comprises materials and devices that can fulfill just this dual ionic-electronic capability. Iontronics utilize the coupling of electrical and ionic signals in conducing polymers, leading to, for example, organic electrochemical transistors (OECTs), [2] electrolyte-gated (also known as electric doublelayer capacitor-gated) organic field-effect transistors (EGOFETs), [3,4] organic electrochemical biosensors, [5,6] and iontronic delivery electrodes and devices. [7][8][9][10][11] In iontronic delivery devices (Figure 1), chemical gradients are created by controlled release of charged biomolecules (ions) at specific locations at specific times. [7,8,12] Ions are transported to these release sites through ionic conductors due to applied electric fields between electrodes. The ionic conductors form the foundation of iontronic resistors (organic electronic ion pumps, OEIPs), diodes, and transistors which can be combined into circuits for, for example, multiplexing, addressing, and signal processing. These iontronic circuits behave analogous to traditional electronics, but use ions as charge carriers rather than electrons, and allow for the development of fully chemical systems generating complex signal patterns at high spatiotemporal resolution and biochemical specificity.There are several other techniques for electronic control of substance release, drug delivery, or ion transport related to this form of iontronics. These include techniques such as microfluidic and microelectromechanical systems (MEMS) based micropumps, [13] iontophoresis, [14,15] and organic electronic redox-mediated controlled release. [11,16] In comparison to these technologies, iontronic drug delivery provides a means of simultaneously achieving high delivery precision, minimal (or zero) liquid transport that could interfere with fragile biochemical microenvironments, continuous resupply of the transported substance, and (in principle) exact control over delivered amounts, even at speeds on par with synaptic signaling. In addition, as they are based on well-established solid-state device manufacturing techniques, iontronic components and systems can be miniaturized, addressed, and integrated with complex electronic systems in a straightforward manner. These features of iontronics combine to enable the lowest dose possible. With other techniques for substance release and transport, larger doses are often distributed (usually in solution phase) with less control, which could result in unwanted side effects. Other technologies have their advantages primarily in potentially simpler device design, the ability to transport larger molecules (e.g., In contrast to electronic systems, biology rarely uses electrons as the signal to regulate functions, but rather ions and molecules of varying size. Due to the unique combination of both electronic and ionic/molecular conductivity in conjugated polymers and polyelectrolytes, these materials have emerged as an excellent tool for translating signals between these two realms, hence the field of organic bioelectroni...

Combining experimental and theoretical approach, we demonstrate practical solutions to limiting currents in capillary-based electrophoretic delivery devices.

techniques. [5,6] This technology can serve as an efficient tool for an application such as biosensing, [7][8][9] electrophysiological recording, [10] and drug delivery. [11][12][13] Organic electronic ion pumps (OEIPs) are a primary example of organic bioelectronics combining electronic and ionic properties of organic electronics materials [14][15][16] to enable release, via electronic addressing, of ionic-biochemical signals for biological applications. OEIPs operate as an "iontronic" resistors [17][18][19] and can be used to electrophoretically deliver charged species through a cation or anion exchange membrane (AEM), resulting in high spatiotemporal delivery resolution and high dosage precision (one electron per delivered monovalent ion). [14] In recent years, these electrophoretic delivery devices have been used to trigger cell signaling in vitro, [1,20] to control epileptiform activity in brain slice models, [21][22][23] to effect sensory function in vivo, [19] to suppress pain sensation in awake animals, [17] and to modulate plant physiology. [24,25] OEIP devices based on glass capillary fibers offer several design advantages for use in freestanding or implantable geometries. OEIPs fiber capillaries, in contrast to planar OEIPs devices, provide less water uptake inside the channel providing a large ion-transport cross-section with high ionic conductivity, which allows transport of relatively large ions such as drugs and neurotransmitters. [26,27] These devices can furthermore be more easily implanted or located in proximity to targeted cells, tissues, or organs.In the present work, we demonstrate OEIPs based on glass capillary fibers that are filled with a polyelectrolyte (polycationic AEM). The AEM is characterized by a high concentration of fixed positive charges that allows for the selective transport of negative ions while blocking coions from drifting in the opposite direction. Nonfixed mobile ions with the same charge with the fixed charges of ion exchange membrane are referred to as coions while ions with opposite charge are referred to as counterions. The permselectivity, according to Donnan exclusion, holds if the ionic concentrations in the adjacent electrolytes are considerably lower than the fixed charge concentration of the AEM. [28] The potential gradient, and associated current through the OEIPs fiber capillaries, is established by applying a potential difference between electrodes (Ag/AgCl) applied in the source and target solutions. [27] The polarizable electrodes in this An organic electronic ion pump (OEIP) delivers ions and drugs from a source, through a charge selective membrane, to a target upon an electric bias. Miniaturization of this technology is crucial and will provide several advantages, ranging from better spatiotemporal control of delivery to reduced invasiveness for implanted OEIPs. To miniaturize OEIPs, new configurations have been developed based on glass capillary fibers filled with an anion exchange membrane (AEM). Fiber capillary OEIPs can be easily implanted in proximi...