Contents I. Introduction
Hydrogel beads are widely employed for biomedical applications including diagnostics [1], drug delivery [2] and tissue engineering [3], owing to the features of hydrophilicity, biocompatibility, and tailorable properties. Compared to their counterparts of solid and non-permeable particles, hydrogels are highly porous and permeable, providing an extended surface area for chemical functionalization for such as detection probes or targeting ligands required in biochemical reactions [4]. Moreover, the high liquid contents presented in hydrogels also provide the benefits of transparency and biocompatibility for the ease of optical detection [5] and aqueous-based biological reactions [6], [7]. Recent developments on microfluidics [8] have enabled the production of hydrogel beads in a high- throughput (> 1,000 drops per second) and controllable (uniform droplet size) manner. Furthermore, the porosity of hydrogel network may also be regulated by the concentration of crosslinkers and polymerization catalysts, for an array of purposes including entrapment of cells or biomolecules [9], DNA barcoding [6], display of proteins [10] and as nanoreactors for enzymatic reactions [11].
