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1. Rapid Micromolding of Sub-100 µm Microfluidic Channels Using an 8K Stereolithographic Resin 3D Printer

   https://doi.org/10.3390/mi14081519  

   Vedhanayagam, Arpith ; Golfetto, Michael ; Ram, Jeffrey L. ; Basu, Amar S. ( August 2023 , Micromachines)

   Nguyen, Nam-Trung ; Munoz, Rodrigo Alejandro ; Kalinke, Cristiane (Ed.)

   Engineering microfluidic devices relies on the ability to manufacture sub-100 micrometer fluidic channels. Conventional lithographic methods provide high resolution but require costly exposure tools and outsourcing of masks, which extends the turnaround time to several days. The desire to accelerate design/test cycles has motivated the rapid prototyping of microfluidic channels; however, many of these methods (e.g., laser cutters, craft cutters, fused deposition modeling) have feature sizes of several hundred microns or more. In this paper, we describe a 1-day process for fabricating sub-100 µm channels, leveraging a low-cost (USD 600) 8K digital light projection (DLP) 3D resin printer. The soft lithography process includes mold printing, post-treatment, and casting polydimethylsiloxane (PDMS) elastomer. The process can produce microchannels with 44 µm lateral resolution and 25 µm height, posts as small as 400 µm, aspect ratio up to 7, structures with varying z-height, integrated reservoirs for fluidic connections, and a built-in tray for casting. We discuss strategies to obtain reliable structures, prevent mold warpage, facilitate curing and removal of PDMS during molding, and recycle the solvents used in the process. To our knowledge, this is the first low-cost 3D printer that prints extruded structures that can mold sub-100 µm channels, providing a balance between resolution, turnaround time, and cost (~USD 5 for a 2 × 5 × 0.5 cm^3 chip) that will be attractive for many microfluidics labs. 

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2. Robust Microfabrication of Highly Parallelized Three-Dimensional Microfluidics on Silicon

   https://doi.org/10.1038/s41598-019-48515-4  

   Yadavali, Sagar ; Lee, Daeyeon ; Issadore, David ( August 2019 , Scientific Reports)

   We present a new, robust three dimensional microfabrication method for highly parallel microfluidics, to improve the throughput of on-chip material synthesis by allowing parallel and simultaneous operation of many replicate devices on a single chip. Recently, parallelized microfluidic chips fabricated in Silicon and glass have been developed to increase the throughput of microfluidic materials synthesis to an industrially relevant scale. These parallelized microfluidic chips require large arrays (>10,000) of Through Silicon Vias (TSVs) to deliver fluid from delivery channels to the parallelized devices. Ideally, these TSVs should have a small footprint to allow a high density of features to be packed into a single chip, have channels on both sides of the wafer, and at the same time minimize debris generation and wafer warping to enable permanent bonding of the device to glass. Because of these requirements and challenges, previous approaches cannot be easily applied to produce three dimensional microfluidic chips with a large array of TSVs. To address these issues, in this paper we report a fabrication strategy for the robust fabrication of three-dimensional Silicon microfluidic chips consisting of a dense array of TSVs, designed specifically for highly parallelized microfluidics. In particular, we have developed a two-layer TSV design that allows small diameter vias (d < 20 µm) without sacrificing the mechanical stability of the chip and a patterned SiO₂etch-stop layer to replace the use of carrier wafers in Deep Reactive Ion Etching (DRIE). Our microfabrication strategy allows >50,000 (d = 15 µm) TSVs to be fabricated on a single 4” wafer, using only conventional semiconductor fabrication equipment, with 100% yield (M = 16 chips) compared to 30% using previous approaches. We demonstrated the utility of these fabrication strategies by developing a chip that incorporates 20,160 flow focusing droplet generators onto a single 4” Silicon wafer, representing a 100% increase in the total number of droplet generators than previously reported. To demonstrate the utility of this chip for generating pharmaceutical microparticle formulations, we generated 5–9 µm polycaprolactone particles with a CV < 5% at a rate as high as 60 g/hr (>1 trillion particles/hour).

    

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3. Area-Specific Positioning of Metallic Glass Nanowires on Si Substrate

   https://doi.org/10.1007/s41871-023-00205-6  

   Theeda, Sumanth ; Kumar, Golden ( July 2023 , Nanomanufacturing and Metrology)

   This paper presents a novel technique to fabricate metallic nanowires in selective areas on a Si substrate. Thermoplastic drawing of viscous metallic glass from cavities etched in Si can produce metallic nanowires. The length and diameter of nanowires can be controlled by adjusting the drawing conditions without changing the Si mold. A thin metal shadow mask is stacked above the Si mold during thermoplastic drawing to fabricate the nanowires only in specific locations. The mask restricts the flow of metallic glass to predefined shapes on the mask, resulting in the formation of nanowires in selected areas on Si. An Al foil-based mask made by a benchtop vinyl cutter is used to demonstrate the proof-of-concept. Even a simple Al foil mask enables the positioning of metallic nanowires in selective areas as small as 200 µm on Si. The precision of the vinyl cutter limits the smallest dimensions of the patterned areas, which can be further improved by using laser-fabricated stencil masks. Results show that a single row of metallic glass nanowires can be patterned on Si using selective thermoplastic drawing. Controllable positioning of metallic nanowires on substrates can enable new applications and characterization techniques for nanostructures.

    

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4. Fabrication of 3D Micro-Blades for the Cutting of Biological Structures in a Microfluidic Guillotine

   https://doi.org/10.3390/mi12091005  

   Koppaka, Saisneha ; Zhang, Kevin ; Kurosu Jalil, Myra ; Blauch, Lucas ; Tang, Sindy ( September 2021 , Micromachines)

   Micro-blade design is an important factor in the cutting of single cells and other biological structures. This paper describes the fabrication process of three-dimensional (3D) micro-blades for the cutting of single cells in a microfluidic “guillotine” intended for fundamental wound repair and regeneration studies. Our microfluidic guillotine consists of a fixed 3D micro-blade centered in a microchannel to bisect cells flowing through. We show that the Nanoscribe two-photon polymerization direct laser writing system is capable of fabricating complex 3D micro-blade geometries. However, structures made of the Nanoscribe IP-S resin have low adhesion to silicon, and they tend to peel off from the substrate after at most two times of replica molding in poly(dimethylsiloxane) (PDMS). Our work demonstrates that the use of a secondary mold replicates Nanoscribe-printed features faithfully for at least 10 iterations. Finally, we show that complex micro-blade features can generate different degrees of cell wounding and cell survival rates compared with simple blades possessing a vertical cutting edge fabricated with conventional 2.5D photolithography. Our work lays the foundation for future applications in single cell analyses, wound repair and regeneration studies, as well as investigations of the physics of cutting and the interaction between the micro-blade and biological structures. 

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5. Electrowetting-based microfluidic operations on rapid- manufactured devices for heat pipe applications

   Hale, R ( June 2017 , Journal of micromechanics and microengineering)

   The heat transport capacity of traditional heat pipes is limited by the capillary pressure generated in the internal wick that pumps condensate to the evaporator. Recently, the authors conceptualized a novel heat pipe architecture, wherein wick-based pumping is replaced by electrowetting (EW)-based pumping of microliter droplets in the adiabatic section. An electrowetting heat pipe (EHP) can overcome the capillary limit to heat transport capacity and enable compact, planar, gravity-insensitive, and ultralow power consumption heat pipes that transport kiloWatt heat loads over extended distances. This work develops a novel technique for rapid, scalable fabrication of EW-based devices and studies critical microfluidic operations underlying the EHP, with the objective of predicting the key performance parameters of the EHP. Devices are fabricated on a printed circuit board (PCB) substrate with mechanically-milled electrodes, and a removable polyimide dielectric film. The first set of experiments uncovers the maximum channel gap (1 mm) for reliable EW-based pumping; this parameter determines the heat transport capacity of the EHP, which scales linearly with the channel gap. The second set of experiments uncovers the maximum channel gap (375 microns) at which EW voltages can successfully split droplets. This is an important consideration which ensures EHP operability in the event of unintentional droplet merging. The third set of experiments demonstrate and study EW-induced droplet generation from an open-to-air reservoir, which mimics the interface between the condenser and adiabatic sections of the EHP. The experimental findings predict that planar, water-based EHPs with a (10 cm by 4 mm) cross section can transport 1.6 kW over extended distances (>1 m), with a thermal resistance of 0.01 K W−1. 

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