Impact of a microfluidic jet on a pendant droplet.

High speed microfluidic jets can be generated by a thermocavitation process: from the evaporation of the liquid inside a microfluidic channel, a rapidly expanding bubble is formed and generates a jet through a flow focusing effect. Here, we study the impact and traversing of such jets on a pendant liquid droplet. Upon impact, an expanding cavity is created, and, above a critical impact velocity, the jet traverses the entire droplet. We predict the critical traversing velocity (i) from a simple energy balance and (ii) by comparing the Young-Laplace and dynamic pressures in the cavity that is created during the impact. We contrast the model predictions against experiments, in which we vary the liquid properties of the pendant droplet and find good agreement. In addition, we assess how surfactants and viscoelastic effects influence the critical impact velocity. Our results increase the knowledge of the jet interaction with materials of well-known physical properties.

Multidirectional In Vivo Characterization of Skin Using Wiener Nonlinear Stochastic System Identification Techniques.

A triaxial force-sensitive microrobot was developed to dynamically perturb skin in multiple deformation modes, in vivo. Wiener static nonlinear identification was used to extract the linear dynamics and static nonlinearity of the force-displacement behavior of skin. Stochastic input forces were applied to the volar forearm and thenar eminence of the hand, producing probe tip perturbations in indentation and tangential extension. Wiener static nonlinear approaches reproduced the resulting displacements with variances accounted for (VAF) ranging 94-97%, indicating a good fit to the data. These approaches provided VAF improvements of 0.1-3.4% over linear models. Thenar eminence stiffness measures were approximately twice those measured on the forearm. Damping was shown to be significantly higher on the palm, whereas the perturbed mass typically was lower. Coefficients of variation (CVs) for nonlinear parameters were assessed within and across individuals. Individual CVs ranged from 2% to 11% for indentation and from 2% to 19% for extension. Stochastic perturbations with incrementally increasing mean amplitudes were applied to the same test areas. Differences between full-scale and incremental reduced-scale perturbations were investigated. Different incremental preloading schemes were investigated. However, no significant difference in parameters was found between different incremental preloading schemes. Incremental schemes provided depth-dependent estimates of stiffness and damping, ranging from 300 N/m and 2 Ns/m, respectively, at the surface to 5 kN/m and 50 Ns/m at greater depths. The device and techniques used in this research have potential applications in areas, such as evaluating skincare products, assessing skin hydration, or analyzing wound healing.

Nonidealities in CO2 Electroreduction Mechanisms Revealed by Automation-Assisted Kinetic Analysis.

In electrocatalysis, mechanistic analysis of reaction rate data often relies on the linearization of relatively simple rate equations; this is the basis for typical Tafel and reactant order dependence analyses. However, for more complex reaction phenomena, such as surface coverage effects or mixed control, these common linearization strategies will yield incomplete or uninterpretable results. Cohesive kinetic analysis, which is often used in thermocatalysis and involves quantitative model fitting for data collected over a wide range of reaction conditions, requires more data but also provides a more robust strategy for interrogating reaction mechanisms. In this work, we report a robotic system that improves the experimental workflow for collecting electrochemical rate data by automating sequential testing of up to 10 electrochemical cells, where each cell can have a different electrode, electrolyte, gas-phase reactant composition, and applied voltage. We used this system to investigate the mechanism of carbon dioxide electroreduction to carbon monoxide at several immobilized metal tetrapyrroles. Specifically, at cobalt phthalocyanine (CoPc), cobalt tetraphenylporphyrin (CoTPP), and iron phthalocyanine (FePc), we see signatures of complex reaction mechanisms, where observed bicarbonate and CO2 order dependences change with applied potential. We illustrate how phenomena such as electrolyte poisoning and potential-dependent degrees of rate control can explain the observed kinetic behaviors. Our mechanistic analysis suggests that CoPc and CoTPP share a similar reaction mechanism, akin to one previously proposed, whereas the mechanism for FePc likely involves a species later in the catalytic cycle as the most abundant reactive intermediate. Our study illustrates that complex reaction mechanisms that are not amenable to common Tafel and order dependence analyses may be quite prevalent across this class of immobilized metal tetrapyrrole electrocatalysts.

Self-assembled, nanostructured polypyrrole films grown in a high-gravity environment.

A simple, novel method of synthesizing self-assembled, nanostructured conducting polymer films has been developed. Applying an increased centrifugal force on the electrodes during the electrochemical deposition process yields high surface area, micro- or nanostructured polymer films. Scanning electron microscopy showed that as the applied g-force increased, the polymers progressed from having smooth, "cauliflower" morphologies, to intermediate microstructured surfaces, to finally dense nanostructured surfaces with pore sizes as small as 50 nm. Cyclic voltammetry revealed that films grown at higher centrifugal accelerations (higher than 500g) exhibited less degradation after electrochemical cycling and more capacitive behavior.

A superhydrophobic to superhydrophilic in situ wettability switch of microstructured polypyrrole surfaces.

We present an electrochemical layered system that allows for the fast, in situ wettability switch of microstructured PPy upon the application of an electric stimulus. We have eliminated the need for PPy to be immersed in an electrolyte to switch between wetting states, laying the groundwork for PPy to be used as a viable material in many applications, including microfluidics or smart textiles. The PPy surface was switched from the superhydrophobic state (contact angle=159) to the superhydrophilic state (contact angle=0) in 3 s. A wettability gradient was also created on a PPy surface using the layered system, causing a 3 µL droplet to travel approximately 2 mm in 0.8 s.

The effect of fin ray flexural rigidity on the propulsive forces generated by a biorobotic fish pectoral fin.

A biorobotic pectoral fin was developed and used to study how the flexural rigidities of fin rays within a highly deformable fish fin affect the fin's propulsive forces. The design of the biorobotic fin was based on a detailed analysis of the pectoral fin of the bluegill sunfish (Lepomis macrochirus). The biorobotic fin was made to execute the kinematics used by the biological fin during steady swimming, and to have structural properties that modeled those of the biological fin. This resulted in an engineered fin that had a similar interaction with the water as the biological fin and that created close approximations of the three-dimensional motions, flows, and forces produced by the sunfish during low speed, steady swimming. Experimental trials were conducted during which biorobotic fins of seven different stiffness configurations were flapped at frequencies from 0.5 to 2.0 Hz in flows with velocities that ranged from 0 to 270 mm s(-1). During these trials, thrust and lift forces were measured, kinematics were recorded in three dimensions, and digital particle image velocimetry was used to evaluate flow hydrodynamics. The results of the trials revealed that slight changes to the fin's mechanical properties or to the operating conditions can have significant impact on the direction, magnitude and time course of the propulsive forces. In general, the magnitude of the 2-D (thrust and lift) propulsive force scaled with fin ray stiffness, and increased as the fin's flapping speed increased or as the velocity of the flow decreased.

Actuation of untethered pneumatic artificial muscles and soft robots using magnetically induced liquid-to-gas phase transitions.

Pneumatic artificial muscles have been widely used in industry because of their simple and relatively high-performance design. The emerging field of soft robotics has also been using pneumatic actuation mechanisms since its formation. However, these actuators/soft robots often require bulky peripheral components to operate. Here, we report a simple mechanism and design for actuating pneumatic artificial muscles and soft robotic grippers without the use of compressors, valves, or pressurized gas tanks. The actuation mechanism involves a magnetically induced liquid-to-gas phase transition of a liquid that assists the formation of pressure inside the artificial muscle. The volumetric expansion in the liquid-to-gas phase transition develops sufficient pressure inside the muscle for mechanical operations. We integrated this actuation mechanism into a McKibben-type artificial muscle and soft robotic arms. The untethered McKibben artificial muscle generated actuation strains of up to 20% (in 10 seconds) with associated work density of 40 kilojoules/meter3, which favorably compares with the peak strain and peak energy density of skeletal muscle. The untethered soft robotic arms demonstrated lifting objects with an input energy supply from only two Li-ion batteries.

Dynamic Fluid-Like Graphene with Ultralow Frictional Molecular Bearing.

Fluid-like sliding graphenes but with solid-like out-of-plane compressive rigidity offer unique opportunities for achieving unusual physical and chemical properties for next-generation interfacial technologies. Of particular interest in the present study are graphenes with specific chemical functionalization that can predictably promote adhesion and wetting to substrate and ultralow frictional sliding structures. Lubricity between stainless steel (SS) and diamond-like carbon (DLC) is experimentally demonstrated with densely functionalized graphenes displaying dynamic intersheet bonds that mechanically transform into stable tribolayers. The macroscopic lubricity evolves through the formation of a thin film of an interconnected graphene matrix that provides a coefficient of friction (COF) of 0.01. Mechanical sliding generates complex folded graphene structures wherein equilibrated covalent chemical linkages impart rigidity and stability to the films examined in macroscopic friction tests. This new approach to frictional reduction has broad implications for manufacturing, transportation, and aerospace.

Artificial Muscles: Mechanisms, Applications, and Challenges.

The area of artificial muscle is a highly interdisciplinary field of research that has evolved rapidly in the last 30 years. Recent advances in nanomaterial fabrication and characterization, specifically carbon nanotubes and nanowires, have had major contributions in the development of artificial muscles. However, what can artificial muscles really do for humans? This question is considered here by first examining nature's solutions to this design problem and then discussing the structure, actuation mechanism, applications, and limitations of recently developed artificial muscles, including highly oriented semicrystalline polymer fibers; nanocomposite actuators; twisted nanofiber yarns; thermally activated shape-memory alloys; ionic-polymer/metal composites; dielectric-elastomer actuators; conducting polymers; stimuli-responsive gels; piezoelectric, electrostrictive, magnetostrictive, and photostrictive actuators; photoexcited actuators; electrostatic actuators; and pneumatic actuators.

Vertically Aligned Niobium Nanowire Arrays for Fast-Charging Micro-Supercapacitors.

Planar micro-supercapacitors are attractive for system on chip technologies and surface mount devices due to their large areal capacitance and energy/power density compared to the traditional oxide-based capacitors. In the present work, a novel material, niobium nanowires, in form of vertically aligned electrodes for application in high performance planar micro-supercapacitors is introduced. Specific capacitance of up to 1 kF m-2 (100 mF cm-2 ) with peak energy and power density of 2 kJ m-2 (6.2 MJ m-3 or 1.7 mWh cm-3 ) and 150 kW m-2 (480 MW m-3 or 480 W cm-3 ), respectively, is achieved. This remarkable power density, originating from the extremely low equivalent series resistance value of 0.27 Ω (2.49 µΩ m2 or 24.9 mΩ cm2 ) and large specific capacitance, is among the highest for planar micro-supercapacitors electrodes made of nanomaterials.

Fast Torsional Artificial Muscles from NiTi Twisted Yarns.

Torsional artificial muscles made of multiwalled carbon nanotube/niobium nanowire yarns have shown remarkable torsional speed and gravimetric torque. The muscle structure consists of a twisted yarn with half of its length infiltrated with a stimuli-responsive guest material such as paraffin wax. The volumetric expansion of the guest material creates the torsional actuation in the yarn. In the present work, we show that this type of actuation is not unique to wax-infiltrated carbon multiwalled nanotube (MWCNT) or niobium nanowire yarns and that twisted yarn of NiTi alloy fibers also produces fast torsional actuation. By gold-plating half the length of a NiTi twisted yarn and Joule heating it, we achieved a fully reversible torsional actuation of up to 16°/mm with peak torsional speed of 10 500 rpm and gravimetric torque of 8 N·m/kg. These results favorably compare to those of MWCNTs and niobium nanowire yarns.

Multidirectional Artificial Muscles from Nylon.

Multidirectional artificial muscles are made from highly oriented nylon filaments. Thanks to the low thermal conductivity of nylon and its anisotropic thermal expansion, bending occurs when a nylon beam is differentially heated. This heat can be generated via a Joule heating mechanism or high power laser pulses.

Needle-free small-volume liquid injection system powered by a rotary actuator.

Controllable jet injection could provide a fast, repeatable method for delivery of small volume, high viscosity drugs to the dermis. We have developed a compact, lightweight jet injector that uses a small commercial rotary motor and differential screw to create a jet of fluid with sufficient force to deliver drug into tissue. Clear tissue analogue and ex vivo tissue were used to demonstrate repeatable delivery of <;10 µL of fluid using the device. The fluid when forced through a narrow orifice (108 µm) reaches a velocity of >250 m/s with an injection time of <;3 ms.

Delivery of immunoreactive antigen using a controllable needle-free jet injector.

Intradermal immunization of mice against hepatitis B surface antigen (HBsAg) using a novel real-time controlled jet injector was assessed by comparison with intradermal and subcutaneous injection of antigen using a 27G needle and syringe. Three doses of aluminium-absorbed HBsAg were delivered at 0, 14, and 28days. Antibodies to HBsAg were detected only in mice injected with antigen with antibody levels increasing with secondary injections. Mice vaccinated by intradermal injection using the jet injector or subcutaneous needle injection exhibited comparable immune responses at day 47. Differences in titer observed between intradermal jet injected and needle injected animals reflect differences in the volume of antigen delivered. With the exception of minor bleeding at the injection site in a few animals injected either by jet injection or needle, no adverse events were observed in any of the mice used in the study.

High-speed X-ray analysis of liquid delivery during jet injection.

The effect of varying velocity during jet injection on the dispersion of fluid into tissue is investigated using a custom-built X-ray imaging system. Injections are performed into ex-vivo porcine abdominal tissue using a voice coil actuated injection device. Single velocity and two-phase velocity injections reveal the complex nature of the dispersion of the fluid jet in layered tissue and highlight the effects of changing the jet velocity following the initial penetration of the liquid into the tissue.

Analysis of Moving-Coil Actuator Jet Injectors for Viscous Fluids.

OBJECTIVE: A jet injector is a device that can be used to deliver liquid drugs through the skin using a fluid jet, without the use of a needle. Most jet injectors are designed and used for the delivery of inviscid liquids, and are not optimized for the delivery of viscous drug compounds. To better understand the requirements for delivering viscous drugs, we have developed a mathematical model of the electromechanics of a moving-coil actuated jet injector as it delivers viscous fluids. METHODS: The model builds upon previous work by incorporating the nonlinear electrical properties of the motor, compliant elements of the mechanical piston and ampoule system, and the effect of viscosity on injector characteristics. The model has been validated by monitoring the movement of the piston tip and measurements of the jet force. RESULTS: The results of the model indicate that the jet speed is diminished with increasing fluid viscosity, but overshoot and ringing in the jet speed is unaffected. However, a stiffer ampoule and piston will allow for a better control of the jet speed profile during an injection, and reduce ringing. CONCLUSION: We identified that the piston friction coefficient, the compliance of the injector components, and the viscous properties of the fluid are important determinants of performance when jet-injecting viscous fluids. SIGNIFICANCE: By expanding upon previous jet injector models, this study has provided informative simulations of jet injector characteristics and performance. The model can be used to guide the design of future jet injectors for viscous fluids.

Adaptive controller for a needle free jet-injector system.

A nonlinear, sliding mode adaptive controller was created for a needle-free jet injection system. The controller was based on a simplified lumped-sum parameter model of the jet-injection mechanics. The adaptive control scheme was compared to a currently-used Feed-forward+PID controller in both ejection of water into air, and injection of dye into ex-vivo porcine tissue. The adaptive controller was more successful in trajectory tracking and was more robust to the biological variations caused by a tissue load.

A needle-free technique for interstitial fluid sample acquisition using a lorentz-force actuated jet injector.

We present a novel method of quickly acquiring dermal interstitial fluid (ISF) samples using a Lorentz-force actuated needle-free jet injector. The feasibility of the method is first demonstrated on post-mortem porcine tissue. The jet injector is used to first inject a small volume of physiological saline to breach the skin, and the back-drivability of the actuator is utilized to create negative pressure in the ampoule and collect ISF. The effect of the injection and extraction parameters on sample dilution and extracted volumes is investigated. A simple finite element model is developed to demonstrate why this acquisition method results in faster extractions than conventional sampling methods. Using this method, we are able to collect a sample that contains up to 3.5% ISF in 3.1s from post-mortem skin. The trends revealed from experimentation on post-mortem skin are then used to identify the parameters for a live animal study. The feasibility of the acquisition process is successfully demonstrated using live rats; the process is revealed to extract samples that have been diluted by a factor of 111-125.

The effect of jet shape on jet injection.

The effects of the dispersion pattern of a needle-free jet injector are explored. The shape of the jets were compared using a high-speed video camera and jet injections of collimated and dispersed fluid jets with a Lorentz-force actuated jet injector were made into acrylamide gel and post-mortem porcine tissue. A custom-built high-speed X-ray imaging system was used in order to observe the dynamics of the dispersion mechanism for each injection in real time. We show that a collimated jet stream results in greater tissue penetration than a dispersed jet stream.

Needle-free delivery of macromolecules through the skin using controllable jet injectors.

INTRODUCTION: Transdermal delivery of drugs has a number of advantages in comparison to other routes of administration. The mechanical properties of skin, however, impose a barrier to administration and so most compounds are administered using hypodermic needles and syringes. In order to overcome some of the issues associated with the use of needles, a variety of non-needle devices based on jet injection technology has been developed. AREAS COVERED: Jet injection has been used primarily for vaccine administration but has also been used to deliver macromolecules such as hormones, monoclonal antibodies and nucleic acids. A critical component in the more recent success of jet injection technology has been the active control of pressure applied to the drug during the time course of injection. EXPERT OPINION: Jet injection systems that are electronically controllable and reversible offer significant advantages over conventional injection systems. These devices can consistently create the high pressures and jet speeds necessary to penetrate tissue and then transition smoothly to a lower jet speed for delivery of the remainder of the desired dose. It seems likely that in the future this work will result in smart drug delivery systems incorporated into personal medical devices and medical robots for in-home disease management and healthcare.