Hierarchical biopolymer-based materials and composites

The mass production and disposal of non-degradable fossil-based plastics is responsible for alarming environmental and social issues when not managed responsibly. Towards manufacturing environmentally-friendly materials, biopolymers, that is, polymers synthesized by living organisms, emerge as promising sustainable alternatives as they combine attractive mechanical properties, compostability, and renewable sourcing. In this review, we analyze the structural and mechanical properties of three of the most studied biopolymer classes: cellulose, chitin, and protein beta-sheet structures. We first discuss the hierarchical structure of the biopolymers and how their rich interaction networks induce appealing mechanical properties. Then, we review different fabrication and processing methods to translate these attractive properties into macroscopic materials and composites. Finally, we discuss a nascent approach, which leverages the direct use of microorganisms, in the form of intact cells, tissues or dissociated biological matter (biomatter), as meso-scale material building blocks. These non- or little pre-processed biomatter building blocks are composed of the biopolymer structural elements (molecular-nano scale), but also inherit the higher-scale hierarchical characteristics. Processing-structure-property relationships for biomatter-based materials are discussed, emphasizing on the role of hierarchical arrangement, processing-induced transformations, and intermolecular bonding, on the macroscopic mechanical properties. Finally, we present a perspective on the role of biopolymers in a circular economy.