Is Paper The New Metal?

In the development of new high performance materials, there are two highly desirable features: strength and toughness. Unfortunately, these have always been considered mutually exclusive. Now, however, materials scientists may have delivered a solution from an entirely unexpected source: paper.

Though strength and toughness are often used interchangeably in everyday conversation, scientists attach a very specific meaning to each term. Strength refers to a material’s ability to resist damage, think cast iron. In contrast, tough materials are those that can bounce back from damage, think of a paper clip. Strong materials tend to be brittle – they resist damage, but when they are damaged it is catastrophic. More deformable materials can recover their form after damage, but their inherent softness makes them more damage-prone to begin with. Scientists and manufacturers have always done their best to balance these two factors depending on the application at hand, but have had limited success in optimizing both simultaneously.

Flexible solar panel: flexible solar cells are just one example of a technology that stands to benefit from this discovery. Image from solopower.com.Flexible solar panel: flexible solar cells are just one example of a technology that stands to benefit from this discovery. Image from solopower.com.

Until now! New research conducted at the University of Maryland and published this month in the Proceedings of the National Academy of Sciences has led to the development of a paper that gets stronger as it gets tougher. How did they do it? They took advantage of the most abundant renewable bio-resource on Earth: cellulose. Cellulose is an important component of cell walls allowing the maintenance of cellular structure. As such, we can easily extract it from any number of green plants, wood, or even algae. In this research, cellulose fibers were used to make paper and the effect of fiber diameter on paper strength was analyzed. The results were completely counter-intuitive and displayed what is termed a reverse scaling law. As the fiber diameter decreased, the paper actually got stronger! Paper made from 10 nm diameter fibers – which are about 1000 times smaller than a red blood cell – was 40 times tougher and a whopping 130 times stronger than standard notebook paper made from 10,000 nm diameter fibers.

Cotton: this abundant bio-resource is one of the best sources of cellulose which makes up about 90% of its fibers.Cotton: this abundant bio-resource is one of the best sources of cellulose which makes up about 90% of its fibers.

Perhaps more important than the discovery that cellulose fibers display a reverse scaling law is the understanding of why. This insight will now allow researchers to custom-design new materials with the same property along with other desirable behaviors like electrical conductivity or resistance to heat and cold. Cellulose fibers, as it turns out, are held together in a network by a large number of hydrogen bonds. Though relatively weak individually, enough hydrogen bonds can lead to a fairly cohesive solid. The thinner the fibers, the more opportunity for hydrogen bonding in a given area. Another fascinating aspect of hydrogen bonds is the relative ease with which they can be broken and reformed. That means materials based on this principle can be self-healing!

A number of industries are expected to benefit from this discovery. In the near term, paper electronics, printable solar cells, and flexible displays will emerge that are lighter, stronger, tougher, and more environmentally sustainable than their predecessors. As a new generation of researchers exploits this discovery to establish novel materials, we may start to see super-lightweight ‘paper’ vehicles. Imagine a tough and strong car that’s made from plants, is so light it consumes minimal fuel, and can heal itself after you accidentally side swipe that mailbox, and you understand the incredible potential of these results.

Via Phys.org and PNAS.