Patent Publication Number: US-2017350806-A1

Title: Multichannel Excitation and Emission for Miniaturized, Planar Fluorescence Activated Cell Sorting

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/345,499, filed Jun. 3, 2016, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Fluorescence activated cell sorting (FACS) is a technique used in cytometry for measuring, sorting and enriching rare cells and particles, such as beads, from large heterogeneous populations. While FACS systems offer desirable multiplexing performance, they can be large and expensive and are typically operated by specially trained staff. As a result, FACS systems are used in only a relatively small number of facilities. To allow for more widespread availability, miniaturized FACS systems (often called “μFACS”) have been developed. However, there are few commercial systems to date and those that exist have limited numbers of both detection and sorting channels compared to traditional FACS. 
     In one example μFACS system, the laser excitation light shares a common path with the flowing cells. This configuration can limit the microfluidic geometry and involves specialized coatings on the channel to permit optical waveguiding. Such coatings can make the chips more costly to fabricate and may not be suitable for all biological samples. Further, this example system only allows for a single excitation wavelength, which is not desirable if more than about ten fluorescent markers are to be identified. Another example μFACS system offers more channels with up to 4 excitation lasers and 8 fluorescent channels (along with two scatter channels), but employs a cuvette such that the excitation lasers are perpendicular to the collection path. Although this approach may have the advantage that the side scatter channel could be efficiently collected, it may be desirable to use widely available, planar microfluidic chips instead of cuvettes. In addition, this approach uses a large number of detectors, such as photomultiplier tubes (PMTs). 
     Accordingly, there is a need for systems that are compatible with existing FACS protocols, that are in a planar geometry such as a microfluidic chip, and that can employ a number of simultaneous fluorescent markers and emission channels on the order of what is possible with traditional FACS systems. 
     SUMMARY 
     The present invention is directed to μFACS systems and related methods, in particular, μFACS systems having at least two excitation lasers each positioned at an angle with respect to each other. 
     In one aspect, a system is provided, comprising: (a) at least two excitation lasers; (b) an objective, wherein the at least two excitation lasers have different orientations relative to the objective such that light from the at least two lasers passes through the objective and intersects a fluidic channel at different positions within an interrogation region, and wherein the fluidic channel directs a flow of a plurality of fluorescently labeled particles through the interrogation region; (c) at least one detector; and (d) at least one optical element optically coupled to the objective and the at least one detector to direct light emitted from the plurality of fluorescently labeled particles and transmitted through the objective to the at least one detector. 
     In another aspect, a method is provided, comprising the steps of: (a) moving a plurality of fluorescently labeled particles through a fluidic channel comprising an interrogation region, wherein the plurality of fluorescently labeled particles move through the interrogation region at a flow speed; (b) directing light from at least two excitation lasers through an objective to the fluidic channel, wherein the at least two excitation lasers have different orientations relative to the objective such that light from the at least two lasers intersects the fluidic channel at different positions within the interrogation region; (c) receiving, by at least one detector, light emitted from each of the plurality of fluorescently labeled particles and transmitted through the objective; and (d) generating, by the at least one detector, a fluorescence signal indicative of intensity of light emitted from each of the plurality of fluorescently labeled particles as the particles move through the interrogation region. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view of an example optical architecture of an embodiment of a μFACS system. 
         FIG. 2  is a top view of an example interrogation region of a fluidic channel, and illustrating a side scatter beam spot and interrogation beam spots generated by the example μFACS system of  FIG. 1 . 
         FIG. 3A  is a side view of an example interrogation region, illustrating side scatter light generated as a particle passes through a side scatter beam generated by the example μFACS system of  FIG. 1 . 
         FIG. 3B  is a side view of an example interrogation region, illustrating forward scatter and fluorescent light generated as a particle passes through an interrogation light beam generated by the example μFACS system of  FIG. 1 . 
         FIG. 4  is a flow chart of an example method. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying figures, which form a part hereof In the figures, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, figures, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     Overview 
     Flow cytometry is an analytical technique used to measure and analyze the physical and chemical characteristics of individual particles, such as cells, as they flow in a fluid stream through a beam of light. The properties measured can include the relative size, relative granularity or internal complexity, and relative fluorescence intensity of each individual particle. Typically, cell components are fluorescently labelled and then excited by a laser to emit light at various wavelengths. The emitted light is received up by one or more detectors, and analyzed, for example, based on how the cell or particle scatters incident laser light and emits fluorescence. FACS is a particular form of flow cytometry that enables a mixture of different particles or cells to be sorted one by one into one or more containers according to their specific light scattering and fluorescence characteristics. 
     Traditional FACS systems, while being generally commercially available and offering desirable capabilities, can be undesirably large and expensive. Further, while there has been an effort to develop miniaturized FACS systems (often described as μFACS systems), very few commercial μFACS systems currently exist. 
     In the present disclosure, an example multi-laser μFACS system includes two or more individual excitation lasers that have different orientations relative to an objective that directs the light from the at least two excitation lasers into a fluidic channel. With this orientation, the light from the at least two excitation lasers intersects the fluidic channel at different locations. The example μFACS system can have a reduced number of detectors, as compared to traditional FACS systems. In some embodiments, the μFACS system is compatible for use with a planar microfluidic chip. 
     Example μFACS Optical Systems 
       FIG. 1  illustrates a schematic of an example optical architecture of a μFACS system  100  having at least two excitation lasers. In this example, three individual excitation lasers  102 ,  104 ,  106  are provided. The use of more than three excitation lasers is contemplated. Light from the excitation lasers  102 ,  104 ,  106  is directed through an objective  120  to an interrogation region  122  of a fluidic channel  116 . The fluidic channel  116 , which may be defined in a planar microfluidic chip  118 , directs a flow of a plurality of fluorescently labeled particles through the interrogation region  122 . In the example shown in  FIG. 1 , the fluidic channel  116  is positioned below the objective  120 , such that excitation beams  108 ,  110 ,  112  from each of the excitation lasers are incident on the fluidic channel in a plane substantially perpendicular to the direction of fluid flow (A) in the channel. 
     One or more cylindrical lenses  114  may be provided to shape each of the excitation beams  108 ,  110 ,  112  so that the desired beam profile is delivered to the fluidic channel  116  positioned below the microscope objective  120 . In flow cytometry applications, it can be desirable to shape the excitation lasers such that they have an elliptical profile and orient the laser spots so that the longer axis of each ellipse is perpendicular to the direction of flow in the fluidic channel. In addition or instead of cylindrical lenses, anamorphic prisms or other optical elements that have different optical powers along the two axes could be used to shape the beam. 
     In this embodiment of system  100 , the excitation lasers  102 ,  104 ,  106  are oriented such that each of the emitted beams of light  108 ,  110 ,  112  is incident on the microscope objective  120  at a different angle. Each of the lasers  108 ,  110 ,  112  has a different angle of incidence on the back aperture of the objective  120  such that each of the beams  108 ,  110 ,  112  intersects the fluidic channel  116  at different points  109 ,  111  and  113  within an interrogation region  122 . The angle at which each of the lasers  102 ,  104 ,  106  is positioned may depend on the number of excitation lasers, the field of view of the microscope objective  120 , and the desired spacing of the laser spots  109 ,  111 ,  113  on the fluidic channel  118 . In some examples, the lasers  102 ,  104 ,  106  may be positioned such that the laser spots are equally spaced apart and do not overlap. The spacing between the spots may, however, be minimized so as to not leave space where particles flowing in the fluidic channel would not be under interrogation of any excitation laser. In other examples, two or more of the laser spots  109 ,  111 ,  113  (shown in  FIG. 2 ) may be permitted to overlap if, for example, the emission spectra of the fluorophores or labels do not or only minimally overlap and the excitation wavelength from one laser doesn&#39;t overlap with the measured emission spectra of the fluorescence channels measured from any of the spatially overlapped lasers. 
       FIG. 2  is a top view of a portion of μFACS system  100 . A sample containing a plurality of fluorescently labelled particles, such as cells, is introduced into the fluidic channel  116 . The system  100  may be designed to detect a plurality of different fluorophores simultaneously by, for example, using lasers emitting at different wavelengths. For example, different types of particles or cells may each be labelled with different fluorophores, thereby allowing each type of particle or cell to be identified or categorized. The lasers  102 ,  104 ,  106  may be selected to emit at a suitable color to excite the fluorophores selected. Individual particles  200 , labeled with one or more fluorophores  202 , move in the fluidic channel  116  in the direction of fluid flow (A). The fluid flows at a particular flow speed, which may be known and may be manually or automatically controlled. The particle  200  is interrogated as it passes through each of the laser spots  109 ,  111 ,  113  in the interrogation region  122 . Also shown is a side scatter beam spot  139 , located upstream of the interrogation region  122 , with respect to the direction of fluid flow (A), which will be discussed in further detail below. 
     Fluorescence  130  emitted from each of the laser spots  109 ,  111 ,  113  is imaged through the objective  120  and tube lens  124  (if needed) onto one or more detectors  126 , such as PMTs, for each of four emission channels (I-IV). The amount of fluorescent light emitted can be correlated with the quantity of a particular cell or particle present in the sample. In the example shown in  FIG. 1 , the detectors  126  may be provided as multi-element detectors, thereby reducing the overall number of detectors in the system to one detector for each fluorescence wavelength channel. The tube lens  124 , or other lenses, may be selected to provide sufficient spacing so that a single multi-element detector is provided for each emission wavelength. In other examples, the detectors  126  may be provided as single pixel detectors. A single pixel detector would, in this case, be provided for each emitting fluorophore excitable by each laser. The fluorescence detectors  126  measure the amplitudes of the fluorescent signals generated by the different fluorescent markers as they pass through the interrogation region  122 . Numerical values are generated based on pulse heights (amplitudes) measured by each of the various detectors  126 . The resulting signals can be input into a processor (not shown) and used to create histograms corresponding to the detected events. 
     In the illustrated embodiment of system  100 , the excitation path shares a common path with the collection path, as they are both coupled through the objective  120 . An optical element  128  directs light that is emitted from the plurality of fluorescently labeled particles after it passes through the objective  120  to the detectors  126 . In one example, the optical element  128  is provided as a dichroic D 1 . The dichroic D 1  can, in one example, have narrow transmission peaks for the excitation laser wavelengths and reflects all other wavelengths to the fluorescence detectors  126 . For simplicity, emission channels II-IV are illustrated with broken lines. Dichroics D 2 , D 3  and D 4  direct each emitted fluorescence wavelength to a detector  126  for each respective emission channel. A bandpass filter F 1 , F 2 , F 3  and F 4  may also be provided for each of emission channels I-IV. While  FIG. 1  illustrates dichroic D 1  transmitting the excitation laser wavelengths and reflecting the emission wavelengths, this could alternatively be switched such that D 1  reflects the excitation laser wavelengths and transmits the emission wavelengths. The remaining optics in the system can be reconfigured as necessary. 
     Alternatively, the optical element  128  may be provided as a dot mirror, in which the excitation laser beams  108 ,  110 ,  112  are reflected by small mirrors in a window (called “dot optic”). Since the emitted fluorescence light will occupy the entire back aperture of the objective  120  and a considerable amount of the area of the dot optic, the percentage of emitted light lost in the collection path due to the small mirrors is small. In the three laser system illustrated in  FIG. 1 , three small mirrors would be used. One possible advantage of this embodiment is that the dot mirror may be less costly to produce and may provide better performance than a multi-band dichroic, such as D 1 . A second advantage is that dot mirrors could be used for alignment during system construction or servicing. Alternatively, the optical element  128  may be a dot optic configured to transmit the excitation laser beams  108 ,  110 ,  112  through small apertures in a mirror. 
     In addition to fluorescence emission, forward and side scattered light may also be detected and measured by the system  100 . A forward scatter detector  134  and a side scatter detector  140  can generate electrical signals corresponding to detected events as the cells or particles are directed through the fluid channel  116 . In one example, shown in  FIG. 1 , one of the incident laser beams  108 ,  110 ,  112  is used for the forward scattering channel. Forward scattered light  132  is scattered through the fluidic channel  116  and is collected by the detector  134 . A bandpass filter  135  and one or more tube lenses  136  may also be provided in the optical path. In some examples, the bandpass filter  135  may be positioned after one of the one or more tube lenses (with respect to the direction of propagation of light). Alternatively, or additionally, an absorptive filter  137  (shown in  FIG. 3B ), may be positioned before or after the bandpass filter  135  for blocking non-absorbed, non-scattered light. A composite filter, with an absorptive dot on a bandpass filter, may also be used. 
     For side scatter measurements, the same or a different one of the incident laser beams  108 ,  110 ,  112  can have its power partially picked off with a beamsplitter BS to provide a side scatter beam  138 . A mirror M 1  directs the side scatter beam  138  to the fluidic channel  116 , and side scattered light  144  is diffusely reflected by particles in the fluidic channel  116 . In one implementation (shown in  FIG. 1 ), the side scatter beam  138  is directed to an area of the fluidic channel  116  that is located upstream of the interrogation region  122 . In that implementation, the side scattered light  144  is detected by the detector  140  through an optical path that is separate from the objective  120 . One or more tube lenses  142  may also be provided in the optical path. In an alternative implementation (not shown), the side scatter beam  138  is directed to the interrogation region  122 . In this alternative implementation, the side scattered light  144  is imaged through the objective and detected by the detector  140 . 
     The laser path used for side scatter shown in  FIG. 1  (side scatter beam  138  directed to the fluidic channel  116  upstream of the interrogation region  122 ) could also be used for forward scatter measurements instead of using one of the incident laser beams through the objective  120 . One advantage of this alternative architecture is that the system  100  may be able to detect particles more quickly if additional synchronization steps are needed, as both the side and forward scatter measurements are taken prior to the fluorescence interrogation area. In an additional alternative embodiment, if the angle of one of the non-normal incidence lasers is large enough, it may be used for side scatter as well, thereby eliminating the need for the beamsplitter BS. 
     The objective  120  may also be used in brightfield detection (not shown). This approach may involve an appropriate substitution of the dichroic and an incoherent light source. Brightfield detection may be used to observe the beam spots  109 ,  111 ,  113  in the field of view in order to ensure proper alignment of the spots  109 ,  111 ,  113  with the fluidic channel  116 . 
       FIG. 3A  and  FIG. 3B  illustrate a side view of a portion of the fluidic channel  116  of the system  100 . In the example shown in  FIG. 3A , side scattered light  144  is emitted from the fluidic channel  116  when a particle  200  passes through the side scatter beam  138 . In the example shown in  FIG. 3A , the system  100  is configured such that the side scatter beam  138  intersects the fluidic channel  116  at a position upstream of the position at which the excitation laser beams  108 ,  110 ,  112  intersect the channel  116 . Alternatively, the side scatter beam  138  could intersect the fluidic channel  116  within the interrogation region  122  so that the side scattered light  144  is imaged through the objective  120 . 
     In the example shown in  FIG. 3B , forward scattered light  132  is transmitted through the fluidic channel  116  when a particle  200  passes through one of the excitation beams in the interrogation region  122 . Fluorescence  130  may be generated, depending on the particular fluorophore(s), as a particle  200  moves through one or all of the excitation beams  108 ,  110 ,  112 . 
     In order to process the output of the system  100 , and ensure that each of the collected signals is assigned to the correct particle in a sample, the side scatter, forward scatter and fluorescence signals from the various laser spots  109 ,  111 ,  113  can be synchronized. To do this, the flow speed of the fluid stream in the fluidic channel  116  is determined. In one embodiment, the flow speed of the fluidic channel may be externally controlled, and therefore has a known value. In another embodiment, the spacing between the individual laser spots  109 ,  111 ,  113  may be used to obtain the flow speed of the individual particles in the fluid stream. If the spacing between each of the laser spots  109 ,  111 ,  113  is known, the speed of the fluid flow may be determined by the time at which one or more of the emission signals from the laser spots  109 ,  111 ,  113  are received at the detectors  126 . The system  100  may combine each of the scatter and emission signals into a single flow event, provide closed-loop flow speed regulation and precisely synchronize actuation of a deflection system, which is used to sort a particle under interrogation into a capture channel of interest. 
     The present μFACS system  100  may provide several advantages over existing approaches. First, this approach is compatible with planar microfluidic chips. Planar microfluidic chips (as compared to other devices, such as cuvettes) can offer more flexibility, for example, by integrating the sorting architecture directly onto the chip. Further, a planar microfluidic chip may be disposable to avoid contamination between sets of samples. Second, this approach may allow for a more compact optical setup, while providing as many illumination inputs and outputs as existing FACS systems. Third, this setup may have a simpler method of alignment as all excitation and emission paths pass through a single element—the objective  120 . Fourth, this approach may allow for a more economical detection approach since multi-element detectors could be used (each detector would be responsible for a single detection wavelength), thereby allowing for more channels with fewer detectors. 
     Example Method 
     A flowchart of an example method  400  for operating a μFACS system having at least two lasers to collect one or more of fluorescent, side scatter or forward scatter light, is shown in  FIG. 4 . In a first step ( 402 ), a plurality of fluorescently labeled particles is moved through a fluidic channel, including an interrogation region, of a μFACS system, such as system  100 . The plurality of fluorescently labeled particles moves through the interrogation region at a flow speed, which may be known or measured during operation of the system. The plurality of particles may be obtained from a sample source, such as a microtiter plate, by, for example, a probe in fluid communication with the fluidic channel. In some examples, the particles are drawn from the sample source into the fluidic channel by means of a pump, such as a peristaltic pump. The light from at least two excitation lasers is directed through an objective to the fluidic channel. ( 404 ). As described above with respect to system  100 , each of the at least two excitation lasers have different orientations relative to the objective, such that light from the at least two lasers intersects the fluidic channel at different positions within the interrogation region. Light emitted from each of the plurality of fluorescently labeled particles is transmitted through the objective, and is received by at least one detector. ( 406 ). The at least one detector generates a fluorescence signal corresponding to the intensity of light emitted from each of the plurality of fluorescently labeled particles, as the particles move through the interrogation region. ( 408 ). 
     The method may also include steps for collecting side and forward scatter light with a μFACS system having at least two lasers, such as system  100 . In some examples, a portion of the light from one of the at least two lasers is directed to a location in the fluidic channel for side scatter measurements. The location could be, for example, within the interrogation region or upstream of the interrogation region with respect to the direction of flow in the fluidic channel. At least one side scatter detector receives side scattered light from each of the plurality of fluorescently labeled particles and generates a side scattered signal. The side scattered light is, in some examples, emitted from within the fluidic channel. In some examples, at least one forward scatter detector receives forward scattered light from each of the plurality of fluorescently labeled particles and generates a forward scattered signal. In some examples, the forward scattered light is transmitted through the fluidic channel. 
     The generated fluorescence, forward scatter, and side scatter signals attributed to a single particle of the plurality of particles can also be synchronized by the system  100 . In some examples, the various signals may be synchronized based, at least in part, on the flow speed of the plurality of particles in the fluidic channel. If, for example, the flow speed is known or determined, the system may identify certain events in the fluorescence, forward scatter, and side scatter signals as being attributable to a single particle. The flow speed may be, in some examples, be set and controlled by a pump. Alternatively, the flow speed may be measured or determined. In one example, the flow speed may be determined based, at least in part, on the timing of generation of fluorescence signals from each excitation laser and the distance between the intersection points of the lasers. Specifically, the flow speed may be determined based, at least in part, on the time that a first fluorescence signal is generated by the at least one detector as a particle of the plurality of particles passes through the position that a first laser of the at least two excitation lasers intersects the fluidic channel, the time that a second fluorescence signal is generated by the at least one detector as the particle passes through a second position that a second laser of the at least two excitation lasers intersects the fluidic channel, and the distance between the first and second positions. Other methods of measuring or otherwise determining the flow speed are contemplated herein. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.