Abstract:
Systems for analyzing target materials of a suspension include a tube and a float in which at least a portion of the float is porous. The at least one pore can allow for the flow of fluids and reagents out of the float and into a space between the outer surface of the float and the inner surface of the tube. The at least one pore can also prevent unwanted particles or material from flowing into the float. The introduction of additional fluids, such as fixing agents, washing agents, detergents, or labeling agents, may aid in further processing or detection of the target analyte. The porosity of the float may also allow for the target analyte to be extracted through the float by introducing a removal device, such as a vacuum, to draw the target analyte through the float and into the vacuum.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
       [0001]    This application claims the benefit of Provisional Application No. 62/036,442, filed Aug. 12, 2014. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to density-based fluid separation and, in particular, to tube and porous float systems for the separation and axial expansion of constituent suspension components layered by centrifugation. 
       BACKGROUND 
       [0003]    Suspensions often include materials of interest that are difficult to detect, extract and isolate for analysis. For instance, whole blood is a suspension of materials in a fluid. The materials include billions of red and white blood cells and platelets in a proteinaceous fluid called plasma. Whole blood is routinely examined for the presence of abnormal organisms or cells, such as fetal cells, endothelial cells, epithelial cells, parasites, bacteria, and inflammatory cells, and viruses, including HIV, cytomegalovirus, hepatitis C virus, and Epstein-Barr virus and nucleic acids. Currently, practitioners, researchers, and those working with blood samples try to separate, isolate, and extract certain components of a peripheral blood sample for examination. Typical techniques used to analyze a blood sample include the steps of smearing a film of blood on a slide and staining the film in a way that enables certain components to be examined by bright field microscopy. 
         [0004]    On the other hand, materials of interest composed of particles that occur in very low numbers are especially difficult if not impossible to detect and analyze using many existing techniques. Consider, for instance, circulating tumor cells (“CTCs”), which are cancer cells that have detached from a tumor, circulate in the bloodstream, and may be regarded as seeds for subsequent growth of additional tumors (i.e., metastasis) in different tissues. The ability to accurately detect and analyze CTCs is of particular interest to oncologists and cancer researchers, but CTCs occur in very low numbers in peripheral whole blood samples. For instance, a 7.5 ml sample of peripheral whole blood that contains as few as 3 CTCs is considered clinically relevant in the diagnosis and treatment of a cancer patient. However, detecting even 1 CTC in a 7.5 ml blood sample may be clinically relevant and is equivalent to detecting 1 CTC in a background of about 50 billion red and white blood cells. Using existing techniques to find, isolate and extract as few as 3 CTCs of a whole blood sample is extremely time consuming, costly and is extremely difficult to accomplish. 
         [0005]    As a result, practitioners, researchers, and those working with suspensions continue to seek systems and methods to more efficiently and accurately detect, isolate and extract target materials of a suspension. 
       SUMMARY 
       [0006]    Systems for analyzing target materials of a suspension include a tube and a float in which at least a portion of the float is porous. The at least one pore can allow for the flow of fluids and reagents out of the float and into a space between the outer surface of the float and the inner surface of the tube. The at least one pore can also prevent unwanted particles or material from flowing into the float. The introduction of additional fluids, such as fixing agents, washing agents, detergents, or labeling agents, may aid in further processing or detection of the target analyte. The porosity of the float may also allow for the target analyte to be extracted through the float by introducing a removal device, such as a vacuum, to draw the target analyte through the float and into the vacuum. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1A-1B  show isometric views of two example tube and porous float systems. 
           [0008]      FIGS. 2A-2B  show a porous float. 
           [0009]      FIGS. 3-5  show examples of different types of porous floats. 
           [0010]      FIGS. 6A-6C  show a porous float. 
           [0011]      FIGS. 7A-7B  show a porous float. 
           [0012]      FIGS. 8A-8B  show a porous float. 
           [0013]      FIGS. 9A-9C  show a porous float. 
           [0014]      FIGS. 10A-10B  show a porous float. 
           [0015]      FIG. 11  shows a fluid being introduced to a tube and porous float system. 
           [0016]      FIG. 12  shows a target analyte being withdrawn from a tube and porous float system. 
           [0017]      FIG. 13A  shows an example tube and porous float system. 
           [0018]      FIG. 13B  shows a cross-sectional view of the tube and porous float system. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The detailed description is organized into two subsections: A general description of tube and porous float systems is provided in a first subsection. Using tube and porous float systems to analyze target materials of a suspension is provided in a second subsection. 
         [0020]    It should be understood that “fluid” includes a gas and a liquid, such as a solution (solute in a solvent) or a suspension (heterogeneous fluid with solid particles suspended within the heterogeneous fluid). 
       General Description of Tube and Porous Float Systems 
       [0021]      FIG. 1A  shows an isometric view of an example tube and porous float system  100 . The system  100  includes a tube  102  and a porous float  104  suspended within a suspension  106 . In the example of  FIG. 1A , the tube  102  has a circular cross-section, a first closed end  108 , and a second open end  110 . The open end  110  is sized to receive a stopper or cap  112 . The tube may also have two open ends that are sized to receive stoppers or caps, such as the example tube and porous float system  120  shown  FIG. 1B . The system  120  is similar to the system  100  except the tube  102  is replaced by a tube  122  that includes two open ends  124  and  126  configured to receive the cap  112  and a cap  128 , respectively. The tubes  102  and  122  have a generally cylindrical geometry, but may also have a tapered geometry that widens, narrows, or a combination thereof toward the open ends  110  and  124 , respectively. Although the tubes  102  and  122  have a circular cross-section, in other embodiments, the tubes  102  and  122  can have elliptical, square, triangular, rectangular, octagonal, or any other suitable cross-sectional shape that substantially extends the length of the tube. The tubes  102  and  122  can be composed of a transparent or semitransparent flexible material, such as flexible plastic or another suitable material. The tube may also include a plug (not shown) at the closed end  108  to permit the removal of a fluid, the suspension, or a suspension fraction, whether with a syringe, by draining, or the like. 
         [0022]    The tube may have a sidewall and a first diameter. The porous float can be captured within the tube by an interference fit. To remove the porous float from the tube after the porous float has been captured, the sidewall, being elastically radially expandable to a second diameter, may be expanded in response to an axial load, pressure due to centrifugation, external vacuum, or internally-introduced pressure, the second diameter being sufficiently large to permit axial movement of the porous float in the tube during centrifugation. 
         [0023]      FIG. 2A  shows an isometric view of the porous float  104  shown in  FIG. 1 . The porous float  104  includes a porous main body  210 , two teardrop-shaped end caps  204 ,  206 , and support members  208  radially spaced and axially oriented on the main body  210 . The porous float can also include two dome-shaped end caps or two cone-shaped end caps. The support members  208  provide a sealing engagement with the inner wall of the tube  102 . The porous float  104  may also include a pierceable segment  202 . The pierceable segment  202  may be located in the top end cap  204  or the bottom end cap  206  through which fluids can be introduced. The porous float  104  may also include a porous layer (not shown) which may surround at least a portion of the porous main body  210  and/or the support members  208 . The porosity of the main body  210  may be greater than the porosity of the porous layer (not shown). 
         [0024]    In alternative embodiments, the number of support members, support member spacing, and support member thickness can each be independently varied. The support members  208  can also be broken or segmented. The porous main body  210  is sized to have an outer diameter that is less than the inner diameter of the tube  102 , thereby defining fluid retention channels between the outer surface of the porous main body  210  and the inner wall of the tube  102 . The surfaces of the porous main body  210  between the support members  208  can be flat, curved or have another suitable geometry. In the example of  FIG. 2A , the support members  208  and the main body  210  form a single structure. The porous main body  210  includes at least one pore. 
         [0025]      FIG. 2B  shows a cross-sectional view of the porous float from  FIG. 2A  taken along the line I-I. The porous float  104  may include a central area  212  that is in fluid communication with the pierceable segment  202  and the main body  210  to permit a fluid to flow through and out of the porous float  104 . The central area  212  may include a porous material or may be a hollow space. The porous float  104  may be pre-filled with a fluid before being introduced into a tube; or, the porous float  104  may be empty and may filled with a fluid at some point in time after being introduced to a tube. 
         [0026]    Embodiments include other types of geometric shapes for porous float end caps.  FIG. 3  shows an isometric view of an example porous float  300  with a dome-shaped end cap  302  and a cone-shaped end cap  306 . A porous main body  308  of the porous float  300  can include the same structural elements (i.e., support members)  310  as the porous float  104 . A porous float can also include a teardrop-shaped end cap. The porous float end caps can include other geometric shapes and are not intended to be limited to the shapes described herein. The porous float  300  may also include a pierceable segment  304  and a central area (not shown), which may be hollow or filled with a porous material. 
         [0027]    In other embodiments, the main body of the porous float  104  can include a variety of different support structures for separating target materials, supporting the tube wall, or directing the suspension fluid around the porous float during centrifugation.  FIGS. 4 and 5  show examples of two different types of main body structural elements. Embodiments are not intended to be limited to these two examples. In  FIG. 4 , a porous main body  408  of a porous float  400  is similar to the porous float  104  except the porous main body  408  includes a number of protrusions  410  that provide support for the tube. In alternative embodiments, the number and pattern of protrusions can be varied. The porous float  400  may also include a pierceable segment  404  and a central area (not shown), which may be hollow or filled with a porous material. In  FIG. 5 , a porous main body  508  of a porous float  500  includes a single continuous helical structure or ridge  512  that spirals around the porous main body  508  creating a helical channel  510 . In other embodiments, the helical ridge  512  can be rounded or broken or segmented to allow fluid to flow between adjacent turns of the helical ridge  512 . In various embodiments, the helical ridge spacing and rib thickness can be independently varied. The porous float  500  may also include a pierceable segment  504  and a central area (not shown), which may be hollow or filled with a porous material. 
         [0028]      FIG. 6A  shows an exploded view of a porous float  600 .  FIG. 6B  shows an isometric view of the porous float  600 . The porous float  600  includes a float  610  and a porous layer  620 . The float  610  includes a main body  616 , a top end cap  612 , and a bottom end cap  614 . The float  610  may also include float support members  618  radially spaced and axially oriented on the main body  616 . When present, the float support members  618  provide support for the porous layer  620 . The top end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. The bottom end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. 
         [0029]    The porous layer  620 , which may be removable from or permanently attached to the float  610 , includes a hole  622 , a porous main body  624 , and layer support members  626  radially spaced and axially oriented on the porous main body  624 . The hole  622  is configured to fit around the float  610 . The layer support members  626  provide a sealing engagement with the inner wall of a tube. 
         [0030]      FIG. 6C  shows a cross-sectional view of the porous float  600  taken along the line II-II. The porous float  600  includes the float  610  and the porous layer  620 . When the float  610  includes the float support members  618 , the float support members  618  may provide support for the porous layer  620 , such that when the porous layer  620  is placed over the float  610 , a space is created between the float  610  and the porous layer  620 . The space provides an area by which a fluid may be introduced into the porous float. The fluid may be introduced in the space, thereby allowing for the fluid to diffuse through the porous layer  620  and into other areas of a tube. When the float  610  does not include the float support members  618 , the space may not be present and the float  610  and the porous layer  620  may be in contact. Alternatively, when the float  610  does not include the float support members  618 , the space may be present by including a spacer between the float  610  and the porous layer  620 . The spacer (not shown) is configured to create a space between the float  610  and the porous  620 . The space can extend any length between the float  610  and the porous layer  620 . The space may also be non-continuous in either one or both of the circumferential or longitudinal directions. 
         [0031]    The top end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. The bottom end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. The float support members and the layer support members may also be a helical ridge which creates a helical groove or may be protrusions. 
         [0032]      FIG. 7A  shows an isometric view of a porous float  700 .  FIG. 7B  shows cross-section view of the porous float  700  along the line III-III. The porous float  700  includes a main body  712  and a porous layer  702 , which includes at least one pore. The porous float  700  includes a teardrop-shaped top end cap  704  and a teardrop-shaped bottom end cap  706 . The porous float  700  may also include a top support member  708 , which extends circumferentially around an upper portion of the main body  712  or porous layer  702 , and a bottom support member  710 , which extends circumferentially around a lower portion of the main body  712  or porous layer  702 . The porous layer  712  surrounds an outer surface of the main body  712 . The porous layer  702  and the main body  712  may be a singular structure, or the porous layer  702  may be wrapped around the main body  712 , and thereby being different structures. The porous layer  702  may extend underneath the top support member  708  to a bottom portion of the top end cap  704 ; the porous layer  702  may extend underneath the bottom support member  710  to a top portion of the bottom end cap  706 ; or, the porous layer  702  may extend from a bottom of the top support member  708  to a top of the bottom support member  710 . The porous layer  702  can surround the entire main body  712 , a portion of the main body  712 , or a plurality of portions of the main body  712 . The porous float  700  may also include at least one support member (not shown) between the top and bottom support members, the at least one support member (not shown) may be vertical, be horizontal, be protrusions, be at least one helical ridge, or any appropriate support member shape or configuration. The top and bottom end caps  704 ,  706  may include a portion of the porous layer  702  or may include at least one porous segment including the same porous material as the porous layer  702  and in fluid communication with the porous layer  702 . The top support member  708  and the bottom support member  710  may be extensions of the porous layer  702 , which extend horizontally out from the porous layer  702 . Alternatively, the top support member  708  and the bottom support member  710  may be extensions of the main body  712  or top and bottom end caps  704 ,  706 , which extend horizontally out from the main body  712  or the top and bottom end caps  704 ,  706 . The porous layer  702  can be layered over the horizontal extensions to further augment the top and bottom support members  708 ,  710 . The porous layer  702  can also be layered over the at least one other support member for augmentation. The porous float  700  may also include a coating (not shown), such as parylene, which at least partially covers the porous float  700  and which may be made porous, at least in areas between the top and bottom support members  708 ,  710  through machining, through the use of a laser, or the like. 
         [0033]      FIG. 8A  shows an isometric view of a porous float  800 .  FIG. 8B  shows cross-section view of the porous float  800  along the line IV-IV. The porous float  800  includes a main body  816  and a porous layer  802 , which includes at least one pore. The porous float  800  includes a teardrop-shaped top end cap  804  and a teardrop-shaped bottom end cap  806 . The porous float  800  may also include a top support member  808 , which extends circumferentially around an upper portion of the main body  816  or porous layer  802 , and a bottom support member  810 , which extends circumferentially around a lower portion of the main body  816  or porous layer  802 . The porous layer  802  surrounds an outer surface of the main body  816 . The porous layer  802  and the main body  816  may be a singular structure, or the porous layer  802  may be wrapped around the main body  816 , and thereby being different structures. The porous layer  802  may extend from a bottom of the top support member  808  to a top of the bottom support member  810 . The top and bottom end caps  804 ,  806  may include a cap gap  812 ,  814  which is a different porous material than the porous layer  802 , though still in fluid communication with the porous layer  802 . The cap gap  812 ,  814  may be segmented, may an individual opening, or may be a continuous ring around at least one of the end caps  804 ,  806 . The porous layer  802  can surround the entire main body  802 , a portion of the main body  816 , or a plurality of portions of the main body  816 . The porous float  800  may also include at least one support member (not shown) between the top and bottom support members, the at least one support member (not shown) may be vertical, be horizontal, be protrusions, be at least one helical ridge, or any appropriate support member shape or configuration. The top support member  808  and the bottom support member  810  may be extensions of the porous layer  802 , which extend horizontally out from the porous layer  802 . Alternatively, the top support member  808  and the bottom support member  810  may be extensions of the main body  816  or top and bottom end caps  804 ,  806 , which extend horizontally out from the main body  816  or the top and bottom end caps  804 ,  806 . The porous layer  802  can be layered over the horizontal extensions to further augment the top and bottom support members  808 ,  810 . The porous layer  802  can also be layered over the at least one other support member for augmentation. The porous float  800  may also include a coating (not shown), such as parylene, which at least partially covers the porous float  800  and which may be made porous, at least in areas between the top and bottom support members  808 ,  810  through machining, through the use of a laser, or the like. 
         [0034]      FIG. 9A  shows an isometric view of a porous float  900 .  FIG. 9B  shows cross-section view of the porous float  900  along the line V-V. The porous float  900  includes a main body  916  and an intermediary layer  918 . The intermediary layer  918  may be porous. The porosity of the intermediary layer  918  may be greater than the porosity of a porous layer  902 . The porous float  900  includes a teardrop-shaped top end cap  904  and a teardrop-shaped bottom end cap  906 . The top end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. The bottom end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. The porous float  900  may also include the top support member  908 , which extends circumferentially around an upper portion of the main body  916  or intermediary layer  918 , and the bottom support member  910 , which extends circumferentially around a lower portion of the main body  916  or intermediary layer  918 . The porous float  900  may also include a coating (not shown), such as parylene, which at least partially covers the porous float  900  and which may be made porous, at least in areas between the top and bottom support members  908 ,  910  through machining, through the use of a laser, or the like. 
         [0035]    The intermediary layer  918  surrounds an outer surface of the main body  916 . The intermediary layer  918  may extend underneath the top support member  908  to a bottom portion of the top end cap  904 ; the intermediary layer  918  may extend underneath the bottom support member  910  to a top portion of the bottom end cap  906 ; or, the intermediary layer  918  may extend from a bottom of the top support member  908  to a top of the bottom support member  910 . The top and bottom end caps  904 ,  906  may include at least one cap gap  912 ,  914  which may be filled with the same porous material as the porous layer, a different porous material than the intermediary layer  918 , though still in fluid communication with the intermediary layer  918 , or may be empty and therefore an open channel to permit fluid communication between the open channel and the intermediary layer  918 . The at least one cap gap  912 ,  914  may be segmented, may be an individual opening, or may be a continuous ring around the respective end cap  904 ,  906 . The intermediary layer  918  can surround the entire main body  916 , a portion of the main body  916 , or a plurality of portions of the main body  916 . The porous float  900  also includes the porous layer  902 . The porous layer  902  surrounds the intermediary layer  918 . The porous layer  902 , intermediary layer  918 , and the main body  916  may be a singular structure; the intermediary layer  918  and. the main body  916  may be a singular structure, with the porous layer wrapped around the intermediary layer  918 ; or the porous layer  902 , the intermediary layer  918  and the main body  916  may all be different structures. The porous float  900  may also include at least one support member (not shown) between the top and bottom support members  908 ,  910 , the at least one support member (not shown) may be vertical, be horizontal, be protrusions, be at least one helical ridge, or any appropriate support member shape or configuration. The top support member  908  and the bottom support member  910  may be extensions of the porous layer  902 , the at least one cap gap  912 ,  914 , or the intermediary layer  918 , which extend horizontally out from each respective layer and each other respective layer may be used to augment the top and bottom support members  908 ,  910 . Alternatively, the top support member  908  and the bottom support member  910  may be extensions of the main body  916 , which extend horizontally out from the main body  916 . The porous layer  902  or intermediary layer  918  can be layered over the horizontal extensions to further augment the top and bottom support members  908 ,  910 . The porous layer  902  can also be layered over the at least one other support member (not shown) for augmentation. 
         [0036]    As seen in  FIG. 9C , there may be a space  920  between the porous layer  902  and the main body  916 . When the space  920  is present, the porous layer  902  still surrounds the space  920 . At least one spacer (not shown) may be used to maintain the spacing of the space  920 , though the spacer need be present, as the porous layer  902  may be fit and held between the top and bottom supper members  908 ,  910 , whether through a pressure fit, an adhesive, or the like. 
         [0037]      FIG. 10A  shows an isometric view of a porous float  1000 . The porous float  1000  includes a main body  1006  and a coating  1012 . The porous float  1000  includes a teardrop-shaped top end cap  1002  and a teardrop-shaped bottom end cap  1004 . The top end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. The bottom end cap may be teardrop-shaped, dome-shaped, cone-shaped, or any other appropriate shape. The porous float  1000  may also include a top support member  1008 , which extends circumferentially around an upper portion of the main body  1006 , and a bottom support member  1010 , which extends circumferentially around a lower portion of the main body  1006 . The porous float  1000  may also include at least one support member (not shown) between the top and bottom support members, the at least one support member (not shown) may be vertical, be horizontal, be protrusions, be at least one helical ridge, or any appropriate support member shape or configuration. 
         [0038]      FIG. 10B  shows a cross-sectional view of the porous float  100  along line VI-VI. The porous float  1000  includes a porous layer  1032  and the coating  1012 . The porous layer  1032  surrounds an outer surface of the main body  1006 . The top support member  1008  and the bottom support member  1010  may be extensions of the porous layer  1032 , which extend horizontally out from the porous layer  1032 . The coating  1012  can be layered over the horizontal extensions to further augment the top and bottom support members  1008 ,  1010 . Alternatively, the top support member  1008  and the bottom support member  1010  may be extensions of the main body  1006 , which extend horizontally out from the main body  1006 . The porous layer  1032  and the coating  1012  can be layered over the horizontal extensions to further augment the top and bottom support members  1008 ,  1010 . 
         [0039]    The porous layer  1032  may extend underneath the top support member  1008  to a bottom portion of the top end cap  1002 ; the porous layer  1032  may extend underneath the bottom support member  1010  to a top portion of the bottom end cap  1004 ; or, the porous layer  1032  may extend from a bottom of the top support member  1008  to a top of the bottom support member  1010 . The porous layer  1032  can surround the entire main body  1006 , a portion of the main body  1006 , or a plurality of portions of the main body  1006 . The porous float  1000  may also include at least one support member (not shown) between the top and bottom support members  1008 ,  1010 , the at least one support member (not shown) may be vertical, be horizontal, be protrusions, be at least one helical ridge, or any appropriate support member shape or configuration. The top and bottom end caps  1002 ,  1004  may include a portion of the porous layer  1032 . The coating  1012  may also be porous. The coating  1012  may be a porous material or may be a material, such as parylene, which is made porous through machining, through the use of a laser, or the like. 
         [0040]    The porous float  1000  may also include a reflective layer  1034 . The reflective layer  1034  can be layered in between the porous layer  1032  and the coating  1012 ; or, the reflective layer  1034  may be layered on top of at least a portion of the coating  1012 . The reflective layer  1034  can reflect light, which includes electromagnetic radiation in the visible portion of the electromagnetic spectrum and radiation in the ultraviolet and infrared portions of the electromagnetic spectrum. The reflective layer  1034  can be made to be reflective by combining the rigid organic and inorganic materials listed below with a white pigment during fabrication; or, can be made to be reflective by adding a highly reflective or white pigment to the material composition. The reflective layer  1034  can have a glossy or matte finish. Examples of white plastics that can be used include, but are not limited to, white Delrin®, moisture resistance polyester, wear-resistant slippery cast nylon 6, impact-resistant slippery UHMW polyethylene, opaque white polypropylene, rigid HDPE polyethylene, UV resistant VHMW polyethylene, acrylic PVC, flame-retardant polypropylene, moisture-resistant LDPE polyethylene, lightweight rigid PVC foam, structural fiberglass, and white polystyrene. The reflective layer  1034  can be made to be reflective by applying a reflective coating. For example, the reflective coating can be a reflective paint, such as white paint, paint with reflective particles or ceramic beads or a reflective polymer. The paint can have a glossy or matte finish. The reflective layer  1034  can be made to be reflective by plating a reflective material on the reflective layer  1034 . For example, the plating can be a shiny reflective metal, ceramic, or a mirror. Suitable reflective metals include, but are not limited to, gold, silver, aluminum, tin, copper, bronze, chromium, cobalt, nickel, palladium, platinum, manganese, zinc, titanium, niobium, molybdenum, tungsten, stainless steel, or a suitable metalloid. The reflective layer  1034  can be made to be reflective by incorporating reflective objects or particles. The reflective layer  1034  may also be porous. The reflective layer  1034  may be a porous material or may be a material, such as parylene, which is made porous through machining, through the use of a laser, or the like. 
         [0041]    At least one pore may extend through the coating  1012 , the reflective layer  1034 , and the porous layer  1032 , thereby stretching from a space between the porous float  1000  and the tube  102  to the main body  1006  of the porous float  1000 . Additionally, the outermost layer (i.e. the coating or the reflective layer, depending on the arrangement of the layers) may be non-porous at portions on or around the top and bottom support members  1008 ,  1010 ; the remaining portions of the outermost layer, such as those between the top and bottom support members  1008 ,  1010  and on the top and bottom end caps  1002 ,  1004 . The outermost layer of the porous float  1000  may also include an overlay, such a chemical or adhesive, to attract and/or hold a target analyte. 
         [0042]    A porous layer, porous intermediary layer, or porous main body may include at least one pore. The at least one pore may be sized to prevent a target analyte from passing through, thereby only allowing at least one molecule, such as a molecule of a suspension or a solute molecule in a solvent, to pass through, such as by passive (i.e. diffusion) or active (i.e. a pressure gradient) action; or, the at least one pore may be sized to permit a target analyte to pass through. The number of pores, pore spacing, and pore size can be varied. When a plurality of layers is present, each layer may have pores which are different in size, number and/or shape than the pores of a different layer. Within a given layer and when a plurality of pores is present, the size, number and/or shape may be different from pore to pore. The at least one pore may be any appropriate shape, including, but not limited to, circular, elliptical, triangular, rectangular, quadrilateral, or polyhedral. The pore size may be less than 1 μm, equal to 1 μm, or greater than 1 μm. The support members may also be porous, including at least one pore. 
         [0043]    A porous float can be composed of a variety of different materials including, but not limited to, metals, including, but not limited to, aluminum, brass, gold, silver, tin, copper, bronze, chromium, cobalt, nickel, lead, iron, steel, manganese, zinc, neodymium, and combinations thereof; rigid organic or inorganic materials; ferrous plastics; sintered metal; machined metal; and rigid plastic materials, such as polyoxymethylene (“Delrin®”), polystyrene, acrylonitrile butadiene styrene (“ABS”) copolymers, aromatic polycarbonates, aromatic polyesters, carboxymethylcellulose, ethyl cellulose, ethylene vinyl acetate copolymers, nylon, polyacetals, polyacetates, polyacrylonitrile and other nitrile resins, polyacrylonitrile-vinyl chloride copolymer, polyamides, aromatic polyamides (“aramids”), polyamide-imide, polyarylates, polyarylene oxides, polyarylene sulfides, polyarylsulfones, polybenzimidazole, polybutylene terephthalate, polycarbonates, polyester, polyester imides, polyether sulfones, polyetherimides, polyetherketones, polyetheretherketones, polyethylene terephthalate, polyimides, polymethacrylate, polyolefins (e.g., polyethylene, polypropylene), polyallomers, polyoxadiazole, polyparaxylene, polyphenylene oxides (PPO), modified PPOs, polystyrene, polysulfone, fluorine containing polymer such as polytetrafluoro ethylene, polyurethane, polyvinyl acetate, polyvinyl alcohol, polyvinyl halides such as polyvinyl chloride, polyvinyl chloride-vinyl acetate copolymer, polyvinyl pyrrolidone, polyvinylidene chloride, specialty polymers, polystyrene, polycarbonate, polypropylene, acrylonitrite butadiene-styrene copolymer, butyl rubber, ethylene propylene diene monomer, others, and combinations thereof. 
         [0044]    The end caps may be manufactured as a portion of the main body, thereby being one singular structure, by machining, injection molding, additive techniques, or the like; or, the end caps may be connected to the main body by a press fit, an adhesive, a screw, any other appropriate method by which to hold at least two pieces together, or combinations thereof 
         [0045]    A porous float can be made to be reflective. A porous layer or a main body of the porous float may be made to reflective. The porous layer or main body can be made to be reflective by combining the rigid organic and inorganic materials listed above with a white pigment during fabrication; or, can be made to be reflective by adding a highly reflective or white pigment to the material composition. The porous layer or main body can have a glossy or matte finish. Examples of white plastics that can be used include, but are not limited to, white Delrin®, moisture resistance polyester, wear-resistant slippery cast nylon 6, impact-resistant slippery UHMW polyethylene, opaque white polypropylene, rigid HDPE polyethylene, UV resistant VHMW polyethylene, acrylic PVC, flame-retardant polypropylene, moisture-resistant LDPE polyethylene, lightweight rigid PVC foam, structural fiberglass, and white polystyrene. A porous layer or main body can be made to be reflective by applying a reflective coating. For example, the coating can be a reflective paint, such as white paint, paint with reflective particles or ceramic beads or a reflective polymer. The paint can have a glossy or matte finish. The porous layer or main body can be made to be reflective by plating a reflective material on the porous layer. For example, the plating can be a shiny reflective metal, ceramic, or a mirror. Suitable reflective metals include, but are not limited to, gold, silver, aluminum, tin, copper, bronze, chromium, cobalt, nickel, palladium, platinum, manganese, zinc, titanium, niobium, molybdenum, tungsten, stainless steel, or a suitable metalloid. The porous layer or main body can be made to be reflective by incorporating reflective objects or particles. 
         [0046]    A porous layer can also include an overlay to attract and/or hold the target analyte. The overlay may be any material which may either attract the target analyte and form a chemical bond with the target analyte, cause the target analyte to adhere to the porous layer (i.e. an adhesive), or both. The overlay, located on an outer surface of the porous layer, may completely cover the outer surface or may only cover portions of the outer surface. The different types of overlays are designed to increase the affinity of the porous layer for the cells through different mechanisms. In the instance in which the overlay attracts the target analyte and forms a chemical bond with the target analyte, the bond, and related attraction, may be covalent, ionic, dipole-dipole interactions, London dispersion forces, van der Waals forces, or hydrogen bonding. The overlay may include a primary antibody that binds to biomarkers, including but not limited to, EpCAM, AMACR, Androgen receptor, CD146, CD227, CD235, CD24, CD30, CD44, CD45, CD56, CD71, CD105, CD324, CD325, MUC1, CEA, cMET, EGFR, Folate receptor, HER2, Mammaglobin, PSMA, or combinations thereof. In the case in which the overlay is an adhesive, including any variation of  Mytilus edulis  foot protein (“Mefp”), biopolymers, or polyphenolic proteins (including those polyphenolic proteins containing L-DOPA), the target analyte adheres to the porous layer. 
         [0047]    The overlay may also be any material which is convertible or releasable to hold the target analyte to the removable layer (i.e. photo-convertible adhesive, photolysible particle). The material may be converted or released after the system undergoes density-based based separation. When the overlay comprises a convertible material, then the overlay is converted by the energy from a given source to form chemical bonds with the cell. When the overlay comprises a releasable material, then a secondary material may be released from the releasable material or the releasable material itself may be released from the outer surface of the removable layer, such that the secondary material or the releasable material form chemical bonds with the target analyte. The secondary material or releasable material may include fixing agents (i.e. formaldehyde, formalin, paraformaldehyde, or glutaraldehyde), detergents (i.e. saponin, polyoxyethylene, digitonin, octyl β-glucoside, octyl β-thioglucoside, 1-S-octyl-β-D-thioglucopyrano side, CHAPS, CHAPSO, (1,1,3,3-Tetramethylbutyl)phenyl-polyethylene glycol or octylphenol ethylene oxide), or staining agents (i.e. fluorescently-labeled antibodies, Pap stain, Giemsa stain, or hematoxylin and eosin stains). The energy from a given source may be in forms such as light, heat, ultrasound, or electromagnetism, such as radio waves and microwaves. For example, the porous layer may be coated with a photo-convertible material. A light source having a wavelength ranging from about 250 nm to about 1200 nm may be used to convert the photo-convertible material. An ultraviolet light source, such as a wavelength of approximately 355 nm, may be used to induce a chemical reaction in the photo-convertible material, thereby creating a covalent bond and causing adhesion of the target analytes to the porous layer. 
         [0048]    A porous layer can also be functionalized using a self-assembled monolayer comprising a head, a tail, and a functional group. The head reacts with and attaches to the porous layer, and may be any chemical having a high affinity for the porous layer. For example, sulfur has a high affinity for metals. The tail can be a carbon backbone that connects the head to the functional group and may be any suitable length and may or may not be branched. The functional group is selected based on the appropriate functionality or reaction desired. Examples of self-assembled monolayers include alkanethiols for metals and silanes for nonmetallic oxides. After the porous layer has been functionalized, materials may be added the suspension to provide better capture of the target analytes. The materials include Mytilus edulis foot protein (“Mefp”); biopolymers; polyphenolic proteins (including those polyphenolic proteins containing L-DOPA); chemo-attractant molecules, such as epidermal growth factor (“EGF”) or vascular endothelial growth factor (“VEGF”); an extracellular matrix protein (“ECM”); maleic anhydride; maleimide activated sulfa-hydryl groups, poly-L-lysine; poly-D-lysine; streptavidin; neutravidin; protein A; protein G; protein A/G, protein L; biotin; glutathione; antibodies; recombinant antibodies: aptamers; RGD-peptides; fibronectin; collagen; elastin; fibrillin; laminin; or proteoglycans. 
         [0049]    Chemo-attractant molecules are ones which will elicit a chemotaxis response from the target analyte, whereby the target analyte is attracted to the chemicals. 
         [0050]    Chemotaxis is an active movement of the target analyte due to a chemical or chemicals present in the environment. The EGF, VEGF, chemo-attractant molecule, or ECM may be used as a layer, either alone or layered in conjunction with a material discussed above. Furthermore, the EGF, VEGF, chemo-attractant molecule, or ECM may be mixed together as one layer on the outer surface of the porous layer. The EGF, VEGF, chemo-attractant molecule, or ECM, when used in combination with one of the other overlays discussed above, may be a sub-layer in which it is layered between the porous layer and the other overlay or may be the overlay where the one or the other materials discussed above is the sub-layer. The overlay may also be a mixture of the EGF, VEGF, chemo-attractant molecule, or ECM with one of the materials discussed above. The overlay of EGF, VEGF, chemo-attractant molecule, or ECM will cause the target analyte to migrate towards the porous layer, where the target analyte can then be captured and held by one of the other overlays discussed above. When the overlay of EGF, VEGF, chemo-attractant molecule, or ECM is used separately, it will be the only overlay and will simply be more attractive to the target analyte than other surfaces within the tube and porous layer system. 
       Methods for Using Tube and Porous Float Systems 
       [0051]      FIG. 11  shows an isometric view of a tube and porous float system  100  having undergone centrifugation. A snapshot  1114  is also included which shows the interaction between a solution  1112  and a target analyte  1110 . Suppose, for example, the suspension includes three fractions. During centrifugation, the suspension may be divided into and settle into the three fractions, including a densest fraction  1106 , a medium density fraction  1104 , and a least dense fraction  1102 . The target analyte  1110  may be found in the medium-density fraction  1104 . The porous float  104  may have a density substantially similar to that of the target analyte  1110 , so that the porous float  104  and the target analyte  1110  align properly within the tube  102 . 
         [0052]    A fluid introducer  1108 , such as a syringe, pump, or the like, may be used to introduce a solution  1112  into the porous float  104  through a pierceable segment  202 . The solution  1112 , passing out of the fluid introducer  1108 , can flow into the porous float  104  and then into a space between the porous float  104  and the tube  102  by leaving the porous float  104  through at least one pore  210 . The solution  1112 , upon exiting the porous float  104 , may diffuse throughout at least the medium-density fraction  1104  and interact with the target analyte  1110 . 
         [0053]      FIG. 12  shows an isometric view of a tube and porous float system  100  having undergone centrifugation. A snapshot  1214  is also included which shows the interaction between a vacuum  1212  and a target analyte  1210 . Suppose, for example, the suspension includes three fractions. During centrifugation, the suspension may be divided into and settle into the three fractions, including a densest fraction  1206 , a medium density fraction  1204 , and a least dense fraction  1202 . The target analyte  1210  may be found in the medium-density fraction  1204 . The porous float  104  may have a density substantially similar to that of the target analyte  1210 , so that the porous float  104  and the target analyte  1210  align properly within the tube  102 . 
         [0054]    A needle  1208  may be used to introduce a vacuum into the porous float  104  through a pierceable segment  202 . The vacuum, created by a vacuum tube  1212 , pump, syringe, or the like, connected to the needle  1208  via tubing  1216 , can pull the target analyte  1210  into the porous float  104  through a pore  210  and then into the vacuum tube  1212  or pump. 
         [0055]    Alternatively, the solution may be added to the tube and may flow through the pores via capillary action. 
         [0056]      FIG. 13A  shows an isometric view of a tube and porous float system  1014  having undergone centrifugation. The reflective surface of a reflective layer or coating of a porous float can increase the intensity of light emitted from fluorescent probes.  FIG. 13B  shows a cross-sectional view of the system  1014  along the line VII-VII shown in  FIG. 13A . In the example of  FIG. 13B , the cross-sectional view reveals that the main body  1006  of the porous float  1000  includes the reflective layer  1034 . A multichannel light source  1310  illuminates a medium-density fraction  1016  in a space between an inner wall of the tube  102  and the outermost-layer of the porous float  1000 , which, in this instance, is the coating  1012  with excitation light to excite fluorescent probes attached to target material particles. The excitation light emitted by the multichannel light source  1310  passes through an objective  1328  to focus the excitation light on particular area.  FIG. 13B  includes a magnified view  1312  of a portion of the channel. Octagon  1304  represents a target analyte and smaller shaded circles, such as circle  1326 , represent six fluorescent probes attached to the target analyte  1304  via ligands. Solid-line directional arrows  1314 - 1318  represent rays of excitation light associated with a channel output from the light source  1310 . As shown in  FIG. 13B , rays  1315 - 1317  pass through the tube  102  to illuminate the fluorescent probes facing the tube  102 . Dashed-line directional arrows  1321 - 1323  represent rays of light emitted from the fluorescent probes that face the tube  102 . Rays  1314  and  1318  represent excitation light that is reflected off of the reflective layer  1034  to illuminate fluorescent probes that face the float  1000 . Dashed-line directional arrows  1320  and  1324  represent rays of excitation light emitted from the fluorescent probes that face the float  1000 . 
         [0057]    Note that without the reflective layer  1034 , much of the light represented by the rays  1314  and  1318  is absorbed by the float  1000  and is not available to excite the fluorescent probes that face the float  1000 . In the example of  FIG. 13B , the light emitted from the fluorescent probes that face the float  1000  is also reflected from the reflective layer  1032  back to the objective  1328  and the sent to a detector  1330 , such as a charge-coupled device (“CCD”). The light emitted from the fluorescent probes that face the float  1000  adds to the intensity of the light emitted from the fluorescent probes that face the tube  102 . As a result, images of the target analyte  1304  appear brighter than the target analyte  1304  would otherwise appear with a dark colored or non-reflective float. Note also that because the excitation light and the emitted light are not absorbed by the porous float  1000 , the porous float  1000  may not heat up and expand. Additionally, because the excitation light and the emitted light are not absorbed by the porous float  1000 , the porous float  1000  may not transfer heat to the surrounding fluid, thereby preventing expansion and/or movement of the liquid. As a result, the target analyte  1304  is less likely to shift, making it easier to identify the location of the target analyte  1304  and the same target analyte  1304  can be relocated when the target material is illuminated a second time. 
         [0058]    In order to identify and determine the presence of a target analyte in a suspension, target analyte particles can be tagged with fluorescent probes. After centrifugation, the tube is illuminated with light that induces photon emission from the fluorescent probes. The fluorescent light can be used to confirm the presence, characteristics, and/or identity of the target analyte. The fluorescent molecules are conjugated with molecules or other particles that bind specifically to the target analyte particles. The fluorescent molecules emit light of a known range of wavelengths, depending of the particular fluorescent molecule, within the electromagnetic spectrum when an appropriate stimulus is applied. As described above, the float has a density selected to position the float at approximately the same level as the target analytes when the tube, float, and suspension are centrifuged together. After centrifugation, the target analytes are located between the outer surface of the float and the inner wall of the tube and the fluorescent molecules fluoresce when an appropriate stimulus is applied. 
         [0059]    The target analyte may be collected, and once collected, the target analyte may be analyzed using any appropriate analysis method or technique, though more specifically intracellular analysis including intracellular protein labeling; nucleic acid analysis, including, but not limited to, protein or nucleic acid microarrays; fluorescent in situ hybridization (“FISH”—a tool for analyzing DNA and/or RNA, such as gene copy number changes); or branched DNA (“bDNA”—a tool for analyzing DNA and/or RNA, such as mRNA expression levels) analysis. These techniques require fixation, permeabilization, and isolation of the target analyte prior to analysis. Some of the intracellular proteins which may be labeled include, but are not limited to, cytokeratin (“CK”), actin, Arp2/3, coronin, dystrophin, FtsZ, myosin, spectrin, tubulin, collagen, cathepsin D, ALDH, PBGD, Akt1, Akt2, c-myc, caspases, survivin, p27 kip , FOXC2, BRAF, Phospho-Akt1 and 2, Phospho-Erkl/2, Erk1/2, P38 MAPK, Vimentin, ER, PgR, PI3K, pFAK, KRAS, ALKH1, Twist1, Snail1, ZEB1, Slug, Ki-67, M30, MAGEA3, phosphorylated receptor kinases, modified histones, chromatin-associated proteins, and MAGE. To fix, permeabilize, or label, fixing agents (such as formaldehyde, formalin, methanol, acetone, paraformaldehyde, or glutaraldehyde), detergents (such as saponin, polyoxyethylene, digitonin, octyl β-glucoside, octyl β-thioglucoside, 1-S-octyl-β-D-thioglucopyranoside, polysorbate-20, CHAPS, CHAPSO, (1,1,3,3-Tetramethylbutyl)phenyl-polyethylene glycol or octylphenol ethylene oxide), or labeling agents (such as fluorescently-labeled antibodies, Pap stain, Giemsa stain, or hematoxylin and eosin stain) may be used. 
         [0060]    It should be understood that the method and system described and discussed herein may be used with any appropriate suspension or biological sample, such as blood, bone marrow, cystic fluid, ascites fluid, stool, semen, cerebrospinal fluid, nipple aspirate fluid, saliva, amniotic fluid, vaginal secretions, mucus membrane secretions, aqueous humor, vitreous humor, vomit, and any other physiological fluid or semi-solid. It should also be understood that a target analyte can be a cell, such as ova or a circulating tumor cell (“CTC”), a circulating endothelial cell, a vesicle, a liposome, a protein, a nucleic acid, a biological molecule, a naturally occurring or artificially prepared microscopic unit having an enclosed membrane, parasites, microorganisms, or inflammatory cells. 
         [0061]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the disclosure. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the systems and methods described herein. The foregoing descriptions of specific embodiments are presented by way of examples for purposes of illustration and description. They are not intended to be exhaustive of or to limit this disclosure to the precise foul&#39;s described. Many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best explain the principles of this disclosure and practical applications, to thereby enable others skilled in the art to best utilize this disclosure and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of this disclosure be defined by the following claims and their equivalents: