Increasing Surface Hydrophilicity in Poly(Lactic Acid) Electrospun Fibers by Addition of Pla-b-Peg Co-Polymers

Poly (lactic acid) – b – poly (ethylene glycol) (PLAb-PEG) co-polymers with block lengths of 1000-750, 5000-1000, 1000-5000, and bulk PEG were added to PLA electrospinning dopes to create hydrophilic but non-water soluble nanofibers. PLA-b-PEG block lengths strongly affected the total amount of PEG that could be incorporated, as well as spinnability and fiber morphology. Solutions containing >1% w/w of the lowest molecular weight co-polymer PLA (1000) – b – PEG (750) formed an unspinnable, cloudy gel. Addition of the PLA (5000) – b – PEG (1000) to the base spinning solution influenced fiber diameters and spinnability in the same manner as simply increasing PLA concentration in the spinning dope. Addition of PLA (1000) – b – PEG (5000) resulted in decreased fiber diameters, and allowed for the highest overall co-polymer loading. In final fiber formulations, maximums of 0.9, 2.9 and 9.3 wt% PEG could be achieved using the PLA-b-PEG 1000-750, 5000-1000 and 1000-5000 respectively. PEG (MW = 3350 g/mol) homopolymer was added to the spinning dopes to prepare fibers with 1.0 and 5.0 wt% PEG. The resulting fibers had non-uniform morphology and more variable diameter size than occurred with the addition of PEG in block co-polymer form. Water absorbance by electrospun nonwoven fabrics increased by four times over the control PLA with the addition of 1.0 wt% PEG, and by eighteen times with the addition of 9.3 wt% PEG with the block copolymers. At similar overall PEG loadings, the addition of PLA-b-PEG resulted in a two to four fold increase in water wicking over the addition of PEG homopolymer. INTRODUCTION Increasing the hydrophilicity of nanofibers can improve their performance in applications involving aqueous media, including biomedical devices, tissue engineering scaffolds, and biosensors [1-14]. Improving surface wetting also leads to a significant improvement in material biocompatibility and functionality [15, 16]. Biosensor applications in particular often involve the use of nanometer-scale structures. Nanoscale materials provide increased surface area over bulk materials, enabling more effective capture and detection of analytes in aqueous media. Therefore, it is particularly important to maximize the surface hydrophilicity of the nanoscale materials. In previous studies, improvements in surface hydrophilicity have been monitored by measuring changes in contact angle and wettability [15, 16]. When using nanofibers to construct a biosensor, there are necessary components that must be considered. Both the surface and bulk polymer properties can influence the strength of the sensor signal, so materials must be chosen carefully [17, 18]. For example, the polymer matrix must be suitably hydrophilic to allow the aqueous media to wet, and then penetrate the material. Additionally, the polymer matrix cannot be so water soluble as to dissolve or swell excessively in the presence of aqueous media. Poly(lactic acid) (PLA) is an easily spinnable, biocompatible, and biodegradable polymer often used in biomedical devices [19]. The surface of PLA is strongly hydrophobic, but in order to improve its biocompatibility, various surface and bulk modifications have been shown to enhance hydrophilicity. These surface modifications have been performed by way of coatings, grafting, and plasma treatments, while bulk modifications have included copolymerization or blending of PLA with more hydrophilic polymers [20-26]. Studies have shown that blending PLA films and particles with hydrophilic poly(ethylene glycol) (PEG) is a