Miniature Fluidic Actuators for Surgical Robotics1

Abstract

Miniaturization of surgical robots for insertion inside the peritoneal cavity has become a new trend in Robotic Minimally Invasive Surgery (R-MIS) [1–4]. Robots for laparo-endoscopic single site surgery (LESS) and natural orifice translumenal endoscopic surgery must be fast enough to react to surgeon input, and be capable of providing high levels of force for effective tissue interactions. Size is a key design factor for in vivo robotics. With conventional actuation methods (electric motors), there exists a tradeoff between the scale of the robot and its load capacity and actuation speed. An alternative actuation scheme is to exploit the high power density of fluidic (pneumatic or hydraulic) actuators. However, fabrication of miniature but powerful fluidic linear and rotary actuators with desired sizes has been a challenge due to difficulties in sealing at high pressures, obtaining necessary surface finishes, and associated cost of fabrication.In this paper we present design, fabrication, and initial testing of miniature pneumatic/hydraulic linear and rotary actuators and a fluid-actuated laparoscopic grasper that are seal-less, easy to fabricate, and inexpensive. The vision of this project is to incorporate these components in a surgical robotic arm.An inexpensive miniature pneumatic/hydraulic cylinder that is seal-less, rod-less, and leak-free is presented, as seen in Fig. 1. It consists of an outer tube, an inner tube, a piston, a pin, and two off-the-shelf latex balloons, as depicted in Fig. 1. The novelty of this concept is in the use of elastomeric balloons in both upper and lower chambers of the inner tube to drive the piston. When the balloon is pressurized, it inflates and pushes the piston up/down. A metal pin and groove system are used to transfer force from the piston to the outer tube. The inner tube is stationary and the outer tube can extend or contract depending upon the direction of the piston motion. The balloon itself is sealed onto a PVC tube with an outer diameter (OD) of 3.9 mm using either heat-shrink tube or tight tolerance between the hole on the inner tube and the PVC tube with the balloon. All parts were made using a 3D printer and/or laser cutting machine. The bore, OD, length, and stroke of the cylinder are 5.9, 11, 22.8, and 6 mm, respectively. In an effort to further miniaturize the actuator, the second-generation design with bore, OD, length, and stroke of 4, 5.5, 26, and 2 mm respectively was built, as shown in Fig. 2. Miniature PVC tubing with an OD of 2 mm attaches to both ends.There is no miniature hydraulic motor (in the range of 15 × 15 mm, small enough for in vivo robots) commercially available, nor in the prior art to our knowledge. Revolute joints in previous LESS robot designs do not require continuous rotation. Accordingly, a limited-motion vane motor with rotational range of motion of 180 deg has been designed and built. The working principle is similar to that explained for the linear actuator. When the balloon is pressurized, it inflates and pushes the blade, causing it to rotate. Figure 3 shows the motor in its two extreme states.Three prototypes were fabricated, shrinking down the overall size of the motor from 25 × 25 mm to 18 × 20 mm to 15 × 15 mm in subsequent steps. The final prototype is approximately the same size as a 15 × 15 mm motor, which has been used in many previous in vivo robots for LESS [1–4]. Most of the components have been laser-cut in acrylic with two miniature PVC tubes (OD = 2 mm) attaching to the motor.A normally closed laparoscopic grasper that is actuated pneumatically/hydraulically was also designed using similar principles. The grasper consists of a preloaded torsional spring that provides grasping force, two elastomeric balloons, two jaws, and a hinge pin. Pressurized air or water can be used to expand the balloon inside the grasper and open the jaws. After prototyping several iterations, as illustrated in Fig. 4, a smaller and more robust version was