Citation: Chan HKY, Archbold L, Lau WM, Ng KW. Validating Otto: a Franz diffusion cell autosampler to automate in vitro permeation studies, Journal of Pharmaceutical Sciences, 2025:103837. https://doi.org/10.1016/j.xphs.2025.103837
We have a new paper out. This one is close to my heart because I personally spent many hands-on hours developing Otto.
Who’s Otto?
Otto is a Franz diffusion cell (FDC) autosampler robot. It replaces manual sampling and refilling of FDCs in a skin (and cornea, mucosal membrane, etc.) permeation experiment. Those who have worked with FDCs before would know how fiddly, time-consuming and labour-intensive that is. It’s a job most suited for a robot.
But Otto is about more than us trying to avoid menial labour. It’s about the quality of the science, too.
Let me rephrase that — it’s primarily about the quality of the science.
For a long time, we have noted many skin drug absorption studies that include unusually large sampling gaps of ≥16 hours, presumably because the researchers were unable to collect samples outside normal working hours. This sampling gap could allow the drug to accumulate in the FDC receptor chamber and, consequently, underestimate drug absorption due to sink condition being violated. We have faced similar logistical challenges ourselves as local rules prevent some researchers from working out of normal working hours. A FDC autosampler would solve these challenges, but we have not been able to afford any of the few commercial FDC automation systems available. When COVID-19 hit, and lab access was further restricted, we finally found the impetus and time to build the FDC autosampler we had always needed, for less than £500, and retrofitted it to our existing FDCs.
Thus, Otto was born.

We have spent the last 2 years validating Otto’s performance. In this paper, we demonstrate that the sampling gap indeed led to violation of sink condition and underestimation of drug absorption. We further show that Otto improved data quality by avoiding the sampling gap. We have benchmarked Otto’s precision and accuracy against a trained researcher. We are pleased to report that it outperforms the researcher on both counts.
Otto is better than the commercial offerings in many ways. It is built on open-source technologies, using inexpensive consumables and 3D-printed custom parts, and is therefore fully customisable. It has a small footprint of just 50 cm × 46 cm. It can be retrofitted to generic FDCs and can collect up to 100 samples per experiment, fully unattended. The samples are collected directly into high-performance liquid chromatography (HPLC) autosampler vials, so it integrates seamlessly with downstream analysis without any further liquid handling, nor modification to the FDCs or analytical equipment.
Logistical, human resource and financial constraints continue to grip many research organisations long after COVID-19 restrictions have ended. Otto should prove itself a valuable asset in many research labs seeking to retrofit an automation solution to their existing FDCs.
The build instructions for Otto are too extensive to include in this paper, so we will be publishing them separately.