Flexible mechanical sensors mimicking human mechanoreceptors

• Main Author: Li, Yanzhen
• Other Authors: Chen Xiaodong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Flexible sensors
• Online Access: https://hdl.handle.net/10356/181537
• Institution: Nanyang Technological University

Mechanoreceptors, specialized sensory cells, are located in the skin and other tissues that respond to mechanical inputs such as pressure, stretch, and vibration. These receptors are essential to human perception, supporting essential functions like sensing touch (tactile sensation) and body position (proprioception). Working together, they enable core human experiences, such as movement coordination and environmental interaction, underscoring their importance in daily life. Tactile sensors, which are designed to replicate the sense of touch, are crucial for collecting data from physical interactions with the surrounding environment. These sensors enable devices and machines to detect and react to various mechanical stimuli, supporting functions such as object manipulation and environmental monitoring. Piezoresistive, capacitive, piezoelectric, and other mechanisms have been explored. However, challenges still exist, particularly hysteresis, which refers to the variation in sensor readings when external forces are applied and then removes. In other words, the output signals are influenced not only by the current stimulus but also by previous ones, leading to errors and inconsistencies in measurements. Hysteresis in tactile sensors is primarily caused by two factors. First, energy dissipation occurs during cyclic mechanical stimuli due to internal friction between molecules or within the material's structures. Second, the viscoelastic materials often used in flexible electronics tend to have slow response times. When the rate of cyclic testing exceeds the material’s ability to recover, a delay in sensor output occurs, which amplifies the hysteresis. To address these issues, elastic metal microtubes have been developed. Metal materials are chosen for their low viscoelasticity, which helps minimize energy dissipation during mechanical deformation. Additionally, the microtube structure enhances elasticity, allowing for faster recovery and reducing the lag in sensor response. By incorporating these elastic metal microtube, a tactile sensor with low hysteresis is achieved, improving both performance and reliability for practical applications. Proprioceptive sensors are designed to replicate the function of proprioceptors, providing critical feedback on the location, orientation, and motion of parts within a system or device. This feedback is indispensable for applications requiring precise and adaptive control, such as robotics and human-machine interfaces. Various proprioceptive sensors exist, including resistive, magnetic, capacitive, and optical sensors. Among these, resistive sensors are valued for their straightforward design and cost-effectiveness. However, significant challenges still remain, like low sensitivity and vulnerability to temperature variations. To address these issues, two distinct resistive proprioceptive sensors have been designed for surface reconstruction. The first sensor focuses on large area reconstruction and temperature-independent performance. It achieves enhanced sensitivity through a differential circuit design rather than directly measuring resistance. Additionally, pairs of resistive elements made from the same materials are positioned on opposite sides of the substrates in identical locations, thereby eliminating temperature influence via signal subtraction. The second proprioceptive sensor is designed for precise local bending detection, providing decoupled information between bending angles and bending axis angles. By integrating this sensor into flexible ultrasound arrays, the system allows real-time imaging correction under bending deformation. In conclusion, the low-hysteresis tactile sensor and the resistive proprioceptive sensors have been designed to mimic the roles of human mechanoreceptors. By addressing critical issues such as hysteresis and sensitivity, these sensors improve the precision and reliability of systems requiring fine control and adaptability. As a result, they hold great potential for a range of applications, including robotics, wearable technology, and human-machine interfaces.