Photoactive Organic Substrates for Cell Stimulation: Progress and Perspectives

Abstract

Modulating the cell membrane potential provides an opportunity to control electrical signaling that is central to cellular physiology and its proper biological function. Among the many technological tools available to enable this modulation, optical stimulation through a photoactive substrate is a powerful strategy that is minimally invasive and wireless. This is critical to avoid disrupting the structural integrity of the cell membrane that is often caused by the use of electrical stimulation electrodes or changing its genetic footprints if optogenetics is the approach taken to photomodulate its electrophysiology. The use of a photoactive substrate such as an organic semiconductor combines optoelectronic properties with mechanical tissue compatibility, which is unachievable with inorganic semiconductors. This research news first discusses the mechanisms of cell photostimulation via these organic photoactive substrates and techniques employed to characterize their photoresponse. Then a summary of the relevant organic materials and their recent applications in cellular photostimulation is presented. Finally, the challenges in the field are described and a perspective on how to address these is provided.

A chronic photocapacitor implant for noninvasive neurostimulation with deep red light

“…Getting sufficient light to devices implanted below skin and other tissues, however, is not as obvious and thus photovoltaic implanted neurostimulators have not been made. We propose that using organic semiconductors as the active optoelectronic component could facilitate light-mediated neurostimulation for such applications 38 due to high absorbance coefficient, mechanical flexibility, and biocompatibility 39 . They can enable ultrathin and minimally invasive form factors inaccessible for traditional inorganic materials.…”

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.…”

Conjugated Polymers in Bioelectronics: Addressing the Interface Challenge

“…Where there is damage to the retinal pigment epithelium and photoreceptors, but the inner retinal layers are intact, the application of a retinal prosthesis is a viable treatment option . Such prosthesis either take the form of an external imaging and processing device connected to an implanted electrode array, or a CP based optoelectronic material . The potential benefits that an organic prosthetic would bring (other than biocompatibility and flexibility for large area coverage) include an operating range close to the normal visible range and a single device that would need no external power source, or processing or imaging capabilities.…”