## 4 Discussion  
The materials used in this study for the fiber-constrained annular flexible actuator were all flexible; this allowed for an overall fully flexible design of the direct ventricular assist device while making the device lighter in weight than conventional direct ventricular assist devices, greatly reducing its size and making it less difficult to implant surgically. In the optimization of the structure of the fiber-constrained annular flexible actuator, a fiber inclination of 30\({}^{\circ}\) allowed the device to fit snugly into the heart during assisted compression, ensuring the stability of the output force and avoiding compression of internal organs outside the heart.  
The direct ventricular assist device studied in this paper is pneumatically driven, thus the range of auxiliary pressure needs to be determined. In this paper, the relationship between displacement and driving pressure of fiber-constrained annular flexible actuators was studied. The displacement and pressure of three kinds of fiber-constrained annular flexible actuators were approximately linear. The slopes of the linear regression equation of the three actuators were 0.6369, 0.5406 and 0.42265, respectively, which indicated that under the same displacement, the relation of driving pressure was actuator 3 \(>\) actuator 2 \(>\) actuator 1. The strain and displacement of each part of the human ventricle have different peak values. According to the 17-segment division method of the American Heart Association [16], the lateral displacement peaks of the three driver positions are 9.43 \(\pm\) 2.68 mm, 7.12 \(\pm\) 1.98 mm and 4.91 \(\pm\) 2.25 mm, respectively. The pressure ranges required by the three actuators to assist the heart in achieving lateral displacement were 14.81 \(\pm\) 4.21 Kpa, 13.17 \(\pm\) 3.66 Kpa and 11.51 \(\pm\) 5.28 Kpa, respectively, and the normal pressure range of the device was 10.6-16.38 Kpa.  
The direct ventricular assist device in this study helps the heart pump blood by squeezing it, so it is very necessary to study its pumping effect. Normal left ventricular stroke volume is 72.5-101.5 mL. When a patient has heart failure, with a declining ejection fraction and a stroke volume of less than 58 mL, the device needs to pump at least 14.5 mL more blood to get the heart pumping properly. In patients with heart failure reserved for ejection fraction, where the volume of each stroke is 58 to 72.5 mL, the device needs to assist the heart to pump at least 0 to 14.5 mL more blood in order to achieve normal pumping volume. The displacement of the device in the normal auxiliary pressure range was 12.83 to 19.82 mL, which can meet the requirements of two types of heart failure patients in order to achieve normal pumping volume.  
The advantages of flexible actuator movements that were closer to natural bio-muscular movements made them of great research interest. However, as the core driving component of direct ventricular assist devices, flexible actuators have wide application prospects in direct ventricular assist devices. With the emergence of new materials and the development  
Figure 11: Relationship between pressure input and water discharge in direct ventricular assist devices. (R2 = 0.9857).  
of driving methods, the design of flexible actuators should also become more diversified. For the fiber-constrained annular flexible actuators used in this study, it is also possible in the future to try to change the chamber distribution, muscle arrangement and fiber inclination, in order to achieve a more ideal and personalized assistance effect.