# Poly(\(\alpha\)-hydroxy Acids)-Based Cell Microcarriers
## 4 The Use of Poly(\(\alpha\)-hydroxyacids) as Cell Microcarriers  
The poly(\(\alpha\)-hydroxyacids) considered in this review have many inherent advantages for use as cell microcarriers--for example, their biodegradability, biocompatibility, tunable mechanical properties/degradation rates, processability, etc. However, due to their intrinsic hydrophobicity and lack of cell binding sites, surface modifications that promote cell adhesion and proliferation are usually necessary for their use as cell microcarriers [74]. These surface modifications, apart from determining cell adhesion and proliferation, also play an important role in the migration of cells from the microcarrier to the tissue/area of interest. Accordingly, it has been reported that functionalization of the surface of the microcarriers with proteins or peptides, via physisorption, facilitates cell migration compared to a surface modification based on a covalent coating [75]. [PERSON] et al. [76] reported that collagen-coated PLA microspheres provided a better support for chondrocyte adhesion and viability compared to non-coated spheres. For the immobilization of collagen, microspheres were first aminolyzed via incubation in a hexanediamine/_n_-propanol solution at 60 \({}^{\circ}\)C. The amount of NH\({}_{2}\) groups on the surface of the microcarriers increased with the incubation time. However, in order to avoid extensive degradation of the polymer matrix, aminolysis treatment was limited to 8 min. After aminolysis, NH\({}_{2}\) groups were transferred into aldehyde groups  
Figure 6: Schematic illustration of the electrospraying process. Reproduced from [71] with permission of The Royal Society of Chemistry, 2014.  
by a treatment with glutaraldehyde, and, finally, collagen type I was covalently coupled via Schiff base formation between the amino groups from collagen and aldehyde groups from the surface of the microcarriers. In order to avoid conformational changes associated with large biomacromolecules (i.e., proteins), short peptides can also be incorporated on the surface of microparticles to provide active domains for cell binding [77; 78]. These peptides can be easily incorporated via the carbodiimide chemistry on the surface of microparticles bearing carboxylic groups [77] or previously functionalized with gelatin [78]. In these two examples, incorporation of RGD peptide clearly improved the initial adhesion and subsequent proliferation of chondrocytes. In another example [79], REDV peptide was incorporated onto the surface of PCL microspheres to enhance the adhesion and proliferation of endothelial cells (Figure 7). To achieve this, NH\({}_{2}\) groups were first incorporated onto the surface of PCL microspheres by aminolysis using 1,6-hexanedamine solution in isopropanol at 40 \({}^{\circ}\)C for 60 min. Afterwards, the NH\({}_{2}\) groups were employed to immobilize \(\alpha\)-bromoisobutyl bromide (BIBB) via a triethylamine (TEA)-catalyzed condensation reaction. Brominated PCL surfaces then underwent a surface-initiated atom-transfer radical-polymerization (ATRP) reaction to graft azido-terminated poly(glycidyl methacrylate) (PGMA-N\({}_{3}\)) polymer brushes on the surface of the microspheres. After carboxylation of PGMA-N\({}_{3}\) polymer brushes via the azide-alkyne click reaction, REDV was covalently immobilized by carbodiimide chemistry. Pristine PCL was found to be an unfavourable substrate for HUVEC attachment and growth. In contrast, after functionalization with REDV the adhesion and proliferation of cells was significantly improved, and similar to that obtained in Cytodex \(1\), which was used as a positive control. Short peptides can also be incorporated so as to trigger the differentiation of stem cells toward specific cell lineages [80]. In one particular case, insulin-like growth factor 1 (IGF-1) was immobilized on the surface of PLGA microparticles.