The development and construction of a mobile robot, driven by tiny ultrasonic piezoelectric motors for minimally invasive cardiac therapy, illustrates the capabilities, challenges and future direction of miniaturized motion systems.
The robot design builds on the basic success of previous prototypes of HeartLander, a miniature mobile robot that moves in an inchworm-like fashion to reach any targeted location on the beating heart surface. It was developed jointly by The Robotics Institute at Carnegie Mellon University and the Div. of Cardiac Surgery at the University of Pittsburgh.
While the current HeartLander design was developed as a proof of concept to demonstrate mobility on the cardiac surface, conclusions point to more product development, and research will be required to make the procedure a longer-term reality. Key areas for improvement include a smaller size control system and the ability to operate the ultrasonic vibration motors at lower voltages. The goal is to develop a miniaturized motion control solution including motors, driver and position sensors to achieve closed-loop control utilizing very small modules.
The HeartLander robot presents an alternative solution to some of the problems surrounding robotic, minimally invasive, cardiac techniques. This miniature tandem-bodied mobile robot travels on the heart surface, or epicardium, by alternating suction and the extension length between two bodies.
"The innovation we are pursuing is replacing the external drive wires with internal motorized actuators," says David Henderson, CEO of New Scale Technologies Inc. "It is feasible and promising, but there is more to be done. We have a proposal pending to continue the research - to make the actuators smaller, improve force, reduce operating voltage, seal the unit and protect the mechanism from body fluids." Henderson says other improvements could provide position sensing to properly calculate motion trajectories and increase the robot's turning capabilities.
Despite the effectiveness of the prototype, problems still exist, especially related to the tether the unit utilizes. The tether consists of several components including a drive-wire mechanism that allows for the extension and retraction motions.
This mechanism, in combination with the other components, increases the stiffness of the tether. As the robot turns, the tether pulls the robot toward its original heading orientation, causing a loss of traction and efficiency. Tether stiffness causes difficulty when moving across areas with high curvature.
"If you want to move on the heart with the current drive wires, it's necessary to back up and turn instead of turning and moving," Henderson says. Increased mobility would provide an ability to follow a trajectory on the heart and potentially do electrocardiogram mapping and treatment.
The HeartLander OMNI (Onboard Motor Navigational Instrument) was demonstrated in 2008 by Peter Allen and Professor Cameron Riviere at The Robotics Institute, Carnegie Mellon University. The design uses ultrasonic piezoelectric motors for robotic actuation to provide high axial forces, compact size and robust operation. Squiggle motors from New Scale are small linear actuators (7.0 x 3.4 x 3.4 mm) that use minute orbital vibrations in the nut to spin a mating threaded rod, providing linear translation of the rod with a high axial force. The motors have no gears or cams, which allows for small construction and efficient operation as a linear actuator.
Because the motors advance a threaded rod, stroke is only limited by the length of the rod. The motors use a flexible, printed circuit board for power and communication with motor control circuitry located in an enclosure.
The design of the front body of the robot is sloped to a point to create a space underneath the pericardium, the sac of fibrous and serous tissue surrounding the heart, as it is advanced by the motors. The front body contains two ball bearings to decouple the rotating rods from the non-rotating front body, and a port for diagnostic and therapeutic tools. The rear body accommodates two SQL-3.4 linear motors placed horizontally side-by-side. Each threaded rod has an attached length of nitinol wire connecting to the miniature ball bearings on the rear portion of the front body. The total HeartLander OMNI robot measures 76.1 x 15.5 x 8.8 mm.
A graphical user interface manages all of the robotic functions, including motor controls and the alternation of suction between the two bodies. The program also allows the physician to modify several parameters of the robotic locomotion, including step length and number of steps to provide control over the robotic movement.
Improvements planned for the HeartLander redesign specifically target further miniaturization. A decrease in height would result in a decrease in friction among the robot, the heart surface and the pericardium sac. A decrease in length would reduce the negative effect of the heart's curvature on the robot's prehension of the surface. The application of a hydrophilic coating to the robot would allow for increased lubricity and is another option to reduce friction.
Miniature Hall Effect sensors could be added to the design to incorporate closed-loop control. Pressure sensors could be added to detect loss of suction seal with the surface, according to Henderson. Future testing will include in vivo porcine testing on a beating heart.
Henderson says New Scale has plans to customize the Squiggle motors for this application. The motor typically pushes the load with the bias force in one direction on the screw, to eliminate backlash in the threads. In this case, the goal would be to both push and pull as an actuator, achieving more force and reducing the height. Since the unit moves under the pericardial sac, the higher the profile of the robot, the harder it is to move around. Keeping it thin reduces the force and makes it more mobile. The target height for the application is 5-6 mm. Today, the unit stands 8-10 mm tall.
New Scale is focused on moving beyond its motor technology to providing complete miniature motion systems. The company has developed a close relationship with austriamicrosystems AG, an analog IC company in Austria. They are working together to develop motor driver and position sensing ASICs. One ASIC, called the Tracker, is a position sensor Henderson says is the world's smallest linear position sensor. It offers a tiny, single chip solution and comes in a chip-on-board version that is very thin. The chip uses a moving magnet's North-South poles to encode the magnet and can measure accurately to two microns. New Scale also has a driver ASIC that drives two SQUIGGLE motors with a built-in dc boost from 2.8 to 40V.
"We also recently announced a strategic relationship with EPCOS, a manufacturer of multi-layer piezo ceramics," says Henderson. "These ceramics can be used in New Scale motors to operate at reduced voltages, so that in the same physical size multiple layers of metal and ceramic can be added to reduce the operating voltage just like a ceramic capacitor."
Henderson says piezoelectric elements are essentially ceramic capacitors that change shape when they charge and discharge. EPCOS makes piezo ceramic capacitors in very thin layers to maximize performance and they have already demonstrated motors working at 2.8V directly from batteries. Lower voltage is a much better solution because running at lower voltage and higher current is preferred in most applications, and the design eliminates the dc boost while also reducing the number of parts, size and overall cost.