Fiber optic wire for pressure detection, in particular for medical applications

Abstract

The fiber optic wire (10) comprises a central core (14) having one or more sensing portions (18) of the type comprising a fiber Bragg grating, in particular of the superstructured type. A cladding (16), which surrounds the central core, has a smaller diameter in the region of said sensing portions (18). One or more coating sleeves (20) comprising an auxetic metamaterial surround the cladding (16) in the region of each sensing portion (18).

Description

[0001] TITLE : "Fiber optic wire for pressure detection, in particular for medical applications"

[0002] * * *

[0003] DESCRIPTION

[0004] Technical field

[0005] The present invention relates to a fiber optic wire for pressure detection, in particular for medical applications . Technical background

[0006] In vivo pressure monitoring provides timely and accurate results that can significantly affect the diagnosis, treatment and prognosis of serious illnesses . Diagnosis and treatment of diseases like coronary heart diseases, as well as any other conditions implying increased intracranial pressure, are of fundamental importance for patient survival and quality of life .

[0007] For the above-mentioned applications, the monitoring techniques currently known in the art normally make use of single-point electrical pressure sensors . In the field of coronary physiology, there are basically five major companies that utilize single-point electrical and optical sensors .

[0008] On the one hand, companies St . Jude Medical and Philips Volcano have developed pressure detecting systems based on piezoresistive technology (respectively known under the commercial names Pressure Wire X and Verrata Plus) .

[0009] On the other hand, companies Opsens Medical, Acist Medical Systems and Boston Scientific have introduced pressure detecting systems based on fiber optic technology (respectively known under the commercial names OptoWire, Navuus microcatheter, and Comet II ) . The pressure detecting systems developed by companies Acist and Boston Scientific provide pressure detection based on Fabry-Perot interferometry anda pressure-sensitive membrane at the end of the optical fiber, whereas company Opsens Medical employs a MEMS-based pressure sensor having a silicon membrane with temperature compensation, which reduces the effect of temperature and improves pressure sensitivity. This latter sensor uses frequency demodulation to determine pressure variations .

[0010] Notwithstanding the recent spread of fiber optic sensors, essentially they are still single-point sensors . They include fiber optic wires for pressure detection, which are used by means of a pullback technique to measure the pressure at specific sites; however re-insertion is necessary whenever a measurement is to be taken at a new site . As a matter of fact, the pullback technique requires the operator to pull back the pressure probe manually, and to determine a pressure profile along a critical point (e . g. a stenosis) .

[0011] The above-mentioned technology uses, therefore, singlepoint electrical and optical sensors, which cannot be easily multiplexed to obtain a multi-point detection. Moreover, such technology requires the use of the above-mentioned pullback techniques and methods to monitor the pressure index in vascular lesions . Furthermore, the electrical and optical signals of commercially available pressure sensors are normally detected by means of complex interrogation techniques (which involve the input luminous source and the detection scheme) . In particular, the optical sensors according to the current state of the art employ complex phase modulation schemes to detect very small pressure variations (on the order of a few mmHg) , since their structure is generally based on Fabry-Perot interferometry and MEMS, which drastically increases the cost of the optical detection system as a whole .

[0012] Optical sensors are also known in the art which utilizefiber optic wires comprising one or more sensing portions of the fiber Bragg grating type (generally referred to by the acronym FBG) . In comparison with the above-described techniques, the use of the fiber Bragg grating, which is essentially a wavelength filter, as the main detection technology results in easier-to-design and much less costly demodulation methods .

[0013] However, the use of one or more fiber Bragg grating sensing portions in fiber optic wires has some drawbacks, especially when used for pressure measurements in medical applications . In particular, such sensing portions have low sensitivity to hydrostatic pressure .

[0014] Some examples of such pressure detecting systems for medical use are described in patent publications WO 2016 / 070099 Al, WO 2016 / 070110 Al, and US 8, 805, 128 B2 . However, also the systems described in such patent publications suffer from a few drawbacks which should be remedied.

[0015] Summary of the invention

[0016] It is one obj ect of the present invention to provide a fiber optic wire which can overcome some drawbacks of the prior art .

[0017] According to the present invention, this and other obj ects are achieved through a fiber optic wire as defined by the appended independent claim.

[0018] It is understood that the appended claims are an integral part of the technical teachings provided in the following detailed description of the present invention. In particular, the appended dependent claims define some preferred embodiments of the present invention that include some optional technical features .

[0019] Further features and advantages of the present invention will become apparent in light of the following detaileddescription, provided herein merely as a non-limiting example and referring, in particular, to the annexed drawings as summarized below.

[0020] Brief description of the drawings

[0021] Figure 1 is a block diagram that represents a pressure detecting system comprising a fiber optic wire made in accordance with an exemplary embodiment of the present invention .

[0022] Figure 2 is a schematic side-sectional view of a fiber optic wire which can be used in the system shown in Figure 1, and which is made in accordance with an exemplary embodiment of the present invention.

[0023] Figure 3 is a detailed magnified view of that part of the fiber optic wire which is comprised in frame III in Figure 2, and which includes a sensing portion of the fiber Bragg grating type .

[0024] Figure 4 is a view of the fiber optic wire showing a plurality of sensing portions made in accordance with the preceding figures .

[0025] Figure 5 is a diagram representing a plurality of sensing portions forming a superstructured fiber Bragg grating enabling distributed pressure detection, which can be implemented in the fiber optic wire shown in the preceding figures, and which consists of a succession of n-phase shifted weak FBGs.

[0026] For completeness' sake, the following is a list of alphanumerical references and names used herein to identify parts, elements and components illustrated in the abovesummarized drawings .

[0027] 10. Pressure detecting system

[0028] 12. Fiber optic wire

[0029] 14. Core16. Cladding

[0030] 18. Sensing portion

[0031] 20. Coating sleeve

[0032] 100. Light source

[0033] 200. Optic detection element

[0034] 300. Processing unit

[0035] Detailed description of the invention

[0036] With reference to Figures 1 to 3, numeral 12 designates as a whole a fiber optic wire for pressure detection, made in accordance with an exemplary embodiment of the present invention. Fiber optic wire 12 is particularly, but not exclusively, intended for medical use . In fact, as will be apparent to those skilled in the art, fiber optic wire 12 can be used not only for medical and sanitary purposes, but also in other technical and industrial applications, such as, by way of non-limiting example, pharmaceutical production and tissue engineering (e . g. for monitoring pressure in bioreactors) , the chemical industry (e . g. for monitoring pressure in chemical reactors) , the aerospace industry (e . g. for monitoring fluid pressure in hydraulic systems) .

[0037] As shown in Figure 1, fiber optic wire 12 is intended to be comprised in a pressure detecting system, designated as a whole by numeral 10. Detecting system 10 may comprise, in a per se known manner, a light source 100 (e . g. a laser, a light-emitting diode, a solid-state laser, or the like) configured to generate a luminous signal to be transmitted through fiber optic wire 12, which acts as a light transmission means and also, as will be further described hereinafter, as a sensitive element, modifying the characteristics of the luminous signal as a function of the pressure to be measured. Moreover, detecting system 10 may also comprise an optic detection element 200 (e . g. a photodiode or aphototransistor) configured to convert the luminous signal going through fiber optic wire 12 into an electric signal . Furthermore, detecting system 10 may comprise a processing unit 300 configured to analyze the electric signal supplied by optic detection element 200, so as to provide information about the detected pressure . As is per se known in the medical field, optic fiber wire 12 can be inserted through the lumen of a catheter (not numbered) and into a patient' s body via a specific access way, e . g. a blood vessel, until it reaches the region where pressure is to be detected. Afterwards, still in a per se known manner, light source 100 can be made to emit the luminous signal through fiber optic wire 12 to take pressure measurements by means of optic detection element 200 and processing unit 300.

[0038] With reference to Figures 2 and 3, fiber optic wire 12 comprises a central core 14 and a cladding 16 that surrounds central core 14.

[0039] As clearly visible in Figure 4, central core 14 has a plurality of sensing portions 18, each one comprising - and, in particular, consisting of - a fiber Bragg grating. In further alternative embodiments, sensing portions 18 may be formed on cladding 16. Therefore, in more general terms, sensing portions 18 are defined on central core 14 and / or on cladding 16.

[0040] In a per se known manner, each fiber Bragg grating forming a respective sensing portion 18 acts as a selective filter upon the Bragg wavelength XB, wherein the coupling between the forward propagation mode and the backward propagation mode occurs by means of periodical perturbations of the refractive index of central core 14.

[0041] Each fiber Bragg grating comprised in a respective sensing portion 18 is a n-phase shifted weak fiber Bragg grating(weak FBG) .

[0042] Furthermore, the fiber Bragg gratings that make up sensing portions 18 form, in particular, a superstructured Bragg grating, which can be multiplexed. Such a Bragg grating is particularly suitable for enabling distributed pressure detection. In particular, as shown in Figure 4, if there are n sensing portions 18i, ..., 18±-i, 18±, 18 , ..., 18nsituated along n different detection positions or regions Pi, ..., Pi i, Pi, Pm, ..., Pn defined longitudinally along central core 14 (and / or along cladding 16) , each sensing portion 18± will have its own fiber Bragg grating with a respective Bragg wavelength XBI different from the other Bragg wavelengths of the fiber Bragg gratings XBi, ..., XBm, XBm, ..., XBnof the other sensing portions 18i, ..., 18±-i, 18m, ..., 18n.

[0043] Figure 5 schematically shows a superstructured Bragg grating, and the optical spectrum (see upper Cartesian graph) and reflection spectrum (see lower Cartesian graph) respectively inputted to and outputted from fiber optic wire 12, wherein only a portion of its components are visible . In other terms, in the superstructured Bragg grating, each fiber Bragg grating of a respective sensing portion 18 can be so designed as to have net resonance peaks corresponding to specific detection positions or regions Pi, ..., Pi-i, Pi, Pm, ..., Pn distributed along core 14 of fiber optic wire 12, acting substantially as a plurality of punctual sensors . In the embodiment illustrated herein, each fiber Bragg grating positioned at each sensing portion 18 is n-phase shifted by a specific ref lection / transmission wavelength peak. Tracking these ref lection / transmission peaks results in improved sensitivity to pressure variations, spatial resolution, and dynamic measurements . As a matter of fact, the typical bandwidth of the reflected spectrum of standard fiber Bragggratings is 0.2-0. 5 nm, while typical n-phase shifted fiber Bragg gratings have a bandwidth which is about 10 times narrower, in particular 0.01-0.05 nm.

[0044] In other less preferable variant implementations of the present invention, it is also conceivable that central core 14 and / or cladding 16 have only one sensing portion 18.

[0045] Furthermore, fiber optic wire 12 comprises also a plurality of coating sleeves 20, each one comprising - and preferably consisting of - an auxetic metamaterial that surrounds cladding 16 in the region of one or more sensing portions 18. The auxetic metamaterial of coating sleeves 20 is configured to interact with sensing portions 18 (e . g. by subj ecting them to compression and / or tensile stress in the axial direction) through cladding 16, particularly in response to a pressure to be measured, which is exerted laterally thereto . For example, a respective single coating sleeve 20 may be associated with each sensing portion 18, surrounding it, and is configured to interact with that sensing portion 18 only. In a per se known manner, the auxetic metamaterial has a negative Poisson coefficient . This implies that, when the pressure acting laterally upon each coating sleeve 20 increases, the auxetic metamaterial will contract axially (instead of expanding axially like conventional materials) . This axial contraction of the auxetic material of coating sleeve 20 will then cause a corresponding axial compression of the associated sensing portion 18 it surrounds, which will translate into a variation in the wavelength reflected by the sensing portion 18 from its Bragg wavelength XB . Vice versa, when the pressure acting laterally upon each coating sleeve decreases, the auxetic metamaterial will expand axially, thus triggering an axial tensile strain, and hence an opposite reflected wavelength variationcompared to the one described above .

[0046] For example, as shown in Figure 4, assuming that there are n sensing portions 18i, ..., 18±-i, 18±, 18i+i, ..., 18nsituated along n different sensing positions or regions Pi, ..., Pi-i, Pi, Pm, ..., Pn, there will be n coating sleeves 20i, ..., 20i-i, 20i, 20i+i, ..., 2 On disposed longitudinally along cladding 16. In other less preferable variant implementations of the present invention, wherein there is only one sensing portion, there will be just a single coating sleeve associated therewith. In further variant implementations (not shown) , wherein there are a plurality of sensing portions, it is also conceivable that :

[0047] there is only one coating sleeve extending over the entire region of the cladding of the fiber optic wire where the sensing portions are arranged; or

[0048] each coating sleeve is situated around the cladding in a region where there is more than one sensing portion.

[0049] As aforesaid, thanks to the technical features mentioned in the appended independent claim, fiber optic wire 10 can be used for monitoring very low fluid pressures, particularly within the human body. Monitoring such pressures, such as blood pressure in the stenotic region (measuring pressure indices during percutaneous coronary intervention) , intracranial pressure, high-resolution manometry, gastrointestinal pressure, and bladder pressure, is of the utmost importance .

[0050] In particular, the facts that the fiber Bragg grating of each sensing portion 18 is a n-phase shifted weak FBG, and that an auxetic metamaterial is used for coating sleeves 20, synergically contribute to improving the signal-to-noise ratio .

[0051] On the one hand, the fact that the fiber Bragg gratingis a n-phase shifted weak FBG makes it possible to simultaneously exploit the very low reflectivity of weak fiber Bragg gratings and the high sensitivity within the reflectivity spectrum of n-phase shifted weak fiber Bragg gratings, thus obtaining very good sensitivity also to weak pressure variations, with a high spectral resolution. On the other hand, the design choice of using coating sleeves 20 intensifies, with a synergically enhanced effect, the pressure sensitivity of the n-phase shifted weak FBG, thereby improving the transduction mechanism, which results in effective translation of pressure / def ormation into Bragg wavelength variations . This solution is optimal for, in particular, multiplexed systems, since it reduces optical noise and undesired multiple reflections in comparison with prior-art technical solutions . Indeed, fiber optic wires using, as sensing portions, traditional fiber Bragg gratings generally have poor pressure sensitivity, amounting to approximately 3. 14 pm / MPa .

[0052] Preferably, as is especially visible in Figure 2, cladding 16 has a smaller diameter in the region corresponding to each sensing portion 18. This further contributes to the sensitivity of the transduction mechanism.

[0053] As far as coating sleeves 20 are concerned, the auxetic metamaterial of each one of them is, preferably, elastomeric .

[0054] Preferably, the auxetic metamaterial of each coating sleeve 20 is biocompatible, which makes it particularly advantageous for medical / sanitary applications .

[0055] In the embodiment illustrated herein, the auxetic metamaterial of each coating sleeve 20 is polydimethylsiloxane or PDMS - which has both elastomer properties and biocompatibility properties . In particular, polydimethylsiloxane has a relatively low Young modulus ( 870kPa-1. 67 MPa) thateffectively translates applied pressure into deformation, and its high Poisson ratio (close to 0.5) translates lateral deformation - caused by the pressure to be measured - into longitudinal deformation, which contributes to the Bragg wavelength shift .

[0056] Each coating sleeve 20 has, advantageously but not necessarily, a diameter in the range of 300 pm to 500 pm (see, in particular, Figure 2 ) , resulting in pressure sensitivity being improved by at least one order of magnitude . In general terms, the thicker the coating sleeve 20, the greater the deformation transduced to the fiber Bragg grating, and hence the higher the sensitivity of sensing portion 18. However, this sensitivity gain is somewhat limited by the catheter into which fiber optic wire 12 equipped with coating sleeves 20 has to be inserted. As a matter of fact, the inside diameter of fiber optic wire 12 necessarily limits the thickness of coating sleeves 20, since pressure guidewires / cath-eters have diameters ranging approximately between 355. 6 pm and 635 pm. The configuration of the auxetic metamaterial, which may have a much higher Poisson ratio (close to -1 ) , advantageously increases - as aforementioned - the signal-to-noise ratio of the system. The above-mentioned exemplary dimensions of these coating sleeves 20 are relatively small, so that they can be integrated into guidewires / catheters of standard size and type, as normally used in medical applications .

[0057] In the embodiment illustrated herein, each coating sleeve 20 has a substantially tubular shape, in particular a cylindrical shape with a circular cross-section. Coating sleeves 20 are mutually spaced apart in the longitudinal direction of fiber optic wire 12.

[0058] In the embodiment illustrated herein, the diameterrestriction of cladding 16 in the region of the fiber Bragg grating of each sensing portion 18 is obtained by etching, e . g. chemical etching, or by tapering. Preferably, the nominal diameter of cladding 16 is approximately 125 pm, while the smaller diameter in the region of sensing portion 18 (fiber Bragg grating) is approximately 80 pm. This results in pressure sensitivity being further improved, particularly by a factor of 4 to 8, because the area of the entire crosssection of coating sleeve 20 increases in comparison with that of cladding 16 of fiber optic wire 12. In fact, cladding 16 typically has a higher Young modulus (79.1 GPa) and a lower Poisson ratio ( 0.17 ) , which limit the actual transduction of pressure into a Bragg wavelength shift .

[0059] In the embodiment illustrated herein, as aforementioned, the nominal diameter of cladding 16 is approximately 125 pm, and the smaller diameter of cladding 16 is approximately 80 pm, while the diameter of core 14 is, preferably, approximately 8 pm. Whereas each coating sleeve 20 has, in particular, a diameter ranging between approximately 300 pm and approximately 500 pm - as previously mentioned herein by way of example .

[0060] Preferably, fiber optic wire 12 may incorporate one or more temperature sensors (not numbered in the drawings) configured to detect the temperature in proximity to one or more sensing portions 18. For example, each temperature sensor may comprise - preferably, may consist of - a fiber Bragg grating, advantageously a n-phase shifted weak fiber Bragg grating. In such a case, coating sleeves 20 cannot interact with the fiber Bragg gratings that constitute the temperature sensors, since the latter are, for example, axially spaced apart from coating sleeves 20. According to one embodiment of the present invention, each fiber Bragg gratingconstituting a respective temperature sensor may be obtained from, or applied to, a core (not shown) which is separate from central core 14. According to another alternative embodiment of the present invention, each fiber Bragg grating constituting each one of the temperature sensors may be obtained from, or applied to, the same core 14 and / or the same cladding 16 where sensing portions 18 are formed, but located immediately downstream or upstream of the position of the associated respective coating sleeve 20, thus belonging to the same superstructured Bragg grating. In this way, in both of the above-described embodiments, each temperature sensor will detect substantially the same temperature to which the respective sensing portion 18 is subj ected, without however being affected by the pressure detected by that sensing portion 18, in particular by the pressure acting upon the respective coating sleeve 20 - which thus will not interact with (being separate and / or spaced apart from) the fiber Bragg grating of the temperature sensor .

[0061] In system 100, the detection made by each temperature sensor can advantageously be supplied to processing unit 300. Thus, processing unit 300 will be able to, when determining the detected pressure, compensate for the effects exerted by temperature on the fiber Bragg gratings of sensing portions 18, which, in addition to being affected by pressure, are also sensitive to temperature .

[0062] By way of non-limiting example, central core 14 of fiber optic wire 12 comprises - and, preferably, consists of - a silica-based glass . Advantageously, central core 14 and / or cladding 16 may comprise - and, preferably, consist of -bioresorbable materials, in particular for medical applications, since such materials are gradually absorbed by the body. For example, central core 14 comprises, as abioresorbable material, a bioactive silicate, e . g. calcium phosphate . Thanks to such technical features, the biocompatibility and biodegradability of fiber optic wire 12 are improved .

[0063] Conveniently, central core 14 and / or cladding 16 may comprise - and, preferably, consist of - (totally) polymeric materials, e . g. biodegradable ones .

[0064] Of course, without prejudice to the principle of the invention, the forms of embodiment and the implementation details may be extensively varied from those described and illustrated herein by way of non-limiting example, without however departing from the scope of the invention as set out in the appended claims .

Claims

CLAIMS1. Fiber optic wire ( 12 ) for pressure detection, in particular for medical applications; said fiber optic wire comprising :a transparent central core ( 14 ) , through which a luminous signal emitted by a light source ( 100) can propagate;a cladding ( 16) surrounding said central core ( 14 ) ; at least one sensing portion ( 18 ) comprising a n-phase shifted weak fiber Bragg grating, and defined in said central core ( 14 ) and / or in said cladding ( 16) ; andat least one coating sleeve (20) comprising an auxetic metamaterial that surrounds said cladding ( 16) in the region of said at least one sensing portion ( 18 ) , and configured to interact with said sensing portion ( 18 ) in response to a pressure exerted laterally on it, which is to be detected.

2. Fiber optic wire according to claim 1, wherein said cladding ( 16) has a restriction with a smaller diameter in the region of said at least one sensing portion ( 18 ) .

3. Fiber optic wire according to claim 2, wherein said restriction with a smaller diameter of said cladding ( 16) in the region of the fiber Bragg grating of each sensing portion ( 18 ) is obtained by etching.

4. Fiber optic wire according to any one of the preceding claims, wherein said auxetic metamaterial of said at least one coating sleeve (20) is elastomeric .

5. Fiber optic wire according to any one of the preceding claims, wherein said auxetic metamaterial of said at least one coating sleeve (20) is biocompatible .

6. Fiber optic wire according to claims 4 and 5, wherein said auxetic metamaterial of said at least one coating sleeve (20) is polydimethylsiloxane or PDMS .

7. Fiber optic wire according to any one of the precedingclaims, wherein the nominal diameter of said cladding ( 16) is approximately 125 pm.

8. Fiber optic wire according to claims 2 and 7, wherein said smaller diameter of said cladding ( 16) is approximately 80 pm.

9. Fiber optic wire according to claim 7 or 8, wherein the diameter of said core ( 14 ) is approximately 8 pm.

10. Fiber optic wire according to any one of claims 7 to 9, wherein the diameter of said coating sleeve (20) is in the range of 300 pm to 500 pm.

11. Fiber optic wire according to any one of the preceding claims, wherein said at least one coating sleeve (20) is substantially tubular .

12. Fiber optic wire according to claim 11, wherein said at least one coating sleeve (20) is substantially cylindrical in shape, with a circular cross-section.

13. Fiber optic wire according to any one of the preceding claims, wherein a plurality of said sensing portions ( 18 ) are provided along said core ( 14 ) and / or said cladding ( 16) .

14. Wire according to claim 13, wherein a corresponding plurality of said coating sleeves (20) are provided around said cladding ( 16) , each one of said coating sleeves (20) being associated - and configured to interact - with a respective sensing portion ( 18 ) .

15. Fiber optic wire according to claim 13 or 14, wherein the fiber Bragg gratings of said sensing portions ( 18 ) are n-phase shifted and form a superstructured grating.

16. Fiber optic wire according to any one of the preceding claims, further comprising at least one temperature sensor configured to detect the temperature in proximity to said at least one sensing portion ( 18 ) .

17. Fiber optic wire according to claim 16, wherein said atleast one temperature sensor comprises a fiber Bragg grating not interacting with said coating sleeve (20) .

18. Fiber optic wire according to any one of the preceding claims, wherein said central core ( 14 ) comprises a silica-based glass .

19. Fiber optic wire according to any one of the preceding claims, wherein at least one of said central core ( 14 ) and said cladding ( 16) comprises a bioresorbable material .

20. Fiber optic wire according to claim 19, wherein said central core ( 14 ) comprises a bioactive silicate .

21. Fiber optic wire according to claim 20, wherein said bioactive silicate is calcium phosphate .

22. Fiber optic wire according to any one of the preceding claims, wherein at least one of said central core ( 14 ) and said cladding ( 16) comprises a polymeric material .

23. Fiber optic wire according to claim 22, wherein said polymeric material is biodegradable .

24. Pressure detecting system ( 10) comprising a fiber optic wire ( 12 ) made in accordance with any one of the preceding claims .

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