ASSOCIATE PROFESSOR & GRADUATE PROGRAM CHAIR Rowan University, United States
Introduction: One of the main polymers used in biomedical applications is polylactic acid (PLA) based polymers. This is due to its excellent biocompatibility and biodegradability. Many PLA-based polymers have been approved by the FDA for direct contact with biological fluids . Electrospinning has been used to make PLA nanofibers. Post drawing and processing nanofibers has been employed in order to improve the fibers’ mechanical properties and morphology. Laser zone drawing has the potential to process polymer nanofiber with extremely high molecular alignment due to the rapid heating/cooling rates. It utilizes a high power density laser in order to heat a localized fiber zone and mechanically draw it. This localized heating allows for high amounts of drawability as well as fast cooling. Fast cooling leaves little time for molecular chain relaxations. That accompanied with the small diameter has the potential to produce nanofibers with great mechanical strength and high thermal conductivity. This processing technique is not limited to PLA but can be used on other polymers such as PCL, PVDF, etc. For all these reasons, laser zone-drawing can produce nanofibers that can be utilized in many biomedical applications such as biosensor and medical devices. Previous work studied the heating kinetic at steady state fiber temperatures. This study aims to explore the transient heating kinetic of the fiber drawing process .
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 under the following parameters: 10kV potential difference, draw ratio (DR) of 1, and 30-40% humidity. Fibers are then mounted and heated using a CO2 laser Figure 1. A high speed 2 axis laser scanner is utilized to quickly move the laser through the fiber heating region. This is to ensure that only transient temperature is reached. Scanning electron microscopy is used to image the fibers post drawing. Fiber dimeters are then measured using scanning electron microscopy (SEM)
Results, Conclusions, and Discussions: Discussion Previous work demonstrated a relation between laser power and final fiber diameter. Figure 2 demonstrates a decrease in diameter as laser intensity increases. Moreover, increasing the exposure time did not result in a significant decrease in diameter for a given power. That means that for a given power density, the fiber reaches a diameter where cooling rates and heating rates are at equilibrium. A FEA simulation of nanofiber heating demonstrated the same result (Figure 3). However, there is a transient region where the fiber reaches a drawing temperature but not steady-state temperature. In order to study the transient heating region, PLA nanofibers are heated with quick laser pulses that target the transient region of the heating-cooling profiles shown in Figure 3.
