Fjola Jonsdottir

Rheology of Perfluorinated Polyether-based MR Fluids with Nanoparticles

Motivated by the use of magneto-rheological (MR) technology in prosthetic devices, the goal of this study is to develop a MR fluid composition that is tailored for the requirements of a prosthetic knee actuator. A MR fluid composition is sought with a suitable balance between the field-induced shear stress, the off-state viscosity, and sedimentation stability for the proposed application. Rheological characteristics are investigated for samples with monodisperse micron-sized particles and bidisperse fluids with a mixture of micron- and nanosized particles. Two types of nanosized particles are used. All fluid samples employ a novel perfluorinated polyether oil as carrier liquid which enhances stability. The samples are investigated with respect to both field-induced and off-state characteristics. The results are compared to analytical and empirical models that exist in the literature. The monodisperse fluids are shown to give a favorable trade-off between field-induced strength and off-state viscosity. The addition of a small concentration of nanoparticles is found to moderately increase the field-induced shear-yield stress. However, for a larger concentration of nanoparticles, the yield stress begins to decrease. Nanoparticles exhibit an undesirable effect on the off-state viscosity. The results reveal valuable information for the designers of MR fluids and designers of prosthetic actuators.

Influence of Parameter Variations on the Braking Torque of a Magnetorheological Prosthetic Knee

Microprocessor-controlled prosthetic knees, which rely on magnetorheological (MR) technology, have the potential to increase the comfort and quality of life of amputees. The focus of this study is on a prosthetic knee which is currently on the market and manufactured by the company Ossur Inc. The knee uses magnetic fields to vary the viscosity of the MR fluid, and thereby its flexion resistance. The torque transmissibility of the knee greatly depends on the magnetic field intensity in the MR fluid. The objective of this study is to investigate the strength of the magnetic field and the braking torque in the knee, for a few selected design parameters, and to determine which changes can be made to the existing design in order to maximize the torque output. The magnetic field in the fluid is evaluated by finite element analysis and the torque is calculated by using a Bingham visco-plastic model. A parametric study is carried out for several design parameters where the effect of variation in each parameter on the braking torque is observed. The results of this study give a valuable insight into which parameters should be prioritized for future changes of the knee, with regard to strength and comfortability.

An Experimental Investigation of Unimodal and Bimodal Magnetorheological Fluids with an Application in Prosthetic Devices

The study investigates the field-induced shear yield stress and the off-state viscosity of selected unimodal and bimodal magnetorheological (MR) fluids. Five grades of commercially available iron powder are used to prepare unimodal MR fluids, one for each powder grade, and bimodal MR fluids, using two grades of micron-sized powder. The iron powder contains particles with a mean diameter ranging from 1 to 8 µm. The solid loading of all fluids is held to a constant value while varying the particle size and the ratio between the coarse and the fine powders. All fluids employ a perfluorinated polyether base fluid whose qualities are described. The bimodal MR fluids are compared to their corresponding unimodal fluids. Results show the bimodal fluids to have a lower off-state viscosity than their corresponding unimodal fluids. An application in prosthetic devices is introduced in which the yielding and the off-state characteristics of the MR fluid are of equal importance. In this application, the shear yield stress determines the rigidness while the off-state viscosity determines the flexibility in the absence of a magnetic field. For this particular application, prominent MR fluids are selected based on the ratio between the on-state yield stress and the off-state viscosity.

Elastic fields and energies of coherent surface islands

We present simple models based on isotropic elasticity for determining the stress fields in the vicinity of, and the elastic energy associated with, coherent surface islands (i.e. a quantum dot or a nanorod). The first method treats the surface island as an areal point of dilatation and does not account for details of the island shape. Next, the finite element method is used to study simple island shapes such as spherical caps and cylinders, with a particular focus on the island aspect ratio. The latter is used in conjunction with the analytic results to develop empirical expressions for stress field and energy as functions of aspect ratio, which are somewhat insensitive to other features of the geometry. The analyses are used to assess the effect of lattice mismatch, dot volume and dot/surface contact area on the induced stresses and elastic energies. Furthermore, the interaction energy between surface islands is determined by finite element analyses. Outputs from these analyses are then used in an optimization of several cases of ordering of islands. The results show that for a range of idealized geometries, the shape of the surface island has little impact on the strain energy, and thereby the interaction energy.

Numerical modelling and experimental investigation of drug release from layered silicone matrix systems

Medical devices and polymeric matrix systems that release drugs or other bioactive compounds are of interest for a variety of applications. The release of the drug can be dependent on a number of factors such as the solubility, diffusivity, dissolution rate and distribution of the solid drug in the matrix. Achieving the goal of an optimal release profile can be challenging when relying solely on traditional experimental work. Accurate modelling complementing experimentation is therefore desirable. Numerical modelling is increasingly becoming an integral part of research and development due to the significant advances in computer simulation technology. This work focuses on numerical modelling and investigation of multi-layered silicone matrix systems. A numerical model that can be used to model multi-layered systems was constructed and validated by comparison with experimental data. The model could account for the limited dissolution rate and effect of the drug distribution on the release profiles. Parametric study showed how different factors affect the characteristics of drug release. Multi-layered medical silicone matrices were prepared in special moulds, where the quantity of drug in each layer could be varied, and release was investigated with Franz-diffusion cell setup. Data for long-term release was fitted to the model and the full depletion of the system predicted. The numerical model constructed for this study, whose input parameters are the diffusion, effective dissolution rate and dimensional solubility coefficients, does not require any type of steady-state approximation. These results indicate that numerical model can be used as a design tool for development of controlled release systems such as drug-loaded medical devices.

A numerical framework for drug transport in a multi-layer system with discontinuous interlayer condition.

Discontinuous boundary conditions arise naturally when describing various physical phenomena and numerically modelling such conditions can prove difficult. In the field of pharmaceutical sciences, two such cases are the partitioning of a compound between different materials and a flux rate membrane controlling mass transfer between materials which both result in a discontinuous jump in concentration across adjacent materials. In this study, we introduce a general one-dimensional finite element drug delivery framework, which along with diffusion, reversible binding and dissolution within material layers, incorporates the partitioning and mass transfer conditions between layers of material. We apply the framework to construct models of experiments, which along with experimental data, allow us to infer pharmacokinetic properties of potential material for drug delivery. Understanding such material properties is the key to optimising the therepeutic effect of a targeted drug delivery system.

Numerical Modelling of Transdermal Delivery from Matrix Systems: Parametric Study and Experimental Validation with Silicone Matrices

A model is presented for transdermal drug delivery from single-layered silicone matrix systems. The work is based on our previous results that, in particular, extend the well-known Higuchi model. Recently, we have introduced a numerical transient model describing matrix systems where the drug dissolution can be non-instantaneous. Furthermore, our model can describe complex interactions within a multi-layered matrix and the matrix to skin boundary. The power of the modelling approach presented here is further illustrated by allowing the possibility of a donor solution. The model is validated by a comparison with experimental data, as well as validating the parameter values against each other, using various configurations with donor solution, silicone matrix and skin. Our results show that the model is a good approximation to real multi-layered delivery systems. The model offers the ability of comparing drug release for ibuprofen and diclofenac, which cannot be analysed by the Higuchi model because the dissolution in the latter case turns out to be limited. The experiments and numerical model outlined in this study could also be adjusted to more general formulations, which enhances the utility of the numerical model as a design tool for the development of drug-loaded matrices for trans-membrane and transdermal delivery.

Large Amplitude Thermal Fluctuations of Confined Semiflexible Biopolymer Filaments

The phenomenon of thermal fluctuations of biopolymers has been of active interest for some time with a view toward understanding the effect of filament confinement, migration, and bonding. In this study, we focus our attention on planar fluctuations of a single filament between parallel confining surfaces. Filament slopes, with respect to the centerline of the channel, commonly exceed 0.1 in magnitude and therefore fall outside the range of small amplitude fluctuations. Consequently, large amplitudes are anticipated from the outset. Determination of the partition function leads to the quantitative dependence of free energy and other thermodynamic parameters on the degree of confinement.

An Experimental Investigation into the Off-State Viscosity of MR Fluids

This study investigates the off-state rheological characteristics of magnetorheological fluids. Various carbonyl iron powder grades are tested, both micron- and nano-sized. The study considers both unimodal and bimodal MR fluid samples. The bimodal MR fluids are both micron-sized only compositions and mixed micron- and nano-sized compositions. All fluid compositions employ a novel PFPE oil as a base fluid. This base fluid is introduced and its qualities discussed. An off-state experimental investigation is conducted, for shear-rates from 0 to 200 s−1. All samples exhibit the well-known shear-thinning property of MR fluids, but with varying proportions. The off-state viscosity is of importance in a particular application, in an MR prosthetic knee. It determines how fast the knee joint can rotate in the absence of a magnetic field. A prominent MR fluid composition is selected for the proposed application which aims to maximize to rotational speed of the MR knee joint in the absence of a magnetic field.

Modeling Perfluorinated Polyether-Based MR Fluids

This study investigates the field-induced rheological characteristics of PFPE-based MR fluids. Four PFPE-based MR fluid samples were prepared, each with a different solid loading. All samples contain a single grade of carbonyl iron powder with an average particle diameter of 2 µm. The PFPE base fluid enhances sedimentation stability and its composition and qualities are introduced in the article. The magnetization characteristics of the iron powder and a fluid sample are measured. Its magnetic properties are compared to known models from the literature. The shear-yield stress in all fluid samples is investigated experimentally, as a function of solid loading and the magnetic flux density. The results are compared to established shear stress models for MR fluids. Model parameters are set to conform to the PFPE-based MR fluids. The PFPE-based MR fluids are shown to have a comparable shear-yield stress, when compared to commercially available MR fluids with a comparable particle loading and particle size. An application for the PFPE-based MR fluids is introduced. This application is an adaptive MR prosthetic knee, utilizing the fluid in shear mode.