Composite yarn structure

1. A composite yarn structure comprising a first element comprising a coaxial flexible bi-component monofilament including a conductive component and a thermoplastic component, the thermoplastic component exhibiting piezoelectric properties; and at least a second conductive element twisted around at least part of the first element to provide a composite yarn wherein said composite yarn exhibits auxetic properties and wherein said second conductive element has a lower elasticity with respect to the elasticity of the first element such that, upon elongation of the composite yarn structure in a first direction starting from an unstretched condition, the composite yarn structure expands in a second direction, in such a way that in an unstretched condition said first element extends in a rectilinear direction and said second element is twisted around said first element and in such a way that, upon elongation of said composite yarn structure, said second element is aligned with the elongation direction and said first element expands by forming a series of curves around the second element, whereby the dimensions of the composite yarn structure is increased both in first and second directions to generate an additional force on said thermoplastic component exhibiting piezoelectric properties of the first element.

2. The composite yarn structure according to claim 1 , wherein said second conductive element is a conductive element or comprises a conductive element and the conductive component of the first element is made of an intrinsically conductive polymer.

3. The composite yarn structure of claim 2 , wherein upon said elongation of the composite yarn structure in said first direction, the second conductive element shapes the first element in such a way as to provide an auxetic effect.

4. The composite yarn structure of claim 3 , wherein said second conductive element shapes the first element in a helix around said second element.

5. The composite yarn structure according to claim 1 , wherein the conductive component is made of an elastic conductive material.

6. The composite yarn structure according to claim 5 , wherein the elastic conductive material is a polymer including conductive metallic impurities.

7. The composite yarn structure according to claim 5 , wherein the elastic conductive material is a material chosen from blends of Low-density polyethylene (LDPE) or blends of High-Density Polyethylene (HDPE) with carbon nanotubes (CNTs), graphite, graphene nano-sheets, amorphous carbon, doped polyaniline, doped polypyrrole, and mixtures thereof.

8. The composite yarn structure according to claim 5 , wherein the elastic conductive material is an intrinsically conductive polymer.

9. The composite yarn structure according to claim 1 , wherein the thermoplastic component comprises a material chosen from polyvinylidene fluoride (PVDF) homopolymer and its copolymers with trifluoroethylene P(VDF-co-TrFE) or tetrafluoroethylene P(VDF-co-TFE) or odd numbered polyamides (PA).

10. The composite yarn structure according to claim 1 , wherein the second conductive element comprises a plurality of filaments, at least one of said plurality of filaments being a metallic filament.

11. The composite yarn structure according to claim 1 , wherein said conductive component and said thermoplastic component of the first element are coextruded.

12. An article comprising at least one composite yarn structure according to claim 1 .

13. The article according to claim 12 , wherein said article comprises at least said composite yarn structure arranged adjacent a further said composite yarn structure to bias each other upon being elongated.

14. The article according to claim 12 , wherein said article comprises one of a fabric, a garment, clothes and a rope.

15. The article according to claim 13 , wherein said article comprises one of a fabric, a garment, clothes and a rope.

16. A composite yarn structure comprising a first element comprising a coaxial flexible bi-component monofilament including a conductive component and a thermoplastic component, the thermoplastic component exhibiting piezoelectric properties; and at least a second conductive element twisted around at least part of the first element to provide a composite yarn wherein said composite yarn exhibits auxetic properties and wherein said second conductive element has a lower elasticity with respect to the elasticity of the first element such that, upon elongation of the composite yarn structure in a first direction, the composite yarn structure expands in a second direction, whereby the dimensions of the yarn is increased both in first and second directions to generate an additional force on said thermoplastic component exhibiting piezoelectric properties of the first element and wherein the tensile strength of the first element is in the range of 15-78 MPa, the tensile strength of the second element is preferably in the range of 500-2,000 MPa, wherein in an unstretched condition said first element extends in a rectilinear direction and said second element is twisted around said first element.