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 the dispensing processes.

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 devices 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 automation systems are specialised for the multitude of different applications and requirements and offer the highest flexibility and precision. In addition to these systems, there is an increasing demand for solutions where, apart from the typical requirements of liquid handling, conditions of production processes have to be taken into account. In these processes, single vessels are often used as product containers. 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 the set-up times for cleaning and refilling typical of batch processes are eliminated and the processes are correspondingly more economical. The production of smaller or limited manufacturing quantities, e.g. with frequently changing reagents or product vessels, on the other hand, is carried out discontinuously in batch operation. This requires appropriate measures when changing products. The HTI X-Tube processors work exclusively in batch mode and, in terms of size, throughput and costs, are also suitable for filling tasks in small and medium-sized laboratory operations.

The filling systems can be supplemented with a labelling module so that the tubes are labelled directly. In this way, the processes of labelling, filling and screwing can be carried out in a single line. © 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 largely 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 this 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, with regard to product stability it may be necessary to provide for active cooling of the receiver containers as well as of the filled product containers to be stored in the device in the case of long process times. It should also be noted that when filling a product, a single reagent usually has to be distributed over many vessels and large storage vessels are therefore required.

Different pump systems for dosing applications

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

Syringe pumps are characterised by a high accuracy of dispensing even with small volumes. In applications where preferably only one reagent is dispensed, syringe pumps are often used as direct-displacement dispensers. The dispensing needle does not have to be moved, so dispensing is very fast. However, the piston and cylinder of the pump become contaminated, so that time-consuming cleaning or decontamination is necessary before the start of production and especially when changing reagents.

Uncapping, filling and screwing 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. In this case, 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 necessary drives, with correspondingly higher costs. The use of disposable tips prevents contamination of the pump and carry-over 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. especially deep, vessel, there are limits to aspiration with disposable tips, as they cannot be immersed in the receiving vessel to any depth.

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

Peristaltic pumps are a useful alternative, especially for filling processes. Aspiration and dispensing take place simultaneously and no dispensing cannula procedure is required. The advantages of these pumps are fast filling even of large quantities, 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. The system costs for peristaltic pumps are lower than for syringe pumps.

Accuracy and precision of filling

Syringe pumps are characterised 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 also have a precise drive. However, the pumping behaviour is initially less accurate due to the pump tubing, which is not true to size due to the production process. The solution to this is calibration before each filling. In this process, 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. What remains is a temperature- and usage-related error of a few microlitres, which is often insignificant in most filling tasks, especially in the case of larger dispensing quantities. In the automatic filling machines of HTI Automation, corresponding routines are available with which the calibration can be carried out quickly.

The precision (scatter) 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. As far as 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. In addition, there are process speed requirements to achieve the necessary throughput and process control tasks. As far as accuracy and especially small dispensing volumes are paramount, syringe pumps are the favoured solution. Peristaltic pumps, on the other hand, are advantageous for fast filling and preferably large dispensing volumes. In the X-Tube processors from HTI, both pump systems are optionally used for dispensing. The user can select the appropriate pump for each filling.