Good results on a plate

When it comes to choosing the right microplate for your automated processes, there is a lot to consider. Here Toby Hampshire gives us a handy guide

GLANCE through manufacturers’ catalogues and you’ll see the wide range of different microplates that are available to scientists in many different disciplines. For high-throughput laboratories considering the use of automated equipment and robotic handling, it is important to select a microplate that is ideally suited to the application throughout the entire workflow in order to ensure maximum efficiency and reproducible results. 

Automation is increasingly used in high-throughput laboratories to reduce the need for time consuming and labour intensive manual manipulations. Automation helps to speed up certain tasks considerably, to enhance precision and, in some instances, to allow processing to continue throughout the day and night without interruption.

Many processes that involve the use of microplates, such as the polymerase chain reaction (PCR), sequencing and microarraying, can now be fully automated, utilising robotic handling equipment to transport plates from one piece of automated equipment to the next. Such equipment may include fully automated workstations, liquid handling systems, automatic plate sealers, centrifuges, thermal cyclers (for both PCR and quantitative PCR applications), plate readers and more.

When choosing the microplate that will help to optimise a specific application, there are a number of factors to consider.

Plate compatibility
In order to eliminate the need for transferring of samples from plate to plate, it is preferable to select a plate that is compatible with each element of the automated process. In addition to being time-consuming, transferring of samples increases opportunities for error and loss of valuable sample.

Microplates can be selected according to various different options, including:
• size
Microwells are available in a number of different formats, from individual tubes and strips to larger microplates (usually 24, 48, 96 and 384-well plates). Most automated, high-throughput applications will use 96 or 384-well plates.
• footprint
Most automated microplate processes are designed to fit the industry standard SBS (Society for Biomolecular Screening) microplate footprint.
• plate skirt
Non-skirted, semi-skirted and fully-skirted microplates are available. Fully skirted plates are normally used for automated and robotic handling procedures since they provide the necessary dimensions and plate stability, in addition to ensuring a flat, even surface for robotic arms to grip easily.
• stackability
Stackability of plates is important in high-throughput applications in order to ensure maximum loading of automated equipment, incubators and storage facilities.
• plate colour/transparency
Microplates are now available in a number of different colours, and in either opaque or transparent plastic, to suit various different applications. Some laboratories prefer transparent plates for optimum sample visibility. Others, using fluorescent applications (such as QPCR), prefer to use opaque white plates, such as Thermo Scientific ABgene White QPCR Plates, in order to optimise the reflection of the fluorescent signal1.

Rigidity options
Plate rigidity is particularly important for automated processes that may involve very high temperatures, such as thermal cycling, or other extremes, such as centrifugation, that may cause standard plates to warp or become wedged in plate carriers. Such issues can be extremely problematic for automated processes, particularly where robotic handling is utilised, causing major interruptions to workflow and potential loss of valuable samples.

Increased rigidity of microplates, to minimise plate warping, can be achieved through the use of strengthening ribs in the frame of the plate, such as in the strengthened Thermo Scientific ABgene Rigid, Skirted, Low Profile PCR Plate. Alternatively, rigidity can be achieved through the thickness and/or choice of polymer used.

The ultra rigid SuperPlates incorporate a strengthened frame designed to virtually eliminate the plate warping and handling problems seen with traditional PCR plates, whilst retaining the thin-walled design of individual wells to ensure rapid and uniform heat transfer. 

The 384-well Diamond PCR Plate is also suited to automation and robotic handling.  Manufactured from a specially selected, low DNA-binding polymer, this is the most rigid microplate available for high-throughput PCR applications, eliminating PCR-related shrinking/warping and ensuring continuous, uninterrupted

Figure 1: Well geometries of a) the flat V-well Thermo Scientific ABgene plate and b) the pyramidal-well plate

Well design
Well design is an important consideration, particularly in the automated dispensing of extremely low volumes (<2µl) into empty plates.

When handling such small volumes, it is essential that the polymer used to manufacture the microplate is non-porous, such as polypropylene, to prevent sample loss through the surface of the well. It should also be inert and extremely smooth to ensure that small droplets run smoothly to the bottom and centre of the well. This is further facilitated by the shape of the well.

Users of microplates for applications, such as high-throughput liquid handling, should be aware that different well shapes can affect results when extremely small volumes are used.  Some manufacturers supply plates with square, pyramidal-shaped wells, whereas others recommend the use of round, V-well microplates - such as the Thermo Scientific ABgene 384 V-well Microarray Plate (Figure 1). 

The ultra-smooth surface and V-well design of this plate ensures that small droplets are dispensed precisely into the centre of the each well, thus minimising well to well variation. This was demonstrated in a comparison of the Thermo Scientific ABgene V-well Microarray Plate with an alternative plate with pyramidal-shaped wells for the dispensing of small volumes of fluorescein. In the automated dispensing of 2µl volumes, the V-well plate demonstrated a lower coefficient of variance of the dispensed volume between wells than the pyramidal-well plate2.

Even a defined volume of 0.9µl could still be readily placed and detected in the V-well plate.  By contrast, problems were encountered with the pyramidal-well plate at this low volume since drops were sometimes deposited in the corner of the well, preventing the fluorescent signal from reaching the sample and thus hindering the analysis of data.

By ensuring that small volumes are dispensed precisely into the centre of wells, this ensures that sample is easily detected and accessed for downstream applications. It also allows high-throughput laboratories to reduce sample volumes, thus lowering the amounts of reagents required and resulting in significant cost savings for the laboratory.

Barcoding
Barcoding facilitates easy and accurate tracking of samples throughout an automated experiment or process. It allows rapid and easy location of samples in individual wells and allows them to be linked with any data generated. Many suppliers now offer a range of pre-barcoded plates. Thermo Scientific ABgene barcoded plates, for example, are labelled with the industry standard 128 barcode. The label can withstand temperatures ranging from -190ºC to 120ºC to ensure its integrity throughout the entire workflow, even when it involves extremes in temperature such as thermal cycling and deep freeze storage. The label also has a special coating to ensure that the barcode is not obscured by wearing during robotic handling of the plate.

Off-the-shelf barcoded plates will normally be supplied with the barcode in a fixed position but some manufactures will also consider custom barcoding of plates, according to the specific needs of individual customers. This is normally subject to a minimum order and often companies offer a range of barcode options on any fully skirted or deep well plate, with labels placed in any position to accommodate variations in automation.

Facilitating automatic plate sealing
Many automated processes will incorporate automatic plate sealing to protect samples from contamination and evaporation. Automatic plate sealers are available for use with both heat seals and adhesive seals.

In order to ensure consistent sealing across the entire plate when using automated plate sealing options, it is important to select microplates that have a raised rim around individual wells. This ensures that a good seal is achieved and minimises cross contamination between wells.

Ensuring consistency with quality
In the processing of multiple plates in high-throughput applications, it is important for scientists to be able to rely on the consistency of the microplates they use. Such assurance can be obtained by sourcing microplates from a manufacturer that enforces rigorous quality control measures throughout the manufacturing and packaging process. Laboratories should be confident that each plate will be of the same high quality as the last that conditions in each well are reproducible and that results from each plate and each well are comparable.

Accuracy, reliability, consistency and cost efficiency are important considerations for high-throughput research applications. An informed choice of microplate can influence these factors considerably and can help scientists to achieve the very best results possible in their different applications.