Surgery – Means for introducing or removing material from body for... – Treating material introduced into or removed from body...
Reexamination Certificate
2001-05-31
2003-09-16
Hayes, Michael J. (Department: 3763)
Surgery
Means for introducing or removing material from body for...
Treating material introduced into or removed from body...
C604S113000, C604S530000, C604S531000
Reexamination Certificate
active
06620126
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to 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 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 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. These materials are commonly called “temperature-activated memory materials” or “shape memory alloys” (SMA), because they move to assume a predetermined shape when heated to a predetermined temperature.
Shape memory among metallic alloys is a result of the fact that the alloy undergoes a reversible solid state phase transformation between an austenitic state and a martensitic state with a change in temperature. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such a SMA, for example a wire, is easily deformed from its original high temperature or austenitic configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as the martensite start (M
s
) temperature, and upon continued cooling the temperature at which it finishes, the martensite finish (M
f
) temperature. The wire changes from a rigid state with a relatively high yield strength, in its austenitic form, to a state in which it is easily deformable, with a relatively low yield strength, in its martensitic form, in which it is able to sustain significant plastic-like deformation, at an almost constant stress level, as the result of the realignment of crystallographic twins which form during cooling from the austenitic to the martensitic state, in a process known as self-accommodation.
When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as the austenite start (A
s
) temperature, the deformed object will begin to return to its original configuration. With continued heating, the object will reach a temperature referred to as the austenite finish (A
f
) temperature, at which the reversion to the high temperature configuration is complete.
Nitinol, a nickel-titanium alloy, is one such SMA that has been formed into memory element strips and deployed in the distal ends of catheters. 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 martensite start temperature, it can be distorted into another arbitrary shape. When, however, the device is heated above its austenite start temperature, the imparted shape is partly recovered and, when it is further heated to its austenite finish temperature, the shape is fully recovered. Devices having a distal tip made from SMA utilize these shape memory characteristics to change the shape of the distal tip. Specifically, the SMA guide element is in the martensitic phase during insertion into the body lumen. Application of heat to the guide element causes a phase transformation from the martensitic to the austenitic phase, resulting in the shape of the distal tip being recovered. This change in shape can be used to redirect the device. Shape memory nitinol has previously been used in “strip” or “rod” form in the construction of steerable and aimable devices. Such nitinol strips and rods are solid core guide elements having a circular, rectangular, or other similar cross-sectional shape. In use, these solid core guide elements are placed on opposing sides of a central lumen formed in the device. Selective activation of these guide elements by conversion from martensite to austenite results in articulation of the device. 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.
Devices are also known in which a variable shape guide is constructed from a tube of SMA. One example of such a device is disclosed in U.S. Pat. No. 5,334,168, which describes heating of the SMA tube to cause a phase transformation from the martensitic to the austenitic phase, resulting in shape recovery. The recovered shape allows the device to be redirected through body lumens. However, the preferred embodiments of this patent emphasize that the change in shape of the guide element is effected by the transition of the guide element from the martensitic phase to the austenitic phase.
In the above-mentioned 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 martensitic 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 shall temperature range from the austenitic start temperature (A
s
) to the austenitic finish temperature (A
f
). Above-mentioned U.S. Pat. No. 5,334,168 also discloses a preferred embodiment in which the tubular guide element comprises superlastic nitinol, which is in the austenitic phase during insertion into the body lumen. The use of superlastic nitinol as the guide element is desirable since such a guidewire has significant axial push and flexibility along its length (bending) while exhibiting excellent torque transmission from the proximal end to the distal end.
However, it is expressly stated in U.S. Pat. No. 5,334,168 at column 4, lines 57 to 64, that heating of the superlastic tubular guide element is not required since superlastic nitinol is already in an activated (austenitic) state. Accordingly, a guide apparatus as contemplated by this patent having a superlastic nitinol guide element would not take advantage of the shape change which occurs during the heat-induced transformation from martensite to austenite.
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.
SUMMARY OF THE INVENTION
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 r
Unsworth John D.
Waram Thomas C.
Hayes Michael J.
Serke Catherine
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