A recent study in Nature Communications introduced an all-optical mechano-sensor employing a thin-walled glass microbubble inspired by mechano-sensitive hair-like sensilla (MSHS). Capable of detecting forces and multidirectional displacements within the range of 0.9 μN and 70 nm, this device showcases advancements in artificial perception and mechano-transduction. Its unique design suggests promising applications for real-time, directional mechano-sensory functions in robotics.
a Schematic illustration of the cat whisker sensory system and whisker mechanoreceptor anatomy. b Schematic diagram of a fish lateral line system and a neuromast cross-section. c Schematic diagram of a spider and the tactile hair protruding from its exoskeleton’s surface. d Schematic diagram of the mechano-sensor. A microbubble with a micro-hair, similar to the whisker of a cat, is used to detect tactile stimuli, analogous to a sensory receptor. A fiber taper is used to transmit signals, which is similar to signal transmission by nerve fibers. The polymer matrix can fix the microbubble and fiber taper and protect them from environmental damage, which is similar to the function of the skin. The inset shows an image of a microbubble coupled with a fiber taper under a microscope. The scale bar represents 200 μm. e Experimental setup of the all-optical mechano-sensor system. The blue arrow indicates the direction of the external force. f Responses of the bioinspired mechano-sensor in different deformation states (i.e., Fr = 0 mN, 0.137 mN (φ = 180°), and 0.128 mN (φ = 0°)). Image Credit: https://www.nature.com/articles/s41467-024-47299-0
Optical Sensing for Environmental Perceptions
Artificial sensing harnesses optics and photonics to perceive the environment, including pressures, visual information, detecting molecules, and more. Optical sensing devices are often superior to electronic devices due to properties such as low loss, noncontact operation, and high sensitivity. Additionally, these systems help detect vibrations and forces in artificial systems and nature because they offer mechanical perception.
Organisms in nature, such as mammals, arthropods, and fish, possess MSHS, which aids them in sensing acoustic and mechanical signals in the environment. MSHS comprises sensory neurons and micro-hair antennae, helping them to sense vibrations and forces from various directions. Hence, this biological framework can be valuable for building artificial systems capable of sensing mechanical signals in multiple directions.
While most biomimetic or biological MSHS utilize conventional setups, such as microhairs and electrode arrays, to convert mechanical stimuli into electronic signals, an optical mechano-sensor micro-hair offers potential advantages like enhanced performance, simplified design, and improved anti-interference capabilities. By employing microbubble structures and integrated optics on a glass-microbubble resonator, a straightforward all-optical configuration can efficiently convert multidirectional acoustic and mechanical signals.
Fabrication of Photonic Mechanic-Sensor
The present study utilized a thin-walled glass microbubble to integrate an all-optical mechano-sensor that mimics MSHS as a flexible whispering-gallery-mode resonator. Using a specific technique, fused silica was used to produce thin-wall microbubbles with the help of a fiber fusion splicer.
Subsequently, it was then positioned carefully by five-dimensional stages, with a fiber taper and covered with a polymer matrix with a low refractive index. This enclosing method helps improve the system's stability, emulates the functional and structural traits of natural MSHS, and reduces external disturbances. Sensing experiments were conducted indoors at 40 % humidity and 22 °C.
Outcomes and Observations
The sensor operates based on optical resonance and effectively detects pressure and touch. For the micro-hair component, a fused silica capillary with a radius of 84 μm and a wall thickness of 8 μm was selected, while a microbubble with a radius of 185 μm and a wall thickness of 1.5 μm was fabricated for mechano-opto-transduction.
The polymer matrix utilized was MY-133-V2000, boasting an elastic modulus of 5.2 MPa and a Poisson's ratio of 0.41. These choices were informed by the actual fabrication conditions and finite element method (FEM) analysis results.
The micro-hair's radial plane exhibits a good 32.31 dB directionality for the mechano-sensor, with maximal displacement and force sensitivities of 0.052 pm μm−1 and 3.994 pm mN−1, respectively. The force and displacement sensitivities in the axial direction of the microhair are 0.986 pm mN−1 and 1.570 pm μm−1, respectively.
The capacity of mechano-sensors to withstand interference from various sources, such as seawater and temperature, was verified. The mechanosensor exhibited outstanding temperature-displacement decoupling capacity, indicating its potential application in fully integrated, all-optical, multifunctional perception systems.
Aperiodic airflows and water droplets are among the many quickly identified stimuli, and details about their frequency, direction, strength, and distinctive spectrum profiles of the pulses and peaks were obtained. The all-optical mechano-sensor was incorporated into a quadruped robot resembling a cat to help further display its capability as a directionally aware, real-time mechano-sensory whisker.
The sensor consistently detects obstacles when the robot moves; it is unaffected by jitters and shows no interference with sensing obstacles. In contrast to most reported electronic whiskers, the novel photonic whisker is cost-effective and attains comparable or even quicker reaction and recovery times.
Conclusions
In summary, this study efficiently demonstrated a proof-of-concept for a bioinspired all-optical mechanosensor utilizing micro-hair to mimic MSHS. This integrated device comprises a glass micro-hair acting as a hair probe, a fiber taper facilitating evanescent coupling, and a thin-walled glass-microbubble WGM (whispering-gallery mode) resonator for mechano-opto-transduction.
The photonic mechano-sensor micro-hair boasts a simpler configuration compared to both its biological counterparts and other artificial electronic hair sensors. This simplicity arises from its mechanically flexible and heterogeneously 3D-integrated micro-optics. As a result, it can effectively detect vibrations, forces, and displacements across a range of waveforms, directions, and driving frequencies.
The research outcomes highlight the fabricated mechano-sensor as a promising candidate for diverse applications, including vibration detection systems, AR/VR technology, robotic perception systems, robotics, and the metaverse. Future works, building upon the prototype configuration, could incorporate innovative 3D top-down nano/micro-fabrication techniques to further enhance its capabilities.
Journal Reference
Li, Y., Guo, Z., Zhao, X. et al. An all-optical multidirectional mechano-sensor inspired by biologically mechano-sensitive hair sensilla. Nat Commun 15, 2906 (2024). https://doi.org/10.1038/s41467-024-47299-0, https://www.nature.com/articles/s41467-024-47299-0