Enhancing the Conductivity of Curli Fibers Using Dyes and Plasticizers for Soft Electronic Applications

Biologically derived polymers have several advantages over conventional polymers for their use in wearable electronics or medical applications, which include biodegradability, bioavailability and cell proliferation. However, their use has been limited due to their unsuitable mechanical properties, inconsistent production quality and risks of an immune response. A biological scaffold that can offset these problems is curli fibers, an amyloid protein produced by <i>Enterobacteriaceae</i>. Curli fibers have been shown to have robust stability, as evidenced through their ability to withstand incubation in detergents, harsh solvents and proteolysis. Similarly, it can be characterized as hard tissue due to their high Young’s modulus, which is between 3 – 20 GPa. In contrast to other protein-based materials, it can be produced through a scalable vacuum filtration protocol. The production of curli fibers can be assessed through Congo Red (CR) assay, which relies on the use CR dye and its ease of binding to amyloid structures.<br/><br/>Biopolymers are generally known to have poor mechanical properties for wearable applications and tend to be mixed with plasticizers to improve this. Some considerations for their use include miscibility, resistance to leaching and cost. While many plasticizers are petroleum-derived and have raised concerns with regards to their toxicity and degradability, bio-based plasticizers have gained prominence. Common bio-plasticizers used for this purpose include glycerol and polyethylene glycol (PEG).<br/><br/>Given the use of dyes to identify the presence of proteins as well as the use of plasticizers to enhance their mechanical properties, we sought to evaluate the conductivity impact of incorporating dyes and select plasticizers on curli fibers and evaluate their feasibility as a conductive bio-based materials.<br/><br/>We mixed curli fibers with CR and other similarly structured dyes, such as Tartrazine, Indigo Carmine and Phenol Red. The blends were cast on silver ink printed electrodes to measure their electrical properties through IV sweeps. Overall, we observed a pattern where the dyes reached a peak increase in conductivity upon reaching an ideal concentration before dropping after adding more dye to curli fibers, where this peak varied according to the dye. Additionally, we observed different responses to the behavior of the dyes when mixed with different proteins. Pili, bovine serum albumin and gelatin were used for their different structures to assess changes in the conductivity response.<br/><br/>On the other hand, we observed a similar trend with the blends of glycerol and PEG with curli fibers, where a peak conductivity was achieved at a particular concentration and subsequently dropped. We also saw with PEG that there is a notable difference on the conductivity impact depending on its molecular weight, where the smallest molecular weight gave the largest increase in conductivity. Lastly, we evaluated the conductivity of a mixture of the best performing dye and plasticizer to determine if there was an additive effect.<br/><br/>Furthermore, we generated a curli-tartrazine-glycerol composite and assessed its suitability for use as a bioink and as thin films. In hydrogel form, the composite exhibited shear thinning behavior and was deemed suitable for extrusion printing applications. On the other hand, thin films were fabricated by casting in a silicon mold, which showed stretchable capabilities between 80 – 150%, far greater than regular curli thin films which are brittle.<br/><br/>The use of dyes and plasticizers to enhance the conductivity of biomaterials contrasts with conventional inorganic conductive fillers, with the advantage that they are sourced in an easier and inexpensive manner. Further exploration could lead to an insight on the binding and organization of the presented dyes and plasticizers to further enhance the conductivity of biological scaffolds.