Bastard Hogberries and Rainbow Fibres

A bright blue berry has inspired the design of a new type of colour-changing fibre.

The term “bastard hogberry” doesn’t conjure up the nicest image, but the berry of this South American plant (Margaritaria nobilis) is quite possibly the prettiest, most impressive little fruit you will ever lay your eyes on. In fact, with its bright blue metallic skin it looks more like a precious jewel to be worn on a brooch than something that grows on a plant with such a name.

As well as attracting birds and insects, this alluring fruit caught the eyes of professor Pete Vukusic at the University of Exeter (UK) and Mathias Kolle at the Harvard School of Engineering and Applied Sciences (US). They were intrigued by the way the surface of the tropical berry interacts with light to produce its iridescent blue colour. So the researchers hit the lab to find out more about the structure of the berry’s skin.

“The secret of the bright blue reflection is found in the cells of the fruit’s skin. In each cell we find a layered cylindrical architecture [see image below],” explained Kolle. “Once we understood the underlying physics we thought it would be cool to build an artificial material that interacts with light like the fruit does.”

The hogberry fruit does not change colour. Once the researchers understood its structure, however, they managed to create fibres that pass through a full rainbow when stretched by combining the plants properties with an elastic material. Not only are these fibres visually appealing, they have a wide range of practical uses too.

“I would like to see these fibres as a component in smart colour-tunable textiles that might even be able to sense some body functions (such as temperature) or be sensitive to environmental conditions (humidity),” said Kolle, lead author of the paper published in Advanced Materials. “Such a material would be bright, shiny and potentially useful on lots of surfaces that we see with our eyes (clothing, car and wall paints, security labels in passports or on money bills) or could be applied in novel optical sensors.”

The bastard hogberry (top left) is bright blue. Through the microscope you can see the skin of the fruit has a repeated cylindrical architecture. Image courtesy of Mathias Kolle

There’s no doubt the hogberry plant has exciting applications for humans, but in nature the fruit’s eye-catching appearance would have evolved for a different, more deceptive purpose. In order to reproduce, many plants rely on animals to disperse their seeds (which are inside the fruit). Some plants lure animals in by offering sweet and tasty fruit. But the hogberry fruit is not very tasty at all, without any pulp or nectar. According to Kolle, “It’s probably a bit like chewing on a shiny glass marble.” So the berries evolved to be this magnificent colour to trick birds  into eating them without the plant having to expend energy on making fleshy fruit.

Source: [Harvard School of Engineering and Applied Sciences]

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Keyword: [Optical sensor]

An optical sensor is a device that changes light rays into electronic signals that can be read by an instrument to measure the amount of light present.