During in-vitro studies, the Na-tattoo was applied to rigid and
elastic substrates in a similar fashion as described earlier (Jia et al.,
2013b). In case of epidermal evaluation of the Na-tattoos, a fluidic
channel and sink (stick-on fluidic channel) was fabricated atop the
Na-tattoo [Fabrication and photograph of stick-on fluidic channel
are given in Section S.2 and Fig. S1 of Supplementary data section].
The stick-on fluidic channel directed the sweat to first flow over the
two electrodes and then to the sink. In order to apply the
Na-tattoo to the epidermis, an inverted U-shaped adhesive layer
was applied onto the skin. The gap present in the U-shaped
adhesive was included for facilitating the replenishment of perspiration over the two electrodes (Jia et al., 2013a). Thereafter, the
transparent protective film present on the stick-on fluidic channel
(attached to the Na-tattoo) was gently peeled and applied to the
aforementioned U-shaped adhesive layer. For continuous sodium
monitoring, the user first applies the tattoo sensor, as was
described earlier (Jia et al., 2013a) followed by wearing of wireless
transceiver such that its contact pads are aligned with that of the
Na-tattoo. The data is then wirelessly transmitted to a notebook
for logging and display. The complete fabrication and application
of the Na-tattoo ISE to the epidermis is shown in Fig. 1. A top view
of the tattoo with the dimensions of the individual electrodes is
shown in Fig. S2.