Stimuli-responsive microgel-based etalons are promising optical and bio-sensors. These sensors play a pivotal role in modern healthcare by enabling rapid biomolecule detection, contributing to organ-on-chip applications and early disease diagnosis. This study investigates the suitability of poly(N-isopropylacrylamide) (pNIPAm)-based microgels for inkjet printing, focusing on optimizing their properties for effective deposition. Key parameters, including surface tension, viscosity, and particle size, are characterized to ensure compatibility with inkjet-printing requirements. The addition of surfactants tunes the suspensions’ properties to be in line with the requirements of inkjet printing. Jetting of pNIPAm-based microgels on gold-coated substrates forms a cohesive drop in a range of a few millimeters. Optical and scanning electron microscopy confirm the formation of a uniform microgel layer. The optical reflectance spectroscopy results indicate that inkjet-printed microgel-based etalons can effectively respond to changes in temperature and glucose concentration. In-liquid atomic force microscopy demonstrates the swelling dynamics of the microgels in different glucose concentrations, shedding light on their response dynamics. Our work demonstrates, for the first time, the feasibility of printing microgels in a controlled way, fabricating biocompatible inkjet-printed microgel-based etalon sensors with precise dimensions. The size precision and the sensitive monitoring capabilities of biomolecules hold great promise for in situ and continuous sensing in a wide range of biological and organ-on-chip applications.

Polymer membranes are prime candidates for separation and purification processes, with their functionality enhanced by nanoparticle incorporation and diverse polymer structures. Poly(ionic liquids) (PILs), highly charged electrolyte-like polymers, are gaining interest as membrane polymer matrices. Embedding photocatalytic nanoparticles enables water purification through filtration and degradation reactions. Graphitic carbon nitride (g-CN), a metal-free semiconductor with visible-light activity, offers a promising approach for photoredox-based environmental remediation, though its powder form poses separation challenges. This work presents g-CN embedded PIL nanocomposite membranes fabricated via UV curing, characterized by structural, filtration, and surface properties. Photocatalytic performance and reusability under visible light are evaluated using methylene blue (model dye) and sulfadiazine (model antibiotic) under static conditions. A continuous filtration module with integrated light is developed to assess simultaneous filtration, degradation, and antifouling properties, demonstrating the membranes’ potential for advanced water treatment.

A new and sustainable membrane manufacturing method is 3D printing, which reduces the number of fabrication steps, waste production, and the corresponding CO₂emissions. It further enables fabricating membranes with well-defined pore size, shape, and configuration. Here, we study 3D printing of microfiltration membranes using a novel dual-wavelength microstereolithography method. Via the gradient descent method, we are able to calculate and control a printable membrane with micrometer precision, enabling the possibility of printing membranes directly. Hydrophilic porous membranes with cylindrical microscale pores (≈10 μm in diameter) are printed from polyethylene glycol diacrylate (PEGDA). Membrane printing procedure and postprocessing steps are thoroughly investigated to print consistent membranes with uniform thickness. The membranes are fully characterized using SEM, FTIR, contact angle, and surface roughness measurements. The pure water permeability and separation performance of the 3D-printed membrane are further investigated and compared with those of commercial hydrophilic PTFE membranes. The 3D-printed membranes show similar permeability values to those of commercial membranes and could successfully separate oil droplets from oil-in-water emulsions. The membranes’ permeability is further predicted using a 1D tube model and numerical modeling. The effect of material’s property (e.g., swelling) and pore deformation during pressurization are studied to understand the discrepancy between the calculated and the experimental permeability values. The results provide valuable insights into the permeability prediction of 3D-printed membranes and the corresponding design optimization.

We introduce Microgel-based Etalon Membranes (MEMs), based on the combination of stimuli-responsive microgels with an etalon, which is an optical device consisting of two reflecting plates and is used to filter specific wavelengths of light. The microgels are sandwiched between two reflective layers and, in response to a stimulus (e.g., temperature, pH, or biomarker concentration), swell or de-swell, thereby changing the distance between the two reflective layers and generating multiple peaks in the reflectance spectra. This property gives a MEM the unique capability of simultaneous separation and tunable responses to environmental changes and/or biomarker concentrations. We propose a design based on gold layers on a silicon nitride wafer membrane. Our comprehensive characterization, employing permeability experiments, in situ optical reflectance spectroscopy, in-liquid atomic force microscopy (AFM) analysis, and captive bubble contact angle measurements, elucidates the dynamic response of MEM to pH, temperature, and glucose stimuli and the corresponding effect of microgel swelling/de-swelling on the membrane properties, e.g., permeability. The AFM results confirm the dynamic changes of the microgel layer’s thickness on the membrane surface in response to the stimuli. Although the microgel’s swelling/de-swelling influences the effective pore radius, the decrease in the membrane’s permeance is limited to less than 10%. In the swollen state of the microgels, the etalon membranes show a prominent hydrophilic behavior, while they become less hydrophilic in the microgels’ de-swollen state. This work introduces MEM and provides novel insights into their behavior. The fundamental understanding that we reveal opens the way to applications ranging from point-of-care testing to continuous environmental monitoring.

Acoustic manipulation of particles in microchannels has recently gained much attention. Ultrasonic standing wave (USW) separation of oil droplets or particles is an established technology for microscale applications. Acoustofluidic devices are normally operated at optimized conditions, namely, resonant frequency, to minimize power consumption. It has been recently shown that symmetry breaking is needed to obtain efficient conditions for acoustic particle trapping. In this work, we study the acoustophoretic behavior of monodisperse oil droplets (silicone oil and hexadecane) in water in the microfluidic chip operating at a non-resonant frequency and an off-center placement of the transducer. Finite element-based computer simulations are further performed to investigate the influence of these conditions on the acoustic pressure distribution and oil trapping behavior. Via investigating the Gor’kov potential, we obtained an overlap between the trapping patterns obtained in experiments and simulations. We demonstrate that an off-center placement of the transducer and driving the transducer at a non-resonant frequency can still lead to predictable behavior of particles in acoustofluidics. This is relevant to applications in which the theoretical resonant frequency cannot be achieved, e.g., manipulation of biological matter within living tissues.

Nature has been a constant source of inspiration for technological developments. Recently, the study of nature-inspired materials has expanded to the micro- and nanoscale, facilitating new breakthroughs in the design of materials with unique properties. Various types of superhydrophobic surfaces inspired by the lotus/rice leaf are examples of nature-inspired surfaces with special wettability properties. A new class of functional surfaces whose design is inspired by the pitcher plant are the slippery liquid-infused porous surfaces (SLIPS). This Review summarizes the properties, design criteria, fabrication strategies, and working mechanisms of both surfaces with specific focus on SLIPS. The applications of SLIPS in the field of membrane technology [slippery liquid-infused membranes (SLIMs)] are also reviewed. These membranes are also known as liquid gating membranes due to the gating functionality of the capillary-stabilized liquid in the membrane pores leading to a smart gating mechanism. Similar to the gating ion channels in biological systems, the pores open and close in response to the ambient stimuli, e.g., pressure, temperature, and ions. Different types of stimuli-responsive smart gating membranes are introduced here, and their properties and applications are reviewed in detail. Finally, challenges and perspectives on both SLIPS and smart gating membranes are discussed. This Review provides a thorough discussion and practical applications of nature-inspired functional surfaces and membranes to pave the way for future research and further developments in this emerging field.

Micro-and nanodroplets have many important applications such as in drug delivery, liquid-liquid extraction, nanomaterial synthesis and cosmetics. A commonly used method to generate a large number of micro-or nanodroplets in one simple step is solvent exchange (also called nanoprecipitation), in which a good solvent of the droplet phase is displaced by a poor one, generating an oversaturation pulse that leads to droplet nucleation. Despite its crucial importance, the droplet growth resulting from the oversaturation pulse in this ternary system is still poorly understood. We experimentally and theoretically study this growth in Hele-Shaw-like channels by measuring the total volume of the oil droplets that nucleate out of it. In order to prevent the oversaturated oil from exiting the channel, we decorated some of the channels with a porous region in the middle. Solvent exchange is performed with various solution compositions, flow rates and channel geometries, and the measured droplets volume is found to increase with the Péclet number, with an approximate effective power law. A theoretical model is developed to account for this finding. With this model we can indeed explain the scaling, including the prefactor, which can collapse all data of the 'porous' channels onto one universal curve, irrespective of channel geometry and composition of the mixtures. Our work provides a macroscopic approach to this bottom-up method of droplet generation and may guide further studies on oversaturation and nucleation in ternary systems.