Harvard University scientists on Wednesday said they had created Smileys, Chinese characters and card-game symbols at scales of billionths of a metre using strands of DNA.

The feat marks the next step in "DNA origami" in which the molecule that provides the genetic code for life is used as a building block at the nanoscale, with potential outlets in engineering and medicine.

DNA is like a twisted ladder with double "rungs" of chemicals which interlock.

By unzipping the ladder and cutting it lengthwise, researchers can create a stretch with a set of single rungs that can partner up with a matching strand.

This is the characteristic harnessed by a team led by Peng Yin of Harvard's Wyss Institute for Biologically Inpired Engineering.

Reporting in the British journal Nature, the team showed off short lengths of DNA, each 42 "rungs" long, that interlocked with complementary stretches of the molecule.

Like Lego tiles, the strands could be programmed to assemble themselves into specific shapes.

To demonstrate the method, the team made a molecular picture featuring 107 designs, from emoticons, Chinese characters, numbers and letters from the Latin alphabet.

The canvas is a rectangle measuring 64 nanometres by 103 nanometres, with 310 pixels.

Scientists have been interested in nanoscale shapes for more than 20 years, and have progressively moved from two dimensional to three dimensional successes.

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DNA strands create tiniest Smileys