Anti-inflammatory polymer electrodes for glial scar treatment: bringing the conceptual idea to future results

Abstract

Conducting polymer films offer a convenient route for the functionalization of implantable microelectrodes without compromising their performance as excellent recording units. A micron thick coating, deposited on the surface of a regular metallic electrode, can elute anti-inflammatory drugs for the treatment of glial scarring as well as growth factors for the support of surrounding neurons. Electro-activation of the polymer drives the release of the substance and should ideally provide a reliable method for controlling quantity and timing of release. Driving signals in the form of a constant potential (CP), a slow redox sweep or a fast pulse are all represented in literature. Few studies present such release in vivo from actual recording and stimulating microelectronic devices. It is essential to bridge the gap between studies based on release in vitro, and the intended application, which would mean release into living and highly delicate tissue. In the biological setting, signals are limited both by available electronics and by the biological safety. Driving signals must not be harmful to tissue and also not activate the tissue in an uncontrolled manner. This review aims at shedding more light on how to select appropriate driving parameters for the polymer electrodes for the in vivo setting. It brings together information regarding activation thresholds for neurons, as well as injury thresholds, and puts this into context with what is known about efficient driving of release from conducting polymer films.

The importance of the film structure during self-powered ibuprofen salicylate drug release from polypyrrole electrodeposited on AZ31 Mg

“…Besides these differences, Avrami's equation describes the trend of the experimental data obtained for the self-powered IBU release from PPy films. The contribution of potential difference and diffusion in self-powered release systems can be expected due to potential variations across the film [4].…”

“…This result is consistent with previous reports in the literature denoting a rapid release at the initial stages for dexamethasone in 10 wt.%. NaCl solutions and other systems [1,4,28]. Self-powered systems rely on the dissolution of the substrate metal to reduce the polymer film and release the incorporated drug.…”

“…Polypyrrole is one of the most common, biocompatible, and mechanically robust conductive polymers (CPs) which has been used for corrosion protection of metals [7,[12][13][14] and local drug delivery [4]. The size, steric effects, functional groups, charge, and chemical structure of the uploaded drug may affect the polymer nucleation and growth processes and its mechanical properties [13].…”

“…The mechanism of a self-powered system relies on galvanic coupling between conductive polymer films deposited on the surface of an active alloy. Therapeutic drugs are uploaded as dopants into the film, while the polymer is growing by attracting counterions to balance the charge [4][5][6]. Once a self-powered system is implanted in a corrosive environment, such as the one encountered inside the human body, or on the surface of the skin, it undergoes corrosion.…”

“…The corroding active alloy pumps electrons to reduce the polymer, contracts, and releases the therapeutic drug. Accordingly, a sufficient corrosion-resistant alloy acts as a biodegradable implant, and in combination with a polymer film, serves as vehicle to deliver drugs such as risperidone, dexamethasone, pain relievers, or antibiotics [1,4].…”