Patent Publication Number: US-2021169358-A1

Title: Pressure sensor and guide wire with hydrophilic material

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 15/053,308, filed Feb. 25, 2016, which claims the benefit of U.S. Provisional Application No. 62/121,243, filed on Feb. 26, 2015, the entire contents of all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to devices for making pressure or other measurements inside a living body, and in particular to a sensor/guide device having a guide wire and a distal sensor for pressure measurement in stenotic vessels. 
     Examples of such devices are described, for example, in Swedish Patents SE 441725, SE 453561, SE 454045, SE 460396, and SE 469454, in European Publication EP 0387453, and in U.S. Pat. No. 6,167,763. 
     U.S. Pat. No. 6,167,763 describes a device in which a pressure sensor is mounted in a cantilevered configuration. In one embodiment, a tube extends around the pressure sensor. An opening is provided in the tube above the pressure sensor to allow the pressure sensor to be exposed to a surrounding medium, the pressure of which is to be measured. While this device has many benefits over previous devices, the present inventors discovered that an unstable pressure column can form above the sensor membrane of the pressure sensor due to insufficient wetting when the device is immersed in a liquid. Without being bound by theory, it is believed that instability in the pressure signal is caused by air remaining in the tube. This air influences the pressure column above the sensor membrane due to time dependent motion as a result of the inverse proportionality between capillary pressure and radius, as shown in the Young-Laplace equation, which describes the capillary pressure difference sustained across the interface between two static fluids due to surface tension. 
     Thus, there is a need for an improved sensor/guide device having a guide wire and a distal sensor for pressure measurement in stenotic vessels. 
     All references cited in this disclosure are hereby incorporated by reference in their entireties for the devices, techniques, and methods described therein, and for any disclosure relating to medical sensors and devices. 
     SUMMARY 
     In another embodiment, a guide wire for biological pressure measurement includes a tube extending along a longitudinal axis of the guide wire; and a pressure sensor for biological pressure measurement, at least a portion of the pressure sensor being mounted within the tube. The pressure sensor comprises a pressure sensor membrane facing a top side of the tube. The tube includes at least six openings, including: a first distal opening located on a top side of the tube, a second distal opening located on a right side of the tube, offset right of the first distal opening in a circumferential direction, a third distal opening located on a left side of the tube, offset left of the first distal opening in a circumferential direction, a first proximal opening located on a top side of the tube proximal of the first distal opening, a second proximal opening located on a right side of the tube, offset right of the first proximal opening in a circumferential direction, and a third proximal opening located on a left side of the tube, offset left of the first proximal opening in a circumferential direction. 
     In one aspect, the tube includes exactly six openings: the first, second, and third distal openings, and the first, second, and third proximal openings, such that a bottom of the tube does not include any openings. 
     In one aspect, the pressure sensor is disposed in the tube such that, in a top view of the guide wire, an entirety of the pressure sensor membrane is visible through the distal opening. 
     In one aspect, the second and third distal openings are each offset by 90° in opposite circumferential directions from the first distal opening, and the second and third proximal openings are each offset by 90° in opposite circumferential directions from the first proximal opening. 
     In one aspect, widths of the first distal opening and first proximal opening are greater than widths of the second and third distal openings and second and third proximal openings. 
     In one aspect, proximal ends of each of the distal openings are at a same longitudinal position of the guide wire, and distal ends of each of the distal openings are at a same longitudinal position of the guide wire, and proximal ends of each of the proximal openings are at a same longitudinal position of the guide wire, and distal ends of each of the proximal openings are at the same longitudinal position of the guide wire. 
     In one aspect, each of the distal and proximal openings has a generally rectangular shape. 
     In one aspect, each of the distal and proximal openings has a generally rectangular shape with rounded corners. 
     In one embodiment, a guide wire for biological pressure measurement comprises a tube extending along a longitudinal axis of the guide wire; and a pressure sensor for biological pressure measurement, at least a portion of the pressure sensor being mounted within the tube. The tube includes at least one concave distal opening extending proximally from a distal edge of the tube. 
     In one aspect, the at least one concave distal opening is configured to allow a fluid to enter the tube from a region outside the tube via the at least one concave distal opening and flow past the pressure sensor before exiting the tube. 
     In one aspect, the tube further includes at least one proximal opening located proximal of the at least one concave distal opening. 
     In one aspect, at least a portion of the at least one concave distal opening is located in a longitudinal cylindrical segment that opposes a side surface of the pressure sensor. 
     In one aspect, the at least a portion of the at least one concave distal opening is located distal of the pressure sensor. 
     In one aspect, the tube further includes at least one proximal opening located proximal of the at least one concave distal opening, and at least a portion of the at least one concave distal opening and at least a portion of the at least one proximal opening are located in a longitudinal cylindrical segment that opposes a side surface of the pressure sensor. 
     In one aspect, the at least one concave distal opening includes at least two concave distal openings. 
     In one aspect, the at least one concave distal opening includes at least two concave distal openings: a first concave distal opening and a second concave distal opening, and the tube further includes at least two proximal openings: a first proximal opening located proximal of the first concave distal opening, and a second proximal opening located proximal of the second concave distal opening. 
     In one aspect, the at least one concave distal opening includes at least two concave distal openings: a first concave distal opening and a second concave distal opening, an entirety of the first concave distal opening is located in a first longitudinal half of the tube that opposes a first side of the pressure sensor, and an entirety of the second concave distal opening is located in a second longitudinal half of the tube that opposes a second side of the pressure sensor. 
     In one aspect, a depth of the at least one concave distal opening is within 10% of a distance between the tube and a side surface of the pressure sensor. 
     In one aspect, the at least one distal opening has a single rounded edge. 
     In one aspect, the at least one concave distal opening has at least one straight edge. 
     In one aspect, the guide wire further comprises a wire on which the pressure sensor is mounted, the wire including a recess over which a distal portion of the pressure sensor extends. The tube further includes at least one proximal opening located proximal of the at least one concave distal opening. The at least one proximal opening is located in a cylindrical section of the tube that is adjacent to a proximal end of the recess. 
     In one aspect, the guide wire further comprises a wire on which the pressure sensor is mounted, the wire including a recess over which a distal portion of the pressure sensor extends. The pressure sensor is cantilevered from a shelf portion of the wire. 
     In one aspect, the pressure sensor is an absolute pressure sensor. 
     In one aspect, the guide wire further comprises a radiopaque tip. 
     In another embodiment, a guide wire for biological pressure measurement comprises a tube extending along a longitudinal axis of the guide wire; and a pressure sensor for biological pressure measurement, at least a portion of the pressure sensor being mounted within the tube. The tube includes at least one distal opening, and at least one proximal opening located proximal of the at least one distal opening. 
     In one aspect, the tube includes a first cylindrical section having a first diameter, and a second cylindrical section having a second diameter that is larger than the first diameter, and at least a portion of the at least one distal opening is located at a distal end of the second cylindrical section. 
     In one aspect, the tube includes a cylindrical section, and a tapered section located at a distal end of the cylindrical section, and at least a portion of the at least one distal opening is located in the tapered section. 
     In one aspect, the tube includes a first cylindrical section having a first diameter, a second cylindrical section having a second diameter that is larger than the first diameter, and a tapered transition section located between the first cylindrical section and the second cylindrical section, and at least a portion of the at least one distal opening is located in the tapered transition section. 
     In one aspect, the tube includes an ovoid section, and at least a portion of the at least one distal opening is located at a distal end of the ovoid section. 
     In one aspect, the tube includes a first cylindrical section having a first diameter, and a second cylindrical section having a second diameter that is larger than the first diameter, at least a portion of the at least one distal opening is located at a distal end of the second cylindrical section, and at least a portion of the at least one proximal opening is located at a proximal end of the second cylindrical section. 
     In one aspect, the tube includes a cylindrical section, a first tapered section located at a distal end of the cylindrical section, and a second tapered section located at a proximal end of the cylindrical section, at least a portion of the at least one distal opening is located in the first tapered section, and at least a portion of the at least one proximal opening is located in the second tapered section. 
     In one aspect, the tube includes an ovoid section, at least a portion of the at least one distal opening is located at a distal end of the ovoid section, and at least a portion of the at least one proximal opening is located at a proximal end of the ovoid section. 
     In one aspect, the at least one distal opening has a single rounded edge. 
     In one aspect, the at least one distal opening has at least one straight edge. 
     In one aspect, the guide wire further comprises a wire on which the pressure sensor is mounted, the wire including a recess over which a distal portion of the pressure sensor extends. The at least one proximal opening is located in a cylindrical section of the tube that is adjacent to a proximal end of the recess. 
     In one aspect, the guide wire further comprises a wire on which the pressure sensor is mounted, the wire including a recess over which a distal portion of the pressure sensor extends. The pressure sensor is cantilevered from a shelf portion of the wire. 
     In one aspect, the pressure sensor is an absolute pressure sensor. 
     In one aspect, the guide wire further comprises a radiopaque tip. 
     In another embodiment, a method comprises providing a guide wire for biological pressure measurement comprising: a tube extending along a longitudinal axis of the guide wire; and a pressure sensor for biological pressure measurement, at least a portion of the pressure sensor being mounted within the tube, wherein the tube includes at least one concave opening extending proximally from a distal edge of the tube; and disposing the guide wire into a vessel of an individual such that a fluid enters the tube from a region outside the tube via the at least one distal opening and flows past the pressure sensor before exiting the tube. 
     In another embodiment, a method comprises providing a guide wire for biological pressure measurement comprising: a tube extending along a longitudinal axis of the guide wire; and a pressure sensor for biological pressure measurement, at least a portion of the pressure sensor being mounted within the tube, wherein the tube includes at least one first opening, and at least one second opening located proximal of the at least one first opening; and disposing the guide wire into a vessel of an individual such that a fluid enters the tube from a region outside the tube via the at least one first opening and flows past the pressure sensor before exiting the tube via the at least one second opening. 
     In another embodiment, a guide wire for biological pressure measurement includes a tube extending along a longitudinal axis of the guide wire; and a pressure sensor for biological pressure measurement, at least a portion of the pressure sensor being mounted within the tube. The tube includes at least one distal opening. A hydrophilic material is coated on at least one of an internal surface of the tube, and a surface of the pressure sensor. 
     In one aspect, the pressure sensor comprises a pressure sensor membrane, and the hydrophilic material is further coated on a surface of the pressure sensor membrane. 
     In one aspect, the hydrophilic material comprises at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, carboxymethyl cellulose, hydroxypropyl cellulose, a polysaccharide, and a salt. 
     In one aspect, the hydrophilic material comprises polyethylene glycol. 
     In one aspect, the hydrophilic material comprises 20,000 g/mol polyethylene glycol. 
     In another embodiment, a method includes providing a guide wire for biological pressure measurement comprising: a tube extending along a longitudinal axis of the guide wire; and a pressure sensor for biological pressure measurement, at least a portion of the pressure sensor being mounted within the tube. The tube includes at least one distal opening. A hydrophilic material is coated on at least one of an internal surface of the tube, and a surface of the pressure sensor. The method further includes providing a liquid within the tube, whereby the hydrophilic material dissolves and causes an influx of the liquid into the tube. 
     In one aspect, the pressure sensor comprises a pressure sensor membrane, and the hydrophilic material is further coated on a surface of the pressure sensor membrane. 
     In one aspect, the hydrophilic material comprises at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, carboxymethyl cellulose, hydroxypropyl cellulose, a polysaccharide, and a salt. 
     In one aspect, the hydrophilic material comprises polyethylene glycol. 
     In one aspect, the hydrophilic material comprises 20,000 g/mol polyethylene glycol. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side cross-sectional view of a prior art guide wire including a tube and pressure sensor. 
         FIG. 2  a side cross-sectional view of a sensor portion of the prior art guide wire shown in  FIG. 1 . 
         FIG. 3  is a right-side perspective view of a guide wire according to an exemplary embodiment. 
         FIG. 4  is a left-side perspective view the guide wire shown in  FIG. 3 . 
         FIG. 5  is a top view of the guide wire shown in  FIG. 3 . 
         FIG. 6  is a top view of a portion of the guide wire shown in  FIG. 3 , with internal elements shown in dashed lines. 
         FIG. 7  is a side view of a guide wire according to another exemplary embodiment of the present invention, in which a tube includes a distal tapered section, with internal elements shown in dashed lines. 
         FIG. 8  is a top view of the guide wire shown in  FIG. 7 . 
         FIG. 9  is a side view of a guide wire according to another exemplary embodiment of the present invention, in which a tube includes a distal tapered section and a proximal tapered section, with internal elements shown in dashed lines. 
         FIG. 10  is a top view of the guide wire shown in  FIG. 9 . 
         FIG. 11  is a side view of a guide wire according to another exemplary embodiment of the present invention, in which a tube includes an ovoid section, with internal elements shown in dashed lines. 
         FIG. 12  is a top view of the guide wire shown in  FIG. 11 . 
         FIG. 13  is a perspective view of the guide wire according to another exemplary embodiment, in which a tube includes six openings. 
         FIG. 14  is a perspective view of the tube of the guide wire shown in  FIG. 13 . 
         FIG. 15  is a top view of the guide wire shown in  FIG. 13   
         FIG. 16  is a side view of the tube of the guide wire shown in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring generally to the figures, a guide wire used for biological pressure measurements is shown and described. The guide wire may generally include a tube (sometimes termed a “jacket”) and pressure sensor. The tube may generally include at least one distal opening, such as a concave distal opening. In various embodiments, the guide wire may include one distal opening and one proximal opening, two concave distal openings and two proximal openings, or otherwise. The use of the concave distal opening may suppress the formation of air pockets in the guide wire surrounding the sensor, thereby permitting a more accurate pressure sensor reading. In other words, the guide wire is “self-wetting” in that the pressure or other sensor is wetted when the guide wire is inserted in a vessel. 
     Referring to  FIG. 1 , a cross section of a prior art guide wire  1  is shown. The guide wire  1  includes a solid wire  16  disposed in a portion of a proximal tubing portion  17 . The solid wire  16  may, for example, be machined by centering grinding. The solid wire  16  may form the distal portion of the wire guide  1  and extend beyond the distal end of the proximal tube portion  17 , where said proximal tube portion  17  is connected to or integrally formed with a spiral portion  18 . 
     A pressure sensor  19  is mounted on the wire  16 . The pressure sensor  19  may be an absolute pressure sensor, such as the one disclosed in Swedish patent application No. 9600334-8. The pressure sensor  19  may include a membrane  29  (shown in  FIG. 2 ) made of, for example, polysilicon and a piezoresistive element. Between the wire  16  and the spiral portion  18 , one or more leads  30  may run from the electronic circuitry of the pressure sensor  19 . The wire  16 , may act as a portion of one of the leads  30 , as in  FIG. 1 . 
     The membrane  29  of the pressure sensor  19  may be mounted such that bending artifacts are minimized or eliminated (e.g., ensuring that the edges of the chip do not come into contact with the surrounding tube). 
     The pressure sensor  19  is protected by a short section of a tube  21 , having an aperture  22  through which a surrounding medium interacts with the pressure sensor  19 . The guide wire  1  further includes, at the very distal end of the guide wire, an X-ray non-transparent spiral  23 , made, for example, of platinum, and used for location purposes, and a safety wire  24  for securing the distal part of the spiral  23 . In one embodiment, the non-transparent spiral  23  is a radiopaque coil. 
     In one embodiment, the wire  16  is made of stainless steel. In other embodiments, the wire  16  may be made of a shape memory metal. The proximal tubing  17  and the spiral  18  may be coupled so as to be utilized as an electrical shield. 
     Referring to  FIG. 2 , a prior art sensor arrangement of the guide wire  1  is illustrated. The solid wire  16  may be machined (e.g., by grinding, spark erosion, or laser techniques) to form a groove in which the sensor element is mounted in a cantilevering fashion. The groove provides a free space for the sensor  19  and membrane  29 , allowing air, blood, or other pressure exerting mediums to enter the interior of the guide wire  1  and act on the sensor, which in turn delivers a signal representative of the exerted pressure. 
     The groove may consist of two portions. A first portion of the groove may serve as a shelf  27  for receiving the proximal part of the sensor chip and holding the sensor in place. The second portion  28  may be an open space over which the cantilever extends, which allows the distal part of the sensor chip to protrude freely, even in a case where the wire tip is bent or deflected. 
     As shown in  FIGS. 1 and 2 , a tube  21  is provided around the guide wire  1 , the tube including an opening  22  used to expose the sensor chip to a surrounding medium (e.g., fluid) in order to measure the pressure of the medium. 
     The prior art embodiments of  FIGS. 1 and 2  illustrate a single aperture or opening  22  in the guide wire through which the pressure sensor  19  is exposed to the surrounding medium. However, as discussed above, the presence of air in and around the guide wire  1  may cause unstable sensor readings. 
     Referring generally to  FIGS. 3-12 , embodiments of a guide wire according to the present invention will be described. The guide wire generally includes a tube that has at least one distal opening and at least one proximal opening. By implementing embodiments of the present invention, air in the vicinity of the sensor may be displaced when the guide wire is immersed in a liquid, thereby leading to more accurate sensor readings. The distal opening may allow a fluid to enter the tube and flow past the pressure sensor before exiting the tube at the proximal opening (or vice versa). 
     In each of the embodiments described below, the guide wire may further include a proximal tube portion  17 , a spiral portion  18 , an X-ray non-transparent spiral  23 , leads  30 , etc., as generally described with respect to  FIG. 1  and discussed in U.S. Pat. No. 6,167,763. 
       FIGS. 3-6  depict a guide wire  2  according to a first embodiment.  FIG. 3  is a right-side perspective view of a guide wire  2  according to the first embodiment.  FIG. 4  is a left-side perspective view the guide wire  2  shown in  FIG. 3 .  FIG. 5  is a top view of the guide wire shown in  FIG. 3 .  FIG. 6  is a top view of a portion of the guide wire shown in  FIG. 3 , with internal elements shown in dashed lines. 
     The guide wire  2  includes a tube  21  and pressure sensor apparatus  19 . 
     The tube  21  includes at least one distal opening  31  (for example, two distal openings  31   a,    31   b  as shown in  FIG. 5 ). In the embodiment shown in  FIGS. 3-6 , the distal opening  31  is a concave distal opening  31  that extends proximally from the distal edge  61  of the tube  21 . The term “concave distal opening” as it is used in this disclosure, means that the distal opening  31  is defined by a concave edge  62  of the tube  21 . The term “concave” does not imply any particular shape, as long as the concave edge  62  is formed inwardly from the distal edge  61 . For example, the concave edge  62  may be rounded, as shown in  FIGS. 3-6 , or may be formed of multiple straight edges, or a combination of rounded and straight edges. Where the tube  21  includes two of the concave distal openings  31 , the concave distal openings  31  separate the distal end of the tube into multiple convex distal sections  63 , as shown in  FIGS. 3-6 . More than two distal openings  31  may be included in the tube  21 . 
     The distal opening  31  may be configured such that a fluid may enter the tube  21  from a region outside of the tube  21  via the distal opening  31  (such that the distal opening  31  is an inlet opening). The fluid may then flow past the pressure sensor  19  before exiting the tube  21  via at least one proximal opening  32 , for example, two proximal openings  32   a,    32   b  as shown in  FIG. 5 , (such that the proximal opening  32  is an outlet opening). The proximal opening  32  may be located proximal of the distal opening  31 . In other embodiments, the proximal opening  32  may be an inlet opening and the distal opening  31  may be an outlet opening, depending on the direction in which fluid is flowing relative to the guide wire. 
     The distal opening  31  and proximal opening  32  are shown located in a longitudinal segment of the tube  21  that opposes a side surface of the pressure sensor  19 . The phrase “longitudinal cylindrical segment” refers to a segment of the tube  21  formed by slicing the tube  21  using a longitudinal plane. For example, when the tube  21  includes two distal openings  31   a,    31   b  and two proximal openings  32   a,    32   b,  a first distal opening  31   a  and a first proximal opening  32   a  may be located in a first longitudinal half of the tube  21  that opposes a first side of the pressure sensor  19 , and a second distal opening  31   b  and a second proximal opening  32   b  may be located in a second longitudinal half of the tube  21  that opposes a second side of the pressure sensor  19 , as shown in  FIGS. 5 and 6 . The two distal openings  31   a,    31   b  may be located in a first longitudinal half and second longitudinal half of the tube  21 , respectively, such that the openings  31  oppose a first side and second side of the pressure sensor  19 . 
     In the embodiment of  FIGS. 3-6 , the pressure sensor  19  is shown oriented upwards, and the distal opening  31  and proximal opening  32  are located such that a side of the pressure sensor  19  is exposed. The two distal openings  31  are opposite one another such that the right side of the pressure sensor  19  is exposed in the first distal opening  31   a,  and the left side of the pressure sensor  19  is exposed in the second distal opening  31   b.  However, in other embodiments, the entirety of the distal opening  31  may be located distal of the pressure sensor  19  (i.e., when the guide wire is inserted in a vessel, the position of opening  31  is deeper than the position of the pressure sensor  19 ). 
     The distal opening  31  is shown with a depth (d) between a major diameter of the tube  21  and a deepest point of the opening  31 , when viewed from the side, as shown in  FIG. 6 . In one embodiment, the depth of the concave distal opening  31  may be approximately equal to a distance between the tube  21  and the side surface of the pressure sensor  19 . The term “approximately equal” may be interpreted as, for example, within plus or minus 10%. 
     Referring to  FIG. 6 , a top view of the guide wire is illustrated, with internal elements shown in dashed lines. The guide wire includes a tube  21  including two concave distal openings  31   a,    31   b,  and two proximal openings  32   a,    32   b.  The openings  31  and  32  are shown to include a single rounded edge. The pressure sensor  19  is positioned such that fluids may enter the guide wire at the concave distal openings  31   a,    31   b  in a direction  40  and leave at the proximal openings  32   a,    32   b,  in a direction  42 , flowing over the pressure sensor  19  along the way. Fluid typically flows in the directions  40 ,  42  when the guide wire is flushed prior to use. However, when the guide wire is guide wire is inserted into a vessel, blood would typically flow in directions opposite the directions  40 ,  42  shown in the Figures. 
     The guide wire further includes a solid wire  16  as described with reference to  FIG. 1 . The pressure sensor  19  is mounted on the solid wire  16  such that a distal portion of the pressure sensor  19  extends beyond the solid wire  16  into the groove. The solid wire  16  may include a shelf  27  (shown in greater detail in  FIGS. 8, 10, and 12 ) configured to hold the pressure sensor  19  in place, proximal from the two openings  31 ,  32 . The pressure sensor  19  is cantilevered from the shelf portion  27  of the wire (i.e., only one end of the pressure sensor  19  is coupled to the guide wire). A second portion  28  of the groove is shown as an open space around the pressure sensor  19  in which fluids may flow through the guide wire. The proximal openings  32  may be located, for example, near the distal end of the shelf  27  (i.e., the distal portion of the second portion  28  of the groove). 
     The guide wire is further shown to include an electrical lead  30  running parallel with the pressure sensor  19 . The electrical lead  30  may run from the electronic circuitry associated with the guide wire to the guide wire components (e.g., the pressure sensor  19 ). The wire  16  may act as a portion of a second electrical lead  30 . 
     While the devices shown in the figures include a cantilevered pressure sensor  19 , embodiments of the present invention are not limited to include such sensors. Rather, in embodiments of the present invention, the pressure sensor  19  may be any appropriate sensor which can be used to take pressure measurements. 
       FIGS. 7 and 8  depict a guide wire according to a second embodiment, in which a tube  73  includes a distal tapered section, with internal elements shown in dashed lines.  FIG. 7  is a side view of a guide wire according to the second embodiment.  FIG. 8  is a top view of the guide wire shown in  FIG. 7 . 
     In the second embodiment, the guide wire includes two distal openings  71   a,    71   b  and two proximal openings  72   a,    72   b,  the distal opening  71   a  and proximal opening  72   a  being located opposite the distal opening  71   b  and proximal opening  72   b.  The openings  71 ,  72  are provided such that a side portion of the pressure sensor  19  is exposed in the openings. Each proximal opening  72  is located proximal from its corresponding distal opening  71 . The two distal openings  71  may be located in a first longitudinal half and second longitudinal half of the tube  73 , respectively, such that the openings  71  oppose a first side and second side of the pressure sensor  19 . For example, the two distal openings  71  are opposite one another such that the right side of the pressure sensor  19  is exposed in the first distal opening  71   a,  and the left side of the pressure sensor  19  is exposed in the second distal opening  71   b.  In other embodiments, the distal openings  71  and proximal openings  72  may be otherwise positioned, and/or additional openings may be included. 
     The second embodiment is similar to the first embodiment except, in the second embodiments, the tube  73  includes a first cylindrical section  73   a  and a second cylindrical section  73   b  that has a diameter larger than a diameter of the first cylindrical section  73   a.  The tube  73  further includes a tapered transition section  73   c  located between the first cylindrical section  73   a  and the second cylindrical section  73   b.    
     At least a portion of each of the distal openings  71   a,  and  71   b  is located at a distal end of the second cylindrical section  73   b,  as shown in  FIGS. 7 and 8 . For example, a portion of each of the distal openings  71   a,    71   b  may be formed in the second cylindrical section  73   b,  and a portion of each of the distal openings  71   a,    71   b  may be formed in the tapered transition section  73   c,  as shown in  FIGS. 7 and 8 . 
     In other embodiments, a portion of each of the distal openings  71   a,    71   b  may also be formed in the first cylindrical section  73   a.  The distal openings  71   a,  and  71   b  may be concave distal openings, as discussed above with respect to  FIGS. 3-6 . In still other embodiments, the first cylindrical section  73   a  may be omitted altogether. 
     As the guide wire is inserted into a vessel, a fluid may flow into the distal openings  71   a,    71   b  in a direction  40 , flow over the pressure sensor  19 , and flow out of the guide wire through the proximal openings  72   a,    72   b  in a direction  42  (or vice versa). 
       FIGS. 9 and 10  depict a guide wire according to a third embodiment, in which a tube  52  includes a distal tapered section and a proximal tapered section.  FIG. 9  is a side view of a guide wire according to the third embodiment, with internal elements shown in dashed lines.  FIG. 10  is a top view of the guide wire shown in  FIG. 9 . 
     In the third embodiment, the guide wire includes two distal openings  81   a,    81   b  and two proximal openings  82   a,    82   b,  the distal opening  81   a  and proximal opening  82   a  being located opposite the distal opening  81   b  and proximal opening  82   b.  The openings  81 ,  82  are provided such that a side portion of the pressure sensor  19  is exposed in the openings. Each proximal opening  82  is located proximal from its corresponding distal opening  81 . The two distal openings  81  may be located in a first longitudinal half and second longitudinal half of the tube  52 , respectively, such that the openings  81  oppose a first side and second side of the pressure sensor  19 . For example, the two distal openings  81  are opposite one another such that the right side of the pressure sensor  19  is exposed in the first distal opening  81   a,  and the left side of the pressure sensor  19  is exposed in the second distal opening  81   b.  In other embodiments, the distal openings  81  and proximal openings  82  may be otherwise positioned, and/or additional openings may be included. 
     The tube  52  of the guide wire in  FIGS. 9 and 10  includes a first cylindrical section  52   a  and a second cylindrical section  52   b  that has a diameter larger than a diameter of the first cylindrical section  52   a.  The tube  52  further includes a distal tapered transition section  52   c  and a proximal tapered transition section  52   d,  the distal tapered transition section  52   c  being located distal of the second cylindrical section  52   b,  and the proximal tapered transition section  52   d  being located proximal of the second cylindrical section  52   b.  The tube  52  may further include a third cylindrical section  52   e,  as shown in  FIGS. 9 and 10 . Where the tube  52  includes a third cylindrical section  52   e,  the second cylindrical section  52   b  may have a diameter that is larger than a diameter of the third cylindrical section  52   e.    
     At least a portion of each of the distal openings  81   a,  and  81   b  is located at a distal end of the second cylindrical section  52   b.  For example, a portion of each of the distal openings  81   a,    81   b  may be formed in the second cylindrical section  52   b,  and a portion of each of the distal openings  81   a,    81   b  may be formed in the tapered transition section  52   c,  as shown in  FIGS. 9 and 10 . At least a portion of each of the proximal openings  82   a,    82   b  is located at a proximal end of the second cylindrical section  52   b.  For example, a portion of each of the proximal openings  82   a,    82   b  may be formed in the second cylindrical section  52   b,  and a portion of each of the proximal openings  82   a,    82   b  may be formed in the tapered transition section  52   d,  as shown in  FIGS. 9 and 10 . 
     In other embodiments, a portion of each of the distal openings  81   a,    81   b  may also be formed in the first cylindrical section  52   a.  The distal openings  81   a,  and  81   b  may be concave distal openings, as discussed above with respect to  FIGS. 3-6 . In still other embodiments, the first cylindrical section  52   a  may be omitted altogether. 
     In yet other embodiments, a portion of each of the proximal openings  82   a,    82   b  may also be formed in the third cylindrical section  52   e.  The proximal openings  82   a,    82   b  may be concave proximal openings, in which case the proximal openings  82   a,    82   b  would be formed similarly to the concave distal openings discussed above with respect to  FIGS. 3-6 , except that the proximal openings  82   a,    82   b  would extend from a proximal edge of the tube  52 , rather than the distal edge. In still other embodiments, the third cylindrical section  52   e  may be omitted altogether. 
     As the guide wire is inserted into a vessel, a fluid may flow into the distal openings  81   a,    81   b  in a direction  40 , flow over the pressure sensor  19 , and flow out of the guide wire through the proximal openings  82   a,    82   b  in a direction  42  (or vice versa). 
       FIGS. 11 and 12  depict a guide wire according to a fourth embodiment, in which a tube  56  includes an ovoid section.  FIG. 11  is a side view of a guide wire according to the fourth embodiment, with internal elements shown in dashed lines.  FIG. 12  is a top view of the guide wire shown in  FIG. 11 . 
     In the fourth embodiment, the guide wire includes two distal openings  91   a,    91   b  and two proximal openings  92   a,    92   b,  the distal opening  91   a  and proximal opening  92   a  being located opposite the distal opening  91   b  and proximal opening  92   b.  The openings  91 ,  92  are provided such that a side portion of the pressure sensor  19  is exposed in the openings. Each proximal opening  92  is located proximal from its corresponding distal opening  91 . The two distal openings  91  may be located in a first longitudinal half and second longitudinal half of the tube  56 , respectively, such that the openings  91  oppose a first side and second side of the pressure sensor  19 . For example, the two distal openings  91  are opposite one another such that the right side of the pressure sensor  19  is exposed in the first distal opening  91   a,  and the left side of the pressure sensor  19  is exposed in the second distal opening  91   b.  In other embodiments, the distal openings  91  and proximal openings  92  may be otherwise positioned, and/or additional openings may be included. 
     The tube  56  of the guide wire in  FIGS. 11 and 12  includes an ovoid section  56   a.  The tube  56  may also include a first cylindrical section  56   b  located distal of the ovoid section  56   a  and a second cylindrical section  56   c  located proximal of the ovoid section  56   a,  as shown in  FIGS. 11 and 12 . The maximum diameter of the ovoid section  56   a  may be larger than the diameters of the first and second cylindrical sections  56   b  and  56   c.  The minimum diameter of the ovoid section  56   a  may be substantially equal to the diameters of the first and second cylindrical sections  56   b  and  56   c.    
     At least a portion of each of the distal openings  91   a,  and  91   b  is located at a distal end of the ovoid section  56   a.  For example, a portion of each of the distal openings  91   a,    91   b  may be formed in the ovoid section  56   a.  At least a portion of each of the proximal openings  92   a,    92   b  is located at a proximal end of the ovoid section  56   a.  For example, a portion of each of the proximal openings  92   a,    92   b  may be formed in the ovoid section  56   a.    
     In other embodiments, a portion of each of the distal openings  91   a,    91   b  may also be formed in the first cylindrical section  56   b.  The distal openings  91   a,  and  91   b  may be concave distal openings, as discussed above with respect to  FIGS. 3-6 . In still other embodiments, the first cylindrical section  56   b  may be omitted altogether. 
     In yet other embodiments, a portion of each of the proximal openings  92   a,    92   b  may also be formed in the second cylindrical section  56   c.  The proximal openings  92   a,    92   b  may be concave proximal openings, in which case the proximal openings  92   a,    92   b  would be formed similarly to the concave distal openings discussed above with respect to  FIGS. 3-6 , except that the proximal openings  92   a,    92   b  would extend from a proximal edge of the tube  56 , rather than the distal edge. In still other embodiments, the second cylindrical section  56   c  may be omitted altogether. 
       FIGS. 13-16  depict a guide wire and a tube  101  thereof according to a fifth embodiment, in which the tube  101  includes six openings  102   a,    102   b,    102   c,    103   a,    103   b,  and  103   c  in its circumferential wall.  FIG. 13  is a perspective view of the guide wire according to the fifth embodiment.  FIG. 14  is a perspective view of the tube  101  of the guide wire of the fifth embodiment.  FIG. 15  is a top view of the guide wire of the fifth embodiment.  FIG. 16  is a side view of the tube  101  of the guide wire of the fifth embodiment. It has been found that the embodiment shown in  FIGS. 13-16  yields a particularly effective flow through the tube  101 , and largely inhibits formation of air pockets/bubbles in the tube  101  during use. 
     In the fifth embodiment, the tube  101  of the guide wire includes three distal openings  102   a,    102   b,    102   c  and three proximal openings  103   a,    103   b,    103   c.  The distal opening  102   a  and proximal opening  103   a  are located on a top side of the tube  101 , which is a side that is faced by the pressure sensor membrane  29 . The distal opening  102   b  and proximal opening  103   b  are on a right side of the tube  101 . The distal opening  102   c  and proximal opening  103   c  are on a left side of the tube  101 . The six openings allow for multiple liquid entrance points and multiple air evacuation points, and also aids in the application of a hydrophilic coating to an interior of the tube  101 , as discussed in more detail below. 
     In a preferred embodiment, the tube  101  includes exactly six openings in its circumferential wall, so that a bottom side of the tube  101  does not include any openings. This allows for a core wire to be located at the bottom side of the guide wire without obstructing any of the openings in the circumferential wall of the tube. 
     The pressure sensor  19  is preferably disposed in the tube  101  such that, in a top view of the guide wire, an entirety of the pressure sensor membrane is visible through the distal opening  102   a.  The distal openings  102   b  and  102   c  are preferably located at the left and right sides of the distal opening  102   a.  Preferably centers of the distal openings  102   b  and  102   c  are each offset by 90° in opposite circumferential directions from a center of the distal opening  102   a.    
     The proximal opening  103   a  is located proximal of the distal opening  102   a.  The proximal openings  103   b  and  103   c  are preferably located at the left and right sides of the proximal opening  103   a.  Preferably, centers of the proximal openings  103   b  and  103   c  are each offset by 90° in opposite circumferential directions from a center of the proximal opening  103   a.    
     The distal opening  102   a  is larger than the distal openings  102   b,    102   c.  The proximal opening  103   a  is larger than the proximal openings  103   b,    103   c.  Preferably, the distal opening  102   a  and proximal opening  103   a  have widths that are greater than widths of the distal openings  102   b,    102   c  and proximal openings  103   b,    103   c,  the widths being defined in directions perpendicular to a longitudinal direction of the guide wire. A width of the distal opening  102   a  and proximal opening  103   a  may be, for example, in a range of 0.14 to 0.32 mm, preferably 0.17 to 0.29 mm, and more preferably 0.178 to 0.278 mm. A width of the distal openings  102   b,    102   c  and proximal openings  103   b,    103   c  may be, for example, in a range of 0.03 to 0.21 mm, preferably 0.06 to 0.18 mm, and more preferably 0.071 to 0.171 mm. 
     A distance between the distal opening  102   a  and the distal openings  102   b,    102   c  may be, for example, in a range of 0.03 to 0.21 mm, preferably 0.06 to 0.18 mm, and more preferably 0.071 to 0.171 mm. A distance between the proximal opening  103   a  and the proximal openings  103   b,    103   c  may be, for example, in a range of 0.03 to 0.21 mm, preferably 0.06 to 0.18 mm, and more preferably 0.071 to 0.171 mm. 
     An overall length of the tube  101  may be, for example, in a range of 1.90 to 2.1 mm, and preferably 1.95 to 2.05 mm. A distance from a proximal end of the tube  101  to proximal ends of the proximal openings  103   a,    103   b,    103   c  may be, for example, in a range of 0.130 to 0.330 mm, and preferably 0.180 to 0.280 mm. A distance from a proximal end of the tube  101  to the distal ends of the proximal openings  103   a,    103   b,    103   c  may be, for example, in a range of 0.785 to 0.985 mm, and preferably 0.835 to 0.935 mm. A distance from a proximal end of the tube  101  to proximal ends of the distal openings  102   a,    102   b,    102   c  may be, for example, in a range of 1.015 to 1.215 mm, and preferably 1.065 to 1.165 mm. A distance from a proximal end of the tube  101  to distal ends of the distal openings  102   a,    102   b,    102   c  may be, for example, in a range of 1.670 to 1.870 mm, and preferably 1.720 to 1.820 mm. 
     The proximal ends of each of the distal openings  102   a,    102   b,    102   c  are preferably at the same longitudinal position of the guide wire, and the distal ends of each of the distal openings  102   a,    102   b,    102   c  are preferably at the same longitudinal position of the guide wire. Similarly, the proximal ends of each of the proximal openings  103   a,    103   b,    103   c  are preferably at the same longitudinal position of the guide wire, and the distal ends of each of the proximal openings  103   a,    103   b,    103   c  are preferably at the same longitudinal position of the guide wire. 
     Each of the openings  102   a,    102   b,    102   c,    103   a,    103   b,    103   c  has a generally rectangular shape, preferably a rectangular shape with rounded corners. 
     The overall length of the guide wire may be, for example, between 1.5 m and 2 m, and preferably about 1.75 meters. The tube  21 ,  52 ,  56 ,  73 ,  101  may have an overall diameter of, for example, between 0.30 and 0.40 mm, and preferably about 0.35 mm. In embodiments that have a reduced diameter tube portion, the diameter of this portion may be, for example, between 0.20 and 0.30 mm, and preferably about 0.25 mm. 
     In another embodiment, surfaces of the tube and/or pressure sensor (optionally including the pressure sensor membrane) can be treated with a hydrophilic material, to promote influx of liquid (such as water, blood, or saline solution) into the tube to contact the pressure sensor membrane. Coating surfaces of the tube and/or pressure sensor with a hydrophilic material can allow for more stable sensor signals, by reducing air moving through the tube. When submerged in a liquid, the hydrophilic material may dissolve within a few seconds, causing an influx of liquid into the tube and thereby removing air from (or preventing the introduction of air into) the tube. 
     For example, an inner surface (or “internal circumferential surface”) of the tube (such as tubes  21 ,  52 ,  56 ,  73 ) may be coated with a hydrophilic material. Alternatively or additionally, a surface of the pressure sensor (such as pressure sensor  19 ) may be coated with a hydrophilic material. Alternatively or additionally, a surface of the pressure sensor membrane (such as pressure sensor membrane  29 ) may be coated with a hydrophilic material. The jacket may be partially or completely filled with the hydrophilic material. The hydrophilic material may be a material comprising, for example, polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), a polysaccharide, and/or a salt. Preferably, the hydrophilic material is 20,000 g/mol PEG, which has been found to be more stable at elevated aging temperatures than lower molecular weight PEG. Preferably, a PEG dose is in a range of 6 μg to 20 μg. The hydrophilic material comprising PEG may be applied by, for example, by dispensing or vacuum filling a mixture of 3%-10% PEG (preferably 5% PEG) dissolved in 90%-97% ethanol (preferably 95% ethanol) inside the tube. A PEG application of 0.2 μL of a 5% PEG solution (which will yield a PEG dose in the desired range of 6 μg to 20 μg) is preferred. The mixture can be dried using, for example, a heat lamp, so that the ethanol evaporates, leaving a PEG coating on an interior of the tube and/or on the sensor membrane. The application of a hydrophilic material to the interior of the six-opening tube  101  of the fifth embodiment, described above, has been found to be particularly effective in terms of preventing air from accumulating in the tube  101 . 
     It should be understood that even more variations of tube shapes and opening alignments are possible. For example, the tubes may include any number of cylindrical sections of different diameters, and may include any number of tapered sections or other sections to transition between the cylindrical sections. The sections of the tubes may be of any shape (e.g., rectangular, ovoid, spherical, etc.). The distal openings and proximal openings may be located in any of the sections of the tube such that fluids flow into the guide wire at a distal end of the guide wire though the distal openings, and flows over the pressure sensor  19  and through the proximal openings (or vice versa). 
     The invention being thus described, it will be clear that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention and all such modifications as would be clear to one skilled in the art are intended to be included within the scope of the following claims.