Effect of vapor-phase glutaraldehyde crosslinking on electrospun starch fibers
Introduction
Starch is an abundant, inexpensive biopolymer widely existing in plant. Apart from as food source, starch has been widely used in pharmaceutical and medical fields (Lingyan and Ziegler, 2014, Zhang and Venugopal, 2006, Zongqiang et al., 2014). For example, ARISTA™ AH an absorbable surgical hemostatic powder is a derivative of starch. Starch is chiefly used in the form of powder, film or blend with other materials. Processing starch into fine fibers could open up novel applications especially as tissue engineering scaffolds, wound dressings, and drug release. However, difficulties still remain in making starch fibers using conventional fiber-making techniques. Starch contains three hydroxyl groups in each repeat unit, which are easy to form inter- and intra-molecular hydrogen bonds. Hydrogen bonding between the O3 and O2 of sequential residues allows the starch to form a helical conformation, which is relatively stiff with a hydrophobic surface. The hierarchical structure also leads to the formation of starch granules, reducing chain mobility and therefore increasing the processing difficulty (Moran, Vazquez, & Cyras, 2013).
Electrospinning is a simple, versatile, and scalable technique to produce fine polymeric fibers. Electrospun fibers possess a large surface to mass ratio and small pore size. They have shown enormous potential for applications in the fields of filtration, tissue scaffolds and wound dressing. Recently, starch nanofiber membranes have been prepared by electrospinning (Huang et al., 2003, Reneker et al., 2002). However, the electrospun starch reported has issues with water solubility and low mechanical strength.
Crosslinking is an effective technique to improve the stability and mechanical properties of polymers. Crosslinking agents, such as formaldehyde, sodium trimetaphosphate (Carmona-Garcia et al., 2009, Mao et al., 2006), epichlorohydrin (Kittipongpatana & Kittipongpatana, 2013), and phosphorus oxychloride (Kim, Hwang, & Byung-Yong, 2012) have already been used to crosslink starch granules, mostly through blending the crosslinking agent with starch in an aqueous solution (Cao et al., 2008, Phattaraporn et al., 2011). However, these methods are unsuitable for crosslinking electrospun starch fibers because the addition of the crosslinking agent into electrospinning solution could affect electrospinning process and fiber morphology.
Glutaraldehyde (GTA) has been used to crosslink hydroxyl-containing polymers (e.g. PVA) and gelatin through a vapor phase crosslinking reaction with high efficiency, short reaction time and low cost (Ramires and Milella, 2002, Zhang et al., 2006). In comparison with other crosslinking agents, GTA has lower cytotoxicity and the crosslinked materials are biocompatible and non-thrombogenic, and have good mechanical properties. However, vapor phase GTA crosslinking of starch nanofibers has not been reported in research literature.
Here, we prove that water resistant starch nanofiber membranes with enhanced thermal and mechanical properties can be prepared by electrospinning and by subsequently crosslinking the as-spun starch nanofibers in GTA vapor. The effect of GTA crosslinking on the structure, thermal and mechanical properties, and hydrophilicity of starch nanofiber membranes was examined. Our study has indicated that GPA is an effective crosslinking agent that can improve the thermal and mechanical properties of starch membranes without altering the fiber morphology. We further studied the cytotoxicity of the nanofiber membranes using Escherichia coli (E. coli) as a model (Azami et al., 2010, Bigi et al., 2001, Scotchford et al., 1998). The starch fibers with improved water resistance and mechanical properties may find applications in biological field.
Section snippets
Materials
Starch HYLON VII (an unmodified high amylase corn starch containing approximately 70% amylose) was purchased from Ingredion Incorporated, USA. Dimethyl sulfoxide (DMSO) from Tianjin Kemiou Chemical Reagent and aqueous glutaraldehyde (50%) from Sigma were used as received. The starch-DMSO solution for electrospinning was prepared by dissolving starch HYLON VII in DMSO, and then stirred at 70 °C for 20 min.
Electrospinning
Electrospinning was performed using a purpose-built apparatus consisting of a syringe with a
Morphology
During electrospinning, the spinning temperature was controlled at 60 °C to decrease the molecular interaction and increase the flexibility of starch chains. Fig. 1a shows a photo of the as-spun starch fiber membrane. The fiber membrane was soft and flexible. SEM imaging indicated that the starch fibers were continuous in length with an average diameter of 200–700 nm (Fig. 1b and c).
After GTA vapor crosslinking, the fiber membrane turned yellow slightly and shrank dimensionally (Fig. 1d). The
Discussion
DMSO is a strong hydrogen bond acceptor capable of breaking associative hydrogen bonds in the starch (Cooreman, van Rensburg, & Delcour, 1995). DMSO dissolves starch because the hydroxyl groups of starch can complex to DMSO anions to break the hydrogen bonds. It was reported that polymer macromolecules after electrospinning orientated along the fiber length as a result of fast and large ratio of stretching. Meanwhile, solvent evaporation leads to the solidification of polymer jet. In this way,
Conclusion
We have proven that starch nanofibrous membranes with high tensile strength, water stability and non-cytotoxicity can be produced by electrospinning of starch solution and post-treatment with GTA in vapor phase. GTA vapor phase crosslinking plays a key role in forming water-stable nanofiber membrane and improving the mechanical properties. Comparing with non-crosslinked starch fibers, the crosslinked fibers is increased by nearly 10 times in tensile strength. The crosslinked starch fibrous
Acknowledgements
Funding support from National Natural Science Foundation of China (Nos. 51103101, 51573136), China Postdoctoral Science Foundation (Nos. 2011M500525, 20110490785), National scholarship fund of China (No. 2011812002), Natural Science Foundation of Tianjin (No. 12JCYBJC17800), State key laboratory of membranes separation and processing (No. M3-201504), and Tianjin Polytechnic University (P. R. China) is acknowledged.
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