Scientists have taken analytical chemistry to such advancements that it can detect the effects of extremely tiny amounts of liquids—triggering the requirement of a need to measure such microflow of liquids. NIST, the National Institute of Standards and Technology, has produced such a microflow measurement device, the size of a nickel, and has filed a provisional patent application for it. The device is capable of measuring movements of nanoliters (nL) of liquid per minute. A nanoliter is a billionth of a liter, a volume best understood with an analogy—if allowed to flow at one nanoliter per minute, a one-liter bottle of water would take 200 years to empty completely.
Micro-fluidics is a rapidly expanding field, where such an invention as above could fill an urgent need for critically measuring tiny flow rates precisely. For instance, medical drug-delivery pumps often need to dispense saline at the rate of tens of nanoliters per minute into the bloodstream of a patient, where 50,000 nL may be required to make up a single drop of water.
Apart from medical applications, continuous-flow micro-manufacturing, cell soring and counting, chemical research, and clinical diagnostics are some applications that require increasingly accurate measurements of very small volumes of liquids.
Current devices available on the market, even the state-of-the-art types that profess to measure flow at that scale, suffer one or numerous operational limitations. Some of them require frequent calibration, some use microscopes and other complex imaging systems, while others average the data collected over several minutes, missing out on tracking dynamic changes. Some devices cannot be traced to the International System of Units.
Greg Cooksey invented the optical microflow measurement device. He is a biomedical engineer in the Physical Measurement Laboratory at NIST. Cooksey’s device avoids the above complications. Fabricated at the Center for Nanoscale Science and Technology at NIST, the optical microflow measurement device monitors the speed of fluorescent molecules within a liquid as they flow down a channel nearly the width of a human hair. Two separate laser pulses help to determine the time interval between the responses of the molecules.
When exposed to a specific wavelength of a blue light laser, the fluorescent molecules in the liquid emit green light. In actual practice, a chemical coating modifies the molecules to prevent them from fluorescence. As the fluid travels down the micro-channel, an ultraviolet laser strips off the chemical coating of some of the molecules. At the same time, some distance away on the channel, a blue laser excites these exposed molecules to make them fluoresce. The flow rate is the time elapsed between the removing of the chemical coating and the molecules beginning to fluoresce.
According to Cooksey, the ultraviolet laser pulse, with a wavelength of 375 nm, marks the start-time reference point. Fired down an optical waveguide into the channel, the pulse hits the chemically protected fluorescent molecules moving with the stream, destroying their protective cage and turning them on to respond to excitation by light.
250 micrometers downstream in the channel, the activated molecules cross the path of a blue laser, which makes them emit green light. An optical power meter measures the change in the light intensity 250,000 times per second to estimate the time interval.