Magnetic stirrer apparatus with guided, floating stirrer

A magnetic stirrer apparatus includes a vessel for liquid and a floating stirrer. The floating stirrer is rotated by a magnet driven by a magnetic field generator laterally of the vessel, and a guide rod through the floating stirrer restricts its movement to rotary movement and substantially vertical movement with change in the liquid level. The magnetic field generator is operative at any level of the liquid in the vessel.

TECHNICAL FIELD 
The present invention relates to a magnetically driven stirrer apparatus, 
wherein the stirrer is buoyant. 
BACKGROUND ART 
Apparatus for stirring liquid materials, including culture mediums, have 
long been known. For example, Scharf et al. U.S. Pat. No. 3,649,465 
provides a flask or vessel having an opening at its upper portion, with a 
closure for the opening, having a spindle extending therethrough, the 
spindle at its lower end having a magnetic stirrer, with a shroud 
extending in surrounding relationship to the spindle. The stirrer is 
driven magnetically by a driving magnet, and the magnetic stirrer, located 
at the bottom of the flask, may be adjusted through a limited vertical 
range by vertically adjusting the spindle within the limits permitted by 
the shroud. 
Harker et al. U.S. Pat. No. 2,958,517 provides a flask having a rod guided 
in a bearing in a closure for the flask, the rod having at its lower end a 
magnetic impeller, which engages the bottom of the flask, the magnetic 
impeller being driven by a magnetic stirring apparatus on which the flask 
is held, the apparatus including an electric motor having a shaft driving 
a magnet which is magnetically coupled to the magnet within the flask. 
Harker U.S. Pat. No. 3,572,651 provides a flask having a closure provided 
with a bearing on its underside, the bearing supporting a spindle having 
at its lower end, near the bottom of the flask, a magnetic stirrer, the 
magnetic stirrer being driven by a conventional magnetic driving 
apparatus. 
Mazowski U.S. Pat. No. 3,622,129 also discloses a magnetic stirrer 
apparatus, in which a flask has an opening, a closure for the opening with 
a rod extending through the opening, the rod supporting at its lower end a 
magnetic stirrer, and the rod being adjustable, vertically, in the 
closure, so as to position the stirrer at different depths in the liquid 
in the flask. 
Sada et al. U.S. Pat. No. 4,310,253 discloses an apparatus in which a 
vessel containing a body of liquid has floating, magnetic particles which 
are caused to rotate by a rotating magnetic field, to rotate the interface 
between, for example, a liquid and a gaseous body in the vessel. 
There have been provided stirrer apparatus, particularly for cell culture 
stirring, in which a linearly extending buoyant stirrer was provided, 
having magnetic means associated with it, so that the stirrer could be 
rotated by a rotating magnetic field. Such apparatus, while having many 
advantages, presented a problem of vertical instability when the liquid 
level was low. Additionally, the stirrer could wander in the flask or 
vessel when stirring is interrupted for sampling, or by loss of 
synchronization with the driving magnetic field. 
There are also known in the prior art various magnetic stirrer apparatus 
constructions in which magnetic field generating elements, such as coils, 
were positioned laterally of a vessel, to drive a rotor located within, 
specifically at the bottom, of the vessel: in Stringham U.S. Pat. No. 
1,242,493, the rotor was, itself, the stirrer element, while in Stainbrook 
U.S. Pat. No. 1,420,773, the rotor was connected to a shaft which caused 
rotation of a stirrer element, generally in the form of a propeller. 
The prior art in which the stirrer is submerged in the liquid was subject 
to various defects and deficiencies, including constructions which were 
difficult to clean and which did not have sufficient cell proliferating 
action. In some cases, obstructions were provided to the liquid motion by 
the stirrers, or stirring action unsuitably vigorous for cell culture was 
required to insure complete stirring action. Those constructions in which 
a stirrer is submerged, also provide difficulty in that the stirring 
action is non-uniform with various amounts of liquid in the vessel, and 
are therefore unsuitable where a culture medium stirring or agitating is 
to be provided. 
Culture medium provides for the growth of cells, from nutrients contained 
in the medium. The stirring action required is not violent, but is gentle, 
and care must be taken to avoid damage to cells, such as by violent 
agitation, and by crushing. 
DISCLOSURE OF INVENTION 
The present invention is directed to a magnetic stirrer apparatus in which 
a floating stirrer is provided, with a magnet associated with the floating 
stirrer, the magnet being rotated by a moving magnetic field. The moving 
magnetic field is generated by a magnetic field generating apparatus which 
is laterally of the vessel. The magnet may be connected to and supported 
by the stirrer, so as to move, generally vertical, with the floating 
stirrer, with any change in volume, and therefore of the liquid level in 
the flask or vessel. The field generating means located laterally of the 
vessel is able to generate a moving magnetic field which will drive the 
magnet in any position thereof, as its position changes vertically with 
the changes in liquid level. A guide rod is provided in the vessel, 
preferably extending downwardly along the vessel axis from the cover, the 
guide rod extending through an opening in the floating stirrer, so as to 
guide the floating stirrer, and restrict its movement to rotational 
movement, upon rotation thereof by the magnet within the vessel, and to 
generally vertical movement by changes in the liquid level. 
The magnetic field generator, in one embodiment, includes a rotating shaft 
extending beside, and generally parallel to the axis of the vessel; on the 
shaft at spaced axial locations are a plurality of bar magnets, the ends 
of the bar magnets orbiting about the axis of the shaft as it is rotated, 
so as to bring the north and south poles of each magnet successively 
adjacent to the vessel, and thereby closer to the magnet located within 
the vessel. In another embodiment, a pair of cores extend in spaced 
relationship, substantially parallel to the axis of the vessel, and on 
each core is a coil; the coils are connected to sources of alternating 
current, so their fields are caused to fluctuate alternately, in known 
manner. In yet another embodiment, a plurality of coils are arranged about 
the vessel, supported by an annulus: the supporting annulus may be 
vertically adjusted, as by being carried by a standard, with a releasable 
connection beteen the standard and the supporting annulus, to thereby 
permit the coils and the supporting annulus to be adjusted along the 
vessel. 
In yet another embodiment, the guide rod not only guides the floating 
stirrer, restricting its movement to rotational movement and movement 
along the axis with changes in liquid level, but the guide rod is also 
rotatable, being supported by a bearing on the underside of the cover, and 
having a bar magnet fixed to it: consequently, as the bar magnet is 
rotated, it causes the shaft to rotate, and the shaft is non-rotationally 
connected to the floating stirrer, so as to rotate it, the guide rod in 
this instance, also, permitting the floating stirrer to move along it, 
with changes in liquid level. 
Another aspect of the present invention is the construction of the floating 
stirrer, in one embodiment there being provided two buoyant elements of 
generally bar-bell shape, each having a magnet extending axially through 
it, two such bar-bell shaped buoyant elements being connected in 
side-by-side relationship with a guide tube extending between them, the 
guide tube having a reduced opening at its upper end, which is supported 
above the liquid level. In another embodiment of the floating stirrer, a 
pair of buoyant spheres are provided, held in laterally spaced 
relationship by upper and lower plates, a guide tube extending through the 
plates; the lower plate supports a pair of magnets. 
Among the advantages of the present invention apparatus are the provision 
of a floating stirrer, enabling gentle stirring action to be achieved, 
with guidance of the stirrer so that its movement is restricted and it 
does not wander on the surface of the liquid, thereby eliminating the 
danger of dislocation of the stirrer and the possibility of crushing cells 
between the stirrer and the vessel walls, where culture medium is being 
stirred. Another advantage of the present invention is the avoidance of 
the possibility that the stirrer will be caused to strike the bottom of 
the vessel or flask, should the liquid level become low, and thereby the 
stirrer be drawn downwardly by a magnetic field generating apparatus 
located beneath the vessel, or that magnetic coupling will be lost, as 
where the magnetic field generator is located above the vessel. With the 
present apparatus, a magnetic field is provided which will be operative 
with the magnet within the vessel at any location of the magnet, as, in 
certain embodiments, the magnet changes its location with the change in 
location of the floating stirrer and the liquid level. The present 
apparatus also has the additional advantage that both vertical and lateral 
instability of the magnet within the vessel are avoided, through use of 
lateral magnetic field generating means and a guide rod for the stirrer, 
and there is the same magnetic force delivered to the magnet within the 
vessel, regardless of its position.

MODES FOR CARRYING OUT THE INVENTION 
Referring now to the drawings, wherein like or corresponding reference 
numerals are used to designate like or corresponding parts throughout 
several views, there is shown in FIG. 1 a magnetic stirrer apparatus 10 
comprising a vessel 12 having an open upper end 14 provided with a closure 
16. The closure 16 may have an opening 18 in it, with a stopper 20 
therein, and a guide rod 22 passes through and is supported by the stopper 
20. Preferably, guide rod 22 is coaxial with the vessel 12. Although the 
vessel 12 is disclosed as a conventional flask used for culture medium, it 
may have a different configuration, and, specifically, may be a flask made 
in accordance with Pearson U.S. Pat. No. 4,382,685, issued May 10, 1983. 
Within the vessel 12 there is provided a body of liquid L, and in 
accordance with normal practices, the volume of liquid L within the vessel 
12 may change, thereby changing the elevation of the liquid level, or the 
surface of the body of liquid L. Floating on the body of liquid L is a 
floating stirrer, generally designated 30, including a buoyant element 32 
having a magnet 34 therein. 
Adjacent vessel 12 is a support apparatus 40 including an upstanding post 
42 having an adjustable clamp 44 supporting a housing 46 in which are 
located reduction gears, there being provided on the housing 46 an 
electric motor 48. A control panel 50 mounted on the housing 40 contains 
control circuitry, and a potentiometer 52 for controlling the speed of 
motor 48. An output shaft 54 of the housing 56 is connected to a shaft 56 
by a coupling 58. On shaft 56 are a plurality of spacer sleeves 60, 
between which are located magnets 62, the shaft 56 extending through each 
of the magnets 62 intermediate the ends thereof, and nuts 64 serve to lock 
the shaft 56, spacer sleeves 60 and magnets 62 against relative rotation. 
The length of the shaft 56 and the number of magnets 62 carried thereon is 
illustrative, the length and the number of magnets being determined so as 
to provide a rotating magnetic field for the full range of movement of the 
surface of the body of liquid L which is anticipated with a particular 
cell culture growth operation. Thus, the vertical array of magnets 62 is 
such as to provide a rotating magnetic field throughout the entire range 
and movement of the floating stirrer 30 with the magnet 34 within it. 
In FIG. 2, there is disclosed not only the vessel 12 shown in FIG. 1, but 
additional, substantially identical vessels 12A and 12B, positioned about 
the shaft 56. The vessels 12A and 12B will contain guide rods 22 and 
floating stirrers 30, and the rotating magnetic field generated by the 
magnets 62 will be in operative relationship with the magnets 34 contained 
in each of the vessels 12A and 12B. The positioning of the additional 
vessels 12A and 12B is illustrative, since such additional vessels may be 
placed in position within the rotating magnetic field generated by the 
rotating magnets 62 so as to cause the floating stirrers 30 therein to be 
rotated. 
In FIG. 3, there is disclosed the construction of the floating stirrer 30, 
there being shown a buoyant element 32 of generally bar-bell shape, having 
spherical end portions 32a, connected by a straight connecting portion 
32b, the magnet 34 being a bar magnet and extending through the connecting 
portion 32b, and into the end portions 32a. Two substantially identical 
buoyant or floating stirrer elements 32 are provided, being held in 
laterally spaced relationship by elastic bands 34. Referring to FIG. 5, 
there may be seen the two elastic bands 34, the two buoyant elements 32 
with bar magnets 34 therein, and there is also shown a guide tube 36, of 
generally hollow, cylindrical configuration and being narrow at its upper 
end with a reduced opening 36a. Opening 36a is the smallest diameter 
portion of guide tube 36. The elastic bands 34 connect the buoyant 
elements 32 to each other, in an assemblage with the guide tube 36, the 
bands 34 being located on the connecting portions 32b where they engage 
the end portions 32a, and cause the buoyant elements 32 to clamp the guide 
tube 36 between them. 
In FIG. 4, the guide tube 36 is shown, with the guide rod 22 extending 
through it, the upper opening 36a of guide tube 36 being above the surface 
of the body of liquid L, and being that portion of guide tube 36 which has 
a guiding and moving relationship with the guide rod 22. 
Referring to FIGS. 6-8, an alternate embodiment of a floating stirrer is 
provided, there being shown in FIG. 7 a floating stirrer 70 having a pair 
of spherical buoyant elements 72 held in spaced apart relationship by an 
upper plate 74a and a lower plate 74b, suitable threaded fasteners 76 
securing the plates and buoyant elements together. The upper plate 74a, as 
shown in FIG. 6, has a relatively small opening 74c therethrough, and as 
shown in FIG. 8 the lower plate 74b has a relatively larger opening 74d 
therethrough. The guide rod 22 extends through the openings 74c and 74d, 
and thus through the floating stirrer 72, the guide rod 22 engaging, in 
sliding fashion, the opening 74c in the upper plate 74a. The lower plate 
74b carries a pair of bar magnets 78. 
In operation, with the flask 12 partially filled with liquid L, the 
floating stirrer 30 will float on the liquid body L, due to the buoyancy 
of the entire stirrer, provided principally by the buoyant elements 32. 
The floating stirrer 30 may move up and down, guided by the guide rod 22. 
The shaft 56 is caused to rotate by motor 48, and thereby rotates the 
vertical array of magnets 62, causing the generation of a rotating 
magnetic field which has cooperative engagement with the magnets 34 
carried by the buoyant elements 32 of the floating stirrer 30. The 
rotating magnetic field thus generated causes the floating stirrer 30 to 
rotate. Its movement is restricted to rotation, generally about the axis 
of guide rod 32, and to movement along guide rod 32, when there are 
changes in the level of the liquid surface of the body of liquid L. Where 
plural vessels 12 are utilized, as in FIG. 2, all of the stirrers 30 will 
be rotated, as above set forth. In both the embodiment of the stirrer 30 
as shown in FIGS. 3-5, and the embodiment of stirrer 70 shown in FIGS. 
6-8, there is an upper opening which has cooperative engagement with the 
guide rod 22, so that liquid is not enabled to enter into the space 
between the bearing surfaces provided by the opening 36a of guide tube 36, 
or the opening 74c of plate 74a, and the guide tube 22. Thus, both danger 
of sticking of the stirrer and crushing of cells are avoided. 
In FIG. 9, there is disclosed a stirrer 12, substantially identical to the 
stirrer 12 of FIG. 1. The laterally positioned moving magnetic field 
generator is provided by a core 80a having a coil 82a thereon, and 
connected to a suitable source of electricity. As shown in FIG. 10, there 
is a T-shaped support 84 comprising a cross bar 84a and a stem bar 84b, 
the latter having an opening 84c therein to receive a post 86. FIG. 10 
discloses the arrangement, which includes the core 80a and coil 82a, the 
core 80a being connected to the cross bar 84a adjacent one end by a 
suitable bolt 86a, there being a similar core 80b secured adjacent the 
opposite end of the cross bar 84a by a bolt 86b. The core 80b has a coil 
82b thereon. Thus, when either of the coils is energized, a magnetic field 
is generated, which collapses when the coil is de-energized, in known 
manner. The cores 80a and 80b extend along the vessel 12, generally 
parallel to its axis, and are in spaced relationship to each other. 
As shown in FIG. 11, in Position 1, when positive current is supplied to 
coil 82a, which may be designated as the left hand coil, it acts as a 
"north" pole, the right hand coil not being energized, so that the north 
pole of the magnets 34 are repelled from the left hand coil, causing 
rotational movement of the magnet and the stirrer 30. In Position 2, the 
left hand coil has negative current supplied, causing it to act as a south 
pole, to attract the north pole of the magnet. In Position 3, the right 
hand coil is supplied with negative current, thereby repelling the south 
pole of the magnet and attracting the north pole of the magnet, while in 
Position 4, the left hand coil is energized positive, causing it to 
function as a north pole, attracting the south pole of the magnet. 
The apparatus disclosed in FIGS. 9 and 10 operates in substantially the 
same manner as the apparatus of FIGS. 1 and 2. The moving magnetic field 
generated by the coils and core cause the magnets and the stirrer to 
rotate. The stirrer is guided, just as described in connection with the 
stirrer of FIG. 1. Further, the vertical extent of the cores is chosen to 
be substantially coextensive with the anticipated range of levels of the 
surface of the liquid L in the vessel 12. As will be understood, such 
construction is provided so that there will be a moving magnetic field 
operatively coupled with the magnets 34 in any position of the stirrer 30 
as it changes position with the change in the surface of the liquid L. 
While there has been disclosed a construction including a pair of coils, 
each provided with a core, it will be understood that the number of cores 
and coils may be increased above the two cores and coils which are shown 
for illustrative purposes in the drawing. Obviously, the supporting 
structure for a greater number of cores and coils would be modified, as 
necessary, and, further, it is contemplated that more than a single vessel 
12 may be provided in position to have the stirrer thereof driven by the 
core and coil arrangement as herein disclosed. 
In FIG. 12, there is disclosed a further embodiment of the present 
invention, including a vessel 12 which may be of the form shown in FIG. 1, 
as illustrated, or as are all of the vessels herein disclosed, may be of 
the construction as disclosed in the above-noted Pearson U.S. Pat. No. 
4,382,685. In the apparatus of FIG. 12, an annular support 90 is provided, 
having within it a plurality of coils, such as the coils 92a-92d. The 
support 90 has connected to it a sleeve 94, which is carried on a post 96, 
having a turn screw 98, so as to enable the support 90 and the cores 
92a-92d carried by it to be vertically adjusted at will. In FIG. 14, the 
coil 92b is shown, having a core 92c, such coils and core being of known 
construction, the support 90 having an upper wall 90a and a side wall 90b, 
with a bolt 94 serving to connect the coil 92b to the housing 90. The 
vessel 12 may be seen, with the coil 92 b laterally thereof. 
In the apparatus of FIGS. 12-14, the support 90 will be adjusted, from time 
to time, for the desired position relative to the float 30, so as to 
achieve the desired magnetic force on the magnets of the floating stirrer. 
The speed of rotation of the floating stirrer 30 may be controlled by 
controlling the current supplied to the several coils 92, or by the 
relative position of the support 90 and coils 92 to the stirrer 30. Thus, 
the position of stirrer 30 shown in FIG. 12 relative to support 90 is to 
be taken as being for illustrative purposes only, and not as an indication 
of the necessary relative positioning as would be used in practice. 
In FIG. 15, there is provided another embodiment of the present invention, 
in which there is a flask 12, the closure 16' therefor having on the 
underside thereof a bearing 102, schematically shown. A head 104 is 
provided, and rests upon and is supported by the bearing 102. Extending 
downwardly from the head 104 is a driving guide rod 106, which is of 
non-circular cross section. Fixedly mounted on the driving guide rod 106 
is a bar magnet 108. A floating stirrer 110 is supported by the body of 
liquid L at its surface, and as shown in FIG. 16, the floating stirrer 110 
is of generally elongate shape, having a non-circular opening 112 therein. 
More specifically, the driving guide rod 106 is of square transverse cross 
section, and the opening 112 in the floating stirrer 110 is of the same 
square cross section, the shapes thereby being congruent. 
Laterally of the vessel 12 there is provided a coil 82a with a core 80a' 
which extends through the coil 82a; that is, it does not extend downwardly 
along the vessel, as does the core 80a (and core 80b) as in the embodiment 
of FIGS. 9 and 10. As will be understood, there is a second core and coil, 
carried by the T-shape support 84, on a post 86. Thus, the core and coil 
apparatus of FIG. 15 differ from that shown in FIGS. 9 and 10 by the 
shorter length of the cores. The magnetic field generated is a rotating 
magnetic field, and will be operatively coupled with the bar magnet 108, 
which does not move vertically. 
Thus, in operation, the moving magnetic field generated by the core 80a', 
the coil 82a and one or more additional cores and coils will cause the 
magnet 108 to rotate. Since the magnet 108 is secured to the driving guide 
rod 106, it will rotate the latter, the head 104 thereof being supported 
by the bearing 102. Due to the non-rotational coupling between the driving 
guide rod 106 and the floating stirrer 110, the latter will be rotated. 
Its movement will be restricted, however, in the same manner as the 
movement of the stirrer 30, being able to rotate and to move along the 
driving guide rod 106, but not being able to wander on the surface of the 
body of liquid L. 
There has been provided an improved magnetic stirrer apparatus, 
particularly for stirring liquid cell culture medium, in a gentle and 
effective manner. The laterally positioned moving magnetic field 
generating means causes rotation of the floating stirrer, and the floating 
stirrer is guided, its movement being restricted to a rotational movement 
and to a movement along a guide rod. The provision of a laterally 
positioned rotating magnetic field generator and a floating, magnetically 
driven and guided stirrer avoids lateral and vertical instabilities found 
in prior art devices, while providing effective coupling between the 
driven magnet and the driving magnetic field, in any position in which the 
magnet will occupy, even where, as in certain embodiments, the magnet is 
associated with the floating stirrer and moves with it. 
It will be obvious to those skilled in the art that various changes may be 
made without departing from the spirit of the invention, and therefore the 
invention is not limited to what is shown in the drawings and described in 
the specification but only as indicated in the appended claims.