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Optofluidic Flow Meter

Patent Number: 10,151,681


An optofluidic flow meter to determine a rate of fluid flow in a flow member includes: the flow member; a primary fluid conduit disposed in the flow member and that receives a fluid; a secondary fluid conduit disposed in the flow member; and a fiber optic comprising a fiber Bragg grating interposed between a first flow region of the primary fluid conduit and a second flow region of the secondary fluid conduit and that: physically distorts relative to a pressure differential between the primary fluid conduit and the secondary fluid conduit; and produces a shift in a Bragg wavelength in response to a physical distortion due to the pressure differential.

New enabling technology

Optofluidics is the marriage of microfluidics and optical technology. The NIST optical flow meter uses a photonic sensor to detect pressure changes inside a microfluidic channel, which is then used to calculate volumetric flow rates. It provides on-chip assessment of flow and heat transfer resulting in improvement in fluid metrology and advances in biological sensing.  Accurately measuring flow rates is critical to various microfluidic applications such as droplet formation, particle sorting, flow cytometry and mixing.

Most current pressure measurements rely on external pressure transducers. However, due to pressure dissipation and delays in transmission, it is difficult to accurately measure local pressure in a microfluidic chip using that approach. In addition, many pressure and flow meters are not compatible with liquids or biofluids. 

Blue and green diagrams show the top and side view of the optofluidic flow meter.
Top and side view of flow meter. Two fluid channels (blue) are separated by a flexible membrane (yellow). Embedded in the membrane is a fiber Bragg grating (gray) that reflects certain wavelengths depending on the grating spacing. When there is a difference in pressure between the two channels, the membrane bends and the fiber grating spacing changes. That results in a shift in the wavelength of reflected light. The degree of shift is a measure of pressure difference between channels.
Credit: NIST

Precision measurement at low flow rates

The NIST invention, by contrast, integrates a pressure sensor into a microfluidic chip and provides measurement of microscale forces (pressure). Flow is calculated by dividing pressure drop by the fluidic resistance of the system. A fiber optic with a fiber Bragg grating in its core that reflects selected wavelengths of light is attached to a flexible membrane of 180 µm thickness. The membrane is mounted between the fluid channel and a rounded receptable above the membrane.

A column of temperature-controlled water, or alternatively air pressure above the column, determines pressure in the fluid conduit. As pressure increases or decreases, it strains the membrane and thus the fiber optic. That displacement shifts the Bragg wavelength. The Bragg shift is monitored either by measuring the change in the peak wavelength or the signal intensity (of either the light reflected from the Bragg grating or peak absorption of the light transmitted through the grating at the exit of the fiber optic). After calibrating the Bragg shift to pressure (for example, using changes in height of a column of water), then it can also be used to report volumetric flow rate when scaled by the fluidic resistance of the system.


  • Small and easy to build
  • Measures pressure changes and reports µL/min flow rates
  • Chip-scale
  • Real-time photonic measurements
  • Wide potential temperature range: −266 °C to 500 °C
  • Sensitive on the order of 700 Pa (4 mN)


Exploded schematic of flow meter. Fluid (red) flows into the upper fluid channel. A pressure drop in the upper fluid channel (due to flow) will bend the flexible membrane into the circular region of the lower fluid channel, embedded in which is a fiber Bragg grating (blue). This action causes a shift in the Bragg wavelength, which can report pressure drop (if calibrated or calculated) or flow rate (given the fluidic resistance of the fluid channel).




  • Biomedical devices that depend critically on fluid pressure
  • Flexible or compliant systems where flows and pressures may involve more complicated relationships or evolve with time (and can be internally calibrated or compensated for using this integrated system configuration)


  • Biomedical devices/tissue constructs, wearable technologies
  • Drug delivery systems
  • Continuous microscale liquid manufacturing (e.g., drug development, nanoparticle manufacturing)
Created April 14, 2020, Updated February 11, 2021