Development of anisotropic polymeric substrates for tendon tissue engineering

Abstract

Tendon injuries and degenerative conditions constitute an unmet clinical need with pharmacological strategies and tissue grafts failing to recapitulate native tendon function. Advancements in bioengineering have enabled the development of various scaffold fabrication technologies, using natural or synthetic in origin polymers that closely imitate the native tendon anisotropic architecture. Anisotropic collagen sponges, extruded collagen fibres, isoelectric focused collagen fibres, electro-spun polymeric fibres and imprinted polymeric substrates are at the forefront of scientific and technological research and innovation. Herein, we ventured to assess whether onestep functionalisation of electro-spun fibres with nano / micro particles is possible and whether anisotropic imprinted substrates can maintain tenogenic phenotype in vitro and promote functional neotissue formation in vivo. Starting with electro-spinning, mechanical evaluation demonstrated that aligned orientated electro-spun fibres exhibited significant higher stress at break values than their random aligned counterparts and random orientated electro-spun fibres exhibited significant higher strain at break values than the aligned orientated scaffolds. While maintaining fibre structure, a co-deposition method of spraying and electro-spinning was developed that enabled the incorporation of microspheres within the threedimensional structure of the scaffold. Of significant importance is that bovine tenocytes aligned perpendicular to the fibre orientation, possibly due to the absence of mechanical tension. With respect to imprinting, it is still unclear whether surface topography can be translated into a clinically functional response in vivo at the tissue / device interface. Herein, we demonstrated that anisotropic substrates with groove depth of ~317 nm and ~1988 nm promoted human tenocyte alignment parallel to underlined topography XVII in vitro. However, none of the topographies assessed (~37 nm, ~317 nm and ~1988 nm groove depth) induced parallel to the substrate cellular orientation in a subcutaneous model and none of the topographies promoted directional tenogenesis in vivo. Further, the rigid poly(lactic-co-glycolic acid) substrate used induced transdifferentiation towards chondrogenic / osteogenic lineage, as evidenced by gene analysis. Collectively, these data indicate that two-dimensional imprinting technologies are useful tools for in vitro cell phenotype maintenance, rather than for directional neotissue formation, should multifactorial approaches that consider both surface topography and substrate rigidity are established. Overall, both electro-spinning and imprinting technologies show great promise for tendon repair and regeneration. Imprinting could be the ideal technology for cell phenotype maintenance in vitro, as we can closely control architectural features. Although it was not investigated here, electro-spinning is the ideal technology for in vivo positive outcomes, as the three-dimensional architecture would allow directional tissue formation within the fibrous construct.