# Multilayered electronic transfer tattoo that can enable the crease amplification effect – About Your Online Magazine

## RESULTS AND DISCUSSION

Figure 1A presents the structure of a three-layer METT containing 1 heater and 15 strain sensors. METT has the same structure as the temporary transfer commercial tattoo. It usually consists of three parts: an adhesive layer, a release layer and circuit modules between the two. The adhesive layer is a thin layer of pressure sensitive acrylic adhesive (~ 8 μm). When the pressure is applied, the adhesive layer allows the METT to form a firm and conformed fixation to the skin. Pressure sensitive acrylic adhesives are non-toxic, as they have been successfully applied to various medical tapes. The release layer is a silicone film that can achieve easy separation of the circuit modules from the release film. The circuit layer (~ 14 µm for each layer) is a thin film of poly (styrene-butadiene-styrene) (SBS) with elastic conductors embedded in it. The circuit module in the three-layer METT contains three circuit layers, with 11 deformation sensors in the first circuit layer, 4 deformation sensors in the second circuit layer and 1 heater in the third circuit layer. We chose MPC to manufacture the circuit module, including interconnects, strain sensors and heaters at METT due to the excellent conductivity and extensibility of the MPC. The purpose of the SBS film is to support the conductors and electrically insulate the conductors in different layers. There are electrical connection points (holes) in the SBS films to connect conductors in different layers; thus, conductors in different layers will have vertical electrical connections. When you put the METT on the skin by pressure and remove the release layer, the circuit modules in the METT are transferred to the skin and form a firm fixation on the skin.

We created a layer-by-layer manufacturing strategy to manufacture METT. Fabrication (film S1) begins at the outermost layer of the tattoo on the skin. We obtained a thin layer of SBS film notched in the release layer by spin coating. The thickness of the SBS is dependent on the spin-coating (from 3 to 30 μm; fig. S2). MPC-based conductors can be printed directly on SBS film (Fig. 1B) If we continue to apply spin-coating to another SBS film, the MPC will be completely sealed by the SBS film, losing the electrical connection with the MPC in other layers. Thus, before the SBS coating, we place the silicone stamps at the electrical connection points of the MPC, so that the SBS does not seal the MPC at these points during the SBS coating. After removing the silicone seal, the MPC at these points can provide vertical electrical connections to other layers. We print conductors based on MPC on the second SBS film; The MPC at the electrical connection points will form electrical connections to the conductors based on the MPC at the bottom layer. We can increase the number of layers in the circuit by repeating the procedures in Fig. 1B. Prior to the pressure sensitive adhesive coating, we applied ~ 50% uniaxial voltage to the METT to activate the circuit modules (make the MPC conductive). The MPC is non-conductive after printing because of the non-conductive oxide layer on the liquid metal particles. When we stretch the METT, stress will be transmitted from the substrate to the particles, breaking the oxide layer in the particles to generate conductive paths (32) METT can adhere firmly to the skin when pressure is applied, where no solvent or heat is needed to activate the adhesive. After removing the release layer, the thin and thin circuit modules will remain on the skin (Fig. 1C) The three-layer METT can monitor 15 degrees of freedom of the hand, which suggests that the dexterity of the human hand can be transferred to the robotic hand if the robotic hand has sufficient degrees of freedom (Fig. 1D)

We tested the electromechanical performance of MPC-based strain sensors in METT. The MPC can be used as strain sensors due to the good elasticity and repeatability of the MPC. The resistance of MPC-based strain sensors will increase with increasing tensile strain (Fig. 2, A and B) They can be easily stretched to a tension of 800% (Fig. 2A), which far exceeds the maximum deformation of the skin. MPC-based sensors in METT also show excellent repeatability after being stretched to a 50% strain for 1000 cycles (Fig. 2C) We measured the stress-strain curve of METT and found that METT with different layers has a similar strain-stress curve when the strain is less than 100% (fig. S3A). The METT module is 345 ± 16 kPa at 50% deformation, which is close to the skin modules.

The extensible METT is shaped and sticky, which can cause the crease effect. METT can be incorporated into the folds of the skin, such as the folds of the fingers (Fig. 2D) and digital printing (Fig. 2E) because it is thin and soft. The skin inside the creases will not be completely covered by METT, leaving the bottom of the creases uncovered. METT will connect the two sides of the crease (Fig. 2, H and I) We call METT on the bridge “suspended part”. The length of the suspended part depends on the thickness of the METT (~ 0.2 mm for the single layer METT; section S1). A finer METT will lead to a deeper embedding of the crease, which will lead to a shorter initial length of the suspended part (an1 < antwo, an1 and antwo present the initial length of the suspended part; Fig. 2I, left). In comparison with the reported conformed electronic tattoo, METT can be firmly attached to the skin (Fig. 2F) inside the creases, which can guarantee that the stresses are concentrated on the suspended parts when bending the fingers (Fig. 2G; the MPC in the creases is more reflective than the other parts; we believe that the liquid metal particles inside the METT under higher pressure will be compressed and will have a flatter surface, looking more reflective). As a result, the deformation focused on the strain sensors will remarkably amplify the strength compared to the strain sensors under medium strain. We call this observation the crease amplification effect. According to the simulation and calculation of the crease model (section S1), we obtained the equation of the crease effect, which can be written as

$R′–RR=εtwo(1The–1)$

in which R is the resistance of the tension sensor under the crease effect, R is the resistance of the strain sensor under medium strain, ε is the average strain of the strain sensors, an is the length of the suspended part, and n is the density of creases. By the formula, we know that compared to the strain sensors under medium strain, the crease effect of the crease can greatly amplify the output resistance if the product of an and n is much less than 1.

We manufacture METT with different layers to see how the thickness affects the output signals from the strain sensors. Inside Fig. 2K, the deformation will be uniformly loaded throughout the substrate during bending (Fig. 2I, right, non-conforming) when using polydimethylsiloxane (PDMS; 200 μm) and Ecoflex (200 μm) as substrates, because such substrates are non-conforming, leading to small changes in the resistance of the strain sensors. In contrast, METT can greatly increase the resistance of voltage sensors due to the crease effect of the crease. We found that with increasing METT thickness, the crease effect decreases. This is because increasing the thickness of METT will decrease the depth of incorporation of METT, causing an increase in the initial length (an) of the suspended part. In addition, METT with a greater thickness requires greater force to stretch to the same deformation (fig. S3B). METT will detach from the skin if the force exceeds the adhesion limit of the adhesives, which is equivalent to increasing the suspended part (an) According to the crease effect equation, we know that increasing the length of the suspended part (an) will decrease the crease effect.

Pressure sensitive adhesives are essential for the crease effect. Inside Fig. 2K, METTs without adhesive (22 μm), PDMS (21 μm) and Ecoflex (28 μm) are compliant; they can be embedded in the folds of the fingers when used as substrates for strain sensors. When you bend your finger, these substrates will come off the skin (Fig. 2I, on the right, compliant, but not adherent), which is equivalent to increasing the initial length of the suspended part. In contrast, METT can stick firmly to the skin, even in deformation, because of the pressure sensitive adhesive, concentrating the tension on the suspended part (Fig. 2I, right, METT). We can use PU (polyurethane) to manufacture METT with the crease amplification effect (Fig. 2K) However, we cannot achieve a METT with the effect of amplifying the crease using silicones like PDMS and Ecoflex. These silicones have an inert surface and cannot adhere to pressure sensitive adhesives and therefore cannot immobilize on the surface of the fingers.

Pressure sensitive adhesives can firmly attach METT to the skin, even during vigorous exercise. Figure 2F shows the removal of METT from the proximal interphalangeal joint (PIP), which indicates that pressure sensitive adhesives can make METT adhere firmly to the skin. The peel strength between the METT and the skin is 0.82 N cm-1, which is much stronger than Ecoflex, PU and PDMS (<0.01 N cm-1) External supports are required to correct PDMS-based and PU-based strain sensors in PIP (Fig. 2J) when bending your finger. Ecoflex-based sensors detach from the finger when bending it (frequency = 2 Hz) while the sensor is facing the ground, because the gravity of the sensor is greater than the adhesion between the finger and the sensor. In contrast, METT can be firmly attached to the PIP without detachment, even during vigorous exercise (bending your finger at a frequency of 4 Hz) while the sensor is facing the ground.

To demonstrate the scalability of METT, we manufactured a seven-layer METT as an extendable heater. Figure 3A presents the top view of the seven-layer heater. Each layer of electrical circuit contains an MPC-based coil heater with two connection points at both ends (Fig. 3A, top). These electrical connection points are used to form vertical electrical connections with heaters in other layers (fig. S4). Therefore, seven heaters in seven different layers are connected in series to the power supply. The thermal image (Fig. 3B) demonstrates that MPC-based heaters in different layers formed electrical connections through the connection points. The MPC in different layers other than the connection points formed good electrical insulations by the SBS, with no short circuits found in the thermal image. We apply 30% uniaxial deformation to the seven-layer extendable heater; the thermal image shows that this extensible device still works in deformations. However, with the increase in the number of layers, the conformability of the tattoo will decrease with increasing thickness. Electronic tattoos with two layers are sufficient for most functions.

To balance the formability and electrical insulation between the different layers of METT, we need to determine the minimum thickness of the SBS. METT needs to be very thin to make the crease conform. However, if the SBS layers for insulation are too thin, conductors based on MPC in different layers will lose electrical insulation, causing a short circuit. We found that when the thickness is less than 13.70 μm, the SBS film has poor insulation capacity. From the characterization by scanning electron microscopy (SEM), we can see the contour of the MPC after the coating of the SBS film (4.8 μm) on the MPC (fig. S5). After stretching the MPC to 50% deformation for 100 cycles, some of the liquid metal droplets will erupt from the SBS film (Fig. 3, C and D, on the left and fig. S5A), causing a short circuit between the different layers. When the thickness of the SBS film is 13.70 μm, there are no eruptions of liquid metals in the SBS film, either at rest or in deformation (Fig. 3, C and D, right and fig. S5B). The conductivity test of the MPC in different layers also shows good insulation. To obtain METT with good conformability and insulation capacity, we need to manufacture layers of SBS with a thickness close to 14 μm.

O n– Layered METT usually contains a n + 1 SBS layer. To ensure good electrical insulation and conformability, the thickness of each layer of SBS should be about 14 μm. As a result, the circuit modules transferred to the skin will have a thickness of 14n + 22 μm. One-layer METT can be embedded in fingerprints. However, increasing the number of circuit layers will sacrifice the formability of the METT. The three-layer METT will lose its conformability in digital printing, but it can still be embedded in the folds of the PIP. We normally manufacture METT with two circuit layers that can meet most circuit design conditions.

We studied the electrical connection points to connect MPC in different layers. Inside Fig. 1B, when the silicone stamp is removed, a shallow blind hole will be left in the SBS film, the MPC at these points will be exposed, while other parts will be sealed by the SBS. These blind holes are called electrical connection points. After printing another layer of MPC on the SBS film, the MPC in different layers will form vertical connections at the electrical connection points. The orifice depth is equal to the thickness of the SBS film. We found that the edge of blind holes with a depth of 13.70 μm will not block the paths of the MPC. From the SEM characterization, we can see that the liquid metal particles can fill the edges of the blind holes, connecting MPC in different layers of SBS (Fig. 3E) However, when we increase the depth of the blind hole to about 18.13 μm, from the SEM characterization, the MPC paths will be blocked by the edge of the holes (fig. S6A). To reconstruct the MPC paths, we usually add a drop of MPC paint (50 μl) to the edge of the holes (fig. S6B). Thus, we can manufacture multilayer circuit modules through the electrical connection points.

We feature the cross section of a three-layer METT. From the SEM characterization (Fig. 3F), we found that the cross section has a three-layer structure. Discontinuous liquid metal particles are incorporated into the SBS film, which suggests that liquid metal particles have formed conductive networks in each layer. The SBS formed a good insulation between the layers of MPC.

We put the three-layer METT in Figure 1 containing 15 strain sensors (Fig. 4D) and 1 heater for the left hand to test its performance in detecting deformation and heating (Fig. 4E) Using the three-layer METT, we can simultaneously measure hand movement at 15 degrees of freedom and adjust the temperature, which is impossible for reported single-layer electronic tattoos. Figure 4F shows the heating performance of the heater; the MPC heater on METT can heat the back of the hand to a temperature of around 45 ° C in 30 s. The serpentine shape of the heater in the thermal image also demonstrates that the SBS layer can form a good insulation between the different layers of the circuit, since no short circuit was found. Although the increase in METT layers decreases the sensitivity of the voltage sensors in METT (Fig. 4C), the three-layer METT is able to monitor hand movements. Figura 4G mostra as mudanças de sinal em tempo real dos 15 sensores de tensão e mudanças de temperatura do aquecedor no METT com diferentes movimentos da mão. Podemos ajustar a temperatura do aquecedor no METT controlando o tempo liga-desliga. A temperatura aumentará gradualmente de 34 ° a 37 ° C em cerca de 1 min (3 s ligado e 3 s desligado). Descobrimos que a medição do PIP é muito sensível. Os sinais de saída no PIP são cerca de três a quatro vezes maiores do que no MCP e OAF. Mudanças em ângulos pequenos no PIP podem causar mudanças óbvias de sinal. Por exemplo, a mudança de sinal do toque PIP pode ser facilmente reconhecida do gesto “1” ao gesto “2” no 15º segundo. Em contraste, a medição de MCP e OAF não é sensível em ângulos pequenos. Os ângulos impostos ao MCP e OAF devem exceder um determinado valor para serem reconhecidos ao estender lentamente a mão da extensão total dos dedos até o punho (25 a 30 s). Em geral, podemos usar METT para medir os movimentos das mãos.

Conseguimos um METT de duas camadas para controlar remotamente uma mão robótica com 6 graus de liberdade. Este METT de duas camadas contém cinco sensores de tensão em PIPs e um sensor de tensão no pulso, que pode monitorar 6 graus de liberdade em tempo real. O METT pode ser transferido diretamente para a mão ou para uma luva descartável, fixando-o na superfície e removendo o filme de liberação (Fig. 5, A e B) O METT pode ser transferido para a maioria dos substratos apenas se os adesivos sensíveis à pressão no METT forem aderentes aos substratos. O METT é controlado por um dispositivo parecido com um relógio através dos pads de contato externos (Fig. 5B) Figura 5C shows the system-level overview of the robot control system, which contains signal transduction, conditioning, processing, and transmission paths. The circuit design for the sensing system is shown in fig. S10. The strain sensors are connected to Wheatstone bridges. Signals caused by bending the fingers will be amplified and transmitted to the robotic hand through Bluetooth. As a result, we can wear the METT to control the robotic hand wirelessly. The robotic hand can imitate the movements of our hand without abnormal vibration (Fig. 5D and movie S2). The signals obtained from the METT attached to the PIP when we make various gestures are shown in Fig. 4G, which suggests that strain sensors on fingers have good repeatability and do not interfere with each other. Signals from each finger are stable when the finger remains stationary, which avoids the abnormal vibration of the robotic hand. This robot control system will have great potential in the medical system and military field for performing dangerous tasks remotely.

### Conclusions

In this work, we have achieved multilayered electronic tattoos that can enable the crease amplification effect. We found that the conformal and sticky structure of the stretchable METT can enable the crease amplification effect, which will lead to the amplification of the output signal of the strain sensors on the METT. We create a layer-by-layer fabrication strategy to fabricate the METT with different layers, while the crease amplification effect can be retained.

The MPC is indispensable in the METT, because the MPC-based strain sensors and interconnects have excellent stretchability and repeatability, which can function in large local deformation (~500% for the one-layered METT on the PIP when bending the finger; section S1) caused by the crease amplification effect.

By contrast, the carbon nanomaterials such as graphene and carbon nanotubes usually have poor conductivity (0.1 to 100 S/m) and low stretchability (<200%) (34, 35) They have potentials as strain sensors, but they are not suitable as stretchable interconnects because their conductivity is several orders of magnitude worse than that of metal. Besides, the large local deformation (~500%) caused by the crease amplification effect will cause the failure of the carbon nanomaterial–based strain sensors.

The nanostructured metal, especially the silver nanowires, has comparable conductivity and satisfactory stretchability (~800%). They have the potential as stretchable interconnects, but they are not suitable as strain sensors because the repeatability of the nanostructured metal is poor. The electrical performance of the silver nanowire will degrade slowly or sharply with the deformation cycles (36, 37) Besides, the electrical performance of the silver nanowire will decrease because of the high oxidation tendency of Ag.

Using the three-layered METT, we can measure 15 degrees of freedom of the hand. We believe that by increasing the degrees of freedom in robotic hands, we can use the robotic control system to perform delicate and complicated tasks remotely, which have a great potential in the medical system, virtual reality, and wearable robots. In the future, we can fabricate creases/cracks on the MPC-based strain sensors, and the sensitivity of the MPC-based strain sensors can be adjusted by the density and the width of the creases/cracks. Thus, the strain sensors based on the crease amplification effect will have broader applications, not limited to skin with creases.

Acknowledgments: Funding: We thank the National Key R&D Program of China (2018YFA0902600 and 2017YFA0205901), the National Natural Science Foundation of China (21535001, 81730051, and 21761142006), Shenzhen Bay Laboratory (SZBL2019062801004), the High-level University Construction Fund from Shenzhen Government (nos. G02386301 and G02386401), and the Tencent Foundation through the XPLORER PRIZE for financial support. Author contributions: X.J. conceived and supervised this project. L.T. designed, fabricated, and characterized the METT. J.S. screened the polymers for fabricating the METT. L.T., J.S., and X.J. wrote the manuscript. All authors discussed the results and commented on the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.