Inspired by biological tissues, a research group has developed a cellulose-based polymeric material that has the property of being moved by the influence of moisture, which gives it the potential to be used as a base material for programmable actuators, much needed for soft robotics and other applications requiring smart films, and was used in the manufacture of a self-healing wrapping paper.
A paper that unwraps could only be possible thanks to a new material. Source: pxhere.com.
We may think that plants are immobile, but in fact they have several functions and mechanisms that have been perfected over thousands of years that allow them to move in response to a stimulus, these movements are strategies used to adapt quickly to environmental conditions, for example, plants detect and respond to stimuli such as light, temperature or contact, changing the way they grow or opening and closing their leaves or petals. Thus we can see that plants such as sunflowers move by turning towards the sun, slowly following the sun on its route from east to west, or how the Venus flytrap responds quickly to contact to catch its prey.
This detection and subsequent actuation of the plants are induced by hormones or substances that act on the cellulose fiber and cell wall, increasing the plasticity of this wall, for example, allowing local swelling or contraction of the cellulose fibrils due to moisture, which causes the pine cones to open and close or the pods to curl up.
This material mimics the response of some plants to certain stimuli. Source: Wikimedia.org.
Now, inspired by this property observed in plants, intelligent sensitive actuators have been designed that are able to respond to various environmental signals, such as light, temperature or humidity, generating a response that is reversible and can be programmable, offering highly desirable functionalities in applications such as the development of sensors, soft electronics and smart packaging. And in this regard, actuators that respond to humidity, in the same way that plant tissues do, have attracted great interest from researchers due to the ubiquity and harmlessness of water, which makes them more sustainable and environmentally friendly.
But despite the advances in the development of moisture-sensitive actuators, their applications have been limited due to several factors, on the one hand this type of actuators have not shown a good adjustment in the speed and direction of the response, on the other hand, the molecular design employed so far based on non-covalent cross-linked networks has resulted in poor mechanical properties and sensitive to defects. So, in general, this type of material is expected to have programmable response, mechanical robustness and be defect tolerant, even self-repairing, three conditions necessary for the design of smart moisture-sensitive actuators, but which have been mutually exclusive so far.
But, a research team comprising scientists from the Royal Institute of Technology in Stockholm (KTH), DESY and the Helmholtz Center for Heavy Ion Research, has succeeded in developing a thin film of cellulose nanofibers by incorporating two types of polymers, polyvinyl alcohol (PVA) and polystyrene sulfonate (PSS), trying to emulate biological tissue. They mixed the polymers with the cellulose fibrils using a melt-evaporation strategy, and then poured the solution onto a glass plate and allowed it to dry. Once dry, it formed a film made up of a dense network of chemical and physical bonds that gave it different properties, among them great elasticity and resistance, as well as an internal tension stored in the polymeric matrix that transforms into reversible deformations in the presence of a humidity gradient.
Illustration of cellulose polymer film bending in response to moisture gradient. Source: image elaborated in Powerpoint.
As a result, the speed, direction and pitch of bending in the film obtained can be programmed by adapting the internal stresses and geometry of the samples, and the multiple hydrogen bonds present in the film contribute to the dissipation of energy during stretching, giving it the ability to self-repair when the film is saturated with moisture.
According to the researchers, this film can be cut into pieces of several centimeters that fold when exposed to moisture, this would allow them, in principle, to produce an intelligent wrapping paper, which when moistened a little, unwraps itself, another option is to manufacture awnings that roll up when it starts to rain.
But for this novel cellulose polymer film, more applications than a smart wrapping paper are envisaged, since the properties obtained, such as bending speed and film curvature, which can be adapted by controlling the geometry of the samples before cutting them, are fundamental for the development of sensors or bionic actuators controlled by a signal, in this case humidity, where water molecules act as switches and can break (during adsorption) or rebuild (during adsorption), are fundamental for the development of sensors or bionic actuators controlled by a signal, in this case humidity, where water molecules act as switches that can break (during adsorption) or rebuild (during desorption) the hydrogen bonds of the sample.
Undoubtedly, the self-repair mechanisms exhibited by this new material make it unique and with wide possibilities.
Well friends, I hope you liked the information about this new material. See you next time!
References
Qing Chen, Benedikt Sochor, Andrei Chumakov, Marie Betker, Nils M. Ulrich, Maria E. Toimil-Molares, Korneliya Gordeyeva, L. Daniel Söderberg, Stephan V. Roth. Cellulose-Reinforced Programmable and Stretch-Healable Actuators for Smart Packaging. Advanced Functional Materials, October 2022