Small wire, big surface: the electrochemical probe patent

Every electrode that touches living tissue faces a trade-off. Make it thinner, and it disturbs the tissue less. But a thinner wire has a smaller tip, and a smaller tip is a worse listener: its impedance rises, signals weaken and noise grows.

US Patent 10,768,139, filed by the Francis Crick Institute with inventors Romeo-Robert Rácz, Mihaly Kollo and Andreas Schaefer, describes a way around that trade-off.^[1] Keep the wire very thin, but rebuild its tip with chemistry, so that a tiny tip behaves like a much larger one.

The problem it addresses

The patent explains that as neural sensors are miniaturised, their impedance in salty body fluids rises, signals weaken and noise increases.^[1] That forces designers to add amplifier stages, filtering and matching circuitry, which consume power, a critical issue for battery-powered implants. Improving signal strength while keeping electrodes at micrometre scale is, in the patent's words, "of paramount importance".

What the patent describes

The first claim sets out the probe.^[1] It is a bundle of wire electrodes lying alongside one another. Each has a conducting core and insulation, a diameter of 25 micrometres or less, and a front end that tapers to a point. On the polished tip sits a layer of metal or metal-oxide nanostructures, each 500 nanometres wide or less, which reduces impedance and does not extend beyond the wire's own diameter. A functionalisation layer sits on top. Along the sides of each wire, the insulation touches the core directly.

Further claims cover separate insulating sheaths or a shared insulating matrix; noble-metal and gold nanostructures; a recessed tip with the layers deposited inside the recess; different groups of wires carrying different functionalisation layers; a harness with threads that can steer the bundle's tip; pairing the probe with an amplifier chip; and methods for making it.

Rebuilding the tip

The patent describes a two-step coating.^[1] First, gold is grown onto the polished tip as a flaky, highly branched layer of nanostructures, which provides a large surface area for charge transfer. Then a spongy, porous layer of iridium oxide is deposited on top. The patent notes that iridium oxide supports pH sensing, and that its high charge storage capacity helps deliver current for stimulation and to detect substances such as dopamine.

The patent compares three tips: bare polished gold, iridium oxide alone, and the two-layer combination, which it labels "JULIE". The two-layer tip had an impedance about an order of magnitude lower than the other two.

Recording from neurons

To test the electrodes, the inventors recorded in the olfactory bulb of anaesthetised mice aged 4 to 6 weeks.^[1] They reliably recorded spikes of up to 1.6 millivolts, and well-isolated signals of 500 to 1,500 microvolts from neurons about 20 to 30 micrometres from the tip. Units could be recorded throughout the olfactory bulb after the probe had been lowered several millimetres and brought back to a shallower position. The patent compares these with recordings from a typical commercially available probe, which show much smaller spikes.

The patent lists further advantages: penetrating wires 2 to 5 times smaller and recording sites up to 50 times smaller than conventional probes, which it says reduce tissue displacement and improve the separation of signals from individual neurons; and tips that can be sharpened to a needle point.

Connecting to a chip

At the back end, gold bumps are grown on each wire. The patent describes pressing these directly onto the contact pads of an amplifier chip, such as a microelectrode array chip or a CMOS-based potentiostat, with no connector in between.^[1] For bundles with more than about 1,000 wires, it notes, wiring each one individually becomes impractical, so direct pressing is the more useful approach.

This is the same principle that was later tested at scale in the CHIME work, where bundles of hundreds of wires were pressed onto CMOS chips to record from the mouse brain.^[3]

One probe, many sensors

Because the gold layer provides a platform for many coatings, the patent describes probes in which different groups of wires carry different functionalisation layers: for example, one group with iridium oxide for pH or electrical signals, another with DNA probes, another with an enzyme to detect alcohol.^[1] It lists potential applications including neural recording and stimulation, neurotransmitter detection, pH, glucose, DNA and immune sensing, and scanning electrochemical microscopy. These are described as possible uses, not demonstrated results.

The patent also describes a practical manufacturing aid: a connector filled with a carbon composite that touches every wire end at once, so a bundle with many wires can be coated in a single step without damaging the gold bumps.

What a patent is, and is not

• What it shows. A specific electrode design and how to make it, with measurements of impedance and recordings from the mouse brain reported in the patent.^[1]
• What it does not show. Long-term implantation, safety, or use in people. Most of the listed sensing applications are described rather than tested.
• What defines it. The scope of protection is set by the granted claims, which should be read in full on the public register.

Why it matters

The idea separates two things that usually compete: the size of the wire, which decides how gently it meets tissue, and the surface of its tip, which decides how well it senses. That principle runs through the work that followed. The electrodes were described in detail, with recording and stimulation experiments, in the jULIEs paper.^[2] Read our guide to that paper. A later patent applied the same electrodes to a high-channel visual implant.^[4]