Abstract:
A two-way actuated shape memory composite material is provided. The composite material includes a shape memory alloy and an elastic metal. The composite material takes a first shape at a lower temperature and a second shape at a higher temperature. At the higher temperature, the shape memory alloy has a “remembered” shape, causing the composite material to take the second shape. The elastic material provides the composite material with elastic properties which cause the composite material to return to the first shape when cooled to the lower temperature.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to two-way actuators. Specifically, the present invention relates to two-way thermal actuators comprising a shape memory alloy, such as nitinol.  
       BACKGROUND OF THE INVENTION  
       [0002]     Shape memory alloys (SMA) are alloys that exhibit the ability to return to a specific shape when brought to a certain temperature. Materials that exhibit shape memory thus have the ability to “remember” and return to a specified shape.  
         [0003]     Nitinol, a class of nickel-titanium alloys, is well known for its shape memory properties. As a shape memory material, nitinol is able to undergo a reversible thermoelastic transformation between certain metallurgical phases. Generally, the thermoelastic shape memory effect allows the alloy to be shaped into a first configuration while in the relative high-temperature austenite phase, cooled below a transition temperature or temperature range at which the austenite transforms to the relative low-temperature martensite phase, and deformed while in the martensitic state into a second configuration. When heated, the material returns to austenite such that the alloy transforms in shape from the second configuration to the first configuration. The thermoelastic effect is often expressed in terms of the following transition temperatures: M s , the temperature at which austenite begins to transform to martensite upon cooling; M f , the temperature at which the transformation from austenite to martensite is complete; A s , the temperature at which martensite begins to transform to austenite upon heating; and A f , the temperature at which the transformation from martensite to austenite is complete.  
         [0004]     Two-way actuation using SMAs is currently achieved in one of two ways. As an example of the first way, a single shape memory alloy is coupled to an elastic bias spring, as shown in  FIGS. 1A and 1B . In  FIG. 1A , at a lower temperature, which is equal to or less than M f , the nitinol spring  10  is compressed by the elastic spring  20 . As the temperature is raised to a temperature equal to or greater than A s , the nitinol spring  10  starts to expand. In  FIG. 11B , at a higher temperature, which is equal to or greater than A f , the nitinol spring  10  takes on the shape as illustrated, compressing the elastic spring  20 . If the temperature is then lowered to a temperature equal to or less than M s , the nitinol spring  10  starts to compress. When the temperature lowers so that it is again equal to or less than M f , the nitinol spring  10  is again fully compressed by the elastic spring  20 , as shown in  FIG. 1A .  
         [0005]     In both  FIGS. 1A and 1B , the combined spring assembly needs to be constrained by a rigid constraint  50 . Rigid constraint  50  has two ends for affixing to opposite ends of the spring assembly as well as a side support to prevent lateral movement of the spring assembly that would otherwise occur due to compression of the spring assembly between the two end constraints. One problem with this arrangement is the size of the assembly, which due to the necessity of constraining the two springs, may only be scaled down to a limited degree.  
         [0006]     The second way of achieving two-way actuation is to laboriously train a SMA material. However, this training may require on average as many as twenty (20) heating, cooling, and constraint cycles. Therefore, since the processing is difficult and has yet to be fully perfected, limited commercial application has been-found for this type of two-way actuation.  
         [0007]     SMA materials and specifically nitinol have been applied to numerous applications. For example, nitinol has been used for applications such as fasteners, couplings, heat engines, and various dental and medical devices. Owing to the unique mechanical properties of nitinol and its biocompatibility, the number of uses for this material in the medical field has increased dramatically in recent years and would increase further if an easier way of forming a two-way actuated SMA can be found.  
       SUMMARY OF THE INVENTION  
       [0008]     If a better way to form a two-way actuated SMA can be found, the possible uses are infinite. For example, any application that requires an actuated device may use a two-way actuated SMA. The present invention provides a two-way actuated composite material, which may be used in numerous actuator systems. In one embodiment of the present invention, a two-way actuated composite material is provided. The composite material comprises a first component comprising a first shape memory alloy, and a second component, which may be selected from the group consisting of a second shape memory alloy, stainless steel, cobalt alloy, refractory metal or alloy, precious metal, titanium alloy, nickel superalloy, and combinations thereof, where the composite material forms a first shape at a temperature equal to or above A f  of the first component and the composite material forms a second shape at a temperature equal to or below M f  of the first component. The first component and second component may be fabricated together to form a metallurgical bond between them by working and/or heating. The second component is elastically deformable, and, during use of the actuator, the second component is elastically deformed between the second shape and the first shape. The two-way actuator may be constructed so that the elastic limit of the second component is not exceeded in the first shape, so that the spring properties cause the two-way actuator to return to the second shape upon cooling to the proper temperature.  
         [0009]     In another embodiment of the present invention, a method is provided for using the two-way actuated composite material described above, comprising cooling the composite material below M f  of the first component, heating the composite material above A f  of the first component, and cooling the composite material below M f  of the first component. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIGS. 1A and 1  show a prior art method of two-way actuation using nitinol.  
         [0011]      FIGS. 2A and 2B  show an embodiment of a composite material of the present invention at both a low temperature and a high temperature.  
         [0012]      FIGS. 3A and 3B  show embodiments of wires formed from composite materials in accordance with the present invention.  
         [0013]      FIGS. 4A  to  4 C show embodiments of tubes formed from composite materials in accordance with the present invention.  
         [0014]      FIG. 5  shows an embodiment of a strip with a rectangular cross-section, the strip being formed from composite material in accordance with the present invention.  
         [0015]      FIGS. 6A and 6B  show an embodiment of the material of the present invention formed into a spring.  
         [0016]      FIGS. 7A and 713  show another embodiment of the material of the present invention formed into a spring.  
         [0017]      FIGS. 8A and 8B  show another embodiment of the material of the present invention formed into a spring.  
         [0018]      FIGS. 9A and 9B  show an embodiment of a wire formed from material of the present invention at a low temperature and a high temperature.  
         [0019]      FIGS. 10A and 101B  show a structure usable as a delivery device formed from material of the present invention.  
         [0020]      FIGS. 11A and 11B  show a structure usable as a gripping device formed from material of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0021]     The present invention provides a composite material that has two-way thermal actuation in the absence of an external bias. As one example, the composite material of the present invention may be used to reduce the profile of invasive medical device systems and improve the performance of these systems.  
         [0022]      FIGS. 2A and 2B  show an embodiment of a composite material according to the present invention. In  FIG. 2A , a first component  26 , which may be an SMA, is layered on a second component  25 , which may be an elastic metal. This layering is not intended to be limiting, but may be reversed or include multiple layers.  
         [0023]     In a, preferred embodiment, component  26  may be nitinol, and component  25  may be selected from biocompatible metals; stainless steels, such as  316 ; Co based alloys, such as MP35N or ELGILOY®; refractory metals, such as Ta, and refractory metal alloys; precious metals, such as Pt or Pd; titanium alloys, such as high elasticity beta Ti, such as FLEXFUM®; nickel superalloys; and combinations thereof. Specific stainless steel may also include austenitic or martensitic stainless steels, precipitation hardenable steels including 17-4PH, 15-4PH and 13-8Mo, or similar materials. Specific refractory metals and alloys may include Ta, Ta-10W, W, W—Re, Nb, Nb1Zr, C-103, Cb-752, FS-85, and T-111. Titanium alloys might be commercially pure, Ti6Al4V, Ti5Al2.5Sn, Beta C, Beta III or similar. In other preferred embodiments, component  26  is nitinol, and component  25  may be selected from high strength 300 Series stainless steel with an elastic recovery of approximately 1%, Beta C or Beta III titanium with an elastic recovery of approximately 1.5%, bulk metallic glass with an elastic recovery of approximately 2%, or High Elasticity Beta Ti; such as FLEXIUM™ with an elastic recovery of approximately 3-4%. The larger the elastic recovery of component  26 , the better.  
         [0024]     Two additional examples of shape memory alloy compositions include Ti—Pt—Ni with approximately 30% Pt and Ti—Pd—Ni with approximately 50% Pd. The Ti—Pt—Ni with approximately 30% Pt has an A f  of approximately 702° C. and an M f  of approximately 537° C., while the Ti—Pd—Ni with approximately 50% Pd has an A f  of approximately 591° C. and an M f  of approximately 550° C.  
         [0025]     The components  25  and  26  may be joined together to form the layered material by a suitable process, including working and/or heating. Suitable metal working practices known in the art include drawing, swaging, rolling, forging, extrusion, pressing, and explosive bonding. In one example of a joining method, one component may be deposited or otherwise placed on or adjacent to the other component, the two components may be fused, for example with a hot isostatic press, and the two components may be rolled to a final thickness. A metallurgical bond is formed between the components, thereby forming the layered composite. A description of composite metal fabrication processing may be found in the  ASM Handbook , Volume 2, Tenth Edition, pages 1043-1059.  
         [0026]     To set the actuator shapes for the two way actuator shown in  FIGS. 2A and 2B , the layered composite is formed into a first configuration ( FIG. 2B ) thereby storing elastic energy in component  25 , the composite is held in the first configuration and heated so that the shape memory component  26  is in the relatively high-temperature austenite phase, and the composite is shaped into that first configuration as shown in  FIG. 2B . The composite is then cooled below a transition temperature at which the shape memory component transforms to the relatively low-temperature martensite phase, and the stored elastic energy in component  25  forces the composite into a second configuration, as shown in  FIG. 2A .  
         [0027]     The layered composite shown in  FIG. 2A  is at a temperature T that is below M f  of component  26 .  FIG. 2B  shows a bent shape achievable by heating the composite material to or above A f  of component  26 . When heated to or above A f , the SMA wants to change to its remembered shape, so the composite material takes the shape shown in  FIG. 2B . To return the composite to its resting state or its initial shape as shown in  FIG. 2A , the temperature of the composite is lowered. The elastic properties of the composite material cause the return to this shape.  
         [0028]      FIGS. 3A  to  5  show additional embodiments of various composite material structures.  FIG. 3A  shows component  26  as a core of a wire with component  25  as cladding around the core.  FIG. 3B  shows the reverse structure, with component  25  as the core and component  26  as the cladding. These composite structures may be formed, for example, by placing a rod or tube within a tube and then drawing down to the illustrated diameter. It will be appreciated that through working and/or heat, a metallurgical bond may be formed between the two components, i.e., the core and the cladding, to form a composite structure.  
         [0029]      FIGS. 4A  to  4 C show examples of different ways of forming the composite material of the present invention into a tube. As shown in  FIG. 4A , the tube may be predominantly one component, such as component  25  with an embedded ring of component  26 . As shown in  FIG. 4B , the tube may comprise an outer tube of component  25  and an inner tube of component  26 . Alternatively, as shown in  FIG. 4C , the tube may comprise discontinuous sections or strips of either component  25  or  26 .  
         [0030]     The structures of  FIGS. 4A and 4B  may be constructed, for example, by placing tubes within other tubes and drawing. The structure of  FIG. 4C  may be constructed, for example, by depositing stripes of component  26  on the outer surface of a tube of component  25 , and then placing that structure inside a larger tube of component  25 , and drawing. It will be appreciated that the material of the inner and outer tubes of component  25  may fuse between the areas of the stripes of material  26 . Alternatively, the structures of  FIGS. 4A-4C  may be constructed by making a composite flat sheet as described above (depositing stripes in the case of  FIG. 4C ), and then rolling and joining to form a tube. It will be appreciated that with these techniques involving working and/or heating, a metallurgical bond is formed between components  25  and  26 .  
         [0031]      FIG. 5  shows another embodiment of the composite material, including a strip having a rectangular cross-section, where component  26  acts as a core and component  25  acts as cladding around the core. As will be appreciated, such a structure may be formed using techniques similar to those described above. Similar to  FIG. 5 , the composite material may also be in the form of a sheet.  
         [0032]     Further methods for forming composite structures are disclosed in U.S. patent application Ser. No. 09/702,226, the disclosure of which is hereby incorporated herein by reference.  
         [0033]     As one skilled in the art no doubt would understand, there are any number of possible configurations and structures that may be constructed to form the composite material of the present invention, including reversing the location and structure of the components shown.  
         [0034]     To illustrate the composite material&#39;s two-way actuation,  FIGS. 6A  to  8 B show embodiments of the present invention formed into various types of springs. To form the springs shown, an embodiment of the composite material of the present invention is formed into a wire and then heat treated. For example, a composite structure as shown in  FIGS. 3A and 3B  may be used. To form the spring, a wire is wound around a mandrel to form a coil or bias spring, and then heat treated at a suitable temperature for a suitable period of time, for example, heated to between approximately 350° C. to 650° C. for approximately 2 to 30 minutes (or longer), to set the spring shape. As an example, the heat treating range is approximately between 450° C. and 550° for between 5 and 15 minutes.  
         [0035]     In  FIGS. 6A and 6B , a spring  30  formed from the composite material of the present invention is affixed to a structure  35 . This embodiment of the present invention illustrates one possible direction of movement for an actuator. In  FIGS. 6A and 6B , the spring  30  may move laterally in a single direction by expanding and contracting. For example, the spring  30  contracts or relaxes when cooled to or below the M f  of component  26 , and it expands when the spring  30  is heated to or above A f  of component  26 . One use for this configuration may be to reduce the size of a two way thermal actuator.  
         [0036]     In  FIGS. 7A and 7B , a spring  30  formed from a composite material of the present invention is illustrated moving laterally in two directions. In  FIGS. 7A and 7B , no external fixation is used, and the spring  30  again expands and contracts based on the temperature applied. Uses for this embodiment may be to engage and release pins in a delivery system or to act as a spring trigger.  
         [0037]     In  FIGS. 8A and 8B , a tight spring  30  is formed, which expands to a larger diameter formation as temperature is applied. This configuration may be used to provide access to an area when the bias spring is enlarged and to block access to the same area by shrinking the bias spring.  
         [0038]      FIGS. 9A-11B  show examples of different geometries the composite material of the present invention may take. For example, FIGS.  9 A-B show a wire  90  formed from an embodiment of the composite material of the present invention. At T 1  (equal to or less than M f ) the wire  90  is straight; however at T 2  (equal to or more than A f ), the wire  90  bends. A use for the wire shown in  FIGS. 9A and 9B  may be as a shapeable guidewire or catheter.  
         [0039]     In  FIG. 10A , a tubular structure  100  formed from an embodiment of the composite material of the present invention has a seam running from one end. The tube  100  is shown in  FIG. 10A  at T 1  (equal to or less than M f ). At T 2  (equal to or more than A f ), as shown in  FIG. 10B , the portion of the tube  100  of  FIG. 10A  that had the seam has opened into two separate portions  100 A and  100 B. One use for this structure may be as a delivery system, where the structure shown in  FIG. 10B  is used to release an item.  
         [0040]     Similar to  FIGS. 10A and 10B ,  FIGS. 11A and 11B  show a structure that may be used as a reversible grasper or ablation grasper. In  FIG. 11A , a tubular structure  120  having finger portions  130 A and  130 B is shown at T 1  (equal to or less than M f ). In  FIG. 11B , the structure changes to an open configuration at T 2  (equal to or more than A f ). Alternatively, the reverse motion, i.e., moving from an open position as shown in  FIG. 11B  at T 1  (equal to or less than M f ) to closure as shown in  FIG. 11A  at T 2  (equal to or more than A f ), can also be obtained through alternative positioning during shape setting. Closure at elevated temperatures could be a useful feature in certain applications.  
         [0041]     Many additional geometries are possible with the composite materials of the present invention. For example, the composite material may be formed into a cantilever beam, a belleville washer, a thin film membrane, a linear wire or rod, a helical spring, or a tension spring.  
         [0042]     To use the composite material of the present invention, a two-way actuation cycle is used. In a preferred embodiment of the present invention, a body temperature/ice water actuation cycle is illustrated. In this method a composite material of the present invention is formed using Nitinol with an A f  of approximately 35° C. and a M f  of approximately 0° C., and one of the following materials: stainless steel, a cobalt alloy, tantalum, platinum, palladium or high elasticity titanium (FLEXIUM®). The composite material is then formed into a wire, strip, or tube. Thermal shaping is next performed, where the composite material structure is heat treated at a suitable temperature for a suitable period of time (for example, the temperatures and times stated above) and held in a particular shape, such as the bent structure shown in  FIG. 2B . When the composite material is bent, the bend strain can be within the elastic range for the non-nitinol component. Following thermal shaping, the composite material may then be cooled below M f , which will soften the nitinol and allow for elastic recovery of the non-nitinol component, and thus straighten the composite material. The composite material may then be heated above A f  in order to activate the memorized configuration. To release or recover from the memorized configuration, the composite material may be cooled to below M f . M f  and A f  may be between −200° C. to 170° C. These heating and cooling cycles may be repeated as often as necessary.  
         [0043]     In another preferred embodiment of the present invention, a reversible two-way actuation cycle may use an elevated temperature and body temperature as the cycling temperatures. For example, a composite material structure as described above may be formed using thermal shaping. However, in this embodiment, the nitinol A f  temperature is approximately 100° C. and the M f  is approximately 40° C. As described above, the temperature cycling may go from cooling the composite material to heating the composite material as many times as required.  
         [0044]     The thermal fluctuations used in these two embodiments may be any type of thermal cycling, such as different temperature fluids, electric resistance heating, induction heating, and conduction heating, in the body or otherwise. In addition, the range of thermal fluctuations may extend beyond the functional temperature range of binary nitinol. For example, if additional alloying elements are used to increase phase transformation temperature, then the upper temperature may be as high as 700° C.  
         [0045]     While the present invention has been described with reference to what are presently considered to be preferred embodiments thereof, it is to be understood that the present invention is not limited to the disclosed embodiments or constructions. On the contrary, the present invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are described and/or shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single embodiment, are also within the spirit and scope of the present invention.