IrOx–carbon nanotube hybrids: A nanostructured material for electrodes with increased charge capacity in neural systems Nina M. Carretero a , Mathieu P. Lichtenstein b , Estela Pérez a , Laura Cabana a , Cristina Suñol b,c , Nieves Casañ-Pastor a,⇑ a Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, E-08193 Bellaterra, Barcelona, Spain b Institut d’Investigacions Biomèdiques de Barcelona (IIBB-CSIC), Institut d’Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), c/Rosselló 161, 08036 Barcelona, Spain c CIBER Epidemiología y Salud Pública (CIBERESP), Spain article info Article history: Received 16 March 2014 Received in revised form 9 May 2014 Accepted 11 June 2014 Available online 18 June 2014 Keywords: Hybrids Neural electrodes Coatings Nanostructuring Nanoscaffold abstract Nanostructured iridium oxide–carbon nanotube hybrids (IrOx–CNT) deposited as thin films by dynamic electrochemical methods are suggested as novel materials for neural electrodes. Single-walled carbon nanotubes (SWCNT) serve as scaffolds for growing the oxide, yielding a tridimensional structure with improved physical, chemical and electrical properties, in addition to high biocompatibility. In biological environments, SWCNT encapsulation by IrOx makes more resistant electrodes and prevents the nanotube release to the media, preventing cellular toxicity. Chemical, electrochemical, structural and surface char- acterization of the hybrids has been accomplished. The high performance of the material in electrochem- ical measurements and the significant increase in cathodal charge storage capacity obtained for the hybrid in comparison with bare IrOx represent a significant advance in electric field application in bio- systems, while its cyclability is also an order of magnitude greater than pure IrOx. Moreover, experiments using in vitro neuronal cultures suggest high biocompatibility for IrOx–CNT coatings and full functional- ity of neurons, validating this material for use in neural electrodes. Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction The development of implanted electrodes in the neural system requires a new generation of electroactive and biocompatible materials that are able to render applied electric fields without the secondary effects related to charge transfer at the electrode– liquid interface, usually related to heating, radical formation and oxidation of the nearby tissue. The cells must continue to be viable, so that implants do not need to be explanted and tissue is not dam- aged. In that sense, the electrode must have good conductivity and low impedance, and allow minimal faradic reactions at the surface involving the biological ionic media. Currently, different types of conductive materials are available for application as electrodes in the neural system [1–8]: gold, platinum, glassy carbon, plati- num–iridium, titanium–nitride or iridium oxide, among others, the latter showing superior performance. Properties such as high electrochemical efficiency, good biostability and significant bio- compatibility have turned IrOx into one of the most promising materials for neural recording and stimulation electrodes [1–7]. These superior characteristics are mainly a consequence of the far- adaic (pseudo-capacitive) charge-injection properties of iridium oxide, instead of the sole capacitive character of other materials such as TiN [8]. During an electrical stimulation pulse within safe potential working limits, IrOx can undergo reversible redox changes itself, with the corresponding ion-intercalation processes, yielding to high charge capacity values and preventing side reac- tions. All faradaic contributions are therefore assumed to be com- ing from the material itself. This redox-intercalation capability prevents electrode alteration/degradation or tissue damage by pos- sible radical formation or irreversible reactions [9]. However, the stability of the coatings based on it has precluded their greater use. This work shows a significant improvement in electrodes con- taining iridium, thanks to the formation of a nanostructured hybrid that increases up to10 times the total charge capacity available from the pure oxide. Carbon nanotubes (CNT) present exceptional low density and high superficial area, in addition to desirable mechanical, thermal and conductive properties [10]. These unique characteristics have made the use of CNT in oxide nanocomposites widely reported for several applications, such as energy storage, electroanalysis or photocatalysis, and also when combined with conducting poly- mers [11–20]. However, nanotubes tend to induce fagocytosis and cell death when released into biological media [21]. Chemical http://dx.doi.org/10.1016/j.actbio.2014.06.019 1742-7061/Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: nieves@icmab.es (N. Casañ-Pastor). Acta Biomaterialia 10 (2014) 4548–4558 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat