Conductive MXene nanosheets infused in protein fiber hydrogels for bioprinting and thin film electrodes

Abstract

Conductive hydrogels are materials of choice for wearable sensors and soft electronics. They are typically engineered by incorporating a conductive filler into a hydrophilic polymer network. MXene (Ti₃C₂T_x) is a remarkable 2D nanomaterial that can be used as a conductive filler with excellent electrical conductivity and high hydrophilicity. However, it has a poor self-supporting organization due to a lack of interactions between the nanosheets. To offset this problem, we used a biological scaffold composed of curli fibers to load conductive MXene nanosheets. Curli fibers are bacterial amyloid fibers that exhibit robust mechanical properties and have been shown to interact with a range of surfaces and nanomaterials. By varying the loading of MXene in curli-MXene nanocomposites (CMXn), we were able to modulate their conductivity and mechanical properties. At the lowest loading (25 wt%), we achieved a conductivity of 44 nS cm⁻¹ with the films able to withstand a strain of up to 84%, in contrast to the highest loading (70 wt%) which reached a considerably higher conductivity of 49 S cm⁻¹ but a strain at break of only 7%. All CMXn hydrogels exhibited shear thinning behavior and G′/G′′ ratios between 2–5, suitable properties for extrusion printing. We conducted shelf-life studies over a month for the highest performing nanocomposite, identifying that storage temperatures had an impact on their conductivity as they retain 36% of its original conductivity when stored at 4 °C but lost 99% when left at room temperature. Overall, fabricating CMXn hydrogels capitalized on the self-assembly of curli fibers and their ability to form hydrogels suitable for bioprinting. By modulating the content of MXene in CMXn hydrogels, we tuned their conductivity and mechanical properties, which could suit different needs for sensing and soft electronic applications.