Plasma sprayed coatings composed of stoichiometric hydroxyapatite have been extensively used to improve integration of metallic implants in the host bone, as hydroxyapatite (HA) is normally regarded as similar to the mineralized phase of bone. However, these coatings exhibited several drawbacks that limited their success. On the one hand biological apatite is a carbonated-HA, containing significant amounts of foreign ions, having low crystallinity and a small crystals size. This means that it differs from stoichiometric HA in terms of composition, stoichiometry, crystallinity degree, crystal size/morphology and, as a direct consequence, solubility, and ions release in the peri-implant environment. On the other hand, thick plasma sprayed coatings can undergo cracking and delamination and are scarcely uniform. For these reasons, research is pushing into two directions: (i) Increasing the similarity of apatite coatings to real bone, and (ii) exploring deposition by alternative plasma assisted techniques, allowing to achieve thin films, and having superior adhesion and a better control over the coating composition. In this article, we review the latest advances in the field of plasma-assisted deposition of ion-substituted hydroxyapatite thin films, highlighting the state of the art, the limitations, potentialities, open challenges, and the future scenarios for their application.

Next‐generation neural interfaces for bidirectional communication with the central nervous system aim to achieve the intimate integration with the neural tissue with minimal neuroinflammatory response, high spatio‐temporal resolution, very high sensitivity, and readout stability. The design and manufacturing of devices for low power/low noise neural recording and safe and energy‐efficient stimulation that are, at the same time, conformable to the brain, with matched mechanical properties and biocompatibility, is a convergence area of research where neuroscientists, materials scientists, and nanotechnologists operate synergically. The biotic–abiotic neural interface, however, remains a formidable challenge that prompts for new materials platforms and innovation in device layouts. Conductive polymers (CP) are attractive materials to be interfaced with the neural tissue and to be used as sensing/stimulating electrodes because of their mixed ionic‐electronic conductivity, their low contact impedance, high charge storage capacitance, chemical versatility, and biocompatibility. This manuscript reviews the state‐of‐the‐art of poly(3,4‐ethylenedioxythiophene)‐based neural interfaces for extracellular recording and stimulation, focusing on those technological approaches that are successfully demonstrated in vivo. The aim is to highlight the most reliable and ready‐for‐clinical‐use solutions, in terms of materials technology and recording performance, other than spot major limitations and identify future trends in this field.

Poly(3,4-ethylentedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most studied materials for organic bioelectronics, supercapacitors and organic photovoltaics. Its low impedance is ascribed to the so-called volumetric capacitance, a property that phenomenologically correlates the capacitive coupling/charge storage in devices to the PEDOT:PSS volume/thickness. Here we investigate the correlation between the capacitance and the electroactive surface area (ESA) for large-volume spin-cast PEDOT:PSS electrodes. We measure the capacitance with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and characterize the surface morphology by atomic force microscopy and X-ray photoelectron spectroscopy. Capacitance of PEDOT:PSS films scales with volume up to ∼5 × 10⁶ μm³ but is saturated at larger volumes. This scaling behavior is paralleled by the scaling of the ESA, hence the ratio between the effective capacitance and ESA remains constant across the whole data set, thus showing that the specific areal capacitance is indeed the relevant material property of PEDOT:PSS. EIS data fit supports the experimental evidence obtained by CV, further revealing that the diffusion time constant is also saturated at high volumes. This supports the scenario where the effective capacitance relates to the ion accessible ESA, and shows that the saturation of the capacitance arises from a change of ion penetration from a diffusive (at small volumes) to a non-diffusive regime at large volumes.