# Journey to the Market: The Evolution of Biodegradable Drug Delivery Systems
## 2 The Evolution of Biodegradable Polymeric Drug Delivery Systems
### Microfluidics Method  
The microfluidics method, similar to the hydrogel template method, was developed in recent years along with availability of microfluidics devices. Apart from the precise control of particle size range and morphology, this method demonstrates significant advantages in building structures and fabricating microencapsulated particles with multiple components in a one-step emulsification by the microfluidic device [246; 247]. Additionally, the particles already formed can be re-encapsulated in the same device to cover them with a double layer with core-shell structures [248]. Recently, based on this ability in fabricating multiple layered structures, particles with a PLGA (poly(lactide-co-glycolide)) surface and a PCL (polycaprolactone) core have been reported [249]. Due to their distinct behaviours in degradation, a potentially superior release profile could be achieved [250]. Generally, the method is summarized in Figure 8.  
Figure 7: Schematic diagram of the hydrogel template fabrication process for producing drug-loaded microparticles: Steps (**a**–**e**): (**a**) PDMS intermediate template with 20 \(\upmu\)m diameter vertical posts was formed using a designed silicon wafer master template with patterns of 20 \(\upmu\)m diameter wells; (**b**) casting hydrogel solution on the surface of the PDMS template; (**c**) hydrogel template with 20 \(\upmu\)m diameter wells; (**d**) casting drug/polymer solution into the wells of the hydrogel template; (**e**) dissolution of the hydrogel template; and (**f**) collection of the microparticles (adapted from [PERSON] et al., 2019 [244]).  
Normally, the microfluidics system is comprised of two main parts, the syringe pump used to precisely control the flow rate of various solvent inputs, and the microchips with multiple channels, in which the continuous phase and the dispersed phase meet and generate particles. For traditional microchips, they are made of PDMS (polydimethylsiloxane), which is less tolerant to high pressure and so vulnerable to breaking [251]. Recently, a glass-PDMS system has been introduced that has broad applications for a wider range of drugs with improved efficiency and higher quality [251, 252].  
Notably, the droplets size can be precisely controlled in the range 20-100 \(\upmu\)m [253] and 100-300 nm [246] along with controlling the flow rates and the concentration of drug and the polymer solution. Particle aggregation in the fabrication and collection process is potentially a serious problem. Aggregation of particles in the microchips could block the channels and even cause breakages. Aggregation of particles in the collection process will affect the size range and generate undesirable particles [254]. The efficiency of particle production is always the main problem with this technique as only one droplet can be generated at a time [212, 255]. Recently, a reliable and versatile system called the Droplet Parallel has been introduced to the market (Dolomite Microfluidics, Royston, UK). With the connection of several microchips, up to 30,000 droplets can be generated per second, which greatly boosts method efficiency and the possibility of large-scale production in a broad range of real applications [256, 257].