Neurostimulation: Direct Electrical Neurostimulation with Organic Pigment Photocapacitors (Adv. Mater. 25/2018)

Abstract

Organic electrolytic photocapacitors transduce light pulses into localized displacement currents that electrically stimulate neurons. These nanoscale devices are freestanding, requiring no wiring, and are stable in physiological conditions. In article number https://doi.org/10.1002/adma.201707292, Yael Hanein, Eric Daniel Głowacki, and co‐workers successfully photostimulate cultured neurons and light‐insensitive explanted retinas, paving the way for minimally invasive retinal prosthetics.

Photo‐Chemical Stimulation of Neurons with Organic Semiconductors

“…The photosensitive component can also be made from a combination of p-type (hole transporting) and n-type (electron transporting) organic semiconductors, to form light-sensitive pn junctions. [24] These p-n junctions are generally able to deliver large photovoltage and photocurrent at the polymer/cell interface due to exciton splitting that minimizes charge recombination. As a result, free electrons interact with cations in the electrolyte and induce a photo-capacitive modulation of the cell membrane.…”

“…Organic semiconductors can be molecularly tuned by chemical synthesis to achieve unique absorption profiles, leading to photo-actuators with sensitivity to specific wavelengths [27] and intensities. [28] For instance, Głowacki group developed organic pn junctions based on conjugated small molecules and used them for the photo-capacitive stimulation of neuronal cultures [24,29] and single cells (i.e., oocytes). [30] These devices are sensitive in the deep-red region of the optical spectrum (i.e., ≈650 nm) and can photo-stimulate cells at light intensities in the range of 100-1000 mW cm −2 .…”

P‐type Semiconducting Polymers as Photocathodes: A Comparative Study for Optobioelectronics

“…[5][6][7][8][9] The aim is to substitute invasive wires and implantable electronics by a single unconnected organic transducer device that is operated by light pulses transmitted through the surrounding tissue. First examples for this concept are organic semiconductor based retinal implants [10][11][12][13] or nerve stimulators [14] as well as optically controlled smart surfaces to guide tissue growth and cell regeneration. [15][16][17][18][19] All these applications rely on the transduction of light pulses to physico-chemical stimuli at the semiconductor/electrolyte interface further effecting on biological processes.…”

“…Different architectures of organic optoelectronic interfaces have been presented in the literature such as planar p-n heterojunctions, [10,[20][21][22] polymeric thin films, [12,23] structured polymeric films, [19,24] and polymeric nanoparticles. [25][26][27] In these architectures, semiconducting polymers offer several advantages compared to other materials such as high optical absorption and multiple photostimulation mechanisms, [21] high biocompatibility, mechanical compliance and synthetic flexibility.…”

“…The first example demonstrating this concept is the bulk heterojunction developed by the Lanzani group [31] which consist of a binary polymeric blend made of the poly-3-Hexylthiophene (P3HT) polymer and [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM). A highly stable junction for photocapacitive stimulations is made by the organic pigments H2PC-PTCDI thin-film molecular planar heterojunction developed by Glowacki and coworkers , [10,21] It has been demonstrated that also a single layer of p-type polymer such as P3HT can buildup photovoltage due to photoelectrochemical reactions. [29,32] This simplification has important implications, as it can be applied to p-type semiconducting nanoparticles interacting directly with the cell membrane to stimulate cells, even though the operating mechanism is still under debate.…”