Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Introduction <s> We describe devices in which optics and fluidics are used synergistically to synthesize novel functionalities. Fluidic replacement or modification leads to reconfigurable optical systems, whereas the implementation of optics through the microfluidic toolkit gives highly compact and integrated devices. We categorize optofluidics according to three broad categories of interactions: fluid–solid interfaces, purely fluidic interfaces and colloidal suspensions. We describe examples of optofluidic devices in each category. <s> BIB001 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Introduction <s> We describe a microfluidic cytometer that performs simultaneous optical and electrical characterisation of particles. The microfluidic chip measures side scattered light, signal extinction and fluorescence using integrated optical fibres coupled to photomultiplier tubes. The channel is 80 μm high and 200 μm wide, and made from SU-8 patterned and sandwiched between glass substrates. Particles were focused into the analysis region using 1-D hydrodynamic focusing and typical particle velocities were 0.1 ms(-1). Excitation light is coupled into the detection channel with an optical fibre and focused into the channel using an integrated compound air lens. The electrical impedance of particles is measured at 1 MHz using micro-electrodes fabricated on the channel top and bottom. This data is used to accurately size the particles. The system is characterised using a range of different sized polystyrene beads (fluorescent and non-fluorescent). Single and mixed populations of beads were measured and the data compared with a conventional flow cytometer. <s> BIB002 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Introduction <s> A modern flow cytometer can analyze and sort particles on a one by one basis at rates of 50,000 particles per second. Flow cytometers can also measure as many as 17 channels of fluorescence, several angles of scattered light, and other non-optical parameters such as particle impedance. More specialized flow cytometers can provide even greater analysis power, such as single molecule detection, imaging, and full spectral collection, at reduced rates. These capabilities have made flow cytometers an invaluable tool for numerous applications including cellular immunophenotyping, CD4+ T-cell counting, multiplex microsphere analysis, high-throughput screening, and rare cell analysis and sorting. Many bio-analytical techniques have been influenced by the advent of microfluidics as a component in analytical tools and flow cytometry is no exception. Here we detail the functions and uses of a modern flow cytometer, review the recent and historical contributions of microfluidics and microfabricated devices to field of flow cytometry, examine current application areas, and suggest opportunities for the synergistic application of microfabrication approaches to modern flow cytometry. <s> BIB003 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Introduction <s> Microfluidic devices offer automation and high-throughput screening, and operate at low volumes of consumables. Although microfluidics has the potential to reduce turnaround times and costs for analytical devices, particularly in medical, veterinary, and environmental sciences, this enabling technology has had limited diffusion into consumer products. This article analyzes the microfluidics market, identifies issues, and highlights successful commercialization strategies. Addressing niche markets and establishing compatibility with existing workflows will accelerate market penetration. <s> BIB004 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Introduction <s> Cellular analysis plays important roles in various biological applications, such as cell biology, drug development, and disease diagnosis. Conventional cellular analysis usually measures the average response from a whole cell group. However, bulk measurements may cause misleading interpretations due to cell heterogeneity. Another problem is that current cellular analysis may not be able to differentiate various subsets of cell populations, each exhibiting a different behavior than the others. Single-cell analysis techniques are developed to analyze cellular properties, conditions, or functional responses in a large cell population at the individual cell level. Integrating optics with microfluidic platforms provides a well-controlled microenvironment to precisely control single cell conditions and perform non-invasive high-throughput analysis. This paper reviews recent developments in optofluidic technologies for various optics-based single-cell analyses, which involve single cell manipulation, treatment, and property detection. Finally, we provide our views on the future development of integrated optics with microfluidics for single-cell analysis and discuss potential challenges and opportunities of this emerging research field in biological applications. <s> BIB005
Since the original attempt in 1934 when researchers first successfully counted particles and cells in a small tube BIB003 , flow cytometry has developed into a powerful technique for cell analysis, sorting and counting. Recently, flow cytometry has been applied in many fields, such as point-of-care (POC) diagnostics, cellular immunophenotyping, rare cell analysis and genomics BIB002 . The commercialization of conventional flow cytometers has been very successful: the market of modern microflow cytometers is expected to reach $3.6-5.7 billion by 2018 at a compound annual growth rate of 18%-29% BIB004 . Compared to a bulky conventional flow cytometer, microchip-based flow cytometers (referred to as microflow cytometers in this paper) are simple to use, time efficient, consume low amounts of expensive reagents and have overall lower associated costs (capital, operation, training, etc). With the rapidly developing demands of POC applications, the growing demands of in situ and in vitro diagnostics in the biomedical field and the need to improve rapid analysis and synthesis in the chemical field, microflow cytometers are poised to facilitate great advancement in these and other fields and allow applications that will change many aspects of everyday life in the near future. The term "optofluidics" was first mentioned in 2003 and was coined to reference new devices that integrated the fields of optics and microfluidics BIB001 . Microfluidics is the technology that manipulates fluids on the nL-fL scale on a microchip platform, whereas optofluidics manipulates both fluids and optics simultaneously in a seamlessly integrated platform. A microchip-based device that is based on the technology of microfluidics is called a microfluidic device, while an optofluidic device is a device based on optofluidics, requiring both fluidic and optical capabilities. A microflow cytometer is a highly integrated system that utilizes a microchip-based device for the fluidic handling and manipulation in a flow cytometry application. An optofluidic microflow cytometer utilizes an optofluidic device to apply flow cytometry using a single device to integrate both the fluidic and optical sub-systems onto a single device. Classification of the terminology and the function of the device is shown in Table 1 . In an optofluidic microflow cytometer, the optical components are integrated into a microfluidic system and vice versa  . Integration allows the benefits of including new optical features on the device, such as built-in optical alignment, beam shaping, high optical sensitivity and tenability-each seamlessly integrated in one platform with fluidics. Previous review papers have already summarized the fundamentals and applications of optofluidic technology BIB001  BIB005 . In this paper, we will discuss the basic principles and components of an optofluidic device-based microflow cytometer in detail, as well as review the performance of a few microflow cytometers developed recently and compare the performance of the devices. 