3D Printing Bioinspired, Bacteria-Embedded Ceramic Composites

Abstract

Our reliance on synthetic structural materials has contributed extensively to environmental degradation, resource depletion, and pollution. The urgent need for sustainable alternatives has sparked interest in bio-based materials that align with Nature's cycles while maintaining high performance. However, finding such alternatives has remained challenging, especially for ceramics, which typically require high-temperature processes to achieve structural integrity. Yet Nature offers a promising blueprint: the nacreous layer, "mother of pearl", found in molluscan shells. Millions of years have allowed these invertebrates to evolve remarkable defect tolerance, achieving strength, stiffness, and toughness - a combination elusive in most conventional ceramics.

Despite extensive efforts, mimicking the intricate microstructure of nacre to achieve new, advanced materials remains challenging. Recent studies have leveraged the outstanding shaping freedom and microstructural control offered by additive manufacturing to provide new opportunities. However, these approaches continue to rely on sintering to improve mechanical properties, thereby undermining their ecological potential. The underlying printing process also remains largely unexplored. This thesis confronts both challenges by exploring a novel bacterial stiffening approach in direct ink writing using various material compositions. For the first time, mineral-depositing bacteria are incorporated into a ceramic-biopolymer suspension in an attempt to achieve sinter-like stiffening of the structure at reduced energy expenditure.

Direct ink writing ceramic-polymer bioinks is challenging due to the sensitivity of colloidal dispersions to changes in electrostatic and steric interactions, further complicated by strongly time-dependent rheology. These aspects hindered exploring the effects of biomineralisation on material performance. A 'printing window' is identified, beyond which bacteria-induced coagulation disrupts extrusion, establishing an essential constraint. Printability may be improved by controlling pH, performing more appropriate rheological experiments, and tuning rheology. Nutrient and bacterial content reduced material strength, meaning that any positive effect of biomineralisation must at least overcome this negative influence to provide a net benefit. Excessively high void content highlights the need to maximise compaction and solid content. Crucially, biomineralisation was achieved in preliminary tests, suggesting that the approach is fundamentally promising. These findings provide critical insights and guidelines for developing shaped, strong, tough, and sustainable ceramic materials, paving the way towards eco-friendly materials inspired and built by Nature.