Electrochemical impedance spectroscopy of human cochleas for modeling cochlear implant electrical stimulus spread.

Cochlear implants (CIs) have tremendously helped people with severe to profound hearing loss to gain access to sound and oral-verbal communication. However, the electrical stimulus in the cochlea spreads easily and widely, since the perilymph and endolymph (i.e., intracochlear fluids) are essentially electrolytes, leading to an inability to focus stimulation to discrete portions of the auditory nerve, which blurs the neural signal. Here, we characterize the complex transimpedances of human cadaveric cochleas to investigate how electrical stimulus spread is distributed from 10 Hz to 100 kHz. By using electrochemical impedance spectroscopy (EIS), both the resistive and capacitive elements of human cochleas are measured and modeled with an electrical circuit model, identifying spread-induced and spread-independent impedance components. Based on this electrical circuit model, we implement a Laplace transform to simulate the theoretical shapes of the spread signals. The model is validated by experimentally applying the simulated stimulus as a real stimulus to the cochlea and measuring the shapes of the spread signals, with relative errors of <0.6% from the model. Based on this model, we show the relationship between stimulus pulse duration and electrical stimulus spread. This EIS technique to characterize the transimpedances of human cochleas provides a new way to predict the spread signal under an arbitrary electrical stimulus, thus providing preliminary guidance to the design of CI stimuli for different CI users and coding strategies.

A Na+ conducting hydrogel for protection of organic electrochemical transistors.

Organic electrochemical transistors (OECTs) are being intensively developed for applications in electronics and biological interfacing. These devices rely on ions injected in a polymer film from an aqueous liquid electrolyte for their operation. However, the development of solid or semi-solid electrolytes are needed for future integration of OECTs into flexible, printed or conformable bioelectronic devices. Here, we present a new polyethylene glycol hydrogel with high Na+ conductivity which is particularly suitable for OECTs. This novel hydrogel was synthesized using cost-effective photopolymerization of poly(ethylene glycol)-dimethacrylate and sodium acrylate. Due to the high water content (83% w/w) and the presence of free Na+, the hydrogel showed high ionic conductivity values at room temperature (10-2 S cm-1) as characterized by electrochemical impedance spectroscopy. OECTs made using this hydrogel as a source of ions showed performance that was equivalent to that of OECTs employing a liquid electrolyte. They also showed improved stability, with only a 3% drop in current after 6 h of operation. This hydrogel paves the way for the replacement of liquid electrolytes in high performance OECTs bringing about advantages in terms of device integration and protection.

Wettability of PEDOT:PSS films.

Conducting polymers such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate ( PEDOT: PSS) are increasingly used to interface electronics with biology. Their wettability, however, remains poorly understood. We show that the frequently-used sessile drop technique yields results that are strongly dependent on the experimental conditions (measurement duration and relative humidity), due to the hydrogel-like behavior of PEDOT: PSS. In contrast, the captive bubble technique on a fully wet film overcomes these limitations. Dynamic wettability measurements provide hysteresis values for this polymer and clarify the physical meaning of the static contact angles measured by sessile drop and captive bubble. Finally we show that aging the films in water leads to loss of PSS chains which makes the films more hydrophobic. The addition of a crosslinker reduces and slows down these effects.

Coverage dependent adsorption dynamics in hyperthermal organic thin film growth.

We have examined the dynamics of adsorption of diindenoperylene (DIP) on SiO(2) and SiO(2) modified with an interfacial organic layer using in situ real time synchrotron x-ray scattering, focusing on the effects of coverage. On both surfaces we observe a substantial increase in the probability of adsorption with increasing coverage, which is most dramatic at the highest incident kinetic energies. On the initially uncovered surfaces, we observe a smooth decrease in the probability of adsorption with increasing incident kinetic energy, indicative of trapping-mediated adsorption. Once both surfaces are covered by DIP, the effects of incident kinetic energy are greatly reduced, and trapping is very efficient over the range of kinetic energies examined. Possible reasons for efficient trapping at high coverage and at high incident kinetic energy include more efficient momentum transfer due to mass matching, and possibly direct molecular insertion. Comparison to results on another small-molecule, pentacene, suggests that this behavior should be common to hyperthermal growth of a variety of other small-molecule thin films.

Mobility-dependent charge injection into an organic semiconductor.

Measurements of charge injection from indium tin oxide (ITO) into the organic semiconductor, tetraphenyl diamine doped polycarbonate (PC:TPD), were carried out. The current injected at the contact was measured as a function of the hole mobility in the organic semiconductor, which was varied from 10(-6) to 10(-3) cm (2)/V x s by adjusting the concentration of the hole transport agent, TPD, in the PC host. These experiments reveal that the current injected at the contact is proportional to the hole mobility in the bulk. As a result, the ITO/PC:TPD contact is found to limit current flow in all samples, regardless of the hole mobility in PC:TPD.