Abstract:
A peristaltic pump wherein the pumping mechanism is enclosed in a vacuum chamber. Placement of the pumping mechanism within a vacuum chamber decreases the pressure differential between the inside and the outside of the pump channel, thereby minimizing changes in trapped fluid volume.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to peristaltic pumps and more specifically to peristaltic pumps used in ophthalmic surgical equipment.  
           [0002]    Peristaltic pumps work by compressing or squeezing a length of flexible tubing (sometimes between a fixed race) using a rotating roller head. As the roller head rotates, the rollers stretch and pinch off a portion of the tubing and push any fluid trapped in the tubing between the roller in the direction of rotation. While it is difficult to achieve high vacuum levels with a peristaltic pump, peristaltic pumps are widely used in medical applications because of their predictable, constant flow properties.  
           [0003]    Many factors influence the efficiency of peristaltic pumps, for example, pump motor torque, pump speed, pump tube flexibility and vacuum levels. Here, efficiency refers to the volume flow rate of a given pump and its relationship to the translational velocity of the pinching forces. The efficiency of a peristaltic pump is also dependent on the compliance or memory of the elastic material used to make the pump tubing. Some compliance in the pump tubing is required to assure that the tubing expands and returns to its undisturbed state after the translating force imparted by the rollers in the pump roller head have passed.  
           [0004]    One disadvantage to peristaltic pumps is that for a given translation velocity (S), the average flow through the pump is adversely affected by a decrease in fluid pressure (P 1 , vacuum) at the input end of the pump. This decrease in average flow results from a decrease in trapped volume (V) within the pump tubing caused by a gradual collapse of the tubing with decreasing P 1 , (increasing gauge vacuum level). At very low P 1  (very high vacuum levels) the pump tubing is completely collapsed, making trapped volume V and the corresponding pump output zero.  
           [0005]    Accordingly, a need continues to exist for a peristaltic pump with increased pumping efficiency, particularly at high vacuum levels.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The present invention improves upon prior art peristaltic pumps by providing a peristaltic pump wherein the pumping mechanism is enclosed in a vacuum chamber. Placement of the pumping mechanism within a vacuum chamber decreases the pressure differential between the inside and the outside of the pump tubing, thereby minimizing changes in trapped fluid volume.  
           [0007]    Accordingly, one objective of the present invention is to provide a high efficiency peristaltic pump.  
           [0008]    Another objective of the present invention is to provide a means for controlling the maximum achievable vacuum of a peristaltic pump.  
           [0009]    Yet another objective of the present invention is to decrease the reliance of peristaltic pump efficiency on the compliance of the pump tubing.  
       
    
    
       [0010]    These and other advantages and objectives of the present invention will become apparent from the detailed description, drawings and claims that follow.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic representation of prior art peristaltic pumping mechanisms.  
         [0012]    [0012]FIG. 2 is a schematic representation of the peristaltic pump mechanism of the present invention.  
         [0013]    [0013]FIG. 3 is a partial elevational view of the peristaltic pump mechanism of the present invention.  
         [0014]    [0014]FIG. 4 is a partial cross-sectional view of the peristaltic pump mechanism of the present invention taken at line  4 - 4  in FIG. 3.  
         [0015]    [0015]FIG. 5 is a schematic representation of the operation of a peristaltic pump.  
         [0016]    [0016]FIG. 6 is a graphic representation comparing the performance of the prior art with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    For purposes of the present invention, the term peristaltic pump mean any type of pump using peristalsis to move fluid. As best seen in FIG. 1, prior art peristaltic pumps operate at barometric external conditions. The pressure surrounding pump tube or channel  10  is barometric pressure P 0 . Fluid flow within pump channel  10  is caused by a sequential, rolling series of pinching forces F along the length of channel  10 . As seen in FIGS. 3, 4 and  5 , these pinching forces are generally supplied by rotating head  14  or other device having a series of spaced rollers  20 . Each of the pinching forces creates a small trapped volume V of fluid that is propelled along channel  10  by the sequential nature of forces F. As shown in FIG. 5, fluid is drawn into pump channel  10  by the return of pump channel  10  to its expanded, unpinched state  18  after pump roller  20  has passed by. While the average flow rate is generally proportional to the speed S of rotating head  14 , average flow rate is adversely affected by a decrease in fluid pressure P 1  within pump channel  10 . This decrease in average fluid flow is due to a decrease in volume V resulting from the gradual collapse of pump channel  10  with decreasing pressure P 1 .  
         [0018]    Also shown in FIG. 5 are the forces involved in producing flow. Force F t  is the return force of compliant pump channel  10 . Force F 0  is the force due to the pressure P 0  surrounding pump channel  10 . Force F 1  is the force due to the pressure P 1  within pump channel  10 . The resultant force F n  is responsible for performing the work of drawing the fluid through pump channel  10  and is the vector sum of all of the forces involved: F n =F t +(F 1 −F 0 ).  
         [0019]    For the existing art, F 0  is due to P 0  being equal to barometric pressure, and internal pressure P 1  work against the ability of the system to draw fluid because of the low levels of F 1 . At some point, P 1  can reach a high level of vacuum (very low levels of F 1 ) such that F n  reaches zero. At this point, pump channel  10  remains collapsed and average flow is zero. The level of P 1  at which the average flow is zero is at the maximum achievable vacuum of the pump V max .  
         [0020]    The above discussion demonstrates that if P 0  is decreased relative to barometric pressure, the average flow at high levels of vacuum (P 1 ) will be improved and the maximum achievable vacuum V max  will also be improved. Conversely, V max  can now be controlled to any desired level by controlling P 0 . In addition, one skilled in the art will recognize that pressures greater than barometric pressure may also be used to lower V max .  
         [0021]    Operation of a peristaltic pump at high vacuum levels places significant design constraints on pump channel  10 . These constraints add to the cost of pump channel  10  as well as limit the selection of materials capable of meeting the design requirements for pump channel  10 . The present invention allows for a relaxation of the design constraints for pump channel  10 , and new types of materials for pump channel  10  which are compressible yet inelastic to expansion. For example, polyester film (e.g. MYLAR®) or other suitable materials can be used where F t  is zero so F n =F 1 −F 0 .  
         [0022]    As illustrated in FIGS. 2, 3 and  4 , the inventors have found that by placing the peristaltic pump mechanism inside pressure or vacuum chamber  22  or  22 ′, the degrading effects of vacuum P 1  inside channel  10  or  10 ″ can be reduced or eliminated. See FIG. 6. By introducing a vacuum inside chamber  22  or  22 ′ relative to P 0 , the collapsing force on channel  10  or  10 ″ caused by P 1  can be negated, and any reduction in trapped volume v caused by partial collapse of channel  10  and  10 ″ as a result of P 1  can also be reduced. One skilled in the art will recognize that the present invention is not limited to peristaltic pumps using a roller head and a pump tube but also encompasses any type pump using peristalsis, such as linear peristaltic pumps.  
         [0023]    As seen in FIG. 6, to test the effectiveness of the present invention, the inventor constructed a system where pump channel  10  or  10 ″ was placed within a vacuum chamber  22  or  22 ′. The internal pressure within chamber  22  or  22 ′ was varied from barometric to 400 mmHg below barometric pressure. The two graphs shown in FIG. 6 demonstrate the time necessary for the pump to evacuate a constant, fluid filled volume for a pump when pump channel  10  or  10 ″ is exposed to barometric pressure and for a pump when pump channel  10 ″ or  10 ″ is exposed to 400 mmHg below barometric pressure. As can be seen, there is a significant decrease in evacuation time for the system when pump channel  10  or  10 ″ is exposed to 400 mmHg below barometric pressure, as well as an increase for V max . One skilled in the art will recognize that lower pressures, as low as 760 mmHg below barometric pressure may also be used. In addition, one skilled in the art will recognize that pressures greater than barometric pressure may also be used.  
         [0024]    This description is given for purposes of illustration and explanation. It will be apparent to those skilled in the relevant art that modifications may be made to the invention as herein described without departing from its scope or spirit.