Surface functionalisation of poly L-lactic acid to control protein organisation and growth factor presentation in tissue engineering

Poly L-Lactic acid (PLLA) is a versatile established biodegradable polymer used in biomedical engineering. The characteristic properties (stiffness, topology and biodegradation) of PLLA are ideal for regeneration applications in vivo, providing a biocompatible microenvironment to in situ cells and allowing for gradual transfer of mechanical stress to the engineering tissue via controllable polymer degradation. However, inefficient adsorption of key biological factors, including the extracellular matrix component fibronectin (FN), hinder its use as a cellular microenvironment. Poly(ethyl acrylate) (PEA) has been shown to induce spontaneous organisation of FN into physiological-like fibrils, exposing binding motifs critical for cell adhesion, differentiation and the binding of growth factors (GF). Thus, here we present a novel chemical process to polymerise functional PEA brushes, able to drive FN fibrillogenesis, onto PLLA. The produced surface initiated atomic transfer radical polymerisation system (SI-ATRP), allows control of surface biofunctionality while maintaining PLLA bulk properties such as degradability profile and mechanical strength. Production of a molecularly thin PEA brush coating onto PLLA providing additional functionality to the widely utilised backbone biomaterial. The process outlined is shown to be highly tuneable, taking place in 3 distinct optimized steps; aminolysis, initiator immobilisation and polymerisation. Initial functionalisation via aminolysis and subsequent immobilisation of the bromine based initiator occur in 2 separate reaction processes, named as a 2-pot system. Here we also present initial production of a streamlined 1-pot SI-ATRP system, performing aminolysis and initiator immobilisation within the same reaction vessel, a 1-pot system. These processes, alongside further potential variations to the system, highlight the resilience and highly modifiable nature of this surface modification technology. Neither of these processes are shown to significantly impact on the bulk physical properties of the backbone PLLA, as measured primarily via enzymatic degradation with proteinase K and atomic force microscopy (AFM) nanoindentation. Alongside surface characterisation utilising AFM, X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA) to measure PEA grafting, we investigated the biological activity of modified surfaces in terms of FN adsorption and cellular response. Produced PEA brushes were shown to retain the functionality of driving fibrillogenesis triggering FN organisation into physiological-like fibrils, which allow for enhanced cellular adhesion, growth factor binding and differentiation of myoblast C2C12 cells and human mesenchymal stem cells. Translation of this system into 3D, by printing medical grade PLLA scaffolds and modifying their surface, has shown that this process is scalable and functional for use in implantable biomaterials. The results demonstrate the potential of this technology to engineer controlled microenvironments to tune cell fate via biologically active surface modification of an otherwise bioinert biodegradable polymer gaining wide use in tissue engineering applications