Static electricity can affect automated instrumentation within the lab.
As a phenomenon commonly encountered in everyday life, the effects of static electricity can be observed in the weather as lightning, in the classroom with a balloon sticking to the ceiling, or simply in your hair and clothing sparking. Perhaps a lesser known problematic effect of static electricity occurs when using automated instrumentation within the laboratory. Pipette tips can hold a static charge due to various friction that is encountered during shipping when the tips rub against one another, for example. When combined with automated liquid handling workstations, this can become highly problematic, causing tip loading errors and inaccurate aspirating and dispensing. Automation is commonly introduced to provide walk-away capabilities and as such, the effects of static need to be significantly reduced in order to maintain a sense of confidence in the integrity of resulting data.
Static electricity
Static electricity is experienced as the result of a build-up of an electrical charge on the surface of an object. This charge commonly results from friction between two materials causing the separation of positive and negative charges. This essentially means that the two materials will either attract or repel one another, an effect that remains present until removed by a conductive material. Since the chemical properties of any atom are determined by the number of negatively charged electrons contained within the outer ‘electron shell’, it is the movement of these that alters the polarity of any material. For example, materials with weakly bound outer shell electrons often lose them to materials with sparsely filled outer shells, since the desired state for all atoms is to have a full outer shell. As a result of this exchange, termed the triboelectric effect, the two materials become polar and thus attract or repel other polar materials.1
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Table 1: The Triboelectric Series |
By definition, the triboelectric effect is a type of contact electrification, in which certain materials become electrically charged following contact, such as rubbing (friction), with another material. The polarity and strength of the charge produced differs greatly across various material types, surface roughness levels, temperature, and strain. It is therefore not very predictable and only broad generalizations can be made as to how strongly this will occur. In order to help predict the results of this effect, materials are often listed in order of the polarity of charge separation when they come into contact with another object (as shown in table 1).
The more negatively charged materials at the bottom of the series will attain a more negative charge when they encounter friction with a material near the top of the series. The further away the two are in this series, the greater the transferred charge.
Static and pipette tips
Commonly composed of polypropylene, pipette tips are located quite low down on the triboelectric scale. This means that, when they come into contact with any materials which are higher up the scale, such as air, human skin, leather or glass, they are susceptible to losing an electron and therefore becoming more positively charged.3 Following manufacture, there are a number of opportunities for pipette tips to become charged. The first of these instances is in the packaging and shipping processes. The rubbing of plastic packaging or shipping boxes/containers against the tips can result in them being charged as soon as they reach the laboratory. This can subsequently be exacerbated during experimentation as more friction can occur upon loading onto the pipette or robotic head, thus increasing the triboelectric effect. Furthermore, the stacking of the tip racks into hotels does not allow for this charge to be neutralized, since there is no manual grinding or conductive material to help return the material to its neutral state.
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Thermo Fisher Scientific employs steps during the production and assembly of the Molecular BioProducts BioRobotix to minimize the occurrence of static charge. (Source: Thermo Fisher Scientific) |
Effect on experimental integrity
Although the pipette tip is a small consumable, it can have a large impact on the integrity of any experiment requiring highly accurate liquid transfer. When statically charged, the tips will either attract or repel various surrounding materials. As such, when stacked in hotels, they are often attracted to the material of the rack above. Since they are of such small mass, they will effectively ‘stick’ to the bottom of it and be removed from the rack in which it should sit. When this rack of tips is then loaded onto an automated dispenser, one pipetting mandrel will be without its pipette tip, which if un-noticed, will miss aspirating and dispensing the liquid in the associated well. What is more, the sample in question will be missed, which will significantly hinder the integrity of the experimental data, resulting in false negatives and the need to repeat the protocol.
As well as being attracted to the rack above, statically charged tips can also be attracted or repelled to the rack in which they sit. As the robot head lowers to mount the pipette tips, it can damage the angled tip, creating gouges in it and rendering it inaccurate and unusable. Alternatively, the tip may be mounted onto the robot head at a slight angle, which can result in missing it’s intended target well or tube, an incorrect dispense or tip ejection, as well as potentially crashing the robot. Since the use of automated instrumentation should provide walk-away functionality, such issues may not be noted as they occur, allowing experimental inaccuracies to be carried forward into downstream applications.
Preventative measures
In order to combat these potentially detrimental effects, there are a number of measures that pipette tip manufacturers can take. Mostly, these are part of the manufacturing process and involve using various types of resins, as well as anti-static mats and de-ionizing air.
One such measure is to use black carbon conducting resin in the racks for all 384-well robotic pipette tips to help prevent the build-up of static during shipment as well as remove any charge. In addition, deionozing fans may be used to blow deionized air onto all highly susceptible parts, such as very small tips. Antistatic floor mats are yet another method to help prevent the buildup of static when handling highligy susceptible parts, such as 384-well pipette tips. Finally, manufacturers that use e-beam sterilization processes also help to remove static during the sterilization period, thus ensuring that all sterile tips are of the highest possible quality, while remaining neutral in charge. These measures all act to ensure that the occurrence of static is kept to a minimum.
Conclusion
Due to the physical composition of various materials, it is a significant challenge to completely eliminate the occurrence of any static electricity within an environment. There are however a number of measures that can be implemented to decrease its affect. As one of the largest manufacturers of robotic pipette tips, Thermo Fisher Scientific employs a number of steps during the production and assembly of the Molecular BioProducts BioRobotix and Thermo Scientific brand automated tips, to minimize the occurrence of static charge which can impact upon the efficient automation of the liquid transfer process.
References
1. Allen, Ryne, C, Triboelectric Generation: Getting Charged
2. http://www.wordig.com/definition/triboelectric_effect
3. http://science.howstuffworks.com/transport/engines-equipment/vdg1.htm