**2. Microfluidic devices**

There are two main types of microfluidic devices for particle production: microchannels and microcapillaries [38]. Microchannel-based devices are commonly manufactured through processes such as micro-milling, micro-machining, lithography, and shape replication. In such devices, minimizing the interfacial environment causes spontaneous droplet formation, and therefore the droplet size is best dependent on the microchannel geometry while maintaining the oil phase flow rate within an optimal range. Total devices based entirely on microchannels are costly and timeconsuming to manufacture, but they enable microsystems to be manufactured with

a particle size of only a few tens of micrometers. In addition, the microchannels in such devices can be properly aligned and, in addition to the uniform flow and strong liquefaction of certain droplets or the splitting of droplets to a uniform size, they can serve their equipment, and the systems can be expanded to produce a large number of products. In addition, structures mainly based on capillaries are usually made of low-cost components available on the market and can become microchannels in particle manufacturing. Importantly, these systems can be manufactured in a shorter time and can operate under harsh process conditions [39]. In a complete device based on microchannels, the dispersed phase is very close to the tool wall before being emulsified by the continuous phase, which may cause phase inversion [40]. Although, the affinity of the dispersed relative substance is greater than that of the continuous phase, the dispersed phase preferentially wets the partitions of the tool. This makes the selection of materials produced by the equipment more important than all other materials. However, phase inversion can be avoided by deciding which equipment is suitable for water droplets or organic droplets [41]. On the other hand, capillaryprimarily based devices are tremendous for such terms. Here, the droplets are stopped from assembling the device's partitions. Capillary-specific devices allow for the manufacture of oil-in-water or water-in-oil emulsions with a single microsystem [42].

Using a variety of materials and shapes in microfluidic devices to allow future and desirable sort of physical activity and features. Each layer of a laminated microfluidic device is cut separately. The cutting process has a considerable impact on the device's dimensions and functionality. For prototyping and laboratory settings, due to the speed and simplicity each tool offers, cutting is usually done with a knife plotter (i.e., xurography) or laser cutter. A knife plotter works by precisely cutting material with a blade to create the geometry, while a laser cutter uses a focused beam (traditionally, CO2 lasers are used) [43, 44]. Under these conditions, the droplet diameter can be reduced by increasing the flow rate, density ratio, and viscosity [45]. Bottom-up


## **Table 1.**

*Advantages and disadvantages of the methods used in invitro drug screening by microfluidics.*
