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Biomedical subjects

Robert H Davis

Publications and source records attributed to Robert H Davis.

13 recordsLinked to original sources
PubMed2006

A water-activated pump for portable microfluidic applications.

An on-chip micropump for portable microfluidic applications was investigated using mathematical modeling and experimental testing. This micropump is activated by the addition of water, via a dropper, to ionic polymer particles that swell due to osmotic effects when wetted. The resulting particle volume increase deflects a membrane, forcing a separate fluid from an adjacent reservoir. The micropump components, along with the microfluidic components, are fabricated using the contact liquid photolithographic polymerization (CLiPP) method. The maximum flow rate achieved with this pump is 17 microL per minute per mg of dry polymer particles of 355-425 microm in diameter. The pump flow rate may be controlled by adjusting the particle size and amount, the membrane properties, and the channel dimensions. The experimental results demonstrate good agreement with an analytical model describing the particle swelling and its coupling with resistive forces from the bending membrane, viscous flow in the microchannel, and interfacial effects. Key features of this micropump are that it can be placed directly on a microdevice, and that it requires only a small amount of water and no external power supply to function. Therefore, this pumping system is useful for applications in which a highly portable device is required.

Microfluidic Analytical Techniques↗
PubMed2006

An effervescent reaction micropump for portable microfluidic systems.

A water-activated, effervescent reaction was used to transport fluid in a controllable manner on a portable microfluidic device. The reaction between sodium bicarbonate and an organic acid, tartaric acid and/or benzoic acid, was modeled to analyze methods of controlling the generation of carbon-dioxide gas for the purposes of pumping fluids. Integration and testing of the effervescent reaction pump in a microfluidic device was made possible by using elastomeric polymers as both photopolymerizable septa and removable lids. These materials combined to enable facile access to otherwise gas-tight devices. Based on theoretical predictions for 0.33 mg of sodium bicarbonate and a stoichiometric amount of organic acid, the pumping flow rate could be varied from 0.01 microL s(-1) to 70 microL s(-1). The flow rate is controlled by adjusting any or all of the particle size of the least soluble reactant, the amount of reactants used, and the type of organic acid selected. The tartaric acid systems rapidly produce carbon dioxide; however, the gas generation rates dramatically decrease over the course of the reaction. In contrast, carbon dioxide production rate in the benzoic acid systems is lower and nearly constant for several minutes. Water activation and direct placement on a microfluidic device are key features of this micropump, which is therefore useful for portable microfluidic applications.

Microfluidics↗
PubMed2005

Pennsylvania State Hospital system's seclusion and restraint reduction program.

OBJECTIVES: This study examined the use of seclusion and mechanical restraint from 1990 to 2000 and the rate of staff injuries from patient assaults from 1998 to 2000 in a state hospital system. METHODS: Records of patients older than 18 years who were civilly committed to one of the nine state hospitals in Pennsylvania were included in the analyses. Two databases were used in each of the nine hospitals: one identified date, time, duration, and justification for each episode of seclusion or restraint and the other identified when a patient was hospitalized and the demographic characteristics and the diagnosis of the patient. Rate and duration of seclusion and restraint were calculated. Reports from compensation claims were used to determine staff injuries from patient assaults. RESULTS: The rate and duration of seclusion and mechanical restraint decreased dramatically during this period. From 1990 to 2000, the rate of seclusion decreased from 4.2 to .3 episodes per 1,000 patient-days. The average duration of seclusion decreased from 10.8 to 1.3 hours. The rate of restraint decreased from 3.5 to 1.2 episodes per 1,000 patient-days. The average duration of restraint decreased from 11.9 to 1.9 hours. Patients from racial or ethnic minority groups had a higher rate and longer duration of seclusion than whites. Seclusion tended to be less likely, but longer, during the night shift. Patients were restrained less often during the night shift, but for a longer duration. The rate of restraint was higher during the week than during weekends and holidays. Younger patients were more likely to be secluded and restrained, but older patients remained secluded and restrained longer. No significant changes were seen in rates of staff injuries from 1998 to 2000. CONCLUSIONS: Many factors contributed to the success of this effort, including advocacy efforts, state policy change, improved patient-staff ratios, response teams, and second-generation antipsychotics.

Adult↗
PubMed2004

In situ fabrication of macroporous polymer networks within microfluidic devices by living radical photopolymerization and leaching.

Novel fabrication techniques and polymer systems are being explored to enable mass production of low cost microfluidic devices. In this contribution we discuss a new fabrication scheme for making microfluidic devices containing porous polymer components in situ. Contact lithography, a living radical photopolymer (LRPP) system and salt leaching were used to fabricate multilayer microfluidic devices rapidly with various channel geometries and covalently attached porous polymer plugs made of various photopolymerizable substrates. LRPP systems offer the advantages of covalent attachment of microfluidic device layers and facile surface modification via grafting. Several applications of the porous plugs are also explored, including a static mixer, a high surface area-to-volume reactor and a rapidly responding hydrogel valve. Quantitative and qualitative data show an increase in mixing of a fluorescein and a water stream for channels containing porous plugs relative to channels with no porous plugs. Confocal laser scanning microscopy images demonstrate the ability to graft a functional material onto porous plug surfaces. A reaction was carried out on the grafted pore surfaces, which resulted in fluorescent labelling of the grafted material throughout the pores of the plug. Homogenous fluorescence throughout the depth of the porous plug and along pore surfaces indicated that the porous plugs were surface modified by grafting and that reactions can be carried out on the pore surfaces. Finally, porous hydrogel valves were fabricated which swelled in response to contact with various pH solutions. Results indicate that a porous hydrogel valve will swell and close more rapidly than other valve geometries made with the same polymer formulation. The LRPP-salt leaching method provides a means for rapidly incorporating porous polymer components into microfluidic devices, which can be utilized for a variety of pertinent applications upon appropriate selection of porous plug materials and surface treatments.

Journal Article↗
PubMed2004

Buoyancy-driven coalescence of spherical drops covered with incompressible surfactant at arbitrary Péclet number.

Collision efficiencies are determined for two surfactant-covered spherical drops in the limits of nearly uniform surface coverage and bulk insolubility for Brownian and/or gravitational motion as a function of drop-size ratio, drop-to-medium viscosity ratio, and retardation parameter. For two equal-sized drops in Brownian motion in the limit of small viscosity ratio, the calculated collision efficiencies agree well with earlier results for bubbles. While the two-sphere relative mobility functions for motion parallel to the drops' line of centers tend to the same values in the limits of infinite viscosity ratio and infinite retardation parameter, the asymmetric mobility functions do not, because the coefficients for the rotational term in Lamb's singular solution are independent of the presence of surfactant. The complex dependence of the transverse mobility functions on the viscosity ratio and retardation parameter makes it possible for the gravitational collision efficiency to increase slightly with viscosity ratio at fixed size ratio and retardation parameter of O(10(3)) or larger. Typical hydrosols are also studied in gravitational motion at arbitrary Péclet number, showing the combined influence of Brownian and gravitational motion.

Journal Article↗
PubMed2004

Electroosmotic flow in channels with step changes in zeta potential and cross section.

We consider the effects that step changes in zeta potential and cross section have on electroosmosis in long-and-narrow channels with arbitrary cross-sectional shapes. The Stokes equation of flow is solved analytically utilizing the thin Debye layer approximation to provide effective slip velocities on the channel walls. The effects of channel dimensions, surface potentials, applied pressure drop, and applied voltage are discussed. One anecdotal case, a two-region rectangular channel, is presented to illustrate the solution. The flow in each region is a combination of a uniform electroosmotic flow and a nonuniform pressure-driven flow. The electroosmotic pumping causes the pressure gradient in each region to adjust so that the flow rate is the same in each region and the overall applied pressure drop is met, resulting in convex velocity profiles in some regions and concave velocity profiles in other regions. By appropriate choice of the applied pressure drop, flat velocity profiles may be achieved in one or more regions.

Journal Article↗
PubMed2004

Secondary membranes for flux optimization in membrane filtration of biologic suspensions.

We employ in situ deposited secondary membranes of yeast (SMYs) to optimize permeate flux during microfiltration and ultrafiltration of protein solutions. The deposited secondary membrane was periodically removed by backflushing, and a new cake layer was deposited at the start of the next cycle. The effects of backflushing time, backflushing strength, wall shear rate, and amount of secondary membrane deposited on the permeate flux were examined. Secondary membranes were found to increase the permeate flux in microfiltration by severalfold. Protein transmission was also enhanced owing to the presence of the secondary membrane, and the amount of protein recovered was more than twice that obtained during filtration of protein-only solutions under otherwise identical conditions. In ultrafiltration, the flux enhancement owing to the secondary membrane was only 50% or less. In addition, the flux for ultrafiltration was relatively insensitive to changes in the concentration of yeast used during deposition of SMY and to the backflushing strength used to periodically remove the secondary membrane.

Biotechnology↗
PubMed2004

Cellulase retention and sugar removal by membrane ultrafiltration during lignocellulosic biomass hydrolysis.

Technologies suitable for the separation and reuse of cellulase enzymes during the enzymatic saccharification of pretreated corn stover are investigated to examine the economic and technical viability of processes that promote cellulase reuse while removing inhibitory reaction products such as glucose and cellobiose. The simplest and most suitable separation is a filter with relatively large pores on the order of 20-25 mm that retains residual corn stover solids while passing reaction products such as glucose and cellobiose to form a sugar stream for a variety of end uses. Such a simple separation is effective because cellulase remains bound to the residual solids. Ultrafiltration using 50-kDa polyethersulfone membranes to recover cellulase enzymes in solution was shown not to enhance further the saccharification rate or overall conversion. Instead, it appears that the necessary cellulase enzymes, including beta-glucosidase, are tightly bound to the substrate; when fresh corn stover is contacted with highly washed residual solids, without the addition of fresh enzymes, glucose is generated at a high rate. When filtration was applied multiple times, the concentration of inhibitory reaction products such as glucose and cellobiose was reduced from 70 to 10 g/L. However, an enhanced saccharification performance was not observed, most likely because the concentration of the inhibitory products remained too high. Further reduction in the product concentration was not investigated, because it would make the reaction unnecessarily complex and result in a product stream that is much too dilute to be useful. Finally, an economic analysis shows that reuse of cellulase can reduce glucose production costs, especially when the enzyme price is high. The most economic performance is shown to occur when the cellulase enzyme is reused and a small amount of fresh enzyme is added after each separation step to replace lost or deactivated enzyme.

Biomass↗
PubMed2003

Solid-solid contacts due to surface roughness and their effects on suspension behaviour.

Solid-solid contacts due to microscopic surface roughness in viscous fluids were examined by observing the translational and rotational behaviours of a suspended sphere falling past a lighter sphere or down an inclined surface. In both cases, a roll-slip behaviour was observed, with the gravitational forces balanced by not only hydrodynamic forces but also normal and tangential solid-solid contact forces. Moreover, the nominal separation between the surfaces due to microscopic surface roughness elements is not constant but instead varies due to multiple roughness scales. By inverting the system, so that the heavy sphere fell away from the lighter sphere or the plane, it was found that the average nominal separation increases with increasing angle of inclination of the plane or the surface of the lighter sphere from horizontal; the larger asperities lift the sphere up from the opposing surface and then gravity at large angles of inclination is too weak to pull the sphere back down to the opposing surface before another large asperity is encountered. The existence of microscopic surface roughness and solid-solid contacts is shown to modify the rheological properties of suspensions. For example, the presence of compressive, but not tensile, contact forces removes the reversibility of sphere-sphere interactions and breaks the symmetry of the particle trajectories. As a result, suspensions of rough spheres exhibit normal stress differences that are absent for smooth spheres. For the conditions studied, surface roughness reduces the effective viscosity of a suspension by limiting the lubrication resistance during near-contact motion, and it also modifies the suspension microstructure and hydrodynamic diffusivity.

Computer Simulation↗
PubMed2003

Large-scale simulations of concentrated emulsion flows.

Applications of the newly developed hybrid of the boundary integral and economical multipole techniques to large-scale dynamical simulations of concentrated emulsion flows of deformable drops are considered. For N = O(10(2)-10(3)) drops in a periodic cell with O(10(3)) boundary elements per drop, the method has two to three orders of magnitude gain over a standard boundary-integral method at each time-step, thus making long-time large-scale dynamical simulations feasible. In the steady shear flow, large systems N >/= O(10(2)) are imperative for convergence at high drop volume fractions c >/= 0.5. At high concentrations, most of the shear thinning occurs for nearly non-deformed drops; at c approximately 0.55 and small capillary numbers, phase transition is observed in dynamical simulations. In sedimentation of deformable drops from a homogeneous initial state, even larger N >/= O(10(3)) are required to accurately describe the Koch-Shaqfeh type of instability in a wide time range with N up to 1200 and ensemble averaging over the initial conditions. The dynamics of the average sedimentation rate is studied versus concentration c for matching viscosities lambda = 1; for a Bond number of 1.75, systems with c approximately 0.25 are found to be most unstable. Additionally, a low drop-to-medium-viscosity ratio system, lambda = 0.1, is more unstable than those with lambda = 0.25 and lambda = 1. In the third application, buoyancy- or gravity-driven motion of a large bubble/drop through a concentrated emulsion of neutrally buoyant drops is studied by simulations. For a size ratio of two, convergent (box-size independent) results for the bubble/drop settling velocity are obtained in simulations with N </= 800.

Algorithms↗
PubMed2002

A three-dimensional boundary-integral algorithm for thermocapillary motion of deformable drops.

A three-dimensional boundary-integral algorithm has been developed to handle the tangential Marangoni stresses in thermocapillary motion of drops. Depending on whether the integration and observation points are on the same or different drops, singularity or near-singularity subtraction is used in the inhomogeneous term of the boundary-integral formulation. Integration is then performed analytically over flat triangles in the subtracted region. Relative trajectories for two deformable drops are calculated for different values of the drop size ratio, drop-to-medium thermal conductivity ratio, and viscosity ratio and compared to those for spherical and slightly deformable drops. Results indicate that deformation increases the minimum separation and inhibits coalescence but is not important enough for appropriate physical parameters to induce the capture or breakup behaviors observed in buoyancy. Interaction times calculated by artificially continuing spherical drop trajectories yield results accurate to within about 10%.

Journal Article↗
PubMed2002

Combined sedimentation and filtration process for cellulase recovery during hydrolysis of lignocellulosic biomass.

A combined sedimentation and ultrafiltration process was investigated for recovering cellulase enzymes during the hydrolysis of lignocellulosic biomass. Lignocellulosic particles larger than approx 50 microm in length were first removed via sedimentation using an inclined settler. Ultrafiltration was then used to retain the remaining lignocellulosic particles and the cellulose enzymes, while transmitting fermentable sugars and other small molecules. The permeate flux from the ultrafiltration step for a feed consisting of 0.22 w/v% cellulase is 64+/-5 L/m2-h, while that for a feed consisting of the settler overflow from a mixture 0.22 w/v% cellulase and 10 wt% lignocellulose fed to the settler is 130+/-20 L/m2-h. The higher permeate flux in the latter case is presumably due to binding of a portion of the cellulase enzymes to the lignocellulosic particles during hydrolysis and filtration, preventing the enzymes from fouling the membrane. A filter paper activity assay shows little loss in enzymatic activity throughout the combined sedimentation/ultrafiltration separation process.

Biomass↗