Abstract:
An implantable fluid-dispensing device having an improved fluid-retainment portion is disclosed as having a housing having a first end and a second end, wherein at least a portion of one of the first end and second end comprises an expandable material, and a thermally activated component associated with the elastic material within the housing wherein the thermally activated component prevents unwanted fluid flow when the housing is exposed to a temperature increase. The thermally activated component can be a shaped single or bimaterial member that expands upon exposure to a temperature increase, or a valve apparatus that halts fluid delivery. Additionally, the entire device can be made from a thermally activated material such that fluid expansion is accommodated. A method for the prevention of the inadvertent release of fluid from a fluid delivery device that is exposed to a temperature increase is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed generally to fluid delivery devices, and specifically to an improved implantable fluid delivery device.  
           [0003]    2. Background of the Art  
           [0004]    Fluid delivery devices have been in use for years for a variety of different applications. From delivery of industrial fluids, to everyday fluids such as gasoline, these devices all provide the force and regulation necessary to deliver a specific amount of fluid as needed.  
           [0005]    One particularly useful category of fluid delivery devices is implantable fluid-delivery devices for delivering medicament to a patient. Implantable fluid-delivery devices are small, biocompatible pumps that contain a small ampoule or reservoir of medicament, as well as any number of other components to deliver that fluid when needed. The technology contained in these devices can regulate flow rate, delivery time, and whether or not medicament is delivered at all. Once properly implanted, these fluid-delivery devices can deliver pain medication, beneficial medicament, and other necessary fluids according to any number of medical needs.  
           [0006]    Essential to these implantable devices is the predictability and consistency of the drug delivery mechanism. Generally, conventional devices have a number of means to regulate the mechanisms that provide the force to deliver the fluid. For example, conventional fluid delivery devices have incorporated gas-generating cells within the devices to generate a gas to drive fluid out of the device. The operational voltage of the gas-generating cell is altered as a function of time in order to deliver different amounts of fluid. Thus, conventionally, fluid-delivery devices have focused on regulating the force-creating mechanism to ensure consistent fluid delivery.  
           [0007]    Such conventional devices have been deficient in addressing a significant factor in biological systems that can and does affect fluid delivery rates: body temperature. As body temperatures increase due to factors such as fever, diurnal cycles, environmental changes, or physical activity, the temperature of the fluid within an implanted device is likewise increased. The temperature increase causes, among other things, the fluid contained within the device to expand. Since the implanted devices are small, and must by their nature be sealed devices, the expanded fluid forces its way out of the device in an amount not predicted or desired by the fluid-flow regulation of the delivery mechanism. The force of the expanding fluid can deliver as much as an additional 0.1% of fluid volume per degree Celsius increase. Thus, by ignoring fluctuations in body temperature, the consistency and predictability of fluid delivery in implanted devices can be drastically affected.  
           [0008]    It is therefore an object of the present invention to provide an improved implantable drug delivery device that can effectively and efficiently account for temperature increases in surrounding tissues, and that can still deliver a predictable and consistent amount of fluid.  
           [0009]    It is additionally an object of the present invention to provide a device that can react to temperature increases within the surrounding environment, and within the device itself, so as to prevent any additional, unwanted drug delivery.  
           [0010]    These and other objects will become apparent to one of ordinary skill in the art in light of the present specification, claims and drawings appended hereto.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is directed to a thermally equilibrated fluid delivery device that is capable of delivering consistent fluid delivery rates over an extended period of time as well as a method for the controllable release of a fluid from the device. The device has an improved fluid control means so as to allow for the consistent delivery operation of device, despite variances in environmental conditions such as changes in temperature. As described herein, the device is shown as having a housing with fluid reservoir, a first end, a second end, and one or more delivery holes, with the second end preferably having an expandable portion. The fluid-dispensing device preferably contains a fluid that will be delivered by the device, and which, when exposed to a temperature increase, will expand in size and affect the operation of the device. The device, however, also has a fluid control means, such as a thermally activated component, which enables the device to prevent inconsistent and unwanted fluid flow due to a fluctuation in external temperature. The thermally activated component will prevent any negative influence of the fluid expansion or contraction on the operation of the device by expanding the volume within the device to accommodate the fluctuating fluid size.  
           [0012]    In a preferred embodiment of the present invention, the fluid control means includes a thermally activated component associated with a portion of the housing that is expandable. The combination of the component and the expandable portion prevents unwanted fluid flow by increasing the overall volume contained within the housing when it is heated up. The volume is increased by the thermally activated component expanding into the expandable portion to extend that portion outward from the housing and into the surrounding environment.  
           [0013]    Preferably, the thermally activated component comprises a material having a coefficient of thermal expansion comparable to the fluid. Alternatively, the thermally activated component could be made up of two or more materials affixed to one another, wherein each of the materials would have a different coefficient of thermal expansion relative to one another, but where the overall thermal expansion coefficient of the component would still be comparable to the fluid. The differences in thermal expansion coefficients between the two affixed materials would allow the thermally activated component to alter its shape in a desired direction once exposed to a temperature increase. If a multi-material component is utilized, the materials may be selected from any of a polymer, a ceramic, a metal, or a combination of those materials. Preferred shapes of the component include a disc, a cantilever, a spiral, a helix, a beam, or a plate.  
           [0014]    In the embodiment where the material is in the shape of a disc, it is preferred that the materials have a top sheet and a bottom sheet, and that each sheet is of approximately 4 thousandths of an inch in thickness. Alternatively, additional sheets could be added to the disc, as needed. Preferably, two or more discs are stacked together to form a stack, which multiplies the effects of the expansion of each disc. Stacks could have any number of discs, but preferably would have at least 10 discs to at least 100 discs.  
           [0015]    Alternatively, the thermally expandable material can be a single material in, for example, the shape of a beam.  
           [0016]    The housing of the device additionally has a delivery hole to allow the flow of the drug from inside the housing to the external environment. In one embodiment, the thermally activated component includes a valve apparatus which, when the component is activated, will come into contact with and close off the delivery hole. Alternatively, the valve could simply begin to curtail the flow of the fluid out of the device, as needed.  
           [0017]    In one other preferred embodiment, the fluid control means could be the housing of the device itself. In this embodiment, the housing expands in response to a temperature increase so as to increase the volume within the housing when needed. Preferably, the housing has a thermal expansion coefficient that is at least comparable to the coefficient of the fluid within the housing so that it expands at a rate comparable to the expansion rate of the fluid within the device. The comparison of the two expansion rates does not have to be exact, but could be greater or lesser for the housing than for the fluid, depending upon the desired application.  
           [0018]    Any of the above embodiments may be utilized to prevent the unwanted flow of fluid from the device upon exposure of the device to an increase in temperature. The flow may be prevented by increasing the overall volume contained within the housing by the use of a thermally activated component, or by making the housing out of a thermally expandable material. Alternatively, the thermally activated component could include a valve mechanism that would seal off the flow of fluid from the device.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a plan view of a drug delivery device according to the present invention;  
         [0020]    [0020]FIG. 1 a  is a plan view of a drug delivery device according to the present invention;  
         [0021]    [0021]FIG. 2 a  shows a perspective view of a single bimaterial disc in a relaxed position as described in the present invention;  
         [0022]    [0022]FIG. 2 b  shows a side elevational view of a single bimaterial disc in a relaxed position as described in the present invention;  
         [0023]    [0023]FIG. 2 c  shows a side elevational view of a stack of bimaterial discs in relaxed positions as described in the present invention;  
         [0024]    [0024]FIG. 2 d  shows a perspective view of a single bimaterial disc in an expanded position as described in the present invention;  
         [0025]    [0025]FIG. 2 e  shows a side elevational view of a single bimaterial disc in an expanded position as described in the present invention;  
         [0026]    [0026]FIG. 2 f  shows a side elevational view of a stack of bimaterial discs in expanded positions as described in the present invention;  
         [0027]    [0027]FIG. 3 a  shows a plan view of a bimaterial cantilever as described in the present invention;  
         [0028]    [0028]FIG. 3 b  shows a plan view of a bimaterial simple rod as described in the present invention;  
         [0029]    [0029]FIG. 3 c  shows a plan view of a bimaterial unshaped rod as described in the present invention;  
         [0030]    [0030]FIG. 3 d  shows a plan view of a bimaterial simple coil as described in the present invention;  
         [0031]    [0031]FIG. 3 e  shows a plan view of a bimaterial helix coil as described in the present invention;  
         [0032]    [0032]FIG. 4 shows a plan view of a drug delivery device according to an alternative embodiment of the present invention;  
         [0033]    [0033]FIG. 5 shows a plan view of a drug delivery device according to another alternative embodiment of the present invention;  
         [0034]    [0034]FIG. 6 shows a plan view of a drug delivery device according to another alternative embodiment of the present invention; and  
         [0035]    [0035]FIG. 7 shows a plan view of a mathematical representation of the area under a disc, as discussed relative to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    While this invention is susceptible of embodiment in many different forms, there is shown in the drawings, and will be described in detail, several specific embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated.  
         [0037]    The present invention is shown and described herein as comprising a thermally equilibrated fluid delivery device, and a thermal equilibrator for the device. Generally, the equilibrator is best utilized with fluid-delivery devices implanted in human beings, such as implanted pharmaceutical pumps or the like. It should be noted, however, that the teachings of the present specification and claims may similarly be transferred to any number of external fluid delivery systems in which the device and the fluid within the device undergo expansion due to external thermal changes. For the sake of clarity and consistency throughout the specification and claims, however, the present invention will be discussed in relation to an implantable device. As will be explained in further detail below, any fluid delivery device outfitted with the teachings of the present invention will ensure constant and/or predictable fluid delivery rates, even after sudden temperature increases.  
         [0038]    The present invention is best shown in relation to the enclosed drawings. One embodiment of the invention is shown in FIG. 1, which depicts thermally equilibrated delivery device  10  comprising housing  12  containing fluid reservoir  22 , delivery mechanism  24 , plug  26  and fluid control means  29 . Housing  12  is shown as a substantially cylindrical vessel with closed first end  14  and closed second end  16 . As would be readily understood to those having ordinary skill in the art, housing  12  may have numerous alternative geometric configurations, such as a disk, cylinder, bellows, sphere, tube, block, etc.—such as would be needed for specific medical applications or implantations.  
         [0039]    Housing  12  is preferably constructed from a rigid or semi-rigid biocompatible material such as stainless steel, titanium, other metals, polymers, ceramics, composites, etc. Once constructed, housing  12  provides a secure containment area for medicament and/or other fluids, so that the fluid is maintained in a pristine condition even if the device is implanted into a human body. Additionally, housing  12  provides an enclosure for the delivery apparatus of drug delivery device  10 .  
         [0040]    First end  14  and second end  16  of housing  12  comprise substantially identically shaped enclosures sealing the inside of housing  12  from the external environment surrounding device  10 . First end  14  is associated with delivery mechanism  24 , which, as will be described further below, drives fluid from fluid reservoir  22  toward and out of delivery hole  20  located in second end  16  of device  10 . In order to help force the fluid out of the device, first end  14  is constructed from a rigid or semi-rigid material, allowing delivery mechanism  24  to operate effectively. Second end  16 , on the other hand, includes expandable member  18 . An expandable member may comprise any number of conventional devices that expand upon the application of force to the device. It is preferred, in this case, that at least a portion of the structure of second end  16  be constructed from an elastic material such as rubber, called elastic portion  18 . Alternatively, expandable member could comprise an accordion shape. In any case, elastic portion  18  is stretched outward by thermally expanded thermal equilibrating member  30  (described further below), which in turn creates a greater volume within housing  12 .  
         [0041]    As will be explained in greater detail, delivery hole  20  enables release of fluid, from fluid reservoir  22  out of device  10  upon operation of delivery mechanism  24 . Generally, delivery hole  20  comprises an opening between interior of housing  12  and the external environmental (e.g. tissue) surrounding device  10 . In a preferred embodiment, hole  20  is open continuously, with the pressure difference and flow between the interior of housing  10  and the surrounding bodily tissue through a long and/or tortuous pathway acting as a barrier to the influx of fluids into housing  10 . Upon activation of mechanism  24 , fluid escapes out of housing  10  through hole  20 .  
         [0042]    Alternatively, device  10  could have more than one delivery hole  20 . It may be necessary to deliver two separate fluids simultaneously, or to deliver a fluid to two different locations in the body, and as such, device  10  may require additional fluid outlets. It is contemplated that the teachings of the present invention may be utilized in conjunction with both the single outlet devices (which will be described herein throughout the specification), and with multiple outlet configurations, as needed.  
         [0043]    Contained within housing  10  are the common components of a drug delivery device, including fluid reservoir  22 , delivery mechanism  24 , and plug  26 . Fluid reservoir  22  comprises a containment area for a fluid/medicament to be delivered by device  10 . Reservoir  22  may comprise a structure as simple as the interior space of housing  10  located between plug  26  and first end  14 , or may comprise any number of conventional fluid-containment vehicles. Preferably, reservoir  22  comprises a fixed-volume polymer-walled section of housing, associated with delivery mechanism  24  on one end, and plug  26  on the other.  
         [0044]    Delivery mechanism  24  comprises one of any number of conventional fluid-delivery components, such as compressed gas or propellant, osmotic engine, electromechanical drive, etc. These devices provide a steady motivational force to drive fluid out of reservoir  22 , to and through plug  26 , and out of hole  20  in second end  16 . As is known in the art, the delivery mechanism  24  may also be adjusted via conventional means, such as directly through a surgical procedure, or transdermally through RF reprogramming signals, to alter the specific delivery rate as needed.  
         [0045]    Plug  26  is shown in FIG. 1 in its preferred embodiment, having channel  28  running circumferentially in a helical fashion around plug  26  from reservoir  22  to second end  16 . Plug  26  separates second end  16  from fluid reservoir  22 , and helps to control the delivery rate of fluid out of fluid reservoir  22 . Channel  28  consists of a shallow groove connecting fluid reservoir  22  to second end  16 , which surrounds plug  26  in a generally spiral shape. Of course, alternative pathways could also be used. The depth and width of channel  28  can be adjusted, as needed, to control the flow rate of fluid out of fluid reservoir  22 .  
         [0046]    All of the components of device  10 , including fluid reservoir  22 , delivery mechanism  24 , and plug  26  with channel  28 , are configured for operation at ideal operational conditions. In other words, under controlled operating conditions, the components deliver a predictable and measured amount of fluid as desired and needed. There are several environmental conditions, however, that can alter the delivery rate, without these standard components changing their operating parameters at all. The present invention is directed towards ensuring that the predictable and measured fluid flow is maintained during the entire operation of device  10 , specifically during changes in temperature in the environment surrounding device  10 .  
         [0047]    In order to achieve a steady and predictable fluid flow, the present device includes fluid control means  29 , which is used in association with the previously described components of fluid delivery device  10  to control the flow of thermally-expanded fluid out of device  10 . In one preferred embodiment shown in FIG. 1, fluid control means  29  comprises a thermal equilibrating member  30  associated between plug  26  and second end  16  of housing  12 . Thermal equilibrating member  30  cooperates with expandable member  18  of housing  12  to, in response to an external temperature change, create additional volume within housing  12  so as to accommodate thermal expansion of the fluid within fluid reservoir  22 .  
         [0048]    Thermal equilibrating member  30  comprises one or more pieces of material having coefficients of thermal expansion such that member  30  begins to expand upon exposure to a temperature increase. Preferably, thermal equilibrating member  30  comprises a thermal expansion coefficient that is comparable to the fluid contained within fluid reservoir  22 , so as to maximize the efficiency of operation of the device. As temperature increases within a body, for example due to fever or sickness, heat from the body is thermally transferred to device  10 , and, in turn, into the fluid within fluid reservoir  22 . Such a transference of heat results in an increase in the temperature of the fluid within the fluid reservoir  22 . Accordingly, this increase in fluid temperature further results in an expansion of the fluid that, in turn, causes unexpected, inconsistent and unwanted release of fluid out of device  10 . Similar effects occur upon cooling with the results being a smaller volume of fluid being delivered or no fluid delivery at all.  
         [0049]    Thermal equilibrating member  30  accommodates the expanding fluid by likewise expanding in response to a temperature increase. Associated at or near second end  16  of housing  12 , thermal equilibrating member  30  expands in a direction toward and into elastic portion  18 —thereby forcing elastic portion to deflect outward, which, in turn, results in an increasing in the overall volume within housing  12 . This increase in volume is shown in FIG. 1 wherein V 1  represents the volume of housing  12  when thermal equilibrating member  30  is in its unexpanded or relaxed state. In this state, thermal equilibrating member  30  does not impact into elastic portion  18  directly. Therefore, housing  12  volume remains static.  
         [0050]    As can be seen in FIG. 1 a,  V 2  represents the volume of housing  12  when thermal equilibrating member  30  is in an expanded state. In that state, thermal equilibrating member pushes into elastic portion  18 , expanding that part of second end  16  of housing  12  outward. The outward expansion creates additional volume within the housing in order to accommodate the increased volume of the enclosed fluid, so that such fluid is retained within housing  12  until operationally pumped out.  
         [0051]    Expansion of thermal equilibrating member  30 , and, in turn, expansion of the housing volume is accomplished by fabricating the thermal equilibrating member from certain materials, in certain shapes, as will be described further below. Preferably, the expansion of thermal equilibrating member  30  is caused by either the thermal expansion differences in a bimaterial member, or by thermal expansion of a single-material member in a specific shape.  
         [0052]    In a preferred embodiment of the present invention, the thermal equilibrating member  30  is fabricated from two materials that are selected for their expansion properties. Specifically, thermal equilibrating member  30  is shown in FIGS.  1 - 2   f  as comprising one or more disc shaped members  32 . In a preferred embodiment, discs  32  comprise a first material  34  affixed to a second sheet of material  36  to form a substantially planar, circular piece. First material  34  and second material  36  comprise materials having a low and a high coefficients of thermal expansion, respectively, causing second material  36  to thermally expand faster than first material  34  when exposed to a common temperature increase. The difference in expansion between first material  34  and second material  36  causes the shape of the associated discs (shown in FIGS. 2 d - 2   e ) to expand from a substantially relaxed configuration (see FIGS. 2 a - 2   c ) to an expanded orientation, such as shown in FIGS. 1 and 2 d - 2   f,  creating volume  35  underneath. As described above, this expanded orientation causes thermal equilibrating member  30  to deflect into elastic member  18 , deflecting the member  18  outward. This deflection, in turn, increases the overall volume within housing  12  from V 1  (FIG. 1) to V 2  (FIG. 1 a ).  
         [0053]    Importantly, the general shape of the deflection of first material  34  and second material  36 , as well as the volume  35  created thereby, material can be accurately predicted using a number of well-known equations, and empirically collected data. Examples of such calculations have been extracted from  Roark&#39;s Formulas For Stress  &amp;  Strain  (Young, W. C.,  Roark&#39;s Formulas for Stress  &amp;  Strain  pp. 446 (6 th  Ed., 1989) and are reproduced herein for convenience as Table I.  
                     TABLE I                       From Table 24, Case 15                                Legend:       Young&#39;s Modulus − E       Coeff. Of Thermal Expansion − γ or α       Poisson&#39;s Ratio − v       Zero Strain Temp. ≡ T o                         Substitute                     6        (       γ   b     -     γ   a       )          (     T   -     T   O       )          (       t   a     +     t   b       )          (     1   +     v   e       )           t   b   2          K     1      p                         for                 the                 term                   γ        (     1   +   v     )                       ΔT   t                                                     K     1      p       ≡     4   +     6        (       t   a       t   b       )       +     4          (       t   a       t   b       )     2       +         E   a            t   a   3          (     1   -     v   b       )             E   b            t   b   3          (     1   -     v   a       )           +         E   b            t   b          (     1   -     v   a       )             E   a            t   a          (     1   -     v   b       )                                                   Now Replace D with D e                           D   e     =           E   a          t   a   3         12        (     1   -     v   a   2       )              K     2      P                                                         Where                   K     2      p         =     1   +         E   b            t   b   3          (     1   -     v   a   2       )             E   a            t   a   3          (     1   -     v   b   2       )           +       3        (     1   -     v   e   2       )            (     1   +       t   b       t   a         )     2          (     1   +         E   a          t   a           E   b          t   b           )             (     1   +         E   at          t   a           E   b          t   b           )     2     -       (       v   a     +       v   b              E   a          t   a           E   b          t   b             )     2                                                           Replace                 v                 with                   v   e       =       v   a            K     3      p         K     2      p                                                           Where                   K     3      p         =     1   +         v   b          E   b            t   b   3          (     1   -     v   a   2       )             v   a          E   a            t   a   3          (     1   -     v   b   2       )           +       3        (     1   -     v   e   2       )            (     1   +       t   b       t   a         )     2          (     1   +         v   b          E   a          t   a           v   a          E   b          t   b           )             (     1   +         E   at          t   a           E   b          t   b           )     2     -       (       v   a     +       v   b              E   a          t   a           E   b          t   b             )     2                                                           In                 Case                 15        :                     y   c       =         -   γΔT       2      t            [       a   2     -     r   0   2     -         r   0   2          (     1   +   v     )          ln                   a     t   0           ]                                             If we let r 0  ≡ 0, assuming uniform temperature, then                                 y   c     =           -       γΔTa   2       2      t                           (     1   +   v     )       (     1   +   v     )         ⇒     y   c       =     -         a   2       2        (     1   +     v   c       )              [         γ        (     1   +   v     )          ΔT     t     ]                       ↖                       Replace                 as                 above                                                        
 
         [0054]    Utilizing these equations, we can calculate the angle of the edge, area under the disc, and volume under the disc, as follows:  
               Angle   :     Θ   2       =       a     (     1   +     v   e       )            [         γ                 Δ                 T     t          (     1   +   v     )       ]                     Area   :   A     =           (     d   Θ     )     2          Cos        [       (       d   Θ     -   y     )       d   Θ       ]         -       (       d   Θ     -   y     )              2        d   Θ        y     -     y   2                           Volume   :   V     =       1   3        π                     y   2          (         3      d     Θ     -   y     )                                     
 
         [0055]    Wherein theta, y and d determine the system dimensions as shown in FIG. 7  
         [0056]    As can be seen, the total deflection of the discs, and the area/volume  35  created under the disc, can be predicted based on the height of the deflection and the angle of deflection, which can in turn be predicted based upon empirically determined constants such as the coefficient of thermal expansion. The constants are accessible from a number of sources, including material handbooks or literature from material suppliers. Some common materials and their constants are listed below.  
                                                     TABLE II                                   COEFF OF                       THERM EXP.           COM-   YOUNG&#39;S   (×10 −6 ° F. −1 )   POISSON&#39;S       NAME   POSITION   MODULUS   α   RATIO                                Ti6Al4V       16.5 MPSI   5.3   0.33       Titanium   CP   15.0 MPSI   5.3   0.33       316L S.S.   Cr 16-18,   28   8.9   0.305?           N: 10-14           Mo 2-3,       Nitinol   50 N:, T:       Aust.       12   11   0.33       Mart.       6   6.6   0.33       Tatalum       27   3.6   0.35       Niobium       15   4.1   0.38       Tungsten       59   2.5   0.28       Zirconium   Zr 702   14.4   3.2   0.34           Uni Alloy Co.       Cobalt   Co35, M: 35,   33.6   8.7       Alloy   Cr20, Mo10       Gold   99.5%-100%   12   7.9   0.42       Platinum   99.85%   25   4.9   0.39       Silver       11   10.9   0.37                  
 
         [0057]    [0057]                                             GROUP BY α (w/e)                Low α   High α                       Tungsten (59)   Nitinol-Aust. (12)           Zirconium (14.4)   Silver (11)           Tantalium (27)   316L S.S. (28)           Niobium (15)   Cobalt (33.6)           Platinum (25)   Gold (12)           Titanium (15)   Nitionl-Mart. (6.6)           Ti6Al4V (16.5)                        
         [0058]    It should be noted that, preferably, any bimaterial member is comprised of two corresponding materials, with one having a higher thermal expansion coefficient, compared to the other bimaterial. Typically, such materials comprise two metals, such as silver and platinum, or gold and platinum. It is possible, however, to have one or more of the materials comprise a ceramic material or a plastic material. As can be seen from the calculations, the important factors are the difference in thermal expansion coefficients between the two materials, and the similarity in the Young&#39;s Modulus. If a combination of materials other than metal/metal is utilized, care should be taken in selecting the bonding agents, as the agent may affect the expansion relationship between the two materials.  
         [0059]    In the following sections of this disclosure, certain elements of the present invention have been identified by primes. The primes have been included in the numbering system so as to more clearly define the relationship between identical elements in the drawings. They are not being used to identify elements of the present device having differing properties. For example, as will be discussed below, first material  34  and first material  34 ′ comprise the same element for purposes of the invention. This convention is continued throughout the specification and the drawings.  
         [0060]    Based on the projection of volume increase due to thermal expansion in a bimaterial disc, it is possible to configure a device that is capable of increasing the volume of the device described above as needed. Preferably, and as shown in FIGS. 2 c  and  2   f,  thermal equilibrating member  30  comprises two or more discs  32  forming stack  38 , wherein the stack comprises alternating pairs  37  of bimaterial discs. Each disc  32  of each pairing  37  comprises a first material  34  and a second material  36 . Preferably, discs  32  of each pairing  37  are aligned with the first material  34  of one disc  32  facing the first material  34 ′ of the next disc  32 ′.  
         [0061]    Upon exposure to a temperature increase, the pairs  37  of discs  32 ,  32 ′ will deflect from normal as anticipated by the above equations, forming expanded cavity  40  therebetween. Cavity  40  is formed by the combination of the volume  35  of one disc  32  in a pairing  37 , and the volume  35 ′ of the adjacent disc  32 ′, upon thermal expansion of each disc  32 ,  32 ′. Cavity  40  provides a reservoir for the retention of expanded fluid within housing  12 , in addition to providing a means to force thermal equilibrating member  30  into elastic portion  18 , pushing it outward to help accommodate the additional volume created by cavity  40 .  
         [0062]    As will be explained further below, as fluid passes through channel  28  of plug  26 , it passes into second end  16  of housing  12 , within which discs  32  are located. To accommodate the passage of fluid through discs  32 , each disc  32  additionally includes bore  42  passing through both first material  34  and second material  36 , providing a fluidic pathway through disc  32 . Each bore  42  of each disc  32  is aligned so that when discs  32  are in a relaxed position (FIG. 2 c ), the bores  42  create a fluid channel through an entire stack  38 . Thus, as the fluid enters second end  16  of housing  12 , it enters bore  42  for passage through stack  38 , and delivery through exit port  20 .  
         [0063]    In an expanded position (as in FIG. 2 f ), bore  42  provides access to cavity  40 , as well as providing a fluidic pathway through stack  38 . Therefore, as fluid enters second end  16 , if stack  38  is in a thermally expanded position, fluid will enter stack  38  through bore  42 , and accumulate in cavity  40 . As the bores  42  will still be substantially aligned, however, fluid will accommodate in one cavity  40  of one pairing  37  until full, and then flow into the next, adjacent cavity  40 ′. Once all of the cavities  40  of pairings  37  are full, fluid will be delivered out of exit port  20 , to the surrounding tissue. Thus, the expansion of thermal equilibrating member  30  accommodates the expansion of fluid out of fluid reservoir  22  by pooling the fluid in cavity  40 , without interrupting the normal delivery functions of device  10 .  
         [0064]    Although the above discussion has been directed to the embodiment of the present invention in which the thermal equilibrator member  30  comprises a disc shape, there are a number of other bimaterial shapes that could similarly provide an increase in volume within the device by thermally expanding into, and stretching outward, the elastic portion  18  of device. For example, and as shown in FIGS. 3 a - e,  thermal equilibrator member  30  could comprise a cantilever ( 3   a ), simple beam ( 3   b ), u-shape ( 3   c ), coil ( 3   d ), or helix coil ( 3   e ). Of course, other shapes that provide the same function could also be used, without deviating from the scope of the present invention.  
         [0065]    The above embodiments are based on the deflection caused in a piece of material when that material is comprised of two materials having varying coefficients of thermal expansion. This deflection could similarly be achieved through the addition of extra layers, such as a third or a fourth layer. For example, a stack of layers having an increasing coefficient of thermal expansion could be used so that, upon an increase in temperature, a more severe or less severe deflection could be achieved.  
         [0066]    In an alternative embodiment, fluid control means  29  comprises a thermal equilibrating member  30  that is formed from a single material. Preferably, the material has a thermal expansion coefficient comparable to the fluid contained within fluid reservoir  22  so as to maximize the efficiency of the operation of the device. In this embodiment, shown in FIG. 4, thermal equilibrator member  30  comprises a single rod-like structure placed between second end  16  of housing and plug  26 . As the temperature within the housing is increased, thermal equilibrating member  30  expands stretching elastic portion  18  of second end  16 , to, in turn, create additional volume within housing  12 .  
         [0067]    Each of the above embodiments operates in essentially the same manner. In operation, device  10  is implanted into a subject, such as a human being, with the conventional mechanisms and programming necessary to deliver an amount of fluid contained within fluid reservoir  22  over a set period of time. Under standard conditions, fluid is delivered from fluid reservoir  22  by delivery mechanism  24 , driving fluid out of reservoir  22  and into contact with plug  26 . Fluid then enters channel  28  of plug  26 , wherein the dimensions and path of channel  28 , in combination with the force provided by delivery mechanism  24 , dictate the rate of fluid flow through plug  26 . Fluid exits channel  28 , enters second end  16  of housing, and passes through thermal equilibrating member  30 , whereafter fluid is delivered to the surrounding environment through delivery hole  20 .  
         [0068]    After implantation and during normal operation, device  10  may be exposed to a variance in environmental temperature due to, for example, a fever or increased metabolic rate due to physical activity. As the environmental temperature increases, device  10  is heated up, expanding the fluid contained within fluid reservoir  22 . At the same time, thermal equilibrating member  30  of device  10  is also heated up, causing thermal equilibrating member  30  to likewise expand. As thermal equilibrating member  30  expands, it makes contact with elastic portion  18  of second end  16 , stretching that portion outward and into the surrounding environment—thereby increasing the total volume within housing  12 . Due to this increased volume, the additional fluid pushed through plug  26  by the increased volume of fluid is allowed to accumulate in second end  16  of housing  12 , without delivering additional fluid to the surrounding environment.  
         [0069]    If the elevated environmental temperature is maintained for a period of time, the fluid expansion and the expansion of the thermal equilibrating member  30  will eventually abate, with both the fluid and the thermal equilibrating member  30  reaching an equilibrated, expanded shape. Thermal equilibrating member  30 , in its final expanded shape, has created additional volume within housing  12  at or near second end  16 , in which the expanded fluid is retained. Delivery mechanism  24 , however, continues to operate throughout this process. Although the additional volume created by thermal equilibrating member  30  is intended to encompass the increased volume of the expanded fluid, it is not intended to overcompensate for that volume. To that end, thermal equilibrating member  30  allows for the free flow of fluid there through via, for example, bore  40  or other means for allowing free fluid flow. Therefore, the fluid is continually delivered through delivery hole  20  at the same rate as before thermal expansion, despite the expansion process of the fluid and thermal equilibrating member  30 .  
         [0070]    Some time after expansion is complete, the temperature of the surrounding environment will eventually cool. For example, either the fever of the patient could break, or the metabolic rates of the body could slow as physical activity decreases. As the environment surrounding device  10  cools, so too will the fluid within device  10 , and thermal equilibrating member  30 . Due to the sealed nature of reservoir  22 , as the fluid cools and contracts in size, a vacuum-like effect is caused within reservoir  22 . Fluid that is retained in the expanded volume in first end  14  caused by thermal equilibrating member  30  is pulled back into reservoir  22  instead of being delivered directly through delivery hole  20 . The amount of fluid returned into reservoir  22  will be proportional to the temperature decrease, so that normal delivery operation can continue. Additionally, as thermal equilibrating member  30  is cooled, and returns to a relaxed state, the elastic portion  18  will similarly retract so that the expanded volume within second end  16  is also decreased, allowing for efficient operation of device throughout both the heating and cooling processes.  
         [0071]    In an alternative embodiment of the present invention, it may be desirous to halt the flow of fluid completely upon exposure to a temperature increase. In such an embodiment, fluid control means  29  comprises a thermal equilibrating member  30  intended to completely halt the flow of fluid out of device  10  upon exposure to a temperature increase. In order to stop the flow of fluid, thermal equilibrating member  30  acts as a fluid valve in this embodiment, actually sealing off delivery hole  20  upon an increase in system temperature. As shown in FIG. 5, thermal equilibrating member  30  can comprise valve  42 , which is associated with bimaterial piston  44 . As with the structures explained above, piston  44  will undergo a deflection upon an increase in temperature, and the degree of deflection can be predicted based upon temperature increases and known empirical information. The deflection will push piston  44  and valve  20  into contact with exit port  20 , sealing off port  20  and therefore fluid flow out of device  20 . Exit port  20  can be a portion of a solid housing, with second end  16  of device no longer including the expandable portion  18 , or exit port  20  may be associated with an expandable portion  18 . Valve  20  should remain in contact with exit port  20 , in any case, throughout operation.  
         [0072]    Piston  44  is shown in FIG. 5 in one preferred shape as a bimaterial helix, but may additionally comprise any number of shapes, including, but not limited to those shapes specifically highlighted above.  
         [0073]    As device  10  of FIG. 5 undergoes a temperature increase due to environmental temperature conditions, piston  44  begins to expand. Upon expansion, piston  44  deflects upward, contacting valve  42  with hole  20 . Thereafter, as fluid in reservoir  22  expands, it enters second end  16  of device  10 , where elastic portion  18  of second end  16  expands to accommodate the additional volume. Piston  44  continues its expansion throughout the entry of fluid into second end  16 , ensuring that valve  42  remains in contact with hole, sealing it. Therefore, no fluid should be delivered from device while the temperature of the surrounding environment is elevated.  
         [0074]    Alternatively, thermal equilibrating member  30  could comprise a single disc  32 , or stack  38  of discs, wherein the bore  42  therethrough is misaligned with exit port  20 . In such an embodiment, a space between discs  32  (in a relaxed position) and exit port  20  enables fluid flow through discs  32  and into the space, and thereafter out of exit port  20 . Upon expansion of thermal equilibrating member  30 , however, bore  42  would move into contact with elastic portion, and not exit port  20 , fluidically sealing bore  42 . Thus, flow would be prevented throughout the high temperature operation.  
         [0075]    An additional alternative embodiment is shown in FIG. 6, wherein device  10  is shown including housing  12  having first end  14  and second end  16 , reservoir  22 , delivery mechanism  24  and plug  26 . Fluid control means  29  comprises the housing  12  being constructed entirely from a material having a thermal expansion coefficient such that housing  12  begins to expand in response to a temperature increase. Preferably, the thermal expansion coefficient is comparable to the fluid to be enclosed in housing  12 . Device  10  does not include thermal equilibrating member  30  in second end  16 , however. The material of housing  12  helps device  10  to accommodate the increased volume of fluid in reservoir  22  upon exposure to a temperature increase.  
         [0076]    In operation, device  10  of FIG. 6 is implanted into a location such as a human body for the delivery of medicament to the patient. Upon exposure to a temperature increase in the surrounding environment, fluid in reservoir  22  begins to expand. Simultaneously, however, housing  12  begins to expand in an outward direction also, increasing the total volume within housing  12 . As the thermal expansion coefficients of housing  12  and the fluid within reservoir  22  are comparable, the expansion of housing  12  should similarly be comparable to the expansion of fluid. Therefore, the increase in fluid volume will be accommodated by the increase in housing  12  volume, ensuring that no additional fluid is forced out of device  10  due to the thermal increase.  
         [0077]    The foregoing description merely explains and illustrates the invention and the invention is not limited thereto except insofar as the appended claims are so limited, as those skilled in the art who have the disclosure before them will be able to make modifications without departing from the scope of the invention.