# Poly(\(\alpha\)-hydroxy Acids)-Based Cell Microcarriers
## 3 Methods for the Fabrication of Microcarriers
Apart from traditional solid microparticles, there is increasing interest in porous microparticles for use as microcarriers due to their larger surface area, reduced density and enhanced transport of nutrients and oxygen within the porous structure. For the fabrication of porous microcarriers, several modifications have been suggested in the emulsion-solvent evaporation method. The incorporation of ammonium bicarbonate in the internal aqueous phase (W\({}_{1}\)) yielded microparticles with high interconnected porosity because of the carbon dioxide and ammonia gas bubbles that are produced during the solvent evaporation [58; 59; 60; 61] (Figure 4). As for solid microparticles, several process parameters (e.g., concentration of ammonium bicarbonate in the internal aqueous phase, mixing time/speed, concentration of the polymer in the oil phase, type of solvent employed in the oil phase, post-treatment with NaOH, etc.) can be modulated to tune the pore size and distribution [58; 59; 60; 61]. Other porogens, such as camphene (which is removed by sublimation, leaving a highly porous structure) [62; 63] or Pluronic F127 [64] (which is extracted from the structure by contact with water) have been also proposed as promising strategies for the fabrication of porous microparticles. Figure 3: Schematic representation of the single emulsion-solvent evaporation method. One of the main drawbacks to the emulsion-solvent evaporation described above is the wide particle size distribution that is obtained. This usually requires post-processing via separation methods (e.g., filtration, centrifugation, etc.) for the homogenization of the particle size, and the batch-to-batch variation. An elegant method to overcome this problem is the use of microfluidic devices to generate polymer droplets that further solidify into uniform-sized microparticles [65; 66; 67]. Apart from synthetic polymers, this method has been satisfactorily exploited for the fabrication of natural polymer-based microparticles [68]. In one of its configurations, known as the flow-focusing device, the dispersed phase (i.e., organic solvent containing the polymer of interest) is injected into the continuous phase (i.e., aqueous solutions with an emulsifier), and, consequently, droplets of uniform size are formed (Figure 5). Finally, by removing the organic solvent, solid particles with narrow particle size distribution are obtained. Changing the polymer concentration in the dispersed phase or modulating the flow-rates allow a precise control over the resulting particle size. Figure 4: Poly(d,l-lactide) (PDLLA) porous microparticles fabricated with the aid of ammonium bicarbonate and further hydrolyzed with NaOH to increase the porosity and pore-interconnectivity. Reprinted from [60] with permission from Wiley Online Library, 2009. Figure 5: Poly(t-lactide-\(\alpha\)-glycolide) PLGA microparticles prepared via a flow-focusing device. Above, an optical micrograph of the orifice of the flow-focusing region generating droplets of the dispersed phase in the continuous phase is displayed. Below, scanning electron micrographs of the resulting particles at low (**A**) and high (**B**) magnification are represented. Reprinted from [66] with permission from Wiley Online Library, 2009. Electrospraying represents a completely different approach for the fabrication of microcarriers made out of biodegradable poly(\(\alpha\)-hydroxyacids), and has received less attention than the aforementioned methods, probably due to the need for more complex equipment for its implementation [69; 70; 71]. In this technique, a polymer solution is loaded into a syringe that flows at a constant rate along a highly charged needle. Then, a high voltage source is used to inject charge of certain polarity into the polymer solution, which is subsequently accelerated in the form of nano- or micro-fiber/droplets toward a collector with opposite polarity once the electrostatic force has overcome the cohesive force of the droplet.