# Parallel Fin Ray Soft Gripper with Embedded Mechano-Optical Force Sensor
## 2 State of the Art  
Like other soft robotic systems, soft grippers are characterized by their intrinsic deformability and compliance [28], providing significant advantages in adaptability and safety when interacting with dynamic and unpredictable environments. However, they also present challenges in terms of sensorization and control due to their high inherent number of Degrees of Freedom (DoFs). The complexity of soft robotic finger control is compounded by the need for precise feedback on parameters such as force, deformation, and position during operation. To address these challenges, sensor integration has emerged as a pivotal area of research in soft robotics. Integrating soft sensors into actuators enables real-time monitoring and adaptation to changing conditions, thereby enhancing both control and functionality. Unlike traditional rigid sensors, soft sensors are specifically designed to provide flexibility, adaptability, and the ability to conform seamlessly to soft robotic systems. These sensors measure physical parameters such as pressure, deformation, and temperature while actively contributing to the real-time control and performance of soft robotic fingers. Recent advancements, including liquid metal-filled microchannels and 3D-printed resistive sensors, have demonstrated significant potential for improving the versatility and precision of soft grippers, particularly in applications involving delicate or irregular objects [29; 30]. One of the significant advancements in soft robotics is the development of 3D-printed resistive soft sensors that can be co-fabricated with the robot bodies themselves, as demonstrated by [29]. This method enables the creation of integrated, multifunctional soft robotic actuators with enhanced sensing capabilities by incorporating sensors directly into the actuator's structure, providing greater design flexibility and customization. The co-fabrication technique also allows for the production of lightweight and flexible sensors, essential for maintaining the compliance of soft robotic systems. Additionally, soft sensors utilizing liquid metal-filled microchannels offer unique advantages in terms of compliance and durability. These sensors can stretch and bend with the soft robotic finger's movements without compromising functionality, making them ideal for dynamic applications [30]. Similarly, embedded barometric sensors can measure internal pressure changes, allowing for the precise control of soft robotic fingers by monitoring deformation and exerted forces. These sensors are cost-effective and provide reliable data, crucial for precise control and feedback in soft robotics. A review in [31], shown in Figure 2, illustrates the range of these soft sensor technologies, highlighting their potential and versatility in the field. Figure 2: Overview of typical touchless sensors that have been used in soft sensing reviewed in [31]. (**A**) Photosensitive soft sensors [32; 33; 34]. (**B**) Magnetic-based sensing [19; 35; 36]. (**C**) Adaptive skin with IR sensors [37]. (**D**) Acoustic sensors [20; 38; 39]. (**E**) UV light sensing modules [40] (**top,middle**). Fluorescence sensor [41] (**bottom**). (**F**) Capacitive bimodal sensor array made entirely of textiles [21] (**top**) and multimodal sensor network integrated with a soft robotic gripper [42] (**bottom**). Therefore, various sensing technologies have been explored in soft robotics to provide tactile and force feedback. Piezoresistive sensors are valued for their low cost and ease of integration, but they suffer from hysteresis and low spatial resolution [43]. Piezoelectric sensors offer a high dynamic range but are unable to measure static forces [44]. Capacitive sensors provide high spatial resolution but require complex filtering electronics and are prone to noise interference [45].