图1 微流控可控制备液滴、颗粒、胶囊[3,28-30]

Fig. 1 Controlled preparation of droplets, particles and capsules by microfluidics[3,28-30]

图2 微流控可控制备液滴及其应用：(a) PDMS和玻璃微管微流控器件可控制备单组分、双组分和三组分单乳液滴[41-42]；(b) 微流控构建水包油食品乳液。通过微流控将葵花籽油乳化成液滴，并利用大豆蛋白稳定液滴[30]；(c) 微流控3D液滴打印可控制备液滴阵列。通过将液滴阵列嵌入弹性体，实现刺激触发的多形状、多模式、多步骤变形[48]；(d) 微流控构建均匀分布的悬浮液滴和微流控3D液滴打印构建有序排列的悬浮液滴[49,51]

Fig. 2 Controlled preparation of droplets by microfluidics and their applications: (a) Controlled preparation of one-, two- and three-component droplets by PDMS and glass-capillary microfluidic devices[41-42]; (b) Oil-in-water food emulsions prepared by microfluidics. Sunflower oil is emulsified into droplets and and stabilized by soybean proteins[30]; (c) Microfluidic 3D droplet printing for the controlled preparation of droplet arrays[48]. Droplet arrays are embedded in the elastomer matrix as functional units. Multi-shape, multi-mode and multi-step deformations can be achieved by different stimuli; (d) Randomly distributed droplet suspension prepared by microfludics and ordered droplet suspension prepared by microfluidic 3D droplet printing[49,51].

图3 以单乳液滴为模板可控制备颗粒及其应用：(a) 以单乳液滴为模板，溶剂挥发制备包裹胡萝卜素的颗粒，用于食品天然色素添加[57]；(b) 离子交联制备包裹细胞的水凝胶颗粒，用于细胞封装培养[59]；(c) 光交联制备负载抗炎药双氯芬酸钠的水凝胶颗粒，用于关节炎缓释治疗[60]；(d) 光交联制备用于负载骨髓干细胞的多孔颗粒，用于骨骼再生[61]；(e) 化学交联制备包裹抗癌药物5-氟尿嘧啶的颗粒，用于肿瘤治疗[29]；(f) 化学交联制备作为细胞生长支架的水凝胶颗粒，用于组织再生[62]。

Fig. 3 Controlled preparation of particles using single-emulsion droplets as templates and their applications: (a) Preparation of carotenoid-loaded particles by solvent evaporation and their applications as natural colorants for food[57]; (b) Preparation of cell-loaded hydrogel particles by ionic crosslinking and their applications for cell culture[59]; (c) Preparation of drug-loaded hydrogel particles by photo-triggered crosslinking and their applications for arthritis treatment[60]; (d) Preparation of stem cell-loaded porous particles by photo-triggered crosslinking and their applications for bone regeneration[61]; (e) to Preparation of drug-loaded particles by chemical crosslinking and their applications for oncology therapy[29]; (f) Preparation of hydrogel particles as cell growth scaffolds by chemical crosslinking and their applications for tissue regeneration[62].

图4 微流控可控制备双乳液滴和胶囊及其应用：(a) PDMS和玻璃微管微流控器件可控制备单核和多核的双乳液滴[42,65]；(b) 微流控3D液滴打印可控制备单核和多核的双乳液滴[71]；(c) 以单乳液滴为模板，溶剂挥发使聚合物在油/水界面沉积，包裹油滴形成胶囊[73]；(d) 以双乳液滴为模板，离子交联制备水核包裹肝细胞、水凝胶壳层包裹成纤维细胞的水凝胶胶囊，构建肝脏类器官模型[75]；(e) 以薄壁双乳液滴为模板，制备通过渗透压控制释放的薄壁水核胶囊[76]。

Fig. 4 Controlled preparation of core-shell droplets and capsules and their applications: (a) Controlled preparation of single- and multi-core droplets by PDMS and glass-capillary microfluidic devices[42,65]; (b) Controlled preparation of single- and multi-core droplets by microfluidic 3D droplet printing[71]; (c) Oil-core capsules prepared using single droplets as templates and by solvent evaporation[73]; (d) Hydrogel capsules with hepatocytes in the core and fibroblasts in the shell prepared using core-shell droplets as templates and by ionic crosslinking, which are used as liver organoids[75]; (e) Thin-shell water-core capsules prepared using thin-shell core-shell droplets as templates and by solvent evaporation, whose release could be triggered by osmotic pressure[76].

图5 两相流与三相流数值模拟单乳和双乳液滴的形成：(a) 空气剪切水相形成液滴的两相流数值模拟[83]。水相为分散相，空气为连续相；(b) 空气流速和内管尺寸影响液滴直径的模拟与实验比较[83]。流体模拟采用与实验相同的内外管直径、水相粘度、水相密度、水相流速、空气粘度、空气密度、空气流速等参数；(c) 水包油胶囊的三相流数值模拟[84]。油相为内相，水相为中间相，空气为连续相；(d) 油/水和水/空气界面张力影响胶囊直径、内核直径和壁厚的模拟结果[84]。流体模拟采用与实验相同的内外管直径、油相粘度、油相密度、油相流速、水相粘度、水相密度、水相流速、空气粘度、空气密度等参数。

Fig. 5 Two- and three-phase computational fluid dynamics (CFD) simulations of the formation of single droplets and core-shell droplets: (a) Two-phase CFD simulations of the droplet formation by shearing water with air[83]. Water is the dispersed phase and air is the continuous phase; (b) Comparison of CFD and experimental results on droplet diameter as a function of air flow rate and inner tube size[83]. CFD simulations use the same parameters as those of experiments, such as inner and outer tube diameters, water viscosity, water density, water flow rate, air viscosity, air density and air flow rate; (c) Three-phase CFD simulations of the formation of core-shell droplets[84]. The oil phase is the inner phase, the aqueous phase is the middle phase, and air is the continuous phase; (d) CFD results on capsule diameter, core diameter and shell thickness as a function of oil/water and water/air interfacial tensions[84]. CFD simulations use the same parameters as those of experiments, such as inner and outer tube diameters, oil viscosity, oil density, oil flow rate, water viscosity, water density, water flow rate, air viscosity and air density.

图6 微流控乳化单元的平行放大：(a) 基于平行树枝状的放大设计[87]；(b) 基于发散树枝状的放大设计[88]。平行树枝状和发散树枝状两种放大设计保证了流体从入口到出口的路径（流阻）完全相同；(c) 基于主枝干结构的平行放大设计[24]。该设计主干通道路径（流阻）不同，枝干通道路径（流阻）完全一致。为保证各单元流量一致，要求枝干通道流阻远大于主干通道流阻，即枝干通道尺寸远小于主干通道尺寸。

Fig. 6 Scale up by parallelization of microfluidic emulsification units: (a) Scale-up design based on parallel dendrites[87]. (b) Scale-up design based on divergent dendrites[88]. Both scale-up designs ensure that the path/resistance of each unit from the inlet to the outlet is identical. (c) Scale-up design based on main-branch structures[24]. The path/resistance of the main channel of each unit is different, but the path/resistance of the branch channel is identical. To ensure the flow rate of each unit to be identical, the resistance of the branch channel should be much larger than that of the main channel, i.e. the dimension of the branch channel is much smaller than that of the main channel.