Linear actuator wiggles to perform heart surgery

Tom Shelley reports on technology within medical micro surgery robots that have already been demonstrated. Linear wire driving motors are key to telerobots that can go into a human chest through a small hole, walk around on the beating heart and deliver an injection into exactly the right place without requiring massive surgery. So far proven and tested on anaesthetised pigs, the robots offer a means of injecting genes, protein, cells or drugs into the patients’ hearts with minimal trauma. The motors, which use ultrasonically distorting tube nuts to drive threaded rods, have applications beyond medical robots, including cryogenic sensors, biometric ID camera systems, night vision systems, locks and latches. Called Squiggle, the motors are made by US company New Scale Technologies and are available in the UK through Orlin Technologies. The heart crawling robot is called the HeartLander OMNI, or Onboard Motor Navigational Instrument, and has been developed by a team led by Professor Cameron Riviere at Carnegie Mellon’s Robotics Institute in the US. The front body of the robot is sloped to a point so as to create a space under the pericardium – the membrane surrounding the heart – as it is pushed forward by the motors. The front body contains two ball bearings to provide rotational degrees of freedom for the linear motors plus a 2mm diameter port for the injection system. The rear body accommodates two Squiggle 3.4-SQL linear motors side by side, which act on threaded rods attached to 0.3mm diameter super elastic, nickel titanium Nitinol wires. The wires connect to the ball bearings on the rear of the front body. The bodies measure 8 x 8 x 5.5mm and were made by stereolithography. The robot attaches itself to the heart’s surface using suction chambers. In order to generate movement, suction is alternated between the front and back parts. Throughout the movement cycle, the control system monitors the vacuum line pressure sensors to ensure that at least one suction pad maintains adhesion on the heart’s surface at all times. The position and orientation of a tracking sensor, located on the front of the crawler, was determined relative to the magnetic transmitter attached to the operating table. Because the heart was beating, its movement had to be filtered out in real time in order to determine position relative to the surface of the heart. A stationary wire frame computer model of the surface of the heart allowed the position of the crawler to be visualised. This was done by capturing the surface of the heart by using a tracking probe. The apex of the left ventricle was marked on the model according to its anatomic location on the heart – visible through the incision used to insert the probe. While the system is not yet certified for human use, it shows the concept of small, crawling robots that go into the human body and perform surgery – imagined in the January 2009 edition of Eureka -- is getting closer. For those with non medical applications, the motors are commercial products capable of speeds of 2 to 7mm/s and resolutions down to 20nm. Actuator travels from 3 to 50mm are possible and forces from 0.3 to 5N can be delivered. Pointers * A crawling robot has been demonstrated that can move over the surface of a beating pig’s heart, and deliver and injection to a positional accuracy of 1mm. * It depends on a commercially available Squiggle motor, with a tubular nut on a thin, screwed shaft, that deforms itself at ultrasonic frequencies to move the shaft relative to itself. * Resolutions are down to a few tens of nm, speeds are mm/s and forces up to 5N