Many of the markets for stretchable conductors is in the area of sensors and many sensors are being developed to be either stuck directly to the human body or integrate into clothing.
Vacuum deposited metal is conducting and can be flexible but tends not to be very stretchable. Vacuum deposited transparent conductors are worse in that they are less flexible than metals. Often they are based on oxides and are brittle ceramic type materials. It is possible to laminate or encapsulate the conducting layer to make it on the neutral axis but this is not always convenient. Other technologies have enabled many different products to be developed. There are now the options of printing metallic inks where acicular or flake metal flakes are incorporated into an ink and the these flakes align parallel to the surface they are printed onto and the lamellar nature of the flakes allows some flexibility and stretch. The conductivity can be further improved if a conducting polymer matrix is used as the carrier for the flake material and so the resistance between the flakes is reduced. Competing with this is the use of fine wires which can be embedded in a polymer and again this polymer may be a conducting polymer. The wires reduce the resistivity that would be possible for the same thickness of conducting polymer without the wires included. Graphene is also starting to be used and this too can be used incorporated into a matrix.
To make sensors or devices it is typical for several layers to be used to make up the required circuits. This requires matching the materials and ensuring that there is sufficient adhesion to allow for not only flexing of the device but also stretching where the elasticity of each material may be very different.
Common to all of these materials is the problem of cleanliness and particulates. The substrates are not generally manufactured in a clean room and despite cleaning using the likes of tacky rolls and then covering up any remaining particulates and surface defects with a planarising layer there are still likely to be some particulates or defects that remain. As these may cause coating problems on any or all layers this can significantly impact the yield of the process.
The problems of managing the defect level and adhesion coupled to maintaining the conductivity whilst being stretched and at a competitive cost continue to be the challenge.