Ultra-flexible microelectrode array nanostructured by FIB: A possible route to lower the device impedance

•Fabrication of ultra-flexible microelectrodes arrays (MEAs) is reported.•The electrodes are nano-patterned by using focused ion beam technique.•In vitro analysis is performed in KCl 100 mM solution to simulate the brain environment.•The impedance behaviour is investigated by changing the pitch of the pattern and the depth of the nano-holes.

In this work we study the impedance behaviour of microelectrodes arrays nanostructured with different patterns by using a dual beam focused ion beam FIB. The devices were first fabricated by embedding a metal tri-layer of Ti/Au/Cr, 250 nm thick, into two ultra-flexible polyimide layers, reaching a final thickness of 8 μm. Then, different patterns of holes (diameters from 100 to 500 nm) were produced by milling single pixels at a current of ∼10 nA at increasing pitch (up to 3000 nm) and at different dwell time (up to 12 ms). The adopted milling parameters (in particular the single pixel milling strategy at rather high current) allow to obtain a nano-pattern onto an electrode area of 150 × 150 μm in a reasonably short time (from 10 s to 5 min for each electrode). Other patterns were also investigated in which the FIB was scanned uniformly to induce surface roughening. Electrochemical impedance spectroscopy analysis was performed in vitro in a KCl solution 100 mM, by using a potentiostat VersaSTAT 4 by PAR, finding an average impedance reduction of about one order of magnitude respect to the impedance of the flat electrodes.

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