Polymeric Nanofibers - American Chemical Society

Chapter 15

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: February 23, 2006 | doi: 10.1021/bk-2006-0918.ch015

Understanding the Effects of Processing Parameters on Electrospun Fibers and Applications in Tissue Engineering 1

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Cheryl L . Casper , Weidong Yang2, Mary C . Farach-Carson , and John F . Rabolt * 1,

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Departments of Materials Science and Engineering and BioIogical Sciences, University of Delaware, Newark, D E 19716

The effects of molecular weight and atmospheric conditions on the electrospinning process have been investigated. Electrospun polymer microfibers with a nanoporous surface texture have been produced in the presence of humidity. The density of pores, their depth, and their shape have been shown to vary with relative humidity, molecular weight of the polymer, and solvent volatility. Although polystyrene (PS) was investigated in detail, other commodity polymers were also shown to exhibit the nanoporous surface structure under a judicious choice of spinning conditions. The effect of molecular weight on electrospun fiber formation was also studied. It was determined that sufficient chain entanglements are necessary for fiber formation. Molecular weight was also found to affect fiber diameters. Understanding the effects of molecular weight and humidity on electrsospun fibers allows for a greater understanding of how to control the electrospinning process to meet specific applications needs. Utilizing this knowledge, we have investigated the use of electrospun collagen and gelatin fibers as tissue engineering scaffolds. This study shows that cells readily attach to this unique fiber morphology.

© 2006 American Chemical Society

In Polymeric Nanofibers; Reneker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction Electrospinning is a fiber processing technique that involves applying a high voltage to a polymeric solution. The diameters of electrospun fibers are in the nanometer to micron range making them applicable in a wide variety of applications. One challenge in the electrospinning field is the ability to understand and control processing parameters. A multitude of parameters directly affect the electrospinning process. The majority of the research has focused on the effects of solution concentration (1), applied voltage (1), electric field (2), and the effect of the polymer/solvent system (2, 3). This initial work focuses on providing some insight into how molecular weight and humidity affect this fiber formation technique. A fundamental understanding of how to control the formation, shape, texture, and morphology of these fibers is essential. Determining the link between electrospinning parameters and electrospun fiber morphology will allow for the design of polymeric fibers to meet specific application needs. Electrospun fibers have been investigated for use in a variety of applications such as filtration (4, 5), electronic and fuel cell applications (6-8). However, much of the current research has focused on using electrospun fibers for biomedical applications such as wound dressings (9), drug delivery vehicles (10), and tissue engineering scaffolds (11-13). Properties such as an interconnected porous network, small fiber diameters, controllable degradation rate, and mechanical integrity make electrospun membranes ideal candidates for tissue engineering constructs (14). The nanometer diameter of electrospun fibers mimics the size scale of fibrous proteins found in the extracellular matrix (ECM) of the body (15). The porous nature of the membrane is vital for cells to move throughout the membrane and transport fluids and waste materials through the membrane. Studies have been done in our laboratory to introduce nanoporous features on the surface of electrospun fibers to eventually aid in fabrication of electrospun tissue engineering scaffolds (3, 16). The goal of this work is to investigate how simple processing parameters, such as humidity and molecular weight, affect the electrospinning process. This knowledge will then be used to study electrospun fibers in applications such as tissue engineering scaffolds.

The effect of molecular weight on fiber formation One important issue in the electrospinning process is the effect of polymer molecular weight on fiber formation. Literature shows that electrospun fibers can vary in diameter from nanometer to micron (1, 3, 17-21), but little is known



207 about what causes these diameter differences. Studies also show that electrospinning equipment can be used to produce freestanding beads, which would make this an electrospray process (22). We believe that molecular weight plays a major role in controlling fiber formation and fiber diameter. To investigate this theory, the molecular weight of polystyrene (PS) was varied while keeping other processing parameters constant.. PS molecular weights from 31,600 g/mol to 1,800",00 g/mol were studied as outlined in Table I. The solutions were electrospun under the following experimental conditions: 35 wt% PS dissovled in tetrahydrofuran (THF), 10 kV, 25% relative humidity, and working distance of 30 cm. The needle diameter increased with molecular weight due to changes in viscosity. Field Emission Scanning Electron Microscopy (FESEM) was used to characterize the samples. Figures l a and l b show that the lower molecular weights PS yielded beads, thus electrospraying had occured. Fiber formation began at the 75,700 g/mol molecular weight and was apparent by the appearance of fiber 'tails' on the end of the beads, Figure l c . However, it was not until a sufficient molecular weight of 171,000 that typical electrospun fibers began to form, Figure Id. The normal ribbon-like shape of PS electrospun fibers was not observed when electrospinning the 560,900 and 1,800,000 molecular weight solutions. Instead, these fibers appeared to have a rippled surface as seen in Figures le and If. Thus, molecular weight affects both fiber formation and surface morphology. We believe that fiber formation is dependent upon the presence of chain entanglements and solution viscosity. Recently, Koski et al. showed that solution concentration plays a major role in the diameter of electrospun polyvinyl alcohol (PVA) fibers (18). Fibers were formed when the solution was in the semi-dilute entangled regime. This semi-dilute region can be noted by [r|]C>4, where C is the solution concentration and [r\] is the intrinsic viscosity calculated by the Mark-Houwink relationship: [t]]= K M . For PS in THF, K = l l x l O " and a= 0.725. Applying this equation to the various molecular weights of PS studied, the [r|]C values are as follows in Table II. a

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Table I. Electrospinning Various Molecular Weight PS in T H F M (g/mol) w

31,600 44,100 75,700 171,000 560,900 1,800,000

Needle Inner Observation Diameter (mm) 0.26 Beads only (15-20 jim) 0.34 Beads only (15-20 jum) 0.51 Beads (15-20 jim), fiber 'tails' (0.5 |Lim) 0.51 No beads, fibers (3-10 |um) 1.60 No beads, fibers (5-40 \xm) 1.60 No beads, fibers (5-30 N M )



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Figure 1. FESEM micrographs of (a)31,600 g/mol, (b) 44,100 g/mol, (c) 75,700 g/mol, (d) 171,000 g/mol, (e) 560,900 g/mol, and (f) 1,800,000 g/mol PS/THF electrospun fibers.

Table II. Applying the Mark-Houwink Equation to Various Molecular Weight PS Studied PS Molecular Weight (g/mol) 31,600 44,100 75,700 171,000 560,900 1,800,000

MC 7 9 13 24 57 132



209 These results show that in order for PS/THF fibers to be formed, the [ T I ] C must be greater than 13. These results agree with the findings of Koski et al. for P V A systems (18). To further test this idea, the 31,600 g/mol PS/THF was electrospun again, but this time the concentration was increased to 80 wt%. A t this concentration and molecular weight, [rj]C= 16 which is in the entangled regime that should produce fibers. Figure 2 shows that increasing the concentration of this low molecular weight sample results in micron diameter fibers. This leads us to believe that sufficient chain entanglements ([r|]C>13) are necessary for fiber formation.

Figure 2. FESEM micrograph of31,600 g/mol PS electrospun from an 80 wt % solution of PS/THF.

The effect of humidity on surface morphology Electrospinning was used to produce PS fibers that exhibit a nanoporous surface texture whose morphology is highly dependent upon processing parameters. Previous work has suggested that humidity plays a critical role in the formation of surface features in electrospun fibers (3, 16). The presence of nanopores on the surface of electrospun fibers increases the surface area and provides sites for incorporating drugs, nanoparticles, or enzymes. Learning how to control the surface properties of these fibers is of prime importance for the fabrication of tissue engineering constructs. The goal of this work is to study the effect of increasing humidity and molecular weight on the surface morphology of electrospun PS fibers. Understanding this link between humidity, molecular weight, and surface morphology will allow for tailoring of polymeric fibers to meet specific application needs.



210 The effect of humidity on the electrospinning process was studied using a 35 wt% PS/THF electrospun under varying humidity ranges (16). The humidity was varied using a humidifier (Holmes HM-1700) placed inside of an enclosed electrospinning box. Five different humidity ranges were studied:

Polymeric Nanofibers - American Chemical Society

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