Method for separating serum or plasma from the formed elements of blood

A self-contained separator assembly comprises a container having at least one end open which is adapted to receive blood for subsequent separation into a light phase comprising plasma or serum and a heavy phase comprising the formed elements of blood. A closure is sealed into the open end of the container which may be penetrated by a cannula through which blood may be received into the container. A piston-like plug which forms a seal against the container wall is characterized by an average specific gravity so as to permit the plug to be centrifuged through a blood sample toward the interface of the heavy phase and the light phase. The plug comprises a valve seating portion having a valve opening therein and a valve head adapted to seat on the seating portion and bias means extending between the seating portion and the valve head to bias the head to the seated position on the seating portion after centrifugation.

BACKGROUND OF THE INVENTION 
This invention relates to the separation of the formed elements or the 
heavy phase of a blood sample, i.e., white cells, red cells and platelets 
of blood, from the serum or plasma or the light phase of the blood. 
Such separation is typically performed by centrifuging a blood sample so 
that the heavier phase is forced to one end of the blood sample and the 
lighter phase is forced to the other end of the blood sample. 
After such separation by centrifugation, it is desirable to physically 
separate the formed elements of the blood from the serum or plasma in 
order to prevent the contamination of the serum or plasma. Such 
contamination can occur as the red blood cells begin to liberate potassium 
and other contaminants which might interfere with tests performed on the 
serum or plasma. It is also possible that any fibrum which remains in the 
sample may produce some contamination. 
In order to achieve this isolation or fluid separation of the formed 
elements of the blood from the serum or plasma, self contained separator 
assemblies of the Vacutainer type have been suggested. Examples of such 
Vacutainer assemblies are shown in U.S. Pat. Nos. 3,891,553, 3,894,951 and 
3,894,952. 
Such blood separation apparatus typically comprises a collection container 
having elastomeric plugs sealing opposite ends of the container with a 
piston-like plug located within the container and forming a seal with the 
walls of the container. A partial vacuum is created on the interior of the 
container so as to assist in drawing the blood sample into the container 
through a cannula which penetrates one of the end plugs. The plug which is 
characterized by an average specific gravity greater than the light phase 
of the blood comprises a pressure-responsive valve which, under the force 
of centrifuging and the pressure of serum or plasma against the valve, 
will open the valve to allow the plasma or serum to pass therethrough. The 
plug typically also comprises a filter member associated with the valve to 
prevent fibrin or formed elements of the blood from clogging or passing 
through the valve. As centrifugation continues, the plug will stop at the 
interface between the light phase and the heavy phase as the filter 
becomes clogged with the elements of the heavy phase. Once centrifugation 
is completed, the light phase should be on one side of the plug and the 
heavy phase should be on the other side of the plug. 
Reliance on the clogged filter to stop the plug at the interface of the 
light phase and the heavy phase can result in lysing of the blood cells 
due to the pressure placed on the cells by the clogged filter. Once lysing 
occurs, any contact between the lysed cells and the serum or plasma will 
produce contamination of the serum or plasma. 
It has been found to be particularly desirable to utilize the separator 
assembly in and of itself as a transport apparatus for transporting the 
blood sample to the laboratory from the point at which the blood sample is 
taken. This requires the valve of the assembly to close tightly after 
centrifugation and remain closed so as to prevent any contamination 
between the two phases of the blood. 
Heretofore, valves of self-contained separator assemblies have in general 
relied upon the elastomeric properties of the plug or valve to return the 
valve to the closed condition, i.e., there is no force or bias acting on 
the valve after centrifugation to hold the valve closed. Rather, the valve 
elements, when properly structured, merely contact one another when 
returning to a natural or unbiased state. Closure of these valves may be 
assisted by the tubular member of the container if the inside diameter of 
the tubular member is slightly smaller than the outside diameter of the 
plug but the tolerances on the inside diameter of the tubular member are 
difficult to control particularly where the tubes are extruded. 
A further difficulty associated with such separator apparatus is a result 
of the necessity for the valve to open with the force generated during 
centrifugation and still close after centrifugation. 
Similar characteristics may be found in the valve structures of my U.S. 
Pat. Nos. 3,661,265 and 3,799,342 where plunger type serum separators are 
adapted to serve as transport devices. U.S. Pat. No. 3,954,614 also 
discloses a valve in a serum separator which is characterized by little or 
no seating forces when the valve is in the closed position. The same is 
true with respect to U.S. Pat. No. 3,962,085 wherein the periphery of the 
disc acts as a valve which is closed when the disc is in the closed 
position. However, there are no substantial seating forces beyond the 
weight of the blood sample itself. Valves such as those shown in U.S. Pat. 
Nos. 1,777,408 and 2,191,636 are biased so as to provide a substantial 
seating force when in the closed position, but such valves are too complex 
for use in serum separators. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a self-contained separator 
assembly with an improved valve and a method of operating the assembly. 
It is a more specific object of this invention to provide such a separator 
assembly with a valve which will maintain itself in the closed or seated 
position after centrifugation. 
It is another specific object of this invention to provide such a separator 
assembly with a valve which will properly open under the influence of 
centrifugation. 
It is a further specific object of this invention to provide such a 
separator assembly with an improved valve which is relatively easy to 
manufacture. 
In accordance with these and other objects of the invention, the separator 
assembly comprises a container having at least one open end which is 
adapted to receive blood for subsequent separation into a light phase and 
a heavy phase and a closure seated at the open end of the container. The 
closure comprises a material which may be penetrated by a cannula through 
which blood may be received into the container. A piston means having an 
average specific gravity permitting the piston means to move toward the 
interface of the heavy phase and the light phase of the blood under the 
influence of specific gravity comprises a peripheral sealing means and a 
valve means. The peripheral sealing means is adapted to sealingly engage 
the container wall at the periphery of the container. The valve means 
comprises a seating portion having an opening therein, a movable valve 
member adapted to seat on the seating portion and bias means coupled to 
the valve member and the seating portion for maintaining substantial 
seating forces between the seating portion and the valve member when the 
valve member is closed. In accordance with this invention, the valve 
opening is effective to allow plasma or serum to pass therethrough under 
the force provided during centrifugation while at the same time returning 
to the closed position after centrifugation with seating forces acting on 
the valve. 
In the preferred embodiment of the invention, the bias means comprises at 
least one flexible strand of elastomeric material which extends from the 
valve member on the under side of the valve opening to an anchoring 
location on the valve seating portion. In the closed position, the valve 
strand is maintained under tension so as to hold the valve member in the 
closed position. As the plasma or serum pushes against the valve member 
during centrifugation, the flexible strand stretches and the valve member 
becomes unseated so as to allow the plasma or serum to flow through the 
valve opening. After centrifugation, the tension on the flexible strand 
returns the head to the seated position at the valve opening while the 
strand remains under tension so as to assure the desired closure of the 
valve. 
In accordance with one important aspect of the invention, the valve means 
in integrally molded on one side of the valve opening and subsequently 
forced through the opening to create the tension on the strand. 
In accordance with another important aspect of the invention, the valve 
member floats above the valve opening during centrifugation so as to be 
separable from the valve opening around the entire periphery thereof. 
In accordance with another important aspect of the invention, the valve 
seating portion may comprise a cylindrical section and a seating section 
at one end of the cylindrical section where the seating section includes 
the valve opening. The flexible strand extends from the valve member 
through the central chamber formed by the central section and the flexible 
strand is anchored on the cylindrical section. Preferably a plurality of 
flexible strands are utilized. 
In one embodiment of the invention, the piston means seeks the interface 
between the light phase and the heavy phase by providing an average 
specific gravity of the piston means than the light phase and less than 
the heavy phase. Preferably, the specific gravity of the piston means 
including a filter is 1.030 to 1.040. One preferred material for the 
elastomeric material which provides the desired specific gravity is Kraton 
with appropriate fillers. In the alternative, the specific gravity may be 
greater when mechanical stop means are provided.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 discloses a self-contained separation apparatus embodying the 
invention which comprises a tubular container member 10 having two open 
ends which are closed respectively by a first closure 12 and a second 
closure 14 which are seated in the open ends. Each of the closures 12 and 
14 comprise a self-sealing, elastomeric material such as rubber or Kraton. 
As shown, closure 14 is penetrated by a cannula 16 through which blood 18 
is received into the container 10. By creating a vacuum within the 
container 10, the blood is readily drawn into the container. Because of 
the elastomeric nature of the closure 14, the cannula 16 may be removed 
from the closure 14 with the hole automatically closing. 
In accordance with this invention, the container 10 encloses a novel piston 
means comprising a plug 20. The plug 20 comprises a substantially 
cylindrical body portion 28 which carries a plurality of annular, 
peripheral sealing beads 30 which form a seal with the walls of the 
container 10. The end of the body member 28 remote from the blood sample 
18 comprises a radially inwardly directed annular flange 34 which is 
adapted to form a valve seat along the surface 36 for a movable valve 
member 38. In FIG. 1 and better shown in enlarged views of FIGS. 3 and 4, 
the valve member 38 comprises a body portion 40 terminated by a sealing 
head portion 42 which is adapted to seat on the surface 36 along a surface 
44 at a valve opening 46. 
In order to provide a closing bias on the valve member 38 which is 
independent of the diameter of the container 10, resilient strand or strut 
members 48 are attached to the interior walls of the cylindrical body 28 
of the plug 20. As shown in FIG. 5, three such strands or struts 48 may be 
utilized which are evenly spaced around the wall 50 and the valve member 
38 at points of attachment 52 so as to assure that the appropriate seal 
will be formed between the sealing surface 44 and the seating surface 36 
as shown in FIGS. 1, 2 and 4. 
In accordance with this invention, the plug 20 is integrally molded from an 
elastomeric material such as Kraton. As originally molded, the head 42 is 
on the other side of the valve opening 46 so that there is no tension on 
the strands 48. However, once the head or sealing portion 42 is forced 
through the opening 46 as shown in FIGS. 1-4, the strands 48 are under 
tension and store energy so as to force the sealing surface 44 into 
contact with the seating surface 36 in the absence of centrifugation 
forces. Of course, the valve member 38 may be pushed back through the 
opening 46 to relieve the tension on the strands 48. The beads 30 are 
capable of providing a seal with the container member 10 while still 
providing an accommodation to variation in the internal diameter of the 
container member 10. In addition, an annular recess 57 is located at the 
flange 34 which spaces the flange 34 from the walls of the container 
member 10 and thereby assures that any variation in the internal diameter 
of the container member 10 will not adversely affect the operation of the 
valve member 38. 
As shown in FIGS. 1-4, a filter 56 is inserted into the central cavity 
formed by the wall 50 of the body member 28. The purpose of the filter is 
to prevent fibrin and any formed elements of the blood from passing 
through the valve opening 44 during centrifugation. To prevent the filter 
56 from riding up in the cavity formed by the wall 50, an annular 
projection or bead 64 is located on the wall 50 just above the filter 56. 
The bead 64 assures that the filter is not forced upwardly so as to 
interfere with the valve member 38. The filter may comprise a variety of 
materials having pores sufficiently smaller than the blood cellular 
material so as to prevent the cellular material from passing through the 
valve opening 46. 
During centrifugation, the plasma or serum of the blood sample forces the 
head 42 upwardly and off the seating surface 36 so as to allow the serum 
or plasma to pass therethrough to achieve a light phase at the end of the 
container 10 adjacent the closure 12 and a heavy phase at the end of the 
container 10 adjacent closure 14 as shown in FIG. 2. It will be understood 
that the strands 48 must provide the appropriate amount of bias on the 
head 42 so as to permit the head 42 to lift off the seating surface 36 
during centrifugation. At the same time, the strands 48 must provide 
sufficient bias on the head 42 to assure closure on the surface 36 after 
centrifugation is completed as shown in FIG. 3. 
In order to assure that the plug 20 will appropriately seek the interface 
between the light phase and the heavy phase as shown in FIG. 2, the plug 
20 must have the appropriate specific gravity. In this connection, it has 
been found that an average specific gravity of 1.030 to 1.040 is 
particularly suitable. One elastomeric material which has been found to be 
well suited for providing the necessary bias on the valve member 38 as 
well as the appropriate specific gravity for the entire plug including the 
filter is Kraton with appropriate fillers. It must be appreciated that 
other elastomeric materials with suitable specific gravities might be 
substituted. 
In the embodiment of FIG. 6, a mechanical stop in the form of a projection 
70 is used to stop the plug 20 rather than the specific gravity relied 
upon in FIGS. 1-5. Accordingly, the average specific gravity of the plug 
20 need only be greater than the light phase, i.e., 1.030 or greater so as 
to be sure that the plug may be centrifuged through the plasma or serum. 
In the plug of FIG. 6, an annular sealing flange 72 has been substituted 
for the beads 30. The flange 70 includes annular relieved areas 76 which 
are provided so that the flange may readily retract and extend to achieve 
the appropriate seal at a radially surface 74 which contacts the wall of 
the container 10. The remainder of the plug 20 is identical to that shown 
in FIGS. 1-5. 
A method of making the plug 20 is disclosed in my copending application 
Ser. No. 793,284 filed May 3, 1977, which is incorporated herein by 
reference, and abandoned in favor of continuation application Ser. No. 
963,298 filed Nov. 24, 1978. Reference is also made to my related 
copending application Ser. No. 793,282 filed May 3, 1977 which discloses a 
serum separator tube assembly utilizing various plugs having features 
incorporated herein by reference. 
Although a specific embodiment of the invention has been shown and 
described, it will be understood that other embodiments and modifications 
may be utilized without departing from the true spirit and scope of the 
invention as set forth in the appended claims.