Flexible heating device and method of making same

1. A flexible heating device comprising one or more flexible heating pad, the flexible heating pad comprising:

a flexible substrate layer;

a flexible heat generating layer configured to generate heat and comprising a number of perforations formed through a thickness thereof; and

a lateral heat transfer layer interposed between the flexible substrate layer and the flexible heat generating layer for receiving and transferring therethrough at least part of the heat generated by the flexible heat generating layer,

wherein the lateral heat transfer layer comprises a number of thermally conductive pouches, at least part of which overlays at least one of the perforations to reduce a temperature gradient between inside and around the at least one perforation.

2. The flexible heating device of claim 1 , further comprising a conductivity layer in contact with the flexible heat generating layer and comprising a number of locally continuous and electrically conductive areas that are discontinuous from one another such that the conductivity layer alone does not provide electrical conductivity throughout the entirety of a two-dimensional area of the conductivity layer while providing electric conductivity within at least part of the number of locally continuous and electrically conductive areas.

3. The flexible heating device of claim 2 , wherein the flexible heat generating layer and the conductivity layer in combination have a surface resistance a range between about 2 ohms/square and about 15 ohms/square.

4. The flexible heating device of claim 1 , wherein the flexible heat generating layer comprises an electrically conductive material having a surface resistance sufficient to generate the heat therein.

5. The flexible heating device of claim 4 , wherein the flexible heat generating layer with the perforations has a surface resistance that is substantially the same as that of the electrically conductive material without the perforations, wherein the flexible heat generating layer with the perforations has a resistance ranging between about 2 and about 50 per unit area of 10 cm2.

6. The flexible heating device of claim 4 , wherein the flexible heat generating layer with the perforations has a surface resistance that is substantially higher than that of the electrically conductive material without the perforations.

7. The flexible heating device of claim 4 , wherein the electrically conductive material comprises carbon black nanoparticles, carbon nanotubes, graphene pieces and a binder, which are mixed together:

such that the carbon black nanoparticles are dispersed in the electrically conductive material,

such that at least part of the carbon nanotubes electrically bridge between carbon black particles, and

further such that at least part of the graphene pieces electrically bridge among at least part of the carbon black particles, at least part of the carbon nanotubes and other graphene.

8. The flexible heating device of claim 1 , wherein the thickness of the flexible heat generating layer is in a range between about 40 μm and about 80 μm.

9. The flexible heating device of claim 1 , wherein the lateral heat transfer layer has a thickness ranging between about 0.1 μm and about 100 μm.

10. The flexible heating device of claim 1 , wherein at least part of the thermally conductive pouches is aligned with at least part of the perforations such that one thermally conductive pouch is placed within one perforation.

11. The flexible heating device of claim 1 , wherein the perforations have a diameter in a range between about 0.1 cm and about 1 cm.

12. The flexible heating device of claim 1 , wherein the thermally conductive pouches contain a liquid metal therein and are liquid-tightly sealed.

13. The flexible heating device of claim 12 , wherein the liquid metal is a eutectic metal alloy comprising gallium, indium and tin.

14. The flexible heating device of claim 1 , wherein the flexible substrate layer is referred to as a first flexible substrate layer, wherein the flexible heating device further comprises a second flexible substrate layer formed over the flexible heat generating layer such that the flexible heat generating layer is interposed between the first flexible substrate layer and the second flexible substrate layer.

15. The flexible heating device of claim 14 , wherein each of the first and second flexible substrate layers comprises a water-proof flexible substrate, wherein the first and second flexible substrate layers are water-tightly bonded such that the flexible heat generating layer is enclosed in a space defined between the first and second flexible substrate layers.

16. A garment comprising a garment body and the flexible heating device of claim 1 , wherein the flexible heating device is attached to the garment body.

17. A method of making a flexible heating device, the method comprising:

providing a film of an electrically conductive material;

printing a metal paste on a surface of the film to form a conductive layer;

forming a number of perforations through a thickness of the film and the conductive layer to provide a perforated flexible heat generating layer;

laminating the perforated flexible heat generating layer with a lateral heat transfer layer comprising a number of thermally conductive pouches, which provide an intermediate device; and

placing the intermediate device over a flexible substrate layer,

wherein laminating is performed such that at least part of the thermally conductive pouches overlays at least one of the perforations to reduce a temperature gradient between inside and around the at least one perforation.

18. The method of claim 17 , wherein the electrically conductive material comprises carbon black nanoparticles, carbon nanotubes, graphene pieces and a binder, which are mixed together:

such that the carbon black nanoparticles are dispersed in the electrically conductive material,

such that at least part of the carbon nanotubes electrically bridge between carbon black particles, and

further such that at least part of the graphene pieces electrically bridge among at least part of the carbon black particles, at least part of the carbon nanotubes and other graphene.

19. The method of claim 17 , wherein the thermally conductive pouches contain a liquid metal therein and are liquid-tightly sealed, wherein the liquid metal is a eutectic metal alloy comprising gallium, indium and tin.

20. The method of claim 17 , wherein the thermally conductive pouches contain a liquid metal therein and are liquid-tightly sealed, wherein at least part of the thermally conductive pouches is aligned with at least part of the perforations such that one thermally conductive pouch is placed within one perforation.