Controlling Aerosol-Jet Printed Mxene Flakes Morphology for Neural and Energy Storage Applications

MXenes are a class of 2D layered materials consisting of carbides and nitrides of transition metals which has found application in a wide range of fields due to their excellent electrical conductivity, charge storage, and biocompatibility. Mainly used as aligned flakes, under certain processing conditions these 2D flakes can also fold in crumpled 3D structures. Despite being occasionally associated with faulty deposition, crumpled MXenes hold untapped potential, harboring intriguing properties that remain largely unexplored. Thus, the main goal of this study was to investigate the uncharted role that crumpled MXenes can play in both neural and energy storage applications.<br/>In this work, the production of crumpled and aligned MXene (Ti₃C₂T_x) flake morphologies was systematically studied by using high-resolution Aerosol Jet 3D printing, while assessing their novel physical properties, such as roughness, conductivity, and optical properties. Furthermore, cell line -derived and primary neurons were plated on either morphology to examine the ability of both forms to support neuronal growth and development. In addition, to assess the potential utility of crumpled MXenes for energy storage applications, crumpled MXene flakes were used as a conductive spacer between aligned MXene layers for developing an all-MXene supercapacitor.<br/>By modifying several 3D printing parameters, MXene flakes could both align and crumple at specific mass and sheath flow regimes, depending on the nozzle size, speed, and solvent mixture, while flake morphology was rapidly monitored using the in-built camera.<br/>To assess biocompatibility, mouse neurons were seeded on printed crumpled and aligned MXene films, revealing distinct behavior depending on the flake morphology: crumpled MXene facilitated higher cell adhesion (as indicated by DNA count and immunostained cells), whereas neurons grown on aligned MXene films possessed longer neurite outgrowth. Further analysis using mouse primary neurons, resulted in healthy neuronal growth on both types of MXene for 8 days, while crumpled surfaces elicited decreased expression of GFAP, as observed in image analysis, suggesting reduced astrocytic reactivity.¹<br/>MXene supercapacitors were aerosol jet printed with the first layer being a highly conductive aligned MXene current collector, followed by alternating layers of crumpled and aligned MXene, resembling a <i>lasagne </i>structure. This hierarchical architecture effectively prevented excessive restacking of the flakes by smartly combining both morphologies, preserving a nanoporous network and achieving up to 65 (F/g) gravimetric capacitance, 193 (F/cm³) volumetric capacitance and 88 (mF/cm²) areal capacitance.<br/>Taken together, these results demonstrate that crumpled and aligned MXenes can be easily 3D printed with Aerosol Jet printing; both MXene morphologies exhibited unique characteristics when supporting neuronal growth, and also powered energy-dense <i>lasagne </i>structured<i> </i>supercapacitors. This study highlights the need for precise management of MXene deposition and customized flake design, encouraging material scientists to intensify the study of crumpled MXenes and other 2D materials, and their application in multiple fields including biomedicine, electronics and energy storage.<br/><br/>References:<br/>[1] Woods, I. <i>et al.</i> Biomimetic Scaffolds for Spinal Cord Applications Exhibit Stiffness-Dependent Immunomodulatory and Neurotrophic Characteristics. <i>Adv. Healthc. Mater.</i> <b>11</b>, 2101663 (2022).