Читать книгу Applications of Polymer Nanofibers - Группа авторов - Страница 17

The effect of process parameters (e.g. flow rate, tip‐to‐collector distance, applied voltage) has been widely studied with conflicting experimental results. An alternative approach to tuning nanofiber and membrane properties has been adjusting parameters of the electrospinning setup.

Ambient conditions (temperature and humidity) affect the electrospinning process and fiber characteristics. The temperature of the spinning solution affects the evaporation rate and the viscosity. At higher temperatures, lower viscosities lead to increased stretching force and result in smaller fibers. Humidity affects solvent evaporation which can affect the resulting fiber characteristics. Using PEO in water as a model system, a twofold monotonic decrease in fiber size was observed with increasing relative humidity. At low humidity, solvent evaporation may be faster than removal of the solution away from the tip of the needle and can lead to needle clogging, especially with volatile solvents. Leveraging humidity to create porous fibers is further discussed in Section 1.6.2. Although, the relative humidity cannot always be readily controlled (Cai and Gevelber 2013), monitoring the ambient temperature and humidity during electrospinning is of practical importance.

Generally, electrospinning is performed in air. Controlling the gaseous environment can be advantageous for affecting fiber diameter. To slow the rate of drying, a gas‐jacketed capillary tip can be used to surround the jet with nitrogen saturated with spinning solvent. With slower solvent evaporation, stable Taylor cones could be achieved by electrospinning poly‐L‐lactic acid in dichloromethane (high volatility). Notably, the flow rate of gas affected the rate of electrospinning. Accelerating the rate of evaporation using an external heat source has also been reported to improve the mat quality of hyaluronic acid fibers spun from water. The improved fiber quality was attributed to increased stretching, enhanced solvent evaporation, and a threefold reduction in viscosity due to the flow of hot air (~60 °C). The composition of the gaseous environment is also an important consideration; it affects leakage of the surface charge on the jet to the surrounding environment and ultimately the fiber size. For example, when using Freon‐12 as the electrospinning environment, the fiber diameter was twofold larger than air at the same conditions. This result was attributed to the higher breakdown voltage of Freon‐12 compared to air. With a higher breakdown voltage, the fiber retained its electric charge for a longer period of time which would increase the jet velocity and ultimately result in a larger fibers (Ramakrishna 2005; Baumgarten 1971).

Polarity of the applied electric field also affects fiber quality and size. For nylon‐6 in formic acid, the average fiber diameter was approximately twofold smaller when the capillary was charged with a negative polarity compared to when a positive polarity at the same conditions. Further, the area over which the fibers deposited was smaller in the case of a positive polarity. The difference in fiber quality and size was attributed to increased charge density in the case of negative polarity (Andrady 2008).

Generally, DC voltage is used in electrospinning. The use of alternating current (AC) potential has also been reported. Since the charging of the solution is very rapid, jet initiation occurs before the voltage alternates. The jet contains positive and negative charged which reduces the repulsive forces and bending instability in the jet. Therefore, using AC the fibers are larger when compared to DC at the same voltage. In AC, there is reduced accumulation of like charges on the deposited fiber. Therefore, thicker layers of electrospun fibers can be achieved, especially when using an insulating collector (Ramakrishna 2005).

Notably, using sharp, pointed needles, i.e. capillary tips results in more efficient charging of the solution. The tip diameter is also an important consideration. Practically, the tip diameter selection is important in avoiding needle clogging due to solvent evaporation. Smaller internal diameters have been observed to reduce beading and reduce the diameter of the fibers (in some cases). As the internal diameter decreases, the surface tension increases and a greater electrostatic force is required for jet initiation leading to smaller fibers. Therefore, the smallest tip that facilitates extrusion of the solution is generally selected. Generally, electrospinning is performed with 16G–27G needles (Andrady 2008; Ramakrishna 2005).

In more complex setups, additional electrodes can be added to tune fiber deposition (Teo and Ramakrishna 2006; Teo et al. 2011). These auxiliary electrodes can be base electrodes, steering electrodes, focusing electrodes, and guiding electrodes (Figure 1.2). The base electrode is usually a conductive plate placed parallel to the collector at the needle tip to improve the uniformity of the electric field and minimize the effect of surrounding objects on the electric field. Since the base electrode increases the stability of the jet, fibers with smaller diameters have been observed. The base electrode should be level with the needle top. Notably, using a base electrode, a higher applied voltage is required to initiate spinning (Teo and Ramakrishna 2006; Teo et al. 2011). Focusing electrodes are used to damp the whipping of the electrostatic jet to achieve more localized deposition. The electrodes are ring‐shaped, cylinder, or conical and placed close to the needle tip. Multiple focusing electrodes can be used to reduce the spread of the fiber deposition. Electrodes with 400 μm diameter holes resulted in ~200 μm diameter nanofiber patches of randomly oriented nanofibers (Teo and Ramakrishna 2006; Teo et al. 2011). Steering electrodes are used to align the electric field in the vicinity of the collector. For example, a pair of parallel electrodes placed near the collector can be used to achieve uniaxially aligned nanofibers. Multiple pairs of steering electrodes are necessary to achieve more complex patterns (Teo and Ramakrishna 2006; Teo et al. 2011).

Figure 1.2 Schematic of various electrodes used to control the electrospinning process.

Source: Adapted from Teo et al. (2011).

The collector influences the electric field and is also an important factor in the electrospinning process (Teo and Ramakrishna 2006; Andrady 2008; Ramakrishna 2005; Teo et al. 2011). The simplest collector is a stationary metal plate placed at a fixed distance from the tip. The fibers generally collect as a symmetric circular batch of nanofibers on the plate. Since the plate is grounded, the residual charges on the deposited fibers are dissipated and the mat has high areal density. Moving the collector surface during processing provides some control in the areal density (Andrady 2008). Collectors with grids or charged needles can be used to create patterned nanofiber membranes which consist of regions of high‐ and low‐fiber density. Low‐fiber density occurs in regions where the collector is insulated. Another common collector is a rotating metal drum/mandrel. The rotating surface leads to an even deposition of fibers and a uniform nanofiber mat. The rotating drum can further stretch the fiber leading to reduced diameters as well as introduce alignment of nanofibers. When using high boiling point solvents, e.g. DMF, a rotating collector can provide a longer time for the solvent to evaporate to prevent fiber fusing. Combining electrospinning and mechanical drawing by collecting on a rotating mandrel can affect fiber size. For example, the diameter of PEO fibers spun from chloroform could be reduced from ~1600 to 600 nm by increasing the velocity of the rotating drum (Ogata et al. 2007). Rotating mandrels are also often used to make tubular constructs for potential application as vascular grafts. For tubular constructs, the wall thickness could be controlled linearly with electrospinning time (Teo and Ramakrishna 2006; Andrady 2008; Ramakrishna 2005; Teo et al. 2011).

The material of the collector is also an important consideration that affects the packing density. When nonconductive materials are used as the collector, charge accumulates, and fewer fibers are deposited resulting in lower‐packing densities when compared to fibers collected on conductive surfaces. Even when using conductive collectors, nonconductive behavior can be observed as the fibers (insulating) collect. Sensitivity analysis indicates that the dielectric properties and surface area of the collector are dominant variables that influence fiber diameter and fiber spacing (porosity). Using an auxiliary electrode supplied with AC voltage minimizes the effect of the material on the collector because it reduces the residual change on the deposited fibers. Collecting in a liquid has also been reported and significantly affects the fiber morphology. The choice of liquid can affect the surface characteristics of the fiber (Ramakrishna 2005).

The porosity of the collector also effects fiber deposition. Fibers collected on metal meshes had lower‐packing densities than smooth surfaces. This effect has been attributed to increased evaporation rate when using a porous collector. As the fibers dry faster, the residual charges persist and repel subsequent fibers. Notably, the topography of the deposited fiber mat will follow the texture of the collector (Ramakrishna 2005). Deposition of two‐dimensional patterned structures or three‐dimensional structures has also been observed. Honeycomb and dimpled structures have been observed using insulating collectors. Two‐dimensional and three‐dimensional patterning is attributed to charge repulsion of deposited fibers. The fibers of the three‐dimensional structures are loosely packed and easily compressed. The conditions to form such three‐dimensional structures are not well understood (Teo et al. 2011).

Controlling fiber deposition to achieve fiber patterning can be achieved using gap electrodes or open frame collectors. The two parallel electrodes cause the electrostatic field lines in their vicinity to align perpendicular to the edges of the electrodes. The jet aligns with the field lines and deposits uniaxially aligned nanofibers. Charge repulsion of the deposited fibers limits collection of aligned fibers to ~minutes so that samples of thick aligned fibers are difficult to achieve. Arrays of multiple electrodes have been used to achieve more complex patterns, e.g. orthogonal fibers (Teo and Ramakrishna 2006; Andrady 2008; Ramakrishna 2005; Teo et al. 2011).

Alternatively, fibers can be aligned by collecting on a rotating mandrel. The fibers align along the circumference of the mandrel. Typically, high rates of rotation ~1000 rpm are used. To achieve alignment, the rotation of the mandrel must be faster than fiber deposition so that the fibers are taken up on the surface of the mandrel and wound rather than randomly deposited. By replacing a solid mandrel with a wire drum, alignment can be achieved at much lower rates of rotation ~1 rpm. In the case of a wire drum, the fibers are thought to align due to the electric field profile created by the parallel wires. Use of a thin disk with a sharp edge as a collector provides more control of the electrostatic field to align fibers. The electrostatic field lines concentrate toward the knife‐edge and the jet tends to follow the direction of the electric field. As the disk rotates (~1000 rpm), the fibers wind continuously along the knife‐edge with a pitch of 1–2 μm (Ramakrishna 2005). To improve the alignment, the fibers must be collected before the onset of the whipping instability. Auxiliary electrodes can be used to suppress the whipping instability (Carnell et al. 2008). Alternatively, using solvents with low dielectric constants and high purity can suppress the whipping instability (Ogata et al. 2007). Practically, the highly aligned fibers are achieved for a short period of time ~ minutes after which alignment decreases, which may be attributed to fiber repulsion due to charge accumulation. Therefore, for complex patterns, mechanical drawing techniques that avoid the electric field and whipping stability are preferable (Nain and Wang 2013).

Nanofiber yarns have also been of interest. To produce yarns, electrospun nanofibers can be deposited on water. As the nanofibers are lifted off the water, the surface of tension bundles the fibers into a yarn. Collecting on water flowing in the form of a vortex is a means to achieving continuous yarn production. The disadvantage of this approach is that the yarn must then be dried. Self‐bundling nanofibers have been achieved using AC power. The jet splits and contains both negative and positive segments which bundle together midflight. Twisting the fibers can improve yarn strength. Such twisting can be achieved by collecting on two parallel ring electrodes and rotating one of the rings. Although simple, the length of the yarn is limited using this approach (Abbasipour and Khajavi 2013).