If you hold a biologist at gun point and yell "Your real-time PCR or your life," be prepared to spend some time waiting for them to weigh the pros and cons before getting an answer. Polymerase chain reaction (PCR) is actually a general technique that allows a researcher to take small amounts of DNA and amplify it so that it can be analyzed. Without it, most of modern biology would be impossible. Despite its utility, you need have some reading material handy while performing PCR, because super-fast real-time PCR instruments take at least an hour to get anything done, and often two. Some recent research, published in Optics Express shows how to reduce this time, and not just by a little bit.
The researchers use a system where the reaction takes place in tiny drops of water that are immersed in oil. These water droplets can be manipulated by lasers, allowing them to be shifted from site to site, split, and combined. Now, researchers have shown that they can use the same laser that controls the drops to control a PCR.
The key is to use laser light that is transparent to oil but strongly absorbed by water. This can rapidly heat the small bubbles when the power is turned up, while the surrounding oil quickly cools the bubble back to ambient temperature when the laser is shut off. This rapid cycling of temperatures is exactly what is required for PCR. The DNA double helix is split into two single strands at high temperature. On cooling to an intermediate temperature, the PCR enzyme gets to work, copying each strand using raw materials in the solution. Heat the solution back up, and the cycle can start again.
A schematic of the PCR reaction cycle
This amplification process results in the number of DNA strands doubling every cycle, so it doesn't take many cycles to go from bugger-all to more than you really need (40-50 cycles does it). What makes the process slow is the time it takes to heat and cool the solutions in a typical PCR machine. There are only two ways to get around this: reduce the amount of water, or use more power.
The system described in the paper does both. The entire reaction takes place in a 20-100 nanoliter volume, which has the added advantage that only tiny amounts of reagent are required. The laser itself is 30mW, which doesn't sound like a lot but, considering that all those milliwatts are pretty much absorbed by a tiny droplet, it is about the equivalent of having a 1kW heater inside a dice. This makes heating pretty much instantaneous. Cooling is also very fast because of the high surface-to-volume ratio—the average droplet has a ratio of about 15,000.
With the time spent ramping temperatures up and down eliminated, the entire cycle time is dictated by the process speeds of each part of the cycle. Ten seconds to activate the enzyme, two seconds to split the DNA, and seven seconds to get set for another round. A 40 cycle PCR takes just 370s. They tested the apparatus on a range of DNA concentrations, and it works well down to 40 picograms per microliter. The authors warn that, given long DNA molecules, this concentration is lower than one molecule per droplet. However, in a basic test, they got amplification anyway because the pipette delivery system breaks the DNA up into shorter strands.
Life imitating art is a favorite cliché of journalists, and I would love to use it here, since PCR reactions seem to take mere minutes on the TV. But, unfortunately, calling CSI art—a show I actually like, though the spinoffs suck—is stretching things somewhat, so I won't go there. In any case, this is a fantastic piece of work, performed at a company, so you can bet it will be on the market soon.
Optics Express, 2009, DOI: 10.1364/OE.17.000218