Electrical Resistance Tomography (ERT) Proves a Reliable Process Analytical Technology (PAT) for Real-Time Monitoring of Solid-Liquid Suspension and Deposition in Pharmaceutical Processing

Introduction: Aluminum-based adjuvants are widely utilized in vaccines; however, they present specific processing challenges due to their tendency to sediment, necessitating thorough mixing and resuspension to ensure homogeneity. Materials and methods: This study investigates the use of Electrical Resistance Tomography (ERT) as a non-invasive monitoring technique for solid-liquid suspensions, overcoming the limitations of optical methods in opaque systems. A laboratory-scale stirred tank was used to analyze the resuspension behavior of hydrous magnesium silicate, which was selected as a mimic solid due to its similar density and particle size distribution to aluminum hydroxide. Discussion: ERT measurements using multiple circular electrode probes demonstrated reliable quantitative detection of solid volume fractions above 5% v/v, consistent with previous research. Below this threshold, only qualitative analysis is reliable at the cost of quantitative accuracy. Notably, in the fully settled state, an unexpected increase in conductivity was observed in the lower electrode planes, suggesting that electrode-solid interactions affected the reconstructed conductivity field. This hypothesis was further validated through experiments with glass beads, where coverage of specific electrodes led to similar conductivity artefacts. Conclusions: The implementation of a linear probe markedly improved the detection of deposited solids at low overall volume fractions but elevated local concentrations, demonstrating a clear correlation between electrode surface coverage and conductivity signal saturation. These findings confirm the viability of ERT for real-time suspension monitoring, demonstrating its potential for detecting solid accumulation and optimizing mixing in industrial and pharmaceutical processes, including vaccine manufacturing.