## 6 Sensors for Tactile Internet Applications
These sensors are being developed with integrated signal processing for low latency and higher accuracy [61]. However, designing the sensor with high durability, reliability, predictive maintenance, and real-time diagnostics is a challenge. * _Optical tactile sensors:_ These sensors are generally designed by using the fiber Bragg grating principle [58; 62] and are immune to EMI interference and capable of distributed sensing. Photonic crystal-based [63] tactile sensors use periodic nanostructures to detect mechanical forces, are ultra-sensitive, have a fast response time, and are non-contact sensing. An inbuilt camera pointing towards the tactile membrane is used to capture the properties of the objects which cause the deformations to the sensor's tactile membrane [64]. They can detect surface texture and pressure, offer high resolution, and are suitable for applications demanding thorough tactile feedback. However, they have lower sensitivity to stress, and their sensitivity decreases with the distance separating two sensing points. * Motion and Inertial Sensors  
Motion and inertial sensors are required to track movement and orientation. They includes accelerometers, gyroscopes, and inertial measurement units. These sensors play a crucial role in Tactile Internet applications, like robotic teleoperation, AR/VR, and exoskeletons [65]. Accelerometers are used to measure changes in acceleration, and, hence, they can track movements of the hand and body. MEMS-based accelerometers with low power consumption and high accuracy have been developed [66]. AI-based accelerometers can play a crucial role in predictive motion sensing. The gyroscopes can detect angular velocity and produce the corresponding electrical output for precise motion tracking in roboticapplications. Recently, MEMS-based gyroscopes with improved stability and reduced power consumption have been designed [67]. Researchers are exploring the possibility of designing AI-based miniaturized gyroscopes with high accuracy and adaptive motion tracking capabilities. The inertial measurement units (IMUs) integrate accelerometers and gyroscopes to provide six degrees of freedom (6 DoF) motion sensing. Furthermore, they also integrate a 3-axis magnetometer to make it a 9-axis IMU. IMUs are widely used for haptic gloves and industrial control systems. Magnetic sensors are also used to detect changes in a magnetic field using the Hall effect [68]. They measure strains based on the received magnetic field and are known for their low hysteresis, robustness, high sensitivity, and good repeatability [69]. Magnetic sensors are used to measure low-range normal and shear forces. * Biometric and Physiological Sensors  
In the Tactile Internet, biometric sensors are crucial for real-time human physiological monitoring for applications, such as health wearables, brain-computer interfaces, and prosthetic controls. For example, their use in prosthetics can provide amputees with a sense of touch in their artificial limbs (physiological sensing). The use of multimodal sensors that combine tactile sensing with other modalities (such as acoustic or visual) could enhance the capabilities of physiological or biometric sensing systems [58]. These sensors include electromyography (EMG), electroencephalography (EEG), and skin conductance sensors. EMG sensors detect muscle movements and are used to control robotic arms and prosthetic devices. The developments of flexible EMG sensors for wearable applications [70] and AI-based adaptive prosthetic control could be instrumental in realizing them in tactile applications. EEG sensors capture real-time brain signals that are electrically useful for brain-computer interaction. These sensors help with the neural control of haptic interfaces, prosthetic devices, VR, and real-time neurofeedback systems. Skin conductance sensors measure emotional responses [71] by detecting changes in skin conductivity and are used in immersive environments for adaptive haptic feedback.