Large area UV casting using diverse polyacrylates of microchannels separated by high aspect ratio microwalls

Disposable microfluidic substrates: transitioning from the research laboratory into the clinic

As more microfluidic applications emerge for clinical diagnostics, the choice of substrate and production method must be considered for eventual regulatory approval. In this review, we survey recent developments in disposable microfluidic substrates and their fabrication methods. We note regulatory approval for disposable microfluidic substrates will be more forthcoming if the substrates are developed with the United States Pharmacopeia's biocompatibility compliance guidelines in mind. We also review the recent trend in microfluidic devices constructed from a hybrid of substrates that takes advantage of each material's attributes.

Stability of high-aspect-ratio micropillar arrays against adhesive and capillary forces

High aspect-ratio (HAR) micropillar arrays have many interesting and technologically important applications. Their properties, such as large mechanical compliance, large surface area, and a topography that is well-separated from the underlying substrate, have allowed researchers to design and explore biomimetic dry adhesives, superhydrophobic, superoleophobic, and tunable wetting surfaces, mechanical sensors and actuators, and substrates for cell mechanics studies. However, the mechanical compliance and large surface area of the micropillars also make these structures susceptible to deformation by adhesive and capillary surface forces. As a result such micropillars, particularly those made from soft polymers, can prove challenging to fabricate and to use in various applications. Systematic understanding of these forces is thus critical both to assemble stable micropillar arrays and to harness these surface forces toward controlled actuation for various applications. In this Account, we review our recent studies on the stability of HAR polymeric micropillar arrays against adhesive and capillary forces. Using the replica molding method, we have successfully fabricated HAR epoxy micropillar arrays with aspect ratios up to 18. The stability of these arrays against adhesive forces is in agreement with theoretical predictions. We have also developed a new two-step replica molding method to fabricate HAR (up to 12) hydrogel micropillar arrays using monomers or monomer mixtures. By varying the monomer composition in the fabrication process, we have fabricated a series of hydrogel micropillar arrays whose elastic moduli in wet state range from less than a megapascal to more than a gigapascal, and we have used these micropillar arrays to study capillary force induced clustering behavior as a function of the modulus. As a result, we have shown that as liquid evaporates off the micropillar arrays, the pillars bend and cluster together because of a much smaller capillary meniscus interaction force while the micropillar structures are surrounded by a continuous liquid body. Previously, researchers had often attributed this clustering effect to a Laplace pressure difference because of isolated capillary bridges. Our theoretical analysis of stability against capillary force and micropillar cluster size as a function of pillar elastic modulus agrees well with our experimental observations. The fabrication approaches presented here are quite general and will enable the fabrication of tall, stable micropillar arrays in a variety of soft, responsive materials. Therefore, researchers can use these materials for various applications: sensors, responsive wetting, and biological studies. The new insights into the capillary force induced clustering of micropillar arrays could improve rational design and fabrication of micropillar arrays that are stable against capillary force. In addition, these results could help researchers better manipulate capillary force to control the assembly of micropillar arrays and actuate these structures within novel devices.

Microfabricating high-aspect-ratio structures in polyurethane-methacrylate (PUMA) disposable microfluidic devices

We recently reported a new UV-curable polyurethane-methacrylate (PUMA) resin that has excellent qualities as a disposable microfluidic substrate for clinical diagnostic applications. This article discusses strategies to improve the production yield of PUMA chips that contain dense and high-aspect-ratio features, which presents unique challenges in demolding and bonding steps. These fabrication improvements were deployed to produce a microfiltration device that contained closely spaced and high-aspect-ratio columns, suitable for retaining and concentrating cells or beads from a highly diluted suspension.

Lab-on-a-chip for multiplexed biosensing of residual antibiotics in milk

A multiplexed immunoassay-based antibiotic sensing device integrated in a lab-on-a-chip format is described. The approach is multidisciplinary and involves the convergent development of a multi-antibiotic competitive immunoassay based on sensitive wavelength interrogated optical sensor (WIOS) technology and a polymer-based self-contained microfluidic cartridge. Immunoassay solutions are pressure-driven through external and concerted actuation of a single syringe pump and multiposition valve. Moreover, the use of a novel photosensitive material in a 'one step' fabrication process allowed the rapid fabrication of microfluidic components and interconnection port simultaneously. Pre-filled microfluidic cartridges were used as binary response rapid tests for the simultaneous detection of three antibiotic families - sulfonamides, fluoroquinolones and tetracyclines - in raw milk. For test interpretation, any signal lower than the threshold value obtained for the corresponding Maximum Residue Limit (MRL) concentration (100 microg L(-1)) was considered negative for a given antibiotic. The reliability of the multiplexed detection system was assessed by way of a validation test carried out on a series of six blind milk samples. A test accuracy of 95% was calculated from this experiment. The whole immunoassay procedure is fast (less than 10 minutes) and easy to handle (automated actuation).

A new USP Class VI-compliant substrate for manufacturing disposable microfluidic devices

As microfluidic systems transition from research tools to disposable clinical-diagnostic devices, new substrate materials are needed to meet both the regulatory requirement as well as the economics of disposable devices. This paper introduces a UV-curable polyurethane-methacrylate (PUMA) substrate that has been qualified for medical use and meets all of the challenges of manufacturing microfluidic devices. PUMA is optically transparent, biocompatible, and exhibits high electroosmotic mobility without surface modification. We report two production processes that are compatible with the existing methods of rapid prototyping and present characterizations of the resultant PUMA microfluidic devices.

Design of experiment for optimization of plasma-polymerized octafluorocyclobutane coating on very high aspect ratio silicon molds

As-fabricated deep reactive ion etched (DRIE) silicon mold with very high aspect ratio (>10) feature patterns is unsuitable for poly(dimethylsiloxane) (PDMS) replication because of the strong interaction between the Si surface and the replica and the corrugated mold sidewalls. The silicon mold can be conveniently passivated via plasma polymerization of octafluorocyclobutane (C4F8), which is also employed in the DRIE process itself, to enable the mold to be used repeatedly. To optimize the passivation conditions, we have undertaken a Box-Behnken experimental design on the basis of three passivation process parameters (plasma power, C4F8 flow rate, and deposition time). The measured responses were fluorinated film thickness, demolding status/success, demolding force, and fluorine/carbon ratio on the fifth replica surface. The optimal passivation process conditions were predicted to be an input power of 195 W, a C4F8 flow rate of 57 sccm, and a deposition time of 364 s; these were verified experimentally to have high accuracy. Demolding success requires medium-deposited film thickness (66-91 nm), and the thickness of the deposited films correlated strongly with deposition time. At moderate to high ranges, increased plasma power or gas flow rate promoted polymerization over reactive etching of the film. It was also found that small quantities of the fluorinated surface were transferred from the Si mold to the PDMS at each replication, entailing progressive wear of the fluorinated layer.

Replica molding of high-aspect-ratio polymeric nanopillar arrays with high fidelity

Polymeric nanostructures with high aspect ratios, so-called nanopillars, are of interest for a wide range of applications. However, it remains a challenge to fabricate high-density, polymeric nanopillars using soft lithography when the feature size is decreased to hundreds of nanometers and the structures are close to each other. Here, we investigate the fidelity of replica molding technique to fabricate polymer nanopillar arrays with diameters ranging from 300 nm to 1 mum, and we compare the experimental results to the theoretical prediction to understand the nature of the instability of nanopillars. Nanopillars molded from soft materials, poly(dimethylsiloxane) (PDMS), mainly ground collapse due to the adhesive force when the aspect ratio is above 6, whereas those from stiffer materials, polyurethane and epoxy, collapse laterally at a much higher aspect ratio (>/=12), of which the critical value is dependent on the nanopillar's feature size, spacing, height, and shape. Further, we attempt to restore the collapsed high-aspect-ratio nanopillars using supercritical CO(2) drying.

UV-embossed microchannel in biocompatible polymeric film: Application to control of cell shape and orientation of muscle cells

This article shows that ultra violet (UV) micro-embossing can be successfully used for fabricating biocompatible micropatterned films with microchannels separated by high aspect ratio microwalls. Eight series of micropatterns were investigated; the width of the microwall was either 10 or 25 microm and that of the microchannel either 40, 80, 120, or 160 microm. The material investigated was principally polyurethane diacrylate. The UV-embossed micropattern was extracted with methanol, converting the micropatterns from cytotoxic to biocompatible. The typical UV embossing method was modified by using a marginally adhesive polyester substrate, which facilitates demolding but is removable before methanol extraction to avoid fragmentation of the embossed micropatterns. The effect of the micropatterns on A7r5 smooth muscle cells and C2C12 skeletal muscle cells was investigated. The dimensions of both channel and wall have significant effects on the elongation of both muscle cells. In the narrower 40-microm channel, the C2C12 cells merged together to form myofibers. These results indicate that UV-embossed micropatterns may present a useful scaffold for in vitro cell shape and orientation control needed in vascular and muscle tissue engineering.