Elastic folding on soft matter surfaces aids high-contrast optical sensing.

Materials World magazine
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3 Jun 2020

Creating folds on the surface of materials has led to a ‘fold-to-glitter’ optical technology. 

With potential applications in flexible wearable, healthcare and or engineering devices, the technology proves, for the first time, that elastic folding on the surface of soft matter can enable high-contrast optical sensing. 

Creating folds on the surface of materials has led to a ‘fold-to-glitter’ optical technology, with potential for application in flexible and wearable healthcare and engineering devices. 

According to researchers from Northumbria University, UK, the technology proves, for the first time, that elastic folding on the surface of soft matter can enable high-contrast optical sensing. 

‘Wrinkles and folds are usually unwanted in engineering terms, as they represent structural failures,’ says Ben Xu, Associate Professor in Mechanical Engineering at the University. ‘In this research, we harness this reversible transition.’ 
The soft multi-layer of the elastomeric material is designed and micro-engineered, and then subjected to mechanical stimuli. The desired wrinkles and fold patterns are, in turn, realised on the surface in a controlled and targeted way, to create switchable optical features and structural colour with dynamic luminescent patterns. 

‘The actuated folds in the soft material can be easily observed from top view under fluorescence microscope with the assistance from an optical indicator layer,’ says Xu. ‘By further applying a thin layer, around 600nm, of functional phosphorescent cyclometallate on the surface, the targeted folding can lead to an ultra-high-contrast optical sensing.’ 

The research consists of two different elements – creating a thin film and a chemical paint. When stimulated with a mechanical or electronic signal, the thin film results in microscopic folds being created on its surface, usually too small to be seen with the naked eye.

This is followed by applying the chemical paint developed by the researchers to the material. When the folds are created in the surface, the resulting change in oxygen levels within the paint leads to a chemical reaction, creating a luminescent effect. This makes the surface appear to light up in the region where the fold exists. 

According to Xu, some applications have already been demonstrated. These include an in-plane topo-optical sensor to detect large surface strains, a dynamic 2D spy barcode that can hide information, an adaptive topo-optical luminescence grid and a flexible bending sensor to detect out-of-the-plane bending degree. The team plans to integrate the technology in a wider range of applications. 

‘Due to the elastic nature of the multi-layer structure and the instability morphology, this technology can be developed into a wearable device with proper [integration of] other electronic units, or combined with signal reading/interpreting mechanism to increase the visibility of generated high-contrast optical signal,’ Xu explains. 

‘We are aiming to develop one or two flexible devices for epidermal diagnosis or therapeutic application next, with proper funding support.’