Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Materials <s> Microfluidics is an emerging field that has given rise to a large number of scientific and technological developments over the last few years. This review reports on the use of various materials, such as silicon, glass and polymers, and their related technologies for the manufacturing of simple microchannels and complex systems. It also presents the main application fields concerned with the different technologies and the most significant results reported by academic and industrial teams. Finally, it demonstrates the advantage of developing approaches for associating polymer technologies for manufacturing of fluidic elements with integration of active or sensitive elements, particularly silicon devices. <s> BIB001 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Materials <s> An optimal excitation beam shape is necessary to perform reliable flow cytometric analysis but has so far not been implemented in a photonic-microfluidic integrated (i.e. optofluidic) device. We have achieved this feature by integrating a 1D lens system with planar waveguides and microfluidic channel on a substrate using one patterning material via a one-shot process. In this paper, we report the method of design and the performance of specifically formed excitation regions shaped to be ideal for reducing double detections, improving SNR, and for reliable detection in a flow cytometry application. Demonstration of different sizes via changes to lens design shows the ability to control the width of the shaped beam according to a targeted detection. <s> BIB002 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Materials <s> We report a method, based on the principles of origami (paper folding), for fabricating three-dimensional (3-D) paper microfluidic devices. The entire 3-D device is fabricated on a single sheet of flat paper in a single photolithographic step. It is assembled by simply folding the paper by hand. Following analysis, the device can be unfolded to reveal each layer. The applicability of the device to chemical analysis is demonstrated by colorimetric and fluorescence assays using multilayer microfluidic networks. <s> BIB003 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Materials <s> A simple method of irreversibly sealing SU-8 microfluidic channels using PDMS is reported in this paper. The method is based on inducing a chemical reaction between PDMS and SU-8 by first generating amino groups on PDMS surface using N2 plasma treatment, then allowing the amino groups to react with the residual epoxy groups on SU-8 surface at an elevated temperature. The N2 plasma treatment of PDMS can be conducted using an ordinary plasma chamber and high purity N2, while the residual epoxy groups on SU-8 surface can be preserved by post-exposure baking SU-8 at a temperature no higher than 95 °C. The resultant chemical bonding between PDMS and SU-8 using the method create an interface that can withstand a stress that is greater than the bulk strength of PDMS. The bond is permanent and is long-term resistant to water. The method was applied in fabricating SU-8 microfluidi-photonic integrated devices, and the obtained devices were tested to show desirable performance. <s> BIB004 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Materials <s> Abstract To demonstrate the ability to efficiently count and identify suspended micron-sized particles by simultaneously detecting their fluorescence emission and light scattering in microfabricated channel, a compact configuration that used a polydimethylsiloxane (PDMS) microfabricated channel as interrogation component, hydrodynamic focusing for particle control, and a simple free-space optical setup for signal detection, was accordingly developed. Subsequently, a quantitative count of 1.013 μm diameter fluorescently labeled beads in suspension was implemented in a microfluidic device employing both fluorescence emission and light scattering at average particle throughput ranging from 83 to 416 particles/s. As a result, the detection efficiencies above 88% for both signals and correlation percentages above 97% between them were routinely achieved. In addition, it was shown that effective differentiation of 1.013 μm fluorescently labeled beads from various unlabeled beads in mixed populations of high mixing ratios had been successfully realized in this microfluidic-device-based instrumentation. Finally, the demonstrated system was used to detect fluorescein isothiocyanate (FITC) labeled nonpathogenic bacteria of Escherichia coli ( E. coli ) DH5α. The results showed the detection efficiencies above 89.7% for fluorescence emission and 94.5% for light scattering signals, and a correlation of 94.9% between the two signals at an average throughput of 350 cells/s have been obtained. As a comparison, the detection accuracies of the dual-channel cytometric detection of the FITC-labeled E. coli DH5α cells in the microfluidic device are approximately 84.3% and 88.8% for fluorescence emission and light scattering respectively when compared against a manual cell count using a haemocytometer as a standard. <s> BIB005 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Materials <s> Microfluidics consist of microfabricated structures for liquid handling, with cross-sections in the 1–500 μm range, and small volume capacity (fL-nL). Capillary tubes connected with fittings,1 although utilizing small volumes, are not considered microfluidics for the purposes of this paper since they are not microfabricated. Likewise, millifluidic systems, made by conventional machining tools, are excluded due to their larger feature sizes (>500 μm). ::: ::: Though micromachined systems for gas chromatography were introduced in the 1970’s,2 the field of microfluidics did not gain much traction until the 1990’s.3 Silicon and glass were the original materials used, but then the focus shifted to include polymer substrates, and in particular, polydimethylsiloxane (PDMS). Since then the field has grown to encompass a wide variety of materials and applications. The successful demonstration of electrophoresis and electroosmotic pumping in a microfluidic device provided a nonmechanical method for both fluid control and separation.4 Laser induced fluorescence (LIF) enabled sensitive detection of fluorophores or fluorescently labeled molecules. The expanded availability of low-cost printing allowed for cheaper and quicker mask fabrication for use in soft lithography.5 Commercial microfluidic systems are now available from Abbott, Agilent, Caliper, Dolomite, Micralyne, Microfluidic Chip Shop, Micrux Technologies and Waters, as a few prominent examples. For a more thorough description of the history of microfluidics, we refer the reader to a number of comprehensive, specialized reviews,3, 6–11 as well as a more general 2006 review.12 ::: ::: The field of microfluidics offers many advantages compared to carrying out processes through bulk solution chemistry, the first of which relates to a lesson taught to every first-year chemistry student. Simply stated, diffusion is slow! Thus, the smaller the distance required for interaction, the faster it will be. Smaller channel dimensions also lead to smaller sample volumes (fL-nL), which can reduce the amount of sample or reagents required for testing and analysis. Reduced dimensions can also lead to portable devices to enable on-site testing (provided the associated hardware is similarly portable). Finally, integration of multiple processes (like labeling, purification, separation and detection) in a microfluidic device can be highly enabling for many applications. ::: ::: Microelectromechanical systems (MEMS) contain integrated electrical and mechanical parts that create a sensor or system. Applications of MEMS are ubiquitous, including automobiles, phones, video games and medical and biological sensors.13 Micro-total analysis systems, also known as labs-on-a-chip, are the chemical analogue of MEMS, as integrated microfluidic devices that are capable of automating multiple processes relevant to laboratory sciences. For example, a typical lab-on-a-chip system might selectively purify a complex mixture (through filtering, antibody capture, etc.), then separate target components and detect them. ::: ::: Microfluidic devices consist of a core of common components. Areas defined by empty space, such as reservoirs (wells), chambers and microchannels, are central to microfluidic systems. Positive features, created by areas of solid material, add increased functionality to a chip and can consist of membranes, monoliths, pneumatic controls, beams and pillars. Given the ubiquitous nature of negative components, and microchannels in particular, we focus here on a few of their properties. Microfluidic channels have small overall volumes, laminar flow and a large surface-to-volume ratio. Dimensions of a typical separation channel in microchip electrophoresis (μCE) are: 50 μm width, 15 μm height and 5 cm length for a volume of 37.5 nL. Flow in these devices is normally nonturbulent due to low Reynolds numbers. For example, water flowing at 20°C in the above channel at 1 μL/min (2.22 cm/s) results in a Reynolds number of ~0.5, where <2000 is laminar flow. Since flow is nonturbulent, mixing is normally diffusion-limited. Small channel sizes also have a high surface-to-volume ratio, leading to different characteristics from what are commonly found in bulk volumes. The material surface can be used to manipulate fluid movement (such as by electroosmotic flow, EOF) and surface interactions. For a solution in contact with a charged surface, a double layer of charge is created as oppositely charged ions are attracted to the surface charges. This electrical double layer consists of an inner rigid or Stern Layer and an outer diffuse layer. An electrostatic potential known as the zeta potential is formed, with the magnitude of the potential decreasing as distance from the surface increases. The electrical double layer is the basis for EOF, wherein an applied voltage causes the loosely bound diffuse layer to move towards an electrode, dragging the bulk solution along. Charges on the exposed surface also exert a greater influence on the fluid in a channel as its size decreases. Larger surface-to-volume ratios are more prone to nonspecific adsorption and surface fouling. In particular, non-charged and hydrophobic microdevice surfaces can cause proteins in solution to denature and stick. ::: ::: We focus our review on advances in microfluidic systems since 2008. In doing this, we occasionally must cover foundational work in microfluidics that is considerably less recent. We do not focus on chemical synthesis applications of microfluidics although it is an expanding area, nor do we delve into lithography, device fabrication or production costs. Our specific emphasis herein is on four areas within microfluidics: properties and applications of commonly used materials, basic functions, integration, and selected applications. For each of these four topics we provide a concluding section on opportunities for future development, and at the end of this review, we offer general conclusions and prospective for future work in the field. Due to the considerable scope of the field of microfluidics, we limit our discussion to selected examples from each area, but cite in-depth reviews for the reader to turn to for further information about specific topics. We also refer the reader to recent comprehensive reviews on advances in lab-on-a-chip systems by Arora et al.10 and Kovarik et al.14 <s> BIB006 </s> Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review <s> Materials <s> Polydimethylsiloxane (PDMS) and SU-8 are currently two very commonly used polymeric materials in the microfluidics field for biological applications. However; there is a pressing need to find a simple, reliable, irreversible bonding method between these two materials for their combined use in innovative integrated microsystems. In this paper, we attempt to investigate the aminosilane-mediated irreversible bonding method for PDMS and SU-8 with X-Ray Photoelectron Spectroscopy (XPS) surface analysis and bonding strength tests. Additionally, the selected bonding method was applied in fabricating a microelectrode array (MEA) device, including microfluidic features, which allows electrophysiological observations on compartmentalized neuronal cultures. As there is a growing trend towards microfluidic devices for neuroscience research, this type of integrated microdevice, which can observe functional alterations on compartmentalized neuronal culture, can potentially be used for neurodegenerative disease research and pharmaceutical development. <s> BIB007
Traditionally, silicon and glass substrates are the most common materials used in microflow cytometers. Recently, inorganic materials, like ceramics, and polymers, like PDMS, and even paper BIB003 have been used to construct microfluidic devices. Silicon and glass technologies, as well as polymer technology have been reviewed in many papers BIB001 BIB003 BIB006 . Wet or dry etching methods are applied to create microstructures on a silicon or glass substrate, but organic long-chain polymers are attracting more and more attention with the growing interest in fabricating multilayered structures. Polymers are less expensive and convenient for mass production. More importantly, most polymers are optically transparent to visible wavelengths of light and adaptable through chemical modification for bonding to glass or silicon substrates. Polymers can be divided into elastomers and thermoplastics. PDMS, as one of the elastomers, was first used as a substrate in the late 1990s. Since then, PDMS has established itself as the most commonly-used elastomer in microfluidics. Chemical modification of PDMS has allowed the diversification of its application in microfluidics. PDMS structures can be cured on molds at room temperature for microchannels or other microstructures, and PDMS can provide good sealing properties after chemical modification. Zhang et al. BIB004 sealed SU-8 microfluidic channels using PDMS after the N 2 plasma treatment. Amino groups generated by N 2 plasma on the PDMS surface reacted with the residual epoxy groups on the SU-8 surface. The bond was long-term resistant to water, and the structure could withstand a high degree of stress. Polystyrene (PS), polycarbonate (PC), poly(methyl methacrylate) (PMMA) and cyclic olefin copolymer (COC) are other thermoplastic polymers that are used in microfluidics. As stated earlier, SU-8 is a commonly-used epoxy-based negative photoresist that can be utilized to form the functional layer (waveguides or lenses) when processed on a substrate BIB005 BIB002 BIB007 . High aspect ratio structures can be obtained in SU-8 by lithography. Researchers now are seeking methods to integrate PDMS and SU-8 together to take advantage of both materials. Ren et al. BIB007 bonded SU-8 and PDMS using the aminosilane-mediated bonding method in a microfluidic device for neuroscience research. Paper is a promising new material with low cost and easy fabrication process. Furthermore, paper is available everywhere and relatively environmentally-friendly. Liu and Crook BIB003 fabricated a 3D paper microfluidic devices simply by hand folding. Colorimetric and fluorescence detection of glucose and protein were achieved.