Conductive collagen/polypyrrole-b-polycaprolactone hydrogel for bioprinting of neural tissue constructs

Abstract

Bioprinting is increasingly being used for fabrication of engineered tissues for regenerative medicine, drug testing, and other biomedical applications. The success of this technology lies with the development of suitable bioinks and hydrogels that are specific to the intended tissue application. For applications such as neural tissue engineering, conductivity plays an important role in determining the neural differentiation and neural tissue regeneration. Although several conductive hydrogels based on metal nanoparticles (NPs) such as gold and silver, carbon-based materials such as graphene and carbon nanotubes and conducting polymers such as polypyrrole (PPy) and polyaniline were used, they possess several disadvantages. The long-term cytotoxicity of metal nanoparticles (NPs) and carbon-based materials restricts their use in regenerative medicine. The conductive polymers, on the other hand, are non-biodegradable and possess weak mechanical properties limiting their printability into three-dimensional constructs. The aim of this study is to develop a biodegradable, conductive, and printable hydrogel based on collagen and a block copolymer of PPy and polycaprolactone (PCL) (PPy-block-poly(caprolactone) [PPy-b-PCL]) for bioprinting of neural tissue constructs. The printability, including the influence of the printing speed and material flow rate on the printed fiber width; rheological properties; and cytotoxicity of these hydrogels were studied. The results prove that the collagen/PPy-b-PCL hydrogels possessed better printability and biocompatibility. Thus, the collagen/PPy-b-PCL hydrogels reported this study has the potential to be used in the bioprinting of neural tissue constructs for the repair of damaged neural tissues and drug testing or precision medicine applications.

Keywords: Conductive scaffolds; Nerve guide conduit; Peripheral nerve injury; Stem cells; Three-dimensional printing; Tissue engineering scaffolds.

Figure 1

(A) Schematic of the bioprinting set-up and (B) image of the actual in-house built bioprinting system.

Figure 2

Samples of (A) collagen, (B) collagen/0.5% polypyrrole-block-poly(caprolactone) (PPy-b-PCL), (C) collagen/1% PPy-b-PCL, and (D) collagen/2% PPy-b-PCL bioink for rheological characterization.

Figure 3

Printing design for fiber fusion test and calculation of fiber width. Two consecutive layers (first layer in blue and second layer in red) of the different bioinks were fabricated layer by layer. The fabricated structure follows zero degree and an orthogonal pattern and increasing fiber to FD. The range of fiber to fiber distance used in this study is 2-10 mm with 2 mm increments.

Figure 4

Influence of (A) printing speed on fiber width at a constant material flow rate of 3 ml/min and (B) material flow rate on fiber width at a constant printing speed of 5 mm/s.

Figure 5

Rheological studies – frequency sweep (A) collagen, (B) collagen/0.5% PPy-block-poly(caprolactone) (PPy-b-PCL), (C) collagen/1% PPy-b-PCL, and (D) collagen/0.5% PPy-b-PCL.

Figure 6

Rheological studies – strain sweep (A) collagen, (B) collagen/0.5% PPy-block-poly(caprolactone) (PPy-b-PCL), (C) collagen/1% PPy-b-PCL, and (D) collagen/2% PPy-b-PCL.

Figure 7

(A-D) Images of fiber fusion tests of (A) collagen, (B) collagen/0.5% PPy-block-poly(caprolactone) (PPy-b-PCL), (C) collagen/1% PPy-b-PCL, (D) collagen/2% PPy-b-PCL, (E) diffusion rate in printed hydrogel constructs of various pore sizes, and (F) cytotoxicity results of bioprinted constructs with PC12 cells using PrestoBlue assay after 48 h.