Robotic assistance can provide surgeons with greater precision and control for minimally invasive procedures. However, the systems themselves are often large, taking up much of the room and still being bigger than the patient anatomy on which they operate. The Wyss Institute for Biologically Inspired Engineering today announced that it has worked with Harvard University and Sony Corp. to build the mini-RCM, a surgical robot the size of a tennis ball that weighs as much as a penny.

Robert Wood, Ph.D., an associate faculty member at the Wyss Institute, and Horoyuki Suzuki, a robotics engineer at Sony, were inspired by origami when they created the “miniature remote center of motion manipulator,” or mini-RCM. Wood and Suzuki began working together in 2018.

Manufacturing the mini-RCM

Mass-producing the tiny, complex structures needed for a folding robotic manipulator would be difficult to do by hand, according to the Wyss Institute. Wood and Suzuki used the Pop-Up MEMS method developed at Wood’s laboratory. It applies techniques from printed circuit-board (PCB) manufacturing to microelectromechanical systems (MEMS). Materials are deposited in layers that are bonded and then laser-cut in a pattern that allows a three-dimensional shape to pop up like a children’s book.

The researchers created a parallelogram for the robot’s base and then three miniature linear actuators (mini-LAs) to control it.  The mini-LAs are built around a piezoelectric ceramic material that deforms when an electrical field is applied. That shape change pushes the mini-LA’s “runner unit” along its “rail unit” like a train on its tracks, and that linear motion moves the robot, said the Wyss Institute.

“We use alumina and composite prepreg, or carbon fiber deposits, as well as PZT [lead zirconate titanate],” said Dr. Suzuki. “The rail unit, including slit patterns for displacement sensing, is made from a 0.38-mm-thick alumina plate.”

“Alumina was used because we need to keep the surface flat during laser machining,” he told The Robot Report. “Prepreg were used for the mini-LA because of their high stiffness and low mass.”

One mini-LA is parallel to the shape’s bottom to raise and lower it, and another is perpendicular to the parallelogram to rotate it. A third actuator at the tip extends and retracts the tool. Wood and Suzuki claimed that the resulting robot is smaller and lighter than other microsurgical devices that academics have developed to date.

Because piezoelectric materials inherently deform, the team also integrated LED-based optical sensors into the mini-LA to detect and correct any deviations from the desired movement, such as those caused by hand tremors.

Testing steadiness with Phantom Omni

To mimic the conditions of a tele-operated surgery, the researchers used a Phantom Omni device, which manipulated the mini-RCM in response to a user manually controlling a pen-like tool. The first test evaluated a human’s ability to trace a tiny square smaller than the tip of a ballpoint pen, looking through a microscope and either tracing it by hand or using the mini-RCM.

“The Phantom Omni device — a commercially available interface tool — does not contribute to human accuracy,” said Suzuki. “Geometric scaling can compensate for human inaccuracy due to hand tremor reduction. The amount of the hand tremor is approximately 156 μm [micrometers].”

The mini-RCM tests reduced errors by 68% in comparison with manual operation. This is especially important when dealing with small structures and delicate tissues during surgical procedures, said the team.

The researchers then created a mock version of a surgical procedure called retinal vein cannulation, in which a surgeon must carefully insert a needle through the eye to inject therapeutics into the tiny veins at the back of the eyeball. They fabricated a silicone tube the same size as the retinal vein — about twice the thickness of a human hair — and successfully punctured it with a needle attached to the end of the mini-RCM without causing local damage or disruption.

The positional precision of 26.4 μm is similar to the benchmark for retinal surgical robots, said Suzuki.

mini-RCM makes surgery more portable

Not only does the mini-RCM help surgeons perform delicate maneuvers, but its small size also makes it easy to install and remove if necessary in the case of a complication or power outage, said the Wyss Institute.

“We believe that how easy the setup is often related to the compactness and weight,” said Suzuki. “Conventional surgical robots, including [Intuitive Surgical’s] da Vinci, are very large and heavy, making setup in small operation rooms complicated.”

“The Pop-Up MEMS method is proving to be a valuable approach in a number of areas that require small yet sophisticated machines, and it was very satisfying to know that it has the potential to improve the safety and efficiency of surgeries to make them even less invasive for patients,” stated Wood. He is also the Charles River Professor of Engineering and Applied Sciences at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS).

The researchers published their results in Nature Machine Intelligence.

Future collaboration

The researchers sad they plan to increase the force of the robot’s actuators to cover the maximum forces experienced during an operation, and improve its positioning precision. They are also investigating using a laser with a shorter pulse during the machining process, to improve the mini-LAs’ sensing resolution.

Suzuki said that it could take three to five years to achieve the improvements in force and positioning. “We think that optimized computational simulation should be required to improve the performance, which would take one to two years,” he said. “And later, fabrication should be necessary, which would take one to two years. This estimation is pretty rough.”

In addition, carbon-composite frames could enable the mini-RCM to be used with magnetic resonance imaging (MRI), noted Suzuki. The ferromagnetic portion of the runner unit could be replaced with a titanium alloy. The mini-RCM robot is also small enough to fit inside the tight space of an MRI system.

“This unique collaboration between the Wood lab and Sony illustrates the benefits that can arise from combining the real-world focus of industry with the innovative spirit of academia, and we look forward to seeing the impact this work will have on surgical robotics in the near future,” said Wyss Institute Founding Director Don Ingber, M.D., Ph.D. He is also the the Judah Folkman Professor of Vascular Biologyat Harvard Medical School and Boston Children’s Hospital, and Professor of Bioengineering at SEAS.

“This was a preliminary study in the hopes of establishing further collaborations between Sony and Harvard, and with clinicians, with the ultimate goal of providing more effective surgical tools in the future,” Wood told The Robot Report.