Electrospun Nanofiber Orientation

By Francisco Chaparro, 7 minutes to read
Industry: Technology: , Material:

The popularity of the electrospinning technique continues to grow, and advances in technology are making the workflow easier and more precisely controllable. One important factor to control when processing electrospun fibers is their orientation as they are collected. The two main fiber orientations obtained during electrospinning are aligned (Figure 1a) and random (Figure 1b). While random fibers are randomly oriented in different angles throughout the collected sample, aligned fibers are typically oriented in the same direction.

Flat plate and roll-to-roll collectors are the most used collection tools for nonwoven, randomly-oriented, electrospun micro- and nanofibers. These randomly-oriented fibers provide high surface area, which is beneficial for many applications such as filtration and desalination, where more contact points are provided so contaminants or salt can be efficiently removed from the filtered media. Randomly oriented fibers are also useful for blood vessel tissue engineering because their structure mimics the native tissue, resulting in enhanced cell growth. However, randomly oriented fibers are unsuitable for some specialized applications like generating nerve conduits. Neural cells have much better mobility and proliferation on aligned electrospun structures. Aligned fiber orientation can also enhance mechanical properties by increasing tensile strength and preventing samples from elongating. One application of this principle is when aligned fibers surround the circumferential axis of a vascular graft. These aligned fibers will prevent the sample from expanding significantly, and will render the sample clinically sufficient to prevent burst pressure.

Polyacrylonitrile electrospun fibers
Figure 1. Electrospun polyacrylonitrile (PAN) nanofibers with different orientation: a) aligned and b) random. PAN is a common electrospun material used in battery separator applications as it is cost-effective and offers good chemical properties.

The most efficient, and increasingly common method of collecting aligned fibers is by using a rotating drum collector (Figure 2a). The degree of fiber alignment is determined by the linear speed of the rotating collector. As linear speed increases, fibers become more aligned. Linear speed on a rotating collector is calculated by , and is typically represented in revolutions per minute (rpm), where = linear velocity, = radius of drum collector, and = angular velocity (Figure 2b). A rotating drum with a 10 cm diameter operating at 500, 1,000, and 2,000 rpm will generate a linear speed of 2.62, 5.24, and 10.47 m s-1 , respectively. A 20 cm drum will achieve these same linear speeds at half the rpm.

Figure 2. a) Schematic diagram illustrating basic electrospinning setup to obtain random or aligned nanofibers.
b) Diagram with variables to consider when calculating linear velocity () on a circular rotating collector with radius, , and angular velocity, .

To demonstrate the effects of changes in linear velocity on fiber alignment, a 10 cm drum collector was used to collect electrospun polycaprolactone (PCL). PCL was selected for this example due to its extensive use in tissue engineering and drug delivery applications. Rotating speed was varied while all other electrospinning conditions were kept constant. When the drum was set to 500 rpm, the collected fibers had random orientation with an average diameter of 3.44 µm (Figure 3a). As rotational speed was increased to 1,000 rpm (Figure 3b) and 1,500 rpm (Figure 3c), a transition towards aligned fibers was observed and fiber diameter only changed marginally. At 2,000 rpm (Figure 3d) the high linear speed stretched the fibers into alignment and reduced fiber diameter to 2.75 µm. Note how an increase in pore length is observed with an increase in rpm; a key aspect to keep in mind when controlling pore length is required.

Figure 3. Microstructure of electrospun PCL collected onto a 10 cm rotating drum at different speeds using the Spinbox technology: a) at 500 rpm ( = 2.62 m s-1), b) at 1,000 rpm ( = 5.24 m s-1), c) at 1,500 rpm ( = 7.85 m s-1), and d) at 2,000 rpm ( = 10.47 m s-1). Fiber diameter (FD) is reported as average ± standard deviation for each condition. Scale bar = 50 µm. Images collected with the Phenom Desktop Scanning Electron Microscope (SEM).

Figure 4 shows the quantitative analysis for fiber orientation on all PCL electrospun samples from Figure 3. Parameters for the analysis were: a distribution of 0 to 180°, a bin size of 5° and 750 measurements performed per sample. As expected from Figure 3a, when the rotational speed was only 500 rpm (Figure 4a), fibers did not have enough linear speed to stretch into aligned orientation. This resulted in an average frequency of aligned fibers below 40 across the sample. When the speed was slowly increased to 1,000 rpm (Figure 4b) and 1,500 rpm (Figure 4c), fiber orientation gradually shifted towards 90°. At 2,000 rpm (Figure 4d) there was a much tighter distribution of aligned orientations between 85° and 95°, reaching a peak frequency of 160.

Figure 4. Fiber orientation analysis (n=750) from electrospun PCL collected at different rotating speeds using a 10 cm diameter drum:
a) at 500 rpm ( = 2.62 m s-1), b) at 1,000 rpm ( = 5.24 m s-1), c) at 1,500 rpm ( = 7.85 m s-1), and d) at 2,000 rpm ( = 10.47 m s-1).

When increasing linear speed of a rotating drum to collect aligned fibers, wind/air turbulence may be generated which can affect fiber orientation during sample collection. The Fluidnatek and Spinbox instruments are designed so an opposite charged voltage is applied in the collector. This improves the electric field between the needle tip and the collector, which mitigates turbulence issues and allows fibers to properly deposit with desired alignment. If processing parameters meet certain specifications, this electrical bias can also create what are known as super aligned fibers. Though super aligned fibers were thought to be difficult to generate in large quantities via electrospinning, they are now necessary structures for some applications and can be repeatably generated using the Fluidnatek or Spinbox systems. The structural and mechanical properties of super aligned fibers are critical for applications such as: batteries, nerve conduit tissue engineering, filters, textiles, and sensors.

In biomedical and pharmaceutical applications, super aligned polyhydroxybutyrate (PHB) fibers are useful because they are highly biocompatible and biodegrade easily. Figure 5 shows the workflow used to obtain a sample of super aligned electrospun PHB. These super aligned fibers were made with the Fluidnatek LE-100 electrospinning system (Figure 5a). Figure 5b shows the super aligned electrospun PHB deposited onto a 20 cm drum in the LE-100. Figure 5c shows the super aligned sample after removal from the drum collector, and Figure 5d is an SEM image showing the microstructure of the super aligned PHB fibers ( FD = 4.48 ± 0.39 µm).

Figure 5. a) Fluidnatek LE-100. b) Super aligned PHB on the drum collector of the LE-100. c) Super aligned PHB fibers. d) SEM image of the super aligned electrospun PHB fibers. Scale bar = 20 µm. Image collected with the Phenom Desktop SEM.

To achieve your desired fiber orientation when using the Fluidnatek or Spinbox systems, the key parameters to optimize during sample deposition are: the diameter of the rotating drum collector, the drum’s linear speed, and the electrical bias. Whether you are generating randomly-oriented fibers to improve filtration properties, or creating aligned fibers to optimize mechanical properties, the Fluidnatek and Spinbox systems simplify the workflow and provide batch-to-batch reproducibility.

Fiber orientation has a big impact on mechanical properties and will affect sample behavior according to application needs and testing to be performed. For example, aligned fibers will have minimal force when suture retention strength (a mechanical test typically done for blood vessel assessment) is used, but a higher force when a stress-strain tensile test is implemented. For this reason, all Fluidnatek systems are engineered to be used up to 2,000 rpm as a standard offered accessory, achieving a linear speed up to 20.94 m s-1, with higher speeds achievable upon request. Figure 6 shows our Fluidnatek LE-500 system with a rotating drum ready to collect random and aligned nanofibers, according to the rpm’s used.

Single Coaxial Emitter on the Fluidnatek LE-500
Figure 6. Fluidnatek LE-500 showing the use of a single coaxial emitter and rotating drum to collect random or aligned electrospun fibers.