Microfluidic devices are composed of microchannels with a diameter ranging from ten to a few hundred micrometers. Thus, quite a small (10⁻⁹–10⁻¹⁸ l) amount of liquid can be manipulated by such a precise system. In the past three decades, significant progress in materials science, microfabrication, and various applications has boosted the development of promising functional microfluidic devices. In this review, the recent progress on novel microfluidic devices with various functions and applications is presented. First, the theory and numerical methods for studying the performance of microfluidic devices are briefly introduced. Then, materials and fabrication methods of functional microfluidic devices are summarized. Next, the recent significant advances in applications of microfluidic devices are highlighted, including heat sinks, clean water production, chemical reactions, sensors, biomedicine, capillaric circuits, wearable electronic devices, and microrobotics. Finally, perspectives on the challenges and future developments of functional microfluidic devices are presented. This review aims to inspire researchers from various fields—engineering, materials, chemistry, mathematics, physics, and more—to collaborate and drive forward the development and applications of functional microfluidic devices, specifically for achieving carbon neutrality.

Those various cross-sectional vessels in trees transfer water to as high as 100 meters, but the traditional fabrication methods limit the manufacturing of those vessels, resulting in the non-availability of those bionic microchannels. Herein, we fabricate those bionic microchannels with various cross-sections by employing projection micro-stereolithography (PµSL) based 3D printing technique. The circumradius of bionic microchannels (pentagonal, square, triangle, and five-pointed star) can be as small as 100 μm with precisely fabricated sharp corners. What’s more, those bionic microchannels demonstrate marvelous microfluidic performance with strong precursor effects enabled by their sharp corners. Most significantly, those special properties of our bionic microchannels enable them outstanding step lifting performance to transport water to tens of millimeters, though the water can only be transported to at most 20 mm for a single bionic microchannel. The mimicked transpiration based on the step lifting of water from bionic microchannels is also achieved. Those precisely fabricated, low-cost, various cross-sectional bionic microchannels promise applications as microfluidic chips, long-distance unpowered water transportation, step lifting, mimicked transpiration, and so on.

Smart windows with tunable optical properties that respond to external environments are being developed to reduce energy consumption in buildings. In the present study, we introduce a new type of 3D printed hydrogel with amazing flexibility and stretchability (as large as 1500%), as well as tunable optical performance controlled by surrounding temperatures. The hydrogel on a PDMS substrate shows transparent-opaque transition with high solar modulation (ΔT_sol) up to 79.332% around its lower critical solution temperature (LCST) while maintaining a high luminous transmittance (T_lum) of 85.847% at 20 ℃. In addition, selective transparent-opaque transition above LCST can be achieved by patterned hydrogels which are precisely fabricated via a projection micro-stereolithography based 3D printing technique. Our hydrogel promises great potential applications for the next generation of soft smart windows.

Hydrogels with stimuli-responsive capabilities are gaining more and more attention nowadays with prospective applications in biomedical engineering, bioelectronics, microrobot, etc. We develop a photothermal responsive hydrogel based on N-isopropylacrylamide that achieved a fast and reversible deformation manipulated only by near-infrared (NIR) light. The hydrogel was fabricated by the projection micro stereolithography based 3D printing technique, which can rapidly prototype complex 3D structures. Furthermore, with the variation of the grayscale while manufacturing the hydrogel, the deformation of the hydrogel structure can be freely tuned within a few seconds by losing and absorbing water through adjusting the intensity and the irradiation direction of the NIR light, showing a potential application in ultra-fast object grabbing and transportation. The present study provides a new method for designing ultrafast photothermal responsive hydrogel based microrobot working in water.

Projection micro stereolithography (PμSL) is a high-resolution (up to 0.6 μm) 3D printing technology based on area projection triggered photopolymerization, and capable of fabricating complex 3D architectures covering multiple scales and with multiple materials. This paper reviews the recent development of the PμSL based 3D printing technologies, together with the related applications. It introduces the working principle, the commercialized products, and the recent multiscale, multimaterial printing capability of PμSL as well as some functional photopolymers that are suitable to PμSL. This review paper also summarizes a few typical applications of PμSL including mechanical metamaterials, optical components, 4D printing, bioinspired materials and biomedical applications, and offers perspectives on the directions of the further development of PμSL based 3D printing technology.