Spinal cord injury (SCI) is a devastating condition that disrupts both sensory and motor function, with very limited prospects of functional recovery. Electrical stimulation (ES) has become a common clinical remedy to lessen the impact of SCI-induced pain after injury. However, regeneration-focused lesion site electrostimulation has not had clinical translation yet, despite promising evidence of directing axonal growth and encouraging cord repair. Furthermore, neural interfaces still face challenges over long implantation times due to delamination, insufficient water barrier properties and inflammatory responses. MXenes, a novel class of 2D electroconductive layered materials, possess unique properties for developing ES systems that can wrap around the injured cord to deliver charge safely and efficiently.
Here we show the aerosol jet 3D printing (AJP) of a neural interface cuff with highly conductive MXene (Ti3C2TX) electrodes, protected and insulated by a polytetrafluoroethylene (PTFE) structure.
MXene films were produced using doctor blade, vacuum-assisted and AJP-printing to assess the effect of fabrication methods on their physical properties and biocompatibility. Conductivity, hydrophilicity and mechanical properties from the films were evaluated.
To assess biocompatibility, NSC-34 mouse motor neurons were seeded on the MXene films to study the morphology influence onto the cells over 3 days and their morphology, proliferation and metabolic rate were studied.
PTFE substrate was spin-coated with a commercial ink followed by 3D printing of the MXene circuit using an Optomec AJP-300 3D printing system. Then, the circuit was passivated and protected with another layer of PTFE by 3D printing with the same system, and then sintered at 360⁰C under non-oxidizing argon atmosphere to create a solid structure.
Results showed that all MXene films were biocompatible, supporting neuron cell viability via similar proliferation and metabolism rate to tissue culture polystyrene controls. Also, neurons grown on AJP-printed films displayed enhanced neuronal neurite outgrowth and cell morphology, possibly, due to their enhanced conductivity (∼12000 S/cm) and higher hydrophilicity (35⁰) in comparison to filtered and doctor blade films. Top and cross-section SEM images of the device showed conformal deposition of MXenes onto a printed PTFE substrate, facilitated by the surfactant-induced hydrophilicity of the PTFE commercial ink. After sintering at 360⁰C, the PTFE nanoparticles coalesced, effectively bounding the printed MXenes onto its otherwise hydrophobic surface. During the thermal treatment, the surfactant and remaining moisture from the PTFE ink evaporated, switching the behaviour of the PTFE surface from hydrophilic (41⁰) to hydrophobic (125⁰) (p<0.0001).
Direct adhesion of 3D printed MXenes after PTFE sintering postulates as a facile method to construct bespoke neural electrode implants for stimulation of the injured spinal cord, while limiting abiotic and biotic faults due to the excellent biocompatibility and pliability of the device.
Zamhuri et al. BioMed. Eng. Online, 20:1-24, 2021
Secor et al. Flexible and Printed Electr., 3:1-12, 2018