# Parallel Fin Ray Soft Gripper with Embedded Mechano-Optical Force Sensor
## 3 Parallel Fin Ray Soft Gripper Design  
Robotic grippers can be designed with various geometries and actuation methods. Among these, parallel grippers are the most common due to their ease of control, suitability for repetitive tasks, cost-effectiveness, and reliability. As a result, they dominate industrial process automation. In this regard, within soft robotics, there are various designs of adaptable parallel-type soft grippers. This stands out compared to another significant group within soft grippers, pneumatically actuated ones, which are more sensitive to potential manufacturing defects and dependent on a constant pneumatic supply line. This reduces their efficiency, as it requires converting electrical energy into the potential energy of compressed air, which increases costs compared to grippers that only need electrical energy. Among adaptable soft grippers, those that leverage the Fin Ray effect stand out for their reliability, ease of manufacturing, and simple control. This bioinspired effect, discovered by biologist [PERSON], mimics the deformation behavior of fish fins. The structure of the actuators that make up these grippers consists of two alternating tension and compression sides flexibly connected by rigid ribs. This structure enables a high degree of movement with minimal force input [50]. Building upon these principles, we designed a parallel soft gripper that not only leverages the Fin Ray effect but also integrates a mechano-optical force sensor for embedded force measurement. To achieve this, we selected a compact and monolithic actuator design (see Figure 3), ensuring scalability and modularity, both critical for industrial applications. To optimize the structural parameters of the Fin Ray fingers, we followed the methodology outlined in [24], which highlights the importance of rib configuration in tuning both stiffness and displacement. A maximum applied force of 150 N was established as the design constraint to ensure that the deformation remained within an optimal range. The ribs were iteratively adjusted to balance mechanical compliance and structural integrity, allowing the gripper to maintain adaptability while preventing excessive deformation under high loads. The gripper was fabricated using two different materials: thermoplastic elastomer (TPE) with a hardness of 70A and polylactic acid (PLA). TPE was selected for the robotic fingers and embedded sensors, as these components directly interact with objects, requiring both flexibility and durability. In contrast, PLA was chosen for structural components that connect the robotic fingers to the wrist of the robotic manipulator, ensuring mechanical stability without interfering with the gripping mechanism. The use of these materials aligns with Fused Filament Fabrication (FFF) technology, enabling the production of complex geometries while maintaining cost-effectiveness. The design of the embedded mechano-optical force sensor was guided by the study conducted in [49], which analyzed the mechanical response of various metamaterial lattice structures, including simple cubic, face-centered cubic (FCC), octahedral, body-centered cubic (BCC), 2.5D X-cell, Schwartz D, and gyroid lattices. Among these, both gyroid and BCC lattices were identified as isotropic, meaning they provide uniform resistance and deformation along all three axes. This isotropic nature is crucial for force sensing applications, as it ensures predictable and repeatable deformation behavior, which is fundamental for achieving a linear force response in soft robotic applications. Figure 3: Parallel Fin Ray soft gripper. (**a**) Frontal view of the soft robotic finger. (**b**) Sectional view. (**c**) Fully assembled soft gripper. All dimensions are in millimeters (mm). (**d**) FEM analysis. The gyroid lattice was ultimately selected due to its favorable balance between stiffness and deformation characteristics. Specifically, it exhibits a gradual transition from a low-stiffness regime to a high-stiffness regime under moderate deformation levels.