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robots at the small scale, that could perform

actions similar to much bigger robots like

Boston Dynamic’s Big Dog, a four-legged,

Saint Bernard-sized robot that runs and

jumps over rough terrain, powered by hy-

draulic actuators.

“For small systems, it’s often expensive to

manufacture tiny moving pieces,” Hosoi

says. “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.”

The general understanding among biologists

has been that water, propelled by surface

tension, travels up a tree’s channels of

xylem, then diffuses through a semiperme-

able membrane and down into channels of

phloem that contain sugar and other nutri-

ents.

The more sugar there is in the phloem, the

more water flows from xylem to phloem to

balance out the sugar-to-water gradient, in a passive process known as

osmosis. The resulting water flow flushes nutrients down to the roots. Trees

and plants are thought to maintain this pumping process as more water is

drawn up from their roots.

“This simple model of xylem and phloem has been well-known for decades,”

Hosoi says. “From a qualitative point of view, this makes sense. 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 mi-

crofluidic pumps, fabricating parts that mimic xylem and phloem. But they

found that these designs quickly stopped pumping within minutes.

It was Hosoi’s student Comtet who identified a third essential part to a tree’s

pumping system: its leaves, which produce sugars through photosynthesis.

Comtet’s model includes this additional source of sugars that diffuse from

the leaves into a plant’s phloem, increasing the sugar-to-water gradient,

which in turn maintains a constant osmotic pressure, circulating water and

nutrients continuously throughout a tree.

Running on sugar

With Comtet’s hypothesis in mind, Hosoi and her team designed their tree-

on-a-chip, a microfluidic pump that mimics a tree’s (continued on page 3>>)

Engineers Design “Tree-On-A-Chip”

Microfluidic device generates passive hydraulic power

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Requires no moving parts or external

pumps, able to pump water and sugars

through chip at steady flow rate for days

March 20, 2017: Trees and other plants, from

towering redwoods to diminutive daisies, are

nature’s hydraulic pumps. They are constantly

pulling water up from their roots to the

topmost leaves, and pumping sugars pro-

duced by their leaves back down to the roots.

This constant stream of nutrients is shuttled

through a system of tissues called xylem and

phloem, which are packed together in woody,

parallel conduits.

Now engineers at MIT and their collaborators

have designed a microfluidic device they call

a “tree-on-a-chip,” which mimics the pumping

mechanism of trees and plants. Like its natu-

ral counterparts, the chip operates passively,

requiring no moving parts or external pumps.

It is able to pump water and sugars through

the chip at a steady flow rate for several

days. The results are published this week in

Nature Plants.

2017 APR/MAY #8-2

Anette “Peko” Hosoi, professor and associate department head for op-

erations in MIT’s Department of Mechanical Engineering, says the chip’s

passive pumping may be leveraged as a simple hydraulic actuator for

small robots. Engineers have found it difficult and expensive to make tiny,

movable parts and pumps to power complex movements in small robots.

The team’s new pumping mechanism may enable robots whose motions

are propelled by inexpensive, sugar-powered pumps.

“The goal of this work is cheap complexity, like one sees in nature,” Hosoi

says. “It’s easy to add another leaf or xylem channel in a tree. In small

robotics, everything is hard, from manufacturing, to integration, to actua-

tion. If we could make the building blocks that enable cheap complexity,

that would be super exciting. I think these [microfluidic pumps] are a step

in that direction.”

Hosoi’s co-authors on the paper are lead author Jean Comtet, a former

graduate student in MIT’s Department of Mechanical Engineering; Kaare

Jensen of the Technical University of Denmark; and Robert Turgeon and

Abraham Stroock, both of Cornell University.

A hydraulic lift

The group’s tree-inspired work grew out of a project on hydraulic robots

Engineers have designed a microfluidic device they call a

“tree-on-a-chip,” which mimics the pumping mechanism

of trees and other plants. Image: news.mit.edu