# Development, Experimental Assessment, and Application of a Vacuum-Driven Soft Bending Actuator
## 2 Materials and Methods
### Design and Development of a Vacuum-Powered Soft Bending Actuator Utilizing Heat-Shrinkable Material
It was found that air trapped between the heat-shrinkable tube and the mold pipe during shrinking prevented shape formation, so holes were drilled along the pipe to allow air to escape. New reinforcing rings were made from steel, 2 mm thick, using a laser cutter. The distancers were made from small steel pipes, and the lids were manufactured from steel pipe with threaded rods welded on. To improve sealing, the outer edges of the lids were wrapped with seal tape, and zip ties were applied around the outer surface of the VSBA. The fabricated VSBA-2 was tested for airtightness, and preliminary performance measurements were conducted (Figure 4a). The experimental results confirmed complete airtightness, a maximum bending angle of 120\({}^{\circ}\), and a maximum force of 50 N at a vacuum pressure of \(-\)0.8 bar. When comparing the results to the VSBA-1, it is observed that the maximum force is 55% lower, and the maximum bending angle is 7% smaller, due to a smaller outer diameter and larger stiffness of the heat-shrinkable material. However, the VSBA-2 still demonstrates impressive performance, achieving a significant maximum bending angle and force. The following fabricated VSBA-3 has the same length and outer diameter as the VSBA-2, but with an increased spacing between the ribs, from the previous 8 mm to a new 20 mm. This modification resulted in an actuator with 5 ribs, compared to the previous 10 (Figure 4b). Preliminary measurements confirmed a maximum bending angle of 130\({}^{\circ}\) and a maximum force of 46 N. These results indicate that increasing the spacing between the ribs allows the VSBA-3 to achieve a larger bending angle (8% increase), but with a slightly reduced force (8% decrease) compared to the VSBA-2. This decrease in force is attributed to the reduced number of ribs, which decreases the surface area of the VSBA-3. Figure 4: Operating principles of (**a**) VSBA-2; (**b**) VSBA-3; (**c**) VSBA-4. As already mentioned, we aimed to develop a smaller-sized actuator for the VSBA-4 than the previous ones, as shown in Figure 4c. A heat-shrinkable tube with an outer diameter of 27 mm was selected, resulting in an outer diameter of 25 mm for the VSBA-4. This represents a 50% reduction compared to the VSBA-2 and VSBA-3 versions, and a 54% reduction compared to the VSBA-1. The new mold was created using a pipe with an outer diameter of 14 mm and 5x14 O-rings, achieving the same rib rounding as the VSBA-2 and VSBA-3. For consistency in comparison, the ten ribs were kept the same as in the VSBA-2. The newly fabricated reinforcing rings have an outer diameter of 23 mm, while the lids were modified for smaller dimensions and designed with side openings for tube connections. Experimental results showed a maximum bending angle of 135\({}^{\circ}\), representing a 4% increase compared to the VSBA-2. The maximum force achieved is 4.6 N, which is expected given the significant reduction in the dimensions of the VSBA-4. Additionally, 60-min cyclic fatigue tests were conducted on the VSBA-2 (660 cycles), VSBA-3 (900 cycles), and VSBA-4 (720 cycles), and the results indicated no signs of material fatigue or changes in their performance. The test was performed using the same procedures as the previously conducted fatigue test for the VSBA-1. Finally, it can be concluded that the VSBA using heat-shrinkable materials yielded encouraging results, which will be further analyzed in the following chapter through detailed experimental assessments.