Advances in printed electronics continuously stimulate the scalable and sustainable fabrication of wearable and flexible devices. Unlike traditional subtractive processes, direct ink printing offers a viable alternative for rapid, large-scale manufacturing due to its relatively facile, cost-effective procedures, and desirable material compatibility and utilization. Nevertheless, with regard to the room-temperature fabrication of flexible electronics, the existing printing approaches are yet far from ideal. The major hurdle comes from ink formulations and printing processes. Most printable inks (metal or carbon-based) either suffer from complex ink formulations (requiring surfactants/rheological modifiers/binders), unsatisfactory intrinsic physical properties (i.e., poor electrical conductivity), or demand lengthy post-treatments (i.e., high-temperature annealing to remove additives). These issues complicate the device's manufacturing process, exclude the low-cost polymeric substrate choices while compromise device printing precision and thereafter properties. On the other hand, the increasing structural complexity of flexible electronics (especially various wireless multi-functional systems) puts higher requirements for direct ink printing technologies, particularly high-precision conformal printing and multi-module integrated manufacturing to avoid time-consuming cumbersome transfer and assembly processes. One promising approach is to combine additive-free aqueous conductive inks with extrusion printing technology. Compared with other printing methods, extrusion printing allows high-throughput additive manufacturing without additional masks and accessories, offering greater opportunities in material/substrate choice and printing extensibility (from co-planar to three-dimensional). Nonetheless, while additive-free aqueous conductive inks have been proven promising in simplifying ink formulation and eliminating post-processing, it remains a challenge to endow functional inks with appropriate rheological and electrical properties to achieve room-temperature fabrication of flexible wireless electronics. In this regard, as an emerging family of 2D transition metal carbides and nitrides, MXenes, which possess unique properties desirable for functional inks (i.e., metallic conductivity, hydrophilicity, and negative surface charges), offer new possibilities. Especially, Ti3C2Tx (Tx denotes surface terminations), as the most widely studied MXene, allows controllable formation of stable additive-free aqueous colloidal dispersions without any additives and thus has been applied in different devices, such as batteries, micro-supercapacitors (MSCs), triboelectric nanogenerators (TENGs), transistors, sensors, etc. However, when it comes to fabricating flexible wireless electronics, little success has been achieved on room-temperature, fine printing precision of component lines with ultrahigh electrical conductivity based on MXene inks. Moreover, feasible protocol of multi-module integrated printing for all-printed wireless devices has been rarely reported so far. This article reports on the realization of direct printing of flexible wireless electronics at room temperature. Additive-free MXene aqueous inks possess desirable rheological and electrical properties, stemming from large single-layer ratio, high ink concentration, and narrow flake size distribution, to achieve metallic conductivity in high-precision extrusion printing, thereby enabling the efficient fabrication of monolithic flexible systems for energy harvesting, wireless communication, and sensing. The complete demonstration of all-MXene functional electronics powerfully reveals the enormous potential of room-temperature direct MXene printing for large-scale integrated manufacturing of next-generation wearable and flexible wireless electronics.

The authors reviewed existing literature on printed electronics, highlighting challenges in room-temperature fabrication due to complex ink formulations, poor conductivity, and the need for post-treatments. They discussed the advantages of additive-free aqueous conductive inks and extrusion printing for high-throughput manufacturing. The potential of MXenes, particularly Ti3C2Tx, for creating functional inks was emphasized, noting their metallic conductivity, hydrophilicity, and negative surface charges. While MXenes have been used in various applications, their use in room-temperature, high-precision printing of flexible wireless electronics with ultrahigh conductivity and multi-module integration remained largely unexplored. The authors cite relevant research on conductive inks, 3D printing, MXene synthesis, and applications in energy storage and sensing.

The study began with the formulation of additive-free MXene aqueous inks using a modified minimally intensive layer delamination route, employing optimized centrifugation and ultrasonic methods to enhance rheological and electrical properties. The resulting inks, with a high concentration (~60 mg mL⁻¹), consisted of predominantly single-layer Ti3C2Tx flakes with an average flake size of ~1.6 µm and a thickness of ~1.5 nm. These inks exhibited shear-thinning viscoelastic properties, enabling continuous extrusion and quick solidification. The ink wettability on substrates was improved through plasma treatments. Room-temperature direct printing was performed using a programmable three-axis pneumatic extrusion dispenser, allowing the creation of various patterns and circuits on planar or curved surfaces with high precision. Uniform MXene lines with line gaps ranging from 3 to 30 µm were printed, with a reported line gap of 3 µm, a state-of-the-art achievement. Lines with varying widths were also printed, exhibiting high spatial uniformity (0.43%). The printed MXene films were characterized using SEM, Raman spectroscopy, and optical profilometry, confirming the dense stacking and interconnectedness of Ti3C2Tx nanosheets, leading to high conductivity and flexibility. Electrical conductivity was measured, reaching up to 6900 S cm⁻¹ under low-humidity conditions. The printing efficiency was evaluated using a figure of merit (FoM = σc), which showed a significantly higher value compared to other printable inks. Near-field communication (NFC) antennas and radio-frequency identification (RFID) temperature sensing systems were fabricated using the MXene inks. The performance of the NFC antennas was evaluated through simulation and measurement of surface current distribution, resistance, Q factor, and mechanical robustness under bending. The RFID tags were tested for radiation patterns and temperature sensing capabilities. Finally, a fully integrated flexible wireless sensing system was constructed, incorporating MXene-based NFC antennas, micro-supercapacitors (MSCs), a temperature sensor, and a humidity sensor. The MSCs were characterized using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements, demonstrating high areal capacitance, energy density, and power density. The temperature and humidity sensors were evaluated for their sensitivity and response time.

This research successfully demonstrated the room-temperature, high-precision printing of flexible wireless electronics using additive-free MXene aqueous inks. The key findings include: 1. **High-performance MXene inks:** The developed MXene inks exhibited desirable rheological and electrical properties due to their high single-layer ratio (>90%), high concentration, and narrow flake size distribution. This resulted in metallic conductivity (up to 6900 S cm⁻¹) in printed tracks with an ultra-narrow line gap of 3 µm and high spatial uniformity (0.43%). This represents a significant advancement in direct printing using nanomaterials at room temperature. 2. **High-resolution printing:** The direct printing method achieved unprecedented high-resolution patterns with line gaps as small as 3 µm, significantly smaller than previously reported values. This enables the fabrication of high-packing density electronic devices. 3. **Versatile substrate compatibility:** The MXene inks demonstrated compatibility with various substrates (glass, PDMS, PET, PI), offering flexibility in device fabrication and cost reduction. 4. **Functional devices:** The study successfully fabricated functional devices including all-MXene-printed NFC antennas capable of wireless communication and energy harvesting, and RFID temperature sensing tags demonstrating high sensitivity and stability. 5. **Integrated system:** A fully integrated flexible wireless sensing system was demonstrated, combining NFC antennas, MSCs, a temperature sensor, and a humidity sensor. The MSCs achieved high areal capacitance (~900 mF cm⁻²), energy density (up to 9.7 µWh cm⁻²), and power density (1.875 mW cm⁻²). The temperature and humidity sensors exhibited good sensitivity and response time. 6. **Room-temperature processing:** All fabrication steps were conducted at room temperature, avoiding high-temperature annealing and making the process compatible with low-cost and temperature-sensitive substrates.

The successful fabrication of all-MXene-printed wireless sensing electronics at room temperature demonstrates a significant advancement in the field of flexible electronics. This work addresses the limitations of existing printing techniques by employing additive-free MXene inks with exceptional rheological and electrical properties. The high-precision printing capabilities allow for the creation of complex, integrated devices with high packing density. The use of room-temperature processing significantly simplifies the manufacturing process, reduces costs, and expands the range of compatible substrates. The demonstration of functional NFC and RFID devices showcases the practical applications of this technology in areas such as IoT, wearable electronics, and environmental monitoring. The integrated sensing system further emphasizes the potential for creating self-powered, multifunctional devices. This research opens up new avenues for large-scale manufacturing of next-generation flexible wireless electronics, particularly in applications where low cost, high performance, and flexible form factors are crucial.

This study successfully demonstrated the room-temperature high-precision printing of flexible wireless electronics using additive-free MXene aqueous ink. The resulting high-performance inks enabled the fabrication of high-resolution, multi-functional devices including NFC antennas, RFID temperature sensors, and micro-supercapacitors, all integrated into a single flexible system. This method offers significant advantages over existing techniques in terms of simplicity, cost-effectiveness, and material compatibility. Future research could focus on optimizing the MSCs for higher energy density, exploring diverse MXene compositions for enhanced performance, and developing more sophisticated sensing modalities. The findings have broad implications for the development of advanced wearable electronics, IoT devices, and other applications.

While the study demonstrates significant advancements, some limitations exist. The long-term stability of the printed devices under various environmental conditions requires further investigation. The scalability of the current printing process for mass production needs to be assessed. The performance of the micro-supercapacitors, while impressive, could be further enhanced through optimization of the electrolyte and device architecture. The sensitivity and response time of the sensors could also be improved through material and design modifications. A more comprehensive analysis of the devices' reliability and lifetime is also warranted.