Cellulose for manufacturing advanced materials

The last decade has seen an increase in scientific publications and patents on cellulose, the most abundant natural polymer. By reviewing these papers, a researcher in the UPV/EHU's Department of Graphic Design and Engineering Projects has explored the level of development of nanohybrid materials made from cellulose nanocrystals combined with organic and inorganic particles. The review focusses on manufacturing methods, types of nanohybrids created, and their applications.

Erlantz Lizundia-Fernandez, who lectures in the UPV/EHU's Department of Graphic Design and Engineering Projects, works with renewable polymers. "We are seeking to drive forward the circular economy so we use renewable materials to substitute the applications that currently come from petroleum, or, for example, so they can be used to substitute scarce elements such as lithium or cobalt. My research focuses on cellulose, and out of all the types of cellulose, I have worked mainly with nanocrystals," he said.

As an expert in the subject, Lizundia has reviewed together with another three researchers from Italy and Canada the main developments and advances that have emerged recently in the area of cellulose nanocrystals. "There is a huge number of research papers explaining the synthesis of materials of this type and which are geared towards what is known as proof of concept, in other words, to show that they can be used for a specific application. Cellulose nanocrystals have been widely used to mechanically strengthen polymers. Yet there are hardly any pieces of work that catalog and explain the applications of hybrid materials produced using cellulose nanocrystals. This is what we have contributed: we have described the state of the art in this area of knowledge by conducting an in-depth review of the papers published in this respect," explained the researcher.

Cellulose crystals can be extracted from any object that contains cellulose, be it a tree or a newspaper, and these crystals are used as the base, like a matrix, to produce multifunctional materials by hybridizing them with other components, such as metal oxide nanoparticles, carbon nanoparticles or others of natural origin. The materials created have numerous interesting properties: they are renewable and biodegradable, they can be obtained simply and cost-effectively, they offer great flexibility, are of low density and high porosity, and have excellent mechanical, thermal and physico-chemical properties, among other things. In the analysis they explored three aspects of hybrid materials in depth: the manufacturing process by which they are formed, the types of hybrid materials produced, and the applications for which they are used.

A whole host of applications in engineering and medicine

Lizundia and the other researchers reviewed the manufacturing methods used to form hybrid materials with a range of morphologies and shapes. "The most widely used method is the simplest of all," they said in the article: cellulose nanocrystals and the other elements destined to form the hybrid material are blended in a solution; this solution is decanted onto a surface and the water is allowed to evaporate." Through this technique the cellulose nanocrystals produce helix-shaped structures, chiral nematic structures. "The special feature of these structures is that they provide the material with structural color. The nanocrystals are organized into layers and, depending on the distance between the layers, the hybrid material will reflect light in one wavelength or another, which is the same as saying that it will be in one color or another," added Lizundia.

Apart from the above-mentioned manufacturing method, the study also took filtering, 3-D printing, layer-by-layer assembly and the sol-gel process into account. In all the cases the degree of development of the method is described and the features of the materials produced by it are quoted. However, an entire chapter is devoted afterwards to the features of the nanohybrids formed in the various studies analyzed; this is followed by a classification in terms of the elements added to the nanocrystals: metals, metal oxides, carbon nanofibres and nanoparticles, graphene layers, luminescent nanoparticles, etc. Finally, the applications proposed for hybrid materials are examined, focussing mainly on the fields of engineering and medicine. Sensors, catalytic converters, wastewater treatment materials and energy applications developed by means of cellulose nanocrystals stand out among engineering applications. And among those geared towards medical applications they quote contributions made by materials to areas, such as tissue engineering, drug delivery, antibacterial solutions or wound dressings.

In each of the parts mentioned they review what has been achieved in the different pieces of research, but as experts in the subject they also provide their own assessment about the potential of the materials and what remains to be developed. Lizundia reached the following conclusion: "This work has served to bring together all the research spread across different locations, and we are offering a complete picture of the level of development of hybrid materials. That way we hope that interest in them will increase and that research in this area will be encouraged to fill the gaps we have found, such as a nanotoxicity study in medical applications or the establishing of the environmental impact of these materials."