Abstract:
A drug delivery device drive that includes a single shape memory alloy wire actuator to advance a lead screw via a ratcheting mechanism and method thereof are disclosed. In one embodiment, a shape memory alloy wire is operably connected to one of a pair of ratchet wheels and configured to drive incrementally the rotation of the connected ratchet wheel via a contraction, which in turn drives the rotation of the other ratchet wheel about a rotational axis which moves a lead screw and advances a plunger to dispense a liquid drug from a drug container. A drug delivery device using the shape memory alloy wire actuator in combination with the ratcheting mechanism to incrementally rotate a shaft, a lead screw or a sleeve provides for a more compact an less complicated design.

Description:
FIELD OF THE INVENTION 
   The present invention is related to drug delivery devices, and in particular to a drug delivery device that includes a single shape memory alloy wire actuator drive to advance a lead screw via a ratcheting mechanism. 
   BACKGROUND OF THE INVENTION 
   Shape memory alloy (SMA) actuators are used for a wide range of applications. One typically application for SMA actuators is to perform limited displacements which generate work. In such displacement applications, the SMA actuator is typically in the form of a wire that transforms linear motion into incremental relative motion. When applying a current to the cold (low temperature or martensitic state) shape memory alloy (SMA) wire the temperature rises until the transformation temperature is reached (high temperature or austenite state) and due to a crystalline restructuring of the material, a contraction occurs. With such a contraction, force or torque is thus generated. However, it is to be appreciated that after the contraction, the SMA wire does not reset itself and therefore a counterforce has to bring the SMA wire into its original position. 
   In the field of drug delivery devices, prior art drug delivery systems which use shape memory alloy actuators are typically reset by having opposed SMA wires. Such a configuration allows higher cycle frequencies. However, such a design is more complex and additional space is required for the second SMA wire. 
   SUMMARY OF THE INVENTION 
   It is against the above background that the present invention provides a drug delivery device that includes a single shape memory alloy wire actuator to advance a lead screw via a ratcheting mechanism which provides for a more compact and less complicated design. 
   In a first embodiment, a drive system used to dispense a liquid drug from a drug container having a plunger is disclosed. The drive system comprises a lead screw having a rotational axis and operably connected to the plunger. A ratcheting mechanism having first and second ratchet wheels provided along the rotational axis is also provided. The first ratchet wheel is connected to the lead screw to move the lead screw. A shape memory alloy wire is operably connected to the second ratchet wheel and configured to drive incrementally the rotation of the second ratchet wheel via a contraction, which in turn drives the rotation of the first ratchet wheel about the rotational axis which moves the lead screw and advances the plunger to dispense the liquid drug from the drug container. 
   In a second embodiment, a method of dispensing a liquid drug from a drug container having a plunger is disclosed. The method comprises operably connecting to the plunger a lead screw having a rotational axis, and providing along the rotational axis a ratcheting mechanism having first and second ratchet wheels. The first ratchet wheel is connected to the lead screw to move the lead screw. The method further includes operably connecting a shape memory alloy wire to the second ratchet wheel, and driving incrementally the rotation of the second ratchet wheel via contracting the shaped memory alloy wire to drive the rotation of the first ratchet wheel about the rotational axis which moves the lead screw and advances the plunger to dispense the liquid drug from the drug container. 
   These and other features and advantages of the invention will be more fully understood from the following description of various embodiments of the invention taken together with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description of the various embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
       FIG. 1  is a partially exploded, perspective view of a drug delivery pump drive embodiment using a single shape memory alloy wire actuator to advance a lead screw via a ratcheting mechanism having a torsion spring which provides tight engagement between facing ratcheting surfaces and a reset function according to the present invention; 
       FIG. 2  is a partially exploded, perspective view of another drug delivery pump drive embodiment using a single shape memory alloy wire to advance a lead screw via a ratcheting mechanism having a torsion spring which provides tight engagement between facing ratcheting surfaces and a spring providing a reset function according to the present invention; 
       FIG. 3  is a partially exploded, perspective view of a drug delivery pump drive embodiment using a single shape memory alloy wire attached to a slotted sliding mechanism which advances a lead screw via a ratcheting mechanism according to the present invention; 
       FIG. 4  is a partially exploded, perspective view of another drug delivery pump drive embodiment using a single shape memory alloy wire to advance a lead screw via a ratcheting mechanism according to the present invention; and 
       FIG. 5  is a perspective view of a miniature drug delivery pump embodiment using a drug delivery pump drive using a single shape memory alloy wire to advance a lead screw via a ratcheting mechanism to advance a syringe plunger to deliver a liquid drug according to the present invention. 
   

   DETAILED DESCRIPTION 
   In the following description of the embodiments of the invention, skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiment(s) of the present invention. Accordingly, the drawings are merely schematic representations, intending to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. The invention will be described with additional specificity and detail through the accompanying drawings. The description of the invention may contain, for example, such descriptive terms as up, down, top, bottom, right or left. These terms are meant to provide a general orientation of the parts of the invention and are not meant to be limiting as to the scope of the invention. 
     FIG. 1  is a partially exploded perspective view of a first embodiment of a drug delivery pump drive  100  that includes a single Shape Memory Alloy (SMA) wire  10 . It is to be appreciated that such a drug delivery pump drive  100  is suitable for use in an infusion pump application to accurately pump a liquid drug. In one embodiment, the pump drive  100  has a 3.5×3.5×15 mm footprint, which is about 20% the size of conventional electrical micro-motors. 
   In the illustrated embodiment of  FIG. 1 , the SMA wire  10  is connected to a ratcheting mechanism, generally indicated by symbol  12 , which in turn is connected to a lead screw  14 . In one embodiment, the ratcheting mechanism  12  comprises first and second ratchet wheels  16 ,  18  having facing ratcheting surfaces  20 ,  22  arranged perpendicularly to the lead screw  14 . As shown, the first ratchet wheel  16  is fixed to the lead screw  14 , and the second ratchet wheel  18  is mounted rotatably to a post  24 . As shown, the post  24  extends through the second ratchet wheel  18  and is accommodated at an end  26  in a cavity  28  of the first ratchet wheel  16 . In this manner, the first ratchet wheel  16  is also rotatable mounted to the end  26  of the post  24 . The post  24  at the other end is fix, such as, to a base  30  of an infusion pump  32 . 
   In one embodiment, the facing ratcheting surfaces  20 ,  22  are opposing axially extended teeth, such that rotation in a first (drive) direction, indicated by arrow D, the teeth of both ratchet wheels  16 ,  18  mesh and rotate together, and in a second (return or reset) direction, indicated by symbol S, the teeth of the second ratchet wheel  18  slip or rides over the teeth of the first ratchet wheel  16 , such that second ratchet wheel  18  rotates relative to the first ratchet wheel  16  about a rotational axis, indicated by symbol X. Accordingly, the ratcheting mechanism  12  provides a unidirectional ratchet like motion, which rotates the lead screw  14  about the rotational axis X in only the first direction D. 
   In the illustrated embodiment of  FIG. 1 , a torsion spring  34  is provided to the post  24  to performs two functions simultaneously, which helps to facilitate a very compact infusion pump design. The first function of the torsion spring  34  is to provide an axial spring force that is applied to the second ratchet wheel  18 . The axial spring force is indicated by arrow Z, and maintains a tight engagement between the surfaces  20 ,  22  of the ratchet wheels  16 ,  18  in the first (drive) direction D, but which can be overcome in the second direction S via a resetting force, which is discussed hereafter, to permit disengagement between the facing ratcheting surfaces  20 ,  22  along the rotational axis X, thereby allowing the unidirectional ratchet like motion. 
   The second function simultaneously provided is that the torsion spring  34  acts as a reset mechanism to reset the SMA wire  10  after a contraction, indicated by symbol C, which rotates the lead screw  14  about the rotational axis X. In particular, the torsion spring  34  stretches the SMA wire  10  back to an original (initial) position via applying a resetting force, which is indicated by symbol R. The resetting force R is provide via a first end (or portion thereof) of the torsion spring  34  being connected to the second ratchet wheel  18 , and a second end (or portion thereof) of the torsion spring  34  being mounted to the post  24 . Accordingly, rotation of the second ratchet wheel  18  in the first (drive) direction D, which also rotates the first ratchet wheel  16  in the same direction, causes the ends of the torsion spring  34  to move (twist) relative to each other, thereby loading the torsion spring  34  with torque. In this manner, upon removal of the excitation to the SMA wire  10 , the torque of the torsion spring  34  counter-rotates the second ratchet wheel  18  about the post  24 , thereby returning the SMA wire  10  to its initial (pre-contraction) position. 
   As shown by  FIG. 1 , the SMA wire  10  is electrically connected at both ends to a controller  36  via electrical conductors  38 . In use, the application of a drive signal  40 , such as for example, a current pulse from the controller  36 , via the electrical conductors  38 , causes the SMA wire  10  to contract due to excitation from the martensitic state to the austenite state. This contraction C causes the second ratchet wheel  18  as well as the first ratchet wheel  16  to rotate incrementally in the first (drive) direction D about the rotational axis X. 
   As the first ratchet wheel  16  of the ratcheting mechanism  12  is fixed to the lead screw  14 , rotation of the ratcheting mechanism in the first (drive) direction D causes the lead screw  14  to also rotate about the rotational axis X. It is to be appreciated that the lead screw  14  is only free to rotate and is prevented from translating axially. 
   A threaded nut  42  engages the lead screw  14 . The threaded nut  42  is provided with a projection or key portion  44  which is situated in a slot or keyway  46  that is mounted, for example, to the base  30  of the infusion pump  32 , both of which are illustrated in block diagram for ease of illustration. In this manner, the threaded nut  42  is prevented by the keyway  46  from rotating about the rotational axis X with the ratcheting mechanism  12  and lead screw  14  in the first direction D, but is free to translate incrementally along axis X. The incremental advancement of the threaded nut  42  along the keyway  46  causes a plunger  48  to be pushed into a syringe-like drug cartridge or container  50 . The pushing of the plunger  48 , via incremental advancement of the threaded nut  42 , causes a liquid drug to be expelled from the container  50  in a controlled manner. 
   In one embodiment, the controller  36  includes capacitor(s)  52  for storing a charge received form a power source  54 , in which the drive signal  40  is a single charge. The signal charge from the controller  36  causes the incremental rotation of the ratcheting mechanism  12  and lead screw  14 . As mentioned previously above, this rotation advances the plunger  48  into container  50  to dispense a predetermined volume of the drug out of the drug container, such as for example, into the subcutaneous tissue of a patient. A hard stop  56  is provided to ensure that the ratcheting mechanism  12  is incremental rotated a desire amount with each contraction of the SMA wire  10 . In this embodiment, the hard stop  56  is also electrically connected to the controller, via electrical conductor  58 , and provides a motion feedback signal of the ratcheting mechanism  12 . In one embodiment, the hard stop  56  provides the controller  36  the motion feedback signal upon a projection  60  provided by the second ratchet wheel  18  contacting the hard stop  56  due to rotation of the ratcheting mechanism  12  in the first (drive) direction D. In this manner, the hard stop  56  helps to provide accuracy in the dispensing of the liquid drug since accurate displacement of the SMA wire  10  is very difficult to achieve. 
     FIG. 2  shows another embodiment of a pump drive  200  according to the present invention wherein the torsion spring  34  is not fixed between the second ratchet wheel  18  and post  24  as in the previous embodiment illustrated by  FIG. 1 . In this alternative embodiment, the second ratchet wheel  18  is rotated by the attached SMA wire  10  in the first (drive) direction D and counter rotated in the second (reset) direction S by a reset spring  62 . In this manner, the reset spring  62  takes over the function of the torsion spring  34  of applying the reset force R to the second ratchet wheel  18  to bring the SMA wire  10  back to the initial position after a contraction C. For brevity, as the pump drive  200  advances the plunger  48  in the same manner as described above with reference to the pump drive  100  shown by  FIG. 1 , no further discussion is provided about this embodiment. 
     FIG. 3  shows still another embodiment of a pump drive  300  according to the present invention. In this embodiment, the pump drive  300  comprises two ratchet wheels  16 ,  18  similar to the embodiment of  FIG. 1 , except that the SMA wire  10  is not directly connected to the second ratchet wheel  18 . In this alternative embodiment, the SMA wire  10  is oriented parallel to the rotational axis X, instead of perpendicular as in the previous embodiments, and directly connected to a side of a sliding mechanism  64 . The reset spring  62  is connected to another side of the sliding mechanism  64  opposite to the side to which the SMA wire  10  is attached, wherein the torsion spring  34  provides the same function as provided in the embodiment shown by  FIG. 2 . In this manner, the sliding mechanism  64  performs parallel reciprocating motion to the rotational axis X upon contraction C of the SMA wire  10 , and resetting via reset force R being provided by the reset spring  62 . This aligned configuration of the SMA wire  10 , the reset spring  62 , and sliding mechanism  64  allows a compact design since all parts are accommodated close to the ratcheting mechanism  12  and lead screw  14 . 
   To move the ratcheting mechanism  12 , via the parallel reciprocating motion of the sliding mechanism  64 , the second ratchet wheel  18  provides a pin  66  that fits into a S-curve shaped slot  68  of the sliding mechanism  64 . The sliding mechanism  64  is guided parallel to the rotational axis X and when the SMA wire  10  contracts, the sliding mechanism  64  moves axially and the S-curve shaped slot  68  displaces the pin  66  angularly about the rotational axis X. This motion results in an incremental rotation of the ratchet wheels  16 ,  18  and lead screw  14  in the first direction D and therefore in an incremental axial advancement of the threaded nut  42 . For brevity, as the pump drive  300  advances the plunger  48  in the same manner as described above with reference to the pump drive  100  shown by  FIG. 1 , no further discussion is provided about this embodiment. 
   Another embodiment of a pump drive  400  according to the present invention is shown by  FIG. 4 , with the same basic arrangement as the embodiment shown by  FIG. 2 , except for the use of the facing ratcheting surfaces  20  and  22 , the post  24 , and the torsion spring  34 . In this embodiment, the first ratchet wheel  16  is provided with an insert portion  70  providing radially flexible teeth, generally indicated by symbol  72 , and the second ratchet wheel  18  is provided as a sleeve having rigid teeth, generally indicated by symbol  74 , provided radially on an interior surface thereof. In an alternative embodiment, the flexible teeth  72  may be provided to the second ratchet wheel  18  and the rigid teeth  74  may be provided to the insert portion  70  of the first ratchet wheel  16 . 
   As shown, the insert portion  70  is accommodated in the second ratchet wheel  18  such that the flexible teeth  72  is engaged by the rigid teeth  74  when the second ratchet wheel is rotated in the first (drive) direction D, and the rigid teeth  74  will slip pass the flexible teeth  72  when the second ratchet wheel  18  is rotated in the opposite direction via the resetting force R provided by reset spring  62  after a contraction C of the SMA wire  10 . An end cap  76  is mounted rotatably to an end of the insert portion  70  adjacent the second ratchet wheel  18  on the side opposed to the first ratchet wheel  16 , to maintain the teeth  72 ,  74  in axial alignment along the rotational axis X. For brevity, as the pump drive  400  advances the plunger  48  in the same manner as described above with reference to the pump drive  200  shown by  FIG. 2 , no further discussion is provided about this embodiment. 
   In all the illustrative pump drive embodiments shown by  FIGS. 1-4 , the power source  54  is a battery provided to power the infusion pump  32 . The power source (i.e., battery)  54  is held between a pair of electrical terminal posts  78  which are wired to provide power to the controller  36 . Other electrical component, such as for example, an on/off button  90  ( FIG. 5 ) and a small/large dose selection switch  92  ( FIG. 5 ) may also be provided as input to the controller  36  to provide the stated function to the infusion pump  32 . 
   As shown by  FIG. 5 , a pump drive  500  according to any one of embodiments of the pump drive  100 ,  200 ,  300 , and  400  shown by  FIGS. 1-4 , is conveniently used in a miniature drug delivery pump  128 . In the illustrated embodiment, the power source  54  is a battery, such as for example a size AAAA, which is about 42.5 mm long and about 8.3 mm in diameter, weighing around 6.5 grams. Output of alkaline batteries in this size is about 1.5 volts, 625 mA·h. Although elements in the figures may be exaggerated in portion to other components, it is to be appreciated that the approximate relative size between the drug deliver pump  128  and the battery is intended to be shown in the embodiment illustrated by  FIG. 5 . Accordingly, as shown, the miniature drug deliver pump  128  is not much larger than the AAAA battery, and is in one embodiment about 61 mm long, about 32 mm wide, and 15.5 mm in height, and weighs about 32 grams, with the container  50  holding about 2 ml of a liquid drug. Such dimensions of the drug deliver pump  128  is about one third the size of existing conventional drug deliver pumps. The small size of the drug delivery pump  128  due to the reduction in size and components of the pump drive  500  as well as the drive&#39;s silent operation, makes it easier for the patient to conceal the drug deliver pump under clothing. 
   In the illustrated embodiment shown by  FIG. 5 , the drug delivery pump  128  provides a scaled window  94  through which a portion of plunger  48  is visible and by which the patient may use to meter/monitor the delivery of the liquid drug from the container  50 . The container  50  includes an injection site  96  which is used to connect a spike connector  98  of an administration set  102  to the drug deliver pump  128 . The spike connector  98  is connected to a fluid conduit  104  which at the distal end connects to a catheter  106 , which enters the patient&#39;s intravenous system through the skin for delivery of the liquid drug. 
   Although not limited to, some of the noted advantages of the present invention are as follows: the inherent precision of the motion from the shape memory alloy wire actuator arrangement which can be used to accurately deliver very small doses (i.e., about 100 nL), nearly silent operation, fewer moving parts, and smaller parts. Such advantages result in an overall compact and low cost drug delivery pump for the consumer. 
   The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The above embodiments disclosed were chosen and described to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.