A team of researchers affiliated with the Center for Compact and Efficient Fluid Power has developed a fully pneumatic surgical robot designed to aid with delicate surgeries that require the use of magnetic resonance imaging. The most obvious and most important challenge to overcome was to ensure that the device is safe for the patient.
“Ultimately, we want medical devices to improve patient outcomes and not, in anyway, make things worse,” said David Comber, a fourth year PhD student at Vanderbilt University who worked on the device.
According to Comber, researchers approached this challenge by designing and building bellow-based, flexible actuators that use a stepper mechanism, similar to how a mechanical pencil works, to move the device in tiny increments. Doing this prevents the system from over-extending and causing harm to the patient in the event of a computer or hardware failure.
Frictional forces within these actuators also had to be dealt with so the system would operate as smoothly as possible. To reduce these forces, designers built the actuators with pistons made of graphite, commonly used as a dry lubricant, and cylinders made of glass. Alignment was also considered when trying to reduce frictional forces and prevent binding in the device.
“The design for robot assembly required adjustable alignment of several mating parts such as journal bearings on linear guide rods; piston rods coupled to sliding plates and piston rods coupled to timing belts,” said Comber. “The solution I used in every case was a loose fit hole with nuts and washers. I adjusted each alignment by feel until the friction seemed to be at its minimum.”
One of the biggest problems facing the device came from its need to operate effectively in the tight spaces and powerful magnetic fields of an MRI machine. Electro-mechanical devices could not be used in the device, as the magnetic fields they create would interfere with the MRI image. Pneumatic devices, however, produce limited magnetic fields and wouldn’t interfere with the image. To further reduce the affects of magnetic fields on the MRI image, and the device itself, the robot was built using mostly non-ferromagnetic materials.
The device is designed to be compact enough to fit inside the MRI with the patient. This was done by maximizing the volume available to the device by positioning it on the bed of the MRI above the head of the patient. The mechanisms within the device were then kinematically coupled to minimize the stroke length, and thus, the length of the piston-cylinder actuator. The bulkier components of the system, including pressure sensors and valves, are kept in a separate room to avoid interfering with the MRI and are connected to the device by long lines of tubing.
While the current iteration of the device is procedure and location specific, the team believes the technology could be used one day in a number of different medical applications.
“The technologies we are developing here could be adapted to other parts of the anatomy,” said Comber. “But it would certainly require a redesign to integrate easily with that new anatomy.”