Further, the polymer skin can also clog the nozzle 106, necessitating pausing the electrospinning process for cleaning the nozzle. For example, when pausing the operation of solution electrospinning, the flow of polymer solution through the nozzle is typically stopped. The polymer solution at the nozzle tip can dry out and the polymer precipitate can form a polymer skin or a clog, which blocks or seals the nozzle tip. Once blocked, it can be challenging to unblock the nozzle tip, and typically it may require changing the nozzle for resuming the operation, which can result in a waste of time/material and prevent automation/scaleup. In some circumstances, the fiber membrane or film may also have local defects from residual solvent interfering with membrane formation.

Additionally, as in melt electrospinning, conventional solution electrospinning also has a large operating distance (e.g., about 15-20 cm) between the nozzle 106 and the substrate 116. Thus, the resulting fiber membrane or film also tends to have non-uniform thickness. Similarly, the large operating distance can complicate scaling up as placement of multiple nozzles close to each other can negatively affect electric field and jetting stability.

Many of the shortcomings of the conventional melt electrospinning and solution electrospinning can be overcome by the heated solution electrospinning technology described herein.

Example Heated Solution Electrospinning System

FIG. 2 depicts an example setup of a heated solution electrospinning device, apparatus, or system 200, according to certain aspects of disclosed technology. Similar to the system 100, the heated solution electrospinning system 200 includes a high voltage power supply 202, a reservoir 204 containing a polymer solution 220, a nozzle 206 configured to receive the polymer solution 220 from the reservoir 204, and a collector 210 configured to hold a substrate 216.

In some examples, the system can also include a pump 208 configured to pump the polymer solution 220 from the reservoir 204 into the nozzle 206, and further extrude the polymer solution 220 out of the nozzle 206. The pump 208 can be any devices configured to apply a hydraulic or pneumatic pressure to the polymer solution 220 contained in the reservoir 204. In some examples, the pump 208 can be connected to a pressure controller 230C, which is configured to apply a desired pressure to the pump 208 so as to extrude the polymer solution 220 through an orifice or opening 218 at a tip portion 205 of the nozzle 206. The applied pressure can be determined based on the size/length of the nozzle 206 and the viscosity of the polymer solution 220. In one specific example, when the polymer solution 220 includes 25-35 wt. % of PCL dissolved in DMF and a ¼-inch nozzle 206 of 32 gauge is used, the applied pressure can vary between 0.15 and 0.30 bar pressure.

As described herein, the polymer solution 220 can be formed by dissolving a polymer in a solvent. The polymer solution 220 can be heated to a higher temperature to increase the solubility and concentration of the dissolved polymer. As in solution electrospinning, the solvent used in heated solution electrospinning is volatile (i.e., the solvent can vaporize at the room temperature sufficiently rapidly so that fibers are formed). For example, the polymer used in the polymer solution 220 can be polycaprolactone and the corresponding solvent can be dimethylformamide. Other polymers and solvents can also be used. In some examples, the polymer solution 220 can include a small amount (e.g., up to 0.4-1 wt. %) of water. Water can be added to the polymer solution 220 through hydration of the pure solvent, and/or from air in contact with the polymer solution 220 as it exits the nozzle 206. Small amount of water content in the polymer solution 220 can change the conductivity of the polymer solution 220 and affect electrospinning quality.

The power supply 202 can be either direct current (DC) or alternating current (AC). Both the nozzle 206 (at least the tip portion 205) and the collector 210 can comprise conductive materials such as metals. For example, the nozzle 206 can comprise stainless steel or have a metal/conductive coating. The power supply 202 can apply a high voltage (also referred to as “accelerating voltage”) between the nozzle 206 and the collector 210. The accelerating voltage can range from 5 kV to 50 kV, inclusive (e.g., about 15 kV in one specific example). In some examples, the nozzle 206 can be positively charged and the collector 210 can be grounded, or vice versa. In some examples, a differential voltage can be applied between the nozzle 206 and the collector 210, while neither the nozzle 206 nor the collector 210 is grounded.

In some examples, the power supply 202 can be connected to a voltage controller 230A which is configured to program and/or adjust the accelerating voltage, thereby generating an electrostatic field between the tip portion 205 of the nozzle and the substrate 216.

Similar to solution electrospinning, at a sufficient voltage differential between the tip portion 205 and the collector 210, the electrostatic force acting on the polymer solution 220 can overcome the surface tension and generate a Taylor cone 212 at the tip portion 205, from which a charged polymer jet 214 is drawn. Rapid evaporation of the solvent consolidates the polymer jet 214 into a fiber 222. Electrostatic repulsion and bending instability can cause chaotic whipping of the fiber 222, leading to fiber drawing and deposition on the substrate 216 with a random orientation.

As shown in FIG. 2, the heated solution electrospinning system 200 can further include a heating mechanism configured to maintain the polymer solution 220 in the nozzle 206 at a predetermined operating temperature. For example, the heating mechanism can be configured to heat the polymer solution 220 contained in both the reservoir 204 and the nozzle 206 to the operating temperature.

As described herein, the operating temperature for heated solution electrospinning is selected to be higher than the room temperature and lower than a boiling temperature of the solvent (thus, there is no boiling bubble of solvent at the operating temperature). In some examples, the operating temperature can be configured to be below a melting point of the polymer. For example, when the polymer comprises polycaprolactone (which has a melting point of 60° C.) and the solvent comprises dimethylformamide (which has a boiling point of 153° C.), the operating temperature can be set within a range between 40° C. and 100° C., inclusive. In some examples, the operating temperature can be set between 40° C. and 60° C., inclusive. In one specific example, the operating temperature can be set between 40° C. and 50° C., inclusive.

As described above, the polymer solution 220 can be prepared by dissolving the polymer of interest in a corresponding solvent. By heating the polymer solution 220, the viscosity of the polymer solution 220 can be reduced. Additionally, heating the polymer solution 220 allows the polymer to be dissolved at a higher concentration (e.g., compared to the polymer solutions used in conventional solution electrospinning). For example, heating the polymer solution 220 can allow the polymer concentration (measured in weight percentage, or wt. %) to be varied from few wt. % all the way up to 100 wt. % (e.g., as practiced in melt electrospinning). In one specific example, the polymer has a concentration between 10-50 wt. %, inclusive, in the polymer solution 220 at the operating temperature. Such polymer concentration ranges can be helpful as they allow substantial decreases in the viscosity of the heated polymer solution 220, which aids its thinning into nanoscale fibers, as described further below.

In the depicted example, the heating mechanism includes a heating container 240 which encloses the reservoir 204 and a body portion 207 of the nozzle 206. The temperature inside the heating container 240 can be adjusted so as to heat the polymer solution 220 inside the reservoir 204 and the nozzle 206 to the operating temperature. In one example, heating of the polymer solution 220 can be achieved via conductive coupling to the reservoir 204 and/or the nozzle 206 (e.g., when the reservoir 204 and/or the nozzle 206 are made of heat conductive materials). In other examples, heating of the polymer solution 220 can be achieved by other heat transfer mechanisms, such as convection or radiation.

In other examples, the heating mechanism can be configured to limit the heating to the polymer solution 220 contained in the nozzle 206. For example, the heating container 240 can be configured to enclose the nozzle 206 but not the reservoir 204. In such cases, the unheated polymer solution in the reservoir 204 can be heated to the operating temperature when received in the nozzle 206.

In some examples, the heating mechanism can be configured to maintain the polymer solution 220 at the operating temperature (T) by using a PID (i.e., proportional, integral, and derivative) temperature control mechanism. For example, a thermostat or other types of temperature sensors can be configured to measure the temperature of the polymer solution 220 and a heat controller 230B can be configured to control the heating container 240 to ensure the temperature of the polymer solution 220 remains as close as possible to the operating temperature (T).

In some examples, the heating mechanism can be configured to vary the operating temperature (T) within a predefined temperature range (e.g., between 40° C. and 60° C., etc.) of during the electrospinning process so as to change the evaporation rate of the solvent. Generally, increasing the operating temperature can increase the evaporate rate of the solvent, and vice versa. Variation of the operating temperature can be linear or nonlinear. In some examples, the pattern of operating temperature variation during the electrospinning process can be user defined. Changing the evaporation rate of the solvent can affect the properties of the produced fibers. For example, slower solvent evaporation allows the polymer solution to stretch and solidify more gradually, leading to thinner and more uniform fibers, whereas faster evaporation can result in thicker fibers and irregular morphologies due to rapid solidification. Slower evaporation can also result in a more porous structure than faster evaporation. Further, controlled evaporation helps maintain certain concentration gradients in the spinning solution, leading to desired fiber properties. Additionally, varying the operating temperature can affect fiber cooling/solidification, thereby controlling degree of fusion between fibers. Thus, by varying the operating temperature while electrospinning the polymer solution, the heated solution electrospinning system 200 can tweak solvent evaporation and fiber solidification dynamics without the need of changing composition of the polymer solution (e.g., using different solvent mixtures, etc.), as typically used in conventional solution electrospinning systems.

As shown in FIG. 2, the tip portion 205 of the nozzle can extend out of the heating container 240. The axial length (L) of the tip portion 205 extending out of the heating container 240 is desirably within a predefined range. On one hand, the axial length (L) of the tip portion 205 is configured to be sufficiently small so as to reduce the temperature drop when the heated polymer solution 220 moves out of the body portion 207 and passes through the tip portion 205. On the other hand, the axial length (L) of the tip portion 205 is configured to be sufficiently large so as to ensure field lines of the electrostatic field between the tip portion 205 and the collector 210 (or substrate 216) has a desired density and uniformity. In certain examples, the axial length (L) of the tip portion 205 is between 0.5 mm and 3 mm, inclusive (e.g., 2 mm).

As described above, heating can reduce the viscosity of the polymer solution 220, thus allowing the heated polymer solution to flow through a very small opening 218 of the nozzle, thereby producing very small diameters of the resulting fibers. For example, the opening 218 of the nozzle can have a sufficiently small inner diameter so as to reduce the diameter of the ejected polymer jet 214, thus ensuring the resulting fibers 222 have a small diameter (e.g., in the nanometer scale). In certain examples, the opening 218 of the nozzle has an inner diameter between 50 μm and 1 mm, inclusive. In one specific example, the tip portion 205 can be a 32-gauge needle with an opening diameter about 108 μm. In some examples, the diameters of the resulting PCL fibers 222 can range between 100 nm and 1000 nm, inclusive. In one specific example, the diameters of the resulting PCL fibers 222 can range between 100 nm and 400 nm, inclusive.

Under the electrostatic field, the charged polymer jet 214 can be drawn toward the substrate 216 situated at an operating distance (D) from the tip portion 205 of the nozzle. As described above, the fiber 222 can be formed from the charged polymer jet 214 after evaporation of at least some of the solvent and collected on the substrate 216. In some circumstances, residual solvent in the collected fibers 222 can facilitate inter-fiber bonding to form a fiber mesh or fiber membrane (or fiber film). The increased polymer concentration in the heated polymer solution 220 can also reduce or even eliminate whipping instability, presumably due to the fact that as the polymer solution cools down (e.g., to room temperature or below), the solution is already viscous enough to prevent breaking of the charged polymer jet 214 or reflow of fibers after deposition. Higher concentration of dissolved polymer also allows the polymer solution 220 to have less solvent, which can also improve robustness of fiber forming process.

As described above, one problem associated with conventional solution electrospinning is the formation of a polymer skin adjacent the opening of the nozzle, which can prevent stable jetting into thin fibers. However, one unexpected and surprising benefit of the heated solution electrospinning technology described herein is that the formation of polymer skin near the opening 218 of the nozzle 206 can be avoided. It is hypothesized that the heating can eliminate the formation of polymer skin by mitigating the local cooling effect of the polymer solution 220 (due to fast solvent evaporation), and/or by reducing the viscosity of the polymer solution 220.

Eliminating the skin formation near the opening 218 of the nozzle can have a number of advantages. For example, the absence of the skin surrounding the nozzle opening can cause less disturbance to the charged polymer jet 214. As a result, fiber diameter and/or porosity of the resulting fiber membrane or fiber film can be more homogeneous. For instance, one experiment shows that the resulting fiber membrane or fiber film (formed by fibers having a diameter between 100 nm and 400 nm) can have a porosity about 50-90% with pore sizes of less than 2 μm. Additionally, the absence of skin formation can also reduce or eliminate the risk of clogging of the opening 218, thus enabling continuous operation of the electrospinning without the need of intermittent pauses (to clean the nozzle).

In some examples, the heated solution electrospinning system 200 can be used to unclog the nozzle 206 if the nozzle 206, for whatever reason, is at least partially clogged by a polymer precipitate. For example, the nozzle 206 may be previously used for electrospinning a polymer solution (which can be the same polymer solution 220 used for heated electrospinning or another polymer solution containing a different polymer used for conventional solution electrospinning). After temporarily pausing the electrospinning, the polymer solution at the tip portion 205 can dry out (e.g., due to evaporation of the solvent) and the polymer in the polymer solution can precipitate and partially/fully clog the nozzle 206. To unclog the nozzle 206, the nozzle 206 can be heated (e.g., by the heating container 240 or another heating apparatus outside the system 200), at least temporarily, to a melting temperature that is above the melting point of the polymer forming the polymer precipitate. As a result, the polymer precipitate can be melted to become a polymer melt, which can be pressured or flushed out of the nozzle 206 through the opening 218 (e.g., by applying a pressure through the pump 208). After the nozzle 206 is unclogged, it can be reused for heated or conventional solution electrospinning. For example, the heating container 240 can maintain the nozzle 206 at the operating temperature (e.g., between 40° C. and 100° C., inclusive) while electrospinning the polymer solution. The operating temperature can be less than, equal to, or greater than the melting temperature used for unclogging the nozzle.

Moreover, the heated solution electrospinning system 200 allows evaporation of the solvent and deposition of the fiber over a shorter operating distance (D). Specifically, higher temperature of the polymer solution 220 and the resulting higher concentration of dissolved polymer can reduce the initial amount of solvent in the polymer solution, cause faster solvent evaporation and faster increase of polymer viscosity as it quickly cools. As a result, the heated solution electrospinning described herein can lead to faster fiber solidification and reduction of operating distance compared to conventional solution electrospinning technologies.

In any of the examples described herein, the operating distance (D) for the heated solution electrospinning can be significantly smaller than the operating distance achievable by the conventional solution electrospinning. For example, the operating distance for the system 100 of FIG. 1 is typically in a range between about 10-20 cm. In contrast, the operating distance (D) for the heated solution electrospinning as depicted in FIG. 2 can be less than 30 mm. For example, the operating distance (D) in heated solution electrospinning can be reduced to 5 mm, or even as short as 1 mm. In one specific example, the operating distance (D) is between 10 mm and 20 mm, inclusive. As a result, the collection of the fibers 222 on the substrate 216 can be very focused, e.g., the fibers 222 can be deposited in a localized spot about 1 mm-10 mm in diameter on the substrate 216).

One unexpected and surprising benefit of reduced operating distance (D) is the improved robustness of the electrospinning process against changing operational and/or environmental conditions. As described above, conventional solution electrospinning is sensitive to changes in operational conditions (e.g., accelerating voltage, pump pressure, etc.) and/or environmental conditions (e.g., temperature and/or humidity of the media between the nozzle and the collector, etc.). In contrast, the reduced operating distance (D) for the heated solution electrospinning can significantly improve the reproducibility and uniformity (e.g., in terms of fiber diameter, porosity, etc.) of the resulting fiber membranes or fiber films, presumably due to higher electric field gradient at a given accelerating voltage. For example, test results show that, when D is between 10 mm and 20 mm, changing the applied voltage from 10 kV to 20 kV or varying the environmental temperature by as much as 10° C. has negligible effect on physical characteristics of the resulting fibers. Experimental results also show that flow rate can be varied in a relatively broad range (factor of >2), e.g., by applying between 0.15-0.3 bar pneumatic pressure to the polymer solution, without causing detrimental effect on the quality of the produced fibers. Additionally, the heated solution electrospinning is less sensitive to ambient humidity than conventional solution electrospinning, presumably due to the fact that relative humidity (RH) drops quickly as the ambient air is heated in the vicinity of the nozzle and jet, resulting in both lower humidity and reduced humidity variation (e.g., when the ambient temperature is at 20° C. and the humidity is varied from 30% to 80% RH, the same air heated to 40° C. would have only 5-15% RH variation).

In some examples, the collector 210 (and the substrate 216 retained thereon) and the nozzle 206 can be configured to be movable relative to one another. In one example, the nozzle 206 can be stationary whereas the collector 210 can be motorized and its movement can be controlled by a motion controller 230D. In another example, the collector 210 can be stationary whereas the nozzle 206 can be configured to be movable (e.g., under the control of a motion controller similar to 230D) relative to the collector 210. The relative movement between the collector 210 and the nozzle 206 can be multi-directional. In one example, the collector 210 can be translated in an X-Y plane that is perpendicular to the longitudinal axis (or Z-axis) of the nozzle 206. In another example, the collector 210 can be moved along the Z-axis (e.g., moving closer to or farther away from the opening 218 of the nozzle 206). In yet additional examples, the collector 210 can be configured to rotate (e.g., in the X-Y plane) relative to the nozzle 206.

The controlled movement of the collector 210 and/or the nozzle 206 allows the fibers 222 to be deposited onto the substrate 216 into homogenous fiber membranes or films (e.g., with consistent fiber diameters, porosity, and/or pore sizes) of desired sizes.

In some examples, the electrical properties of the substrate 216 can be varied to impart specific properties on the fiber membranes of films deposited thereon. The electrical properties of the substrate 216 can directly affect the quality of the collected fibers 222 due to the difference of electrical charge accumulation or relaxation of the fibers 222 upon deposition. For example, the substrate 216 on which the fibers 222 are collected can comprise an electrically insulating material (e.g., glass, plastic, etc.), an electrically semiconductive material (e.g., silicon wafer, etc.), and/or an electrically conductive material (e.g., metals such as aluminum, aluminum foil, stainless steel, etc., or conductive polymers). In some examples, different parts of the substrate 216 can comprise different materials so as to allow local control on electrospun membrane.

Thus, by varying the electrical properties of the substrate 216, the same operational conditions of electrospinning may lead to different membrane properties. For example, FIG. 3 shows SEM images (magnification 1000×) of two different fiber membranes 310, 320 formed when the substrate 216 comprises a glass and a metal, respectively. As shown, when electrospinning on glass, thinner and more straight fibers can be obtained, and the fiber fusion can be reduced. As a result, the membrane 310 has a higher membrane porosity and a larger average pore size (compared to the membrane 320). However, at the same operating conditions but when electrospinning on metal, the collected fibers can have a larger diameter and be more wavy. Additionally, the fibers can arrive the substrate less solidified and thus have more inter-fiber fusion. As a result, the membrane 320 has a lower porosity and smaller average pore size (compared to the membrane 310), suggesting better barrier properties. Thus, the selection of the substrate material can be used to further optimize the membrane properties for specific applications.

In some examples, the formed fiber membranes or films can have uniform or varied thickness. Specifically, local thickness of the fiber membrane can be controlled by varying or adjusting translation path of the collector 210 or substrate 216 while electrospinning the heated polymer solution. In some examples, the thickness of the formed fiber membranes can vary from a few hundred nanometers to tens of micrometers or more, depending on the number of programmed deposition passes over the same location.

To illustrate, FIG. 4 schematically depicts two example membranes 410, 420 that can be fabricated using the heated solution electrospinning system 200. The membrane 410 is produced when translation of the substrate relative to the nozzle causes the polymer jet to deposit along a plurality of parallel deposition lines 412 with uniform spacing. The resulting membrane 410 has a uniform thickness (H) because the same amount of fiber is formed across the deposition lines 412. In contrast, the membrane 420 is produced when translation of the substrate relative to the nozzle causes the polymer jet to deposit along a plurality of parallel deposition lines 422 with progressively reduced spacing. The thickness of the resulting membrane 420 progressively increases from H2 to H1 because more fiber is locally formed as the density of deposition lines 422 increases.

Thus, by allowing relative movement between the collector 210 and the nozzle 206, the heated solution electrospinning system 200 can be used to 3D print polymer structures (e.g., by controlling the membrane thickness locally and/or introducing gradients of thickness of the structures). As an example, one test shows that one nozzle can deposit a fiber membrane of 25 mm×25 mm in planar size and 10 μm of thickness with nanofibers of 150 nm or 300 nm in diameter in about 10-15 minutes.

Additionally, the speed and pattern of motion of the collector 210 relative to the nozzle 206 can affect the fiber properties and deposition spot size. For example, faster motion of the collector 210 can result in smaller spot size, presumably due to less charge accumulation on the collector 210 when faster passes are made, thus causing less fiber repulsion and narrower deposition spot (e.g., less fiber repulsion allows fibers to land closer to the original jet trajectory).

Moreover, the smaller operating distance (D) also allows the usage of an array of nozzles to increase the throughput efficiency. In conventional solution electrospinning, placement of multiple nozzles close to each other can negatively affect jetting stability because the neighboring electrostatic fields can interfere with each other over a relatively long operating distance. By reducing the operating distance (D), a plurality of nozzles (similar to 206) can be used simultaneously for electrospinning of heated polymer solution because the likelihood of interference between neighboring electrostatic fields is reduced by confining the electrostatic field within the smaller gap between the nozzles and the substrate. The plurality of nozzles can be arranged in one dimensional (e.g., along X or Y axis) or in two dimensional (e.g., in both X and Y axes). The nozzle spacing, i.e., the distance between two adjacent nozzles, can be between 50% and 250%, inclusive, of the operating distance (D). For example, the smallest nozzle spacing can be about 5 mm for D=10 mm, or 10 mm for D=20 mm. In one specific example, when the operating distance (D) is varied from 10 mm to 20 mm, the nozzle spacing between array of nozzles can be set to 25 mm without observable interference between neighboring electrostatic fields.

Straight jet with no or minimal whipping along its trajectory is a distinctive feature of the disclosed heated electrospinning compared to standard (non-heated) solution electrospinning. Experimental results shows that whipping might be observed very locally just above the collector (e.g., 1 mm or less). This local whipping is arguably due to accumulated charges on collector. This straight jet trajectory suggests that spacing could be significantly reduced without jet interference compared with the nozzle spacing used.

When multiple nozzles are used, the polymer solution feeding each nozzle can be provided by the same reservoir (e.g., 204) or multiple reservoirs (e.g., each nozzle can receive the polymer solution from a designated reservoir). In some examples, all nozzles and/or reservoirs can share and be heated by the same heating container (e.g., 240). Alternatively, each nozzle and the corresponding reservoir can be enclosed within a corresponding heating container.

As an example, FIG. 5 schematically depicts a heated solution electrospinning system 500 including multiple nozzles. As shown, the heating container 240 can enclose and heat multiple reservoirs 204a, 204b, . . . , 204n, which are in fluid communication with respective nozzles 206a, 206b, . . . , 206n. The polymer solution 220 in each reservoir can be individually pressurized by respective pumps 208a, 208b, . . . , 208n to control the flow rate. In the depicted example, the pressure controller 230C is configured to apply equal pressure to all pumps 208. In other examples, different pressure control can be applied to different pumps.

Although FIGS. 2 and 5 shows multiple controllers (e.g., 230A, 230B, 230C, and 230D), it should be understood that any number of the depicted controllers can be integrated within one controlling module. For example, a unitary controller can be configured to control the applied voltage between the nozzle 206 and the collector 210, the pressure applied to the pump 208, the operating temperature of the heating container 240, and/or the relative movement between the collector 210 and the nozzle 206.

Example Methods for Heated Solution Electrospinning

FIG. 6 is a flowchart describing an overall method 600 for heated solution electrospinning, which can be performed, for example, by using the heated solution electrospinning system 200 of FIG. 2.

- At 610, a polymer solution can be heated to an operating temperature. The operating temperature can be greater than or equal to 40° C. The polymer solution includes a polymer dissolved in a solvent at the operating temperature.
- At 620, the heated polymer solution can be electrospun as a charged jet escaping from a nozzle, e.g., under an electrostatic field established between the nozzle and a substrate.
- At 630, a polymer fiber can be collected on the substrate. The polymer fiber can be formed from the charged jet after evaporation of at least some of the solvent.

FIG. 7 is a flowchart depicting a specific example method 700 for heated solution electrospinning.

- At 710, a polymer solution including PCL dissolved in DMF is prepared.
- At 720, the polymer solution can be heated to an operating temperature that is between 40° C. and 100° C.
- At 730, the heated polymer solution can be fed to a nozzle.
- At 740, an electrostatic field between a tip portion of the nozzle and a substrate spaced apart from the tip portion of the nozzle can be applied. The electrostatic field is configured to draw a charged jet of the polymer solution out of the tip portion of the nozzle toward the substrate.
- At 750, a fiber can be collected on the substrate. The fiber is formed from the charged jet after evaporation of at least some of the DMF.

FIG. 8 is a flowchart depicting an example method 800 for unclogging a nozzle.

- At 810, a polymer solution is provided to a nozzle which is at least partially blocked by a polymer precipitate. For example, the polymer solution can contain a polymer dissolved in a solvent. The nozzle can be previously used for electrospinning the polymer solution. After temporarily pausing the electrospinning, the solvent may evaporate, and the polymer can precipitate and clog the opening.
- At 820, the nozzle can be heated (at least temporarily) to a melting temperature (e.g., above the melting point of the polymer) that is sufficient to melt the polymer precipitate to a polymer melt.
- At 830, the polymer melt can be pressured out of the nozzle through the opening so that the nozzle is unclogged.
- At 840, the polymer solution can be electrospun as a charged jet escaping from the nozzle. At this step, the polymer solution can be maintained at an operating temperature (e.g., between 40° C. and 100° C., inclusive) while electrospinning the polymer solution.
- At 850, a polymer fiber can be collected on a substrate. The polymer fiber can be formed from the charged jet after evaporation of at least some of a solvent in the polymer solution.

Example Operating Variables

The table below lists example ranges and specific examples of certain operating variables that can be used in heated solution electrospinning described herein.

Variables
Example Ranges
Specific Example(s)

Polymer
Natural or synthetic
PCL, PLA, PLGA,

organic polymers
PDO, etc.

Solvent
Volatile solvent
DMF, HFIP, TFE,

DMSO, etc.

Operating
Above room
40° C.-100° C.

temperature (T)
temperature and

below boiling

temperature of

solvent

Operating distance
5 mm-30 mm
10 mm-20 mm

(D)

Applied voltage
5 kV-50 kV
10 kV-20 kV

Pump pressure
0.05-1.00 bar
0.15-0.30 bar

Tip portion length
0.5 mm-3 mm
1-2 mm

(L)

Nozzle opening
50 μm-1 mm
108 μm (32 gauge)

diameter

Fiber diameter
100 nm-1000 nm
100-200 nm, 200-400 nm

Fiber membrane
20%-95%
70%-90%

porosity

Fiber membrane
<5 μm
<2 μm

pore size

Environmental
15° C.-35° C.
19° C.-22° C.

temperature

Environmental
0-100% RH
20-40% RH

humidity

Solution
10-100 wt. %
25-35 wt. %

concentration

Translation speed
0.1-20 mm/s
1-5 mm/s

Example Advantages

A number of advantages can be achieved via the technologies described herein. As described above, conventional solution electrospinning generally lacks reproducibility due to its sensitivity to multiple operational and environmental conditions, whereas conventional melt electrospinning cannot produce thinner nanofibers that are achievable with solution electrospinning. Further, in both melt electrospinning and solution electrospinning, the relatively large operating distance can result in non-uniform thickness of the resultant fiber membrane or film and complicate scaling up of electrospinning by using multiple nozzles. The heated solution electrospinning technology described herein overcomes many of the above-mentioned disadvantages of the conventional melt and solution electrospinning technologies.

Specifically, by heating the polymer solution to a predefined operating temperature, the heated solution electrospinning can eliminate the skin formation near the opening of the nozzle and reduce the operating distance. Preventing skin formation can eliminate the need for regular cleansing and maintenance of the nozzle. Even with a previously clogged nozzle, the heating can automatically unclog the nozzle. Due to smaller operating distance, the resulting fibers can be uniformly thin (e.g., consistently between 100 nm and 300 nm in diameter) and precisely controlled. As a result, a number of unique advantages can be achieved. For example, the resulting fiber membrane or fiber film can have fewer defects, have homogenous fiber diameters, have consistent and/or controllable membrane thickness, and have controllable fusion between fibers (e.g., by varying operating temperature while electrospinning the polymer solution).

Importantly, this heated solution electrospinning approach can produce high-quality polymer membranes with improved consistency and process stability over a wide range of operating and/or environmental conditions, as described above. Improved process stability and reproducibility can positively impact the quality of the collected fibers and allows for the automation of this technique.

Further, the heated solution electrospinning system described herein is scalable by incorporating multiple nozzles (without affecting the stability of the electrospinning process). Automation of relative movement between the collector and the nozzle(s) can further convey additional advantages, such as controlling the membrane thickness locally and introducing gradients of thickness (e.g., by adjusting translation path of the collector while electrospinning the heated polymer solution), thereby allowing precise 3D printing of polymer structures.

Example Computing Systems

FIG. 9 depicts an example of a suitable computing system 900 in which the described innovations can be implemented. For example, the computing system 900 can be used in any of the controllers (e.g., 230A-230D) depicted in FIG. 2. The computing system 900 is not intended to suggest any limitation as to scope of use or functionality of the present disclosure, as the innovations can be implemented in diverse computing systems.

With reference to FIG. 9, the computing system 900 includes one or more processing units 910, 915 and memory 920, 925. In FIG. 9, this basic configuration 930 is included within a dashed line. The processing units 910, 915 execute computer-executable instructions, such as for implementing the features described in the examples herein. A processing unit can be a general-purpose central processing unit (CPU), processor in an application-specific integrated circuit (ASIC), or any other type of processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. For example, FIG. 9 shows a central processing unit 910 as well as a graphics processing unit or co-processing unit 915. The tangible memory 920, 925 can be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some combination of the two, accessible by the processing unit(s) 910, 915. The memory 920, 925 stores software 980 implementing one or more innovations described herein, in the form of computer-executable instructions suitable for execution by the processing unit(s) 910, 915.

A computing system 900 can have additional features. For example, the computing system 900 includes storage 940, one or more input devices 950, one or more output devices 960, and one or more communication connections 970, including input devices, output devices, and communication connections for interacting with a user. An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computing system 900. Typically, operating system software (not shown) provides an operating environment for other software executing in the computing system 900, and coordinates activities of the components of the computing system 900.

The tangible storage 940 can be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium which can be used to store information in a non-transitory way and which can be accessed within the computing system 900. The storage 940 stores instructions for the software implementing one or more innovations described herein.

The input device(s) 950 can be an input device such as a keyboard, mouse, pen, or trackball, a voice input device, a scanning device, touch device (e.g., touchpad, display, or the like) or another device that provides input to the computing system 900. The output device(s) 960 can be a display, printer, speaker, CD-writer, or another device that provides output from the computing system 900.

The communication connection(s) 970 enable communication over a communication medium to another computing entity. The communication medium conveys information such as computer-executable instructions, audio or video input or output, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media can use an electrical, optical, RF, or other carrier.

The innovations can be described in the context of computer-executable instructions, such as those included in program modules, being executed in a computing system on a target real or virtual processor (e.g., which is ultimately executed on one or more hardware processors). Generally, program modules or components include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functionality of the program modules can be combined or split between program modules as desired in various examples. Computer-executable instructions for program modules can be executed within a local or distributed computing system.

For the sake of presentation, the detailed description uses terms like “determine” and “use” to describe computer operations in a computing system. These terms are high-level descriptions for operations performed by a computer and should not be confused with acts performed by a human being. The actual computer operations corresponding to these terms vary depending on implementation.

Example Computer-Readable Media

Any of the computer-readable media herein can be non-transitory (e.g., volatile memory such as DRAM or SRAM, nonvolatile memory such as magnetic storage, can be implemented by storing in one or more computer-readable media (e.g., computer-readable storage media or other tangible media). Any of the things (e.g., data created and used during implementation) described as stored can be stored in one or more computer-readable media (e.g., computer-readable storage media or other tangible media). Computer-readable media can be limited to implementations not consisting of a signal.

Any of the methods described herein can be implemented by computer-executable instructions in (e.g., stored on, encoded on, or the like) one or more computer-readable media (e.g., computer-readable storage media or other tangible media) or one or more computer-readable storage devices (e.g., memory, magnetic storage, optical storage, or the like). Such instructions can cause a computing device to perform the method. The technologies described herein can be implemented in a variety of programming languages.