‘E-tattoos’ function as breathable sensors

Technology News | March 2, 2018

By Rich Pell

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.

A 1.5μm-thick multifunctional e-tattoo transferred
on a human chest, connected for data acquisition.

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.

The 75×40mm e-tattoo incorporates two ECG electrodes,
two hydration sensors, and a resistance temperature
detector, all in filamentary serpentine layout.

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.

In their paper, the researchers note that such a cut-and-paste method can be used to pattern other material combinations, including Al/PET, carbon-doped thin film PDMS, graphene/PMMA and even indium tin oxide (ITO) on PET.

Once transferred, the 1.5μm-thick tape-free and open-mesh e-tattoo can adhere on the skin purely via van der Waals forces. For improved durability, it was sprayed with a transparent encapsulation layer about 1μm thick (except for the hydration sensors which had been covered by a paper stencil).

Due to their high conformability, these low cost e-tattoos ensure a large contact area between the sensors and the skin, lowering the contact impedance and facilitating signal transfer across the sensor–skin interface. Measuring 75×40mm, the multifunctional e-tattoo was shown to operate reliably even under various skin deformations.