Use of two-step grafting to fabricate dual-functional films and site-specific functionalized scaffolds.

Polycaprolactone (PCL) is a widely utilized bioresorbable polymer in tissue engineering applications. However, the absence of intrinsic functional groups in the polymer backbone necessitates the incorporation of functional chemistries to enable the further addition of bioactive molecules to PCL-based surfaces and scaffolds. The current study aimed to incorporate two different functional groups, amine and carboxylate, first on two-dimensional (2D) spin-coated PCL films and, thereafter, throughout all surfaces within three-dimensional (3D) porous PCL-based scaffolds, produced using the thermally induced phase separation (TIPS) method, but in a spatially separated manner. Specifically, gamma irradiation induced grafting of acrylic acid (AA) and 2-aminoethyl methacrylate hydrochloride (AEMA) onto PCL was performed in selected solvents and the resulting substrates were characterized using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements to determine the surface free energy. Results demonstrated that stepwise graft copolymerization of AEMA and AA allows the fabrication of dual-functional surfaces, with chemistry depending on the order of grafting of the two monomers. In addition, 3D scaffolds could be decorated exclusively with carboxylate groups in the interior, while the outer surface displayed dual-functionality. This simple surface modification methodology, with the ability to create spatially separated surface functional groups throughout 3D porous scaffolds post their fabrication, has the potential to be applied to many current and future scaffold systems being investigated in the field of tissue engineering.

Functionalised polycaprolactone films and 3D scaffolds via gamma irradiation-induced grafting.

Polycaprolactone (PCL) is frequently used as the base polymer in scaffolds targeted for tissue engineering applications. However, in the absence of further surface modification, the lack of functional moieties on the PCL chain results in non-ideal surface properties of such scaffolds, especially in terms of the inability to tailor the presentation of functional ligands for directed cell adhesion and growth. The current study investigates gamma irradiation-induced grafting as a means of improving the biofunctionality of the PCL surface. The surface presentation of carboxylic acid groups on 2D PCL films could be tailored by changing the acrylic acid (AAc) concentration and/or the solvent during grafting, as evaluated from X-ray photoelectron spectroscopy (XPS). From data obtained using Raman spectroscopy, it was concluded that the penetration depth of the grafted pAAc was affected by the solvent system with a mixed water-methanol system yielding high penetration. Grafted samples displayed a decreased elastic modulus of the surface correlating with pAAc penetration depth, as shown by nano-indentation using atomic force microscopy (AFM). The most promising grafting conditions found for the 2D PCL films were then applied to 3D thermally induced phase separation (TIPS) scaffolds and it was demonstrated using XPS that equivalent levels of grafting of pAAc could be achieved throughout the whole depth of the scaffold. The scaffolds maintained their overall integrity after grafting, even though we observed a decrease in the compressive modulus by 20% after surface modification. These combined studies confirm the utility of this surface modification methodology for scaffolds targeted at tissue engineering and cell culture applications.