Towards electroconductive polymer scaffolds: polycaprolactone nanofiber membranes with Ti3C2 MXene surface coating.

  Polycaprolactone (PCL) is a biocompatible and biodegradable polymer, widely used in design of scaffolds for regenerative medicine. Versatility of this polymer enables the fabrication of scaffolds with various architecture, porosity and mechanical properties tailored to specific tissue engineering needs. They combine mechanical strength with flexibility, mimicking the natural extracellular matrix of tissues. Such scaffolds are intensively explored in design of e.g. neuronal guidance conduits and cardiac patches. However, given the electrochemical nature of these tissues, it was suggested that their efficient regeneration requires electro-conductive scaffolds. Yet, design of electro-conductive polymer scaffolds is still a challenging task given the dielectric properties of the most used biocompatible and biodegradable polymers, including PCL.

  We and others suggested that MXenes, new 2D nanomaterials with intriguing properties, can be used to render the polymer scaffolds electro-conductive. We previously showed that electrospun PCL nanofiber membranes can be turned electro-conductive by a straightforward dip-coating technique. We also suggested a number of procedures to increase hydrophilicity of the PCL nanofibers, e.g. by treatment with oxygen plasma. Here we investigated morphological, electrical and mechanical properties of the electrospun PCL nanofiber membranes coated with 1 to 3 layers of Ti3C2 MXene after oxygen plasma surface conditioning.

  Optical profilometry was performed by confocal microscopy followed by 3D image deconvolution. Measurement of the electro-conductivity was done with the HIOKI IM7585 Impedance Analyzer. The tensile strength was measured using strips of the membranes 3 mm x 20 mm in size with Instron tensile stress system.

  We found that the membranes with MXene surface layers had enriched surface topography and higher surface roughness in comparison to the nascent membranes. Pre-conditioning of the surface with oxygen plasma further contributed to enrichment in surface topography and increase in roughness. Increasing the number of MXene layers to 2 and 3 did not lead to further increases in these parameters. 

  Electrical impedance was the lowest in the case of PCL-2 coat membranes. Moreover, the increase in the number of layers did not lead to further decrease in impedance. Assessment of phase angle suggested that the MXene layers were deposited on both sides of the membrane with few contacts between them. 

  The tensile strength at the point of tearing off was the highest with 1 coating layer of MXene. Further increase in the number of MXene layers to 2 and 3 led to noticeable decrease in the tensile strength down to values lower than the non-coated membrane. Pre-conditioning of the fibers with oxygen plasma had no effect on the tensile strength of the membranes. 

  Overall, research in PCL scaffold design hold promise for advancing the field of regenerative medicine.