Research on the release of drugs from neural electrodes has been ongoing for some time. So far, no sustainable and universal concept has been found. Currently, platinum is used as an electrode material in neural electrodes, such as in the cochlear implant (CI). By provision of neuroprotective substances or neuronal growth factors it is possible to restore the balance in the inner ear and thus stabilize the remaining spiral ganglion neurons (SGNs). Furthermore, the outgrowth of neurites from these cells can be induced to improve the electrode-nerve contact and increase signal resolution for an improved hearing perception. Although platinum can also be produced in porous form, controlled release from its pore system is not possible due to its chemical inertness and consequently the difficulty of surface modifications for interactions with drug molecules.
To nevertheless achieve local implant-associated drug delivery, we devised a novel nanocomposite material of nanoporous silica nanoparticles (NPSNPs) embedded in the pores of nanoporous platinum (NPPt) which can be employed as a coating on the platinum surfaces of the CI electrode. NPPt exhibits high conductivity and favorable electrochemical properties. NPSNPs provide large surface area, permanent porosity and high versatility in terms of easily tunable surface properties to achieve high drug loading.

To fabricate the NPSNPs@NPPt material on the surface of platinum substrates, unmodified and sulfonic acid modified NPSNPs were first encapsulated in a polystyrene shell. After the 100 nm sized silica-polystyrene core-shell nanoparticles were applied on the surface of the substrates, platinum was electrolytically deposited between the particles. In the final step, the polystyrene was removed by extraction to obtain the dual porous nanocomposite coating on platinum.
Scanning electron microscopy images showed the successful incorporation of the modified and unmodified NPSNPs into the NPPt. Impedance and cyclic voltammetric measurements demonstrated improved electrochemical properties compared to dense platinum due to the enhanced surface of the NPPt. The release experiments indicate that the sulfonic acid modified particles in the composite coating lead to higher loading and release of methylene blue compared to the unmodified particles.