WO2015014950A1

WO2015014950A1 - Textile pressure sensor and method for fabricating the same

Info

Publication number: WO2015014950A1
Application number: PCT/EP2014/066519
Authority: WO
Inventors: Jose Francisco SAENZ COGOLLO; Annalisa Bonfiglio; Beatrice Fraboni
Original assignee: Universita' Degli Studi Di Cagliari; Alma Mater Studiorum Universita' Di Bologna
Priority date: 2013-07-31
Filing date: 2014-07-31
Publication date: 2015-02-05

Abstract

The present invention relates to a pressure sensor including: A single non-conductive textile layer defining a first and a second surface; A first and a second conductive yarn sewed or embroidered in said non-conductive textile layer, said first and second yarn being so arranged that there is no contact among said first and said second yarn and that they overlap in a crossing point so that a portion of said textile layer is sandwiched between said first and said second yarns; Said non-conductive textile layer including a treated portion comprising said sandwiched portion, said treated portion of textile layers being impregnated with a conductive polymer. Furthermore, the invention discloses a method for realizing such a sensor.

Description

Textile pressure sensor and method for fabricating the same

DESCRIPTION

Field of the invention

The present invention relates to an "all-textile" pressure sensor, which is soft and flexible. More specifically, it relates to a pressure sensor made of textile materials and thin enough to be integrated in garments and clothes without sacrificing comfort and usefulness. Furthermore, the invention relates to a method to fabricate such a pressure sensor.

Technical background

Textile pressure sensors are ideal to determine strength or pressure upon soft objects and can be used in a wide range of applications, for instance, to measure the interface pressures of a person sitting on a chair or laying on a bed. In such applications it is necessary for the sensor to be flexible and thin in order to adjust to the shape of the interfaces and to adequately measure the exerted forces. Moreover, in order to measure pressure at different points on a surface, the sensor has to cover the area of interest with several sensing points distributed usually as an array of n x m small elements.

Existing textile pressure sensors can be classified in three categories: contact, capacitive and resistive sensors. Contact sensors are usually based on a configuration of two conductors separated by a non-conductive material that, when a force or pressure is applied, allows the two conductive parts to become in contact and thus closing an electric circuit. For instance, these types of sensors are described in patent application US20030119391A1 which discloses a textile sensor where the conductors are normally biased apart at crossover points with an air gap between them. Another example with a different architecture is presented in patent application WO2005073685 which discloses a lineal sensor formed by conducting wires laid on two layers of fabric, one layer in a lengthwise sense and the other layer on a crosswise sense and the conduction is obtained when the surface is pressured and the wires on both faces get into contact. Another relevant example of contact type textile pressure sensors is presented in patent application US20020180578A1 . The disadvantage of contact sensors is that they are basically on-off devices and do not give much information about the intensity of the stimulus, in this case the pressure.

The operating principle of capacitive sensors is based on the change in capacitance that occurs between two parallel conductive plates between which there is a nonconductive elastomeric material, when a force or pressure is applied upon them. An example of this kind of sensors is presented in patent application WO2005121729 A1. Capacitive sensors give a continuous response to pressure intensity but have the drawback of requiring very precise and highly sensitive and stable electronics, since the changes in measured capacitance are usually less than few pico-faradays.

Resistive sensors are based on the change in electric resistance that takes place in a piezoresistive material when a force or pressure is applied onto them. Since changes in resistance are usually of several orders of magnitude and relatively fast, this type of sensors requires a simple electronics; this is important in case arrays of many sensor elements have to be built. One example of resistive sensors is presented in patent application US20070202765 which discloses a textile pressure sensor formed by three layers: two conductive layers (typically polypyrrole coated polyester) and one non-conductive intermediate layer coated with a piezoresistive material (like Teskcan® piezoressitive ink) distributed in a non-continuous way. In patent US7770473 a textile pressure sensor based on a multilayer thread having a piezoresistive layer (like polyaniline) exhibiting a pressure-dependent electrical resistance, and a conductive layer in contact with the pressure sensitive layer is presented. A pressure sensor is constructed by incorporating conductive threads that contact the multilayer thread at specific points of a textile structure. Another example of resistive pressure sensors is presented in patent US8393229 which discloses a multilayered device where an elastic conductive element is sandwiched between two grooved surfaces that impose a deformation of the conductive element when a pressure is applied. The strain applied to the elastic conductive element changes the electric resistance of a portion of it included between two conductive yarns. In patent application US20090272197 another multilayer textile sensor is presented where piezoresistive fluid tracks (containing metallic particles, carbon or conductive polymers) are printed on top of a base fabric and are encapsulated and protected with other polymeric layers. Patent US8294226 discloses a distributed pressure sensor made of at least two flexible sheets (plastic films or textiles) coated with polythiophene-like compounds (like PEDOT) and assembled in different configurations using insulating spacers.

Most of the existing textile pressure sensors imply the use of multilayered configurations of fabrics and films that limit the minimum thickness of the devices compromising the comfort and usefulness if used as part of a garment or clothes. Therefore a solution that overcomes the drawbacks of the state of the art is needed.

Summary of the invention

The present invention discloses an "all-textile" pressure sensor made from a single layer of a non-conductive textile embroidered with highly conductive yarns and treated at specific areas with an intrinsically conducting polymer. It can include one or more force sensing elements that can be arranged in an array of n x m elements for measuring the pressure distribution in large areas with high space resolution.

This sensor is extremely thin and precise and can be embedded in any garment to be worn or to any other piece of textile used to measure the pressure. Moreover, the usage of a single textile layer on which yarns are directly coupled allows using a relatively low amount of expensive materials such as conductive polymers or metals.

Another goal of the invention is to render available a pressure sensor which has a high spatial resolution.

Preferably, the system includes a substrate that can be worn by, or fitted directly or indirectly to the body of the user being monitored, and said sensors are fixed or moveably connected to the substrate.

Alternatively, the pressure sensor is located in a textile in contact with the user such as for example the textile wrapping a chair, an armchair, a sofa, or tablecloths, a carpet, etc. ; or, it could be in contact with other objects (as for instance a wrapped chair in contact with a box). According to an embodiment of the present invention there is provided a pressure sensor that can be worn by a user to monitor or sense pressure either separately or in combination with any one or more of temperature, stress, strain, angulation or a physiological condition.

The pressure sensor could be embedded in any form of garment depending on the particular application and body part being monitored such as socks, stockings, underpants, long johns, a singlet or a tubular sleeve. The garment may be made from any suitable textile material and have any structure including knitted, woven or non-woven structures. It is also possible that the pressure sensor may be in the form of an insert, bandage, sleeve, flexible planar materials, pads or inner garments that cover a portion of the body of a user.

In a first aspect, the invention relates to a pressure sensor including:

A single non-conductive textile layer defining a first and a second surface;

A first and a second conductive yarns fixed to said non-conductive textile layer, said first and second yarn being so arranged that there is no contact among said first and said second yarns within said textile layer and that they overlap in a crossing point so that a portion of said non-conductive textile layer is sandwiched between said first and said second yarns;

Said non-conductive textile layer including a treated portion comprising said sandwiched portion, said treated portion of textile layers being impregnated with a conductive polymer and forming a conductive path between said first and second conductive yarn.

In a second aspect, the invention relates to a method to realize a pressure sensor including the steps of:

Providing a single non-conductive textile layer defining a first and a second surfaces;

Fixing a first and a second conductive yarns having in said non- conductive textile layer;

Arranging said first and said second yarns so that there is no contact among them within said non-conductive textile layer; Overlapping said first and said second yarns in a crossing point so that a portion of said non-conductive textile layer is sandwiched between said first and said second yarns; and

Impregnating a portion of said non-conductive textile layer including said sandwiched portion with a conductive polymer so that a conductive path is formed between said first and second conductive yarns. The non-conductive textile material of the invention includes a base fiber material which is selected among the fibers which are textile fibers, i.e. fibers used to create textiles. In the context of the present invention, as textile fiber it is intended a unit in which many complicated textile structures are built up. A textile fiber is suitable for making a fabric or cloth, woven or non-woven.

Preferably, the non-conductive textile layer of the invention is made by a base fiber material which belongs to the class of natural fibers, which include those produced by plants, animals, and geological processes, or to a sub class of man-made fibers, the regenerated fibers from natural cellulose, the mineral fibers such as fiberglass or carbon fibers. Alternatively, the textile layer is made by synthetic fibers. It can additionally include a mixture of the above mentioned fibers.

The group of natural fibers includes as sub categories vegetable fibers, which are generally based on arrangements of cellulose: examples include cotton, hemp, jute, flax, ramie, and sisal. Animal fibers consist largely of particular proteins; possible examples are silk, wool and hair such as cashmere, mohair and angora, fur, etc. Mineral fibers comprise asbestos. Synthetic fibers comprise nylon, polyester, etc.

Preferably, the non-conductive textile layer includes a base fiber material which comprises cellulose, regardless whether it is a natural or a regenerated or a mineral fiber. The base fabrics, yarns or fibers can be made from standard textile materials like cotton, polyester, polyamide, spandex, silk, wool and their blends.

Even more preferably, the non-conductive textile layer includes a base fiber material which comprises cotton.

A non-conductive textile layer formed by any of the above mentioned fibers is a flexible material consisting of a network of fibers belonging to the above mentioned groups, often referred to as thread or yarn. Yarn is produced by spinning raw fibers to produce long strands.

Textiles, as the non-conductive textile layer present in the sensor of the invention, are formed by weaving, knitting, crocheting, knotting, or pressing fibers together, non-woven fabrics are also included. Any network of fibers is therefore included in the present invention.

In any case, preferably the non-conductive textile layer includes a base fiber which is either a single fiber or a yarn, i.e. it has an elongated structure along one direction.

The textile layer which functions as a "substrate" for the construction of the pressure sensor is not-conductive.

The non-conductive textile layer defines two opposite surfaces, a first and a second surface which are on the two opposite sides of the non-conductive textile layer.

The non-conductive textile layer, although flexible so that it can have any shape due to bending and folding, may define a plane when extended on a surface or additional substrate. Such a plane, called in the following the (X, Y) plane, is for description purposes the reference plane for the positioning of the various elements on the non-conductive textile layer. It is to be understood that in the normal usage of the sensor, the non-conductive textile layer may not keep a planar form but may be deformed, for example it can adapt and follow the contour of a body (such as a human body) wearing a garment including the sensor of the invention.

Furthermore, on the non-conductive textile layer at least a first and a second yarns are fixed. These yarns are conductive and preferably intertwined into the non-conductive textile layer. The "highly conductive yarns" of the invention can be also made of the base fibers above described and listed. The difference is that the base fibers included in each of the conductive yarn are then treated so that they become conductive, while the base fibers forming the non-conductive textile layer remain non-conductive.

In order to render such yarns conductive, according to an embodiment of the invention, they can include nanoparticles. As an example, nanoparticles can be deposited on the external surface of the base fiber material. More preferably, the deposition is substantially uniform. In more details, the nanoparticles form a "layer with holes" i.e. the nanoparticles do not form a continuous layer in which all nanoparticles are in contact with each other, but they present an average distance of 1 nm - 200 nm, however, the deposited nanoparticles have preferably substantially a uniform thickness, which means that when nanoparticles are present on the surface of the base fiber all these "clusters" of local presence have the same height. The "layer with holes" thickness is preferably comprised between 5 nm - 50 nm. The nanoparticles include one or more metal or metal oxide. Preferred metals are those of Groups IV-XII, more preferred those of Groups XI, even more preferred Au and Ag. Examples of metal oxides are ZnO, Ti0₂, SnO.

The process of the treatment of the base fiber material in order to deposit the nanoparticles and the deposition of the nanoparticles on the base fiber material is preferably made according to the disclosure of the article H. Dong, J. P. Hinestroza, ACS Appl. Mater. Inter, 2009, 1 , 797, more in detail according to the teaching of the patent applications WO2009129410 and WO2010120531 , the teaching of which is hereby incorporated by reference.

However, the deposition of the nanoparticles on the surface of the base fiber material can be realized for example using the method disclosed in US2006278534. Preferably this deposition is used for non-cellulosic base fiber materials.

In addition, the conductive yarns include the fiber material and a conductive polymer layer which is deposited on top of the nanoparticles. In particular, the base fiber material on which the nanoparticles have been deposited undergoes a second deposition process, preferably a conformal coating of a conductive polymer. Applicants have found that the presence of two materials, the nanoparticles and a conductive polymer's layer provides a synergistic effect that enhances the conductivity of the modified fiber material by at least one order of magnitude compared to specimens that were merely coated with the conductive polymer or on which only the nanoparticles are deposited. The process of deposition is disclosed in WO2012120006. Alternatively, the conductive yarns include a base fiber without nanoparticles, but only including the conductive polymer deposited on the base fiber. The conductive polymer is deposited on top of the yarn which becomes the conductive yarn. Preferred conductive polymers are comprised in the classes of polythiophenes, polypyrroles and polyacetylenes.

Alternatively, the yarn is rendered conductive by a metal coating deposited on top of it. Preferably, the yarn is coated with a silver layer.

The pressure sensor of the invention includes at least two conductive yarns which are fixed, for example are intertwined, e.g. sewn or embroidered, on the textile layer. The fixing, such as the sewing or embroidering, follows a given pattern so that the two conductive yarns never come into contact to each other in the area which "works" as a pressure sensor. In other words, there is no "closure" of a circuit due to a contact between the two conductive yarns. On the contrary, there is always a gap between them, gap either in the (X, Y) plane (i.e. for some portions the first and second yarns are both laying on the same surface but spaced apart) or perpendicularly to it (the first and second yarns are facing for some portions opposite surfaces of the textile layer). The yarns thus may lay alternatively facing the first or facing the second surface of the non-conductive textile layer. For example, the first yarn for some of its portions faces the first layer and for some other of its portions faces the second surface, depending on the specific pattern.

Alternatively, instead of sewing or embroidering, the yarns can be fixed onto the textile layer by other means, for example locally gluing the yarns. However, they are not fixed to additional layer(s) and then onto the non-conductive textile layer.

The conductive yarns may be everywhere in contact with the first and/or second surface or raised at least for some portion(s) from the first and/or the second surfaces, depending on the pattern formed and/or on the type of intertwining and/or on the tightening of the sewing/embroidering of the first and/or second yarn.

In this way, a pattern at the first surface and a pattern at the second surface of the non-conductive textile layer are made. Alternatively, the first yarn faces the first surface only and the second yarn faces the second surface only of the non-conductive textile layer.

Moreover, in at least one location, the two conductive yarns overlap. Although not in direct contact, in at least a portion of the non-conductive textile layer, the first conductive yarn and the second conductive yarn are positioned at corresponding areas of the first and second surfaces of the textile layer, so that a portion of the non-conductive textile layer is sandwiched between the first and the second conductive yarns. In other words, there is at least a location within the non- conductive textile layer in which the two yarns superimpose. In this location of superposition, the first yarn is facing a first area of the first surface and the second yarn is facing a second area of the second surface of the textile layer and the first area and the second area are corresponding to each other, i.e. these two first and second areas of the first and second surface correspond to the two opposite sides of the same portion of textile. This portion having the first and second area as opposite sides is called in the following portion of overlap or sandwiched portion. The two conductive yarns in the overlapping portion substantially have the same (X, Y) coordinates in the (X, Y) plane at least locally defined by the non-conductive textile layer.

Furthermore, a portion of the non-conductive textile layer, portion that includes the sandwiched portion of textile layer above defined, is treated using a conductive polymer. The conductive polymer is substantially embedded in the textile, i.e. it does not form a uniform layer on top of the textile, but it enters within the fibers itself forming the textile, impregnating the textile layer. In other words, the conductive polymer penetrates within the textile layer in such a way that a conductive path, preferably a continuous conductive path, is formed between the first yarn located on or facing one surface and the second yarn located on or facing the other surface of the textile layer. Thus, the conductive polymer is present in the first surface, in the second surface and in between.

Preferably, this treated portion of the non-conductive textile layer is wider than the overlapping portion.

As mentioned, according to the present invention, a conductive polymer is used in order to treat this portion. There is virtually no limits on the conductive polymer suitable for the purposes of the present invention; preferably the surface resistivity of the textile layer treated with such a polymer is between 100 k/ sq and 1 k/sq. Generally, conductive polymers are well-known in the art. Examples are polymers based on aromatic cycles, such poly(fluorine), polypyirenes, polyazulenes, polynaphthalenes, polypyrroles, polycarbazole, polyindoles, polyazepines, polyanilines, polythiophenes, poly(p- phenylene sulfide). Examples are polymers based conjugated unsaturated bonds, such as polyacetylenes or combinations of aromatic cycles and conjugated unsaturated bonds, such as poly(p-phenylene vinylenes). Preferred polymers are poly(3,4- ethylenedioxythiophene) poly(styrene sulfonate (PEDOT:PSS), poly(4-(2,3- dihydrothieno[3,4-b]-[1 ,4]dioxin-2-yl-methoxy)-butanesulfonic acid) (PEDOT-S) and PEDOT:tosylate.

An even more preferred embodiment of the present invention is the conductive polymer PEDOT:PPS.

Conductive polymers are generally well-known in the art, see for example G. Inzelt "Conductive Polymers" Springer 2008.

However, the textile layer could be treated with a mixture of substances, not only with the conductive polymer.

The thickness of the conductive polymer used to treat the portion of non- conductive textile layer does not need to be uniform.

Treating a portion of non-conductive textile layer with a conductive polymer makes the treated portion conductive.

The portion to be treated can be made conductive impregnating the portion of textile layer in many different ways. According to a preferred embodiment, the treatment is performed by directly patterning the textile layer with a conductive polymer solution. The patterning can be of any dimension; however a small dimension is preferred in case many force sensing elements (defined below) are created in the non-conductive textile layer. Regardless of the method in which the conductive polymer comes into contact with the fibers forming the textile layer, the conductive polymer "enters" within the fibers.

Preferred methods of treating such a portion of non-conductive textile layer with the conductive polymer are for example: printing, such as ink-jet printing, screen printing, etc.;

dip coating in liquid phase;

vapour deposition such as chemical vapour deposition (CVD);

brushing;

dripping;

etc. ;

either manually or automatically performed.

The treated portion has the function of a force sensing element within the pressure sensor. A force sensing element is a portion of the textile layer where the pressure exerted over the textile layer can be measured. Each of such force sensing elements is a portion of the treated textile layer sandwiched between two conductive yarns. The treated portion of textile layer treated with the conductive polymer becomes piezoresistive. When pressure increases over one of such force sensing elements, both the textile layer transversal resistance and the yarn-textile contact resistance decrease which results in a high sensitivity of the overall sensor. It is important to underline that the working mechanism of the sensor is the piezoresistivity of the sandwich conductive yarn - treated portion of textile layer - conductive yarn.

Thus applying a force onto the treated portion of textile layer, where also the yarns intersect without contact, changes the piezoresistivity of the sandwich yarn-treated textile-yarn and thus allows measuring the pressure exerted by the force onto the textile layer.

In other words, it is desired to use the variation of the electrical resistance measured between the two conductive yarns facing the two opposite sides of the textile layer to estimate the pressure value. This measurement can be done by means of a dedicated electronic circuit or a resistance measuring instrument plugged at the termination of the conductive yarns by means of a connector or contact pad.

The sensor of the invention may include one or more of such force sensing elements. The more numerous, e.g. packed, they are within a given unit of area of the textile layer, the higher the spatial resolution that is achieved by the pressure sensor. Therefore, the distance and number of force sensing elements is determined on the basis of the application and specific positioning of the pressure sensor of the invention.

Many force sensing elements can be present in the pressure sensor of the invention in a relatively small area: being the conductive elements realized as yarns, the volume occupied by them is relatively very small and they can be put one close to the other easily without any contact. A dense amount of force sensing elements could be realized.

The first and/or second aspect of the invention may include, in alternative or in combination, any of the following characteristics.

Preferably, said first and/or second conductive yarns have a linear resistivity lower than 30 /cm.

For a proper measuring, preferably the yarns are "highly conductive".

In a preferred embodiment, said first and/or conductive yarns include a metal coated yarn or a yarn including metal nano-particles.

As mentioned, the yarns can be rendered conductive by metal, either in form of nanoparticles or of a coating. The preferred metal is for example Silver.

Alternatively or in addition, said first and/or second conductive yarns include a yarn treated with a conductive polymer.

A different or additional approach for rendering the yarn conductive is to apply a layer of conductive polymer. The yarn can be soaked or otherwise drenched with such conductive polymer which penetrates into the fibers forming the yarn(s).

Advantageously, said treated portion includes a portion of textile layer having an area defined in said first and/or second surface larger than 1 mm².

The size of the area which is treated so as to form the force sensing element of the sensor of the invention is larger than 1 mm². Smaller sizes are difficult to achieve not only because of the small area of the fabric to treat but also because it implies the use of very thin yarns that could be very problematic in intertwining, such as for sewing with conventional machines.

Preferably, the distance between said first and said second conductive yarns in said sandwiched portion is larger than 100 μιτι. As mentioned, the first and second conductive yarns do not come into contact. In the sandwiched portion of textile, the two yarns overlap and there is always a gap between them in a direction substantially perpendicular to the plane (X, Y) defined by the textile layer. Gaps smaller than 100 μιτι may cause unwanted short circuits, i.e. unwanted contacts between the two yarns, for example due to the fibers themselves which might be "woolly" so that fibers of two different yarns can easily come into contact to each other if there is not enough distance among them. The preferred minimal distance depends on the type of fibers forming the yarn which has been used.

Preferably, said non-conductive textile layer has a surface resistivity larger than 10⁶ /sq .

It is preferred to have a very low conductivity of the textile layer so that the forcing sensing elements, in case more than one is present, are substantially independent one from the others, i.e. the detection made by one element does not affect the other(s).

Preferably, said conductive polymer is PEDOT:PSS.

In a preferred embodiment, said non-conductive textile layer includes natural fibers.

In a preferred advantageous embodiment of the invention, the present sensor is incorporated in garments for everyday use. Therefore, it is preferably realized on a substrate, such as the textile layer, made of the same fibers of the garment.

More preferably, said natural fibers include cotton.

Cotton is one of the preferred fibers for garments; it is relatively cheap and easy to treat. Moreover, Applicants have found that the various treatments applied on the textile layer are very well controlled if the textile layer is made of cotton. Not least, the desired conductivity is easily achieved.

In an advantageous embodiment, the pressure sensor includes N conductive yarns sewed or embroidered in said textile layer which do not contact each other, arranged in a matrix configuration and forming a plurality of crossing points so that a plurality of portions of said textile layer are sandwiched between two different yarns of said N conductive yarns. In order to have a good spatial resolution of the force applied onto the textile layer, a plurality of force sensing elements is included in the same pressure sensor. Including a plurality of force sensing elements allows having a spatial resolution of the pressure applied onto the sensor, so that a force field can be analysed. The higher the number of force sensing elements in the same sensor, the higher spatial resolution is achieved.

More preferably, a distance between two different crossing points is larger than 1 mm.

A smaller distance would cause an overlap of the treated portions.

Advantageously, the pressure sensor includes one or more additional layer(s) deposited or attached on top of said non-conductive textile layer and said first and second yarns.

The additional layers can have any function. An additional layer could be a protective layer to protect the pressure sensor from any element from the outside, or an insulating layer to better insulate the pressure sensor, or a coupling layer to better mechanically couple the pressure sensor to additional elements, or a sealing layer to protect the pressure sensor from humidity, etc. The material of this/these additional layer(s) depend(s) on the specific function of the layer(s) itself/themselves.

In said second aspect, advantageously said step of treating a portion of said textile layer includes:

- Soaking or printing or covering said portion with said conductive polymer.

In said second aspect, alternatively in a preferred embodiment, said step of treating a portion of said textile layer includes:

- Evaporating or depositing said conductive polymer onto said portion. Preferably, the deposition of the conductive polymer is made while the conductive polymer is in the liquid phase because the deposition of the conductive polymer can be controlled with high accuracy and precision and in normal ambient conditions.

According to a third aspect, the invention relates to a method to measure the pressure applied, said method including: - Providing a sensor according to the first aspect or realized according to the second aspect;

- Measuring the variation of the contact resistance at the treated portion between said first and said second conductive yarns when pressure is applied at said treated portion.

As mentioned, said pressure sensor according to the invention or realized according to the invention is used to measure the pressure applied on it using the piezoresistive effect of the force sensing element including the yarn-treated textile portion - yarn.

Brief description of the drawings

With reference to the attached drawings, further features and advantages of the present invention will be shown by means of the following detailed description of some of its preferred embodiments. According to the above description, the several features of each embodiment can be unrestrictedly and independently combined with each other in order to achieve the advantages specifically deriving from a certain combination of the same.

In the said drawings,

- fig. 1 is a schematic top view of a pressure sensor realized according to a first embodiment invention;

- fig. 2 is a schematic lateral view in cross-section of the sensor of fig. 1 ;

- fig. 3 is a schematic top view of a second embodiment of a detail of the pressure sensor realized according to the invention;

- fig. 4 is a schematic lateral view in cross-section of the detail of the pressure sensor of fig. 3;

- fig. 5 is a schematic lateral view in cross-section of the detail of the pressure sensor according to a third embodiment of the present invention;

- fig. 6 is a schematic lateral view in cross-section of the detail of the pressure sensor according to a fourth embodiment of the present invention;

- fig. 7 is a top view is a schematic top view of a fifth embodiment of a detail of the pressure sensor realized according to the invention; - fig. 8 is a schematic lateral view in cross-section of the detail of the pressure sensor of fig. 7;

- fig. 9 is a schematic lateral view in cross-section of the detail of the pressure sensor of the invention when pressure is applied on it;

- fig. 10 is a photograph of a pressure sensor realized according to the invention;

- fig. 1 1 is a graph of the sensor resistance vs. pressure; and

- fig. 12 are two graphs of the pressure (curve above) and current (curve below) vs. time.

Description of preferred embodiments of the invention

An "all-textile" pressure sensor realized according to the invention is globally indicated with 100 in the appended drawings.

The pressure sensor 100 includes a single layer of a base fabric or textile 1 . The single layer of textile includes a plurality of fibers, preferably natural fibers, which form a network among them. The textile layer 1 is shown always in a planar configuration, i.e. defining a (X, Y) plane (see for example fig. 1 and 3), however it is to be understood that this is one of the possible configurations of the textile layer, which can be bent or folded in numerous ways.

The textile layer is non-conductive.

The textile layer 1 defines a first and a second surface 7a, 7b, one opposite to the other (e.g. they are the two sides of the textile layer 1 ).

A first and a second yarns 2, 3 are fixed onto the textile layer 1 . The first and second yarn 2, 3 form patterns on the first and/or second surface 7a, 7b of the textile layer 1 in such a way that they never contact each other, i.e. there is always a gap in between. The gap can be given by a distance between the two yarns when they are located on the same surface of the textile layer, or by the presence of the textile layer itself which keeps the two yarns 2, 3 separated when they are on opposite surfaces but at the two opposite sides of the same textile portion.

In figure 1 and 2, the first yarn 2 is always on the first surface 7a, while the second yarn 3 is always on the second surface 7b. However different embodiments are possible, for example the yarns can be both for some length in the first and in the second surface. This configuration is particularly advantageous when the conductive yarns are embroidered or sewed onto the textile layer 1 as shown in figures 3 and 4.

Each of the highly conductive yarns 2, 3 used for the invention can be a metal-coated yarn, preferably a silver-coated yarn, suitable for sewing/embroidery with a linear resistance of less than 30 /cm. However for each y arn a different method to make it conductive can be used.

Furthermore, the pattern formed by the first and second yarns 2, 3 on the two surfaces 7a, 7b of the textile layer 1 is such that there is at least a portion of the layer 1 which is sandwiched between the two yarns 2, 3. In other words there is at least a portion of textile layer 1 in which the yarns 2, 3 have - for a small length - the same (X, Y) coordinates on the two opposite surfaces 7a, 7b.

The two yarns 2, 3 thus form a crossing point, but they do not contact each other because the textile layer 1 is located between them. This crossing point or sandwiched portion of textile is indicated with 5 in figure 1 and 2 and highlighted with a circle.

In addition to the first and second yarns 2, 3, many other conductive yarns could be embroidered or sewed onto the textile layer 1 , as depicted in fig. 1 . In addition, many crossing points can be formed, again as shown in figures 1 and 2.

Furthermore, other yarns, such as non-conductive yarns, could be also sewed or embroidered or otherwise fixed onto the textile layer 1 . As shown in fig. 5, non-conductive yarns 7 and 8 are also sewed onto the textile 1 . This embodiment is particularly preferred when a sewing/embroidery machine is used, because the standard stitching lines are made from two yarns, one on one side and one on the opposite side of the textile layer 1 . In this case, for the fabrication of a pressure sensor 100 according to the invention, one yarn - either the one on the first surface 7a or the one on the second surface 7b - should be a conductive yarn 2, 3, which is sewed with a non-conductive yarn 7, 8. It is preferable that the additional non-conductive yarns 7, 8 are made from cotton fibers.

The sandwiched portion of textile 5, or crossing point, is then treated so as to form a treated portion of textile layer 4. The treated portion 4 includes a conductive polymer pattern. The conductive polymer pattern on the textile 1 can be made by directly patterning the fabric with a conductive polymer solution. The base fabrics, yarns or fibers forming the textile layer can be made from standard textile materials like cotton, polyester, polyamide, spandex, silk, wool and their blends. The conductive polymer used for treating the textile layer 1 preferably belongs to the class of poly(3,4- ethylenedioxythiophene) poly(styrene sulfonate (PEDOT SS), poly(4-(2,3- dihydrothieno[3,4-b]-[1 ,4]dioxin-2-yl-methoxy)- butanesulfonic acid) (PEDOT-S) and PEDOT:tosylate.

The treated portion 4 includes the crossing point. The treated portion 4 is shown in the appended figures as a circle covering an area of the first and of the second surface 7a, 7b.

If a plurality of crossing points is present, as shown in figure 1 and 2, also a plurality of treated portions 4 is realized, one for each crossing point 5 as again shown in figures 1 and 2. The realization of a treated portion 4 creates a piezoresistive zone or force sensing element.

A single piezoresistive zone is shown in figure 3. A piezoresistive zone thus includes the two conductive yarns 2, 3 in the portion having identical (X, Y) coordinates, the textile layer 1 and the conductive polymer patterned onto the treated portion 4.

In some embodiments of the invention, the treated portion can include more than only one stitching line, i.e. more than a couple of conductive yarns 2, 3. In Figures 7 and 8, a force sensing element with three stitching lines is illustrated, where three pairs of conductive yarns are used (all indicated with 2 and 3 in the drawings).

Other embodiments of the pressure sensor 100 of the invention can include other layers in addition to the base fabric textile layer 1 and the conductive yarns 2, 3. Such additional layers can be selected to provide protection of the active layer, electrical insulation from conductive surfaces, better mechanical coupling, etc. In Figure 6, an example of a sensor 100 with additional layers 10, 1 1 formed on the first and second surfaces 7a, 7b of the textile layer 1 is shown.

In addition, in any of the above described embodiments, the textile layer 1 includes one or more pads 6. The pads 6 are used to perform a measurement of the electrical resistance between first and second conductive yarns 2, 3 to estimate the pressure value. Pad 6 is in electrical contact with first and second conductive yarns. Instead of pad 6, a connector could be used.

If pressure is applied to any of the force sensing elements, as schematically depicted in fig. 9 where arrow F indicates the force applied to the textile layer 1 , the resistance between the first and the second yarns 2, 3 changes, and such a variation can be measured by an appropriate instrument or electronic circuit 50 at the ends of the yarns (see figure 10). Any approach for measuring electrical resistance can be used, for instance a fixed voltage can be applied between the terminals of yarns 2 and 3 and, by measuring the current that flows through each of the force sensing element, the resistance can be calculated using the Ohm's law. Alternatively, a fixed current can be applied to each of the force sensing element and by measuring the resulting voltage the resistance of the force sensing element can be obtained.

Example

In the preferred embodiment, the base fabric 1 is made from 100% cotton fibers. The treated portion 4 is made by treating the fabric with the conducting polymer poly-3,4-ethylenedioxythiophene doped with poly(styrene sulfonate) (PEDOT SS). The treatment is done with an aqueous solution of PEDOT:PSS mixed with a pure poly-alcohol (such as glycerol, ethylene glycol or sorbitol) which acts as second dopant.

Fig. 10 shows an example of the invention implemented as an array of 5 x 5 sensing elements. In this case the cotton fabric is batiste type and has been treated with a solution of PEDOT:PSS (Clevios PH 500 from Heraeus, Leverkusen, Germany) with 2% of sorbitol which gives a surface resistivity between 10 k/sq and 100 k/sq. The treatment can be done manually but the use of a fabric stamping machine, a numerically-controlled drop-casting machine or an inkjet printer is preferred in order to deposit the conductive polymer specifically on the areas of interest for pressure sensing.

Yarns 2, 3 are made of nylon coated with Silver, specifically a Shieldex 1 17/17 dtex 2-ply from Statex (Bremen, Germany). Sensing elements are 10 mm apart so there is approximately one sensing element per square centimeter. Yarns 2, 3 are sewed to the base fabric 1 with a digital embroidering machine using a 100% cotton yarn n. 60 as a bottom thread. The ends of yarns 2, 3 are sewed to a printed circuit board (PCB) which has a flat cable connector to bring the signals to a measuring circuit.

In fig. 1 1 the calibration curve of a typical sensor of the example illustrated in fig. 1 0 is shown. It can be seen that sensor resistance changes from hundreds of kilo-ohms when pressure is close to zero to tens of kilo-ohms when pressure increases near to 1 000 kPa which cover the range of pressure produced in human-body interactions (sitting, laying, leaning against something) and static and dynamic foot pressures. It's important to note that the response of the sensor in terms of the resistance (R) is not linear with pressure (P) but shows a behavior approximately of the kind:

R K ~ .

p

This behavior is due because the physical phenomenon behind the piezoresistivity of the sensor is more likely the resistance of electrically-conductive contacting surfaces. In this case the fibers of the yarns 2,3 and the fibers of the treated portion 4 of the base fabric become in an increasingly close contact as the pressure increases. It has been found that the total contact resistance R can be approximated as follow:

p ~ _ P v

~ F

where is the resistivity of the contacting surfaces (fibers of yarns 2, 3 and base fabric 1 ), F is the force resultant from the pressure and applied normal to the contact surfaces and K \s a function of the roughness and elastic properties of the surfaces (fibers). It is clear that for a constant set of surface roughness and elastic properties K, the lower the resistivity of the contacting fibers (in particu lar the treated base fabric fibers) the bigger the variation of the total resistance R for a given force (pressure) F. Thus, by tuning the conductivity obtained with the polymeric treatment, for instance by selecting a less conductive dispersion of PEDOT:PSS or by using a different second dopant, it is possible to make the textile pressure sensor of the invention more or less sensitive in a given range of pressures. The difficulty of treating with the non-linearity of the sensor can be reduced by measuring the current / that flows through the sensor when a constant voltage V is applied and using that current as the sensor output. This is because by the Ohm's law / = V/R, therefore / is proportional P. In other words, the current that flows through the sensor is proportional to the pressure P applied therefore it increases more or less linearly with the increase of pressure and can be used as sensor output.

Fig. 12 shows a typical time response of a textile pressure sensor (one single force sensing element is considered) in terms of current flowing when a fixed voltage of 1 V is applied between yarns 2, 3.

Claims

1 . A pressure sensor (100) including:

- A single non-conductive textile layer (1 ) defining a first and a second surface (7a, 7b);

- A first and a second conductive yarns (2, 3) fixed to said non-conductive textile layer (1 ), said first and second yarns being so arranged that there is no contact among said first and said second yarns (2, 3) within said non- conductive textile layer (1 ) and that they overlap in a crossing point (5) so that a portion of said non-conductive textile layer (1 ) is sandwiched between said first and said second yarns (2, 3);

- Said non-conductive textile layer (1 ) including a treated portion (4) comprising said sandwiched portion (5), said treated portion (4) of textile layers being impregnated with a conductive polymer and forming a conductive path between said first and second conductive yarns (2, 3).

2. The pressure sensor (100) according to claim 1 , wherein said first and second yarns (2, 3) are sewed or embroidered onto said non-conductive textile layer (1 ).

3. The pressure sensor (100) according to claim 1 or 2, wherein said first and/or second conductive yarns (2, 3) have a linear resistivity lower than 30 /cm .

4. The pressure sensor (100) according to any of the preceding claims, wherein said first and/or second conductive yarns (2, 3) include a metal coated yarn or a yarn including metal nano-particles.

5. The pressure sensor (100) according to any of the preceding claims, wherein said first and/or second conductive yarns (2, 3) include a yarn treated with a conductive polymer.

6. The pressure sensor (100) according to any of the preceding claims, wherein said treated portion (4) includes a portion of textile layer (1 ) having an area defined in said first and/or second surface larger than 1 mm².

7. The pressure sensor (100) according to any of the preceding claims, wherein the distance between said first and said second conductive yarns (2, 3) in said sandwiched portion (4) is larger than 100 μιτι.

8. The pressure sensor (100) according to any of the preceding claims, wherein said non-conductive textile layer (1 ) has a surface resistivity larger than 10⁶ /sq .

9. The pressure sensor (100) according to any of the preceding claims, wherein said conductive polymer is any of the following: poly(3,4- ethylenedioxythiophene) poly(styrene sulfonate (PEDOT:PSS), poly(4-(2,3- dihydrothieno[3,4-b]-[1 ,4]dioxin-2-yl-methoxy)-butanesulfonic acid) (PEDOT- S) and PEDOT:tosylate.

10. The pressure sensor (100) according to any of the preceding claims, wherein said non-conductive textile layer (1 ) includes natural fibers.

1 1 . The pressure sensor (100) according to claim 10, wherein said natural fibers include cotton.

12. The pressure sensor (100) according to any of the preceding claims, including N conductive yarns (2, 3, 7, 8) sewed or embroidered in said non- conductive textile layer (1 ) which do not contact each other, arranged in a matrix configuration and forming a plurality of crossing points (5) so that a plurality of portions of said non-conductive textile layer (1 ) are sandwiched between two different yarns (2, 3, 7, 8) of said N conductive yarns.

13. The pressure sensor (100) according to claim 12, wherein the distance between two different crossing points is larger than 1 mm.

14. The pressure sensor (100) according to any of the preceding claims, including one or more additional layer(s) (10, 1 1 ) deposited or attached on top of said non-conductive textile layer (1 ) and said first and second yarns (2, 3).

15. A method to realize a pressure sensor (100) including the steps of:

- Providing a single non-conductive textile layer (1 ) defining a first and a second surface (7a, 7b);

- Fixing a first and a second conductive yarn (2, 3) to said non-conductive textile layer (1 );

- Arranging said first and said second yarns (2, 3) so that there is no contact among them within said non-conductive textile layer (1 ); - Overlapping said first and said second yarns (2, 3) in a crossing point (5) so that a portion of said non-conductive textile layer (1 ) is sandwiched between said first and said second yarns (2, 3);

- Impregnating a portion (4) of said non-conductive textile layer (1 ) including said sandwiched portion with a conductive polymer, so that a conductive path is formed between said first and second conductive yarns (2, 3).

16. The method according to claim 15, wherein said step of impregnating a portion (4) of said textile layer (1 ) includes:

- Soaking or printing or covering said portion (4) with said conductive polymer.

17. The method according to claim 15, wherein said step of impregnating a portion (4) of said textile layer (1 ) includes:

- Evaporating or depositing said conductive polymer onto said portion (4).

18. The method according to any of claims 1 5-17, wherein said step of fixing a first and a second conductive yarns (2, 3) to said non-conductive textile layer (1 ) includes:

- Sewing or embroidering said first and/or second conductive yarns (2, 3) to said non-conductive textile layer (1 ).

19. A method to measure the pressure applied to a sensor (100), said method including:

- Providing a sensor (100) according to claims 1 -14 or realized according to claims 15-18;

- Measuring the variation of the contact resistance at the treated portion (4) between said first and said second conductive yarns (2, 3) when pressure is applied at said treated portion (4).