Digital Microfluidic Biochips
Microfluidics is rapidly emerging as one of the most advanced technology for the design of biochip for medical, pharmaceutical and environmental monitoring applications. These sensor systems depend on devices which are miniaturized and can manipulate fluids at microliter and nanoliter scales. Such devices are referred to as microfluidic biochips or lab-on-a-chip because micro/nano-liter droplets are controlled on a miniatured lab or manipulated to perform intended biochemical operations.
The first generation of microfluidic biochip consists of several micrometer scale components having channels, valves, actuators, sensors, pumps, and so on. This generation shows applications like DNA probing successfully, but it is not suitable for building a complex and large biochip, as it uses continuous liquid flows through fabricated microchannels, as like continues voltages in analog VLSI design. They were basically designed for simple biochemical analyses or assays.
The new or second generation paradigm has emerged based on a recent technology breakthrough where the continuous liquid flow is sliced or digitized into discrete nanoliter droplets and are manipulated on a two-dimensional array of electrodes. These droplets are processed independently by an electric field. Following the analogy of digital electronics, this technology is referred to as digital microfluidics. As of their inherent properties of dynamic reconfigurability and architectural scalability, digital biochips can be used as programmable microfluidic processors. This new generation of biochip is referred to as a digital microfluidic biochip.
Due to such a digital nature of a digital microfluidic biochip, any operation on droplets can be accompanied with a set of library operations like VLSI standard library, controlling a droplet by applying a sequence of preprogrammed electric signals. In this circumstance, it can be said easily that a large scale complex digital microfluidic biochip can be designed as done in VLSI, once strong CAD frameworks are ready.
The first methodology which is top down for a digital microfluidic biochip is proposed, which mainly consists of architecture-level synthesis and geometry-level synthesis. Geometry-level synthesis can be further divided into module placement and droplet routing.
At the time of module placement, the time interval and location of each module are determined to minimize response time or chip area. As different modules can be on the same spot during different time intervals depending on reconfigurability, module placement is equivalent to a 3D packing problem.
Sometimes, in droplet routing, the path of each droplet is found to transport the droplet without any unexpected mixture under the requirements of design. Similarly to modulate placement, a spot can be used during different time intervals (simply in a time-multiplexed manner) to transport different droplets, which actually increases the complexity of the routing. Routability is the most critical goal of droplet routing as in VLSI, at the time of satisfying timing constraint and maximizing fault-tolerance.
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