The micro encapsulation of mammalian cells within a semi-permeable hydrogel matrix is an attractive procedure for many biomedical and biotechnological applications. Thanks to controlled and automated systems it is possible to encapsulate cells in 3D microenvironments constituted by different biomaterials that can act as models to predict how cells respond. This would allow creating a library of materials to correlate the composition of the artificial extra cellular matrix to specific cell responses. A possible strategy to encapsulate cells is through the adoption of a microfluidic approach to create microdroplets of a hydrogel precursor in a non-miscible continuous phase (oil and surfactant). The droplets must be then crosslinked, removed from the oil and collected. This procedure is laborious given the presence of oil and the droplets are difficult to handle in the successive processing and characterization steps. We hereby propose a new oil-free approach compatible with the most common polymers crosslinking strategies (chemical, ionic, thermal and UV) and that surpass the constraints of the emulsion systems. Fibres embedding different cell laden droplets were fabricated with a flow-focusing microfluidic chip by pulsatile flow of polymeric solutions in the inner channel in anti-phase with the outer channels. The composition of the cell laden droplets was controlled by programmable flow sensors that allow an accurate regulation of the amount of the different components. The length of the droplets embedded in the fibre was also controlled by adjusting the flow rates of the solutions and their pulse periods. The fibres were extruded directly into an isotonic calcium chloride crosslinking bath for polymerisation. Ionically crosslinkable gellan gum was blended with marine derived collagen, chondroitin sulphate and hyaluronic acid at different concentrations to form the cell laden droplets whose compositions linearly change along the axis of the fibre. Alginate was used as the acellular portion of the fiber which allowed its degradation in alginase or dissolution with EDTA to release the individual compartments on demand. Human adipose stem cells encapsulated in the different compartments remained viable up to 21 days of culture mainly due to the width of the droplets, smaller than 400 μm, which does not compromise the diffusion of nutrients and removal of metabolites. Cell-laden fibres were also cultured in osteogenic differentiation medium and the degree of differentiation analysed, after immunocytochemistry, by confocal microscopy. Image analysis methods were employed to assess cell response to the different materials and the results were confirmed by quantitative standard methods after release of the individual droplets from the fibers.

Overall the herein proposed approach allows the fabrication of multiple 3D cell-laden hydrogel-based platforms that can be used for the screening of cell-materials interactions and selection of conditions for the development of improved tissue engineering approaches.

This work was supported by European Research Council grant agreement ERC-2012-ADG 20120216- 321266 for project ComplexiTE.

(European Cells and Materials)