In the near future, it may be possible to reveal numerous properties of individual biological cells suspended in a stream of fluid by using ultra-small probes and sensors packed into a device the size of a nickel. That is the goal of several groundbreaking microfluidics projects now underway at NIST that could have a major impact on medical diagnostics, pharmaceutical development and more.
Researchers are investigating how various properties of biological samples can be revealed by exposing them to electric fields and/or electromagnetic radiation. Observing the response of cells to varying probe frequencies uncovers important information about cell membranes as well as their external and internal chemistry.
In one approach, researchers are using microwave radiation to sensitively measure important properties of complex fluids containing cells, DNA, proteins and other biological materials. Using micrometer-scale microfluidic channels, researchers place extremely small volumes of fluids at precise locations within microwave measurement systems. With this approach, known as microwave microfluidics, researchers can accurately measure a biofluid’s electromagnetic response to microwave frequencies over six orders of magnitude, up to 100 GHz and beyond. This emerging technique opens up exciting new capabilities: Researchers can now measure distributions of electrical charge in biofluids with the same degree of accuracy achieved in solid-state integrated circuits for computing and wireless communications applications.
Alternatively, measuring the way electrons in a fluid-borne substance react to changing magnetic fields yields key information. The process, similar to magnetic resonance imaging, can detect, for example, the presence of specific proteins.
In addition, NIST researchers are experimenting with placing optical fiber, around 200 µm thick, next to the fluid channels. The fiber responds to conditions in the fluid channel, such as temperature (which expands or contracts the fiber) and pressure (which pushes a flexible membrane, bending the fiber). Those reactions, in turn, change the wavelength of the light that exits the fiber, providing a sensitive measure of key features of the fluid.
Fibers are also being used to get light into even smaller integrated waveguides on a chip, which deliver and collect light from “reporter” molecules in the flow channel, such as fluorescently labeled biomarkers. These reporters can sense various components in the medium (e.g., ions and oxygen concentration) and can indicate the number of molecules bound to a particular cell as it flows past waveguides. Thus fluorescently tagged antibodies that bind to specific cancer proteins can be used to indicate the number of cancer markers on a cell. Importantly, these integrated waveguides make it feasible to measure the cell multiple times, thus greatly improving the measurement precision and enabling better discrimination of rare events, such as circulating tumor cells or activated lymphocytes, which are being targeted as potential therapeutic agents.
At present, diagnostic analysis of blood or other biofluids entails a host of test procedures typically performed on samples in off-site laboratories. NIST microfluidics technologies could enable easily deployable, real-time tests on-site.
They can also provide information about the physical properties of a wide range of complex fluids — such those that include cells, biomolecules and nanoparticles in solution — many of which behave very differently at the microscale than, for example, an equivalent flow of plain water. NIST teams are exploring methods to measure and understand how those flows respond to changes in force, channel configuration and other variables.
In parallel, NIST researchers are working to design a miniaturized “common platform” for a wide range of different sensors and measurements by combining a microfluidic chip with other micro- and nanofabricated components and electrical circuits. That would make it possible to compare results from different laboratories that now use a variety of instruments and techniques.
Continued progress will enable researchers in biochemistry, pharmaceuticals and other fields to study the characteristics and interactions of biological materials at microscopic length scales in their native fluid environments. Microfluidic systems can transfer cells through a branching channel network into many different biochemical environments at once and then gauge differences in the way the cells interact with each environment.
For maximum utility and range of measured properties, NIST scientists are developing designs in which a number of probes and sensors can be mounted on a single chip-scale platform. All components must fit on the chip, along with detectors of temperature and many other variables. And the devices must be capable of identifying properties at the scale of individual cells. Several NIST projects are exploring ways to make such observations possible with the accuracy needed for medicine and industry. These new instruments promise to spur innovation broadly in many scientific and technical disciplines, from biology and chemistry to electrical and mechanical engineering.
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