Liquid handling in automated filling

Product filling on a laboratory scale can be automated with dispensing systems. Dr. Wolfgang Heimberg, HTI Automation, sheds light on the process and describes the characteristics and influences of different types of pumps on metering operations.

 

Devices of the “X-TubeProcessor®” series from HTI Automation can be used to automate filling processes. © HTI Automation

Liquid handling can be described as one of the fundamental processes in laboratories and the equipment and tools for it as the classics of laboratory equipment and automation. There are numerous applications and just as many manual and automated systems on the market that support liquid handling. Modern liquid handling laboratory automats are specialized for the multitude of different applications and requirements and offer highest flexibility and precision. In addition to these systems, there is an increasing demand for solutions that take into account the conditions of production processes in addition to the typical requirements of liquid handling. Within the scope of these processes, single vessels are often used as product vessels. The “X-TubeProzessor” systems from HTI are specially designed for the automatic processing of screw containers (tubes).

Process requirements for automated production

The production of test kits involves numerous process steps, for which sterile and strict regulatory conditions with individual sample tracking and dedicated in-process control (IPC) must often be ensured. In automated production, a fundamental distinction must be made between continuous operation and batch operation. Continuous processes are preferred, for example, in pharmaceutical production when processing large quantities with few product changes, as this eliminates the setup times for cleaning and refilling typical of batch processes and the processes are correspondingly more economical. On the other hand, the production of smaller or limited manufacturing quantities, e.g. with frequently changing reagents or product vessels, is carried out discontinuously in batch operation. This requires appropriate measures when changing products. The HTI X-Tube processors operate exclusively in batch mode and, in terms of size, throughput and cost, are also suitable for filling tasks in small and medium-sized laboratory operations.

The filling systems can be supplemented by a labeling module so that the tubes are labeled directly. In this way, the labeling, filling and screwing operations can be carried out in a single draw frame. © HTI Automation

Automated filling

Dispensing, i.e. the distribution of liquids from a large container into many small product transport vessels, is a central functionality of automated filling. The requirements are in large parts comparable to those of typical liquid handling tasks. These include avoiding contamination, ensuring sterility, and filling accuracy. With regard to automated production, there is also the fact that it must take into account the requirements of a planned throughput and product stability. While only the process time of the filling operation is decisive for the throughput, it may be necessary to provide active cooling of the feed vessels as well as of the filled product vessels to be stored in the unit with regard to product stability in the case of long process times. It should also be noted that product filling usually involves distributing a single reagent to many vessels and therefore requires large receiver vessels.

Various pump systems for metering applications

For automated filling, both the syringe pumps familiar from liquid handling machines and peristaltic pumps are used.

Syringe pumps are characterized by high accuracy of dispensing even at small volumes. In applications where only one reagent is preferably dispensed, syringe pumps are often used as direct-displacement dispensers. The dispensing cannula does not need to be moved, thus dispensing is very fast. However, the piston and cylinder of the pump become contaminated, so that extensive cleaning or decontamination is required before the start of production and especially when changing reagents.

Uncapping, filling and capping of micro screw tubes with a fully automatic feeding of tubes and screw caps via a tube or cap feeder as well as a syringe pump with exchangeable tips. © HTI Automation

For flexible dispensing applications with e.g. batchwise changing reagents, the syringe pumps are preferably used as air displacement pipettors with disposable tips. Aspiration and dispensing are carried out separately in terms of time and space, so that the pipette tips have to be moved from the receiver vessel to the product vessel. The consequences are longer process times and a high level of robotic effort due to the required drives, with correspondingly higher costs. The use of disposable tips prevents contamination of the pump and carryover of reagent. Conditional process control is possible by means of liquid level detection. However, if the reagent to be dispensed is placed in a large, i.e. in particular deep, vessel, there are limits to aspiration with disposable tips, as these cannot be immersed to any depth in the receiver vessel.

Precise dispensing is possible with the aid of an integrated peristaltic pump and automatic walltouch. © HTI Automation

Peristaltic pumps are a sensible alternative, particularly for filling processes. Aspiration and dispensing occur simultaneously and no dispensing cannula procedure is required. The advantages of these pumps are fast filling even of large volumes, no contamination of the pump and – as long as a new pump tubing is used for each filling batch – no reagent carry-over from batch to batch. System costs for peristaltic pumps are lower than for syringe pumps.

Accuracy and precision of filling

Syringe pumps are characterized by a defined geometry of pump piston and cylinder and a precise drive. The different properties of the various reagents to be filled are taken into account by predefined liquid classes. This results in a high accuracy of the dispensing. Peristaltic pumps equally have a precise drive. However, the pumping behavior is initially less accurate due to the pump tubes not being dimensionally accurate for production reasons. The solution to this is calibration before each filling. All systematic influences such as the reagent, pump and pump tubing properties are recorded. The actual dispensing quantities are compared with the target quantities and subsequently corrected via the pump control. Calibration largely eliminates systematic dispensing errors. An error of a few microliters due to temperature and use remains, which is often insignificant for most filling tasks, especially in the case of larger dispensing volumes. In the filling machines of HTI Automation, corresponding routines are available with which the calibration can be carried out quickly.

The precision (dispersion) of the dispensing describes the statistical error, which is not caused by the systems, but often by the application. One of the causes is alternating large droplet accumulations on the pipette tips or dispensing tips. Where possible for contamination reasons, precision can be significantly improved by surface or wall touch, i.e. immersion in the liquid or touching the vessel wall in the target vessel. The automated X-Tube processor systems also offer appropriate solutions for this.

Conclusion

The liquid handling of automated filling processes differs from classic liquid handling applications in terms of the specific conditions of process automation.

The most important difference is that usually only one or a few reagents are distributed to a large number of production vessels. Added to this are process speed requirements to achieve the necessary throughput and process control tasks. As far as accuracy and especially small dispensing volumes are a priority, syringe pumps are the favored solution. Peristaltic pumps, on the other hand, are advantageous for rapid filling and preferably large dispensing volumes. In HTI’s X-Tube processors, both pump systems are optionally used for dispensing. The user can select the appropriate pump for each filling.