Table 1.
A Review of Materials for Neural Electrical Stimulation, Their Properties, Advantages, and Disadvantages
| Electrode materials | Fabrication process | Geometry | Electrical resistivity/conductivity | Charge injection density | Biocompatibility | Flexibility | Advantages | Disadvantages | References |
|---|---|---|---|---|---|---|---|---|---|
| PPy | Template-assisted electro-deposition | Flat planar design | 190 S/cm; doped with PSS; 19.84 S/cm | 5 mC/cm2 | Positive biocompatibility profile in vivo; increased neuron adhesion | Highly flexible | Flexibility; high electrical conductivity; biocompatible | Fragile mechanical properties; coating is thin; degradation possible | 76–78 |
| PEDOT:PSS | Crosslinked | 3D printed micropillars | 5.8 S/m | 1.2–3.9 mC/cm2 | Indirect and direct cytotoxic tests ISO 10993-5 | Highly flexible | High electrical conductivity; transparency; biocompatible; neural stimulation demonstrated | Water soluble; long-term unstable | 64,79 |
| PPy/PSS layered with MWNTs | Layering and codeposition | N/A | 30 S/cm | 7.5 mC/cm2 | Cell growth inhibition assay | Highly flexible | Electrochemically stable; high electrical conductivity | Requires process optimization; toxicity needs to be further verified | 78,80 |
| CNT | Low-pressure chemical vapor deposition | Vertically aligned pillars | 1.8 × 107 S/m | 1.0 − 1.6 mC/cm2 | Uncertain | Flexible | Highly electrically conductive; versatile | Surface modification required for biocompatibility; poor dispersion in composites | 70,71 |
| Porous graphene | Laser reduction | Coating | 303 S/m | 3.1 mC/cm2 | Live/dead cell analysis | Flexible | Mechanically flexible; biocompatible; high electrical conductivity | May be fragile; requires surface modification to enhance hydrophilicity | 73,81,82 |
| Pt | Extrusion/drawing | Cylindrical and wire | 9.6 × 106 S/m | 0.10–0.30 mC/cm2 | MTT proliferation assay; smaller scar thickness | Rigid | Good mechanical properties; good biocompatibility; used extensively; chemically inert; high electrical conductivity | Cell death possible; possible corrosion; may undergo irreversible dissolution producing toxic by-product | 83,84 |
| Chemical vapor deposition | Circle | 295.07 μC/cm2 | 83,85,86 | ||||||
| Fractal | 510.50 μC/cm2 | ||||||||
| Serpentine | 318.82–359.53 μC/cm2 | ||||||||
| Iridium oxide | Electrodeposition | Thin film | 0.75 × 10−3 to 1.67 × 10−3 Ω cm | 1.2 mC/cm2 | Glial scar assay; neuron adhesion; MTT cell viability | Rigid | Good mechanical and electrical properties; high charge injection capacity | Over-pulsing can cause degradation; chronic usage may lead to inconsistency; less biocompatible compared to Pt | 86–89 |
| Titanium nitride | Sputtering | Thin film, coating | 25 × 10−6 to 800 × 10−6 Ω cm | 0.87 mC/cm2 @ 0.2 ms | MTT proliferation assay; live/dead cell analysis | Flexible coating | High surface areas; ease of fabrication | Potentially increased cell death; oxidation possible | 90–93 |
3D, three-dimensional; CNT, carbon nanotube; MTT, 3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide; MWNTs, multiwalled carbon nanotubes; PEDOT:PSS, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; PPy, polypyrrole; PPy/PSS, polypyrrole polystyrene sulfonate; Pt, platinum.