As microfluidic devices continue to decrease in size and increase in complexity, the ability to monitor the passage of material throughout them becomes ever more important. Efficient sensing of the position of discrete fluidic elements in microfluidics may eventually lead to the creation of viable routing and scheduling systems for the microscale. Many current microfluidic sensing techniques commonly employ optical methods based on differences in the refractive index of fluids. These techniques, though accurate, can often be complicated and difficult to incorporate with the rest of the microfluidic fabrication process. Optical sensors for microfluidics are also very difficult to scale down as microchannels approach the nanometer regime. A promising alternative to optical sensors involves the use of electrical elements (either resistors, capacitors, or conduction gaps) embedded in microfluidic devices. Electrical sensors can be less demanding in space, less expensive, and can offer a greater number of sensors per unit area than their optical counterparts.

We have used thin film resistive heaters and coplanar capacitive sensors to demonstrate the detection of discrete plugs of alternating fluids in a microfluidic networks. Resistive sensors detect position by applying a small, constant potential across a resistor and continuously monitoring the current through it. Changes in current are caused by changes in the thermal conductivity of the fluid surrounding the resistor, and thereby signify the presence of a new fluid plug at the position of the resistor. In the capacitive case, a potential is applied across the gap between two coplanar electrodes, and the current is monitored. Changes in the fluid bridging the gap result in a sudden and sharp induced current spike in the output, and indicates the detection a fluid plug.

To fully realize the goal of a microfluidic control system, these individual electrical sensors must be integrated into a large array spanning an entire microfluidic network. A major problem associated with this is that as the number of sensors increases, so too does the number of electrical leads necessary to connect the sensors with external monitoring equipment. This rapid growth can make the fabrication and implementation of the sensor array exceedingly difficult. The creation of a large array of sensors that also minimizes the number of electrical leads necessary is therefore desirable. The present work proposes a solution to this problem by introducing a multiplexing approach that allows an array of m×n sensors to be controlled by only m+n+1 electrical leads. Each sensing element is connected to two electrical leads, and each electrical lead is connected to multiple sensing elements. When a change in fluid occurs at some sensor (m,n), a change in the monitored output value is displayed in the trace of both lead m and lead n at the same time. This allows one to pinpoint exactly where a fluid element is in a microfluidic system with only m+n+1 leads.

As a proof of the multiplexing concept, 4×4 arrays of resistive and coplanar capacitive sensors were used to monitor the passage of discrete fluid plugs throughout a microfluidic network. This presentation will focus on the fabrication, design rules, and applicability of these sensors to different types of fluid transport detection in various microfluidic architectures.