Silke Musa

Fully integrated silicon probes for high-density recording of neural activity

Sensory, motor and cognitive operations involve the coordinated action of large neuronal populations across multiple brain regions in both superficial and deep structures. Existing extracellular probes record neural activity with excellent spatial and temporal (sub-millisecond) resolution, but from only a few dozen neurons per shank. Optical Ca2+ imaging offers more coverage but lacks the temporal resolution needed to distinguish individual spikes reliably and does not measure local field potentials. Until now, no technology compatible with use in unrestrained animals has combined high spatiotemporal resolution with large volume coverage. Here we design, fabricate and test a new silicon probe known as Neuropixels to meet this need. Each probe has 384 recording channels that can programmably address 960 complementary metal–oxide–semiconductor (CMOS) processing-compatible low-impedance TiN sites that tile a single 10-mm long, 70 × 20-μm cross-section shank. The 6 × 9-mm probe base is fabricated with the shank on a single chip. Voltage signals are filtered, amplified, multiplexed and digitized on the base, allowing the direct transmission of noise-free digital data from the probe. The combination of dense recording sites and high channel count yielded well-isolated spiking activity from hundreds of neurons per probe implanted in mice and rats. Using two probes, more than 700 well-isolated single neurons were recorded simultaneously from five brain structures in an awake mouse. The fully integrated functionality and small size of Neuropixels probes allowed large populations of neurons from several brain structures to be recorded in freely moving animals. This combination of high-performance electrode technology and scalable chip fabrication methods opens a path towards recording of brain-wide neural activity during behaviour.

Validating silicon polytrodes with paired juxtacellular recordings: method and dataset

Recording in vivo from the same neuron with two different methods is difficult. It requires blindly moving each probe to within ∼100 μm of one another and for this reason such “dual-recordings” are rare. However, comparing the signals measured by different techniques is necessary to understand what they measure. We developed a method to precisely align the axes of two manipulators and used it to gather a “ground truth” dataset for dense extracellular polytrodes.

Bottom-Up SiO2 Embedded Carbon Nanotube Electrodes with Superior Performance for Integration in Implantable Neural Microsystems

The reliable integration of carbon nanotube (CNT) electrodes in future neural probes requires a proper embedding of the CNTs to prevent damage and toxic contamination during fabrication and also to preserve their mechanical integrity during implantation. Here we describe a novel bottom-up embedding approach where the CNT microelectrodes are encased in SiO(2) and Parylene C with lithographically defined electrode openings. Vertically aligned CNTs are grown on microelectrode arrays using low-temperature plasma-enhanced chemical vapor deposition compatible with wafer-scale CMOS processing. Electrodes with 5, 10, and 25 μm diameter are realized. The CNT electrodes are characterized by electrochemical impedance spectroscopy and cyclic voltammetry and compared against cofabricated Pt and TiN electrodes. The superior performance of the CNTs in terms of impedance (≤4.8 ± 0.3 kΩ at 1 kHz) and charge-storage capacity (≥513.9 ± 61.6 mC/cm(2)) is attributed to an increased wettability caused by the removal of the SiO(2) embedding in buffered hydrofluoric acid. Infrared spectroscopy reveals an unaltered chemical fingerprint of the CNTs after fabrication. Impedance monitoring during biphasic current pulsing with increasing amplitudes provides clear evidence of the onset of gas evolution at CNT electrodes. Stimulation is accordingly considered safe for charge densities ≤40.7 mC/cm(2). In addition, prolonged stimulation with 5000 biphasic current pulses at 8.1, 40.7, and 81.5 mC/cm(2) increases the CNT electrode impedance at 1 kHz only by 5.5, 1.2, and 12.1%, respectively. Finally, insertion of CNT electrodes with and without embedding into rat brains demonstrates that embedded CNTs are mechanically more stable than non-embedded CNTs.

Planar 2D-Array Neural Probe for Deep Brain Stimulation and Recording (DBSR)

Implantable micromachined probes with planar electrode arrays for neural stimulation and recording were designed, fabricated and evaluated. Probes have been realized with distributed electrode sites and different electrode configurations (i.e. size and geometry). These were tested for their ability to selectively record and stimulate cortical brain areas. The probes were stereotactically implanted into the cortical region of the rat brains without breaking and were successfully used to measure neural signals and evoke limb contraction in response to electrical stimulation.

Coulometric detection of irreversible electrochemical reactions occurring at Pt microelectrodes used for neural stimulation.

The electrochemistry of 50 μm diameter Pt electrodes used for neural stimulation was studied in vitro by reciprocal derivative chronopotentiometry. This differential method provides well-defined electrochemical signatures of the various polarization phenomena that occur at Pt microelectrodes and are generally obscured in voltage transients. In combination with a novel in situ coulometric approach, irreversible H(2) and O(2) evolution, Pt dissolution and reduction of dissolved O(2) were detected. Measurements were performed with biphasic, charge-balanced, cathodic-first and anodic-first current pulses at charge densities ranging from 0.07 to 1.41 mC/cm(2) (real surface area) in phosphate buffered saline (PBS) with and without bovine serum albumin (BSA). The extent to which O(2) reduction occurs under the different stimulation conditions was compared in O(2)-saturated and deoxygenated PBS. Adsorption of BSA inhibited Pt dissolution as well as Pt oxidation and oxide reduction by blocking reactive sites on the electrode surface. This inhibitory effect promoted the onset of irreversible H(2) and O(2) evolution, which occurred at lower charge densities than those in PBS. Reduction of dissolved O(2) on Pt electrodes accounted for 19-34% of the total injected charge in O(2)-saturated PBS, while a contribution of 0.4-12% was estimated for in vivo stimulation. These result may prove important for the interpretation of histological damage induced by neural stimulation and therefore help define safer operational limits.

In vitro and In vivo electrochemical characterization of a microfabricated neural Probe

The electrochemical behavior of neural implants with 50 µm-diameter platinum electrodes was tested during acute implantations in the motor cortex of anesthetized rats. Custom Ag|AgCl reference electrodes were prepared that could be co-implanted with the probes. The results obtained in vivo are compared with in vitro measurements performed in buffered saline solution (PBS) with and without the addition of bovine serum albumin (BSA). The presence of BSA clearly altered the performance of the electrodes which was studied by means of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), voltage transient measurements (VT) and monitoring of the open circuit potential (OCP). We found that hydrogen gas evolved at 1.22 A/cm2 in BSA-free PBS whereas in BSA-containing PBS it occurred already at 0.51 A/cm2.

Towards a noise prediction model for in vivo neural recording

The signal-to-noise ratio of in vivo extracellular neural recordings with microelectrodes is influenced by many factors including the impedance of the electrode-tissue interface, the noise of the recording equipment and biological background noise from distant neurons. In this work we study the different noise sources affecting the quality of neural signals. We propose a simplified noise model as an analytical tool to predict the noise of an electrode given its geometrical dimensions and impedance characteristics. With this tool we are able to quantify different noise sources, which is important to determine realistic noise specifications for the design of electronic neural recording interfaces.

Time Multiplexed Active Neural Probe with 1356 Parallel Recording Sites

We present a high electrode density and high channel count CMOS (complementary metal-oxide-semiconductor) active neural probe containing 1344 neuron sized recording pixels (20 µm × 20 µm) and 12 reference pixels (20 µm × 80 µm), densely packed on a 50 µm thick, 100 µm wide, and 8 mm long shank. The active electrodes or pixels consist of dedicated in-situ circuits for signal source amplification, which are directly located under each electrode. The probe supports the simultaneous recording of all 1356 electrodes with sufficient signal to noise ratio for typical neuroscience applications. For enhanced performance, further noise reduction can be achieved while using half of the electrodes (678). Both of these numbers considerably surpass the state-of-the art active neural probes in both electrode count and number of recording channels. The measured input referred noise in the action potential band is 12.4 µVrms, while using 678 electrodes, with just 3 µW power dissipation per pixel and 45 µW per read-out channel (including data transmission).

22.7 A 966-electrode neural probe with 384 configurable channels in 0.13µm SOI CMOS

In vivo recording of neural action-potential (AP) and local-field-potential (LFP) signals requires the use of high-resolution penetrating probes. Driven by the need for large-scale recording and minimal tissue damage, a technology roadmap has been defined for next-generation probes aiming to maximize the number of recording sites while minimizing the probe dimensions [1]. In this paper we present a 384-channel configurable active neural probe for high-density recording which implements in situ buffering under each electrode to minimize the crosstalk between adjacent metal lines along the shank and other parasitic effects inherent to traditional passive probes [2]. Up to 966 selectable, neuron-sized electrodes (12×12μm2) were densely packed along a narrow (70μm) and thin (20μm) implantable shank using integrated CMOS. With twice the number of electrodes compared to state-of-the-art neural probes [2], our design achieves the highest electrode count in a single shank reported so far.

High-density optrode-electrode neural probe using SixNy photonics for in vivo optogenetics

Moore s law in neural sciences: we present an optical neural probe (optoprobe) with the highest integration density of optrodes-electrodes using a CMOS process platform in 193 nm lithography. We designed, developed, and packaged an ultrathin (30 μm) optical neural probe, co-integrating silicon nitride (SixNy) photonics and biocompatible titanium nitride (TiN) electrodes (1). Functionality was verified in vivo by optically evoking and electrically recording neuronal activity in a mouse brain. Our design takes advantage of CMOS technology and incorporates 12 miniaturized optical outputs (optrodes) placed symmetrically next to 24 recording electrodes on a narrow 100-μm wide shank. We achieved an unprecedented optrode density by integrating grating couplers (GCs) instead of traditional end-butt coupling. The size of each optrode and electrode is 6 × 20 μm2 and 10 × 10 μm2 respectively, which is the typical size of a neuron. The system was capable of local excitation and recording of transduced neurons, a breakthrough achieved by the high-density integration.