Multilayered electronic transfer tattoo that can enable the crease amplification effect

Abstract

Electronic tattoos have great potential in health and movement sensing applications on the skin. However, existing electronic tattoos cannot be conformal, sticky, and multilayered at the same time. Here, we have achieved multilayered integration of the electronic tattoo that is highly stretchable (800%), conformal, and sticky. This electronic tattoo can enable the crease amplification effect, which can amplify the output signal of integrated strain sensors by three times. The tattoo can be transferred to different surfaces and form a firm attachment, where no solvent or heat is needed. The tattoo fabrication is straightforward and scalable; a layer-by-layer strategy and two materials (metal-polymer conductors and the elastomeric block copolymer) are used to fabricate the circuit module with desirable numbers of layers within the tattoo. A three-layered tattoo integrating 1 heater and 15 strain sensors is developed for temperature adjustment, movement monitoring, and remote control of robots.

Fig. 1. Schematic illustrations and optical images of the three-layered METT.

(A) Exploded schematics of the METT containing three circuit layers. (B) Schematic illustrations of the layer-by-layer fabrication of the METT. (C) Optical image of the METT after transferring onto the skin; inset, the METT can be embedded into the creases on the finger joints. (D) Optical image of the METT for remotely controlling a robotic hand. Photo credit: Lixue Tang, Southern University of Science and Technology.

Fig. 2. The METT is conformal and sticky, which can enable the crease amplification effect.

(A) ΔR/R of the MPC in METT versus different tensile strains from 0 to 800%. Error bars in this paper represent SE. (B) ΔR/R of the MPC in METT versus tensile strains from 0 to 150%. (C) Real-time monitoring of the strain sensor in METT by stretching the METT from a strain of 0 to 50% for about 100 cycles. (D) Photograph of the METT embedded in the creases of fingers. (E) The METT can be embedded in the fingerprint. (F) Photograph of peeling off the METT from the skin. (G) Enlarged view of the METT attaching to the proximal interphalangeal joints (PIPs) during bending. (H) Schematic illustrations of the crease amplification effect; “a” presents the initial length of the suspended part. Dashed box, the crease model. (I) Schematic illustrations of different substrates with different thicknesses on the crease. The initial length of the suspended part, a₁ < a₂. Strain when bending, red > orange > yellow. (J) Photographs of the strain sensors on the skin of finger joints with reference. (K) A comparison of the output signals of the MPC strain sensors on different substrates with different thicknesses when bending the index finger to 105°. Photo credit: Lixue Tang, Southern University of Science and Technology.

Fig. 3. The scalability of the METT.

(A) Optical image of the seven-layered heater. (B) The thermal image of the heater without deformation (left) and with 30% strain (right). (C) The numbers of the erupted liquid metal droplets depend on the thickness of the SBS layer after the stretch cycles. (D) Scanning electron microscopy (SEM) characterization of the surface of the SBS corresponding to (C); the thickness of the SBS in I (left) and II (right) is 4.8 and 18.13 μm, respectively. (E) SEM characterizations of the electric connection point. The dotted lines present the edge of the electric connection point, which is covered by liquid metal particles. (F) Cross section of a three-layered METT.

Fig. 4. The METT can monitor the movements of the hand.

(A) ΔR/R of strain sensors in different position versus angles. Inset: The schematic illustration of the measurement positions of the strain sensors. (B) Resistance response of the METT attached to PIP in different bending angles. (C) ΔR/R of strain sensors in the METT with different layers depending on the bending angles of the index PIP. (D) Schematic illustration of the measurement positions of the strain sensors. (E) Optical images of the three-layered METT attaching to the hand. (F) The thermal image of the three-layered METT on hand. (G) The real-time signal changes of the 15 strain sensors and temperature changes of the heater on the METT with different hand movements. Photo credit: Lixue Tang, Southern University of Science and Technology.

Fig. 5. The METT can control the robotic hand remotely.

(A) Photograph of transferring the tattoo onto the hand. (B) Photograph showing METT on the skin (left) and disposal glove (right). Dotted frame, the external contact pads. (C) System-level block diagram of the robot controlling system. (D) The METT can remotely control the movements of the robotic hand. Photo credit: Lixue Tang, Southern University of Science and Technology.