Liquid metal (LM) with high electrical conductivity, thermal conductivity, excellent biocompatibility, and extraordinary fluidity has emerged as a promising class of functional materials. However, such materials still encounter many practical challenges due to the rather limited forms available so far. As a promising remedy, LM composites in synergy with other materials would open tremendous opportunities for fundamental research or practical applications. This is because controllable integration of base LM with functional materials (e.g., metal nanoparticles, polymers, and drug molecules) would significantly tune the intrinsic properties of LM as desired, enabling it to offer further major potential in tackling various sectors' challenging issues, including thermal management, biomedicine, chemical catalysis, flexible electronics, and soft robots. Here, we systematically summarize and review the fundamental progress in pursuing LM composites. The basic composite strategies are outlined in three categories: LM composites with core-shell structure, LM-polymer composites, and LM-particle composites. The effectiveness of the composite strategy is illustrated via the typical applications of LM composites in representative fields. The challenges and perspectives in developing LM composites are also identified and interpreted to better guide future research. It is expected that the coming LM era will witness a new world of fruitful composites thereby discovered or invented.

A novel core-shell microcapsule system is developed in this study to mimic the miniaturized 3D architecture of pre-hatching embryos with an aqueous liquid core of embryonic cells and a hydrogel-shell of zona pellucida. This is done by microfabricating a non-planar microfluidic flow-focusing device that enables one-step generation of microcapsules with an alginate hydrogel shell and an aqueous liquid core of cells from two aqueous fluids. Mouse embryonic stem (ES) cells encapsulated in the liquid core are found to survive well (> 92 %). Moreover, ~ 20 ES cells in the core can proliferate to form a single ES cell aggregate in each microcapsule within 7 days while at least a few hundred cells are usually needed by the commonly used hanging-drop method to form an embryoid body (EB) in each hanging drop. Quantitative RT-PCR analyses show significantly higher expression of pluripotency marker genes in the 3D aggregated ES cells compared to the cells under 2D culture. The aggregated ES cells can be efficiently differentiated into beating cardiomyocytes using a small molecule (cardiogenol C) without complex combination of multiple growth factors. Taken together, the novel 3D microfluidic and pre-hatching embryo-like microcapsule systems are of importance to facilitate in vitro culture of pluripotent stem cells for their ever-increasing use in modern cell-based medicine.