Towards simultaneous determination of tablet porosity and height by terahertz time-domain reflection spectroscopy.

The quality control of pharmaceutical tablets is still based on testing small sample numbers using at- and off-line testing methods. Traditional in-process controls, such as tablet mass, height, mechanical strength, and disintegration time are time- and resource-consuming and poorly suited to support an effective transition towards continuous manufacturing. Another suitable parameter to monitor during production would be tablet porosity. Porosity can be linked to mechanical strength and disintegration but typically requires knowledge of tablet dimensions and mass. Tablet porosity measurements based on terahertz time-domain spectroscopy (THz-TDS) offer a fast and non-destructive approach to in-process control testing for physical tablet properties. This study presents THz-TDS reflection measurements as an alternative to the previously reported transmission setup. It is shown that the proposed method can determine porosity based on the reflected amplitude from the tablet surface, but also allows for precise determination of tablet height in the same measurement. The tablet mass can be estimated by combining the height and porosity measurements. This opens up for the opportunity to determine the tablet's mechanical strength by using the possible correlation to the determined porosity.