For the probe, various materials and associated processes were tested, in order to obtain the critical functional parameters: conductivity, adhesion, electrochemical window, charge storage and injection capacities. These trials were done first on a planar replica of the targeted cylindrical geometry, aiming, however, at developing the technologies seamlessly adapted for direct transfer to the needle-type structure. The electrochemical parameters obtained for the ~1 mm2 carbonic (graphitic paste) distal contacts were promising, showing the viability of the materials and of the optimized processes.  

Work on the wireless headstage progressed with the development of the recording, stimulation, wireless transmission, and battery management modules.

At this stage, a first integration of the two demonstrator components was done. The in-vitro (saline solution) recording and stimulation tests showed that:
- current-controlled biphasic-symmetric stimulation signals of 150 Hz and 120 ms half-period requested an unbiased voltage excursion still in the symmetric water window at 10 mA amplitude. Also, the voltages developed evolve smoothly with the current amplitude.
- when emulating a LFP recording in a floating output configuration, the results showed low baseline noise and no spectral alteration.

Optimizations were started for transferring the planar processes to the long (~25 cm) and thin (~0.4 mm radius) cylindrical geometry.  

Next, we will concentrate on:
- advanced integration, optimization and in-vitro testing
- in-vivo tests

Phase 3

In the last phase of the project, the developed technology was employed to manufacture various probe samples for in vitro and in vivo tests. The samples realized on 550, and 770 mm diameter shanks of 10 to 23 cm length comprised 2, and 3 longitudinal channels, with the distal graphitic macrocontacts of ~1 mm2 GSA being circumferentially segmented towards 2 and 3 radial views (180° or 120° orientations, respectively).

To assess the behavior under realistic current-controlled stimulation pulses, stress endurance tests were performed using cathode-leading, biphasic, pulsed current with 150 Hz frequency and 120 ms phase duration. Even the most stressed channel (pulse amplitude: 10 mA; test duration: 2 hours) was not affected chemo-physically or morphologically, or in terms of adherence, validating the stability specifications.

The in vitro chronopotentiometry characterizations with biphasic current pulses (150 Hz, 150 ms phase duration) of 1 to 9 mA amplitude provided the confirmation of a stable capacitive behavior of the carbonic contacts and led to a charge injection limit of more than 135 mC/cm2 for 9 mA current amplitude, hat covers the operational demands during test stimulation for DBS electrophysiological mapping.

The in vivo tests in adult rats in hippocampal and thalamic positions showed that segmented carbonic contacts can be used to record LFPs directionally and with high signal quality. The wireless system is capable of data transmission for one microelectrode recording channel sampled at 32 kHz and 4 macro recording channels sampled at 1 kHz, resulting in a total transmission rate of 792 kbps. In constant current stimulation mode, the system allows sustaining current pulses

with a peak-to-peak voltage of 20 V, and a minimum pulse duration of 10 μs, which is an advantage in setting up the usual stimulation pulse durations (between 60 μs and 120 μs). The total consumption is low (100 mAh), resulting in a battery life of more than 6 hours, which covers the operational demands for DBS electrophysiological mapping.

The project concludes at the targeted TRL 4 level.