Flexible polymer neural probes are an attractive emerging approach for invasive brain recordings, given that they can minimize the risks of brain damage or glial scaring. However, densely packed electrode sites, which can facilitate neuronal data analysis, are not widely available in flexible probes. Here, we present a new flexible polyimide neural probe, based on standard and low-cost lithography processes, which has 32 closely spaced 10 μm diameter gold electrode sites at two different depths from the probe surface arranged in a matrix, with inter-site distances of only 5 μm. The double-layer design and fabrication approach implemented also provides additional stiffening just sufficient to prevent probe buckling during brain insertion. This approach avoids typical laborious augmentation strategies used to increase flexible probes’ mechanical rigidity while allowing a small brain insertion footprint. Chemical composition analysis and metrology of structural, mechanical, and electrical properties demonstrated the viability of this fabrication approach. Finally, in vivo functional assessment tests in the mouse cortex were performed as well as histological assessment of the insertion footprint, validating the biological applicability of this flexible neural probe for acquiring high quality neuronal recordings with high signal to noise ratio (SNR) and reduced acute trauma.

The demand for affordable tools for recording extracellular activity and successfully isolating single units from different brain preparations has pushed researchers and companies to invest in developing and fabricating new recording devices. However, depending on the brain region of interest, experimental question or type of preparations, different devices are required thus adding substantial financial burden to laboratories. We have developed a simple and affordable tetrodebased device that allows interchangeable extracellular recordings of neural activity between in vivo and ex vivo preparations and can be easily implemented in all wet-bench laboratories. Spontaneous activity from several putative single neurons could be easily recorded and isolated by lowering the device into ex vivo cerebellum brain slices. The same device was also used in vivo, lowered into primary auditory cortex of adult anesthetized transgenic mice expressing channelrhodopsin in cortical neurons. Acoustic stimulation of the contralateral ear or direct laser optogenetic stimulation successfully evoked cortical activity at the recording site. Several isolated putative single neurons presented time-locked activity response to the different stimuli. In summary, we developed an affordable, versatile and modular device to facilitate tetrode extracellular recordings interchangeably between in vivo anaesthetized animals and ex vivo brain slice recordings. HighlightsDeveloped a versatile and modular device to facilitate tetrode acute brain recordings interchangeably between in vivo and ex vivo preparations.Conducted ex vivo extracellular recordings in acute cerebellar slices.Conducted in vivo extracellular recordings in auditory cortex of anaesthetized mice.Recorded and isolated multiple single units in both acute slices and anaesthetized mice recordings using the same device.Device can be easily extended to accommodate optic fiber and cannula.

The demand for affordable tools for recording extracellular activity and successfully isolating single units from different brain preparations has pushed researchers and companies to invest in developing and fabricating new recording devices. However, depending on the brain region of interest, experimental question or type of preparations, different devices are required thus adding substantial financial burden to laboratories. We have developed a simple and affordable tetrodebased device that allows interchangeable extracellular recordings of neural activity between in vivo and ex vivo preparations and can be easily implemented in all wet-bench laboratories. Spontaneous activity from several putative single neurons could be easily recorded and isolated by lowering the device into ex vivo cerebellum brain slices. The same device was also used in vivo, lowered into primary auditory cortex of adult anesthetized transgenic mice expressing channelrhodopsin in cortical neurons. Acoustic stimulation of the contralateral ear or direct laser optogenetic stimulation successfully evoked cortical activity at the recording site. Several isolated putative single neurons presented time-locked activity response to the different stimuli. In summary, we developed an affordable, versatile and modular device to facilitate tetrode extracellular recordings interchangeably between in vivo anaesthetized animals and ex vivo brain slice recordings. HighlightsDeveloped a versatile and modular device to facilitate tetrode acute brain recordings interchangeably between in vivo and ex vivo preparations.Conducted ex vivo extracellular recordings in acute cerebellar slices.Conducted in vivo extracellular recordings in auditory cortex of anaesthetized mice.Recorded and isolated multiple single units in both acute slices and anaesthetized mice recordings using the same device.Device can be easily extended to accommodate optic fiber and cannula.

Flexible probes for brain activity recordings are an attractive emerging approach that reduces mechanical mismatch between probe and neuronal tissue, thus minimizing the risk of brain damage or glial scaring. Although promising, flexible probes still present some technical challenges namely: i) how to overcome probe buckling during brain insertion given its intrinsically low mechanical rigidity; ii) how to fabricate closely spaced electrode configurations for high density recordings by standard lithography techniques in the flexible substrate. Here, we present a new flexible probe based solely on standard and low-cost lithography processes, which has closely spaced 10 μm diameter gold electrode sites on a polyimide substrate with inter-site distances of only 5 μm. By using a double-layer design and fabrication approach we were able to accommodate closely spaced electrode sites at two different depths from probe surface while also providing additional stiffening, just sufficient to prevent probe buckling during brain insertion. Detailed probe characterization through metrology of structural and electrical properties and chemical composition analysis, as well as functional assessment through in vivo high density recordings of neuronal activity in the mouse cortex, confirmed the viability of this new fabrication approach and showed that this probe can be used for obtaining high quality brain recordings with excellent signal to noise ratio (SNR).

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