Shrinking Down Soft Robots

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A novel manufacturing process opens the door for tiny, complex, soft robots

By Liz Sheeley

Credit: Wyss Institute for Biologically Inspired Engineering

When a traditional robot built from hard plastic or metal suffers damage, it loses functionality, making it unadaptable to a dynamic environment. But a soft, flexible robot offers resiliency in unpredictable situations. Rigid robots continue to be scaled down, but scaling down soft robots in the same way has proved difficult.

A novel multi-step process has been developed by researchers including Assistant Professor Tommaso Ranzani (ME, MSE) and Assistant Professor Sheila Russo (ME, MSE) to construct microfluidic origami for reconfigurable pneumatic/hydraulic (MORPH) systems. Their work, initiated when they were postdoctoral fellows at Harvard’s Wyss Institute for Biologically Inspired Engineering and the Harvard John A. Paulson School of Engineering and Applied Sciences and now published in Advanced Materials, details the process and demonstrates its capabilities with the fabrication of a highly complex soft structure.

“If you have a soft robot you can very easily deal with a highly structured environment and also better deal with the uncertainties of the real world than with traditional, rigid robots,” says Ranzani. “Soft microscale robots for applications like surgery do exist, but they are really simple, just one degree of freedom and they also do not apply that much force to the tissue.”

The MORPH system the researchers built to demonstrate their new process is a 12-layer silicone structure that looks like a peacock spider. The microstructure begins as a 2D object that is then transformed into a 3D structure. The process begins with soft lithography when the 12 separate layers are manufactured, after which laser micromachining precisely cuts the material to form the spider shape before the layers are bonded together.

MORPH: A new soft material microfabrication process. Credit: Wyss Institute at Harvard University

Each layer’s architecture is unique and when placed on top of each other, can form a soft spider with 3D channels running through it—almost like simplified blood vessels. These channels are what allow the spider to go from 2D to 3D.

“The second innovation that we present here is that we can actually program this structure to bend when injected with a fluid that you can cure then once inside, and it will generate the three-dimensional structure you want,” says Russo.

“This technique is called injected-induced self-folding,” she says. “It takes inspiration from kirigami and origami art because we are cutting and then folding the structure in the shape that we want it and it can be reconfigured. If you think about origami artists, they can make a swan out of paper, but then you could take that swan and re-unfold it or fold it in a different way and have another animal.”

This manufacturing method can be used to build other shapes, or tweaked to fold the spider in a different way.

“You can reconfigure its shape and this has a lot of potential to deal with any complications during the situation you’re using the robot, everywhere you’re dealing with something that it’s dynamically changing,” Ranzani adds. “The whole system is much more damage resilient. If it undergoes a shape deformation, the soft robot will still work—with a rigid system if it becomes deformed, it just gets stuck.”

One potential application of this new manufacturing method is to build soft end effectors for surgical robots. Ranzani says that in the past he’s worked with larger scale soft robots for surgery, like an octopus-like tentacle on the centimeter scale. “This whole process actually started with the struggle to fabricate a soft end effector for surgical robot that was a few millimeters wide,” he says. “I started to realize that something like that is really hard to fabricate, especially if you want it to have a complex shape, effectively manipulate, but still be delicate to the body’s tissues.”

Then when the group realized there weren’t any processes to manufacture their desired structure, they began to put together a fabrication strategy, which turned into this four-step technique.

“If we can come up with manufacturing technologies, then maybe 10 years from now, someone can use our technologies to make better tools,” says Ranzani. “One possibility being a new surgical tool constructed from soft materials with embedded intelligence and highly sophisticated control to make difficult and complex surgical procedures easier to perform.”