Monolithic functional structures and method for the production thereof

Abstract

The present invention belongs to the field of electrically powered functional structures, specifically monolithic functional structures comprising at least one functional structure (such as a resonator, sensor, energy collector, etc.) and at least one piezoelectric material. In addition, the at least one functional structure and the at least one piezoelectric material form part of a single piece and are electrically connected together. The invention relates to the monolithic functional structure and to a method for 3D printing the monolithic functional structure.

Description

[0001] DESCRIPTION

[0002] MONOLITHIC FUNCTIONAL STRUCTURES AND METHOD FOR THEIR PRODUCTION

[0003] Field of the invention

[0004] The present invention pertains to the field of electrically powered functional structures, specifically to monolithic functional structures comprising at least one functional structure (such as a resonator, a sensor, an energy harvester, etc.) and at least one piezoelectric material. The invention relates both to said monolithic functional structure and to a method for 3D printing said monolithic functional structure.

[0005] Background

[0006] There is a need to provide reliable and efficient sources of electrical power for functional structures (such as resonators, sensors, energy harvesters, etc.) that require the use of electrical power and are heavily dependent on such electrical power source.

[0007] For example, the rapid increase in the Internet of Things (IoT) has led to a massive growth in the development of ultra-low-power sensors that communicate with each other and form a large wireless network. The biggest challenge in expanding the IoT is the energy dependence of the sensors, which is limited by their battery usage. For this reason, we face a significant barrier due to the high cost of replacing batteries and the resulting environmental impact. Other monolithic functional structures (resonators, sensors, energy harvesters, etc.) face the same shortcomings due to their dependence on battery power.

[0008] The current solution is to provide batteries as a source of electrical power for these functional structures (such as sensors). However, batteries present a disadvantage due to their high cost, their need for replacement, and the associated environmental impact. Therefore, an alternative is required to supply electrical power to these functional structures (such as sensors) in the field that overcomes these shortcomings. Such an alternative, with its corresponding advantages, is the subject of this patent application and will be evident throughout its description.

[0009] Summary of the invention

[0010] On the one hand, there are materials such as piezoelectric materials, which allow us to harness residual environmental energy (such as motion, vibrations, magnetic fields, light, heat, etc.) and convert it into electrical energy. An example of a device that can convert mechanical energy into electrical energy is piezoelectric transducers. In vibrational energy harvesting, piezoelectric materials are one of the most widely used transducers. An example of such piezoelectric materials are piezoceramics, such as lead zirconate titanate (PZT). PVDF and its copolymers are advantageous because they do not contain toxic materials such as lead.

[0011] On the other hand, there is three-dimensional (3D) printing or additive manufacturing (AM), a process for creating solid, three-dimensional objects from a digital file. This technique enables the production of many custom-designed products in a wide variety of fields, including agriculture, healthcare, automotive, aerospace, and more.

[0012] In summary, AM technologies are based on three types of processes:

[0013] 1) Synthesis, in which the temperature of the material is raised without liquefying it to create high-resolution prototypes.

[0014] 2) Fusion, in which powders are fused using electron beams.

[0015] 3) Stereolithography, which uses a method called photopolymerization, which employs an associated ultraviolet laser.

[0016] The present invention proposes to integrate a monolithic functional structure (resonators, sensors, energy harvesters, etc.; preferably a low-power sensor) and at least one piezoelectric material (which converts ambient energy into electrical energy usable by the monolithic functional structure) to be produced by 3D printing, in order to create monolithic 3D piezoelectric devices. The functional structure performs the intended function of the device (resonator, sensor, energy harvester, etc.), while the piezoelectric material converts residual energy from the device and the environment into electrical energy to be used by the functional structure.

[0017] The term "functional structure" according to the present invention refers to a mechanical structure capable of exerting a mechanical stress on the piezoelectric layer, typically a resonator or a mechanical stress generator, which causes a reaction in the electrical device so that the device performs its corresponding function, such as a sensor, a low power sensor, an energy harvester or an actuator device.

[0018] The term "low-power sensor" is well-known in the art and refers to sensors that operate using little energy or power. When multiple low-power sensors are networked together, they operate as a type of wireless technology. This means they have the benefits of wide-area cellular networks at a lower cost. They do not allow for large data transfers, so they are not used for human communications (voice, video, and other communications that transfer large volumes of data). However, their ability to connect devices over long distances makes them useful for Internet of Things (IoT) technologies and other machine-to-machine communications. Long-range, low-power sensor networks can be deployed over large areas to collect data that can be analyzed and developed into predictive models for strategic decision-making.They can also be integrated into existing products to add smart features that make them more appealing to users and collect usage data to improve their performance over time.

[0019] The term "functional structure material" according to the present invention refers to the type of material that is used to manufacture the functional structure.

[0020] The term "piezoelectric material" according to the present invention refers to any material capable of transforming residual or ambient energy into electrical energy. The term "monolithic" in "monolithic functional structure" according to the present invention refers to the fact that the components of said structure, including at least one functional structure and at least one piezoelectric material, are integrated into a single part or structure, or are part of a single part or structure.

[0021] The term "compact" according to the present invention refers to the fact that the different components are connected with direct physical contact between them, without the need for loose parts or assembled parts to connect them.

[0022] A non-limiting example of such a monolithic piezoelectric device is a functional sensor structure (such as a low-power sensor) in conjunction with a PVDF piezoelectric material. For example, a compact, electrically connected sensor and a PVDF material, printed as a single piece using 3D printing. Another non-limiting example is a piezoelectric energy harvester, such as an energy harvester in conjunction with a PVDF piezoelectric material.

[0023] Examples of piezoelectric materials that can be used according to the invention are lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) or their copolymers, such as PVDF-TrFE (polyvinylidene fluoride and trifluoroethylene copolymer) and PVDF-TeFE (polyvinylidene fluoride and tetrafluoroethylene copolymer), and combinations thereof.

[0024] Therefore, this solution allows, through 3D printing in a single printing step, to obtain a compact part comprising both components (at least a functional structure and a piezoelectric material) integrated, which in addition to comprising and performing the function of the functional structure, also allows the collection of ambient energy and its transformation into electrical energy by the piezoelectric material.

[0025] The 3D printing method for obtaining monolithic functional structures according to the invention has the advantages of being an easier, cheaper and highly customizable method (compared to other methods of manufacturing functional structures, such as a sensor) and allows the functional structure and the piezoelectric material to be produced simultaneously in the same printing process and as a single piece, without the need to assemble different parts.

[0026] In fact, 3D printing of monolithic functional structures according to the invention is simpler as it requires no additional assembly steps or tools, and no molds. Furthermore, it allows for rapid and customizable design changes and the easy manufacturing of complex, monolithic functional structures. Furthermore, the 3D printing method according to the invention also allows for the integration of computer-implemented design software (such as CAD), enabling complete customization of the monolithic functional structures, for example, allowing them to be fully adapted to the environmental conditions in which the structure is intended to be used.

[0027] The resulting monolithic functional structure is compact, more economical, and has a very low environmental impact, since its source of electrical energy (the piezoelectric material) does not need to be replaced or disposed of, and the structure is capable of reusing residual energy from the structure or its surroundings. Furthermore, the compactness of the monolithic functional structure is advantageous in making the piezoelectric material contained within it more efficient in converting the part's residual energy (mechanical, light, thermal) into electrical energy. This is because its compactness and integration prevent losses due to electrical transmission, as the device itself has its own support and does not need to be externally connected to a support. In other words, the improved bonding between the functional structure and the piezoelectric material helps convert and transmit more of the residual energy into electrical energy that will be used by the functional structure.

[0028] Figure 1 schematically illustrates the advantage of integrating piezoelectric materials by 3D printing to create monolithic 3D piezoelectric devices, according to the present invention. In this way, it is possible to adapt the functional structure (such as a resonator, an energy harvester or a sensor, etc.) to different needs, and with its own self-integrated electrical power source, in a fully customizable manner and without the need for subsequent assembly, which entails a simpler and cheaper production method, as well as a more economical final device. The invention therefore relates to the manufacture of monolithic functional structures (such as resonators, sensors, energy harvesters) by using 3D printing and by printing the layers of the functional material and the piezoelectric material in the same process.Preferably, the three-dimensional (3D) printing technique used to produce the monolithic functional structures according to the present invention is Fused Deposition Modeling (FDM), which is a material extrusion process used to manufacture thermoplastic parts by extrusion and deposition of materials layer by layer.

[0029] A first aspect of the invention relates to a monolithic functional structure, characterized by comprising:

[0030] • at least one functional structure made of a functional structure material; and

[0031] • at least one piezoelectric material;

[0032] • in which the functional structure and the piezoelectric material are part of a single piece;

[0033] • in which the functional structure and the piezoelectric material are electrically connected; and

[0034] • in which the piezoelectric material is not located along the neutral axis of the monolithic functional structure.

[0035] The "neutral axis" in the present invention refers to the axis along the monolithic functional structure along which there are no longitudinal stresses or deformations.

[0036] The functional structure is a mechanical structure capable of exerting mechanical stress on the piezoelectric layer, typically a resonator or a mechanical stress generator.

[0037] The monolithic functional structure preferably includes a substrate (e.g., a glass substrate) on which the functional structure and the piezoelectric material are located. 3D printing of at least one functional structure and of at least one piezoelectric material is performed on the substrate. At least one piezoelectric material can collect residual ambient energy (e.g., motion, vibrations, magnetic fields, light, heat, etc.) and transform it into electrical energy, which can be used by the functional structure of the monolithic functional structure to power it so that it can carry out its function. This allows the functional structure performing the function of interest of the device (e.g., a sensor) to be supplied with power without the need for batteries or external power supplies.

[0038] In a preferred embodiment, the functional framework material of the monolithic functional structure is selected from PLA (polylactic acid), ABS (acrylonitrile butadiene styrene), PVA (polyvinyl alcohol), PP (polypropylene), Tough PLA (tough polylactic acid), CPE (copolyester), PET-G (polyethylene terephthalate modified with glycol), nylon (polyamide), PC (polycarbonate), and any combination thereof. Preferably, the functional framework material is selected from PLA or Tough PLA or combinations thereof, preferably Tough PLA.

[0039] The term "tough PLA" as used here refers to a special type of PLA, called tough PLA, which combines ease of printing with improved mechanical performance. Specifically, tough PLA has greater impact resistance, avoiding the typical brittle failure of regular PLA. Tough PLA is used for functional prototyping, tooling, and manufacturing aids.

[0040] In another preferred embodiment, the functional structural material is a printed material. In another even more preferred embodiment, the functional structural material comprises at least one layer of printed structural material. In another even more preferred embodiment, the functional structural material has a region of said layer of functional structural material that is left empty, such as a depression. In another more preferred embodiment, the functional structural material comprises more than one layer of printed structural material, where said empty region is located in the upper and last printed structural layer.In another preferred embodiment, the piezoelectric material of the monolithic functional structure is zirconium titanate (PZT), polyvinylidene fluoride (PVDF) or their copolymers, such as PVDF-TrFE (copolymer of polyvinylidene fluoride and trifluoroethylene), PVDF-TeFE (copolymer of polyvinylidene fluoride and tetrafluoroethylene), or combinations thereof.

[0041] In another preferred embodiment, the substrate material is selected from glass, metals such as gold, silver, copper, aluminum, iron, nickel, titanium, platinum, cobalt, or lithium. Preferably, the substrate is glass. Furthermore, once the monolithic functional structure is manufactured, the substrate can be retained as part of the monolithic functional structure or removed from it.

[0042] In another preferred embodiment, the material of the functional structure of the monolithic functional structure is a low-power sensor.

[0043] In another preferred embodiment, the monolithic functional structure is a monolithic energy harvesting structure with PVDF as a piezoelectric material.

[0044] In another preferred embodiment, at least one functional structure and at least one piezoelectric material of the monolithic functional structure are at least partially in physical contact with each other. The advantage of this embodiment is that, by being in direct contact, the conversion of residual energy, and therefore its transformation into electrical energy, is more efficient.

[0045] In another preferred embodiment, the monolithic functional structure further comprises a conductive material, preferably selected from an electrically conductive wire, a conductive track, and a metal layer, such as gold, silver, copper, iron, nickel, titanium, platinum, or cobalt; more preferably, the conductive material is a metal layer, and even more preferably, a copper layer. In another more preferred embodiment, the conductive material is in contact with the functional structure.

[0046] In another preferred embodiment, the monolithic functional structure is characterized in that it comprises from bottom to top: • at least a first layer of functional structure material comprising at least one layer of printed structural material, in which a region of said layer of functional structure material is left empty as a depression, where a layer of metal, preferably copper, is applied in said region;

[0047] • at least one second layer of functional structural material comprising at least one layer of printed structural material on the first layer of functional structural material, wherein at least the top of the metal layer is left uncovered;

[0048] • a layer of piezoelectric material on the first layer of functional structure material, at least partially in contact with the metal layer;

[0049] • at least a third layer of functional structure material comprising at least one layer of printed structural material on the layer of piezoelectric material and on the second layer of functional structure material, wherein at least one region on top of the metal layer and one region on top of the piezoelectric material layer are left uncovered;

[0050] • an electrical connector selected from a metal wire or a metal spring-loaded connector, located in both exposed regions on top of the metal layer and on top of the piezoelectric material layer.

[0051] The electrical connector can be any type of connector that allows a connection between the metal layer and the piezoelectric material layer.

[0052] The term "structural layer or first, second, or third structural layer" in the present invention refers to a layer comprising at least one layer of structural layer material, a layer obtained by any method, whether by printing, by the additive method, or by any deposition technique known in the state of the art. It obviously contains as many layers of material as necessary in any layer of functional structure material or in the indicated first, second, or third structural layers; and where in the first and second layers, the last layer of material making up each structural layer has an area without material that leaves a space to house the metallic layer or the corresponding piezoelectric. The combination of the different layers of structural material together constitutes the functional structure of the device.

[0053] The monolithic functional structure may comprise a substrate at the bottom, in the event that the substrate on which the 3D printing of the monolithic functional structure was carried out is not finally removed from said structure.

[0054] A second aspect of the invention relates to a method for 3D printing monolithic functional structures, characterized in that it comprises the steps of: a. providing a substrate, preferably a glass substrate; b. printing at least one layer of a first functional structure material comprising at least one layer of printed structural material on the substrate of step (a), preferably PLA, without applying functional structure material at least in a region of the last printed layer; c. applying metal in said region of step (b), preferably copper, d. optionally, printing at least one layer of a second functional structure material comprising at least one layer of printed structural material, preferably PLA, without applying functional structure material at least on top of the region where the metal was applied in step (c), e.applying a piezoelectric material, preferably polyvinylidene fluoride, on top of the layer where the metal was applied in step (c), preferably at least partially in direct contact with the metal, f. printing at least one layer of a third functional structure material comprising at least one layer of printed structural material, preferably PLA, without applying functional structure material on top of at least one region of the metal of step (c) and on top of at least one region of the piezoelectric material of step (e), and g. placing at least one electrical connector in both regions left uncovered on top of the metal and on top of the piezoelectric material of step (f).

[0055] In a preferred embodiment according to the second aspect of the invention, the 3D printing is carried out using Fused Deposition Modeling (FDM). In another preferred embodiment according to the second aspect of the invention, the method uses computer-aided design (CAD) software to design the monolithic functional structure to be 3D printed.

[0056] In another preferred embodiment according to the second aspect of the invention, a model created by computer-aided design (CAD) software is divided into layers by means of slicing software.

[0057] In another preferred embodiment according to the second aspect of the invention, the metal applied in step c) of the method is selected from gold, silver, copper, aluminum, iron, nickel, titanium, platinum, cobalt, lithium, or their alloys; preferably copper.

[0058] In another preferred embodiment according to the second aspect of the invention, at least one electrical connector placed in both regions, in step g) of the method, is selected from a metal wire or a metal spring-loaded connector.

[0059] In another preferred embodiment according to the second aspect of the invention, the metal of the metal wire or the metal spring-loaded connector is selected from gold, silver, copper, aluminum, iron, nickel, titanium, platinum, cobalt, lithium, or their alloys; preferably copper.

[0060] In another preferred embodiment according to the second aspect of the invention, the functional structure material is selected from PLA (polylactic acid), ABS (acrylonitrile butadiene styrene), PVA (polyvinyl alcohol), PP (polypropylene), PLA Tough, CPE (co-polyester), PET-G (glycol-modified polyethylene terephthalate), Nylon (polyamide) and PC (polycarbonate) and any combination thereof; preferably PLA, and even more preferably PLA Tough.

[0061] In another preferred embodiment, the substrate is selected from glass, metals such as gold, silver, copper, aluminum, iron, nickel, titanium, platinum, cobalt, or lithium. Preferably, the substrate is glass.

[0062] In another preferred embodiment, the process further comprises an additional step of removing the substrate from the monolithic material obtained in step (g). A third aspect of the invention relates to the use of the monolithic functional structure described above as a sensor, a low-power sensor, a self-powered sensor, an energy harvester, or an actuator.

[0063] In another preferred embodiment, the use of the monolithic structure is as a low-power sensor for creating a large wireless network in which the low-power sensors communicate with each other.

[0064] In 3D printing, as can be seen in Figure 2, the process begins with the design of the desired part using 3D design software, typically CAD software (e.g., Tinkercad, https: / / www.tinkercad.com). This CAD software design allows for the optimization of both the device's function and the collection and conversion of residual ambient energy into electrical energy by the piezoelectric material. Once the design is complete, the next step is to export the file in STL (Stereolithography) format. The STL file contains all the necessary information about the 3D object. The CAD model is then sliced ​​into layers using slicing software, such as Ultimaker Cura 4.13.1. Finally, the object is printed by transferring the file, positioning, and printing the design layer by layer. This process is summarized schematically in Figure 2.

[0065] The 3D printing manufacturing process according to the present invention includes two additional steps, in order to introduce the lower electrode and the piezoelectric material (such as PVDF material).

[0066] The piezoelectric material preferably comprises two metal electrodes, referred to in this description as the "lower electrode" and the "upper electrode." The lower electrode and the upper electrode may be made of any electrically conductive metal, for example, gold, silver, copper, aluminum, iron, nickel, titanium, platinum, cobalt, lithium, etc.; preferably copper.

[0067] The first step (as shown in Figure 3a) consists of providing a substrate (e.g., a glass substrate) on which to print the monolithic functional structure. First, a few layers of a "functional structure material" (such as PLA) are printed on top of the substrate, layers that will serve as the bottom of the final structure that will encapsulate the piezoelectric material within the structure (Figure 3a). Therefore, the material of this encapsulation layer will be referred to as "functional structure material". It was observed that the thinner the printing layer of the functional structure material, the higher the power generated by the printed monolithic functional structure. Preferably, the layer thickness to be printed with the nozzle for each printing layer is between 60-150 µm, even more preferably 100 µm, which allows to guarantee good quality and avoid exceeding the limits of the printer's print head.

[0068] The lower part of the encapsulation layer preferably comprises two or three printed layers. This low number of layers (2-3 layers) will help maintain the high quality of the encapsulation layer and also increase the stress received by the piezoelectric material, which increases the electricity generated by the piezoelectric material. The higher the stress, the more charge is generated in the piezoelectric material. In addition, a small region (such as a hole) on the surface of the material is left uncovered, at least unprinted in the last printing layer. In this small region (such as a hole), the metal that will serve as the electrode (e.g., a lower copper electrode) will be placed.

[0069] Subsequently, metal or a piece of metal (e.g., a piece of copper) is applied to establish the connection with the lower electrode, in that small region (as shown in Figure 3b).

[0070] At this stage, optionally and additionally, one or more layers of the encapsulation material (such as PLA) can be applied on top of the previously applied layers of the encapsulation material, provided that the small region (such as a hole) where the metal for the electrode was applied is left uncovered (Figure 3c).

[0071] Then, a layer of piezoelectric material is printed using the nozzle and deposited onto the metal electrode material such that the piezoelectric material is at least partially in direct contact with the metal electrode material (as shown in Figure 3d). The remainder of the encapsulation material is then printed layer by layer such that the remainder of the monolithic functional structure is printed (as shown in Figure 3e). In the final printing step, at least a portion of the area of ​​both the electrode material (e.g., copper) and the piezoelectric material (e.g., PVDF) is left uncovered (as shown in Figure 3e). This allows electrical connectors (metallic connectors such as metal wires or metal springs, preferably gold connectors) to be inserted upon completion of the printing to facilitate the connection between at least one copper electrode and the piezoelectric material (as shown in Figure 3f).

[0072] The material to be printed layer by layer for the encapsulation layer according to the invention can be any polymeric material printable by 3D printing. Non-limiting examples of such 3D printing materials are PLA (polylactic acid), ABS (acrylonitrile butadiene styrene), PVA (polyvinyl alcohol), PP (polypropylene), PLA Tough, CPE (co-polyester), PET-G (glycol-modified polyethylene terephthalate), Nylon (polyamide) and PC (polycarbonate); preferably PLA and even more preferably PLA Tough. PLA has the advantage of being a biodegradable and recyclable polymer that is a reliable and easy-to-print material that can be printed at low temperatures. It has a low shrinkage factor and does not require the use of a heated build platform.

[0073] In summary, the present invention involves 3D printing 2 parts (a functional structure and a piezoelectric material that can be printed or integrated within the monolithic functional structure) as a single, compact structure, such that the resulting monolithic functional structure can perform the function of the functional structure (resonator, sensor, energy harvester), while taking advantage of residual ambient energy to convert it into electrical energy that is transmitted to the functional structure for use, providing the aforementioned advantages, which can be summarized as follows:

[0074] - No batteries or other non-renewable and wasteful energy sources are required.

[0075] - Use of residual energy from both the monolithic functional structure and its surroundings.

[0076] - Greater efficiency in converting waste energy into electrical energy (due to the improved mechanical and electrical connection between the piezoelectric material and the functional structure). - Greater compactness and integration of the structure (compared to the functional structure and piezoelectric material as two individual components).

[0077] - Manufacturing of the entire monolithic functional structure using a single technique (3D printing) and in a single printing process.

[0078] - Both structures (functional structure and piezoelectric material) are printed as a single structure.

[0079] - No assembly is required between the two structures (since both structures are printed as a single piece).

[0080] This invention represents a contribution to current and future trends in self-powered functional structures, such as sensors. By harnessing ambient energy, self-powered sensors have the potential to reduce the environmental impact associated with batteries.

[0081] Last but not least, the scope of the present invention is not limited to the embodiments explicitly disclosed herein, and the person skilled in the art understands that other alternative or equivalent embodiments, including modifications that are equivalent or obvious, are within the scope of protection of the present invention.

[0082] Description of the figures

[0083] Figure 1 schematically represents the integration of a functional structure and a piezoelectric material as a single monolithic functional structure produced by 3D printing.

[0084] Figure 2. schematically represents the basic 3D printing process performed by 3D printers and their software, in order to create 3D objects.

[0085] Figure 3 schematically represents the different steps af of the 3D printing process by Fused Deposition Modeling according to the present invention. The sequences of steps to create a particular embodiment of a monolithic functional structure according to the invention are: (a) printing a base layer of PLA for the encapsulation of the piezoelectric material; (b) placement of copper material to make the connection of the lower electrode; (c) printing a layer of PLA for the subsequent assembly of the piezoelectric material; (d) placement of the PVDF piezoelectric material; (e) printing the rest of the PLA structure leaving two holes for subsequent connections and (f) insertion of gold connectors for the connection of the two electrodes of the piezoelectric material.

[0086] Figure 4 shows images of a monolithic energy harvesting structure with PVDF as a piezoelectric material, manufactured by 3D printing using Fused Deposition Modeling, according to the invention.Figure 4 shows the result of the different steps of the 3D printing process according to the invention to create the energy harvesting monolithic structure: (a) printing of a PLA base layer for the encapsulation of the piezoelectric material (also shown schematically in Figure 3a), and laying of copper material to make the bottom electrode connection (also shown schematically in Figure 3b); (b) laying of the PVDF piezoelectric material (also shown schematically in Figure 3d); (c) a PLA layer is then printed after the laying of the PVDF material, leading to its partial encapsulation (also shown schematically in Figure 3e); and (d) printing of the rest of the PLA structure leaving two small uncovered regions (holes) for subsequent electrical connections (shown schematically in Figure 3e).

[0087] Figure 5 shows a schematic representation of (a) a cross-section of the energy harvester with the specified dimensions; (b) a view of the 3D design used for the energy harvester; and (c) the stress distribution when the energy harvester resonates at its resonant frequency, and a detail of the cross-section showing the neutral axis in COMSOL Multiphysics® software.

[0088] Figure 6. graphically represents the power distribution (in pW) obtained from the resonant piezoelectric energy harvester as a function of its dimensions (Figure 6a: Lb / Lc dimensions; Figure 6b: Tb dimensions) and as a function of the resistance (in MQ) of an external circuit including a resistor, allowing to determine the optimal load resistance to achieve the maximum power of the resonant piezoelectric energy harvester. Figure 7a. graphically shows the simulation of the power density, power, cost and volume of the resonant piezoelectric energy harvester as a function of the beam thickness (Tb).

[0089] Figure 7b graphically shows the simulation of the power generated by the resonant piezoelectric energy harvester as a function of the depth of the piezoelectric material (Pd) with respect to the surface of the energy harvesting device.

[0090] Figure 8. shows the first electrical characterization, namely a) the setup for the first electrical characterization using a stepper motor performing movements of the resonant piezoelectric energy harvester, movements in position from 1 mm to 6 mm on the Z axis; b) graphical representation of the maximum voltage reached in the setup of Figure 8a. with a stepper motor displacement of 1 mm to 6 mm on the Z axis, relative to the displacement in the Z axis, measured with a “Sourcemeter” multimeter for 6 different 3D printed energy harvesters, three of these harvesters with their piezoelectric material at 200 pm from the surface of the energy harvesting device, while the other three remaining harvesters have their piezoelectric material at 300 pm.For each of the energy harvesting devices, the generated voltage is shown as a function of displacement from 1 mm to 6 mm on the Z axis; c) Same graphical representation of the same 6 harvesters and displacement of 1 -6 mm, as in Figure 8b, but where the Y axis instead represents the force (N) measured with the displacement by a force sensor attached to the stepper motor and recorded by a dynamometer.

[0091] Figure 9 shows the second electrical characterization, namely (a) setup for the second electrical characterization using an electrodynamic shaker at 50 Hz to simulate ambient vibrations, at different input acceleration levels; (b) Optimal load resistance for maximum power generation at 0.1 G acceleration; (c, d) Maximum voltage and power for an acceleration sweep from 0.1 G to 1 G and using a frequency of 50 Hz and an optimal load resistance of 4 MQ. Figure 10 shows the physical characterization of the PVDF piezoelectric material, namely the thickness and the piezoelectric coefficient, and more precisely an SEM image of the piezoelectric material layers thicknesses.

[0092] Figure 11. graphically represents a simulation of the output voltage (V) variation with frequency (Hz) of the resonant piezoelectric energy harvester comprising the PVDF piezoelectric material, using COMSOL Multiphysics® software. More precisely, Figure 11a represents a comparison of the simulated electrical characterization of the harvesters (curves plotted from 48 to 54 Hz) with the real electrical characterization of the harvesters (curves plotted from 45 to 55 Hz), for the maximum open circuit voltage (Y axis) that can be generated by the piezoelectric generator, for the frequency sweep represented in the figure with an acceleration of 0.5 G. Figure 11 b) represents a comparison of the maximum generated power in relation to the depth of the piezoelectric material (200 pm and 300 pm) and in relation to the load impedance (MQ).

[0093] Figure 12 shows the simulation results for the piezoelectric current sensor using COMSOL Multiphysics® software: (a) Structure and dimensions used in the simulation; (b) 3D finite element (FEM) simulation of the stress distribution in the piezoelectric current sensor beam.

[0094] Figure 13 shows an image (a) and a schematic representation (b) of a piezoelectric current sensor manufactured using 3D printing technology mounted inside a sensor node (current sensor provided by the startup Energiot); (c) graphical representation of the open-circuit voltage generated by the piezoelectric sensor, as a function of the selected power of two stoves, which can obtain up to four different currents (1500 W, 3000 W, 4500 W and 6000 W). EXAMPLES

[0095] A resonant energy harvester has been designed, manufactured and electrically characterized. . In terms of electrical characterization, it was determined that the device can achieve a maximum output power of 1.46 mW when operating with an optimal load impedance of 4 MQ and subjected to an acceleration of 1 G.

[0096] The piezoelectric material and the final resonator were also physically characterized. Furthermore, the device was studied and optimized using finite element modeling (COMSOL Multiphysics®).

[0097] Finally, a proof-of-concept device (case resonator) was designed and manufactured. in order to measure the current passing through a cable.

[0098] Materials and methods

[0099] 1.1. Piezoelectric and magnetic materials

[0100] The piezoelectric material used in the collector is a commercially available PVDF film with two parallel electrodes from TE Connectivity. According to the manufacturer's specifications, the PVDF material has a thickness of 110 µm and 6 µm for each silver ink electrode. The piezoelectric coefficient of the commercial PVDF is -33x10 -12 m / V, according to the data sheet. The material selected for the structural collector is Ultimaker's "Ultimaker Tough PLA green," with a Young's modulus of 2.8 ± 0.15 GPa and a density of 1.22 g / cm 3 A neodymium (Nd) mass with a density of 7 g / cm was used. 3 , from the Nd mass supplier "Supermagnete", as a magnetic material. This Nd magnetic material can be seen, for example, in Figure 5a (Nd magnetic mass).

[0101] 1.2. Finite element modeling

[0102] Finite element modeling (FEM) of the energy harvester was performed using COM-SOL Multiphysics® 6.0 software. The software's AC / DC (Electrostatics, Electric Circuits, and Piezoelectricity) and Structural Mechanics (Solid Mechanics and Piezoelectricity) modules were used for the simulation. The model was created using a fine triangular mesh, consisting of over 25,000 elements.

[0103] 1.3. Manufacturing process and characterizations

[0104] An "Ultimaker S3" printer with a 0.25 mm AA nozzle was used for the fabrication process, allowing printing layers with thicknesses ranging from 60 µm to 150 µm. Initial characterization involved a Zaber Technologies LSQ075B-T3-MC03 stepper motor (Vancouver, BC, Canada), capable of performing movements of 1 mm to 6 mm in the Z axis. At the same time, the voltage generated by the harvester was measured using a Keithley 2470 Sourcemeter (Cleveland, OH, USA). A Mark-10 (Copiage, NY, USA) MR03-20 force sensor fixed to the stepper motor recorded the data with the help of a Mark-10 M5I dynamometer. An ad-hoc LabVIEW® program controlled the setup that included the aforementioned devices.

[0105] 1.4. Device for electrical characterizations

[0106] Electrical characterization was performed using a Vibration Testing Controller (Jenison, MI, USA) VR9500 electrodynamic vibrator to emulate ambient vibrations at different input acceleration magnitudes. This was controlled using a custom MATLAB® program that allowed for automation, acquisition, and processing of voltage measurements. All data were measured using a National Instruments (Austin, TX, USA) PCI-6132 acquisition system and a BNC-2110 terminal block.

[0107] The piezoelectric coefficient of the material was measured using a wide range d33 piezometer from APC International, Ltd (Mackeyville, PA, USA).

[0108] 1.5. Collector design

[0109] 1.5.1. Definition of fixed parameters

[0110] There are various sources for energy harvesting, such as vibrations, human motion, residual magnetic fields, and so on. In this case, the magnetic fields generated by a live conductor were chosen. For this reason, the resonant frequency was set at 50 Hz (European grid frequency). To study the optimization of the structure, the effective area and resonant frequency of the device must be determined. In this case, the dimensions are 50 mm x 40 mm (length x width).

[0111] 1.5.2. Device structure

[0112] There are different types of structures for vibratory energy harvesters, such as cantilever beams, double-supported beams, or diaphragms. The cantilever beam produces the highest energy output when used as a piezoelectric energy harvester. Furthermore, the cantilever structure leads to the lowest resonant frequency for a given size. For this reason, the cantilever structure was chosen.

[0113] The resonant frequency of a spring-mass system can be expressed as:

[0114] Here, k ef fy and rrieff represent the effective values ​​of suspension stiffness and effective mass, respectively.

[0115] The neutral axis of a beam is the layer in the beam's cross section that does not experience longitudinal deformation under bending. For composite beam structures, the effect of the different materials in each layer must be summed to find the neutral axis:

[0116] Where Z is the position of the neutral axis of the composite beam, Z¡ is the height of the centroid of layer i, h¡ is the thickness of layer i, and Y¡ is the Young's modulus of layer i.

[0117] 1.6. Shape optimizations and finite element modeling

[0118] 1.6.1. Obtaining the resonance frequency

[0119] In addition to reducing manufacturing costs, the monolithic device avoids resonance pinning losses due to the improved resonator pinning structure. To achieve the desired resonant frequency, a very thick mass was required due to the low density of PLA. Furthermore, to achieve actuation with magnetic fields using the Lorentz force, a ferromagnetic material is required as the cantilever's inertial mass. For this reason, neodymium (Nd) magnets were used as the inertial mass.

[0120] As shown in Figure 5a, the collector consists of a PLA cantilever with a length composed of the beam length (L b ), the mass (L m ) and the length of the overhang (L c ). In addition, the thickness of the beam was defined as (T b ), the thickness of the dough as (T m ) and the depth of the piezoelectric material with respect to the surface as (Pd).

[0121] The 3D model shown in Figure 5b consists of the energy harvester with the piezoelectric layer inside the structure and an external circuit with a load resistor to calculate the power.

[0122] A piezoelectric material produces an electric charge from mechanical stress. The coefficient d33 represents the piezoelectric charge coefficient along a specific direction, reflecting the material's response to applied stress. It can be expressed as:

[0123] From Equation (2), it can be deduced that the highest stress resulting from a beam deformation in a cantilever is located at the surface. In Figure 5c, the stress distribution is shown when the device resonates at its resonant frequency. It can be observed that the stress at the surface of the material is at its maximum, while it is zero at the center of the beam. Therefore, during the manufacturing process, it is crucial to place the encapsulated piezoelectric material as close to the surface as possible. However, there is a trade-off; if the piezoelectric layer is too close to the surface, it will not be properly embedded and may delaminate.

[0124] A parametric simulation of L was performed b and L mto find the different combinations that obtain a resonance frequency of 50 Hz. The dimensions of the magnets are more restrictive because they are defined by the manufacturer. A parametric study of L was performed m from 10 mm to 40 mm. Table 1 shows the different combinations of dimensions obtained to achieve a resonant frequency of 50 Hz.

[0125] Lb (mm) Lm (mm) Lc (mm) Lb / L Ratio (%) Frequency (Hz)

[0126] 41.20 10.00 51.20 80 50.04

[0127] 33.10 15.00 48.10 69 49.95

[0128] 26.80 20.00 46.80 57 50.02

[0129] 21.60 25.00 46.60 46 50.00

[0130] 17.15 30.00 47.15 36 50.01

[0131] 13.35 35.00 45.35 29 50.05

[0132] 10.20 40.00 50.20 20 49.99

[0133] Table 1. Comparison with different combinations of parameters to obtain the fixed resonance frequency at 50 Hz.

[0134] 1 .6.2. Study of the generated power

[0135] The voltage and power generated by the piezoelectric resonant energy harvester, depending on its dimensions, were studied using an external circuit including a resistor. By performing a sweep of the external load resistance, the optimal value of the load resistance can be determined to achieve maximum power generation for the piezoelectric resonant energy harvester. As shown in Figure 6a, the smaller the ratio of Lb to L c , the greater the power generated by the piezoelectric resonant energy harvester.

[0136] A parametric thickness of the beam under the piezoelectric sheet was simulated to improve the power generated by the piezoelectric generator. Due to the increase in Lb, the natural frequency of the device increases. To compensate for this increase, the density or T must be increased. m. In this case, the mass density was increased to compensate for the frequency. This way, the mass dimension always remains the same and simplifies the meshing process during the simulation. As can be seen in Figure 6b, the higher the T b , the greater the power generated by the collector. In the following Table 2, different values ​​of T can be seen m to compensate for the increase in Tb- In addition, the data on the maximum generated power, volume and power density of each collector obtained from the simulation are shown.

[0137] Tb (mm) Tm (mm) Power (pW) Volume (cm 3 ) Power density (pW / cm 3 )

[0138] 1.00 1.00 95 4.10 23.08

[0139] 1.50 3.48 324 10.20 31.77

[0140] 2.00 8.13 620 20.80 29.85

[0141] 2.50 15.46 938 36.80 25.50

[0142] 3.00 26.12 1261 59.60 21.15

[0143] 3.50 40.40 1534 89.90 17.06

[0144] Table 2. Comparison with the different mass thicknesses to compensate for the increase in beam thickness and data on the maximum power generated, volume and power density of each collector.

[0145] Using the data obtained from the simulation shown in Table 2, the volume and cost of each collector are calculated. As seen in Figure 7a, the cost and volume of the collector are closely related because most of the volume is mass. Furthermore, it can be observed how the highest power density is obtained with a T b of 1.5 mm.

[0146] Finally, a parametric simulation of the depth of the piezoelectric material from the collector surface was performed. As can be seen in Figure 7b and as mentioned earlier in the design section, the closer the material is to the surface, the more power it will generate, as it will be further from the beam's neutral line.

[0147] With the results obtained from the simulations, it can be concluded that the best specification for the collector is the design with a dimension of 10.2 mm x 40 mm (L c x L m ). In addition, to achieve higher power density, the beam should be 1.5 mm thick and the piezoelectric material should be as close to the surface as possible.

[0148] 1 .7. Characterization

[0149] 1.7.1. Electrical Characterization Once the piezoelectric resonant energy harvesters, which include an energy harvester as a functional structure and PVDF as a piezoelectric material, have been fabricated according to the 3D printing method of monolithic functional structures according to the present invention, we proceed to the electrical characterization of such 3D printed devices. Three harvesters were fabricated with the piezoelectric material at 200 pm from the surface and another three harvesters were fabricated with the piezoelectric material at 300 pm.

[0150] Two different electrical characterizations were performed.

[0151] The first characterization, shown in Figure 8a, involves a stepper motor moving from 1 mm to 6 mm in distance along the Z axis. Simultaneously, the voltage generated by the harvester is measured using a multimeter (“Sourcemeter”). A force sensor, connected to the stepper motor, records the force data through a dynamometer. An ad hoc LabVIEW program controls the setup comprising these devices. As can be seen in Figure 8b, a 6 mm displacement along the Z axis produces an average voltage of 71.7 V for the harvester with the piezoelectric material positioned 300 pm from the surface. Reducing the distance of the piezoelectric material from 300 pm to 200 pm increases the average voltage to 85.2 V, representing a 19% increase in the maximum generated voltage.According to the force data represented in Figure 8c, the depth of the piezoelectric material does not affect the force required to achieve 6 mm of travel at the cantilever tip.

[0152] For the second electrical characterization, as seen in Figure 9a, a magnetic mass is attached to tune the resonant frequency to 50 Hz. An electrodynamic shaker is used to emulate ambient vibrations at different input acceleration levels. This is controlled through a dedicated MATLAB program that allows the voltage measurements to be automated, acquired, and processed. All of this data is captured by an acquisition system. Subsequently, by setting a certain acceleration magnitude, a load resistance sweep is performed. This allows us to determine the optimal load resistance value for maximum power generation. Due to the voltage limitations of the acquisition system, which cannot measure voltages higher than 10 V, the load resistance sweep is initially performed with a small acceleration of 0.1 G, ensuring that it remains below 10 V.Figure 9b shows that the optimal load resistance for maximum power generation is 4 MΩ), resulting in a maximum power of 4.4 pW.

[0153] Once the optimum load resistance is known, a voltage divider consisting of a set of 500 K and 3.5 M resistors is used. It is possible to measure the voltages generated by the collectors with accelerations greater than 0.1 G.

[0154] The voltage generated by the piezoelectric generator was measured while performing an acceleration sweep ranging from 0.1 G to 1 G as shown in Figure 9c. For an acceleration of 1 G, the collector with the piezoelectric material positioned 300 pm from the surface achieved an average voltage of 62.3 V. By reducing the distance from the piezoelectric material to 200 pm, an average voltage of 75.6 V is achieved, which represents a 21.4% increase in the maximum generated voltage. Using the aforementioned voltage data and the optimal load resistance of 4 Mí), the maximum power generated by the collector can be calculated for each acceleration.

[0155] 1 .7.2 Physical Characterization of PVDF Material

[0156] An inspection using a scanning electron microscope (SEM) was performed to validate the thickness of the materials provided by the manufacturer and, as can be seen in Figure 10a, the measured thickness closely matches the manufacturer's specifications.

[0157] Using a piezometer, we can measure the exact coefficient of our PVDF part. As shown in Figure 10b, the coefficient is the same as the manufacturer's specifications.

[0158] 1.7.3. Measurement of Young's Modulus of the Functional Structure Material (PLA)

[0159] The Young's modulus of the PLA used to 3D print the piezoelectric resonant energy harvesters in the examples is 2.8 ± 0.15 GPa, as indicated in the data sheet. By introducing a material into the PLA, the overall effective Young's modulus can change, depending on the resulting material combination. Therefore, the Young's modulus of the harvesters will be calculated based on force data measured with a stepper motor moving, as shown in the previous section.

[0160] The displacement of a cantilever beam 8 is related to the applied load

[0161] (P) and Young's modulus (E), according to the following Equation (4):

[0162] 1 PL 3

[0163] 8 = - , (4)

[0164] 3 Where L is the cantilever length and I the second moment of inertia. The second moment of inertia (I) quantifies the cross-section's resistance to bending, relative to that transverse axis.

[0165] For a prismatic beam with a rectangular section (depth h and width w), the value of the second moment of inertia of the cantilever beam I is given by: 0.04 m x 0.0015 m 3 . - - 1 9 - - = ll,25xl0 -12 m 4 , (5) 12 v 7

[0166] From Equation (4) and using the force data measured with the displacement of the stepper motor, the Young's modulus of the collectors is calculated:

[0167] 1 PL 3 4.5 N x 0.05 m 3 E = 3 T 3 x 0.006 mx ll,25%10- 12 m4 2.78 GPa'

[0168] From the Young's modulus obtained from Equation (6) above, it can be seen that the modulus is within the manufacturer's specifications and that the piezoelectric material has little effect on the final Young's modulus of the collectors.

[0169] 1.8 Finite Element Simulation

[0170] The energy harvesters were simulated again using COMSOL Multiphysics® to compare the electrical characterization results with a theoretical model. Physical parameters such as Young's modulus, thickness, piezoelectric material coefficient, and harvester dimensions were included in the simulation to achieve a realistic representation. A parametric sweep ranging from 48 Hz to 54 Hz was performed to determine the resonant frequency of the resonators with an optimal load resistance of 4 MQ. Figure 11a shows a comparison of the simulated electrical characterization of the harvesters (curves plotted from 48 to 54 Hz) with the actual electrical characterization of the harvesters (curves plotted from 45 to 55 Hz), for the maximum open-circuit voltage (Y-axis) that the piezoelectric generator can generate, for the frequency sweep represented in the figure with an acceleration of 0.5 G.The resonant frequency of the harvesters simulated using COMSOL Multiphysics® software was approximately 50.5 Hz, while electrical characterization of the fabricated harvesters yielded a resonant frequency of 49.5 Hz. The harvesters were initially tuned to resonate at 50 Hz with an acceleration of 0.1 G. However, due to spring softening with increasing acceleration, the resonant frequency shifted from 0.5 Hz to 49.5 Hz.

[0171] As in the electrical characterization with the stirrer, a load resistance sweep is performed to calculate the maximum power. As shown in Figure 11 b, which graphically represents a comparison of the maximum power generated in relation to the depth of the piezoelectric material (200 pm and 300 pm) and in relation to the load impedance (MQ), the maximum power generation is around 5 MQ, with a maximum power of 283 pW for the collector at a distance of 300 pm from the piezoelectric material and 464 pW for the distance of 200 pm. These values ​​are very similar to the maximum powers measured in Figure 9 c, with 288 pW and 462 pW, respectively.

[0172] 1 .9. Application Example

[0173] As mentioned above, the energy harvester can be customized using computer-aided design (CAD) software, such as Tinkercad (https: / / www.tinkercad.com), to adapt its specifications to various application scenarios. An exemplary application for the inventive monolithic functional structure, with PVDF as the piezoelectric material, could be to use it as a sensor (instead of an energy harvester). In this particular case (using a sensor as the functional structure), the sensor can be used to measure the current flowing through the cable of a power pylon. When an electric current flows through the cables, it generates a magnetic field that is directly proportional to the current flowing through the cable.Furthermore, by using a magnetic mass (e.g. two neodymium magnetic masses with dimensions of 10 mm x 10 mm x 2.7 mm), the monolithic sensor structure with PVDF as the piezoelectric material can be made to resonate and, depending on the current passing through it, we can measure an instantaneous proportional voltage on our sensor.

[0174] In this application example, the maximum sensor dimensions are 30 mm x 20 mm with a height of 10 mm. The resonant frequency for this resonator is the same as the frequency of the current flowing through the wire being measured, which is 50 Hz. The magnets for the magnetic mass have dimensions of 10 mm x 10 mm and a thickness of 2.7 mm. The support will have dimensions of 20 mm x 10 mm and a height of 10 mm to ensure good anchoring and easy mounting. A simulation is performed to obtain the resonant frequency at 50 Hz. As shown in Figure 12a, measurements of 12 mm x 20 mm and a thickness of 700 pm are obtained for the cantilever beam. Two magnets, for example, two neodymium magnetic masses with dimensions of 10 mm x 10 mm x 2.7 mm, will be required for the magnetic mass. Figure 12b shows the stress distribution in the beam when the cantilever beam is resonating with an acceleration of 0.1 G.

[0175] As can be seen in Figure 13a and Figure 13b, the simulated sensor is mounted inside a sensor node provided by the startup company "Energiot Devices SL" (www.energiot.com). To test the sensor in the laboratory, two heaters with two different power settings have been connected. This configuration allows the generation of four different currents depending on the selected power: 1500 W (6.5 A), 3000 W (13 A), 4500 W (19.5 A), and 6000 W (26 A). As can be seen in Figure 13c, the different current cases passing through the cable can be measured by the corresponding voltage readings, demonstrating a linear response.

[0176] In addition, the magnetic field can also be used to power wireless sensors that measure various parameters, such as magnetic field, ambient and line temperature, acceleration, tilt, humidity, and light intensity, and transmit the collected information to the cloud.

[0177] Example Summary

[0178] In this example, a resonant piezoelectric energy harvester was designed, fabricated, and physically and electrically characterized using 3D printing technology and PVDF encapsulation within the structure, resulting in a monolithic harvester. This resulted in an economically viable energy harvester that mechanically improves resonator anchoring thanks to its structure bonded to the fixing support. The device was optimized using COMSOL Multiphysics® 6.0 finite element simulation software.

[0179] In addition, a practical application example of the resonator was demonstrated by designing and manufacturing a specific use case for measuring the current flowing through a wire using magnetic fields. It was shown that 3D printing enabled the fabrication of a compact, monolithic functional structure (i.e., a monolithic collector) and also facilitated its customization and optimization for specific requirements.

[0180] Overall, this example provides a practical approach to designing and manufacturing resonant harvesters using 3D printing. The demonstrated use case shows the potential for using such devices in a variety of real-world scenarios.

Claims

CLAIMS 1. A monolithic functional structure, characterized in that it comprises: • at least one functional structure made of a functional structure material; and • at least one piezoelectric material; • in which the functional structure and the piezoelectric material are part of a single piece; • in which the functional structure and the piezoelectric material are electrically connected; and • in which the piezoelectric material is not located along the neutral axis of the monolithic functional structure.

2. Monolithic functional structure according to claim 1, wherein the functional structure material is selected from PLA, ABS, PVA, PP, Tough PLA, CPE, PET-G, Nylon, PC and any combination thereof.

3. Monolithic functional structure according to claims 1 or 2, wherein the piezoelectric material of the monolithic functional structure is selected from zirconate titanate, polyvinylidene fluoride or copolymers thereof, polyvinylidene trifluoroethylene copolymer (PVDF-TrFE), polyvinylidene tetrafluoroethylene copolymer (PVDF-TrFE), or combinations thereof.

4. Monolithic functional structure according to any of the preceding claims, further comprising at least one substrate on which the functional structure and the piezoelectric material are located, wherein the substrate material is selected from glass.

5. Monolithic functional structure according to any of the preceding claims, wherein the functional structure of the monolithic functional structure is a low power consumption sensor.

6. Monolithic functional structure according to any of the preceding claims, wherein the monolithic functional structure is a monolithic energy harvesting structure with PVDF as a piezoelectric material.

7. Monolithic functional structure according to any of the preceding claims, wherein at least one functional structure and at least one piezoelectric material are in at least partial physical contact with each other.

8. Monolithic functional structure according to any of the preceding claims, characterized in that it comprises from bottom to top: • at least one first layer of functional structure material comprising at least one layer of printed structural material, wherein a region of said layer of functional structure material is left empty as a depression, wherein a metallic layer, preferably copper, is applied in said region; • at least one second layer of functional structural material comprising at least one layer of printed structural material on the first layer of functional structural material, where at least the upper part of the metal layer is left uncovered; • a layer of piezoelectric material on the first layer of functional structure material, at least partially in contact with the metal layer; • at least a third layer of functional structure material comprising at least one layer of printed structural material on the layer of piezoelectric material and on the second layer of functional structure material, where at least one region on top of the metal layer and one region on top of the piezoelectric material layer are left uncovered; • an electrical connector selected from a metallic wire or a metallic spring-loaded connector, located in both uncovered regions, on top of the metallic layer and on top of the piezoelectric material layer.

9. Method for 3D printing monolithic functional structures according to any of the preceding claims, characterized in that it comprises the steps of: a. providing a substrate, preferably a glass substrate; b. printing on the substrate of step (a), at least one layer of a first functional structure material comprising at least one layer of printed structural material, preferably PLA, without applying functional structural material to at least one region of the last printed layer; c. applying metal to said region of step (b), preferably copper; d. optionally, printing at least one layer of a second functional structural material comprising at least one layer of printed structural material, preferably PLA, without applying functional structural material at least on top of the region where the metal has been applied in step (c); e. applying a piezoelectric material, preferably fluorinated polyvinylidene, on top of the layer where the metal has been applied in step (c), preferably at least partially in direct contact with the metal; f.printing at least one layer of a third functional structure material comprising at least one layer of printed structural material, preferably PLA, without applying functional structure material on top of at least one region of the metal of step (c) and on top of at least one region of the piezoelectric material of step (e), and g. placing at least one electrical connector in both uncovered regions on top of the metal and on top of the piezoelectric material of step (f).

10. Method according to claim 9, wherein the 3D printing is carried out by Fused Deposition Modeling.

11. A method according to claim 9 or 10, wherein computer-implemented computer-aided design (CAD) software is used to design the monolithic functional structure to be 3D printed.

12. Method according to any one of claims 9 to 11, wherein a model created by the computer-aided design (CAD) software is divided into layers using a division software.

13. Method according to any of claims 9 to 12, wherein the metal applied in step c) of the method is selected from gold, silver or copper, preferably copper.

14. Method according to any of claims 9 to 13, wherein at least one electrical connector placed in both regions, in step g) of the method, is selected from a metallic wire or a metallic spring-loaded connector.

15. The method of claim 14, wherein the metal of the metal wire or the metal spring-loaded connector is selected from gold, silver or copper, preferably copper.

16. Method according to any one of claims 9 to 15, wherein the functional structure material is selected from PLA (polylactic acid), ABS (acrylonitrile butadiene styrene), PVA (polyvinyl alcohol), PP (polypropylene), PLA Tough, CPE (copolyester), PET-G (glycol modified polyethylene terephthalate), Nylon (polyamide) and PC (polycarbonate); preferably PLA, and even more preferably PLA Tough.

17. Use of the monolithic functional structure according to any of claims 1 to 5 or 7 to 8, as a sensor, low-power sensor, energy harvester or actuator.

18. Use of the monolithic functional structure according to claim 17 as a low-power sensor for creating a large wireless network in which the low-power sensors communicate with each other.

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