Room-temperature high-precision printing of flexible wireless electronics based on MXene inks

The study addresses the need for room-temperature, high-precision, and conformal printing of flexible electronics suitable for IoT, wearable, and biomedical applications. Conventional direct ink printing faces challenges including complex ink formulations requiring additives, poor intrinsic conductivity of many printable materials, and the necessity for high-temperature post-treatments that are incompatible with low-cost polymer substrates. Increasing device complexity, especially for wireless multifunctional systems, also demands high-precision conformal printing and integrated multi-module manufacturing to avoid transfer and assembly steps. The authors propose combining additive-free aqueous conductive inks with extrusion printing to overcome these limitations. MXenes, particularly Ti3C2Tx, offer metallic conductivity, hydrophilicity, and negative surface charge, enabling stable additive-free aqueous dispersions. Despite prior applications of MXenes in various devices, achieving room-temperature, fine-resolution printing of highly conductive components and integrated wireless devices has been limited. This work demonstrates direct printing of flexible wireless electronics at room temperature using optimized MXene inks to fabricate and integrate antennas, micro-supercapacitors, and sensors with high resolution and conductivity.

The authors review printed electronics progress and limitations of existing inks and processes, highlighting that many metal and carbon-based inks require surfactants, binders, or post-annealing, complicating fabrication and limiting substrate choices. Extrusion printing offers high-throughput, mask-free additive manufacturing adaptable to diverse substrates and 3D geometries. Additive-free aqueous conductive inks can simplify processing but often lack balanced rheological and electrical properties for room-temperature fabrication of wireless electronics. MXenes, an emerging class of 2D carbides/nitrides, provide metallic conductivity and hydrophilicity; Ti3C2Tx in particular enables stable additive-free aqueous dispersions and has been used in batteries, micro-supercapacitors, triboelectric nanogenerators, transistors, and sensors. Prior reports demonstrated MXene inks and printed components but lacked room-temperature, fine-line, high-conductivity printing and fully integrated, all-printed wireless systems. The authors position their approach as addressing these gaps by optimizing MXene ink composition and printing to enable high-precision, room-temperature fabrication and integration of wireless modules.

Ink formulation and characterization: Additive-free Ti3C2Tx MXene aqueous inks were prepared via a modified minimally intensive layer delamination (MILD) route with optimized centrifugation and ultrasonic steps to tune rheology and electrical properties. The inks achieved high concentration (~60 mg mL-1) and consisted predominantly of ultrathin single-layer Ti3C2Tx flakes (>90% single-layer), with average lateral size ~1.6 µm and thickness ~1.5 nm. Rheology exhibited shear-thinning viscoelastic behavior with viscosity ~2.5×10^2 Pa·s, enabling continuous extrusion and rapid solidification. Stability: inks stored in Ar-sealed bottles in the dark at <4 °C were stable without sedimentation for at least two years; after oxygen removal, inks remained stable under ambient conditions. Substrate wettability was enhanced by plasma treatments to form continuous films and improve adhesion. Direct printing: A programmable three-axis pneumatic extrusion dispenser enabled room-temperature printing of digitally predefined patterns with specific line gaps and widths on planar and curved substrates (e.g., leaves, fruits). Uniform MXene lines with precise line gaps from 3 to 30 µm and various widths were printed with ultrahigh spatial uniformity (0.43%). Optical profilometry showed elliptical cross-sections due to suitable rheology (high G′). Raman spectra confirmed preservation of MXene features across substrates and over two years. Printed films consisted of densely stacked, shear-aligned Ti3C2Tx nanosheets forming metallic networks. Film thickness scaled linearly with the number of printing passes (N), and sheet resistance decreased with increasing thickness. Conductivity reached 6260 S cm-1 immediately after printing at N=2 and improved to 6900 S cm-1 after 4 h in low humidity (~10% RH). A figure of merit FoM=σc reached ~414,000 S cm-1 mg mL-1 (N=2). NFC antennas: Coil geometry was pre-simulated to maximize quality factor Q; selected design used 5 turns, 2.5 mm line width, and 0.5 mm spacing for 13.56 MHz operation. Antennas were printed on substrates including PVA, ferrite, PET, PDMS, and others. Finite element analysis assessed surface current distribution and strain under bending. Encapsulation studies and low-humidity storage longevity were evaluated. RFID antennas and sensing: Folded dipole with closed-loop was designed/printed for UHF (peak near 920 MHz). Antenna radiation patterns were measured in an anechoic chamber and compared to simulations. Bending durability (500 bends) and current density distribution were assessed. RFID temperature tags interfaced with a laptop-connected reader for backscatter-based sensing were demonstrated on plant leaves and human body. Integrated system: An all-MXene-printed integrated wireless sensing system was assembled by printing MXene modules (NFC antenna for wireless power/data, micro-supercapacitors, temperature/humidity sensors) on PDMS and integrating with a flexible printed circuit board (FPCB) control module (MCU, ADC, power management, NFC chip). The NFC-harvested energy was stored in printed MSCs to power sensors when the smartphone was removed. MSCs: interdigitated design (finger gap ~200 µm) using PVA/H3PO4 gel electrolyte; electrochemical performance evaluated by CV and GCD across scan rates and current densities, Ragone analysis, and cycling (up to 3000 cycles). Temperature sensing relied on PDMS thermal expansion affecting MXene network resistance; humidity sensing used a thin MXene film responding to interlayer spacing changes under humidity. Dynamic response, response/recovery times, and repeatability were measured.

- High-precision room-temperature extrusion printing of additive-free Ti3C2Tx MXene inks achieved ultrafine tracks with line gaps down to 3 µm and spatial uniformity of 0.43% across printed line widths. - Printed films exhibited metallic conductivity: up to 6260 S cm-1 immediately after printing (N=2) and up to 6900 S cm-1 after 4 h in ~10% RH without any annealing. - Film thickness increased linearly with the number of printing passes; sheet resistance decreased with thickness, indicating sharp printing edges and controlled deposition. - Record-high ink figure of merit FoM=σc ≈ 414,000 S cm-1 mg mL-1 (at N=2), surpassing other reported printable ink systems. - NFC antennas (13.56 MHz) with pre-simulated geometry (5 turns, 2.5 mm line width, 0.5 mm spacing) printed on diverse substrates demonstrated robust wireless energy/data transfer; energy harvested from a smartphone could light 168 parallel LEDs; tags functioned after two years of low-humidity storage and showed excellent bending robustness. - RFID UHF dipole antennas (~920 MHz) showed omnidirectional H-plane and dipole-shaped E-plane radiation patterns in simulations/measurements; wide frequency band with a peak near 920 MHz maintained after 500 bends; sufficient current density to power a microchip; enabled passive backscatter temperature sensing on plants and human skin. - All-MXene-printed micro-supercapacitors delivered high areal capacitance up to ~900 mF cm-2 (from CV and GCD), areal energy density up to 9.7 µWh cm-2, and power density up to 1.875 mW cm-2; cycling stability ~90% capacitance retention after 3000 cycles (7-unit module within 3 V); capability to power LEDs. - Temperature sensor exhibited positive temperature coefficient with sensitivity ~0.066% °C-1 and rapid response; humidity sensor showed repeatable resistance changes with response time ~25 s over 20–80% RH. - An integrated monolithic system combining NFC power/data, MSC energy storage, and T/H sensors on PDMS with an FPCB enabled wireless monitoring (e.g., plant microenvironment) and energy buffering.

The results demonstrate that carefully engineered, additive-free Ti3C2Tx MXene inks enable room-temperature, high-precision extrusion printing of flexible wireless electronics. Achieving metallic conductivity without thermal post-processing addresses a critical bottleneck of conventional inks that require annealing, thus broadening compatibility with low-cost, temperature-sensitive substrates and facilitating conformal printing on curved surfaces. The ultrafine line gap (3 µm) and high spatial uniformity support high-density circuit integration. The high figure of merit underscores efficient printing with high conductivity at practical concentrations. Functional demonstrations of NFC and RFID modules validate robust wireless power and data transfer, mechanical durability, and practical utility (e.g., lighting LEDs, passive temperature sensing). The integrated system highlights seamless on-device energy harvesting, storage, and sensing, pointing to applications in IoT, smart labels, and wearable/biomedical monitoring. The authors note scope for performance enhancements, including optimizing micro-supercapacitor architectures and improving sensor structures/materials. Considering the broad MXene family and rapidly evolving printing/wireless technologies, the platform could support more advanced modules and systems, potentially replacing conventional metal-based tags and reducing e-waste.

This work introduces a room-temperature, high-precision direct printing strategy for flexible wireless electronics using additive-free Ti3C2Tx MXene aqueous inks. The inks’ tailored rheological and electrical properties allow ultrafine printing (3 µm gaps), high spatial uniformity (0.43%), and metallic conductivity (up to 6900 S cm-1) without annealing. Distinct all-MXene functional modules—NFC antennas, UHF RFID antennas, micro-supercapacitors, and temperature/humidity sensors—were fabricated and integrated into a monolithic flexible system enabling wireless power harvesting, data transmission, and environmental sensing. The approach offers a scalable route for high-density, eco-friendly, printed electronics suitable for IoT, smart packaging, environmental monitoring, agriculture, healthcare, and beyond. Future work may focus on optimizing MSC electrode geometry and thickness, surface chemistry, asymmetric configurations, and sensor designs, as well as exploring other MXenes and energy/sensing modules to further enhance performance and functionality.

The reported devices exhibit promising but preliminary performance; the authors note considerable room for improvement. Micro-supercapacitors could benefit from further optimization of printed gap/thickness, surface chemistry, and asymmetric configurations to increase energy/power densities. Sensing modules (temperature and humidity) may be enhanced through structural design and nanomaterial modification to improve sensitivity, stability, and response time. While MXene stability was improved (e.g., low-humidity storage and encapsulation), long-term environmental durability under varied conditions and broader reliability testing remain to be fully established.