Strain gauges and tourniquets incorporating such strain gauges

Abstract

A strain gauge has a fixed-length framework that conforms to a curved surface and an elastically extendable strain sensor 12 that has a portion of its length mounted on the framework along a path that

Description

04 02 25 Field of the Invention The present invention relates to strain gauges generally, and to the incorporation of such strain 5 gauges in inflatable or pneumatic tourniquet assemblies. Background of the Invention Inflatable tourniquets used in surgery are designed to occlude blood flow to a body part (most usually a limb) during the surgical procedure. They are also used in emergency trauma situations to reduce blood loss from a traumatic injury. 10 Tourniquets can be manually inflated, or can be inflated under the control of an automated controller. For conventional automated tourniquets the surgeon or operator determines what pressure is required in the tourniquet and then the electronic controller maintains this pressure in the tourniquet. There is also a category of tourniquets known as smart tourniquets. A smart tourniquet makes a decision in firmware or software about the required tourniquet pressure 15 based on operator inputs, sensor inputs and an algorithm. The electronic controller for a smart tourniquet (and indeed any automated tourniquet) may be integrated into the tourniquet, or may be an external controller. One type of sensor used in automated or smart tourniquets is a strain gauge. A strain gauge which conforms circumferentially to the limb surface at the site of the tourniquet can detect 20 minute volumetric changes which arise from systolic or diastolic blood flow, i.e. blood pulsing below the strain gauge. A typical strain gauge comprises a length of silastic tubing through which a current flows. As the tubing is elastically stretched under strain, the electrical resistance increases, and the measurement of this resistance (detectable as a change in current) provides a measure of the 25 extension in length of the sensor. Typical strain gauges used in the industry have a length of silastic tubing which is doubled back at a halfway point, so that the two endpoints of the tubing lie alongside one another for connection to terminals or electrodes of a measurement system. A drawback with such sensors, when used in tourniquets, is that their elastic limit is relatively 30 low, e.g. about 10%. At the elastic limit, moreover, the elastic restoring force of the tubing is sufficiently constrictive on the limb that it may provide a non-negligible tourniquet effect itself, occluding blood flow, while the ideal sensor will not contribute to occlusion (which should be the function of the tourniquet, and should simply monitor the volumetric changes without inhibiting them. 04 02 25 If the sensor length is tailored to the normal circumference of the limb where the sense=or is being applied, the 10% elastic limit will not normally be reached, However, neither tourniquets nor sensors are individually tailored to patients in a commercial tourniquet system. Commercially available tourniquets are provided to fit a range of limb sizes, with each 5 tourniquet being provided as an adjustable cuff (e.g. with Velcro (trade mark) or similar attachment means to accommodate a range of limb circumferences between lower and upper limits. The range of adjustment is usually of the order of about 7.5cm to 10.5 cm in typical tourniquets found on the market. A strain gauge which in its relaxed state fits around a limb with a circumference of e.g. 30 cm (if 10 this is the lower limit of the range of adjustment for the tourniquet) cannot stretch to fit a 40cm circumference limb without breaching the working limit (whether this is the elastic limit of the tubing or the limit at which the stretched tubing unduly constricts the limb to occlude blood flow). There is therefore a need for alternative strain gauges which can accommodate a larger range of limb circumferences when used in tourniquet applications. 15 There is further a need more generally for strain gauges which have a larger working range, when used in other diverse applications. Summary of the Invention In accordance with one aspect of the invention there is provided a strain gauge comprising: a sensor having an elastically extendable length, the sensor being adapted to provide a 20 sensor output which varies in dependence on the length thereof; and a framework on which the elastically extendable sensor is mounted, wherein the framework comprises a body which is conformable to a curved surface, the body having a fixed length in a first direction; the framework comprising a plurality of sensor mounting points along its length, the 25 plurality of sensor mounting points defining a sensor path having a length greater than the fixed length of the framework body; and the sensor being mounted on the plurality of sensor mounting points and movable past the sensor mounting points such that under elastic strain the sensor is free to extend or contract along the length of the sensor path. 30 By providing an elastically extendable sensor, which is mounted on a framework providing a sensor path which is longer than the framework itself, the sensor can be stretched more while remaining within its elastic limit (which is typically a fixed percentage of the full sensor length). This allows the strain gauge to be used over a larger range of lengths than conventional 04 02 25 extendable strain gauges, since the additional length of sensor “stored” in the elongated sensor path effectively allows the sensor’s length to be increased for a given length of strain gauge. Preferably, the body of the framework has a width which is perpendicular to the length of the body, and the plurality of sensor mounting points are provided in a zigzag arrangement such 5 that the sensor path follows a zigzag path back and forth across the width of the body along a portion of the length of the framework. In this way, the length of the sensor path may be a multiple of the length of the framework. In preferred embodiments, the body of the framework comprises a plurality of segments arranged adjacent to one another along the length of the body, with flexible connections 10 between pairs of adjacent segments to enable the framework to conform to a curved surface. Preferably, when the body of the framework comprises a plurality of such segments and the sensor path follows a zigzag path, at least one of the segments is provided with at least two of said mounting points spaced apart from one another in the width-wise direction of the body. Preferably, the at least one segment has first and second ends which are spaced apart in the 15 width-wise direction, and has a first mounting point disposed towards the first end and a second mounting point disposed towards the second end. It will be appreciated that it is preferred that each of the segments has the same configuration, possibly apart from the first and last segments which may have a different configuration where the sensor enters or leaves the framework. 20 Preferably, the sensor path traverses across the at least one segment from the first mounting point to the second mounting point, and traverses from the second mounting point to a mounting point of an adjacent segment disposed towards the first end of the adjacent segment. Preferably, a plurality of said segments are provided adjacent one another, each with respective first and second mounting points disposed towards the first and second ends of the segment on 25 which they are provided, and wherein the sensor path traverses each of said adjacent segments from the first mounting point to the second mounting point thereof, and traverses between adjacent segments from the second mounting point of one segment to the first mounting point of another segment. Optionally, at least one segment can have three or more mounting points, each of which is 30 disposed towards either the first end or the second end, and wherein the sensor path traverses said at least one segment in a zig-zag path between mounting points alternately disposed towards either end. So, for example, the sensor path can traverse a series of alternately inverted “W” shapes on each adjacent segment. 04 02 25 Preferably, at least one of the mounting points is a pulley around which the sensor is mounted, the pulley being free to turn to accommodate the elastic movement of the sensor. The pulley is preferably provided as a cylindrical roller rotatable on a mounting post, where the mounting post is used as a structural member of the framework. 5 Preferably, at least one of the segments comprises upper and lower structural members having a space therebetween, wherein the sensor is mounted to the segment within the space. Preferably, each segment comprises respective upper and lower members, such that the plurality of segments together provide a framework body having an upper framework assembly composed of the upper members and a lower framework assembly composed of the lower 10 members, and wherein the sensor path lies within a space defined between the upper and lower framework assemblies. Preferably, in this arrangement, at least one of the mounting points in each segment is a pulley comprising a rotatable member mounted on a post, and the post connects the upper and lower members of the segment. 15 Optionally, at least one of the mounting points is a post around which the sensor is mounted, the post being of a material chosen such that the coefficient of friction between the post and the sensor is sufficiently low that the sensor can elastically deform substantially uniformly along the sensor path by sliding past the mounting point. Preferably, the framework is sufficiently flexible to conform to a cylindrical surface having a 20 circumference of at least 25 cm. In preferred embodiments, the framework is sufficiently flexible to conform to a cylindrical surface having a circumference of at least 12.5 cm. Preferably, the length of the sensor path is at least 3 times the fixed length of the framework body. 25 More preferably, the length of the sensor path is at least 5 times the fixed length of the framework body. More preferably, the length of the sensor path is at least 6 times the fixed length of the framework body. More preferably, the length of the sensor path is at least 7 times the fixed length of the 30 framework body. Even more preferably, the length of the sensor path is between 6 and 15 times the fixed length of the framework body, more preferably between 7 and 10 times. In another aspect there is provided a tourniquet assembly comprising: an inflatable tourniquet adapted to be applied to and encircle a limb; and 04 02 25 a strain gauge according to the first aspect provided on the inflatable tourniquet such that when the tourniquet encircles a limb, the strain gauge contacts the limb circumferentially and extends around the majority of the limb’s circumference, 5 conforming to the surface of limb. By using a strain gauge as described previously, the tourniquet can be used with limbs of different sizes, the strain gauge stretching to accommodate the different limb circumferences, and the elongated sensor path within the framework taking up the extension without reaching the elastic limit of the sensor. 10 Preferably, the tourniquet assembly comprises a limb stocking which is disposed in use between the inflatable tourniquet and the limb, and wherein the strain gauge is attached to the limb stocking and thereby provided on the inflatable tourniquet. Preferably, the inflatable tourniquet is adjustable to fit closely around a range of limb circumferences between a lower limit and an upper limit, and wherein the limb stocking has a 15 circumference below the lower limit and the limb stocking is elastically stretchable to fit around a limb within the range of limb circumferences. Preferably, the strain gauge extends substantially around the entirety of the circumference of the limb stocking, and wherein the elastic stretching of the limb stocking to the upper limit causes the length of the sensor of the strain gauge to be elastically extended by no more than 20 10%. Brief Description of the Drawings The invention will now be further illustrated by the following description of embodiments thereof, given by way of example only with reference to the accompanying drawings, in which: Fig.1 is a perspective view of a strain gauge according to the invention; 25 Fig. 2 is a perspective view of the strain gauge of Fig. 1 with the upper framework structure removed; Fig. 3 is an enlarged plan view of three segments of the strain gauge of Fig. 1 with the upper framework structure and the sensor removed; Fig. 4 is a side elevation of three segments of the strain gauge, with the sensor 30 removed; Fig 5 is a side elevation, similar to Fig. 4 but with the segments flexed relative to one another; Fig. 6 is an enlarged plan view, similar to Fig. 3, with the components of Fig. 3 shown with broken lines, and with the sensor shown as a solid grey line; 04 02 25 Fig. 7 is a perspective view of the strain gauge of Fig. 1 with the upper and lower framework structures removed; Fig. 8 is an enlarged plan view, similar to Fig. 7, showing a second embodiment of strain gauge. 5 Detailed Description of Preferred Embodiments In Fig. 1 there is shown, generally at 10, a perspective view of a strain gauge. The strain gauge 10 comprises a sensor, in the form of a thin-walled Silastic tubing of 1 mm external diameter, filled with an Indium-Gallium alloy (“Silastic” is a trade mark of Dow Corning, referring to an elastically deformable silicone elastomer). 10 The sensor 12 has a first end 14 and a second end 16. The sensor passes through and is supported on a framework of connected segments 18, each comprising an upper segment member 20 and a lower segment member 22. The upper and lower segment members are rectangular in plan view and are spaced apart by and connected by a post located at either end, as described further below. The sensor 12 is supported in the spacing between the upper and 15 lower framework structures which are constituted by the set of upper segment members and the set of lower segment members, respectively. The ends of the sensor may be terminated by a conductive terminal, electrode or connector (not shown), to facilitate the connection of the sensor to a circuit such as an electrical measurement system which applies a voltage across the sensor to measure the current and hence the 20 resistance of the sensor. In practice the sensor will typically have longer ends than are shown, i.e. extend further from the framework than shown in Fig. 1. For example, when used as a strain gauge for a tourniquet, the framework may have a length that is e.g. approximately half of the circumference of the limb to which the tourniquet is applied, while the ends of the sensor extending from the framework 25 by a sufficient length to encircle the remainder of the limb circumference. Referring additionally to Fig. 2, the framework is shown with the upper segments removed. The sensor 12 can be seen to be wrapped around post-and-roller structures 24 at either end of each segment in a zig-zag manner so that it is threaded from a first end of a segment to a second end of the same segment, and then to the first end of the next adjacent segment, and so on. 30 Referring additionally to Fig. 3, three of the segments are shown, with upper segment members removed and the sensor removed. The post-and-roller structures 24 can be seen to each comprise a central post 26, which connects the upper and lower segments and holds them spaced apart, and a cylindrical roller or bearing 28 which is designed to freely rotate with minimal friction about the post. 04 02 25 Referring next to Fig. 4, three segments are shown, with the sensor removed but including both upper and lower segment sections. It can be seen that the adjacent lower segment members are connected by flexible strips 30 which allow the segments to flex relative to one another. Referring to Fig. 5, the three segments of Fig. 4 are shown flexed relative to one another. The 5 curvature achievable, due to the flexible strips 30, is significantly greater than is required e.g. to conform to a patient’s limb circumference, so that the framework as a whole can easily conform to the limb surface. Referring additionally to Fig. 6, a similar view is seen to that of Fig. 4, but with the sensor shown threaded around the rollers 28. Because the rollers can rotate with low friction around the posts 10 26, a tensile force on the ends of the sensor 12 results in the sensor stretching uniformly along its length. Referring additionally to Fig. 7, the sensor and arrangement or rollers and posts is shown without either upper or lower segment members. The section of sensor 12 shown in Fig. 6 has a length that is approximately 7 or 8 times the 15 width W of the portion of framework about which this section is threaded. Because of this, the overall length of the sensor will be a multiple of the circumference around which the sensor may be wrapped e.g. when applied to a patient limb. This means that the sensor may be stretched to conform to a range of patient limb circumferences, such as the range of circumferences typically accommodated by a given size of tourniquet, without approaching either the elastic 20 limit of the sensor or the limit above which the sensor will exert a constrictive force on the limb that would interfere with blood flow. Fig. 8 shows an alternative embodiment in which each segment 22 is provided with a pair of rollers 32 towards one end and a single roller 34 towards the opposite end. The rollers are shown without the posts present for simplicity. However, the skilled person will 25 be aware that any low-friction structure to provide a sensor path can be used (i.e. no moving parts might be required if the rollers were replaced by posts with a low coefficient of friction, or by e.g. channels through which the sensor was routed. The sensor 12 is threaded around one of the pair of rollers 32, then around the single roller 34, then around the other of the pair of rollers 32 in each segment 22 before passing to the next 30 adjacent segment where the arrangement is reversed with the pair of rollers 32 at the opposite end. In this way the sensor path describes a series of alternately inverted “W” shapes on each adjacent segment. The skilled person will appreciate that many other configurations can provide a sensor path whose length within the framework is a multiple of the length of the framework itself. The invention is not limited to the specific embodiments herein which may be varied within the scope of the appended claims. 04 02 25

Claims

1. A strain gauge comprising:a sensor having an elastically extendable length, the sensor being adapted to provide a sensor output which varies in dependence on the length thereof; anda framework on which the elastically extendable sensor is mounted,wherein the framework comprises a body which is conformable to a curved surface, the body having a fixed length in a first direction;the framework comprising a plurality of sensor mounting points along its length, the plurality of sensor mounting points defining a sensor path having a length greater than the fixed length of the framework body; andthe sensor being mounted on the plurality of sensor mounting points and movable past the sensor mounting points such that under elastic strain the sensor is free to extend or contract along the length of the sensor path.

2. A strain gauge according to claim 1, wherein the body of the framework has a width which is perpendicular to the length of the body, and wherein the plurality of sensor mounting points are provided in a zigzag arrangement such that the sensor path follows a zigzag path back and forth across the width of the body along a portion of the length of the framework.

3. A strain gauge according to claim 1 or 2, wherein the body of the framework comprises a plurality of segments arranged adjacent to one another along the length of the body, with flexible connections between pairs of adjacent segments to enable the framework to conform to a curved surface.

4. A strain gauge according to claim 3 when dependent on claim 2, wherein at least one of the segments is provided with at least two of said mounting points spaced apart from one another in the width-wise direction of the body.

5. A strain gauge according to claim 4, wherein the at least one segment has first and second ends which are spaced apart in the width-wise direction, and has a first mounting point disposed towards the first end and a second mounting point disposed towards the second end6. A strain gauge according to claim 5, wherein the sensor path traverses across the at least one segment from the first mounting point to the second mounting point, and traverses from the second mounting point to a mounting point of an adjacent segment disposed towards the first end of the adjacent segment.

7. A strain gauge according to claim 6, wherein a plurality of said segments are provided adjacent one another, each with respective first and second mounting points disposed towardsthe first and second ends of the segment on which they are provided, and wherein the sensor path traverses each of said adjacent segments from the first mounting point to the second mounting point thereof, and traverses between adjacent segments from the second mounting point of one segment to the first mounting point of another segment.

8. A strain gauge according to claim 5, wherein at least one segment has three or more mounting points, each of which is disposed towards either the first end or the second end, and wherein the sensor path traverses said at least one segment in a zig-zag path between mounting points alternately disposed towards either end.

9. A strain gauge according to any preceding claim, wherein at least one of the mounting points is a pulley around which the sensor is mounted, the pulley being free to turn to accommodate the elastic movement of the sensor.

10. A strain gauge according to any of claims 3-9, wherein at least one of the segments comprises upper and lower structural members having a space therebetween, wherein the sensor is mounted to the segment within the space.

11. A strain gauge according to claim 10, wherein each segment comprises respective upper and lower members, such that the plurality of segments together provide a framework body having an upper framework assembly composed of the upper members and a lower framework assembly composed of the lower members, and wherein the sensor path lies within a space defined between the upper and lower framework assemblies.

12. A strain gauge according to claim 11, wherein at least one of the mounting points in each segment is a pulley comprising a rotatable member mounted on a post, and the post connects the upper and lower members of the segment.

13. A strain gauge according to any preceding claim, wherein at least one of the mounting points is a post around which the sensor is mounted, the post being of a material chosen such that the coefficient of friction between the post and the sensor is sufficiently low that the sensor can elastically deform substantially uniformly along the sensor path by sliding past the mounting point.

14. A strain gauge according to any preceding claim, wherein the framework is sufficiently flexible to conform to a cylindrical surface having a circumference of at least 25 cm.

15. A strain gauge according to any preceding claim, wherein the framework is sufficiently flexible to conform to a cylindrical surface having a circumference of at least 12.5 cm.

16. A strain gauge according to any preceding claim, wherein the length of the sensor path is at least 3 times the fixed length of the framework body.

17. A strain gauge according to claim 16, wherein the length of the sensor path is at least 5 times the fixed length of the framework body.

18. A strain gauge according to claim 17, wherein the length of the sensor path is between 7 and 15 times the fixed length of the framework body.

19. A tourniquet assembly comprising:an inflatable tourniquet adapted to be applied to and encircle a limb; anda strain gauge according to any preceding claim provided on the inflatable tourniquet such that when the tourniquet encircles a limb, the strain gauge contacts the limb circumferentially and extends around the majority of the limb’s circumference, conforming to the surface of limb.

20. A tourniquet assembly according to claim 19, wherein the tourniquet assembly comprises a limb stocking which is disposed in use between the inflatable tourniquet and the limb, and wherein the strain gauge is attached to the limb stocking and thereby provided on the inflatable tourniquet.

21. A tourniquet assembly according to claim 20, wherein the inflatable tourniquet is adjustable to fit closely around a range of limb circumferences between a lower limit and an upper limit, and wherein the limb stocking has a circumference below the lower limit and the limb stocking is elastically stretchable to fit around a limb within the range of limb circumferences.

22. A tourniquet assembly according to claim 21, wherein the strain gauge extends substantially around the entirety of the circumference of the limb stocking, and wherein the elastic stretching of the limb stocking to the upper limit causes the length of the sensor of the strain gauge to be elastically extended by no more than 10%.

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