Lisa Sprenger

A micromechanical comparison of human and porcine skin before and after preservation by freezing for medical device development.

Collecting human skin samples for medical research, including developing microneedle-based medical devices, is challenging and time-consuming. Researchers rely on human skin substitutes and skin preservation techniques, such as freezing, to overcome the lack of skin availability. Porcine skin is considered the best substitute to human skin, but their mechanical resemblance has not been fully validated. We provide a direct mechanical comparison between human and porcine skin samples using a conventional mechano-analytical technique (microindentation) and a medical application (microneedle insertion), at 35% and 100% relative humidity. Human and porcine skin samples were tested immediately after surgical excision from subjects, and after one freeze-thaw cycle at −80 °C to assess the impact of freezing on their mechanical properties. The mechanical properties of fresh human and porcine skin (especially of the stratum corneum) were found to be different for bulk measurements using microindentation; and both types of skin were mechanically affected by freezing. Localized in-plane mechanical properties of skin during microneedle insertion appeared to be more comparable between human and porcine skin samples than their bulk out-of-plane mechanical properties. The results from this study serve as a reference for future mechanical tests conducted with frozen human skin and/or porcine skin as a human skin substitute.

Thermodiffusion in concentrated ferrofluids: Experimental and numerical results on magnetic thermodiffusion

Ferrofluids consist of magnetic nanoparticles dispersed in a carrier liquid. Their strong thermodiffusive behaviour, characterised by the Soret coefficient, coupled with the dependency of the fluids parameters on magnetic fields is dealt with in this work. It is known from former experimental investigations on the one hand that the Soret coefficient itself is magnetic field dependent and on the other hand that the accuracy of the coefficients experimental determination highly depends on the volume concentration of the fluid. The thermally driven separation of particles and carrier liquid is carried out with a concentrated ferrofluid (φ = 0.087) in a horizontal thermodiffusion cell and is compared to equally detected former measurement data. The temperature gradient (1 K/mm) is applied perpendicular to the separation layer. The magnetic field is either applied parallel or perpendicular to the temperature difference. For three different magnetic field strengths (40 kA/m, 100 kA/m, 320 kA/m) the diffusive ...

Thermodiffusion in concentrated ferrofluids: A review and current experimental and numerical results on non-magnetic thermodiffusion

Ferrofluids are colloidal suspensions consisting of magnetic nanoparticles dispersed in a carrier liquid. Their thermodiffusive behaviour is rather strong compared to molecular binary mixtures, leading to a Soret coefficient (ST) of 0.16 K−1. Former experiments with dilute magnetic fluids have been done with thermogravitational columns or horizontal thermodiffusion cells by different research groups. Considering the horizontal thermodiffusion cell, a former analytical approach has been used to solve the phenomenological diffusion equation in one dimension assuming a constant concentration gradient over the cells height. The current experimental work is based on the horizontal separation cell and emphasises the comparison of the concentration development in different concentrated magnetic fluids and at different temperature gradients. The ferrofluid investigated is the kerosene-based EMG905 (Ferrotec) to be compared with the APG513A (Ferrotec), both containing magnetite nanoparticles. The experiments prov...

Experimental, numerical, and theoretical investigation on the concentration-dependent Soret effect in magnetic fluids

Applying a temperature gradient to a layer of a binary fluid establishes a diffusive transport mechanism called thermophoresis or Soret effect which separates the two fluid’s components and is measured by the Soret coefficient. Recent investigations carried out on concentrated magnetic fluids showed that the intensity of the Soret effect depends on the concentration of the nanoparticles transported. The present article, therefore, deals with the concentration-dependence of the Soret coefficient using five equally composed magnetic fluids only varying in the concentration of the particles from 2 vol. % to 10 vol. % of magnetic material. The current investigations point out that the determination of the Soret coefficient and especially its dependence on the particles’ concentration is based on the determination of the thermal and particle diffusion coefficient. The article, therefore, presents a theoretical approach for the determination of the thermal diffusion coefficient and adapts a commonly used Ansatz for the particle diffusion coefficient for the present case of concentrated magnetic fluids. It is thereby possible to determine a theoretical Soret coefficient in dependence on an empirical parameter α. The coefficient is compared with the experimental approaches which have been previously used, these will be referred to as “analytical approach” throughout the text. A second comparison is achieved with a hybrid Soret coefficient which fits the experimentally detected separation curves numerically. Within the investigations, the hydrodynamic concentration of the particles is used, assuming a surfactant layer’s thickness of 2 nm per magnetic particle which leads to concentrations between approximately 11 vol. % and 47 vol. %. The diffusion coefficient ranges from 0.6 × 10−11 m2/s to 2.5 × 10−11 m2/s depending on the analytical model used. The theoretical Soret coefficient decreases with increasing particles’ concentration; the experimental values derived from the analytical approach decrease from 0.06 K−1 to 0.01 K−1 for increasing particles’ concentration. The numerically determined coefficient ranges from 0.11 K−1 to 0.022 K−1. The experimental values are smaller than former experimental results suggest (0.16 K−1), which is due to the fact that in former works, only magnetic concentrations had been considered. All three current investigations prove what could also be partly seen in former experiments that the higher the particles’ concentration the weaker is thermophoresis. The particle diffusion coefficient has to be known for the determination of the Soret coefficient. It is carried out a proof of principle in the article showing that the horizontal thermophoresis cell can also be used to determine the rehomogenisation process which takes place after separating the fluid by applying a homogeneous temperature to the fluid. The diffusion coefficients that could be determined experimentally range from 1 × 10−11 m2/s to 6 × 10−11 m2/s.

Simulation and experimental determination of the online separation of blood components with the help of microfluidic cascading spirals.

Microfluidic spirals were used to successfully separate rare solid components from unpretreated human whole blood samples. The measured separation ratio of the spirals is the factor by which the concentration of the rare component is increased due to the Dean effect present in a flow profile in a curved duct. Different rates of dilution of the blood samples with a phosphate-buffered solution were investigated. The diameters of the spherical particles to separate ranged from 2 μm to 18 μm. It was found that diluting the blood to 20% is optimal leading to a separation ratio up to 1.97. Using two spirals continuously placed in a row led to an increase in separation efficacy in samples consisting of phosphate-buffered solution only from 1.86 to 3.79. Numerical investigations were carried out to display the flow profiles of Newtonian water samples and the shear-thinning blood samples in the cross-section of the experimentally handled channels. A macroscopic difference in velocity between the two rheologically different fluids could not be found. The macroscopic Dean flow is equally present and useful to help particles migrate to certain equilibrium positions in blood as well as lower viscous Newtonian fluids. The investigations highlight the potential for using highly concentrated, very heterogeneous, and non-Newtonian fluidic systems in known microsystems for screening applications.

Continuous form-dependent focusing of non-spherical microparticles in a highly diluted suspension with the help of microfluidic spirals

Microfluidic spirals are able to focus non-spherical microparticles in diluted suspension due to the Dean effect. A secondary flow establishes in a curved channel, consisting of two counter-rotating vortices, which transport particles to an equilibrium position near the inner wall of the channel. The relevant size parameter, which is responsible for successful focusing, is the ratio between the particle diameter of a sphere and the hydraulic diameter, which is a characteristic of the microfluidic spiral. A non-spherical particle has not one but several different size parameters. This study investigated the minor and major axes, the equivalent spherical diameter, and the maximal rotational diameter as an equivalent to the spherical diameter. Using a polydimethylsiloxane (PDMS)-based microfluidic device with spirals, experiments were conducted with artificial peanut-shaped and ellipsoidal particles sized between 3 and 9 μm as well as with the bacteria Bacillus subtilis. Our investigations show that the equi...