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Abstract Induced‐charge electroosmosis (ICEO) offers a practical approach to drive microscale flows by application of AC electric fields across polarizable surfaces, enabling diverse functions including microfluidic pumping, active cargo transport, and biosensing. While ICEO along pristine surfaces is well‐understood, practical applications of ICEO often require surface modifications that affect ICEO flows in a manner that is poorly understood. Here, this study introduces dielectrophoretic (DEP) polarizability measurement, DPM, as a method to study effects of surface modifications on surface polarizability and ICEO flows. The method entails DEP trapping of probe particles and analysis of their equilibrium motions to measure polarizability. This DPM‐generated polarizability data is then used to predict effects of surface modifications on ICEO flows and reveal the contribution of additional factors affecting ICEO. It compares predictions with experimentally observed changes to the speed of Janus particles traveling by ICEO‐driven induced‐charge electrophoresis. This study shows that DPM enables prediction of decreased particle speed upon protein capture by functional Janus particles and reveals that increased speed of polymer‐modified Janus particles likely arises from hydrodynamic factors. Overall, this work lays the foundation for investigating new ICEO‐driven systems with applications in complex environments, potentially including those encountered in biosensing, remediation, or cargo delivery. 

Abstract Conventional dogma suggests that decompression sickness (DCS) is caused by nitrogen bubble nucleation in the blood vessels and/or tissues; however, the abundance of bubbles does not correlate with DCS severity. Since immune cells respond to chemical and environmental cues, we hypothesized that the elevated partial pressures of dissolved gases drive aberrant immune cell phenotypes in the alveolar vasculature. To test this hypothesis, we measured immune responses within human lung‐on‐a‐chip devices established with primary alveolar cells and microvascular cells. Devices were pressurized to 1.0 or 3.5 atm and surrounded by normal alveolar air or oxygen‐reduced air. Phenotyping of neutrophils, monocytes, and dendritic cells as well as multiplexed ELISA revealed that immune responses occur within 1 h and that normal alveolar air (i.e., hyperbaric oxygen and nitrogen) confer greater immune activation. This work strongly suggests innate immune cell reactions initiated at elevated partial pressures contribute to the etiology of DCS. 

Conventional strategies for biological sensing require complex workflows to capture, isolate, and prepare biomolecules for specific and high-sensitivity detection. Instruments that facilitate these processes are usually cumbersome, reducing user-friendliness and accessibility. Here, we present a multichannel acoustic separator with biospecific and acoustically responsive microparticles to simplify workflows and shorten the time needed to isolate and detect biomarkers from complex fluids. The multichannel acoustic separator is 3D-printed and supports 12 acoustofluidic trapping channels that isolate the biospecific particles from off-target contaminants in the fluid. Fluid flow through the channels is driven by a semi-continuous siphon, which eliminates the need for fluid pumps. We tested the system for purifying three disparate biomolecules in individual and multiplexed formats, as well as the purification of IgA from whole blood for detection in approximately 70 min. 

Most biosensing techniques require complex processing steps that generate prolonged workflows and introduce potential points of error. Here, we report an acoustic pipette to purify and label biomarkers in 70 minutes. A key aspect of this technology is the use of functional negative acoustic contrast particles (fNACPs), which display biorecognition motifs for the specific capture of biomarkers from whole blood. Because of their large size and compressibility, the fNACPs robustly trap along the pressure antinodes of a standing wave and separate from blood components in under 60 seconds with >99% efficiency. fNACPs are subsequently fluorescently labeled in the pipette and are analyzed by both a custom, portable fluorimeter and flow cytometer. We demonstrate the detection of anti-ovalbumin antibodies from blood at picomolar levels (35 to 60 pM) with integrated controls showing minimal nonspecific adsorption. Overall, this system offers a simple and versatile approach for the rapid, sensitive, and specific capture of biomolecules. 

Swimming at the microscale typically involves two modes of motion: mechanical propulsion and propulsion due to field interactions. During mechanical propulsion, particles swim by reconfiguring their geometry. When propelled by field interactions, body forces such as phoretic interactions drive mobility. In this work, we employ slender-body theory to explore how a bent rod actuator propels due to a mechanical mode of swimming via hinge articulations and due to a chemical mode of swimming via diffusiophoretic interactions with a solute field. Although previous theoretical studies have examined mechanical and chemical modes of swimming in isolation, the simultaneous investigation of both modes has remained unexplored. For the mechanical mode of swimming, our calculations, both numerical and analytical, recover Purcell’s scallop theorem and show that the bent rod actuator experiences zero net displacement during reciprocal motion. Additionally, we calculate the trajectories traced by a bent rod actuator under a non-reciprocal hinge articulation, revealing that these trajectories are influenced by the amplitude of the hinge articulation, geometric asymmetry, and the angular velocity distribution between the two arms of the bent rod actuator. We provide intuitive explanations for these effects using free-body diagrams. Furthermore, we explore the motion induced by simultaneous hinge articulations and self-diffusiophoresis. We observe that hinge articulations can modify the effective phoretic forces and torques acting on the bent rod actuator, either supporting or impeding propulsion. Additionally, during self-diffusiophoretic propulsion, reciprocal hinge articulations no longer result in zero net displacement. In summary, our findings chart a new direction for designing micron-sized objects that harness both mechanical and chemical modes of propulsion synchronously, offering a mechanism to enact control over trajectories. 

Active particles, or micromotors, locally dissipate energy to drive locomotion at small length scales. The type of trajectory is generally fixed and dictated by the geometry and composition of the particle, which can be challenging to tune using conventional fabrication procedures. Here, we report a simple, bottom-up method to magnetically assemble gold-coated polystyrene Janus particles into “locked” clusters that display diverse trajectories when stimulated by AC electric fields. The orientation of particles within each cluster gives rise to distinct modes of locomotion, including translational, rotational, trochoidal, helical, and orbital. We model this system using a simplified rigid beads model and demonstrate qualitative agreement between the predicted and experimentally observed cluster trajectories. Overall, this system provides a facile means to scalably create micromotors with a range of well-defined motions from discrete building blocks. 

Microrobots are being explored for biomedical applications, such as drug delivery, biological cargo transport, and minimally invasive surgery. However, current efforts largely focus on proof-of-concept studies with nontranslatable materials through a "design-and-apply" approach, limiting the potential for clinical adaptation. While these proof-of-concept studies have been key to advancing microrobot technologies, we believe that the distinguishing capabilities of microrobots will be most readily brought to patient bedsides through a "design-by-problem" approach, which involves focusing on unsolved problems to inform the design of microrobots with practical capabilities. As outlined below, we propose that the clinical translation of microrobots will be accelerated by a judicious choice of target applications, improved delivery considerations, and the rational selection of translation-ready biomaterials, ultimately reducing patient burden and enhancing the efficacy of therapeutic drugs for difficult-to-treat diseases. 

Diffusiophoresis refers to the phenomenon where colloidal particles move in response to solute concentration gradients. Existing studies on diffusiophoresis, both experimental and theoretical, primarily focus on the movement of colloidal particles in response to one-dimensional solute gradients. In this work, we numerically investigate the impact of two-dimensional solute gradients on the distribution of colloidal particles, i.e. , colloidal banding, induced via diffusiophoresis. The solute gradients are generated by spatially arranged sources and sinks that emit/absorb a time-dependent solute molar rate. First we study a dipole system, i.e. , one source and one sink, and discover that interdipole diffusion and molar rate decay timescales dictate colloidal banding. At timescales shorter than the interdipole diffusion timescale, we observe a rapid enhancement in particle enrichment around the source due to repulsion from the sink. However, at timescales longer than the interdipole diffusion timescale, the source and sink screen each other, leading to a slower enhancement. If the solute molar rate decays at the timescale of interdipole diffusion, an optimal separation distance is obtained such that particle enrichment is maximized. We find that the partition coefficient of solute at the interface between the source and bulk strongly impacts the optimal separation distance. Surprisingly, the diffusivity ratio of solute in the source and bulk has a much weaker impact on the optimal dipole separation distance. We also examine an octupole configuration, i.e. , four sinks and four sources, arranged in a circle, and demonstrate that the geometric arrangement that maximizes enrichment depends on the radius of the circle. If the radius of the circle is small, it is preferred to have sources and sinks arranged in an alternating fashion. However, if the radius of the circle is large, a consecutive arrangement of sources and sinks is optimal. Our numerical framework introduces a novel method for spatially and temporally designing the banded structure of colloidal particles in two dimensions using diffusiophoresis and opens up new avenues in a field that has primarily focused on one-dimensional solute gradients.