This work aims to increase the compliance of PLA nonwoven fabrics to better mimic the structure of the ... properties of the electrospun fabrics are important.
ELECTROSPUN FIBER MORPHOLOGY AND NONWOVEN FABRIC COMPLIANCE Jenna Puckett 1, Margaret Frey 2, and Mary Rebovich2 1
Department of Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, 2 Department of Fiber Science and Apparel Design, Cornell University, Ithaca, New York
Electrospinning is a technique that shows great promise for tissue engineering applications. Micrometer diameter fibers arranged in a highly porous fabric have been synthesized in the biocompatible polymer polylactic acid (PLA). This work aims to increase the compliance of PLA nonwoven fabrics to better mimic the structure of the extracellular matrix by investigating the relationship between fiber morphology (fiber diameter, fiber porosity) and nonwoven material compliance. Applied voltage was varied (10 kV, 15 kV, 20 kV) to create PLA fibers ranging in diameter from 1.35 ± 0.48 um to 1.59 ± 0.51 um and compliance values ranging from .00987 - .01864 MPa-1. The highest applied voltage produced large fibers with the highest compliance, while the lowest applied voltage produced the smallest fibers with the lowest compliance.
Introduction Polylactic acid (PLA) is a biocompatible and biodegradable polymer [7] that is promising for biomedical engineering applications, including tissue engineering and wound dressing. Electrospun fabrics show great promise as scaffolds in tissue engineering for their nanometer to micrometer sized fibers arranged in interconnected, porous nonwoven fabrics. These fabrics provide a biomimetic environment similar to the extracellular matrix of cells [5]. The fiber structure and overall material properties of the electrospun fabrics are important components in successfully mimicking the extracellular matrix to allow for cell growth. Electrospinning produces polymer fibers by the attraction of a charged drop of polymer solution from a needle to a grounded collector [6]. As the drop is attracted from the needle, a charged jet extends straight and then bends and whips rapidly, greatly extending the length and reducing the diameter of the fiber. A nonwoven fabric is formed by the accumulation of the electrospun fibers. Operating parameters such as applied voltage distance between needle and substrate pump speed, and needle diameter can affect the resulting fibers. [1] Environmental conditions like humidity and
temperature can be important. Furthermore, solution properties like molecular weight of the polymer, polymer conductivity, solution concentration, and solution viscosity are relevant. Many types of fibers can be produced through electrospinning. With certain polymer solutions, surface tension in the charged jet is minimized by producing “beads on a string”, as shown in Figure 1.[3] This picture shows beads formed from a solution of 3.85 wt% poly(ethylene oxide) in water. The edge of the image is 20 microns long.
Figure 1 [3]
Fibers can be porous, as shown in Figure 2. [4] These fibers are partially crystalline poly-L-lactide (PLLA) fibers spun from a 5 wt% of PLLA in dichloromethane. The pores are attributed to the faster evaporation of solvent rich regions during the electrospinning process.
Materials Polylactic acid (MW = 143,000) was obtained from Cargill Dow LLC (Minnetonka, MN). [2] Chloroform (EM Science, Gibbstown, NJ) and analytical-reagent-grade acetone (J.T. Baker, Phillipsburg, NJ) were used to prepare an 8 wt% solution of PLA with a 3:1 ratio of chloroform to acetone. Electrospinning A schematic of the electrospinning setup is shown in Figure 4.
Figure 2 [4]
Hollow fibers can also be produced, as shown in Figure 3. [1] The hollow structure of these poly(vinyl phenol) / TiO2 fibers came from the dissolution of mineral oil with octane. Figure 4
The polymer solution was pumped at a rate of 0.01 ml/min through a 22 G needle that was two inches in length. The distance between the needle and the collector was 10 cm. The collector was a copper plate, 3” by 3”, covered in aluminum foil. Three different voltage differences were applied between the needle and the collector: 10 kV, 15 kV, and 20 kV. Samples were collected for one hour. Figure 3 [1]
This work will investigate the relationship between fiber morphology and nonwoven material compliance. For cell growth to occur, the mechanical properties of the material must match the mechanical properties of the extracellular matrix. This work aims to increase compliance (decrease the elastic modulus) by lowering the applied voltage to produce fibers with reduced volume.
Experimental
Characterization Morphology was investigated through Scanning Electron Microscopy with a Leica 440 SEM. Fiber diameter was measured from SEM images with Image J software. Tensile testing was performed using the ASTM D638-02A Standard, using a consistent thickness of 0 .01 mm. The obtained elastic modulus was then used to calculate compliance, where compliance is the inverse of elastic modulus. Compliance was then normalized by weight, C = C’ x W’ / W,
where C is the normalized compliance value of each individual sample, C’ is the non normalized compliance value of each individual sample, W’ is the average weight of the tensile test sample, and W is the weight of the individual tensile test sample. An ANOVA statistical analysis (from EXCEL) was performed on fiber diameter and compliance values to determine whether results were statistically significant. A sample size of 100 diameters per voltage condition was used for the fiber diameter measurements. A sample size of 17 was used in the compliance analysis. Results were determined to be statistically significant if p-value < .05.
Figure 6
Results and Discussion Fiber Morphology The fiber morphology of the 10 kV sample, (Figure 5) is highly porous. The average fiber diameter was found to be 1.35 ± 0.48 um. The fiber morphology of the 15 kV sample (Figure 6) has smaller pores, and the overall fiber is much smoother. The average fiber diameter was found to be 1.59 ± 0.51 um. The fiber morphology of the 20 kV sample (Figure 7) is much more varied, with large diameter and small diameter fibers present. Some scaling and porous fibers are seen. The average fiber diameter was found to be 1.58 ± .48 um. Fiber morphology was influenced by change in applied voltage difference.
Figure 7
The 10 kV fibers were the smallest, and the 15 and 20 kV fibers were larger but similar in size. The 10 kV fibers were found to statistically different from both the 15 kV and 20 kV fibers through ANOVA analysis. The 15 kV and 20 kV fibers were not found to be statistically different from each other. However, all three conditions produced fibers within one standard deviation of each other. The lower applied voltage difference is theorized to cause a decrease in jet stability, as shown by the porosity, but also resulting in smaller diameter fibers.
Figure 5
Compliance Compliance values are similar for all applied voltage differences, as shown in Figure 8. The compliance values were not found to be statistically different by ANOVA analysis. However, only seventeen compliance values were used in the
ANOVA analysis. At least thirty samples are desired for an accurate normal distribution.
A more decisive analysis of the affect of applied voltage on fiber morphology and material compliance, as well as the relationship between the two, could be obtained by increasing the response of the polymer to changes in applied voltage difference. This could be accomplished by increasing the conductivity of PLA (PLA is nonconductive) with additives.
Conclusion
Figure 8
Despite these compliance values, a relationship between standard error and applied voltage difference is apparent from Figure 8. The standard error increases as applied voltage increases. In other words, while the average compliance does not change, the variability of nonwoven fabrics produced increases as applied voltage difference increases. The 20 kV applied voltage difference creates an unstable polymer jet, resulting in nonwoven fabrics with high variability. If the 10 kV applied voltage difference produced samples with reduced volume, why is there a slight increase in compliance as applied voltage difference increases? With stable jet formation, high applied voltage difference results in high whipping leading to smaller diameter fibers with more molecule alignment. Thus, with stable jet formation, an increase in applied voltage difference results in a decrease of compliance. However, the porous 10 kV sample shows that erratic jet formation was present. There appears to be a balance between the porosity of the fibers and the diameter of the fibers. The larger fibers formed in a more stable jet formation at a higher voltage had higher compliance, while smaller fibers formed from more erratic jets had a lower compliance. A more concise measurement of porosity compared to fiber diameter as applied voltage difference varies would be useful in investigating this relationship.
In conclusion, fiber morphology varied as the applied voltage difference changed. When the applied voltage difference was 10 kV, the smallest diameter of fiber (1.35 ± 0.48 um) was formed. All samples produced comparable compliance values (.00987 - .01864 MPa-1), despite changes in fiber morphology. Further investigation of the relationship between porosity and fiber diameter is essential to understanding the effect of fiber morphology on nonwoven material compliance.
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