Wet chemical treatment

Figure 1. Functionalization of electrospun fibers through wet chemical treatment.

Covalent bonding of functional molecules on the substrate is probably the best method for ensuring long-term material functionality. In wet chemical treatment, the relatively inert polymer nanofiber is treated with a chemical reagent to introduce reactive groups such as carboxylic, amine or hydroxyl depending on the presence of sites susceptible to electrophilic or nucleophilic attack on the polymer molecule. Soaking the nanofiber in an alkaline medium is often used to generate carboxylic and/or hydroxyl group on the nanofiber surface. A cross-linking or bonding agent such as glutaraldehyde or 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride may be used for attachment of molecules onto the nanofiber [Ghasemi-Mobarakeh et al 2010, Liu et al 2003]. However, chemical treatment condition needs to be optimized so that the fiber does not melt completely [Xuyen et al 2009] and the deterioration of its strength remains acceptable. Given the large surface area of the nanofiber, it is expected that a milder hydrolytic or chemical reaction condition is necessary compared to larger structures.

Table 1. Selected examples of chemically treated electrospun fibers.

Polymer	Chemical reagent	Activated Group	Reference
Cellulose acetate	Hydrolyzed in NaOH/ethanol; treated with methacrylate chloride; graft polymerization with methyl methacrylate, acrylamide and N-isopropylacrylamide	- OH	Liu et al 2003
Polyacrylonitrile	Sodium hydroxide	- COOH	Chiu et al 2011
Polycaprolactone	Potassium hydroxide (KOH)	- COOH	Xuyen et al 2009
Poly(L-lactide-co-caprolactone) copolymer	Aminolysis using 1,6-hexanediamine followed by bonding using glutaraldehyde	- NH2	Zhu et al 2007
Polysulfone	Formaldehyde solution under acidic conditions	- CH2OH	Ma Z et al 2006
Polyimide	Sodium hydroxide followed by bonding using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride	- OH, - COOH	Ghasemi-Mobarakeh et al 2010

Biomolecules may be attached to electrospun fibers after the scaffold has been formed by covalent bonding. Cross-linking technique has been used to bond peptides to electrospun fibers. Laboy-López et al (2022) covalently bonds adhesive peptides KRSR, RGD, and growth factor BMP-2 on the surface of electrospun porous cellulose acetate (CA) fiber mat. A deacetylation procedure followed by oxidation to form carboxylic terminals for further bioconjugation on the electrospun CA fiers. Carbodiimide crosslinking technique was used to attach the peptides to the activated CA fibers. Laboy-López et al (2022) were able to couple the peptides individually on the CA fibers and all three peptides on the CA fibers.

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