temperature - controlled actuators described herein use an inhomogeneous core - wire that , when subjected to a pulling force , stretches by different amounts at different locations . these different amounts depend , in part , on temperatures at various sections of the core - wire . at least one portion of the core - wire includes a shaped memory alloy that has been pre - heated to take a pre - defined shape when in its austenite state . this portion of the wire is attached to and controls the shape of a flexible portion of the actuator . a weight or other force applicator coupled to the proximal section of the core - wire maintains tension along the core - wire . referring to fig1 , a first embodiment of an actuator 10 incorporating the principles of the invention includes a housing 12 having a proximal portion and a distal portion . in the illustrated embodiment , the housing 12 is a flexible tube made of articulating segments . however , the housing 12 can also be a tube having a flexible distal portion and a rigid proximal portion . a housing 12 has an equilibrium compressed state in which it defines a pre - selected path . additionally , the housing 12 can be a tube having a rigid distal portion coupled to a rigid proximal portion by one or more hinges to allow movement of the distal portion relative to the proximal portion . in other embodiments , the housing 12 need not be tubular at all , but can instead be open to its surroundings . a sleeve 14 enclosing the proximal portion of the housing 12 provides rigid support to the proximal portion . the distal portion of the housing 12 , however , is free to change its shape . in particular , the distal portion is free to change between a relaxed shape , shown in fig1 , and a tensioned shape , shown in fig2 . in fig1 and 2 , the relaxed shape is a coil and the tensioned shape is straight . however , the invention is not constrained to these two particular configurations . as indicated by fig1 , the housing 12 can be a segmented structure capable of articulation between its constituent segments . however , the housing 12 can also be any flexible section capable of freely making the required transition between the curved state of fig2 and the extended state of fig1 . the housing 12 may be a close wound coil , with or without preload , or it may be an open wound coil . the housing 12 can include baffles , bellows , or any such flexible and compressible member . a cross - sectional view of the actuator 10 , shown in fig3 and 4 , reveals a portion of the structure that enables a change in temperature to toggle the housing 12 between its relaxed state and its tensioned state . referring to fig3 , a core - wire 16 anchored at an end cap 19 at the distal end of the housing 12 extends through a lumen between the distal and proximal ends thereof . the end cap 19 provides mechanical coupling between the core - wire 16 and the housing 12 so that a change in the path traced out by the core - wire 16 results in a corresponding change in the path traced out by the housing 12 . coupling between the housing 12 and the core - wire 16 can also be provided by a direct connection between the housing 12 and the core - wire 16 . in addition , the point of coupling need not be at the tip of the housing 12 as shown in fig3 . by proximally displacing the coupling point , for example , the tip can be made floppy . a proximal end of the wire 16 is operably connected to a tensioning element 20 that applies a constant force , denoted by the force vector { right arrow over ( f )}, to the proximal end of the core - wire 16 . because the core - wire 16 is anchored to the end cap 19 , this constant force does not move the core - wire 16 . instead , it places the wire 16 under tension . this tension is manifested as a stress field throughout the core - wire 16 . in response to the stress field , the core - wire 16 stretches . the design of the core - wire 16 is such that at a particular temperature , different portions of the core - wire 16 stretch by different amounts . the tensioning element 20 is represented in fig3 as a weight . however , any mechanism for applying a force can be used as a tensioning element 20 . for example , a pulley may be used to direct the force at an angle relative to the force vector . the magnitude of the force need not be constant . in other embodiments , the weight can be replaced by a spring mechanism . a distal section 22 of the core - wire 16 is made from a shaped - memory alloy . a suitable alloy from which the core - wire 16 can be manufactured is a nickel - titanium alloy sold under the trade name nitinol ™. such an alloy has the property that when deformed and heated past a critical temperature , which is on the order of 700 degrees fahrenheit for nitinol , it “ remembers ” its deformed shape . a distal section 22 is formed by deforming a distal section of the core - wire 16 , heating it past a critical temperature , and then cooling it . the shape into which the distal section 22 is deformed then becomes the remembered shape . when treated in this manner , the distal section 22 acquires temperature - dependent mechanical properties . in particular , the distal section 22 has the property that it can be in one of two states : an austenite state , in which it reverts to its remembered shape , and a martensite state , in which it is super - elastic . the state in which the distal section 22 of the core - wire 16 finds itself depends on its temperature . when heated past an austenite transformation temperature , the distal section 22 reverts to its austenite state . in this state , the distal section 22 has a tendency to recover its remembered shape . in addition , when the distal section 22 is stressed , it yields reluctantly . an applied stress on the distal section 22 in its austenite state results in comparatively little elongation of that section . in contrast , when cooled below a martensite transformation temperature , the distal section 22 becomes super - elastic . in its martensite state , the distal section 22 yields readily . thus , an applied stress results in considerable strain , and hence considerable elongation of the distal section 22 . a proximal section 24 of the core - wire 16 is made of a rigid material , for example stainless steel , whose strain response is only weakly dependent on temperature . alternatively , the proximal section 24 can be made of a super - elastic alloy having an austenite transformation temperature that is less than the austenite transformation temperature of the distal section 22 . in operation , the force applied by the tensioning element 20 urges the core - wire 16 to stretch . when the distal section 22 of the core - wire 16 is below its martensite transformation temperature , the distal section 22 loses its tendency to assume its remembered shape . in addition , the distal section 22 becomes super - elastic . as a result , most of this stretching occurs at the distal section 22 . the proximal section 24 , being more rigid than the super - elastic distal section 22 , stretches very little . because the distal end of the core - wire 16 is anchored to the end cap 19 , there is a tendency for the core - wire 16 to straighten the distal section of the housing 12 , as shown in fig4 . in contrast , when the distal section 22 of the core - wire 16 is above its austenite transformation temperature , it loses its super - elastic properties and assumes its remembered shape . as a result , it stretches very little . in this case , what stretching occurs is borne by the proximal section 24 . in addition , the distal section 22 reverts to its remembered shape . because the core - wire 16 is mechanically coupled to the housing 12 by the end cap 19 , the distal section of the housing 12 likewise assumes this remembered shape . as noted above , a material such as nitinol becomes super - elastic when it transitions from its austenite form to its martensite form . this can occur when the nitinol , in its austenite form , is cooled to below its martensite transition temperature . another way to cause a transition from austenite to martensite , however , is to pull so hard on an austenite wire that it turns into martensite . martensite formed in this way is referred to as “ stress - induced martensite ”. additional embodiments of the invention , described below , make use of stress - induced martensite . in a second embodiment , shown in fig5 and 6 , the core - wire 16 has a distal section 22 , a proximal section 24 , and an intermediate section 26 between the distal and proximal sections 22 , 24 . the distal section 22 and the intermediate section 26 are similar to the distal section 22 and proximal section 24 described above in connection with the first embodiment . as was the case with the first embodiment , a tensioning element 20 coupled to the proximal end applies a constant force that places the core - wire 16 in tension . the resulting tension causes a stress field throughout the core - wire 16 , including within its proximal section 24 . the strain experienced by the proximal section 24 in response to that stress depends in part on whether the distal section 22 is in its austenite state or in its martensite state . referring to fig5 , when the distal section 22 is below its martensite transition temperature , it becomes super - elastic . as a result , most of the stress imposed by the tensioning element 20 is relieved by the stretching of the distal section 22 . because the stress is relieved primarily by stretching of the distal section 22 , the proximal section 24 undergoes comparatively little strain . as a result , the proximal section 24 remains in its austenite form . referring now to fig6 , when the distal section 22 is above its austenite transition temperature , it loses its super - elastic properties and reverts to its remembered shape . as a result , the distal section 22 no longer contributes so generously toward relieving the stress present throughout the core - wire 16 . in this case , the stress strains the proximal section 24 and thereby causes it to transition into its martensite form . once in its martensite form , the proximal section 24 becomes super - elastic . in its super - elastic form , the proximal section 24 stretches sufficiently to relieve the stress in the core - wire 16 . the proximal section 24 and the intermediate section 26 can be different materials . however , to avoid having to join the proximal and middle sections , it is convenient to make them integral with each other . in the illustrated second embodiment , the proximal section 24 is formed by grinding down a section of the core - wire 16 . in this case , the proximal section 24 is that portion of the wire 16 whose diameter has been reduced by grinding and the intermediate section 26 is that portion of the wire 16 that retains its original diameter . because the proximal section 24 has a smaller diameter than the intermediate section 26 , it yields more to stress than does the intermediate section 26 . this , in turn , ensures that the intermediate section 26 can remain in its austenite form even when the proximal section 24 has transitioned into its martensite form . in a third embodiment , shown in fig7 and 8 , the roles of the proximal and distal sections of the core - wire 16 are opposite those in the second embodiment . in this case , a nitinol core - wire 16 has a reduced - diameter distal section 22 . as a result , the distal end responds to sufficient stress by transitioning into stress induced martensite . in so doing , it acquires super - elastic properties and stretches as shown in fig7 . because the core - wire 16 is coupled to the housing 12 by the end cap 19 , this causes the housing 12 to straighten . in the absence of such stress , the distal end reverts to austenite and recovers a remembered shape . again , because the core - wire 16 is coupled to the housing 12 by the end cap 19 , this causes the housing 12 to assume that remembered shape . the proximal section 24 of the core - wire 16 has an austenite transition temperature that is higher than the austenite transition temperature of the intermediate section 26 of the core - wire 16 . as was the case with the second embodiment , a tensioning element 20 applies a pulling force to the proximal end . referring to fig8 , when the proximal end of the core - wire 16 is below its martensite transition temperature , it becomes martensite . as a result , it stretches considerably , so much so that it manages to relieve most of the stress applied throughout the core - wire 16 . the proximal section 24 thus isolates the distal section 22 from stress sufficient to turn it into stress induced martensite . because the distal section 22 remains austenite , it assumes its remembered shape . because of the coupling between the core - wire 16 and the housing 12 , the housing 12 likewise assumes the remembered shape . referring to fig7 , when the proximal section 24 is above its austenite transition temperature , it becomes austenite , and therefore does not stretch significantly in response to the applied stress . as a result , the stress must be borne by the remainder of the core - wire 16 . because of its reduced diameter , the distal section 22 of the core - wire 16 experiences considerable stress , enough to cause it to transition into stress - induced martensite . in doing so , it loses its remembered shape and straightens . because of the coupling between the core - wire 16 and the housing 12 , the housing 12 also straightens . the tensioning element 20 shown in fig6 - 8 is a collar having a slot for accepting the sleeve 14 and a central opening for attachment to the core - wire 16 . the slot enables the tensioning element 20 to move axially along the sleeve 14 , thereby changing the tension applied to the core - wire 16 . the axial position of the slot can be adjusted by , for example , by a rack and pinion arrangement . however , no particular form of tensioning element 20 is required . what is important is that the core - wire 16 be constantly sufficient tension to stretch a portion of the core - wire when the temperature provides an opportunity to do so . another embodiment of a tensioning element 20 is a screw that mounted across the diameter of the housing . the screw has a hole in its shaft that engages the core - wire 16 . as the screw turns , the core - sire 16 can be tightened or loosened in the same manner that a string is tuned on a guitar or other stringed instrument . the austenite transformation temperature and the martensite transformation temperature can be adjusted by known methods such as heat treating the alloy or doping the alloy . it is to be understood that while the invention has been described in conjunction with the detailed description thereof , the foregoing description is intended to illustrate and not limit the scope of the invention , which is defined by the scope of the appended claims . other aspects , advantages , and modifications are within the scope of the following claims .
a temperature - controlled actuator has a housing with a proximal end and a moveable distal portion . a core - wire extends along the housing , with its distal section anchored to the distal portion of the housing . the core - wire &# 39 ; s distal section has an austenite state and a martensite state . the distal section moves the distal portion by transitioning between the austenite and martensite states in response to a temperature change along a thermometric section of the core - wire . a proximal section in mechanical communication with the core - wire &# 39 ; s distal section transmits tension , provided by a tensioning element , to the distal section . the tensioning element , which is coupled to the proximal section of the core - wire , constantly applies a tensioning force to the core - wire .