Untangling Photofaradaic and Photocapacitive Effects in Organic Optoelectronic Stimulation Devices

Abstract

Light, as a versatile and non-invasive means to elicit a physiological response, offers solutions to problems in basic research as well as in biomedical technologies. The complexity and limitations of optogenetic methods motivate research and development of optoelectronic alternatives. A recently growing subset of approaches relies on organic semiconductors as the active light absorber. Organic semiconductors stand out due to their high optical absorbance coefficients, mechanical flexibility, ability to operate in a wet environment, and potential biocompatibility. They could enable ultrathin and minimally invasive form factors not accessible with traditional inorganic materials. Organic semiconductors, upon photoexcitation in an aqueous medium, can transduce light into (1) photothermal heating, (2) photochemical/photocatalytic redox reactions, (3) photocapacitive charging of electrolytic double layers, and (4) photofaradaic reactions. In realistic conditions, different effects may coexist, and understanding their role in observed physiological phenomena is an area of critical interest. This article serves to evaluate the emerging picture of photofaradaic vs. photocapacitive effects in the context of our group's research efforts and that of others over the past few years. We present simple experiments which can be used to benchmark organic optoelectronic stimulation devices.

A chronic photocapacitor implant for noninvasive neurostimulation with deep red light

“…The organic PN pixel forms an electrolytic closed circuit with the back electrode. 42,50 The displacement current which flows around the device during photocharging and discharging generates transient potentials in the solution which affect the electrophysiology of nearby cells, as established in our earlier in vitro studies 43,44 . To visualize this effect, we measure the transient potential (VT) above the PN pixel in electrolyte (Figure 2e).…”

Organic Bioelectronics: From Functional Materials to Next‐Generation Devices and Power Sources

“…These hole-electron pairs (i.e., excitons) can be separated into free charge carriers, typically in the presence of electron-accepting molecules or with an electrical field. At the CP film/electrolyte interface, depending on the materials used, the device dimensions, and the intensity and duration of applied light stimulus, the generated charges interact with the electrolyte (and adherent cells) in any or a combination of these three ways: [113,114] 1) photocapacitive coupling, where one type of electronic charge (holes or electrons) of the CP film accumulates at the electrolyte/film interface and couples with the electrolyte ions of opposite charge, hence generating a transient electric field that (de)polarizes the membrane; [115] 2) photoelectrochemical coupling, which involves free charge carriers transferred to the electrolyte that cause a Faradaic reaction, which then interferes with the physiology of the cell; [116] and 3) photothermal coupling, where the absorption of light leads to localized heating in the polymer, which then propagates to the interface and causes physical changes in the lipid bilayer membrane of the cells. [117] For more insight into these processes and the types of planar or nanostructured surfaces used, we refer readers to recent reviews on the topic.…”

“…[117] For more insight into these processes and the types of planar or nanostructured surfaces used, we refer readers to recent reviews on the topic. [113,114,118] Photocapacitive coupling has been employed to achieve a high temporal resolution and immediate response after stimulation, which are required especially for chronic implants, and to avoid redox byproducts or heating that may cause irreversible damage to cells. [115] Overall, light can be a noninvasive stimulus to modulate cellular behavior with its localized action and an immediate response.…”

Wireless Bioelectronic Devices Driven by Deep Red Light

“…The different mechanisms through which the stimulation can happen are photothermal, photochemical, photofaradaic, and photocapacitive, or some combination. 59,67 Many systems working on various wavelengths of light have been reported, both for in vitro and in vivo applications. 59,[68][69][70][71][72] The latter is possible either by implantation of optical fibers and light emitting diodes or by transmitting light within the so-called tissue transparency window.…”