Driven by the needs of modern transportation and the clean energy transition, the demand for sustainable and lightweight materials is increasing. Composite materials incorporating natural fibers such as flax fibers have gained attention due to their carbon-capturing potential and good specific mechanical properties. However, when embedded in hydrophobic polymer matrices, flax fibers exhibit inferior mechanical performance primarily due to their hydrophilic composition and discontinuous fiber architecture. Biological materials such as nacre have developed useful strategies through mineralization to distribute localized stresses and develop extrinsic toughness that could inspire a solution to enhance stress transfer in natural fiber composites. Here, we report a biomineralization strategy to introduce an additional hierarchy to flax composites. By tuning salt concentrations in the process, we achieve controlled deposition of microbe-mediated mineral particles on flax yarns. With controlled biomineralization, we show that the minerals can enhance the compressive toughness by 178% and compressive strength by 30%. The findings highlight a novel bio-inspired pathway for tailoring composite performance through sustainable processing, offering a scalable and environmentally friendly approach to enhance natural fiber composites for structural applications.

Correction to: Scientific Reportshttps://doi.org/10.1038/s41598-023-43682-x, published online 13 October 2023 The original version of this Article contained errors in the Discussion section, where reference 34 was incorrectly cited as reference 25. Consequently, “Spirito et al.²⁵ also observed a decreased conversion rate when feeding a chain-elongating reactor microbiome with mainly ethanol. This could explain why previous studies with shorter incubation times reported no metabolic activity in the absence of acetate^25,35. The mechanism that underlies this change in rate remains elusive. Spirito et al.²⁵ proposed a thermodynamic constraint on the rate due to increased hydrogen partial pressures, but in our experiments pH₂ was low due to continuous sparging and we still observed low rates.” now reads: “Spirito et al.³⁴ also observed a decreased conversion rate when feeding a chain-elongating reactor microbiome with mainly ethanol. This could explain why previous studies with shorter incubation times reported no metabolic activity in the absence of acetate^34,35. The mechanism that underlies this change in rate remains elusive. Spirito et al.³⁴ proposed a thermodynamic constraint on the rate due to increased hydrogen partial pressures, but in our experiments pH₂ was low due to continuous sparging and we still observed low rates.” The original Article has been corrected.

Since the inception of fibre-reinforced composite materials, they have been widely acknowledged for their unparalleled weight-to-performance ratio. Nonetheless, concerns are escalating regarding the environmental impact of these materials amidst global warming and pollution. This perspective explores a ground-breaking shift towards harnessing living organisms to produce composite materials. Living composites not only offer sustainable, carbon-capturing alternatives but also afford an unprecedented level of control over shape and anisotropy. Recent advancements in biology, particularly genetic engineering and sequencing, have provided extraordinary control over living organisms. Coupled with ever-evolving additive manufacturing techniques, these breakthroughs enable the construction of engineered living materials from the ground up. Here, we explore the key factors propelling the emergence of engineered living materials for structural applications and delves into the capabilities of living organisms that can be harnessed for creating functional materials, including harvesting energy, forming structures, sensing/adapting, growing and remodelling. Incorporating living organisms can revolutionise manufacturing for renewable and sustainable composite materials, unlocking previously unattainable functionalities.

Hexanoate is a valuable chemical that can be produced by microorganisms that convert short-chain- to medium-chain carboxylic acids through a process called chain elongation. These microorganisms usually produce mixtures of butyrate and hexanoate from ethanol and acetate, but direct conversion of ethanol to hexanoate is theoretically possible. Steering microbial communities to ethanol-only elongation to hexanoate circumvents the need for acetate addition and simplifies product separation. The biological feasibility of ethanol elongation to hexanoate was validated in batch bioreactor experiments with a Clostridium kluyveri-dominated enrichment culture incubated with ethanol, acetate and butyrate in different ratios. Frequent liquid sampling combined with high-resolution off-gas measurements allowed to monitor metabolic behavior. In experiments with an initial ethanol-to-acetate ratio of 6:1, acetate depletion occurred after ± 35 h of fermentation, which triggered a metabolic shift to direct conversion of ethanol to hexanoate despite the availability of butyrate (± 40 mCmol L⁻¹). When only ethanol and no external electron acceptor was supplied, stable ethanol to hexanoate conversion could be maintained until 60–90 mCmol L⁻¹ of hexanoate was produced. After this, transient production of either acetate and butyrate or butyrate and hexanoate was observed, requiring a putative reversal of the Rnf complex. This was not observed before acetate depletion or in presence of low concentrations (40–60 mCmol L⁻¹) of butyrate, suggesting a stabilizing or regulatory role of butyrate or butyrate-related catabolic intermediates. This study sheds light on previously unknown versatility of chain elongating microbes and provides new avenues for optimizing (waste) bioconversion for hexanoate production.