A critical review on lignin structure, chemistry, and modification towards utilisation in additive manufacturing of lignin-based composites

The complex but highly abundant and renewable biopolymer lignin has gained recent attention as a promising candidate towards utilisation of naturally occurring substances for advanced material applications. This review provides a comprehensive overview of lignin in terms of structure, chemistry, and modifications towards utilisation in additive manufacturing, especially for the manufacturing of lignin-based composites. The heterogeneous and complex nature of lignin presents challenges for its integration and functionalisation in various additive manufacturing processes, but its diverse properties entail promising opportunities in the development of environmentally friendly and renewable components for high-value applications. Hence, this review discusses various routes of lignin isolation and modifications that enable the obtainment of different lignins with target-specific properties that improve its processability and enhances its compatibility with various polymer matrices. Additionally, this article presents recent developments in lignin-based composite formulations in different additive manufacturing technologies, such as material extrusion, direct ink writing, digital light processing, stereolithography, and selective laser sintering. The great advancements made within this field of science in recent years highlight the importance of lignin processing to unlock its full potential in additive manufacturing, paving the way for the development of high-value and sustainable composite materials with tuneable application-specific properties. Finally, material- and additive manufacturing technology related challenges and future directions regarding the use of lignin in additive manufacturing of composites are discussed, emphasising the importance of source and processing methods to overcome current challenges in this emerging field of research.