Abstract:
A glass tube is positioned over a heater and stretched until drawn apart into two drawn portions. A controlled pulling force is applied to the tube by a powered driver such as a linear motor. After separation, one of the drawn glass portions is repositioned over the heater and reciprocated in a controlled fashion by the driver to refine the geometry of the fine tip formed on the resulting pipette.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to the fabrication of glass pipettes and relates in particular to an apparatus which forms fine tipped glass pipettes during a pulling operation and which is adapted to modify the geometry of the pipette tips during a heat treating operation. 
     2. Description of Prior Developments 
     Equipment for producing pipettes has been available in numerous configurations for producing various types of fine tipped glass tubes such as injection pipettes and patch pipettes. Although such apparatus has generally functioned satisfactorily, most require significant knowledge and skill to operate properly so as to produce uniform results and consistent product quality. 
     That is, conventional pipette pulling apparatus typically apply a tensile load to a thin glass tube as the tube is heated to a point where the tensile strength of the glass decreases. The tube then stretches or elongates under the applied load as the heated and softened region necks down and breaks in two. At this point, the fine tipped section created at the break is typically removed from the pulling apparatus and further conditioned in a secondary heat treating operation. 
     More particularly, the fine tipped pipettes are placed in a secondary heating apparatus known as a “forge” where the fine tips of the tubes are heated under controlled conditions to modify the geometry of the glass tips. During these heating operations, generally referred to as “fire polishing”, the tips of the pipettes are typically moved back and forth over a heating element or within an oven so as to modify the shape and size of the opening within and around the tip of the pipette. 
     It can be appreciated that the secondary forging or heating operation is not only time consuming, since it necessitates removal of the pipette from a pulling apparatus and placement in a forge, but is also expensive insofar as a separate forging apparatus must be purchased to carry out additional heating operations. 
     Another problem encountered with conventional pipette pulling apparatus is the relatively complex process involved in producing larger diameter patch pipettes. While small diameter injection pipettes, also known as intracellular pipettes, can be fabricated with a single pulling step, larger diameter patch pipettes typically require a two stage heating and pulling operation which is more labor intensive and subject to more process variables than a single stage pulling operation. 
     During a two stage pulling operation, a glass tube is initially heated at a first predetermined temperature while being subjected to a first predetermined tensile load. Once the tube elongates a predetermined amount, the tube elongation and the travel of that half of the pipette being pulled (typically by a dead weight) is limited by a preset travel stop. 
     At this point, the heating element, which was initially centered over the necked down region between the two halves of the heated tube, is deactivated and manually moved several millimeters to a second preset location which coincides with the new location of the center of the necked down region of the glass tube. The heater is then reactivated to a second preset temperature which is typically higher than that of the first heating step. 
     Once the heater is reactivated for a preset period of time, a second tensile load is applied and the travel stop is manually removed to allow the tube to stretch even further to the point where it breaks in two. The second tensile load can be applied as a simple dead weight which operates purely by gravity, or by a positively driven load produced by a solenoid. Again, once the tube is pulled apart, the tip of the pipette tube is typically removed from the pulling apparatus and heated in a separate heating apparatus, such as a forge, where a reciprocatory fire-polishing operation is effected. This entire operation requires considerable expertise, and is subject to numerous process variables leading to non-uniform pipette tip geometry. 
     Accordingly, a need exists for a method and apparatus for producing glass pipettes which reduces the level of operator skill required to consistently produce high quality pipettes having uniform dimensions and uniform functional characteristics. 
     A further need exists for such a method and apparatus which obviates the need for a separate heating apparatus or “forge” for carrying out secondary heating operations on a pipette, such as fire polishing operations. 
     Yet another need exists for such a method and apparatus which is highly automated so as to reduce the amount of labor and time required to produce high quality glass pipettes. 
     The aforementioned objects, features and advantages of the invention will in part, be pointed out with particularity, and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawings, which formn an integral part thereof. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed to fulfill the needs noted above and therefore has as an object the provision of a method and apparatus for producing fine tipped glass pipettes with relatively low skilled operators. 
     A further object of the invention is the provision of a method and apparatus for consistently producing high quality glass pipettes in a highly automated system which promotes uniform product features and product performance. 
     Still a further object of the invention is to provide a highly automated method and apparatus for producing high quality glass pipettes using a minimum amount of labor and time. 
     Yet a further object of the invention is to provide a method and apparatus which obviates the need for separate heating apparatus for carrying out secondary heating operations on a pipette tip, such as fire polishing operations. 
     Another object of the invention is the provision of a method and apparatus for producing both single stage and two stage pulled pipettes which are loaded and drawn with a highly controlled positive driver which is not limited to acceleration provided by gravity. 
     Yet another object of the invention is the provision of a method and apparatus for producing pipettes using a linear motor for generating highly controlled tensile loads on glass tubes as they are stretched into fine tipped pipettes. 
     These and other objects are met by the present invention which is directed to a method and apparatus for producing high quality glass pipettes having uniform dimensions and consistent product performance. Rather than applying a tensile load to a pipette tube with a dead weight during a pulling operation, the present invention uses a highly accurate and highly controllable linear motor to provide a carefully controlled tensile pulling load to such a glass tube as it is being heated and stretched. 
     Because a linear motor can apply a controlled variable stroke length and a controlled driving force in two opposing axial directions, a pipette can be reintroduced into the same heater as that used during pulling for subsequent heating as required, for example, to finely finish the geometry of the pipette tip. Moreover, because a linear motor can provide high acceleration to a pipette as it is being elongated, highly customized pipette tip configurations can be produced. 
     A particular advantage of a linear motor, as used in the present invention, is the ability to produce a carefully timed sequence of controlled movement, such as the reciprocatory movement often used during fire polishing. Such finely controlled variable length movements are not possible with conventional fixed stroke solenoid-actuated pipette pullers. 
     The aforementioned objects, features and advantages of the invention will, in part, be pointed out with particularity, and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawings, which form an integral part thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a front elevation view of a pipette pulling and heating apparatus constructed in accordance with the invention; 
     FIG. 2 is a schematic side view, partially in section along section line  2 — 2  of the apparatus of FIG. 1 showing in block diagram the major components of a control system; 
     FIG. 3 is a partial view in section taken along section line  3 — 3  of FIG. 2; 
     FIG. 4 is a view in central section of the linear motor of FIG. 2; 
     FIG. 5 is a schematic view of a glass tube clamped in the apparatus of FIGS. 1 and 2; 
     FIG. 6 is a view of FIG. 5 after the glass tube has been heated and stretched to a first length; 
     FIG. 7 is a view of FIG. 6 showing a second heating and stretching operation; 
     FIG. 8 is a view of FIG. 7 after the glass tube has been stretched apart and showing a first variation of the invention; 
     FIG. 9 is a view of FIG. 7 after the glass tube has been stretched apart and showing a second variation of the invention; and 
     FIG. 10 is a view showing a further heating operation being performed on the pipette produced according to FIG. 8 or FIG.  9 . 
    
    
     In the various views of the drawings, like reference characters designate like or similar parts. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in conjunction with the drawings, beginning with FIG. 1 which shows a pipette pulling and heating apparatus  10  constructed in accordance with the invention. The apparatus  10  includes a rectangular box-shaped cabinet  12  which includes two or four leveling pads  14  located at the bottom comers of the cabinet. A control panel  16  is mounted to a sidewall  18  of cabinet  12 . 
     A support such as a rigid metal planar support plate  20  formed of aluminum alloy or steel is rigidly fixed between the sidewalls  18 ,  18 . A linear slideway rail  22  is mounted along the center of the top or outer surface  24  of plate  20  and extends along substantially the full length of plate  20 . A first carriage  26  is mounted on the rail  22  with high accuracy linear bearings for free smooth sliding movement over the rail. 
     A clamp plate  28  is adjustably secured to carriage  26  with a threaded clamp screw  30 . A recessed groove is formed in the upper surface of the carriage  26  and in the lower or underside surface of clamp plate  28  to define a generally cylindrical clamping pocket within which the end of a glass pipette tube may be securely clamped in a known fashion. Alternatively, radially adjustable collets can be used in place of the clamps. 
     Carriage  26  includes an extension arm  32  which extends transversely over a guide slot  34  formed through support plate  20 . Slot  34  extends along plate  20  parallel to rail  22 . An adjustable winged clamp screw  36  extends through the extension arm  32  and into slot  34 . A clamp plate is located beneath slot  34  and is threaded to clamp screw  36  to allow the extension arm  32  and the carriage  26  to be linearly adjusted along rail  22  and clamped in place in a desired position along rail  22 . 
     A heater for heating a glass tube is adjustably mounted on support plate  20 . The heater can take the form of a coil  40  of resistance wire  42  through which a glass tube is mounted as discussed below. Alternatively, a flat resistance heating ribbon can be used in place of coil  40 . The heater wire  42  is connected by clamps  43  to a pair of electrodes  44  which are mounted on an electrical insulator block  46  which extends transversely over rail  22 . Coil  40  is centered over rail  22 . 
     The insulator block  46  is mounted to a second carriage  48  which is mounted for sliding movement on rail  22  with a linear bearing in a known fashion. Electrical power wires  50  extend through plate  20  from within cabinet  12  via a pair of clearance slots  52  which extend parallel to and symmetrically about rail  22 . Wires  50  are attached to electrodes  44  to power the heater, i.e. coil  40 . 
     A driver arm  54  is connected at one end to the second carriage  48  and at its other end to an actuator  56  of a powered driver  58 . Driver  58  can take the form of an electrically powered solenoid or a fluid driven cylinder such as an air cylinder or motor which operates on external pressurized shop or laboratory air. 
     Actuator  56  has a preset throw or travel, such as for example, 3 millimeters. This throw can be adjusted with an adjustable stop such as the indexed rotary cam wheel  60  which engages the driver arm  54  and stops the travel of the second carriage  48 . When the actuator  56  is powered, it drives the second carriage  48  away from the first carriage  26  and holds the second carriage in a fixed position as set by cam wheel  60 . When the actuator  56  is depowered, a return spring or other return force applicator returns the second carriage  48  to its original predetermined home position. 
     A conventional known lateral adjustment may be provided on the second carriage  48  for adjusting the sideward or transverse position of the coil  40  on the insulator block  46 . A threaded rotary lead screw  62  journaled to the insulator block engages fixed teeth on the second carriage  48 . The electrodes  44  are mounted and fixed on the insulator block  46 . Turning knob  66  back and forth causes the insulator block  48  and the electrodes  44  to slide back and forth across the carriage  48  so as to accurately position circular coil  40  coaxially around a glass tube as described below. 
     A third carriage  70  is mounted in a known fashion to the slideway rail  22  with linear bearings  71  for free accurate sliding movement along the rail. Each carriage  26 ,  48  and  70  may have the same type of mounting to rail  22 . A clamp plate  72  is adjustably secured to the third carriage  70  with a threaded clamp screw  74  which is threaded through the clamp plate and into the carriage body. 
     A cylindrical clamping pocket is formed between the clamp plate  72  and carriage  70  as discussed above with respect to the other clamp plate  28 . As seen in FIG. 2, carriage  70  has a lower or base portion  75  slidably attached to rail  22  and an upper cantilevered portion  77  fixed to the base  75  and spaced above plate  20  so as to be slidable over the top surface of a portion of carriage  48 . 
     As seen in FIGS. 2 and 3, the third carriage  70  is connected by a yoke  78  to a powered driver  80  located within cabinet  12 . Yoke  78  has a pair of arms  82  which respectively extend through a pair of parallel slots  84  formed through the support plate  20 . Slots  84  are aligned parallel to rail  22  to allow the arms  82  to move the third carriage  70  smoothly along rail  22 . Arms  82  may be connected to the underside of carriage  70  with screws  86 . 
     As further seen in FIGS. 2 and 3, the yoke  78  is connected to the sliding actuator rod  88  of the driver  80  by a flange  90 . A threaded fastener  92  passes through flange  90  and into the end of the rod  88  to form a secure interconnection therebetween. A mounting bracket  96  securely mounts the driver  80  to the underside or rear surface of plate  20 . 
     Although any controllable powered reciprocating driver can be used for driver  80 , it has been found preferable to use a linear motor of the type commercially available under the brand LinMot P linear motors. As seen in FIG. 4, such a linear motor includes a series of alternating north (N)  98  and south (S)  100  stator windings encircling a sliding actuator rod  88 . Actuator rod  88  is formed as a hollow chromium steel tube which houses a series of axially spaced neodymium magnets  102 . Position sensors  104  are mounted in a housing  106  for providing a position feedback signal to microelectronics  108  also held within housing  106 . 
     Plain bearings are housed in the stator windings  98 ,  100  for guiding rod  88 . There is no electrical connection between the sliding rod  88  and the stator formed by windings  98 ,  100 . Referring again to FIGS. 2 and 4, a control and power cable  110  supplies power and control signals to the linear motor driver  80 . Control signals supplied by a commercially available microprocessor-controlled electronic controller  112  causes the power from a commercially available power supply  114  to positively drive the actuator rod  88  back and forth according to a preselected pattern of movement. The movement of rod  88  directly translates into movement of the third carriage  70 . 
     Virtually any pattern or sequence of controlled powered movement can be imparted to actuator rod  88  and the third carriage  70  by appropriate programming of a standard off-the-shelf microprocessor  116  which is powered by a standard power supply  118 . Microprocessor  116  can also control another power supply  120  for selectively supplying power to the electrodes  44  of the heater coil  40 . 
     The driver  58  which drives the heater coil  40  and insulator block  46  back and forth along rail  22  is also controlled by the microprocessor  116  via a conventional electrically-actuated valve assembly  121 . The controller  112 , power supply  114 , microprocessor  116 , power supply  118 , and power supply  120  are all mounted within cabinet  12  and operated by switches on the control panel  16  (FIG.  1 ). 
     The operation of the apparatus  10  is schematically shown in FIGS. 5 through 10. Beginning with FIG. 5, a glass tube  122  is clamped at one end to the upper or first carriage  26  with clamp plate  28  and at its other end to the lower or third carriage  70  with clamp plate  72  after being inserted through heater coil  40  on the center or second carriage  48 . Once the glass tube  122  is clamped in place, a start button  124  on control panel  16  is pushed or actuated to begin a preprogrammed pipette pulling and heating process in accordance with the invention. 
     Upon such actuation of the pulling process, the heating coil  40  is powered by power supply  120  to reach a first predetermined temperature and the linear motor driver  80  is powered by power supply  114  and controlled by controller  112  to apply an axial pulling force on the third carriage  70  via rod  88  and yoke  78 . This pulling force is applied to glass tube  122  via clamp plate  72 . As the heating coil  40  heats the glass tube  122  and causes it to weaken, the third carriage  70 , as shown in dashed lines in FIG. 5, moves axially downwardly and independently away from the first carriage  26  as the heated portion  126  of the glass tube  122  begins to stretch and form a necked down region  130 , as seen in FIG.  6 . 
     Once the third carriage  70  moves a predetermined distance, such as six millimeters, the linear motor driver  80  is programmed to stop and the heater coil  40  can be, and preferably is, deactivated. At this point, the microprocessor  116  energizes driver  58  causing actuator  56  to reposition the second carriage  48  and heater coil  40  over the center of the necked down region  130 . The movement of the second carriage  48  represented in dashed lines in FIG. 6 is limited and preset by the engagement of driver arm  54  with cam wheel  60 . A typical movement of about 3 millimeters will reposition coil  40  over the center of the necked down region  130  as shown in FIG.  7 . 
     At this point, the heater coil  40  is reactivated to a second predetermined temperature and the driver  80  is repowered to again apply an axial pulling force on the glass tube  122 . As the glass tube stretches further, the third carriage  70  independently moves further down along rail  22 , as shown in dashed lines in FIG.  7 . Eventually, the glass tube  122  breaks into two pieces or halves  132 ,  134  as shown in FIG.  8 . 
     At a predetermined length of travel on rail  22 , the travel of the third carriage is stopped by deactivating driver  80  according to the program set by the microprocessor. At this point the upper half  132  of the glass tube  122  is removed from coil  40  by one of several possible steps. As shown in FIG. 8, the first or upper carriage  26  can be manually retracted upwardly away from the second carriage  48  by manually loosening clamp screw  36  and the underlying clamp plate and manually sliding carriage  26  upwardly along rail  22 . It is also possible to provide another driver similar to driver  58  for automatically moving the first carriage  26  in the same fashion that driver  58  moves the second carriage. 
     Another step for removing the upper half  132  of the glass tube  122  is to allow driver  58  to drive the second carriage  48  further downwardly toward the third carriage  70  as represented by the dashed lines in FIG.  9 . If this option is used, the cam wheel  60  is moved or removed to allow for the additional travel stroke of actuator  56 . 
     Whether the upper half  132  of the glass tube  122  is removed from the heater coil by the step of FIG. 8 or FIG. 9, the resulting relative position of the upper carriage  26  and upper half  132  of the glass tube is shown in FIG.  10 . Once the upper half of the glass tube is removed from the coil  40 , the driver  80  drives the third carriage  70  upwardly toward the second carriage a preset distance so that the tip  136  of the lower half  134  of the glass tube is repositioned within the heater coil  40 . 
     At this point, the driver  80  is programmed to effect a back and forth reciprocatory movement to the third carriage  70 , thereby causing the tip  136  of the lower half  134  of the glass tube  122  to pass in and out of the coil  40  with coil  40  being energized at a third predetermined temperature. This heating of tip  136  effects a desirable shaping of the end of tip  136  as well as the opening formed within the tip. This last heating of tip  136  is conventionally carried out in a separate heater called a forge. Because a linear motor is used to drive the pipette back over the heater after completion of the pulling operation, no forge is required. 
     The process described above is a two step pulling process typically used for producing patch type pipettes. However, the apparatus  10  can be easily programmed to effect a single pulling process for producing intracellular pipettes. In this case, the pulling step of FIG. 6 is extended until the glass tube  122  is broken in half as shown in FIG.  8  and the second pulling step of FIG. 7 can be eliminated. Final heating of tip  136  can then be carried out as described above in connection with FIG.  10 . 
     There has been disclosed heretofore the best embodiment of the invention presently contemplated. However, it is to be understood that the various changes and modifications may be made thereto without departing from the spirit of the invention. For example instead of employing clamps such as clamp plates to hold the glass tube on the apparatus  10 , any type of holder such as a chuck or collet could be used.