A Different Kind of Wood Chip

Passive hydraulic pump that mimics trees could make small robots move.

A microfluidic device designed by engineers at MIT, Cornell, and the Technical University of Denmark could serve as a simple, low-cost hydraulic actuator for small robots. 

“For small systems, it’s often expensive to manufacture tiny moving pieces,” says mechanical engineering professor Anette “Peko” Hosoi. “So we thought, ‘What if we could make a small-scale hydraulic system that could generate large pressures, with no moving parts?’ And then we asked, ‘Does anything do this in nature?’ It turns out that trees do.”

Trees and other plants pull water up from their roots to the leaves, and pump sugars produced by their leaves back down to the roots. Propelled by surface tension, water travels up a tree’s channels of vascular tissue called xylem; then it diffuses through a semipermeable membrane and down into channels of another vascular tissue, phloem, containing sugar and other nutrients.

The more sugar in the phloem, the more water flows from the xylem to balance the concentrations on either side of the membrane, a passive process known as osmosis. The water flow flushes nutrients down to the roots, and this pumping process continues as the roots draw up more water.

“This simple model of xylem and phloem has been well-known for decades,” Hosoi says. “But when you actually run the numbers, you realize this simple model does not allow for steady flow.” In fact, engineers have previously attempted to design tree-inspired microfluidic pumps, but such designs stopped pumping within minutes.

As a grad student in Hosoi’s lab, Jean Comtet, SM ’15, identified a third essential part of a tree’s pumping system: its leaves, which produce sugars through photosynthesis. Comtet hypothesized that the sugars that diffuse from the leaves into a plant’s phloem increase the concentration of sugar there, creating a constant osmotic pressure that allows water and nutrients to circulate continuously.

To create a pump that mimics the entire system, Hosoi’s team sandwiched together two plastic slides, through which they drilled small channels to represent xylem and phloem. They filled the xylem channel with water and the phloem channel with water and sugar, separating the two slides with a semipermeable material. They placed another membrane over the phloem slide, and set a sugar cube on top to replicate the sugar from a tree’s leaves. When they attached the chip to a tube leading to a water tank, water passively pumped through the chip and out into a beaker at a constant rate for several days.

“As soon as we put this sugar source in, we had it running for days at a steady state,” Hosoi says. “If you design your robot in a smart way, you could absolutely stick a sugar cube on it and let it go.”