Focusing Electrodes

Various cylindrical electrode design. The design at the bottom give rise to the most stable spinning jet [Kim et al 2007].

The primary purpose of the focusing electrode is to restrict the deposition area of the electrospinning jet. The focusing electrode usually comes in the form of ring that is charged with the same polarity as the spinneret tip. However, it is also possible to use an insulating (non-conducting) cylinder [Ying et al 2005]. Deitzel et al used several ring electrodes to narrow the fiber deposition area from a diameter of 7 cm to 1 cm [Deitzel et al 2001]. However, if the applied voltage to the rings are not controlled properly, the electrospinning jet will either be repelled or deposit on the the ring electrodes.

Having a single ring [Buttafoco et al 2006] or cylindrical [Stankus et al 2004; Kim 2006; Kim et al 2006] electrode to restrict the deposition area is much simpler although the amount of control it can exert is more limited. Kim et al tested various cylindrical electrode design (See diagram) and found that the design that is opening wider away from the spinneret give rise to the most stable spinning jet [Kim et al 2007]. For purpose such as directing the electrospinning jet onto a tube collector or just to restrict the deposition location of the fibers, this setup is sufficient.

The main challenge with using the focusing electrode is to optimize the charges on it such that the electrospinning jet is not totally repelled from the collector but to direct the jet to deposit fibers on specific spots. With sufficient control and optimization, it is possible to narrow down the deposition area to about 0.5 mm. Huang et al (2010) demonstrated this capability using a Al-coated shadow mask to narrow the deposited electrospun fiber to a 600 µm diameter area. The material used was alginate with polyethylene oxide and the distance between the tip and the collector was brought 3 cm. Such close proximity is probably necessary for the electrospinning jet to easily "detect" the irregularity of the electric potential field. A similar setup was used by Santos et al (2014) to electrospin tin oxide precursors for deposition onto a sensor of diameter 10 mm. In their setup, the spinning needle protrudes from a copper plate and the masking plate is placed on the collector with a single hole in the middle where the sensor is placed. The distance between the needle tip and the sensor is 40 mm. They were able to accurately and precisely deposit the fibers on the sensor.

Multiple-ring electrodes [Deitzel et al 2001]	Single-ring electrode [Buttafoco et al 2006]
Single-cylinder electrode [Stankus et al 2004; Kim 2006; Kim et al 2006]	Shadow mask to narrow the area of deposited electrospun fiber [Huang et al 2010]
Single mask to localized fiber deposition area [Santos et al 2014]