WonderFil Specialty Threads Black InvisaFil is a remarkable silk-like 100wt, 2-ply cottonized polyester. This ultra-fine but surprisingly strong thread has cotton-like characteristics and negligible stretch while also feeling soft in the hand. InvisaFil makes for super low bulk stitches and virtually disappears, making it the ultimate choice for English paper piecing, delicate lace designs, fine fabrics, creating texture when micro quilting, or whenever you don't want your stitching to show.