Microfabricated Sampling Probes for Monitoring Brain Chemistry at High Spatial and Temporal Resolution

Abstract

Monitoring neurochemical dynamics has played a crucial role in elucidating brain function and related disorders. An essential approach for monitoring neurochemicals is to couple sampling probes to analytical measurements; however, this approach is inherently limited by poor spatial and temporal resolution. In this work, we have developed miniaturized sampling probes and analytical technology to overcome these limitations.

Conventional sampling probes were handmade and have several disadvantages, including large sizes (over 220 µm in diameter) and limited design flexibility. To address these disadvantages, we have used microfabrication to manufacture sampling probes. By bulk micromachining of Si, microchannels and small sampling regions can be fabricated within a probe, with an overall dimension of ~100 µm. For development of a dialysis probe, nanoporous anodic aluminum oxide was adapted for monolithically embedding a membrane. Coupling the probe to liquid chromatography-mass spectrometry, multiple neurochemicals were measured at basal conditions, including dopamine and acetylcholine. Comparing to conventional dialysis probes, the microfabricated dialysis probe provided at least 6-fold improvement in spatial resolution and potentially had lower tissue disruption.

Furthermore, we have continued the development of a microfabricated push-pull probe. We enhanced functionality of the probe by integrating an additional channel into the probe for chemical delivery. Further, we demonstrated that the probe can feasibly be coupled to droplet microfluidic devices for improved temporal resolution. Nanospray ionization mass spectrometry was used for multiplexed measurements of neurochemicals in nanoliter droplet samples. Utility of the integrated system was demonstrated by monitoring in vivo dynamics during potassium stimulation of 4 neurochemicals, including glutamate and GABA. The probe provided unprecedented spatial resolution and temporal resolution as high as ~5 s. Additionally, we highlighted versatility of the method by coupling the probe to another high-throughput assay, i.e., droplet-based microchip capillary electrophoresis for rapid separation (less than 3 s) and measurement of multiple amino acid neurochemicals.

This collection of work illustrates that development of the microfabricated sampling probes and their compatible microfluidic systems are highly beneficial for studying brain chemistry. The integrated miniaturized analytical technology can potentially be useful for solving other problems of biological significance.