Mixing Enhancement in Serpentine Micromixers with a Non-Rectangular Cross-Section

In this numerical study, a new type of serpentine micromixer involving mixing units with a non-rectangular cross-section is investigated. Similar to other serpentine/spiral shaped micromixers, the design exploits the formation of transversal vortices (Dean flows) in pressure-driven systems, associated with the centrifugal forces experienced by the fluid as it is confined to move along curved geometries. In contrast with other previous designs, though, the use of non-rectangular cross-sections that change orientation between mixing units is exploited to control the center of rotation of the transversal flows formed. The associated extensional flows that thus develop between the mixing segments complement the existent rotational flows, leading to a more complex fluid motion. The fluid flow characteristics and associated mixing are determined numerically from computational solutions to Navier–Stokes equations and the concentration-diffusion equation. It is found that the performance of the investigated mixers exceeds that of simple serpentine channels with a more consistent behavior at low and high Reynolds numbers. An analysis of the mixing quality using an entropic mixing index indicates that maximum mixing can be achieved at Reynolds numbers as small as 20 in less than four serpentine mixing units

Passive Micromixers

Micro-total analysis systems and lab-on-a-chip platforms are widely used for sample preparation and analysis, drug delivery, and biological and chemical syntheses. A micromixer is an important component in these applications. Rapid and efficient mixing is a challenging task in the design and development of micromixers. The flow in micromixers is laminar, and, thus, the mixing is primarily dominated by diffusion. Recently, diverse techniques have been developed to promote mixing by enlarging the interfacial area between the fluids or by increasing the residential time of fluids in the micromixer. Based on their mixing mechanism, micromixers are classified into two types: active and passive. Passive micromixers are easy to fabricate and generally use geometry modification to cause chaotic advection or lamination to promote the mixing of the fluid samples, unlike active micromixers, which use moving parts or some external agitation/energy for the mixing. Many researchers have studied various geometries to design efficient passive micromixers. Recently, numerical optimization techniques based on computational fluid dynamic analysis have been proven to be efficient tools in the design of micromixers. The current Special Issue covers new mechanisms, design, numerical and/or experimental mixing analysis, and design optimization of various passive micromixers

A Review of Cooperative Actuator and Sensor Systems Based on Dielectric Elastomer Transducers

This paper presents an overview of cooperative actuator and sensor systems based on dielectric elastomer (DE) transducers. A DE consists of a flexible capacitor made of a thin layer of soft dielectric material (e.g., acrylic, silicone) surrounded with a compliant electrode, which is able to work as an actuator or as a sensor. Features such as large deformation, high compliance, flexibility, energy efficiency, lightweight, self-sensing, and low cost make DE technology particularly attractive for the realization of mechatronic systems that are capable of performance not achievable with alternative technologies. If several DEs are arranged in an array-like configuration, new concepts of cooperative actuator/sensor systems can be enabled, in which novel applications and features are made possible by the synergistic operations among nearby elements. The goal of this paper is to review recent advances in the area of cooperative DE systems technology. After summarizing the basic operating principle of DE transducers, several applications of cooperative DE actuators and sensors from the recent literature are discussed, ranging from haptic interfaces and bio-inspired robots to micro-scale devices and tactile sensors. Finally, challenges and perspectives for the future development of cooperative DE systems are discussed

Micro total analysis system for in-situ and autonomous spectrophotometric monitoring of iron in groundwater

We have developed a compact micro total analysis system (μTAS) to serve as a platform for in-situ spectrophotometric water quality monitoring. Individual fluidic, optical, and electrical components were designed, developed, and characterized. These components were combined in both an integrated (single lithographic “chip-based” platform) and a modular manner. The microfluidic components include a modular microfabricated filter chip, a miniature reagent storage bag, an integrated micromixer & absorbance flow cell, and modular micropumps. Optical and electrical components include a miniature spectrometer, a white LED, fiber optics, SMA fiber couplers, a processor, motor & LED drivers, spectrometer circuitry, EEPROM memory, and AAA batteries. Both a partial (fluidics but not all optics or electronics) and complete system were tested in the laboratory, by determining total iron (1,10-orthophenanthroline method), with environmental and surrogate samples. These measurements compared well to results from the same samples measured with a benchtop (cuvette and spectrometer) system (calibration slopes 3.4 x 10-3 AU/μM; 10% difference for an environmental sample). For the complete μTAS, the detection limit for iron in the laboratory samples was estimated to be 1 μM. For the partial system, the RSD of the absorbance of a 38.4 μM standard was 2.8% (n = 13) and the estimated detection limit for a wetland sample was 2 μM. To demonstrate the utility of the device, a groundwater monitoring well, designed both for environmental sampling and device deployment, was installed in a seasonally wet prairie to measure iron. The device was deployed in the well and samples were taken to verify system performance. Due to a software malfunction, the device batteries were drained after 2.5 days of operation. During this time the device performed adequately, although all iron levels were below the detection limits of both the μTAS and the benchtop system. This work is the first demonstration of an in-situ μTAS for water quality monitoring, the first microfluidic measurement of environmental iron, the first use of an internal dye standard to determine the dilution factor, and one of the smallest complete μTAS developed thus far

Analysis, Design and Fabrication of Micromixers, Volume II

Micromixers are an important component in micrototal analysis systems and lab-on-a-chip platforms which are widely used for sample preparation and analysis, drug delivery, and biological and chemical synthesis. The Special Issue "Analysis, Design and Fabrication of Micromixers II" published in Micromachines covers new mechanisms, numerical and/or experimental mixing analysis, design, and fabrication of various micromixers. This reprint includes an editorial, two review papers, and eleven research papers reporting on five active and six passive micromixers. Three of the active micromixers have electrokinetic driving force, but the other two are activated by mechanical mechanism and acoustic streaming. Three studies employs non-Newtonian working fluids, one of which deals with nano-non-Newtonian fluids. Most of the cases investigated micromixer design

Towards rapid 3D direct manufacture of biomechanical microstructures

The field of stereolithography has developed rapidly over the last 20 years, and commercially available systems currently have sufficient resolution for use in microengineering applications. However, they have not as yet been fully exploited in this field. This thesis investigates the possible microengineering applications of microstereolithography systems, specifically in the areas of active microfluidic devices and microneedles. The fields of micropumps and microvalves, stereolithography and microneedles are reviewed, and a variety of test builds were fabricated using the EnvisionTEC Perfactory Mini Multi-Lens stereolithography system in order to define its capabilities. A number of microneedle geometries were considered. This number was narrowed down using finite element modelling, before another simulation was used to optimise these structures. 9 × 9 arrays of 400 μm tall, 300 μm base diameter microneedles were subjected to mechanical testing. Per needle failure forces of 0.263 and 0.243 N were recorded for the selected geometries, stepped cone and inverted trumpet. The 90 μm needle tips were subjected to between 30 and 32 MPa of pressure at their failure point - more than 10 times the required pressure to puncture average human skin. A range of monolithic micropumps were produced with integrated 4 mm diameter single-layer 70 μm-thick membranes used as the basis for a reciprocating displacement operating principle. The membranes were tested using an oscillating pneumatic actuation, and were found reliable (>1,000,000 cycles) up to 2.0 PSIG. Pneumatic single-membrane nozzle/diffuser rectified devices produced flow rates of up to 1,000 μl/min with backpressures of up to 375 Pa. Another device rectified using active membrane valves was found to self-prime, and produced backpressures of up to 4.9 kPa. These devices and structures show great promise for inclusion in complex, fully integrated and active microfluidic systems fabricated using microstereolithography alone, with implications for both cost of manufacture and lead time

Development of Innovative Microfluidic Polymeric Technologies for Point-of-care & Integrated Diagnostics Devices

This thesis presents the development of four different microfluidic technologies that can be used as stand-alone devices or integrated in point-of-care systems. The first technology is a rapid, low-cost, portable microfluidic system for assessing the somatic cell count and fat content of milk in 15 min using a “sample-in, answer-out” approach. The system consists of twelve independent microfluidic devices, essentially flattened funnel structures, fabricated on the footprint of a plastic compact disc (CD). The assay separates cells and fat globules based on their densities (by differential sedimentation), concentrating white cells in the closed-end channel near the outer rim of the CD for estimation of total “cell pellet” volume, while fat globules move toward the center of disc rotation, forming a fat “band” in the funnel. The closed-end channel provides accurate cell counts over the range 50,000 to over 3,000,000 cells per mL. A technique is also presented to recirculate liquids in a microfluidic channel by alternating the predominance of centrifugal and capillary forces. With this technique, liquid volumes of μL to mL can be sampled with many sizes of microfluidic channels that contain only a fraction of the sample at one time, provided the channel wall with greatest surface area is hydrophilic. We present a theoretical model describing the balance of centrifugal and capillary forces in the device and validate the model experimentally. Towards the development of an integrated pathogen identification system, two other technologies are demonstrated and implemented. The design, fabrication, and characterization of a polymer centrifugal microfluidic system for the specific detection of bacterial pathogens is presented. This single-cartridge platform integrates bacteria capture and concentration, supernatant solution removal, lysis, and nucleic-acid sequence-based amplification (NASBA) in a single unit. The unit is fabricated using multilayer lamination and consists of five different polymer layers. Bacteria capture and concentration are accomplished by sedimentation in five minutes. Centrifugation forces also drive the subsequent steps. A wax valve is integrated in the cartridge to enable high-speed centrifugation. Oil is used to prevent evaporation during reactions requiring thermal cycling. Device functionality was demonstrated by real-time detection of E. coli cells from a 200-μL sample. Finally, the laser-printer-based fabrication of pressure-resistant microfluidic single-use valves is reported, along with their implementation on pressure-driven and centrifugal microfluidic platforms. A laser printer is used to selectively deposit toner on a plastic substrate in the form of circular dots. After assembly into a microfluidic device, the valve is opened (melted) with a pulse of laser light. This is an easy approach to connect multiple fluidic levels. This simple technology is compatible with a range of polymer microfabrication technologies and should facilitate the development of fully integrated, (re)configurable, and automated lab-on-a-chip systems, particularly when reagents must be stored on chip for extended periods, e.g. for medical diagnostic devices, lab-on-a-chip synthetic systems, or hazardous bio/chemical analysis platforms

Towards rapid 3D direct manufacture of biomechanical microstructures

The field of stereolithography has developed rapidly over the last 20 years, and commercially available systems currently have sufficient resolution for use in microengineering applications. However, they have not as yet been fully exploited in this field. This thesis investigates the possible microengineering applications of microstereolithography systems, specifically in the areas of active microfluidic devices and microneedles. The fields of micropumps and microvalves, stereolithography and microneedles are reviewed, and a variety of test builds were fabricated using the EnvisionTEC Perfactory Mini Multi-Lens stereolithography system in order to define its capabilities. A number of microneedle geometries were considered. This number was narrowed down using finite element modelling, before another simulation was used to optimise these structures. 9 × 9 arrays of 400 μm tall, 300 μm base diameter microneedles were subjected to mechanical testing. Per needle failure forces of 0.263 and 0.243 N were recorded for the selected geometries, stepped cone and inverted trumpet. The 90 μm needle tips were subjected to between 30 and 32 MPa of pressure at their failure point - more than 10 times the required pressure to puncture average human skin. A range of monolithic micropumps were produced with integrated 4 mm diameter single-layer 70 μm-thick membranes used as the basis for a reciprocating displacement operating principle. The membranes were tested using an oscillating pneumatic actuation, and were found reliable (>1,000,000 cycles) up to 2.0 PSIG. Pneumatic single-membrane nozzle/diffuser rectified devices produced flow rates of up to 1,000 μl/min with backpressures of up to 375 Pa. Another device rectified using active membrane valves was found to self-prime, and produced backpressures of up to 4.9 kPa. These devices and structures show great promise for inclusion in complex, fully integrated and active microfluidic systems fabricated using microstereolithography alone, with implications for both cost of manufacture and lead time.EThOS - Electronic Theses Online ServiceEngineering and Physical Sciences Research Council (EPSRC)GBUnited Kingdo