Sensing Light with ‘Liquid Lego’

Note: The following article was adapted from a news release issued by the University of Oxford.

c4.jpgScientists at Oxford University and Duke University's Pratt School of Engineering have used tiny water droplets to build a unique microscopic light sensor. Their approach turns water droplets into protocells: empty artificial cells that can be filled with different cellular components. In theory, networks of protocells could be used to simulate biological systems – such as heart muscle or brain tissue.

“Each millimetre-sized water droplet in our network acts as a protocell. Chains of droplets are put together like liquid Lego, and are just as easily taken apart or reorganised,” said Matthew Holden of Oxford University’s Department of Chemistry who conducted the research with Oxford’s Professor Hagan Bayley and Professor David Needham at Duke.

"When you bring the two water droplets together in oil, they just kiss rather than coalesce," Needham added. The stability of the network results from the lipid bilayer between them, a structure like that surrounding living cells. By inserting particular proteins into the linked water droplets, they can be made to function as linear or branched networks, acting as wires, batteries or light sensors, the researchers report in the Journal of the American Chemical Society. Stream a video of the kissing droplets (requires Real Player). Some additional online images and videos of the droplet networks here.

In the new study, the scientists took a protein (bacteriorhodopsin) normally used by bacteria to produce energy and incorporated it into the network. This protein reacts to green light by pumping protons across a cell membrane, which creates a positive electrical charge. By piercing the droplets with hair-thin electrodes this current can be measured with a sensitive amplifier. In the future such droplets could be arranged to form ‘pixels’ in an imaging array – acting as an artificial eye, as reported earlier by the London Financial Times.

The networks might also have broader applications to cell biology. "Using protocells to simulate biological systems offers significant advantages to working with live cells, where there is far less control over their contents, size and function," said Holden. "Many living cells do not survive outside a narrow temperature range and are extremely sensitive to environmental conditions, such as pH. In the long run protocell-based systems could reduce the cost and complexity of biological experiments and, in some cases, might be used in place of animal testing."

Although protocells have provided biological and evolutionary insights before this is the first time they have been shown working together to perform a function. The researchers hope to create increasingly complex systems as a stepping-stone towards understanding a variety of biological functions ranging from nerve impulses to heartbeats.

Collaboration in the Making

The Duke-Oxford collaboration began after a chance encounter at a 2005 scientific meeting in Vancouver, Canada, Needham recalled. Bayley presented some evidence that a genetically engineered potassium channel could insert itself into a suspended lipid bilayer, called a black lipid film because they are so hard to see. The system offered a path toward biosensing, Bayley suggested.

But as a post-doctoral student in Cambridge in the early 1980’s, Needham had worked with black lipid films, examining them as a "quintessential model" for cell membranes Needham's interest in membranes originated with his desire to improve the delivery of drugs to cancerous tumors, a passion ignited by his mother's bout with breast cancer. (For more on his cancer-fighting nanowaxes, known as liposomes, which are now showing promise in early clinical trials, see http://pratt.duke.edu/news/?id=919.) He had made careful measurements of the tension and free energies associated with lipid bilayers' formation and he knew that the films are notoriously unstable, lasting just a half an hour on average.

He suggested to Bayley that he might consider a droplet-droplet system, earlier shown to stabilize emulsions by forming a lipid bilayer between water droplets in oil. Why not add a membrane protein to such a two-droplet system?

Back in Oxford, Bayley put his American post-doctoral student Matthew Holden on the case. He put two droplets together, connected electrodes and inserted a protein, which was immediately incorporated into the bilayer. Rather than a fleeting half-hour that the black lipid films would last, the system continued running through the weekend and then over Christmas break.

"Now we had the basis for a new technology," Needham said.