Enhanced Oxidation-Resistant Cu@Ni Core-Shell Nanoparticles for Printed Flexible Electrodes

Printable and flexible integrated sensing systems for wireless healthcare

The rapid development of wearable sensing devices and artificial intelligence has enabled portable and wireless tracking of human health, fulfilling the promise of digitalized healthcare applications. To achieve versatile design and integration of multi-functional modules including sensors and data transmission units onto various flexible platforms, printable technologies emerged as some of the most promising strategies. This review first introduces the commonly utilized printing technologies, followed by discussion of the printable ink formulations and flexible substrates to ensure reliable device fabrication and system integration. The advances of printable sensors for body status monitoring are then discussed. Moreover, the integration of wireless data transmission via printable approaches is also presented. Finally, the challenges in achieving printable sensing devices and wireless integrated systems with competitive performances are considered, so as to realize their practical applications for personalized healthcare.

3D printed electronics with nanomaterials

A large variety of printing, deposition and writing techniques have been incorporated to fabricate electronic devices in the last decades. This approach, printed electronics, has gained great interest in research and practical applications and is successfully fuelling the growth in materials science and technology. On the other hand, a new player is emerging, additive manufacturing, called 3D printing, introducing a new capability to create geometrically complex constructs with low cost and minimal material waste. Having such tremendous technology in our hands, it was just a matter of time to combine advances of printed electronics technology for the fabrication of unique 3D structural electronics. Nanomaterial patterning with additive manufacturing techniques can enable harnessing their nanoscale properties and the fabrication of active structures with unique electrical, mechanical, optical, thermal, magnetic and biological properties. In this paper, we will briefly review the properties of selected nanomaterials suitable for electronic applications and look closer at the current achievements in the synergistic integration of nanomaterials with additive manufacturing technologies to fabricate 3D printed structural electronics. The focus is fixed strictly on techniques allowing as much as possible fabrication of spatial 3D objects, or at least conformal ones on 3D printed substrates, while only selected techniques are adaptable for 3D printing of electronics. Advances in the fabrication of conductive paths and circuits, passive components, antennas, active and photonic components, energy devices, microelectromechanical systems and sensors are presented. Finally, perspectives for development with new nanomaterials, multimaterial and hybrid techniques, bioelectronics, integration with discrete components and 4D-printing are briefly discussed.

Surface-passivated Cu conductors for high-temperature sulfurous environments

Advanced materials capable of withstanding extreme environments garner extensive interest in the development of next-generation advanced anti-corrosion electronics. Herein, we report that the surface passivation of printed copper conductors imparts corrosion resistance in high-temperature sulfurous environments while maintaining a high electrical conductivity of 4.42 MS m^-1 when subjected to a sulfur-containing environment at 350 °C for 12 h. This study provides potential for the development of surface-passivated copper conductors that are resistant to the sulfidizing environments found in several applications of modern technology.

Copper inks for printed electronics: a review

Conductive inks have attracted tremendous attention owing to their adaptability and the convenient large-scale fabrication. As a new type of conductive ink, copper-based ink is considered to be one of the best candidate materials for the conductive layer in flexible printed electronics owing to its high conductivity and low price, and suitability for large-scale manufacturing processes. Recently, tremendous progress has been made in the preparation of cooper-based inks for electronic applications, but the antioxidation ability of copper-based nanomaterials within inks or films, that is, long-term reliability upon exposure to water and oxygen, still needs more exploration. In this review, we present a comprehensive overview of copper inks for printed electronics from ink preparation, printing methods and sintering, to antioxidation strategies and electronic applications. The review begins with an overview of the development of copper inks, followed by a demonstration of various preparation methods for copper inks. Then, the diverse printing techniques and post-annealing strategies used to fabricate conductive copper patterns are discussed. In addition, antioxidation strategies utilized to stabilize the mechanical and electrical properties of copper nanomaterials are summarized. Then the diverse applications of copper inks for electronic devices, such as transparent conductive electrodes, sensors, optoelectronic devices, and thin-film transistors, are discussed. Finally, the future development of copper-based inks and the challenges of their application in printed electronics are discussed.

Self-Organizing, Environmentally Stable, and Low-Cost Copper-Nickel Complex Inks for Printed Flexible Electronics

Cost-effective copper conductive inks are considered as the most promising alternative to expensive silver conductive inks for use in printed electronics. However, the low stability and high sintering temperature of copper inks hinder their practical application. Herein, we develop rapidly customizable and stable copper-nickel complex inks that can be transformed in situ into uniform copper@nickel core-shell nanostructures by a self-organized process during low-temperature annealing and immediately sintered under photon irradiation to form copper-nickel alloy patterns on flexible substrates. The complex inks are synthesized within 15 min via a simple mixing process and are particle-free, air-stable, and compatible with large-area screen printing. The manufactured patterns exhibit a high conductivity of 19-67 μΩ·cm, with the value depending on the nickel content, and can maintain high oxidation resistance at 180 °C even when the nickel content is as low as 6 wt %. In addition, the printed copper-nickel alloy patterns exhibit high flexibility as a consequence of the local softening and mechanical anchoring effect between the metal pattern and the flexible substrate, showing strong potential in the additive manufacturing of highly reliable flexible electronics, such as flexible radio-frequency identification (RFID) tags and various wearable sensors.

Ultrahigh Temperature Copper-Ceramic Flexible Hybrid Electronics

Advanced high-temperature materials, metals and ceramics, have been widely sought after for printed flexible electronics under extreme conditions. However, the thermal stability and electronic performance of these materials generally diminish under extreme environments. Additionally, printable electronics typically utilize nanoscale materials, which further exacerbate the problems with oxidation and corrosion at those extreme conditions. Here we report superior thermal and electronic stability of printed copper-flexible ceramic electronics by means of integral hybridization and passivation strategies. High electric conductivity (5.6 MS/m) and thermal stability above 400 °C are achieved in the printed graphene-passivated copper platelet features, while thermal management and stability above 1000 °C of printed electronics can be achieved by using either ultrathin alumina or flexible alumina aerogel sheets. The findings shown here provide a pathway toward printed, extreme electronic applications for harsh service conditions.

Laser Erasing and Rewriting of Flexible Copper Circuits

Integrating construction and reconstruction of highly conductive structures into one process is of great interest in developing and manufacturing of electronics, but it is quite challenging because these two involve contradictive additive and subtractive processes. In this work, we report an all-laser mask-less processing technology that integrates manufacturing, modifying, and restoring of highly conductive Cu structures. By traveling a focused laser, the Cu patterns can be fabricated on the flexible substrate, while these as-written patterns can be selectively erased by changing the laser to a defocused state. Subsequently, the fresh patterns with identical conductivity and stability can be rewritten by repeating the writing step. Further, this erasing-rewriting process is also capable of repairing failure patterns, such as oxidation and cracking. Owing to the high controllability of this writing-erasing-rewriting process and its excellent reproducibility for conductive structures, it opens a new avenue for rapid healing and prototyping of electronics.

Recent advances in printed flexible heaters for portable and wearable thermal management

Flexible resistive heaters (FRHs) with high heating performance, large-area thermal homogeneity, and excellent thermal stability are very desirable in modern life, owing to their tremendous potential for portable and wearable thermal management applications, such as body thermotherapy, on-demand drug delivery, and artificial intelligence. Printed electronic (PE) technologies, as emerging methods combining conventional printing techniques with solution-processable functional ink have been proposed to be promising strategies for the cost-effective, large-scale, and high-throughput fabrication of printed FRHs. This review summarizes recent progress in the main components of FRHs, including conductive materials and flexible or stretchable substrates, focusing on the formulation of conductive ink systems for making printed FRHs by a variety of PE technologies including screen printing, inkjet printing, roll-to-roll (R2R) printing and three-dimensional (3D) printing. Various challenges facing the commercialization of printed FRHs and improved methods for portable and wearable thermal management applications have been discussed in detail to overcome these problems.

Highly Conductive, Flexible, and Oxidation-Resistant Cu-Ni Electrodes Produced from Hybrid Inks at Low Temperatures

Recently, Ni and Ni-Cu nanoparticle-based inks have gained considerable research interest because of their high corrosion resistance as conductors in electronic devices. However, reported inks based on Cu-Ni nanoparticles need to be sintered at high temperatures above 300 °C to obtain electrodes with high conductivity on the order of 10^-5 Ω·cm. This study proposes a new conductive Cu-Ni-based hybrid ink that could be sintered at only 150-180 °C for producing Cu-Ni electrodes with low electrical resistance, high oxidation resistance, and flexibility. The hybrid ink contains Cu flakes and a complex of nickel formate and 1-amino-2-propanol (NiF-AmIP complex). At 150-180 °C, the Cu flakes catalyze the self-reduction of the NiF-AmIP complex, and Cu-Ni electrodes with high conductivity (on the order of 10^-5 Ω·cm) are formed on flexible polymer substrates at temperatures exceeding 150 °C. Analysis indicates that metallic Ni was decorated on the Cu flakes (especially on the edge) to improve the electrode's conductivity, oxidation resistance, and flexibility by forming bridging interconnections between the Cu flakes. The Cu-Ni electrodes demonstrated high stability against oxidation up to approximately 400 °C in air, as well as at 80 °C and 80% RH after 7 days. In addition to the excellent oxidation stability, the Cu-Ni electrode showed high durability under mechanical bending stress. Such sintered Cu-Ni electrodes obtained from hybrid inks have great potentials in printed/flexible devices due to their oxidation resistance and cost-effectiveness.

Recent Advancement of Emerging Nano Copper-Based Printable Flexible Hybrid Electronics

Printed copper materials have been attracting significant attention prominently due to their electric, mechanical, and thermal properties. The emerging copper-based flexible electronics and energy-critical applications rely on the control of electric conductivity, current-carrying capacity, and reliability of copper nanostructures and their printable ink materials. In this review, we describe the growth of copper nanostructures as the building blocks for printable ink materials on which a variety of conductive features can be additively manufactured to achieve high electric conductivity and stability. Accordingly, the copper-based flexible hybrid electronics and energy-critical devices printed by different printing techniques are reviewed for emerging applications.

Synthesis of silver nanoparticles embedded with single-walled carbon nanotubes for printable elastic electrodes and sensors with high stability

Soft electronic devices that are bendable and stretchable require stretchable electric or electronic components. Nanostructured conducting materials or soft conducting polymers are one of the most promising fillers to achieve high performance and durability. Here, we report silver nanoparticles (AgNPs) embedded with single-walled carbon nanotubes (SWCNTs) synthesized in aqueous solutions at room temperature, using NaBH4 as a reducing agent in the presence of highly oxidized SWCNTs as efficient nucleation agents. Elastic composite films composed of the AgNPs-embedded SWCNTs, Ag flake, and polydimethylsiloxane are irradiated with radiation from a Xenon flash lamp within a time interval of one second for efficient sintering of conductive fillers. Under high irradiation energy, the stretchable electrodes are created with a maximum conductivity of 4,907 S cm-1 and a highly stretchable stability of over 10,000 cycles under a 20% strain. Moreover, under a low irradiation energy, strain sensors with a gauge factor of 76 under a 20% strain and 5.4 under a 5% strain are fabricated. For practical demonstration, the fabricated stretchable electrode and strain sensor are attached to a human finger for detecting the motions of the finger.

Surface and Interface Designs in Copper-Based Conductive Inks for Printed/Flexible Electronics

Silver (Ag), gold (Au), and copper (Cu) have been utilized as metals for fabricating metal-based inks/pastes for printed/flexible electronics. Among them, Cu is the most promising candidate for metal-based inks/pastes. Cu has high intrinsic electrical/thermal conductivity, which is more cost-effective and abundant, as compared to Ag. Moreover, the migration tendency of Cu is less than that of Ag. Thus, recently, Cu-based inks/pastes have gained increasing attention as conductive inks/pastes for printed/flexible electronics. However, the disadvantages of Cu-based inks/pastes are their instability against oxidation under an ambient condition and tendency to form insulating layers of Cu oxide, such as cuprous oxide (Cu2O) and cupric oxide (CuO). The formation of the Cu oxidation causes a low conductivity in sintered Cu films and interferes with the sintering of Cu particles. In this review, we summarize the surface and interface designs for Cu-based conductive inks/pastes, in which the strategies for the oxidation resistance of Cu and low-temperature sintering are applied to produce highly conductive Cu patterns/electrodes on flexible substrates. First, we classify the Cu-based inks/pastes and briefly describe the surface oxidation behaviors of Cu. Next, we describe various surface control approaches for Cu-based inks/pastes to achieve both the oxidation resistance and low-temperature sintering to produce highly conductive Cu patterns/electrodes on flexible substrates. These surface control approaches include surface designs by polymers, small ligands, core-shell structures, and surface activation. Recently developed Cu-based mixed inks/pastes are also described, and the synergy effect in the mixed inks/pastes offers improved performances compared with the single use of each component. Finally, we offer our perspectives on Cu-based inks/pastes for future efforts.

Control of thermal deformation with photonic sintering of ultrathin nanowire transparent electrodes

Development of electronic devices on ultrathin flexible plastic substrates is of great value in terms of portability, cost reduction, and mechanical flexibility. However, because thin plastic substrates with low heat capacity can be more easily damaged by thermal energy, their use is limited. Highly flexible nanowire (NW) transparent conductive electrodes on ultrathin (∼10 μm) low cost polyethylene terephthalate (PET) substrates are fabricated. The control of intense pulsed light (IPL) irradiation process parameters to induce NW welding for maximum conductivity and minimal thermal damage of the PET substrate is explored. For this purpose, trends in temperature variation of NW thin films irradiated by IPL under various operating conditions are numerically analyzed using commercial software. Simulations indicate that irradiating light operated at a higher voltage and for a shorter time, and use of multiple pulses of low frequency can reduce thermal deformation of the PET substrate. Furthermore, we experimentally confirm that NW transparent electrodes can be successfully fabricated with less thermal deformation of the ultrathin plastic substrate when light is irradiated under well-controlled conditions derived from the simulation. The highly flexible NW transparent conducting electrode exhibits excellent mechanical flexibility to withstand severe deformation and can be successfully implemented in flexible organic light-emitting diodes (OLEDs).

Construction of Core-Shell Nanowire Arrays in a Cu-Cu2O Film Electrode for High Efficiency in Heat Dissipation

Thermal engineering dramatically impacts the efficiency of microelectronics, but the corresponding technology lags far behind the need. For energy-efficient thermal management, a Cu-Cu₂O film with highly ordered core-shell nanowire arrays and a good self-protection property was successfully fabricated using the magnetron sputtering method. The dense arrangement of nanowires in the films enhances the electronic transport property (220 mΩ sq^-1), while the modified stable Cu₂O layer maintained its perfect heat dissipation property, along with long-term thermal stability. The core-shell and nanogaps structure imparted an anisotropic thermal conductivity, where the out-plane electronic thermal conductivity (321 ± 16 W m^-1 K^-1) was 33.6 times higher than the in-plane value. To study the role of anisotropic properties in heat dissipation, a boiling experiment and thermal simulation were undertaken. The Cu-Cu₂O core-shell electrode was beneficial to elevate the heat transfer coefficient, which would cause a fast directional transport and reduction of interfacial superheating. We demonstrated that an advancement of microelectronics could be achieved by integrating Cu electrodes with an ordered architecture.

The laser writing of highly conductive and anti-oxidative copper structures in liquid

Flexible conductive structures are essential for the fabrication of commercial integrated electronic devices. Developing efficient processes for manufacturing these structures with high conductivity and stability is significant. Based on a modifiable cost-effective Cu-based ionic liquid precursor, here we present an in situ laser patterning technique to manufacture flexible electrodes. The fabricated Cu structure has excellent conductivity, approximately comparable to bulk Cu, while its oxidation resistance could be further enhanced through introducing an additional carbon source to form a Cu@C microstructure. The chemical and electrical stabilities are evaluated. This method provides a possible bottom-up route for manufacturing microelectronic devices in one step, as we demonstrated through a flexible heater.

Grab and Heat: Highly Responsive and Shape Adaptive Soft Robotic Heaters for Effective Heating of Objects of Three-Dimensional Curvilinear Surfaces

Soft actuators have received great research attention because of the recent rise of soft robotics. However, these actuators could perform only relatively simple deformations (such as bending, twisting, etc.) for manipulation, limiting their functionality. Here, we develop highly responsive and shape adaptive soft robotic heaters which not only can achieve large degree of deformation but also can grab and heat objects of three-dimensional (3D) curvilinear surfaces. With intentionally synthesized and selected materials for device fabrication, a U-shaped soft robotic heater exhibits a deformation angle of more than 860° and a curvature of 4.0 cm^-1 at a very low voltage of 2 V, and its curvature can quickly reach 1.31 cm^-1 within 6 s. Moreover, the device can also function as a stable heat source with temperature of 203 °C upon actuation, demonstrating a maximum energy efficiency of 7.44% as a heater. Importantly, the soft robotic heaters can deform to enclose 3D curvilinear surfaces with pressure to enable intimate contact for more effective heat transfer. The unique utility of the soft robotic heaters is illustrated through the heating of objects of various 3D shapes, showcasing their potential applications in soft robotics, advanced thermal therapy, food handling and processing, etc.

Conductive nanomaterials for 2D and 3D printed flexible electronics

This review describes recent developments in the field of conductive nanomaterials and their application in 2D and 3D printed flexible electronics, with particular emphasis on inks based on metal nanoparticles and nanowires, carbon nanotubes, and graphene sheets. We present the basic properties of these nanomaterials, their stabilization in dispersions, formulation of conductive inks and formation of conductive patterns on flexible substrates (polymers, paper, textile) by using various printing technologies and post-printing processes. Applications of conductive nanomaterials for fabrication of various 2D and 3D electronic devices are also briefly discussed.

All-Solution-Processed Thermally and Chemically Stable Copper-Nickel Core-Shell Nanowire-Based Composite Window Electrodes for Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have recently attracted tremendous attention because of their excellent efficiency and the advantage of a low-cost fabrication process. As a transparent electrode for PSCs, the application of copper nanowire (CuNW)-network was limited because of its thermal/chemical instability, despite its advantages in terms of high optical/electrical properties and low-cost production. Here, the copper-nickel core-shell nanowire (Cu@Ni NW)-based composite electrode is proposed as a bottom window electrode for PSCs, without the involvement of a high-cost precious metal and vacuum process. The dense and uniform Ni protective shell for CuNWs is attainable by simple electroless plating, and the resulting Cu@Ni NWs exhibit outstanding chemical stability as well as thermal stability compared with bare CuNWs. When the Ni layer with the optimal thickness is introduced, the Cu@Ni NW electrode shows a high transmittance of 80.5% AVT at 400-800 nm, and a sheet resistance of 49.3 ± 5 Ω sq^-1. Using the highly stable Cu@Ni NWs, the composite electrode structure is fabricated with sol-gel-derived Al-doped zinc oxide (AZO) over-layer for better charge collection and additional protection against iodine ions from the perovskite. The PSCs fabricated with AZO/Cu@Ni NW-based composite electrode demonstrate a power conversion efficiency (PCE) of 12.2% and excellent long-term stability maintaining 91% of initial PCE after being stored for 500 h at room temperature. Experimental results demonstrate the potential of highly stable Cu@Ni NW-based electrodes as the cost-effective alternative transparent electrode, which can facilitate the commercialization of PSCs.