An Injectable Conductive Hydrogel Electrode That Matches Neural Tissue's Softness

Implantable neural interfaces face a fundamental materials problem. Platinum and iridium are widely used for their electrical performance, but their stiffness differs substantially from that of soft neural tissue, and implanting them typically requires invasive surgery. 

Researchers in the Department of Bioengineering at Imperial College London investigated an alternative: an injectable conductive hydrogel designed to provide electrical functionality while better matching the mechanical properties of the tissue it sits against.

The material is built from PEDOT:PSS particles that aggregate and jam together under physical rather than chemical crosslinking, so the gel can be extruded through a hypodermic needle and re-settle once in place.

The team formulated it at five different ratios of PEDOT:PSS to acetic acid and found that composition let them separate two properties that usually trade off against each other: formulations with more PEDOT:PSS relative to acetic acid performed better electrochemically, while higher acetic acid content lowered viscosity and made the gel easier to extrude, though it also weakened structural integrity and complicated handling during fabrication. That gave the team a real design choice rather than a single fixed formulation, and they selected the highest-PEDOT:PSS ratio (1.5:1) for the ex vivo work as the best balance of the two.

Batch emulsion was used to break the gel into an aggregate size of roughly one micron across all formulations, which the researchers report helped the particles pack more closely while maintaining favourable charge transfer between them.

Benchmarking against platinum

Characterising the hydrogel's electrochemical performance required a consistent reference point, and this is where Advent's material entered the work. 

The paper specifies a platinum sheet counter electrode — 99.99% purity, as-rolled temper, 0.10 mm thick — purchased from Advent Research Materials, used in the three-electrode setup for the standard in vitro electrochemical characterisation (geometric surface area 1.92 cm²) and again, at a larger geometric surface area of 6.431 cm², in the ex vivo brain measurements. 

The same three-electrode arrangement was also used with a platinum rod as the working electrode, and separately with a coated-wire electrode alone, as controls — the platinum sheet counter electrode was common to that full set of comparisons, though the study's voltage-transient measurements used a separate two-electrode setup. 

What the gel measured against platinum

Across the frequency spectrum, every hydrogel formulation showed impedance roughly two orders of magnitude below the platinum control, and at the biologically relevant frequency of 1 kHz all formulations sat significantly lower than platinum. Charge storage capacity showed a similar pattern but not a uniform one: the platinum control recorded 1.08 mC cm⁻², and all five hydrogel formulations recorded substantially higher values than that — but only three of the five (the higher-PEDOT:PSS formulations) also exceeded the 154.1 mC cm⁻² recorded for a conductive-elastomer-coated wire control; the other two, at 104.2 and 144.2 mC cm⁻², fell short of that second benchmark.

Extrusion through a 21-gauge needle stayed below the study's 30 N safety threshold for every formulation, ranging from 27.3 ± 0.6 N for the highest-PEDOT:PSS gel down to 11.1 ± 0.8 N as acetic acid content increased. All five formulations were non-cytotoxic in direct contact with Schwann cells, with viability reaching 99.2% for one formulation and remaining above 95% across the rest.

Testing in ex vivo brain tissue

Injected into ex vivo rat brain tissue, the gel stayed anatomically localised on CT imaging, with no evidence of diffusion into the ventricular system. Electrochemically, impedance measured at 1 kHz was approximately 540 Ω with the gel in contact with a conductive-elastomer-coated wire, against 730 Ω for that same wire measured alone — roughly a 200 Ω reduction attributable to the gel.

For materials research of this kind, the performance of the test system matters alongside the material under investigation. Here, that meant a platinum counter electrode whose purity, temper and thickness were specified and held constant, so that the differences the researchers measured could be attributed to the hydrogel formulations rather than to variation in the reference electrode.

Source: 
Title: Injectable conductive hydrogel electrodes for minimally invasive neural interfaces
Journal: Journal of Materials Chemistry B, Vol. 12, Issue 36, pp. 8929–8940
Authors: Ines Kusen, Aaron Lee, Estelle A. Cuttaz, Zachary K. Bailey, Joshua Killilea, Shirine Merlo-Nikpay Aslie, Josef A. Goding, Rylie A. GreenPublished: 30 July 2024
DOI: 10.1039/d4tb00679h

Abstract: Soft bioelectronic neural interfaces have great potential as mechanically favourable alternatives to implantable metal electrodes. In this pursuit, conductive hydrogels (CHs) are particularly viable, combining tissue compliance with the required electrochemical characteristics. Physically-aggregated…