The earliest mass-produced electronic circuits used point-to-point wiring, where components were mounted and then connected together with wires. Printed circuit boards (PCBs) dramatically improved manufacturing efficiency and reduced the size of devices. The first PCBs had a single layer and only had traces on one side. Then both sides, and finally with many layers like we see today — making it possible to create densely-populated PCBs. Now, researchers from the University of California San Diego are bringing that same functionality to flexible circuits.
Until now, flexible circuits have only reached a level akin to early PCBs, with just a single layer. That makes it difficult to create complex circuits, and also limits the kinds of components that can be used. Many modern processors, for example, are designed with the assumption that traces can be overlapped in multi-layer PCBs. This new development makes it possible to use those components in flexible, stretchable circuits by stacking layers in three dimensions.
In their design, each flexible layer is populated with components, just like any other flexible circuit. The breakthrough is in how the layers are connected to allow electrical signals to pass between them. The flexible silicone elastomer that makes up the substrate of the layer is mixed with black organic dye. Once the layers are stacked, they use a laser to heat precise points in order to create a conductive pathway between the layers.
As a proof of concept, they created a 3D stretchable smart bandage that wirelessly monitors biological electrical signals, so it can be used to track heart rate like an ECG, or brain waves like an EEG. In the future, they believe this same fabrication method could be used to create wearable flexible circuits that are just as complex as circuits built on traditional PCBs.