(B-68) Effect of Laser Zone Drawing on the Mechanical Properties and Morphology of Preloaded Electrospun Polylactic Acid (PLA) Nanofibers.

Introduction:: Electrospinning is fabrication technique capable of creating fibers on the nanoscale [1]. It has been employed in a variety of biomedical fields including tissue engineering, wound healing, drug delivery, biosensors, and many more [2]. One of the main polymers that has been used in such applications is polylactic acid (PLA), which is due to its excellent biocompatibility and biodegradability [3]. Moreover, it is a nontoxic polymer FDA approved for direct contact with biological fluids [4]. In addition to its biological properties, PLA has desirable mechanical properties [5]. Nanofiber post-drawing and processing plays an important role in improving their mechanical properties and morphology [6]. One of the methods of post drawing fibers is zone drawing. It is done by applying a localized source of heat to a fiber segment. Laser zone drawing is when the source of heat is a laser [7]. This study aims to explore the effect of laser zone drawing on the mechanical properties and morphology of electrospun PLA nanofibers.


Materials and Methods:: Electrospinning solution is made by dissolving PLA in a dichloromethane (DCM) and dimethylformamide (DMF) mixture (17% PLA in 3:1 DCM:DMF). PLA nanofibers are then produced using an electrospinning device (figure 1) under the following parameters: 10kV potential difference, draw ratio (DR) of 1, and 30-40% humidity. Fibers are collected on frames and a known mechanical load is a applied to them. Finally, fibers are zone drawn with a CO₂ laser (figure 2) with a wavelength of 9.3µm and power ranging between 4-20 Watt. Scanning electron microscopy (SEM) is used to take images of drawn fibers, and Raman spectroscopy is used to analyze the morphology of the fibers.


Results, Conclusions, and Discussions:: It was demonstrated that the laser is capable of drawing the fibers without destroying them. Table 1 demonstrates a reduction of the diameter of unloaded fibers as the power of the laser is increased. Another important factor to be determined was whether zone drawing is due to localized heating from the laser or other factors like laser absorption. If the zone-drawing is due to absorption at the specific laser wavelength, it means that the current method is specific to PLA and could be difficult to replicate for other polymers. In order to determine absorption at the specific wavelength, an IR spectrum of a PLA film was taken. Figure 4 shows very high transmission (94%) at the wave number corresponding to 9.3 µm (1075 cm^-1)

Mechanical testing will be performed using universal mechanical and atomic force microscopy. It was shown that Raman spectroscopy can be used to extract morphological properties, such as crystallinity, of polymers by looking at the depolarization ratio of Raman scattering [8]. Figure 5 shows a Raman spectrum for a single PLA fiber in the . The depolarization ratio is 0.16 which indicates amorphicity. On of the challenges to overcome is determining the actual zone drawing region when obtaining Raman spectra. At the moment the heating zone is equal to the laser beam diameter. We plan on introducing a cylindrical lens in order expand the beam into a line. This will increase the size of the zone drawing area and will make it easier for the fibers to be analyzed under different instruments.





Table 1: Change in PLA fiber diameter as the laser power is increased.

Laser Power (Watt)	PLA fiber diameter (µm)
4	1.7
10	0.58
20	0.34

Acknowledgements (Optional): : The project is funded by the National science foundation NSF 2110027

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