Small liquid droplets with volumes on the microliter scale are used in many applications today. Areas where these droplets can play a particularly important role include: liquid handling in life sciences and pharmaceutical research, diagnostics, and microfluidic applications. Two liquid handling techniques which are typically employed are pipetting (a contact technique) and dispensing (which can be a non-contact approach).
For many applications, such as the dispensing of buffer solutions or media for cell cultures, a bulk dispenser can be a good option. One method of obtaining microliter-sized droplets is to use fast switching solenoid valves. To allow the liquid to be dispensed, these valves can be fed by a fluidic line, for example, from a pressurized reservoir. A schematic illustration of this simple type of system is provided below:
Pipetting, on the other hand, can be considered a more sophisticated method of liquid handling. In its most basic form, this type of liquid transfer device obtains a volume of liquid from a source container by means of aspiration, before being repositioned over a target destination and subsequently dispensing the liquid. The sample liquid is typically held in a structure referred to as the “tip.” Tips can comprise stationary equipment, such as thin tubes or large hypodermic needles made of metal or plastic, or disposable conical pieces, also known as disposable tips. The use of disposable tips can eliminate carryover risk while also reducing dead volume when handling precious reagents as compared to bulk dispensing. Many systems have multiple tips, which allows a system to speed up the transfer of liquids by simultaneously aspirating from or dispensing into multiple containers. Typically, each working tip in a system is referred to as a channel.
At the present time, a large portion of automated liquid handling systems operates using volumes within the microliter to milliliter range. One current trend that can be observed is the increasing miniaturization of assays – something that allows laboratories to lower the cost of reagents and samples while also increasing throughput. An intelligent combination of pipetting and bulk dispensing can help optimize the automation of liquid handling processes.