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Low-cost cellulose sensor breakthrough

Courtesy Silvia Vignolini Department of Chemistry

Dr Silvia Vignolini and colleagues have created sustainable, low-cost photonic sensors based on a cellulose derivative for the first time.

In research published 30 August in Advanced Materials, the researchers explain how they developed biocompatible pressure-sensitive photonic sensors, which have many potential commercial applications.

These types of ‘strain sensors’ measure the amount of pressure which is being applied, and can be used in areas as diverse as medical procedures to high-tech security systems.

Traditional crystal strain sensors have disadvantages because they cannot be scaled or used in dry environments.  Similarly, electric strain sensors cannot make fine distinctions between different types of deformations, namely strain and pressure.  The new cellulose sensors developed by Vignolini and her colleagues overcome these drawbacks, with the additional advantages of being renewable, biocompatible and, crucially, much less expensive to produce.

“What is great about these sensors is their simplicity,” said lead author Dr Gen Kamita, a postdoctoral researcher in the Vignolini Group.  “The active component of the sensor consists of only two materials, HPC (hydroxypropyl cellulose) and water, and that is all you need.”

Cellulose and its derivatives are receiving increasing interest in sensor development and other uses, because they are an environmentally friendly alternative to plastics.  For these sensors, Vignolini and her colleagues used hydroxypropyl cellulose (HPC), a liquid crystal polymer which is an extraordinarily multifunctional and versatile material. 

“HPC encompasses all the desirable properties of cellulose,” said Vignolini.  “It is non-toxic, water-soluble and responsive.” In fact, Vignolini and Kamita started the project when they realised Kamita’s inspiration to use liquid crystals for strain sensing could be combined with her ideas about creating coloured materials by encapsulating HPC. 

The team chose HPC because it can form a cholesteric liquid-crystalline phase. These types of cholesteric liquid crystals are a promising class of optical materials because they respond to various external stimuli such as mechanical stress, electric fields, temperature and light.  Additionally, HPC’s reflected colour can also be adjusted by changing various parameters, such as the type of solvent being used or the operative temperature.

To form the sensors, the team created a water-based HPC mesophase, in which the HPC is suspended in a state between liquid and solid form. They sandwiched the mesophase between polymer sheets to obtain robust, large-area and flexible strain sensors, which can detect different types of pressure such as compression, shear and extension, based on the optical signature.

“These devices are manufactured using an extremely simple fabrication process with low-cost materials, which makes the sensors suitable for both miniaturisation and large-scale manufacture,” said Vignolini.

The sensors also demonstrate that biopolymers can be used for responsive large area coatings, which can be used in a wide range of applications such as smart fabrics, displays and even security devices. 

“Because the sensors are so simple, they can be scaled-up for industrial production very easily,” said Kamita.  “Imagine how sachets of ketchup are manufactured in factories.  Our sensors can be manufactured in a very similar way, because we simply need to inject the paste of HPC into a polymer packaging.  It’s as simple as that.”

The sensor is demonstrated in a video, which shows a hand pressing onto the HPC strain sensor.  As the strain pattern of the hand is applied, the pressed region of the sensor shows a vivid blue colour.  The different colours produced correspond to varying amounts of pressure.  The colour change is reversible when the pressure is removed. 

Gen Kamita, Bruno Frka-Petesic, Antoine Allard, Marielle Dargaud, Katie King, Ahu Gumrah Dumanli and Silvia Vignolini, Bio-compatible and Sustainable Optical Strain Sensors for Large-area applications, Advanced Optical Materials, DOI 10.1002/adom.201600451