Arrested fungal biofilms as low-modulus structural bio-composites: water holds the key

Biofilms are self-assembling structures consisting of rigid microbial cells embedded in a soft biopolymeric extracellular matrix (ECM), and have been commonly viewed as being detrimental to health and equipment. In this work, we show that biofilms formed by a non-pathogenic fungus Neurospora discreta, are fungal bio-composites (FBCs) that can be directed to self-organize through active stresses to achieve specific properties. We induced active stresses by systematically varying the agitation rate during the growth of FBCs. By growing FBCs that are strong enough to be conventionally tensile loaded, we find that as agitation rate increases, the elongation strain at which the FBCs break, increases linearly, and their elastic modulus correspondingly decreases. Using results from microstructural imaging and thermogravimetry, we rationalize that agitation increases the production of ECM, which concomitantly increases the water content of agitated FBCs up to 250% more than un-agitated FBCs. Water held in the nanopores of the ECM acts a plasticizer and controls the ductility of FBCs in close analogy with polyelectrolyte complexes. This paradigm shift in viewing biofilms as bio-composites opens up the possibility for their use as sustainable, biodegradable, low-modulus structural materials.