FPGAs Yield Virtual Laser Valves for Microfluidics
- Tuesday, 01 May 2007
In today’s “micro world,” complex electrical systems, including analog and digital components, can fit on integrated circuits smaller than a fingernail. Microfluidics, a subset of microelectro-mechanical systems (MEMS) technology, is emerging as a new technical niche within microelectronics with widespread application in the health, chemical, and food industries.
Microfluidics deals with the precise control of microliters (sometimes nano-liters) of fluid. The benefits of scaling chemical processes to this level include extremely low volumes of samples and reagents that can be very expensive, highly parallel processes for massive throughput, and safer testing through decreased volumes of dangerous samples and reactions. Nicknamed lab-on-a-chip (LOC), these types of experiments still face many challenges. First, when scaled to microliters, surface tension, capillary forces, and other fluid dynamics become major considerations. Microfluidics applications require external pressure sources through pumps, centrifugal force, or electrokinetics for flow. Liquids must be precisely manipulated, necessitating some type of micro-valve to divert flow. Practical applications require LOCs with thousands of independently controlled valves to achieve true LOC functionality. Until now, the process for manufacturing entire chips, including microfluidic chambers, valves, and channels, has been cost-prohibitive for commercial viability.
Conventional thinking is that micro-valves must be pre-manufactured into the fabric of the chip and controlled externally. However, with the Virtual Laser Valve (VLV) from SpinX Technologies (Geneva, Switzerland), you can place a valve anywhere on the lattice to perform ultra-precise user-programmable liquid handling.
VLV works in the following manner:
- A thin film separates two microfluidic structures,
- An FPGA-based controller moves and triggers a laser pulse,
- The focused laser creates a microscopic perforation large enough for fluid flow, and
- The two structures connect, and fluid flows from one to the other.
This concept scales upward to control thousands of microfluidic structures for highly parallel processes. The gCard, SpinX Technologies’ version of the microfluidic chip, contains the potential for more than 200,000 discrete valve placements for flexibility and throughput.
VLVs require ultra-precise laser control. Controlling the laser in two dimensions is difficult enough. However, triggering the laser becomes the most challenging component. The entire chip is on a spindle moving at high RPM, which applies centrifugal force to overcome surface tension and capillary action at the microlevel. The application requires a laser to strike a defined spot, plus or minus a few microns, on a swiftly moving target. Angular accuracy must exceed 20 microradians (about 0.001 degrees) to achieve this precision.
The only way to achieve this spatial and temporal accuracy for the laser pulse was to develop prediction algorithms to trigger the laser at a precise point in time. Based on the instantaneous speed sensed with a series of Hall-effect sensors and the desired time to fire, a reconfigurable field-programmable gate array (FPGA) controls the laser trigger. Since the application requires precise hardware timing, reliability, and determinism, FPGAs edge out microprocessor-based solutions by allowing truly parallel control loops. With true parallelism, the same programmable chip controls the laser positioning and triggering, but also handles other components of the SpinX machine, including spinning speed control, readout lasers, single-photon detection and imaging on the rotating cards, and laser focusing onto the film. The entire machine design — consisting of more than 13 control loops, at a single clock cycle period of 25 ns — is implemented on one off-the-shelf programmable FPGA device.