CONTROLLING MXENES FLAKE MORPHOLOGY BY AEROSOL JET PRINTING FOR NEURAL INTERFACES AND ENERGY STORAGE APPLICATIONS

Abstract

MXenes are 2D layered nanomaterials composed of transition metal carbides and nitrides, renowned for their outstanding electrical conductivity, charge storage capabilities and biocompatibility. MXenes (specifically Ti3C2Tx) are typically employed as aligned flakes to maximise their conductivity and flexibility, however, under specific processing conditions the 2D flakes can also adopt a crumpled or more complex 3D structure. Despite being occasionally associated with faulty deposition, crumpled MXenes may hold untapped potential, harbouring properties that remain largely unexplored due to the complexity of existing methods or additional processing steps. Therefore, the objective of this study was to develop a method to precisely control MXene flake morphology through 3D printing at micron-level resolution, using Aerosol Jet printing (AJP) technology to explore the use of both aligned and crumpled MXene morphologies for neural interface and energy storage applications. Chapter 2 describes the systematic analysis of the AJP printing parameters necessary to print crumpled and aligned MXene morphologies. Mass flow (MF) emerged as the main factor influencing flake morphology, working in tandem with nozzle size to produce crumpled structures at low MF, whereas higher MF and higher printing speeds facilitated the alignment of flakes. Additionally, an in-situ inspection method was developed to rapidly evaluate printed flake morphology based on the printed appearance: aligned networks exhibited a metallic lustre, whereas crumpled networks appeared darker and duller. Flake morphology also influenced the material properties; while aligned MXene films obtained high conductivities 2672 ±125 (S/cm), crumpled flake networks showed an enhanced hydrophilicity and higher roughness. Finally, the applicability of the developed method was tested with other 2D materials, including two forms of graphene and also molybdenum disulfide, demonstrating the versatility of this approach. To assess the potential of the two MXene morphologies to be incorporated into neural interfaces, Chapter 3 studied the biocompatibility of different neural cell types, including induced pluripotent stem cells to grow on the two printed morphologies. Neuron and astrocyte morphology, proliferation, cellular marker expression and cytokine release were assessed to determine whether flake morphology could influence the growth and behaviour of neurons and astrocytes. Results showed a distinctive response depending on the flake and cell type, with aligned MXenes promoting the extension of neurites from neurons, while both MXene types induced a resting state on the astrocytes. Collectively, these findings demonstrated the neurocompatibility of MXenes, with potential for their selective use in the design of future neural interfaces. Chapter 4 explored the development of miniaturised supercapacitors (MSCs), typically employed for powering small devices. While 2D materials and MXenes have significantly enhanced the performance of MSCs, their potential is constrained by the dense stacking of flakes, which impedes the utilization of their full theoretical surface area. To address this limitation, AJP was employed to increase the surface area of MXenes through the controlled deposition of crumpled and aligned flakes. After designing a current collector with conductive aligned flakes, and testing several configurations, a lasagne architecture with alternating crumpled and aligned layers showed improved areal and volumetric performance at 5 mV·s-1 (51.35 mF·cm-2 and 113.19 F·cm-3). Moreover, the fine-tuning of porosity by controlled mass deposition of crumpled MXene layers led to further enhancement of the volumetric capacitance, reaching up to 130.57 F·cm-3. In conclusion, overall, this thesis has developed a method to 3D print at high- resolution crumpled and aligned MXenes using AJP, demonstrating its potential use in the development of neural interfaces with a modulated foreign body response, and also porous all-MXene micro-supercapacitors with enhanced energy storage.