Localized Dielectric Loss Heating in Dielectrophoresis Devices

Abstract

Temperature increases during dielectrophoresis (DEP) can affect the response of biological entities, and ignoring the effect can result in misleading analysis. The heating mechanism of a DEP device is typically considered to be the result of Joule heating and is overlooked without an appropriate analysis. Our experiment and analysis indicate that the heating mechanism is due to the dielectric loss (Debye relaxation). A temperature increase between interdigitated electrodes (IDEs) has been measured with an integrated micro temperature sensor between IDEs to be as high as 70 °C at 1.5 MHz with a 30 V_pp applied voltage to our ultra-low thermal mass DEP device. Analytical and numerical analysis of the power dissipation due to the dielectric loss are in good agreement with the experiment data.

Figure 1

Device overview. (a) Isometric view of the device. IDEs, RTD, and electrical contacts are integrated on the top surface of the SiN membrane. Inlet/outlet tubing is mounted on the inlet/outlet holes (not shown). IDEs, RTD, and electrical contacts are electrically isolated from the channel by the 500 nm thick SiN membrane. (b) Channel side of the device. Silicon is etched to form the main channel and inlet/outlet with feed through. A 1 mm thick PDMS is placed to complete the channel. (c) Cross-sectional view of the device showing the channel and IDEs/RTD. (d) Close-up optical photograph of the top view of IDEs and RTD on the SiN membrane.

Figure 2

Measured temperature data as a function of time. The channel is filled with PBS. (a) Measured temperature data. (b) Close-up view of the dotted box in (a). Voltages are applied to IDEs for 1.5 s, and then the RTD measures the temperature for 1.5 s. The measurement cycle is repeated for the frequency range from 100 kHz to 2 MHz at frequency increments of 100 kHz. (c) Close-up view of the dotted box in (b). The temperature data is fitted with $T=A\cdot e^{-t/{\tau }_{th}}+B$, which is the temperature cooling of a first order system. The thermal time constant, τ_th, is 0.301 s when the channel is filled with PBS. The τ_th is ~0.2 s when the channel is empty.

Figure 3

Measured ΔT per unit power (~V²) of PBS, Salt (NaCl) water (4%), and DI water. No obvious difference between three samples is noticeable, indicating that no Joule heating mechanism is involved. COMSOL is used to calculate the power dissipation (loss power) in the SiN dielectric located between the DEP electrodes. Since the power dissipation is proportional to the temperature, the calculated results are in good agreement with the measured data.

Figure 4

Temperature increase per applied voltage squared (unit power) due to the dielectric loss in a DEP device as a function of frequency. The maximum ΔT for PBS and no media in the channel is 9 °C and 45 °C, respectively, at an applied voltage of 26 V_pp. The maximum ΔT with PBS in the channel is lower due to the larger thermal mass of PBS compared to that of air. (a) The temperature is measured with PBS solution in the channel. The measured data is in good agreement with the dielectric loss analysis (Eq. 8). The blue circles are measured data. The red dotted line is the power dissipated in the capacitor (Eq. 8). The black dotted line is the dielectric loss factor (Eq. 3). The regression of the data fit is higher than 0.999. (b) The same plot with no media in the channel (empty). The data clearly shows that the temperature increase is due to the SiN dielectric loss since no media is in the channel.

Figure 5

COMSOL simulation results. As shown in Fig. 3, the numerical analysis agrees with the measured results. (a) A perspective view of the DEP device in Fig. 1(d). The temperature increase between the IDEs is shown. (b) Cross-sectional view along AA’ in (a). The highest temperature increase occurs on the SiN membrane between the IDEs. (c) Temperature profile from the center of the SiN membrane (0 μm) to water in the z-direction.

Figure 6

Numerical simulation of the electric field and potential in the DEP device. (a) Electric field distribution. (b) Enlarged electric field distribution of the red box area in (a). (c) Electric field along BB’ in (b). (d) Electric potential along BB’.