Electrospinning is a nanofiber fabrication technique that has grown in popularity due to its potential in numerous biomedical applications. The process uses an electrical charge to draw ultrafine fibers from liquid polymer solution to form a non-woven fiber scaffold. The polymer fiber diameters can range from millimeters to as small as nanometers in scale.

Figure 1. Electrospinning schematic. A high voltage is applied to a polymer solution dispensed from a syringe pump. Electrostatic forces stretch the droplet at the needle tip, and a thin jet of polymer is ejected from the cone, which dries and whips as it travels toward the grounded collector plate.

The primary elements of an electrospinning system include: a dispensing needle (or spinneret), a high-voltage power supply (5 to 50 kV), a syringe pump, and a grounded collector. The polymer solution in the syringe is extruded from the spinneret at a constant rate. As high voltage is applied at the spinneret, a cone is formed at the tip. This phenomena, known as the “Taylor cone”, is caused by the balance between electrostatic forces and the surface tension of the polymer. With sufficient voltage, electrostatic forces will overcome the surface tension, ejecting a thin jet of polymer from the spinneret (Figure 1). As the polymer stream travels to the collector, the solvent dries and the stream experiences whipping instabilities, which promotes the elongation and thinning of the fiber before deposition on the collecting surface.

The electrospinning hardware consists of three sub-systems: the gantry components, the syringe pump, and the high voltage power supply. Control of the three sub-systems is achieved using a personal computer (PC) running custom software developed in LabVIEW™.

Three axes of translational movement were required to adjust the position of the dispensing needle relative to the collecting surface. Each axis consists of a stepper motor that rotates a lead screw to move a plate along the track of a linear actuator. The x-axis and y-axis are coupled and move the collector plate, while an independent z-axis moves the dispensing needle.

The linear actuators have a lead screw, with a pitch of 10 turns per inch, rigidly attached to a mounting plate. The assembly lies inside a track enclosed on three sides. Each slide has adjustable limit switches on either end of travel to provide a soft emergency stop prior to the slide reaching a hard physical limit. The x- axis and z-axis slides have a travel distance of 15" and the y-axis slide has a travel distance of 20".

The stepper motors used in the electrospinning system each have an integrated driver and controller. The motors have a 1.8° full step resolution and are capable of generating microsteps as small as 1/256 of a full step. The encoder generates 1250 counts per revolution and allows the position of the gantry to be controlled and measured with a resolution of ~ 2 microns. The motor assembly is also programmable with 4kB of memory, and prewired to receive inputs from the limit switches at either end of the travel distance.

Figure 2. Sample trajectory for patterned spinning. The operator specifies the gap distances (Δx, Δy), speed, pause time (if desired), and coordinates for locations that require a pause or a change in direction.

An RS232 kit provides a communication link between the motors and the PC. The RS232 kit converts serial communication from the PC to an indexed RS485 protocol, which allows a single PC serial port to communicate with multiple stepper motors independently. A 36V power supply is used exclusively for the gantry motors. The power supply input is 120VAC, and it outputs 36V at 8.5A (300W).

Polymer solution is delivered to the energized needle with a high-precision syringe pump with dual syringes. The instrument can be programmed to operate as a single dispenser (one solution and one syringe), dual dispenser (two solutions in two separate syringes), or continuous dispenser (two linked syringes and one solution). The syringe pump uses positive displacement providing better than 99% volumetric accuracy. The step resolution is 48,000 steps per stroke of the pump, independent of syringe size.

The high voltage power supply provides the necessary voltage differential between the needle tip and the collector plate to eject the polymer fiber from the Taylor cone. The voltage required to spin fibers varies widely depending on the particular polymer and solvent combination; typical values from the literature range between 5 – 50 kV. Communication with the high voltage power supply was established with a data acquisition unit (DAQ), which receives commands from the LabVIEW™ interface and sends a scaled control signal to the high voltage supply. The high voltage output is connected to the needle using a charged adapter disc held in place with a setscrew, and the power supply ground is connected to the collector. A second wire with an alligator clip connects the dispensing needle to ground whenever the power supply is off, to ensure the system is fully discharged when not in use.

At any point after initialization, the gantry can be moved in x, y or z directions. From the interface, the operator enters a distance value in the dialog box and presses the control indicating the desired direction of movement. Once the gantry is initialized, the operator also has the option to load a spreadsheet with patterning parameters to instruct the gantry to move along a specified path. In a simple example of its application, the gantry can traverse a rectangular space by moving along one dimension, alternating direction with each row and pausing at intervals (Figure 2). The operator enters x and y distances values, which define the arbitrary coordinate grid containing the node locations of a pause or change in direction. Other input parameters include speed and time to run, which is linked to the syringe pump. Once the gantry has reached the last node specified in the routine, the program executes the path in reverse. The collector plate will run this forward and reverse sequence continuously until stopped by the operator or at the end of a pre-defined period of time.

This work was done by Bridget Endler, MS; Roy Dory, MS; Tony Yuan, MS; and Mauris DeSilva, PhD for the Naval Medical Research Unit. NRL-0066

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(reference NRL-0066) is currently available for download from the TSP library.

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