Hybrid flexible-rigid robotic arm can go where flexible endoscopes can’t

August 8, 2017
Though flexible endoscopes can snake through narrow passages to treat difficult to reach areas of the body, once they arrive at their target, these devices rely on rigid surgical tools to manipulate or remove tissue. These tools offer surgeons reduced dexterity and sensing, limiting the current therapeutic capabilities of the endoscope.

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences and the Wyss Institute for Biologically Inspired Engineering at Harvard University have jointly developed a solution: a hybrid rigid-soft robotic arm for endoscopes with integrated sensing, flexibility, and multiple degrees of freedom.

This arm — built using a manufacturing paradigm based on pop-up fabrication and soft lithography — lies flat on an endoscope until it arrives at the desired spot, then pops up to assist in surgical procedures

Soft robots promising tools

Soft robots are promising tools for surgical applications, because they can match the stiffness of the body, meaning they won’t accidentally puncture or tear tissue. But at small scales, soft materials cannot generate enough force to perform surgical tasks.

“At the millimeter scale, a soft device becomes so soft that it can’t damage tissue but it also can’t manipulate the tissue in any meaningful way,” explains Tommaso Ranzani, a postdoctoral fellow at SEAS and the Wyss Institute and coauthor of a paper on the research in Advanced Materials Technologies.  “That limits the application of soft microsystems for performing therapy.”

The question then is how to develop soft robots that are still able to generate the necessary forces without compromising safety. The answer was inspired by biology. The joint research team developed a hybrid model that used a rigid skeleton surrounded by soft materials.

Origami-inspired pop-up fabrication

The manufacturing method drew on previous work in origami-inspired, pop-up fabrication, developed by Robert Wood, Charles River Professor of Engineering and Applied Sciences and co-author of the paper.
Previous pop-up manufacturing techniques — such as those used with the Robobees — rely on actuation methods that require high voltages or temperatures to operate, something that wouldn’t be safe in a surgical tool directly manipulating biological tissues and organs. To circumvent this, the team integrated soft actuators into the pop-up system.

“We found that by integrating soft fluidic microactuators into the rigid pop-up structures, we could create soft pop-up mechanisms that increased the performance of the actuators in terms of the force output and the predictability and controllability of the motion,” said Sheila Russo, postdoctoral fellow at SEAS and Wyss and lead author of the paper. “We basically obtained the best of both worlds by combining soft robotic technologies with origami-inspired rigid structures.”

This way, the researchers were able to design a device that can lie flat when the endoscope is navigating to the surgical area, and when the surgeon reaches the area they want to operate on, they can deploy a soft system that can safely and effectively interact with tissue.

The soft actuators are powered by water and connected to the rigid components with an irreversible chemical bond, without the need of any adhesive. The team demonstrated the integration of simple capacitive sensing that can be used to measure forces applied to the tissue and to give the surgeon a sense of where the arm is and how it’s moving.

The fabrication method allows for bulk manufacturing, which is important for medical devices, and allows for increased levels of complexity for more sensing or actuation. Furthermore, all materials used are biocompatible.

Suction cup for safe interaction

The arm is also equipped with a suction cup — inspired by octopus tentacles — to safely interact with tissue. The team tested the device ex vivo, simulating a complicated endoscopic procedure on pig tissue. The arm successfully manipulated the tissue safely.

“The ability to seamlessly integrate gentle yet effective actuation into millimeter-scale deployable mechanisms fits naturally with a host of surgical procedures,” said Wood. “We are focused on some of the more challenging endoscopic techniques where tool dexterity and sensor feedback are at a premium and can potentially make the difference between success and failure.”

The researchers demonstrated that the device could be scaled down to 1 millimeter, which would allow it to be used in even tighter endoscopic procedures, such as in lungs or the brain. Next, the researchers hope to test the device in vivo. Harvard’s Office of Technology Development has filed a patent application on the technology and is actively seeking commercialization opportunities.