Choosing a conductive thread or yarn can be a challenge, as it's not always clear what one should be looking for in a conductive filament.
After inspecting a few threads on the market, we came up with a criteria for choosing a "good" thread or yarn.
1.) Electronic Functionality
You should aim for the thread that will apply to the most applications possible. To us, this means the highest conductivity. Even for resistive heating, you can create a large surface area heating element with a more conductive thread.A less conductive thread restricts the surface you can cover for just about any application.
The yarn or thread you choose should be sewable, weave-able or knit-able in an industrial setting. We optimize for manufacturing at Loomia, so if the material cannot be used in a scalable way, it's not a good choice. When it comes to sewing, the thread should have a small diameter - something under .2" (based on the guide below from Superior Threads). For knitting, choose something that won't damage an industrial machine, like Jameco 92 thread. Some manufacturers won't run course metallic threads through their CNC knitting machines, so ensure that you choose something that is safe for the threading system.
Ideally, the yarn or thread should should have an outer metallic coating, making it solderable. Of course, there are many ways to make connections, but having a solderable thread opens up how you can connect the thread to components.
As a whole, we haven't seen conductive thread or yarn used widely in consumer products. Aside from Sensoria Socks (which have conductive yarn knit in) or Project Jacquard (which uses a cap touch embroidered grid, many products we've seen online still use traditional wiring or use electronics techniques. For example, the image below shows normal wires with an overmolded "winged" component that allows you to sew the wires to a fabric.