A biocompatible polypyrrole membrane for biomedical applications

Abstract

Polypyrrole (PPy) is the most widely investigated electrically conductive biomaterial. However, because of its intrinsic rigidity, PPy has only been used either in the form of a composite or a thin coating. This work presents a pure and soft PPy membrane that is synergically reinforced with the electrospun polyurethane (PU) and poly-l-lactic acid (PLLA) fibers. This particular reinforcement not only renders the originally rather fragile PPy membrane easy to manipulate, it also prevents the membrane from deformation in an aqueous environment. Peel and mechanical tests confirmed the strong adhesion of the fibers and the significantly increased tensile strength of the reinforced membrane. Surface electrical conductivity and long-term electrical stability were tested, showing that these properties were not affected by the reinforcement. Surface morphology and chemistry were analyzed with scanning electron spectroscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Material thermal stability was investigated with thermogravimetric analysis (TGA). Finally, the adhesion and proliferation of human skin keratinocytes on the membrane were assessed by Hoechst staining and the methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay. In conclusion, this membrane proves to be the first PPy-based soft conductive biomaterial that can be practically used. Its electrical conductivity and cytocompatibility promise a wide range of biomedical applications.

Fig. 1. Schematic illustration of how the electrospun fibers are assembled on top of the bubble surface of PPy membrane.

Fig. 2. SEM images of the fibers and the cross section of the reinforced membranes. (A– C) PLLA reinforced membrane, showing the PLLA fibers before (A) and after (B) wash, and the cross-section of the washed membrane. (D–F) PU reinforced membrane, showing PU fibers before (D) and after (E) wash, and the cross-section of the washed membrane. (G–I) PLLA/PU reinforced and washed membrane wPPy-PU/PLLA, showing the straight PLLA fibers (G), compliant PU fibers (H), and the cross-section of the membrane (I).

Fig. 3. Peel test of the electrospun fibers on PPy, showing the weak adhesion of the PLLA fibers and the strong adhesion of the PU fibers. (A) illustration of the peel test; (B) PLLA fibers were easily peeled off without damaging the PPy membrane; (C) PU fibers cannot be peeled off without breaking the PPy membrane.

Fig. 4. Stress–strain curves of the membranes, before (A) and after (B) 7 day wash, and the SEM photos of the stretched and broken wPPy-PU/PLLA specimen at low (C) and high (D) magnifications.

Fig. 5. Electrical stability of the wPPy-PU/PLLA membrane.

Fig. 6. Curve fittings of the high resolution XPS spectra of N_1s, PPy membrane before 7 day wash (A) and after 7 day wash (B).

Fig. 7. Infrared spectra of (a) MO; (b) PPy; (c) PPy-PU/PLLA; (d) wPPy; (e) wPPy-PU/PLLA.

Fig. 8. The TGA (solid black curve) and DTG (dash red curve) analyses of the membranes. (A) PPy; (B) PPy-PU/PLLA; (C) wPPy; (D) wPPy-PU/PLLA.

Fig. 9. Adhesion of human skin keratinocytes on wPPy-PU/PLLA membrane at 24 h (A), 48 h (B) and 72 h (C), showing a comparable cell density to the controls on glass slide (D–F). The histograms show the proliferation of the keratinocytes, showing a comparable or higher number of cells on the wPPy-PU/PLLA membrane compared to that in the tissue culture plate. **p < 0.01.