Graphene electronic tattoos 2.0 with enhanced performance, breathability and robustness

The study addresses key limitations of first-generation graphene electronic tattoos (GETs) for wearable healthcare: lack of breathability to water vapor/sweat, average electronic performance with high variability due to defects, and susceptibility to transfer-induced cracks and grain boundaries. The purpose is to engineer GETs 2.0 with improved electrical interface (lower tattoo–skin impedance and sheet resistance), enhanced robustness and reproducibility, and added breathability via microperforations, while maintaining the ultra-thin, conformal, and skin-like properties of graphene. Given that atomically thin electrodes rely heavily on in-plane conductivity for charge transfer at the skin interface, the authors hypothesize that multilayer stacking and incorporation of graphene nanoscrolls (GNS) can mitigate monolayer defects, reduce device-to-device variability, and improve bioelectrode performance. They further seek to enable sweat permeability and evaluate additional functionalities such as heating and temperature sensing.

Graphene has emerged as a promising material for wearable biomedical applications due to its electrical, mechanical, and optical advantages. Earlier GETs (first reported in 2017) demonstrated ~85% transparency, >40% stretchability, and self-adhesion via van der Waals forces, enabling operation on skin with reduced motion artifacts. However, prior GETs used research-grade graphene at small scale and did not explicitly optimize interface impedance, sheet resistance, or water vapor transmission rate (WVTR). Monolayer GETs exhibited performance degradation at centimeter scale and significant variability due to grain boundaries, cracks, and folds. Conventional medical electrodes (e.g., Ag/AgCl gels) achieve low impedance via conformal ionic contact but lack the ultrathin, imperceptible form factor of GETs. The work positions multilayer graphene and GNS as strategies to overcome monolayer limitations, improve robustness, and add breathability compared to earlier GETs and rigid metal-based electrodes.

Graphene growth and normal GET preparation: Single-layer CVD graphene on copper was used. PMMA (495 A3) was spin-coated at 2500 rpm for 1 min, baked at 200 °C for 20 min, then Cu was etched in 0.1 M ammonium persulfate for ~12 h. The PMMA/graphene film was rinsed in DI water and transferred onto temporary tattoo paper with a resin-based release coating, dried overnight, and mechanically patterned (typical active area ~25 mm²).

Multilayer GET preparation: Multilayer GETs (2L and 3L) were fabricated by sequential wet transfer stacking of individual CVD monolayers (piece A and piece B), with intermediate annealing at 200 °C for 10 min to reflow PMMA and improve adhesion. For 3L, the transfer/etch/transfer cycle was repeated. Raman spectroscopy indicated stacked monolayers (I2D/IG ~1.2–2.0), low defect density (ID/IG < 0.04), and occasional regions consistent with near-"magic-angle" stacking (~12°) in polycrystalline films.

Graphene nanoscrolls (GNS): During Cu wet etching, unsupported backside graphene rolls into GNS that electrostatically attach to PMMA-supported graphene, forming conductive features. SEM and AFM characterized GNS as tangled networks with lengths up to tens of micrometers and average diameter ~32 ± 11 nm.

Holey GETs: Microperforations were embossed mechanically using a plotter on the tattoo-supported GETs. Triangular holes (50–100 µm sides; ~3000 µm² area; ~1 mm pitch) were introduced (material forms flaps rather than complete removal).

On-skin and electrical measurements: Skin impedance was measured with a Hioki IM3536 LCR meter (10 Hz–1 MHz, 50 mV AC, no DC bias). Sheet resistance was measured using 4-probe TLM structures on substrates with gold contacts; additional tests included "reversed" GET placement (graphene facing up) exploiting edge folds and hole flaps for biplanar contact. Ecoflex-coated glass (2–4 mm) served as skin-mimicking substrates for some resistance tests. Conductive adhesive tapes (10 µm) with evaporated Ti/Ni (10/90 nm) were used for skin contacts.

Evaporation experiments (WVTR): Normal and holey GETs (N=7 each; ~10×10 mm²) were transferred as caps on water-filled plastic vials; three vials were left open. Water loss was monitored up to 2 weeks (qualitative at ~50 °C; quantitative daily averages reported).

GET heaters: Tattoos (~5.8 mm² stripes) on 4-terminal gold contacts were driven with constant DC voltages (commonly 5–20 V, occasionally up to 30 V) for 1 min on/1 min off. Surface temperatures were monitored by an optical thermal camera (FLIR E320/ET320). Heating efficiency was computed as power per °C.

GET temperature sensing: TLM devices on a hotplate were cycled from room temperature to ~60 °C and cooled, with resistance sampled at ~1 Hz; TEC α was calculated from ΔR/R0 vs ΔT. Surface temperature was cross-checked with a thermocouple.

Human subject procedures were approved by UT Austin IRB (2018-06-805), with informed consent.

• Multilayer stacking and GNS significantly improve electrical performance and reduce variability compared to monolayer GETs.
• 1L GETs with GNS: ~20% lower skin impedance and ~45% lower sheet resistance on average vs 1L without GNS (sheet resistance drops from 2686 to 1456 Ω/sq). GNS add minor disorder increasing SD in sheet resistance slightly, but overall averages improve markedly.
• 2L GETs: Performance saturates with two stacked monolayers; adding GNS does not produce distinct further improvements. The two layers provide redundancy: carriers bypass defects via the alternate monolayer. Average sheet resistance around ~500 ± 100 Ω/sq (for various channel sizes).
• 3L GETs (with GNS): Lowest tattoo–skin impedance and sheet resistance among tested configurations, with impedance 6.8 ± 0.6 kΩ at 10 kHz and sheet resistance 0.5 ± 0.25 Ω/sq (as reported). The improvement from 2L to 3L is ~17%, whereas 2L is nearly 2× better than 1L on average.
• Variability reduction: GETs 2.0 (2L/3L or 1L+GNS) show up to 5-fold reduced standard deviations in sheet resistance and skin impedance versus 1L.
• Phase behavior: 1L GETs show phase >45° at high frequencies; 2L/3L GETs exhibit more non-polarizable, efficient interfaces (improved phase response across frequency).
• Transfer robustness: 2L and 3L GETs transfer with almost ~100% yield and are easier to handle than 1L GETs, especially those without GNS.
• Contact engineering and reversed placement: Reversed GETs (graphene up) can still make electrical contact due to edge folds and hole flaps; measured sheet resistance on TLM comparable to normal placement (example 638 Ω/sq; average 555 ± 116 Ω/sq, N=2). Contact resistance decreases with channel width (~350 Ω at 3 mm width, ~260 Ω at 5 mm, ~190 Ω at 10 mm).
• Breathability (WVTR): Holey GETs allow efficient evaporation nearly matching open vials: 2770 ± 494 g/m²/day vs open 3600 ± 600 g/m²/day; normal GETs still permit 1400 ± 350 g/m²/day, likely via micro-defects. Introducing holes causes only minor impedance increase for 1L GETs and negligible impact for 2L/3L GETs.
• Heating: All multilayer configurations function as heaters. Average heating efficiencies are similar across stacks: 1L 6.2 ± 1.2 mW/°C; 2L 6.2 ± 1.3 mW/°C; 2L reversed 6.5 ± 1.0 mW/°C; 3L 5.6 ± 1.0 mW/°C. 3L shows highest temperature rise per applied voltage (up to ~4 °C/W effect). Device area affects areal efficiency: smaller (3×3 mm²) channels show higher mW/°C per cm² after normalization.
• Temperature sensing: TEC α during heating is ~1–3×10⁻³ °C⁻¹; during cooling ~0.2–0.5×10⁻³ °C⁻¹, attributed to current-induced annealing. 3L GETs show tighter distributions (lower SD) than 1L/2L, indicating more uniform sensing behavior.

The study demonstrates that addressing in-plane conductivity limitations of atomically thin electrodes via multilayer stacking and incorporation of graphene nanoscrolls effectively mitigates defects (cracks, grain boundaries, folds) inherent to monolayer CVD graphene and transfer processes. This reduces tattoo–skin interface impedance and sheet resistance while dramatically lowering device-to-device variability—critical for reliable bioelectrode performance across users and sessions. The improved phase characteristics and lower impedance at relevant bioimpedance frequencies suggest more efficient, less polarizable interfaces, translating into higher-fidelity electrophysiological recordings. The introduction of microperforations solves a major barrier to long-term wear—sweat and vapor impermeability—by enabling high WVTR with minimal impact on electrical performance, particularly for 2L/3L GETs. Discovery of robust biplanar contacting through edges and hole flaps simplifies interconnection strategies and may extend device lifetime by reducing mechanical stress at contacts. Furthermore, the GETs’ capability as conformal heaters and their stable, uniform temperature coefficient in multilayers broaden application scope (skin warming, clinical therapies, thermal management). Collectively, these advances directly address the initial challenges of performance, robustness, and breathability, enhancing the practicality of GETs for ambulatory health monitoring and hybrid wearable systems.

This work introduces GETs 2.0—multilayer, microperforated graphene electronic tattoos with enhanced electrical performance, robustness, and breathability. Stacked monolayers (2L/3L) and GNS incorporation reduce sheet resistance and tattoo–skin impedance while significantly lowering variability. Holey morphology achieves WVTR approaching open conditions without compromising electrical performance, and also enables reliable biplanar electrical contact. GETs 2.0 operate as efficient, skin-conformal heaters with average heating efficiency ~6 mW/°C and demonstrate viable temperature sensing, with 3L devices offering the most uniform response. Given the modest incremental improvement from 2L to 3L versus increased fabrication complexity, 2L GETs (with or without GNS) are identified as an optimal balance for next-generation wearable electrodes. Future work may include scalable single-crystal multilayer stacking (e.g., controlled twist angles), extended on-body trials for long-term perspiration compatibility, and integration with rigid electronics (e.g., smartwatches) for power, sensing, and wireless communication.

• Fabrication scalability and complexity: 3L assembly requires significantly longer, more complex processing, yet yields only ~17% improvement over 2L devices.
• Material variability: While reduced, residual variability remains, particularly for 1L and 2L devices in temperature sensing (higher SD than 3L), reflecting dependence on CVD graphene quality and transfer-induced defects.
• Breathability evaluation: WVTR and evaporation were assessed using capped vials rather than in vivo perspiration tests; translation to on-skin, dynamic sweating conditions may differ.
• Electrical benchmarking: Performance comparisons emphasize impedance at 10 kHz; broader physiological frequency band performance on-body under motion and perspiration is not fully detailed.
• Special stacking phenomena: Potential advantages of controlled twist ("magic-angle") stacking could not be explored due to the lack of large-area single-crystal stacked graphene.
• Limited sample sizes for some configurations (e.g., reversed GET TLM, N=2) constrain statistical power.