Temporal and spatial temperature measurement in insulator-based dielectrophoretic devices

Abstract

Insulator-based dielectrophoresis is a relatively new analytical technique with a large potential for a number of applications, such as sorting, separation, purification, fractionation, and preconcentration. The application of insulator-based dielectrophoresis (iDEP) for biological samples, however, requires the precise control of the microenvironment with temporal and spatial resolution. Temperature variations during an iDEP experiment are a critical aspect in iDEP since Joule heating could lead to various detrimental effects hampering reproducibility. Additionally, Joule heating can potentially induce thermal flow and more importantly can degrade biomolecules and other biological species. Here, we investigate temperature variations in iDEP devices experimentally employing the thermosensitive dye Rhodamin B (RhB) and compare the measured results with numerical simulations. We performed the temperature measurement experiments at a relevant buffer conductivity range commonly used for iDEP applications under applied electric potentials. To this aim, we employed an in-channel measurement method and an alternative method employing a thin film located slightly below the iDEP channel. We found that the temperature does not deviate significantly from room temperature at 100 μS/cm up to 3000 V applied such as in protein iDEP experiments. At a conductivity of 300 μS/cm, such as previously used for mitochondria iDEP experiments at 3000 V, the temperature never exceeds 34 °C. This observation suggests that temperature effects for iDEP of proteins and mitochondria under these conditions are marginal. However, at larger conductivities (1 mS/cm) and only at 3000 V applied, temperature increases were significant, reaching a regime in which degradation is likely to occur. Moreover, the thin layer method resulted in lower temperature enhancement which was also confirmed with numerical simulations. We thus conclude that the thin film method is preferable providing closer agreement with numerical simulations and further since it does not depend on the iDEP channel material. Overall, our study provides a thorough comparison of two experimental techniques for direct temperature measurement, which can be adapted to a variety of iDEP applications in the future. The good agreement between simulation and experiment will also allow one to assess temperature variations for iDEP devices prior to experiments.

Figure 1

(a) Schematic of the iDEP microfluidic device (not to scale). The arrow represents the direction of applied electric field (E). Sizes shown are the actual device dimensions (without reservoirs for simplification) used for the experiments and applied to the numerical modeling. The dimensions are the following: L (device length) = 1 cm, W (device width) = 2 cm, W_c (channel width) = 100 μm, H_c (channel depth) = 10 μm, H_glass (thickness of the bottom glass slide) = 150 μm for method A and 1.15 mm for method B, and H_PDMS (thickness of the top PDMS wall) = 0.5 mm. (b) Numerically simulated electric field distribution at 3000 V/cm inside of the channel where the insulating triangular posts are integrated to create an inhomogeneous electric field necessary for DEP.(c) The result of mitochondria DEP experiment, providing a fluorescence microscopy image of mitochondria obtained under DC conditions at 3000 V/cm. White dash lines indicate the edges of the channel and that of a row of posts, and the other rows of posts are indicated by triangles. Scale bar is 30 μm.

Figure 2

Schematic representation of two methodologies employed to measure temperature in iDEP microfluidic devices. The 2D schematics correspond to the cross-section view of the iDEP device shown in Figure 1 (not to scale). (a) In method A, the channel (dotted line) is filled with the RhB containing buffer (pink). (b) In method B, a thick and thin glass slide sandwich the RhB doped thin PDMS film located 150 μm below the channel. Channel is filled with the desired working buffer which does not contain RhB (light blue).

Figure 3

Temperature surface plot at various times after applying a DC potential of 3000 V for a 1 cm channel using the same buffer as previously used for mitochondria DEP experiments. Temperature evolution within the iDEP channel reveals that the temperature does not exceed 34 °C. White lines indicate the edges of the channel and that of triangular insulating posts. The scale bar is 20 μm.

Figure 4

(a,b) Experimentally and (c) numerically obtained temperature resulted from Joule heating inside of the iDEP channel, tested with various conductivities and applied potentials. (a,b) Experimentally measured temporal temperature variations using a phosphate buffer with conductivity of (a) 100 μS/cm (∼0.6 mM) and (b) 1 mS/cm (∼5 mM). Three different potentials were tested for each conductivity of 100 μS/cm (triangles) and 1 mS/cm (dots): 100 V (green), 1000 V (red), and 3000 V (blue) for a 1 cm long channel. (c) Numerical simulation results showing the steady state temperatures as a function of applied potential for the buffer conductivity of 100 μS/cm (tringles) and 1 mS/cm (dots). Inset shows the spatial temperature variations, revealing that the temperature variation is ∼1.5 °C within the channel. (d) Temporal temperature variations obtained numerically for 100 μS/cm (triangles) and 1 mS/cm (dots) when 3000 V is applied.

Figure 5

(a, b) Experimentally and (c) numerically obtained temperature resulted from Joule heating with various conductivities and applied potentials. Temperature was measured on thin PDMS film located ∼150 μm below the iDEP channel. (a, b) Experimentally measured temporal temperature variations using a phosphate buffer with conductivity of (a) 100 μS/cm (∼0.6 mM) and (b) 1 mS/cm (∼5 mM). Three different potentials were tested for each conductivity: 100 V (green), 1000 V (red), and 3000 V (blue) for a 1 cm long channel. (c) Temporal temperature variations obtained numerically for 100 μS/cm (triangles) and 1 mS/cm on the film (filled circles) and in the channel (nonfilled circles) when 3000 V is applied.