## 6 Sensors for Tactile Internet Applications  
The Tactile Internet requires ultra-low latency, high reliability, and real-time haptic feedback. The sensors are the most important components as they sense physical interactions and capture environmental data and physiological signals for the realization of natural experience for the users to whom they are directly connected. The sensors used in Tactile Internet applications can be broadly classified into five types, namely haptic sensors, motion and inertial sensors, biometric and physiological sensors, environmental and proximity sensors, and bio-integrated and implantable sensors. In this section, we discuss the various types of sensors related to the Tactile Internet and their technical aspects. * Haptic sensors are critical for capturing tactile sensations, such as force, pressure, vibration, and texture [56]. These sensors generate realistic touch feedback for applications. The haptic sensors can be capacitive, piezoelectric, MEMS-based force, or optical. * _Resistive and piezoresistive tactile sensors_: Resistive tactile sensors detect variation in electrical resistance upon the application of external force or pressure. They typically consist of conductive layers separated by a porous material. When pressure is applied, the layers come into contact, changing the resistance and, hence, the current flowing through it. They are simple, flexible, and cost-effective; however, their accuracy is low. They can be used as robotic grippers and prosthetic devices. Piezoresistive tactile sensors operate based on the piezoresistive effect, where the electrical resistance of a material changes under applied mechanical stress or pressure [57; 58]. These sensors require external power to work, and, generally, they are fabricated using silicon,graphene, and carbon nanotubes. They give higher sensitivity and are capable of sensing static as well as dynamic forces. They can be used in medical wearables to measure blood pressure, prosthetic control, and robotic applications [58]. * _Capacitive sensors_: Capacitive sensors measure variation in capacitance due to pressure or touch. When pressure is applied, the distance between the capacitor plates varies; hence, the capacitance also varies. Due to advancements in materials, such as graphene and carbon nanotubes, flexible capacitive sensors can play a crucial role in wearable applications. Recent research shows that graphene-based capacitive sensors have high sensitivity and flexibility, making them suitable for Tactile Internet applications [59]. These sensors are stretchable and are less temperature-sensitive than resistive and piezoresistive sensors. They can have high spatial resolution and force sensitivity and can also be used to measure lateral strains or shear forces. However, it is challenging to read very slight changes in capacitance values and they are sensitive to humidity and metallic parts [58]. * _Piezoelectric sensors:_ Piezoelectric sensors convert mechanical stress into electrical signals and are used for high-speed haptic feedback due to their fast response time. They do not require electric power to operate and to achieve high spatial resolution; they can be integrated with high-density CMOS [60]. They have good stability and are less prone to electromagnetic interference compared to resistive and piezoresistive sensors [58]. The challenges are complex signal conditioning circuits and the nonlinear response of piezoelectric materials for electrical fields. The research involving the use of polyvinylidene fluoride for flexible and wearable haptic sensors shows encouraging results [48]. * _MEMS-based force sensors:_ Micro-electromechanical systems (MEMS) force sensors are used for real-time pressure sensing in biomedical applications, like telemedicine and remote surgery. They enable high-density integration of piezoelectric sensors with high spatial resolution. They have low power consumption and are suitable for portable and wearable devices [58].