Piezoresistive ink, methods and uses thereof

Abstract

The present disclosure relates to a piezoresistive ink composition for sensors production. This ink, change linearly their electrical resistivity with an applied deformation and can easily recover when the external applied stress is released. The composition comprises flexible polymers as thermoplastic elastomers from the styrene-butadiene-styrene family (SBS, SEBS or others), nanostructures of carbon or metal, polar solvents and dispersive agents. With this ink, the user can print the sensor with any desired geometry and use different printing techniques, including drop casting, spray, screen and inkjet printing.

Claims

1. A printable piezoresistive ink comprising: .[.30-90%.]. .Iadd.2.5-30% .Iaddend.wt/wt ink of a thermoplastic elastomer copolymer selected from the group consisting of styrene-butadiene-styrene (SBS) and styrene ethylene butylene styrene (SEBS) and mixtures thereof; a filler comprising conductive nanostructures of carbon or metal or a combination thereof; .[.2.5-30%.]. .Iadd.30-90% .Iaddend.wt/wt ink of a solvent selected from the group consisting of: chloroform, methoxycyclopentane, 1,3-dioxolane, dimethylformamide, or combinations of the foregoing; and a dispersive agent comprising, as .[.surfactants.]. .Iadd.a surfactant.Iaddend., a compound selected from the group consisting of: sodium dodecyl sulfate, cetyl trimethylammonium bromide, citric acid, or combinations of the foregoing, wherein a ratio of solvent/thermoplastic elastomer copolymer (v/v) is between 3/1 and 12/1.

2. The printable piezoresistive ink according to claim 1, wherein the amount of thermoplastic elastomer copolymer is between 10-30% wt/wt ink.

3. The printable piezoresistive ink according to claim 1, wherein the amount of solvent is between 50-80% wt/wt ink.

4. The printable piezoresistive ink according to claim 1, wherein the amount of conductive nanostructures is between 0.5-6% wt/wt copolymer.

5. The printable piezoresistive ink according to claim 1, wherein the ratio of solvent to copolymer (v/v) is between 5/1-10/1.

6. The printable piezoresistive ink according to claim 1, wherein the surfactant is sodium dodecyl sulfate.[., or mixture thereof.]..

7. The printable piezoresistive ink according to claim 1, wherein the amount of dispersive agent is up to 20% wt/wt filler.

8. The printable piezoresistive ink according to claim 1, wherein an amount of .[.surfactants.]. .Iadd.the surfactant .Iaddend.in the dispersive agent is up to 15% wt/wt filler.

.[.9. The printable piezoresistive ink according to claim 1, wherein the amount of the surfactants in the dispersive agent is up to 15% wt/wt filler..].

10. The printable piezoresistive ink according to claim 1, wherein the relation of surfactant with respect to the conductive filler is between 1:2 to 1:10 wt filler/wt surfactant.

11. The printable piezoresistive ink according to claim 1, wherein the metal is silver, gold, platinum, or mixtures thereof.

12. The printable piezoresistive ink according to claim 1, wherein at least one of: a ratio (wt/wt) of butadiene/styrene in SBS varies between 45/55 to 80/20; and a ratio (wt/wt) of (ethylene/butylene)/styrene in SEBS varies between 70/30 and 67/33.

13. The printable piezoresistive ink according to claim 1, wherein the conductive nanostructures are nanoparticles, nanotubes, nanowires, or nano-plates.

14. The printable piezoresistive ink according to claim 13, wherein the carbon conductive nanostructures is selected from a group consisting of: carbon black, graphite, single carbon nanotubes, multi-walled carbon nanotubes, graphene, or combinations thereof.

15. A sensor, comprising a piezoresistive ink according to claim 1 configured to provide a deformation or force sensor.

16. The printable piezoresistive ink according to claim 1, wherein the printable ink is formulated to have a viscosity range between 4000-40000 cP (m.Math.Pa.Math.s) at 20° C. for screen printing, a viscosity range between 4-30 cP (m.Math.Pa .Math.s) at 20° C. for inkjet printing, and a viscosity range between 1-50 cP (m.Math.Pa.Math.s) at 20° C. for spray printing.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of present disclosure.

(2) FIG. 1—Experimental procedure for the preparation of the piezoresistive inks, with carbon nanostructures dispersed and TPEs dissolution to produce the ink (center of the image) and different printing techniques that can be used to print the piezoresistive sensors, including drop casting, inkjet, screen and spray printing, among others. 1—represents the solvent; 2—represents the carbon nanotubes; 3—represents the polymer; 4—represents the piezoresistive ink; 5—represents the spray printing; 6—represents the drop casting; 7—represents the inkjet printing; 8—represents the screen printing and 9—represents the substrate.

(3) FIG. 2—Illustration of piezoresistive sensors response, for both stretching (left) and bending (right). 10—represents the conductive pattern; 11—represents a clamp; 12—represents the stretching direction.

(4) FIG. 3—Piezoresistive response during stress-strain cycles for inks with certain amount of carbon nanostructures. A) Linearity during 10 stress-strain cycles and B) GF values from 0.1 to 20% strain for the same piezoresistive inks.

DETAILED DESCRIPTION

(5) The present subject-matter discloses a piezoresistive ink based on polymer matrices from thermoplastic elastomers of the styrene-butadiene-styrene family or hydrogenated rubber styrene-ethylene/butylene-styrene having different butadiene/styrene ratio and copolymer block structure. Their exceptional mechanical properties with large maximum strain and low mechanical hysteresis in mechanical stress-strain cycles provide the materials with suitable properties for large strain and soft pressure sensors.

(6) The experimental method processing the piezoresistive inks follows the following steps (FIG. 1):

(7) In an embodiment, specific amounts of carbon nanotubes (CNT) are added to solvent and placed in ultrasonic bath for CNT dispersion. This step is important to tune the electrical properties of the piezoresistive ink. The CNTs can be chemically functionalized by covalent attachment of functional groups or the non-covalent adsorption of functional molecules onto the surface of the CNTs in order to tailor ink electrical conductivity and dispersion within the ink formulation.

(8) In an embodiment, the CNT content relatively to the polymer matrix depends on the specific application, generally varying between 0.5 to 6 weight percentage (wt %) in the CNT/polymer ratio.

(9) In an embodiment, the solvent used is toluene or related solvent and the solvent/polymer ratio ranges from 3/1 to 12/1 depending on the physical properties (including density, viscosity and electrical conductivity, among others) of the piezoresistive ink and the specific requirements of the printing techniques.

(10) In an embodiment, after keeping solvent and CNT in ultrasonic bath the TPE are added and placed in magnetic stirring until complete dissolution.

(11) In an embodiment, a dispersive agent involving an affinity functional groups containing block copolymer solution as sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB), citric acid or Triton, or mixtures thereof as surfactants is further used in order to allow proper ink rheology.

(12) A proper ink rheology is important because it is related to the application technology of the ink and the final performance of the sensor.

(13) The control of the viscosity of the inks depend on the application. The homogeneity of the solution and the sedimentation of the nanoparticles on the ink is crucial for their piezoresistive properties.

(14) The viscosity of the ink varies between 4000 and 40000 cP (m.Math.Pa.Math.s) for screen printing, between 4 and 30 cP (m.Math.Pa.Math.s) for inkjet printing and between 1 and 50 cP (m.Math.Pa.Math.s) for spray printing.

(15) Before use of the inks, magnetic stirring and/or ultrasonic bath are recommended.

(16) These inks can be easily spread or printed by different techniques directly in different substrates (FIG. 2) with excellent adhesion and with a short curing time. The curing time depends on temperature. It can be performed at room temperature, but the curing time strongly decreases with increasing temperature, being less than 10 min at 60-80° C., which can be less if the temperature is increased or with specific drying procedures, such as infrared treatment, down to less than 1 minute. The curing time also depend on the solvent and the applied spread/printed technique.

(17) The stretchable sensor sensibility is determined by the piezoresistive response (FIG. 3) that is quantified by the gauge factor (GF) which is defined as the percentage change in resistance per unit strain:

(18) $\begin{array}{cc}\mathrm{GF}=\frac{\mathrm{dR}/R_0}{\mathrm{dl}/l_0}& \left(1\right)\end{array}$

(19) In equation 1, the strain is represented by dl/l.sub.0 (ε), where dl means the relative change in mechanical deformation and l.sub.0 means the initial length. The resistance change with strain is represented by dR/R.sub.0, where dR means relative change in electrical resistance and R.sub.0 means the initial electrical resistance. The change in resistance is affected by the geometric effect and by the intrinsic piezoresistive effect of the material with applied strain. So, the fractional resistance change is dependent on two effects:

(20) $\begin{array}{cc}\mathrm{dR}/R=1+2x+\frac{d\rho /\rho }{\epsilon }& \left(2\right)\end{array}$
where the geometrical effect is represented by (1+2ν), on which ν represents the Poisson's ratio (usually between 0.35 and 0.5 for TPEs matrixes), and the intrinsic piezoresistive effect by

(21) $\left(\frac{d\rho /\rho }{\epsilon }\right).$
The geometric effect by itself contributes to a GF in the range of 1.7-2.0.
Characterization of the Obtained Inks

(22) Elastomers and thermoplastics elastomers are known for their capability to exhibit high deformation capability and high electrical resistance.

(23) The mechanical properties of CNT/SBS and CNT/SEBS inks show the typical curves of thermoplastic elastomers. The maximum strain is larger than 700%, reaching close to 2000% for some inks. The large deformation behavior of both used TPEs and their composites is the required for large strain sensors applications. The mechanical properties of the final sensors can be tailored by matrix composition (S/B ratio) and carbon nanostructures content. Furthermore, these materials show low mechanical hysteresis and easy recovery, with the mechanical hysteresis increasing with the applied stain and styrene content.

(24) The electrical conductivity increases several orders of magnitude with increasing high aspect ratio (nanotubes and/or nanowires) carbon or metallic content in the composites and the typical electrical percolation threshold of these composites inks is between 0.5 to 1.5 wt % of metallic or carbon nanostructures. The electrical conductivity shows a large increase on these inks materials until 2 wt % of filler content, after that concentration the electrical conductivity shows a slight increase. This allows to tailor the electrical resistivity (inverse of the electrical conductivity) of the inks from hundreds of Ohm to few MOhms.

(25) The piezoresistance is determined by the linear correlation between the electrical resistances and strain (FIG. 3A). The piezoresistive sensibility (GF value) determines the piezoresistive properties of inks, the higher this value, the larger the sensitivity of the sensor to the applied strain (FIG. 3B).

(26) The carbon nanostructure content is critical in determining the printed materials piezoresistive properties, as well as the sensibility or maximum deformation of the sensor application.

(27) The piezoresistive inks involved in this allow printing sensors with GF values that can reach up to GE˜10 (FIG. 3B). Further, they are suitable for low and large deformations with higher sensibility. Strains up to 40% and above can be accurately measured adapting the inks to a specific application. Furthermore, the TPEs matrices can be adapted to application too, changing butadiene/styrene ratio to provide specific elasticity to the piezoresistive sensor. The biocompatibility can be offered by the SEBS matrices, being FDA approved.

(28) Embodiment

(29) In an embodiment, the thermoplastic elastomer Calprene CH-6120, a Styrene-Ethylene/Butylene-Styrene (SEBS) copolymer with a ratio of Ethylene-Butylene/Styrene of 68/32 and a molecular weight of 245.33 g/mol, supplied by Dynasol. Multi-walled carbon nanotubes (MWCNT) were supplied by Nanocyl: reference NC7000, purity of 90%, length of 1.5 μm and outer mean diameter of 9.5 nm. Cyclopentyl methyl ether (CPME) was supplied from Carlo Erba with a density of 0.86 g/cm3 at 20° C., and Sodium dodecyl sulfate (SDS) (sigma).

(30) MWCNT/SEBS composites with 0, 0.5, 1, 2, 4, and 5 MWCNT weight percentage (wt %) were prepared by dispersing the respective mass loading in the CPME solvent within an ultrasound bath (ATU, Model ATM40-3LCD) for 5 h at room temperature. The solutions were prepared with the dispersing agent Sodium dodecyl sulfate (SDS) in proportions with respect to the MWCNT (MWCNT:SDS) of 1:2; 1:6 and 1:10. This solvent was selected in order to replace toluene, which has been previously used for MWCNT dispersion and SEBS dissolution (Costa, P.; Ribeiro, S.; Botelho, G.; Machado, A. V.; Mendez, S. L. Polymer Testing 2015, 42, 225-233.). Toluene is consider a health and environmentally dangerous solvent by the American Chemistry Society (ACS). In a scale of 1 to 10, toluene shows a classification of 5 for safety, 7 for health, and 6, 6 and 2 for air, water and soil environmental risks (ACS Green Chemistry Institute, P. R. S. S. G., 2011). For these reasons and due to the increase application of these materials by solvent based printing technologies, it is necessary to replace toluene by a greener solvent. CPME is a green solvent and a valid replacement for toluene due to its physico-chemical characteristics. Further, it shows a boiling point of 106° C., lower than toluene (111° C.), allowing a faster evaporation of the solvent. Sodium dodecyl sulphate (SDS) is used as dispersing agent though others such as Triton or CTBA can be also used. After a good dispersion of the nanofillers was achieved, SEBS was added with a polymer/solvent ratio of 1:6 (grams of polymer to ml of solvent) and the solution was magnetically stirred at room temperature until complete dissolution of the copolymer. The MWCNT/SEBS films were then prepared by solution casting on a clean glass substrate and let to dry, at room temperature, for 24 h until total solvent evaporation. The films thickness can be tailored between 50 and 300 μm.

(31) After the optimization of the materials, piezoresistive sensors were prepared by screen and spray printing techniques and deposited on solvent casted (1 g of SEBS to 6 ml of CPME) SEBS films.

(32) Screen Printing

(33) Screen printing was performed with a home-made set-up with a metallic base structure supporting the screen. With respect to the printing procedure, the piezoresistive and conductive inks are pressed using a squeegee over the screen placed at 100 mm distance of the polymer substrate. The screen (from Sefar) shows 62 monofilaments by cm with a tension of 17 N.

(34) The screen printed sensors .[.(FIG. 3A).]. .Iadd.(FIG. 2) .Iaddend.are formed by three interdigitated geometries formed by 11 conductive digits each with 0.8 mm width and 0.8 mm distance between them. The piezoresistive ink was deposited over the conductive interdigitated patters with an area of 3×3 cm.

(35) Screen printable ink formulations for the conductive and active piezoresistive layers are formulated as follows: The piezoresistive ink is prepared with a polymer/solvent (SEBS/CPME) ratio of 1:6 (g:ml) and the conductive ink with a relation of 1:13 (g:ml). The viscosity of the piezoresistive and conductive inks can be in the 744-1490 cP and 881-1615 cP range, respectively, for the development of sensors with suitable characteristics, under the present fabrication conditions.

(36) Spray Printing

(37) In an embodiment, spray printing was achieved with a commercial air pressure pistol with a pressure between 3 and 5 bar and at 100 to 200 mm distance between pistol and substrate.

(38) The piezoresistive ink for spray printing was prepared with a polymer/solvent (SEBS/CPME) ratio of 1:8 (g:ml) and the conductive ink with a relation of 1:19 (g:ml). Thus, the viscosity ranges between 244-407 cP and 89-166 cP, for the piezoresistive and the conductive inks, respectively.

(39) The spray printing pattern is formed by 7 conductive lines with 5 mm distance between them, the area of each piezoresistive sensor being 5×20 cm.

(40) The microstructure of the obtained composites and the dispersion of the MWCNT was analyzed by scanning electron microscopy (SEM-FEI-NOVA NanoSEM 200).

(41) TABLE-US-00001 TABLE I Characterization of the ink of the present disclosure namely: strain, conductivity and GF Conductivity Strain (Ωm).sup.−1 GF (%) Multiwall Carbon Nanotubes/SEBS; 0.1 1.5 80% 5% wt MWCNT content; MWCNT:SDS of 1:2 Multiwall Carbon Nanotubes/SEBS; 1E−2 2  5% 2% wt MWCNT content; MWCNT:SDS of 1:2 Multiwall Carbon Nanotubes/SBS; 1E−2 18 20% 5% wt MWCNT; MWCNT:SDS of 1:2

(42) The term “comprising” whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

(43) It will be appreciated by those of ordinary skill in the art that unless otherwise indicated herein, the particular sequence of steps described is illustrative only and can be varied without departing from the disclosure. Thus, unless otherwise stated the steps described are so unordered meaning that, when possible, the steps can be performed in any convenient or desirable order.

(44) The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.

(45) The above described embodiments are combinable.

(46) The following claims further set out particular embodiments of the disclosure.