Molecular engines that walk along strands of DNA are nothing new, but none has featured as many successful features as the Oxford team's device. Unlike earlier attempts, their nanobot doesn't wander aimlessly back and forth, fall off its track or destroy its track as it walks. The team have also devised an ingenious way of powering the nanobot that allows it to move freely.
The walker consists of two connected feet, each made of a short sequence of DNA bases that attach to a complementary sequence on the DNA track. However, the sequence of bases on the track is designed so that the feet have to compete for a foothold. That means that as one foot steps down, the other is forced to lift off.
The power for this process is supplied by molecules floating nearby, which react together to release energy as long as a specific catalyst is there. The clever part of the design is that the DNA feet themselves act as the catalyst when they lift off the track.
The new walker is designed so that only the back foot can lift at any one time. The walker can put its foot back in the same place or move it forwards but it cannot take a backward step. This also ensures that one foot is always attached to the track.
This design solves some long-standing problems with walking molecules. In some designs, both feet can become detached at the same time, allowing the walker to float away; in others, the feet are just as likely to step backwards as forwards and so end up going nowhere.
There are challenges ahead, however. One is that the DNA track easily gets tangled, preventing the walker from moving. "At the moment, the nanobot has taken a single step but our ambition is to make it move 100 nanometres or more," says Andrew Turberfield, a physicist at the University of Oxford who led the research. To do that, the team will have to find a way to straighten the tracks.
So what else could the nanobot be coaxed into doing? "We can already stop and start our motor by controlling the amount of fuel we add, but we could add other control signals to make walkers interact with each other, and could easily attach a cargo to the region that links the two legs."
