Scanning electrochemical microscopy as a novel proximity sensor for atraumatic cochlear implant insertion.

A growing number of minimally invasive surgical and diagnostic procedures require the insertion of an optical, mechanical, or electronic device in narrow spaces inside a human body. In such procedures, precise motion control is essential to avoid damage to the patient's tissues and/or the device itself. A typical example is the insertion of a cochlear implant which should ideally be done with minimum physical contact between the moving device and the cochlear canal walls or the basilar membrane. Because optical monitoring is not possible, alternative techniques for sub millimeter-scale distance control can be very useful for such procedures. The first requirement for distance control is distance sensing. We developed a novel approach to distance sensing based on the principles of scanning electrochemical microscopy (SECM). The SECM signal, i.e., the diffusion current to a microelectrode, is very sensitive to the distance between the probe surface and any electrically insulating object present in its proximity. With several amperometric microprobes fabricated on the surface of an insertable device, one can monitor the distances between different parts of the moving implant and the surrounding tissues. Unlike typical SECM experiments, in which a disk-shaped tip approaches a relatively smooth sample, complex geometries of the mobile device and its surroundings make distance sensing challenging. Additional issues include the possibility of electrode surface contamination in biological fluids and the requirement for a biologically compatible redox mediator.

Scanning electrochemical microscopy of living cells: different redox activities of nonmetastatic and metastatic human breast cells.

Electrochemical methods have been widely used to monitor physiologically important molecules in biological systems. This report describes the first application of the scanning electrochemical microscope (SECM) to probe the redox activity of individual living cells. The possibilities of measuring the rate and investigating the pathway of transmembrane charge transfer are demonstrated. By this approach, significant differences are detected in the redox responses given by nonmotile, nontransformed human breast epithelial cells, breast cells with a high level of motility (engendered by overexpression of protein kinase Calpha), and highly metastatic breast cancer cells. SECM analysis of the three cell lines reveals reproducible differences with respect to the kinetics of charge transfer by several redox mediators.

Voltammetry at micropipet electrodes.

The use of micropipet electrodes for quantitative voltammetric measurements of ion-transfer (IT) and electron-transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) requires knowledge of geometry of the liquid interface. The shape of the meniscus formed at the pipet tip was studied in situ by video microscopy under controlled pressure. The shape of the interface can be changed from a complete sphere to a concave spherical cap by varying the pressure applied to the pipet, and the diffusion current to the pipet changes accordingly. With no external pressure applied, the water/organic interface turned out to be flat, and the voltammetric response of a pipet must follow the well-known theory for a microdisk electrode. The large deviations from this theory observed previously can be attributed to a small amount of the filling aqueous solution which escapes from the pipet and forms a thin layer on its outer wall. This effect can be eliminated by making the outer pipet wall hydrophobic. Procedures have been developed for independent silanization of the inner and outer walls of the pipet. Pipets with a silanized inner wall can be filled with an organic solvent (e.g., 1,2-dichloroethane) and be used for voltammetric measurements in aqueous solutions. Another mode of voltammetry is based on trapping of a thin layer of organic solvent in the narrow shaft of a pipet between the filling solution and the aqueous outer phase. This arrangement is potentially useful for electrochemical catalysis and sensor applications.

Direct electrochemical measurements inside a 2000 angstrom thick polymer film by scanning electrochemical microscopy.

An extremely small, conically shaped Pt microelectrode tip (with a radius of 30 nanometers) and the precise positioning capabilities of the scanning electrochemical microscope were used to penetrate a thin (200 nanometers) polymer film and obtain directly the standard potential and kinetic parameters of an electrode reaction within the film. The thickness of the film was determined while it was immersed in and swollen by an electrolyte solution. The film studied was the perfluorosulfonate Nafion containing Os(bpy)(3)(2+) (bpy, 2,2'-bipyridine) cast on an indium tin oxide surface. The steady-state response at the ultramicroelectrode allowed direct determination of the rate constant for heterogeneous electron transfer K(o) and the diffusion coefficient D without complications caused by transport in the liquid phase, charge exchange at the liquid-polymer interface, and resistive drop.