## 1 Introduction  
The direct heart assist device allows the heart to pump more blood by squeezing the ventricles or aorta. Since there is no direct contact with blood, the direct ventricular assist device avoids many biocompatibility issues and reduces the risk of infection and medication for patients. Early direct ventricular assist devices were limited by the lack of flexibility, dexterity and biocompatibility of their rigid and semi-rigid materials, which made clinical application of direct ventricular assist devices difficult, for example, due to high implantation trauma, destruction of myocardial cells and difficulty in adjusting the fit. These devices included a cup-shaped epicardial compression device for CPR designed by [PERSON] [1], and a non-blood contact pneumatic ventricular assist device designed by [PERSON] [2] and [PERSON] [3]. In 2017, [PERSON] et al. designed a non-blood contact ventricular assist device, the soft robotic sleeve [4], which is made of multiple artificially customized soft inflatable muscles moulded in silicone, and could be controlled by inflating and deflating the left and right sides separately. The fully flexible design of this direct ventricular assist device, which can also accommodate cardiac torsion, provided a new idea and demonstration for the design of a fully flexible direct ventricular assist device. Pneumatic artificial muscles are manufactured by using flexible materials, and their good safety and suppress compensate for the lack of rigid materials. There are two main types of pneumatic artificial muscle structures available: fluid elastic actuators (FEA) and fiber-constrained actuators [5]. In recent years, several design types of fiber-constrained actuators have been derived from the updated designs of researchers in various countries. The fiber winding angle has been extensively studied as an important design parameter for fiber-constrained actuators, and the actuator's action pattern is directly related to its fiber winding angle. Toshiba has developed a three-degrees-of-freedom FMA [6] (Flexible Micro actuator), which limited the deformation of the air chamber by varying the angle between the direction of the fibers and the circumference, thus achieving three degrees of freedom in extension, bending and torsion. However, few researchers have investigated the feasibility of a fiber-constrained actuator as a drive unit for direct ventricular assist devices. Fiber-constrained flexible actuators enable multi-degree-of-freedom movements, and have the right flexibility to meet the requirements of direct ventricular assist devices for their actuators. However, few researchers have investigated the feasibility of fiber-constrained flexible drives as direct ventricular assist device drive units. The first aim of this study was to design a fiber-constrained flexible actuator, taking into account the specific location of the heart within the thoracic cavity, the peripheral environment and other physiological characteristics. In addition, several researchers have investigated the effect of the drive's structural parameters on the performance of flexible actuators. [PERSON] et al. [7] analyzed the effect of balloon wall thickness, cross-sectional area and length on the deformation effect of the actuator, and optimized the design of the actuator in terms of bending angle, maximum stress generated and radial expansion rate of the actuator. It showed that the structural parameters of the actuator were important factors affecting its performance.