known as denticles that are thought to be multifunctional. It has been hypothesized that these surface features prevent biofouling and, more importantly, reduce fluid friction drag, which enables more efficient swimming. This has inspired the development of commercially successful products such as the Fastskin swimming suit by Speedo. Shark skin structures could also be applied to ships, underwater vehicles, airplanes, and pipelines to reduce energy waste from friction drag.Many studies have been conducted over the past few decades to understand the mechanism of drag reduction over shark skin-inspired surfaces. The physical causes responsible for drag reduction are reasonably well understood. [2][3][4] However, fabrication of such multifunctional surfaces remains a challenge. Although various methods have been used to reproduce shark skin geometry, a trade-off between throughput and resolution always exists and severely hinders the progress in this field. Some high-throughput methods include textured liners, [5,6] molding from real shark skin samples, [7] microcasting and wax printing, sampling from real shark skin, [8] and laser cutting. [9] However, these methods suffer from low resolution or difficulty in accurately reproducing the 3D geometry of shark skin, comprising dermal denticles attached to the surface. More importantly, given the vast design spaces involved, it is difficult to flexibly and quickly fabricate many different modified shapes and sizes that may be required in biomimetic studies. High-resolution digital fabrication methods such as two-photon lithography [10] have successfully fabricated highly detailed and accurate 3D shark skin denticles. [9] However, such fabrication processes can only produce samples with small areas. The small building extent, coupled with extremely low throughput, limits the value of such fabrication techniques for research purposes. As an emerging manufacturing technology, 3D printing has been playing an increasingly important role in various industries due to its advantages in high customization and fast prototyping, as well as the ability to fabricate complex shapes. [11][12][13][14][15][16][17] The flexibility of 3D printing also makes it very appealing for biomimetic studies. For example, 3D printing enables the development of artificial shark skin with textures of varying shape and size on arbitrary surface shapes. Indeed, some recent studies have successfully utilized 3D printing to fabricate shark skin structures. [9,18,19] Nevertheless, Additive manufacturing has many advantages in creating highly complex customized structures. In this study, a low-cost multiscale stereolithography technology that can print a macroscale object with microscale surface structures with high throughput is demonstrated. The developed multiscale stereolithography is realized by dynamic switching of laser spot size and adaptively sliced layer thickness. An optical filter based on subwavelength resonance grating is used to modify laser spot size for lasers with different wavelengths an...

While vat photopolymerization has many advantages over soft lithography in fabricating microfluidic devices, including efficiency and shape complexity, it has difficulty achieving well-controlled micrometer-sized (smaller than 100 μm) channels in the layer building direction. The considerable light penetration depth of transparent resin leads to over-curing that inevitably cures the residual resin inside flow channels, causing clogs. In this paper, a 3D printing process — in-situ transfer vat photopolymerization is reported to solve this critical over-curing issue in fabricating microfluidic devices. We demonstrate microchannels with high Z-resolution (within 10 μm level) and high accuracy (within 2 μm level) using a general method with no requirements on liquid resins such as reduced transparency nor leads to a reduced fabrication speed. Compared with all other vat photopolymerization-based techniques specialized for microfluidic channel fabrication, our universal approach is compatible with commonly used 405 nm light sources and commercial photocurable resins. The process has been verified by multifunctional devices, including 3D serpentine microfluidic channels, microfluidic valves, and particle sorting devices. This work solves a critical barrier in 3D printing microfluidic channels using the high-speed vat photopolymerization process and broadens the material options. It also significantly advances vat photopolymerization’s use in applications requiring small gaps with high accuracy in the Z-direction.

Adaptive slicing is an important computational task required in the layer-based manufacturing process. Its purpose is to find an optimal trade-off between the fabrication time (number of layers) and the surface quality (geometric deviation error). Most of the traditional adaptive slicing algorithms are computationally expensive or only based on local evaluation of errors. To tackle these problems, we introduce a method to efficiently generate the slicing plans by a new metric profile that can characterize the distribution of deviation errors along the building direction. By generalizing the conventional error metrics, the proposed metric profile is a density function of deviation errors, which measures the global deviation errors rather than the in-plane local geometry errors used in most prior methods. Slicing can be efficiently evaluated based on metric profiles in contrast to the expensive computation on models in boundary-representation. An efficient algorithm based on dynamic programming is proposed to find the best slicing plan. Our adaptive slicing method can also be applied to models with weighted features and can serve as the inner loop to search the best building direction. The performance of our approach is demonstrated by experimental tests on different examples.

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