Stronger, tougher paper may lead to flexible electronics

Cellulose fibres can be used to make tougher and stronger paper that may pave the way for flexible electronics such as paper smartphones, printable solar cells and 'green' vehicles.
Researchers at the University of Maryland have discovered that paper made of cellulose fibres is tougher and stronger the smaller the fibres get.
"Strength and toughness are often exclusive to each other. For example, a stronger material tends to be brittle, like cast iron or diamond," said Teng Li, associate professor of mechanical engineering at UMD.
The researchers explored the mechanical properties of cellulose, the most abundant renewable bio-resource on Earth.

They made papers with several sizes of cellulose fibres - all too small for the eye to see - ranging in size from about 30 micrometres to 10 nanometres.
The paper made of 10-nanometre-thick fibres was 40 times tougher and 130 times stronger than regular notebook paper, which is made of cellulose fibres a thousand times larger.

"These findings could lead to a new class of high performance engineering materials that are both strong and tough, a Holy Grail in materials design," said Li.
High performance yet lightweight cellulose-based materials might one day replace conventional structural materials (ie metals) in applications where weight is important.

This could lead, for example, to more energy efficient and "green" vehicles, researchers said.
In addition, researchers said, transparent cellulose nanopaper may become feasible as a functional substrate in flexible electronics, resulting in paper electronics, printable solar cells and flexible displays that could radically change many aspects of daily life.
Cellulose fibres can easily form many hydrogen bonds. Once broken, the hydrogen bonds can reform on their own - giving the material a 'self-healing' quality.
The researchers discovered that the smaller the cellulose fibres, the more hydrogen bonds per square area. This means paper made of very small fibres can both hold together better and re-form more quickly, which is the key for cellulose nanopaper to be both strong and tough.

"It is helpful to know why cellulose nanopaper is both strong and tough, especially when the underlying reason is also applicable to many other materials," said Liangbing Hu, assistant professor of materials science at UMD.
To confirm, the researchers tried a similar experiment using carbon nanotubes that were similar in size to the cellulose fibres.
The carbon nanotubes had much weaker bonds holding them together, so under tension they did not hold together as well. Paper made of carbon nanotubes is weak, though individually nanotubes are arguably the strongest material ever made.
The study was published in the Proceedings of the National Academy of Sciences.