A step closer to continuous buffer preparation from solids: Predicting powder compaction and how to prevent it

The preparation of buffer solutions used in the biopharmaceutical industry is typically performed manually by the addition of one or multiple buffering reagents to water. Recently, the adaptation of powder feeders for continuous solid feeding was demonstrated for continuous buffer preparation. However, the intrinsic characteristics of powders can change the stability of the process, due to the hygroscopic nature of some substances and humidity induced caking and compaction behavior, but there is no simple and easy methodology available predicting this behavior for buffer species. To predict which buffering reagents are suitable without special precautions and investigate their behavior, force displacement measurements were conducted with a customized rheometer over 18hours. While most of the eight investigated buffering reagents indicated uniform compaction, especially sodium acetate and dipotassium hydrogen phosphate (K₂HPO₄) showed a significant increase in yield stress after 2hours. Experiments conducted with a 3D printed miniaturized screw conveyor confirmed the increased yield stress measurement by visible compaction and failure of the feeding. By taking additional precautions and adjusting the design of the hopper, we demonstrated a highly linear profile of all buffering reagents over a duration of 12 and 24hours. We showed that force displacement measurements accurately predict the behavior of buffer components in continuous feeding devices for continuous buffer preparation and are a valuable tool to identify buffer components that need special precautions. Stable, precise feeding of all tested buffer components was demonstrated, showing the importance of identifying buffers that need a specialized setup with a fast methodology.

In-situ gradient formation by direct solid addition of buffer components

Buffer preparation and storage requires a significant facility footprint in large scale bioprocessing and together with the costs of supply chain management can have a substantial economic impact. In-line buffer mixing in chromatography is commonly performed by blending different buffer solutions using at least two pumps and a static or dynamic mixer. We developed a device for an in-line gradient delivery of buffering agents directly from solids to be applied for chromatographic separation processes. A solid feeding device with a screw conveyor and a hold tank for the solids was designed and a miniaturized system was 3D printed. The coefficient of variation for the precision of the solid feeding of 5 different buffering agents was below 5% even for very small solid flow rates necessary for lab-scale chromatography. Stability was demonstrated by a constant linear solid feed at a very low dosing rate of 0.05 g.min^-1 over 24 hours. We demonstrated the suitability for chromatography by directly connecting the system to a standard chromatography workstation for protein chromatography. The solids were fed into a miniaturized continuously stirred tank reactor connected to an ÄKTA purification system. The performance of the in-line gradient delivery of buffering agents directly from solids was compared to conventional in-line buffer mixing. We were able to achieve highly linear gradients for elution using only one pump of a chromatographic system, generating the gradient by the direct addition of solids avoiding the necessity of additional pumps and hold tanks. By direct conditioning of buffers and the addition of solids a simple, just in time, at site preparation of buffers was possible. The design of the feeding unit for solid addition for buffer preparation is easily scalable and adaptable to work with or as a replacement for already existing in-line dilution or conditioning units.