Hybrid soft circuits connecting small islands of hard silicon-based electronic components is an approach that has been under investigation for some time. Though the main difficulty in these hybrid approaches is to make reliable connections between the soft rubbery circuit areas and the hard rigid pins of silicon devices. Sometimes, this is done by creating progressively thicker and firmer pad areas which will receive the hard components, from there, meandering circuit patterns can alleviate the stress on the connection and on the conductive tracks too. Typically, the soft printed circuit parts are first manufactured, and then placed under a pick-and-place machine to receive the hard silicon parts.
A new approach taken by researchers from the Wyss Institute for Biologically Inspired Engineering, in collaboration with the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the US Air Force Research Laboratory consists in layering all the soft circuit compounds using a dedicated 3D printer which can also pick-and-place the hard parts in a last pass.
Key to this hybrid 3D printing is the use of a stretchable conductive ink made of thermoplastic polyurethane (TPU) mixed with silver flakes. Both pure TPU and silver-TPU inks are co-printed to create the devices’ underlying soft substrate and conductive electrodes and ensuring a very strong adhesion and integration. Simple electrode patterns can be printed to create sensors (by monitoring their change electrical conductivity upon stretching) whose data can be processed by a programmable microcontroller.
The novelty is that the same conductive ink dispenser is used as a vacuum nozzle (when empty) to pick-and-place electronic components whose contact pads have first received a blob of rubbery conductive ink. The TPU dots applied beneath each component serve to anchor them and to distribute the stress throughout the entire stretchable circuit matrix.
This trick allows the fully assembled devices to be stretched up to 30% while still maintaining their function, the researchers reported in the Advanced Materials journal. Their paper "Hybrid 3D Printing of Soft Electronics" relates a number of experiments and a proof-of-concept, with the realization of two soft electronic devices.