Ultrasensitive and selective real-time detection of neurotransmitters for brain-on-a-chip applications

Neurological diseases account annually up to 10 million deaths and the number is expected to increase as the population ages. Up to date, drug discovery has had 0% success rate to find disease modifying treatment for central nervous system diseases, mainly arising from extremely poor translation of results from animal studies to us humans. To overcome this limitation, human cellbased brain models are being rapidly adopted into academic neuroscientific research but also in the pharmaceutical industry. Unfortunately, the use of these brain models is limited by lack of highthroughput characterization methods. Electrochemical measurements can be used to examine the condition of specific cell types in the brain models, however this is severely complicated by the highly-fouling nature of cell culture medium. As complex in vitro brain models are extremely delicate, electrochemical recordings must be performed in the culture medium instead of highly clean electrolytes. By using electrochemical sensors prepared from single-walled carbon nanotube (SWCNT) films, real-time detection of neurotransmitters at nanomolar sensitivity was demonstrated in culture medium for the first time. Arising from the sensitivity of brain model cultures, biocompatibility of the sensor materials is essential because otherwise the brain-on-a-chip will lack the "brain" component. Healthy development of all the most-sensitive and complex in vitro brain models were observed with the SWCNT electrodes, including primary dopaminergic neurons, induced human pluripotent stem cell-derived neurons, but also human brain organoids. Lastly, many biological characterization techniques are based on inverse microscopy and thus, optical transparency is necessary. While transparent substrates could be used to obtain partial visibility for non-transparent microelectrode array, the electrode areas and electrical wiring remain out of sight. Contrasting that, the SWCNT films are highly conductive and can be made with 90% transparency to obtain complete visibility. While many materials are claimed to meet the requirements of brain-on-a-chip devices, more thorough investigation reveals that such claims are mistakenly made and actually only concern individual components from the ensemble of strict specifications. Furthermore, these earlier claims have been made based on highly simplified and misleading experimental settings. SWCNTs are thus a very unique electrode material, as they are currently the only material fulfilling all the requirements of brain-on-a-chip devices, paving way for broader adoption of the technology.