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A facile gravity-induced sliding droplets method is reported for deposition of nanoliter sized droplets on hydrophilic/superhydrophobic patterned surface. The deposition process is parallel where multiple different liquids can be deposited simultaneously. The process is also high-throughput, having a great potential to be scaled up by increasing the size of the substrate.

Targeting good user experiences, softness and stretchability are essential features for epidermal devices in body signal monitoring and body area stimulation. A highly soft, stretchable and sticky polydimethylsiloxane based elastomer (S3-PDMS) is achieved by a simple process with a widely used siloxane precursors, the properties of which are tuned by adding small fractions of an amine-based polymer, ethoxylated polyethylenimine (EPEI). This allows formation of a thick unobstrusive patch and may ease implementation of epidermal electronics in wearable healthcare applications.

A tunable process of polydimethylsiloxane (PDMS) has been created with a tiny amount of a polyethylenimine, ethoxylated (PEIE) solution. With varying the PEIE fraction, the softness, stretchablility and stickiness of a commercial PDMS (Sylgard®184) are enhanced at the same time. We controlled the curing process of the tuned PDMS to achieve desired mechanical properties. A soft strain sensor for finger movement monitoring is successfully demonstrated with a liquid alloy resistor in the soft, stretchable and sticky (S3-PDMS) packaging.

Unique and beneficial gallium based liquid alloys [1,2] have high potential to a compliant sensor,power source and wireless communication unit. There are several types of patterning techniques forliquid alloys have been reported such as injection [3], printing [4], selective wetting [5], stamping [6]and direct writing [7]. Our recently developed liquid alloy printing technique employing atomizationand tape transfer masking for batch type and large area fabrication is practically useful for micrometerscale conductor patterning to realize a compliant microsystem. The process provides a quick and easyfabrication scheme for soft and stretchable, liquid alloy applied microsystems.On the other hand, an elastomer as a packaging material has an important role in compliantmicrosystem performance. Polydimethylsiloxane (PDMS) based elastomer and silicone have beenwidely used due to their good processability with soft-lithography and versatile behaviors such asstretchability, inertness and transparency for microfluidics, soft robotics and stretchable electronics.Still, the PDMS based elastomer needs to be tuned in many properties, in order to meet requirementsof applications. Mechanical properties tuning of the PDMS based elastomers have been reported withdifferent approaches [10,11,12] to make more compliant and stretchable one. Here, we present ourrecently developed multi-functional elastomers, which are based on PDMS. One is a thermalelastomer composite (TEC) and the other is a soft, stretchable and sticky PDMS based elastomer(S3-PDMS). The former is useful for stretchable thermal systems such as a temperature sensor or athermoelectric generator to achieve higher sensitivity and higher efficiency. Nanometer scale liquidalloy droplet fillers to tune the elastomer to be softer or stiffer can be dispersed in the PDMS basedelastomer by high speed mixing. The thermal conductivity reaches twelve fold higher than the originalelastomer with keeping 100% stretchability. The latter is

In order to overcome the current limitation in the resolution of the previous developed, liquid alloy patterning method, a shrinking substrate is, for the first time, employed. The current printing technique combining spraying a liquid alloy and transferring a tape mask can be processed with a 100 μm level resolution. This is limited by the resolution of a mask fabricated by using a cutting plotter. By shrinking a substrate, a smaller scale of the liquid alloy pattern can be created without changing the current printing process with a tape mask, which has the advantage of quick and easy processing. We have found that the shrinking process by relaxing a radially 200% pre-strained substrate makes to increases twofold in the resolution of the liquid alloy line pattern.

A liquid alloy allows large strain due to its liquid state in a stretchable and deformable system and thus,a resistive strain sensor made of liquid alloy can measure large strain of human motion. Liquid alloy-based strainsensors have been demonstrated with various designs for a better understanding of human motion dynamics and toimprove the design of wearable robotics. So far, the sensors have a large size of several centimeters.To expand the applicability of liquid alloy-based strain sensors, a high resolution patterning technique would enable asmall size and high density of the strain sensor. This allows strain measurements in area-limited situations ordirectional strain measurement with high density arrays. A small-size, high-density stretchable strain sensor array canbenefit many DOF (degree of freedom) dynamics measurements on, e.g., a shoulder, hand, foot, or neck.For realizing a highly stretchable strain sensor, a novel soft, highly stretchable and sticky elastomer was used toencapsulate the liquid alloy. A quick and simple patterning technique for high density design has been developed. Thiscombined our previously developed spray printing technique of the liquid alloy with a stretched elastomer substrateand encapsulation of it. The substrate could be stretched either one dimensionally or radially, depending on thedesign.High resolution patterns that were radially stretched and uniformly shrunken on an elastomer substrate weresuccessfully achieved with the stretch-shrink-printing technique. This technique enables large area fabrication withone step processing. High density patterns that have a low resistance and large stretchability were realized with theliquid alloy for a small and highly stretchable strain sensor. A high density array of small strain sensors wasdemonstrated in measuring strains in different directions of neck movements of the human body.

A microscale ceramic high-temperature combustor with a built-in temperature sensor and source of oxygen has been designed, manufactured and characterized. The successful in situ electroplating and oxidation of copper, and the use of copper oxide as the source of oxygen were demonstrated. It was shown that residual stresses from electroplating, copper oxidation and oxide decomposition did not cause much deformation of the substrate but influenced mainly the integrity and adhesion of the metal films. The process had influence on the electrical resistances, however. Calibration of the temperature sensor and correlation with IR thermography up to 1000°C revealed a nearly linear sensor behavior. Demonstration of combustion in a vacuum chamber proved that no combustion had occurred before release of oxygen from the metal oxide resource.

Supercritical carbon dioxide (scCO2) is often used to replace harmful solvents and can dissolve a wide range of organic compounds. With a favorable critical point at 31 °C and 7.4 MPa, reaching above the critical point for scCO2 is fairly accessible. Because of the compressible nature of scCO2 and the large changes of viscosity and density with temperature and pressure, there is a need to determine the behavior of scCO2 in microfluidic systems. Here, the influence of how parameters such as flow rate, temperature, pressure, and flow ratio affects the length of parallel flow of water and scCO2 and the length of the created CO2 segments are investigated and modeled using multivariate data analysis for a 10 mm long double-y channel. The parallel length and segment size were observed in the laminar regime around and above the critical point of CO2. The flow ratio between the two fluids together with the flow rate influenced both the parallel length and the segment sizes, and a higher pressure resulted in shorter parallel lengths. Regarding the segment length of CO2, longer segments were a result of a higher Weber number for H2O together with a higher temperature in the channel.

Fast photoswitching of wetting properties is important for the development of micro/nanofluidic systems and lab-on-a-chip devices. Here, we show how structuring the surface amplifies photoswitching properties. Atomic layer-deposited titanium dioxide (TiO2) has phototunable hydrophilic properties due to its surface chemistry, but microscale overhang pillars and additional nanoscale topography can override the chemistry and make the surface sup erhydrophobic. Three switching processes are achieved simply by controlling the UV exposure time: from (1) rolling superhydrophobic to sticky superhydrophobic (Cassie-Baxter to Wenzel), (2) sup erhydrophobic to hydrophilic, and (3) superhydrophobic to superhydrophilic after 1, 5, and 10 min of UV exposure, respectively. We report the fastest reversible switching to date: 1 min of UV exposure is enough to promote a rolling-to-sticky transition, and mild heating (30 min at 60 degrees C) is sufficient for recovery. This performance is caused by a combination of the photoswitching properties of TiO2, the micropillar overhang geometry, and surface nanostructuring. We demonstrate that the switching also can be performed locally by introducing microwriting under a water droplet., Funding: Academy of Finland (PROWET project nos. 263538, 263560, and 266820)

A production scalable technique is presented to make hemispherical coil antennas by using a stretchable printed silver paste conductor and plastic thermoforming. To ease the fabrication process an unbalanced feed-structure was designed for solderless mounting on conductive materials. The manufactured antenna had a resonance frequency of 2.467 GHz with a reflection coefficient of -33.8 dB. The measured and simulated radiation patterns corresponded to that of monopole structure and the measured efficiency was 40%.

Recently, the shrinking of the personal computer market has given a clear signal that it is time to divert our focus from the strategy of miniaturization of transistors to a different strategy with emerging technologies. As a new form of electronics, stretchable electronics has significantly advanced in the past few years by micro/nanofabrication of thin films of traditional stiff and hard materials such as silicon, metals, and ceramics, and especially subsequent transfer process on an elastic substrate. However, such a thin structure often suffers from high resistance that leads to low performance when long structures are required. This is particularly true for antennas in radio-frequency (RF) electronics. By introducing microfluidics into RF electronics, we found out that it was an excellent way to make high-performance stretchable RF electronics. Apart from antennas, the microfluidic approach was also adopted and further developed to various devices with integrated wireless communication. This fusion of microfluidics with RF electronics brings not only a lot of opportunities for researchers as a radically new research field, but also potentially commercial benefits for industry. As a new emerging field, a huge effort, ranging from fundamental science to technology development, is required to realize it. This paper illustrates the fundamentals in processing and relevant applications, and highlights recent advances in microfluidic RF electronics. The authors would like to inspire the electronics community to further exploit the advantages of this approach and accelerate innovations in this field.

Stretchable electronics offers unsurpassed mechanical compliance on complex or soft surfaces like the human skin and organs. To fully exploit this great advantage, an autonomous system with a self-powered energy source has been sought for. Here, we present a new technology to pattern liquid alloys on soft substrates, targeting at fabrication of a hybrid-integrated power source in microfluidic stretchable electronics. By atomized spraying of a liquid alloy onto a soft surface with a tape transferred adhesive mask, a universal fabrication process is provided for high quality patterns of liquid conductors in a meter scale. With the developed multilayer fabrication technique, a microfluidic stretchable wireless power transfer device with an integrated LED was demonstrated, which could survive cycling between 0% and 25% strain over 1,000 times.

There is growing interest in using microdialysis (MD) for monitoring larger and more complexmolecules such as neuropeptides and proteins. This promotes the use of MD membranes withmolecular weight cut off (MWCO) of 100 kDa. Hence, the hydrodynamic property of themembrane goes to ultrafiltration, making the sampling more sensitive to pressure changes. Also,despite the large membrane pore size, studies have shown that membrane biofouling still leads tounstable catheter performance. Our objective is to study in vitro how four kinds of surfacemodifications (Pluronic L31, L44, F87 and F127+L31) affect the fluid recovery (FR) andextraction efficiency (EE) of 100 kDa MWCO MD catheters, under controlled pressure. Apressure chamber was employed to facilitate the tests, using as MD sample a protein standardwith proteins of similar concentrations as in human cerebral spinal fluid. The collected dialysatefractions were examined for FR and EE. Targeted mass spectrometry analysed the EE ofindividual proteins and peptides. The thicker the pluronic adsorption layer, the less thehydrodynamic diameter of the membrane pores, leading to lower and more stable FR. The foursurface modifications had three different behaviours: Pluronic F127 + L31 showed similarbehavior to the Pluronic F127 and the native original membrane; Pluronic F87 showed acontinuous EE increase with pressure; Pluronic L31 and L44 showed similar EE values, whichwere stable with pressure. Different surface modifications are clearly selective to differentproteins and peptides. We conclude that a pluronic surface modification could provide MDsampling with more stable FR, and more stable or enhanced EE with high FR, depending on theobjective of the sampling.

Alignment of microchips with receptors is an important process step in the construction of integrated micro- and nanosystems for emerging technologies, and facilitating alignment by spontaneous self-assembly processes is highly desired. Previously, capillary self-alignment of microchips driven by surface tension effects on patterned surfaces has been reported, where it was essential for microchips to have sufficient overlap with receptor sites. Here we demonstrate for the first time capillary self-transport and self-alignment of microchips, where microchips are initially placed outside the corresponding receptor sites and can be self-transported by capillary force to the receptor sites followed by self-alignment. The surface consists of hydrophilic silicon receptor sites surrounded by superhydrophobic black silicon. Rain-induced microscopic droplets are used to form the meniscus for the self-transport and self-alignment. The boundary conditions for the self-transport have been explored by modeling and confirmed experimentally. The maximum permitted gap between a microchip and a receptor site is determined by the volume of the liquid and by the wetting contrast between receptor site and substrate. Microscopic rain applied on hydrophilic-superhydrophobic patterned surfaces greatly improves the capability, reliability and error-tolerance of the process, avoiding the need for accurate initial placement of microchips, and thereby greatly simplifying the alignment process.

Stretchable electronics and soft robotics have shown unsurpassed features, inheriting remarkable functions from stretchable and soft materials. Electrically conductive and mechanically stretchable materials based on composites have been widely studied for stretchable electronics as electrical conductors using various combinations of materials. However, thermally tunable and stretchable materials, which have high potential in soft and stretchable thermal devices as interface or packaging materials, have not been sufficiently studied. Here, a mechanically stretchable and electrically insulating thermal elastomer composite is demonstrated, which can be easily processed for device fabrication. A liquid alloy is embedded as liquid droplet fillers in an elastomer matrix to achieve softness and stretchability. This new elastomer composite is expected useful to enhance thermal response or efficiency of soft and stretchable thermal devices or systems. The thermal elastomer composites demonstrate advantages such as thermal interface and packaging layers with thermal shrink films in transient and steady-state cases and a stretchable temperature sensor.

There is a need for thermal elastomer composites (TEC) which are stretchable, electrically insulating and easily processablefor soft and stretchable sensor or actuator systems as a thermal conductor or heat spreader at an interface or in a package.A novel TEC was made by embedding a gallium based liquid alloy (Galinstan) as a droplet in polydimethylsiloxane (PDMS,Elastosil RT 601) matrix with a high speed mechanical mixing process.

Epidermal electronics and soft robotics are harnessing the advantages of the adaptable and compliantsoft contacts of soft materials. Soft materials can conformally match the morphology of the contact surfaces, which isthe vital point in the implementation of epidermal sensors or soft actuators (S. Xu et al., Soft Microfluidic Assemblies ofSensors, Circuits, and Radios for the Skin, Science, 2014, 344, 70-74). Unfortunately, this conformality can suffer fromdelamination or air trapping at the interface during contact movement. Here, adhesion of soft material surfaces is thecritical parameter. For example, an epidermal sensor on human internal organs and skin, or soft-robotic fingers forgrabbing or climbing needs proper adhesion to its targeted contact surface. Mechanical softness of elastomermaterials provides a good ensemble with surface adhesion because the conformal contact of the soft materials assiststhe adhesion on the target surfaces. Alas, when the soft device is thicker, with its inherent adhesion its compliancemay not suffice but an adhesive layer is needed to ensure good contact.Sticky surfaces of soft materials will significantly help to improve adhesion on target surfaces by preventing sliding.Therefore, more reliable immobilization and manipulation of contacted objects can be secured. Physical and chemicaladhesion forces of the soft material surfaces can be utilized for this purpose. We have developed a sticky elastomercomposite based on PDMS, which has a tape-like adhesive surface after curing. This sticky elastomer composite isstretchable and compliant. The processability of it is compatible with PDMS processes for microfluidic stretchabledevices. It can be easily shaped with laminating, spinning and casting before curing. And, it is reusable several timeswithout leaving residues on the adhered surfaces after detaching and its adhesive strength is tunable with differentmixing ratios with the additive.The sticky elastomer composite showed high enough a

This work focuses on desalination of a protein sample in a lab-on-chip device using the ion concentration polarization (ICP) technique. It was demonstrated with a salt containing buffer with four proteins and two peptides of concentrations typical to cerebrospinal fluid (CSF). Not only was the output desalinated but its protein concentration with large molecular weight (MW) was as much as 3 times higher for the largest protein compared to the original. We conclude that ICP based microfluidic chips have great potential for desalination and protein concentration in microdialysis sampling coupled to mass spectroscopy (MS).

This study reports a novel approach to quantitatively investigate the antibacterial effect of antibiotics on bacteria using a three-dimensional microfluidic culture device. In particular, our approach is suitable for studying the pharmacodynamics effects of antibiotics on bacterial cells temporally and with a continuous range of concentrations in a single experiment. The responses of bacterial cells to a linear concentration gradient of antibiotics were observed using time-lapse photography, by encapsulating bacterial cells in an agarose-based gel located in a commercially available microfluidics chamber. This approach generates dynamic information with high resolution, in a single operation, e. g., growth curves and antibiotic pharmacodynamics, in a well-controlled environment. No pre-labelling of the cells is needed and therefore any bacterial sample can be tested in this setup. It also provides static information comparable to that of standard techniques for measuring minimum inhibitory concentration (MIC). Five antibiotics with different mechanisms were analysed against wild-type Escherichia coli, Staphylococcus aureus and Salmonella Typhimurium. The entire process, including data analysis, took 2.5-4 h and from the same analysis, high-resolution growth curves were obtained. As a proof of principle, a pharmacodynamic model of streptomycin against Salmonella Typhimurium was built based on the maximal effect model, which agreed well with the experimental results. Our approach has the potential to be a simple and flexible solution to study responding behaviours of microbial cells under different selection pressures both temporally and in a range of concentrations.

The use of supercritical carbon dioxide (scCO2) as an apolar solvent has been known for decades. It offers a greener approach than, e.g., hexane or chloroform, when such solvents are needed. The use of scCO2 in microsystems, however, has only recently started to attract attention. In microfluidics, the flow characteristics need to be known to be able to successfully design such components and systems. As supercritical fluids exhibit the exciting combination of low viscosity, high density, and high diffusion rates, the fluidic behavior is not directly transferrable from aqueous systems. In this paper, three flow regimes in the scCO2–liquid water two-phase microfluidic system have been mapped. The effect of both total flow rate and relative flow rate on the flow regime is evaluated. Furthermore, the droplet dynamics at the bifurcating exit channel are analyzed at different flow rates. Due to the low viscosity of scCO2, segmented flows were observed even at fairly high flow rates. Furthermore, the carbon dioxide droplet behavior exhibited a clear dependence on both flow rate and droplet length.

This paper presents a reusable microdispenser intended for continuous flow dispensing of variable and controlled volumes of liquid against high back-pressures. The microdispenser consists of two active valves and a dispenser chamber, all actuated by the volume change associated with the solid-to-liquid phase transition of paraffin wax. It is fabricated using stainless steel sheets, a flexible printed circuit board, and a polyimide membrane. All are covered with Parylene C for insulation and fusion bonding at assembly. A finite element method (FEM) model of the paraffin actuator is used to predict the resulting flow characteristics. The results show dispensing of well-defined volumes of 350 and 540 nL, with a good repeatability between dispensing sequences, as well as reproducibility between devices. In addition, the flow characteristics show no back-pressure dependence of the dispensed flow in the interval 0.5--2.0 MPa. The FEM model can be used to predict the flow characteristics qualitatively

Cerebral microdialysis (MD) was introduced as a neurochemical monitoring method in the early 1990s and is currently widely used for the sampling of low molecular weight molecules, signaling energy crisis, and cellular distress in the neurointensive care (NIC) setting. There is a growing interest in MD for harvesting of intracerebral protein biomarkers of secondary injury mechanisms in acute traumatic and neurovascular brain injury in the NIC community. The initial enthusiasm over the opportunity to sample protein biomarkers with high molecular weight cut-off MD catheters has dampened somewhat with the emerging realization of inherent methodological problems including protein-protein interaction, protein adhesion, and biofouling, causing an unstable in vivo performance (i.e., fluid recovery and extraction efficiency) of the MD catheter. This review will focus on the results of a multidisciplinary collaborative effort, within the Uppsala Berzelii Centre for Neurodiagnostics during the past several years, to study the features of the complex process of high molecular weight cut-off MD for protein biomarkers. This research has led to new methodology showing robust in vivo performance with optimized fluid recovery and improved extraction efficiency, allowing for more accurate biomarker monitoring. In combination with evolving analytical methodology allowing for multiplex biomarker analysis in ultra-small MD samples, a new opportunity opens up for high-resolution temporal mapping of secondary injury cascades, such as neuroinflammation and other cell injury reactions directly in the injured human brain. Such data may provide an important basis for improved characterization of complex injuries, e.g., traumatic and neurovascular brain injury, and help in defining targets and treatment windows for neuroprotective drug development.

Restrictor valves allow proportional control of fluid flow but are rarely integrated in microfluidic systems. In this study, an optically actuated silicon membrane restrictor microvalve is demonstrated. Its actuation is based on the phase transition of paraffin, using a paraffin wax mixed with a suitable concentration of optically absorbing nanographite particles. Backing up the membrane with oil (the melted paraffin) allows for a compliant yet strong contact to the valve seat, which enables handling of high pressures. At flow rates up to 30 mu L min(-1) and at a pressure of 2 bars, the valve can successfully be closed and control the flow level by restriction. The use of this paraffin composite as an adhesive layer sandwiched between the silicon valve and glass eases fabrication. This type of restrictor valve is best suited for high pressure, low volume flow silicon-based nanofluidic systems.