Valor razonable de los activos y pasivos del balance

Over the years the term “MEAs” has become an umbrella term that encompasses a wide variety of different forms of recording device. These may be differentiated based on the type of transducers (multi-transistor array, capacitive-coupled ar- ray, micro-electrode array), substrates (active array, passive array, silicon array, CMOS array), shape of the device (tetrodes, needle, polytrodes) or the applica- tion (in vivo - implantable, in vitro - non implantable). Hence, it is important to briefly set the context for the terminologies used with regards to MEAs in this chapter and the whole thesis in general.

The generic term MEA covers both implantable neural probes (see Figure2.7) and electrode-integrated planar substrates (see Figure 2.8). Implantable neural probes can be placed directly on the region of interest in a living organism to record extracellular neural activity. Electrode-integrated substrate MEAs are not implantable and generally used for in vitro neuronal cultures. Such MEAs have a cell culture dish or a chamber to hold the culture medium (see Figure2.8). The term array refers to the spatial layout of the array of electrodes, though the MEAs refers to the whole device. For the context of this thesis work, MEAs refers to substrate based MEAs and not the implantable neural probes.

The work of [51] describing a planar Micro-Electrode Arrays (MEA), also referred as Multi-Electrode Arrays - and a successful demonstration of recording and stimulation of network of neuronal culture by [52], were important milestones

Figure 2.7: Fifty-four site polytrodes. The polytrodes have electrodes or recording sites in 2 or 3 columns spaced at 43 − 75µm [50]

in in-vitro electrophysiology. These landmark works opened up new avenues in the area of in-vitro electrophysiology to study a wide variety of neural functions. MEAs are essentially an array of multiple low impedance electrodes laid out on a planar substrate which are sensitive enough to detect slight changes in the membrane potential of contacted cells.

In the early 1990s, Fromhertz et al [53] mounted a cell on the surface of a Field-Effect Transistor(FET) on Silicon(Si) allowing for direct coupling of neu- rons and Si without the use of interfering metal electrodes. Since then with the development in CMOS (Complementary Metal Oxide Semiconductor) technology, it is possible to record and stimulate simultaneously from an integrated circuit packed with thousands of electrodes which is especially beneficial to study neural plasticity and neural dynamics.

In the current technology, with the few exception of high density MEAs, MEAs are usually composed of a few hundred electrodes with an electrode pitch of 100- 500 µm embedded on a glass substrate as shown in Figure2.8. The electrodes are typically made of Gold (Au), Indium-tin Oxide (ITO), Titanium Nitride (TiN) or platinum(Pt) which are bio-compatible and have low impedance (less than 500Ω at 1kHz).

(a) 60 electrodes MEA with electrode layout (b) 256 electrodes MEA with electrode layout

Figure 2.8: MEA devices with electrode layout

limitation in terms of substantial spatial under-sampling of the neuronal network. This is not enough to unlock the full capability of MEAs; specifically studying electrophysiological activity to study correlation/synchrony at both cellular and network level. MEA electrodes with size and distance approximating that of neuronal cells dimensions can provide a much better spatial resolution and help provide a much better insight into the neuronal activity [54].

Active pixel sensor (APS) CMOS based MEAs allow for a higher density of electrodes with reduced electrode size and electrode pitch distance. APS devices consider each electrode as a pixel which can register changes in electrophysiolog- ical activity.

The experimental setup for this work is based on a CMOS MEA (3Brain AG, Switzerland). This MEA has 4096 electrodes laid out in a 64 x 64 grid. The dimension and spatial layout is designed to capture as much detail as possible from the neuronal culture. The system comes with a recording device, BioCam, with BrainWaveX software to visualise, record and do preliminary data analysis. The device can record at up to 18kHz sampling frequency. Figure 2.9(A) shows a CMOS based MEAs (3Brain AG) with a zoomed in section of the active area (B) where 4096 electrodes are laid out in a 64 x 64 grid, along with neuronal responses recorded with the BrainWaveX software (C).

Figure 2.9: A. High density CMOS-based MEA B. Zoomed in section of the active area of the MEA chip C. Electrical activity recorded by 4096 electrodes of the APS device.

rectly at the point where the biological signals are measured. Due to multiplexing, multiple channels can be read from a single wire which results in minimisation of the number of physical wires and an increase in the density of the electrodes.

The MEAs used are of three different types with different area size and elec- trode layout.

• HD-MEA Prime: Prime is an electrode array of 4096 electrodes laid out in an area of 2.67 mmx 2.67 mm Figure2.10(A). The electrode size 21µmx21µm with pitch distance of 42µm.

• HD-MEA Arena: Arena has the same recording array as the Prime, but it offers a larger surrounding flat area(6mmx6mm) Figure 2.10(B). This can be useful for placing a larger tissue slice on the chip for optimum coupling.

• HD-MEA Stimulo: The Stimulo, as the name suggests, incorporates in addition stimulating electrodes laid out in a 4x4 uniform grid. The Stimulo

has a large sensing area of 5.12mmx5.12mm with electrode pitch distance of 81µm Figure2.10(C).

Figure 2.10: (A). HD-MEA Prime (B). HD-MEA Arena. (C). HD-MEA Stimulo