Mother Nature has already developed an intricate and awesome system for moving biological fluids around inside of organisms. Exploiting the physics of these biological situations is inspiring for further research, so that physics and biomicrofluidics can one day (hopefully) expand beyond natural bounds.

In an article in the August 30, 2007 Physics of Fluids Khatavkar and friends (sounds better than et al, doesn't it?) propose a design for a micromixer inspired by the cilia of bacteria like E. coli The article presents an outline for this bio-inspired scheme. The cilia can sit unobtrusively in a microchannel and—powered by an electromagnetic field—nicely stir up a heterogeneous mixture.

Also taking a nod from human biological process, Yoon and friends published this article in the 2007 SPIE Proceedings of Nanosensors, Microsensors, and Biosensors and Systems.The researchers discuss and build artificial cilia that measure flow rate, direction, and other flow properties at microscopic levels. Once they take these measurements, the researchers can change the geometry of the flow channel to better accommodate or dissipate unwanted flow patterns. The researchers note that the idea came from observing how cilia are used in the kidney to measure how fluids are flowing. If there is an electrochemical or other change in the flow, the cilia can then give feedback, and the body will regulate tubule diameter. All of this is crucial for proper kidney function.

Biomimicry, as this type of bio-inspired design is sometimes called, has a lot of room to expand. An interesting related website is the Biomimicry Database, an open-source tool sponsored by the Rocky Mountain Institute, a non-profit in Boulder, Colorado. The database touts itself as: "A tool to cross-pollinate biological knowledge across discipline boundaries." Browsing their database is sort of amazing, and there's plenty of significant information, including a list of challenges and strategies for researchers, organisms with interesting and inspiring biological processes, and lots more. Although the information is not geared towards micro- or nano-processes, it still offers insight into physical systems that sometimes go unnoticed in the animal kingdom.

Even further from microfluidics: Daimler-Chrysler looked to the boxfish—a funky fish known for it's boxy shape and thick skin—for inspiration on automotive design, claiming that "the boxfish is... an ideal example of rigidity and aerodynamics." The result is a little odd looking. Regardless of the car's peculiar look, it purportedly performed remarkably well in the wind tunnel, as well as in fuel efficiency and emissions. Here nature proves that this counter-intuitive structure actually provides a useful product. It's remarkable to see biological processes like these inspiring researchers.

Just the thought of swimming fills me with dread. I don't mind hordes of children at a public pool, nor does the thought of swallowing several quarts of chorine-infested water bother me—it's the actual swimming that makes me nervous. Despite my future success in physics classes, I could never get the physics of swimming to work for me. If only Edward Purcell had been my swimming instructor...

In a lecture given by Purcell in 1974 and published in the American Journal of Physics in 1977, the Nobel laureate elegantly describes "Life at low Reynolds number."

The low Reynold's number (R) that an E. Coli experiences means that its motion is dominated by viscous forces while inertial are mostly ignored. In his lecture, Purcell paints a detailed picture of the different methods microorganisms use to swim. It turns out that any type of reciprocal motion isn't effective for dealing with Reynolds numbers on the order of 10^-2. The kind of motion a scallop uses to move, where its shell opens slowly and slams shut—spitting out water and propelling it forward—is useless in such a viscous fluid. This is Purcell's "Scallop Theorem" and concludes that if R is much less than one, the pattern of motion will be the same regardless of whether it is slow or fast, or even forward or backward in time.

Purcell continues on to describe two imaginary microorganisms; one with two hinges and another with a toroidal body, as well as the real bacterium Spirillum volutans, which uses a corkscrew motion to swim. It is not often that an expert in nuclear resonance and NMR such as Purcell could come up with this amazing description of fluid flow. I guess all that work with glycerin, a viscous fluid that readily responds to nuclear resonance, really paid off. Also consider that, at the time, this was considered kind of a useless problem; or at least just one that biologists would think about. The world is always in need of great thinkers who understand how biology and physics work together.

Purcell's lecture is more relevant than he could have imagined. As bacteria are coerced through micro-channels on a lab-on-a-chip device and organic fluids are coaxed to flow through MEMS, the method of propagation becomes a major factor in how things get done.

If physicists and biologists want to eventually send nanorobots through our bodies to repair damaged synapses (in the future, this is how a night of binge drinking will be treated), Purcell will most likely be thanked for inspiring the method of their locomotion.