About the antimicrobial behavior and biocompatibility of polypyrrole and its polymeric hybrid materials toward cardiac tissue engineering

Polypyrrole (PPy) is a well-known intrinsically conductive polymer broadly used in cardiac tissue engineering due to its excellent biocompatibility, radical scavenging capacity and antimicrobial activity. However, the lack of proper processability of PPy motivated the development of hybrid biomaterials mainly by in situ polymerization for coating and composite designs. Much less studied has been the direct mixing of previously synthesized PPy with polymers mainly through solution mixing. However, although melt mixing is a well-known technology to produce polymer composites for TE, this method has been barely reported for PPy. In this context, a thermoplastic polyurethane (TPU) was used as a matrix for preparing melt-mixed composite films using as filler PPy particles synthesized in presence of FeCl₃ acting as an initiator and further providing dopant agents. A PPy-coated 3D-printed TPU patch was also produced for comparison. The as-synthesized PPy particles exhibited a conductivity of 1000 S/m, while coated PPy/TPU presented a conductivity of 1 S/m, and the percolated TPU composite containing 25 w/w% of PPy showed a value of 10⁻⁴ S/m. Regarding the mechanical properties, although coated samples did not present a relevant change in the tensile mechanical behavior as compared with pure TPU, composites drastically increased the stiffness by up to 1000%, although increasing the brittleness. Our results show that the antioxidant activity of PPy particles was transferred to the coated and composited TPU samples. Regarding the antibacterial activity, PPy displayed high antimicrobial activity against E. coli and S. aureus as measured by inhibition zone tests. Under antibacterial contact tests, coated TPU samples presented high activity against both strains, while the composite was only active against S. aureus. Regarding the biocompatibility of these hybrid samples toward cardiac TE, coated TPU samples presented similar cardiomyocytes' viability as compared to pure TPU, whereas the composites showed reduced viability. These results underscore the potential of PPy-based hybrid materials for the design of biomaterials, giving key antecedents about the differences between PPy coatings and PPy composite materials. These findings can open up strategies to develop PPy biocompatible materials having conductivity, radical scavenging capacity, antimicrobial activity, and cardiomyocyte viability.