Recently electronic tattoo sensors have attracted immense interest for health monitoring mainly due to their higher sensing performance than conventional dry sensors, owing to the ultra-low thickness which results in their conformability to the skin. However, their performance is worse than wet sensors. Further, these electronic tattoo sensors are not durable and reusable when free-standing because of their low thickness and being too delicate. Here, we report a remarkably high-performance freestanding, reusable, ultrathin and ultra-soft electronic tattoo sensor made of parylene-hydrogel double layer system with high water retention over extended periods that can be used for the extended period of 6 months. The hydrogel electronic tattoo (HET) sensors consist of electrically conductive self-adhesive hydrogel with a thickness of 20 µm and Young’s modulus of only 31 kPa at 37 °C, allowing for ultra-conformal contact to the skin microscopic features. Our HET sensors are fabricated using a scalable cost-effective method on ordinary tattoo papers and are laminated on the skin like temporary tattoos and were used for electrophysiological signals recording such as electrocardiography (ECG), electromyography (EMG), and skin hydration, temperature sensing. The HET sensors, for the first time, show 234% lower sensor-skin interface impedance (SSII) and significantly lower susceptibility to motion than gold standard medical grade silver/silver chloride wet gel electrodes which are known to have the lowest SSII and susceptibility to motion. Further, the low HET-skin interface impedance leads to a considerably larger signal amplitude and signal-to-noise ratio (SNR) of the electrophysiological signals recorded using HET sensors in comparison with those obtained using gold standard medical grade silver/silver chloride wet gel electrodes. The SNR of some types of electrophysiological signals such as EMG recorded using HET is up to 19 dB higher than gold standard medical grade electrodes due to higher signal amplitude, significantly lower susceptibility of HET to motion and lower motion artifacts. Also, the HET sensor is the first free-standing ultrathin tattoo sensor that can be transferred from the skin to tattoo paper and vice versa many times and the electrophysiological sensing quality remained high during repeated use for over 6 months.

Electroencephalography has garnered interest for applications in mobile healthcare, human–machine interfaces, and Internet of Things. Conventional electroencephalography relies on wet and dry electrodes. Despite favorable interface impedance of wet electrodes and skin, the application of a large amount of gel at their interface with skin limits the electroencephalography spatial resolution, increases the risk of shorting between electrodes, and makes them unsuited for long‐term mobile recording. In contrast, dry electrodes are better suited for long‐term recordings but susceptible to motion artifacts. In addition, both wet and dry electrodes are non‐adhesive to the hairy scalp and mechanical support, or chemical adhesives are used to hold them in place. Herein, a conical microstructure array (CMSA) based sensor made of carbon nanotube‐polydimethylsiloxane composite is reported. The CMSA sensor is fabricated using the innovative, cost‐effective, and scalable method of viscosity‐controlled dip‐pull process. The sensor adheres to the hairy scalp by generating negative pressure in its conical microstructures when it is pressed against scalp. Aided by the application of a trace amount of gel, CMSA sensor establishes good electrical contact with the skin, enabling its applications in mobile electroencephalography over extended periods. Notably, the signal quality of CMSA sensors is comparable to that of medical‐grade wet gel electrodes.

The remarkable mechanical robustness and excellent electrical/thermal properties make graphene a promising candidate for future flexible, stretchable and bio-integrated electronics. In practice, many soft electronics such as the graphene electronic tattoos (GETs) demand the chemical vapor deposited (CVD) graphene to be supported by a deformable substrate. Moreover, various conductive overlayers need to directly laminate on graphene to make electrical contacts. To investigate the mechanical reliability of CVD graphene in these situations, we fabricated CVD monolayer graphene supported by ultrathin poly(methyl methacrylate) (PMMA) substrate and also placed gold/polyethylene terephthalate (Au/PET) and graphene/PMMA (Gr/PMMA) overlayers on graphene. The stretchability of the Gr/PMMA and the overlayer-Gr/PMMA interface was characterized by electrical resistance change during uniaxial tensile tests. Combined with in situ microstructure and Raman investigation, we identified four deformation/fracture stages of Gr/PMMA—pre-cracking elastic deformation, limited micro-cracking in graphene, extensive cracking in graphene, and macro-cracking in PMMA. While micro-cracks emerged in graphene at very small strain (~0.9%), the electrical conductivity of the Gr/PMMA specimen remained up to tensile strains of ~14.5%. In contrast, 100 nm-thick Au film supported by the same PMMA substrate fully ruptured after tensile strains of ~1%. When laminating Au/PET and Gr/PMMA over Gr/PMMA, we found that the Au/PET- Gr/PMMA interface is very vulnerable but the Gr/PMMA- Gr/PMMA interface behaves very similar to intact Gr/PMMA electromechanically. The cyclic behaviour of Gr/PMMA, the effects of PMMA thickness and adhesion are also briefly discussed. The present experimental study provides fundamental insight into the failure of ultrathin polymer-supported graphene and its electrical contacts, which is critical for designing future graphene-based soft electronics.

Here, we report an ultrasoft extra long-lasting, reusable hydrogel-based sensor that enables high-quality electrophysiological recording with low-motion artifacts. The developed sensor can be used and stored in an ambient environment for months before being reused. The developed sensor is made of a self-adhesive electrical-conductivity-enhanced ultrasoft hydrogel mounted in an Ecoflex-based frame. The hydrogel’s conductivity was enhanced by incorporating polypyrrole (PPy), resulting in a conductivity of 0.25 S m⁻¹. Young’s modulus of the sensor is only 12.9 kPa, and it is stretchable up to 190%. The sensor was successfully used for electrocardiography (ECG) and electromyography (EMG). Our results indicate that using the developed hydrogel-based sensor, the signal-to-noise ratio of recorded electrophysiological signals was improved in comparison to that when medical-grade silver/silver chloride (Ag/AgCl) wet gel electrodes were used (33.55 dB in comparison to 22.16 dB). Due to the ultra-softness, high stretchability, and self-adhesion of the developed sensor, it can conform to the skin and, therefore, shows low susceptibility to motion. In addition, the sensor shows no sign of irritation or allergic reaction, which usually occurs after long-term wearing of medical-grade Ag/AgCl wet gel electrodes on the skin. Further, the sensor is fabricated using a low-cost and scalable fabrication process.

Carbon nanotube (CNT)-based nanocomposites have found applications in making sensors for various types of physiological sensing. However, the sensors’ fabrication process is usually complex, multistep, and requires longtime mixing and hazardous solvents that can be harmful to the environment. Here, we report a flexible dry silver (Ag)/CNT/polydimethylsiloxane (PDMS) nanocomposite-based sensor made by a solvent-free, low-temperature, time-effective, and simple approach for electrophysiological recording. By mechanical compression and thermal treatment of Ag/CNT, a connected conductive network of the fillers was formed, after which the PDMS was added as a polymer matrix. The CNTs make a continuous network for electrons transport, endowing the nanocomposite with high electrical conductivity, mechanical strength, and durability. This process is solvent-free and does not require a high temperature or complex mixing procedure. The sensor shows high flexibility and good conductivity. High-quality electroencephalography (EEG) and electrooculography (EOG) were performed using fabricated dry sensors. Our results show that the Ag/CNT/PDMS sensor has comparable skin–sensor interface impedance with commercial Ag/AgCl-coated dry electrodes, better performance for noninvasive electrophysiological signal recording, and a higher signal-to-noise ratio (SNR) even after 8 months of storage. The SNR of electrophysiological signal recording was measured to be 26.83 dB for our developed sensors versus 25.23 dB for commercial Ag/AgCl-coated dry electrodes. Our process of compress-heating the functional fillers provides a universal approach to fabricate various types of nanocomposites with different nanofillers and desired electrical and mechanical properties.