# Poly(\(\alpha\)-hydroxy Acids)-Based Cell Microcarriers
## 3 Methods for the Fabrication of Microcarriers  
Microcarriers made out of biodegradable poly(\(\alpha\)-hydroxyacids) have traditionally been fabricated via the single (oil-in-water (O/W)) or double emulsion-(water-in-oil-in-water (W\({}_{1}\)/O/W\({}_{2}\))) solvent evaporation method. In the former, the polymer of interest is dissolved in an organic solvent (e.g., dichloromethane, trichloromethane, etc.) and subsequently added to an aqueous phase containing an emulsifier (e.g., polyvinyl alcohol, sodium dodecyl sulfate, Tween 80, etc.) to form an emulsion by \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Polymer** & **Composition** & **Degradation Rate (Day\({}^{-1}\))** & **Reference** \\ \hline poly(L-lactide) (PLLA) & N/A & 0.0009 & [31] \\  & N/A & 0.00043 & [33] \\ \hline  & (DL: 0.2\%) & 0.0008 & [33] \\ poly(D,L-lactide) (PDLLA) & (DL: 1.2\%) & 0.00142 & [33] \\  & (DL: 50\%) & 0.0088 & [31] \\  & (DL: not reported) & 0.00386 & [36] \\ \hline  & (LA: 80/GA: 20) & 0.0127 & [31] \\ poly(L-lactide-co-glycolide) (PLGA)/ & (DL: 53/GA: 47) & 0.0506 & [31] \\ poly(D,L-lactide-co-glycolide) (PDLGA) & (LA: 75/GA: 25) & 0.0219 & [36] \\  & (LA: 85/GA: 15) & 0.0106 & [36] \\ \hline  & (LA: 74/CL: 26) (random) & 0.034 & [41] \\ poly(L-lactide-co-\(\varepsilon\)-caprolactone) (PLCL) & (LA: 74/CL: 26) (blocky) & 0.022 & [41] \\  & (LA: 62/CL: 38) & 0.036 & [41] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Some of the reported degradation rates for the poly(\(\alpha\)-hydroxyacids) considered herein, (DL: D-lactide; LA: L-lactide; GA: Glycolide; CL:caprolactone). mixing with a stirrer, homogenizer or sonicator (Figure 3). In the latter, a primary W\({}_{1}\)/O emulsion is first prepared and then emulsified in a W\({}_{1}\)/O/W\({}_{2}\) emulsion. This method has been widely employed for the fabrication of drug/gene/protein micron- or nanosized delivery vehicles thanks to the possibility of incorporating water soluble entities in the internal aqueous phase (W\({}_{1}\)) [50; 51]. The size of the particles developed needs to be considered carefully, since it will play a pivotal role in the application of microparticles as cell microcarriers. Thus, smaller microparticles will provide a larger surface area, and, accordingly, will provide more area for the cells to adhere to. However, very small particles (i.e., in the nano-size range) may be taken up by cells but not be useful as cell microcarriers [52]. Many studies are still being carried out to clarify the mechanism of uptake, but it has been suggested that particles below 200 nm are internalized by receptor-mediate endocytosis, whereas larger ones are internalized by phagocytosis [53]. On the other hand, the size of the microparticles will also determine the release rate and profile of the encapsulated entity (i.e., protein, drug, etc.) [50]. Several process parameters affect the size and size distribution of the resulting microcarriers, such as the concentration of the polymer in the oil phase, concentration/nature of the emulsifier, viscosity of the internal aqueous phase (W\({}_{1}\)), volume of the external aqueous phase (W\({}_{2}\)), mixing time/speed, etc. [54; 55; 56; 57]. Of these, the concentration of the polymer in the oil phase and the mixing time/speed of the second emulsion play a major role in determining the particle size and distribution. Accordingly, when the concentration of PCL in dichloromethane was increased from 0.08 to 0.33 g\(\cdot\)mL\({}^{-1}\), the resulting size of the particles increased from 4.2 to 11.6 \(\upmu\)m. Conversely, when the stirring speed of the second emulsion was increased from 6500 to 21,500 rpm, the particle size decreased from 37.9 to 7 \(\upmu\)m [54]. Similarly, [PERSON] et al. [55] reported an increase in particle size from 2.12 to 5.88 \(\upmu\)m when the concentration of PCL was increased from 0.5% to 6%. In contrast, the particle sized decreased from 4.40 to 3.26 \(\upmu\)m when the mixing time was increased from 2 to 15 min.