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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-
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
powered by pumping fluids. Hosoi was interested in designing hydraulic
Engineers have designed a microfluidic device they call a
¡°tree-on-a-chip,¡± which mimics the pumping mechanism
of trees and other plants. Image: