The microwire 'brush' array electrodes that are now commonly used in the SCNI were initially developed in collaboration with Igor Bondar and first reported by Bondar et al., 2009. The concept is to take a bundle of insulated microwires and insert them into the brain through a guide tube. As the electrode tip is lowered, the individual wires splay out in the tissue, allowing for stable isolation of extracellular spiking activity for a local population of neurons over long time periods. Each microwire consists of a nickel-chromium-aluminum core of 12.5 µm diameter, insulated with polyimide by the manufacturer (‘IsaOhm’, Isabellenhuete, Germany). These electrodes are currently commercially available from MicroProbes, MD in lengths from 2.5 mm to 120 mm.
Figure 1. Chronic "microwire brush array" manufactured by MicroProbes, MD.
While this basic electrode design continues to be used in the SCNI, we are continuing to develop the implant hardware that houses the electrode, mircrodrive, and connectors. A chronically implantable microdrive designed by David Ide and David McMahon at the NIH was first published by McMahon et al., 2014 and is currently available commercially from Hybex.
Figure 2. Coronal MR image and micro-CT showing an implanted chronic microwire multielectrode and an illustration of the 'brush' tip (MR image courtesy of McMahon et al., 2014).
Microwire brush tip development notes
In order to improve our cell yield per implant, we attempted moving from 64-channel bundles to 128-channel bundles several years ago. However, our experience was that 128-channel bundles do not produce comparable yield to 64-channel bundles, possibly due to the mechanical properties of the increased bundle size. We subsequently reverted to using 64-channel bundles, but also miniaturized the microdrive design in order to obtain between 128-256 channels per implant using multiple 64-channel bundles.
During our tests to discern why the 128-channel bundles might not be working as well as we had hoped, Dr Kenji Koyano performed a series of tests, producing microwire bundles with a variety of tip designs, and inserting them into gelatin as a simulated brain tissue medium. His observations of how the individual microwires deflect upon entering the gelatin are show below (Figure 3). The results suggest that a non-flat tip shape and active pre-splaying of the wires prior to insertion in the guide tube both improve splaying of the wires in tissue.
Figure 3.Simulations of microwire brush tip trajectories in tissue. Columns show different brush tip preparations. Top row shows the state of the array tip prior to insertion in the guide tube. The second and third rows show the gradual deflection of individual microwires with distance emerged from the guide tube tip. (Figure courtesy of Dr. Kenji Koyano).
- Bondar IV, Leopold DA, Richmond BJ, Victor JD, Logothetis NK (2009). Long-Term Stability of Visual Pattern Selective Responses of Monkey Temporal Lobe Neurons.
- McaMahon DBT, Afuwape OAT, Ide DC, Leopold DA (2014). One month in the life of a neuron: longitudinal single-unit electrophysiology in the monkey visual system.
- Mulliken GH, Bichot NP, Ghadooshahy A, Sharmab J, Kornblith S, Philcock M, Desimone R (2015). Custom-fit radiolucent cranial implants for neurophysiological recording and stimulation.
- Englitz B, David S, Depireux DA, Shamma SA (2011). The Array Drive : Optimizing the Yield and Flexibility of Chronic, Multielectrode Array Recordings