Electrospun Ordered 3D Structures

Electrospinning is a well-known method for producing randomly oriented fibrous membranes. It's ability to easily produce nanofibers and its resemblance to natural extracellular matrix (ECM) has given it widespread attention in the late 1990s for the production of biological scaffolds. Further research has led to the development of electrospun 3D scaffolds although these are generally made out of disordered fibers. Another area of electrospinning development is in precise and accurate placement of the fiber. With progress in this area, some researchers have shown that it is possible to use electrospinning to construct 3D ordered structures.

Electrospinning melt polymers at close distance to the collector (30 mm or less) have been carried out to construct highly ordered structures. 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].

3D scaffold has also been constructed using electrospinning melt and with ordered fiber deposition. Hochleitner et al (2014) was able to build a 2 mm thick structure out of ordered poly(2-ethyl-2-oxazoline) fibers. 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. This creates new applications and opportunities for nanofiber structures where greater order brings benefits or better performance. 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 the 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.

Near-field electrospinning has emerged as a possible method to get a highly ordered structure as the fiber is deposited while the electrospinning jet is still at its stable phase. When combined with a guiding electrode and a corresponding translating collector, researchers were able to construct an ordered three-dimensional structure. Zheng et al (2021) used near field electrospinning and a sharp pin guiding electrode below the collector as a means of getting highly precise and accurate fiber deposition.Using polyethylene oxide (PEO) as the model polymer, they were able to stack up to layers of fibers to create 3D complex structures with 10 to 80 fibers layers and height of 10 to 110 µm. In melt electrospinning, stacking of fiber layers has been limited to 50 layers [Brown et al 2014, Hochleitner et al 2015] due to the presence of residual charges which limits the accuracy of fiber placement. However, with solution electrospinning, residual solvents present in the deposited fibers may also facilitate migration of charges on the fiber to the ground which allow more layers of fiber to be stacked. A parameter that affects fiber placement is the collector motion velocity. A higher velocity would increase fiber placement accuracy as the polymer jet is stretched and reduces fiber diameter. A higher applied voltage also led to better fiber alignment due to greater focusing of the electric field between the nozzle and the guiding electrode. Although up to 80 fiber layers have been constructed, the effect of residual charges and the shielding of the electric field by the higher fiber layer cannot be prevented. Hence the accuracy and precision of the fiber deposition will be reduced as the layers build up. For a 60-layer fiber wall, Zheng et al (2021) reported a width of 94.3 µm and height of 102.2 µm although the fiber thickness is about 1.8 µm.

Guiding electrode setup for stacking layers of electrospun polyethylene oxide (PEO) fibers [Zheng et al 2021]

Mechanism of a straight jet. Digital images of Taylor cones for (A) a single-phasic microfiber and (B) a biphasic microfiber. PLGA was used for the single-phasic microfiber; PLGA and TPU were used for the biphasic microfiber. Schematic diagrams of the forces applied to the polymeric jet for (C) the conventional jetting system and (E) the CREW system. SEM images of grid patterns that are produced by (D) the conventional jetting system and (F) the CREW system. Scale bars, 300 µm. (Moon et al 2021)

With the capability of maintaining a precise and accurate deposition of the electrospun fibers, they were able to construct ordered 3D structures using electrospinning. Although a diameter of 10 µm is large for electrospun fibers, this diameter is much smaller than filament-based 3D printing and may be used together with it to achieve more complex architecture.

Tandem scaffold.(A) Schematic diagram of the design of tandem scaffolds using PMMA and PLGA, where the PLGA was precisely positioned onto a PMMA pattern; (B) schematic diagram of the mechanism of bending motion of a PLGA/PMMA bilayered pattern at elevated temperature (here, 80°C); schematic diagram and digital photo of before and after heat treatment depending on its PLGA pattern, (C) perpendicular pattern, (D) horizontal pattern, (E) combined pattern, and (F) PMMA only [Moon et al 2021].

Another form of directing the electrospinning jet by modifying the electric field is using steering electrodes. Unlike the guiding electrode which is located at the collector and its effect weakened with a buildup of fibers, the steering electrode is located between the spinneret tip and the collector. It works by pushing the electrospinning jet at mid-flight hence it is not shielded by deposited fibers. Karisson et al (2021) employed a steering electrode in the direction perpendicular to the main printing direction. The deposition point of the fiber is dictated by the mechanical movement of the spinneret relative to the collector in one axis, and the steering electrode in the other axis. A CCD camera connected to a stereo microscope was used to monitor the deposition position of the fibers in real time such that the voltage to the steering electrode can be actively adjusted to steer the electrospinning jet to the correct position. Polycaprolactone was melted electrospun to determine the performance of the setup. The distance between fibers was found to affect the accuracy of the fiber placement. When the gap was 75 µm, there was little deviation between the PCL jet in flight and the fiber on the collector with and without active steering. The distance between the fibers was sufficiently large to limit the effect of residual charges on the fibers. At closer distances such as with 50 µm and 40 µm separation, active steering was able to maintain the accuracy of the fiber deposition to within a few micrometers. However, without active steering, the accuracy of the fibers deposition reduced to more than 10 µm. When the fibers were stacked to form walls, there was much less error in the positioning for active steered fibers at a fiber gap of 50 µm. However, it is not possible to get an ordered 3D structure when the fiber gap is reduced to less than 50 µm.

Secondary electron SEM images of printed 3D structures comparing active steering disabled in (a) and (c) to active steering enabled in (b) and (d). The images are tilted to 45°. In (a) and (b), the distance between lines was set to 75 µm, and in (c) and (d), the distance between lines was set to 50 µm. The white arrows in (b) and (d) indicate the direction of control applied from the control loop [Karisson et al 2021].

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