Proof‐of‐concept modular fluid handling prototype integrated with microfluidic biochemical assay modules for point‐of‐care testing

Large populations around the world suffer from numerous but treatable health issues, caused by either lifestyle choices or environmental factors. Over the past decades, point-of-care testing kits have been developed to circumvent the reliance on laboratories, by allowing users to perform preliminary health or environmental testing from the privacy of their homes. However, these kits heavily rely on the precision of the user to perform the procedures, leading to increased variability in final assessments. To eliminate user-induced errors, we present an integrated, completely sealed, and disposable point-of-care testing prototype that exploits the benefits of microfluidics and 3D-printing fabrication techniques. The palm-sized modular prototype consists of a manually operated fluid handling device that allows precise mixing, filtration, and delivery of fluids to an on-board microfluidic assay unit for subsequent detection of specific biochemical analytes, with a minimized risk of contamination

Steady viscoelastic flow around high-aspect-ratio, low-blockage-ratio microfluidic cylinders

We employ a state-of-the-art microfabrication technique (selective laser-induced etching, SLE) to produce microfluidic cylinder geometries that explore new geometrical regimes. Using SLE, two microchannels are fabricated in monolithic fused silica substrate with height H = 2 mm and width W = 0.4 mm (aspect ratio α = H/W = 5) containing cylinders of radius r = 0.02 mm (blockage ratio β = 2r/W = 0.1), centered at the channel mid-width, W/2. An ‘sc’ channel contains a single cylinder, while a ‘dc’ channel contains two axiallyaligned cylinders separated by a distance L = 1 mm (L = 50r). Compared with cylinder geometries fabricated by soft lithography (which typically have α ≪ 1 and β ≳ 0.5), these rigid glass devices provide a quasi-two-dimensional flow along the direction of the cylinder axis and also more clearly reveal the effects of the strong extensional wake regions located at the leading and trailing stagnation points. Using flow velocimetry and quantitative birefringence measurement techniques, we study the behaviour of a well-characterized viscoelastic polymer solution in flow around the cylinders. The small cylinder radii result in low inertia and very high elasticity numbers El ≈ 2400. For the sc device, we report strong flow modification effects around the cylinder as the flow rate is incremented. This is associated with the deformation of polymer molecules primarily in the upstream wake region, leading to the onset of a purely elastic flow asymmetry upstream of the cylinder. Stretched polymer molecules are advected around the cylinder and relax downstream of the cylinder, resulting in an extremely long elastic wake extending for > 300r downstream. In the dc channel, at lower flow rates, similar flow modification effects are observed to develop around, and downstream of, both cylinders. However, at higher flow rates the wake of the first cylinder extends > 50r downstream, and begins to interact with the second cylinder. The second cylinder becomes encapsulated by the wake of the first and is effectively obviated from the flow field. The results will be of relevance to understanding practical applications of viscoelastic fluids, for example in particle suspension and porous media flows, and also for benchmarking against numerical simulations using viscoelastic constitutive models

Particle trapping in merging flow junctions by fluid-solute-colloid-boundary interactions

Merging of different streams in channel junctions represents a common mixing process that occurs in systems ranging from soda fountains and bathtub faucets to chemical plants and microfluidic devices. Here, we report a spontaneous trapping of colloidal particles in a merging flow junction when the merging streams have a salinity contrast. We show that the particle trapping is a consequence of nonequilibrium interactions between the particles, solutes, channel, and the freestream flow. A delicate balance of transport processes results in a stable near-wall vortex that traps the particles. We use three-dimensional particle visualization and numerical simulations to provide a rigorous understanding of the observed phenomenon. Such a trapping mechanism is unique from the well-known inertial trapping enabled by vortex breakdown [Proc. Natl. Acad. Sci. USA 111, 4770 (2014)], or the solute-mediated trapping enabled by diffusiophoresis [Phys..Rev. X 7, 041038 (2017)], as the current trapping is facilitated by both the solute and the inertial effects, suggesting a new mechanism for particle trapping in flow networks

Glioblastoma adhesion in a quick-fit hybrid microdevice

Translational research requires reliable biomedical microdevices (BMMD) to mimic physiological conditions and answer biological questions. In this work, we introduce a reversibly sealed quick-fit hybrid BMMD that is operator-friendly and bubble-free, requires low reagent and cell consumption, enables robust and high throughput performance for biomedical experiments. Specifically, we fabricate a quick-fit poly(methyl methacrylate) and poly(dimethyl siloxane) (PMMA/PDMS) prototype to illustrate its utilities by probing the adhesion of glioblastoma cells (T98G and U251MG) to primary endothelial cells. In static condition, we confirm that angiopoietin-Tie2 signaling increases the adhesion of glioblastoma cells to endothelial cells. Next, to mimic the physiological hemodynamic flow and investigate the effect of physiological electric field, the endothelial cells are pre-conditioned with concurrent shear flow (with fixed 1 Pa shear stress) and direct current electric field (dcEF) in the quick-fit PMMA/PDMS BMMD. With shear flow alone, endothelial cells exhibit classical parallel alignment; while under a concurrent dcEF, the cells align perpendicularly to the electric current when the dcEF is greater than 154 V m(-1). Moreover, with fixed shear stress of 1 Pa, T98G glioblastoma cells demonstrate increased adhesion to endothelial cells conditioned in dcEF of 154 V m(-1), while U251MG glioblastoma cells show no difference. The quick-fit hybrid BMMD provides a simple and flexible platform to create multiplex systems, making it possible to investigate complicated biological conditions for translational research

3D-printed glass microfluidics for fluid dynamics and rheology

Microfluidics provides a versatile platform for handling small volumes of fluids at small length scales. From a fluid dynamics perspective, microfluidics gives access to a regime of very high deformation rates at moderate to negligible Reynolds numbers Re. For viscoelastic fluid flows, the resulting high Weissenberg numbers Wi = tau, where tau is the fluid characteristic time, means the flow occurs at high elasticity number El = Wi/Re. Consequently, microfluidics supports a burgeoning interest in the experimental study of purely elastic flow instabilities and elastic turbulence. However, for rheological studies, typical microfluidic fabrications by soft lithography in poly (dimethyl siloxane) suffer from a number of limitations arising from the low elastic modulus and poor optical properties of the material. In this review, we summarise a few recent studies from our group in which we have experimented with microdevice fabrications using the subtractive three-dimensional (3D)-printing technique of selective laser-induced etching (SLE). SLE can be used to fabricate arbitrary 3D geometries with micron precision in fused silica: a high modulus, highly transparent material, which is robust and resistant to organic solvents. Apart from high elasticity number flows, we have found that SLE fabricated devices can sustain very high deformation rates without device failure, providing new access to little-explored inertio-elastic regimes in extremely dilute polymer solutions. Furthermore, it is possible to visualize flows from multiple planes of observation, allowing the quantitative study of 3D flow instabilities and vortex dynamics in both Newtonian and non-Newtonian fluids. SLE fabrication offers many new opportunities to those involved in fluid dynamics and rheology research at the microscale, and we highlight what we perceive as potentially fruitful ideas for future studies using this technique

Microcontact printing with aminosilanes: creating biomolecule micro- and nanoarrays for multiplexed microfluidic bioassays

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

Volumetric evolution of elastic turbulence in porous media

Viscoelastic flow instability, which is compelled by elastic effects rather than inertia, can be driven to a chaotic state termed elastic turbulence (ET) manifested as strong velocity fluctuations with an algebraic decay in the frequency spectrum and increased mixing. We report the first spatiotemporally complete description of ET by considering a broad volume within a novel three-dimensional ordered porous medium, reconstructing flow at a micrometre characteristic length scale (Reynolds numbers≪1) via time-resolved microtomographic particle image velocimetry. Beyond a critical Weissenberg number of 2, we observe an elastic flow instability accompanied by an enhanced pressure drop with spectral characteristics typical of ET. Polymer chains in the ET flow state are advected along increasingly curved streamlines between pores such that they accumulate strain and generate a local flow instability evaluated per an established instability criterion based on local evaluation of elastic tensile stress and streamline curvature. The onset of ET leads to increased pore-scale resistance and positive feedback on upstream streamline curvature. ET is thus characterized by a continuous evolution between states of laminar and unstable flow: pores with unstable flow flood their adjacent peers and thus encourage straightened streamlines and flow stability across the array, while positive feedback from flow resistance on streamline curvature results in the instability propagating upstream along the array. By employing a geometrically ordered medium, we permit flow state communication between pores, yielding generalized insights highlighting the significance of spatial correlation and flow history, and thus provide new avenues for explaining the mechanisms of ET

Extensional rheometry of mobile fluids. Part I: OUBER, an optimized uniaxial and biaxial extensional rheometer

We present a numerical optimization of a "6-arm cross-slot" device, yielding several three-dimensional shapes of fluidic channels designed to impose close approximations to ideal uniaxial (or biaxial) stagnation point extensional flow under the constraints of having four inlets and two outlets (or two inlets and four outlets) and Newtonian creeping flow conditions. Of the various numerically-generated geometries, one is selected as being most suitable for fabrication at the microscale, and numerical simulations with the Oldroyd-B and Phan-Thien and Tanner models confirm that the optimal flow fields in the chosen geometry are observed for both constant viscosity and shear thinning viscoelastic fluids. Fabrication of the geometry, which we name the optimized uniaxial and biaxial extensional rheometer (OUBER), is achieved with high precision at the microscale by selective laser-induced etching of a fused-silica substrate. Employing a viscous Newtonian fluid with a refractive index matched to that of the optically transparent microfluidic device, we conduct microtomographic-particle image velocimetry in order to resolve the flow field at low Reynolds number (< 0.1) in a substantial volume around the stagnation point. The flow velocimetry confirms the accurate imposition of the desired and predicted flows, with pure extensional flow at an essentially uniform deformation rate being applied over a wide region around the stagnation point. In Part II of this paper [Haward et al., J. Rheol. submitted (2023)], pressure drop measurements in the OUBER geometry will be used to assess the uniaxial and biaxial extensional rheometry of dilute polymeric solutions, in comparison to measurements made in planar extension using an optimized-shape cross-slot extensional rheometer (OSCER, Haward et al, Phys. Rev. Lett., 2012)

Flow of wormlike micellar solutions around microfluidic cylinders with high aspect ratio and low blockage ratio

We employ time-resolved flow velocimetry and birefringence imaging methods to study the flow of a well-characterized shear-banding wormlike micellar solution around a novel glass-fabricated microfluidic circular cylinder. In contrast with typical microfluidic cylinders, our geometry is characterized by a high aspect ratio alpha = H/W = 5 and a low blockage ratio beta = 2r/W = 0.1, where H and W are the channel height and width, and the cylinder radius r = 20 mum. The small cylinder radius allows access up to very high Weissenberg numbers 1.9 </= Wi = lambdaMU/r </= 3750 (where lambdaM is the Maxwell relaxation time) while inertial effects remain entirely negligible (Reynolds number, Re < 10-4). At low Wi values, the flow remains steady and symmetric and a birefringent region (indicating micellar alignment and tensile stress) develops downstream of the cylinder. Above a critical value Wic approximately 60 the flow transitions to a steady asymmetric state, characterized as a supercritical pitchfork bifurcation, in which the fluid takes a preferential path around one side of the cylinder. At a second critical value Wic2 approximately 130, the flow becomes time-dependent, with a characteristic frequency f0 approximately 1/lambdaM. This initial transition to time dependence has characteristics of a subcritical Hopf bifurcation. Power spectra of the measured fluctuations become complex as Wi is increased further, showing a gradual slowing down of the dynamics and emergence of harmonics. A final transition at very high Wic3 corresponds to the re-emergence of a single peak in the power spectrum but at much higher frequency. We discuss this in terms of possible flow-induced breakage of micelles into shorter species with a faster relaxation time