阅读说明：本技术 用于高度可拉伸的软电子器件的高传导性可印刷油墨及可由其获得的高传导性超可拉伸导体 (High conductivity printable inks for highly stretchable soft electronic devices and high conductivity super-stretchable conductors obtainable therefrom ) 是由 孙红叶 诺伯特·威伦巴赫 于 2020-02-14 设计创作，主要内容包括：本发明涉及用于高度可拉伸的软电子器件的高传导性可印刷油墨、其制造方法以及可由其获得的高传导性超可拉伸导体。(The present invention relates to high conductivity printable inks for highly stretchable soft electronic devices, methods of making the same, and high conductivity super stretchable conductors obtainable therefrom.)

1. A high conductivity printable ink comprising:

(i) from 1.5 to 21.0% by volume of conductive hydrophobic silver particles as a conductive solid phase, based on the total volume of the ink,

(ii) a liquid major phase comprising as a polymer matrix a Thermoplastic Polyurethane (TPU) dissolved in an organic polar solvent, wherein the volume content of TPU in said organic polar solvent is from 25% to 50%, wherein said liquid major phase constitutes from 76.90% to 98.49% by volume of the total volume of the ink,

(iii) an ionic liquid-based liquid secondary phase in a volume content of from 0.0015 to 2.1 vol.%, based on the total volume of the ink, with a volume ratio p between the liquid secondary phase and the conductive solid phase of from 0.001 to 0.1,

wherein the liquid minor phase is immiscible with the liquid major phase and does not wet the conductive solid phase such that a three-phase system produces a capillary suspension.

2. The ink according to claim 1, wherein the conductive hydrophobic silver particles have a median particle diameter d50 of 0.1 to 50 μ ι η measured by laser diffraction according to DIN EN 725-5, ISO 13320.

3. The ink according to claim 1 or 2, wherein the thermoplastic polyurethane is selected from polyester-based or polyether thermoplastic polyurethanes.

4. The ink of any of claims 1-3, wherein the thermoplastic polyurethane is characterized as having a tensile strain to failure ε of 50% to 2500%_r。

5. The ink according to any one of claims 1 to 4, wherein the polar solvent of the liquid main phase is selected from Tetrahydrofuran (THF), Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc) or a combination thereof.

6. The ink of claim 5, wherein the polar solvent is mixed with a co-solvent selected from acetone, Methyl Ethyl Ketone (MEK), cyclohexanone, toluene, or ethyl acetate (ETAc).

7. The ink according to any one of claims 1 to 6, wherein the ionic liquid contains a substituted or unsubstituted imidazolium cation, wherein the imidazolium cation of the salt preferably has a (C1-C6) alkyl group in the 1-and 3-position or in the 1-, 2-and 3-position and the anion of the ionic liquid is a halide, perchlorate, pseudohalide, sulfate, phosphate, alkylphosphate and/or C1-C6 carboxylate ion.

8. The ink of claim 7, wherein the imidazolium cation is selected from a 1-ethyl-3-methylimidazolium cation, a 1, 3-dimethylimidazolium cation, or a 1-butyl-3-methylimidazolium cation.

9. The ink according to any one of claims 1 to 8, wherein the ionic liquid comprises 1-butyl-3-methylimidazolium iodide, 1-butyl-3-methylimidazolium chloride, or 1-butyl-3-methylimidazolium bromide:

10. a method for producing the high conductivity printable ink of any one of claims 1 to 9, the method comprising:

mixing the following:

(i) from 1.5 to 21.0% by volume of conductive hydrophobic silver particles as a conductive solid phase, based on the total volume of the ink,

(ii) a liquid major phase comprising as a polymer matrix a Thermoplastic Polyurethane (TPU) dissolved in an organic polar solvent, wherein the volume content of TPU in said organic polar solvent is from 25% to 50%, wherein said liquid major phase constitutes from 76.90% to 98.49% by volume of the total volume of the ink,

(iii) an ionic liquid-based liquid secondary phase in a volume content of from 0.0015 to 2.1 vol.%, based on the total volume of the ink, with a volume ratio p between the liquid secondary phase and the conductive solid phase of from 0.001 to 0.1,

wherein the liquid minor phase is immiscible with the liquid major phase and does not wet the conductive solid phase such that a three-phase system produces a capillary suspension.

11. The method of claim 10, wherein the silver particles are dispersed in the liquid major phase by mechanical agitation, followed by addition of the liquid minor phase and a subsequent mixing step to break the liquid minor phase into droplets.

12. Method for producing a highly stretchable soft electronic device in the form of an elastic composite by applying the ink of any one of claims 1 to 9 onto a soft substrate, preferably by means of spot coating, screen printing, slot die coating, spray coating or direct write molding, and then evaporating the solvent to produce a stretchable and deformable electrical device.

13. A high conductivity stretchable conductor obtained by the method of claim 12 comprising:

(i) 5 to 30 vol.%, preferably 8 to 20 vol.%, of conductive hydrophobic silver particles as a conductive solid phase of the resulting solid conductor,

(ii)67 to 94.99 volume%, preferably 78.80 to 91.99 volume%, of a Thermoplastic Polyurethane (TPU), and

(iii) a liquid (secondary) phase based on an ionic liquid, the volume fraction of which is between 0.005% and 3.0% by volume, preferably between 0.08% and 1.20% by volume, of the resulting solid conductor, while the volume ratio p between the liquid secondary phase and the conductive solid phase is between 0.001 and 0.1.

14. Use of the high conductivity printable ink of any one of claims 1 to 9 for the production of sensors, soft robots, wireless devices, flexible solar cells or soft electronics.

Examples

In one embodiment, Ag flakes (great wall precious metals, china) with an average size of 1 μm were used as conductive particles and Thermoplastic Polyurethane (TPU) was used as soft polymer. Thermoplastic Polyurethane (TPU) Elastollan 35A (22.5 wt%, BASF SE, germany) was dissolved in N, N-Dimethylformamide (DMF) for 18 hours, and the solution was further diluted with acetone in a volume ratio of 4:5 between DMF and acetone. Ag flakes were added to the TPU solution by: mix in planetary mixer at 2000rpm for a total of 15 minutes, in 5 minute increments, waiting 5 minutes between subsequent mixing steps. Room temperature ionic liquid (IL, 1-butyl-3-methylimidazolium iodide, Sigma-Aldrich) was added as a secondary fluid to the Ag-TPU suspension and mixed using a planetary mixer at 1700rpm for 1 minute.

The presence of Ag-network in TPU was observed by Scanning Electron Microscope (SEM) imaging (fig. 1). The Ag flakes are uniformly distributed in a regular binary mixture (fig. 1 a). In contrast, Ag agglomerates were observed in capillary Ag-TPU conductors (fig. 1 b). The volume ratio between the secondary fluid and the Ag flakes was set to 2%. As shown in fig. 2, further increase of the secondary fluid will decrease the conductivity. At low percolation thresholdAt volume%, the strong capillary forces between the particles will drive the Ag flakes to self-organize into a conductive network, as shown in fig. 3. For comparison, the corresponding binary Ag-TPU mixtures showVolume% as reported previously using similarly sized and shaped Ag flakesThe values are consistent (Valentine, A.D. et al, Hybrid 3D printing of soft electronics, Advanced Materials,2017,29 (40)). Notably, the initial conductivity (EC) was as high as 1300S/cm at 15 vol%, while the corresponding conventional ink without capillary bridging remained insulating at this particle concentration. The self-assembly of Ag particle network is caused by the capillary force in the ternary systemA sharp drop and a sharp improvement in EC. According to lowHigh stretchability of capillary ink based elastomeric conductors can be expected. The respective stress-strain diagrams obtained from the tensile test are shown in fig. 4. For capillary Ag-TPU conductors with Ag loadings up to 15% by volume, a strain at break ε was observed_r>1600 percent. Notably, the strain at break is independent of Ag content, in contrast to previous reports that indicated that increasing Ag loading resulted in significantly reduced stretchability (Guo, S.Z. et al, 3D printed Stretchable sensors, Advanced Materials,2017,29 (27); Larmagnac, A. et al, Stretchable electronics based on Ag-PDMS composites, Scientific reports,2014,4: p.7254).

The electromechanical properties of the capillary 15 volume% Ag-TPU conductor and the binary 38 volume% Ag-TPU conductor are shown in FIG. 5 a. When strained to 111%, the conductivity EC of the binary Ag-TPU system drops to 0.1S/cm and electrical failure occurs at-125% strain. However, capillary composites exhibit EC of 0.1S/cm at 205% strain and electrical failure occurs at-215% strain. These results clearly demonstrate the benefits of the capillary suspension concept of the present invention in terms of conductivity under strain and significantly reduced silver consumption.

The sensitivity of 10 vol% Ag and 15 vol% Ag in the TPU to tensile strain was further evaluated during repeated stretching up to 50% strain in 8 cycles after the two initial cycles. As shown in FIGS. 5b and 5c, for 10 volume% Ag-TPU, R/R₀Is-8, R/R for 15 vol% Ag-TPU₀Is-2.5. In other words, low Ag loading samples are suitable for sensing applications, while high Ag loading samples are suitable for wiring.

To evaluate the resistance recovery, the R/R of a 15 vol% Ag-TPU was recorded at four different strains during a single cycle test₀Time evolution of (fig. 5 d). During stretching, the resistance change follows the same path regardless of the maximum strain applied. When the strain is released to zero, the resistance is continuously recorded during and after the strain cycle until it reaches a steady state (within 5% variation). The resistance increases with stretching and recovers with strain release without delay. For small 20% strain, the resistance is fully recovered. The larger strain produced up to R/R after 100% strain₀A resistance of 2 remains. It is noteworthy that the capillary Ag-TPU conductor according to this invention exhibits almost complete reversibility of resistance even when it is subjected to high tensile strain of 100% and the change in resistance R/R₀So as to reach 200. To further investigate reversibility, samples of 15 vol% Ag-TPU were subjected to 100% and 200% strain. R/R to be measured after two initial cycles at 100% and 200% cyclic strain₀Shown in fig. 5e and 5 f. Samples exposed to 100% strain are in transition between conducting and non-conducting, e.g. R/R₀And the change in peak resistance from one cycle to another. At 200% strain, R/R₀Seven orders of magnitude increase and the sample was clearly non-conductive at peak strain。R/R₀Always in phase with the applied strain. From cycle to cycle, the sample switches between conducting and non-conducting states upon stretching and releasing without delay. To the best of the inventors' knowledge, this unique reversibility has not been reported previously.

3D printed sensor and wiring made from capillary ink

As a proof of the concept of elastic conductors based on capillary suspension, strain sensors were made from low Ag loading inks and conductive wires were made from high Ag loading inks by direct write molding. Fig. 6a shows a serpentine strain sensor printed with a capillary Ag-TPU ink with 10 vol% Ag. The copper foil serves as an electrode. To avoid delamination, the substrate used for printing comprises the same soft polymer as in the ink. Figure 6b shows the performance of the sensor. After two initial cycles, the relative resistance change Δ R/R for 6 triangular strain cycles is shown₀. The sensor exhibits repeatable and in-phase response to applied strain. The sensitivity of the sensor is determined by the sensitivity coefficient GF ═ DeltaR/R₀) And/epsilon. The sensor obtained according to the invention showed a GF of 7.2 at 30% strain for 10% by volume Ag-TPU.

These GF values are in the same range as previously reported (Valentine, A.D. et al, Hybrid 3D printing of soft electronics, Advanced Materials,2017,29 (40); Kim, I. et al, A photosite ceramic driven Ag flap/nanoparticle-based high sensitive strip sensor for human motion monitoring, Nanoscale,2018,10(17): p.7890-7897), however, this is achieved at much lower silver consumption.

A circuitous wavy wiring of TUP-type capillary ink containing 15 vol% Ag was printed on the same soft polymer substrate by direct write molding. The amplitude and wavelength were 1mm and 2mm, respectively. The wires were tested at 50% triangular strain for 10 cycles. A microscopic image of the patterned wiring before stretching is shown in fig. 7 a. FIG. 7b shows the relative resistance change Δ R/R10 cycles after 2 initial cycles₀。

Compared with the prior art, the method has the advantages that,Ag-TPU wiring also has a low Δ R/R at lower silver contents₀～1.2。

To fully demonstrate the wiring ability of the conductive elastomers of the present invention, printed stretchable circuits made from 15 volume% Ag-TPU are shown in fig. 8a in combination with LEDs, resistors and button cells. Fig. 8b shows a photograph of the circuit during its stretching to 100% strain. It is fully functional at 50% and fails at 100% as shown by the on/off of the LED lamp. However, when the strain is released to 70%, the circuit immediately functions again. This demonstrates the protruding reversibility of the capillary Ag-TPU conductor.