Nanopaper made of gently processed natural cellulose nanofibers is found to have remarkable strength; it has a tensile strength almost equaling that of structural steel.

Lars Berglund from the Swedish Royal Institute of Technology in Stockholm, Sweden found that the mechanical processes used to pulp wood damages the natural fibers, weakening them. Berglund developed a process to extract the fibers, keeping their properties intact.

The secret to the nanopaper's performance is not only the strength of the undamaged cellulose fibres, but also they way they are arranged into networks. Although strongly bound together, they are still able to slip and slide over each other to dissipate strains and stresses.

The individual cellulose fibres are also much smaller than in conventional paper. "A regular paper network has fibres 30 micrometres in diameter, here we are at a scale three orders of magnitude smaller," says Berglund. "The material [has] very small defects compared with a conventional paper network."

Mechanical testing shows it has a tensile strength of 214 megapascals, making it stronger than cast iron (130 MPa) and nearly as strong as structural steel used in buildings and bridges (250 MPa). Normal paper is flimsy; it has a tensile strength less than 1 MPa. The tests used strips 40 millimeters long by 5mm wide and about 50 micrometers thick.

Science fiction readers may recall the material used in Jules Verne's 1866 classic Robur the Conqueror.

...Unsized paper, with the sheets impregnated with dextrin and starch and squeezed in hydraulic presses, will form a material as hard as steel. There are made of it pulleys, rails, and wagon-wheels, much more solid than metal wheels, and far lighter. And it was this lightness and solidity which Robur availed himself of in building his aerial locomotive...
(Read more about Verne's paper steel)

See also this attempt to combine two plastics to create 'metal', as well as efforts to spin webs strong as steel.

From New 'super-paper' is stronger than cast iron.

(This Science Fiction in the News story used with permission of Technovelgy.com)