Transpiration-induced self-growth of texture hydrogel surfaces.

Trees grow by coupling the transpiration-induced nutrient absorption from external sources and photosynthesis-based nutrient integration. Inspired by this manner, we designed a class of polyion complex (PIC) hydrogels containing isolated liquid-filled voids for growing texture surfaces. The isolated liquid-filled voids were created via irreversible matrix reconfiguration in a deswelling-swelling process. During transpiration, these voids reversibly collapse to generate negative pressures within the matrices to extract polymerizable compounds from external sources and deliver them to the surface of the samples for photopolymerization. This growth process is spatial-controllable and can be applied to fabricate complex patterns consisting of different compositions, suggesting a new strategy for making texture surfaces.

Anisotropic Diffusion of Elongated Particles in Active Coherent Flows.

The study of particle diffusion, a classical conundrum in scientific inquiry, holds manifold implications for various real-world applications. Particularly within the domain of active flows, where the motion of self-propelled particles instigates fluid movement, extensive research has been dedicated to unraveling the dynamics of passive spherical particles. This scrutiny has unearthed intriguing phenomena, such as superdiffusion at brief temporal scales and conventional diffusion at longer intervals. In contrast to the spherical counterparts, anisotropic particles, which manifest directional variations, are prevalent in nature. Although anisotropic behavior in passive fluids has been subject to exploration, enigmatic aspects persist in comprehending the interplay of anisotropic particles within active flows. This research delves into the intricacies of anisotropic passive particle diffusion, exposing a notable escalation in translational and rotational diffusion coefficients, as well as the superdiffusion index, contingent upon bacterial concentration. Through a detailed examination of particle coordinates, the directional preference of particle diffusion is not solely dependent on the particle length, but rather determined by the ratio of the particle length to the associated length scale of the background flow field. These revelations accentuate the paramount importance of unraveling the nuances of anisotropic particle diffusion within the context of active flows. Such insights not only contribute to the fundamental understanding of particle dynamics, but also have potential implications for a spectrum of applications.

An optically fabricated gradient nanochannel array to access the translocation dynamics of T4-phage DNA through nanoconfinement.

It has been widely recognized that nanostructures in natural biological materials play important roles in regulating life machinery. Even though nanofabrication techniques such as two-photon polymerization (TPP) provide sub-100 nm fabrication resolution, it remains technologically challenging to produce 3D nanoscale features modeling the complexity in vivo. We herein demonstrate that a nanochannel array carrying different sizes and nanostructures with gradually transitioning dimensions can be easily produced on a slightly tilted nano-stage. Using the gradient nanochannel array, we systematically investigate the factors affecting the dynamics of DNA translocation through nanoconfinement, including the size of biomolecules and geometrical features of the physical environment, which resembles the selectivity of nanopores in the cell membrane. It is observed that T4-phage DNA shows distinctive conformational transition dynamics during translocation through nanochannels driven by electric field or flow, and the deformation energy required for DNA to enter the nanochannels depends on both chemical environmental conditions, i.e., the ionic strength regulating DNA persistence length and nanochannel dimension. In the electric field, DNA repeatedly gets stretched and compressed during its migration through the nanochannel, reflected by elevated fluctuation in extension, which is substantially greater than the thermal fluctuation. However, driven by flow, DNA remains stretched during translocation through nanochannels, and shows variances in extension of merely a few hundred nanometers. These results indicate that the optically fabricated gradient nanochannel array is a suitable platform for optimizing the experimental conditions for biomedical applications such as gene mapping, and verify that production of complex three dimensional (3D) nanostructures can be greatly simplified by including slight inclination during TPP fabrication.

Direct Observation of Nanotracer Transport in Swarming Bacteria during Antibiotic Adaptation.

Microorganisms inevitably encounter environmental variations and thus need to develop necessary strategies to adjust the colonies for survival. Here, we use cooperating Serratia marcescens bacteria to reveal how the whole population responds to a gradually deteriorating habitat. When subjected to antibiotics with increasing doses, the swarming bacteria transform weak homogeneous turbulent flows to nematic jet flows with defects and vortices on a large scale, by which bacteria exploit these coherent flows to transfer material and/or information. We elucidate a complete view of detailed spatiotemporal transport behavior in such microscale active turbulence with single-nanoparticle tracking. The nanoparticles in these active flows are brought into the state with the up limit of superdiffusion by the bacterial collective response to the stronger antibiotic stimulation. Strikingly, we found that, under the strengthening stimulation from antibiotics, bacteria with only a small fraction of their community get elongated and facilitate the drastic turbulence transition and an enhanced superdiffusion. These findings imply a possible collective response mechanism against environmental variations.

A fluidic platform for mobility evaluation of zebrafish with gene deficiency.

Introduction: Zebrafish is a suitable animal model for molecular genetic tests and drug discovery due to its characteristics including optical transparency, genetic manipulability, genetic similarity to humans, and cost-effectiveness. Mobility of the zebrafish reflects pathological conditions leading to brain disorders, disrupted motor functions, and sensitivity to environmental challenges. However, it remains technologically challenging to quantitively assess zebrafish's mobility in a flowing environment and simultaneously monitor cellular behavior in vivo. Methods: We herein developed a facile fluidic device using mechanical vibration to controllably generate various flow patterns in a droplet housing single zebrafish, which mimics its dynamically flowing habitats. Results: We observe that in the four recirculating flow patterns, there are two equilibrium stagnation positions for zebrafish constrained in the droplet, i.e., the "source" with the outward flow and the "sink" with the inward flow. Wild-type zebrafish, whose mobility remains intact, tend to swim against the flow and fight to stay at the source point. A slight deviation from streamline leads to an increased torque pushing the zebrafish further away, whereas zebrafish with motor neuron dysfunction caused by lipin-1 deficiency are forced to stay in the "sink," where both their head and tail align with the flow direction. Deviation angle from the source point can, therefore, be used to quantify the mobility of zebrafish under flowing environmental conditions. Moreover, in a droplet of comparable size, single zebrafish can be effectively restrained for high-resolution imaging. Conclusion: Using the proposed methodology, zebrafish mobility reflecting pathological symptoms can be quantitively investigated and directly linked to cellular behavior in vivo.

Gradient Nanoconfinement Facilitates Binding of Transcriptional Factor NF-κB to Histone- and Protamine-DNA Complexes.

Mechanically induced chromosome reorganization plays important roles in transcriptional regulation. However, the interplay between chromosome reorganization and transcription activities is complicated, such that it is difficult to decipher the regulatory effects of intranuclear geometrical cues. Here, we simplify the system by introducing DNA, packaging proteins (i.e., histone and protamine), and transcription factor NF-κB into a well-defined fluidic chip with changing spatical confinement ranging from 100 to 500 nm. It is uncovered that strong nanoconfinement suppresses higher-order folding of histone- and protamine-DNA complexes, the fracture of which exposes buried DNA segments and causes increased quantities of NF-κB binding to the DNA chain. Overall, these results reveal a pathway of how intranuclear geometrical cues alter the open/closed state of a DNA-protein complex and therefore affect transcription activities: i.e., NF-κB binding.

Glycerol Droplet Spreading on Growing Bacillus Subtilis Biofilms.

Bacterial biofilm is a three-dimensional matrix composed of a large number of living bacterial individuals. The strong bio-interaction between the bacteria and its self-secreted matrix environment strengthens the mechanical integrity of the biofilm and the sustainable resistance of bacteria to antibiotics. As a soft surface, the biofilm is expected to present different dynamical wetting behavior in response to shear stress, which is, however, less known. Here, the spreading of liquid droplet on Bacillus subtilis biofilm at its different growing phases was experimentally investigated. Due to the viscoelastic response of the biofilm to fast spreading of the droplet, three stages were identified as inertial, viscous stages, and a longer transition in between. The physical heterogeneity of growing biofilm correlates with the spreading scaling within the inertial stage, followed by the possible chemical variation after a critical growing time. By using the duration of inertial spreading, the characteristic time scale was successfully linked to the shear modulus of the elastic dissipation of the biofilm. This measurement suggests a facile, non-destructive and in vivo method to understand the mechanical instability of this living matter.

Swimming of Buoyant Bacteria in Quiescent Medium and Shear Flows.

Gravity has an unavoidable effect on all living organisms inhabiting fluidic surroundings. To investigate the spatial distribution of bacteria in quiescent fluids and their rheotactic behavior in shear flows under buoyancy, we adjust the buoyant force to regulate bacterial swimming in a microfluidic channel. It is found that swimming bacteria of Escherichia coli exhibit an obvious vertical separation when exposed to a medium with high density and gradually gather close to the up wall within minutes. The bacterial population presents a net upward number flux, which enhances the trapping of motile bacteria onto the up surface as a result of buoyancy force apart from the hydrodynamic and kinematic interactions in quiescent fluids. When flow is imposed into the channel, the buoyancy effect is however significantly suppressed. Additionally, the drift velocity perpendicular to the buoyancy vector as a result of chirality-induced transverse swimming decreases with buoyancy force. However, this transverse drift capability is recovered after excluding the intrinsic swimming motility in a quiescent medium. Failing to escape from the trapping as a result of buoyant force allows for a facile separation of bacteria along the vertical direction. The findings also offer a controllable way to redisperse and homogenize the bacteria distribution close to walls by imposing a weak shear flow.

A Stand-Alone Microfluidic Chip for Long-Term Cell Culture.

Live-cell microscopy is crucial for biomedical studies and clinical tests. The technique is, however, limited to few laboratories due to its high cost and bulky size of the necessary culture equipment. In this study, we propose a portable microfluidic-cell-culture system, which is merely 15 cm×11 cm×9 cm in dimension, powered by a conventional alkali battery and costs less than USD 20. For long-term cell culture, a fresh culture medium exposed to 5% CO2 is programmed to be delivered to the culture chamber at defined time intervals. The 37 °C culture temperature is maintained by timely electrifying the ITO glass slide underneath the culture chamber. Our results demonstrate that 3T3 fibroblasts, HepG2 cells, MB-231 cells and tumor spheroids can be well-maintained for more than 48 h on top of the microscope stage and show physical characters (e.g., morphology and mobility) and growth rate on par with the commercial stage-top incubator and the widely adopted CO2 incubator. The proposed portable cell culture device is, therefore, suitable for simple live-cell studies in the lab and cell experiments in the field when samples cannot be shipped.

Escaping Behavior of Sperms on the Biomimetic Oviductal Surface.

Before fertilization, sperms adhere to oviductal epithelium cells, and only a restrictive number of winner sperms can escape to reach the egg. To study the sperm escape behavior from the oviductal surface, we developed a microfluidic chip to fabricate an adhesive surface and to create a gradient of progesterone (P4) for mimicking the oviduct microenvironment in vivo. We identified three sperm motion patterns in such a microenvironment─anchored spin, run-and-spin, and escaped mode. By using kinetic analysis, we verified the hypothesis that the responsive rotation energy anchored with the adhered sperm head determines whether the sperm is trapped or detaching, which is defined as the hammer flying strategy of successful escape after accumulating energy in the process of rotating. Intriguingly, this hammer-throw escaping is able to be triggered by the P4 biochemical stimulation. Our results revealed the tangled process of sperm escape before fertilization in the ingenious microfluidic system.

Onset of Nonlinear Electroosmotic Flow under an AC Electric Field.

Nonlinearity of electroosmotic flows (EOFs) is ubiquitous and plays a crucial role in ion transport, specimen mixing, electrochemistry reaction, and electric energy storage and utilization. When and how the transition from a linear regime to a nonlinear one occurs is essential for understanding, prohibiting, or utilizing nonlinear EOF. However, due to the lack of reliable experimental instruments with high spatial and temporal resolutions, the investigation of the onset of nonlinear EOF still remains in theory. Herein, we experimentally studied the velocity fluctuations of EOFs driven by an alternating current (AC) electric field via ultrasensitive fluorescent blinking tricks. The linear and nonlinear AC EOFs are successfully identified from both the time trace and energy spectra of velocity fluctuations. The transitional electric field (EA,C) is determined by both the convection velocity (U) and AC frequency (ff) as EA,C ∼ ff0.48-0.027U. We hope the current investigation could be essential in the development of both theory and applications of nonlinear EOFs.

Activity-Induced Enhancement of Superdiffusive Transport in Bacterial Turbulence.

Superdiffusion processes significantly promote the transport of tiny passive particles within biological fluids. Activity, one of the essential measures for living matter, however, is less examined in terms of how and to what extent it can improve the diffusivity of the moving particles. Here, bacterial suspensions are confined within the microfluidic channel at the state of bacterial turbulence, and are tuned to different activity levels by oxygen consumption in control. Systematic measurements are conducted to determine the superdiffusion exponent, which characterizes the diffusivity strength of tracer particles, depending on the continuously injecting energy converted to motile activity from swimming individuals. Higher activity is quantified to drastically enhance the superdiffusion process of passive tracers in the short-time regime. Moreover, the number density of the swimming bacteria is controlled to contribute to the field activity, and then to strengthen the super-diffusivity of tracers, distinguished by regimes with and without collective motion of interacting bacteria. Finally, the non-slip surfaces of the microfluidic channel lower the superdiffusion of immersed tracers due to the resistance, with the small diffusivity differing from the counterpart in the bulk. The findings here suggest ways of controlled diffusion and transport of substances within the living system with different levels of nutrition and resources and boundary walls, leading to efficient mixing, drug delivery and intracellular communications.

Cervix chip mimicking cervical microenvironment for quantifying sperm locomotion.

As the gate for sperm swimming into the female reproductive tract, cervix is full of cervical mucus, which plays an important role in sperm locomotion. The fact that sperm cannot pass through the cervical mucus-cervix microenvironment will cause the male infertility. However, how the sperm swim across the cervix microenvironment remains elusive. We used hyaluronic acid (HA), a substitute of cervical mucus to mimic cervix microenvironment and designed a cervix chip to study sperm selection and behavior. An accumulation of sperm in HA confirmed that HA served as a reservoir for sperm, similar to cervical mucus. We found that sperm escaping from HA exhibited higher motility than the sperm accessing into HA, suggesting that HA functions as a filter to select sperm with high activity. Our findings construct a practical platform to explore the sophisticated interaction of sperm with cervix microenvironment, with elaborate swimming indicators thus provide a promising cervix chip for sperm selection with kinematic features on-demand. What's more, the cervix chip allows the convenient use in clinical infertility diagnosis, owing to the advantage of simple, fast and high efficiency.

Drops Sliding on Non-SLIPS Structures.

Inspired by a plant leaf, a slippery liquid-infused porous surface (SLIPS) exhibits attractive nonwetting and self-cleaning abilities. However, rigorous requirements for the infused liquid layer and its inevitable loss limit its practical use. Here, we propose a model structure defined as a non-SLIPS by introducing solid nanostructures covered with a discontinuous lubricant film. This non-SLIPS tuned by solid wettability achieves the excellent self-cleaning feature with a small sliding angle comparable to the counterpart of a typical SLIPS. This sliding angle α* can be further reduced to a saturated plateau by a slight enhancement of hydrophobicity of the solid nanostructures. Interestingly, the sliding velocity remains almost constant for all of these non-SLIPS samples at a given tilt angle, independent of solid wettability. We formulate the slippery mechanism by defining an energy barrier responsible for the sliding initiation on the non-SLIPS. This energy barrier of the non-SLIPS is correlated, with a qualitative agreement, to the molecular adsorption on the solid nanostructures. The antibiological contamination is confirmed for this non-SLIPS, indicating its excellent self-cleaning ability. The findings suggest that the new surfaces, even with the gradual depletion of the infused oil layer, exhibit the nondegradation of the self-cleaning performance.

Dynamic intracellular mechanical cues facilitate collective signaling responses.

Collective behavior emerges in diverse life machineries, e.g., the immune responses to dynamic stimulations. The essential questions that arise here are that whether and how cells in vivo collectively respond to stimulation frequencies higher than their intrinsic natural values, e.g., the acute inflammation conditions. In this work, we systematically studied morphological and signaling responses of population fibroblasts in an interconnected cell monolayer and uncovered that, besides the natural NF-κB oscillation frequency of 1/90 min-1, collective signaling response emerges in the cell monolayer at 1/20 min-1 TNF-α input periodicity as well. Using a customized microfluidic device, we independently induced dynamic chemical stimulation and cytoskeleton reorganization on the stand-alone cells to exclude the effect of cell-cell communication. Our results reveal that, at this particular frequency, chemical stimulation is translated into dynamic intracellular mechanical cues through RAC1-medicated induction of dynamic cell-cell connections and cytoskeleton reorganizations, which synergize with chemical input to facilitate collective signaling responses.

Chirality-induced bacterial rheotaxis in bulk shear flows.

Interaction of swimming bacteria with flows controls their ability to explore complex environments, crucial to many societal and environmental challenges and relevant for microfluidic applications such as cell sorting. Combining experimental, numerical, and theoretical analysis, we present a comprehensive study of the transport of motile bacteria in shear flows. Experimentally, we obtain with high accuracy and, for a large range of flow rates, the spatially resolved velocity and orientation distributions. They are in excellent agreement with the simulations of a kinematic model accounting for stochastic and microhydrodynamic properties and, in particular, the flagella chirality. Theoretical analysis reveals the scaling laws behind the average rheotactic velocity at moderate shear rates using a chirality parameter and explains the reorientation dynamics leading to saturation at large shear rates from the marginal stability of a fixed point. Our findings constitute a full understanding of the physical mechanisms and relevant parameters of bacteria bulk rheotaxis.

Air-Bubble Induced Mixing: A Fluidic Mixer Chip.

In this study, we report the design and fabrication of a novel fluidic mixer. As proof-of-concept, the laminar flow in the main channel is firstly filled with small air-bubbles, which act as active stirrers inducing chaotic convective turbulent flow, and thus enhance the solutes mixing even at a low input flow rate. To further increase mixing efficiency, a design of neck constriction is included, which changes the relative positions of the inclusion bubbles significantly. The redistribution of liquid volume among bubbles then causes complex flow profile, which further enhances mixing. This work demonstrates a unique approach of utilizing air bubbles to facilitate mixing in bulk solution, which can find the potential applications in microfluidics, fast medical analysis, and biochemical synthesis.

A hillock-like phenomenon with low friction and adhesion on a graphene surface induced by relative sliding at the interface of graphene and the SiO2 substrate using an AFM tip.

Graphene demonstrates high potential as an atomically thin solid lubricant for sliding interfaces in industry. However, graphene as a coating material does not always exhibit strong adhesion to any substrates. When the adhesion of graphene to its substrate weakens, it remains unknown whether relative sliding at the interface exists and how the tribological properties of the graphene coating changes. In this work, we first designed a method to weaken the adhesion between graphene and its SiO2 substrate. Then the graphene with weakened adhesion to its substrate was rubbed using an AFM tip, where we found a novel phenomenon: the monolayer graphene not only no longer protected the SiO2 substrate from deformation and damage, but also prompted the formation of hillock-like structures with heights of approximately tens of nanometers. Moreover, the surface of the hillock-like structure exhibited very low adhesion and a continuously decreasing friction force versus sliding time. Comparing the hillock-like structure on the bare SiO2 surface and the proposed force model, we demonstrated that the emergence of the hillock-like structure (with very low adhesion and continuously decreasing friction) was ascribed to the relative sliding at the graphene/substrate interface caused by the mechanical shear of the AFM tip. Our findings reveal a potential failure of the graphene coating when the adhesion strength between graphene and its substrate is damaged or weakened and provide a possibility for in situ fabrication of a low friction and adhesion micro/nanostructure on a SiO2/graphene surface.

Macroscopic Self-Evolution of Dynamic Hydrogels to Create Hollow Interiors.

A solid-to-hollow evolution in macroscopic structures is challenging in synthetic materials. A fundamentally new strategy is reported for guiding macroscopic, unidirectional shape evolution of materials without compromising the material's integrity. This strategy is based on the creation of a field with a "swelling pole" and a "shrinking pole" to drive polymers to disassemble, migrate, and resettle in the targeted region. This concept is demonstrated using dynamic hydrogels containing anchored acrylic ligands and hydrophobic long alkyl chains. Adding water molecules and ferric ions (Fe3+ ) to induce a swelling-shrinking field transforms the hydrogels from solid to hollow. The strategy is versatile in the generation of various closed hollow objects (for example, spheres, helix tubes, and cubes with different diameters) for different applications.

Ultrasonication-Triggered Ubiquitous Assembly of Magnetic Janus Amphiphilic Nanoparticles in Cancer Theranostic Applications.

The ultrasonication-triggered interfacial assembly approach was developed to synthesize magnetic Janus amphiphilic nanoparticles (MJANPs) for cancer theranostic applications, where the biocompatible octadecylamine is used as a molecular linker to mediate the interactions between hydrophobic and hydrophilic nanoparticles across the oil-water interface. The obtained Co cluster-embedded Fe3O4 nanoparticles-graphene oxide (CCIO-GO) MJANPs exhibited superior magnetic heating efficiency and transverse relaxivity, 64 and 4 times higher than that of commercial superparamagnetic iron oxides, respectively. The methodology has been applicable to nanoparticles of various dimensions (5-100 nm), morphologies (sphere, ring, disk, and rod), and composition (metal oxides, noble metal and semiconductor compounds, etc.), thereby greatly enriching the array of MJANPs. In vivo theranostic applications using the tumor-bearing mice model further demonstrated the effectiveness of these MJANPs in high-resolution multimodality imaging and high-efficiency cancer therapeutics. The ubiquitous assembly approach developed in the current study pave the way for on-demand design of high-performance Janus amphiphilic nanoparticles for various clinical diagnoses and therapeutic applications.