Optical particle detection in liquid suspensions with a hybrid integrated microsystem

In this work, we present an optical microsystem for particle flow detection, based on hybrid integration and silicon micromachining. Sensing principle is light obscuration. We demonstrate the feasibility of integrating commercial components such as light sources (850 nm near infrared Vertical-Cavity-Surface-Emitting Lasers VCSELs) and optics (an array of microlenses) into a silicon fabricated structure to create an optoelectronic package of reduced dimensions (14 mm × 12 mm) and low power consumption (≈7.2 mW) that satisfies optical requirements and provides four collimated laser beams with reduced diameter spots (minimum radius of 124 μm measured as 1/e2 standard). Besides this microoptical system, the detection system is completed with a double linear array imaging sensor of 256 pixels, designed and fabricated in 0.35 μm CMOS technology, and a microfluidic platform. The designed set-up allows the simultaneous optical detection of singly aligned microbeads flowing in liquid suspensions through several parallel microfluidic channels, which share the pitch of the laser emitting sources, i.e. 250 μm. While the throughput of the system is increased up to 5000 particles/s thanks to the possibility of performing parallel measurements, the double array of the imaging sensor improves the detection process discarding false detected events. Experimental results prove that the system is able to detect and distinguish microparticles of several sizes in a mixed solution, with detection limit set at 10 μm diameter. In conclusion, the fabricated microsystem is presented as a miniaturized and robust alternative to current optical microflow cytometers based on the use of optical fibre, and the complete system, including fluidics and imaging sensor stages, represents a portable solution packaged in less than 5 cm3. These features make it an interesting choice to consider in the field of point-of-care biosensing applications.