I just read this in Nature:
The researchers designed the structure of the nanorobots using open-source software, called Cadnano, developed by one of the authors — Shawn Douglas, a biophysicist at Harvard's Wyss Institute for Biologically Inspired Engineering. They then built the bots using DNA origami. The barrel-shaped devices, each about 35 nanometres in diameter, contain 12 sites on the inside for attaching payload molecules and two positions on the outside for attaching aptamers, short nucleotide strands with special sequences for recognizing molecules on the target cell. The aptamers act as clasps: once both have found their target, they spring open the device to release the payload.
These robots may be able to identify and target cancer cells.