Real nanobots !!!! (???)

With fancy chemicals being developed for new medicines, a question remains: how can we deliver the most bang for the buck? It would be ideal to have a microscale delivery truck, capable of delivering the drug to the precise location needed. This would also eliminate many side effects experienced through a general delivery of the drug.

 Catalytic nanoswimmers are being researched as potential cargo vehicles. They are tiny spheres, with one half coated with a reactive material. This side reacts with chemicals in the environment, and the energy of reaction is transmitted into motion forward. Attach a blob of drug to them, and they can move medicine. But questions remain about how to get them to go a specific direction, without having to babysit them. Can we make these trucks driverless?

One way to direct these particles is to establish a fuel concentration gradient. The particles will be 'attracted' to the hot spot, though they have no free will.

In a recent paper, researchers say while this approach is interesting, it can't be the solution. After all, the fuel molecules are not necessarily always the signal you want the swimmers to react to.

They explore another signal: a pH gradient.

Hydrogel particles can swell or shrink in size when the acidity of the environment changes. Particles are  smaller in high acidity, and larger in low acidity. By exposing hydrogel catalytic swimmers to a pH gradient, the team was able to accumulate the swimmers in high pH regions. Cancer cells, as an example, can cause acidity changes.

So why does this accumulation happen? First, the high pH makes the particles larger, which makes their diffusion - the random motions experienced at the microscale - slower. Thus (smaller) particles in low pH regions will be kicked out more often, and settle in the high pH region. The smaller particles also undergo more (random) rotational motion, and this ends up being a double whammy for remaining happy in low pH regions.

I was struck by this paper as it was in nice contrast-and-compare to the run-and-tumble motion of E. Coli. bacteria, as is mentioned in the paper. Both systems are governed by the same physics, at the same scales. E. Coli turns its motor off to tumble and rotate, and figure out where the food is. It then turns the motor back on when it finds somewhere to go.  The catalytic swimmers have no such on/off switch, but can still get to where they need to go.