Drops of red and blue liquid move along the upper and lower surface of the vibrating UW platform at speeds up to 1 inch per second. This combined image shows drops as they move toward the center and merge. Image: Karl Bohringer/University of Washington
As medical researchers and engineers try to shrink diagnostics
to fit in a person’s pocket, one question is how to easily move and mix small
samples of liquid.
of Washington researchers
have built and patented a surface that, when shaken, moves drops along certain
paths to conduct medical or environmental tests.
“This allows us to move drops as far as we want, and in
any kind of layout that we want,” said Karl Böhringer, a UW professor of
electrical engineering and bioengineering. The low-cost system, published in Advanced Materials, would require very
little energy and avoids possible contamination by diluting or electrifying the
samples in order to move them.
The simple technology is a textured surface that tends to push
drops along a given path. It’s inspired by the lotus effect—a phenomenon in
which a lotus leaf’s almost fractal texture makes it appear to repel drops of
“The lotus leaf has a very rough surface, in which each
big bump has a smaller bump on it,” Böhringer said. “We can’t make
our surface exactly the same as a lotus leaf, but what we did is extract the
essence of why it works.”
The UW team used nanotechnology manufacturing techniques to
build a surface with tiny posts of varying height and spacing. When a drop sits
on this surface, it makes so little contact with the surface that it’s almost
perfectly round. That means even a small jiggle can move it.
A drop of liquid sits on the textured silicon surface that has arced rungs to guide the drop, and a grid of pillars to keep the drop in the channel. Image: Karl Bohringer/University of Washington
Researchers used an audio speaker or machine to vibrate the
platform at 50 to 80 times per second. The asymmetrical surface moves
individual drops along predetermined paths to mix, modify, or measure their
contents. Changing the vibration frequency can alter a drop’s speed, or can
target a drop of a certain size or weight.
“All you need is a vibration, and making these surfaces
is very easy. You can make it out of a piece of plastic,” Böhringer said.
“I could imagine this as a device that costs less than a dollar—maybe much
less than that—and is used with saliva or blood or water samples.”
In testing, different versions of the UW system could move the
drops uphill, downhill, in circles, upside down, or join two drops and then
move the combined sample.
The type of system is known as a “lab in a drop”: All
the ingredients are inside the drop, and surface tension acts as the container
to keep everything together.
A student tried using a smartphone’s speaker to vibrate the
platform, but so far a phone does not supply enough energy to move the drops.
To better accommodate low-energy audio waves, the group will use the UW’s
electron beam lithography machine to build a surface with posts up to 100 times
“There’s good evidence, from what we’ve done so far, that
if we make everything smaller then we will need less energy to achieve the same
effect,” Böhringer said. “We envision a device that you plug into
your phone, it’s powered by the battery of the phone, an app generates the
right type of audio vibrations, and you run your experiment.”