3D Printing and Stretchable Electronics

Stretchable electronics are electronic systems that comply with dimensional changes of the substrate without the loss of functionality. Stretchable electronic technologies are developed for the manufacturing of wearable electronics, which require soft and elastic substrates. Thus, stretchable electronics have normally (I) a deformable thermoplastic polyurethane (TPU) substrate, (II) rigid printed circuit board (PCB) module islands, and (III) stretchable interconnections between the islands. In the system, the mechanical and electrical features have a linkage, where the electrical performance is affected by the mechanical difference of the components.

In this thesis, manufacturing methods are researched to control and decrease the mechanical differences in stretchable electronics. The joining of rigid and stretchable substrates with structural and non-structural adhesives is studied with peel tests. Also, TPU 3D printing on the TPU substrate is explored to optimize shaped stretchable interconnections. The 3D printing is further adapted for the stretchable 3D-printed interconnections and sensors. Finally, the interconnection deformations close to the islands are studied in hybrid stretchable circuit board (SCB) technology.

The results show that the structural adhesives are incapable of complying with the elongation of TPU substrate, which induces irregular peeling. With the nonstructural adhesive, the adhesive stretches, producing stable peeling. In the case of small and local interconnection supports, the adhesives are replaced by 3D printing directly molten TPU plastic on the screen-printed TPU substrate. The good adhesion of ~ 1,9 N/mm allows the supports to increase the safe elongation of interconnections by ~ 27%. Moreover, the 3D printing of TPU is usable for stretchable 3D printed structures. Unlike with current conductive 3D printing materials, permeable carbon fiber nonwovens enable stretchable and conductive composites. Thin ~ 53 μm fiber layer deforms evenly with cyclic 50 % elongation, and thick > 150 μm fiber layers have the best resistance results 4 Ω□. Lastly, with various protective structures, the interconnection transition area close to the SCB islands is used to stabilize the samples at 10 % and 20 % cyclic elongation.

Demonstrated new manufacturing methods advance stretchable electronics by balancing the mechanical differences of the system, which increases the stretchability and integration level of stretchable systems. More durable and smaller systems build a foundation for indistinguishable structural electronics and maintainable wearable electronics for casual use, military, sports, and healthcare sectors.