Aerospace & Mechanical
Engineering

StarBot

Waves pounding a rocky shoreline always create a scenic view, relieving our minds of the stresses of daily living. Life remains stressful, however, for the creatures living in this intertidal zone where, among other challenges, they must survive the onslaught of these waves.

Sea stars foraging on barnacles.
Sea stars (Pisaster ochraceus) foraging on barnacles in a rocky intertidal habitat. Photo by Stephen Pavlov

The sea star (starfish) resides in this environment. It crawls over the rocky substrate, remaining attached despite the large fluid dynamic forces developed from waves with flow velocities reaching the order of 30 knots. Sea stars accomplish this by manipulating the hundreds of soft tissue tube feet lining the underside of their arms. They have no brain, so this array of tube feet is managed by a complex nervous system.

A university based group, headed by AME's Eva Kanso, has just won an Ofice of Naval Research Basic Research Challenge grant inspired by the capabilities of sea stars. The $2 million grant will fund research by Kanso, USC AME professor Mitul Luhar, UC Irvine professor Matthew J. McHenry, and UC San Diego professors Shenqiang Cai and Michael Tolley. The goals of the program are 1)to discover the control principles governing the sea star's use of tube feet to move; 2)develop actuation and sensing systems using soft materials; and 3)combine the results of the first two goals to build a a robotic sea star. Such a robotic sea star could be useful to perform inspections or to control biofouling on the submerged hull of a moving ship or other surfaces.

Yohanna Hanna describes his research.
A very blue Phataria_unifascialis. Others are purplish, reddish or tan. Photo by Gunter Eichhorn.

The group will take advantage of each institution's strengths to achieve the three goals. At UCI a live sea star, a Phataria unifascialis, will be enticed to move over transparent surface while researchers video record the action of its tube feet from below and the motion and movement of its arms from above. Tracking the tube feet and the arms this way will give various position measurements for use in developing a model of decentralized control of the tube feet. Later they will record similar video as the sea star crawls over trasparent obstacles. At in the water channel at USC the researchers will investigate how flow changes the sea star's strategy for moving over flat surfaces and obstacles. Also at USC they will measure lift and drag over the body.

Similar video and force measurements will be done with species of sea stars having different shapes, e.g., shorter arms, and with individuals which have lost one or more of their five arms.

Proposed tube feet for Starbot.
Proposed tube feet for Starbot.

While kinematics are investigated at USC and UCI, the group at UCSD will design and fabricate active structures similar to a sea star's tube feet. These "feet" will be tubes of soft, deformable material with hydraulically activated artificial muscle fibers attached to their surfaces. These will extend and pivot the tube. The end of the tube will be closed with plug constructed of nanofabricated polymer pillars coated with thin layers of synthetic musselmimetic polymer. When the pressure is reduced in this plug, its porous bottom will attach to the underwater surface. By pivoting the tube foot forward from the vertical, then extending it, dropping the pressure in the end plug, and, finally, pivoting back to the vertical, the Starbot will move forward.

After the tube foot is operational, its capabilities will be tested in the water channel. This will begin with determining the static and then the hydrodynamic loads an individual tube foot can tolerate while still remaining attached to the submerged surface. Next, similar tests will be performed for groups of tube feet on a robotic Starbot arm, with a goals of determining the optimal tube foot distribution, the coordination between sensors and tube feet needed to move the arm, and characterize the overall robustness of the sensor-tube foot-arm system. Finally, a complete Starbot will be assembled and tested crawling over irregular surfaces in high speed, unsteady flows.