GB2614259A

GB2614259A - Elongate instrument

Info

Publication number: GB2614259A
Application number: GB2118753.9A
Authority: GB
Inventors: Benjamin Simpson Brown Andrew
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2021-12-22
Filing date: 2021-12-22
Publication date: 2023-07-05
Also published as: GB202219524D0; CN118414492A; GB2615881A

Abstract

An elongate instrument 100 comprises a plurality of superelastic Shape Memory Alloy (SMA) wires 110 arranged around an axis between proximal and distal ends. The superelastic SMA wires are arranged, on selective energization, to bend the distal end relative to the proximal end. The instrument will then remain in the bent position while the wires are not energised, due to hysteresis of the superelastic SMA wires, which constrains the distal end from returning to an unbent position when the plurality of superelastic SMA wires are not energized. There may be a flexible core 120, and SMA wires 110 may be separated by insulators 130.

Description

ELONGATE INSTRUMENT

Field

The present application relates to an elongate instrument comprising a plurality of superelastic SMA wires.

Background

Shape memory alloy (SMA) actuators are used in a variety of applications to effect motion of a movable part relative to a static part. For example, WO 2013/175197 Al describes a camera with an SMA actuator assembly SMA wires are configured to, on contraction, move the movable part in directions perpendicular to an optical axis to provide optical image stabilization (015). This actuator assembly includes flexure arms that provide a lateral biasing force that biases a lens assembly towards a central position. Holding the lens assembly in a given position may require continuously energizing the SMA wires over a prolonged time. Such an arrangement not only consumes energy during the holding period, the stability of the lens carriage may also be susceptible to sudden movements and other external factors.

In general, it is desirable to provide SMA actuators that may hold a movable portion in place without requiring continuous power supply to the SMA wires.

Summary

According to an aspect of the present invention, there is provided an elongate instrument comprising a proximal end and a distal end; a plurality of superelastic SMA wires arranged around an axis between the proximal and distal ends, wherein the superelastic SMA wires are arranged, on selective energization, to bend the distal end relative to the proximal end, and wherein the hysteretic properties of the superelastic SMA wires constrain the distal end from returning to an unbent position when the plurality of superelastic SMA wires are not energized.

Selective energization of the superelastic SMA wires may thus effect movement of the distal end relative to proximal end. Upon ceasing energization, the hysteretic properties of the superelastic SMA wires constrain the distal end from moving back to its previous position. As such, the distal end may be moved and subsequently retained in position without continuous energization of the superelastic SMA wires, reducing power consumption compared to a situation in which SMA wires need to be provided with continuous energization to hold the distal end in place.

Brief description of the drawings

Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic perspective view of an elongate instrument according to an embodiment of the present invention; Figure 2 is a graph showing the stress-strain relation of SMA material; and Figures 3A-F are cross-sectional views of embodiments of the elongate instrument.

Detailed description

The present invention provides various means for retaining a movable part at a desired position when SMA wires are not energised, thereby eliminating the need for continuously energising the SMA wires as required by known techniques. The present invention relies on hysteresis of SMA wires to retain the movable part at the desired position.

Elongate instrument Figure 1 schematically depicts an elongate instrument 100 according to embodiments of the present invention. The elongate instrument 100 comprises a proximal end 100a and a distal end 100b. The elongate instrument 100 extends between the proximal end 100a and the distal end 100b. Figures 3A-3E depict cross-sections perpendicular to the axis A of embodiments of the elongate instrument 100.

The elongate instrument 100 comprises a plurality of superelastic SMA wires 110. The superelastic SMA wires 110 are arranged around an axis A. The superelastic SMA wires 110 extend between the proximal end 100a and the distal end 100b.

The superelastic SMA wires 110 are arranged, on selective energization (i.e. heating), to bend the distal end 100b relative to the proximal end 100a. SMA material has the property that on heating it undergoes a solid-state phase change which causes the SMA material to contract. At lower temperatures the SMA material enters the Martensite phase. At higher temperatures the SMA enters the Austenite phase which induces a deformation causing the SMA material to contract. The phase change occurs over a range of temperature due to the statistical spread of transition temperature in the SMA crystal structure. Thus, heating of the superelastic SMA wires 110 may cause them to decrease in length. Selective energization of the superelastic SMA wires 110 may thus lead to selective contraction of the superelastic SMA wires 110, which may be used to bend the distal end of the elongate instrument 100.

The superelastic SMA wires may each be electrically connected to a control circuit (not shown) which may be implemented in an integrated circuit (IC) chip, for example. The control circuit in use applies drive signals to the superelastic SMA wires which resistively heat the superelastic SMA wires, causing them to be energized and to contract. The plural superelastic SMA wires may be driven independently or otherwise. The control circuit may also measure the resistance of the superelastic SMA wires, and use the measured resistance to calculate/determine the position of the distal end 100b. In general, however, the superelastic SMA wires may be heated so as to contract by any other suitable means, such as via an external heat source, radiative heating or inductive heating.

The superelastic SMA wires exhibit hysteretic properties that constrain the distal end from returning to an unbent position when the plurality of superelastic SMA wires are not energized.

In general, the superelastic SMA wires, when unenergized, exhibit pseudo-elastic properties in which the superelastic SMA wires are in a state between a full austinite phase and a full martensite phase. With reference to Figure 2, for example, the superelastic SMA wires may be in the pseudo-elastic range (ii) between the full Martensite (i) and the full Austenite (iii) phase. The superelastic SMA wires thus have a phase transition (from the full Martensite to the full Austenite phase) within a temperature range that is below the average operating temperature at which the superelastic SMA wires normally operate. The average operating temperature of the superelastic SMA wires may be the ambient temperature of an environment within which the superelastic SMA operate (e.g. the body temperature of about 37°C of a human for operation within the human body), or may be greater than the ambient temperature due to heating effects of operating the superelastic SMA wires. The superelastic SMA wires may have a phase transition within a temperature range that is below 80°C, preferably below 50°C, for example.

The superelastic SMA wires 110 may be in tension. The superelastic SMA wires may be stretched to a strain that places them halfway across the pseudo-elastic region or more than halfway across the pseudo-elastic region depicted in Figure 2.

Alternatively, the superelastic SMA wires 110 may be in compression. The superelastic SMA wires may be compressed to a strain that places them halfway across the pseudo-elastic region or more than halfway across the pseudo-elastic region.

Arrangement of superelastic SMA wires The plurality of superelastic SMA wires may comprises two superelastic SMA wires 110. This is schematically depicted in Figure 3A, for example. The two superelastic SMA wires 110 are configured, on selective energization, to bend the distal end 110b in opposite directions. The two superelastic SMA wires 110 may be in tension and strained in an unbent starting position, i.e. in a position in which the elongate instrument is straight and extends along a straight axis between the proximal and distal ends 100a, 100b. Energization (so heating) of one of the superelastic SMA wires 110 will lead to contraction of that superelastic SMA wire 110. With reference to Figure 3A, for example, contraction of the right superelastic SMA wire 110 will bend the distal end 100b of the elongate instrument 100 towards the right. Contraction of the left superelastic SMA wire 110 will bend the distal end 100b of the elongate instrument 100 towards the left. Selective energization and so selective contraction of the superelastic SMA wires 110 may thus be used to bend the distal end 110b in opposite directions and in one degree of freedom.

Upon ceasing energization of the superelastic SMA wires 110, the hysteretic properties of the superelastic SMA wires 110 will prevent the distal end from returning to its unbend position. The elongate instrument will thus not automatically straighten when energy supply to the superelastic SMA wires ceases. This allows the distal end to maintain its bent position without requiring power supply to the superelastic SMA wires, reducing the overall power consumption of the elongate instrument.

Figure 3B depicts an alternative embodiment in which the plurality of superelastic SMA wires comprises three superelastic SMA wires 110. The superelastic SMA wires are configured, on selective energization, to bend the distal end in any direction. Energizing any one of the superelastic SMA wires 110, for example, will bend the distal end in the angular direction in which the one superelastic SMA wire 110 is arranged relative to the axis A. So, energizing the bottom superelastic SMA wire 110 in Figure 3B will bend the distal end of the elongate instrument 100 towards the bottom. Energizing two superelastic SMA wires will bend the distal end in an angular direction between the angular directions in which the two superelastic SMA wires 110 are arranged. In this manner, the distal end of the elongate instrument 100 may be bent in any direction.

Figure 3F depicts another alternative embodiment in which the plurality of superelastic SMA wires comprises four superelastic SMA wires 110. The superelastic SMA wires are configured, on selective energization, to bend the distal end in any direction. The four superelastic SMA wires 110 are arranged in two pairs of superelastic SMA wires no. Within each pair, the superelastic SMA wires 110 are configured, on selective energization, to bend the distal end 110b in opposite directions. The two pairs of superelastic SMA wires 110 bend the distal end 110b in directions that are perpendicular to one another. So, the superelastic SMA wires 110 of a first pair may bend the distal end in opposite first directions, and the superelastic SMA wires 110 of a second pair may bend the distal end in opposite second directions, where the first and second directions are perpendicular to one another. Using four superelastic SMA wires 110 may thus effectively decouple bending in perpendicular directions, which may make control of the position of the distal end 100b simpler (at the cost of an extra superelastic SMA wires 110) compared to using three superelastic SMA wires 110.

Flexible core As shown in Figures 3A, 3B, 3C and 3F, the elongate instrument 100 may further comprise a flexible core 120. The flexible core 120 is not necessarily required. The superelastic SMA wires 110 surround the flexible core 120 The superelastic SMA wires 110 may be evenly distributed around the flexible core 120.

The flexible core 120 may carry the tensile or compressive force in the superelastic SMA wires 110. The superelastic SMA wires 110 may be arranged to be in tension and strained in an unbent position of the distal end 100b. The flexible core 120 may be in compression and resisting buckling of the elongate instrument 100. Alternatively, the superelastic SMA wires 110 may be arranged to be in compression and strained in an unbent position of the distal end 100b. The flexible core 120 may be in tension. So, in the unbent position the flexible core 120 may be in compression and the plurality of superelastic SMA wires 110 in tension, or wherein the flexible core 120 may be in tension and the plurality of superelastic SMA wires 110 in compression.

The plurality of superelastic SMA wires 110 are coupled to the flexible core 120. The plurality of superelastic SMA wires 110 may be glued or otherwise affixed to the flexible core 120. Alternatively, a sleeve may surround the plurality of superelastic SMA wires 110 so as to keep the superelastic SMA wires coupled to the flexible core 120. The plurality of superelastic SMA wires 110 may be coupled to the flexible core 120 along the entire extent of the superelastic SMA wires 110, or may at least be coupled at separated points. The superelastic SMA wires 110 may be coupled at least at the distal end to the flexible core 120.

Optionally, the flexible core comprises one or more electrical conductors connected at the distal end 100b to the plurality of superelastic SMA wires 110. The flexible core may be made from an electrically conductive material. The electrical conductors may be used to selectively energize the plurality of superelastic SMA wires 110. For example, the electrical conductor may be used to provide a common ground connected to the SMA wires 110 at the distal end 100b, and In some embodiments, the flexible core 120 comprises superelastic SMA. Superelastic SMA may be particularly suitable to carry the relatively large tensile or compressive forces that may arise in the plurality of superelastic SMA wires 110.

Insulator wires As shown in Figures 3C and 3D, the elongate instrument 100 may further comprise insulator wires 130. The insulator wires 130 coextend with the plurality of superelastic SMA wires 110. The insulator wires 130 are provided between adjacent superelastic SMA wires 110. The insulator wires 130 may serve to electrically and/or thermally insulate the superelastic SMA wires 110 from one another. Electrical shorting between the superelastic SMA wires 110 may thus be avoided.

Providing the insulator wires 110 may be provided as spacers between adjacent superelastic SMA wires 110, making assembly of the superelastic SMA wires 110 easier. Direct contact between the superelastic SMA wires 110 may thus be reliably avoided.

In some embodiments, the superelastic SMA wires 110 may be coated with an insulating coating, thus further reducing the risk of direct contact between adjacent superelastic SMA wires 110.

Compressive and tensile superelastic SMA wires As explained above, the superelastic SMA wires 110 may be arranged to be in tension or in compression, and are strained in an unbent position of the distal end. In the embodiments of Figures 3A-D, the tensile or compressive force in the superelastic SMA wires 110 is carried by the flexible core 120 and/or the insulator wires 130.

In another embodiment, schematically depicted in Figure 3E, the plurality of superelastic SMA wires 110 comprises a first set 110 of superelastic SMA wires that are arranged to be in tension and strained in an unbent position of the distal end and a second set 110' of superelastic SMA wires that are arranged to be in compression and strained in an unbent position of the distal end. The superelastic SMA wires of the first set 110 thus carry the forces in the superelastic SMA wires of the second set 110', and vice versa.

As shown in Figure 3E, the superelastic SMA wires of the second set 110' may be arranged diametrically opposite the axis to the superelastic SMA wires of the first set 110. Bending of the distal end 100b of the elongate instrument 100 may be effected by energizing diametrically opposite superelastic SMA wires 110. With reference to Figure 3E, for example, energizing the bottom SMA wire 110 (which is in tension) and concurrently energizing the top SMA wire 110' (which is in compression) will result in the bottom SMA wire 110 contracting and the top SMA wire 110' extending. The distal end 100b thus bends downward.

Using superelastic SMA wires both in compression and in tension may allow the tensile/compressive forces in the superelastic SMA wires to be balanced more easily. Furthermore, bending of the distal end is achieved by both extending and concurrently contracting selected superelastic SMA wires, improving movement control of the distal end 100b.

In preferred embodiments, there are at least three superelastic SMA wires in the first set of superelastic SMA wires. The three superelastic SMA wires 110 are arranged as already described in relation to Figure 3B. This allows the distal end to be bent in any direction.

Relative diameters of wires Although Figures 3A-E depict the superelastic wires 110, the flexible core 120 and the insulator wires as having substantially equal diameters, in general the diameters of these elements may differ, in some cases significantly.

For example, the diameter of the flexible core 120 may be larger than the diameter of the superelastic wires 110. This may be particularly beneficial to allow the flexible core to carry the relatively large tensile or compressive forces in the superelastic SMA wires 110. For example, the diameter of the flexible core 120 may be at least twice, or at least three times, or at least five times the diameter of the superelastic SMA wires 110.

The diameter of the insulator wires 130 may be similar to the diameter of the superelastic SMA wires 110. Alternatively, the diameter of the insulator wires 130 may be greater than the diameter of the superelastic SMA wires 110. For example, the diameter of the insulator wires 130 may be at least twice, or at least three times, or at least five times the diameter of the superelastic SMA wires 110.

The elongate instrument may be a guide wire. Such a guide wire can be inserted into a patient and is then used to guide the insertion of larger tools such as surgical tools, endoscopes, probes etc into patients. The elongate instrument may thus have a maximum extend, perpendicular to the axis A, that is less than 2mm, preferably less than 1mm. The elongate instrument may have a maximum extend, perpendicular to the axis A, in the range from 0.3 to 0.9mm, for example.

Assembly of elongate instrument As described above, the superelastic SMA wires 110 may be stretched or compressed to a strain that is approximately halfway across the pseudo-elastic region or more than halfway across the pseudo-elastic region.

This may be achieved during assembly of the elongate instrument, for example, by tensioning and stretching the (first set of) superelastic SMA wires 110 with an external force prior to coupling the superelastic SMA wires 110 to the flexible core 120, insulator wires 130 and/or second set of superelastic SMA wires 110'. The superelastic SMA wires 110 (of the first set) may be stretched more than halfway across the pseudo-elastic region. After coupling, the external tensile force may be removed to allow the superelastic SMA wires 110 (of the first set) to contract until the tensile force in the superelastic SMA wires 110 is balanced by the compressive force in the flexible core 120, insulator wires 130 and/or second set of superelastic SMA wires 110'.

Alternatively, during assembly of the elongate instrument the flexible core 120 and/or insulator wires 130 may be tensioned and stretched with an external force prior to coupling to the superelastic SMA wires 110. After coupling, the external tensile force may be removed to allow the flexible core 120 and/or insulator wires 130 to contract until the tensile force in the flexible core 120 and/or insulator wires 130 is balanced by the compressive force in the superelastic SMA wires 110.

SMA wire The above-described SMA actuator assemblies comprise an SMA wire. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire.

It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires.

Claims

Claims 1. An elongate instrument comprising: a proximal end and a distal end; a plurality of superelastic SMA wires arranged around an axis between the proximal and distal ends, wherein the superelastic SMA wires are arranged, on selective energization, to bend the distal end relative to the proximal end, and wherein the hysteretic properties of the superelastic SMA wires constrain the distal end from returning to an unbent position when the plurality of superelastic SMA wires are not energized.
2. An elongate instrument according to claim 1, wherein the plurality of superelastic wires are arranged to be in tension or in compression, and are strained in an unbent position of the distal end.
3. An elongate instrument according to claim 2, wherein at the unbent position of the distal end the plurality of superelastic SMA wires are strained equally.
4. An elongate instrument according to any one of claims Ito 3, wherein the plurality of superelastic SMA wires comprises two superelastic SMA wires, wherein the two superelastic SMA wires are configured, on selective energization, to bend the distal end in opposite directions.
5. An elongate instrument according to any one of claims Ito 3, wherein the plurality of superelastic SMA wires comprises three or more superelastic SMA wires, wherein the superelastic SMA wires are configured, on selective energization, to bend the elongate instrument in any direction.
6. An elongate instrument according to any one of claims Ito 3, wherein the plurality of superelastic SMA wires comprises two pairs of superelastic SMA wires, wherein the superelastic SMA wires within each pair are configured, on selective energization, to bend the elongate instrument in opposite directions and wherein the pair of superelastic SMA wires are configured, on selective energization, to bend the elongate instrument in penpendicular directions.
7. An elongate instrument according to any preceding claim, wherein the plurality of superelastic SMA wires are evenly distributed around the axis.
8. An elongate instrument according to any preceding claim, further comprising a flexible core extending along the axis, wherein the plurality of superelastic SMA wires are coupled to the flexible core.
9. An elongate instrument according to claim 8, wherein the flexible core comprises one or more electrical conductors connected at the distal end to the plurality of superelastic SMA wires.
10. An elongate instrument according to claims or 9, wherein the flexible core comprises superelastic SMA.
11. An elongate instrument according to any one of claims 8 to 10, wherein in the unbent position the flexible core is in compression and the plurality of superelastic SMA wires are in tension, or wherein the flexible core is in tension and the plurality of superelastic SMA wires are in compression.
12. An elongate instrument according to any preceding claim, further comprising insulator wires that coextend with the plurality of superelastic SMA wires and are provided between adjacent superelastic SMA wires.
13. An elongate instrument according to any preceding claim, wherein the plurality of superelastic SMA wires comprises a first set of superelastic SMA wires that are arranged to be in tension and strained in an unbent position of the distal end and a second set of superelastic SMA wires that are arranged to be in compression and strained in an unbent position of the distal end.
14. An elongate instrument according to claim 13, wherein the superelastic SMA wires of the second set are arranged diametrically opposite the axis to the superelastic SMA wires of the first set.
15. An elongate instrument according to claim 13 or 14, wherein the superelastic SMA wires of the first set are comprises three or more superelastic SMA wires that are configured, on selective energization, to bend the elongate instrument in any direction.