Optoelectronic Trajectory Reconfiguration and Directed Self-Assembly of Self-Propelling Electrically Powered Active Particles.

Self-propelling active particles are an exciting and interdisciplinary emerging area of research with projected biomedical and environmental applications. Due to their autonomous motion, control over these active particles that are free to travel along individual trajectories, is challenging. This work uses optically patterned electrodes on a photoconductive substrate using a digital micromirror device (DMD) to dynamically control the region of movement of self-propelling particles (i.e., metallo-dielectric Janus particles (JPs)). This extends previous studies where only a passive micromotor is optoelectronically manipulated with a translocating optical pattern that illuminates the particle. In contrast, the current system uses the optically patterned electrode merely to define the region within which the JPs moved autonomously. Interestingly, the JPs avoid crossing the optical region's edge, which enables constraint of the area of motion and to dynamically shape the JP trajectory. Using the DMD system to simultaneously manipulate several JPs enables to self-assemble the JPs into stable active structures (JPs ring) with precise control over the number of participating JPs and passive particles. Since the optoelectronic system is amenable to closed-loop operation using real-time image analysis, it enables exploitation of these active particles as active microrobots that can be operated in a programmable and parallelized manner.

Switching from Chemical to Electrical Micromotor Propulsion across a Gradient of Gastric Fluid via Magnetic Rolling.

To address and extend the finite lifetime of Mg-based micromotors due to the depletion of the engine (Mg-core), we examine electric fields, along with previously studied magnetic fields, to create a triple-engine hybrid micromotor for driving these micromotors. Electric fields are a facile energy source that is not limited in its operation time and can dynamically tune the micromotor mobility by simply changing the frequency and amplitude of the field. Moreover, the same electrical fields can be used for cell trapping and transport as well as drug delivery. However, the limitations of these propulsion mechanisms are the low pH (and high conductivity) environment required for Mg dissolution, while the electrical propulsion is quenched at these conditions as it requires low conductivity mediums. In order to translate the micromotor between these two extreme medium conditions, we use magnetic rolling as means of self-propulsion along with magnetic steering. Interestingly, electrical propulsion also necessitates at least the partial consumption of the Mg, resulting in a sufficient geometrical asymmetry of the micromotor. We have successfully demonstrated the rapid propulsion switching capability of the micromotor, from chemical to electrical motions, via magnetic rolling within a microfluidic device with the concentration gradient of the simulated gastric fluid. Such triple-engine micromotor propulsion holds considerable promise for in vitro studies mimicking gastric conditions and performing various bioassay tasks.

Non-wetting Liquid-Infused Slippery Paper.

Liquid-infused slippery surfaces have replaced structural superhydrophobic surfaces in a plethora of emerging applications, hallmarked by their favorable self-healing and liquid-repelling characteristics. Their ease of fabrication on different types of materials and increasing demand in various industrial applications have triggered research interests targeted toward developing an environmental-friendly, flexible, and frugal substrate as the underlying structural and functional backbone. Although many expensive polymers such as polytetrafluoroethylene have so far been used for their fabrication, these are constrained by their compromised flexibility and non-ecofriendliness due to the use of fluorine. Here, we explore the development and deployment of a biodegradable, recyclable, flexible, and an economically viable material in the form of a paper matrix for fabricating liquid-infused slippery interfaces for prolonged usage. We show by controlled experiments that a simple silanization followed by an oil infusion protocol imparts an inherent slipperiness (low contact angle hysteresis and low tilting angle for sliding) to the droplet motion on the paper substrate and provides favorable anti-icing characteristics, albeit keeping the paper microstructures unaltered. This ensures concomitant hydrophobicity, water adhesion, and capillarity for low surface tension fluids, such as mustard oil, with an implicit role played by the paper pore size distribution toward retaining a stable layer of the infused oil. With demonstrated supreme anti-icing characteristics, these results open up new possibilities of realizing high-throughput paper-based substrates for a wide variety of applications ranging from biomedical unit operations to droplet-based digital microfluidics.

Flow and deformation characteristics of a flexible microfluidic channel with axial gradients in wall elasticity.

Axial gradients in wall elasticity may have significant implications in the deformation and flow characteristics of a narrow fluidic conduit, bearing far-reaching consequences in physiology and bio-engineering. Here, we present a theoretical and experimental framework for fluid-structure interactions in microfluidic channels with axial gradients in wall elasticity, in an effort to arrive at a potential conceptual foundation for in vitro study of mirovascular physiology. Towards this, we bring out the static deformation and steady flow characteristics of a circular microchannel made of polydimethylsiloxane (PDMS) bulk, considering imposed gradients in the substrate elasticity. In particular, we study two kinds of elasticity variations - a uniformly soft (or hard) channel with a central strip that is hard (or soft), and, increasing elasticity along the length of the channel. The former kind yields a centrally constricted (or expanded) deformed profile in response to the flow. The latter kind leads to increasingly bulged channel radius from inlet to outlet in response to flow. We also formulate an analytical model capturing the essential physics of the underlying elastohydrodynamic interactions. The theoretical predictions match favourably with the experimental observations and are also in line with reported results on stenosis in mice. The present framework, thus, holds the potential for acting as a fundamental design basis towards developing in vitro models for micro-circulation, capable of capturing exclusive artefacts of healthy and diseased conditions.

Anomalous diffusion in an electrolyte saturated paper matrix.

Diffusion of colored dye on water saturated paper substrates has been traditionally exploited with great skill by renowned water color artists. The same physics finds more recent practical applications in paper-based diagnostic devices deploying chemicals that react with a bodily fluid yielding colorimetric signals for disease detection. During spontaneous imbibition through the tortuous pathways of a porous electrolyte saturated paper matrix, a dye molecule undergoes diffusion in a complex network of pores. The advancing front forms a strongly correlated interface that propagates diffusively but with an enhanced effective diffusivity. We measure this effective diffusivity and show that it is several orders of magnitude greater than the free solution diffusivity and has a significant dependence on the solution pH and salt concentration in the background electrolyte. We attribute this to electrically mediated interfacial interactions between the ionic species in the liquid dye and spontaneous surface charges developed at porous interfaces, and introduce a simple theory to explain this phenomenon.

Electro-kinetically driven route for highly sensitive blood pathology on a paper-based device.

Enhancing the sensitivity of colorimetric detection in paper-devices is a quintessential step in achieving frugal diagnosis. Here, we demonstrate an effective way of improving the detection sensitivity of paper-based devices, as mediated by electro-kinetic mechanisms. By directly employing blood plasma, we investigate the electro-kinetic clustering of glucose, a neutral molecule in paper devices. Under the influence of uniform electric field, dispersed glucose gets accumulated in the paper strips. Due to the combination of EOF and electrophoretic migration, we achieve twofold increase in the colour intensity for both normal and diabetic samples. This approach is robust and possesses better sensitivity than conventional colorimetric assays and can be easily extended to other body fluid based diagnosis. These results may turn out to be of profound importance in improving the quality of pathological diagnosis in low-cost paper-based point-of-care devices deployed in resource-limited settings.

Electrical Power Generation from Wet Textile Mediated by Spontaneous Nanoscale Evaporation.

Developing low-weight, frugal, and sustainable power sources for resource-limited settings appears to be a challenging proposition for the advancement of next-generation sensing devices and beyond. Here, we report the use of centimeter-sized simple wet fabric pieces for electrical power generation by deploying the interplay of a spontaneously induced ionic motion across fabric nanopores due to capillary action and simultaneous water evaporation by drawing thermal energy from the ambient. Unlike other reported devices with similar functionalities, our arrangement does not necessitate any input mechanical energy or complex topographical structures to be embedded in the substrate. A single device is capable of generating a sustainable open circuit potential up to ∼700 mV, which is further scaled up to ∼12 V with small-scale multiplexing (i.e., deploying around 40 numbers of fabric channels simultaneously). The device is able to charge a commercial supercapacitor of ∼0.1 F which can power a white light-emitting diode for more than 1 h. This suffices in establishing an inherent capability of functionalizing self-powered electronic devices and also to be potentially harnessed for enhanced power generation with feasible up-scaling.

Hydroelectric power plant on a paper strip.

We exploit the combinatorial advantage of electrokinetics and tortuosity of a cellulose-based paper network on laboratory grade filter paper for the development of a simple, inexpensive, yet extremely robust (shows constant performance for 12 days) 'paper-and-pencil'-based device for energy harvesting applications. We successfully achieve harvesting of a maximum output power of ∼640 pW in a single channel, while the same is significantly improved (by ∼100 times) with the use of a multichannel microfluidic array (maximum of up to 20 channels). Furthermore, we also provide theoretical insights into the observed phenomenon and show that the experimentally predicted trends agree well with our theoretical calculations. Thus, we envisage that such ultra-low cost devices may turn out to be extremely useful in energizing analytical microdevices in resource limited settings, for instance, in extreme point of care diagnostic applications.

Electroosmosis of Viscoelastic Fluids: Role of Wall Depletion Layer.

We investigate electroosmotic flow of two immiscible viscoelastic fluids in a parallel plate microchannel. Contrary to traditional analysis, the effect of the depletion layer is incorporated near the walls, thereby capturing the complex coupling between rheology and electrokinetics. Toward ensuring realistic prediction, we show the dependence of electroosmotic flow rate on the solution pH and polymer concentration of the complex fluid. In order to assess our theoretical predictions, we have further performed experiments on electroosmosis of an aqueous solution of polyacrylamide (PAAm). Our analysis reveals that neglecting the existence of a depletion layer would result in grossly incorrect predictions of the electroosmotic transport of such fluids. These findings are likely to be of importance in understanding electroosmotically driven transport of complex fluids, including biological fluids, in confined microfluidic environments.