GB2457096A

GB2457096A - Strain gauge having support structure formed integrally with flexible substrate and method of manufacture thereof

Info

Publication number: GB2457096A
Application number: GB0802010A
Authority: GB
Inventors: Tony Corless
Original assignee: University of Newcastle, The; Newcastle University of Upon Tyne
Current assignee: University of Newcastle, The; Newcastle University of Upon Tyne
Priority date: 2008-02-04
Filing date: 2008-02-04
Publication date: 2009-08-05
Also published as: GB0802010D0

Abstract

A strain gauge comprises: flexible film substrate 2, strain sensing element 16 supported by substrate 2, and support structure 20 integrally formed with substrate 2. An array of such strain gauges is formed on a common substrate and is preferably cylindrical. Preferably, support structure 20 is substantially rigid, formed of an electro-plated material and is coplanar with and formed on the opposite surface of substrate 2 from sensing element 16. Preferably substrate 2 comprises a polymeric layer and an additional polymeric layer is preferably formed on element 16. The strain gauge is manufactured by electro-deposition or photolithography, the method comprising the steps of: coating flexible substrate 2 with first thin metal film 4 and second thin metal film 6 of a different metal; etching selected portions of thin metal films 6, 4 to expose substrate 2 to form strain sensing element 16 and an interconnect structure; and applying a third metal film to substrate 2 to provide support structure 20. Further methods of manufacturing the strain gauge are described detailing the order in which layers are deposited and removed.

Description

1 2457096 Strain Gauge and Method of Manufacture Thereof The present invention relates to a strain gauge and method of manufacture thereof.

It is desirable to provide a sensor device that may be used in medical applications, such as examining the function of the oesophageal sphincter. In this case, there is a clinical need to determine: whether the sphincter closes with a sufficient pressure; whether the sphincter closes uniformly or has regions exerting little or no pressure; and whether the peristaltic wave which transfers foodstuff is operating correctly. A similar sensor may be used to monitor the function of the urethral sphincter.

The sensor must be small, the more so since it must be loaded into the body for example through the nasal passages, to avoid the gag reflex that would occur were it simply loaded through the mouth. In the urethral application the device is loaded through the urethra, and again small size is desirable. It will be further appreciated that it is desirable that such devices are "single use" and must be low cost.

Thin film strain gauges on polyimide film carriers have been developed with these requirements in mind. A known failure mode for the oesophageal sphincter is for one section of the sphincter to fail to close effectively. In the case of an oesophageal sensor therefore, if the operation of portions of the sphincter is to be determined, then it is desirable to form the gauges into a cylindrical array of strain gauges, the individual gauges being deformed by the operation of the sphincter. In such an arrangement, it is necessary to isolate the measurement of one gauge from another, that is to reduce gauge "crosstalk" In order to form a cylindrical array of strain gauges it is desirable to mount the gauges onto a pre-formed or formable, carrier. Such a carrier may produce an overall cylindrical form, in which the previously flat film of the gauges is formed to a cylinder, so that each gauge is an arch, or may produce an approximation to a cylinder in which each gauge is flat with a folded or bent section of film between gauges.

In the first case, in which the gauges are formed into an arch, stress is applied to the gauge which has produced cracking or buckling of the thin film in some cases This cracking has been mitigated by the use of very thin support film. This is apparent from consideration of the mechanics, in folding or rolling the film the interior surface is compressed and the exterior extended. The degree of compression or extension depends on the thickness of the film.

In the second case, the gauges remain flat and are thus not stressed. However, the bend radius in the sections of film between gauges is consequently tighter. This means that the stress on any tracks over that section is higher.

In the case of an oesophageal sensor, gauges are typically applied as one or more cylindrical arrays, with the cylindrical arrays arranged along the length of the tract at convenient intervals. Each cylinder of the array may comprise any suitable number of gauges. Conveniently, each cylinder may comprise 4, 6, 8, 12, 16, 24, or 32 gauges.

The task of forming the gauges and attaching them to a former is difficult. This is partly due to the small size and partly as the material is inherently and essentially flimsy. The gauges may be attached to a former using an adhesive. However, it is difficult to apply the adhesive, and more difficult to prevent adhesive from covering the film in the region of the gauge, where it alters the mechanical properties and prevents accurate operation. Moreover, analysis of the resulting array shows that the adhesive material is not mechanically strong. Adhesives typically have a low modulus of elasticity. The effect of the low modulus is that when stress is applied to a first gauge the adhesive will to an extent deform and in so deforming will reduce the stress at the first gauge and transfer some part of said stress to a second, adjacent gauge. That is, the low modulus adhesive results in a high level of inter-gauge crosstalk. Further known adhesives of high modulus would be difficult to apply in this device structure.

For sensing of the function of the urethral sphincter, a single gauge may suffice. In this case the gauge may be used in its flat form.

Conveniently, a sensor will include at least one so-called "dummy" gauge. This dummy gauge is of similar construction to the gauge or gauges which provide the sensing function, the so-called "active" gauge or gauges, but is substantially insensitive to the mechanical stress applied. However, the dummy gauge does experience similar changes in temperature to the active gauges and is thus a convenient means of supplying a temperature compensation. Conveniently the dummy gauge will be positioned under a part of the support structure that is in a substantially rigid part of the device The number and disposition of dummy gauges will typically depend on the electronic read out means used. For example, if an analogue read out means is used then it may be convenient to have one dummy gauge for each active gauge so that a so-called half-bridge configuration can be utilised. If the read out means includes a first digitising stage, so that readings of each gauge resistor are stored as digitised values, then a single dummy gauge may be used and the compensation applied by subsequent computation.

According to an embodiment of a first aspect of the present invention there is provided a strain gauge comprising: a flexible film substrate; a strain sensing element supported by the substrate; and a support structure integrally formed with the substrate.

According to a preferred embodiment of the invention, the support structure is a substantially rigid support structure. The substantially rigid support structure enables the gauge to deform in a defined way in response to an applied force.

For the purposes of this specification, substantially rigid includes within its meaning the possibility that the support structure may be deformed intentionally, for example during the manufacture or assembly of the strain gauge.

An example of an embodiment of a substantially rigid support structure is a support structure that, during the use of the strain gauge provides a sufficient degree of constraint to the free movement of the film in response to an applied force that the film is deformed by the applied force, allowing determination of the force in such a manner and to the extent required for a specific application.

The mechanical modulus of the support structure may be high relative to that of the flexible film substrate.

The flexible film substrate may comprise a polymeric layer. which may be a polyimide film and the thickness of the polymeric layer may be less than 26pm and preferably less than 10pm, for example 8pm.

An additional polymeric layer may be formed on the strain sensing element. The thickness of the additional layer may be approximately 2pm or less. The thickness and mechanical modulus of the flexible film substrate may be much less than those of the additional polymeric layer, such that the behaviour of the strain gauge is dominated by the mechanical behaviour of the additional polymeric layer.

According to an embodiment of a second aspect of the present invention there is provided a strain gauge array comprising: a flexible film substrate; a plurality of strain sensing elements supported by the substrate; and a plurality of support structures integrally formed with the substrate The support structures may be substantially rigid support structures. The support structures may be integrally formed on a surface of the substrate that is opposite to the surface on which the strain sensing element is supported. The opposed free ends of the substrate may be brought into and retained in proximity such that the array is of substantially circular form. The array may further comprise a support operable to impart a substantially cylindrical form to the array.

According to an embodiment of a third aspect of the present invention there is provided a method of manufacturing a strain gauge or strain gauge array according to the first or second aspect of the present invention, wherein the or each support structure is integrally formed with the flexible film substrate by an electro-deposition technique.

The electro-deposition technique may comprise electroplating or electroless plating.

According to an embodiment of a fourth aspect of the present invention there is provided a method of manufacturing a strain gauge or strain gauge array according to the first or second aspect of the present invention, wherein the or each support structure is defined by a photolithographic technique.

According to an embodiment of a fifth aspect of the present invention there is provided a method of manufacturing a strain gauge comprising the steps of: (a) coating a flexible substrate with a first thin metal film and a second thin metal film of a different metal; (b) etching selected portions of the second and first thin metal films to expose the substrate to form a strain sensing element and an interconnect structure; and (c) applying a third metal film to the substrate to provide a support structure.

The third metal film may be locally thickened by means of selective electroplating.

According to an embodiment of a sixth aspect of the present invention there is provided a method of manufacturing a strain gauge comprising the steps of: (a) coating a carrier substrate with a release layer; (b) depositing a first polymeric film layer on said release layer; (c) depositing at least one metallic layer over the first polymeric layer and patterning the at least one metallic layer to form a strain sensing element and track; (d) depositing a second polymeric film layer over the strain sensing element and track; (e) forming via contacts through the second polymeric film, (f) depositing a thick metal layer over selected portions of the second polymeric film layer to form a support structure; and (g) removing the release layer to separate the said first polymeric layer from the carrier substrate.

The carrier substrate may be a silicon wafer, glass wafer or tile of smooth or glazed ceramic. Step (f) may also include the formation of thickened tracks or connection pads. The thickened tracks that may be formed at stage (f) may be used to reduce the resistance of the tracking and/or to provide a robust region for interconnection to a subsequent interconnection, such as a wiring harness or printed circuit board.

According to a seventh aspect of the invention there is provided a method of manufacturing a strain gauge comprising the steps of: (a) depositing a first polymeric film layer on a carrier substrate that is substantially transparent to radiation of a predetermined wavelength; (b) depositing a metallic layer over the first polymeric layer and patterning the metallic layer to form a strain sensing element and track; (c) depositing a second polymeric film layer over the strain sensing element and track; (d) depositing a thick metal layer over selected portions of the second polymeric film layer to form a support structure; and (e) separating the said first polymeric layer from the carrier substrate by means of a radiation pulse.

The radiation may be selected to facilitate ablation or otherwise detaching of the first polymeric layer. The radiation pulse may be short pulse laser radiation, such as excimer laser radiation. The substrate may be a quartz substrate. The radiation pulse that separates the first polymeric layer from the carrier substrate may comprise intense short-duration radiation pulses such as to cause local ablation of that portion of the thin polymer of stage (b) which is adjacent to the substrate.

According to an eighth aspect of the present invention there is provided a method of investigating a function of a sphincter of a human or animal body using a strain gauge according to the first aspect of the present invention or a strain gauge array according to a second aspect of the present invention.

According to a ninth aspect of the present invention there is provided a method of diagnosis of a human or animal body using a strain gauge according to the first aspect of the present invention or a strain gauge array according to the second aspect of the present invention.

Thus, in a first embodiment of the present invention, rather than bonding the device to a support structure, a support structure is formed integral'y with the device. In one embodiment of the present invention, on the (polyimide) support film on which the gauges have been, or will subsequently be, formed a support structure is electroplated in a material of relatively high modulus. To facilitate electroplating an adhesion and seed layer material (by way of non-limiting example, a thin layer of Titanium with a copper, nickel or gold overlayer) is first sputtered onto a substrate. Subsequent to this a method known as "pattern plating" is used to form a thick layer of electroplated metal such as nickel. Preferably the thin layer of adhesion and seed layer is removed after support structure formation except in the regions covered by the support structure. By virtue of the intimate contact between the support structure and the film the crosstalk is minimised. Moreover, it will be apparent that the method allows for photolithographic definition of a support structure of high complexity, considerable accuracy, and with close alignment to the strain gauge structures. Further, judicious use of the opportunity to form a complex support allows the metal structure to be designed and manufactured to facilitate subsequent forming of the gauge array into the required 3-dimensional form. Conveniently, but not essentially, the support structure is formed on the opposite major surface of the thin substrate to the gauges. This facilitates the incorporation of tracking between the gauges without requiring additional insulating layers. Typically the gauge array will have an outer diameter of no more than 4mm and will be short. The support film is approximately 12mm x 4mm.

Reference will now be made, by way of example, to the accompanying drawings, wherein: Figure 1 is a schematic sectional representation of a strain gauge embodying the present invention; Figure 2 is a plan view of a strain gauge embodying the present invention, the flexible film substrate being omitted for clarity; Figures 3A to 31 are respective cross-sectional views taken on line A-A' in Fig. 2 illustrating a process of manufacturing a strain gauge embodying the present invention; and Figures 4A to 4C show respective images of strain gauges formed using an alternative manufacturing process.

A method for the cost-effective fabrication of sensor elements on flexible substrates is disclosed. This method is advantageous in providing a first level of interconnection and lead out 14 from a sensor element 16 integrated with same with a single vacuum metalisation step and optionally within a single photoresist coating stage.

Advantageously, this method is compatible with roll-to-roll vacuum metalisation systems and, in sufficient volume, with subsequent roll-to-roll photolithographic patterning and etching.

According to a first aspect of the method, illustrated in Figures 1 to 3, an electrically insulating or insulated flexible substrate 2 is coated with a first thin film of a metal 4 and a second thin film of a different metal 6. It is subsequently coated with a positive working photoresist 8 which is dried and imaged with a first pattern 10 which is transferred by etching through metal films 6 and 4, exposing substrate 2 before being imaged a second time to a second pattern 12, being a subset of the first pattern 10, which exposes further material of metal film 6 for subsequent etching. Following this the photoresist 8 is removed. Alternatively, the photoresist 8 may be removed after the completion of the first etching stage and then a new photoresist coat applied for the second image. It will be apparent that the sensor element 16 is composed only of the first thin metal film 4 whereas the first level of interconnection lead out 14 is composed both of first thin metal film 4 and of second thin metal film 6. The formation of the sensor elements is thus completed.

The reverse face of the substrate 2 is then processed to provide a support frame structure. A first strike layer 18, comprising a combination of metals having good adhesion to the substrate and utility as a plating strike layer, is deposited by thin film technique such as sputtering or evaporation. A thick photoresist film 22 is applied and apertures are opened in the resist in the pattern of the support frame structure 20.

Electrodeposition, e g. of a nickel, is then employed to form the structure of the frame 20. The thick resist is removed and optionally that portion of the strike layer 18 that is not covered by the frame may be removed, for example by wet etching. It will be appreciated that the sensors and interconnect formed on the first face of the substrate may be protected from the processing to form the frame, for example by the application of an overall resist material, which is removed on completion of the processing.

Metal films 4 and 6 are chosen according to selection rules, such that metal film 4 has good adhesion to the flexible substrate 2 and serves as a strain sensing layer, for which it is desirable that the material has a low temperature coefficient of resistance. It also serves to adhere the metal film 6 to the substrate. Metal film 6 is typically of a low resistance to facilitate electrical connection. Moreover, the combination of metals 4 and 6 is chosen such that they may be chemically etched with adequate selectivity. By way of non-limiting example the combination may be Nichrome with gold Those skilled in the art will recognise that chemical etchant materials may be selected that etch Nichrome but not gold, and that etch gold and not Nichrome. Similarly the metal combination for the strike layer 18 is chosen to facilitate etching of the strike layer without damage to the plated nickel frame. By way of non-limiting example the strike layer 18 may be formed of titanium and copper.

In a currently preferred embodiment the substrate 2 is a film of polyimide, metal 4 is Nichrome, metal 6 is gold, photoresist 8 is S1813, a product of Rohm and Haas, and patterns 10 and 12 combine to form a strain gauge sensor structure in the metal film 4 with lead out tracking and connection pads in the metal film 6. By way of non-limiting example the structure may take the form illustrated in Figure 2 in which patterns for the sensor element 16 and the first level of interconnection and lead out 14 are illustrated.

It is recognised that a common feature of strain gauge designs is that there is a compromise between the elements which are aligned approximately along the strain axis and elements which are generally orthogonal to this axis. This situation naturally occurs due to the meandering pattern typical of such gauges. It is advantageous to minimise the resistance of the sections orthogonal to the strain. In the manufacturing sequence illustrated it is possible so to design the photolithographic mask that the low resistance second metal film 6 overlays the first metal film in all of the areas orthogonal to the intended axis of strain sensitivity. This approach is not illustrated in the appended drawings for reasons of clarity of illustration. The line A-A' in Figure 2 is used to reference the position of a section which is used to illustrate an example process sequence in Figures 3A-31. Fig. 3A illustrates the substrate with first and second metal films 4, 6 Fig. 3B illustrates the photoresist 8 imaged with first pattern prior to etching of metal films 4, 6. Fig. 3C shows the section after etching of metal films 4, 6 to the first pattern 10. Fig. 3D shows the photoresist 8 imaged with second pattern 12 after etching of the second metal film 6. Fig. 3E shows the completed sensor, with photoresist 8 removed, but prior to the processing to form the frame on the opposing face of substrate 2. Figure 3F shows the adhesion metal I strike layer 18 coated on the reverse surface of the film. Figure 3G illustrates the addition of a thick resist film 22 with apertures formed therein. Figure 3H shows an electrodeposited frame material (20) added to the structure. Figure 31 shows the complete structure following removal of the resist 22 and the optional etching of exposed portions of adhesion I strike layer 18.

An alternative, wafer-based, processing method is described below: 1. Silicon wafers are coated with a material providing a release layer, in the current process this is the sequential deposition of a chrome layer and an aluminium layer. The thicknesses of these layers are not critical, but conveniently the chrome is of order 50 to lOOnm and the aluminium is of order 1 pm. Preferably an evaporation method is used for the aluminium layer to provide a smoother surface than is achieved by sputter deposition.

2. A first polymeric layer, such as a polyimide layer is deposited. This layer is preferably thin, such as less than 2pm, with 1pm or less being more preferred.

This layer serves as a barrier between the deposited aluminium and the subsequent layers. It is desirable that it is not a mechanically strong component. This layer is typically thermally cured before further process layers are deposited.

3. A metal bi-layer comprising for example Nichrome and gold is deposited and patterned essentially as was described for the film-based process. This provides Nichrome sensor elements and gold tracks.

4. A second polymer layer is deposited. This layer plays the role of the film of the previous process and hence may be several microns, such as 8pm, in thickness This layer is patterned to provide contact vias through the polymer layer to allow connections to the tracks. This patterning is well-known, and may take the form of wet-etch processing prior to polymer curing or of reactive ion etching after the polymer curing.

5. A plating seed or strike layer is deposited. This conveniently comprises titanium and copper, but other materials may be selected from a well-known range.

6. A thick photoresist is deposited and apertures are imaged therein in the pattern of at least a frame support element, but preferably also of electrical contact pads which are in communication with the tracks of step 3 through the vias of step 4.

7. A thick metal layer is electroplated, typically a nickel layer with optionally a thin overlayer of electroplated gold. This thick metal layer forms the "frame" in the same way as for the film-based gauge but in this construction there is the option for this same plating layer also to form thick electrical conductor and pads.

Following plating the photoresist is stripped, and the plating seed/ strike layer of step 5 is etched away in those regions in which it has not been over-plated.

8. A further photoresist layer may be applied and a further reactive ion etch, or plasma etch, step applied to cut through the polymer (polyimide) layers to separate wholly the individual gauges from each other (while they remain on the wafer) or to largely separate the gauges while leaving convenient multiples of gauges mechanically connected by tabs of polymer (polyimide).

9. The release layer material of step us removed and the gauges are then separated from the silicon wafer with the end result being similar to gauges fabricated on film. In a currently preferred process the release layer material is removed by the method of anodic dissolution in a saline (brine) solution.

Alternatively the aluminium, or other release, may be dissolved in a more aggressive chemistry without the need for an anodic connection. In a variation of the process, the gauges may be released by so-called laser ablation release.

In this process the substrate would be chosen to be transparent to a chosen laser radiation, such as a quartz substrate. No specific release layer is required but rather a high energy laser pulse, such as an excimer laser pulse, applied through the substrate ablates an extremely thin portion of the polymer of step 2 and thereby releases the film.

The gauge is within the polymer multi-layer (bi-layer) and this necessarily reduces the sensitivity n essence the closer the mechanical thickness (by which is meant the appropriate combination of the thickness and modulus) of the two polymer layers of step 2 and step 4 the nearer the gauge is moved to a neutral axis for the strain of the overall polymer. Ideally, therefore, one layer (preferably that of step 2) is made as thin and mechanically low modulus as is practical and the other layer (preferably that of step 4) then dominates the mechanical behaviour. Hence the gauge sees a strain which approximates to that which would be seen by a gauge on a film comprising only the polymer of step 4.

The wafer-based process flow offers advantages especially for more complex device types. These advantages are: 1) The dimensional stability of the wafer compared to the film, which allows better inter-layer registration and thus new options in the positioning of the gauges. For example gauges can be placed close to the edge of the membrane in the region of highest sensitivity. In the film based process it is hard to maintain the stability of the film sufficient to allow accurate placement, and the variation in gauge response may be too great if the gauge was placed in the most sensitive region. This is because the sensitivity to gauge placement error is maximised alongside the maximisation of the sensitivity.

2) The use of silicon wafers facilitates the use of the standard processing equipment, and is in particular better-adapted to the rinsing and drying stages than is the film substrate.

3) The relative ease of forming inter-level vias through the polyimide with the commonly available equipment. This opens the possibility to provide substantially more robust bond pads allowing a wider range of subsequent packaging options. It also permits thicker tracking thereby reducing the parasitic resistance of the track, which in turn reduces the cross-sensitivity to temperature variation.

4) The gauge sensitive element is electrically insulated from the body by being between two polymer layers. This may provide both an electrically more satisfactory approach and assured biocompatibility (although neither is an especially serious issue in the film process).

Figures 4A to 4C show microscope images of wafer-based devices still on the wafer.

Fig. 4A shows the pad section with the pads plated with nickel. The structure of the tracking and dummy gauge can be seen as a topography on the nickel at the right hand side. For reference, the Y-axis of the overall device is approximately 1.7mm. The pads are 200pm wide (in the Y-axis). Fig. 4B shows the active gauge section and is an image of the area of the structure to the right of that shown in Fig. 4A. Fig. 4C shows the alternative gauge position that is viable using the wafer-based process. The small active gauge structure in the upper part of the window extending below the centre of the nickel frame. The corresponding dummy gauge is buried under the frame in the lower part of the window and can be discerned as a surface feature even through the thick nickel plating.

Although the strain gauge disclosed above has been described with reference to use in determining the function of the oesophageal sphincter, a strain gauge or strain gauge array embodying the present invention will be suitable for use in other medical applications.

Claims

1 A strain gauge comprising: a flexible film substrate; a strain sensing element supported by the substrate; and a support structure integrally formed with the substrate

2 A strain gauge as claimed in claim 1, wherein the support structure is substantially rigid.

3 A strain gauge as claimed in claim 1 or 2, wherein the support structure is adjacent to, and extends substantially parallel with, the strain sensing element.

4 A strain gauge as claimed in claim 1, 2 or 3, wherein the support structure is integrally formed on a surface of the substrate that is opposite to the surface on which the strain sensing element is supported.

A strain gauge as claimed in any one of the preceding claims, wherein the support structure is formed of an electroplated material.

6 A strain gauge as claimed in any one of the preceding claims, wherein the flexible film substrate comprises a polymeric layer having a working temperature of 90°C or higher.

7 A strain gauge as claimed in claim 6, further comprising an additional polymeric layer formed on the strain sensing element.

8 A strain gauge as claimed in any one of the preceding claims, wherein the total thickness of the polymer is less than 51 pm.

9 A strain gauge as claimed in any one of the preceding claims, wherein the total thickness of the polymer is less than 26pm.

A strain gauge as claimed in any one of the preceding claims, wherein the total thickness of the polymer is less than 9pm.

11 A strain gauge as claimed in any one of the preceding claims, wherein the strain sensing element comprises a deposited thin film.

12 A strain gauge as claimed in claim 11, wherein the said thin film is formed of a material of low temperature coefficient of resistance.

13 A strain gauge as claimed in claim 12, wherein the said thin film is formed from a material chosen from the group comprising nichrome, constantan and sichrome.

14 A strain gauge array comprising a plurality of strain gauges as claimed in any one of the preceding claims the elements of the array being formed on a common substrate.

A strain gauge array as claimed in claim 14, wherein opposed free ends of the flexible film substrate are brought into and retained in proximity, such that the array is of substantially cylindrical form.

16 A strain gauge array as claimed in claim 15, wherein the array has an outer diameter of approximately 4mm.

17 A method of manufacturing a strain gauge or strain gauge array as claimed in any preceding claim, wherein the or each support structure is integrally formed with the flexible film substrate by an electro-deposition technique.

18 A method of manufacturing a strain gauge or strain gauge array as claimed in any one of claims 1 to 16, wherein the or each support structure is defined by a photolithographic technique.

19 A method of manufacturing a strain gauge comprising the steps of: (a) coating a flexible substrate with a first thin metal film and a second thin metal film of a different metal; (b) etching selected portions of the second and first thin metal films to expose the substrate to form a strain sensing element and an interconnect structure; and (c) applying a third metal film to the substrate to provide a support structure.

A method as claimed in claim 19, wherein the etching step (b) is preceded by the step of coating the second thin metal film with a photoresist and patterning the photoresist with a first pattern and followed by the steps of patterning the photoresist with a second pattern, which is a subset of the first pattern, etching the second thin metal film, and removing the photoresist.

21 A method of manufacturing a strain gauge comprising the steps of: (a) coating a carrier substrate with a release layer; (b) depositing a first polymeric film layer on said release layer; (c) depositing at least one metallic layer over the first polymeric layer and patterning the at least one metallic layer to form a strain sensing element and track; (d) depositing a second polymeric film layer over the strain sensing element and track; (e) forming via contacts through the second polymeric film; (f) depositing a thick metal layer over selected portions of the second polymeric film layer to form a support structure; and (g) removing the release layer to separate the said first polymeric layer from the carrier substrate.

22 A method of manufacturing a strain gauge comprising the steps of: (a) depositing a first polymeric film layer on a carrier substrate that is substantially transparent to radiation of a predetermined wavelength; (b) depositing a metallic layer over the first polymeric layer and patterning the metallic layer to form a strain sensing element and track; (c) depositing a second polymeric film layer over the strain sensing element and track; (d) depositing a thick metal layer over selected portions of the second polymeric film layer to form a support structure; and (e) separating the said first polymeric layer from the carrier substrate by means of a radiation pulse.

23 A method as claimed in claims 21 or 22, further comprising the steps of: depositing a plating seed layer on the second polymeric layer before deposition of the thick metal layer; and removing the plating seed layer in those regions not covered by said thick metal layer before separating the first polymeric layer from the carrier substrate.

24 A method as claimed in any one of claims 21 to 23, wherein the thick metal layer is also deposited to form an electrical conductor and or contact pads.

A method as claimed in any one of claims 21 to 24, wherein a plurality of strain gauges are formed on the same carrier substrate and an etching step is carried out to separate one or more of the said strain gauges from the or each other strain gauge of the plurality.

26 A method of investigating a function of a sphincter of a human or animal body using a strain gauge as claimed in any one of claims 1 to 13 or a strain gauge array as claimed in any one of claims 14 to 16.

27 A method of diagnosis of a human or animal body using a strain gauge as claimed in any one of claims 1 to 13 or a strain gauge array as claimed in any one of claimsl4tol6.