Soft Microchannel Research Articles

Artificial Vascular with Pressure-Responsive Property based on Deformable Microfluidic Channels.

In vitro blood vessel models are significant for disease modeling, drug assays, and therapeutic development. Microfluidic technologies allow to create physiologically relevant culture models reproducing the features of the in vivo vascular microenvironment. However, current microfluidic technologies are limited by impractical rectangular cross-sections and single or nonsynchronous compound mechanical stimuli. This study proposes a new strategy for creating round-shaped deformable soft microfluidic channels to serve as artificial in vitro vasculature for developing in vitro models with vascular physio-mechanical microenvironments. Endothelial cells seeded into vascular models are used to assess the effects of a remodeled in vivo mechanical environment. Furthermore, a 3D stenosis model is constructed to recapitulate the flow disturbances in atherosclerosis. Soft microchannels can also be integrated into traditional microfluidics to realize multifunctional composite systems. This technology provides new insights into applying microfluidic chips and a prospective approach for constructing in vitro blood vessel models.

Swelling-induced patterning in soft microchannels.

We study the effect of inflation on the swelling-induced wrinkling of thin elastic membranes in a set-up that is commonly used to create microchannels in lab-on-chip applications. Using a combination of experiments and associated numerical simulations, we demonstrate that the out-of-plane deformation of the inflated membrane and the resulting anisotropic stress lead to two distinct instabilities as the swelling progresses. The membrane first develops small-amplitude wrinkles that retain the cross-channel symmetry. Their wavelength depends on the pressure and is set in a process similar to the axisymmetric buckling of pressurised, uni-axially compressed cylindrical shells. As swelling increases, the membrane undergoes a secondary instability during which the wrinkles coarsen into large-amplitude folds whose morphology can be controlled by the degree of pre-inflation. We elucidate the fundamental mechanisms responsible for this behaviour and explain how inflation can be used as a control mechanism in the manufacture of microchannels.

Smartphone generated electrical fields induce axon regrowth within microchannels following injury

Axonal injury is a major pathology of central nervous system injury associated with rapid rupture of axonal projections and resulting in the disconnection from their targets and impaired function. While electrical stimulation can promote axon sprouting in in vitro models, the study of electrical stimulation for axon regrowth following a stretch-induced axonal injury has not been reported. In this work, a simple electrostimulation-on-a-chip was developed to investigate the effect of electrical fields on morphological changes of axons ruptured. A pulse generator application in a smartphone was used to generate the electrical stimulation in microchannels via sterile needles. The results indicated that electrical stimulation promoted axon outgrowth 72% of the injured axons within microchannels. The length of regenerated axons showed a growth rate of 46 µm/day. In comparison, unstimulated controls had 100% axonal degeneration within 3 days and no axon regrowth was observed within 9 days after injury. The modified methods were developed to demonstrate axon regrowth after stretch-induced injury using electrical stimulation in a microchannel culture system. Results encourage an engineering concept of combining soft microchannel scaffold with stimulating electrodes towards developing potential medical devices for traumatic brain and spinal cord injury.

Microstructured biphasic hydrogels for highly sensitive and asymmetric sensors with temperature‐dependent sensitivity

AbstractMicrofabrication of advanced architectures in flexible polymers has been a prevalent strategy to achieve high‐performance stretchable sensors. However, such microfabrication is usually challenging, requiring delicate synthesis with expensive instruments. This work utilizes the microphase separation in biphasic hydrogels (BHs) to conveniently achieve microstructured high‐performance sensors. Different microphase separated structures were observed in BH sensors depending on the oil/water ratios and temperature, resulting in many unexpected features. The BH sensors show excellent extensibility (1000%) and phenomenal mechanical durability (>1000 compressive cycles). The microphase separated non‐conductive phase (oil phase) and conductive phase (water phase) lead to structured conductive microchannels of water phase, providing the high resistive sensitivity (gauge factor > 50), superior than many reported flexible sensors. Interestingly, the sensory behaviors of BHs are drastically varied around body temperature due to the melting point of oil phases (eicosane, 36.8°C), making BH sensors highly suitable to monitor human bodily motions. The BH sensors are able to discern a series of subtle writings via resistive change attributed by the asymmetric deformation in soft microchannels from microphase separation. In this work, the BH sensors are successfully utilized to discern bodily motions, voice and writings around body temperature.

Mixing in a rotating soft microchannel under electrical double layer effect: A variational calculus approach

In this work, we investigate the mixing dynamics of fluid streams in the rotating narrow-fluidic channel having grafted polyelectrolyte layer on its inner wall surfaces. We invoke the variational calculus approach for solving the coupled nonlinear system of transport equations, which is integrated with the non-homogeneous boundary conditions pertinent to this analysis. We obtain the velocity distribution in the asymptotic limit of geostrophic plug flow and then demonstrate the mixing dynamics from the perspective of qualitative assessment as well as quantitative evaluation. Performing the Poincaré map analysis, we predict the mixing of fluid streams from the qualitative assessment, while for the quantification of underlying mixing, we focus on the entropy of mixing analysis. We show that the grafted polyelectrolyte layer at the channel walls modulates the electrical double layer phenomenon following the involved electrostatics. This phenomenon in the presence of an external electric field strengthens the electroosmotic pumping in the fluidic channel non-trivially. Results show that the effects stemming from a larger thickness of the grafted polyelectrolyte layer, that is, the stronger electroosmotic pumping together with a relatively larger magnitude of friction drag, modulate the rotational force-driven primary as well as the secondary flows in the channel. The correlative–cooperative effects of the grafted polyelectrolyte layer on the rotational electrohydrodynamics lead to the formation of the dumbbell-shaped vortex and results in an enhancement in the underlying mixing.

Polyelectrolyte layer grafting effect on the rotational electroosmotic flow of viscoplastic material

Due to growing importance of study of bio-fluids in rotational microfluidic platforms, discussion of flow of viscoplastic materials in such situations becomes very important. Bio-fluids also generally flow in soft channel microfluidic environment and therefore studying the effect of softness of channels on flow of such viscoplastic bio-fluids becomes critical. We, in the present study, investigate the effect of the rotation of the channel and yield stress of the fluid on the flow of viscoplastic material in a soft microchannel for three constitutive models viz. Bingham, Casson and Herschel–Bulkley models. It is found that the soft grafted layer, the channel rotation and the fluid yield stress play vital roles in determining the flow behaviour. We find that that larger rotational speeds tend to destabilize the flow. We also find the critical rotational speed for which secondary flow magnitude is maximum. We also find that this non-linear interaction between rotation of channel and fluid yield stress leads to the generation of multiple yield planes within the flow domain. Our study will help in obtaining good designs of rotational microfluidic platforms for studying bio-fluids with yield stress.

Dynamics of viscoelastic fluid in a rotating soft microchannel

In this study, we numerically investigate the effects of rotational forces, viz., centrifugal force and Coriolis force, on the flow dynamics of a viscoelastic fluid in a polymeric layer grafted microchannel. The viscoelastic fluid is represented by the Oldroyd-B model, and the effect of viscoelasticity on the underlying transport is studied. A numerical procedure consistent with the finite difference method is used to solve the system of partial differential equations. The numerical model takes into consideration, among many others, the drag effects of the “soft layer” and the transiences in the flow dynamics leading to the steady state. The complex interplay between the effect of rotational forcing and the presence of the soft layer is observed to lead to vital conclusions that could improve the design of many lab-on-a-compact disc based microfluidic devices. In addition, the effect of elasticity on the flow dynamics in the presence of rotational forces and soft layer induced drag force is studied. The in-house numerical code employs the finite difference numerical scheme to discretize the equations and consequently solves the obtained system of linear algebraic equations using the Gauss–Seidel iterative scheme. By demonstrating the velocity profiles, we discuss the effect of the various rheological parameters on the underlying transport feature. Finally, the effect of the rotation on the net throughput is studied extensively.

Hydrodynamic dispersion by electroosmotic flow in soft microchannels: Consideration of different properties for electrolyte and polyelectrolyte layer

We theoretically study the hydrodynamic dispersion by fully developed electroosmotic flow in soft slit microchannels of dense polyelectrolyte layer (PEL). When the PEL is dense, the smallness of its permittivity with respect to the fluid leads to the rearrangement of ions, a phenomenon known as ion partitioning. Moreover, the fluid viscosity gets much higher within the PEL while the opposite happens to molecular diffusivity. Adopting the Debye-Hückel linearization, we obtain analytical solutions for electric potential and velocity distributions by considering different values of permittivity and viscosity inside and outside the PEL. The velocity profile is then used to derive expressions for long-term analyte concentration and dispersion coefficient. Full numerical simulations are also performed to track the broadening of an analyte band from the time of injection. The results indicate that ion partitioning gives rise to the amplification of fluid velocity and, hence, the hydrodynamic dispersion, whereas the opposite is true for the increase in the dynamic viscosity within the PEL. Moreover, even though the reduction of the molecular diffusivity inside the PEL leads to the increase of the dispersion coefficient, its influence is much weaker than those of the ion partitioning and the viscosity variations.

Reversible Mechanical Deformations of Soft Microchannel Networks for Sensing in Soft Robotic Systems

Microfluidics has enabled numerous applications in, for example, analytical chemistry, medical diagnostics, microelectronics, and soft robotics. In most of these applications, the geometries of the microchannels are of fixed dimensions that (ideally) remain invariant during operation. In soft robotics, however, the geometries of the microchannels contained in soft actuation systems are inherently dynamic, and the specific dimensions are expected to change during operation, and, by extension, the fluid transport properties of the system are variable. If this characteristic is not properly considered, or if methods are not developed to control it, the progress of soft robotic devices with distributed fluid transport systems can be hampered. Herein, the deformation of soft microchannel networks is investigated using a finite‐element method and experimental observations, and the understandings are applied to imbibe sensing capabilities in soft robots that have integrated microfluidic networks with dynamic channel geometries of predictable dimensions. This approach enables the fabrication of soft fluid transport systems with deterministic deformation characteristics—a capability that is specifically applied to touch and actuation sensing in soft actuators. This work provides insight into the channel deformation processes expected in soft robotic systems with embedded networks of microchannels, enabling devices with reliable/predictable transport properties.

Electroosmotic flow in soft microchannels at high grafting densities

This paper reports the results of a theoretical modeling of the fully developed electroosmotic flow in a rectangular microchannel with a high-density polyelectrolyte layer (PEL) attached to the walls. At these conditions, the ions are partitioned between the PEL and the fluid outside the PEL owing to the difference between the permittivities of the two media. It is taken into account that the dynamic viscosity is higher within the PEL because of hydration effects. Solutions are obtained for the electric potential and velocity distributions as well as the mean velocity by making use of a variational approach, applied to the linearized form of the governing equations, which treats the whole area under consideration as a single domain with variable physical properties. The resulting equations are solved using a spectral method. Closed-form analytical expressions are obtained for a slit geometry, representing the case of high aspect ratios. The solutions obtained are validated by comparing to finite-element simulations of the full nonlinear equations. It is shown that the electric potential drop inside the channel increases due to the depletion of the counterions within the PEL caused by the ion partitioning effect. This effect, surprisingly, magnifies the electroosmotic flow rate because of the increase of the space charge outside the PEL. As expected, the hydration effects reduce the flow rate, especially for thick PELs.

Rotating electroosmotic flows in soft parallel plate microchannels

We present a theoretical investigation of rotating electroosmotic flows (EOFs) in soft parallel plate microchannels. The soft microchannel, also called as the polyelectrolyte-grafted microchannel, is denoted as a rigid microchannel coated with a polyelectrolyte layer (PEL) on its surface. We compare the velocity in a soft microchannel with that in a rigid one for different rotating frequencies and find that the PEL has a trend to lower the velocities in both directions for a larger equivalent electrical double layer (EDL) thickness λFCL (λFCL = 0.3) and a smaller rotating frequency ω (ω < 5). However, for a larger rotating frequency ω (ω < 5), the main stream velocity u far away from the channel walls in a soft microchannel exceeds that in a rigid one. Inspired by the above results, we can control the EOF velocity in micro rotating systems by imparting PELs on the microchannel walls, which may be an interesting application in biomedical separation and chemical reaction.

Surface Tension Driven Filling in a Soft Microchannel: Role of Streaming Potential

The filling characteristics of a Newtonian electrolyte through a microchannel with grafted polyelectrolyte layer on its inner walls have been investigated. A reduced order model has been deployed f...

Stick–slip motion and controlled filling speed by the geometric design of soft micro-channels

HypothesisLiquid can move by capillary action through interconnected porous materials, as in fabric or paper towels. Today mass transport is controlled by chemical modification. It is, however, possible to direct mass transport by geometrical modifications. It is here proposed that it is possible to tailor capillary flow speed in a model system of micro-channels by the angle, size and position of attached side channels. ExperimentsA flexible, rapid, and cost-effective method is used to produce micro-channels in gels. It involves 3D-printed moulds in which gels are cast. Open channels of micrometre size with several side channels on either one or two sides are produced with tilting angles of 10–170°. On a horizontal plane the meniscus of water driven by surface tension is tracked in the main channel. FindingsThe presence of side channels on one side slowed down the speed of the meniscus in the main channel least. Channels having side channels on both sides with tilting angles of up to 30° indicated tremendously slower flow, and the liquid exhibited a stick-slip motion. Broader side channels decreased the speed more than thinner ones, as suggested by the hypothesis. Inertial forces are suggested to be important in branched channel systems studied here.

Theoretical modeling of electroosmotic flow in soft microchannels: A variational approach applied to the rectangular geometry

Modeling of fluid flow in polyelectrolyte layer (PEL)-grafted microchannels is challenging due to their two-layer nature. Hence, the pertinent studies are limited only to circular and slit geometries for which matching the solutions for inside and outside the PEL is simple. In this paper, a simple variational-based approach is presented for the modeling of fully developed electroosmotic flow in PEL-grafted microchannels by which the whole fluidic area is considered as a single porous medium of variable properties. The model is capable of being applied to microchannels of a complex cross-sectional area. As an application of the method, it is applied to a rectangular microchannel of uniform PEL properties. It is shown that modeling a rectangular channel as a slit may lead to considerable overestimation of the mean velocity especially when both the PEL and electric double layer (EDL) are thick. It is also demonstrated that the mean velocity is an increasing function of the fixed charge density and PEL thickness and a decreasing function of the EDL thickness and PEL friction coefficient. The influence of the PEL thickness on the mean velocity, however, vanishes when both the PEL thickness and friction coefficient are sufficiently high.

Directed Self-Assembly of Topological Defects of Liquid Crystals.

One of the alluring aspects of liquid crystals (LCs) is their readily controllable self-assembly behavior, leading to comprehension of complex topological structures and practical patterning applications. Here, we report on manipulating various kinds of topological defects by adopting an imprinted polymer-based soft microchannel that simultaneously imposes adjustable surface anchoring, confinement, and uniaxial alignment. Distinctive molecular orientation could be achieved by varying the surface anchoring conditions at the sidewall polymer and the rubbing directions on the bottom layer. On this pioneering platform, a common LC material, 8CB (4'-n-octyl-4-cyano-biphenyl), was placed where various topological defect domains were generated in a periodic arrangement. The experimental results showed that our platform can change the packing behavior and even the shape of topological defects by varying the rubbing condition. We believe that this facile tool to modulate surface boundary conditions combined with topographic confinement can open a way to use LC materials in potential optical and patterning applications.

Solute dispersion by electroosmotic flow through soft microchannels

Abstract We study the hydrodynamic dispersion (HD) by electroosmotic flow in soft microchannels. Considering a fully developed flow in a slit microchannel of low surface potential and adopting the Taylor dispersion theory, we derive analytical solutions for the solute concentration field and the effective dispersion coefficient. We also conduct numerical analyses to broaden the paper’s scope to high surface potentials and to specify a criterion for the validity of the Debye-Huckel linearization in soft microconduits as well as to investigate the broadening of an analyte band from the time of injection. It is demonstrated that the effective dispersion coefficient of a neutral solute band is generally larger for thicker polyelectrolyte layers (PEL). This means that the hydrodynamic dispersion in electroosmotic flow usually increases by grafting a PEL to the surface. It is, however, possible to reduce the HD dispersion by appropriately altering the channel surface. Furthermore, it is found that higher PEL frictions lead to smaller HD. In addition, unlike rigid channels, a smaller HD is not necessarily achieved for a PEL-grafted microchannel by decreasing the surface potential. Finally, anomalies observed in the electrical potential and velocity distributions of soft microchannels are shown to be caused due to the creation of a triple-EDL within and outside the PEL.

A soft microchannel decreases polydispersity of droplet generation.

We study the effect of softness of the microchannel on the process of droplet generation in two-phase flows in a T-junction microchannel. One side of the microchannel has a flexible thin PDMS layer, which vibrates naturally while droplets are generated; the deformation frequency coincides with the frequency of droplet formation. Furthermore, we compare the polydispersity of water-in-oil droplets formed with a microchannel with one soft wall with those formed in a conventional rigid microchannel. We show that deformation of the soft wall reduces the polydispersity in the droplet size.