Micropyramid structured photo capacitive interfaces.

Optically driven electronic neuromodulation devices are a novel tool in basic research and offer new prospects in medical therapeutic applications. Optimal operation of such devices requires efficient light capture and charge generation, effective electrical communication across the device's bioelectronic interface, conformal adhesion to the target tissue, and mechanical stability of the device during the lifetime of the implant-all of which can be tuned by spatial structuring of the device. We demonstrate a 3D structured opto-bioelectronic device-an organic electrolytic photocapacitor spatially designed by depositing the active device layers on an inverted micropyramid-shaped substrate. Ultrathin, transparent, and flexible micropyramid-shaped foil was fabricated by chemical vapour deposition of parylene C on silicon moulds containing arrays of inverted micropyramids, followed by a peel-off procedure. The capacitive current delivered by the devices showed a strong dependency on the underlying spatial structure. The device performance was evaluated by numerical modelling. We propose that the developed numerical model can be used as a basis for the design of future functional 3D design of opto-bioelectronic devices and electrodes.

Spontaneous wrinkle emergence in nascent eumelanin thin films.

Self-patterning processes originated by physical stimuli have been extensively documented in thin films, whereas spontaneous wrinkling phenomena due to chemical transformation processes are, to the best of our knowledge, unprecedented. Herein we report a case of spontaneous polymerization-driven surface nano-patterning (∼500 nm) that develops in smooth thin solid films of 5,6-dihydroxyindole (DHI), a major precursor of eumelanin polymers, over a time scale of 30 to 60 days in air at room temperature. The phenomenon can be observed only above a critical film thickness of ∼250 nm and it is affected by exposure to ammonia vapors causing acceleration of the oxidation process. The thickness-dependent onset of wrinkling can be attributed to non-homogeneous rates of oxidation through the film causing slow swelling/expansion of the inner layers followed by fast stiffening and cross-linking in the outer layer exposed to higher oxygen levels.

Shedding Light on Cardiac Excitation: In Vitro and In Silico Analysis of Native Ca2+ Channel Activation in Guinea Pig Cardiomyocytes Using Organic Photovoltaic Devices.

OBJECTIVE: This study aims to explore the potential of organic electrolytic photocapacitors (OEPCs), an innovative photovoltaic device, in mediating the activation of native voltage-gated Cav1.2 channels (ICa,L) in Guinea pig ventricular cardiomyocytes. METHODS: Whole-cell patch-clamp recordings were employed to examine light-triggered OEPC mediated ICa,L activation, integrating the channel's kinetic properties into a multicompartment cell model to take intracellular ion concentrations into account. A multidomain model was additionally incorporated to evaluate effects of OEPC-mediated stimulation. The final model combines external stimulation, multicompartmental cell simulation, and a patch-clamp amplifier equivalent circuit to assess the impact on achievable intracellular voltage changes. RESULTS: Light pulses activated ICa,L, with amplitudes similar to voltage-clamp activation and high sensitivity to the L-type Ca2+ channel blocker, nifedipine. Light-triggered ICa,L inactivation exhibited kinetic parameters comparable to voltage-induced inactivation. CONCLUSION: OEPC-mediated activation of ICa,L demonstrates their potential for nongenetic optical modulation of cellular physiology potentially paving the way for the development of innovative therapies in cardiovascular health. The integrated model proves the light-mediated activation of ICa,L and advances the understanding of the interplay between the patch-clamp amplifier and external stimulation devices. SIGNIFICANCE: Treating cardiac conduction disorders by minimal-invasive means without genetic modifications could advance therapeutic approaches increasing patients' quality of life compared with conventional methods employing electronic devices.

Downsizing the Channel Length of Vertical Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronic devices such as sensors and neural interfaces. While the majority of OECTs use simple planar geometry, there is interest in exploring how these devices operate with much shorter channels on the submicron scale. Here, we show a practical route toward the minimization of the channel length of the transistor using traditional photolithography, enabling large-scale utilization. We describe the fabrication of such transistors using two types of conducting polymers. First, commercial solution-processed poly(dioxyethylenethiophene):poly(styrene sulfonate), PEDOT:PSS. Next, we also exploit the short channel length to support easy in situ electropolymerization of poly(dioxyethylenethiophene):tetrabutyl ammonium hexafluorophosphate, PEDOT:PF6. Both variants show different promising features, leading the way in terms of transconductance (gm), with the measured peak gm up to 68 mS for relatively thin (280 nm) channel layers on devices with the channel length of 350 nm and with widths of 50, 100, and 200 µm. This result suggests that the use of electropolymerized semiconductors, which can be easily customized, is viable with vertical geometry, as uniform and thin layers can be created. Spin-coated PEDOT:PSS lags behind with the lower values of gm; however, it excels in terms of the speed of the device and also has a comparably lower off current (300 nA), leading to unusually high on/off ratio, with values up to 8.6 × 104. Our approach to vertical gap devices is simple, scalable, and can be extended to other applications where small electrochemical channels are desired.

Photo-Chemical Stimulation of Neurons with Organic Semiconductors.

Recent advances in light-responsive materials enabled the development of devices that can wirelessly activate tissue with light. Here it is shown that solution-processed organic heterojunctions can stimulate the activity of primary neurons at low intensities of light via photochemical reactions. The p-type semiconducting polymer PDCBT and the n-type semiconducting small molecule ITIC (a non-fullerene acceptor) are coated on glass supports, forming a p-n junction with high photosensitivity. Patch clamp measurements show that low-intensity white light is converted into a cue that triggers action potentials in primary cortical neurons. The study shows that neat organic semiconducting p-n bilayers can exchange photogenerated charges with oxygen and other chemical compounds in cell culture conditions. Through several controlled experimental conditions, photo-capacitive, photo-thermal, and direct hydrogen peroxide effects on neural function are excluded, with photochemical delivery being the possible mechanism. The profound advantages of low-intensity photo-chemical intervention with neuron electrophysiology pave the way for developing wireless light-based therapy based on emerging organic semiconductors.

Direct measurement of oxygen reduction reactions at neurostimulation electrodes.

Objective. Electric stimulation delivered by implantable electrodes is a key component of neural engineering. While factors affecting long-term stability, safety, and biocompatibility are a topic of continuous investigation, a widely-accepted principle is that charge injection should be reversible, with no net electrochemical products forming. We want to evaluate oxygen reduction reactions (ORR) occurring at different electrode materials when using established materials and stimulation protocols.Approach. As stimulation electrodes, we have tested platinum, gold, tungsten, nichrome, iridium oxide, titanium, titanium nitride, and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). We use cyclic voltammetry and voltage-step amperometry in oxygenated versus inert conditions to establish at which potentials ORR occurs, and the magnitudes of diffusion-limited ORR currents. We also benchmark the areal capacitance of each electrode material. We use amperometric probes (Clark-type electrodes) to quantify the O2and H2O2concentrations in the vicinity of the electrode surface. O2and H2O2concentrations are measured while applying DC current, or various biphasic charge-balanced pulses of amplitude in the range 10-30µC cm-2/phase. To corroborate experimental measurements, we employ finite element modelling to recreate 3D gradients of O2and H2O2.Main results. All electrode materials support ORR and can create hypoxic conditions near the electrode surface. We find that electrode materials differ significantly in their onset potentials for ORR, and in the extent to which they produce H2O2as a by-product. A key result is that typical charge-balanced biphasic pulse protocols do lead to irreversible ORR. Some electrodes induce severely hypoxic conditions, others additionally produce an accumulation of hydrogen peroxide into the mM range.Significance. Our findings highlight faradaic ORR as a critical consideration for neural interface devices and show that the established biphasic/charge-balanced approach does not prevent irreversible changes in O2concentrations. Hypoxia and H2O2can result in different (electro)physiological consequences.

Chronic electrical stimulation of peripheral nerves via deep-red light transduced by an implanted organic photocapacitor.

Implantable devices for the wireless modulation of neural tissue need to be designed for reliability, safety and reduced invasiveness. Here we report chronic electrical stimulation of the sciatic nerve in rats by an implanted organic electrolytic photocapacitor that transduces deep-red light into electrical signals. The photocapacitor relies on commercially available semiconducting non-toxic pigments and is integrated in a conformable 0.1-mm3 thin-film cuff. In freely moving rats, fixation of the cuff around the sciatic nerve, 10 mm below the surface of the skin, allowed stimulation (via 50-1,000-µs pulses of deep-red light at wavelengths of 638 nm or 660 nm) of the nerve for over 100 days. The robustness, biocompatibility, low volume and high-performance characteristics of organic electrolytic photocapacitors may facilitate the wireless chronic stimulation of peripheral nerves.

Organic electrolytic photocapacitors for stimulation of the mouse somatosensory cortex.

Objective.For decades electrical stimulation has been used in neuroscience to investigate brain networks and been deployed clinically as a mode of therapy. Classically, all methods of electrical stimulation require implanted electrodes to be connected in some manner to an apparatus which provides power for the stimulation itself.Approach. We show the use of novel organic electronic devices, specifically organic electrolytic photocapacitors (OEPCs), which can be activated when illuminated with deep-red wavelengths of light and correspondingly do not require connections with external wires or power supplies when implanted at various depthsin vivo. Main results. We stimulated cortical brain tissue of mice with devices implanted subcutaneously, as well as beneath both the skin and skull to demonstrate a wireless stimulation of the whisker motor cortex. Devices induced both a behavior response (whisker movement) and a sensory response in the corresponding sensory cortex. Additionally, we showed that coating OEPCs with a thin layer of a conducting polymer formulation (PEDOT:PSS) significantly increases their charge storage capacity, and can be used to further optimize the applied photoelectrical stimulation.Significance. Overall, this new technology can provide an on-demand electrical stimulation by simply using an OEPC and a deep-red wavelength illumination. Wires and interconnects to provide power to implanted neurostimulation electrodes are often problematic in freely-moving animal research and with implanted electrodes for long-term therapy in patients. Our wireless brain stimulation opens new perspectives for wireless electrical stimulation for applications in fundamental neurostimulation and in chronic therapy.

Untangling Photofaradaic and Photocapacitive Effects in Organic Optoelectronic Stimulation Devices.

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.

Eumelanin Precursor 2-Carboxy-5,6-Dihydroxyindole (DHICA) as Doping Factor in Ternary (PEDOT:PSS/Eumelanin) Thin Films for Conductivity Enhancement.

The integration of the pristine not-doped commercial poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) PH1000 with eumelanin, the brown to black kind of melanin pigment, was achieved by dissolving the melanogenic precursors 2-carboxy-5,6-dihydroxyindole (DHICA) in the PH1000 suspension. Solid state oxidative polymerization of the catecholic indole allowed obtaining the ternary blend PEDOT:PSS/eumelanin. The introduction of DHICA into PH1000 produced a noticeable increase in the conductivity of PEDOT thin films akin to that produced by dimethyl sulfoxide (DMSO) treatment, opening up novel strategies for the simultaneous integration of eumelanin polymer and conductivity enhancement of PEDOT containing coatings, as well as the long term goal of replacing PSS by DHICA eumelanin for PEDOT pairing.

Evidence of Unprecedented High Electronic Conductivity in Mammalian Pigment Based Eumelanin Thin Films After Thermal Annealing in Vacuum.

Melanin denotes a variety of mammalian pigments, including the dark electrically conductive eumelanin and the reddish, sulfur-containing, pheomelanin. Organic (bio)electronics is showing increasing interests in eumelanin exploitation, e.g., for bio-interfaces, but the low conductivity of the material is limiting the development of eumelanin-based devices. Here, for the first time, we report an abrupt increase of the eumelanin electrical conductivity, revealing the highest value presented to date of 318 S/cm. This result, obtained via simple thermal annealing in vacuum of the material, designed on the base of the knowledge of the eumelanin chemical properties, also discloses the actual electronic nature of this material's conduction.

Eumelanin Coating of Silica Aerogel by Supercritical Carbon Dioxide Deposition of a 5,6-Dihydroxyindole Thin Film.

Eumelanin integration in silica aerogel (SA) was achieved via supercritical adsorption of 5,6-dyhydroxyindole (DHI) from CO2. Notably, after the supercritical treatment, DHI evolved towards spontaneous polymerization, which resulted in uniform pigment development over the SA. The new material was characterized for its morphological and physicochemical properties, disclosing the formation of a eumelanin-like coating, as confirmed by UV⁻vis and electron paramagnetic resonance (EPR) spectroscopy.