Low-cost, rapid patterning of liquid metal on various substrates is a key processing step for liquid metal-based soft electronics. Current patterning methods rely on expensive equipment and specific substrates, which severely limit their widespread applications. Based on surface adhesion adjustment of liquid metal through thermal transferring toner patterns, we present a universal printing technique of liquid metal circuits. Without using any expensive processing steps or equipment, the circuit patterns can be printed quickly on thermal transfer paper using a desktop laser printer, and a toner on the thermal transfer paper can be transferred to various smooth substrates and polymer-coated rough substrates. The technique has yielded liquid metal circuits with a minimum linewidth of 50 μm fabricated on various smooth, rough, and three-dimensional substrates with complex morphology. The liquid metal circuits can maintain their functions even under an extreme strain of 800%. Various circuits such as LED arrays, multiple sensors, a flexible display, a heating circuit, a radiofrequency identification circuit, and a 12-lead electrocardiogram circuit on various substrates have been demonstrated, indicating the great potential of such a technique to rapidly achieve large-area flexible circuits for wearable health monitoring, internet of things, and consumer electronics at low cost and high efficiency.

to curved surfaces such as roof-tops, [5][6][7] car ceilings, [8][9][10] airships, [11][12][13] and satellites. [14][15][16] To ensure the reliability and robustness of flexible solar cells, they are typically connected with semi-rigid metallic electrodes and sandwiched between stainless steel backing layers and thick polymer passivation layers. As a result, the overall thicknesses of the flexible solar cells are larger than 200 µm, resulting in a declined capability to adapt to biological tissues as well as surfaces with small radii of curvature. As the maximum bending strain is inversely proportional to the thickness of the devices, techniques to thin and repackage the flexible solar cells are highly demanded.Among various mechanical and chemical thinning approaches, [17][18][19] chemical thinning is one of the favorable methods for reducing chip thickness due to the introduction of almost no intrinsic stress, high yield and rapid etch rates. Chemical thinning approaches have yielded multiple ultrathin flexible devices, such as field-effect transistors, [20] silicon integrated circuits, [21] piezoelectric generators [22] and resonators. [23] However, it has seldomly been used to thin the encapsulated photovoltaic devices, and the performance before and after thinning has not yet been systematically studied. In addition to superior flexibility, some scenarios may also require certain levels of stretchability, leading to the development of stretchable solar cells that have been commonly achieved Integration of photovoltaic devices on the deformable surfaces of plants and animals is challenging, as most of the photovoltaic devices possess no stretchability which severely restricts the underneath deformable substrates and causes potential delamination of the devices from the substrates due to large shear stress. Here, techniques to achieve stretchable photovoltaic devices through chemically thinning flexible solar cells and intermediate hardsoft transition layers are proposed. The resulting photovoltaic devices are only 25 µm in thickness with a radius of curvature of 1 mm, offering excellent adaptability to leaves and many curved surfaces. The intermediate layer that is based on silicone adhesive with low modulus provides an interface between non-stretchable photovoltaic devices and deformable substrates, resulting in a stretchability of the overall structures larger than 140%. Key parameters of the photovoltaic device such as open-circuit voltage, short circuit current, fill factor, and conversion efficiency indicate the excellent performance of thinned devices. Potential applications of ultra-thin photovoltaic devices in energy harvesting and light intensity sensing on stretchable substrates are demonstrated, suggesting the practical use of the devices in constructing stretchable self-powered wearable systems for agriculture and healthcare monitoring.

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