Introduction: Polypyrrole (PPy), due to its inherent electrical conductivity, easy for chemical modification and biocompatibility, has been extensively investigated for biomedical applications such as in tissue engineering, neural prostheses and biosensors[1]. However, with its rigid molecular structure, usually PPy is polymerized either as a coating layer on the surface of a metallic or polymeric substrate, or as powders serving as fillers to form conductive composites. Recently, highly conductive free-standing PPy film was prepared through interfacial polymerization[2]. However, such a film just has limited flexibility and is very fragile. In fact, highly flexible pure PPy membrane has not been reported. In this work, we report for the first time a highly flexible PPy membrane prepared through chemical polymerization. Such a membrane has asymmetric surface structures, is semi-conductive and highly porous, making it a good candidate for investigating the mammalian cell behavior under electrical stimulation.
Materials and Methods: The flexible PPy membranes were fabricated through interfacial polymerization in the presence of a molecular template. Firstly, FeCl3 as oxidant was mixed with a water solution of template[3]. Then a solution of pyrrole monomer in chloroform was prepared and slowly added into the solution of FeCl3. The polymerization happened at the chloroform/water interface resulting in a uniform membrane. The specimens were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and four-point probe resistivity measurement. To test the cytotoxicity of the membrane, human skin fibroblasts were seeded on both sides of the membrane and cultured for 24 hours. Cell adhesion was observed and photographed after Hoechst staining.
Results and Discussion: The PPy membrane is about 1 mm in thickness and has a surface conductivity about 1.0 S/cm. The membrane has an asymmetric surface structure showing bubble-like morphology at the chloroform side and nano-tubular morphology at the aqueous side. Surprisingly, a PPy membrane at this thickness is highly flexible and can be repeatedly bent to almost 180 degree. FTIR revealed the characteristic C=O stretching at 1715 cm-1 from over-oxidation, the C=C double bond stretching of the aromatic ring at 1545cm-1, and the C-N stretching at 1454 cm-1, showing no significant difference in chemistry between the two sides of the membrane, In contrary, the XPS survey scans showed a remarkably richer Fe element at the nano-tubular surface than at the bubble surface. Figure 1 demonstrates similar number of cells at both sides of the membrane at 8 hours and a marginal increase at 24 hours. Because both the nano-tubular surface and bubble-like surface exhibited 3D morphology, a number of cells migrated into the substrate at 24h. These data clearly demonstrated that the designed PPy film is not cytotoxic suggesting its safe use as a conductive membrane to electrically stimulate the cells.
Conclusion: A highly flexible free-standing conductive PPy membrane with asymmetric surface structures was fabricated for the first time using interfacial polymerization and molecular template. The two sides of the membrane have similar chemistry but different morphology that might have contributed to the remarkable membrane flexibility. The absence of cytotoxicity, the conductivity and the 3D morphology may find this PPy membrane useful in biomedical applications.
Natural Sciences and Engineering Research Council of Canada; CHU Research Center of Quebec
References:
[1] Bendrea A-D, Cianga L, and Cianga I. Review paper: progress in the field of conducting polymers for tissue engineering applications. Journal of biomaterials applications 2011;26(1):3-84
[2] Qi, Guijin, Zhenglong Wu, and Huiliang Wang. Highly conductive and semitransparent free-standing polypyrrole films prepared by chemical interfacial polymerization. Journal of Materials Chemistry C. 2013; 42: 7102-7110
[3] Zhao Cui‐E, Jiansheng Wu, Staffan Kjelleberg, Joachim Say Chey Loo, and Qichun Zhang. Employing a Flexible and Low-Cost Polypyrrole Nanotube Membrane as an Anode to Enhance Current Generation in Microbial Fuel Cells. Small. 2015;11: 3440-3443