SYNTHETIC ENROUTES TO ENGINEER ELECTROSPUN SCAFFOLDS FOR STEM CELLS AND TISSUE REGENERATION - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 125

low immunogenicity, and better mechanical properties 53-55 and at low cost. How-
ever, it is poorly electrospinnable and forms aggregates with non-electrospinnable
HAp nanoparticles. Therefore, formulating a robust chitosan solution to generate
nanofibrous HAp-chitosan biocomposite scaffolds is difficult. Because of these
limitations in electrospinning of chitosan, 56,57 there are only a few reports on
nanofibrous hydroxyapatite (HA)-chitosan composites for bone tissue engineering.
Using ultrahigh-molecular-weight poly(ethylene oxide) (UHMWPEO) as a support
polymer, Zhang et al. 58 fabricated composite chitosan nanofibers by a modified two-
step approach. 59 In short, an in situ co-precipitation synthesis route was designed to
overcome the problem of nanoparticles agglomeration and electrospinning process
was carried out for the preparation of HAp-chitosan nanocomposite nanofibers with
a higher (30 wt%) loading of HAp nanoparticles. It was confirmed with electron
diffraction and X-ray diffraction analysis that the acetic acid used for chitosan
dissolution had minor or no influence on the crystallinity of HAp nanoparticle
incorporated within the nanocomposite nanofibrous structure. Bone regeneration
ability of the scaffold was assessed on these HAp-chitosan nanocomposite nano-
fibrous scaffolds, and the results confirmed that the scaffolds had significantly
enhanced bone formation compared with the pure chitosan scaffold.
5.2 SYNTHETIC ENROUTES
Multiple procedures and method combinations are used for the successful fabri-
cation of a nanofibrous construct for stem cell or tissue regeneration. The scaffold
needs to be stable in culture media; hence, natural polymeric scaffolds have
limitations in direct application, highlighting the need for cross-linking of the
electrospun natural protein-based scaffolds, which makes it stable during incuba-
tion in culture media.
5.2.1 Chemistry of Cross-Linking
Cross-linking is the process of chemically joining two or more molecules by a
covalent bond. Cross-linking of proteins or carbohydrates depends on the availability
of particular chemical groups that are capable of reacting with the specific kinds of
functional groups that exist in proteins.
Despite the complexity of protein or carbohydrate structure, four major functional
groups constitute for the vast majority of cross-linking and chemical modifications:
1. Primary Amine Functionality (-NH 2 ): The amine group exists at the N-
terminus of each polypeptide chain and in the side chain of some amino acid
residues.
2. Carboxyl Groups (-COOH): The carboxylic acid group exists at the C-
terminus of each polypeptide chain and in the side chains of some amino acid
residues.