Design and Preliminary Characterisation of a New Soft Steerable Sheath for Cardiovascular Interventions

Mitral regurgitation is the second most common of all cardiovascular diseases in Europe. Nowadays, minimally invasive approaches such as the MitraClip system are becoming popular to treat this disease. The MitraClip system relies on a steerable catheter and sheath to navigate through the vasculature. However, this flexible instrument is generally prone to large friction and backlash, highly nonlinear force-displacement relations, and significant hysteresis effects. These limitations call to investigate new designs of actuators and flexible structures. Flexible instruments are commonly actuated with cables, but other options such as soft materials and pneumatic muscles exist. In this paper, a novel soft steerable sheath design is proposed. A silicone mixture is used as the backbone for six McKibben muscles which are set up antagonistically. This results in low cost and high force per volume ratio for actuation. Due to the soft nature of the instrument, the device possesses some intrinsic safety. This paper addresses some of the bigger challenges of using flexible instruments, namely miniaturization and hysteresis. A first prototype of outer diameter 12 mm and inner diameter 6 mm is manufactured and characterized. This shows a bending performance up to 68°. The hysteresis on the sheath is calculated to be around 8%.