The rise of flexible electronics in neuroscience, from materials selection to in vitro and in vivo applications

Figures & data

Table 1. All the values are referred to transistors tested at room temperature for a fixed W/L geometry.

	Mobility (cm^2/Vs)	Roll-to-roll process	Max temperature (°C)	Device type
Organics	0.1–10	Yes	150	Mainly p-type
Amorphous Silicon	1	Unlikely	300	Both n-type and p-type
Polycrystalline Silicon	100	Unlikely	350	Both n-type and p-type
Crystalline Silicon	1000	No	850	Both n-type and p-type
Metal Oxide	50	Possible	350	Mainly n-type

Mobility values indicate the average number for the specific technology, without taking into account complex procedure that in same case can enhance these standards. The process temperatures indicate the maximum value usually reached also if it is possible to fabricate devices at lower temperatures with lower quality.

Figure 1. (a) A schematic representation of a neural interface divided in its main components; (b) some critical aspects to be considered for the designing of a performing neural interface.

Figure 2. A sketch showing the parts of head where grids can be placed to collect signals from the cortex: subdural devices, epidural devices and penetrating electrodes. Non-invasive electrodes have not been taken into account in this scheme.

Figure 3. (a) A sketch showing the fundamental elements of a passive grid: a conductive film embedded into two layers of flexible polymeric materials; (b) the five properties that enhance grid functionalities to overcome challenges for a high-density recording/stimulating active grid.

Figure 4. (a) Typical penetrating electrodes (Utah array); (b) Nanomesh electrodes copyright science advances [89]; (c) Ultra-flexible polyimide-based MEAs covering the whole hemisphere of a rat brain, copyright scientific reports [96]; (d) Syringe-injectable electronics copyright nature nanotechnology [87]; (e) Pure PEDOT:PSS hydrogels copyright nature communications [97]; (f) Electronic dura made in stretchable PDMS Copyright Science [31] .

Figure 5. (a) Flexible high-density active electrode array copyright nature neuroscience [16]; (b) Organic electrochemical transistor–based electronics copyright nature communications [151]; (c) Freely deformed stretchable array of CMOS inverters in PDMS copyright national academy of sciences 2008 [28]; (d) Ultra-thin imperceptible e-skin copyright IEEE Spectrum [120]; (e) Colorized SEM image of an epidermal electronic systems copyright advanced materials [121] .

Table 2. A list of the main features of the hybrid electronics systems used in neuroscience in terms of total bandwidth, sampling frequency, number of recording channels, power consumption, level of noise and ADC characteristics.

Bandwidth	Sampling frequency	Number of channels	Power consumption	Input referred noise	ADC resolution	ADC	Stimulation	Tested subject	Ref.
[0–200 Hz]	51.2 kHz	1	28.7 µW	1.2 µV [25.4–25.6 kHz]	12 bits	Σ-Δ	No	No	[154]
[0.5 Hz–5 kHz]	625 KS/s shared	32	134 µW per channel	0.7 µV [0.5–300 Hz]	12 bits	SAR	No	Rat	[155]
0 – 10 kHz	31.25 kSa/s	96	6.4 mW	2.2 µV [0 10 kHz]	10bits	SAR	No	Rhesus macaque	[156]
[0.5–100 Hz]	500 kSa/s shared	16	3.26 µW per channel	2.09 µV [0.5–100 Hz]	10 bits	SAR	No	No	[157]
[0.1 Hz–6 kHz]	200 kS/s shared	16	8.1 µW per channel	2.0 µV [n.a.]	12 bit	SAR	No	Rat	[158]
[0.1–1000 Hz]	1 kHz	128	39 µW per channel	3.0 µV [n.a.]	12 bit	–	No	Human phantom	[159]
[0.5–300 Hz]	1 kHz	64	15.5 µW per 32 channels	1.0 µV [0.5–300 Hz]	12 bits	–	No	Rhesus macaque	[160]
[2–175 Hz]	–	64	0.69 µW per channel	2.3 µVRMS	No adc	–	No	Human	[161]
[12–190 Hz]	–	64	0.219 µW per channel	2.19 µVRMS	No adc	–	No	Human	[161]
[0.1 Hz–20 kHz]	1 MS/s shared	32	158 µW per channel	2.4 µVRMS	16 bit	–	Yes	Mouse and ferret slices	[^Citation96,Citation162]
[1 Hz–7.5 kHz]	25 KHz for channel	512	< 6 μW per channel	7.5 ± 0.67 μVRMS [300 Hz–7.5 kHz]	12 bits	–	No	Rat	[100]
[0.01 Hz–10 kHz]	7.25 kS/s per channel	24	886 μW with ADC	4.7 μVRMS (10–5 KHz)	8 bits	SAR	Yes	Rat	[163]
[0.5 Hz–7 kHz]	1 MS/s shared	16	2.5 µW per channel	2.85 μVRMS	10 bits	SAR	No	Rhesus macaque	[164]
[20 Hz–15 KHz]	25 KHz for channel	32	11.7 µW per channel	3 μVRMS	10 bits	SAR	Yes	Rat	[165]

To compare the different systems, the power consumption of the low noise amplifiers of the systems has been taken into consideration.

Figure 6. Possible technologies to be integrated in a future neural interface: local temperature and pH sensing, magnetic stimulation, drug delivery embedded functionalities, infrared-based communication module, reliable bidirectional communication and even brain-to-brain connectivity. Flexible electronics represents a promising ally to make it possible.

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