Candidate
Youssef Helwa
Title
Fabrication of µ-pH Biosensor for Implantable Medical Devices and Applications in Detecting Post-Operative Complications
Supervisor
Bo Cui
Abstract
The monitoring of the pH milieu inside the body is critical to the functions associated with implantable medical devices. By monitoring the variation of pH in real-time inside the body, we are capable of identifying the body response to the implant, the probability of infection, calibrating sensors and monitoring complications such as internal bleeding or anastomotic leakage. In this work, a pH sensor is presented consisting of a working electrode, a counter electrode and reference Ag/AgCl fabricated to allow the signals to be collected and compared to a reference value. For the working electrode, we chose Polyaniline (PANI) as sensing material. Upon exposure to different pH solutions, PANI (the active sensing material) acts as an ion-selective membrane, the concentration gradient of ions across the membrane generates a potential difference that can be measured.
We first fabricate microscale interdigitated electrodes by photolithography, e-beam deposition, etching, and lift-off. Then we coated a conducting hydronium-sensitive layer of PANI or PU by electropolymerization onto the active electrode. Then we placed the second electrode into a solution of KCl to apply a thin layer of AgCl on the Ag electrode, creating the Ag/AgCl reference electrode. The potential for the polymerization to provide the most stable active layer and the Nernstian potential was optimized. Moreover, the porosity of the active layers has been modified to allow the highest concentration of hydronium ions to diffuse to the electrodes, maximizing the signal stability. This bio-compatible electrodeposited polymer layer also protects the electrode from cellular attacks and biofouling.
The fabricated device was used to monitor changes in pH in biological fluids such as gastric juice, simulated blood, and peritoneal fluid. The device was capable of monitoring changes in pH with a Nernstian potential of 68mV/pH. In addition, the active layer demonstrated an active lifetime of five weeks where the electrodes were capable of collecting data continuously during the active period. Figure 2 showed the data collected from the pH sensing electrodes, within the region of pH 2 and pH 12.