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Organ-on-chip systems are promising new in vitro research tools in medical, pharmaceutical, and biological research. Their main benefit, compared to standard cell culture platforms, lies in the improved in vivo resemblance of the cell culture environment. A critical aspect of these systems is the ability to monitor both the cell culture conditions and biological responses of the cultured cells, such as proliferation and differentiation rates, release of signaling molecules, and metabolic activity. Today, this is mostly done using microscopy techniques and off-chip analytical techniques and assays. Integrating in situ analysis methods on-chip enables improved time resolution, continuous measurements, and a faster read-out; hence, more information can be obtained from the developed organ and disease models. Integrated electrical, electrochemical, and optical sensors have been developed and used for chemical analysis in lab-on-a-chip systems for many years, and recently some of these sensing principles have started to find use in organ-on-chip systems as well. This perspective review describes the basic sensing principles, sensor fabrication, and sensor integration in organ-on-chip systems. The review also presents the current state of the art of integrated sensors and discusses future potential. We bring a technological perspective, with the aim of introducing in-line sensing and its promise to advance organ-on-chip systems and the challenges that lie in the integration to researchers without expertise in sensor technology.

Left-right asymmetry is a conserved property in nature and observed in the human body, a property known as cell chirality. Cell chirality is often studied using micropatterned

Introduced for rapid prototyping in 1980, 3D printing has evolved in the last decade as a versatile technology with applications in various fields, including aerospace, medical devices, and optics [1]. Among the different 3D printing techniques, two-photon polymerization (2PP) generates 3D structures with resolution and feature size down to nanometric scale while removing the need of supports for full freedom in sample design [2,3]. With regards to the recent upsurge of commercial 2PP printing systems, this work aims at benchmarking two printers and provide an overview of the capabilities of those two systems.

Photosynthetic dinoflagellates in the family Symbiodiniaceae engage in symbiosis with scleractinian corals. As coral ‘bleaching’ is partly governed by the thermal sensitivity of different Symbiodiniaceae lineages, numerous studies have investigated their temperature sensitivity. However, the systematic identification of single-cells with increased temperature resistance among these dinoflagellates has remained inaccessible, mostly due to a lack of technologies operating at the microscale. Here, we employed a unique combination of microfluidics, miniaturized temperature control, and chlorophyll fluorometry to characterize the single-cell heterogeneity among five representative species within the Symbiodiniaceae family under temperature stress. We monitored single-cell maximum quantum yields (Fv/Fm) of photosystem (PS) II under increasing temperature stress (22‒39 °C, + 1 °C every 15 min), and detected a significant Fv/Fm reduction at lineage-specific temperatures ranging from 28 °C to 34 °C alongside a 40- to 180- fold increase in intraspecific heterogeneity under elevated temperatures (>31 °C). We discovered that the initial Fv/Fm of a cell could predict the same cell’s ability to perform PSII photochemistry under moderate temperature stress (

The generation of hydrogel droplets using droplet microfluidics has emerged as a powerful tool with many applications in biology and medicine. Here, a microfluidic system to control the position of particles (beads or astrocyte cells) in hydrogel droplets using bulk acoustic standing waves is presented. The chip consisted of a droplet generator and a 380 µm wide acoustic focusing channel. Droplets comprising hydrogel precursor solution (polyethylene glycol tetraacrylate or a combination of polyethylene glycol tetraacrylate and gelatine methacrylate), photoinitiator and particles were generated. The droplets passed along the acoustic focusing channel where a half wavelength acoustic standing wave field was generated, and the particles were focused to the centre line of the droplets (i.e. the pressure nodal line) by the acoustic force. The droplets were cross-linked by exposure to UV-light, freezing the particles in their positions. With the acoustics applied, 89 ± 19% of the particles (polystyrene beads, 10 µm diameter) were positioned in an area ± 10% from the centre line. As proof-of-principle for biological particles, astrocytes were focused in hydrogel droplets using the same principle. The viability of the astrocytes after 7 days in culture was 72 ± 22% when exposed to the acoustic focusing compared with 70 ± 19% for samples not exposed to the acoustic focusing. This technology provides a platform to control the spatial position of bioparticles in hydrogel droplets, and opens up for the generation of more complex biological hydrogel structures.

Some of the most fundamental chemical building blocks of life on Earth are the metal elements. X-ray absorption spectroscopy (XAS) is an element-specific technique that can analyse the local atomic and electronic structure of, for example, the active sites in catalysts and energy materials and allow the metal sites in biological samples to be identified and understood. A microfluidic device capable of withstanding the intense hard X-ray beams of a 4th generation synchrotron and harsh chemical sample conditions is presented in this work. The device is evaluated at the

The reliability of conventional cell culture studies to evaluate biomaterials is often questioned, as in vitro outcomes may contradict results obtained through in vivo assays. Microfluidics technology has the potential to reproduce complex physiological conditions by allowing for fine control of microscale features such as cell confinement and flow rate. Having a continuous flow during cell culture is especially advantageous for bioactive biomaterials such as calcium-deficient hydroxyapatite (HA), which may otherwise alter medium composition and jeopardize cell viability, potentially producing false negative results in vitro. In this work, HA was integrated into a microfluidics-based platform (HA-on-chip) and the effect of varied flow rates (2, 8 and 14 µl/min, corresponding to 0.002, 0.008 and 0.014 dyn/cm2, respectively) was evaluated. A HA sample placed in a well plate (HA-static) was included as a control. While substantial calcium depletion and phosphate release occurred in static conditions, the concentration of ions in HA-on-chip samples remained similar to those of fresh medium, particularly at higher flow rates. Pre-osteoblast-like cells (MC3T3-E1) exhibited a significantly higher degree of proliferation on HA-on-chip (8 μl/min flow rate) as compared to HA-static. However, cell differentiation, analysed by alkaline phosphatase (ALP) activity, showed low values in both conditions. This study indicates that cells respond differently when cultured on HA under flow compared to static conditions, which indicates the need for more physiologically relevant methods to increase the predictive value of in vitro studies used to evaluate biomaterials. Statement of significance There is a lack of correlation between the results obtained when testing some biomaterials under cell culture as opposed to animal models. To address this issue, a cell culture method with slightly enhanced physiological relevance was developed by incorporating a biomaterial, known to regenerate bone, inside of a microfluidic platform that enabled a continuous supply of cell culture medium. Since the utilized biomaterial interacts with surrounding ions, the perfusion of medium allowed for shielding of these changes similarly as would happen in the body. The experimental outcomes observed in the dynamic platform were different than those obtained with standard static cell culture systems, proving the key role of the platform in the assessment of biomaterials.

Introduction Human mesenchymal stem cells (hMSCs) have a great clinical potential for tissue regeneration purposes due to its multilineage capability. Previous studies have reported that a single addition of 5-azacytidine (5-AzaC) causes the differentiation of hMSCs towards a myocardial lineage. The aim of this work was to evaluate the effect of 5-AzaC addition frequency on hMSCs priming (i.e., indicating an early genetic differentiation) using two culture environments. Methods hMSCs were supplemented with 5-AzaC while cultured in well plates and in microfluidic chips. The impact of 5-AzaC concentration (10 and 20 μM) and addition frequency (once, daily or continuously), as well as of culture period (2 or 5 days) on the genetic upregulation of PPARγ (adipocytes), PAX3 (myoblasts), SOX9 (chondrocytes) and RUNX2 (osteoblasts) was evaluated. Results Daily delivering 5-AzaC caused a higher upregulation of PPARγ, SOX9 and RUNX2 in comparison to a single dose delivery, both under static well plates and dynamic microfluidic cultures. A particularly high gene expression of PPARγ (tenfold-change) could indicate priming of hMSCs towards adipocytes. Conclusions Both macro- and microscale cultures provided results with similar trends, where addition frequency of 5-AzaC was a crucial factor to upregulate several genes. Microfluidics technology was proven to be a suitable platform for the continuous delivery of a drug and could be used for screening purposes in tissue engineering research.

Acoustic trapping is a useful method for handling biological samples in microfluidic systems. The aim of this work is twofold: first to investigate the physics behind acoustic trapping in a glass capillary and secondly to perform binary acoustic trapping. The latter is achieved by increasing the density of the fluid in the trapping channel. The trapping device consisted of a glass capillary with a rectangular inner cross-section (height 200 µm × width 2000 µm) equipped with a small piezoelectric transducer. The piezoelectric transducer was actuated at 4 MHz to generate a localised half-wavelength acoustic standing-wave-field in the capillary, comprising of a pressure field and a velocity field. Under acoustic actuation, only particles with higher density than the fluid, i.e. having a positive dipole scattering coefficient, were trapped in the flow direction. The numerical and analytical modelling of the system show that the trapping force which retains the particles against the flow depends only on the dipole scattering coefficient in the pressure nodal plane of the acoustic field. The analytical model also reveals that the retention force is proportional to the dipole scattering coefficient, which agrees with our experimental findings. Next, we showed that in a mixture of melamine particles and polystyrene particles in a high-density fluid it is possible to selectively trap melamine particles, since melamine particles have higher density than polystyrene particles.

The development of organs-on-chip (OoC) has revolutionized in vitro cell-culture experiments by allowing a better mimicry of human physiology and pathophysiology that has consequently led researchers to gain more meaningful insights into disease mechanisms. Several models of hearts-on-chips and vessels-on-chips have been demonstrated to recapitulate fundamental aspects of the human cardiovascular system in the recent past. These 2D and 3D systems include synchronized beating cardiomyocytes in hearts-on-chips and vessels-on-chips with layer-based structures and the inclusion of physiological and pathological shear stress conditions. The opportunities to discover novel targets and to perform drug testing with chip-based platforms have substantially enhanced, thanks to the utilization of patient-derived cells and precise control of their microenvironment. These organ models will provide an important asset for future approaches to personalized cardiovascular medicine and improved patient care. However, certain technical and biological challenges remain, making the global utilization of OoCs to tackle unanswered questions in cardiovascular science still rather challenging. This review article aims to introduce and summarize published work on hearts- and vessels-on chips but also to provide an outlook and perspective on how these advanced in vitro systems can be used to tailor disease models with patient-specific characteristics.

In this work, a two-channel, water-based cooling system was integrated into a polydimethylsiloxane (PDMS)-glass microfluidic device for application in single-cell biological studies. This system is designed to cool living cells to single-digit temperatures in situ, without requiring any features of the electron-beam fabricated master mould to be changed, and without interfering either biologically or optically with the cells themselves. The temperature profile inside the device was mapped using multiple thermocouples mounted inside the device, over time. A parametric study including coolant flow rate, distance between the cooling channel and the fluidic channel, and number of active cooling channels was performed to evaluate the performance of the system. By using ice water as the coolant, we have demonstrated stable on-chip cooling reaching an average temperature of 4.9 °C when operated at a coolant flow rate of 23 ml min−1 and using two active cooling channels, positioned only 400 µm away from the cell trapping sites. The maximum observed temperature deviation during an 80 min stability test was ±0.2 °C. We have observed that flowing room temperature culture media through the device with active cooling had no influence on the temperature inside the chip, demonstrating its suitability for use in live cell culture experiments. Finally, we have also demonstrated that the active cooling system successfully decreased the cell metabolism of trapped Escherichia coli resulting in a decreased growth rate of the bacteria.

Subcutaneous injection is one of the most common approaches for administering biopharmaceuticals unsuitable for oral delivery. However, there is a lack of methods to predict the behavior of biopharmaceuticals within the extracellular matrix of the subcutaneous tissue. In this work, we present a novel miniaturized microfluidic-based in vitro method able to investigate interactions between drug molecules and the polymers of the subcutaneous extracellular matrix. To validate the method, microgels consisting of, respectively, covalently cross-linked hy-aluronic acid, polyacrylic acid, and commercially available DC BeadTM, were exposed to three model substances: cytochrome C, protamine sulfate and amitriptyline hydrochloride. These components were chosen to include systems with widely different physiochemical properties (charge, size, self-assembly, etc.) The experimental results were compared with theoretical predictions from a gel model developed earlier. The results show that the method is suitable as a rapid screening method for automated, large-scale, probing of interactions between biopolymers and drug molecules, with small consumption of material.

This Feature article describes the different particle manipulation techniques available in the droplet microfluidics tool-box to handle particles encapsulated inside droplets and to manipulate whole droplets. We address the advantages and disadvantages of the different techniques to guide new users.

We have developed a straightforward technique for fabricating user-friendly and biomimetic microfluidic devices out of a gelatin/agar gel cross-linked with 1,1′-carbonyldiimidazole. The fabrication procedure requires only inexpensive starting materials such as glass capillaries and wires to mold 3D cylindrical channels into the gel with the possibility of achieving channel diameters of 375 μm and 1000 μm. We demonstrate that the channel absent of gel injury can retain fluid within its dimensions for at least 7 h. We also show that the device material does not autofluoresce nor provide hindrances with fluorescent imaging. A discussion of the chemical linkage identities of cross-linked gelatin/agar is included via ATR-FTIR studies. Crosslinking of the gelatin/agar is further confirmed by the lack of a gel to sol transition at physiological temperature as assessed by DSC measurements. SEM micrographs that demonstrate the 100 nm mean pore width of the cross-linked gelatin/agar are provided. This device is considered biomimetic because it represents components present in the natural extracellular matrix such as collagen and proteoglycans in the form of cross-linked gelatin/agar.

Interfacing solid-state nanopores with biological systems has been exploited as a versatile analytical platform for analysis of individual biomolecules. Although clogging of solid-state nanopores due to nonspecific interactions between analytes and pore walls poses a persistent challenge in attaining the anticipated sensing efficacy, insufficient studies focus on elucidating the clogging dynamics. Herein, we investigate the DNA clogging behavior by passing double-stranded (ds) DNA molecules of different lengths through hafnium oxide(HfO2)-coated silicon (Si) nanopore arrays, at different bias voltages and electrolyte pH values. Employing stable and photoluminescent-free HfO2/Si nanopore arrays permits a parallelized visualization of DNA clogging with confocal fluorescence microscopy. We find that the probability of pore clogging increases with both DNA length and bias voltage. Two types of clogging are discerned: persistent and temporary. In the time-resolved analysis, temporary clogging events exhibit a shorter lifetime at higher bias voltage. Furthermore, we show that the surface charge density has a prominent effect on the clogging probability because of electrostatic attraction between the dsDNA and the HfO2 pore walls. An analytical model based on examining the energy landscape along the DNA translocation trajectory is developed to qualitatively evaluate the DNA-pore interaction. Both experimental and theoretical results indicate that the occurrence of clogging is strongly dependent on the configuration of translocating DNA molecules and the electrostatic interaction between DNA and charged pore surface. These findings provide a detailed account of the DNA clogging phenomenon and are of practical interest for DNA sensing based on solid-state nanopores.

To bring novel biomaterials to clinical use, reliable in vitro models are imperative. The aim of this work was to develop a microfluidic tool to evaluate the biological properties of biomaterials for bone repair. Two approaches to embed medical grade titanium (Ti6Al4V) on-chip were explored. The first approach consisted of a polydimethylsiloxane microfluidic channel placed onto a titanium disc, held together by an additively manufactured fixture. In the second approach, a titanium disc was assembled onto a microscopic glass slide, using a double-sided tape microfluidic channel. Both approaches demonstrated potential for maintaining MC3T3-E1 preosteoblast-like cell cultures on-chip, as was shown by the vast majority of living cells after 1 day. In addition, the cells cultured on-chip showed a more elongated morphology compared to cells grown under static conditions and a tendency to align to the direction of the flow. For longer-term (i.e. 10 days) studies, the glass-based chip was selected. Assessment of cell viability showed a high number of living cells during the entire experimental period. Cell proliferation and differentiation studies revealed an increase in cell proliferation on-chip, suggesting that proliferation was the dominating process at the detriment of differentiation in this micrometric dynamic environment. These results illustrate the importance of optimizing in vitro cell culture conditions and how these may affect biomaterial testing outcomes. Overall, this work provides a step towards more in vivo-like microfluidic testing platforms, which are expected to provide more reliable in vitro screening of biomaterials.

Polydimethylsiloxane (PDMS) is among the most widely used materials for organ-on-chip systems. Despite its multiple beneficial characteristics from an engineering point of view, there is a concern about the effect of PDMS on the cells cultured in such devices. The aim of this study was to enhance the understanding of the effect of PDMS on cellular behavior in a context relevant for on-chip studies. The focus was put on an indirect effect of PDMS, namely leaching of uncrosslinked oligomers, particularly for bone regeneration applications. PDMS-based chips were prepared and analyzed for the potential release of PDMS oligomers within the microfluidic channel when kept at different flow rates. Leaching of uncrosslinked oligomers from PDMS was quantified as silicon concentration by inductively coupled plasma - optical emission spectrometry and further confirmed by mass spectrometry. Subsequently, PDMS-leached media, with a silicon concentration matching the on-chip experiment, were prepared to study cell proliferation and osteogenic differentiation of MC3T3-E1 pre-osteoblasts and human mesenchymal stem cells. The silicon concentration initially detected in the media was inversely proportional to the tested flow rates and decreased to control levels within 52 h. In addition, by curing the material overnight instead of 2 h, regardless of the curing temperature (65 and 80 °C), a large reduction in silicon concentration was found, indicating the importance of the PDMS curing parameters. Furthermore, it was shown that PDMS oligomers enhanced the differentiation of MC3T3-E1 pre-osteoblasts, this being a cell type dependent effect as no changes in cell differentiation were observed for human mesenchymal stem cells. Overall, this study illustrates the importance of optimization steps when using PDMS devices for biological studies, in particular PDMS curing conditions and extensive washing steps prior to an experiment.

This review article describes microfabrication techniques to define chemical, mechanical and structural patterns in hydrogels and how these can be used to prepare in vivo like, i.e. biomimetic, cell culture scaffolds. Hydrogels are attractive materials for 3D cell cultures as they provide ideal culture conditions and they are becoming more prominently used. Single material gels without any modifications do however have their limitation in use and much can be gained by in improving the in vivo resemblance of simple hydrogel cell culture scaffolds. This review article discusses the most commonly used cross-linking strategies used for hydrogel-based culture scaffolds and gives a brief introduction to microfabrication methods that can be used to define chemical, mechanical and structural patterns in hydrogels with micrometre resolution. The review article also describes a selection of literature references using these microfabrication techniques to prepare organ and disease models with controlled cell adhesion, proliferation and migration. It is intended to serve as an introduction to microfabrication of hydrogels and an inspiration for novel interdisciplinary research projects.

In this paper an acoustofluidic system to trap hydrogel droplets is shown. The presented trapping method is label-free, biocompatible and operated in non-contact mode. The results show that the droplets can be trapped at flow rates up to 76 µL/min which corresponds to an average flow speed of 3.2 mm/s. Moreover, it is shown that the droplets can be trapped for several hours, thus allowing for studies of the encapsulated cells over time. An application of the system is shown by performing on-chip cell nuclei staining.

This paper presents a microfluidic platform for on-chip droplet dilution where the bead recovery also can be controlled. The droplets containing 10 µm polystyrene beads can be diluted with high bead recovery. This platform involves 5 steps for on-chip dilution of the droplets: droplet generation, bead focusing, droplet splitting, pico-injection and serpentine mixing. Background signal in the droplets is significantly reduced with maintained bead recovery by this on-chip dilution method. The technology is applicable to many types of samples and does not require any labelling of the bioparticles.

We have characterised three droplet split designs for acoustic particle enrichment in water-in-oil droplets. The microfluidic channel design included a droplet generation junction, acoustic focusing channel and a trident-shaped droplet split. The microfluidic channels were dry-etched in silicon and sealed with glass lids by anodic bonding. To each microfluidic chip a piezoelectric transducer was glued, and at actuation of the transducer at the fundamental resonance frequency of the acoustic focusing channel (1.91–1.93 MHz), a half wavelength standing wave field was created between the channel walls. The acoustic force focused the encapsulated particles (3.2 μm, 4.8 μm and 9.9 μm diameter polystyrene microbeads) to the centre-line of the droplets, and when the droplets reached the droplet split the particles were directed into the centre daughter droplets. The results show that the design of the droplet split and the flow ratio between the centre and side outlet channels are the main factors that affect the particle enrichment and particle recovery in the centre daughter droplets. The highest particle enrichment was achieved in the droplet split design having the smallest centre channel (38 μm wide). Using this microfluidic chip design, we demonstrate up to 16.7-fold enrichment of 9.9 μm diameter polystyrene microbeads in the centre daughter droplets. This is almost three times higher particle enrichment than what has previously been presented using other intra-droplet particle enrichment techniques. Moreover, the acoustic technique is label-free and biocompatible.

In this work we present high-aspect ratio carbon pillars (1.4 μm in diameter and ∼11 μm in height) on top of interdigitated electrode arrays to be used for electrochemical applications. For this purpose, different types of 2D and 3D pyrolysed carbon structures were fabricated and characterised including surface- and microstructure, electrical and electrochemical properties. A pre-treatment of oxidised Si wafers is introduced to eliminate electrode delamination and ensure structure stability in water during long time-experiments. Additionally, a heat treatment method is reported for regeneration of pyrolysed carbon films with increased film resistance due to oxidation during storage.

In this paper, we utilize bulk acoustic waves to control the position of microparticles inside droplets in two-phase microfluidic systems and demonstrate a method to enrich the microparticles. In droplet microfluidics, different unit operations are combined and integrated on-chip to miniaturize complex biochemical assays. We present a droplet unit operation capable of controlling the position of microparticles during a trident shaped droplet split. An acoustic standing wave field is generated in the microchannel, and the acoustic forces direct the encapsulated microparticles to the center of the droplets. The method is generic, requires no labeling of the microparticles, and is operated in a noncontact fashion. It was possible to achieve 2+-fold enrichment of polystyrene beads (5 μm in diameter) in the center daughter droplet with an average recovery of 89% of the beads. Red blood cells were also successfully manipulated inside droplets. These results show the possibility to use acoustophoresis in two-phase systems to enrich microparticles and open up the possibility for new droplet-based assays that are not performed today.

The aim of this paper is to study resonance conditions for acoustic particle focusing inside droplets in two-phase microfluidic systems. A bulk acoustic wave microfluidic chip was designed and fabricated for focusing microparticles inside aqueous droplets (plugs) surrounded by a continuous oil phase in a 380-μm-wide channel. The quality of the acoustic particle focusing was investigated by considering the influence of the acoustic properties of the continuous phase in relation to the dispersed phase. To simulate the system and study the acoustic radiation force on the particles inside droplets, a simplified 3D model was used. The resonance conditions and focusing quality were studied for two different cases: (1) the dispersed and continuous phases were acoustically mismatched (water droplets in fluorinated oil) and (2) the dispersed and continuous phases were acoustically matched (water droplets in olive oil). Experimentally, we observed poor acoustic particle focusing inside droplets surrounded by fluorinated oil while good focusing was observed in droplets surrounded by olive oil. The experimental results are supported qualitatively by our simulations. These show that the acoustic properties (density and compressibility) of the dispersed and continuous phases must be matched to generate a strong and homogeneous acoustic field inside the droplet that is suitable for high-quality intra-droplet acoustic particle focusing.

The effect of applied potential pulses on DNA functionalization (thiolated single stranded DNA) and hybridization processes has been monitored in-situ on gold surfaces using electrochemical quartz crystal microbalance with dissipation (EQCM-D). The applied potentials were chosen with respect to the potential of zero charge (Epzc) of the gold surfaces: a positive potential to attract the negatively charged DNA molecules and a negative potential to enhance the vertical alignment due to electrostatic repulsion. The obtained results clearly show that both DNA modification and hybridization are strongly enhanced by applying potential pulses. Based on the EQCM-D results, we present a model to explain the influence of the potential pulsing. Aside from the effect of applied potentials on DNA related processes, this work also demonstrates the versatility of the combination of electrochemistry and quartz crystal microbalance with dissipation in facilitating real-time in situ monitoring of such processes. Keywords: Potential pulse assisted functionalization, Thiolated DNA functionalization, DNA hybridization, Vertically aligned DNA conformation, EQCM-D

This work presents an acoustic trap with integrated thin film sensors to monitor temperature variations during operation. The acoustic trap is wet-etched in glass with a thermally bonded glass lid ...

Production of cell-laden hydrogel droplets as miniaturized niches for 3D cell culture provides a new route for cell-based assays. Such production can be enabled by droplet microfluidics and here we present a droplet trapping system based on bulk acoustic waves for handling hydrogel droplets in a continuous flow format. The droplet trapping system consists of a glass capillary equipped with a small piezoelectric transducer. By applying ultrasound (4 MHz), a localized acoustic standing wave field is generated in the capillary, trapping the droplets in a well-defined cluster above the transducer area. The results show that the droplet cluster can be retained at flow rates of up to 76 μl/min, corresponding to an average flow speed of 3.2 mm/s. The system allows for important operations such as continuous perfusion and/or addition of chemical reagents to the encapsulated cells with in situ optical access. This feature is demonstrated by performing on-chip staining of the cell nuclei. The key advantages of this trapping method are that it is label-free and gentle and thus well-suited for biological applications. Moreover, the droplets can easily be released on-demand, which facilitates downstream analysis. It is envisioned that the presented droplet trapping system will be a valuable tool for a wide range of multistep assays as well as long-term monitoring of cells encapsulated in gel-based droplets.

To transfer cell- and bead-assays into droplet-based platforms typically requires the use of complex microfluidic circuits, which calls for methods to switch the direction of the encapsulated particles. We present a microfluidic chip where the combination of acoustic manipulation at two different harmonics and a trident-shaped droplet-splitter enables direction-switching of microbeads and yeast cells in droplet microfluidic circuits. At the first harmonic, the encapsulated particles exit the splitter in the center daughter droplets, while at the second harmonic, the particles exit in the side daughter droplets. This method holds promises for droplet-based assays where particle-positioning needs to be selectively controlled.

Herein we present a chimeric recombinant spider silk protein (spidroin) whose aqueous solubility equals that of native spider silk dope and a spinning device that is based solely on aqueous buffers, shear forces and lowered pH. The process recapitulates the complex molecular mechanisms that dictate native spider silk spinning and is highly efficient; spidroin from one liter of bacterial shake-flask culture is enough to spin a kilometer of the hitherto toughest as-spun artificial spider silk fiber.

SU-8 is the preferred photoresist for development and fabrication of high aspect ratio (HAR) three dimensional patterns. However, processing of SU-8 is a challenging task, especially when the film thickness as well as the aspect ratio is increasing and the size of the features is close to the resolution limit of photolithography. This paper describes process optimization for the fabrication of dense SU-8 micropillar arrays (2.5@mm spacing) with nominal height >=20@mm and nominal diameter = =8). Two approaches, differing in temperature, ramping rate and duration of the baking steps were compared as part of the photolithographic processing, in order to evaluate the effect of baking on the pattern resolution. Additionally, during the post-processing, supercritical point drying and hard baking were introduced yielding pillars with diameter 1.8@mm, AR=11 and an improved temporal stability.

The design and fabrication of a combined electrochemical-cantilever microfluidic system is described. A chip integrating cantilevers with electrodes into a microchannel is presented with the accompanying polymer flow cell. Issues such as electrical and fluid connections are addressed, electromechanical behavior in ionic solution is investigated, and two uses of the system are demonstrated. First, all cantilevers are functionalized with cysteine, to facilitate detection of Cu2+ ions, then one cantilever is electrochemically cleaned in situ to generate a reference cantilever for differential measurements. Two concentrations of Cu2+ ions are successfully measured in this way. Clean cantilevers are used to probe a solution with and without [Fe(CN)6]3−/4− redox couple present, demonstrating the combined voltammetric and deflection readout.

In this work we have used simple microfabrication techniques and chemical de-alloying of co-sputtered AgAu alloys to create nanoporous gold (np-Au) electrodes. The physical properties of the np-Au electrodes were investigated using scanning electron microscopy with energy dispersive X-ray analysis, X-ray photo-electron spectroscopy and profilometer. The electrochemical performance of the np-Au electrodes was measured by cyclic voltammetry and electrochemical impedance spectroscopy. We have fabricated np-Au electrodes with pore sizes between 10nm and 60nm, directly related to the Ag:Au ratio. The electrochemical results reveal that np-Au electrodes have much lower impedance than the conventional Au electrodes, due to the significantly higher surface area to volume ratio of np-Au. The np-Au electrodes made from Ag"6"6Au"3"4 and Ag"6"0Au"4"0 show more than 10-fold magnitude reduction in impedance compared to conventional Au electrodes. These results show that np-Au electrodes have a great potential for electrochemical applications.

This work investigates methods for obtaining reliably clean gold film surfaces. Nine gold cleaning methods are investigated here: UV ozone photoreactor; potassium hydroxide-hydrogen peroxide; potassium hydroxide potential sweep; sulfuric acid hydrogen peroxide; sulfuric acid potential cycling; hydrochloric acid potential cycling; dimethylamine borane reducing agent solutions at 25 and 65^oC; and a dilute form of Aqua Regia. Peak-current potential-differences obtained from cyclic voltammetry and charge transfer resistance obtained from electrochemical impedance spectroscopy, as well as X-ray photo-electron spectroscopy are used to characterize surface cleanliness. A low peak-current potential-difference and charge transfer resistance indicates a cleaner surface, as does a higher percentage of elemental gold on the electrode surface. The potassium hydroxide potential sweep method is found to leave the gold surface the cleanest overall.