They had initially developed the cut-and-paste method (whereby sensors and circuits are simply cut out of commercially available metalized polymer sheets using a benchtop programmable mechanical cutter plotter) as a low cost alternative to lab-based photolithography circuit patterning and transfer printing to tattoo paper.
The original cut-and-paste method, although cheaper and faster to implement than conventional microfabrication and transfer printing methods, was limited by the thickness of commercially available metalized polymer sheets (at least 13μm) and also required a medical-grade tape onto which the electronic tattoo sensors had to be pasted, further increasing thickness and reducing their breathability.
directly transferrable from commercially available tattoo paper to the skin. Describing their approach in a paper titled "Low-cost, μm-thick, tape-free electronic tattoo sensors with minimized motion and sweat artifacts" published in npj Flexible Electronics, the researchers reported simple tattoo-like sensors capable of measuring electrocardiograms (ECG), skin temperature, skin hydration but also heart rate and respiratory rate (both extracted from the ECG signals). They also demonstrated minimized motion artefacts compared to signals recorded by thicker tape-based sensors or more commonplace gel-based electrodes.
The fabrication process starts with commercial tattoo paper whose protective liner is peeled off. After laminating a 1.4µm-thick transparent PET film directly on the tattoo paper (which already has a thin layer of water-soluble adhesive), the researchers proceeded to metallize the whole sheet with 10nm-thick chromium and 100nm-thick gold before using a programmable mechanical cutter plotter to shape profile serpentine-shaped circuits and electrode patches. Slightly wetting the tattoo paper allows to peel off the unwanted cut-outs, leaving only the filamentary-serpentine-shaped stretchable sensors. The e-tattoos can then be pasted directly on human skin.