Controlling the fate of particles and cells in microfluidic devices is critical in many biomedical applications, such as particle and cell alignment and separation. Recently, viscoelastic polymer solutions have been successfully used to promote transversal migration of particles and cells toward fixed positions in straight microchannels. When inertia is negligible, numerical simulations have shown that strongly shear-thinning polymer solutions (fluids with a shear viscosity that decreases with increasing flow rates) promote transversal migration of particles and cells toward the corners or toward the centerline in a straight microchannel with a square cross section, as a function of particle size, cell deformability, and channel height. However, no experimental evidence of such shifting in the positions for particles or cells suspended in strongly shear-thinning liquids has been presented so far. In this work, we demonstrate that particle positions over the channel cross section can be shifted "from the edge to the center" in a strongly shear-thinning liquid. We investigate the viscoelasticity-induced migration of both rigid particles and living cells (Jurkat cells and NIH 3T3 fibroblasts) in an aqueous 0.8 wt % hyaluronic acid solution. The combined effect of fluid elasticity, shear-thinning, geometric confinement, and cell deformability on the distribution of the particle/cell positions over the channel cross section is presented and discussed. In the same shear-thinning liquid, separation of 10 and 20 μm particles is also achieved in a straight microchannel with an abrupt expansion. Our results envisage further applications in viscoelasticity-based microfluidics, such as deformability-based cell separation and viscoelastic spacing of particles/cells.
Large-scale plasmonic substrates consisting of metal-insulator nanostructures coated with a biorecognition layer can be exploited for enhanced label-free sensing by utilizing the principle of localized surface plasmon resonance (LSPR). Most often, the uniformity and thickness of the biorecognition layer determine the sensitivity of plasmonic resonances as the inherent LSPR sensitivity of nanomaterials is limited to 10-20 nm from the surface. However, because of time-consuming nanofabrication processes, there is limited work on both the development of large-scale plasmonic materials and the subsequent surface functionalizing with biorecognition layers. In this work, by exploiting properties of reactive ions in an SF plasma environment, we are able to develop a nanoplasmonic substrate containing ∼10/cm mushroom-like structures on a large-sized silicon dioxide substrate (i.e., 2.5 cm by 7.5 cm). We further investigate the underlying mechanism of the nanoassembly of gold on glass inside the plasma environment, which can be expanded to a variety of metal-insulator systems. By incorporating a novel microcontact printing technique, we deposit a highly uniform biorecognition layer of proteins on the nanoplasmonic substrate. The bioplasmonic assays performed on these substrates achieve a limit of detection of 10 g/mL (∼66 zM) for biomolecules such as antibodies (∼150 kDa). Our simple nanofabrication procedure opens new opportunities in fabricating versatile bioplasmonic materials for a wide range of biomedical and sensing applications.
Microfluidic systems integrated with protein and DNA micro- and nanoarrays have been the most sought-after technologies to satisfy the growing demand for high-throughput disease diagnostics. As the sensitivity of these systems relies on the bio-functionalities of the patterned recognition biomolecules, the primary concern has been to develop simple technologies that enable biomolecule immobilization within microfluidic devices whilst preserving bio-functionalities. To address this concern, we introduce a two-step patterning approach to create micro- and nanoarrays of biomolecules within microfluidic devices. First, we introduce a simple aqueous based microcontact printing (μCP) method to pattern arrays of (3-aminopropyl)triethoxysilane (APTES) on glass substrates, with feature sizes ranging from a few hundred microns down to 200 nm (for the first time). Next, these substrates are integrated with microfluidic channels to then covalently couple DNA aptamers and antibodies with the micro- and nanopatterned APTES. As these biomolecules are covalently tethered to the device substrates, the resulting bonds enable them to withstand the high shear stresses originating from the flow in these devices. We further demonstrated the flexibility of this technique, by immobilizing multiple proteins onto these APTES-patterned substrates using liquid-dispensing robots to create multiple microarrays. Next, to validate the functionalities of these microfluidic biomolecule microarrays, we perform (i) aptamer-based sandwich immunoassays to detect human interleukin 6 (IL6); and (ii) antibody-based sandwich immunoassays to detect human c-reactive protein (hCRP) with the limit of detection at 5 nM, a level below the range required for clinical screening. Lastly, the shelf-life potential of these ready-to-use microfluidic microarray devices is validated by effectively functionalizing the patterns with biomolecules up to 3 months post-printing. In summary, with a single printing step, this aminosilane patterning technique enables the creation of functional microfluidic micro- and nano-biomolecule arrays, laying the foundation for high-throughput multiplexed bioassays.
labeled assays tend to be constrained by:(1) the cellular expression of a given cell type, and (2) the availability of tags for specific cell expression. In most of the labeled assays, "signal quenching" is often associated with false experimental positives, [11] which decreases assay reliability. Furthermore, labeled assays are laborious, costly, unsuitable for real-time cell analysis, and sometimes require the usage of toxic reagents, such as radioactive labels. [12] Label-free biosensors utilize biophysical properties of a given analyte, such as its mass (e.g., in quartz crystal microbalance), refractive index (e.g., in plasmonic nanobiosensors), or molecular charge (e.g., in potentiometric and amperometric sensors) to monitor the response of the analyte in real-time. [13] Recently, nanomaterialbased label-free photonic biosensors have revealed unprecedented information on DNA and protein molecular interactions. [14] However, few attempts have been made to apply label-free photonic biosensors to cellular assays, as it is challenging to develop nanostructured substrates with large surface areas that promote both sensing and long-term cell survival. Meanwhile, photonic techniques have been coupled with microscopy tools to enhance live cell imaging and distinguish different types of cell behavior such as cell activation, adhesion, proliferation, migration, and apoptosis. [15] In comparison to these "imaging"-based techniques, "sensor"-based techniques generate average responses proportional to the concentrations of a given analyte. Such an average response captures real-time kinetics of cell behavior. In this work, we develop novel nanomushroom (NM) structures (NM-based sensors) for labelfree, long-term cell proliferation detection with high sensitivity, on a large interrogation area, specifically suitable for clinically relevant cell experiments (e.g., drug testing) that require large interrogation areas involving standard 96-well assay plates. Moreover, the unique features of our NM-based sensors can be complementary to existing imaging-based tools to probe cell kinetics in real-time with high accuracy.The developed NM structures are 45-60 nm in height and ≈20 nm in width, evenly distributed with ≈10 nm spacing on a standard glass slide (25 mm × 75 mm). Each NM consists of a silicon dioxide (SiO 2 ) stem of 30-40 nm in height, covered by a gold (Au) cap of 15-20 nm in thickness (Figure 1a). These structures were fabricated by a simple, high-throughput, three-step process based on well-established principles of gold Innovative sensing materials have enabled the discovery of cell biology principles at the nanoscale. In order to evaluate cell behavior and responses, it is necessary to accurately monitor cell proliferation. However, it remains challenging to develop nanomaterials possessing pertinent properties for sensing, while ensuring long-term cell survival and unaltered cellular responses. This work develops highly sensitive, large-scale, and biocompatible nanoplasmonic biosensors for long-term monitoring of ...
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