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1. Fabrication of Functional Microdevices in SU-8 by Multi-Photon Lithography

   https://doi.org/10.3390/mi12050472  

   Golvari, Pooria; Kuebler, Stephen M. (May 2021, Micromachines)

   null (Ed.)

   This review surveys advances in the fabrication of functional microdevices by multi-photon lithography (MPL) using the SU-8 material system. Microdevices created by MPL in SU-8 have been key to progress in the fields of micro-fluidics, micro-electromechanical systems (MEMS), micro-robotics, and photonics. The review discusses components, properties, and processing of SU-8 within the context of MPL. Emphasis is focused on advances within the last five years, but the discussion also includes relevant developments outside this period in MPL and the processing of SU-8. Novel methods for improving resolution of MPL using SU-8 and discussed, along with methods for functionalizing structures after fabrication. 

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2. Pressure‐assisted binder jetting for additive manufacturing of mock energetic composites

   https://doi.org/10.1002/prep.202300175  

   Kirby, Levi; Udaykumar, H. S.; Song, Xuan (November 2023, Propellants, Explosives, Pyrotechnics)

   Abstract Additive manufacturing (AM) has emerged as a promising approach to achieve energetic materials (EMs) with intricate geometries and controlled microstructures, which are crucial for safety and performance optimization. However, current AM methods still face limitations such as limited densities and inadequate solids loading. To overcome these limitations, we have developed a pressure‐assisted binder jet (PBJ) process that has the potential to allow for the fabrication of intricate EMs while preserving their desired properties. This study aims to investigate the effects of printing parameters on the microstructures and properties of EMs, including density, solids loading, mechanical properties, and heterogeneity. Our results demonstrate that the PBJ process achieves exceptional properties in EMs, including densities up to 83.4 % and solids loading up to 95.4 %, surpassing those achieved by existing AM processes. Furthermore, the mechanical properties of the fabricated EMs are comparable to those achieved using conventional fabrication techniques, including a compressive strength of 3.32 MPa, a Young's modulus of 16.68 MPa, a Poisson's ratio of 0.45, a shear modulus of 5.73 MPa, and a bulk modulus of 21.01 GPa. Various test cases were printed to showcase the ability of the PBJ process to create EMs with complex structures and exceptional properties. Micro‐computed tomography was employed to analyze the influence of printing parameters on the internal composition and microstructures of the printed specimens. 

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3. Microsphere photolithography using reusable microsphere array mask for low-cost infrared metasurface fabrication

   https://doi.org/10.1116/6.0002557  

   Zhu, Chen; Kinzel, Edward C. (May 2023, Journal of Vacuum Science & Technology B)

   Microsphere photolithography (MPL) is an alternative low-cost technique for the large-scale fabrication of periodic structures, such as metasurfaces. This technique utilizes the photonic nanojet generated in the photoresist (PR), by microspheres in near proximity, which are exposed to collimated ultraviolet (UV) flood illumination. In the basic approach, a microsphere array is self-assembled on, or transferred to, the substrate prior to exposure. After exposure, the microspheres are washed away in the development step. The process to recover and clean these microspheres for reuse is complicated. This paper investigates the use of reusable microsphere masks created by fixing the microspheres on a UV transparent support. This is then brought into contact with the photoresist with controlled pressure. There is a trade-off between the quality of the fabricated samples and the wear of the mask determined by the contact pressure. The system is demonstrated using a digital micromirror device (DMD)-based direct-write exposure system to fabricate infrared (IR) metasurfaces. These metasurfaces are characterized and compared to simulation models. Finally, a series of 50 hierarchically patterned IR metasurfaces was fabricated using a single reusable mask. These samples had a <3% coefficient of variance when viewed with a thermal camera. This work shows the potential of mask-based MPL and other contact microlens array-based photolithography techniques for low-cost large-scale fabrication. 

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4. Robust Bicontinuous Elastomer–Metal Foam Composites with Highly Tunable Stiffness

   https://doi.org/10.1002/adem.202101533  

   Sharifi, Siavash; Mohammadi_Nasab, Amir; Chen, Pei-En; Liao, Yiliang; Jiao, Yang; Shan, Wanliang (January 2022, Advanced Engineering Materials)

   Herein, a new class of robust bicontinuous elastomer–metal foam composites with highly tunable mechanical stiffness is proposed, fabricated, characterized, and demonstrated. The smart composite is a bicontinuous network of two foams, one metallic made of a low melting point alloy (LMPA) and the other elastomeric made of polydimethylsiloxane (PDMS). The stiffness of the composite can be tuned by inducing phase changes in its LMPA component. Below the melting point of the LMPA, Young's modulus of the smart composites is ≈1 GPa, whereas above the melting point of the LMPA it is ≈1 MPa. Thus, a sharp stiffness change of ≈1000× can be realized through the proposed bicontinuous foam composite structure, which is higher than all available robust smart composites. Effective medium theory is also used to predict the Young's modulus of the bicontinuous smart composites, which generates reasonable agreement with experimentally measured Young's modulus of the smart composites. Finally, the use of these smart materials as a smart joint in a robotic arm is also demonstrated. 

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5. The Effect of the Stiffness of Soft Materials on Hemiwicking Performance

   https://doi.org/10.1109/ITherm45881.2020.9190180  

   Germain, Thomas; Putnam, Shawn A. (July 2020, 2020 19th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm))

   Fabricating micro and nanosized structures to induce hemiwicking on a heated surface has risen in popularity due to the higher heat flux the surface can experience. Recent studies have focused on the effects on the pillar geometry and spacing on the wicking velocity and the critical heat flux. As a result, a majority of the models that have been derived focus on the fluid properties and the wicking structure geometry and spacing. This study presents changes to the wicking performance when the stiffness of a soft material is taken into effect. Multiple similar wicking structures were fabricated using a negative mold method utilizing an in-house stamping apparatus. Using the mold, multiple polydimethylsiloxane (PDMS) samples were created, where the stiffness of the samples was varied by altering the mixing ratio and the curing time. The wicking velocity of ethanol, isopropyl alcohol, and isooctane did not vary for the samples that had a Young's Modulus greater than 1 MPa, but a notable decrease in the wicking velocity for all three fluids were observed for samples with a Young's Modulus less than 1 MPa. This study provides insight to the importance of the stiffness of the material is for hemiwicking on soft materials and that deformation effects have to be taken into account for Young's Moduli less than 1 MPa. 

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