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Increasing demand for high-energy batteries necessitates a revisit of the most energy-dense negative electrode, lithium (Li) metal, which was once abandoned in the 1990s because of safety risks associated with inhomogeneous deposition and stripping (i.e., dendrite growth) during battery cycling. In recent years, to better understand and overcome the Li metal dendrite problem, great efforts have been made to reveal dendrite growth processes using various imaging modalities. However, because of being almost invisible to electrons and X-rays, directly imaging Li metal with the required contrast, spatial and temporal resolutions have always been the challenge. Here, we show that by exploiting photoacoustic effect, microscale-resolution threedimensional structure of Li protrusions can be clearly visualized within minutes by photoacoustic microscopy (PAM). PAM enables high contrast as well as depth information of Li metal inside the glass fiber separator of a Li/Li liquid electrolyte symmetric cell. Our proof-of-principle experiment introduces a new imaging tool to the Li metal battery community, which could greatly benefit the study of fundamental mechanisms of not only the Li metal dendrite growth in conventional and solidstate batteries, but also sodium and magnesium metals. We believe PAM is a promising in-operando tool for battery diagnostic and prognostic. I. INTRODUCTIONLithium (Li)-ion batteries are ubiquitous in present-day technological applications, ranging from portable devices, electric vehicles to grid-scale stationary energy storage. Li-ion batteries are composed of positive and negative electrodes (two Li reservoirs with different concentrations) which are separated by a polymeric membrane, i.e., a separator. The separator is immersed in Li-ion conducting liquid electrolyte which permits only Li-ion shuffling between the positive and negative electrodes during battery cycling. Simultaneously, electrons flow through the external circuit powering electronic devices.With increasing demand for higher-energy batteries, it is now a common consensus that graphite anode in Li-ion batteries must be replaced with the most energy-dense Li metal in the next-generation Li metal batteries [1]. Ironically, Li metal anode was the choice when the first rechargeable Li battery was invented in the 1970th [2]. Soon afterwards, however, safety hazards associated with Li metal anode were identified, which halted the development of Li metal batteries. The problem is

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Angiogenesis occurs during both physiological and pathological processes. In this study, a microfluidic chip for the development of angiogenesis was utilized to assess angiogenic sprouting and functional vessel formation. We also found that vascular endothelial growth factor (VEGF) was a determinant of the initiation of vascular sprouts, while the direction of these sprouts was greatly influenced by interstitial flow. Isoforms of VEGF such as VEGF121, VEGF165, and VEGF189 displayed different angiogenic properties on the chip as assessed by sprout length and number, vessel perfusion, and connectivity. VEGF165 had the highest capacity to induce vascular sprouting among the three isoforms assessed and furthermore, also induced functional vessel formation. This chip could be used to analyze the effect of different angiogenic factors and drugs, as well as to explore the mechanism of angiogenesis induced by such factors.

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