nature, and harsh processing conditions of the ITO and the scarcity of the indium limit the further application for the THs. [3,4] As such, recent studies have proposed several emerging materials for the next-generation TCF to replace ITO, including carbon-based materials, [5][6][7][8] metal nanowires (NWs) or nanofibers (NFs), [9][10][11][12][13][14][15] metal meshes, [16][17][18] conductive polymers, [19] and hybrid materials. [1,[20][21][22] However, the cost, mechanical robustness, and trade-off between transmittance (T) and sheet resistance (R s ) of these TCFs remain limited and inconsistent across applications.The R s of carbon-based materials and conductive polymers is higher than that of ITO, [23] which restricts their use in high-performance TGHs. [4] Metal NWs or NFs and metal meshes have been studied extensively in recent years and have been described as the most promising TCF materials due to excellent electrical and optical properties (in some cases superior to ITO). Metal NWs and NFs have been demonstrated as an ITO substitute in flexible optoelectronic applications because of their mechanical flexibility and preferable T-R s trade-off. However, the resulting THs or TGHs have struggled to achieve T > 90% and R s < 10 Ω sq −1 . [24] Hsu et al. reported a high-performance TCF with R s = 0.36 Ω sq −1 at T = 92% by combining mesoscale NFs with metal NWs, [25] and An et al. produced a copper NF with T > 90% and R s < 0.5 Ω sq −1 using electrospinning and electroplating methods. [21] These NWs and NFs exhibit limitations such as excessive surface roughness, low uniformity, high material cost, and unavoidable haze. [13,26] In addition, adhesion between NWs and commonly-employed substrates is often poor, which makes it difficult to use NWs in harsh environments for extended time periods. [27] These characteristics hinder the production of low-cost high-performance TGHs, which require low R s , high T, and strong TCF adhesion.Metal mesh is considered to be an ideal TCF because of its inherently high T, low haze, high electrical conductivity, good mechanical properties, and low cost. [17] The T-R s trade-off in metal meshes can be further optimized by increasing the intrinsic conductivity or the aspect-ratio (AR) of the metal wire Great challenges remain concerning the cost-effective manufacture of highperformance metal meshes for transparent glass heaters (TGHs). Here, a high-performance silver mesh fabrication technique is proposed for TGHs using electric-field-driven microscale 3D printing and a UV-assisted microtransfer process. The results show a more optimal trade-off in sheet resistance (R s = 0.21 Ω sq −1 ) and transmittance (T = 93.9%) than for indium tin oxide (ITO) and ITO substitutes. The fabricated representative TGH also exhibits homogeneous and stable heating performance, remarkable environmental adaptability (constant R s for 90 days), superior mechanical robustness (R s increase of only 0.04 in harsh conditions-sonication at 100 °C), and strong adhesion force with a negligible increase in R...

performance, transparent conductive oxides, as represented by indium tin oxide (ITO), have been mostly used widely for transparent electrodes (TEs) in the past few decades. However, the inherent properties of ITO such as brittleness and poor flexibility hinder its applications for flexible and stretchable optoelectronic products. [10] Hence, many alternatives to ITO have been developed to address these challenges for the next generation of FTEs, such as graphene, [11] carbon nanotubes, [12,13] conductive polymers, [14][15][16][17][18] transparent thin metal films, [19,20] random metal nanowires, [2,21] regular metalmesh, [22,23] and hybrid materials. [24,25] Among these FTEs, metal-mesh possesses excellent mechanical flexibility and optoelectronic properties. In particular, the trade-off between low sheet resistance and the high transmittance of TEs can be parametrically designed and further optimized by simply changing the line width, pitch, aspect ratio (AR), shape, and arrangement of the mesh. The metal mesh can be fabricated with a low-cost and large-area manufacturing process (such as solution-process), which can be carried out in a vacuum-free environment and usually requires only a low temperature process. [26][27][28][29][30] So far, FTEs based on metal mesh Flexible transparent electrodes (FTEs) with an embedded metal mesh are considered a promising alternative to traditional indium tin oxide (ITO) due to their excellent photoelectric performance, surface roughness, and mechanical and environmental stability. However, great challenges remain for achieving simple, cost-effective, and environmentally friendly manufacturing of high-performance FTEs with embedded metal mesh. Herein, a maskless, templateless, and plating-free fabrication technique is proposed for FTEs with embedded silver mesh by combining an electric-field-driven (EFD) microscale 3D printing technique and a newly developed hybrid hot-embossing process. The final fabricated FTE exhibits superior optoelectronic properties with a transmittance of 85.79%, a sheet resistance of 0.75 Ω sq −1 , a smooth surface of silver mesh (R a ≈ 18.8 nm) without any polishing treatment, and remarkable mechanical stability and environmental adaptability with a negligible increase in sheet resistance under diverse cyclic tests and harsh working conditions (1000 bending cycles, 80 adhesion tests, 120 scratch tests, 100 min ultrasonic test, and 72 h chemical attack). The practical viability of this FTE is successfully demonstrated with a flexible transparent heater applied to deicing. The technique proposed offers a promising fabrication strategy with a cost-effective and environmentally friendly process for high-performance FTE.

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Flexible transparent electrodes (FTEs) with embedded metal meshes play an indispensable role in many optoelectronic devices due to their excellent mechanical stability and environmental adaptability. However, low-cost, simple, efficient, and environmental friendly integrated manufacturing of high-performance embedded metal meshes remains a huge challenge. Here, a facile and novel fabrication method is proposed for FTEs with an embedded metal mesh via liquid substrateelectric-field-driven microscale 3D printing process. This direct printing strategy avoids tedious processes and offers low-cost and high-volume production, enabling the fabrication of high-resolution, high-aspect ratio embedded metal meshes without sacrificing transparency. The final manufactured FTEs with 80 mm × 80 mm embedded metal mesh offers excellent optoelectronic performance with a sheet resistance (R s ) of 6 𝛀 sq −1 and a transmittance (T) of 85.79%. The embedded metal structure still has excellent mechanical stability and good environmental suitability under different harsh working conditions. The practical feasibility of the FTEs is successfully demonstrated with a thermally driven 4D printing structure and a resistive transparent strain sensor. This method can be used to manufacture large areas with facile, high-efficiency, low-cost, and high-performance FTEs.

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Transparent glass with metal mesh is considered a promising strategy for high performance transparent glass heaters (TGHs). However, the realization of simple, low‐cost manufacture of high performance TGHs still faces great challenges. Here, a technique for the fabrication of high performance TGHs is proposed using liquid sacrificial substrate electric‐field‐driven (LS‐EFD) microscale 3D printing of thick film silver paste. The liquid sacrificial substrate not only significantly improves the aspect ratio (AR) of silver mesh, but also plays a positive role in printing stability. The fabricated TGHs with a line width of 35 µm, thickness of 12.3 µm, and pitch of 1000 µm exhibit a desirable optoelectronic performance with sheet resistance (Rs) of 0.195 Ω sq−1 and transmittance (T) of 88.97%. A successful deicing test showcases the feasibility and practicality of the manufactured TGHs. Moreover, an interface evaporator is developed for the coordination of photothermal and electrothermal systems based on the high performance TGHs. The vapor generation rate of the device reaches 10.69 kg m−2 h−1 with a voltage of 2 V. The proposed technique is a promising strategy for the cost‐effective and simple fabrication of high performance TGHs.

Thermally responsive shape memory polymers (SMPs) used in 4D printing are often reported to be activated by external heat sources or embedded stiff heaters. However, such heating strategies impede the practical application of 4D printing due to the lack of precise control over heating or the limited ability to accommodate the stretching during shape programming. Herein, we propose a novel 4D printing paradigm by fabricating stretchable heating circuits with fractal motifs via electric-fielddriven microscale 3D printing of conductive paste for seamless integration into 3D printed structures with SMP components. By regulating the fractal order and printing/processing parameters, the overall electrical resistance and areal coverage of the circuits can be tuned to produce an efficient and uniform heating performance. Compared with serpentine structures, the resistance of fractal-based circuits remains relatively stable under both uniaxial and biaxial stretching. In practice, steady-state and transient heating modes can be respectively used during the shape programming and actuation phases. We demonstrate that this approach is suitable for 4D printed structures with shape programming by either uniaxial or biaxial stretching. Notably, the biaxial stretchability of fractal-based heating circuits enables the shape change between a planar structure and a 3D one with double curvature. The proposed strategy would offer more freedom in designing 4D printed structures and enable the manipulation of the latter in a controlled and selective manner.

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