Silicone Membranes Create Micron-Scale Temperature Maps

11 September 2018

 

Silicone Membranes Create Micron-Scale Temperature Maps

Temperature-sensitive membranes could improve microfluidic devices used for point-of-care diagnostics

 

WASHINGTON — By integrating multiple laboratory functions on a chip, microfluidic devices allow lab tests to be performed in a doctor’s office or in settings with limited healthcare services. A newly developed silicone-based membrane makes it possible to create micron-scale temperature maps that could help scientists understand how to keep sensitive cells and fragile DNA at optimal temperatures in microfluidic devices.

“Temperature is important for most biological applications and can be critical for achieving accurate biomedical test results,” Juan Hernández-Cordero, a member of the Universidad Nacional Autónoma de México research team that developed the new membranes. “The membranes we created can be placed on the microchannels in a device so that a technique known as laser-induced fluorescence thermometry can be used to create a detailed temperature map.”  

In The Optical Society (OSA) journal Optical Materials Express, the researchers report their new composite polymer membranes, which are fabricated by simple mixing, and show that they can detect temperature changes as small as 1 degree Celsius with a spatial resolution of 2.7 microns per pixel without any damage to the sample. This resolution was high enough to observe heating by light from the tip of an optical fiber.

“We previously developed a simple way of mixing nanostructured materials into polymers,” said Hernández-Cordero. “By following this same procedure we were able to use off-the-shelf materials to make temperature sensitive membranes of any thickness that can be easily replicated in any laboratory.”

 

Measuring microheater temperature
The researchers developed the new membranes to solve a problem they encountered when working with colleagues at the University of California, Riverside, to develop new ways to use optical techniques for treating brain ailments.

“We were developing fiber optic devices for photothermal therapy, which uses light to generate heat for treating tumors,” said Hernández-Cordero. “We realized that we didn't have a reliable way of measuring the temperature produced by a fiber optic microheater, which is roughly the size of human hair in thickness.”

     
     

Image Name: Mapping heat from optical fiber microheater  

Caption: Researchers created new composite polymer membranes that can be used to measure temperature changes as small as 1 ⁰ Celsius at micron scales. They used the membranes to measure the temperature generated by a fiber optic microheater. The fluorescence of the membranes (top row) changes depending on the heat. This information is then converted into temperature maps (bottom row)

Credit:  Reinher Pimentel-Domínguez, Universidad Nacional Autónoma de México

They examined laser-induced fluorescence thermometry, which measures temperature by using a temperature sensitive fluorescent dye, rhodamine B. Although it can provide measurements at small scales, the rhodamine must be dissolved, requiring the measurements to be performed in liquid solvents such as water or ethanol. For the fiber optic microheaters, taking measurements in a liquid would cause problematic bubbles to form due to the high heat generated.  

The researchers developed the new temperature-sensitive membranes by simply mixing a type of silicone known as polydimethylsiloxane (PDMS) with rhodamine B. The membranes could be used with laser-induced fluorescence thermometry but without the requirement for taking measurements in liquid solvent. After shining laser light on the membrane, the varying brightness of the fluorescence from the membrane can be imaged and then converted into a map of temperatures across the membrane.

“For photothermal therapy there are certain temperature ranges that trigger cellular events that are important for therapy to work,” said Hernández-Cordero. “So, in addition to measuring the temperature in the device, the membranes could make it possible to measure the temperature in the tissue samples with high accuracy.”

To test the membranes, the researchers used a hot plate with a precise temperature control system to see how the hot plate temperatures compared with those obtained with the membranes. After this test was successful, they used the membranes to create temperature maps of the fiber optic microheaters with a heated area of 750 microns by 650 microns.

 

     
     

Image Name: Temperature map of a microheater

Caption:  Temperature sensitive membranes were used to create temperature maps of heat created by a fiber optic microheater. The left image shows a fiber with no heating, while the right image shows the heat generated by the microheater.

Credit:  Juan Hernández-Cordero, Universidad Nacional Autónoma de México

Improving measurement quality
For the most part, the researchers followed established procedures for laser-induced fluorescence thermometry. However, they did make some improvements in the process used to obtain reference measurements, which are necessary to account for laser fluctuations that introduce noise into the measurements.

Although two dyes are typically used to cancel out noise sources, the researchers developed a way to use a portion of the laser beam as a reference. By incorporating that reference information into the image processing scheme, they could directly eliminate any laser fluctuations that would lead to noisy images. This helped improve the quality of measurements that could be obtained with the temperature-sensitive membranes.

“The capabilities of our setup depend heavily on the image analysis that is used,” said Hernández-Cordero. “One advantage of our system is that once you obtain a temperature map with the membrane, any processing technique can be used to analyze the images.” The researchers are now working to optimize the polymer to speed up reaction time. “Although for most small-scale applications, fast response times aren’t necessary, a thinner membrane will make it possible to read more dynamic temperature changes,” said Hernández-Cordero. “We want to find out the minimum thickness of the membrane that we can have and still achieve an accurate reading.”


Paper: F. González-Martínez, O. González-Cortez, R. Pimentel-Domínguez, J. Hernández-Cordero, G. Aguilar. “Composite polymer membranes for laser-induced fluorescence thermometry,” Opt. Mater. Express 8, 10, 3072--3081(2018). DOI: https://doi.org/10.1364/OME.8.003072.

 

 

About Optical Materials Express
Optical Materials Express (OMEx) is an open-access journal focusing on the synthesis, processing and characterization of materials for applications in optics and photonics. OMEx, which launched in April 2011, primarily emphasizes advances in novel optical materials, their properties, modeling, synthesis and fabrication techniques; how such materials contribute to novel optical behavior; and how they enable new or improved optical devices. The editor-in-chief for OMEx is Alexandra Boltasseva from Purdue University. For more information, visit: OSA Publishing.

About The Optical Society
Founded in 1916, The Optical Society (OSA) is the leading professional organization for scientists, engineers, students and business leaders who fuel discoveries, shape real-life applications and accelerate achievements in the science of light. Through world-renowned publications, meetings and membership initiatives, OSA provides quality research, inspired interactions and dedicated resources for its extensive global network of optics and photonics experts. For more information, visit osa.org.

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