The transport of particles through a fluid by an electric current, known as electrokinetics, is a process used in a number of well-known applications such as gel electrophoresis and drug delivery systems. These types of ionic conductors operate based on the interaction of a direct current (DC), applied between metal electrodes and charged ions suspended in a fluid. This process, however, can have a number of critical drawbacks such as the production of chemical side products or gases that may impede particle movement. Moreover, the charge limitations of typical metal electrodes present the largest handicap to current technology in this field.
As reported in the August 13 issue of Advanced Materials (DOI: 10.1002/adma.201401258; p. 5143), Magnus Berggren and his research team from Linköping University in Sweden have built a four-diode full-wave rectifier for the transport of ionic species using conducting polymer electrodes to overcome this considerable restriction. Conducting polymers can be used to improve electrode capacity by increasing effective electrode area; however, most conducting polymers cannot withstand prolonged DC, necessitating alternating current (AC) operation. The AC acts just as it would in an electrical circuit, producing a periodic reversal in the direction of particle flow that would produce no net movement. Conventional electronics typically use a circuit configuration called a full-wave rectifier to convert AC to DC. This circuit is arranged in such a way that an AC signal current may be used while still maintaining a forward flow of current. For ionic currents, this allows for extended periods of operation with charge capacities surpassing that of conventional metal electrodes.
Berggren’s group constructed a bipolar membrane-type ion current rectifier using conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes. A typical bipolar membrane contains oppositely charged ion-selective membranes; this functions like a diode for ionic currents. The group then puts this material through the rigors of a typical four-diode full-wave rectifier and found that it performed well and was able to maintain an overall rectifying ionic current efficiency of 86%, with higher efficiencies of 95% during steady-state operation.
To demonstrate its suitability as a drug delivery system, the researchers constructed a cation-selective channel inside the ionic four-diode bridge. They then used this channel to deliver a common neurotransmitter called acetylcholine (ACh) from a source to a target electrolyte. They find that this system permits a nearly undisturbed delivery of ACh over an extended period of time without the production of adverse side reactions.
While their device suffers from some voltage and frequency limitations, Berggren and colleagues have demonstrated that this type of four-diode bridge could be used to improve select types of electrokinetic devices. Particularly when compared to similar systems that utilize moving parts for ion transport, this simple approach paves the way for smaller ionic circuits with no moving parts, perfect for implantable devices.
