Variable shape guide apparatus

Details Variable shape guide apparatus Variable shape guide apparatus

2000-01-05

2003-03-18

Casler, Brian L. (Department: 3763)

Surgery

Means for introducing or removing material from body for...

Treating material introduced into or removed from body...

C604S530000, C604S531000

Reexamination Certificate

active

06533752

ABSTRACT:

FIELD OF THE INVENTION
The field of the invention is steerable catheters, cannulae, guides and the like, that are designed to be steered through body cavities and aimable at obstructions, organs, or tissues within the body from a position external from the body.
BACKGROUND AND SUMMARY OF THE INVENTION
A great deal of research has been directed at developing a catheter or guide having a distal end which, when inserted into a body, is readily steerable and aimable to advance the device through the body cavities and passageways. It has been observed that materials exhibiting mechanical memory properties triggered by heat are particularly useful for enhancing the maneuverability of catheters of like devices. The materials are commonly called “temperature-activated memory materials” or “shape memory alloys” or simply “SMA”, because they move to assume a predetermined shape when heated to a predetermined temperature. Nitinol, a nickel-titanium alloy, is one such SMA that has been formed into memory element strips and deployed in the distal end of a catheter. Heating the nitinol memory element strips to a given temperature using an electric current provided by a power supply causes the memory elements to deform to assume a predetermined shape, thereby deflecting the distal end of the catheter. See, for example, U.S. Pat. Nos. 4,543,090; 4,601,705; and 4,758,222 for descriptions of known memory element systems for steering and aiming catheters, cannulae, and the like.
The shape that is recovered by heating is first imparted into the device at high temperature during the manufacturing process. When the device is cooled below its martensitic start temperature, it can be distorted into another arbitrary shape. When however the device is heated above its austenitic start temperature, the imparted shape is partly recovered and when it is further heated to its austenitic finish temperature, the shape is fully recovered. These devices utilize these characteristic to change the shape of the distal tip of the devices by heating the distal tip of the device while it is in its martensitic phase. When it transformed into its austenitic phase, the shape is recovered, and the changed shape can be used to redirect the device.
Use of shape memory nitinol previously has been used in “strip” or “rod” form in the construction of steerable and aimable apparatus. Such nitinol strips and rods are solid core elements having a circular, rectangular, or other similar cross-sectional shape. In use, these solid core memory elements strips or rods are placed on opposing sides of a central lumen formed in an apparatus about the circumference of the apparatus. Selective activation of these memory element strips or rods results in articulation of the apparatus. See, for example U.S. Pat. No. 4,601,705 for a disclosure of a four-memory element strip steering and aiming system and U.S. Pat. No. 4,758,222 for a disclosure of a steering and aiming system using a spring and one temperature activated memory element strip. Similarly, U.S. Pat. No. 5,334,168 describes a variable shape guide apparatus that is constructed from a tube of SMA that is heated in its martensitic form to recover an imparted shape as it transforms into austenite at a higher temperature. This recovered shape then allows the device to be redirected down the body lumen. The preferred embodiments that utilize the shape memory effect from martensite to austenite do not disclose a biasing element and make clear that the shape change is effected by the transition from the martensitic phase to the austenitic phase. The only preferred embodiment of U.S. Pat. No. 5,334,168 4 that includes superlastic nitinol (which is in the austenitic phase) is expressly not heated to effect a shape change but rather uses a guide wire to do so:
“In this embodiment, nickel titanium tube 122 is made, for example, of superelastic nitinol. It will be understood that control means is not needed to heat superelastic nitinol since it is already in an activated (above transition) temperature. For example, a superelastic nitinol tube 122 is formed in a present curved shape and a guide wire (not shown) is used to return the nitinol tube 122 to a straight shape.”[Column 4, line 57 to 64]
In all previous steerable SMA systems the steering means is achieved by heating the SMA while it is in its martensitic form and recovering a different shape as it transforms into its austenitic form. The difficulty with utilizing the transformation from martensite to austenite, or visa versa, to effect shape change and thereby allow for steering is two-fold: firstly the device in its matensitic state is not as springy as in its austenitic state, which makes it more difficult for the operator to manipulate the device from outside the body; secondly, it is difficult to partly transform the SMA to allow for a partial change in shape of the steerable portion of the device. The second shortcoming is due to the fact that shape recovery occurs over the relatively small temperature range from the austenitic start temperature (A
s
) to the austenitic finish temperature (A
f
).
What is needed is a system that allows for steering but at the same time maintains the springy qualities of SMA in its austenitic phase. What is also needed is a system that allows for partial changes in the shape of the steerable portion of the device to permit a greater range of steerability.
The invention herein disclosed is a steerable device that effects shape change entirely while above the austenitic finish temperature and does not rely on recovering the imparted shape at the transition between austenite and martensite. This invention herein disclosed relies on the fact that a tube of SMA in its austenitic form becomes stiffer as the temperature of the material is increased. This increase in stiffness or modulus is approximately linear as a function of increased temperature and therefore allows for a gradual increase in stiffness in response to increases in heat energy applied to the device. For example, if an appropriate force is applied normal to the longitudinal axis of a tube (a bending force) and the tube is made of SMA material that remains in the austenitic phase, when the tube is heated, the tube will become stiffer, partly overcoming the bending force and thereby changing its shape or radius of curvature. This shape change can be used to steer the device. Depending upon the shape of the tube along its longitudinal axis in its unloaded mode and the nature of the impending biasing forces, the tube can be designed to assume many different shapes as it is heated and cooled. The unloaded shape of the SMA tube and the shape of the biasing element can be simple or complex. For example the unloaded SMA tube could be helical, and the biasing element could be contoured to exert the appropriate forces to hold the unit in a straight configuration; upon heating the unloaded helical shape of the SMA tube could come to dominate the shape of the unit.
In summary, the shape change is not due to recovering a shape by heating the SMA between the A
s
and the A
f
temperature; it is instead a result of the stiffening of the SMA that occurs solely above the A
f
while it is in the austenitic phase. It should be noted that this shape change is dependent upon first, a biasing force distorting the SMA device from its unloaded shape (that being the shape it would have at or above the A
f
temperature if no biasing force was applied), and second the application of heat to the device causing it to overcome the biasing force somewhat; and moving the device from its distorted shape closer to unloaded shape. The stiffening of austenite due to heating has been described by Unsworth and Waram in U.S. Pat. No. 5,904,657 to stiffen guides wires but without a biasing force that allows it to steered.
In the case of such a device being introduced into the bloodstream, the heat applied to the tube would for example be above the temperature of the blood and as more heat is applied to the tube it would become stiffer and with the said appropriate bia

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