Whipping fluids along in microlabs

Scientists detail one way for hairlike structures to move liquid in a “lab on a chip”

Taking a cue from nature, scientists have shown one way that microscopic, hairlike structures could move liquid through tiny channels in a “lab on a chip” — essentially a miniaturized biology lab etched into a device roughly the size of a computer chip.

CILIUM BY DESIGN Cells inside the lungs beat microscopic cilia back and forth to move mucus up the trachea toward the mouth, thus helping to flush out dust and other debris. Researchers want to mimic this design for moving fluids through tiny channels in a biological “lab on a chip.” Charles Daghlian

Developing microlabs is a burgeoning area of research, and one of the principle challenges is controlling the flow of fluids at the minute scale of micrometers. In these chips, tiny volumes of blood or other biological fluids are moved between chambers where automated tests are performed. Smaller versions of pumps that work in the human-scale world would be useless in these chips because in this microscopic world even water acts thick and viscous, like honey.

“Swimming at this scale is like swimming in a pool of syrup,” says Patrick Onck, who researches the micromechanics of materials at the University of Groningen in the Netherlands. Motions that would propel the fluid in the macroscopic world would only move the nearby fluid back and forth in these microscale channels.

Several research groups have tried using vibrating magnetic fields to drive microscopic, synthetic “hairs” impregnated with magnetic particles in a rhythmic, beating motion that will propel fluids. Now Onck and colleagues in Romania and the Netherlands have crunched the numbers, working out the math that describes the complex interactions among the magnetic field, the flexible hairlike structures and the surrounding fluid. Their results, posted online at arxiv.org and submitted to the Journal of the Mechanics and Physics of Solids, show that the scheme should work. The researchers also reduce the complex physics problem to a few simple guidelines that engineers can use to design such systems.

Bacteria and other single-celled organisms solved this fluid-movement problem long ago, either by spinning corkscrew-shaped flagella or whipping hairlike cilia back and forth. Because the cilia motion is asymmetric — the cilia are straighter during the power stroke and more bent during the recovery — many cilia working in concert can propel the surrounding fluid. Cilia also move fluids in larger animals such as humans. For example, the cilia that line the internal surface of a person’s lungs move mucus (and the dust it carries) up toward the mouth. Rather than trying to invent an entirely new solution, researchers are borrowing nature’s solution.

Other techniques such as strong electric fields are also being developed for moving fluids in microlabs, but this technique has pros and cons. Electric fields can interfere with biological fluids by altering their acidity through electrochemical reactions. Magnetic fields driving artificial cilia could be less disruptive.

“This is still a very emerging area, so there is no established technique that’s been proven as being the best” for propelling fluids on the microscale, comments Orlin Velev, an expert in manipulating microscopic particles using electric fields at North Carolina State University in Raleigh. “The importance of this [work] is that it pushes forward the theoretical development of this field.”

Onck’s team proposes making the cilia from polymers and embedding the hairs with nanoscale magnets. Tiny electric circuits on a silicon chip beneath the lab chip could serve as electromagnets to provide the small-scale, vibrating magnetic fields needed to drive the cilia.

The research was funded as part of ARTIC, a European Union–funded project for making fluid-propelling artificial cilia. Other research teams participating in this project are working on building these magnetically driven cilia, Onck says.

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