Pressure sensors and electrical circuits made with liquids

Microfluidic technology plays an essential role in diverse chemical, biomedical, and optical applications by making it possible to manipulate and analyze small volumes of material very quickly.^{1, 2} The goal of work in this area is to shrink entire laboratory processes and control elements (valves and pumps) to fit on a single microfluidic device, analogous to an integrated circuit.^3,4 Achieving fully automated ‘labs-on-a-chip’ requires integrated actuators and sensors, which has spurred the development of practical microfluidic actuation schemes in recent decades.^{3, 5–7} However, monitoring what is happening inside the device is still done with conventional macroscale transducers (which involve tedious interconnections) and direct observation under microscopes (which requires additional image analysis). Finally, traditional sensors do not lend themselves to the basic circuitry functions needed to extract information from microfluidic systems.

We recently proposed a new concept called ionic liquid electrofluidic circuits.^{8, 9} Instead of solid metals, these circuits use channels filled with ionic liquid—electrolyte composed entirely of ions—also known as molten salt.¹⁰ The liquid is conductive and thermally stable. Moreover, it has extremely low vapor pressure, which can alleviate evaporation problems that occur with microfluidic devices due to the extremely small liquid volumes. Because the electrofluidic circuits trace the fluidic channel networks, the circuits can be seamlessly fabricated and integrated into microfluidic systems. Specifically, we are working on systems made of an elastomer material, polydimethylsiloxane (PDMS), which is optically transparent and mechanically compliant. As a result, the entire device can be fabricated using multilayer soft lithography with no additional assembly, thereby facilitating large-scale integration.

We developed three basic types of electrofluidic components: a constant resistor, a pressure-regulated variable resistor, and a pressure-controlled switch: see Figure 1(a–c). Transferring pressure inside microfluidic channels to vary the geometries of the electrofluidic circuits changes their electrical characteristics.

Figure 1. Three basic electrofluidic circuit components: (a) constant resistor, (b) pressure-regulated variable resistor, and (c) pressure-controlled switch. (d) Photo of a fabricated polydimethylsiloxane microfluidic device with an integrated electrofluidic circuit. V: Voltage.

The fabricated devices consist of two PDMS layers with one PDMS membrane in the middle: see Figure 1(d). The top layer incorporates electrofluidic channels for the desired circuitry, whereas the microfluidic channels of the bottom layer serve to measure pressure. We then inject ionic liquid (1-ethyl-3-methylimidazolium dicyanamide) into the upper channels. According to Ohm's law and linear solid mechanics theory, the resistance change of the variable resistor is linearly related to the pressure applied below the membrane. The microfluidic channel of the logic switch has a round-shaped cross section that can be closed entirely by applying enough pressure, thereby turning off the switch. Using three electrofluidic constant resistors and a pressure-regulated electrofluidic variable resistor, we demonstrated a pressure sensor with high sensing linearity and temperature (25–100^°C), and long-term (up to 8 days) stability due to the unique material properties of ionic liquid.⁸

We also integrated basic circuitry functions into the electrofluidic devices. Figure 2 shows examples of analog and digital designs. The analog function is implemented by the electrical characterization of the Wheatstone bridge circuit. The change in output voltage V_M is linearly proportional to addition of pressures P_A and P_C and subtraction of P_B (i.e., P_A−P_B+P_C). For the digital function, we demonstrated AND operation. When both channels are closed by applying pressure (state 1), output voltage V_M equals applied voltage V_S (state 1). Otherwise, V_M is approximately zero (state 0). Figure 3 shows the results of analog operation of various pressure combinations. Here, the output voltages of the horizontal dotted lines correspond to addition and subtraction. Figure 4 shows results for digital operation. The output voltage shows that the device works as designed in performing the AND operation. We have also developed electrofluidic circuits capable of performing other operations, including OR and XOR.

Figure 2. Electrofluidic circuit design for (a) analog (P_A-P_B+P_C)and (b) digital (AND) circuitry functions of applied pressures. V_S: Applied voltage. V_M: Output voltage. R: Constant electrofluidic resistor.

Figure 3. Results of analog operation of various pressure combinations. The output voltage amplitude is linearly related to (P_A-P_B+P_C), and the horizontal dotted lines correspond to pressures with different values.

Figure 4. Results of digital operation for two applied pressures. The output voltage is non-zero (state 1) only when both pressures are applied, which amounts to an AND operation.

In summary, we have developed and built electrofluidic circuits that not only convert physical pressure to electrical signals but also perform analog and digital operations. Experimental characterization shows the ability of these systems to bridge the gap between microfluidic systems, pressure sensors, and basic electrical circuits. Our concept holds promise for the large-scale integration that will be needed to make electrofluidic circuit-based systems capable of precise monitoring and advanced feedback control for the next generation of microfluidic devices. We are currently working on electrofluidic circuit-embedded cell culture platforms for monitoring pressure in real time to enable better control of the cellular microenvironment for biomedical studies.