A Microtiter plate (spelt Microtitre in Europe) or microplate is a flat plate with multiple "wells" used as small test tubes. The microplate has become a standard tool in analytical research and clinical diagnostic testing laboratories. A very common usage is in the enzyme-linked immunosorbent assay (ELISA), the basis of most modern medical diagnostic testing in humans and animals.
A microplate typically has 6, 24, 96, 384 or even 1536 sample wells arranged in a 2:3 rectangular matrix. Some microplates have even been manufactured with 3456 or even 9600 wells, and an "array tape" product has been developed that provides a continuous strip of microplates embossed on a flexible plastic tape.
Each well of a microplate typically holds somewhere between tens of nanolitres to several millilitres of liquid. They can also be used to store dry powder or as racks to support glass tube inserts. Wells can be either circular or square. For compound storage applications, square wells with close fitting silicone cap-mats are preferred. Microplates can be stored at low temperatures for long periods, may be heated to increase the rate of solvent evaporation from their wells and can even be heat-sealed with foil or clear film. Microplates with an embedded layer of filter material were developed in the early 1990s by several companies, and in 1992, the world's first Solid Phase Extraction (SPE) microplate was launched by Porvair Sciences. This allowed simple column chromatography to be carried out in a microplate footprint for the first time. Today there are microplates for just about every application in life Science research which involves filtration, separation, optical detection, storage, reaction mixing or cell culture.
The enormous growth in studies of whole live cells has led to an entirely new range of microplate products which are "tissue culture treated" especially for this work. The surface of these products is modified, using a plasma discharge, to make it easier for adherent cells to grow on.
A number of companies have developed robots to specifically handle SBS microplates. These robots may be liquid handlers which aspirate or dispense liquid samples from and to these plates, or "plate movers" which transport them between instruments, plate stackers which store microplates during these processes, plate hotels for longer term storage or microplate incubators to ensure constant temperature during testing.
Instrument companies have designed plate readers which can detect specific biological, chemical or physical events in samples stored in these plates.
Microplates are manufactured in a variety of materials. The most common is polystyrene, used for most optical detection microplates. It can be coloured white by the addition of titanium dioxide for optical absorbance or luminescence detection or black by the addition of carbon for fluorescent biological assays. Polypropylene is used for the construction of plates subject to wide changes in temperature, such as storage at -80C and thermal cycling. It has excellent properties for the long-term storage of novel chemical compounds. Polycarbonate is cheap and easy to mould and has been used for disposable microplates for the polymerase chain reaction (PCR) method of DNA amplification. Cyclo-olefins are now being used to provide microplates which transmit ultraviolet light for use in newly developed assays.
The most common manufacturing process is injection moulding, used for polystyrene, polypropylene and cyclo-olefin. Vacuum forming can be used with softer plastics such as polycarbonate. Composite microplates, such as filter plates and SPE plates and even some advanced PCR plate designs use multiple components which are moulded separately and later assembled into a finished product. ELISA plates may now be assembled from twelve separate strips of eight wellss, making it easier to only partially use a plate. This saves cost for the scientist.
In 1996, the Society for Biomolecular Screening (SBS) began an initiative to create a standard definition of a microtiter plate. A series of standards was proposed in 2003 and published by the American National Standards Institute (ANSI) on behalf of the SBS. The standards govern various characteristics of a microplate including well dimensions (e.g. diameter, spacing and depth) as well as plate properties (e.g. dimensions and rigidity).