Steerable catheters are not available for neurosurgery because of how small the brain's blood vessels are. Specifically, devices need to be less than one millimeter in diameter--that’s roughly the diameter of a few human hairs--and about five feet long (160 cm). Industrial fabrication methods struggle at this scale. That’s partially because gravity, electrostatics, and the van der Waals force are all similar at this size. So once you pick something up with tweezers, you cannot drop it. If you coax it from the tweezers, it may leap into the air from opposing forces and disappear, never to be found again.
“Unfortunately, many of the most important blood vessels we need to treat are among the most tortuous and fragile in the body,” said James Friend, a professor at the UC San Diego Jacobs School of Engineering and School of Medicine and the paper’s corresponding author. “Although robotics is rising to the need in addressing many medical problems, deformable devices at the scales required for these kinds of surgeries simply do not exist.”
Bioinspiration
To solve this problem, researchers turned to inspiration both from nature and from soft robotics.
“We were inspired by flagella and insect legs, as well as beetles mating, where microscale hydraulics and large aspect deformation are involved,” said Gopesh Tilvawala, who recently earned a Ph.D. in Friend’s research group and the paper’s first author. “This led us to developing [a] hydraulically actuated soft robotic microcatheter.”
Computer simulations and new fabrication methods
The team had to invent a whole new way of casting silicone in three dimensions that would work at those scales, by depositing concentric layers of silicone on top of one another with different stiffnesses. The result is a silicone rubber catheter with four holes inside its walls, each about one half the diameter of a human hair.
The team also conducted computer simulations to determine the configuration of the catheter; how many holes it should include; where these should be placed; and the amount of hydraulic pressure needed to actuate it. To guide the catheter, the surgeon compresses a handheld controller to pass saline fluid into the tip to steer it. Saline is used to protect the patient:; if the device should fail, then saline harmlessly enters the bloodstream. The catheter’s steerable tip is visible on X-rays.
A new way of doing neurosurgery
“This technology is ideal for situations when I need to make a 180 degree turn from the catheter position in the parent artery, and maintaining position and reducing kick-out is critical,” said Dr. David Santiago-Dieppa, neurosurgeon at UC San Diego Health. “This advance may ultimately allow us to treat aneurysms, other brain pathologies and even strokes that we haven’t been able to in the past.”
