with advanced functions, [6] can be attributed to two aspects. 1) The exquisite control and manipulation abilities of fluids at a microscale have reached a new height as droplet microfluidic technology is leaping forward. The precisely tunable interface characteristics of emulsion templates, determined by the optimization of fluids composition and interfacial complexity, contribute to the inherent interface properties of MPRs. [7][8][9][10] 2) The introduction of advanced materials expands the interface characteristics of the MPRs. Considerable natural polymers, synthetic polymers, and stimuli-responsive biomaterials have been employed continuously with the development of materials science. As a result, a series of performances possessed by the materials can significantly improve the inherent interface characteristics of the MPRs under a combination of the mentioned advanced materials. [11][12][13][14][15][16] Overall, researchers have stressed the improvement of the development process of interface characteristics and functions (such as substance encapsulation and controlled release) by providing novel technical innovations in every aspect to support biomedical clinical applications related to cells and drugs. Recently, the types, preparation methods, and biomedical applications of MPRs have been systematically presented in several excellent reviews. [3][4][5][8][9][10][17][18][19][20] The thermodynamic properties of liquid-liquid droplet reactors have been analyzed more specifically in some reviews. [7,17] However, the origin of interface characteristics of MPRs, as well as the function and cuttingedge cell-and drug-related biomedical applications determined by the interface characteristics, are deficient in a systematic summary. This review highlights that different emulsion templates endow the inherent interface characteristics of MPRs. Besides, the methods of converting emulsion templates into MPRs are summarized by introducing specific functional biomaterials, followed by the flexible and adjustable interface characteristics expanded by the mentioned materials. Moreover, the biomedical applications related to cells and drugs are analyzed based on the mentioned unique interface characteristics of the MPRs. Furthermore, the existing challenges regarding the current status are presented, and the subsequent development of the MPRs is illustrated from various perspectives (Scheme 1).

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