Biomedical subjects

David J Beebe

Publications and source records attributed to David J Beebe.

At least 19 recordsLinked to original sources

PubMed2006

PDMS absorption of small molecules and consequences in microfluidic applications.

Microfluidic devices made out of polydimethylsiloxane (PDMS) have many physical properties that are useful for cell culture applications, such as transparency and gas permeability. Another distinct characteristic of PDMS is its ability to absorb hydrophobic small molecules. Partitioning of molecules into PDMS can significantly change solution concentrations and could potentially alter experimental outcomes. Herein we discuss PDMS absorption and its potential impact on microfluidic experiments.

Chemical Phenomena↗

PubMed2006

Adaptive liquid microlenses activated by stimuli-responsive hydrogels.

Despite its compactness, the human eye can easily focus on different distances by adjusting the shape of its lens with the help of ciliary muscles. In contrast, traditional man-made optical systems achieve focusing by physical displacement of the lenses used. But in recent years, advances in miniaturization technology have led to optical systems that no longer require complicated mechanical systems to tune and adjust optical performance. These systems have found wide use in photonics, displays and biomedical systems. They are either based on arrays of microlenses with fixed focal lengths, or use external control to adjust the microlens focal length. An intriguing example is the tunable liquid lens, where electrowetting or external pressure manipulates the shape of a liquid droplet and thereby adjusts its optical properties. Here we demonstrate a liquid lens system that allows for autonomous focusing. The central component is a stimuli-responsive hydrogel integrated into a microfluidic system and serving as the container for a liquid droplet, with the hydrogel simultaneously sensing the presence of stimuli and actuating adjustments to the shape--and hence focal length--of the droplet. By working at the micrometre scale where ionic diffusion and surface tension scale favourably, we can use pinned liquid-liquid interfaces to obtain stable devices and realize response times of ten to a few tens of seconds. The microlenses, which can have a focal length ranging from -infinity to +infinity (divergent and convergent), are also readily integrated into arrays that may find use in applications such as sensing, medical diagnostics and lab-on-a-chip technologies.

Biomedical Engineering↗

PubMed2006

Dissolvable membranes as sensing elements for microfluidics based biological/chemical sensors.

We demonstrate a chemical and biological sensing mechanism in microfluidics that transduces chemical and biological signals to electrical signals with large intrinsic amplification without need for complex electronics. The sensing mechanism involves a dissolvable membrane separating a liquid sample chamber from an interdigitated electrode. Dissolution of the membrane (here, a disulfide cross-linked poly(acrylamide) hydrogel) in the presence of a specific target (here, a reducing agent-dithiothreitol) allows the target solution to flow into contact with the electrode. The liquid movement displaces the air dielectric with a liquid, leading to a change (open circuit to approximately 1 kOmega) in the resistance between the electrodes. Thus, a biochemical event is transduced into an electrical signal via fluid movement. The concentration of the target is estimated by monitoring the difference in dissolution times of two juxtaposed sensing membranes having different dissolution characteristics. No dc power is consumed by the sensor until detection of the target. A range of targets could be sensed by defining membranes specific to the target. This sensing mechanism might find applications in sensing targets such as toxins, which exhibit enzymatic activity.

Acrylic Resins↗

PubMed2006

Mammary stem and progenitor cells: tumour precursors?

Several groups have proposed that mammary epithelial cell (MEC) populations, in common with other epithelia, have stem and progenitor sub-populations that are long-lived and provide most of the growth potential during ductal (and perhaps lobuloalveolar) outgrowth. In this review, we describe what is known about normal development, particularly with respect to the growth potential and regenerative capacity of mouse MEC populations. We have developed a theoretical model in order to understand how the activity of the somatic stem/progenitor cell compartment during mammary gland development could affect the demographic of adult MEC populations. This demographic is likely to be key to understanding tumour risk, since long-lived cells provide great advantages in the process of cancer development.

Animals↗

PubMed2006

A method for dynamic system characterization using hydraulic series resistance.

The pressure required to drive flow through a microfluidic device is an important characteristic of that device. We present a method to measure the flow rate through microfluidic components and systems, including micropumps and microvalves. The measurement platform is composed of two pressure sensors and a glass tube, which provides series resistance. The principle of the measurement is the fluid dynamical equivalent of Ohm's law, which defines the relationship between current, resistance, and voltage that are analogues to flow rate, hydraulic resistance, and pressure drop, respectively. Once the series resistance is known, it is possible to compute the flow rate through a device based on pressure alone. In addition, the dynamic system characteristics of the device-resistance and capacitance-can be computed. The benefits of this method are its simple configuration, capability of measuring flow rate accurately from the more easily measured pressure, and the ability to predict the dynamic response of microfluidic devices.

Microfluidic Analytical Techniques↗

PubMed2006

Characterization of a membrane-based gradient generator for use in cell-signaling studies.

This paper describes a method to create stable chemical gradients without requiring fluid flow. The absence of fluid flow makes this device amenable to cell signaling applications where soluble factors can impact cell behavior. This device consists of a membrane-covered source region and a large volume sink region connected by a microfluidic channel. The high fluidic resistance of the membrane limits fluid flow caused by pressure differences in the system, but allows diffusive transport of a chemical species through the membrane and into the channel. The large volume sink region at the end of the microfluidic channel helps to maintain spatial and temporal stability of the gradient. The chemical gradient in a 0.5 mm region near the sink region experiences a maximum of 10 percent change between the 6 and 24 h data points. We present the theory, design, and characterization of this device and provide an example of neutrophil chemotaxis as proof of concept for future quantitative cell-signaling applications.

Chemotaxis↗

PubMed2006

Capillary inserts in microcirculatory systems.

Microfluidic loops (i.e. closed fluid paths) pose specific practical challenges such as priming, introducing analytes or reagents in a controlled way and sampling products. In this technical note we address these three issues using a removable part of the microchannel that we call a 'capillary insert'.

Biomimetic Materials↗

PubMed2006

Steady flow generation in microcirculatory systems.

In this paper we explore the mechanical generation of steady-non pulsatile-flow in microfluidic systems. The rationale of the paper is inspired in the example of cardiovascular systems where at the microscale (i.e. capillaries) the flow is steady rather than pulsatile to optimize performance. We present a solution to the generation of steady flow in engineered microfluidic systems either in open or closed loop configurations via the use of disc pumps. The disc pump consists of a flat rotating disc and utilizes both viscous drag and centrifugal force to achieve pumping. Experiments using single loop and double loop microfluidic systems are presented to characterize the disc pump. Continuous flow generated by the disc pumps can be used to separate particles based on size using recirculating loops and for extraction of small particles without disturbing the concentration of bigger particles. The potential impact of this technology includes sample separation and extraction techniques into portable microfluidic labs-on-a-chip, and long term culture systems for cells in suspension.

Biomimetic Materials↗

PubMed2006

Do-it-yourself microelectrophoresis chips with integrated sample recovery.

We present a microelectrophoresis chip that is simple to fabricate using the microfluidic tectonics (microFT) platform (Beebe, D. J. et al., Proc. Natl. Acad. Sci. USA 2000, 97, 13488-13493; Agarwal, A. K. et al.,. J. Micromech. Microeng. 2006, 16, 332-340). The device contains a removable capillary insert (RCI) for easy sample collection after separation (Atencia, J. et al.,. Lab Chip 2006, DOI: 10. 1039/b514068d). Device construction is accomplished in less than 20 min without specialized equipment traditionally associated with microelectrophoresis chip construction. microFT was used to build a PAGE device utilizing two orthogonal microchannels. One channel performs standard separations, while the second channel serves as an access point to remove bands of interest from the chip via the RCI. The RCI contains an integrated electrode that facilitates the removal of bands using electrokinetic techniques. The device was characterized using prestained proteins (Pierce BlueRanger and TriChromRanger). Samples were loaded into the microelectrophoresis device via a standard micropipette. An electrical field of 40 V/cm was used to separate and collect the proteins. The microPAGE device is simple to fabricate, benefits from microscale analysis, and includes an on-chip collection scheme that interfaces the macroworld with the microworld.

Electrophoresis, Microchip↗

PubMed2006

Polarity effect in electrovibration for tactile display.

Electrovibration is the tactile sensation of an alternating potential between the human body and a smooth conducing surface when the skin slides over the surface and where the current is too small to stimulate sensory nerves directly. It has been proposed as a high-density tactile display method, for example to display pictographic information to persons who are blind. Previous models for the electrovibration transduction mechanism are based on a parallel-plate capacitor in which the electrostatic force is insensitive to polarity. We present experimental data showing that electrovibratory perceptual sensitivity to positive pulses is less than that for negative or biphasic pulses and propose that this disparity may be due to the asymmetric electrical properties of human skin. We furthermore propose using negative pulses for insulated tactile displays based on electrovibration because their sensory thresholds were found to be more stable than for waveforms incorporating positive pulses.

Adult↗

PubMed2006

An oral tactile interface for blind navigation.

An oral tactile interface was designed and evaluated to provide directional cues through the tactile channel, which may be utilized by a blind traveler to obtain directional guidance in outdoor navigation. The device was implemented as a mouthpiece with a microfabricated electrotactile display on top for tactile presentation onto the roof of the mouth and a tongue touch keypad at the bottom for simultaneous operation by the tongue. An experimental system allowed a user to communicate with a computer tactilely by using the oral interface. Directional cues were presented to the user as line or arrow patterns with four moving directions (leftward, rightward, forward, and backward). Electrotactile presentation on the roof of the mouth was evaluated in experiments of threshold measurement and identification of directional cues. Experimental results from six human subjects showed that the roof of the mouth required stimulation intensities around 15 V for threshold sensation, and around 25-30 V for comfortable and well-perceived stimulation. Furthermore, identification of leftward or rightward movements was highly accurate while performance on forward or backward moving patterns was mixed and varied considerably among subjects.

Adult↗

PubMed2005

Application of sexed semen technology to in vitro embryo production in cattle.

Use of sexed semen in conjunction with in vitro embryo production is a potentially efficient means of obtaining offspring of predetermined sex. For thousands of years, livestock owners have desired a methodology to predetermine the sex of offspring for their herds. The ability to sort individual sperm cells into viable X- and Y-chromosome-bearing fractions made producers' sex selection dreams reality in the 1990s and now semen can be sexed with greater than 90% accuracy with use of a flow cytometric cell sorter. Several concerns regarding the implementation of sexed semen technology include the apparent lower fertility of sorted sperm, the lower survival of sorted sperm after cryopreservation and the reduced number of sperm that could be separated in a specified time period. These issues are discussed in this review. There are also a number of issues that appear to influence the success rates of using sexed semen to produce bovine embryos in vitro. These issues include reductions in fertilization rates, lower cleavage rates, blastocyst rates and pregnancy rates, partial capacitation of the sperm, dilute sperm samples and sire variation. These subjects are also addressed in this paper. Finally, we will describe a recent field trial in which female Holstein embryos produced using the combined technologies of sex-selected semen and microfluidics were transferred either as single or bilateral twin embryos into beef cattle recipients, demonstrating these technologies' contributions to viable embryo production. The results indicate that large-scale transfer of in vitro produced, Holstein heifer embryos to beef recipients is a feasible production scheme.

Animals↗

PubMed2005

Controlled microfluidic interfaces.

The microfabrication technologies of the semiconductor industry have made it possible to integrate increasingly complex electronic and mechanical functions, providing us with ever smaller, cheaper and smarter sensors and devices. These technologies have also spawned microfluidics systems for containing and controlling fluid at the micrometre scale, where the increasing importance of viscosity and surface tension profoundly affects fluid behaviour. It is this confluence of available microscale engineering and scale-dependence of fluid behaviour that has revolutionized our ability to precisely control fluid/fluid interfaces for use in fields ranging from materials processing and analytical chemistry to biology and medicine.

Diffusion↗

PubMed2005

Reduction of polyspermic penetration using biomimetic microfluidic technology during in vitro fertilization.

Efforts to improve the in vitro embryo production process in pigs have included modifying culture medium and number of spermatozoa inseminated in order to reduce the incidence of polyspermy. Polyspermy is a pathological condition which results in aberrant embryonic development. The microchannels are designed to more closely mimic the function of the oviduct and create a flow pattern of spermatozoa past the oocytes similar to the pattern in the oviduct. In vitro fertilization of porcine oocytes in the microchannels has produced a higher incidence of monospermic penetration (p<0.05) as compared to the oocytes fertilized in the traditional microdrop system with comparable penetration and male pronucleus formation rates. Additionally, cleavage rates of the embryos as well as development to the blastocyst stage are similar. Here we demonstrate that the biomimetic microchannel in vitro fertilization system can reduce polyspermy and, therefore, increase the number of potentially viable embryos without reducing the overall in vitro production efficiency.

Animals↗

PubMed2005

Diffusion dependent cell behavior in microenvironments.

Understanding the interaction between soluble factors and cells in the cellular microenvironment is critical to understanding a wide range of diseases. Microchannel culture systems provide a tool for separating diffusion and convection based transport making possible controlled studies of the effects of soluble factors in the cellular microenvironment. In this paper we compare the proliferation kinetics of cells in traditional culture flasks to those in microfluidic channels, and explore the relationship between microchannel geometry and cell proliferation. PDMS (polydimethylsiloxane) microfluidic channels were fabricated using micromolding methods. Fall armyworm ovarian cells (Sf9) were homogeneously seeded in a series of different sized microchannels and cultured under a no flow condition. The proliferation rates of Sf9 cells in all of the microchannels were slower than in the flask culture over the first 24 h of culture. The proliferation rates in the microchannels then continuously decreased reaching 5% of that in the flasks over the next 48 h and maintained this level for 5 days. This growth inhibition was reversible and influenced only by the cell seeding density and the channel height but not the channel length or width. One possible explanation for the observed dimension-dependent cell proliferation is the accumulation of different functional molecules in the diffusion dominant microchannel environment. This study provides insights into the potential effects of the diffusion of soluble factors and related effects on cell behavior in microenvironments relevant to the emerging use of microchannel culture systems.

Animals↗

PubMed2005

Continuous fabrication of biocatalyst immobilized microparticles using photopolymerization and immiscible liquids in microfluidic systems.

We report a novel technique for manufacturing polymeric microparticles containing biocatalysts by the behavior of immiscible liquids in microfluidic systems and in situ photopolymerization. The approach utilizes a UV-polymerizable hydrogel/enzyme solution and an immiscible oil solution. The oil and hydrogel solutions form emulsions in pressure-driven flow in microchannels at high values of the dimensionless capillary number (Ca). The resultant hydrogel droplets are then polymerized in situ via exposure to 365 nm UV light. This technique allows for the generation of monodisperse particles whose size can be controlled by the regulation of flow rates. In addition, both manufacturing microparticles and immobilizing biocatalysts can be performed simultaneously and continuously.

Biosensing Techniques↗

PubMed2005

Development of a disposable infusion system for the delivery of protein therapeutics.

This paper describes the development and optimization of a low flow open-loop infusion device for continuous delivery of protein therapeutics. Specifically, a non-electronic polymer device is actuated with responsive hydrogels to infuse at 2 microL/hr for 12 hours. Hydrogel actuators transduce a chemical signal (change in pH of the local environment) into a mechanical response (swelling) generating the pressure to drive the infusion. The hydrogel actuators are separated from the drug reservoir by an elastomeric impermeable membrane. As the hydrogel actuators expand, the expansion deflects the flexible membrane down and reduces the volume of the drug reservoir causing the infusion of drug through the needle that is the only outlet for the reservoir.

Diffusion↗

PubMed2004

Magnetically-driven biomimetic micro pumping using vortices.

Planar micropumps utilizing vortices shed by an oscillating ferromagnetic bar are presented. The movement of the bar is induced by magnetic coupling with an external spinning magnet. Thus, energy transfer is achieved without physical contact or need of any on-chip power source. To create vortices inside the chip, the Reynolds number is locally increased with the oscillation of the bar. The utilization of the vortices as a tool for efficient transfer of energy is inspired by the behaviour of swimming animals and flying insects in nature. The pumps operate in two different scales (milli-scale and micro-scale) in order to take advantage of both. Experiments are presented characterizing the pumps and their flow patterns. The range of operation of the pumps is from 3 microl min(-1) to 600 microl min(-1), though smaller flow rates are also possible.

Biomimetics↗