Abstract:
An improved system and method for stirring suspended solids in a liquid media to enhance sample growth and improve sample detection results. The system and method employs a sample vessel holder which adapted to receive at least one sample vessel which contains the solids and liquid media and a stirrer, such as a ferrous metal filled stirrer, and maintain the sample vessel in a position such that the longitudinal axis of the sample vessel extends at an angle substantially less than 90 degrees with respect to the horizontal, such as within the range of about 15 degrees to about 25 degrees with respect to the horizontal. The system and method further employs a magnet driver, adapted to move a magnet, such as a rare earth magnet, proximate to an outer surface of the sample vessel to permit the magnet to impose a magnetic influence on the stirrer to move the stirrer in the sample vessel. Specifically, the magnet driver is adapted to move and, specifically, rotate the magnet such that the magnetic influence moves the stirrer along a side wall of the sample vessel. The magnet driver is further adapted to move the magnet away from said outer surface of the sample vessel to allow gravity to move the stirrer toward the bottom of the sample vessel. This technique therefore provides a more gentle and controlled stirring of the suspended solution.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved system and method for stirring suspended solids in a liquid media. More particularly, the present invention relates to a system and method employing a stirrer, in particular, a magnetic ferrous metal-filled polymer, which is deposited in a vessel containing a liquid media that includes a suspended solid, and is manipulated by a moving magnet outside the vessel to stir the suspended solid in an optimal manner. 
     2. Description of the Related Art 
     Many medical diagnoses require that a fluid sample, such as a blood sample, be taken from a patient, cultured in a growth medium, and then examined for the presence of a pathogen believed to be causing the patient&#39;s illness. The growth medium provides nutrients that allow the pathogen, such as bacteria, virus, mycobacteria, mammalian cells or the like, to multiply to a sufficient number so that their presence can be detected. 
     In some cases, the pathogen can multiply to a large enough number so that it can be detected visually. For example, a portion of the culture can be placed on a microscope slide, and visually examined to detect for the presence of a pathogen of interest. 
     Alternatively, the presence of a pathogen or other organism can be detected indirectly by detecting for the presence of byproducts given off by the microorganism during its growth. For example, certain microorganisms such as mammalian cells, insect cells, bacteria, viruses, mycobacteria and fungi consume oxygen during their growth and life cycle. As the number of microorganisms increases in the sample culture, they naturally consume more oxygen. Furthermore, these oxygen consuming organisms typically release carbon dioxide as a metabolic byproduct. Accordingly, as the number of organisms present increases, the volume of carbon dioxide that they collectively release likewise increases. 
     Alternatively, instead of detecting for the presence of carbon dioxide to detect the presence of an oxygen consuming microorganism, it is possible to detect for a depletion in the concentration of oxygen in the sample of interest. The presence of oxygen consuming organisms can also be detected by detecting for a change in pressure in a sealed sample vial containing the sample of interest. That is, as oxygen in a closed sample vial is depleted by oxygen consuming organisms, the pressure in the sealed sample vial will change. The pressure will further change in the sample vial as the organisms emit carbon dioxide. Therefore, the presence of such organisms can be detected by monitoring for a change in pressure in the closed sample vial. 
     Several methods exist for detecting the presence of carbon dioxide in a sample to determine whether organisms are present in the sample. For example, an instrument known as the BACTEC® 9050 manufactured by Becton Dickinson and Company detects for the change in color of an indicator to determine whether carbon dioxide is present in a sample. That is, each sample is collected in a respective sample vial containing an indicator medium having a chemical that reacts in the presence of carbon dioxide to change color. A light sensor detects the color of the indicator medium in the sample vial when the sample vial is loaded into the instrument. If the sample contains an organism which emits carbon dioxide, the reflected or fluorescent intensity of the indicator medium will change in response to the presence of carbon dioxide. The light sensor will therefore detect this change in intensity, and the instrument will thus indicate to an operator that an organism is present in the sample contained in the sample vial. Other examples of instruments for detecting the presence of organisms in a sample by detecting for the change in carbon dioxide in the sample are described in U.S. Pat. Nos. 4,945,060, 5,164,796, 5,094,955 and 5,217,876, the entire contents of each of these patents are incorporated herein by reference. 
     An instrument employing an oxygen detecting technique is described in U.S. Pat. No. 5.567,598, the entire content of which is incorporated herein by reference. Instruments that are capable of detecting changes in pressure in the sample vial are described in U.S. Pat. Nos. 4,152,213, 5,310,658, 5,856,175 and 5,863,752, the entire contents of each of these patents are incorporated herein by reference. In addition, an instrument capable of detecting changes in carbon dioxide concentration, changes in oxygen concentration, and changes in pressure in the vessel is described in a U.S. patent application of Nicholas R. Bachur et al. entitled “System and Method for Optically Monitoring the Concentration of a Gas, or the Pressure, in a Sample Vial to Detect Sample Growth”, Ser. No. 09/892,061, filed on Jun. 26, 2001, and another instrument capable of detecting changes in carbon dioxide concentration or changes in oxygen concentration in the vessel is described in a U.S. patent application of Nicholas R. Bachur et al. entitled “System and Method for Optically Monitoring the Concentration of a Gas in a Sample Vial Using Photothermal Spectroscopy to Detect Sample Growth”, Ser. No. 09/892,012, filed on Jun. 26, 2001, the entire contents of both of said applications being incorporated herein by reference. 
     It is noted that the results obtained by organism detection techniques described above can be improved if the growth of the organism is enhanced to cause a greater production of carbon dioxide, a greater depletion of oxygen, and a greater change in pressure in the vessel. It is known that the biological activity of a solid sample in a liquid media can be enhanced by maintaining the solid sample in a suspended state. This can be accomplished by continuously stirring the solid-liquid mixture, which improves nutrient, waste and gas exchange in the mixture. 
     Examples of stirring techniques are described in U.S. Pat. Nos. 5,586,823, 4,483,623 and 4,040,605, the entire contents of each are incorporated herein by reference. Each of these techniques employs a magnetic stirrer that is placed in the vessel containing the sample and manipulated by a magnet to stir the sample in the vessel. 
     Although these stirring techniques may be somewhat effective in enhancing sample growth, they each suffer from certain disadvantages. For example, because each of the techniques require that the vessel be maintained in a vertical configuration, the fluid-gas interface is minimized, especially in vessels that are not shallow. This minimal fluid-gas interface inhibits biological performance in the vessel. 
     In addition, the vertical configuration of the vessel allows for the magnets to lose their influence over the magnetic stirrer in the vessel, especially if the magnetic influence on the stirrer is weak as in the case of gentle stirring. Also, the vertical configuration causes the stirrer in the vessel to follow a semi-random stirring path, which results in a stirring action that is inefficient and potentially damaging to the sample. Furthermore, in order to change the intensity of the stirring in these known arrangements, the physical size of the stirrer or the apparatus needs to be changed. 
     A need therefore exists for an improved system and method for stirring suspended solids in a liquid media to enhance sample growth and thus improve sample detection results. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved system and method for stirring suspended solids in a liquid media to enhance sample growth and improve sample detection results. 
     Another object of the present invention is to provide an improved system and method for stirring suspended solids in a liquid media which is capable of changing stirring intensity without changing the size of the stirrer in the media or the size or operation of the system. 
     These and other objects are substantially achieved by providing a system and method for stirring a solid suspended in a liquid in a sample vessel that includes a stirrer, such as a ferrous material filled polymer stirrer. The system and method employs a sample vessel holder which adapted to receive at least one sample vessel and maintain the sample vessel in a position such that the longitudinal axis of the sample vessel extends at an angle substantially less than 90 degrees with respect to the horizontal, such as within the range of about 15 degrees to about 25 degrees with respect to the horizontal. The system and method further employs a magnet driver, adapted to move a magnet, such as a rare earth magnet, proximate to an outer surface of the sample vessel to permit the magnet to impose a magnetic influence on the stirrer to move the stirrer in the sample vessel. Specifically, the magnet driver is adapted to move the magnet such that the magnetic influence moves the stirrer along a side wall of the sample vessel. The magnet driver is further adapted to move the magnet away from said outer surface of the sample vessel to allow gravity to move the stirrer toward the bottom of the sample vessel. The magnet driver device can comprise a magnet shaft assembly having a magnet coupled thereto, and a motor, adapted to move the magnet shaft assembly to move the magnet proximate to the outer surface of the sample vessel and away from the outer surface of the sample vessel. The magnet shaft assembly can be rotatable, and the motor rotates the magnet shaft assembly to move the magnet proximate to the outer surface of the sample vessel and away from the outer surface of the sample vessel. The motor can be directly or magnetically coupled to the magnet shaft assembly. Additionally, the sample vessel holder can be adapted to receive a plurality of the sample vessels and maintain each of the sample vessels in a respective position such that the longitudinal axis of each sample vessel extends at a respective angle substantially less than 90 degrees with respect to the horizontal. Furthermore, the magnet driver can be adapted to move each of a plurality of magnets proximate to an outer surface of a respective one of the sample vessels to permit the magnet to impose a magnetic influence on the stirrer in the respective sample vessel to move the stirrer in the respective sample vessel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, advantages and novel features of the invention will be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an example of a system employing an improved system and method for stirring solid samples suspended in liquid media contained in a plurality of sample vessels according to an embodiment of the present invention; 
         FIG. 2  is a detailed perspective view of an example of a panel in the system shown in  FIG. 1  for housing sample vessels; 
         FIG. 3  is a side view of the panel shown in  FIG. 2 ; 
         FIG. 4  is a diagrammatic view of a belt and pulley arrangement employed in the panel shown in  FIG. 2 ; 
         FIG. 5  is a detailed perspective bottom view of the bottom two rows of the panel shown in  FIG. 2 ; 
         FIG. 6  is a bottom view of the panel shown in  FIG. 2 ; 
         FIG. 7  is a detailed exploded view of the drive arrangement of the panel shown in  FIG. 2 ; 
         FIG. 8  is a detailed view of the motor of the drive arrangement shown in  FIG. 7 ; 
         FIG. 9  is a conceptual view showing an example of the relationship between the position and motion of a magnet in the panel show in  FIG. 2  and a respective sample vessel containing a stirrer in accordance with an embodiment of the present invention; 
         FIG. 10  is a detailed perspective view of an example of a stirrer as shown in  FIG. 9 ; 
         FIG. 11  is a side view of the stirrer shown in  FIG. 10 ; and 
         FIG. 12  is a cross-sectional view of the stirrer shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An example of an incubation and measurement module  100  employing a system for stirring solid samples suspended in liquid media contained in sample vessels  102  according to an embodiment of the present invention is shown in  FIG. 1 . Further details of the system  100  and the stirring system are shown in  FIGS. 2–9 . As illustrated, the incubation and measurement module  100  in this example includes a housing  104  and two panels  106  that can be slid into and out of the housing  104  along respective rail arrangements  108  in a direction along arrow A. 
     Each panel  106  includes a plurality of openings  110 , each of which is adapted to receive a sample vessel  102 . As discussed in more detail below, each vessel  102  includes a solid sample suspended in a liquid media, and a stirrer. The openings  110  are tilted with respect to the horizontal so that the sample vessels  102  received in the openings  110  are also tilted for reasons discussed in more detail below. In this example, the openings  110  are tilted at 15° or about 15° with respect to the horizontal, so that the sample vessels  102  received in the openings  110  are also tilted at 15° or about 15° with respect to the horizontal. As discussed in more detail below, this tilting creates a large air to liquid interface in the sample vessels  102 . Furthermore, the openings  110  and hence the sample vessels  102  need not be tilted at 15° with respect to the horizontal, but rather, can be tilted at an angle within the range of at or about 15° to at or about 25°, with the range of at or about 15° to at our about 20° being preferred. However, the openings  110  and the sample vessels  102  can be tilted at any practical angle with respect to the horizontal that will create a sufficient air to liquid interface. 
     The openings  110  are arranged in a plurality of rows and columns as shown, and each panel  106  can have any practical number of openings. For example, the openings  110  can be arranged in ten rows and nine columns, thus totaling 90 openings 110 per panel  110 . The incubation and measurement module  100  further includes one or more doors (not shown) for closing the housing  104  after the panels  106  have been received in the housing  104 . 
     When a sample culture is to be analyzed by the incubation and measurement module  100 , the sample culture is placed in a sample vessel  102 , and the sample vessel  102  is loaded into a respective opening  110  in a respective panel  106  in the incubation and measurement module  100 . The sample vessel  102  is a closed sample vial in this example. The incubation and measurement module  100  can further include a keyboard  112 , a barcode reader (not shown), or any other suitable interface that enables a technician to enter information pertaining to the sample into a database stored in a memory in the incubation and measurement module  100 , or in a computer (not shown) which is remote from the module  100  and controls operation of the module  100 . The information can include, for example, patient information, sample type, the row and column of the opening  110  into which the sample vessel  102  is being loaded, and so on. The module  100  can include the type of detecting devices as described in U.S. patent applications Ser. Nos. 09/892,061 and 09/892,012, referenced above. 
     As further shown in  FIGS. 2–8 , each panel  106  includes a drive assembly  114  for driving a plurality of magnet shaft assemblies  116  as described in more detail below. The drive assembly  114  includes a drive motor assembly  118  and a pulley arrangement comprising a drive pulley  120 , shaft driving pulleys  122 , idler pulleys  124  and  126 , and a serpentine belt  128  that passes around the drive pulley  120 , shaft driving pulleys  122 , and idler pulleys  124  and  126  as shown in  FIG. 4 . The drive motor assembly  118  includes a drive motor  130  that is controlled by, for example, a controller  132 , such as a microcontroller or the like. The drive shaft  134  of the drive motor  130  is coupled to a magnet plate  136  as shown in detail in  FIG. 8 . The magnet plate  136  includes a plurality of magnets  138 , which are generally strong magnets such as rare earth magnets. The drive motor  130  is mounted inside the housing  104  by, for example, a mounting bracket  140 . 
     As shown in detail in  FIGS. 2 and 7 , a magnet plate  142  having a plurality of strong magnets  144  such as rare earth magnets is coupled to drive pulley  120 . The drive motor  130  is positioned inside housing  104  so that when its corresponding panel  106  is fully inserted in the housing  104 , magnet plate  142  aligns with or substantially aligns with magnet plate  136 . It is further noted that the drive motor  130  and magnet plate  136  are located outside of the rear wall  146  of the incubation chamber that is housed inside housing  104  and receives the panels  106 . Accordingly, as shown in  FIG. 6 , magnet plate  136  and magnet plate  142  are on opposite sides of the rear wall  146  of the incubation chamber. However, the magnets  138  and  144  on magnet plates  136  and  142  are strong enough to magnetically couple with each other through the rear wall  146  so that when the drive motor  130  rotates magnet plate  136 , the magnetic coupling causes the rotation of magnet plate  136  to rotate magnet plate  142 . The rotation of magnet plate  142  drives drive pulley  120 , which drives the serpentine belt  128  to drive shaft driving pulleys  122  and idler pulleys  124  and  126 . It is noted that by locating drive motor  130  outside of the incubation chamber, the heat emitted by drive motor  130  during operation does not influence the temperature within the incubation chamber. Furthermore, the drive motor  130  is not influence by the heat of the incubation chamber, which can damage the drive motor  130 . 
     As can be appreciated from  FIGS. 2–7  and, in particular,  FIGS. 5 and 6 , each of the shaft driving pulleys  122  is coupled to a respective magnet shaft assembly  116 . In this example, panel  106  includes ten shaft driving pulleys  122  and ten corresponding magnet shaft assemblies  116 , each corresponding to a respective row of openings  110 . Each magnet shaft assembly  116  includes a shaft  148  that is coupled at one end to a respective shaft driving pulley  122 , extends along the width of the panel  104  and is rotatably coupled at its other end to a mounting assembly  150 . A plurality of magnet assemblies  152 - 1  through  152 - 5  are coupled to each shaft  148  and rotate in unison with the shaft  148  when the shaft  148  is rotated about its longitudinal axis by its respective shaft driving pulley  122 . As shown in  FIG. 5 , each magnet assembly  152 - 1  through  152 - 5  has one or two strong magnets  154 , such as rare earth magnets, which can be received into corresponding openings  156 - 1  through  156 - 5 , respectively, in the panel  106  as the shaft  148  rotates. Specifically, the total number of magnets  154  of a magnet shaft assembly  116  corresponds to the number of openings  110  in the row of openings corresponding to the magnet shaft assembly  116 . In this example, magnet shaft assembly  116  includes ten magnets  154  corresponding to the ten openings  110  in the row of openings corresponding to the magnet shaft assembly  116 . 
     It is further noted that the magnet or magnets  154  of adjacent magnet assemblies (e.g., magnet assemblies  152 - 1  and  152 - 2 ) are oriented at 180° or approximately 180° with respect to each other about the shaft  148 . That is, when the magnets  154  of magnet assembly  152 - 1  are positioned outside of opening  156 - 1 , the magnets  154  of magnet assembly  152 - 2  are positioned inside opening  156 - 2  as shown in  FIGS. 5 and 6 . The magnets  154  are arranged in this manner to improve the overall balance of the magnet shaft assembly  116 . 
     The stirring operation performed by the magnet shaft assembly  116  will now be described.  FIG. 9  shows an example of the relationship between a magnet  154  of magnet assembly  152 - 1  and a sample vessel  102  that has been loaded into the opening  110  corresponding to the magnet  154 . As discussed above, each magnet  154  corresponds to an opening  110  in the row of openings corresponding to the magnet shaft assembly  116 . As shown, when a sample vessel  102  is received in an opening  110 , it is tilted with respect to the horizontal at the angle at which the opening  110  is tilted with respect to the horizontal, which is 15° or about 15° in this example. As mentioned above, this tilting creates a significantly large air-liquid interface in the sample vessel  102 . 
     As further discussed above, each sample vessel  102  includes a solid sample  158 , such as an organism of the type described above, that is suspended in a liquid media  160 , such as a growth media for enhancing growth- of the organism. Each sample vessel  102  further includes a stirrer  162  which is preferably a magnetic, ferrous metal filled polymer. It is noted that the term “magnetic” in this context refers to a type of ferrous metal, such as magnetic stainless steel, that responds to the magnetic fields of the magnet  154 . The ferrous material employed in the stirrer according to this embodiment is not itself a magnet, nor is it magnetized. Further details of the stirrer  162  are shown in  FIGS. 10–12 . The stirrer  162  can be rod shaped or cylindrical as shown, or have any other suitable shape. The stirrer  162  can have an overall length within a range of, for example, 0.500 or about 0.500 inches to 0.750 or about 0.750 inches, and can have an overall diameter of 0.120 or about 0.120 inches. 
     The stirrer  162  can include about 50% to about 80% of polymer by weight, with the remaining 50% to 20% of the weight being ferrous metal. However, any ratio of polymer to ferrous metal can be used as long as it provides sufficient cohesiveness to hold the stirrer  162  together and to allow sufficient responsiveness to the magnet  154 . The polymer material is preferably a biologically inert polymer, such as nylon or polypropylene, which reduces the overall surface hardness of the stirrer  162 , and thus reduces potential damage to the solid sample  158  in the suspension as well as to the sample vessel  102 . The ferrous material is preferably stainless steel, but can be any suitable material that can respond to magnetic influence from magnet  154 . The stirrer  162  can be color coded with colors such as blue, gray, red, green, orange, and so on, to provide an indication of the type and percentage content of the polymer and ferrous material. The stirrer  162  can be provided in the sample vessel  110 , or can be added to the sample vessel  110  prior to or after adding the solid sample  158  and liquid media  160  to the sample vessel  110 . 
     As further shown in  FIG. 9 , the stirring action is created by controlling the motor  130  (see  FIGS. 2 ,  3  and  5 – 8 ) to rotate the magnet shaft assembly  116  in a direction R. The rotation of the magnet shaft assembly  116  thus rotates the shaft  148  about it longitudinal axis, which in turn rotates the magnets  154  about the longitudinal axis of the shaft  148 . As the magnets  154  rotate, they are brought into their respective openings  156 - 1  through  156 - 5  in panel  106  (see  FIGS. 5 and 6 ). That is, when shaft  148  is rotated, magnet  154  of magnet assembly  152 - 1  cyclically enters opening  156 - 1  to come proximate to sample vessel  102  in its corresponding opening  110 , and exits opening  156 - 1  to become distant from sample vessel  102 . This movement causes a rhythmic agitation of the stirrer  162  to occur. That is, as magnet  154  swipes proximate to the outer surface of sample vessel  102 , its magnetic force attracts stirrer  162  to pull stirrer  162  away from the bottom edge  164  of the sample vessel  102  upward along wall  166  of the sample vessel  102 . As the magnet  154  begins to rotate away from the sample vessel  102 , the stirrer  162  becomes less influenced by the magnetic force of magnet  164 , and due to gravity falls along wall  166  of sample vessel  102  toward the bottom edge  164 . This movement is repeated each time magnet  154  swipes along the outer surface of sample vessel  102 . It is desirable for the magnet  154  to be rotated in the direction R so that the stirrer  162  is first moved up along wall  166  and then allowed to fall back toward the bottom edge  164 . Also, the motor  130  can be rotated at a speed of, for example, 150 rotations per minute, which causes the stirrer  162  to travel through the stirring path described above 150 times per minute. However, the motor  130  can be controlled to rotate at any practical speed to achieve the desired stirring action. 
     It is further noted that by increasing the ferrous fill content of the stirrer  162 , the magnetic influence that magnet  154  has on the stirrer  162  will increase. Likewise, by decreasing the ferrous fill content of the stirrer  162 , the magnetic influence that magnet  154  has on the stirrer  162  will decrease. Accordingly, the intensity of the stirring can be varied by simply replacing stirrer  162  with a stirrer having a different ferrous fill content Furthermore, the size and shape of the stirrer  162  need not be changed. 
     The above arrangement provides several advantages over the conventional stirring devices discussed in the Background section above. For example, because the tilted openings  110  maintain the sample vessel  102  at a shallow angle (e.g., 15°) with respect to the horizontal to facilitate maximum exposure of liquid phase to gas phase. This therefore provides an improved dissolved gas exchange as a function of the angle. Furthermore, the angled orientation of the sample vessel  102  increases the probability that the magnet  154  will maintain magnetic influence over the stirrer  162 . Also, the stirring action can be gentler than in conventional methods since the path of the stirrer  162  is constrained by the wall  166  of the sample vessel  102 . All of these improved characteristics of the stirring system enhances the growth of the sample in the liquid media  160  and thus increases the overall carbon dioxide production, oxygen depletion and pressure variation in the sample vessel  110 , thereby improving sample detection results. 
     Table 1 below shows an example of the sample detection results obtained by agitating various samples according to the embodiments of the present invention discussed above in comparison to the sample detection results obtained by agitating the same types of samples according to a conventional “rocking” method in which the vessel containing the sample is agitated or rocked to thus agitate the sample therein. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sample Detection Comparison Data 
               
             
          
           
               
                   
                 Magnetic Agitation 
                 Rocking Agitation 
               
               
                   
                 Time to Detection 
                 Time to Detection 
               
               
                 Organism and Strain 
                 in Hours 
                 in Hours 
               
               
                   
               
             
          
           
               
                   Candida glabrata  231 
                 62 
                 &gt;120 
               
               
                   Candida glabrata  550 
                 58 
                 &gt;120 
               
               
                   Candida glabrata  15545 
                 51 
                 &gt;120 
               
               
                   Candida glabrata  66032 
                 52 
                 112 
               
               
                   N. meningitidis  13113 
                 47 
                 &gt;120 
               
               
                   S. pneumoniae  6305 
                 19 
                 21 
               
               
                   
               
             
          
         
       
     
     As illustrated, for each type of sample, the duration of time that elapses from the beginning of agitation in accordance with the embodiments described above until a detectable amount of sample has been grown is far less that the duration of time that elapses from the beginning of the conventional “rocking” agitation technique until a detectable amount of sample has been grown. Accordingly, the agitation techniques according to the embodiments of the invention described above are far superior to the conventional rocking technique. 
     Although only a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.