3D Printed and In-Situ Poled Flexible Piezoelectric Pressure Sensor

Abstract

A piezoelectric artificial artery can be 3D printed to provide the real-time precise sensing of blood pressure and vessel motion patterns enabling early detection of partial occlusions. An electric-field assisted 3D printing method allows for rapid printing and simultaneously poled complex ferroelectric structures with high fidelity and good piezoelectric performance. The print material consists of ferroelectric potassium sodium niobite (KNN) particles embedded within a ferroelectric polyvinylidene fluoride (PVDF) polymer matrix.

Claims

1. A tubular pressure gauge comprising: a tube comprising: a piezoelectric substrate wherein the substrate is poled perpendicular to inner and outer opposed surfaces of the substrate; a first electrode on the inner surface of the substrate; a second electrode on the outer surface of the substrate; and a voltage meter electrically connected to the first and second electrodes and configured to receive a voltage between the first and second electrodes.

2. The pressure gauge of claim 1 wherein the piezoelectric substrate is ferroelectric potassium sodium niobite (KNN) piezoelectric particles embedded in a ferroelectric polyvinylidene fluoride (PVDF) polymer matrix.

3. The pressure gauge of claim 2 wherein an amount of KNN piezoelectric particles is 30-40 vol %.

4. The pressure gauge of claim 3 wherein the KNN piezoelectric particles have a Curie temperature of at least 300 degrees Celsius.

5. The pressure gauge of claim 3 wherein the KNN piezoelectric particles have a piezoelectric coefficient of at least 80 pC/n at room temperature.

6. The pressure gauge of claim 1 wherein the piezoelectric substrate generates a voltage output of at least 2.5 V under an external force load of 30N.

7. The pressure gauge of claim 1 wherein the piezoelectric substrate has a thickness between 0.1-0.2 millimeters.

8. The pressure gauge of claim 1 wherein the piezoelectric substrate in a lattice.

9. The pressure gauge of claim 8 wherein the lattice is at least 60% openings.

10. The pressure gauge of claim 1 further comprising a sheath encapsulating the piezoelectric substrate and the first and second electrode and forming an outermost and innermost layer of the tube.

11. The pressure gauge of claim 10 wherein the sheath is an inner and outer layer of polydimethylsiloxane (PDMS).

12. The pressure gauge of claim 11 wherein the layers of PDMS have a combined thickness of 1-2 mm.

13. The pressure gauge of claim 1 wherein the inner and outer electrodes are layers of silver ink.

14. The pressure gauge of claim 1 wherein the tube has a tensile modulus of at least 5 MPa.

15. The pressure gauge of claim 1 wherein the tube has a flexural modulus of at least 10 MPa.

16. A method of manufacturing a pressure gauge comprising a tube having a substrate of piezoelectric material wherein the substrate is poled perpendicular to a first and second opposed surfaces of the substrate; a first electrode on the first surface of the substrate; a second electrode on the second surface of the substrate; and a voltage meter configured to receive a voltage between the first and second electrodes, the method comprising the steps of: heating a composite piezoelectric material at a temperature of at least 250 degrees Celsius to form a molten material; extruding the molten material through a 3D printer nozzle; applying an electrical field between the 3D printer nozzle and a lower build plate to pole the molten material as the molten material is being extruded; and depositing the molten material onto the lower build plate to form the substrate of piezoelectric material.

17. The method of claim 16 wherein the electrical field applied is in the range of 0.5-4 kV/mm and the electric field is increased with an increase in build height.

18. The method of claim 16 wherein the molten material is deposited at a print speed of less than 20 mm/s.

19. The method of claim 16 further comprising the step of forming the tube by rolling the substrate of piezoelectric material wherein the tube has a diameter between 5 mm to 1.5 cm and a length of at least 1 cm.

20. The method of claim 19 further comprising the step of covering inner and outer surfaces of the tube with a layer of polydimethylsiloxane (PDMS).

21. A method of 3D printing a substrate of piezoelectric material, the method comprising the steps of: heating a composite material of ferroelectric potassium sodium niobite (KNN) piezoelectric particles embedded in a ferroelectric polyvinylidene fluoride (PVDF) polymer matrix at a temperature of at least 250 degrees Celsius to form a molten material; extruding the molten material through a 3D printer nozzle; applying an electrical field between the 3D printer nozzle and a lower build plate to pole the molten material as the molten material is being extruded; and depositing the molten material onto the lower build plate to form the substrate; wherein the substrate is poled perpendicular to first and second opposed surfaces of the substrate.

22. The method of claim 21 wherein an amount of KNN piezoelectric particles is 30-40 vol %.

23. The method of claim 21 wherein the substrate generates a voltage output of at least 2.5 V under an external force load of 30N.

24. The method of claim 21 wherein the substrate has a piezoelectric coefficient of at least 10 pC/N at room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 is a schematic cross-sectional representation of one embodiment of the present invention showing a method of printing a piezoelectric print where the molten polymer composite is poled simultaneous with 3D printing extrusion to form the piezoelectric print on the print bed;

[0043] FIG. 2 is a schematic representation of the polymer composite of FIG. 1 with piezoceramic particles of ferroelectric potassium sodium niobite (KNN) particles being combined with a polymer matrix of ferroelectric polyvinylidene fluoride (PVDF);

[0044] FIG. 3 is a perspective view of a tubular piezoelectric artificial blood vessel formed by the method of FIG. 1 and responding to blood pressure to produce an electric charge and showing an enlarged view of the complex lattice structure of the piezoelectric artificial blood vessel;

[0045] FIG. 4 is a cross-sectional view of the piezoelectric artificial blood vessel of FIG. 3 showing the layers of the piezoelectric artificial blood vessel responding to changes in blood pressure and producing a voltage; and

[0046] FIG. 5 is an enlarged cross-sectional view of the piezoelectric material having a dipole moment causing a voltage in response to pressure applied on the piezoelectric material by blood pressure changes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0047] Referring to FIG. 1, a 3D printing system 10 of the present invention for fused deposition modeling (FDM) may be used with an electric poling process in the manufacture of piezoelectric substrates 12, for example, that may be used to manufacture artificial biological systems such as artificial arteries within the body.

[0048] The 3D printing system 10 may include a build plate 14 providing a substantially planar surface on which the extruded molten material is deposited, layer by layer, during 3D printing. The build plate 14 may be moved vertically up and down, for example, lowered as the layers are added, and translated in all directions within a plane of the build plate 14 to assist with building the shape of the printed object as understood in the art. The build plate 14 includes multiple layers of materials that are used to heat and/or support the extruded material 15 but also to produce an electric field between the build plate 14 and a print nozzle to pole the extruded material 15 as the molten material is deposited onto the build plate 14.

[0049] The build plate 14 may include a heat bed 16, forming a lowermost layer of the build plate 14, which provides a conductive material that keeps the extruded material 15 on the build plate 14 warm to prevent warping if the extruded material 15 is cooled too quickly. The heat bed 16 may be electrically connected to an electric resistance heater 18 as understood in the art that conducts heat to the conductive heat bed 16. In some embodiments, the heat bed 16 may be an aluminum plate or a glass plate.

[0050] An upper surface of the heat bed 16 may be optionally covered by a glass plate 20 that does not warp or bend under heat and provides a smooth upper surface, particularly when the heat bed 16 itself does not provide a smooth upper surface. In some embodiments, the heat bed 16 may be glass, for example, a tempered glass, removing the necessity for the glass plate 20 to cover the heat bed 16. Generally, the desired characteristics of the material of the upper surface of the heat bed 16, or glass plate 20 if included, are hard and rigid, high modulus of elasticity, and thermally stable (low thermal conductivity).

[0051] An upper surface of the heat bed 16, or glass plate 20 if included, will support an electrode layer 22, for example a copper foil, serving as a cathode of the electrical field assembly connected to a voltage source 39. A nozzle 24 serving as an anode of the electrical field assembly is also connected to the voltage source 39. The nozzle 24 and the electrode layer 22 produce an electric field as further described below.

[0052] An upper surface of the electrode layer 22 includes an adhesion layer 26 to assist with the adhesion of the extruded material 15 to a top of the uppermost layer of the build plate. The adhesion layer 26 may be a thin layer of polyvinyl alcohol (PVA) hydrogel, Kapton tape, masking tape or the like enhancing the adhesion of the lowermost print layer to the build plate 14. The adhesion layer 26 may have a thickness that is less than 50 micrometers.

[0053] The build plate 14 may have a surface area of at least 5 by 5 by 5 inches providing a sufficient surface area to build the printed object.

[0054] The 3D printing system 10 may include a filament extruder 30 including feeding rollers 32 that are driven by a motor 35 to drive the filaments 34 downward through the extruder 30 and into a conical nozzle 24 of the extruder 30. The nozzle 24 may be heated by a heater 36, such as a thermistor, thermocouple, or the like mounted to the extruder 30 and heating the nozzle 24 to further melt the filaments 34 as it flows through the nozzle 24 to dispel the extruded material 15 in molten form. The heater 36 may heat the nozzle 24 and filaments 34 to a temperature of at least 230 degrees Celsius, and at least 250 degrees Celsius, and between 230-290 degrees Celsius to cause the filaments 34 to properly melt and be properly extruded through the nozzle 24 without clogging.

[0055] The extruded material 15 may be expelled through a lower opening 38 of the nozzle 24 and deposited onto a top of the build plate 14. The lower opening 38 may have a diameter ranging from 0.1 mm to 1 mm.

[0056] The nozzle 24 may be manufactured of a heat and electrically conductive material such as brass, stainless steel, and hardened steel allowing for melting of the filament 34 and poling processes described below.

[0057] The filament extruder 30 of the 3D printing system 10 may be moved vertically up and down, for example, raising the nozzle 24 as the build layers are added, and translating in all directions parallel to the plane of the build plate 14 to assist with building the shape of the printed object as understood in the art.

[0058] As the extruded material 15 is extruded through the nozzle 24, an electrical field is created between the electrode layer 22 and the nozzle 24 to pole the extruded material 15 as it is being extruded, causing polarization of the extruded material 15 as it is deposited onto the build plate 14.

[0059] To pole the extruded material 15, an adjustable electric field in the range of 0.5 to 4 kV/mm is applied between the electrode layer 22 and the nozzle 24 extending along a poling axis 37 perpendicular to the build plate 14. The electric field is created by a voltage source 39 having an output voltage up to 30 kV (10 W). A positive electrode of the high voltage source 39 may connected to the nozzle 24 while a negative electrode of the high voltage source 39 may connected to the electrode layer 22. The direction of poling along the poling axis 37 dictates the polarity of the extruded material 15, however, the anode and cathode may be switched without affecting the piezoelectric effect of the resulting printed piezoelectric substrate 12. The poling of the extruded material 15 causes the dipoles of the printed piezoelectric substrate 12 to orient in the same direction, leading to a piezoelectric effect of the material.

[0060] The electric field can be applied at elevated temperatures, for example, at least 230 degrees Celsius, and at least 250 degrees Celsius, and between 230-290 degrees Celsius, and at temperatures required to properly melt the filament 34. At these elevated temperatures, it would normally be expected that the extruded material 15 would exhibit a significantly lower coercive force, therefore requiring the extruded material 15 to cool prior to effective poling application. However, the concentrated electric field and high processing temperature are favorable for aligning the ferroelectric dipoles in the extruded material 15. Thus, the extruded material 15 is poled simultaneous with extrusion onto the build plate 14 and eliminating the need for separate 3D printing and poling steps.

[0061] The printing rate of the extruded material 15 is desirable at less than 20 mm/s, and less than 15 mm/s, and less than 10 mm/s which allows for sufficient poling time and improved dipole alignment than would occur at a faster printing rate, for example, higher than 20 mm/s.

[0062] The extruded material 15 may be deposited onto the build plate 14 layer-by-layer with each layer having a thickness of about 0.1 mm to 0.2 mm (100 μm to 200 μm) and the printed piezoelectric substrate 12 having a desired thickness of between 0.1 mm to 0.2 mm (100 μm to 200 μm) for a rolled substrate as described below. The electric field applied to the extruded material 15 and the poling voltage is increased as each layer is added. In one embodiment, the poling voltage may be increased by 100 volts per additional layer. This allows for the effective construction of multilayer piezoelectric structures having greater sensitivity or mechanical action.

[0063] Referring to FIG. 2, the filament 34 used in the printing process described above to produce the piezoelectric substrate 12 may be a ferroelectric composite 40 formed of ferroelectric potassium sodium niobite (KNN) or KNN-based piezoceramic particles 42 embedded in a ferroelectric polyvinylidene fluoride (PVDF) or PVDF-based polymer 44. The KNN-based piezoceramic particles 42 may include KNN, Li/Sn/Ta-doped KNN, KNN-ABO3 solid solution, and the like. The PVDF-based polymer 44 may include PVDF, PVDF-TrFE, PVDF-HFP, PVDF-CTFE, other PVDF-copolymers, and the like.

[0064] The KNN-based piezoceramic particles 42 are a group of high performance, lead-free ferroelectric ceramics. The KNN-based piezoceramic particles 42 possess high piezoelectric properties and have a piezoelectric coefficient (d.sub.33) of about 100 pC/N and between 80-400 pC/N at room temperature, and a high Curie temperature of at least 270 degrees Celsius and at least 300 degrees Celsius and at least 400 degrees Celsius and between 435-445 degrees Celsius and about 440 degrees Celsius, which assist the composite material in maintaining its ferroelectric phase during high temperature 3D printing. The KNN-based particles piezoceramic 42 have a high dielectric permittivity of about 200 at 10 kHz at room temperature. The KNN-based piezoceramic particles 42 may generate a stable piezoelectric peak-to-peak voltage output (V.sub.pp) of about 20V and a short-circuited current of about 30 μA under an external force load of 30 N peak load. The KNN-based piezoceramic particles 42 are also biocompatible making them a suitable material for artificial blood vessels.

[0065] The KNN-based piezoceramic particles 42 may be prepared to further include 3-(trimethoxysilyl)propyl methacrylate (MPS) 46 that is covalently grafted on the outer surface of the KNN piezoceramic particles 42 to improve the interface compatibility and maximize the dispensability of the KNN piezoceramic particles 42.

[0066] The PVDF-based polymers 44 are a group of soft thermoplastic polymers with high printability, flexibility, and acceptable piezoelectricity. The PVDF-based polymers 44 possess piezoelectric properties and have a piezoelectric coefficient (d.sub.33) of about 20 pC/N at room temperature, and a Curie temperature of between 190 and 200 degrees Celsius and about 195 degrees Celsius, which assist the composite material in maintaining its ferroelectric phase during high temperature 3D printing. The PVDF-based polymers 44 have a high dielectric permittivity of about 12 at 10 kHz at room temperature. The PVDF-based polymers 44 may generate a stable piezoelectric peak-to-peak voltage output (V.sub.pp) of about 3V and a short-circuited current of about 100 nA under an external force load of 30N peak load. The PVDF-based polymers 44 are also biocompatible making them a suitable material for artificial blood vessels. The PVDF-based polymers 44 have low electrical conductivity.

[0067] In one embodiment of the present invention, the filament 34 of ferroelectric composite 40 may be manufactured by the following process.

[0068] First, the KNN-based piezoceramic particles 42 may be synthesized using a solid-state reaction. Raw materials of potassium carbonate (K.sub.2CO.sub.3, >99%, Sigma), sodium carbonate (Na.sub.2CO.sub.3, >99.5%, Sigma), and niobium oxide (Nb.sub.2O.sub.5, >99.9%, Alfa Aesar) are uniformly mixed in a molar ratio of 1:1:2 and dried at 210 degrees Celsius for 2 hours to remove absorbed moisture. The dried mixture is then milled for 4 hours and then then calcined at 1100° C. for 4 h followed by slowly cooling the mixture to room temperature to form high-purity KNN ceramics. The KNN ceramics are milled for another 1 hour to form microparticles. The microparticles are refluxed with H.sub.2O.sub.2 solution (30%) at 105 degrees Celsius for 2 hours to add a hydroxyl group onto the surface of the microparticles. The samples are collected through centrifuge and rinsed with deionized water three times and dried in an oven. The dried samples are further surface modified through refluxing with 3-(trimethoxysilyl)propyl methacrylate (MPS) (2%) in methanol solution at 70 degrees Celsius for 4 hours. Surface-treated samples are collected, rinsed with methanol, and dried in oven at 80 degrees Celsius overnight.

[0069] Second, the surface modified KNN-based piezoceramic particles 42 are mixed with PVDF-based polymers 44, for example, polymer pellets of PVDF (Kynar 720 PVDF), at a volume ratio of 3.5 to 6.5. The mixture is grinded into uniform compositions by a SPEX 6875 Cryogenic Grinder, or added into N,N-dimethylformamide (DMF, >99.8%, Sigma) solvent at 80 degrees Celsius for 1 hour with stirring to form a uniform solution (25 wt % concentration) which is then casted onto a glass dish to form a composite film after evaporating the solvent at 70 degrees Celsius overnight and the films are grinded into fine powders by a SPEX 6875 Cryogenic Grinder.

[0070] Third, the ferroelectric composite powder is extruded to form the ferroelectric filament 34 with constant diameter of between 1.7 mm to 3.5 mm, and about 1.7 mm or 3 mm, at 170 degrees Celsius and collected by a winder with a spool collecting the wound filament 34. A laser sensor may be used for filament diameter control.

[0071] It is understood that other processes may be used in the manufacture of the filament 34 of ferroelectric composite 40.

[0072] Referring to FIGS. 1 and 2, the filament 34 of ferroelectric composite 40 may be used to 3D print a piezoelectric substrate 12 according to the method shown and described with respect to FIG. 1. The ferroelectric composite 40 contains uniform distribution of KNN piezoceramic particles 42 inside the PVDF-based polymer 44 matrix. The ferroelectric composite 40 may consist of 30-40 vol % and about 35 vol % functionalized KNN-based piezoceramic particles 42 and 60-70 vol % and about 65 vol % PVDF-based polymer 44.

[0073] The piezoelectric substrate 12 has a piezoelectric coefficient (d.sub.33) of at least 10 pC/N and at least 12 pC/n and at least 12.4 pC/N at room temperature, and a Curie temperature of at least 250 degrees Celsius. The printed piezoelectric substrate 12 has a permittivity of about 40 at 10 k Hz at room temperature. The printed piezoelectric substrate 12 may generate a stable piezoelectric peak-to-peak voltage output (V.sub.pp) of at least 2.5 V and about 2.7 V, and a short-circuited current of at least 1 μA and about 1.25 μA, under an external force load of 30 N peak load showing a high piezoelectric output. The piezoelectric peak-to-peak voltage output (V.sub.pp) remains at at least 2.5 V and about 2.7 V under a repeated force application of 30 N for over 10000 cycles showing the stable piezoelectric output of the piezoelectric substrate 12.

[0074] The present inventors believe that the improved piezoelectric properties of the piezoelectric substrate 12 is attributed to the synergistic contribution of both the KNN piezoceramic particles 42 and the PVDF-based polymer 44 (see Table 1 below). The PVDF-based polymer 44 plays an important role in assisting the overall polarization and printability of the composite whereas the KNN piezoceramic particles 42 makes the dominant contribution to the polarization of the composite.

[0075] The present inventors have found that the use of alternative composites does not achieve the same favorable piezoelectric characteristics. For example, in polyvinylidene fluoride (PVDF) and barium titanate (BTO), or PVDF-BTO composite, BTO has a low Curie temperature of about 120 degrees Celsius, preventing it from being used for high-temperature printing processes. As a result, the printed film yielded a piezoelectric peak-to-peak voltage output (V.sub.pp) of 0.3 V, which is much lower than the high piezoelectric output of the present invention and owing to the small amount of β-phase PVDF. In another comparative example, polylactic acid (PLA) and ferroelectric potassium sodium niobite (KNN), or PLA-KNN composite, also displays a lower piezoelectric peak-to-peak voltage output (V.sub.pp) of 1.7 V.

TABLE-US-00001 TABLE 1 Piezoelectric Peak-to-Peak coefficient Voltage (d.sub.33) at room (V.sub.pp) under temperature Curie Temperature (T.sub.c) 30N load PVDF ~20 pC/N 190-200° C. ~3 V KNN ~100 pC/N 435-445° C. ~20 V BTO ~190 pC/N 100-120° C. ~4 V PVDF-KNN ~12.4 pC/N Determined by KNN ~2.7 V PVDF-BTO <1 pC/N Determined by BTO ~0.3 V (lower than KNN composites) PLA-KNN ~7 pC/N Determined by KNN ~1.7 V

[0076] Referring now to FIG. 3, the above described method of 3D printing a piezoelectric substrate 12 may be used to print a tubular structure 50 with self-powered sensing capability. In this respect, the tubular structure 50 may also be used as a transducer using an electrical signal to actuate or produce a vibration at the tubular structure 50 and/or receiving the mechanical changes in the tubular structure 50 to produce an electrical signal detected by an external sensor.

[0077] In one embodiment of the present invention, the tubular structure 50 may serve as a pressure gauge detecting a pressure flow through the tubular structure 50. In one embodiment of the present invention, the tubular structure 50 may serve as an artificial artery that can be quickly printed for individual patients with no external battery or power source needed to sense the blood pressure and blood vessel motion patterns of the blood vessel.

[0078] The tubular structure 50 may be a hollow cylinder 52 with an inner tube 54 formed by the planar, rectangular piezoelectric substrate 12 rolled into a tube, for example, rolled around a mandrel with the outer edges joined along a length of the tube. The inner tube 54 has circular open ends extending along a longitudinal axis 61. The piezoelectric substrate 12 is poled along the poling axis 37 that is normal to the inner and outer surfaces of the rolled piezoelectric substrate 12 and normal to the longitudinal axis 61.

[0079] The piezoelectric substrate 12 may be a printed sinusoidal lattice pattern 55. The sinusoidal lattice pattern 55 of the piezoelectric substrate 12 reduces the surface area of the inner tube 54 thus making it more flexible and providing a lower mechanical modulus. The rectangular piezoelectric substrate 12 forming the inner tube has a total area of approximately 18 cm.sup.2 but a reduced surface area of approximately 13 cm.sup.2 attributed to the openings formed within the substrate by the sinusoidal lattice pattern 55. The sinusoidal lattice pattern 55 may include 30% printed area and 70% openings. The increased flexibility provided by the sinusoidal lattice pattern 55 allows the piezoelectric substrate 12 to be rolled into a flexible tube and without breaking.

[0080] As seen in the enlarged inset of FIG. 3, the sinusoidal lattices 55 may comprise of a repeated pattern of equally spaced circles 57 connected by bridges 59 formed by two opposed semicircular arches forming an “S” shape, and which interconnect each of the circles 57 so that each circle 57 is attached to four additional circles 57. The attachment of the bridges 59 are equally spaced on each circle 57 at approximately 90-degree intervals at, for example, 45°, 135°, 225°, and 315° positions around the circles 57. The circles 57 have a radius of about 2 mm. The bridges 59 have two arches with a radius of curvature of about 2.1 mm and an angle of 180 degrees. The bridges 59 may have a total length of about 4 mm and a width of about 0.8 mm.

[0081] Referring also to FIGS. 4 and 5, the inner tube 54 may further include an inner and outer electrode 56, 58 forming layers covering the inner and outer surfaces, respectively, of the piezoelectric substrate 12. The inner and outer electrodes 56, 58 may be a biocompatible silver paste, for example, EPO-TEK® H20E, printed by a paste extruder. The inner and outer electrodes 56, 58 are biocompatible.

[0082] The inner tube 54 of the hollow cylinder 52 may have a thickness of at least 0.1 and at least 0.15 mm and between 0.1 mm and 0.2 mm forming the flexible inner tube 54. The inner tube 54 is biocompatible.

[0083] A sheath 60 of the hollow cylinder 52 may surround the inner and outer surfaces of the inner tube 54, respectively. The sheath 60 may be a silicone material such as polydimethylsiloxane (PDMS) having an inner layer 63 surrounding an inner surface of the inner tube 54, an outer layer 65 surrounding an outer surface of the inner tube 54, and ends of the sheath 60 surrounding the ends of the inner tube 54, to therefore completely encapsulate the inner tube 54 such that the inner tube 54 does not contact the exterior of the hollow cylinder 52.

[0084] The inner layer 63 and outer layer 65 of the sheath 60 may have a total thickness of at least 1 mm and at least 1.5 mm and between 1 mm and 2 mm to form a durable artificial artery structure encapsulating the inner tube 54. The sheath 60 is biocompatible.

[0085] The hollow cylinder 52 may have an inner diameter of 0.8-1.2 cm and about 1.2 cm and an outer diameter of 1.2-1.5 cm and 1.2-1.7 cm and about 1.6 cm. The total length of the hollow cylinder 52 measured along the axis 61 may be at least 1 cm, and at least 2 cm, and at least 3 cm, and at least 4 cm, and between 5 mm and 1.5 cm.

[0086] It is understood that in an alternative method, the hollow cylinder 52 may be printed directly, without requiring the rolling of the planar substrate to form the tube, by printing the tube on a rotating mandrel (not shown) whereby the poling axis 37 is radial with respect to the rotating mandrel. The inner and outer electrode 56, 58 may be 3D printed onto the piezoelectric substrate or added separately. The outer tube 60 may also be 3D printed on the inner tube 54 or added separately.

[0087] The hollow cylinder 52 exhibits a tissue-comparable elasticity and flexibility with a significantly lower tensile modulus of at least 5 MPa, and at least 5.5 MPa, and about 5.68 MPa (elongation of about 38%) and flexural modulus of at least 10 MPa, and at least 10.3 MPa, and about 10.35 MPa. The tensile modulus and flexural modulus are within comparable ranges as human arteries. The hollow cylinder 52 is composed of a bio-composite and non-toxic materials.

[0088] The inner and outer electrode 56, 58 are attached to an electrical or voltage meter 62 via electrical connectors 64, 66. The voltage meter 62 may be a wireless transmitter that can transmit the voltage signals to an external receiver, such as a mobile phone, to monitor changes to blood pressure. The voltage meter 62 may be a pressure sensor that is able to measure the changes in voltage, which is linearly related to the changes in pressure represented by arrows 70 on the piezoelectric substrate 12 caused by changes in blood flow through the hollow cylinder. The mechanical changes to the piezoelectric substrate 12 of the inner tube 54 are caused by the changes in blood flow through the hollow cylinder 52 during vasodilation (widening of the blood vessels) and vasoconstriction (narrowing of the blood vessels). For example, an average adult has a diastolic pressure of 80 mmHg and a systolic pressure of 120 mmHg, and the pressure fluctuates within pressure changes around 40 mmHg (5.33 kPa).

[0089] The mechanical changes in the piezoelectric substrate 12 are converted to an electrical charge detected by the voltage meter 62. For example, the changes in blood pressure 70 will result in a positive or negative voltage difference. The polarization of voltage is consistent with the poling direction of the piezoelectric substrate 12 during the 3D printing method described above with respect to FIG. 1.

[0090] The tubular structure 50 has a pressure/strain sensitivity of at least 0.3 mV/kPa, R.sup.2>0.99 and at least 0.306 mV/mmHg, R.sup.2>0.99 (at least 12 mV/kPa and at least 12.2 mV/kPa). Linearity is displayed between electricity and pressure (2.295 mV/kPa, R.sup.2>0.99) which shows the capability of accurate real-time monitoring of blood pressure 70 by the tubular structure 50.

[0091] The sensing capability of the tubular structure 50 is sensitive enough to detect partial occlusion (e.g., thrombosis) of the artery which can be revealed by the voltage signals. For example, between 0 and 80% occlusion of the artery can be detected. By viewing the voltage profile, asymmetric artery movement and pressure patterns may indicate the dissimilar blood flows at opposite sides of a thrombosis. Therefore, implant failure can be detected early.

[0092] The tubular structure 50 may be used in vascular bypass surgeries that redirect blood flow from one area of the body to another by reconnecting the blood vessels. Vascular grafting is most commonly done to bypass a complete or partial blockage in an artery in order to improve blood flow to the organ or extremity supplied by the diseased artery. Vascular grafts are most commonly used in the heart during a coronary artery bypass or in the legs to help treat vascular disease and poor blood flow. During a vascular bypass, the ends of the tubular structure 50 are sewn onto the source and target vessels, respectively, to attach the vessels and bypass the blockage.

[0093] In one embodiment of the present invention, the voltage meter 62 may be a transducer used to transmit an electrical signal through the electrical connectors 64, 66 to actuate or produce a vibration response in the piezoelectric substrate 12 and tubular structure 50.

[0094] Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context.

[0095] When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0096] References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network.

[0097] It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.