In this article the authors present techniques which allow the microfluidic design of alginate microgels with layer composition on a chip. The hydrogel is created by combining two laminar flows of the gel precursor solutions—a calcium solution and an alginate solution—in a microchannel. The alginate solution is loaded with particles and by employing a certain fluid handling protocol involving several alginate solutions with different types of particles, a gel bar composed of many layers, each layer filled with a certain particle type, is formed. This method allows to produce defined lamellae of gel with extraordinarily small size and large aspect ratios. The minimal width attainable for a single layer by this technique is determined by the experimental conditions and for the conditions of the present article layer widths on the order of 10 μm have been realized at a gel thickness of 100 μm. Another method described is based on the finding that the degree of particle incorporation in the gel varies with the particle speed in the alginate flow. Altering the alginate flow rate thus allows to form a gel bar with an inner structure due to varying particle density. The authors believe that alginate gel patterning technology, which relies on easily available equipment and involves gentle particle immobilization conditions, could offer a novel approach toward the engineering of artificial tissues on the micrometer range or to cell micropatterning for analytical purposes.
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