Electrodes are indispensable parts of some electronic devices. It is easy to prepare them with common structures but difficult to fabricate them with special structures by traditional methods. In order to meet the requirements of precision forming and high performance of electronic devices, the researchers use DIW to print electrodes with special structures.

Zhu et al. used the GO/silica powders suspension as the ink to print 3D periodic structures (Zhu et al., 2015). After supercritical drying and carbonization, this printed GO structure was converted to a 3D periodic graphene aerogel microlattice with high specific surface area and conductivity of 704 m² g⁻¹ and 278 S m⁻¹, respectively. Lacey et al. added holey graphene oxide (hGO) to green solvents, such as water, to make additive-free inks with appropriate rheology, which was employed to print Li-O₂ cathodes with the similar structures to Zhu et al. (2015), as shown in Figures 1A,B (Lacey et al., 2018). As the hGO was inherently nanoporous with 4–25 nm through-holes, the printed Li-O₂ cathodes had multi-sized pores from nanoscale (through-holes on hGO sheets) to microscale (lyophilization-induced micropores on the printed filaments) to macroscale (pores of mesh design) (Figure 1B). This hierarchical porosity was benefit for transfer of oxygen, electrolytes and ions. After the thermal reduction, the printed 3D electrode based on reduced hGO showed a much higher areal capacitance of 13.3 mAh cm⁻² at 0.1 mA cm⁻² than the traditional 2D electrodes (Lu et al., 2020; Yuan et al., 2020). Tang et al. printed graphene and its mixed-dimensional hybrid aerogels using graphene-based mixed-dimensional (2D + nD, n = 0, 1, or 2) hybrid materials as inks, overcoming the limitations of multi-components inhomogeneity and harsh post-treatments for additives removal (Tang et al., 2018). The coin-type symmetric supercapacitor with the printed hybrid aerogels as electrodes exhibited the maximum areal capacitance of 639.56 mF cm⁻² at 4 mA cm⁻² and capacitance retention of 71.4% at 40 mA cm⁻². The good capacitance and rate performance are attributed to the unique structure of 3D scaffolds of the printed electrodes, which can provide sufficient ion- and electron-transport paths from the bottom to top surface.

The interdigitated electrodes, being suitable for microelectronic devices, also can be prepared by the DIW technology. Fu et al. developed LFP (LiFePO₄)/GO and LTO (Li₄Ti₅O₁₂)/GO composite inks to printed the interdigitated cathode and anode electrodes for Li-ion microbattery, respectively (Figures 1C–E) (Fu et al., 2016). The LFP and LTO nanoparticles distributed homogeneously in the GO matrix, ensuring good contact, electrical conductivity and mechanical integrity of electrodes. After thermal treatment, the two printed electrodes delivered stable cycling performance with specific capacities of about 160 mAh g⁻¹ for LFP cathode and 170 mAh g⁻¹ for LTO anode, respectively. Sun et al. reported patterning viscous GO-based ink as interdigitated microelectrodes via DIW on the PET (polyethylene terephthalate) film, and forming the symmetrical micro-supercapacitor (Sun et al., 2015). It should be noted that the formed supercapacitor on PET showed a capacitance of up to 19.8 mF cm⁻² and excellent flexibility, mechanical robustness and cycling stability.

In order to further facilitate the portability of electronic devices, the development of light, thin, flexible and wearable devices has become a hot research topic. Therefore, it is very important to prepare fiber electrodes easily and efficiently. Wang et al. carried DIW to print the fiber cathode and anode with mushy inks composed of LFP or LTO particles, polyvinylidene fluoride (PVDF, as binder) and carbon nanotubes (CNTs, as conductor) (Wang et al., 2017). As shown in Figures 1F-J, the printed fibers were twisted together with gel polymer as the electrolyte to assemble an all-fiber quasi-solid-state Li-ion battery, which demonstrated high flexibility, strength and electrochemical stability. Zhao et al. assembled a fiber asymmetric supercapacitor by using the DIW-printed CNTs/V₂O₅ fiber cathode and CNTs/VN (vanadium nitride) fiber anode (Zhao et al., 2018). Its highest areal energy density (41.28 μWh cm⁻²) and power density (480 μW cm⁻²) obviously exceeded some fiber supercapacitors produced by other method (Kou et al., 2014; Wang et al., 2015; Xu et al., 2015). The above mentioned fiber electrodes can be potentially integrated into textile fabrics for future wearable electronic applications. In short, DIW provides a facile, fast, cost-effective and scalable strategy to prepare the electrodes which are difficultly fabricated by traditional methods, opening up new opportunities for smart, sophisticated, miniaturized, 3D integrated, and wearable electronic devices.

As a promising technology, printed electronics can fabricate electronic devices used in various fields, such as energy conversion and storage, flexible electronics, sensors and so on, attracting wide attentions. DIW represents an advanced and interesting method to replace traditional lithographic circuits due to its simple process, low cost, and digital patterning ability.

Zhang et al. melt-blended reduced graphene oxide (rGO) and poly(lactic acid) (PLA) as printable inks for printing 3D circuits, shown in Figures 2A–D (Zhang et al., 2016). The rGO/PLA composite ink had much improved mechanical properties in favor to maintain the printed structure and increase resolution, which were proved by the printed circuits with clear boundaries and sharp corners (Figures 2B,C). The printed circuits displayed excellent flexibility, as well as adhesion to the substrate without peeling off even under the bending state, and could be used to light up LEDs. Similarly, Abas et al. carried polyvinyl alcohol to modify the commercial carbon paste ink to improve its mechanical strength and adhesiveness, achieving high-quality micro-printing (Abas et al., 2019). Unfortunately, addition of the functional polymer greatly decreases the overall conductivity of the printed circuits. Hu et al. developed a particle-free composite ink which mainly consists of soluble silver salt and adhesive rubber for DIW-preparing highly conductive stretchable circuits (Hu et al., 2016). The ink was transparent solution with good chemical stability, and could be directly handwrote on various substrates to form well adhesive written traces. After in situ reduction, the traces turned into highly conductive circuits because of the formation of silver nanoparticles from high-loading silver salt in the ink. A heart-like circuit and pressure-sensitive switch were fabricated to present their working capability. In order to evaluate the printing quality, Tricot and coworkers developed a DIW equipment with a screw dispenser and silver paste ink (Electrodag 418SS from Henkel) to print circuits on commercial substrates or 3D objects, and compared with the conventional screen printing process (Tricot et al., 2018). The results showed DIW permitted printing patterns with a similar quality to the screen printing process but with a better regularity of the edges of printing lines. Besides, DIW saved more inks because of some ink remaining on the screen meshes after the squeegee stroke during screen printing. In the near future, integrating complex and fine circuitry on any 3D object will be realized.

Since high conductivity and nontoxicity, room-temperature liquid metals namely gallium and its alloys are important raw materials in the printed electronics field. Because of the wettability limitation of common gallium-based alloys to paper, only the polymer can be used as the printed substrate in various printing technology (Zheng et al., 2013; Gozen et al., 2014; Parekh et al., 2016; Wang et al., 2016). However, common paper is a low-cost platform for manufacturing flexible, lightweight and biodegradable devices. So far, there are still few methods to print gallium-based alloys on common paper, creating complex pattern of liquid metal on paper reserves a challenging. If the liquid metal can be stably and evenly spread on the paper surface, it can be effectively used to print paper-supported circuits. Recently, researchers have exploited two directions to achieve direct writing of liquid metal on paper. One is tuning the properties of alloy inks by adding additives. Chang et al. mixed liquid Ga_75.4In_24.5 alloy and nickel powder to form a sticky ink, which possessed excellent plasticity and stable adhesivity to printer paper (Chang et al., 2018). The oxide film wrapping the nickel powder formed a skeleton for additional oxidation products, resulting in superior adhesion to paper. The other is modifying paper surface so that the printed liquid alloy can be firmly attached on paper. Wang et al. wetted the paper surface by microdroplets to reduce the roughness of paper and enhance the interface adhesion between liquid alloy (Ga_75.4In_24.5) and paper, ensuring a smooth direct writing process (Wang et al., 2019). The two works provide a simple avenue to design and manufacture paper-based electronics.

Besides, other types of conductive materials are also used to make inks. Chen et al. introduced polypyrrole into graphene/carbon black composite, using a mixture of alcohol, ethylene glycol, glycerol and deionized water as solvent, to obtain a conductive ink with excellent water and acid resistance (Chen Y. et al., 2019). Uniform and flexibility 3D circuits were prepared by directly writing this ink on high-gloss photographic paper. It has been confirmed that, through distributing on graphene surface, carbon black particles can effectively reduce the agglomeration of graphene sheets as well as bridge them to form conductive paths, further significantly improving the conductivity of the ink (Li et al., 2015; Shukla et al., 2015; Ji et al., 2018). Gogotsi Y et al. developed additive-free Ti₃C₂ MXene-in-water inks which can be written directly on various substrates, including paper, textile, wood and plastics (Quain et al., 2019). The written circuits could be dried at room temperature in air and displayed relatively high conductivity between carbon-based and metal nanoparticle inks.

In addition to preparing electrodes and circuits mentioned above, DIW is often employed to prepare other electronic functional components such as sensors, transistors, light emitting diodes, antennas, and so on. Vatani and Abas et al. all adopted the method of direct writing first and then packaging to fabricate layered resistance sensors (Vatani et al., 2015; Abas and Rahman, 2016). They first printed the designed sensing arrays with corresponding CNTs and carbon paste inks, and then encapsulated them in photocurable resin and PET to assemble the sensors, respectively. Based on this technology, the geometry of the sensing arrays can be controlled accurately so as to realize stable sensing performances. As a result, high-quality soft sensors with consistent sensing arrays could be manufactured repeatedly with liquid metals as the printable inks (Parekh et al., 2016; Kim et al., 2018). Especially for Kim et al. (2018), they directly printed a one-piece glove-type sensor containing 10 strain gauges to measure flexion and extension of the five fingers (Figures 2E–H), presenting a compact sensor system and short production time (within 15 min). Shi et al. prepared aqueous ink based on polydimethylsiloxane (PDMS) submicrobeads/GO nanocomposite, which enabled high-resolution 3D DIW of strain sensors (Shi et al., 2019). Through finely tuning rheology, the ink can be easily extruded from nozzles as small as 50 μm allowing for printing sensing arrays with unrivaled resolution. The obtained micron-scale pressure sensors exhibited high structure resolution (100 μm), excellent sensitivity (0.3 kPa⁻¹), and broad operating range (0–500 kPa).

Recently, some other types of sensing components have been developed via DIW. Chen and colleagues created a precursor ink, containing acrylamide, an ultraviolet initiator, and the poly(butyl acrylate) spheres (160 nm) swollen by 2-ethylhexyl acrylate monomers, to construct a mechanochromic sensor by a direct-writing technique coupled with UV-curing (Chen J. et al., 2019). The precursor was cured under UV-irradiation to form the photonic crystal hydrogel, an interesting photonic soft material. It showed a high elasticity and excellent mechanochromic property of which structural color changed reversibly with different strains (Figures 2I–L). Therefore, this hydrogel can be ink-written into various shapes for wearable optical devices, colorimetric tactile sensors, and full-color displays. Siebert et al. firstly directly wrote mixed metal microparticles arrays of commercially available Fe and Cu particles onto the glass substrate for sensing acetone gas (Siebert et al., 2020). After thermal annealing, the mixed metal microparticles formed open, highly porous nanostructures based on bridging CuO/Cu₂O/Cu-Fe₂O₃/Fe heterojunction networks, achieving high selectivity and sensitivity to acetone vapor at an operating temperature of 300°C and operating power as low as 0.26 μW.

In addition, Kong et al. utilized DIW to printed quantum dot light emitting diodes (QDLEDs), in which every component, including cathode, anode, transport layer, and light-emitting layer, is 3D printed (Figures 2M,N), exhibiting pure and tunable color emission properties (Kong et al., 2014). The QDLEDs could be directly constructed on the substrates with various shapes by incorporating a 3D light-scanning technique. The combination of DIW and 3D scanning technology makes it easier to copy and improve the existed structure, which paves a new way for efficiently preparing complex 3D integrated electronic devices. Fan et al. reported fully 3D printed organic electrochemical transistors (OECTs) by DIW with the poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) composite and silver paste as the channel material and source/drain material, respectively (Figure 2O) (Fan et al., 2019). These printed devices demonstrated high transconductance and current ON/OFF ratio, low operating voltage, as well as excellent stability and robustness after several measurement cycles, being applicable for affordable and customized biosensing and biowearables. Through photomask-assisted DIW, Chen et al. designed a multilevel triboelectric nanogenerator as an energy harvester to enlighten LEDs under external force, for self-powered wearable devices (Chen K. J. et al., 2019). With the Ag paste or Ga/In alloy ink, a hemispherical antenna with tunable resonant frequency could be DIW-printed, showing potential for wireless sensing and dynamic communication (Adams et al., 2011; Boley et al., 2019).

Obviously, using DIW technology to print functional components is expected to realize preparation and assembly of various components in the electronic equipment, simultaneously. The DIW-assembled components have higher molding accuracy and better compatibility, which will help to promote the development of fully printed electronic devices.