## 3 Devices for Flexible and Organic Neural Interfaces
### Organic Field-Effect Transistors
Figure 10: (**a**) Schematic view of cardiomyocyte-like cells integrated with an OECT; (**b**) Photographs of a flexible OECT array device in bent condition; (**c**) Representative traces of continuous and single APs recorded by an OECTs (on rigid substrate). Current traces were converted to voltage through a 10 k\(\Omega\) resistor and amplified by a factor 100. Reproduced with permission from [128], [PERSON], copyright 2015. Similarly to OECT technology, electrolyte-gated organic field-effect transistors (EGOFETs)--also known as water-gated OFETs--are worth mentioning. EGOFETs can operate either with pure water or saline solutions as gate medium and they can be fabricated using many different semiconductors (both p- and n-type) [47; 131; 132; 133]. Such transistors are characterized by currents that are much smaller than the OECT counterpart [134; 135]. However, in an OECT, the on/off switch is produced by ions transfer from the electrolyte and the semiconductor (doping/de-doping) [136], whereas only capacitive processes occur for EGOFETs, without charge transfer [137]. Consequently, EGOFETs are intrinsically faster and more stable than OECTs [134], hence they can be successfully employed as sensors for different kind of analytes (for a review see [137]). [PERSON] et al. deposited a thin film of pentacene, with a thickness of 9 nm, to fabricate EGOFET devices to perform in vitro recording and stimulation (using the liquid gate as stimulating electrode) of neural network activity (Figure 12a) [138]. They showed that murine neural stem cells can be adhered on top of functional devices without the need of an additional layer of cell-adhesive molecules (e.g., PDL) and then differentiated into neuronal networks (Figure 12b). The EGOFET-based sensor exhibited a very good stability under standard cell culture conditions for nine days and, after liquid-gate stimulation, it was able to successfully detect the neural network activity (Figure 12c). The same group recently showed that EGOFETs devices with incorporated gold nanoparticles (Figure 12d) can behave both as a transistor and as a memristor [139], hence opening the door to new multifunctional bio-sensing devices. Figure 11: (**a**) Optical micrograph (left) and schematic layout (right) of the channel of OECT and surface electrode (left), in which the Au films that act as source (S), drain (D) and electrode pad (E) are identified; (**b**) Optical micrograph of the ECoG probe placed over the somatosensory cortex (left) with the craniotomy surrounded by dashed lines; and wiring layout of the transistor (right) with the blue box indicating the brain of the animal; (**c**) Recordings from an OECT (pink), a PEDOT:PSS surface electrode (blue) and an Ir-penetrating electrode (black). The transistor was biased with V\({}_{\text{DS}}=-0.4\) V and V\({}_{\text{GS}}=-0.3\) V and the scale of 10 mV is for both surface and penetrating electrodes; (**d**) Layout of the entire probe showing the parylene and the shuttle parts (top), corresponding optical micrograph (bottom) and principle of delamination (right). (**a–c**) Reprinted by permission from Macmillan Publisher Ltd.: Nature Communications [129], copyright 2013; (**d**) Reproduced with permission from [130], Wiley, copyright 2015. Graphene solution-gated field-effect transistors (G-SGFETs) depicted in Figure 13a, are another alternative to stiff inorganic neural interfaces. [PERSON] et al. fabricated an array of graphene transistors onto a rigid substrate (Figure 13b) and demonstrated that G-SGFETs are capable to record cell signals with SNRs surpassing state-of-the-art devices for bioelectronic applications [140]. Subsequently, the same research group reported the fabrication of flexible arrays of G-SGFETs on polyimide, showing that the electronic properties of their devices do not degrade during repeated bending [141]. After cell culture, they demonstrated that it is possible to perform recording of cell action potentials from cardiomyocyte-like cells with a high SNR.