Miniaturized metachronal magnetic artificial cilia.

Biological cilia, hairlike organelles on cell surfaces, often exhibit collective wavelike motion known as metachrony, which helps generating fluid flow. Inspired by nature, researchers have developed artificial cilia as microfluidic actuators, exploring several methods to mimic the metachrony. However, reported methods are difficult to miniaturize because they require either control of individual cilia properties or the generation of a complex external magnetic field. We introduce a concept that generates metachronal motion of magnetic artificial cilia (MAC), even though the MAC are all identical, and the applied external magnetic field is uniform. This is achieved by integrating a paramagnetic substructure in the substrate underneath the MAC. Uniquely, we can create both symplectic and antiplectic metachrony by changing the relative positions of MAC and substructure. We demonstrate the flow generation of the two metachronal motions in both high and low Reynolds number conditions. Our research marks a significant milestone by breaking the size limitation barrier in metachronal artificial cilia. This achievement not only showcases the potential of nature-inspired engineering but also opens up a host of exciting opportunities for designing and optimizing microsystems with enhanced fluid manipulation capabilities.

Highly motile nanoscale magnetic artificial cilia.

Among the many complex bioactuators functioning at different scales, the organelle cilium represents a fundamental actuating unit in cellular biology. Producing motions at submicrometer scales, dominated by viscous forces, cilia drive a number of crucial bioprocesses in all vertebrate and many invertebrate organisms before and after their birth. Artificially mimicking motile cilia has been a long-standing challenge while inspiring the development of new materials and methods. The use of magnetic materials has been an effective approach for realizing microscopic artificial cilia; however, the physical and magnetic properties of the magnetic material constituents and fabrication processes utilized have almost exclusively only enabled the realization of highly motile artificial cilia with dimensions orders of magnitude larger than their biological counterparts. This has hindered the development and study of model systems and devices with inherent size-dependent aspects, as well as their application at submicrometer scales. In this work, we report a magnetic elastomer preparation process coupled with a tailored molding process for the successful fabrication of artificial cilia with submicrometer dimensions showing unprecedented deflection capabilities, enabling the design of artificial cilia with high motility and at sizes equal to those of their smallest biological counterparts. The reported work crosses the barrier of nanoscale motile cilia fabrication, paving the way for maximum control and manipulation of structures and processes at micro- and nanoscales.

Nanomagnetic Elastomers for Realizing Highly Responsive Micro- and Nanosystems.

Evolution has produced natural systems that generate motion and sense external stimuli at the micro- and nanoscales. At extremely small scales, the intricate motions and large deformations shown by these biosystems are due to a tipping balance between their structural compliance and the actuating force generated in them. Artificially mimicking such ingenious systems for scientific and engineering applications has been approached through the development and use of different smart materials mostly limited to microscale dimensions. To push the application range down to the nanoscale, we developed a material preparation process that yields a library of nanomagnetic elastomers with high magnetic particle concentrations. Through this process, we have realized a material with the highest magnetic-to-elastic force ratio, as is shown by an extensive mechanical and magnetic characterization of the materials. Furthermore, we have fabricated and actuated micro- and nanostructures mimicking cilia, demonstrating the extreme compliance and responsiveness of the developed materials.

Integrated Bioluminescent Immunoassays for High-Throughput Sampling and Continuous Monitoring of Cytokines.

Immunoassays show great potential for the detection of low levels of cytokines, due to their high sensitivity and excellent specificity. There is a particular demand for biosensors that enable both high-throughput screening and continuous monitoring of clinically relevant cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNFα). To this end, we here introduce a novel bioluminescent immunoassay based on the ratiometric plug-and-play immunodiagnostics (RAPPID) platform, with an improved intrinsic signal-to-background and an >80-fold increase in the luminescent signal. The new dRAPPID assay, comprising a dimeric protein G adapter connected via a semiflexible linker, was applied to detect the secretion of IL-6 by breast carcinoma cells upon TNFα stimulation and the production of low concentrations of IL-6 (∼18 pM) in an endotoxin-stimulated human 3D muscle tissue model. Moreover, we integrated the dRAPPID assay in a newly developed microfluidic device for the simultaneous and continuous monitoring of changes in IL-6 and TNFα in the low-nanomolar range. The luminescence-based read-out and the homogeneous nature of the dRAPPID platform allowed for detection with a simple measurement setup, consisting of a digital camera and a light-sealed box. This permits the usage of the continuous dRAPPID monitoring chip at the point of need, without the requirement for complex or expensive detection techniques.

An artificial aquatic polyp that wirelessly attracts, grasps, and releases objects.

The development of light-responsive materials has captured scientific attention and advanced the development of wirelessly driven terrestrial soft robots. Marine organisms trigger inspiration to expand the paradigm of untethered soft robotics into aqueous environments. However, this expansion toward aquatic soft robots is hampered by the slow response of most light-driven polymers to low light intensities and by the lack of controlled multishape deformations. Herein, we present a surface-anchored artificial aquatic coral polyp composed of a magnetically driven stem and a light-driven gripper. Through magnetically driven motion, the polyp induces stirring and attracts suspended targets. The light-responsive gripper is sensitive to low light intensities and has programmable states and rapid and highly controlled actuation, allowing the polyp to capture or release targets on demand. The artificial polyp demonstrates that assemblies of stimuli-responsive materials in water utilizing coordinated motion can perform tasks not possible for single-component devices.

A stirring system using suspended magnetically-actuated pillars for controlled cell clustering.

Controlled stirring of a solution is a household task in most laboratories. However, most stirring methods are perturbative or require vessels with predefined shapes and sizes. Here we propose a novel stirring system based on suspended magnetically-actuated pillars (SMAPs), inspired by the ability of biological flagella and cilia to generate flow. We fabricated flexible, millimeter-scale magnetic pillars grafted on transparent polydimethylsiloxane (PDMS) substrates and built a simple actuation setup to control the motion of the pillars remotely. We tested the system with a standard 24-well plate routinely used in most research laboratories and demonstrate that the magnetic actuation results in robust bending of the pillars and large-scale fluid flow in the wells. Quantitative analysis using computational fluid dynamics modeling indicates that the flow profile in the well can be tuned by modulating the applied magnetic field and the geometries of the well and the pillar. Finally, we show that, by employing the stirring system in a standard cell culture plate, we were able to obtain controlled clustering of cells. The SMAP stirring system is therefore a promising cost-effective and scalable stirring approach for various types of studies involving colloids as well as soft and biological materials.

Photoresponsive Passive Micromixers Based on Spiropyran Size-Tunable Hydrogels.

Microfluidic devices allow the manipulation of fluids down to the micrometer scale and are receiving a lot of attention for applications where low volumes and high throughputs are required. In these micro channels, laminar flow usually dominates, which requires long residence times of the fluids, limiting the flow speed and throughput. Here a switchable passive mixer has been developed to control mixing and to easily clean microchannels. The mixer is based on a photoresponsive spiropyran based hydrogel of which the dimensions can be tuned by changing the intensity of the light. The size-tunable gels have been used to fabricate a passive slanted groove mixer that can be switched off by light allowing to change mixing of microfluidics to non-mixed flows. These findings open new possibilities for multi-purpose microfluidic devices where mixers and valves can be tuned by light.

A membrane-based microfluidic device for mechano-chemical cell manipulation.

We introduce a microfluidic device for chemical manipulation and mechanical investigation of circulating cells. The device consists of two crossing microfluidic channels separated by a porous membrane. A chemical compound is flown through the upper "stimulus channel", which diffuses through the membrane into the lower "cell analysis channel", in which cells are mechanically deformed in two sequential narrow constrictions, one before and one after crossing the stimulus channel. Thus, this system permits to measure cell deformability before and after chemical cues are delivered to the cells within one single chip. The validity of the device was tested with monocytic cells stimulated with an actin-disrupting agent (Cytochalasin-D). Furthermore, as proof of principle of the device application, the effect of an anti-inflammatory drug (Pentoxifylline) was tested on monocytic cells activated with Lipopolysaccharides and on monocytes from patients affected by atherosclerosis. The results show that the system can detect differences in cell mechanical deformation after chemical cues are delivered to the cells through the porous membrane. Diffusion of Cytochalasin-D resulted in a considerable decrease in entry time in the narrow constriction and an evident increase in the velocity within the constriction. Pentoxifylline showed to decrease the entry time but not to affect the transit time within the constriction for monocytic cells. Monocytes from patients affected by atherosclerosis were difficult to test in the device due to increased adhesion to the walls of the microfluidic channel. Overall, this analysis shows that the device has potential applications as a cellular assay for analyzing cell-drug interaction.

Stretchable and transparent electrodes based on patterned silver nanowires by laser-induced forward transfer for non-contacted printing techniques.

Silver nanowires (AgNWs) are excellent candidate electrode materials in next-generation wearable devices due to their high flexibility and high conductivity. In particular, patterning techniques for AgNWs electrode manufacture are very important in the roll-to-roll printing process to achieve high throughput and special performance production. It is also essential to realize a non-contact mode patterning for devices in order to keep the pre-patterned components away from mechanical damages. Here, we report a successful non-contact patterning of AgNWs-based stretchable and transparent electrodes by laser-induced forward transfer (LIFT) technique. The technique was used to fabricate a 100% stretchable electrode with a width of 200 µm and electrical resistivity 10-4 Ωcm. Experiments conducted integrating the stretchable electrode on rubber substrate in which LED was pre-fabricated showed design flexibility resulting from non-contact printing. Further, a patterned transparent electrode showed over 80% in optical transmittance and less than 100 Ω sq-1 in sheet resistance by the optimized LIFT technique.

A microfluidic device based on an evaporation-driven micropump.

In this paper we introduce a microfluidic device ultimately to be applied as a wearable sweat sensor. We show proof-of-principle of the microfluidic functions of the device, namely fluid collection and continuous fluid flow pumping. A filter-paper based layer, that eventually will form the interface between the device and the skin, is used to collect the fluid (e.g., sweat) and enter this into the microfluidic device. A controllable evaporation driven pump is used to drive a continuous fluid flow through a microfluidic channel and over a sensing area. The key element of the pump is a micro-porous membrane mounted at the channel outlet, such that a pore array with a regular hexagonal arrangement is realized through which the fluid evaporates, which drives the flow within the channel. The system is completely fabricated on flexible polyethylene terephthalate (PET) foils, which can be the backbone material for flexible electronics applications, such that it is compatible with volume production approaches like Roll-to-Roll technology. The evaporation rate can be controlled by varying the outlet geometry and the temperature. The generated flows are analyzed experimentally using Particle Tracking Velocimetry (PTV). Typical results show that with 1 to 61 pores (diameter = 250 µm, pitch = 500 µm) flow rates of 7.3 × 10(-3) to 1.2 × 10(-1) µL/min are achieved. When the surface temperature is increased by 9.4°C, the flow rate is increased by 130 %. The results are theoretically analyzed using an evaporation model that includes an evaporation correction factor. The theoretical and experimental results are in good agreement.

The correlation between mineralization degree and bone tissue stiffness in the porcine mandibular condyle.

The aim of this study was to correlate the local tissue mineral density (TMD) with the bone tissue stiffness. It was hypothesized that these variables are positively correlated. Cancellous and cortical bone samples were derived from ten mandibular condyles taken from 5 young and 5 adult female pigs. The bone tissue stiffness was assessed in three directions using nanoindentation. At each of three tested sides 5 indents were made over the width of 5 single bone elements, resulting in a total number of 1500 indents. MicroCT was used to determine the local TMD at the indented sites. The TMD and the bone tissue stiffness were higher in bone from the adult animals than from the young ones, but did not differ between cancellous and cortical bone. In the adult group, both the TMD and the bone tissue stiffness were higher in the center than at the surface of the bone elements. The mean TMD, thus ignoring the local mineral distribution, had a coefficient of determination (R(2)) with the mean bone tissue stiffness of 0.55, p < 0.05, whereas the correlation between local bone tissue stiffness and the concomitant TMD appeared to be weak (R (2) 0.07, p < 0.001). It was concluded that the mineralization degree plays a larger role in bone tissue stiffness in cancellous than in cortical bone. Our data based on bone from the mandibular condyle suggest that the mineralization degree is not a decisive determinant of the local bone tissue stiffness.

Metachronal Motion of Biological and Artificial Cilia.

Cilia are slender, hair-like cell protrusions that are present ubiquitously in the natural world. They perform essential functions, such as generating fluid flow, propulsion, and feeding, in organisms ranging from protozoa to the human body. The coordinated beating of cilia, which results in wavelike motions known as metachrony, has fascinated researchers for decades for its role in functions such as flow generation and mucus transport. Inspired by nature, researchers have explored diverse materials for the fabrication of artificial cilia and developed several methods to mimic the metachronal motion observed in their biological counterparts. In this review, we will introduce the different types of metachronal motion generated by both biological and artificial cilia, the latter including pneumatically, photonically, electrically, and magnetically driven artificial cilia. Furthermore, we review the possible applications of metachronal motion by artificial cilia, focusing on flow generation, transport of mucus, particles, and droplets, and microrobotic locomotion. The overall aim of this review is to offer a comprehensive overview of the metachronal motions exhibited by diverse artificial cilia and the corresponding practical implementations. Additionally, we identify the potential future directions within this field. These insights present an exciting opportunity for further advancements in this domain.

Curved Surfaces Induce Metachronal Motion of Microscopic Magnetic Cilia.

Cilia are hair-like organelles present on cell surfaces. They often exhibit a collective wave-like motion that can enhance fluid or particle transportation function, known as metachronal motion. Inspired by nature, researchers have developed artificial cilia capable of inducing metachronal motion, especially magnetic actuation. However, current methods remain intricate, requiring either control of the magnetic or geometrical properties of individual cilia or the generation of a complex magnetic field. In this paper, we present a novel elegant method that eliminates these complexities and induces metachronal motion of arrays of identical microscopic magnetic artificial cilia by applying a simple rotating uniform magnetic field. The key idea of our method is to place arrays of cilia on surfaces with a specially designed curvature. This results in consecutive cilia experiencing different magnetic field directions at each point in time, inducing a phase lag in their motion, thereby causing collective wave-like motion. Moreover, by tuning the surface curvature profile, we can achieve diverse metachronal patterns analogous to symplectic and antiplectic metachronal motion observed in nature, and we can even devise novel combinations thereof. Furthermore, we characterize the local flow patterns generated by the motion of the cilia, revealing the formation of vortical patterns. Our novel approach simplifies the realization of miniaturized metachronal motion in microfluidic systems and opens the possibility of controlling flow pattern generation and transportation, opening avenues for applications such as lab-on-a-chip technologies, organ-on-a-chip platforms, and microscopic object propulsion.

Interplay of electrokinetic effects in nonpolar solvents for electronic paper displays.

HYPOTHESIS: Electronic paper displays rely on electrokinetic effects in nonpolar solvents to drive the displacement of colloidal particles within a fluidic cell. While Electrophoresis (EP) is a well-established and frequently employed phenomenon, electro-osmosis (EO), which drives fluid flow along charged solid surfaces, has not been studied as extensively. We hypothesize that by exploiting the interplay between these effects, an enhanced particle transport can be achieved. EXPERIMENTS: In this study, we experimentally investigate the combined effects of EP and EO for colloidal particles in non-polar solvents, driven by an electric field. We use astigmatism micro-particle tracking velocimetry (A-µPTV) to measure the motion of charged particles within model fluidic cells. Using a simple approach that relies on basic fluid flow properties we extract the contributions due to EP and EO, finding that EO contributes significantly to particle transport. The validity of our approach is confirmed by measurements on particles with different magnitudes of charge, and by comparison to numerical simulations. FINDINGS: We find that EO flows can play a dominant role in the transport of particles in electrokinetic display devices. This can be exploited to speed up particle transport, potentially yielding displays with significantly faster switching times.

Bio-inspired multimodal learning with organic neuromorphic electronics for behavioral conditioning in robotics.

Biological systems interact directly with the environment and learn by receiving multimodal feedback via sensory stimuli that shape the formation of internal neuronal representations. Drawing inspiration from biological concepts such as exploration and sensory processing that eventually lead to behavioral conditioning, we present a robotic system handling objects through multimodal learning. A small-scale organic neuromorphic circuit locally integrates and adaptively processes multimodal sensory stimuli, enabling the robot to interact intelligently with its surroundings. The real-time handling of sensory stimuli via low-voltage organic neuromorphic devices with synaptic functionality forms multimodal associative connections that lead to behavioral conditioning, and thus the robot learns to avoid potentially dangerous objects. This work demonstrates that adaptive neuro-inspired circuitry with multifunctional organic materials, can accommodate locally efficient bio-inspired learning for advancing intelligent robotics.

Cancer-on-chip models for metastasis: importance of the tumor microenvironment.

Cancer-on-chip (CoC) models, based on microfluidic chips harboring chambers for 3D tumor-cell culture, enable us to create a controlled tumor microenvironment (TME). CoC models are therefore increasingly used to systematically study effects of the TME on the various steps in cancer metastasis. Moreover, CoC models have great potential for developing novel cancer therapies and for predicting patient-specific response to cancer treatments. We review recent developments in CoC models, focusing on three main TME components: (i) the anisotropic extracellular matrix (ECM) architectures, (ii) the vasculature, and (iii) the immune system. We aim to provide guidance to biologists to choose the best CoC approach for addressing questions about the role of the TME in metastasis, and to inspire engineers to develop novel CoC technologies.

Programmable metachronal motion of closely packed magnetic artificial cilia.

Despite recent advances in artificial cilia technologies, the application of metachrony, which is the collective wavelike motion by cilia moving out-of-phase, has been severely hampered by difficulties in controlling closely packed artificial cilia at micrometer length scales. Moreover, there has been no direct experimental proof yet that a metachronal wave in combination with fully reciprocal ciliary motion can generate significant microfluidic flow on a micrometer scale as theoretically predicted. In this study, using an in-house developed precise micro-molding technique, we have fabricated closely packed magnetic artificial cilia that can generate well-controlled metachronal waves. We studied the effect of pure metachrony on fluid flow by excluding all symmetry-breaking ciliary features. Experimental and simulation results prove that net fluid transport can be generated by metachronal motion alone, and the effectiveness is strongly dependent on cilia spacing. This technique not only offers a biomimetic experimental platform to better understand the mechanisms underlying metachrony, it also opens new pathways towards advanced industrial applications.

Dual-Wavelength Volumetric Microlithography for Rapid Production of 4D Microstructures.

4D microstructured actuators are micro-objects made of stimuli-responsive materials capable of induced shape deformations, with applications ranging from microrobotics to smart micropatterned haptic surfaces. The novel technology dual-wavelength volumetric microlithography (DWVML) realizes rapid printing of high-resolution 3D microstructures and so has the potential to pave the way to feasible manufacturing of 4D microdevices. In this work, DWVML is applied for the first time to printing stimuli-responsive materials, namely, liquid crystal networks (LCNs). An LCN photoresist is developed and characterized, and large arrays of up to 5625 LCN micropillars with programmable shape changes are produced by means of DWVML in the time span of seconds, over areas as large as ∼5.4 mm2. The production rate of 0.24 mm3 h-1 is achieved, exceeding speeds previously reported for additive manufacturing of LCNs by 2 orders of magnitude. Finally, a membrane with tunable, micrometer-sized pores is fabricated to illustrate the potential DWVML holds for real-world applications.

A Cell Pre-Wrapping Seeding Technique for Hydrogel-Based Tubular Organ-On-A-Chip.

Organ-on-a-chip (OOC) models based on microfluidic technology are increasingly used to obtain mechanistic insight into (patho)physiological processes in humans, and they hold great promise for application in drug development and regenerative medicine. Despite significant progress in OOC development, several limitations of conventional microfluidic devices pose challenges. First, most microfluidic systems have rectangular cross sections and flat walls, and therefore tubular/ curved structures, like blood vessels and nephrons, are not well represented. Second, polymers used as base materials for microfluidic devices are much stiffer than in vivo extracellular matrix (ECM). Finally, in current cell seeding methods, challenges exist regarding precise control over cell seeding location, unreachable spaces due to flow resistances, and restricted dimensions/geometries. To address these limitations, an alternative cell seeding technique and a corresponding workflow is introduced to create circular cross-sectioned tubular OOC models by pre-wrapping cells around sacrificial fiber templates. As a proof of concept, a perfusable renal proximal tubule-on-a-chip is demonstrated with a diameter as small as 50 µm, cellular tubular structures with branches and curvature, and a preliminary vascular-renal tubule interaction model. The cell pre-wrapping seeding technique promises to enable the construction of diverse physiological/pathological models, providing tubular OOC systems for mechanistic investigations and drug development.

Cell-based drug combination screening with a microfluidic droplet array system.

We performed cell-based drug combination screening using an integrated droplet-based microfluidic system based on the sequential operation droplet array (SODA) technique. In the system, a tapered capillary connected with a syringe pump was used for multistep droplet manipulations. An oil-covered two-dimensional droplet array chip fixed in an x-y-z translation stage was used as the platform for cell culture and analysis. Complex multistep operations for drug combination screening involving long-term cell culture, medium changing, schedule-dependent drug dosage and stimulation, and cell viability testing were achieved in parallel in the semiopen droplet array, using multiple droplet manipulations including liquid metering, aspirating, depositing, mixing, and transferring. Long-term cell culture as long as 11 days was performed in oil-covered 500 nL droplets by changing the culture medium in each droplet every 24 h. The present system was applied in parallel schedule-dependent drug combination screening for A549 nonsmall lung cancer cells with the cell cycle-dependent drug flavopiridol and two anticancer drugs of paclitaxel and 5-fluorouracil. The highest inhibition efficiency was obtained with a schedule combination of 200 nM flavopiridol followed by 100 µM 5-fluorouracil. The drug consumption for each screening test was substantially decreased to 5 ng-5 µg, corresponding to 10-1000-fold reductions compared with traditional drug screening systems with 96-well or 384-well plates. The present work provides a novel and flexible droplet-based microfluidic approach for performing cell-based screening with complex and multistep operation procedures.