Mechanical properties and fatigue analysis on poly(ε-caprolactone)-polydopamine-coated nanofibers and poly(ε-caprolactone)-carbon nanotube composite scaffolds

Recent trends in tissue engineering have focused on the development of electrically conductive composite scaffolds (i.e. with carbon nanotubes) and on increasing the cell interaction of electrospun synthetic polymers by means of incorporation of biological molecules (i.e. via functionalition with polydopamine). In this study the electrospinning process was first optimized for the processing of poly(epsilon-caprolactone) and the use of formic acid allowed mats of 100-200 nm nanofibers with (0.5% of CNT) or without CNT to be created. Then, the PCL nanofibers were successfully coated by a polydopamine layer of approximately 15 nm thick, confirmed by XPS analysis and SEM images. This coating did not add mechanical strength and decreased the Young's modulus of the mats from 25.5 MPa to 13.6 MPa at 21 degrees C. Consequently, the PCL-PDA coated scaffolds showed higher deformation values when the equilibrium was reached (13.7%) during the fatigue tests (at 37 degrees C in aqueous medium, 0.5 MPa of stress) under conditions that simulate the heart beats (1 Hz), experiments in which both PCL and the modified mats (with PDA or CNT) exhibited a fatigue life of more than 10(6) cycles. However, at a cyclic stress of 1.5 MPa the PDA treated mats underwent fatigue failures by plastic deformation while those of non-modified PCL failed after partial tearing (only 40% of them exceeded 10(3) cycles). The incorporation of CNTs within the PCL fibers was confirmed by TEM and improved the fatigue performance by means of preventing the fraying of the fibers (80% of the specimens reached at least 10(3) cycles). Both the PDA layer and the presence of embedded stiff carbon nanotubes, hindered fiber orientation in the tensile tests and their elongation at break were lower than those of PCL scaffolds.