Compression force measurement device with adjustable sensitivity

Abstract

Adjustable Sensitivity Compressive Force Measurement Device This description relates to a force measurement device (10), intended to be subjected to compressive forces, comprising a substrate (100) on which is disposed at least one sensor comprising an organic piezoelectric film (120) arranged between a first electrode (110) and a second electrode (130), the device (10) further comprising an adaptability layer (150) made of a material having a Young's modulus lower or higher than the Young's modulus of the organic piezoelectric film (120). Figure for the abstract: Fig. 1

Description

Title of the invention: Compressive force measurement device with adaptable sensitivity. Technical field

[0001] This description relates generally to force measurement devices, and in particular to compressive force measurement devices. More specifically, it relates to devices having an organic piezoelectric film. Such devices can be used, for example, for measurements on complex and / or deformable surfaces. The devices have a sensitivity that can be easily adjusted, either increased or decreased, depending on the intended applications. Previous technique

[0002] Piezoelectric sensors can be used to measure forces, particularly compressive forces. Generally, the sensor is positioned on a surface such that the mechanical stress is not only uniform over the sensor surface but also unidirectional in the direction transverse to the sensor. Thus, it is possible to have a proportionality between the electrical charge Q (in pC) generated by the sensor and the applied force F (in N): Q = -d³F, where d³F is the piezoelectric coefficient (in pC / N) of the piezoelectric material, characterizing the compressive performance of the piezoelectric material.

[0003] To create lightweight devices, making them compatible with numerous applications, particularly medical ones, piezoelectric materials are used in the form of thin films (typically between 1 and 100 pm). Furthermore, to obtain flexible and / or biocompatible devices, piezoelectric polymers are generally used rather than piezoelectric ceramics, which are rigid and toxic, especially lead-based ceramics such as lead zirconate titanoates (known by the abbreviation PZT).

[0004] However, polymers are less efficient in compression than ceramics (d33= 30 pC / N for PVDF polymer, d33= 590 pC / N for PZT ceramic).

[0005] In order to obtain a sensor with good mechanical strength and easy handling, piezoelectric thin films are generally positioned on a rigid substrate that serves as a mechanical support. However, these substrates can influence the piezoelectric properties of the piezoelectric thin films.

[0006] For example, a very rigid substrate can decrease piezoelectric performance under compression. This has been demonstrated, for example, for a 200 µm thick PZT film on an alumina substrate with a thickness of 0.64 mm, in the article by Torah et al. ('Experimental investigation into the effect of substrate clamping on the piezoelectric behaviour of thick-film PZT elements', J. Phys. Appl. Phys. 37, 1074 (2004)). The model used only concerns the extreme case where the rigidity of the piezoelectric film is negligible compared to that of the substrate, and the latter therefore imposes its mechanical properties on the overall system.

[0007] Conversely, it has been demonstrated by Chakhchaoui et al. ('An enhanced power harvesting from woven textile using piezoelectric materials', IOP Conf. Ser. Mater. Sci. Eng. 827, 012046 (2020)) that a very flexible textile substrate can improve the piezoelectric performance of a PVDF piezoelectric film. Here too, the model concerns the extreme case where the mechanical properties of the piezoelectric film are negligible compared to those of the substrate in the overall system. Moreover, the proposed model shows limitations and is not fully validated by experimental data. The thickness of the textile substrate is 500 µm; that of the PVDF film is not mentioned. Summary of the invention

[0008] There is therefore a need to provide a measuring device whose sensitivity can be adapted according to the compressive forces to be measured.

[0009] This goal is achieved by a force measurement device, intended to be subjected to compressive forces, comprising a substrate on which is disposed at least one sensor comprising an organic piezoelectric film disposed between a first electrode and a second electrode, the device further comprising an adaptability layer having a Young's modulus lower or higher than the Young's modulus of the organic piezoelectric film, the Young's modulus of the adaptability layer being at least 10 times lower or at least 10 times higher than the Young's modulus of the organic piezoelectric film.

[0010] According to a particular embodiment, the adaptability layer is disposed between the substrate and the first electrode.

[0011] According to a particular embodiment, the adaptability layer covers the second electrode.

[0012] According to a particular embodiment, the substrate is made of polyimide.

[0013] According to a particular embodiment, the adaptability layer is in PDMS or in TPU.

[0014] According to another particular embodiment, the adaptability layer is made of epoxy or acrylate.

[0015] According to a particular embodiment, the adaptability layer has a thickness between 10 pm and 100 pm.

[0016] According to a particular embodiment, the device comprises two substrates and two adaptability layers.

[0017] According to a particular embodiment, the device comprises two piezoelectric films, separated by an intermediate electrode, the adaptability layers being arranged on either side of the assembly formed by the first electrode, the first piezoelectric film, the intermediate electrode, the second piezoelectric film and the second electrode.

[0018] According to a particular embodiment, an encapsulation layer covers the sides of the adaptability layers and the piezoelectric film(s).

[0019] According to a particular embodiment, the device comprises several sensors, which can be arranged in lines or in the form of a matrix on the substrate.

[0020] This objective is also achieved by a method for determining the compression performance of a force measuring device intended to be subjected to compressive forces, the device comprising a substrate on which is disposed at least one sensor comprising an organic piezoelectric film disposed between a first electrode and a second electrode, the device further comprising an adaptability layer, disposed between the substrate and the first electrode or covering the second electrode, the method comprising a step in which the generated electric charge Q is calculated from the following equation: O— orf Yp (hp(li>in}ifp+h„£l-ifp)v11} Vp\\ ~\a33~ ^a3n-Vp 1 hdd-vn^YP+hniYvP)Ym ~ Yp ) with Q, the electrical charge, in pC, generated by the sensor, F is the force (in N) applied to the sensor. d jjet d 3i the piezoelectric coefficients of the piezoelectric film, in pC / N, Yp and Ym are the elastic moduli, in Pa, respectively, of the piezoelectric film and the adaptability layer, vp and vm are the Poisson coefficients, respectively, of the piezoelectric film and the adaptability layer. Brief description of the drawings

[0021] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0022] Fig. 1, Fig. 2, Fig. 3, Fig. 4 and Fig. 5 represent, in a schematic and cross-sectional, a device according to different particular embodiments of the invention;

[0023] Figure 6 represents, schematically and in three dimensions, a film piezoelectric arranged on an adaptability layer on which compressive forces (represented by arrows) are applied, according to another particular embodiment of the invention;

[0024] [Fig.7] represents, schematically and in top view, a device according to another particular embodiment of the invention;

[0025] [Fig. 8] is a graph representing the sensitivity of a sensor on a substrate in PI, given for comparison, and a sensor on a PI substrate, an adaptability layer being positioned between the substrate and the sensor, according to another particular embodiment of the invention; and

[0026] [Fig.9] is a photographic image of different piezoelectric sensors, with from left to right: a sensor printed on PI, a sensor printed on TPU and a sensor, according to the invention, printed on TPU and then positioned on PI. Description of the implementation methods

[0027] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0028] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been represented and are detailed.

[0029] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements coupled together, this means that these two elements can be connected or linked through one or more other elements.

[0030] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0031] Unless otherwise specified, the expressions "approximately", "roughly", and "in the order of" mean within 10%, preferably within 5%.

[0032] By between X and Y, we mean that the bounds are included.

[0033] The invention is particularly interesting for applications requiring lightweight, flexible, and / or deformable force sensors to measure small forces. The sensors are wearable on a person. This is of interest, for example, in the medical field, for sensors measuring coaptation forces in a heart valve, or for tactile sensors on the skin.

[0034] We will now describe the device in more detail with reference to figures 1, 2, 3, 4 and 5.

[0035] The piezoelectric device 10 comprises at least one substrate 100 on which at least one sensor is disposed. The substrate comprises a first face 100a, on which the sensor(s) is disposed, and a second face 100b. Each sensor comprises a piezoelectric film 120 disposed between a first electrode 110 (also called the lower electrode) and a second electrode 130 (also called the upper electrode).

[0036] Each piezoelectric film 120 has a thickness, for example, between 1 and 11 µm, preferably between 2 and 5 µm, for example 3 µm. Such thicknesses make it possible to obtain layers 120 with good flexibility.

[0037] Preferably, each piezoelectric film 120 is an organic piezoelectric film. Each piezoelectric film 120 preferably comprises a polymeric matrix of PVDF, a PVDF copolymer, or a PVDF terpolymer. This may be a copolymer of vinylidene fluoride and at least one other monomer copolymerizable with VDF. Advantageously, the copolymer comprises at least 50 mol%, preferably at least 70 mol%, and even more preferably at least 90 mol% of VDF.

[0038] By way of illustration, the copolymerizable monomer(s) are, for example, chosen from chlorotrifluoroethylene (CTFE), chlorofluoroethylene (CFE), hexafluoropropylene (HFP), trifluoroethylene (VF3), methyl methacrylate (MMA), tetrafluoroethylene (TFE), and perfluoro(alkyl vinyl) ethers such as perfluoro(methyl vinyl) ether (PMVE).

[0039] For example, the copolymer is a copolymer of poly(vinylidene fluoride-trifluoroethylene) PVDF / TrFe, also noted P(VDF-TrFe) or PVDF-CTFE.

[0040] It may also be a terpolymer. For example, a PVDF / CTFE / CFE terpolymer will be chosen.

[0041] According to another embodiment, the polymer is not a ferroelectric polymer: it may be PVDF-HFP.

[0042] According to another embodiment, the polymer is polylactic acid (also noted as PLA or PLLA).

[0043] Preferably, the piezoelectric films 120 are piezoelectric polymers selected from PVDF, P(VDF-TrFE) and PLLA.

[0044] Each organic piezoelectric film 120 can be a composite material. For example, the film 120 can comprise, in addition to a piezoelectric or non-piezoelectric polymer matrix, piezoelectric particles and / or PEDOT:PSS particles. The particles will be small enough not to alter the roughness.

[0045] For example, ferroelectric particles are in BaTiO3 (BTO), PZT (lead zirconate titanoate), AIN, ZnO, or in SBN (Sr-Ba-Nb oxide) or SBT (Sr-Ba-Ti oxide).

[0046] For example, piezoelectric films 120 are formed from composite materials comprising, for example, a polymer (piezoelectric type PVDF, or non-piezoelectric type TPU, PDMS, PVDF HFP) and piezoelectric nanoparticles or nanofibers (PVDF, PZT, BaTiO3, AIN).

[0047] The piezoelectric films 120 are arranged between electrodes 110, 130. The electrodes 110, 130 are flexible electrodes.

[0048] Preferably, the electrodes 110, 130 are made of an electrically conductive polymer or a metal such as gold or silver. Electrically conductive polymers provide a better interface with P(VDF-TrFE). Preferably, this is PEDOT-PSS (poly(3,4-ethylenedioxythiophene).

[0049] They can be deposited by screen printing methods to ensure good mechanical cohesion, without risk of detachment, at the interface between the sensor and the adaptability layer.

[0050] The electrodes 110, 130 preferably have a thickness between 0.1 and 3 pm, even more preferably between 0.5 and 1.5 pm, for example about 1 pm).

[0051] The metal electrodes 110, 130 have, for example, a thickness between 30 and 200 nm.

[0052] Preferably, the mechanical properties of the electrodes 110, 130 are close to those of the piezoelectric film 120. Preferably, their Young's moduli vary by less than 30%. There is then no particular constraint on the thickness of the electrodes 110, 130. Preferably, however, electrodes 110, 130 will be chosen with a thickness less than that of the piezoelectric film 120.

[0053] In cases where the mechanical properties of the electrodes 110, 130 are very different from those of the piezoelectric film 120, particularly where their Young's moduli vary by more than 30%, the thickness of the electrodes 110, 130 shall be chosen so that it is negligible compared to that of the piezoelectric film. The thickness of the electrodes shall be at least 10 times less than that of the piezoelectric film 120.

[0054] The substrate 100 preferably has a modulus of elasticity greater than IGPa and / or a thickness greater than 25 pm, for example between 10 and 500 pm, preferably between 20 and 250 pm, to have a device with sufficient mechanical strength.

[0055] The sensor thickness is relatively small (typically less than 10 pm). The substrate 100 ensures the mechanical strength of the device during its manufacture and use.

[0056] The nature of the substrate 100 will depend on the applications. The substrate 100 can be a polymer material, preferably chosen from polyimide (PI), poly(ethylene naphthalate) (PEN), polyethylene terephthalate (PET), thermoplastic polyurethane (TPU) or polydimethylsiloxane (PDMS). Preferably, it is PI.

[0057] The substrate 100 can also be made of glass or steel.

[0058] The device further comprises an adaptability layer 150. The adaptability layer 150 is made of a different material from that of the substrate 100.

[0059] The adaptability layer 150 can be made of polymer, sol-gel material or oxide. For example, it can be made of epoxy, (meth)acrylate, thermoplastic polyurethane (TPU), polydimethylsiloxane (PDMS), alumina, silica, titanium oxide, or zirconium oxide.

[0060] According to a first embodiment, shown in [Fig. 1], the adaptability layer 150 modifying the piezoelectric performance of the sensor is arranged between the substrate 100 and the first electrode 110. In other words, the device 10 comprises successively: the substrate 100, the adaptability layer 150, the first electrode 110, the piezoelectric film 120 and the second electrode 130.

[0061] This configuration allows for a wide choice of substrate 100.

[0062] According to a second embodiment, shown in [Fig.2], the adaptability layer 150 is arranged on the second electrode 130. In other words, the device 10 comprises successively: the substrate 100, the first electrode 110, the piezoelectric film 120, the second electrode 130 and the adaptability layer 150.

[0063] The adaptability layer 150, which modifies the piezoelectric performance, is thus arranged above the sensor. It can also act as an encapsulation layer, protecting it from the external environment.

[0064] In the devices described above, the adaptability layer 150 is positioned on or below the piezoelectric film 120. The material of the adaptability layer 150 is different from that of the piezoelectric film 120. Its mechanical properties differ from those of the piezoelectric film 120, which allows control of the piezoelectric film 120's compression performance. It is the mechanical interactions between the adaptability layer 150 and the piezoelectric film 120 that modify the piezoelectric performance.

[0065] It is thus possible to increase the piezoelectric performance in compression (d33 eff) by choosing an adaptability layer 150 made of a material having a Young's modulus lower than that of the piezoelectric film material 120 (in other words, by choosing an adaptability layer 150 that is more flexible than the piezoelectric film 120). This improves the sensitivity of the piezoelectric film 120, particularly for compressive forces. For example, its performance is multiplied by a factor of between 2 and 10.

[0066] Preferably, the Young's modulus of the adaptability layer 150 is between 10 and 1000 times lower than the Young's modulus of the piezoelectric film 120.

[0067] For example, for a 100 PI substrate, the 150 adaptability layer can be made of TPU or PDMS.

[0068] It is also possible to reduce the piezoelectric performance in compression (d33 eff) by choosing an adaptability layer 150 made of a material having a Young's modulus higher than that of the piezoelectric film material 120 (in other words, by choosing an adaptability layer 150 that is stiffer than the piezoelectric film 120). This makes it possible, for example, to adapt the performance of the piezoelectric film 120 to the measurement range of a measuring electronic connected to the piezoelectric device 10.

[0069] Preferably, the Young's modulus of the adaptability layer 150 is between 10 and 1000 times greater than the Young's modulus of the piezoelectric film 120.

[0070] For example, for a PI substrate 100, the adaptability layer 150 can be made of epoxy or acrylate.

[0071] Furthermore, to increase the piezoelectric performance, it is possible to increase the thickness of the adaptability layer 150 and / or the difference in elastic moduli between the adaptability layer 150 and the piezoelectric film 120.

[0072] The adaptability layer 150 has a greater thickness than the piezoelectric film 120. The thickness of the adaptability layer 150 is between 10 µm and 1 mm, preferably between 10 µm and 100 µm. Such a thickness allows the properties of the piezoelectric film 120 to be modified while remaining flexible (even if its elastic modulus is quite high).

[0073] The adaptability layer 150 has a surface area greater than or equal to the surface area of ​​the piezoelectric film 120 (surface area in the plane - directions 1 and 2 of [Fig.6]). Preferably, the surfaces are identical.

[0074] As shown in Figures 3, 4 and 5, the device 10 can comprise two substrates 100, 170 and two adaptability layers 150, 160. The device 10 comprises successively: the first substrate 100, the first adaptability layer 150, the first electrode 110, the piezoelectric film 120, the second electrode 130, the second adaptability layer 160 and the second substrate 170.

[0075] The two substrates 100, 170 may be made of the same material or of different materials. The two substrates 100, 170 may have the same dimensions or different dimensions. Preferably, the two substrates 100, 170 are made of the same material. They preferably have the same dimensions.

[0076] The two adaptability layers 150, 160 may be made of the same material or of different materials. Preferably, each adaptability layer 150, 160 has a Young's modulus greater (or conversely, smaller) than that of the Piezoelectric film 120. The two adaptive layers 150, 160 may have the same dimensions or different dimensions. Preferably, the two adaptive layers 150, 160 are made of the same material and have the same dimensions.

[0077] The presence of an adaptability layer 150, 160 on either side of the piezoelectric film 120 allows its piezoelectric performance to be increased or decreased further.

[0078] The presence of a substrate 150, 160 on each face of the sensor allows it to be well protected.

[0079] Furthermore, if the device is perfectly symmetrical with the axis of symmetry passing through the piezoelectric film 120, it is possible to divide the piezoelectric film into two parts 120, 125 by adding an intermediate electrode 140 as shown in [Fig. 5]. In other words, the device 10 comprises two piezoelectric films 120, 125. More specifically, the device 10 comprises successively: the first substrate 100, the first adaptability layer 150, the first electrode 110, the first piezoelectric film 120, the intermediate electrode 140, the second piezoelectric film 125, the second electrode 130, the second adaptability layer 160, and the second substrate 170.

[0080] A piezoelectric bimorph is thus formed and it is possible to take advantage of the benefits of this type of device (in particular by polarizing the piezoelectric films in the same way or in opposite ways).

[0081] The device 10 may, in addition, include an encapsulation layer (Figures 4 and 5). It covers the sides of the elements arranged between the substrates 100, 170 (i.e. the sides of the adaptability layers 150, 160, the electrodes 110, 130 and the piezoelectric film(s) 120, 125).

[0082] The encapsulation layer 180 completely isolates the sensor from the outside. Such a layer can be biocompatible and / or waterproof. It can be a dielectric material such as non-polarized PVDF or the product marketed under the reference ED AG 455B by Loctite.

[0083] The encapsulation layer 180 is preferably made of PDMS.

[0084] The device 10 may include a layer (not shown), forming a ground plane between two dielectric layers to reduce measurement noise.

[0085] The ground plane makes it possible to greatly reduce measurement noise, improve measurement accuracy and therefore open up a wider range of applications.

[0086] The ground plane is flexible. It has a thickness between 3 pm and 10 pm, for example 5 pm.

[0087] The site plan is preferably printed. It is integrated into the device during its manufacture by printing.

[0088] To create the ground plane, any flexible conductive ink can be used.

[0089] The ground plane is preferably made of carbon. Carbon is highly conductive and exhibits good biocompatibility, which is advantageous when it comes into contact with the human body, as it is one of the layers closest to the external environment.

[0090] Alternatively, the ground plane can be made of gold.

[0091] The dielectric layers serve to electrically isolate the sensor from the ground plane. The dielectric layers are also flexible. They have a thickness of, for example, between 3 pm and 30 pm, preferably between 3 and 10 pm, for example 5 pm.

[0092] They are thick enough not to snap during polarization, nor too thick so as not to stiffen the sensor.

[0093] The dielectric material presents few constraints. Preferably, a printable and curable material should be chosen. For example, it could be non-piezoelectric crosslinked PVDF or EDAG. Preferably, EDAG is preferred because, during printing, it does not contain a solvent that could damage the underlying layers (unlike the TEP solvent in PVDF, which damages the silver).

[0094] Such layers have mechanical properties close to those of the piezoelectric film 120: their presence in the device does not alter the predictive capacity of the model.

[0095] The device 10 can include several sensors arranged on the same face 100a of the substrate 100. They can be arranged in a line or in the form of a matrix.

[0096] For example, a line may include sensors between X and Y.

[0097] The matrices are matrices of nxm sensors with n and m positive integers, greater than or equal to 2, and preferably between 2 and 10.

[0098] The adaptability layer 150 can be common to all sensors in the matrix or line, or specific to each sensor.

[0099] As shown in [Fig. 7], the device 10 comprises electrical tracks 190 arranged to carry the electrical signal from the sensors, preferably to amplifiers, and then to measuring devices, preferably electronic measuring devices. Voltage measuring devices can also be used to measure the signals from the sensors.

[0100] The electrical tracks 190 are electrically conductive tracks. They can be printed on the substrate. Any highly conductive ink that can be printed with a sufficiently fine resolution (typically the track width is less than 100 pm for high-density matrices) can be used.

[0101] The tracks 190 may be made of a metal (gold, for example), or of an electrically conductive polymer material. For example, the electrically conductive polymer material is PEDOT:PSS. It may also be a polymer in which electrically conductive particles are dispersed, for example, particles of carbon. Silver can also be used in the form of ink, preferably formulated with a solvent-resistant polymer matrix (for example with a silicone, TPU or acrylate base).

[0102] Preferably, a silver-type ink (in particular a silicone matrix with silver particles) is used for the electrical connections between the sensor and the measuring electronics.

[0103] Gold electrical tracks 190 are not only highly conductive but also biocompatible. The thickness of such tracks is preferably less than 200 nm to obtain flexible tracks. The thickness is preferably between 10 nm and 200 nm, for example 100 nm.

[0104] Gold can be deposited by photolithography, which makes it possible to achieve very high resolution and thus obtain high density sensor matrices.

[0105] Preferably, gold is used with a PI substrate, which is resistant to the solvent used during photolithography (typically acetone).

[0106] The manufacturing process for such a device may include the following steps: - providing a substrate 100, - depositing an adaptability layer 150 on the substrate 100, - forming the different layers of the sensor (electrodes, piezoelectric film).

[0107] Alternatively, the process may include the following steps: - form the different layers of the sensor (electrodes, piezoelectric film) on an adaptability layer 150, - place the resulting assembly onto a substrate 100.

[0108] The process may further include a step in which an encapsulation layer is formed.

[0109] When the device 10 comprises two substrates, it is possible to form the layers successively from the first substrate 100 to the second substrate 170. Alternatively, it is possible to position a second assembly (comprising the second substrate 170 and the second adaptability layer 160) on a first assembly (comprising the first substrate 100, the first adaptability layer 150, the first electrode 110, the piezoelectric film 120, and the second electrode 130). The second assembly may optionally also include the intermediate electrode 140 and the second piezoelectric film 125.

[0110] Preferably, two identical assemblies (substrate 100 or 170, adaptability layer 150 or 160, electrode 110 or 130, piezoelectric film 120 or 125) are manufactured and then assembled to the electrode 140, for example by means of a conductive adhesive.

[0111] The process also includes a step of crystallizing (or aligning the dipoles) the layer in piezoelectric material, to improve its performance piezoelectrics. This irradiation is carried out, for example, with a UV flash light, with a flash duration, or pulse, of approximately 500 ps to 2 ms, a fluence (energy delivered per unit area) of approximately 15 J / cm² to 25 J / cm², and with a light wavelength of approximately 200 nm to 380 nm. The number of UV flashes, or pulses, produced during this irradiation varies depending on the thickness to which the piezoelectric material must be crystallized. For example, for a P(VDF-TrFe) thickness of approximately 2 pm, the irradiation can be carried out with a fluence of approximately 17 J / cm², a pulse duration of approximately 2 ms, and 5 pulses.

[0112] The piezoelectric material, possibly having undergone previous crystallization, is then subjected to an annealing step, for example, carried out at about 130°C for about 60 min, to finalize the total crystallization of the piezoelectric material.

[0113] The crystallization of the piezoelectric material can therefore be carried out in two stages: firstly, irradiation by UV light pulse to properly crystallize the second face of the layer in piezoelectric material in order to increase its thermal conductivity, then a thermal annealing completing the crystallization for the rest of the piezoelectric material not crystallized by the previous irradiation.

[0114] When the piezoelectric material is a P(VDF-TrFe)-based copolymer, a biasing step is performed before use. This step can be carried out, for example, by applying a direct current voltage across its terminals via the electrodes to improve the piezoelectric coefficient of the material. This biasing is performed only once for the entire lifetime of the piezoelectric material. This electric field biasing can be done at room temperature or at high temperatures (up to approximately 100°C). When the biasing is performed at room temperature, it is possible to apply a direct current voltage of up to approximately 150 V / m of piezoelectric layer thickness for a duration, for example, of a few seconds to a few minutes. For example, a voltage of 120 V / m could be applied for 20 seconds.When polarization is performed hot, for example at a temperature of approximately 90°C, a DC voltage of, for example, between approximately 50 V and 120 V per micron of piezoelectric layer thickness can be applied for a duration of, for example, between approximately 1 and 5 minutes. The temperature is then lowered to ambient temperature, and the electric field applied to the piezoelectric material via the applied DC voltage is then switched off. Such polarizations allow PVDF to achieve piezoelectric coefficients of approximately 10 to 40 pC / N.

[0115] The molecules inside the piezoelectric layer remain oriented in this way, even when the material is no longer subjected to this electric field. The material can thus be polarized by applying an initial polarization voltage across the electrode terminals. Preferably, a piezoelectric material thickness of approximately 3 pm or less is chosen to promote the polarization of the piezoelectric material of this capacitance, and the level of the electrical voltage applied between the electrodes to achieve the initial polarization of the piezoelectric material (when the piezoelectric material must be initially polarized).

[0116] For example, an ideal remanent polarization of 8 pC / cm2 will be chosen.

[0117] Annealing is advantageously carried out at the end of the process, or between the different steps. Annealing is, for example, at a temperature between 100°C and 150°C, preferably around 100°C to remove residual traces of solvent and / or finalize the crystallization of the piezoelectric material.

[0118] Each sensor of the device is preferably screen-printed onto substrate 100. The printing manufacturing method simplifies and reduces the cost of the manufacturing process.

[0119] All sensors on the same side of the substrate can be printed at the same time.

[0120] The performance, more specifically the compression performance, of a device 10 as described above can be determined. For this purpose, the conditions of [Fig. 6] are assumed, and the contacting piezoelectric film 120 and adaptability layer 150 are considered to be elastic, linear, and isotropic materials. This is almost always the case for low compressive forces, typically less than 100 kPa.

[0121] The following variables are defined: - mechanical quantities: T the stress in the piezoelectric film in N.m2 (T 3 therefore corresponds to the uniform stress applied in compression), S the deformation of the piezoelectric film; - Electrical quantity: D the electrical displacement in the piezoelectric film in pC.m2; - geometric parameters \hp and hm thicknesses in m, respectively of the piezoelectric film 120 and of the material of the adaptability layer 150 allowing to modify the performance of the device; - mechanical properties: Yp and Ym are the elastic moduli of the piezoelectric film 120 and the material of the adaptability layer 150 respectively in Pa, vp and vm are their Poisson's ratio. - piezoelectric properties: d 33 and d 31 are the piezoelectric coefficients of the piezoelectric film 120.

[0122] In the configuration of [Fig.6], the compression performance of the piezoelectric film 120 is characterized by an effective d33 (d33:

[0123] Q=-d33ef^

[0124] [Math.l] a33ëff — a33"ZC(311-Pp ~ yp /

[0125] Indeed, according to the piezoelectric theory for a compressive stress:

[0126] [Math 3] D3 = d31.(T\ + T2) + d33.T3 (1)

[0127] By isotropy of the piezoelectric film, we set T1 = T2 = T and S2 = S2 = S. According to Hooke's law in the piezoelectric film:

[0128] [Math 4] c _ t (2) 5 “ Yp-1 ~ Yp-1 3

[0129] According to Hooke's law in the equivalent material consisting of the piezoelectric film and the contact material:

[0130] [Math 5] S= _^. T (3) J eq û

[0131] where Y eq and veq are respectively the modulus of elasticity in direction 3 and the Poisson's ratio of the equivalent material.

[0132] By combining equation (2) and equation (3):

[0133] [Math 6] Yp_ / JY \ t* (4) 1 ~ " 1-Pp ( Yeq ~ Yp )-1 3

[0134] By combining equation (4) and equation (1):

[0135] [Math 7] jo _ / ri yp„ / .¾¾. _221 ] 1 T" -^3- (u33 zc / 311-yp ( Yeq Yp} J'1 3 (5)

[0136] Y eq and v eq are calculated using the improved version of the Reus equations for a composite material under iso-constraint conditions (equations from the article by Y. Luo ('Inproved Voigt and Reuss Formulas with the Poisson Effect'. Materials. (2022)):

[0137] [Math.8] y = ___________________ eq YpY^YVnWfaYvp^hphd^ (6)

[0138] [Math.9] Veq” (7)

[0139] By substituting equations (6) and (7) into (5) and then normalizing by the surface of the piezoelectric film, we obtain the charge Q:

[0140] [Math. 10] D— (rl Irl YP ( ^'^Vp+h^l-Vp)^ »p H p V- -^33- / 33^^^ - Yp} jr

[0141] This model makes it possible to determine the compression performance of the piezoelectric film 120 by varying the modulus of elasticity of the material of the adaptability layer 150.

[0142] In the case of a piezoelectric polymer for which d33 and dM have opposite signs, the model shows that: - piezoelectric performance is improved if Ym < Yp (adaptability layer 150 more flexible than piezoelectric film 120), - piezoelectric performance decreases if Yp < Yin (adaptability layer 150 is more rigid than the piezoelectric film 120), - piezoelectric performance does not vary or varies little if Yp ~ Y(adaptability layer 150 mechanically close to piezoelectric film 120), - the greater the thickness of the material of the adaptability layer 150 compared to that of the piezoelectric film 120, and the more the elastic moduli are different, the more the piezoelectric performance is altered.

[0143] The dimensions and materials of the electrodes 110, 130 will be chosen so as not to affect the model: - the thickness of the electrodes 110, 130 is negligible compared to that of the piezoelectric film 120 (typically at least 10 times smaller), - the mechanical properties of electrodes 110, 130 are close to those of the piezoelectric film 120 (less than 30% difference between the moduli of elasticity) so that hp can be considered the thickness corresponding to the sum of the thickness of the piezoelectric film 120 and the thickness of electrodes 110, 130.

[0144] From this model, it is possible to predict whether an adaptability layer 150 will decrease or increase the performance of the piezoelectric device 10.

[0145] Illustrative and non-limiting example

[0146] Several devices were made.

[0147] Initially, a comparative device was fabricated. It comprises a sensor on a PI substrate (Ym = 6 GPa, hm = 50 pm, UBE's Upilex). The geometry is that shown in [Fig. 7] (except for the presence of the adaptability layer). The sensors have a diameter between 2 mm and 3 cm, preferably less than 1 cm, to allow them to be easily subjected to a uniform compressive force.

[0148] The sensor is screen-printed onto the substrate. The lower electrode is printed first. This consists of a layer of PEDOT:PSS (a product marketed by Heraeus). A 30-minute annealing at 135°C evaporates the solvent (final thickness approximately 1 µm). The piezoelectric material, P(VDF-TrFE) 80 / 20 (marketed by Arkema Piezotech), is then printed, followed by a 3-minute annealing at 135°C under vacuum (3 µm thickness). The upper PEDOT:PSS electrode is then deposited in the same way as the lower electrode. Traces are printed with silver ink (marketed by Taiyo Ink) and then annealed for 10 minutes at 135°C. These traces will connect the sensor to the measurement electronics.

[0149] The piezoelectric film is then polarized. It is connected to a voltage source via its electrodes, and a first low DC voltage (electric field in the piezoelectric material ~ 30 V / pm) is applied for 3 minutes to discharge as much electrical charge as possible from the sensor. A first AC voltage ramp-up is then performed at 10 Hz (from approximately 30 V / pm to 120 V / pm) to eliminate certain sensor defects. The same voltage ramp-up is then repeated at 1 Hz to finalize the alignment of the dipoles.

[0150] P(VDF-TrFE) and PEDOT:PSS have very similar mechanical properties and can therefore be considered as a single layer in the model (Yp = 2.5 GPa). To experimentally test the device under compression, it is attached to a rigid glass plate with double-sided adhesive (sold by Teraoka under reference 9030W). Then, different forces are applied at 1 Hz using a flat indenter perfectly parallel to the sensor surface and with an area greater than that of the sensor. The electrical charges generated by the piezoelectric film under compressive stress are measured by a Kistler 5015 charge amplifier with a 10 Hz low-pass filter to eliminate noise.

[0151] The same device was made by adding a TPU adaptability layer 150 (Ym= 50 MPa, hm = 100 pm, DuPont Intexar TE-1 IC) between the substrate 100 and the sensor ([Fig.7]).

[0152] The TPU used has a thin layer of adhesive on its underside that activates at high temperature. After screen printing, the assembly formed by the sensor and the TPU layer 150 is placed on the PI substrate (larger than the sensor), and then a heating plate at 150°C is applied for 10 seconds to bond the PI and the TPU. The sensor printed on TPU and then bonded to the PI exhibits better performance (approximately 3 times greater) than the sensor printed directly onto the PI ([Fig. 8]). The various devices are shown in [Fig. 9].

[0153] It is likely that the high-temperature process slightly degrades the performance of P(VDF-TrFE). The expected performance was an improvement of one The factor is 10, but the one obtained represents a 3-fold improvement. Optimizing the process will further improve the sensor's performance. Nevertheless, feasibility has been demonstrated.

[0154] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0155] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.

Claims

Demands

1. A force measuring device (10), intended to be subjected to compressive forces, comprising a polymer substrate (100) on which is disposed at least one sensor comprising an organic piezoelectric film (120) arranged between a first electrode (110) and a second electrode (130), the device (10) further comprising an adaptability layer (150) having a Young's modulus lower or greater than the Young's modulus of the organic piezoelectric film (120), the Young's modulus of the adaptability layer (150) being at least 10 times lower or at least 10 times greater than the Young's modulus of the organic piezoelectric film (120), the adaptability layer (150) being made of a material different from that of the substrate (100), the device comprising successively the substrate (100), the first electrode (110), the piezoelectric film (120), the second electrode (130) and the adaptability layer.

2. Device according to claim 1, characterized in that the substrate (100) is made of polyimide.

3. Device according to any one of claims 1 to 2, characterized in that the adaptability layer (150) is made of PDMS or TPU.

4. Device according to any one of claims 1 to 2, characterized in that the adaptability layer (150) is made of epoxy or acrylate.

5. Device according to any one of the preceding claims, characterized in that the adaptability layer (150) has a thickness between 10 pm and 100 pm.

6. Device according to any one of the preceding claims, characterized in that it successively comprises: the substrate (100), the adaptability layer (150), the first electrode (110), the piezoelectric film (120), the second electrode (130), a second adaptability layer (160) and a second substrate (170).

7. Device according to claim 6, characterized in that it comprises two piezoelectric films (120, 125), separated by an intermediate electrode (140), the adaptability layers (150, 160) being arranged on either side of the assembly formed by the first electrode (110), the first piezoelectric film (120), the electrode intermediate (140), the second piezoelectric film (125) and the second electrode (130).

8. Device according to any one of claims 6 and 7, characterized in that an encapsulation layer (180) covers the sides of the adaptability layers (150, 160) and of the piezoelectric film(s) (120, 125).

9. Device according to any one of the preceding claims, characterized in that it comprises several sensors, which can be arranged in lines or in the form of a matrix on the substrate (100).

10. A method for determining the compression performance of a force measuring device (10) intended to be subjected to compressive forces, the device comprising a substrate (100) on which is disposed at least one sensor comprising an organic piezoelectric film (120) arranged between a first electrode (110) and a second electrode (130), the device (10) further comprising an adaptability layer (150) having a Young's modulus lower or higher than the Young's modulus of the organic piezoelectric film (120), the Young's modulus of the adaptability layer (150) being at least 10 times lower or at least 10 times higher than the Young's modulus of the organic piezoelectric film (120), the adaptability layer (150) being made of a material different from that of the substrate (100), the device comprising successively the substrate (100), the first electrode (110), and the piezoelectric film. (120),the second electrode (130) and the adaptability layer, the process comprising a step in which the generated electric charge Q is calculated from the following equation: [Math. 10] n— (ri 3H Yp ( hp^n^p+hnil-^Vn, Pp \ \ V - - «33- 2«311-pp1 hp(lv„^Yp+hn£l-tfp)Ym - Yp J with Q electric charge, in pC, generated by the sensor (10), F the force (in N) applied to the sensor (10), d jjet d 3i the piezoelectric coefficients of the piezoelectric film (120), in pC / N, Ypet Ym the moduli of elasticity, in Pa, respectively, of the piezoelectric film (120) and the adaptability layer (150), hp and hm are the thicknesses, in m, respectively, of the piezoelectric film (120) and the adaptability layer (150), vp and vm are the Poisson's ratios, respectively, of the piezoelectric film (120) and the adaptability layer (150).

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