Fabrication of Polypyrrole Coated Electrically Conductive Nanofibers for Tissue Engineering Applications

Background: It has been demonstrated that Electrical stimulationcan enhance cellular behaviors such as proliferation, migration,elongation and even differentiation especially in electricallyactive cells such as neural and muscle cells. To this end,electrically conductive polymers have been explored to facilitatedirect electrical stimulation of cells. Meanwhile, electrospinningmethod has been widely used to produce nanofibrousscaffolds capable of providing anchorage sites for cells to adherewhile allowing transport of metabolites. In this study wedemonstrate how an electrically conductive nanofibrous scaffoldcan be attained by polymerization of pyrrole (PY) monomerson the surface of an electrospun base scaffold.Materials and Methods: The base scaffold was prepared byelectrospinning of a poly-l-lactide (PLLA) solution in a mixedsolvent of chloroform (CHL) and dimethylformamide (DMF).Then the base scaffold was dipped in an aqueous solution of PYand the oxidant was added to initiate the polymerization reaction.To enhance the quality of coating, the reaction was slowedby lowering the temperature to ۴ºC and gentle stirring was appliedovernight. To increase hydrophilicity and activate the surface,vacuum air plasma was used both prior and after pyrroledeposition to obtain more uniform coating and enhanced cellularattachment. Morphological features of the scaffold before andafter coating were examined using scanning electron microscopy(SEM). The depth of the coated layer was derived usingthe change in mean fiber diameter due to the coating procedure.Chemical bonds of Polypyrrole in the coated scaffold were verifiedby Fourier-transform infrared (FTIR) spectroscopy using attenuatedtotal reflectance (ATR) technique. To obtain hydrophilicity,water droplet contact angle assay was performed. To assesselectrical conductivity, two parallel electrodes were placed at afixed distance and a gradual increase in voltage was applied. Theslope of the I-V plot was used to calculated conductivity.Results: SEM images revealed that the best morphology wasobtained when a ۹%w/v PLLA was dissolved in a ۱۷:۳ CHLto DMF mixture. For the coating stage a solution of ۱۰mM PYwith a ۱-۱ molar ratio of ammonium persulfate as the oxidantwas selected. Significantly higher concentrations lead to fillingof the pores while lower concentrations can result in sparse andinadequate coating, however, the conductivity of the scaffoldcan still be tailored by changing the depth of the coated layerwhich in turn can be controlled by tuning the monomer andoxidant concentration near the selected value. Electrical conductivityof around ۶۰mS.m was measured for the final scaffold.Multiple rounds of coating can be applied if a significantincrease in conductivity is required for other applications. Tomeasure contact angel, a ۶μl droplet was placed on the scaffoldand the angle was measured ۵s after contact. Angles of ۱۲۷°and ۱۲۰° was measured for the base and the coated scaffoldsrespectively, while complete absorption of water into the latticeof both scaffolds were observed after plasma treatment.Conclusion: In conclusion a nano-fibrous electrically conductivescaffold was introduced. This scaffold is capable of applyingelectrical stimulation to cells directly, eliminating the needfor a metal electrode to be in direct contact with culture medium,thus phasing out the possibility of electrode toxicity. Theplasma treatment can lead to higher cellular attachment due tothe increased hydrophilicity of the surface.