Handbook ofOptofluidics Handbook ofOptofluidics Edited byAaron R. Hawkins Holger Schmidt CRC Press Taylor & F. Optofluidics is an emerging field that involves the use of fluids to modify Handbook of Optofluidics DownloadPDF MB Read online. Read e-book online Handbook of Optofluidics PDF. By Aaron R. Hawkins,Holger Schmidt. Optofluidics is an rising box that includes using fluids.
|Language:||English, Spanish, Hindi|
|Genre:||Fiction & Literature|
|Distribution:||Free* [*Sign up for free]|
It provides brief reviews of those. Download PDF Handbook of Optofluidics [ Related] [PDF] [cornell biological engineering handbook] [Books] downloaded. Additionally, some important optofluidic components, principles of their operation . refractive index of the fluid in the channel to guide the light. By Aaron R. Hawkins. Optofluidics is an rising box that includes using fluids to switch optical houses and using optical units to notice flowing media. finally.
This original vision has largely held true, although recent reviews show that applications of this technology have gravitated away from classical optical communication devices toward biological and chemical sensing — in other words, away from emphasizing the photonic reconfigurability toward focusing on the contents of the fluids that made up the device [ 6 — 9 ]. Moreover, additional potential application areas such as energy harvesting were identified and a number of topical reviews on various aspects of the field are now available [ 10 — 16 ].
Bioanalysis applications have received so much attention due to the much larger efforts that have been initiated in the lab-on-chip community for creating miniaturized instruments that help deal with the ever-increasing need for biomedical testing. Infectious diseases, for example, are constantly threatening humans and are among the leading causes of deaths across the globe [ 17 ].
Integrated virus detection systems need to fulfill several requirements to satisfy the needs of the medical community. These include low limits of detection of viral or bacterial loads, large dynamic range, and the ability for multiplex detection of multiple targets at once [ 18 ]. Other secondary factors, such as time-to-result, cost, ease-of-use, and experimental complexity, also need to be considered, and the latest optofluidic solutions can now address all these challenges successfully.
Similarly, recent progress in optofluidic particle manipulation and trapping promises another type of powerful and easy-to-use instrument for use in laboratory and clinical diagnostics.
It is, therefore, the perfect time to pause and take stock of the most recent trends and achievements in optofluidics. In this review, we emphasize optofluidic approaches that involve optical waveguides, either planar or fiber-based, as this provides the ultimate level of integration between optical and fluidic elements and functions.
We will first highlight fundamental work in devices that implement canonical optofluidic principles and functions. For an in-depth review of the scientific underpinnings of micro-fluidic and integrated optics, see [ 8 ]. Although the study of flows that meet this criteria dates back at le ast hundreds of years, and technologies that knowingly exploit the properties of such flows e. As a result of this resurgence, when we think about microfluidics now, we think of the well-known technology that evolved from this capability, namely, lab-on-a-chip.
In the 15 years that it has been developing, the lab-on-a-chip technology has been applied to a diverse range of appl ications from chemical and biological analytics to s mall-scale energy production.
W hile analytical improvements associated with the scaling down of the size were originally thought to be the biggest advantage of these devices, further developments revealed other significant advantages including minimized consumption of reagents, increased automation, and reduced manufacturing costs Kock et al.
The latter two of these have benefited largely from the development of on-chip flow control elements, such as valves, pumps, and mixers Unger et al. Labs-on-a-chip are covered in more detail in Chapter 7 by Chung et al. Fu ndamentally go verning the op eration o f t hese de vices i s, o f c ourse, m icroscale flow a nd t ransport. P ut si mply, flow a t t hese scales greatly differs from that at large scales in that it is characterized by a strongly laminar behavior, Handbook of Optofluidics a greater relevance of surface tension, an ability to exploit electrokinetic effects, and small volumes that can be manipulated.
L aminar behavior makes t hese flows easier to c ontrol but i ntroduces challenges such as difficulties in mixing different streams together to conduct a re action.
The greater importance of surface tension enables one to create discrete droplets, which can be independently manipulated onchip and used as ultrasmall reaction vessels Pollack et al. Electrokinetics, where the coupling of an externally applied electric field w ith surface charges at fluid—solid interfaces can be used to very precisely drive fluid motion Li , is too weak an effect to impact large-scale flows, but at the microscale provides a much more facile way of manipulating small volumes of fluids without the need for on-chip valves or pumps.
The small volumes associated with microfluidics imply that smaller amounts o f re agents a re re quired to c onduct a c hemical re action a nd t hat t hese re actions c an o ccur much mo re qu ickly. E qually i mportant f or t his b ook, t hese sm all vol umes me an t hat opt ical f orces, which are usually too small to have any significant effect on large-scale flow, can now play a significant role in driving and affecting the motion of fluids and particles contained therein.
In t his c hapter, w e w ill f ocus o n i ntroducing s ome o f t he f undamentals a nd p ractical a spects o f microfluidic flow and transport in a context relevant for optofluidics with the goal of enabling the reader to approach a proposed experiment in an informed way.
We begin in Section 1. We discuss commonly used transport methods at these scales, including pressure-driven flow and electrokinetics. The fluid dynamical equations governing microfluidic flow are also presented here.
In Section 1. In this section, we begin with a brief literature survey followed by a description of the fundamental equations that govern the coupling between optical and hydrodynamic forces.
Methods for both manipulating particles within flows and microfluidic flows themselves are discussed.
Broadly speaking, two kinds of systems are used to perform micro- and nanofluidic transport, namely, continuous microchannel—based flows a nd d iscrete d roplet—based m anipulation Pollack e t a l. Devices based on continuous microchannel flow, driven by either pressure or electrokinetics, are those most commonly encountered, and, therefore, this will be the focus of this chapter.
The alternative paradigm of digital- or droplet-based microfluidics is usually carried out on open substrates and involves the manipulation of many discrete fluid droplets. The most common method of actuating these droplet flows is through electrowetting Pollack et al. Although we do not cover this here explicitly, its effect does have an importance to optofluidics due to t he success of adaptable electrowetting lenses Kuiper and Hendriks For more details on different microscale transport mechanisms, see a re cent comprehensive review by Stone et al.
Lithographic techniques along with the use of soft polymers are the most common methods used to fabricate micro- and nanochannels. These channels usually have rectangular, rather than circular, cross sections because of the orthogonal nature of l ithographic processing. Microfluidic channels range in size from a few microns across to a few hundred microns. Nanochannels are often very planar with large widths compared to their heights, though this is not always the case. Usually, a chip with nanochannels is fabricated with microscale paths to feed the flow to t he sm aller c hannels.
For more de tails a bout t he f abrication of m icro- a nd nanoscale c hannels, re aders a re re ferred to M cDonald e t a l. Introduction to Microfluidic and Optofluidic Transport 1. Pressure-driven flow is the most commonly used because it is the most robust technique, having very little dependence on fluid and surface properties, and requires very little external infrastructure.
Additionally, very efficient and precise flow valving and pumping methods have been developed for pressure-driven flow at the microscale using techniques such as multilayer soft lithography Unger et al. The parabolic velocity profi le characteristic of pressure-driven flow can result in dispersion of a transported chemical sample or tumbling of larger objects like cells because the flow in the middle is faster than that near the wall.
As we show below, electrokinetic flow exhibits a flat velocity profile often referred to as plug flow leading to minimal dispersion and flow vorticity. Because of this, it also tends to downscale much better than pressure, having an average velocity that is largely independent of channel height.
While the actual manipulation of fluids on-chip using electrokinetic transport is conceptually very easy requiring only the manipulation of external voltages , this technique is much less robust than pressure-driven transport, as appreciable flow velocities can only be obtained for low-ionicconcentration aqueous solutions and certain surface conditions.
Liquid flow at the length scales encountered in both microfluidic a nd na nofluidic devices c an be well described by continuum mechanics Israelachvili , and t he N avier—Stokes e quations, u sed i n t raditional fluid me chanics, rem ain t he go verning e quations for fluid transport.
Physically, this means that the transient period between when flow conditions are changed is very short, and, in many cases, it can be assumed that the flow response to changes in external conditions e. For an electroosmotic flow, suitable modifications can be made to Equations 1. Usually, off-chip pneumatics or syringe pumps are used to ac tuate the fluid in a microchannel directly and on-chip valves are used to manipulate flow locally.
At present, the most common on-chip valve designs are those based on the use of multilayer soft lithography Unger et al.
These valves consist of a second layer of microchannels, which sit on top of the main microfluidic layer all of which are fabricated in a soft polymer, such as poly dimethylsiloxane , or PDMS.
When pneumatically actuated, these valves inflate and press down on the main microchannel, collapsing it and stopping the flow. Figure 1. Two different mechanisms to drive this flow are shown in the figure, a syringe pump and a pressure manifold. The syringe pump works by pushing fluid at a g iven flow rate out of a syringe fitted on t he pump. W hile, i n principle, a de vice u sing such a s yringe pump t hat produces a constant flow rate can be advantageous, it has the significant disadvantage that if a clogging problem occurs on-chip, the pressure will build up u ntil eventual failure of the chip.
Generally, therefore, it is preferential to use a pneumatic or constant-pressure technique, such as that from the pressure manifold shown in the figure. In this case, a c onstant-pressure air source is hooked up to t he valve bank, and a tube runs from the outlet of the valve to a fluid reservoir which is the scintillation vial in this figure.
Open image in new window Fig. Summarized in September For optofluidics, the guidance of light within the device is one of the most important features shared by many applications involving detection of beads, cells, and molecules [ 10 ].
Optical waveguides confine the light by the effect of total internal reflection. A majority of optofluidic devices are made of polydimethylsiloxane PDMS because PDMS is easy to process by the soft lithography process [ 11 , 12 , 13 ]. As for some applications where the samples are illuminated within the microfluidic channel, a liquid-core and liquid-cladding waveguide has been demonstrated to deliver light through the microchannel [ 16 ].
In this approach, the choice of the liquids determines the waveguide characteristics. Wolfe et al. Because of the mixing effect between both liquid layers, the stability of the waveguide is hard to maintain although its properties can be adjusted dynamically by controlling the flow rates of the liquids. To avoid the problem of flow control to assure good waveguide stability, people have developed liquid-core and solid-cladding waveguides using a low refractive index solid material, Teflon AF DuPont Inc.
The refractive index of Teflon AF is 1. Moreover, Teflon AF is chemically stable and optically transparent from UV to IR, so a coating layer of Teflon AF as the cladding layer would not influence cell viability and signal detection. Cho et al. The Teflon AF solution is introduced through the PDMS microchannels by vacuum, and the thickness of the Teflon AF coating can be controlled by the viscosity, strength of vacuum, and channel geometry.
The cross-section of microchannel having its wall coated with the Teflon AF cladding layer shows optical confinement in Fig. For comparison, the light output on the cross-section of the channel without the Teflon AF coating is shown in Fig. The results in Fig. One salient feature of the optofluidic waveguide design is to allow multiple points of detection because the guided excitation light follows the travel of particles.
In the conventional setup, laser beam is focused to only one excitation spot, and multipoint detection not only reduces the light intensity by power splitting but also increases the complexity, size, and cost of optics tremendously. The dotted line signifies the wall of the microchannel and the solid line marks the liquid core, with the space in between occupied by the Teflon AF-coated cladding layer.
The purposes of lab-on-a-chip LOC devices are to reduce the required amount of samples and reagents for analysis, to increase the sensitivity of detection, to lower the cost and size of the system, to simplify and expedite the test procedures, and to minimize chances for sample contaminations and infections for biosafety improvement.
The feature dimensions of LOC devices are typically tens or hundreds of micrometers and the entire device is portable. Combining with the electrical, optical, acoustic, and magnetic components, LOC devices are able to detect, concentrate, and isolate chosen subpopulations of samples cells, beads, molecules from a mixed population. This chapter will mainly focus on those LOC devices that utilize optical detection.
Many research groups have been applying optical detection on the analysis of biological samples. Among these applications, flow cytometer is among the most powerful tools for characterization of biological cells because flow cytometer supports single-cell analysis at high data throughput and is capable of simultaneous detection of multiple biomarkers important for investigations of immunology, cancer, and various diseases.
In a flow cytometer system, external light sources are used to interrogate the flowing sample within the channel.
The physical and biological properties of samples are investigated from their forward scattering FS , side scattering SS , and fluorescence FL. The optical signals produced by each single particle are processed to detect and classify the particle individually. Figure 3 shows a generic design of a traditional benchtop flow cytometer, consisting of a fluidic system with sheath flow confinement; an optical system to illuminate and collect signals; a sorting component, if necessary, to isolate the desired samples from the original sample mixture; and an electronic system for data analysis [ 21 ].
The system contains 1 a fluidic system, 2 an optical system, 3 a sorting system, and 4 an electronic control system for data collection and processing Reprinted with permission [ 21 ] In order to address the needs for various clinical tests, the miniaturization of flow cytometer is required.
Most LOC flow cytometers use hydrodynamic flow confinement to confine the particles to the center of the channel to facilitate the optical detection and to reduce the signal variations characterized by the coefficient of variation CV [ 21 ].
However, unlike the benchtop flow cytometers where the samples are confined to the cylindrical core in the quartz tube, a microfabricated LOC flow cytometer provides only 2-dimensional flow confinement, lacking a mechanism to confine the sample flow in the direction normal to the flow plane. As a result, there exists a large variation of particle velocities in the channel since, in a laminar flow, the particle velocity is related to its position in the channel.
Thus, surface roughness in the mold is transferred to the PDMS microfluidic channel and contributes to the background scattering signals, affecting the detection of forward and side scattering of the particle. In the later sections, the latest developments of improving the performance of LOC flow cytometers by employing the techniques of optical coding, surface treatments, 3-dimensional flow confinement, etc.
Labeled and Label-Free Detection As in any in vitro biosensing techniques, optofluidic devices for biomedical detection can be classified into two categories, labeled and label-free detection. The label-free technique produces signals from samples without attaching fluorescent dyes, quantum dots, or beads to the samples [ 22 , 23 , 24 ].
On the other hand, labeled detection binds fluorescent dyes, proteins, or beads to samples so the signals are generated from the labels rather than the samples themselves [ 16 , 25 , 26 , 27 , 28 ]. The label-free technique offers lower cost and faster results and is generally more desirable as long as feasible.
However, the labeled detection still dominates the field today because it provides more specific and accurate detection for most applications. The following contents will cover these two approaches in the context of optofluidics.
Label-Free Detection Based on the properties of samples one wants to detect, label-free detection mainly includes light scattering, Raman scattering, and surface plasmon resonance. Each detection technique can be utilized individually or in combination to collect differentiable signals. Light Scattering The measurement of light scattering is relatively straightforward for optofluidic sensors.
When particles suspended in the fluid are interrogated with an external light source, the refractive index difference between particles and fluid medium or the refractive index difference of organelles within the cells can generate scattering signals. The intensity of scattering light is determined by the refractive index difference as well as the size, shape, and orientation of cells [ 29 , 30 , 31 ].
Therefore, light scattering signals can be used to identify cells or particles. The scattering signals are usually collected at two different angles. The forward scattering signal can be used to retrieve the particle size since it is measured at small angles 0. In some scenarios, the shape of cells in the fluid medium is not spherical so the orientation of cells might cause different forward scattering signals.
The intensity of side scattering reveals information about the intracellular structures and is particularly sensitive to the granularity and internal structures e.