SU-8 as a material for lab-on-a-chip-based mass spectrometry

Recent Trends of Microfluidics in Food Science and Technology: Fabrications and Applications

The development of novel materials with microstructures is now a trend in food science and technology. These microscale materials may be applied across all steps in food manufacturing, from raw materials to the final food products, as well as in the packaging, transport, and storage processes. Microfluidics is an advanced technology for controlling fluids in a microscale channel (1~100 μm), which integrates engineering, physics, chemistry, nanotechnology, etc. This technology allows unit operations to occur in devices that are closer in size to the expected structural elements. Therefore, microfluidics is considered a promising technology to develop micro/nanostructures for delivery purposes to improve the quality and safety of foods. This review concentrates on the recent developments of microfluidic systems and their novel applications in food science and technology, including microfibers/films via microfluidic spinning technology for food packaging, droplet microfluidics for food micro-/nanoemulsifications and encapsulations, etc.

Physiochemically Distinct Surface Properties of SU-8 Polymer Modulate Bacterial Cell-Surface Holdfast and Colonization

SU-8 polymer is an excellent platform for diverse applications due to its high aspect ratio of micro/nanostructure fabrication and exceptional physicochemical and biocompatible properties. Although SU-8 polymer has often been investigated for various biological applications, how its surface properties influence the interaction of bacterial cells with the substrate and its colonization is poorly understood. In this work, we tailor SU-8 nanoscale surface properties to investigate single-cell motility, adhesion, and successive colonization of phytopathogenic bacteria, Xylella fastidiosa. Different surface properties of SU-8 thin films have been prepared using photolithography processing and oxygen plasma treatment. A more significant density of carboxyl groups in hydrophilic plasma-treated SU-8 surfaces promotes faster cell motility in the earlier growth stage. The hydrophobic nature of pristine SU-8 surfaces shows no trackable bacterial motility and 5-10 times more single cells adhered to the surface than its plasma-treated counterpart. In addition, plasma-treated SU-8 samples suppressed bacterial adhesion, with surfaces showing less than 5% coverage. These results not only showcase that SU-8 surface properties can impact the spatiotemporal bacterial behavior but also provide insights into pathogens' prominent ability to evolve and adapt to different surface properties.

Sample-to-analysis platform for rapid intracellular mass spectrometry from small numbers of cells

Real-time, advanced diagnostics of the biochemical state within cells remains a significant challenge for research and development, production, and application of cell-based therapies. The fundamental biochemical processes and mechanisms of action of such advanced therapies are still largely unknown, including the critical quality attributes that correlate to therapeutic function, performance, and potency and the critical process parameters that impact quality throughout cell therapy manufacturing. An integrated microfluidic platform has been developed for in-line analysis of a small number of cells via direct infusion nano-electrospray ionization mass spectrometry. Central to this platform is a microfabricated cell processing device that prepares cells from limited sample volumes removed directly from cell culture systems. The sample-to-analysis workflow overcomes the labor intensive, time-consuming, and destructive nature of existing mass spectrometry approaches for analysis of cells. By providing rapid, high-throughput analyses of the intracellular state, this platform enables untargeted discovery of critical quality attributes and their real-time, in-process monitoring.

Metrology and characterization of SU-8 microstructures using autofluorescence emission

Sophisticated three-dimensional microstructures fabricated using the negative tone SU-8 photoresist are used in many biomedical and microfluidic applications. Scanning electron microscopy (SEM) and profilometry are commonly used metrological techniques for the dimensional characterization of fabricated SU-8 microstructures but are not viable for non-destructive measurements and characterization of subsurface features like hidden microchannels. In this study, we report a unique methodology for the non-destructive dimensional characterization of SU-8 microstructures using the emitted autofluorescence radiation from fabricated SU-8 microstructures to generate depth profiles. The relationship between autofluorescence emission intensities and the thicknesses of the microstructures measured using SEM was determined and used to characterize the dimensions of unknown SU-8 microstructures based on their autofluorescence intensities. Lateral dimensions were also measured. This relationship was used to create highly accurate depth profiles for different types of microstructures including hidden subsurface features. These results were validated by comparison with SEM. The results suggest a feasible and accurate non-destructive, low cost, metrological technique to characterize SU-8 surface and subsurface microstructures using autofluorescence emission intensities.

A 2D Waveguide Method for Lithography Simulation of Thick SU-8 Photoresist

Due to the increasing complexity of microelectromechanical system (MEMS) devices, the accuracy and precision of two-dimensional microstructures of SU-8 negative thick photoresist have drawn more attention with the rapid development of UV lithography technology. This paper presents a high-precision lithography simulation model for thick SU-8 photoresist based on waveguide method to calculate light intensity in the photoresist and predict the profiles of developed SU-8 structures in two dimension. This method is based on rigorous electromagnetic field theory. The parameters that have significant influence on profile quality were studied. Using this model, the light intensity distribution was calculated, and the final resist morphology corresponding to the simulation results was examined. A series of simulations and experiments were conducted to verify the validity of the model. The simulation results were found to be in good agreement with the experimental results, and the simulation system demonstrated high accuracy and efficiency, with complex cases being efficiently handled.

Novel imaging technique for non-destructive metrology and characterization of ultraviolet-sensitive polymeric microstructures

The negative photoresist SU-8 has attracted much research interest as a structural material for creating complex three-dimensional (3D) microstructures incorporating hidden features such as microchannels and microwells for a variety of lab-on-a-chip and biomedical applications. Achieving desired topological and dimensional accuracy in such SU-8 microstructures is crucial for most applications, but existing methods for their metrology, such as scanning electron microscopy (SEM) and optical profilometry, are not practical for non-destructive measurement of hidden features. This paper introduces an alternative imaging modality for non-destructively characterizing the features and dimensions of SU-8 microstructures by measuring their transmittance of 365 nm ultraviolet (UV) light. Here, depth profiles of SU-8 3D microstructures and thin films are determined by relating UV transmittance and the thicknesses of SU-8 samples imaged in the UV spectrum through the Beer-Lambert law applied to the images on a pixel-by-pixel basis. This technique is validated by imaging the UV transmittance of several prototype SU-8 3D microstructures, including those comprising hidden hollow subsurface features, as well as SU-8 thin-films, and verifying the measured data through SEM. These results suggest that UV transmittance imaging offers a cost-effective, non-destructive technique to quickly measure and identify SU-8 microstructures with surface and hidden subsurface features unlike existing techniques.

Integrated Piezoelectric AlN Thin Film with SU-8/PDMS Supporting Layer for Flexible Sensor Array

This research focuses on the development of a flexible tactile sensor array consisting of aluminum nitride (AlN) based on micro-electro-mechanical system (MEMS) technology. A total of 2304 tactile sensors were integrated into a small area of 2.5 × 2.5 cm². Five hundred nm thick AlN film with strong c-axis texture was sputtered on Cr/Au/Cr (50/50/5 nm) layers as the sacrificial layer coated on a Si wafer. To achieve device flexibility, polydimethylsiloxane (PDMS) polymer and SU-8 photoresist layer were used as the supporting layers after etching away a release layer. Twenty-five mM (3-mercaptopropyl) trimethoxysilane (MPTMS) improves the adhesion between metal and polymers due to formation of a self-assembled monolayer (SAM) on the surface of the top electrode. The flexible tactile sensor has 8 × 8 channels and each channel has 36 sensor elements with nine SU-8 bump blocks. The tactile sensor array was demonstrated to be flexible by bending 90 degrees. The tactile sensor array was demonstrated to show clear spatial resolution through detecting the distinct electrical response of each channel under local mechanical stimulus.

Multi-layering of SU-8 exhibits distinct geometrical transitions from circular to planarized profiles

The negative tone photoresist SU-8 permits the creation of micrometer-scale structures by optical lithography. It is also the most used photoresist in soft lithography for the fast-prototyping of microfluidic devices. Despite its importance, the effect of capillary forces on SU-8 multi-layering onto topographical features has not been thoroughly studied. In particular, the profile of the added layer has not been examined in detail. The overlaying process exhibits a set of distinct behaviors, or regimes, depending on the relative thickness of the overlay and the underlying rectangular pattern. We demonstrate how capillary effects control the profile of multi-layer microchannels in a predictable manner. We derive a simple static model to describe the evolution of the overlay as a function of dimensionless geometric parameters. Our study provides a critical understanding of the parameters that govern multi-layer spin coating.

Capillary liquid bridge soft lithography for micro-patterning preparation based on SU-8 photoresist templates with special wettability

Patterned micro-nano arrays have shown great potential in the fields of optics, electronics and optoelectronics. In this study, a strategy of interface-induced dewetting assembly based on capillary liquid bridges and SU-8 photoresist templates is proposed for patterning organic molecules and nanoparticles. First, photoresist templates with chemical stability were prepared via a simplified lithography method. Then the interface wettability and the contact angle hysteresis of water droplets on the fluorosilane modified templates were adequately studied and discussed. Subsequently, a sandwich structure, composed of a superhydrophilic target substrate, a hydrophobic high adhesive photoresist template and a growth solution were introduced for the confined space dewetting assembly. The related mechanism was investigated and revealed, with the assistance of in situ observation via a fluorescence microscope. Finally, the patterned arrays of water-soluble organic small molecules and aqueous dispersed nanoparticles were successfully obtained on the target substrates. This method is simple and easy, and the SU-8 photoresist templates possess a series of advantages such as low processing cost, short preparation periods and reusable performance, which endow this strategy with potential for application in molecular functional devices.

Physico-chemical characterization of caesium and strontium using fluorescent intensity of bacteria in a microfluidic platform

Recently, the impact of radioactive caesium (Cs) and strontium (Sr) on human health and the ecosystem has been a major concern due to the use of nuclear energy. However, this study observed changes in green-fluorescent (GFP)-tagged Pseudomonas aeruginosa PAO1 biofilms by injecting non-radioactive caesium chloride (CsCl) and strontium chloride (SrCl₂) into microstructures embedded in polydimethylsiloxane microfluidic devices, which were used due to their strong toxicity limitations. Four types of microstructures with two different diameters were used in the study. The change of biofilm thickness from fluid velocity and wall shear stress was estimated using computational fluid dynamics and observed throughout the experiment. The effect of pore space became a significant physical factor when the fluid was flowing through the microfluidic devices. As the pore space increased, the biofilm growth increased; therefore, triangular microstructures with the largest pore space showed the best growth of biofilm. Caesium chloride (CsCl) and strontium chloride (SrCl₂), less toxic than radioactive caesium (Cs) and strontium (Sr), completely eradicated the P. aeruginosa PAO1 biofilm with low concentrations. The combined effect of toxicity, fluid velocity, wall shear stress and microstructures increased the efficiency of biofilm eradication. These findings on microfluidic chips can help to indirectly predict the impact on human public health and ecosystems without using radioactive chemicals.

Interfacing Digital Microfluidics with Ambient Mass Spectrometry Using SU-8 as Dielectric Layer

This work describes the interfacing of electrowetting-on-dielectric based digital microfluidic (DMF) sample preparation devices with ambient mass spectrometry (MS) via desorption atmospheric pressure photoionization (DAPPI). The DMF droplet manipulation technique was adopted to facilitate drug distribution and metabolism assays in droplet scale, while ambient mass spectrometry (MS) was exploited for the analysis of dried samples directly on the surface of the DMF device. Although ambient MS is well-established for bio- and forensic analyses directly on surfaces, its interfacing with DMF is scarce and requires careful optimization of the surface-sensitive processes, such as sample precipitation and the subsequent desorption/ionization. These technical challenges were addressed and resolved in this study by making use of the high mechanical, thermal, and chemical stability of SU-8. In our assay design, SU-8 served as the dielectric layer for DMF as well as the substrate material for DAPPI-MS. The feasibility of SU-8 based DMF devices for DAPPI-MS was demonstrated in the analysis of selected pharmaceuticals following on-chip liquid-liquid extraction or an enzymatic dealkylation reaction. The lower limits of detection were in the range of 1⁻10 pmol per droplet (0.25⁻1.0 µg/mL) for all pharmaceuticals tested.

A piezo-ring-on-chip microfluidic device for simple and low-cost mass spectrometry interfacing

Mass spectrometry (MS) interfacing technology provides the means for incorporating microfluidic processing with post MS analysis.

Three-dimensional hydrodynamic focusing microfluidic emitter: a strategy to inhibit sample ion expansion in nanoelectrospray ionization

A proposed 3D HFNE was used to generate a wrapped electrospray to achieve the inhibition of sample ion expansion, thus restraining injection loss in electrospray ionization.

A reliable and simple method for fabricating a poly(dimethylsiloxane) electrospray ionization chip with a corner-integrated emitter

Monolithically integrated emitters have been increasingly applied to microfluidic devices that are coupled to mass spectrometers (MS) as electrospray ionization sources (ESI). A new method was developed to fabricate a duplicable structure which integrated the emitter into a poly(dimethylsiloxane) chip corner. Two photoresist layers containing a raised base which guaranteed the precise integration of the electrospray tip emitter and ensured that the cutting out of the tip exerted no influence even during repeated prototyping were used to ease the operation of the process. Highly stable ESI-MS performance was obtained and the results were compared with those of a commercial fused-silica capillary source. Furthermore, chip-to-chip and run-to-run results indicated both reliability and reproducibility during repeated fabrication. These results reveal that the proposed chip can provide an ideal ion source for MS across many applications, especially with the perspective to be widely used in portable MS during on-site analysis.