INDUSTRY FOCUS CHEMICALS & PHARMACEUTICALS A COST-EFFECTIVE DISPENSING ALTERNATIVE
Pressure-controlled liquid handling systems are a simple, cost- effective way to dose nanolitre-scale fluid volumes, says Paul Kendall, Industry Sector manager – Lifetech at Festo GB
I
nfectious diseases are an ever-present threat to our society – as evidenced by
the current Coronavirus outbreak. Today’s laboratories are increasingly reliant on automated liquid handling systems to develop and produce new vaccines rapidly to protect the human population. The advantages of automated liquid
handling systems are well known: but these systems don’t come cheap, and they are a significant investment for a laboratory. Furthermore, they come with a host of features that are surplus to requirements in many applications, which means unnecessary costs. There is another, more cost-effective
alternative in these situations: pressure- controlled liquid dispensing systems. They provide a simple and quick way to dose nanolitre- to millilitre-scale volumes of fluid with precision, reliability and scalability. Pressure-controlled liquid dispensing
systems have the design flexibility to overcome wide variants in fluid properties and composition and are suitable for
fluid dispensed primarily influenced by the valve’s cycle time. Valves with short, highly repeatable response times offer much better dispense precision at higher speeds. It is important to note that solenoids generate energy when open, and their performance often changes as they warm up. Operating a higher-flow valve for shorter durations reduces this heat but requires a valve with excellent repeatability, such as Festo’s VTOE (see image below). One of the biggest benefits of pressure-
modified via software – the right tip should be selected from the outset.
2. PRESSURE Pressure has a significant effect on dosing volumes and – more importantly – helps control the velocity of the fluid as it passes through the dispense tip. It can also be used as a control for fluid viscosities, achieving clean, splash-free dispenses. Pressure can be generated in several ways, for example by using an
Pressurised liquid dispensing systems can help optimise throughput in lab automation applications at a fraction of the cost of complex robotics
based dispense systems is their scalability. Individual dispense heads can be combined easily to create multi-channel dispense heads that handle different aliquot volumes, fluids and pressures. Fluids with different classes can even be dispensed by mounting multi-channel dispense heads on an electric axis – creating a gantry system. When it comes to multi-channel
‘One of the biggest benefits is scalability. Individual dispense heads can be combined easily to create multi-channel dispense heads that handle different aliquot volumes, fluids and pressures’
substances as thin as alcohol or as thick as honey. They can also cope with corrosive and acidic substances. Mechanically simple, these dispensing systems require a minimum number of components, which makes cleaning simple. A pressure regulator, paired with a safety valve, pressurises the reservoir containing the liquid. This pressure then drives the liquid through the tubing and solenoid dispense valve. Finally, the nozzle dispenses the liquid into the vessel via a needle tip with a calibrated orifice (see figure 1). It is important to consider the following
factors when designing a pressure-based system, as they all affect the quantity and quality of the doses:
1. FLOW RESISTANCE The system’s flow rate is defined by the pressure, as well as the overall resistance of the fluid path. This resistance depends on the tube’s inner diameter (ID), length, dispense valve geometry and fittings. Dispense tip size directly affects the velocity of liquids. Unlike pressure and time – which can be controlled and
22 JUNE 2020 | PROCESS & CONTROL
external gas source such as nitrogen, or a compressor to pump air into the closed liquid reservoir. The pressure ranges found in automated dispense systems are generally low – only 100–250 millibars. Nevertheless, it is important to have safety measures in place that can relieve the pressure in case of a leak or other technical failure.
3. DISPENSE TIME The dispense quantities are controlled by solenoid valve controls, with the volume of
The VTOE enables non- contact
dispensing of the smallest of quantities
Figure 1: Mechanically simple, these systems require a minimum number of components, which makes cleaning simple
systems, however, small differences among the inlets, valves and nozzles can cause some channels to dispense higher volumes than others – with intrinsic tip-to-tip variability of the order of 4%. At resolutions of 1 ms or higher, it is difficult to compensate for these small variations using the processor that is controlling the solenoid dose time. A better course of action is to calibrate the separate channels by varying the
pull-in and hold-in
solenoid current so that all channels dispense the same quantity using the same
dispense times. Festo makes this
easy with the VAEM valve control module, which utilises user-friendly
software to reduce the tip-to-tip variability to less than 1% in most cases. Improvements in the scale, speed and
quality of pharmaceutical and biotechnology processes enabled by automated liquid handling have helped to reduce timescales and deliver faster results, while also making drug discovery more efficient. Pressurised liquid dispensing systems can help optimise throughput in lab automation tasks at a fraction of the cost of complex robotics. For further reading, download the free
Festo whitepaper:
http://www.festo.co.uk/controlunderpre ssurewp
Festo
www.festo.co.uk/liquidhandling
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