Turning up the heat, ever so slightly
Many experiments call for precise control of the temperature during biological and biochemical reactions. The slightest changes can impact cellular processes such as ion channel functioning, synaptic transmission and secretion. Thus, researchers can induce the desired cellular response through the initiation or modulation of pinpoint temperature changes, especially when localized to specific regions of the cell. Such control would be especially helpful with microchips that commonly are used for culturing and analysis of single cells.
Researchers with Institute for Analytical Sciences (ISAS) in Dortmund, Germany, recently reported an improved means by which to achieve such changes. Typically, scientists use thin-film resistive heaters or Peltier devices to heat localized areas within the microchips -- and in this way to achieve precise control of temperature change in single cells; however, this requires a direct connection with a control device, which can be difficult to achieve in complex microfluidic structures. The ISAS investigators addressed this issue by implementing nondirect optical heating by infrared light.
Researchers have described a nondirect optical heating method with which to achieve precise control over temperature change during biological experiments -- thus enabling, for example, cell growth control and lysis of single cells for single-cell analysis. Traditionally, investigators have used Peltier devices or thin-film resistive heaters to this end. However, these require a direct connection with the control device, which can be especially difficult in complex microfluidic structures. The inset shows the underlying principle: Patterned microstructures known as μ-hot plates absorb focused laser light, which then dissipates, giving off heat. The researchers used carbon and gold to construct the hot plates. Reprinted with permission of Lab on a Chip.
“Nondirect optical heating is very flexible and easily implemented in any microfluidic system,” said Helke Reinhardt, the first author of the study. “This may be useful for cell growth control and for lysis of single cells for single-cell analysis, which were the first problems we were seeking to address.”
They induced the temperature changes by irradiating strongly absorbing microstructures on the surface of the microchip (known as μ-hot plates), made of either carbon or gold, with 830-nm infrared light from a focused laser diode, which does not cause damage to the living cells. The arrangement sidesteps the need for a direct connection with a control device and, moreover, facilitates imaging of the cells with standard fluorescence microscopy.
The researchers demonstrated the μ-hot plates by manipulating the size and temperature of a heated volume and thus controlling the growth of colon cancer cells adhered to the patterned surfaces. They used a setup based on a modified inverted microscope made by Olympus Corp. of Melville, N.Y. Light from an 830-nm continuous-wave diode laser made by Sanyo of Osaka, Japan, was transmitted into the microscope and reflected toward a 60× objective, also made by Olympus. Light emitted from the sample was collected by the same objective and imaged by an electron-multiplying CCD camera made by Andor Technology of Belfast, UK. The researchers showed that both the carbon and gold microstructures enabled precise localized control over changes in the temperature and, furthermore, that both allowed growth of the adhered cells.
Thus, the technique can contribute to improved cell growth control and to lysis of single cells for single-cell analysis. Reinhardt noted several other possible advances. “Further investigation and very small μ-hot plates (smaller than 3 μm) are necessary to achieve temporal cell membrane permeability,” she said. In this way, introduction of microparticles -- typically achieved by electroporation -- may be possible. She added that controlled heating of a very small volume inside a cell is feasible with the μ-hot plates, as is the analysis of single cells by applying high laser power and thus rupturing the cells.
Lab on a Chip, November 2007, pp. 1509-1514.
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