PhD defence by Daniel Maher

pH sensing at the microscale is important for the study, monitoring, and control of biological and microfluidic systems. Despite its importance, pH sensing at the microscale is not as straightforward as on the macroscale. Currently, there are several techniques for performing microscale pH sensing. However, they all have their own limitations, so there is a need for new approaches to microscale pH sensing. Here, microrobots are promising solutions because of their small size and ability to maneuver in confined spaces. 

In this Ph.D. project, we developed microrobots for localized pH sensing. Micro 3D printing was used to fabricate the multimaterial microrobots and optical trapping was used to manipulate the microrobots in microchannels. The microrobots are composed of two materials: a hard polymer backbone and a pH-responsive hydrogel. The pH-responsive hydrogel acts as a sensing element, increasing or decreasing in size depending on pH. The hard polymer backbone enables precise transportation using optical trapping. 

From the results of this project, the swelling response, reusability, and high mobility of the microrobots signify that they are promising tools for pH sensing in microfluidic devices. When compared with other techniques for microscale pH sensing, the microrobots are a stable, sustainable, and cost-effective approach as opposed to electrochemical sensors, which are susceptible to fouling over time, or fluorescent probes, which are susceptible to photobleaching. However, the microrobots still lack the precision and reproducibility that other techniques provide, so further improvements are needed to unlock their potential. Nevertheless, the microrobots developed during this project have potential for pH monitoring in organ-on-a-chip devices and other microfluidic systems.