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Droplet-based microfluidics is used to fabricate thin shell hydrogel microcapsules for the removal of methylene blue (MB) from aqueous solutions. The microcapsules composed of a poly(methacrylic acid) hydrogel shell exhibit unique properties, including permeation, separation, purification, and reaction of molecular species. Photocatalytic TiO 2 and ZnO nanoparticles encapsulated in the microcapsules, i.e. photocatalyst in capsule (PIC), are used to remove organic pollutants using an adsorption–oxidation mechanism. A prototype flow microreactor is assembled to demonstrate a controllable water purification approach in short time using photocatalysts. Our studies of aqueous and homogeneous hydrogel environments for the photocatalysts provide important insights into understanding the effectiveness of MB removal. Hydrogel capsules have MB removal rate comparable to homogeneous particles. Further reduction of both capsule and photocatalyst sizes can potentially aid in quicker water purification. 

Abstract Conventional assembly of biosystems has relied on bottom‐up techniques, such as directed aggregation, or top‐down techniques, such as layer‐by‐layer integration, using advanced lithographic and additive manufacturing processes. However, these methods often fail to mimic the complex three dimensional (3D) microstructure of naturally occurring biomachinery, cells, and organisms regarding assembly throughput, precision, material heterogeneity, and resolution. Pop‐up, buckling, and self‐folding methods, reminiscent of paper origami, allow the high‐throughput assembly of static or reconfigurable biosystems of relevance to biosensors, biomicrofluidics, cell and tissue engineering, drug delivery, and minimally invasive surgery. The universal principle in these assembly methods is the engineering of intrinsic or extrinsic forces to cause local or global shape changes via bending, curving, or folding resulting in the final 3D structure. The forces can result from stresses that are engineered either during or applied externally after synthesis or fabrication. The methods facilitate the high‐throughput assembly of biosystems in simultaneously micro or nanopatterned and layered geometries that can be challenging if not impossible to assemble by alternate methods. The authors classify methods based on length scale and biologically relevant applications; examples of significant advances and future challenges are highlighted.