IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 54, NO. 1, JANUARY 2007 99 Hybrid Integration of an Active Pixel Sensor and Microfluidics for Cytometry on a Chip Lee Hartley, Karan V. I. S. Kaler, Member, IEEE, and Orly Yadid-Pecht, Senior Member, IEEE Abstract—Reported are motivations and approaches for the integration of custom sensors with microfluidic devices for cy- tometry on a chip and related fluid metering applications. To demonstrate, details of a digital 16-element mixed-signal CMOS active pixel optical sensor with adaptive spatial filtering is first described. The 0.18- m CMOS fabricated sensor is then shown coupled to a microfluidic channel via a polymer encapsulated chip-on-board approach as well as a preferred flip-chip-on-glass hybrid integration approach. However, both approaches discussed possess attributes that are well suited for reliable high-volume production. Utilizing these two disparate assembly topologies, the intelligent sensor’s general behavior, optical input dynamic range, and near-field sensitivity to polymer beads being transported in a microfluidic channel is explored. The findings suggest that discrete near-field sensor integration with microfluidics is a well-positioned integration approach for helping to obviate the need for precision analog-to-digital conversion, optical fiber microchannel coupling, and conventional microscopy for a set of relevant micro total analysis system applications. By opting instead for a hybrid mul- tichip module approach to system integration, this study marks a slight departure in strategy relative to many common monolithic system-on-chip integration efforts. Index Terms—Cytometer, flip-chip devices, intelligent sensors, microassembly, microelectromechanical devices, microfluidics, spatial filters. I. INTRODUCTION T O DATE, the most commercially successful lab-on-a-chip (LOC) systems manifest as devices fundamentally tar- geting bioinformatics (gene chips), whereas flexible micro total analysis system ( TAS) devices embedding optics and real-time control for applications outside molecular diagnos- tics [1] have been slower to materialize. In time, it is likely that many such systems will augment or entirely replace existing clinical laboratory instrumentation platforms which are prevalently relied upon for sample preparation, analysis, and diagnosis. Outside the laboratory, ubiquitous commercial TAS solutions will, in time, provide affordable, reliable, and disposable means for rapidly detecting and responding to bioterrorism agents, implementing chronic environmental monitoring protocols and realizing cost-effective point-of-care diagnostics. Technologies for embarking on the design and fabrication of such physiologically integrated devices abound Manuscript received February 24, 2006; revised August 19, 2006. This work was supported in part by NSERC, CIPI, CIHR and Chinook Microsystems. Microfabrication support for the project was provided by the Canadian Micro- electronics Corporation (CMC). This paper was recommended by Guest Editor D. Wilson. The authors are is with the Department of Electrical and Computer En- gineering, University of Calgary, Calgary, AB T2N-1N4 Canada (e-mail: lfhartle@ucalgary.ca; kaler@ucalgary.ca; orly@atips.ca). Digital Object Identifier 10.1109/TCSI.2006.887456 in a field that is advancing rapidly [2] and is one which displays an inherent reliance on advanced packaging technologies. Conventional flow cytometry is considered a mature tech- nology, having being developed as a result of research effort in the late 1960s and 1970s. In flow cytometry, single parti- cles (biological cells) in an aqueous media that have been suit- ably tagged with fluorescence markers are hydrodynamically focused and made to traverse a small region of space across which a focused laser beam is shone. Depending on the optical characteristics of the cell traversing the beam, and the chem- ical markers it carries, the incident laser light will be scattered and/or fluorescence generated light will be emitted. The optical signals emitted from the particles are then collected as spec- tral bands of predominantly visible light. Using chromatic fil- ters, photomultipliers, and analog-to-digital (A/D) conversion, the acquired signals can be used to identify and quantify the bio- physical or biochemical characteristics of the cell sample popu- lation. The real advantage of flow cytometry is that a very large number of particles can be evaluated in a very short time pe- riod. For example, some such systems can analyze particles at rates up to 100 000 particles per second. Additionally, the prin- ciple of cell sorting in flow cytometry allows this technology to be employed to physically separate single particles/cells from a heterogeneous population. Thus, fluorescence-activated cell sorters (FACSs) represent an incumbent technology in clinical cytometry. While com- monplace in larger microbiology centers, FACS machines are large and expensive pieces of equipment that typically require highly trained personnel for operation and maintenance. The technology combines flow cytometry, fluorescent tagging, and a reliance on electric field-charged particle interactions to achieve precise sorting and counting of target populations. Fu [3] reports a microscale FACS ( FACS) embodying T-junction micro-channels in glass, laser illumination, beam splitters, optical objective gain, photomultipliers, and high-voltage actu- ation electronics. Seeking alternatives to conventional microscopy, which im- pacts portability and cost, various topologies utilizing passive micro-channels, waveguides, fiber optics, and/or integrated photodetectors have been demonstrated to measure forward or side scattered radiation as particles moving through microflu- idic channels interact with light beams [4]–[8]. Nieuwenhuis reports two near-field optical sensors in a 1- m bipolar process for particle shape-based flow cytometry measurements [9]. The first sensing device consists of a two-photodiode strip sensor that requires particles to pass over the geometric center of the structure (implying very precise hydrodynamic focusing). The second device consists of a 2 20 photodiode array which, although capable of greater resolution, is plagued by a high 1057-7122/$25.00 © 2007 IEEE