Delivering chemical agents to specific locations in the body has been an elusive goal ever since Paul Erhlich described his concept of the 'magic bullet' nearly a century ago. Whether the objective is delivering drugs to sites of injury and disease, or the infusion of imaging agents that allow physicians to see what is going on, one would like to be able to target molecules to the places where they are needed most. Ideally, one would be able to pick out a single cell (such as a cancer cell) in the midst of many others, and deliver a molecular payload to that cell alone.
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