Ultrathin Conformable Electronic Tattoo for Tactile Sensations

Conformable electronics has emerged in recent years as an innovative research field and the ability of conformable devices to monitor human physiological signals has been extensively explored. Therefore, in this study, the possibility of using conformable electronics as active devices capable of providing stimuli to the human body is investigated. In particular, a new approach is proposed to elicit tactile sensations on human skin using an operating principle based on the generation of localized heat in correspondence with a closed volume of air. This latter consequently expands causing the deformation of a thin membrane. The use of fast prototyping fabrication techniques, i.e., inkjet printing, and commercially available materials, i.e., transfer tattoo paper, allow the device to be produced quickly and easily transferred on the target substrate. Despite the ultrathin thickness (few micrometers), it is possible to observe forces and displacements thanks to localized heating at very low working powers (<300 mW). A pilot test on a voluntary subject demonstrates how it is possible to discriminate the tactile sensation elicited by the active tattoo device. While the working principle on a single taxel is demonstrated, these results show the potential of the new approach for developing wearable tactile displays.


Introduction
Tactile sensation is essential for performing grasping and manipulation tasks, for experiencing the surrounding world, and for enabling new communications between machines and the human body, virtual reality, and immersive experience. [1][2][3] Over the years, numerous examples of stimulation technologies have been proposed with the aim of reproducing the sense of touch. Some fields of application concern the possibility There is, therefore, growing interest in the possibility of upgrading common medical tools-such as adhesive patches that allow intimate coupling with human skin-into advanced medical technologies for diagnosis and therapies. [48] In addition to the most studied sensorized devices, the advent of new materials and new manufacturing technologies paved also the way for the development of conformable technologies able to provide some sensations to the human body via thermal, mechanical, and electrical stimulation. [17,49] These devices are becoming increasingly popular: the uniform and constant adhesion between the device and the skin during stimulation can certainly favor the intensity of the sensation provided-especially in the case of pneumatic actuators-as well as avoid burns due to the presence of air gaps and consequent hotspots in the case of electrical stimulation. [50][51][52] We present in this work a conformable and ultrathin electronic device for electro-thermo-pneumatic stimulation, capable of providing localized tactile sensations thanks to its slight expansion that causes pressure sensations on the human skin. Since vibrotactile sensation has been widely explored as a way to restore tactile sensations through the use of haptic displays, [53] the device can be activated thanks to a pulsed driving voltage in order to generate tactile stimuli. In particular, we developed a tattoo-based device consisting of a silver electrode printed on tattoo paper. The device can be easily transferred directly on human skin by simply wetting it with some water. The size of the tattoo is such that it can be attached to the subject's fingertips, where the mechanoreceptors are known to be densely packed, [54,55] but also to other parts of the human body, so as to have a wide range of applicability. Moreover, biocompatibility is fully ensured thanks to two layers of parylene C-one at the top and one at the bottom-that insulate the device from the skin. [56,57] Thanks to the very low thermal capacity of the ultrathin conductive film, it can be activated with low power (145 mW peak) and in a short time interval (in the order of 100 ms). The device has been widely characterized in terms of exerted force, displacement, and heat generation. Moreover, we assessed its capability of providing tactile sensations with a very preliminary test conducted on a voluntary subject.

Tattoo Structure and Working Principle
A notable advantage of ultrathin conductive films is their low thermal capacity. This aspect allows the generation of a fast, controlled, and localized heat flow through a substrate. The schematic in Figure 1a represents the new approach that we propose for eliciting localized pressure sensations on the human skin. This approach mainly consists of using very thin layers where a controlled volume of air could be enclosed. It is possible to place a resistance at the lower level of the air volume which, thanks to the Joule heating effect, provides a very localized and fast increase in temperature once powered. The heat generated causes the expansion of the volume of air and consequently the thin tattoo to locally deflect, thus exerting slight pressure on the human skin. The device can be activated by using a pulsed power and by adjusting parameters like the pulse width (PW) and the period-and consequently the duty cycle (D). The use of pulsed power first allows to generate a tactile sensation-since the thin device expands and deflates following the pulsed signal-and, second, it helps to limit the increase of temperature when the device is directly in contact with human skin as only the average power has to be considered as effective activation power of the device.
As unconventional substrate for the fabrication of our ultrathin and conformable device we chose to use a temporary transfer tattoo paper, as already explored for other ultrathin and conformable electronic devices recently demonstrated, such as bio-electrodes [42,58] OLEDs, [59] and transistors. [60] The use of temporary transfer tattoo paper grants the device to be efficiently transferred to several different substrates -also nonplanar surfaces, it is readily available commercially, and, finally, allows for the production of conformable electrodes and electric paths perfectly adhering to human skin. [42,58] Inkjet printer technique allows the deposition of ultrathin films of the conductive silver ink directly on tattoo paper (covered by 1 µm thin parylene layer to improve mechanical strength), as represented in Figure 1b and as reported in detail in Section 4.2. The dimensions of the printed electric circuit were chosen to allow the tattoo to be worn on a human fingertip (4 × 11 mm 2 ). Furthermore, the electric path presents a restricted section (0.5 × 1 mm 2 ) to achieve a higher resistance in this point (Figure 1c and a close up of the restricted section in Figure S1 in the Supporting Information), thus producing localized joule heating. In order to create and enclose a controlled volume of air at the top of the narrow path, polystyrene microparticles (MP, 260 nm nominal diameter) are added as single drop casting from the solution, followed by a further coating of 1 µm thin parylene layer. We specifically chose these particles because of their dimensions: they were big enough to actually create an accessible volume of air on the target thickness with limited number of layers (good for air expansion), but small enough to obtain full external surface encapsulation with 1 µm thin parylene layer (thanks to the limited diffusion of parylene inside the particle assembly). The final fabricated device is shown in Figure 1d. In a bunch of 20 devices manufactured, only 2 were found to be nonconductive (90% yield), but 100% of those that worked were still conductive when transferred to a flat surface.

Morphological Characterization
Morphology of the device was characterized once transferred on a glass substrate (Figure 2a) by means of focused-ion beam (FIB) and profilometry. We acquired FIB pictures of all the layers of the device once it was transferred on a glass slide.   thickness of 3.14 ± 0.04 µm. This measure is highly homogeneous and fully consistent with single-layer parylene thickness of 1.28 ± 0.04 µm (measured independently by optical confocal profilometry, see Figure S3 in the Supporting Information) and the thickness of tattoo ethyl cellulose substrate form literature (300-400 nm, as reported in ref. [42]).

Thermomechanical Characterization
All the measurements carried out for the thermomechanical characterization were performed on a complete device transferred on a glass slide. The fabricated devices had an overall resistance of 7 ± 2 Ω, (averaged over 30 samples). Excitation voltage has been finely adjusted in the experiment to obtain the specific power of 145 mW of peak power during the first tests (obtained by applying a voltage in the range 0.85-1.15 V). The applied power was then modulated through different ON/OFF temporal sequences. Thermal measurements revealed how, as expected, the heat flow was localized at the level of the restricted section-where the resistance is higher-once the device was activated with a pulsed power. The sequence of thermal images in Figure 3a-i shows how the temperature values accordingly increase with increasing duration of the PW of the applied power. By applying pulsed power, the temperature values that the device reached at each peak were found to be stable throughout the duration of the thermal test, with only small variations, as can be observed by the graph in Figure 3a-ii. This aspect revealed how the increase of the pulsed temperature did not damage the ultrathin electrode, even reaching temperatures of 80-100 °C. One should notice that the temperature on the substrate side should be much lower when in contact with a material with higher heat capacity, i.e., human skin, and, as a result, it cannot cause any thermal pain to the user.
After the thermal assessment, we tested the device under a load cell to ensure that the temperature increase causes the air expansion and, consequently, the ultrathin device to expand and deflate. The tattoo sample was transferred on a glass slide and placed horizontally. In this way, thanks to a micropositioner it was possible to align and put in contact the actuated part of the device and the circular tip of the customized end effector which was mounted on the load cell. We measured the force exerted by the device at an acquisition frequency of 100 Hz and by applying the same peak power used for thermal   characterization. However, different activation signals were considered, keeping constant power and the duty cycle (20%) and varying the activation frequency, as shown in Figure 3b-i. These data revealed that even with the same duty cycle, signals with longer PW could exert higher pressures, made exception for 50/200 signal, where no visible variations of forces have been measured. This is due to the signal-to-noise ratio, defined as the ratio between the peak value of the force obtained (at specified condition) and the RMS (root means square) of the background signal. The calculated ratios were 1.45, 1.86, 5.11, and 12 respectively for 10, 4, 2, and 1 Hz signals. The first two conditions had a signal-to-noise ratio lower than 2, indicating poor efficiency in detecting forces, while 2 and 1 Hz signals appeared to be well defined and reliable.
As can be observed from Figure 3b-ii, variation of PW highly influences the force with a percentage difference of ≈150% between the peak of 50 and 200 ms pulse duration. The average force reached on five different samples (resistance of 8 ± 3 Ω) with the corresponding standard deviation is reported in Figure 3b-iii and it shows how the force reaches peaks of almost 1.5 mN on average when applying 200 ms PW pulse at 145 mW power. Intra-and inter-reproducibility of the force measurements performed on the devices are shown in Figure 3b-ii,biii respectively (average ± standard deviation is reported). Noteworthy, the device responds rapidly to the applied pulsed power, following the PW of the signals, in the order of hundreds and tens of milliseconds. Raw data of the recorded force for the three different pulses and for the 5 different samples used respectively for intra and inter-reproducibility are reported in Figures S4 and S5 in the Supporting Information.
Force measurements show that in the case of 200 ms PW signal it is possible to obtain on average 2 mN of the exerted force, that corresponds to a pressure of 325 Pa, calculated considering the area of the flat tip of the end effector used for the measurements (6.15 mm 2 ). A single pulse of 50 ms PW reached the force of 0.8 mN (130 Pa), while 100 ms PW generated a force of 1.3 mN (211 Pa). These results are in line with the tactile sensitivity thresholds found in the literature, where tactile sensitivity tests revealed an average detection threshold of approximately 0.3 mN for a force applied to the fingertip (on a specific population). [61] Analysis of the achievable displacement of the active area (in the absence of mechanical load) was then performed by using a system comprising a laser sensor with high resolution (0.3 µm).
The results obtained are shown in Figure 3c. As for the force measurements, the duty cycle was fixed at 20% and different activation frequencies were considered. As expected, for very short PW duration at higher frequencies no measurable displacement was observed, while the movements increased with increasing duration of the signal. Notably, a displacement of up to 2 µm can be achieved by the expansion of the small amount of air present within the active area, as it is possible to directly observe in Video S1 in the Supporting Information.
Despite the force exerted by the tattoo device was above the range of thresholds of the touch sensitivity, thus resulting more than satisfactory for tactile devices to be used on the fingertip, different parts of the human body might require higher forces so that the tactile sensation can be perceivable. For achieving this, one can think that a larger amount of MPs (and air) could be beneficial. Instead, we noticed that the presence of too many MPs tends to decrease the performance (data not shown), probably due to an increase of thermal inertia of the tattoo system. Beyond that, other methods are being explored to increase performances, relying mainly on the use of different materials that can withstand higher heat pulses, as well as more controlled and reproducible volume of air.
As a final characterization test, we assessed the lifetime of the device by exciting the tattoo sample continuously for 4 h. The combination of parameters used for the applied voltage was the one that could possibly cause larger damages to the device among the tested signals, i.e. 200 ms PW and 1000 ms Period. In this case, a power of 150 mW was used and the results of the durability test are reported in Figure S6 in the Supporting Information. The tattoo sample exerted force pulses continuously for all the duration of the test and maintained constant force values, consistent with the results previously obtained, showing that the device can work continuously for more than 4 h. The resistance of the device was recorded at the beginning and at the end of the test: a variation of only 2.3 Ω was observed after 4 h of continuous use (from 3.5 to 5.7 Ω). Based on the test, we believe that the eventual causes of failure may be related to the continuous use that could affect the performance of the device, i.e. slow increase of resistance can lead over a relatively long time to a nonactive device (if driven by voltage). From a structural point of view, we did not notice significant changes in the overall structure of the device after 4 h of activation. In general, these performances are in line with targeted practical operational time for tattoo electronic devices, that generally last from minutes up to some hours. [58,62]

Test on a Voluntary Subject
As final demonstration, the electronic tattoo has been transferred on the fingertip of a voluntary subject in order to prove its capability of eliciting tactile sensations (Figure 4a). First, it was necessary to find the activation power at which the subject could perceive a distinguishable sensation. Hence, the test started with a voltage of 0.8 V (resistance of the considered tattoo: 6 Ω), with two pulses of 100 ms PW delayed by 500 ms in time. The voltage was increased by step of 0.1 V until the subject referred to clearly perceive the pulsed pressure sensation. The resulted pulsed voltage capable of providing a detectable pressure sensation was 1.3 V, corresponding to 250 mW peak. Once the perception threshold was found, the aforementioned pulses were applied for 10 trials, spaced out in time and alerting the subject each time the following test was started but sending the pulses randomly over time, based on the experimenter's decisions.
The subject was asked to press a button connected to the control electronics system when he perceived the tactile sensation. The graph in Figure 4b shows how the subject perceived 9 stimulations over 10, reacting within a time delay comparable with standard response to stimulations (within half a second from the stimulation end, see Figure S7 in the Supporting Information for detailed view). One stimulation was not perceived. Therefore, an accuracy of 90% has been recorder for this very preliminary demonstrative test. Notably, the subject reported he did not perceive absolutely any thermal pain neither temperature increase during all the tests, confirming that the elicited pressure sensation was absolutely predominant compared to the thermal one. Concerning the transfer-on-skin yield, around 60% of tattoos worked once released on the fingertip (detecting at least one pulse; the percentage is the result of few transfers attempts thus not statistically significant).
Based on the results obtained by testing the tattoo on the subject, we further characterized the device at higher applied power. In particular, 250 mW of nominal power was considered, since it was the power at which our specific subject clearly perceived the tactile sensation. Following the same protocol already used for characterization, force and displacement values were recorded on a tattoo sample activated by using 200 ms PW and 1000 ms Period pulses. As clearly visible in Figures S8 and  S9 in the Supporting Information, the device provided higher values of both the considered quantities, reaching ≈6 mN (975 Pa) in terms of force (pressure) and ≈5 µm in terms of displacement, almost double the values recorded with 145 mW. Finally, also the specific conditions of subject experiment are reproduced (2 pulses of 100 ms PW delayed by 500 ms over time), to evaluate the perceived force. The graph in Figure S10 in the Supporting Information shows that force peaks around 4 mN can be easily reached, highly compatible with the perception of a touch sensation.

Conclusion
In this work we reported the fabrication and demonstration of an active tattoo capable of providing tactile sensations to human skin thanks to thermo-mechanical coupling. Such ultrathin device shows superior characteristics both in terms of power consumption and applicability. Conformable electronics indeed could be employed as an advantageous way to stimulate the human body due to the high level of adhesion and reduced weight that allow the continuous contact between the device and the skin. In some applications-one could think about the possibility to restore tactile feedback in amputees-also the cosmetics aspect of the device is of fundamental importance to improve its acceptability by the user, as well as the possibility to be noiseless.
Compared to the most investigated methods developed to provide tactile sensations to the human skin, the active tattoo presented in this work has the advantages of being soft and lightweight with a fast time response-as dielectric elastomer actuators technologies-as well as having the possibility of activating it with low driving voltages-as piezoelectric and electromagnetic actuators. A pilot test conducted on a voluntary subject revealed the actual capability of the device to provide tactile sensations by exerting a very localized pressure. In this work, we focused on the development and characterization of a single taxel. However, the approach, the materials and the fabrication techniques could be extended to the realization of more complex devices, such as larger tattooable tactile displays. Even if this is an explorative study and more development/ experiments have to be carried out (also on human subjects), we think this approach is promising toward the development of lightweight and portable haptic displays and devices.

Experimental Section
Materials: A commercial Silverjet DGP-40LT-15C (Sigma-Aldrich, US) Ag nanoparticles dispersion with a conductivity of 11 µΩ cm was purchased from Merck. Interconnection tracks between the printed layer and conductive wires were achieved by screen printing of a stretchable Ag conductor paste (CI-1036, Engineered Materials Systems). Temporary transfer tattoo kit Tattoo 2.1, by The Magic Touch Ltd., UK, was employed as the printing substrate and it comprised glue sheets and a paper liner with a decal coating layer that allows the prints to be transferred on the skin. [42,59] Commercial polystyrene microparticles of 0.260 µm diameter were purchased from Microparticles GmbH (Germany). Parylene C dimer was purchased from Specialty Coating System Inc. (IN, USA).
Fabrication of the Conformable Electrode: All the fabrication steps are summarized in Figure 1b. First, the tattoo paper was placed in a Parylene Coater PDS2010 (Specialty Coating Systems) in which 0.5 g of parylene C were loaded in order to obtain a thin parylene layer of about 1 µm over the tattoo paper. The backside of the tattoo paper has been covered with aluminum foil to allow the device to be released by wetting it with some water during the next steps.
The covered tattoo paper was directly used as the substrate for inkjet printing the specific design. Inkjet printing was performed with a Dimatix Materials Printer DMP-2800 (Fujifilm Corp., Japan) by using Dimatix disposable cartridges with 10 pL nozzle volume (DMC-11610). The Ag dispersion acting as ink was loaded into the cartridge. All the printing process was carried out in a clean room facility at room temperature. During the printing, a drop spacing-i.e., center-to-center distance from one drop to the next in X and Y position-of 25 µm was set for the resolution of the specific design and one layer of Ag ink was printed obtaining high conductivity of the conductive path (7 ± 2 Ω averaged on 30 samples). The printer platen temperature was set to 60 °C, for rapid solvent evaporation. After the printing, the samples were left in the oven at 120 °C for 10 min for annealing and complete removal of solvent traces.
Once the electrodes dried, they were screen printed with the Ag paste thanks to a custom-made mask obtained from a tablet screen protector foil cut with a CO 2 laser cutter (model VLS3.50 by Universal Laser System Inc., US) and by using a blade for uniformly distributing the paste. These conductive paths were required for the subsequent wiring step. The Ag paste was let drying in the oven for 10 min at 120 °C and, subsequently, used again for creating the contact between the fabricated electric path and conductive wires, so as to connect the device to the rest of the electronics later. The samples were baked again for 10 min at 120 °C in the oven and then on a hot plate at 70 °C where polystyrene microparticles were added at the level of the restricted section by using a micropipette (set to 5 µL).
As a final fabrication step, the samples were placed again in the parylene coater with 0.5 g of parylene in order to deposit the biocompatible layer also at the top of the device, which is the portion that will be in contact with the human skin.
The final device is then transferred on different substrates such as glass slides or human skin, as shown in Figures 1d and 4a. The transfer procedure consists first of applying the glue sheet from the tattoo paper kit-laser cut according to the need-on the device; then, the sample is placed on the target substrate and transferred by gently wetting the backside with a wetted sponge in order to remove the paper liner while leaving the device on the target substrate.
Ethical review and approval were waived for this study. Participants in study were chosen among the authors of the study and informed consent was obtained. Tattoo paper, parylene C and acrylic glue, materials in contact with the skin, are commercially available for this specific use.
Characterization of Devices-Morphological Characterization: Thickness measurements were carried out with a DCM 3D Confocal Profilometer (Leica Microsystems), on purposely prepared samples (parylene on a glass slide, codeposited together functional samples). Optical microscopic images have been acquired by using a Digital Microscopy Hirox KH-8700 digital microscope (Hirox, Japan).
Focused ion beam-milled cross-sections and scanning electron microscopy imaging of the complete transferred device has been obtained with a Dual Beam FIB/SEM Helios Nano-Lab 600i (FEI).
Scanning electron microscopy images of cross-sections were obtained under various sample tilt angle as reported (accelerating voltage 10 kV).

Characterization of Devices-Electrothermal Characterization:
The resistance values, expressed in Ohm, were obtained by a two-point measurement across the device contacts with a Fluke "187 True RMS Multimeter" precision multimeter.
Thermal experiments were performed using a FLIR A300 thermal imaging camera (FLIR) and related ResearchIR 4 software (used for analysis and post-processing) acquired at 9 Hz. The sample was in the air during registration and activation. Pulsed power was applied to the tattoo device to record its response to different signals. In particular, different PWs of the pulsed voltage have been considered: 10, 20, 50, 100, and 200 ms.
Characterization of Devices-Mechanical Characterization: Force measurements were performed using a customized system incorporating a load cell model LRF400 (0.25 lb full scale, by Futek, US), connected with a load cell amplifier (X200 gain, 10 V bridge excitation) and read out by with a NI-UBS 6009 DAQ card (National Instrument, US) and a custom developed software (developed with Visual Studio .NET, by Microsoft, US); the acquisition has been performed at 100 Hz, the root mean square noise of the measure was estimated to be in the order of 0.36 mN. The tattoo was transferred onto a glass slide and then placed horizontally. The actuated part of the device was, by means of a manual micropositioner, placed in contact with the circular flat tip of the end effector (6.15 mm 2 area) mounted on the load cell (see Figure S11 in the Supporting Information) in order to record the exerted force. As for thermal measurements, different pulsed powers were applied to the device to investigate the force response characteristics of the device. In particular, a fixed frequency of 1 Hz with different PW was considered (10, 20, 50, 100, and 200 ms) in a first series of experiments. Moreover, signals with duty cycle fixed to 20% were tested with different pulse widths and periods (P) (PW = 20 ms, P = 100 ms; PW = 50 ms, P = 250 ms; PW = 100 ms, P = 500 ms; PW = 200 ms, P = 1000 ms).
Displacement measurements were performed using a ILD 1401-10 Laser Sensor (Micro-Epsilon Optronic GmbH, DE) interfaced through RS232 over USB connection with a custom developed software (developed with Visual Studio. NET, by Microsoft, US) acquiring at 1 kHz. The test has been performed on a tattoo transferred on a glass slide, placed under the laser at working distance, focusing the spot directly onto the actuated region (a black permanent mark is applied on top of working area to have correct laser reflection for measurement) as shown in Figure S11 and Video S1 in the Supporting Information. To characterize the expansion movements of the devices, the same pulsed power signals used for the force measurements were applied.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.