Hans Kaim

Electrode Materials: State-of-the-Art and Experiments

Platinum is the most commonly used electrode material. The charge injection limit for platinum electrodes was found to be 400 μC∕cm2 in [4]. Tim Boretius et al. have reported a value of only 75 μC∕cm2 [3]. Platinum electrodes have proven success in practice, for example in many cochlear implants. Because of their relatively low charge injection capacity, they are usually used where large electrodes are applicable as in intracortical implant [8]. The limit for neural stimulation regarding tissue safety has been determined to be 1 mC∕cm2. In order to increase the charge injection capacity of platinum, various approaches have been proposed in the past few decades, like the galvanization of platinum black or gray. Although platinum black possesses a highly porous layer and therefore high charge injection capacity, its deposition often requires a lead containing electrolyte which limits its application because of cytotoxicity concerns.

Experiments Hardware and Methods

Three materials were extensively experimented with throughout the research here: TiN, iridium and iridium oxide (IrOx). The solution used in the experiments to mimic the surrounding tissue in the application is phosphate buffered saline (PBS). Before continuing with the investigation of different electrode materials and their characteristics, the hardware and methods developed and used here are explained briefly.

Stimulation and Recording Electrodes: General Concepts

In prosthetic devices, electrodes are the interfaces between the implant system and the body. Electrodes may be used for neural stimulation or neural signal recording according to the application. The signals to be recorded are typically small, i.e. some tens of microvolts for single pulse activity to a maximum amplitude of around 80 mV for intracellular potentials measured for example by fine-tipped electrolyte-filled glass micropipettes in cognitive studies in brain machine interface. On the opposite, neural stimulation sometimes needs relatively high electrode voltages and current densities, sometimes as high as several volts, so it may lead to a high enough energy transfer triggering chemical reactions that involve corrosion and changes in electrode properties.

Pixel array with 5×5 spatial highpass filter for a retinal implant

Vision of patients with retinal implants is diminished by spatial lowpass behaviour of the interface between electrode and stimulated cells. Spatial highpass filtering can compensate the interface lowpass to a certain degree and thus improve visual perception. The retinal implant consists of an array of photodiodes, each with an amplification circuit and an electrode. The proposed spatial highpass filter is integrated into that topology as a system of current mirrors, connecting each pixel bidirectionally to 12 adjacent pixels. A fully functional prototype was fabricated in a 180 nm CMOS process. We present pixel based measurements and fixed pattern noise analysis (FPN) in various operation modes. For further verification a sharp edge was projected onto the array. The recorded data is visualised and indicates promising improvements regarding the perceptibility of patterns.

Capacitive coupled full-wave rectifier for rectangular power signals

In this paper, a fully integrated bridge rectifier is presented. It is specialized for fully differential rectangular power supply waveforms. The rectifier is implemented in AMS 0.18 μm 6M/1P high voltage CMOS process with metal insulator metal (MIM) capacitors. Combining self driven switches with capacitive coupled switching gates the proposed rectifier shows very little output ripple without the need for a big output capacitance (Cout). By introducing coupling capacitors (CC), switching time is significantly reduced and the losses during polarity change can almost be eliminated. Simulation results show that the output voltage ripple (AVout) becomes mostly a function of the load current. The output capacitance can be reduced at least by a factor of 2 and still achieve the same output ripple as compared to non-capacitive coupled architectures.

Primary Current Distribution and Electrode Geometry

The current density pattern on the surface of an electrode depends on the electrode shape and position [9, 11, 12, 14, 17]. It affects the corrosion behavior of the electrodes considerably. If electrode polarization is ignored, it was shown in [12] that on a disk electrode, with the surface in the same level as the surface of the surrounding insulator, the current density increases from the center of the disk while approaching the edge, with theoretically an infinite value at the edge. This assumption (no electrode polarization) can be made if the potential on the electrolyte side of the double layer is equal to that of the electrode. The current density under this condition is called primary current distribution. This state prevails at high frequency when the double layer capacitance behaves as a short circuit [14].

Charge Balance and Safe Operation Conditions

In order to ensure long electrode lifetime, charge balance is necessary for electrostimulation. If charge balance is obeyed, the reversible reactions are used to prevent the electrode state to change over time. In the literature, it is usually assumed that when biphasic, charge-balanced current pulses are used, ideal charge balance is achieved. Figure 3.1 shows three common types of biphasic charge balanced current pulses mentioned in the literature. The monophasic capacitor-coupled waveform is generated through a capacitor discharge circuit [1].

The Effect of the Counter Electrode on Stimulation Electrode Lifetime

In electrostimulation, if redox reactions occur, they always happen in two distinct but simultaneous oxidation and reduction half-reaction groups at anode and cathode, which are the working and the counter electrodes depending on the injected signal polarity. A half-reaction cannot happen without the occurrence of its complementary opposite counterpart. Therefore, the size and the material of the counter electrode affects the water window.

Electrode Modeling for Titanium Nitride, Iridium and Iridium Oxide Microelectrodes

As stated before, electrodes are not standard linear electrical elements. Electrode properties depend on the electrode potential. Especially, in stimulation electrodes, the electrode potential fluctuates over a relatively wide range, thus enhancing the nonlinear characteristics of the electrode-electrolyte interface. However, an approximate electrical model can be helpful in designing the interface circuits to the electrodes, like signal recording or driver circuits. As explained in Chap. 5, impedance spectroscopy is one of the methods to extract the electrode model. In the following this and other methods are explained through practical examples.

Irreversible and Reversible Redox Reactions: Water Window

As mentioned above, capacitive charging cannot deliver enough current if current density exceeds certain limits. The potential difference between the active (working) and the counter electrodes (used to close the electrical circuit) must remain low enough so that (almost) no redox reactions occur, if only capacitive charge injection is to follow.