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Paper could be the basis for inexpensive diagnostic devices

29 May 2013

Georgia Tech researchers have created a new type of paper that is capable of repelling a wide variety of liquids, including water and oil.

Georgia Tech professor Dennis Hess and graduate research assistant Lester Li observe oxygen plasma treatment that exposes the cellulose nanofibrils on their superamphiphobic paper samples
Georgia Tech professor Dennis Hess and graduate research assistant Lester Li observe oxygen plasma treatment that exposes the cellulose nanofibrils on their superamphiphobic paper samples

Paper is known for its ability to absorb liquids, making it ideal for products such as paper towels. But by modifying the underlying network of cellulose fibres, etching off surface 'fluff' and applying a thin chemical coating, researchers have created a new type of paper that repels a wide variety of liquids, including water and oil.

The paper takes advantage of the so-called 'lotus effect' – used by leaves of the lotus plant – to repel liquids through the creation of surface patterns at two different size scales and the application of a chemical coating.

The material, developed at the Georgia Institute of Technology, uses nanometer- and micron-scale structures, plus a surface fluorocarbon, to turn ordinary paper into an advanced material.

The modified paper could be used as the foundation for a new generation of inexpensive biomedical diagnostics in which liquid samples would flow along patterns printed on the paper using special hydrophobic ink and an ordinary desktop printer.

This paper could also provide an improved packaging material that would be less expensive than other oil- and water-repelling materials, while being both recyclable and sustainable.

“Paper is a very heterogeneous material composed of fibres with different sizes, different lengths and a non-circular cross-section,” says Dennis Hess, a professor in the Georgia Tech School of Chemical and Biomolecular Engineering. “We believe this is the first time that a superamphiphobic surface – one that repels all fluids – has been created on a flexible, traditional and heterogeneous material like paper.”

The new paper, which is both superhydrophobic (water-repelling) and super oleophobic (oi-repelling), can be made from standard softwood and hardwood fibres using a modified paper process. 

Producing the new paper begins with breaking up cellulose fibres into smaller structures using a mechanical grinding process. As in traditional paper processing, the fibres are then pressed in the presence of water – but then the water is removed and additional processing is done with the chemical butanol. Use of butanol inhibits the hydrogen bonding that normally takes place between cellulose fibres, allowing better control of their spacing.

The second step involves using an oxygen plasma etching process – a technique commonly used in the microelectronics industry – to remove the layer of amorphous 'fluffy' cellulose surface material, exposing the crystalline cellulose nanofibrils. The process thereby uncovers smaller cellulose structures and provides a second level of 'roughness' with the proper geometry needed to repel liquids.

Finally, a thin coating of a fluoropolymer is applied over the network of cellulose fibres. In testing, the paper was able to repel water, motor oil, ethylene glycol and n-hexadecane solvent.

The researchers have printed patterns onto their paper using a hydrophobic ink and a desktop printer. Droplets applied to the pattern remain on the ink pattern, repelled by the adjacent superamphiphobic surface.

That capability could facilitate development of inexpensive biomedical diagnostic tests in which a droplet containing antigens could be rolled along a printed surface where it would encounter diagnostic chemicals. If appropriate reagents are used, the specific colour or colour intensity of the patterns could indicate the presence of a disease. Because the droplets adhere tightly to the printed lines or dots, the samples can be sent to a laboratory for additional testing.

“We have shown that we can do the operations necessary for a microfluidic device,” Hess said. “We can move the droplet along a pattern, split the droplet and transfer the droplet from one piece of paper to another. We can do all of these operations on a two-dimensional surface.”

Previous examples of superamphiphobic surfaces have been made on rigid surfaces through lithographic techniques. Such processes tend to produce fragile surfaces that are prone to damage. The principal challenge in this current work has been to create high-performance in a material that is anything but geometrically regular and consistent.

“Working with heterogeneous materials is fascinating, but it’s very difficult not just to control them, because there is no inherent consistent structure, but also to change the processing conditions so you can get something that, on average, is what you need,” he said. “It’s been a real learning experience for us.”

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