Fabrication and characterization of droplets generation for brain fluid microsampling

Parkinson’s disease (PD) and Alzheimer’s disease (AD) represent big medical challenges which modern medicine has to face with, nowadays. Due to the increase of life expectancy of older population, the number of people in the world suffering from dementia is expected to reach 75 million, by 2030, and 131 million, by 2050. The economic impact on public health is not secondary and represents, each year, an investment of hundreds of billions of dollars. Own to the lack of understanding the causes of these neurodegenerative conditions, the existing solutions provided by medicine only alleviate the symptoms. Brain microdialysis is a powerful tool for research in Neuroscience and pharmacokinetics monitoring, that enables to get a better understanding of the chemical processes taking place in the brain. The analysis of nanoliter-sized droplets, constituted of cerebral extracellular fluid (ECF), coupled with mass spectroscopy techniques, allows to retrieve the concentration of neurotransmitters in time, thus studying drug effects or the mechanisms behind neurodegeneration . The final goal of this project is to realize a biocompatible neural probe able, ultimately, to electrically and optically stimulate the brain (e.g. photobiomodulation), collect and store samples from the substantia nigra, as droplets, and analyze the evolution of neurotransmitter concentrations, in time. The introductory part of this thesis is dedicated to the economical, social and physiological contextualization of neurodegenerative diseases; a state-of-the-art of microdialysis techniques is performed and the main objectives, specifically of this thesis, are set. The first part of the thesis is dedicated to the design of an on-chip storage solution, able to collect the nanodroplet samples; a Matlab model of the fluidics, taking into account the multiphase flow nature of the problem and the interface among the droplets, is performed both to validate the design and to establish the working point of the experimental set-up. Regarding the droplet generation, the performances and the limitations of T-junction vs flow-focusing design are illustrated. The second part is related to the nanodroplets detection and characterization , through the fabrication of Pt-Ti (Platinum-Titanium) microelectrodes at the CMI (Center of MicroNanotechnology) facilities. An electrochemical model derived for the electrodes is presented, the critical design parameters are discussed and compared with respect to the experimental results, derived through impedance spectroscopy measurements. The correlation of the nanodroplet size (nl range) and differential capacitance signal retrieved is demonstrated; furthermore, the characterization of the electrodes is performed with different media, such as Phosphate-Buffered Saline (PBS) and artificial cerebrospinal fluid. This experiment has shown a different response in the Nyquist plot, which is associated to the different ion species present in the solutions. Finally, in order to detect and count in time the number of nanodroplets in the channel, the employment a Capacitance-to-Digital-Converter electronic board is presented: the differential capacitance peak board and the transient time over the electrode show a correlation with respect to the volume and the flow rate of the nanodroplet. The hope of this thesis work is to have contributed to one part of an ambitious project, which might change in future the perspectives of Neuroscience.