Miniaturized head-mounted microscope for whole-cortex mesoscale imaging in freely behaving mice

The advent of genetically encoded calcium indicators, along with surgical preparations such as thinned skulls or refractive index matched skulls, have enabled mesoscale cortical activity imaging in head-fixed mice. However, neural activity during unrestrained behavior substantially differs from neural activity in head-fixed animals. For whole-cortex imaging in freely behaving mice, we here present the “mini-mScope,” a wide-field, miniaturized, and head-mounted fluorescence microscope compatible with transparent polymer skull preparations. With a field of view of 8 mm x 10 mm and weighing less than 4 g, the mini-mScope can image most of the mouse dorsal cortex with resolution ranging from 39 to 56 μm. We have used the mini-mScope to record mesoscale calcium activity across the dorsal cortex during sensory-evoked stimuli, open field behaviors, social interactions, and transitions from wakefulness to sleep.

Passive water ascent in a tall, scalable synthetic tree

The transpiration cycle in trees is powered by a negative water potential generated within the leaves, which pumps water up a dense array of xylem conduits. Synthetic trees can mimic this transpiration cycle, but have been confined to pumping water across a single microcapillary or microfluidic channels. Here, we fabricated tall synthetic trees where water ascends up an array of large diameter conduits, to enable transpiration at the same macroscopic scale as natural trees. An array of 19 tubes of millimetric diameter were embedded inside of a nanoporous ceramic disk on one end, while their free end was submerged in a water reservoir. After saturating the synthetic tree by boiling it underwater, water can flow continuously up the tubes even when the ceramic disk was elevated over 3 m above the reservoir. A theory is developed to reveal two distinct modes of transpiration: an evaporation-limited regime and a flow-limited regime.

Self-Stabilizing Transpiration in Synthetic Leaves

Over the past decade, synthetic trees have been engineered to mimic the transpiration cycle of natural plants, but the leaves are prone to dry out beneath a critical relative humidity. Here, we create large-area synthetic leaves whose transpiration process is remarkably stable over a wide range of humidities, even without synthetic stomatal chambers atop the nanopores of the leaf. While the water menisci cannot initially withstand the Kelvin stress of the subsaturated air, they self-stabilized by locally concentrating vapor within the top layers of nanopores that have dried up. Transpiration rates were found to vary nonmonotonically with the ambient humidity because of the tradeoff of dry air increasing the retreat length of the menisci. It is our hope that these findings will encourage the development of large-area synthetic trees that exhibit excellent stability and high throughput for water-harvesting applications.