Diabetes Therapies and Liquid Memory May Develop
from Micro Fluid Research, Science Researchers Say

Alex Groisman

Polymer solutions pulsing through miniaturized plumbing may control implanted drug delivery devices and provide memory for liquid computers, according to a new study.

The researchers created two small-scale fluid drive devices, one for flow control and the other for fluidic memory, that rely on the special behavior of polymer liquids to function. These findings are described in the 9 May issue of the journal Science, published by the AAAS.

"One option for this research would be an artificial pancreas for patients with diabetes. The device could measure glucose levels and dispense appropriate amounts of insulin," said Stephen Quake, an author from the California Institute of Technology in Pasadena, California.

Implantable devices employing this technology may eventually monitor aspects of human physiology—from the inside—and dole out drugs accordingly, explained Quake.

"Although we have only built pilot devices, there may be some practical drug delivery applications for them," said Alex Groisman, the first author on the paper and a researcher from the University of California–San Diego in La Jolla, California.

Groisman used the pharmaceutical application model to explain the operation of this new generation of fluid contraptions.

"Imagine an implanted pill with an inflatable chamber to hold a drug. If you connect this reservoir to the drug's drop-off spot in the body with a microfluidic channel, you can achieve relatively constant flow of medicine despite decreases in pressure," said Groisman.

This consistency of flow, despite changing pressure, is a special feature of this new device called a "flux stabilizer."

"You could discharge 90 percent of your medicine without an intolerable decrease in flow rate…and you don't need moving parts," said Groisman.

Inside the flux stabilizer's curved chambers, made of a silicon rubber, the mechanical properties of polymers liquids are responsible for the functioning of the little gadget.

The current version of the flux stabilizer is shaped like a bunch of shrunken macaroni noodles lined up end-to-end to form a snaking pattern. The researchers added a bottleneck at the point where each of the noodles touch, thus creating a series of expansions and contractions.

As the dissolved polymer molecules move through each contraction, these molecules—which are made of repeating segments linked together like a chain—move through each contraction they lengthen and unravel. Above a threshold flow rate, the polymers unravel extremely quickly, making the fluid thick and resistant to movement.

It is this resistance that stabilizes the output of the polymer liquid from this new fluid circuit, despite changes in pressure.

Replacing the polymers used in this research with a biocompatible polymer that could regulate drug flow would be an important step in moving this technology into medical devices, according to Quake.

The researchers also used the non-linear properties of polymers and an innovative fluid chamber design to construct a fluidic memory device called a "flip flop." The tiny fluid trinket can maintain a "high" or "low" state, comparable to the ones or zeros used in electronic memory.

"These devices are an option for the basic elements that one would like to use as control circuits for microfluidic computers," said Quake. "There are lots of valves, tubes and other pieces around that could be incorporated into such devices. However, we are missing good control systems to pull it all together."

Inside the flip flop, the researchers' new memory device, the flow of the polymer liquid mimics the binary guts of a computer.

"This is a quite new and rather unexpected design. Construction of fluidic memory devices using polymer liquids has not been suggested before," said Groisman.

The team designed and redesigned channel layouts, always looking for patterns that meshed with the special properties of the polymer solution.

"If you squint you can see the channels," said Quake as he described the fluid chambers.

The polymers start down a long hallway before threading themselves through one bottleneck and then another.

Imagine resizing a pair of swimming goggles. The polymers are the rubber strap and the bottlenecks are the plastic buckle used to change the strap length.

As you pull on the strap to tighten it, the part that has already moved past the buckle stretches. When the polymers stretch out like this past the bottleneck, they unravel. The stream of unraveled polymers moves through a second bottleneck and the flow pattern locks into formation.

The flow is now in one the two stable "states." The fluid will maintain this pattern of flow until a large change in pressure bumps the polymers to their second stable state.

In this way, the polymer flow remembers where the last pressure kick directed its path. The polymer flow will not change direction until the system intentionally delivers another pressure punch. In much the same way, a "one" in computer memory will remain a "one" until directed to become a "zero".

"This is a memory device that can be integrated into medical, biological and biochemical systems," said Groisman.

The incorporation of liquid polymers into fluidics has enabled researchers to escape the demands of inertia and shrink the size of circuits. This miniaturization trend will continue, according to the authors.

—Daniel Kane

8 May 2003