Covalent organic frameworks (COFs) are an emerging class of organic crystalline polymers with welldefined molecular geometry and tunable porosity. COFs are formed via reversible condensation of lightweight molecular building blocks which dictate its geometry in two or three dimensions. Among COFs, two-dimensional COFs (2D COFs) have garnered special attention due to their unique structure composed of two-dimensionally extended organic sheets stacked in layers generating periodic columnar π-arrays, functional pore space and their ease of synthesis. These unique features in combination with their low density, high crystallinity, large surface area, and biodegradability have made them an excellent candidate for a plethora of applications ranging from energy to biomedical sciences. In this article, the evolution of 2D COFs is briefly discussed in terms of different types of chemical linkages, synthetic strategies of bulk and nanoscale 2D COFs, and their tunability from a biomedical perspective. Next, the recent advances of these 2D nanomaterials in biomedicine and biotechnology are summarized emphasizing the principles and strategies involved. In addition, current challenges and emerging approaches of 2D COFs for the advanced biomedical applications are discussed.

Hydrogel microparticles (HMPs) are an emerging bioink that can allow three-dimensional (3D) printing of most soft biomaterials by improving physical support and maintaining biological functions. However, the mechanisms of HMP jamming within printing nozzles and yielding to flow remain underexplored. Here, we present an in-depth investigation via both experimental and computational methods on the HMP dissipation process during printing as a result of (i) external resistance from the printing apparatus and (ii) internal physicochemical properties of HMPs. In general, a small syringe opening, large or polydisperse size of HMPs, and less deformable HMPs induce high resistance and closer HMP packing, which improves printing fidelity and stability due to increased interparticle adhesion. However, smooth extrusion and preserving viability of encapsulated cells require low resistance during printing, which is associated with less shear stress. These findings can be used to improve printability of HMPs and facilitate their broader use in 3D bioprinting.

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