Electrode materials and the foreign body response

The array-quality grid from the intracortical data project showed a chronic array with a fraction of its channels still recording clean neurons after years. The reason is the foreign body responseForeign body response The scarring the body builds around an implant, which insulates electrodes and degrades recordings over months to years. Related: Intracortical recordingIntracortical recording Recording from tiny electrodes inserted into the cortex itself, close enough to hear individual neurons.. Glossary entry: the brain’s reaction to a stiff, alien object that moves relative to it with every heartbeat. Understanding the response is the first step to the materials that might beat it, and the materials spine is largely the search for those.

What happens

Insertion. The electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry cuts through capillaries and neurons on its way in. Bleeding, immediate cell death along the track, and the blood-brain barrier is breached.

Weeks. Microglia, the brain’s immune cells, arrive and coat the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry. Astrocytes activate and begin to form a sheath. Inflammatory signalling continues as long as the barrier leaks.

Months to years. A glial scar, tens of micrometres thick, encapsulates the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry. Neurons within about 50 to 100 µm are lost or retreat. The scar raises the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry’s impedance and pushes the recordable neurons beyond the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry’s reach (recallRecall A correction or removal of a marketed device; FDA's public database of them is a catalogue of how designs fail people. Glossary entry that spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry amplitude falls off within a hundred micrometres). Channels go from single units to multi-unit hash to nothing.

Meanwhile, the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry. Insulation cracks, metal corrodes, connectors fail, and the tether to the skull transmits every micromotion of the brain as shear at the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry tip, which sustains the inflammation.

Strategies

Smaller. ElectrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry with cross-sections near the size of a cell (carbon fibres at 7 µm; polymer threads a few micrometres thick) displace less tissue and provoke less response. Insertion becomes the problem: they buckle, and need stiffeners that dissolve or retract, or robotic insertion.

Softer. Polyimide, parylene, and SU-8 substrates are far more compliant than silicon; hydrogels and shape-memory polymers softer still. Flexible arrays conform to the cortical surface for ECoGElectrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. Related: Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. , Intracortical recordingIntracortical recording Recording from tiny electrodes inserted into the cortex itself, close enough to hear individual neurons. , ECoG gridECoG grid A flexible sheet of electrodes laid on the brain's surface, used clinically for epilepsy and in research for speech decoding. . Glossary entry and thread into tissue for penetrating designs. The Neuralink threads and several academic mesh electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry are this strategy.

Coated. Anti-inflammatory drugs released from the coating, proteins that mimic the extracellular matrix, or conductive hydrogels that blur the mechanical boundary. Encouraging in animals; durability over years unproven.

Untethered. Free-floating electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry with wireless links, so that skull motion is decoupled from the brain. Small wireless “motes” and the stentrodeStentrode Synchron's electrode array mounted on a stent and delivered through a blood vessel to sit against the motor cortex without open surgery. Glossary entry’s placement in a blood vessel are versions of the idea.

Recording strategy. Accept the loss of single units and design decodersDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. Related: ClassifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. , Kalman filterKalman filter An estimator that blends a prediction from a motion model with a noisy measurement, weighting each by how much it trusts it; the decoder behind most intracortical cursor control. , Brain-computer interface (BCI)Brain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. . Glossary entry for threshold crossings, spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry-band power, and LFPLocal field potential (LFP) The slow, summed electrical activity of many neurons near an electrode, below a few hundred hertz, as opposed to individual spikes. Related: Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry, which persist much longer. This is what the human BCIBrain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. Related: DecoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. , Closed loopClosed loop A system in which the output feeds back to influence the input, such as a user adjusting their brain activity while watching the cursor it controls. . Glossary entry programs did, and it is why they still work.

Materials you will meet

Platinum and platinum-iridium (stable, biocompatible, the clinical default). Iridium oxide (high charge capacity for stimulation). Silicon (the Utah ArrayUtah Array The 10-by-10 grid of 1.5 mm silicon needles invented at the University of Utah, the electrode in nearly every human intracortical BCI study to date. Related: Intracortical recordingIntracortical recording Recording from tiny electrodes inserted into the cortex itself, close enough to hear individual neurons. , Utah Slanted Electrode Array (USEA)Utah Slanted Electrode Array (USEA) A version of the Utah Array with needles of graded lengths, designed to reach different depths of a peripheral nerve. . Glossary entry and NeuropixelsNeuropixels Silicon probes with hundreds to thousands of recording sites along a thin shank, the current standard for large-scale animal recording. Related: Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. . Glossary entry: manufacturable at scale, stiff). Polyimide and parylene (flexible substrates and insulation). PEDOT:PSSPEDOT:PSS A conductive polymer coating that drops a microelectrode's impedance by an order of magnitude and improves charge injection for stimulation. Related: Electrochemical impedance spectroscopy (EIS)Electrochemical impedance spectroscopy (EIS) Measuring an electrode's impedance across many frequencies to characterize its interface, standard in electrode development. , Charge densityCharge density The charge injected per pulse divided by electrode area; keeping it below safe limits prevents electrode damage and tissue injury. . Glossary entry (impedance and charge coating). Carbon fibre and glassy carbon. Gold (traces; poor as a recording surface uncoated). Each has a biocompatibility record, a fabrication route, and a failure mode, and the materials spine’s job is knowing all three.

Measuring the response

Histology at endpoint (staining for microglia, astrocytes, and neurons at graded distances from the track) is the definitive measure and requires animal work under protocol. Impedance spectroscopy over time is the non-invasive proxy: the scar shows as a rising impedance and a changing phase, which is why chronic implants log impedance and why your EISElectrochemical impedance spectroscopy (EIS) Measuring an electrode's impedance across many frequencies to characterize its interface, standard in electrode development. Related: Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , PEDOT:PSSPEDOT:PSS A conductive polymer coating that drops a microelectrode's impedance by an order of magnitude and improves charge injection for stimulation. . Glossary entry skills matter here. Recording yield (units per channel over time) is the outcome that matters for a BCIBrain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. Related: DecoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. , Closed loopClosed loop A system in which the output feeds back to influence the input, such as a user adjusting their brain activity while watching the cursor it controls. . Glossary entry.