ACADEMIC RESEARCH • 學術研究 2023 UMAGAZINE 28 • 澳大新語 61 Electronic skin (E-skin) is a novel, lightweight, and flexible wearable device that mimics the sensory function of human skin. By collecting mechanical stimuli and converting them into electrical readouts, E-skin acts as an important link connecting human and electronic terminals. It has attracted increased attention due to its potential applications in various fields, ranging from healthcare monitoring to intelligent perceptions. Therefore, our research group has developed a lightweight, self-powered, and wearable patch by incorporating the principles of ‘electromagnetic induction’ and ‘intrinsic oscillation of elastomer’. This innovative patch offers a fresh perspective on designing future wearable human-machine interaction (HMI) interfaces in a more concise and convenient approach. A Law of Classical Physics: Electromagnetic Induction Faraday’s law of electromagnetic induction is a fundamental law in physics that serves as the basic operating principle of various inductors and generators. According to this law, a change in magnetic flux can induce an electromotive force (EMF) when a conductive coil layer is present. This classical law inspired us to explore the conversion of mechanical energy into electrical energy by altering the spatial distribution of magnetic flux. By leveraging this principle, flexible E-skin can operate in a self-powered manner, reducing reliance on external power consumption, provided that it is comprised of flexible magnetised components and conductors. Nature-Inspired Intrinsic Oscillation of Elastic Microstructures According to Faraday’s law, the time scale also plays a role in determining the magnitude of the EMF. A more significant EMF signal can be obtained if the magnetic flux variation is completed within a shorter duration. This is essential to improving accuracy and avoiding noise interference in real-world applications. Inspired by natural phenomena, many elastic systems exhibit eigenfrequency, which is primarily determined by the properties of their structure. For example, the frequency of a block-spring system is determined by the spring constant and block mass, while the period of a simple pendulum depends on the length of the string. Our research group found that artificial elastic micro-structures (micropillars) also follow rules similar to those of natural behaviour. The micropillars are mainly composed of elastomer gel and microparticles (NdFeB) that can produce a permanent magnetic moment after magnetisation. In subsequent studies, we demonstrated that the eigenfrequency of the elastic micropillars can be tuned by simply adjusting their dimensional parameters or physical properties. These findings enable us to significantly shorten the oscillation period of the magnetised elastic micropillars, thereby generating more pronounced electrical signals and ensuring accurate communication. Magnetic Dipole for Direction-Aware Sensing In nature, magnets have two poles, the north pole and the south pole, which form a magnetic dipole. If you break a permanent magnet, the two daughter magnets will each have a north pole and a south pole. The same rule applies to even smaller magnets such as the micron-sized micropillars, which, once magnetised, also have a pair of north and south poles. Based on this observation, our research group believes that the magnet’s north and south poles could serve as a mechanism to perceive the force direction. When a magnetised micropillar is deformed towards the north or south side, a negative or positive variation of the magnetic flux in the conductive coil will be generated. These ultimately become EMF signals of ‘+/-’ or ‘-/+’ that can be clearly distinguished and accurately identified by the electrical terminal. Designing a Wearable Patch for In-Plane Force Perception A single magnetised micropillar can only respond to two opposite directions of force in one dimension based on the positive and negative signals of the induced EMF. To address this limitation, our research group discovered that the number of voltage peaks can identify the in-plane force from different axial directions (+X, -X, +Y, and -Y). We developed an interface for measuring directional in-plane force by precisely designing
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