The chronic challenge—new vistas on long-term multisite contacts to the central nervous system

Citation: Hofmann UG and Krüger J (2015) The chronic challenge—new vistas on long-term multisite contacts to the central nervous system. This Special Research Topic at hand is a collection of contributions from eminent research groups shedding light on several aspects of the still unresolved problem of a truly chronic cortical interface to enable long term brain-machine interfacing to human patients. The hypothesis article of Fernandez et al. (2014) adds to the three generally agreed-on features for biocompatibility (bio-safety, bio-stability, and bio-functionality) with a fourth one that mirrors the demand for " bio-tolerability. " Sommakia et al. (2014a) study aims to reduce the almost-immediate adsorption of non-cellular tissue components upon insertion by dip-coating polyethy-lene glycol (PEG) as a " stealth " cover. It points toward a beneficial alteration of adsorption on the probe, but cautions PEG's immediate use for long term implants in the brain. In fact, in a second contribution based on a mixed-brain culture (Sommakia et al., 2014b), they show evidence for a complex response of glia cells on micro-wires dip-coated with PEG and/or lipopolysaccharides (LPS), but not of neurons, which is somewhat contradictory to pure in vivo findings. In the same context of passive probe coatings, De Faveri et al. (2014) moved toward a more " natural " method by coating glass-insulated micro-wires with fibrin hydrogel, as a biological cushion between brain and probe. Using immunofluorescence techniques, they were able to demonstrate a beneficial effect on longer term astrocytic responses and successful encapsulation of brain cells in fibrin as in Richter (2012). Beyond the modulatory effects of passive surface coatings, two articles review organic coatings for micro-contacts in the nervous system. The contribution by Asplund et al. (2014) concisely reviews electrodes based on conductive polymers, not only for improving site-tissue coupling, but also for electrically eluting anti-inflammatory drugs using various stimulation patterns. They go to great depth on how to apply this elution process to a living being, since the active elution technique inherently requires compliance with demanding bio-compatibility issues. The review of Aregueta-Robles et al. (2014) addresses the topic of organic and nanoscopic coatings with a wider perspective, and thus provides an excellent overview regarding a huge variety of the various reported approaches including the " living electrode " of Ochiai et al. (1980) and their adaptation by Richter et al. (2010, 2011). Extending the time frame of all above mentioned studies, Prasad et al. (2014) investigate …