Scientists Found a Way to Print Full-Colour Images Using Completely Transparent Ink

Scientists Found a Way to Print Full-Colour Images Using Completely Transparent Ink
Image: Yanlin Song

In what sounds like a completely backwards way to resolve some of the many issues with printers, researchers from the Chinese Academy of Sciences’ Institute of Chemistry have come up with a way to use transparent inks to print images with a full spectrum of colours.

You don’t need to be an environmental scientist or even an engineer to understand why printers are problematic. The chemical processes needed to produce the pigments and inks used in the devices aren’t exactly environmentally friendly, and then there’s the issue of endlessly having to replace multiple ink cartridges if you do a lot of colour printing, even when the cartridges don’t actually appear to be completely empty. Where’s all that leftover ink actually going? Is it being responsibly disposed of? There are lots of questions and concerns around the printing industry that have encouraged researchers to come up with alternative ways to create colour imagery.

One approach takes inspiration from the natural world, where creatures like butterflies and peacocks produce shimmering colour by manipulating how light is reflected through microscopic structures, a phenomenon known as structural colour. In 2015, researchers from the Missouri University of Science and Technology created in inkless printer alternative that used a laser to perforate a metal material with thousands of tiny holes to create a microstructure that produced the same effect, resulting in a small range of visible colours.

In a recently published paper, however, a team from the Chinese Academy of Sciences detail a new approach to structural-colour technology that sounds a lot more promising for real-world applications, because it can reproduce a wide spectrum of colours and uses existing printer hardware. An inkjet — the type of colour printer most have at home — creates full-colour images by shooting microscopic droplets of ink, in different sizes and colours, onto a piece of paper. When viewed up close, the patterns of coloured dots appear completely random, but when viewed from afar, a full-colour image can be seen.

The researchers modified an inkjet printer to use just a single polymer ink that appears transparent to the human eye. Instead of paper, which will readily absorb most liquids, causing them to spread and bleed, they used glass with a hydrophobic surface, so that when the water-based ink droplets were laid down, they were repelled, causing them to form a structure that looked like a tiny dome. Leveraging the surface tension properties of liquids and the hydrophobic effect of the glass allowed the researchers to modify a printer so that it could create these microdomes in varying sizes and shapes, covering a surface in what are essentially thousands and thousands of tiny lenses.

With the microdome structures printed on just one side of the glass, the imagery is visible only from that side; it becomes completely transparent when viewed from the other side. (Image: Institute of Chemistry, Chinese Academy of Sciences) With the microdome structures printed on just one side of the glass, the imagery is visible only from that side; it becomes completely transparent when viewed from the other side. (Image: Institute of Chemistry, Chinese Academy of Sciences)

Depending on its size and shape, each microdome reflects a different wavelength of light, causing the human eye to perceive various colours as a result. With thousands grouped together, a larger, full-colour image can be created the same way coloured ink dots create images on paper (also like the coloured pixels on an LCD screen). The effect only works in one direction, however, so when the printed piece of glass is viewed from the other side, the ink is again transparent. One day, entire skyscrapers could be covered in imagery, be it artistic designs, massive billboards, or protection for birds, without obstructing the view from the inside.

Images of Audrey Hepburn and Isaac Newton demonstrate the high fidelity of this new approach to printing. (Image: Yanlin Song) Images of Audrey Hepburn and Isaac Newton demonstrate the high fidelity of this new approach to printing. (Image: Yanlin Song)

Through careful manipulation of the shape and patterned structure of the microdomes, the researchers believe they can fully control the saturation, lightness, and other aspects of the colours being produced. For the time being, however, they’ve focused on recreating detailed, recognisable images with limited colour palettes to demonstrate the potential of the new approach.

In addition to being compatible with existing printing hardware, including the printers used in larger commercial print shops, the new structural-colour method could potentially also help reduce printing costs, since it relies on just a single ink. It also promises a much longer lifespan for prints than when using dyes and pigments that will naturally fade over time. Assuming light continues to obey the laws of physics, and as long as nothing affects the printed microdome structures, the visible colours will always look as vibrant and saturated as they did when first printed.