## 3 Materials and Methods
### Computer-Assisted Design (CAD) Model Design and SMA Actuation  
One of the main systems of actuation within the soft robotic is the SMA, which is an alloy of nickel and titanium (hence the commercial name Nitinol), developed by the Naval Ordnance laboratory in the United States in 1962. After the material deformation and taking advantage of the change of state in the composition of its crystalline structure because of the change in temperature, the material recovers its original form. From this date, research and prototypes have been carried out using SMA wires as actuators instead of conventional engines [42]. The importance of using SMAs as actuators lies in their small diameter, quick action and low weight, which facilitate the generation of complex movements in small spaces, which with conventional mechanisms, servomotors or other intelligent materials, such as piezoelectric materials, will be complex to replicate [43; 44]. The CAD model developed for analysis and simulations was generated in Solidworks based on a behavior study for jellyfish species Chrysaora hysoscella, which is typical of the Mediterranean Sea and has several representative characteristics of most jellyfish (bell delimited and pronounced tentacles) [45]. Figure 2 shows the analysis carried out on an adult-sized Chrysaora jellyfish (diameter of approximately 23 cm), for which 10 uniformly distributed points were established along its bell to analyze the evolution of the extreme areas (4 points) and middle zone (6 points) during the process of relaxation and contraction. The described movement consists mainly of two stages: the first one of relaxation, Figure 2a,b, at which stage the central bell is extended, and a second phase of contraction, which happens in a range of reduced time with respect to relaxation (around 21% of the relaxation time); the contraction phase is from the state shown in Figure 2c,d, which is in the range of 0.7 s and allows it to expel the fluid inside the bell, achieving movement. The points in Figure 2 [P1-P10] represent distributed points on the surface of the jellyfish body and have been placed to have a visual reference of the states of the body during the contraction and relaxation phases. Additionally, it has been identified that the tentacles contribute with an additional fluid displacement that generates vertical movement. The implemented actuators have been selected based on their desirable characteristics for the movement, specifically to generate rapid contractions, which are their reduced size and their weight which does not represent a significant load, as well as the characteristics of the Nitinol used: activation temperature 50-65 \({}^{\circ}\)C + \(-5\)\({}^{\circ}\)C, diameter 500 and 375 \(\upmu\)m, alloy material: nickel titanium, acquired in \"smartwires\" [46]. The robot body has been made with a highly flexible Ecoflex(r) silicone base, and it has an average diameter of 22 cm. Figure 3a shows a central cross-section of the model, and Figure 3b is a photograph of the final prototype. In the first state, a mold (Figure 3c) made with a 3D printer in ABS plastic generated from the jellyfish CAD model was made to develop such tests as silicone elasticity resistance and temperature resistance of the SMA + silicone set. Finally, the body of the implemented robot (Figure 3d) had to be dried for 2 h in a flat glass surface, and during this process, the central SMA was added, as well as the tentacles, which were taken from the mold. The robot consists of 5 SMA spring actuators placed so that one SMA is in the center of the body (350 _microns_) with a ring shape of an 80 mm diameter and 4 lateral SMAs (500 _microns_) of 75 mm length are located radially, anchored in the center and on the periphery with 90 degrees of separation, as reflected in the cross-section of Figure 3a. In this way, the controller can determine the degree and the time of contraction to produce the relaxation and contraction to expel water.