POROUS MATERIAL THUS OBTAINED AND USES THEREOF

DESCRIPTION

The present invention relates to a process of functionalization of a porous material having resilience or being resilient, such as a natural or synthetic sponge or a porous polymer, implemented by means of applying a continuous conductive polymeric film thereby obtaining a functionalized porous material. Within the present description under the term resilience the capacity of the material, after deformation, to re-assume the original shape is meant. Such application is capable of modifying the surface properties thereof for the whole functionalized porous structure, by making it conductive without modifying the elasticity, flexibility and fluid-absorption features of the material itself.

State of art

The invention belongs to the field of the sensoristics made of integrable materials, allowing to make it active materials which were passive (smart materials, active materials, internet of things) without distorting the structure and functionality thereof. Such devices have to be convenient, not expensive, durable and stable. Typically the traditional sensors are made of materials used in electronics, semiconductors, and they have not a direct mechanical integration with the different materials of which the products are constituted. The integrated sensors made of materials of common use represent a technical field in rapid evolution for the properties and the use possibilities thereof. However, the devices known from the state of art show a series of disadvantages thereamong the main one is the fact of not being actually integrable in the materials and processes, thereby not making simple, cheap and industriazable the process of the implementation thereof.

WO2012120006 Conductive Fiber Materials discloses applications of conductive polymers on textile fibre with a functionalization process similar for uses in the electronics and sensoristics, however the used textile fibre, for the anisotropic nature thereof, has not the elasticity requirements in the three sizes and it has a very limited elastic linear deformation which jeopardizes the sensor performances. This limits the use thereof in the implementation of pressure sensors and other applications involving elasticity and then reversibility of the deformation. On the contrary, the present invention exploits the elasticity and anisotropy properties of the sponge-like structure and it combines them with the electric properties of the polymer. WO2015014950 Textile pressure sensor and method for fabricating the same discloses, in a way similar to the previous case, a pressure sensor implemented by means of the contact between two textile fibres. This application has still the disadvantage that the textile fibres have a limited elasticity and have no reversible and repeatable compression and stretching. The present invention differs as it uses a sponge-like porous polymer and it exploits the elasticity and the complete reversibility thereof, and it is not based upon the simple chance contact between different fibres, which cannot be wholly controlled nor it is reversible.

Adv. Mater. 2013, 25, 6692-6698 discloses a conductive sponge implemented with a graphene nanostructure based process. This implementation has an intrinsic limit as the nanostructures have a discreet structure by their very nature: the conductive layer results to be formed by discreet elements which have adhesion problems. The possible detachment of nanostructured material further involves possible biotoxicity problems by considering that it deeply penetrates the epidermis and the human tissues of the nanostructures. At last the costs for the process for preparing the carbon-based nanostructures are high. These drawbacks are overcome in the present invention by the use of a cheap conductive polymer capable of forming a uniform thin film over the whole surface by giving features of greater uniformity and adhesion. Moreover, the sponge functionalized with determined conductive polymers has the property of reducing the conductivity thereof in presence of positive ions, by making implementable applications of electrochemical transistor type and sensoristics in liquid.

Therefore it was felt the need in the state of art of making available porous and resilient conductive materials (hereinafter sometimes designated as sponges) not having the disadvantages of the state of art and being suitable to be used in sensoristics in a more advantageous way than those known by the state of the art.

The electrochemical transistor structure is known for devices implemented on plastics, silicon glass or tissue, but not that on sponge allowing to adjust in optimum way the absorption of fluids and it allows to integrate the device in an additional class of materials.

Therefore, an object of the present invention is a process of functionalization of a natural or synthetic sponge or a porous polymer implemented by applying a continuous conductive polymeric film, the functionalized porous material obtainable from such process and the use thereof in devices in sensoristics, a matrix of different sensors implemented on resilient material, connected to a control electronics, so as to make the material sensible (artificial skin)

Brief description of the figures

Four drawings are enclosed to the present description, showing: In figure 1 : a sponge produced according to the process of the invention

In figure 2: a pressure sensor comprising a conductive porous material (sponge) obtained according to the process of the present invention. In figure a sponge is represented having a conductive portion (represented as the dark area in the centre) which could cover even the whole sponge or only a portion thereof as in figure. In the upper and lower portion of the conductive sponge there are two contacts extending on the active area of the pressure sensor. A potential difference is applied to the electrodes and the current Ids is measured with an amperometer represented on the right. Upon squeezing the sponge, the pores collapse by putting in contact the conductive walls and by increasing the current proportionally to the increase in the active area of the device and then to the pressure.

Figure 3 shows the response of the sensor shown in figure 1 and 2 with the response in current versus time (on the top), subjected to a pressure with steps increasing from 500 g/cm2 to 6 Kg/cm2 with step of 500 g/cm2. Figure 3 (on the bottom) shows the graph of the sensor signal in current versus the exerted pressure.

Figure 4 shows the scheme of a transistor organic electrochemical sensor based upon the the conductive sponge, capable of analyzing fluids and determining the content of salts and ionic species thereof. At the centre a sponge with a (darker) conductive portion on the right is shown. Two electrodes are present above and below, thereto a potential difference and a current measurer are applied. A third electrode (gate) is applied in the not conductive area of the sponge, on the left. The liquid to be analyzed is absorbed by the sponge and the measurement can be performed when the gate electrode comes in contact, through the liquid, with the conductive sponge. A positive potential is applied to the gate electrode, which potential moves the ions towards the conductive polymer. If the conductive polymer is of PEDOT:PSS type the presence of positive ions reduces the conductivity thereof, by modulating the Ids current flowing therethrough. Since the current decrease is proportional to the number of ions, the device can be used as sensor in the different ionic species in the liquid. Description of the invention

The present invention relates to a process of functionalization of a resilient porous material, such as a natural or synthetic sponge or a porous polymer, implemented through the application of a continuous and conductive polymeric film. Such application is capable of modifying the surface properties thereof by protecting the porous structure of the sponge and by making it conductive without modifying the features thereof of elasticity, flexibility and absorption of the polymer itself. In this way, the surface electric properties of the material are modified, by making it from insulating to conductive or semiconductive.

The process of the present invention is described as follows.

The conductive polymer which will have to be applied on the porous material (on the sponge) is selected from the class consisting of poly- oligo- thiophenes such as poly(3,4-ethylendioxythiophenes) (PEDOT); poly(pyrroles) (PPY), polycarbazoles, polyindoles, polyazepine, polyaniline (PANI) polyacetylenes (PAC), poly(p- phenylene vinylene) (PPV)), polymers additioned with conductive micro or nano particles, with argentum and/or gold, with carbon nanotubes, with graphene and/or graphite. Such compounds can be doped according to techniques known to the state of art to modify the conductivity thereof. Advantageously, doped compounds are used such as those belonging to the following classes: PEDOT: PSS (poly(3,4- ethylenedioxythiophene : polystyrene sulfonate) in case doped with ethylene glycol and dodecylbenzenesulfonic acid (DBSA); PEDOT OS poly(3,4- ethylenedioxythiophene : tyosilate); PEDOT :PPS poly(3,4-ethylenedioxythiophene : poly(p-phenylene sulfide. Advantageously PEDOT: PSS is used which is dissolved in aqueous solution (1 % by weight), additioned with ethylene glycol by 13 - 17, preferably 15% in volume and DBSA by 0.5 - 1.5, preferably 1 % in volume. a. The resilient porous material (or sponge) which should be functionalized is selected from the class consisting of natural sponges, artificial sponges, porous polymers, low density polyethers, derivatives of polyvinyl alcohol (PVA), polyesters, polyurethanes, polypropylenes, microfiber and microporous material and generally materials which should have resiliency. The selected material is dipped into the aqueous solution obtained from stage a) and pressed so as to allow the absorption of the liquid and the contact of the solution with the whole outer and inner (pores) surface of the porous material until obtaining a complete and uniform impregnation. b. Once impregnated, the sponge is extracted from the solution, made it to return to its original shape, the excess liquid is drained and it is subjected to a thermal treatment, with different modes depending upon the film thickness which one wants to obtain, substantially by passing through an oven, vacuum or both. Typically the functionalized porous material with the conductive polymer is subjected to a thermal treatment at temperatures in the range from

40°C to 140° C for a time period between 10 and 75 minutes.

This treatment allows to dry up the solvent and at the same time to make the conductive polymer to adhere to the surface of the porous material. Consequently, the resilient porous material is made conductive through the implementation of a thin conductive film with few hundreds of nanometers, extended on the whole outer and inner surface of the porous material, including that of all pores.

The conductive polymer can be put in contact by means of metallic pastes (argentum paste) or conductive gels, in order to connect it to possible electrical cables and carry out an electrical contact, connecting the surface of the functionalized porous material. By putting in contact two sides of the sponge the measurement of the current passing through the sponge is performed. The spongy material thus is made electrically active, even if it remains deformable and elastic, and it can be the starting material for the production of devices wholly integrated in a product which has a lot of industrial applications. The spongy material made conductive will be even designated as "conductive sponge".

The elastic structure of the material does not damage the conductive film even if it is subjected to stress, thus it is possible to implement specific devices exploiting the elastic, conductive and fluid-absorbing properties, yet remaining integrated and cheap. In this way the sponge is made conductive through the implementation of a continuous thin film with few hundreds of nanometers on the whole sponge surface, including the inner one of all pores. The spongy material thus is made electrically active and sensible to the ions dissolved in water.

The devices implemented starting from the conductive sponge according to the invention form an additional object of the invention.

An example of polymeric material is PEDOT:PSS, obtained by treating the sponge as described previously, by dipping it into aqueous solution of PEDOT:PSS and then by performing a treatment at 130° for 60 minutes, a film thickness of 200 nm is obtained which implements the conductive sponge. Such functionalization keeps flexibility and reversibility capable of implementing devices for measuring the pressure or integrated electro-chemical organic sensors.

Examples of application of the conductive polymeric sponge:

Example 1 Pressure Sensor

The first shown example of device is a pressure sensor of resistometric type. The sensor is implemented simply by a conductive sponge portion with determined volume and by two contacts placed on opposite sides of the sponge. By applying a potential difference to the conductive sponge through the two contacts, there is a current passage determined by the sponge geometry and porosity and, obviously, by the conductive polymer resistivity. By deforming the conductive sponge, the air outgoes and the inner walls of the pores collapse, the ones onto the other ones, by multiplying the contact surface, by decreasing the series resistance and then by increasing the current passing through the sponge. The more the sponge is compressed the more the current signal increases, until saturating when all porous cavities are squeezed and the material results to be compact. When the sponge is released and the pressure decreases, the cavities open again, the electrical resistance increases and the current decreases again, by making the process perfectly reversible. This sensor is cheap and it can be easily integrated in already existing objects.

By means of very simple current-reading electronics and in case data transmission electronics, sensors can be integrated in a lot of products. Such sensors make the active products capable of detecting and transmitting information about the deformation status thereof.

Application examples can be: a plantar which reads a person's weight and sees how it is distributed on the shoe, or a padding which reads the posture of the seated or sleeping person, a moquette which detects the presence of objects or people thereon. Such sensors make active the products.

By referring to figure 2, a pressure sensor is shown comprising a. a porous resilient material 1 , advantageously a sponge, thereon a conductive porous resilient material 2 is placed partially or totally overlapped, which can be obtained according to the process of the invention. Two electrodes 3,4, arranged inside a circuit 6, are connected to said conductive material 2. Such circuit 6, even has a potential difference generator 5 capable of generating inside the circuit 6, a current having intensity i ¹ , the circuit 6 having a ground connection 6bis, and a device 7 for measuring current intensity, advantageously a amperometer. An increase in pressure on the material 2 causes a squeezing of the same material (sponge) with consequent contact between the pores coated with conductive polymer and an increase in the current intensity inside the circuit 6 with consequent increase in the current intensity. Such intensity passes from a value i ¹ to a value i ² > i ¹ and this increase detected by the device 7 can be correlated proportionally to the increase in pressure on said material 2 and it allows a weight measurement.

Example 1 a

A plantar is described which reads the person's weight and analyzes how it is distributed on the shoe. A plantar of this type is implemented simply by functionalizing a common polymeric plantar, in porous or foam and resilient polymer (polypropylene, polyurethane), in determined selected points. In particular in the points of the sole whereon the foot is resting and one wants to detect the pressure exerted by the body weight (heel, toes, sole) a drop of conductive polymeric solution is deposited. The solution is deposited so that only the area of interest of that point (for example on the heel) is functionalized. Subsequently it is treated thermally according to the previous instructions for fastening the conductive polymer. By putting in contact the upper portion and the lower portion of the sole in the functionalized point a sensor integrated in the sole is obtained. By measuring the current passing through the sole in the designated points, through an integrated electronics, a discreet network of sensors integrated in a continuous material is obtained. The operation of the sensor is the same: when the body weight squeezes the sole in the several points, the current increases at higher pressures, due to the squeezing of the interstices of the sponge which increase the contact area. The logic is to implement the sensor by functionalizing the spongy material portion which one wants to measure and to put it in contact separately. It is possible to mould a series of contact tracks on the lower and upper portion of the sponge with a conductive ink based upon argentum or other conductive materials. The contact on the lower portion arrives until the sponge sole is functionalized for a determined area (for example 1 cm ² ), whereas on the upper portion a second moulded contact can be found. Both contacts bring the signal outside the sole. When the sponge is squeezed in the point wherein there is functional material in the area of 1 cm2 there is a current variation between the contacts. In this way it is possible implementing a simple, cheap and functional device.

Example 1 b

A mattress or a sofa are described which read the posture of the seated or sleeping person. As in the preceding case a layer of continuous porous material is inserted, which can even already belong to the mattress. Such porous material is functionalized with the solution only in the points wherein one wishes to detect the pressure. The conductive polymer is then fastened with the thermal treatment described previously and put in contact, thus by implementing a discreet network of sensors in the mattress. Such sensors can monitor, in real time, the position of a patient in bed by determining the correctness of his/her posture and by reconstructing the sleep phases.

Example 1 c

A moquette is described which reads the presence of objects or people thereon. It requires a polymeric porous layer wherein the sensors are to be inserted by simply functionalizing points of interest and as previously.

Example 1 d

Artificial skin: a matrix is described formed by different sensors implemented by functionalizing different points of a porous resilient material (like sponge) and connected through separate contacts and a control electronics which measures the current passage thereof in each one of them. Such material applied as a continuous more or less thin layer can cover different products or objects, by constituting a skin sensible to touch (to the pressure variations). The artificial skin can be used in robotics, on artificial limbs, prostheses or objects to make them active from the tactile point of view.

Example 2 Integrated electrochemical sensor:

The conductive sponge 9 of the present invention is functionalized with a conductive polymer (PEDOT:PSS) which if dipped into the liquid can vary its conductivity proportionally to the concentration of positive ions existing in the solution. Thanks to this property the material can be used even as electrochemical sensor, as described hereinafter. The electrochemical transistor is based upon the properties of absorbing liquids by the conductive sponge and upon the capability of varying its conductivity upon doping the conductive polymer by the positive ions existing in the solution. The active channel of the transistor is constituted by the conductive sponge 9 placed between two metal contacts 10 and 1 1 ; even a not conductive sponge 8 is provided, totally or partially coupled with the sponge 9. Through the application of a fixed potential 12 a current through the conductive sponge i ¹ is measured. By putting the sponge in contact with a liquid in presence of ions in solution (ionic liquid introduced through the inlet area 16 and by putting the liquid in contact with a third electrode 15 acting as gate (other conductive sponge, metal electrodes, Ag, Pt or other conducting material), a transistor is implemented. By referring to figure 4, a potential difference generator 12 is shown placed inside a first circuit 14 comprising the electrodes 10 and 11 through which a current having intensity i ¹ runs, and having a ground connection 14bis. A device (13) for measuring the current intensity inserted into said circuit 14. Moreover, a third gate electrode 15 is applied to the not conductive spongy material 8 and inserted into a second circuit (17) comprising a potential difference generator (18) and a voltmeter (19).

By applying a positive potential to the gate electrode 15 the positive ions are pushed towards the conductive polymer in the sponge, which reduces its conductivity proportionally to the quantity of incoming positive ions. This is a circuit element of transistor type integrated in the material, useful for a possible integrated electronics, or it is a sensor measuring the concentration of ionic species in the liquid absorbed by the sponge. Subsequent functionalizations of the conductive polymer on the sponge can make the sensor selective to different ionic species. Such sensors can be used to make active products such as cushion beds, which can signal the presence and the concentration of fluids inside thereof. This device is an integrated electrochemical transistor sensor, based upon the properties of absorbing liquids by the conductive sponge and upon the capability of varying the conductivity upon doping positive ions by the conductive polymer.

Example 2a

A wrist sponge is described for reading the saline concentration dissolved in the human sweat. In this case the sponge is functionalized in one or more points with two electrodes for measuring the current and a third gate electrode for modulating the quantity of ions in the polymer. When the sweat reaches the sponge, based upon the salt content thereof, it modifies the conductivity of the polymer acting as sensor of the sweat salinity. This, with a combined electronics, allows to monitor the person's physiological status by monitoring his/her dehydration during sport or rehabilitation activities. Furthermore, by specifically modulating the sizes of the pores in the sponge it is possible to favour the sweat inflow towards the active area of the device by minimizing the liquid quantity necessary for the analysis.

Advantages of the invention · Possibility of exploiting the wide elastic deformation regime of the sponge combined with the polymer conductivity

• For the pressure sensors there is the advantage of being wholly integrated in the material, easy to be implemented, cheap and possible distribution in different portions of the product to be monitored. · For the electrochemical sensor there is the possibility of integrating in products a sensor which can determine different properties of fluids (not limited to the sweat salinity), in a direct and wholly integrated simple manner.

• The conductive polymer is also biocompatible and suitable to biological and biomedical applications.