Polydopamine/carboxylic graphene oxide-composited polypyrrole films for promoting adhesion and alignment of Schwann cells

Highlights

• PDA & CGO-composited PPy film was prepared via electrodeposition & DA polymerization.

• Good adhesion and expression of neural proteins in rat Schwann cell (RSC) on films.

• Electrical stimulation facilitated ∼31 % of RSC alignment on this film.

Abstract

Polydopamine (PDA)- and carboxylic graphene oxide (CGO)-composited polypyrrole (PPy) films were prepared via electrospinning of poly-l-lactic acid (PLLA), electrodeposition of PPy blended with CGO and polymerization of dopamine (PDA/CGO/PPy-PLLA), and its surface conductivity and elastic modulus reached to ∼17.3 S/cm and ∼260 MPa, respectively. PDA composition could maintain the higher conduction and elastic modulus of this film after the immersion of 3 w, due to the stronger force between PDA molecule and CGO sheets or PPy particles. The results of cell experiments indicated the better adhesion and higher expression of neural proteins in rat Schwann cells (RSCs) on PDA/CGO/PPy-PLLA films, because the composition of PDA provided more absorption and immobilization site for proteins and the regular coating of PDA particles on the CGO sheets reduced the potential damage of Graphene derivatives on the cell membrane integrity. Electrical stimulation (ES) could facilitate ∼31 % of RSCs to align along the current direction on PDA/CGO/PPy-PLLA films, significantly higher than those on PPy-PLLA and CGO/PPy-PLLA films. A mechanism about the cell alignment on films with ES was proposed: the movement of the cytomembrane proteins under ES and their linkage with serum proteins immobilized by PDA facilitated the extension of growth cone along the ES direction.

Introduction

The repair of long-distance peripheral nerve defects remained an enormous challenge due to the restricted axonal regeneration potential [1]. Nerve guidance conduits facilitated axonal regeneration by connecting two stumps of nerve defects, retarding degeneration of stumps and increasing myelination of Schwann cells [2]. However, the clinical performance of artificial nerve conduits was still inferior to autologous nerve graft due to the lack of directional guidance signals. The regenerative nerve fibers in artificial nerve conduits were less and slender than those in autologous nerve grafts [3]. Electrical stimulation (ES) was proved effectively to promote nerve repair by facilitating cell alignment and protein secretion [[4], [5], [6]]. ES also retarded the degeneration of the nerve regeneration in the early period of crush injury, transected injury, especially in immediate nerve repair in animal models [7,8], recently even in clinic [9,10].

Polypyrrole (PPy) was a common conductive biomaterial and applied in tissue engineering scaffold [11,12]. Our group reported the PPy nanoparticles (NPs)-coated poly-l-lactic acid (PLLA) films for the peripheral nerve regeneration because of the flexibility and biodegradability of PLLA and the semi-conductivity of PPy [[13], [14], [15]]. However, the electrical conductivity of PPy sheath on PLLA films was lower and unsustainable. Recently, graphene and its derivatives received the attention of the biomaterial researchers. Graphene was composed of layers of sheets with the 2-dimensional structure of carbon sp² hybridization, leading to their superior conductivity [16]. As graphene derivatives, graphene oxide and carboxylic graphene oxide (CGO) had better dispersion and more functional groups [17], favoring their application in the biomedicine field. Graphene derivatives could improve the conductivity of composite materials [18,19] and regulate cell behavior, such as enhancing the migration and proliferation of in vitro Schwann cell (SC, also known as neurilemmal cells), promoting the in vivo formation of myelinated axons and functional blood vessels [20,21]. Our group found that the composite of graphene derivatives improved the persistent electrical conductivity of PPy coated PLLA films [22,23]. However, the biocompatibility of the GO-composited conductive films was insufficient, due to their strong π bond possessed potential damages to the cell membrane integrity [24].

Polydopamine (PDA) was one of mussel-inspired materials with abundant catechol groups [25,26], and could be coated tightly onto various solid surfaces through oxidative-polymerization of dopamine [27]. Moreover, PDA possessed excellent hydrophilicity and biocompatibility [28], leading to its application in various biomaterial modifications, including nerve scaffold surface [29,30]. However, there was no research on PDA- and CGO-composited PPy conductive scaffold.

In this study, CGO was composited into PPy-coated PLLA films to increase their persistent conductivity, and PDA was coated on their surface to improve cytocompatibility. Primary rat Schwann cells (RSCs), as the peripheral glial cells with the function of supporting and protecting neuron and guiding axon location [31,32], were cultured on these composited films to evaluate their viability, adhesion and nerve protein expression, and the RSCs alignment was analyzed when ES was exerted on cells through the prepared conductive films.