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3D in vitro model systems such as spheroids and organoids provide an opportunity to extend the physiological understanding using recapitulated tissues that mimic physiological characteristics of in vivo microenvironments. Unlike 2D systems, 3D in vitro systems can bridge the gap between inadequate 2D cultures and the in vivo environments, providing novel insights on complex physiological mechanisms at various scales of organization, ranging from the cellular, tissue‐, to organ‐levels. To satisfy the ever‐increasing need for highly complex and sophisticated systems, many 3D in vitro models with advanced microengineering techniques have been developed to answer diverse physiological questions. This review summarizes recent advances in engineered microsystems for the development of 3D in vitro model systems. The relationship between the underlying physics behind the microengineering techniques, and their ability to recapitulate distinct 3D cellular structures and functions of diverse types of tissues and organs are highlighted and discussed in detail. A number of 3D in vitro models and their engineering principles are also introduced. Finally, current limitations are summarized, and perspectives for future directions in guiding the development of 3D in vitro model systems using microengineering techniques are provided.

The electrochemical CO₂reduction reaction (CO₂RR) to syngas represents a promising solution to mitigate CO₂emissions and manufacture value‐added chemicals. Palladium (Pd) has been identified as a potential candidate for syngas production via CO₂RR due to its transformation to Pd hydride under CO₂RR conditions, however, the pre‐hydridized effect on the catalytic properties of Pd‐based electrocatalysts has not been investigated. Herein, pre‐hydridized Pd nanocubes (PdH_0.40) supported on carbon black (PdH_0.40NCs/C) are directly prepared from a chemical reduction method. Compared with Pd nanocubes (Pd NCs/C), PdH_0.40NCs/C presented an enhanced CO₂RR performance due to its less cathodic phase transformation revealed by the in situ X‐ray absorption spectroscopy. Density functional theory calculations revealed different binding energies of key reaction intermediates on PdH_0.40NCs/C and Pd NCs/C. Study of the size effect further suggests that NCs of smaller sizes show higher activity due to their more abundant active sites (edge and corner sites) for CO₂RR. The pre‐hydridization and reduced NC size together lead to significantly improved activity and selectivity of CO₂RR.

The electrochemical conversion of carbon dioxide (CO₂) into value‐added chemicals is regarded as one of the promising routes to mitigate CO₂emission. A nitrogen‐doped carbon‐supported palladium (Pd) single‐atom catalyst that can catalyze CO₂into CO with far higher mass activity than its Pd nanoparticle counterpart, for example, 373.0 and 28.5 mA mg⁻¹_Pd, respectively, at −0.8 V versus reversible hydrogen electrode, is reported. A combination of in situ X‐ray characterization and density functional theory (DFT) calculation reveals that the PdN₄site is the most likely active center for CO production without the formation of palladium hydride (PdH), which is essential for typical Pd nanoparticle catalysts. Furthermore, the well‐dispersed PdN₄single‐atom site facilitates the stabilization of the adsorbed CO₂intermediate, thereby enhancing electrocatalytic CO₂reduction capability at low overpotentials. This work provides important insights into the structure‐activity relationship for single‐atom based electrocatalysts.