# Parallel Fin Ray Soft Gripper with Embedded Mechano-Optical Force Sensor
## 1 Introduction  
The advancement in soft robotics in various sectors, such as the industrial [1, 2, 3, 4, 5], medical [6, 7], and agricultural [8] domains, is consolidating at the beginning of this century. This type of robotics, which uses soft materials like silicone, rubber, or thermoplastic elastomers (TPEs), is characterized by its ability to adapt to various objects, interact with them without causing damage, and enable safe interactions in robot-environment, robot-robot and robot-human scenarios. One of the standout applications of this type of robotics is object manipulation, particularly through the use of soft grippers. In the literature, various designs of these robotic grippers have been proposed [9, 10, 11, 12], employing different geometries, materials, and actuation methods to achieve adaptable, firm, and non-damaging robot-object interactions. The design of these grippers often draws inspiration from nature, such as those that mimic the grip of octopuses [13, 14] or geckos [15, 16]. However, a major challenge in soft robotics remains the integration of force sensors capable of real-time force measurement without compromising the inherent flexibility and compliance of soft materials. Sensorization in soft robotics is still an emerging field, and most studies have relied on sensors originally designed for traditional robotics. These sensors, being rigid, are often incompatible with the flexibility of soft materials [17, 18]. Thismismatch undermines the core advantages of soft robotics, such as their ability to distribute forces uniformly and conform to objects' shapes for improved adaptability. To overcome this limitation, researchers have explored alternative sensing methods compatible with the elastic and flexible properties of soft materials, leveraging technologies such as magnetics [19], acoustics [20], and textile-based sensors [21]. One of the most widely recognized designs in soft robotics is based on the Fin Ray effect [22; 23; 24; 25; 26], inspired by the anatomy of fish fins, which exhibit both flexibility and adaptability. Fin Ray-inspired grippers, however, often lack integrated force sensors due to technical difficulties [22; 23; 24; 25], as shown in Figure 1. Although some studies have attempted to incorporate sensors into these grippers [26], they frequently rely on conventional rigid sensors that compromise the gripper's ability to maintain its inherent adaptability. This adaptability is crucial for distributing forces over larger contact areas and conforming to objects without requiring active position control [27]. Therefore, advancing sensor technologies that seamlessly integrate with the soft and flexible nature of these materials remains a key focus in soft robotics research. For this reason, this article proposes a Fin Ray-inspired parallel soft gripper with an integrated mechano-optical force sensor, designed to provide real-time force measurement while preserving adaptability. Unlike conventional soft grippers that rely on external force sensors or resistive/capacitive sensing elements, our approach achieves direct force sensing integration within the gripper structure itself. The gripper is entirely 3D printed using thermoplastic elastomer (TPE) filaments, ensuring a cost-effective, scalable, and versatile design. By leveraging a gyroid lattice structure, the integrated sensor achieves a near-linear force response, enabling precise contact force control in robotic manipulation. Compared to existing soft grippers, which often suffer from nonlinear sensing behavior, external sensor placement limitations, and high manufacturing costs, our design provides an all-in-one solution that combines structural adaptability with embedded force-sensing capability. The proposed soft gripper, with its adaptable gyroid lattice structure and integrated force sensor, allows for the gentle grasping of fragile objects, such as small fruits or vegetables, minimizing the risk of bruising.