## 3 Devices for Flexible and Organic Neural Interfaces
### Flexible Microelectrodes
Reproduced with permission from [103], Frontiers Research Foundation, copyright 2017. Notice that, in order to allow minimal implantation damage and to fit the curved surface of the neuronal tissues, device thickness, flexibility and bio-compatibility are the criteria to design functional ECoG electrodes that allow in vivo recording during behaviour. However, another important criterion is the electrochemical impedance of the implanted electrodes that should be as small as possible. For this reason, [PERSON] et al. fabricated a 32 channel MEAs with Cr/Ag/Cr electrodes (Figure 5d) showing that the less expensive silver electrodes feature a much smaller impedance as respect to standard gold and platinum electrodes [111]. Despite all the advantages with respect to standard electrodes, one major failure mechanism of flexible MEAs is the delamination of the metal electrodes (e.g., gold) on top of the organic substrate. [PERSON] et al. solved this issue by encapsulating the electrodes using polydimethylsiloxane (PDMS) [112]. Figure 5e shows the comparison between gold electrodes with and without PDMS encapsulation. Remarkably, despite the increase of the electrode impedance, the PDMS-coated electrode is still able to correctly detect the biological signals through capacitive coupling with the living tissue while preventing the delamination of the gold electrode. Figure 5: (**a**) Spatial layout of the 256 electrodes array for multisite recording [101]. The three sections outlined by the dashed lines, were designed for the medial wall (26 electrodes), the lateral surface (212 electrodes) and the supratemporal plane of the lateral sulcus (18 electrodes, STP array), respectively. A pair of electrodes designed for bipolar recording is magnified in the insert; (**b**) Schematic representation of a section of the array reported in (**a**). The total thickness of the electrode is 53–56 \(\upmu\)m; (**c**) Photograph and schematic cross section of the 16 electrodes array reported in [110]; (**d**) Demonstration of the flexibility of the 32 electrodes array reported in [111]; (**e**) PDMS-encapsulated (left) and non-encapsulation (right) electrodes, retrieved 4 weeks after subcutaneous in vivo implantation. (**a,b**) Reprinted from [101]. Copyright 2014, with permission from Elsevier; (**c**) Reproduced with permission from [110], MDPI, copyright 2013; (**d**) Reprinted from [111]. Copyright 2017, with permission from Elsevier; (**e**) Reproduced from [112]. Copyright 2017, with permission of Springer. Instead of using PDMS as a dielectric insulator, a trade-off between low impedance electrodes and long-term stability can be achieved by means of conductive polymers like polypyrrole and poly(3,4-ethylenedioxythiophene) (PEDOT), that are typically doped with poly(styrene sulfonate) (PSS) [113; 114]. The organic coatings help to prevent delamination and corrosion of the metal layer that is no-longer in direct contact with the ionic solution. At the same time, the increased roughness of the coated electrodes allows the realization of micro-electrodes with a larger effective area and with the capability of transport both electronic and ionic charges, hence reducing the overall electrodes impedance up to two orders of magnitude with respect to bare gold electrodes. Following this idea, [PERSON] et al. developed an array of 256 PEDOT:PSS electrodes onto a parylene C substrate [115] (Figure 6). Featuring an interelectrode spacing of 30 \(\upmu\)m, such flexible and organic ECoG MEA was able to carry out in vivo recording of both local field and action potentials with an excellent implant stability, paving the way to all organic conformable neural interfaces. Another outstanding result was achieved by [PERSON] et al. who created a mesh of 30 gold electrodes fabricated on top of a silk bio-resorbable substrate [27].