MNFs with PDMS. i) Micrograph of a microfluidic chip embedded MNF; ii) Bent PDMS packaged MNF in free space; iii-iv) PDMS packaged MNF attached on human skin (iii) and curved surface (iv), respectively. Figures reproduced with permission from: i) ref.⁵³, the Royal Society of Chemistry; ii-iii) ref.²⁸, under a Creative Commons Attribution 4.0 International License; iv) ref.⁵⁴, the Royal Society of Chemistry.

Fig. 3. Sensing mechanisms of MNF enabled tactile sensors. (a) Biconical MNF enabled tactile sensor. i) Schematic of biconical MNF embedded in a thin layer of PDMS; Inset: Optical field intensity distributions of 900-nm-wavelength light guiding along a 1-µm-diameter glass MNF embedded in a 5°-bent MNF sensor; ii) Optical images of MNFs with different shapes; iii) Optical response of an MNF tactile sensor to weak pressure. Figures reproduced with permission from ref.²⁸, under a Creative Commons Attribution 4.0 International License. (b) MNF ring resonator enabled tactile sensor. i) Schematic of the fabrication process of the MNF ring resonator. ii) Microscope image of the MNF ring resonator on the gold film. Scale bar: 200 µm; iii) Resonant wavelength shift of the MNF sensor. Figures reproduced with permission from ref.²⁷, © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) MNF coupler enabled tactile sensor. i) Schematic of MNF coupler sensor; ii) SEM image of an MNF coupler; iii) Optical response of the coupler sensor in a stretch and release cycle. Figure reproduced with permission from ref.⁹³, under a Creative Commons Attribution 4.0 International License.

Fig. 4. MNF tactile sensors for pulse monitoring. (a) Typical wrist pulse (i) and fingertip pulse (iii) sensors and corresponding waveforms (ii and iv). Figures reproduced with permission from: i) ref.²⁸, under a Creative Commons Attribution 4.0 International License; ii-iv) ref.²⁷, © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Single MNF enabled wrist pulse sensor for simultaneous temperature and pulse measurement. i) Schematic of the pulse sensor; ii) Typical spectral response of the pulse sensor before and after exercise; iii) Comparison of the artery pulse pressure waveform variation with wrist temperature of 22.5 °C and 40.0 °C. Figures reproduced with permission from ref.¹⁰², © 2022 Chinese Laser Press. (c) Spatiotemporal hemodynamic monitoring via configurable skin-like microfiber Bragg grating group. i) Schematic diagram of the measurement positions for the three cases; ii) The details of the BCG signal and pulse wave within one cardiac cycle. Case 1: simultaneous measurement of BCG and pulse wave at the carotid artery (CA). Case 2: simultaneous measurement of BCG and pulse wave at the radial artery (RA). Case 3: simultaneous measurement of BCG and pulse wave at the pedal artery (PA). Figures reproduced with permission from: ref.¹⁰⁴, under a Creative Commons Attribution 4.0 International License.

Fig. 5. MNF tactile sensors for human machine interaction. (a) MNF bending sensor enabled data gloves. i) Image of an MNF glove with five bending sensors; ii) Bending-angle-dependent output of a typical MNF bending sensor; iii) Image of a virtual hand controlled by an MNF data glove; iv) Image of a robotic hand controlled by an MNF data glove. Figures reproduced with permission from ref.²⁸, under a Creative Commons Attribution 4.0 International License. (b) MNF pressure sensor enabled wristband. i) Schematic of the MNF enabled wristband; ii) Schematic of the MNF pressure sensor; iii) Cross-section of a wrist with a three-sensor wristband, indicating the position of the sensors and tendons; iv) Photograph of a three-sensor wristband; v) Application of the wristband for controlling a robotic hand based on hand gesture recognition. Figures reproduced with permission from ref.¹²⁰, under a Creative Commons Attribution License. (c) MNF pressure sensor enabled smart textiles. i) Schematic of optical MNF enabled smart textiles. ii) Optical image of the as-fabricated smart textiles. iii) Logic control of a robotic hand via the smart textile. iv) Machine learning enabled emotional human machine interface. Figures reproduced with permission from ref.¹²², © 2022 Donghua University, Shanghai, China. (d) MNF humidity and pressure sensor enabled contact and proximity sensing. i) Schematic diagram of contact and proximity interaction; ii) Schematic of the contact and proximity sensor; iii) Responses of the sensor under finger approaching and contacting; iv) Photograph of the contact and proximity switch system for controlling the on/off of the LEDs. Figures reproduced with permission from ref.¹²⁵, © 2022 American Chemical Society.

Fig. 6. MNF tactile sensors for robots. (a) Finger-skin-inspired MNF sensor for force sensing and slip detection. i) Schematic of the finger-skin-inspired MNF sensor; ii) Photograph of a robotic gripper equipped with an MNF sensor; iii) Discrete wavelet transform of the response curves with three peaks. Each peak indicates a slip of the object grasped by the robotic hand. Figures reproduced with permission from ref.¹²⁸, © 2021 Wiley-VCH GmbH. (b) Multimodal and modular MNF sensors for multifunctional humanoid tactility. i) Schematic of the multimodal and modular MNF sensors; ii) A robotic hand equipped with the MNF sensor is ready to recognize and pick up a cup of warm coffee. iii) The responses of the MNF sensor for the texture recognition of CD, Fresnel lens and jean. Figures reproduced with permission from ref.¹²⁹, under a Creative Commons Attribution License. (c) Twisted MNFs enabled ultrasensitive temperature sensor. i) Schematic of the twisted MNFs sensor; ii) Response of hand proximity with different distances; iii) The robot with temperature feedback can effectively avoid undesired collisions. Figures reproduced with permission from ref.⁹², © 2023 American Chemical Society. (d) MNF-enabled compact tactile sensor for hardness discrimination. i) Schematic and photograph of the U-shaped MNF-enabled compact tactile sensor; ii) Optical response of the sensor to objects with different hardness; iii) Intensity curves corresponding to objects with different hardness; iv) Intensity curves corresponding to pork liver and an adductor muscle. Figures reproduced with permission from ref.¹³⁰, © 2021 American Chemical Society.

Fig. 7. MNF enabled photoactuators. (a) Schematic of the structure and driving mechanism of the photoactuator. (b) Response time of the photoactuator. (c) Photographs showing the light-driven bending of a photoactuator under different laser powers. (d) One-arm and two-arm OPA gripper capture and move ant (i), glued balls (ii), cuboid (iii, and iv). Figures reproduced with permission from ref.³⁶, under a Creative Commons Attribution 4.0 International License.