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1. Dynamic Actuation of Soft 3D Micromechanical Structures Using Micro‐Electromechanical Systems (MEMS)

   https://doi.org/10.1002/admt.201700293  

   Jayne, Rachael_K; Stark, Thomas_J; Reeves, Jeremy_B; Bishop, David_J; White, Alice_E (January 2018, Advanced Materials Technologies)

   Abstract Direct laser writing (DLW) is an advanced fabrication technique that allows users to create complex 3D microstructures from polymer precursors. These microstructures can be integrated with micro‐electromechanical systems (MEMS) actuators. MEMS actuators provide a convenient platform for interacting with the intricate microstructures, either to characterize their mechanical properties or cause them to deform. Structures are fabricated directly onto electrostatic comb drives and chevron thermal actuators that are produced using a commercial foundry process. By applying a voltage to the MEMS actuators, highly controlled deformation of these microstructures is observed. Mechanical behaviors of microstructures produced with different materials and fabrication conditions are compared. MEMS–DLW integration is a convenient approach to characterizing the mechanics of DLW microstructures and may well lead to a new class of dynamic 3D devices for applications ranging from tissue engineering to imaging. 

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2. Assembly of multicomponent structures from hundreds of micron-scale building blocks using optical tweezers

   https://doi.org/10.1038/s41378-021-00272-z  

   Melzer, Jeffrey E.; McLeod, Euan (June 2021, Microsystems & Nanoengineering)

   Abstract The fabrication of three-dimensional (3D) microscale structures is critical for many applications, including strong and lightweight material development, medical device fabrication, microrobotics, and photonic applications. While 3D microfabrication has seen progress over the past decades, complex multicomponent integration with small or hierarchical feature sizes is still a challenge. In this study, an optical positioning and linking (OPAL) platform based on optical tweezers is used to precisely fabricate 3D microstructures from two types of micron-scale building blocks linked by biochemical interactions. A computer-controlled interface with rapid on-the-fly automated recalibration routines maintains accuracy even after placing many building blocks. OPAL achieves a 60-nm positional accuracy by optimizing the molecular functionalization and laser power. A two-component structure consisting of 448 1-µm building blocks is assembled, representing the largest number of building blocks used to date in 3D optical tweezer microassembly. Although optical tweezers have previously been used for microfabrication, those results were generally restricted to single-material structures composed of a relatively small number of larger-sized building blocks, with little discussion of critical process parameters. It is anticipated that OPAL will enable the assembly, augmentation, and repair of microstructures composed of specialty micro/nanomaterial building blocks to be used in new photonic, microfluidic, and biomedical devices. 

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3. Improved Femtosecond 3D Light Field Lithography With a Phase-Controlled Spatial Light Modulator

   https://doi.org/10.1115/HT2022-85681  

   Jakkinapalli, Aravind; Wen, Sy-Bor (September 2022, ASME SHTC 2022)

   Abstract Through years of development, we have successfully demonstrated 3D light field lithography with UV continuous light. We recently combined this approach with femtosecond laser sources as two-photon femtosecond 3D light lithography. It is found that consistent results can happen under limited conditions with this direct combination. Our theoretical analysis reported last year shows that the experimental difficulty can be attributed to digital micro-mirror devices (DMD) and microlens arrays (MLA) used in the current 3D light field projection. Though they can control the propagation directions and interact at designed 3D locations, rays from such a system diverge with respect to the propagation distance. As a result, 3D voxel intensity in the 3D projection changes as a function of the separation distance with respect to the MLA in the 3D projection. To solve this problem, we replace the combination of DMD and MLA with a phase-controlled spatial light modulator. With a lab-developed algorithm, a single femtosecond laser pulse can generate up to a million sub-rays through the phase-controlled spatial light modulator. These sub-rays with a precisely controlled propagation direction can intersect at designed 3D locations as voxels for 3D virtual object constructions. Moreover, these sub-rays have minimum divergence angles to ensure that the voxel intensities are maintained consistently at each 3D location. We also demonstrated that versatile 3D patterns could be generated with two-photon femtosecond 3D light field lithography based on this innovative approach. 

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4. Two-dimensional active sensing system for bicyclist-motorist crash prediction

   https://doi.org/10.23919/ACC.2017.7963298  

   Jeon, Woongsun; Rajamani, Rajesh (May 2017, American Control Conference (ACC), 2017)

   This paper develops an active sensing system for a bicycle to accurately track rear vehicles that can have two-dimensional motion. The active sensing system consists of a single-beam laser sensor mounted on a rotationally controlled platform. The sensing system is inexpensive, small, lightweight, consumes low power, and is thus ideally suited for the bicycle application. The rotational orientation of the laser sensor needs to be actively controlled in real-time in order to continue to focus on a rear vehicle, as the vehicle’s lateral and longitudinal distances change. This tracking problem requires controlling the real-time angular position of the laser sensor without knowing the future trajectory of the vehicle. The challenge is addressed using a novel receding horizon framework for active control and an interacting multiple model framework for estimation. The features and benefits of this active sensing system are illustrated first using simulation results. Then, preliminary experimental results are presented using an instrumented bicycle to show the feasibility of the system in tracking rear vehicles during both straight and turning maneuvers. 

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5. 3D Printed Bioinspired Hierarchical Surface Structure With Tunable Wettability

   https://doi.org/10.1115/1.4064051  

   Rahman, M. M.; Joyee, Erina Baynojir (December 2022, Journal of Micro- and Nano-Manufacturing)

   Abstract Nature has examples of impressive surfaces and interfaces with diverse wettability stemming from superhydrophilicity to superhydrophobicity. The multiscale surface structures found in biological systems generally have high geometric complexity, which makes it challenging to replicate their characteristics, especially using traditional fabrication techniques. It is even more challenging to fabricate such complex microstructures with tunable wettability. In this paper, we propose a method to tune the wettability of a microscale surface by changing the geometrical parameters of embedded microstructures in the surface. By taking inspiration from an insect (springtails), we designed micropillar arrays with different roughness by adjusting geometric parameters such as reentrant angle, pitch distance, and the number of spikes and pillars. This study shows that, by changing geometrical parameters in microscale, the apparent contact angle, and hence the surface wettability can be calibrated. The microscale pillars were fabricated using a precise microdirect light processing (μDLP) three-dimensional (3D) printer. Different printing parameters were studied to optimize the geometric parameters to fabricate 3D hierarchical structures with high accuracy and resolution. The largest apparent contact angle in our experiments is up to 160 deg, with pillars of 0.17 mm height and 0.5 mm diameter, 55 deg reentrant angle, and a spacing of 0.36 mm between pillars. The lowest contact angle is ∼35 deg by reducing the pillar size and spacing. By controlling the size of different features of the pillar, pillar number, and layout of the mushroom-shaped micropillars, the wettability of the surface is possible to be tuned from a highly nonwetting liquid/material combination to highly wetting material. Such wettability tuning capability expands the design space for many biomedical and thermofluidic applications. 

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