Advanced paper could allow for inexpensive biomedical and diagnostic devices
June 4, 2013
By modifying the underlying network of cellulose fibers, etching off surface “fluff” and applying a thin chemical coating, Georgia Institute of Technology 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 “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 uses nanometer- and micron-scale structures and a surface fluorocarbon.
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 (water-repelling) 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 … composed of fibers with different sizes, different lengths and a non-circular cross-section,” said 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.”
How to create the new paper
The new paper, which is both superhydrophobic (water-repelling) and super oleophobic (oil-repelling), can be made from standard softwood and hardwood fibers using a modified paper process:
- Break up cellulose fibers into smaller structures using a mechanical grinding process.
- As in traditional paper processing, press the fibers in the presence of water.
- Remove the water and add the chemical butanol, which inhibits the hydrogen bonding that normally takes place between cellulose fibers, allowing better control of their spacing.
- Use 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. This uncovers smaller cellulose structures and provides a second level of “roughness” with the proper geometry needed to repel liquids.
- Apply a thin coating of a fluoropolymer over the network of cellulose fibers.
“The desirable properties we are seeking are mainly controlled by the geometry of the fibers,” Hess explained.
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. In testing, the paper was able to repel water, motor oil, ethylene glycol and n-hexadecane solvent.
That capability could facilitate development of inexpensive biomedical diagnostic tests in which a droplet containing antigens could be rolled along a printed surface where the droplet would encounter diagnostic chemicals.
If appropriate reagents are used, the specific color or color 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.
Creating a superhydrophobic suface was relatively straightforward because water has a high surface tension. For oils, which have a low surface tension, the key to creating the repellent surface is to create re-entrant — or undercut — angles between the droplets and the 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, Hess said.
The new paper has so far been made in samples about four inches on a side, but Hess sees no reason why the process couldn’t be scaled up.
The research has been supported by the Institute for Paper Science and Technology (IPST) at Georgia Tech.