Melt Electrospinning Writing

Electrospinning is known to be a simple method of producing fibers with diameters ranging from a few microns down to tens of nanometers. In conventional electrospinning, the electrospinning jet go into bending instability phase to achieve nanometer scale diameter and the resultant configuration is a nonwoven mesh of fibers. Due to the simplicity of forming ultrafine fibers using electrospinning, there is a great interest in controlling electrospun fiber deposition to widen its potential application. Melt electrospinning writing is an emerging technique which uses polymer melts for controlled deposition of the electrospun fiber. High viscosity of the electrospinning melts jet and relative proximity of the spinning tip to the deposition collector meant that the electrospinning melt jet remains in the stable phase upon contact with the collector. Melt writing has enabled complex structures to be constructed in situ or assembled. Agarwal et al (2018) used precision melt electrospinning to align polycaprolactone (PCL) beside one another. Plates made out of these aligned PCL fibers were stacked on top of one another in a hierarchical helicoidal design to mimic the basic building blocks found in shells of mantis shrimp dactyl club. Such arrangement of fibers gave the structure superior impact toughness. The tensile strength of hot-pressed unidirectionally oriented fibers between plates were compared with aligned fibers plates stacked in a 15° rotations between each layer to form a 3D helicoidal structure. Their result showed that the toughness of helicoidal PCL sample was 5 times and 1.5 times greater than bulk PCL block and unidirectionally oriented PCL fibers sample respectively.

Methodology, design, and SEM images of one layer of 3D Helicoidal Structure. (a) Schematics of near-field melt electrospinning (NFES)-the process used to first lay down patterns of electrospun fibres on top a collector; (b) illustration of a top view of the hierarchical helicoidal design of the basic building blocks found in shells of mantis shrimp dactyl club, as reported in the literature; (c) scanning electron microscope (SEM) image of a layer of highly aligned, dense fibre-structured samples of melt electrospinning fibres with diameters around 10µm at 65x magnification [Agarwal et al 2018].

There are several parameters that need to be controlled in order to perform melt electrospinning writing. First, the temperature of the melt needs to be optimized such that the viscosity is sufficiently low for the flow characteristic to be stable [Brown et al 2011]. Generally, a higher temperature above the melting point of the polymer would reduce its viscosity however, it should not be that high to cause degradation of the polymer. With melt electrospinning writing, a minimum viscosity is necessary to maintain a stable jet from the spinning tip to the collector. When the viscosity is too low, the electrospinning jet may undergo bending instability before it reaches the collector which reduces the accuracy of the writing. Further, at higher temperature, the fiber may not be sufficiently solidified upon hitting the collector and this will result in "flattened" fibers.

In melt electrospinning writing, it is vital that the electrospinning jet reaches the collector before bending instability sets in. As viscosity of electrospinning melt is relatively high compared to solution electrospinning, the stable jet portion of the electrospinning jet is generally longer than solution electrospinning. A distance of 30 mm or less between the spinning tip and collector is shown to be sufficient for melt electrospinning writing of polycaprolactone (PCL) [Brown et al 2011].

A translation stage is necessary to match the spinning speed so as to lay the fibers along a straight line. Electrospinning of polycaprolactone with an applied voltage of 10 kV, a translational collector speed of 2.5 x 10^-2 m/s was able to achieve this. The resultant structure has controlled pore size of 46 µm and was demonstrated to support good infiltration of cells [Farrugia et al 2013]. Increasing the tip to collector distance to allow more stretching of the electrospinning jet will cause a corresponding loss in the precision of fiber deposition as the jet enters the bending instability stage [Hellmann et al 2009].

SEM images of 3D melt printed PCL deposited in 120 degree turns taken at A) low magnification and B) higher magnification. C) SEM image of 3D melt printed PCL deposited in 90 degree turns. Images courtesy of Prof Paul Dalton.

The steering electrode may be used in tandem with the movement of the collector for precise deposition of fibers with the collector moving in one direction and the steering electrode controlling movement of the jet in the perpendicular direction. This is particularly useful when the distance between the spinneret tip and the collector is small such as in the case of near field electrospinning. Karisson et al (2021) demonstrated the precise deposition of near field, melt electrospun polycaprolactone (PCL) with the use of a single steering electrode. With the aid of a feedback control from a CCD camera and programmed voltage applied to the steering electrode, they were able to space fibers at 50 µm and 40 µm apart and maintain the accuracy of the fiber deposition to within a few micrometers.

A challenge in building elevated structure using melt electrospinning writing is the accumulation of residual charge. In the work by Brown et al (2014), stacking of the electrospun polycaprolactone fibers was limited to 50 layers. Beyond this, the accurate fiber placement is lost and this was attributed to build-up of internal residual charge within the fiber. Incidentally, Hochleitner et al (2015) also limit the polycaprolactone fiber layer to 50. It was hypothesized that the preference of the fibers to stack on top of one another is due to shorter distance between the nozzle tip and the top of the fiber as compared to the ground and that residual charges are trapped inside the fiber instead of at the surface. Thus the electrospinning jet preferentially deposits on top of existing fiber layer. The impact of accumulated residual charges is significant for precision electrospinning as it restricts the number of fibers that can be stacked up continuously and it probably will affect the distance tolerance between fibers.

A constraint of melt electrospinning is that the fibers are typically in the micrometer diameter dimension although there are a few reports of successfully constructing structures with fiber diameter less than 1 µm. In an attempt to create a 3D scaffold to study dimensional metrology of cell-matrix interactions between 3D fibrous and 2D fibrous substrates, Tourlomousisa et al (2017) used melt electrospinning writing to construct the substrates. Using PCL, the average fiber diameter is about 10 µm and the average interfiber spacing was about 150 µm to 75 µm. The implication of the fiber diameter and the pore size is significant in the study of cell migration and behavior. At such large fiber diameter, a large interfiber spacing would see the cells attaching on a single fiber. Although the scaffold is in 3D, the cell on the surface of a fiber may see it as 2D. When the pore spacing was reduced, it was observed that neonatal human dermal fibroblasts (NHDFs) prefer to straddle across fibers [Tourlomousisa et al 2017]. Hochleitner et al (2015) was able to use melt electrospinning at close distance between the tip and the collector (1 to 10 mm) to produce polycaprolactone fibers with diameter of about 800 nm and stack them on top of one another to form an array of box-structures with periodicity of about 100 µm and height of 80 µm.

Youtube video on melt electrospinning

In near field melt electrospinning writing, the high viscosity and proximity to between the spinneret tip and the collector meant that there is a certain stiffness in the electrospinning jet. This stiffness resists buckling from the electrical charges but it also generates a tensile drag force on the jet as the collector moves faster than the ejection of the polymer melt. When this translational motion is sufficiently high, it helps to straighten the deposited fibers on the collector although this also introduces a lag effect as the jet gets pulled away from the spinneret tip [Jin et al 2020]. Jin et al (2020) used this "lag effect" to introduce a higher tensile drag force which will resist chaotic bending from the electrical charges and improve accuracy of the deposited fibers. However, it may also be argued that in this case, it may be "tensile drawing" of the jet from the spinneret instead of "electrospinning" that pulls the fiber into the desired diameter since the "lag" due to the tension is greater than the effect of the electrostatic charges on the spinning jet. Unfortunately, the study did not verify whether the fiber diameter is due to tensile stretch or electrostatic stretch.

There are several potential applications of melt electrospinning writing. Su et al (2021) combined techniques of melt electrospinning writing, micromolding, and skiving process to mass produce tadpole-like magnetic polycaprolactone/Fe₃O₄ (PCL/Fe₃O₄) microrobot. Melt electrospun PCL fibers were used as a sacrificial template to form PDMS channels. These PDMS channels were formed by layers of melt electrospun fibers stacked on top of one another. By varying the speed of the collector, melt electrospinning writing was able to produce fibers of different diameters. When the collector speed was 100 mm/min, the channel width after removing the fiber was 159 µm. When the collector speed was increased to 1500 mm/min, the channel width after removing the fiber was reduced to 25 µm. Injection molding was used to fill the PDMS channels with PCL/Fe₃O₄ solution. After solidification of the PCL/Fe₃O₄, the billets were removed and embedded in a frozen, water-soluble polymer for skiving into microslides. The tadpole-like microrobots were then released from the polymer by washing.

Characterization of the polydimethylsiloxane (PDMS) channels and magnetic polycaprolactone (PCL)/Fe3O4 assymetric billet. (A) Scanning electron microscopy (SEM) images of the cross-section of PDMS channels fabricated with different printing speeds using 0.9 mm needles. (B) The width and depth of the channels against the printing speeds. (C) SEM images of the cross-section of assymetric PDMS channels with designed shapes. (D) Optical microscope image of channel 2. (E) SEM image of magnetic PCL/Fe3O4 assymetric billet from channel 2. The values of speed were marked on each image and the unit was mm min-1 [Su et al 2021].

[+]

[-] comments powered by Disqus

Content [Melt spinning]