Thin-film shape memory alloy actuators and processing methods

Details Thin-film shape memory alloy actuators and processing methods Thin-film shape memory alloy actuators and processing methods

1998-05-15

2002-09-10

Casler, Brian L. (Department: 3763)

Surgery

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

Treating material introduced into or removed from body...

C604S531000, C600S151000

Reexamination Certificate

active

06447478

ABSTRACT:

FIELD OF THE INVENTION
The present invention is generally directed to apparatus and related methods for providing highly maneuverable shape memory alloy devices. More particularly, the present invention relates to devices with separately addressable thin-film shape memory alloy actuators.
BACKGROUND OF THE INVENTION
Shape memory alloys are a unique group of materials that exhibit memory retentive properties. A shape memory alloy element may be trained with a high temperature shape, and may also have a relatively deformable low temperature shape. Changes in surrounding temperatures result in a phase transformation in its crystalline structure. At lower temperatures, shape memory alloys are relatively deformable and exist in what is known as a martensitic phase. Meanwhile, at higher temperatures, these materials experience a phase transformation towards an austenitic phase which is more rigid and inflexible. The temperature at which the phase transition occurs is referred to as the activation temperature. A shape memory alloy element may be initially imprinted or trained with a particular configuration when heated to a temperature much higher than the transition temperature. Shape memory alloys have been observed to repeatedly recover their memory shape when heated above their respective transition temperatures very rapidly, and with great constant force over a wide range of retentive strain energy. The ability of shape memory alloys to remember their high temperature trained shape makes them particularly suitable for actuating devices that provide useful work and directional movement.
The basis for selecting shape memory alloys in the construction of conventional steerable elements such as a flexible catheters is primarily their ability to reversibly change shapes during their microstructural transformation. At lower temperatures, shape memory alloys are relatively soft and may exhibit a Young's modulus of approximately 3000 MPa. In a martensitic phase, the shape memory alloy may be readily deformed up to about 5% in any direction without adversely affecting its memory properties. When heated just beyond its activation temperature, the transformation process commences, and the material becomes a harder, inflexible material that may have a Young's modulus of approximately 6900 MPa in an austenite or parent phase. When the shape memory alloy material is not excessively deformed or constrained, it attempts to reorganize its structure to a previously trained or memorized shape. Upon cooling, the shape memory alloy again, becomes soft and may be mechanically deformed to begin another cycle. The mechanical deflections produced by activating the memorized state can produce useful work if suitably configured in apparatus such as actuation devices. Although the measurable recovery deflections may be relatively small, the recovery forces and energy have been observed to be extremely high and constant.
A common example of a shape memory alloy includes nickel titanium alloys (NiTi), also known as nitinol, which may vary in relative percentages of composition. The activation temperature of a particular shape memory alloy may be changed according to its elemental composition. When the alloy is heated through its transformation temperature, it reverts back to its austenite phase, and recovers its shape with great force. The temperature at which the material remembers its high temperature form may be adjusted by changes in alloy composition and specific heat treatment. For example, activation temperatures for NiTi alloys may be readily altered from 100° C. above or below zero. The shape recovery process, however, may be controlled and occur over a range of just a few degrees or less if necessary. A wide variety of shapes may be programmed into an shape memory alloy actuator element by physically constraining the piece while heating it to an appropriate annealing temperature. NiTi is commercially available in sheet, tube and wire forms, and may have a wide range of transformation temperatures. The memory transformation of an shape memory alloy element is dependent upon temperature. However, the rate of deformation is largely dependent upon the rate of cooling and heating. The rate at which temperature changes take place often dictates the relative speed at which the actuator can operate. A faster actuating shape memory alloy actuator must often be heated and cooled more readily, and has been known to consume more power and generate an excess amount of dissipated heat.
Shape memory alloy actuators have been used in numerous steerable devices such as catheters. These devices are limited in dexterity, however, and movement is often limited to a single plane and not in a rotational direction. Shape memory alloy elements must also be mechanically deformed to begin another cycle. Each shape memory element is often coupled to a biasing element or at least one other shape memory element. When one of the elements is heated and moves towards its predetermined shape, it is returned to an original position or shape by the biasing element or the activation of another memory element. This generally enables controlled motion but only in a single plane, and may provide only up to two degrees of freedom. Moreover, the relative dimensions of actuator joints are often excessively large and cumbersome since an opposite force is needed to return the shape memory alloy element to its initial martensitic shape. In general, complex linkages are also required to rotate these steerable devices. The range of maneuverability is severely limited by the linkages which are necessary to return the element to its martensitic shape after it has been activated and cooled. Conventional steerable devices using shape memory alloys are also relatively large and have a severely constrained lower size limit. The relatively large size of present actuators is mainly attributed to sizeable control arms, linkages or other elements needed to return the shape memory actuator to its initial state. This severely constrains the geometry of such a conventional steerable device. Available shape memory alloy devices today also lack the precise control necessary to maneuver into very small, geometrically complex spaces. Moreover, current actuators are often too slow for many medical applications where quick, dexterous movement is required. Large steerable devices with shape memory alloy elements often require an increased amount of current in order to produce the activation temperature needed for a quick transition from the martensitic state to the programmed or memorized austenitic phase. A conventional shape memory alloy actuator consumes a great deal of power, thus dissipating a large amount of heat. This necessarily slows down the cooling to the activation threshold, and slows down the transition from the austenitic state back to the martensitic state resulting in a slower acting device.
There is a need for an efficient actuator device that is capable of unrestricted yet highly precise and dexterous maneuvers in three-dimensional space. It would be advantageous to reduce the need for control arms, linkages, or other extraneous mechanical devices for returning conventional shape memory alloy elements to a first position after deactivation, and their transition from the parent phase back to the martensitic state. There is a further need for shape memory alloy actuators that provide unrestricted linear and rotational movement. These devices should be saleable to provide increased dexterity and maneuverability in very small, geometrically constrained areas which are presently inaccessible by conventional steerable devices. An effective heating system is further required to activate highly detailed actuator patterns formed from shape memory alloys or any other material with memory capability. It would further desirable to form a variety of actuator arrays from a minimum number of shape memory alloy sheets to simplify the production and the assembly process.
SUMMARY OF THE INVENTION
The present invention provides shape memory alloy actuat

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