Tag Archives: NIST

Tracking Micro-Fluidic Flows

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.

Mimicking Nerves with Memristors

Researchers are planning to build a computer mimicking the monumental computational power of the human brain. For this, they prefer to use memristors, because these devices vary their electrical resistance on the basis of the memory of their past activity. Memristors are semiconductor devices, and at NIST, the National Institute of Standards and Technology, researchers demonstrate the long and mysterious manner of the inner workings of memristors, explaining their ability to behave as the short-term memory of human nerve cells.

Nerve cells signal one another, but how well they do so depends on the frequency of their recent past communication. In the same way, the resistance of a memristor also depends on the current flow that went through it very recently. The best part is memristors remember even with their electrical power switched off.

Researchers read the memristor with the help of an electron beam. As the beam impinges on various parts of the memristor, it induces currents depending on the resistance value of that part. Traversing the entire device, this yields a complete image of variations of current throughout the device. By noticing the nature of the current variations, it is possible to indicate the places that may fail, as these show overlapping circles within the titanium dioxide filament.

So far, during their study of memristors, scientists have not been able to understand their working, and neither could they develop standard tool-sets for studying them. Now, for the first time, scientists at NIST have been able to create a tool-set that can probe the working of memristors deeply. They envisage their findings will pave the way for operating memristors more efficiently, and minimize current leaks from them.

For exploring the electrical functioning of memristors, the scientists focused a beam of electronics at various locations on the device. The beam was able to knock some of the electronics from the titanium dioxide surface of the device. The free electrons formed an ultra-sharp image of each of the locations. The beam also caused four clear-cut levels of currents to flow through the device. According to the researchers, several interfaces of materials within the memristor were the cause. Typically, a memristor has an insulating layer separating two conducting metal layers. As the researchers could control the position of the electron beam inducing the currents, they were able to know the location of each of the currents.

By imaging the device, researchers located several dark spots on the memristor. They surmised these spots to be regions of enhanced conductivity. These were the places from where there was a greater probability of currents leaking out of the memristor during its normal operations. However, they found the leaking pathways to be beyond the core of the memristor, and at points where it could switch between high and low resistance levels.

Their finding opened up a possibility of reducing the size of the device to eliminate some of the unwanted current leaking pathways. Until now, the researchers were only able to speculate on the current leakages, but had no means of quantifying the size reduction necessary.