Sandwich covers (background, working principles)
The most critical aspect of microbial and cell growth in microtiter plates is the closure system of the individual wells. The closure system must: i) prevent (cross)-contamination even during vigorous shaking, ii) permit a reproducible and defined exchange rate of headspace air, iii) limit evaporation. Equal physical conditions in all wells is a further requirement: the wells in the extremities should have exactly the same characteristics as the wells in the centre of the microtiter plate.
Inspired by the general principle used for multi-litre bioreactors (an average aerobic microbial culture needs an air supply of one working volume per minute), we designed the sandwich covers to have a headspace refreshment rate of 1-2 culture volumes per minute. Such a headspace refreshment rates secures oxygen concentrations in the headspace of at least 18% (v/v), even at an oxygen consumption rate by the culture of 40 mmol O
2/L/ minute. At the same time, such a headspace refreshment rate limits the evaporation to about 2% of the culture volume per day (at an ambient humidity of 50% and 30ºC, see
hydrodynamics, oxygen transfer rates): this degree of water loss is acceptable for most projects, and so avoids the need for air humidification in the incubator for most applications. The latter additionally prevents fungal growth in the incubator and so contributes to a safe working environment.
To increase the gas-liquid transfer, orbital shaking is our method of choice since it is proven to be effective (see
literature), and gives rise to exactly the same hydrodynamic conditions in each well (see
hydrodynamics, oxygen transfer rates).
