Abstract:
A shape memory alloy (SMA) actuator includes a groove formed in a surface of a shape memory alloy (SMA) substrate establishing a trace pattern for a layer of conductive material formed over an electrically insulative layer. The trace pattern includes a first end, a second end, and a heating element disposed between the first and second ends. The SMA substrate is trained to deform at a transition temperature achieved when electricity is conducted through the conductive material via first and second interconnect pads terminating the first and second ends of the trace pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/410,546, filed Apr. 9, 2003 entitled “SHAPE MEMORY ALLOY ACTUATORS”, herein incorporated by reference in its entirety. 
         [0002]    Cross-reference is hereby made to commonly assigned related U.S. Pat. No. 6,832,478 to David Anderson, et al., entitled “Shape Memory Alloy Actuators” (Attorney Docket No. P0009579.00). 
     
    
     FIELD OF THE INVENTION 
       [0003]    Embodiments of the present invention relate generally to shape memory alloy (SMA) actuators and more particularly to means for forming SMA actuators and incorporating such actuators into elongated medical devices. 
       BACKGROUND 
       [0004]    The term SMA is applied to a group of metallic materials which, when subjected to appropriate thermal loading, are able to return to a previously defined shape or size. Generally an SMA material may be plastically deformed at some relatively low temperature and will return to a pre-deformation shape upon exposure to some higher temperature by means of a micro-structural transformation from a flexible martensitic phase at the low temperature to an austenitic phase at a higher temperature. The temperature at which the transformation takes place is known as the activation temperature. In one example, a TiNi alloy has an activation temperature of approximately 70° C. An SMA is “trained” into a particular shape by heating it well beyond its activation temperature to its annealing temperature where it is held for a period of time. In one example, a TiNi alloy is constrained in a desired shape and then heated to 510° C. and held at that temperature for approximately fifteen minutes. 
         [0005]    In the field of medical devices SMA materials, for example TiNi alloys, such as Nitinol, or Cu alloys, may form a basis for actuators designed to impart controlled deformation to elongated interventional devices. Examples of these devices include delivery catheters, guide wires, electrophysiology catheters, ablation catheters, and electrical leads, all of which require a degree of steering to access target sites within a body; that steering is facilitated by an SMA actuator. An SMA actuator within an interventional device typically includes a strip of SMA material extending along a portion of a length of the device and one or more resistive heating elements through which electrical current is directed. Each heating element is attached to a surface of the SMA strip, in proximity to portions of the SMA strip that have been trained to bend upon application of thermal loading. A layer of electrically insulating material is disposed over a portion of the SMA strip on which a conductive material is deposited or applied in a trace pattern forming the heating element. Electrical current is directed through the conductive trace from wires attached to interconnect pads that terminate each end of the trace. In this way, the SMA material is heat activated while insulated from the electrical current. It is important that, during many cycles of activation, the insulative layer does not crack or delaminate from the surface of the SMA strip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  is a plan view including a partial section of an elongated medical device including an SMA actuator. 
           [0007]      FIG. 1B  is a plan view of the exemplary device of  FIG. 1A  wherein a current has been passed through heating elements of the SMA actuator. 
           [0008]      FIG. 1C  is a plan view including a partial section of another embodiment of an elongated medical device including an SMA actuator. 
           [0009]      FIG. 1D  is a plan view of the exemplary device of  FIG. 1C  wherein a current has been passed through heating elements of the SMA actuator. 
           [0010]      FIG. 2A  is a perspective view of an SMA substrate or strip that would be incorporated in an SMA actuator. 
           [0011]      FIG. 2B  is a plan view of a portion of a surface of an SMA actuator. 
           [0012]      FIG. 3  is a section view through a portion of an SMA actuator according an embodiment of the present invention. 
           [0013]      FIG. 4  is a section view through a portion of an SMA actuator according to an alternate embodiment of the present invention. 
           [0014]      FIGS. 5A-D  are section views illustrating steps, according to embodiments of the present invention, for forming the SMA actuator illustrated in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIGS. 1A-D  illustrate two examples of elongated medical devices each incorporating an SMA actuator, wherein each actuator serves to control deformation of a portion of each device.  FIG. 1A  is a plan view with partial section of an elongated medical device  300  including an SMA actuator  56 . As illustrated in  FIG. 1A , medical device  300  further includes a shaft  305 , a hub  303  terminating a proximal end of shaft  305 , and conductor wires  57  coupled to SMA actuator  56 . SMA actuator  56 , positioned within a distal portion  100  of shaft  305 , includes a plurality of heating elements (not shown), electrically insulated from an SMA substrate, through which current flows fed by wires  57 ; wires  57 , extending proximally and joined to electrical contacts (not shown) on hub  303 , carry current to heat portions of the SMA substrate to an activation temperature. At the activation temperature, portions of the SMA substrate revert to a trained shape, for example a shape  200  as illustrated in  FIG. 1B .  FIG. 1B  is a plan view of the exemplary device  300  of  FIG. 1A  wherein a current has been passed through heating elements of SMA actuator  56 , locations of which heating elements correspond to bends  11 ,  12 , and  13 . When the current is cut, either an external force or a spring element (not shown) joined to shaft  605  in proximity of SMA actuator  56  returns distal portion  100  back to a substantially straight form as illustrated in  FIG. 1A . Device  300 , positioned within a lumen of another elongated medical device, may be used to steer or guide a distal portion of the other device via controlled deformation of actuator  56  at locations corresponding to bends  11 ,  12 , and  13 , either all together, as illustrated in  FIG. 1B , or individually, or in paired combinations. 
         [0016]      FIG. 1C  is a plan view including a partial section of another embodiment of an elongated medical device  600  including an SMA actuator  10  embedded in a portion of a wall  625  of a shaft  605 . As illustrated in  FIG. 1C , medical device  600  further includes a hub  603  terminating a proximal end of shaft  605 , a lumen  615  extending along shaft  605 , from a distal portion  610  through hub  603 , and conductor wires  17  coupled to SMA actuator  10 . SMA actuator  10 , positioned within distal portion  610  of shaft  605 , includes a plurality of heating elements (not shown), electrically insulated from an SMA substrate, through which current flows fed by wires  17 ; wires  17 , extending proximally and joined to electrical contacts (not shown) on hub  603 , carry current to heat portions of the SMA substrate to an activation temperature. At the activation temperature, portions of the SMA substrate revert to a trained shape, for example a bend  620  as illustrated in  FIG. 1D .  FIG. 1D  is a plan view of the exemplary device  600  of  FIG. 1C  wherein a current has been passed through a heating element of SMA actuator  10 , a location of which heating element corresponds to bend  620 . When the current is cut, either an external force or a spring element (not shown), for example embedded in a portion of shaft wall  625 , returns distal portion  610  back to a substantially straight form as illustrated in  FIG. 1C . Lumen  615  of device  600 , may form a pathway to slideably engage another elongated medical device, guiding the other device via controlled deformation of distal portion  610  by actuator  10  resulting in bend  620 . 
         [0017]      FIGS. 2A-B  illustrate portions of exemplary SMA actuators that may be incorporated into an elongated medical device, for example device  300  illustrated in  FIGS. 1A-B .  FIG. 2A  is a perspective view of an SMA substrate or strip  20  that would be incorporated into an SMA actuator, such as SMA actuator  56  illustrated in  FIG. 1A . Embodiments of the present invention include an SMA substrate, such as strip  20 , having a thickness between approximately 0.001 inch and approximately 0.1 inch; a width and a length of strip  20  depends upon construction and functional requirements of a medical device into which strip  20  is integrated. As illustrated in  FIG. 2A  strip  20  includes a surface  500 , which according to embodiments of the present invention includes a layer of an inorganic electrically insulative material formed or deposited directly thereon, examples of which include oxides such as silicon oxide, titanium oxide, or aluminum oxide, nitrides such as boron nitride, silicon nitride, titanium nitride, or aluminum nitride, and carbides such as silicon carbide, titanium carbide, or aluminum carbide. Means for forming the inorganic material layer are well know to those skilled the art and include vacuum deposition methods, such as sputtering, evaporative metalization, plasma assisted vapor deposition, or chemical vapor deposition; other methods include precipitation coating and printing followed by sintering. In an alternate embodiment an SMA substrate, such as strip  20 , is a TiNi alloy and a native oxide of the TiNi alloy forms the layer of inorganic electrically insulative material; the native oxide may be chemically, electrochemically or thermally formed on surface  500 . In yet another embodiment, a deposited non-native oxide, nitride, or carbide, such as one selected from those mentioned above, in combination with a native oxide forms the layer of electrically insulative material on surface  500 . 
         [0018]    According to embodiments of the present invention, an SMA substrate, such as strip  20 , is trained to bend, for example in the direction indicated by arrow A in  FIG. 2A , after deposition or formation of an inorganic electrically insulative layer upon surface  500 , since the inorganic insulative layer will not break down under training temperatures. Training temperatures for TiNi alloys range between approximately 300° C. and approximately 800° C. Alternately an SMA substrate, such as strip  20 , may be trained to bend before deposition or formation of the inorganic insulative layer if a temperature of the substrate, during a deposition or formation process, is maintained below an activation temperature of the substrate. Furthermore, according to an alternate embodiment, an additional layer of an organic material is deposited over the inorganic layer to form a composite electrically insulative layer. Examples of suitable organic materials include polyimide, parylene, benzocyclobutene (BCB), and fluoropolymers such as polytetrafluoroethylene (PTFE). Means for forming the additional layer are well known to those skilled in the art and include dip coating, spay coating, spin coating, chemical vapor deposition, plasma assisted vapor deposition and screen printing; the additional layer being formed following training of the SMA substrate and at a temperature below an activation temperature of the substrate. An activation temperature for an SMA actuator included in an interventional medical device must be sufficiently high to avoid accidental activation at body temperature; a temperature threshold consistent with this requirement and having a safety factor built in is approximately 60° C. This lower threshold of approximately 60° C. may also prevent accidental activation during shipping of the medical device. An activation temperature must also be sufficiently low to avoid thermal damage to body tissues and fluids; a maximum temperature consistent with this requirement is approximately 100° C., but will depend upon thermal insulation and, or cooling means employed in a medical device incorporating an SMA actuator. 
         [0019]      FIG. 2B  is a plan view of a portion of a surface of an SMA actuator  50 .  FIG. 2B  illustrates a group of conductive trace patterns; portions of the conductive trace patterns are formed either on a first layer, a second layer, or between the first and second layer of a multi-layer electrical insulation  1  formed on a surface of an SMA substrate, such as strip  20  illustrated in  FIG. 2A . As illustrated in  FIG. 2B , conductive trace pattern includes heating element traces  2 , which are formed on first layer of insulation  1 , signal traces  4 ,  5 , which are formed on second layer of insulation  1 , and conductive vias  3 ,  9 , which traverse second layer in order to electrically couple heating element signal traces  2  on first layer with signal traces  4 ,  5  on second layer. Each signal trace  4  extends from an interconnect pad  6  through via  3  to heating element trace  2 , while signal trace  5  extends from all heating element traces  2  through vias  9  to a common interconnect pad  7 . According to embodiments of the present invention, multi-layer insulation  1  is formed of an inorganic electrically insulative material, examples of which are presented above, deposited or formed directly on the SMA substrate. Portions of conductive trace pattern deposited upon each layer of multi-layer insulation  1 , according to one embodiment, are formed of a first layer of titanium, a second layer of gold and a third layer of titanium and each interconnect pad  6 ,  7  is formed of gold deposited upon the second layer of insulation  1 . Details regarding pattern designs, application processes, thicknesses, and materials of conductive traces that may be included in embodiments of the present invention are known to those skilled in the arts of VLSI and photolithography. 
         [0020]    Section views in  FIGS. 3 and 4  illustrate embodiments of the present invention in two basic forms.  FIG. 3  is a section view through a portion of an SMA actuator  30  including one segment of a conductive trace  32  that may be a portion of a heating element trace, such as a heating element trace  2  illustrated in  FIG. 2B . As illustrated in  FIG. 3 , SMA actuator  30  further includes an SMA substrate  350 , a first insulative layer  31 , electrically isolating conductive trace  32  from SMA substrate  350 , and a second insulative layer  33  covering and surrounding conductive trace  32  to electrically isolate conductive trace  32  from additional conductive traces that may be included in a pattern, such as the pattern illustrated in  FIG. 2B . According to embodiments of the present invention, first insulative layer  31 , including an inorganic material, is deposited or formed directly on substrate  350 , as described in conjunction with  FIG. 2A . Conductive materials are deposited or applied on insulative layer  31 , creating conductive trace  32 , for example by etching, and then second insulative layer  33 , including an inorganic material, is deposited or applied over conductive trace  32 . In an alternate embodiment, second insulative layer  33  includes an organic electrically insulative material; examples of suitable organic materials include polyimide, parylene, benzocyclobutene (BCB), and fluoropolymers such as polytetrafluoroethylene (PTFE). Means for forming insulative layer  33  include dip coating, spray coating, spin coating, chemical vapor deposition, plasma assisted vapor deposition and screen-printing. Training of SMA substrate  350  may follow or precede formation of first insulative layer  31 , as previously described in conjunction with  FIG. 2A . 
         [0021]      FIG. 4  is a section view through a portion of an SMA actuator  40  including one segment of a conductive trace  42 . According to alternate embodiments of the present invention, a groove in a surface of an SMA substrate  450  (reference  FIG. 5A ) establishes a pattern for conductive trace  42 , the pattern including a heating element trace disposed between signal traces, similar to one of heating element traces  2  and corresponding signal traces  4 , 5  illustrated in  FIG. 2B . As illustrated in  FIG. 4 , an insulative layer  41  is disposed between conductive trace  42  and SMA substrate  450  electrically isolating conductive trace  42  from an SMA substrate  450 . According to embodiments of the present invention, insulative layer  41  includes an inorganic material, examples of which are given in conjunction with  FIG. 2A , formed directly on SMA substrate  450 . Training of SMA substrate  450  may follow or precede formation of first insulative layer  41  including an inorganic material, as previously described in conjunction with  FIG. 2A . According to alternate embodiments of the present invention, insulative layer  41  includes an organic material, formed directly on SMA substrate  450  following training of substrate  450 . Selected organic materials for insulative layer  41  include those which may be deposited or applied at a temperature below an activation temperature of SMA substrate  450  and those which will not degrade at the activation temperature of SMA substrate  450 ; examples of such materials include polyimide, parylene, benzocyclobutene (BCB), and fluoropolymers such as polytetrafluoroethylene (PTFE). Means for forming insulative layer  41  include dip coating, spray coating, spin coating, chemical vapor deposition, plasma assisted vapor deposition and screen-printing. 
         [0022]      FIGS. 5A-D  are section views illustrating steps, according to embodiments of the present invention, for forming SMA actuator  40  illustrated in  FIG. 4 .  FIG. 5A  illustrates SMA substrate  450  including a groove  510  formed in a surface  515 ; groove  510  is formed, for example by a machining process.  FIG. 5B  illustrates a layer of electrically insulative material  511  formed on surface  515  and within groove  510 .  FIG. 5C  illustrates a layer of conductive material  512  formed over layer of insulative material  511 .  FIG. 5D  illustrates insulative layer  41  and conductive trace  42  left in groove  510  after polishing excess insulative material  511  and conductive material  512  from surface  515 . As illustrated in  FIG. 5D , conductive trace  42  is flush with surface  515  following polishing; in one example, according to this embodiment, groove  510  is formed having a width of approximately 25 micrometer and a depth of approximately 1.2 micrometer approximately matching a predetermined combined thickness of insulative layer  41  and conductive trace  42 . According to alternate embodiments of the present invention, groove  510  is formed deeper than a resultant combined thickness of the insulative layer  41  and conductive trace  42  so that conductive trace is recessed from surface  515 . 
       EXAMPLES 
       [0023]    Minimum theoretical thicknesses having sufficient dielectric strength for operating voltages of 100V, 10V, and 1V applied across conductive traces on SMA actuators were calculated for insulating layers of Silicon Nitride, Aluminum Nitride, Boron Nitride, and polyimide according to the following formula: 
         [0000]      Thickness=voltage/dielectric strength. 
         [0024]    A dielectric strength for Silicon Nitride was estimated to be 17700 volts/millimeter; a dielectric strength for Aluminum Nitride was estimated to be 15,000 volts/millimeter; a dielectric strength for Boron Nitride was estimated to be 3,750 volts/millimeter; a dielectric strength for polyimide was estimated to be 157,500 volts/millimeter. Results are presented in Table 1. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Thickness, 
                 Thickness, 
                 Thickness, 
               
               
                   
                 100 V 
                 10 V 
                 1 V 
               
               
                   
                 (micrometer) 
                 (micrometer) 
                 (micrometer) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Silicon Nitride 
                 5.65 
                 0.56 
                 0.06 
               
               
                 Aluminum Nitride 
                 6.67 
                 0.67 
                 0.07 
               
               
                 Boron Nitride 
                 26.7 
                 2.67 
                 0.27 
               
               
                 Polyimide 
                 0.64 
                 0.064 
                 0.0064 
               
               
                   
               
             
          
         
       
     
         [0025]    Finally, it will be appreciated by those skilled in the art that numerous alternative forms of SMA substrates and trace patterns included in SMA actuators and employed in medical devices are within the spirit of the present invention. For example, SMA actuators according to the present invention can include conductive trace patterns on two or more surfaces of an SMA substrate or an additional layer or layers of non-SMA material joined to an SMA substrate, which serve to enhance biocompatibility or radiopacity in a medical device application. Hence, descriptions of particular embodiments provided herein are intended as exemplary, not limiting, with regard to the following claims.