Two ultrasonic piezoelectric motors are contained in the rear body (toward the bottom of the heart model). Small linear motors (7.0 x 3.4 x 3.4 mm) use minute rotational motions to spin a threaded rod, providing linear translation of the rod with a high axial force. The motors have no gears or cams which allows for smaller construction and more efficient operation as a linear actuator. Because the motors advance a threaded rod, step length is limited only by the length of the rod. The motors use a flexible printed circuit board for communication with control circuitry located in an offboard enclosure.

A mobile robot driven by miniature ultrasonic piezoelectric motors open one route to minimally-invasive cardiac therapy. The robot is a product of the Robots Institute at Carnegie Mellon University, Pittsburgh, and builds upon previous prototypes of its HeartLander. The system includes motor selection, body, and controllers, and was developed as a proof-of-concept to demonstrate mobility on the cardiac surface. Designers say the robot turns tighter and has more traction than a previous model, and represents the first step in designing a wireless mobile robot for cardiac therapy.

“The HeartLander robot presents solutions to some of problems surrounding robotic minimally invasive cardiac techniques,” says development team member Peter Allen. “The miniature tandem-bodied mobile robot is intended to travel on a heart’s surface, or epicardium, by alternating suction and the extension length between two bodies. Using this inchworm-like locomotion, the robot can reach any location on a beating heart surface.” Because the robot adheres to the beating heart, it is located in the same moving reference frame. This passive stabilization avoids cardiopulmonary bypass or tissue stabilizers, which may cause electrophysiological damage. After reaching a target, a surgeon would use an on board therapeutic endeffector for an intervention.

A few shortcomings on the first prototype included a stiff tether, which made turning difficult, and made it difficult to move across areas of high curvature. A design goal is to reduce tether stiffness by using small onboard motors, which would result in more efficient turns by eliminating the drive-wire mechanism from the tether.

The wedge-shaped front body of the robot slips underneath the pericardium, the tissue surrounding the heart. The rear body accommodates two horizontally and side-by-side Squiggle 3.4-SQL linear motors from New Scale Technologies Inc., Victor, N.Y. Each body contains a suction chamber to grip the heart. To move, suction alternates between the two bodies. Suction comes from an offboard pump.

The robot is capable of extending the front body forward and retracting the back body, as well as turning underneath the pericardium. The curved surface of the beating heart presented some difficulty in terms of maintaining contact to create a suction seal, specifically for the rear body.

The team says the robot can be made smaller. A lower height would decrease the friction between robot and heart surface. A shorter unit would reduce the negative effect of the heart’s curvature. And a hydrophilic coating would increase lubricity. Adding miniature Hall-effect sensors could allow closed-loop control. Pressure sensors could detect loss of suction-seal with the surface. Future testing will include in vivo testing on a beating pig heart.