Magnetic stirring apparatus
TECHNICAL FIELD 
     The present invention relates to magnetic stirring devices. More particularly, it discloses a stirrer having a novel bearing, adjustable in height. The disclosed invention is particularly useful in applications where solids must be suspended in a liquid medium with a minimum of shear force, such as in microcarrier tissue cell culture. 
     BACKGROUND ART 
     The present inventors are not the first to disclose a suspended magnetic stirrer wherein the stirrer, i.e., shaft and impeller, is totally enclosed within a vessel. An early disclosure is U.S. Pat. No. 2,932,493 to Jacobs. In FIG. 3, Jacobs describes a beverage mixer having a magnetic stirrer suspended from a lid by means of a ball and socket joint. 
     A second disclosure of interest is U.S. Pat. No. 3,854,704 to Balas. A gimbaled bearing (ball and socket joint) attached to a closure is illustrated in FIGS. 8 and 9. As used in cell culture, this stirrer also is envisioned to operate in an arcuate manner. 
     Finally, a recently issued patent, U.S. Pat. No. 4,289,854 to Tolbert et al., relates an allegedly novel stirrer using a flexible sail impeller. Used in the suspension culturing of mammalian cells, the impeller centrally rotates about a bearing suspended from a stopper. The bearing consists of two parts: a downwardly projecting, rigid stationary shaft which flares outwardly to provide a lower bearing surface; and a rotatable sleeve member internally journalled to form an upper, rotating bearing surface. 
     While all of these disclosures are relevant to any discussion of enclosed magnetic stirrers, they do not disclose or suggest the novel and non-obvious features of the present invention. 
     DISCLOSURE OF THE INVENTION 
     The present invention comprises an improved magnetic stirring apparatus capable of suspending solids in a liquid medium with a minimum of shear force. It is particularly useful in suspended microcarrier cell culture, where low shear forces and low revolution per minute (RPM) operation are desirable. 
     The instant stirrer has conventional stirrer elements such as: a vessel for containing fluids and solids having walls, a bottom, and an opening at the top; a stirrer having a shaft and a finned magnetic impeller; and a closure. However, the present invention has additional elements which, when combined with the above elements in a novel and non-obvious way, create an effective, yet inexpensive stirrer. 
     These additional elements invention are a variable thickness shaft and an elongated shaft support member, which are shaped and assembled in the following manner. 
     The shaft has an upper and a lower portion. The cross-sectional area of the lower portion is smaller than that of the upper portion. An upper bearing surface is formed on the shaft where the lower portion extends into the upper portion. 
     A lower bearing surface is formed by an elongated and apertured shaft support or sling member. The aperture is dimensioned and configured to receive the upper shaft portion when force-fitted or otherwise distorted. The stirrer is assembled by distorting the sling member, sliding the upper shaft portion through the aperture, and suspending that portion on the shoulders or margins of the aperture. 
     In an alternative embodiment, the aperture also is dimensioned and configured in another part to receive the upper shaft portion, forming a keyhole. The upper portion of the shaft is inserted into the keyhole part of the aperture, the stirrer is suspended onto the sling member at a point where only the narrower lower portion is received, and when the upper bearing surface engages the lower bearing surface in the direction of the vessel bottom, a sling bearing is formed. 
     The above stirrer can provide optimal conditions for the growth of cells either in suspension or attached to microcarrier beads known to the art. An important advantage to this configuration is the ease of assembly and the inexpensive production costs which make it ideal for a pre-sterilized, disposable cell growth system. Conventional suspended cell culture systems have had to deal with bulky, complicated designs which were not economically conducive to a throw-away mode of operation. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the assembled magnetic stirrer. 
     FIG. 2 is a side view of the impeller, along the (2--2) plane of FIG. 1. 
     FIG. 3 is an overhead view of the closure. 
     FIG. 4 is a cross-sectional view of the stirrer sling bearing and the adjustable stirrer height means. 
     FIG. 5 is an end-on view of the adjustable stirrer height means along the (5--5) plane of FIG. 4. 
     FIG. 6 is an overhead view of the elongated shaft support member when not attached to the closure. 
     FIG. 7 is a close-up view of the sling bearing of FIG. 1. 
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     In a preferred embodiment for use in suspended microcarrier cell culture, the present invention begins, in FIG. 1, with a vessel (10) having walls, a bottom, and an opening at the top. A projection (12) rises from the center of the bottom to a height of about a half inch. The corners are radiused where the walls meet the bottom. On opposite upper wall surfaces are side ports (14) covered with closures (16) made from materials which restrict bar cell contaminants from entering the vessel. 
     The opening is covered by a snap-fitting closure (18), having incorporated therein, an oxygen and carbon dioxide gas-permeable, cell contaminant impermeable membrane (20) made, for example, from polyethylene. As best seen in FIG. 3, a preferred closure comprises a structural array of members capable of suspending a stirrer and the gas-permeable membrane (20). The array has a central disc (22) connected by radial arms (24) to an annulus (26). A distinct advantage to this design is that respiratory gasses generated during cell culturing can be exchanged without fear of contamination. Such an exchange is especially desirable where non-ambient gas concentrations could cause a media pH shift. 
     A sling bearing suspends a magnetic stirrer from the closure. As seen in FIGS. 1, 4, and 7, the bearing comprises two opposing surfaces, an upper bearing surface (34) at the upper end of the shaft, and a lower bearing surface (36) at the point of suspension of the shaft on an appropriately apertured and elongated shaft support or sling member (38). Bearing assembly is easy and quick, the upper shaft end being snapped into the aperture of the support member. 
     More particularly, FIG. 6 reveals a shaft support member dimensioned and configured from a strip of low friction material, such as Teflon®, with a T-slotted aperture (40) capable of receiving the shaft therethrough, yet not allowing the shaft to slip through the aperture when suspended. Of course, this requires that the upper shaft end be correspondingly dimensioned at the upper bearing surface, e.g., beveled, such that while an upper portion of the shaft can slip through the T-slot, it is wide enough to rest upon at least opposed margins or shoulders of that portion of the aperture not having the T-slot, i.e., the lower bearing surface. 
     An alternative embodiment of the bearing does not call for a T-slot aperture in the shaft support. Instead, either the shaft support and/or the shaft is made of a pliable material. In one case, the upper bearing surface of a pliable shaft deforms enough to be push-fitted into a narrower aperture, but once through, regains its original shape. On the other hand, the support can be sufficiently pliable to permit a shaft to be push-fitted into a narrower aperture which also regains its original form once the upper bearing surface is through the aperture. In either case, no T-slot is required. 
     Another advantage to the present invention can be found in the way the sling bearing is connected to the closure. In a preferred embodiment, the bearing attaches to an externally-controlled, vertically adjustable member or bushing (42). This permits a suspended microcarrier cell culture system wherein one can adjust the clearance of the impeller to the vessel bottom to suit a particular cell line without having to expose the vessel interior to ambient air. 
     FIGS. 1 and 4 detail a preferred stirrer height adjustment means. The sling bearing support (38) has serrated edges (44) on the lateral edges of both ends. A cylindrical, circumferentially threaded adjustment member (42) is correspondingly slotted on its bottom face (46) (FIG. 5) whereby the serrated edges may be inserted into spaced slots. Proper slot spacing ensures a symmetrical, arcuately-shaped sling support upon which the inserted shaft easily rotates. The adjustment member (42) is received by a correspondingly threaded aperture in the central disc portion (22) of the closure. 
     The stirrer impeller height can be adjusted by turning the adjustment member. For convenience, the upper end of the member can be either grooved (48) for a screwdriver or in the shape of a knurled cap (not shown) which can be turned by hand. Another optional item is a lock nut (50) which threads onto the external portion of the adjustment member. When snugged against the closure, inadvertent adjustment of the stirrer is avoided. 
     The impeller (32) features a single vertical blade with a magnet implanted or encased within. Lying in the same plane as the impeller and centered under the sling bearing, the stirrer operates in a central rotating manner when coupled with a conventional magnet rotating means. The shaft/impeller assembly can be either a snap-fitting arrangement, as shown in FIG. 2, where the impeller is pushed into a bottom-slotted shaft, or it can be a unitary construction (not shown). 
     As claimed, the present invention provides an inexpensive, presterilizable, and disposable magnetic stirrer capable of very low RPM operation and low shear forces. This can be critical when culturing sensitive cell lines. Suspended fibroblast cells such as MRC-5 and HFF are extremely sensitive to shear, dying when they become detached from support surfaces. The following examples demonstrate the present stirrer&#39;s performance. 
     EXAMPLE 1 
     MRC-5 Cell Culturing 
     The MRC-5 cell line as deposited in the American Type Culture Collection (ATCC) of Rockville, Md., was cultured in the present invention as shown in the Figures. An indented spinner apparatus made by Bellco, see U.S. Pat. No. 3,622,129, was used as a control for comparison purposes. In both cases the MRC-5 cells were attached to Cytodex 3® microcarrier beads made by Pharmacia. Conventional culturing practices and media were used throughout. 
     
                       TABLE 1                                                     
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         Impeller        Impeller Cell                                    
Run No.  Clearance (inches)                                               
                         Size.sup.+                                       
                                  Yield                                   
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1        1/2             R        6.74                                    
2        1/2             L        6.34                                    
3        1/8             R        4.38                                    
4        1/8             L        6.02                                    
 5*      1/8             R        6.08                                    
 6*      1/8             R        6.26                                    
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 *designates a Bellco stirrer                                             
 L means a 2.6 cm × 8.0 cm paddle                                   
 R means a 3.5 cm × 8.95 cm paddle                                  
 All runs were made for 7 days at 24 RPM                                  
 Cell yield is in a ratio of amount recovered divided by amount seeded.   
 
    
     EXAMPLE 2 
     MDCK Cell Culturing 
     The MDCK cell line as deposited in the ATCC was cultured for 5 days with the method of Example 1, using a similar control. The results were as follows except the impeller speed was 28 RPM. 
     
                       TABLE 2                                                     
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         Impeller        Impeller Cell                                    
Run No.  Clearance (inches)                                               
                         Size     Yield                                   
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1        1/8             R        16.00                                   
2        1/8             R        16.35                                   
3        1/8             R        15.91                                   
4        1/8             L        15.45                                   
5        1/8             L        17.15                                   
6        1/8             L        16.21                                   
7        1/2             R        16.95                                   
8        1/2             R        14.83                                   
9        1/2             R        18.08                                   
10       1/2             L        18.06                                   
11       1/2             L        16.81                                   
12       1/2             L        17.87                                   
13*      1/8             R        16.51                                   
14*      1/8             R        15.14                                   
15*      1/8             R        19.35                                   
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 *designates a Bellco stirrer                                             
 L means a 2.6 cm × 8.0 cm paddle                                   
 R means a 3.5 cm × 8.95 cm paddle                                  
 Cell yield is in a ratio of the amount recovered divided by the amount   
 seeded.                                                                  
 
    
     Having described the invention with particular reference to preferred form, it will be obvious to those skilled in the art to which the invention pertain, that, after understanding the invention, various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.