A portable device for temperature control along microchannels

Birgit Hausmann

Overview

A design for a conductive path enabling external heaters consisting of a silver-filled epoxy that can be injected and solidified in a microfluidic chip is presented. Both sides of a microchannel can be heated - either at a constant temperature or allowing a temperature gradient along the channel- by the Joule effect. In this way nonequilibrium studies in a microfluidic geometry can be performed.

Results and Discussion

Performing internal thermal control is challenging since the thermal design must be small and follow the channel geometry as closely as possible without hindering visibility while providing ease in operation.

Polydimethylsiloxane (PDMS) was used to fabricate devices. Its low thermal conductivity makes it difficult to design an efficient and fast external temperature control. However the authors claim "it turns out to be a significant advantage for internal heating strategies since, by minimizing undesired energy losses, efficient heat transfer to a channel closely contacted to the heat source is guaranteed." A conductive strip has been designed which follows and surrounds the microchannel at a fixed distance. Temperature is controlled by applying a voltage across the device. The PDMS channels were fabricated utilizing replica molding and softlithography. These channels were filled with a silver two component epoxy. Once the channel is carefully filled the epoxy is solidified. Fig. 1 displays the device containing two separate epoxy-filled ducts (2 mm width, 25 mm height and 40 mm length) that surround, at a distance of 100 mm, a thin central channel (75 mm width and 25 mm height) where the sample to be investigated is injected. While the epoxy is cured, each channel is contacted by thin wires allowing for independent operation of the two heaters during use.

Fig. 1 Schematic of the device: (a) Empty channel filled with a conductive epoxy (b) Wires for electrical contact added (c) Epoxy solidifies during heating.
Fig. 2 Dissipated power (left) and temperature (right) as a function of current (Position of measurements indicated).
Fig. 3 Change in temperature as indicated by dots along the (filled) channel for two different geometries: (a) Heating device shown in Fig. 1 and (b) a device in which one heater surrounds the microchannel. In each design the temperature exhibits a non-uniform distribution. Error bars fall inside the dots.
Fig. 4 (a) Device that is able to maintain a constant temperature in the test channel. (b–d) Device performance for a range of temperature for different currents. (e) 1 $\,^{\circ}\mathrm{C}$ sensitive liquid crystal sheet ranging from 35–36 $\,^{\circ}\mathrm{C}$: only a slight variation of temperature along the microchannel was observed.

The temperature range of operation spans over T=-55 $\,^{\circ}\mathrm{C}$ to 200 $\,^{\circ}\mathrm{C}$ (or intermittently till 300 $\,^{\circ}\mathrm{C}$). Compared to solder metals like In - which has also been used as conductive material in the channel - a lower current is required. The device shown here has a resistance of R=$6\Omega$ and each heater is calibrated by plotting both the dissipated power, P=I$^2$R and temperature, T, reached by the conductive strip versus the input current I. To measure the latter, we used a thermistor in thermal contact with the thin (0.17mm) glass cover slip, which was thermally insulated from the external environment by an additional PDMS layer. The difference in temperature due to the resistance across the glass thickness, $\,\Delta$T was estimated to be in between 0.3 $\,^{\circ}\mathrm{C}$ to 2 $\,^{\circ}\mathrm{C}$ when the temperature of the heater reaches 30 $\,^{\circ}\mathrm{C}$ and 75 $\,^{\circ}\mathrm{C}$, respectively. Typical calibration curves are shown in Fig. 2. Different geometries allow regulating the temperature variations in different ways. Fig. 3 shows two different geometries of the conductive strip: the first (a) is one of the two linear conductive channels that lie next to the central microchannel shown in Fig. 1, while the second (b) is designed to maintain the channel at a constant temperature with a single voltage input by surrounding it with the heater. At high current, the temperature distribution is not uniform along the device and it seems to have a maximum in proximity to the connection wires. A similar profile has been observed with a linear strip of epoxy discarding effects associated with the corners. Other explanations might be a non-uniform filling of the epoxy inside the channel or non-negligible contact resistance between the epoxy and the leads. To avoid the longitudinal thermal gradient the conductive strip has been winded back to its starting point, and the sample microchannel has been placed in between the back and forth paths of the heater (Fig. 4a). This design produces a large and fairly constant temperature region. The temperature profile along the channel has been studied using Mylar liquid crystal sheets with a spatial resolution of about 3–5 mm. The color bar ranges red to dark blue, with color nuances representing temperature steps of 5 $\,^{\circ}\mathrm{C}$ (black is outside the working temperature range). Fig. 4b–d show the temperature distribution for different input currents. The relation between I and T is the same as presented in Fig. 2 with similar absolute values. Furthermore, nearly isothermal regions can be mapped accurately by monitoring the color pattern using images obtained with a higher sensitivity sheet where the whole chromatic range occurs for a temperature change of just 1 $\,^{\circ}\mathrm{C}$ (see Fig. 4e for results from 35–36 $\,^{\circ}\mathrm{C}$). The results show an almost uniform temperature along the channel (within 2–3 $\,^{\circ}\mathrm{C}$ in the zone highlighted by the box in Fig. 4a) for all of the investigated temperature ranges. The stability of the temperature versus time is about 2% over several hours.