Paramagnetic levitational assembly of hydrogels.

Figure 1 . (A) Schematic of hydrogel fabrication process. Hydrogel units were fabricated by photolithography. 20 μ L of gel precursor solution was Tissues and organs are composed of repeating functional units. [ 1–3,48 ] Among these repeating basic cellular structures are the hexagonal lobule in liver, the nephron in kidney, and the islets in pankreas. [ 4–6 ] Tissue engineering aims to mimic the native 3D tissue architecture. [ 7 , 47 ] Engineered tissue constructs have broad applications in regenerative medicine [ 8–10 ] and physiological systems for pharmaceutical research. [ 11 ] In vivo , cells are surrounded by extracellular matrix (ECM), and exist in well-defi ned spatial organization with neighboring cells. Tissue functionality depends on these components, their interactions and relative spatial locations. [ 12–14 ] A high level of control over 3D tissue architecture has applications in identifying structure–function relationships in order to resolve underlying mechanisms and to model biological phenomena and diseases in vitro. Scaffolding and existing top-down approaches offer limited control over recapitulating 3D architecture and complex features of native tissues. [ 15 , 16 ] On the other hand, bottom-up methods aim to generate complex tissue structures by assembling building blocks, such as cell encapsulating microscale hydrogels. [ 4 , 17–22 ]