Soft Passive Valves for Serial Actuation in a Soft Hydraulic Robotic Catheter1

Abstract

Joseph L. McKibben devised soft robotic actuators in the 1950s when he developed arm orthotics for his daughter who suffered from poliomyelitis [1]. The resulting McKibben actuator consists of an elastomeric tube encased by a mesh of equal and opposite fiber orientations allowing for axial expansion or contraction. Recent research of fiber-reinforced enclosed elastomers (FREEs) has focused on varying wrap angles to produce twisting, bending, spiral, and screw motions [2]. Using the FREE model, we propose a soft robot that can locomote through human vasculature as shown in Fig. 1. The robot will be powered by a single hydraulic line connected to a saline pump, and the robot will be comprised of two parts: a locomotion section and an orientation section, much like typical robotics arms. The locomotion section will move through human vasculature with two spiral sections and an extender in the sequences shown in Fig. 2. The orientation section will contain two degrees-of-freedom allowing for a theranostic tip to sense its surroundings and perform procedures. To create these two sections and link the actuators together, new valves must be created which allow for sequential actuation of the locomotion segments with a single pressure line (Fig. 2). These cracking pressure valves must be tunable and have asymmetric hydraulic resistances.Although much research has been performed in soft robotics, less attention has been paid to the hydraulic valves used. Napp et al. have developed a passive bandpass valve that consists of two moving diaphragms [3]. Unlike previous works, our valve consists of a single diaphragm. The valve acts like a two-way check valve. Low-pressure differentials are blocked out while pressures above the cracking pressure are able to flow through the valve wall, allowing each segment to fill up to a desired pressure. This paper contributes a design of asymmetrical cracking pressure valves with an empirical design table in an aim to achieve the intravascular locomotion proposed in Fig. 2. The design is scalable to catheter dimensions and magnetic resonance imaging (MRI) compatible.The design draws inspiration from the mitral valve which consists of three flaps which form a cone. To create bidirectionality, the flaps were removed allowing the cone to invert on itself. The inversion of the cone created asymmetric cracking pressures which was the desired design parameters. The cracking pressure was defined as the pressure that first generated noticeable flow through the valve. This was determined manually by measuring the pressure when water first cracked through the valve. The valves created in Fig. 3 show the design and flow path for the forward and reverse flow directions.The valves were created through a silicone molding process. A two-part mold was 3D printed (Objet260 Connex, Stratasys, Inc., Eden Prairie, MN), which contained six varying cone angles from 0 deg to 50 deg as shown in Fig. 4. A 3/8 in. diameter bushing with a 3/8-16 UNC hole was placed into the mold and served as the outer casing for the silicone valve. To mold the valve within the body, 8A durometer silicone (TAP Platinum Silicone, TAP Plastics, San Leandro, CA) was injected into each valve casing and the mold was degassed. The top-half of the mold was placed and a set of adjustable parallels was used to set the thickness of the valves.An empirical study was performed to test the bidirectional cracking pressure of the soft valves in both the forward and reverse directions. The valve thickness and cone angle ranged from 0.025 in. to 0.100 in. and 0 deg to 50 deg, respectively. To test the cracking pressure of each valve, the pressure of the hydraulic source was gradually increased. The pressure that first cracked the valve, resulting in flow, was recorded based on the reading from a pressure transducer (MLH100PGL06A, Honeywell International, Inc., Morris Plains, NJ). The test setup, as shown in Fig. 5, consisted of a pressure source connected to a nee