• Interface energy regulation directly on skin was applied to achieve ultra-conformable, substrate-free, strongly adhesive PEDOT: PSS E-tattoo for the first time.

• Novel transfer approach to transfer thin, substrate-free PEDOT: PSS E-tattoo onto skin.

• ECG measurement performance of E-tattoo is better that of commercial Ag/AgCl gel electrodes.

Electronic tattoo has great potential application in mobile health. Fully conformal contact between E-tattoo and skin is critical for reliable monitoring. In this paper, we reported a substrate-free, ultra-conformable PEDOT: PSS (3, 4-ethylenedioxythiophene): poly (styrene-sulfonate) E-tattoo achieved by interface energy regulation on skin. The controllable gel/dry electrode mutual transformation property of PEDOT: PSS was carefully studied and reported. Then a novel transfer approach was studied to transfer thin, substrate-free PEDOT: PSS E-tattoo onto skin. Meanwhile, PEDOT: PSS E-tattoo was gelled, then dried directly on skin, regulating its bending energy, contact area, and interface adhesion energy with skin. Through this method, the critical thickness of the after-transformation dry E-tattoo that could form fully conformal contact with skin was increased by 4 times. The electrode-skin interface impedance and ECG measurement performance of the reported E-tattoo were on par with commercial Ag/AgCl gel electrodes, while offering superior comfort and reliability. The substrate-free, ultra-conformable PEDOT: PSS E-tattoo could be applied as sensing electrode for reliable monitoring in mobile health.