Collagen/poly(ethylene glycol) diacrylate cryogels prepared by UV irradiation: tunable macroporous structures for tissue engineering applications.

UV irradiation of acrylate-based precursors in water solutions is a fast and versatile method to obtain hydrogels with well-defined crosslink densities and mechanical properties, that show potential for cell encapsulation and tissue engineering. The formation of an interconnected, macroporous network within the hydrogel is fundamental to cell-material interactions and can be achieved by combining the UV crosslinking with several pore-forming strategies. In this work, macroporous poly(ethylene glycol) diacrylate (PEGDA)–based cryogels were synthesized by UV irradiation of frozen precursor solutions. PEGDA was chosen due to its peculiar non-fouling character, which allows the engineering of substrates with tunable bioactivity. In this regard, several cryogel formulations were prepared by adding different amounts of Type I collagen to the PEGDA solutions, with collagen providing extracellular matrix (ECM) mimicry. PEGDA solutions (700 Da, 10% w/v), containing VA-086 as a photoinitiator, were frozen under controlled freezing conditions (-20°C, with a freezing rate of -1°C/min) in a freeze-dryer (Virtis Advantage). After 1 hour at -20°C, the frozen samples were exposed to UV light (365 nm) for 3 minutes and finally swollen in distilled water for the removal of unreacted precursors. Cryogels containing collagen were synthesized as described above, by adding the appropriate amount of collagen (0.1 or 1% w/v) to the starting PEGDA solution. The gelation yield, the swelling ratio, the degradation rate and the mechanical properties of the cryogels were evaluated. Morphological analyses were also performed on both swollen and dry cryogels by means of fluorescence microscopy and/or micro-computed tomography, to analyse the effect of collagen on the cryogelation-induced porous structure. The experimental findings showed that the gelation yield was slightly increased for higher collagen concentrations, while the swelling ratio, the compressive elastic modulus of the cryogels and the weight loss after 28 days in PBS at 37°C seemed to be independent of the collagen amount. The morphological analysis demonstrated that a macroporous structure, with well-interconnected pores and robust pore walls, was formed for each cryogel formulation, and notably the pore size and pore size distribution were found to progressively decrease and get narrow as the collagen concentration was increased (Fig. 1). Preliminary cell seeding studies with 3T3 fibroblasts also showed the cytocompatibility of the developed cryogels, which thus hold promise for the development of 3D stiff microenvironments with tunable pore sizes, suitable for in vitro tissue engineering.