Scientific publications

The fibrotic encapsulation, which is mainly accompanied by an excessive proliferation of fibroblasts, is an undesired phenomenon after the implantation of various medical devices. Beside the surface chemistry, the topography plays also a major role in the fibroblast-surface interaction. In the present study, one-dimensional aluminium oxide (1D Al2O3) nanostructures with different distribution densities were prepared to reveal the response of human fibroblasts to the surface topography. The cell size, the cell number and the ability to form well-defined actin fibres and focal adhesions were significantly impaired with increasing distribution density of the 1D Al2O3 nanostructures on the substratum.

We propose a theory based on non-equilibrium thermodynamics to describe the mechanical behavior of an active polymer gel created by the inclusion of molecular motors in its solvent. When activated, these motors attach to the chains of the polymer network and shorten them creating a global contraction of the gel, which mimics the active behavior of a cytoskeleton. The power generated by these motors is obtained by an ATP hydrolysis reaction, which transduces chemical energy into mechanical work. The latter is described by an increment of strain energy in the gel due to an increased stiffness. This effect is described with an increment of the cross-link density in the polymer network, which reduces its entropy. The theory then considers polymer network swelling and species diffusion to describe the transient passive behavior of the gel. We finally formulate the problem of uniaxial contraction of a slab of gel and compare the results with experiments, showing good agreement.

This paper outlines an elastoplastic design approach for beam and plate structures subjected to transverse pressure loads and thermal stresses. The purpose of this study is to overcome the limitations of yield-limited designs by exploiting plastic design theorems. The feasible design space of a clamped beam/plate structure subjected to combined thermomechanical loads is explored considering shakedown (stabilized plasticity) as the design criteria. Analytic and numerical solutions are developed that show that allowing shakedown to occur extends the design space and acceptable loading range. In addition, the structures considered here are also prone to buckling due to thermal loads. In this work, interactions between thermal buckling and shakedown are investigated using numerical parametric studies. It is found that buckling enhances elastoplastic shakedown performance which expands the feasible design domain significantly when high aspect ratio beams are considered. In particular it is shown that the enhancement is 2–4 times for the range of aspect ratios examined.

An electro-chemo-mechanical formulation of ion transport in solid electrolytes, in particular for binary systems, is presented. Starting with conservation laws and the second law of thermodynamic, we state a consistent Helmholtz-energy-based framework taking electrostatics, component transport and nonlinear elastic mechanical interaction into account. With the help of finite strain continuum mechanics, we include the effect of geometry changes on ion transport. Changes of local concentration cause swelling and shrinkage and hence stress assisted diffusion. Further coupling originates via an osmotic pressure. Since binary systems are of special interest in battery applications, we formulate both, a fully resolved and an electroneutral model for ion transport. The latter turns out to be an extended version of Newman’s concentrated solution theory taking mechanical effects into account. We demonstrate the importance of these mechanical effects by means of double layers adjacent to blocking electrodes and concentration profiles during galvanostatic charging. Further, we investigate the effect of external deformation as, e.g. found in dendrite growth.

The Butler-Volmer equation is widely used to describe ion-transport across an interface in electrochemical systems. In recent years, a strong focus has been placed on solid state batteries with Li-metal electrodes which promise an increase of energy density and safety, but also introduce new complexity, for example, due to the process of material deposition and stripping which is conceptually different to intercalation and de-intercalation. Especially the understanding of the heterogeneous growth of lithium, in the form of dendrites, requires a consistent model taking all mechanical effects into account. In this work, we use transition state theory based on a purely energetic concept to derive the Butler-Volmer equation for a monovalent reaction M⇌M++e− that is also consistent with the Nernst equation and discuss the energetic contribution due to deposition and stripping. With the help of the Bronsted-Evans-Polanyi principle, we generalize several approaches to include mechanical stress in the Butler-Volmer equation, discuss the underlying assumptions and suggest, through theoretical considerations, a fairly simple extended version of the Butler-Volmer equation. Beside addressing the novel aspects of the effects of mechanics, which impacts both open circuit potential and exchange current density, this work also sharpens the need for consistent use of the Butler-Volmer equation.

Recent work has shown that sharp spectral edges in acoustic stimuli might have advantageous effects in the treatment of tonal tinnitus. In the course of this paper, we evaluate the long-term effects of spectrally notched hearing aids on the subjective tinnitus distress. By merging recent experimental work with a computational tinnitus model, we modified the commercially available behind-the-ear hearing aids so that a frequency band of 0.5 octaves, centered on the patient's individual tinnitus frequency, was blocked out. Those hearing aids employ a steep notch filter that filters environmental sounds to suppress the tinnitus-related changes in neural firing by lateral inhibition. The computational model reveals a renormalization of pathologically increased neural response reliability and synchrony in response to spectrally modified input. The target group, fitted with spectrally notched hearing aids, was matched with a comparable control group, fitted with standard hearing aids of the same type but without a notch filter. We analyze the subjective self-assessment by tinnitus questionnaires, and we monitor the objective distress correlates in auditory evoked response phase data. Both, subjective and objective results show a noticeable trend of a larger therapeutic benefit for notched hearing correction.

Thrombosis and bacterial infection are major problems in cardiovascular implants. Here we demonstrated that a superhydrophobic surface composed of poly(bis(2,2,2-trifluoroethoxy)phosphazene) (PTFEP)–Al2O3 hybrid nanowires (NWs) is effective to reduce both platelet adhesion/activation and bacterial adherence/colonization. The proposed approach allows surface modification of cardiovascular implants which have 3D complex geometries.

During charging of a battery with a lithium metal electrode and a solid electrolyte, a crack in the electrolyte adjacent to the metal electrode will be infiltrated by lithium, forming a dendrite. As further lithium is inserted into the crack, pressure in it will build up. The pressure in the lithium in the crack rises very rapidly during normal rates of charging, reaching 1 GPa within seconds. Such high pressure may cause the crack to propagate; crack extension will relax the pressure in the crack, but high pressure will be restored quickly in the longer crack, which will again propagate. This process continues until the crack touches the counter electrode, causing a short-circuit. Alternatively, the high pressure in the crack can block the redox reaction that injects lithium into it, making the crack non-propagating. We find that this situation occurs for cracks shorter than a critical length. Therefore, to avoid short-circuits of this type it is a requirement that the pressure be sufficient to block the redox reaction before dendrite extension takes place. This places restrictions on allowable lengths for pre-existing cracks and on permissible charging rates. We also find that dendrite cracks can grow subcritically due to cyclic fatigue.

Roughening of metal electrodes in batteries is detrimental as it can lead to metal dendrites. Such dendrites can cause short circuits when they grow from the metal electrode to the other one, as can happen during battery operation when metal is plated onto the surface of an electrode. It has been suggested that solid electrolytes of sufficient elastic stiffness can suppress electrode surface roughening and dendriting, although experimental evidence is now emerging that this possibility is not valid. To investigate whether metal electrode surfaces will roughen during battery charging we carry out a linear perturbation analysis. Our calculations explore whether an electrode surface with one-dimensional sinusoidal roughness will experience growth of its amplitude. We assess a linear elastic electrolyte that is a single ion conductor bonded to a metal electrode being plated by a cathodic ionic current. We find that long wavelength perturbations will always increase in roughness. High current densities during battery charging are found to permit growth of the amplitude of small wavelength roughness. The stiffness of the solid electrolyte is found to play a role in limiting the growth of roughness, but its effect can always be overcome at high current densities and for long wavelength protrusions.

An elementary 1-dimensional model is developed for a solid state lithium-ion battery having a single ion conductor electrolyte, a lithium metal negative electrode and a composite positive electrode. The battery topology is assumed to be of the layered variety, thereby justifying the 1-dimensional formulation. The governing equations for the electrochemical kinetics at the interface between the negative electrode and the electrolyte separator are stated, as are those for ion transport in the electrolyte. The positive electrode is assumed to be a particulate composite of storage material within a matrix of electrolyte. A mixture theory is developed for the positive electrode encompassing ion transport in the electrolyte matrix and storage and unstorage of lithium in the active material subject to electrochemical kinetics at the perimeter of the storage particles. Many simplifying assumptions are made with the advantage of them leading to closed-form or semi-closed-form solutions, including linearization of the equations governing the redox kinetics at interfaces in the battery between electrolyte and active material. An approximation is given for the concentration of lithium in the positive electrode of the battery during discharge, with the details depending on a length scale parameter that depends on the competition between the rate of lithium insertion into/extraction from the storage particles and the rate at which lithium ions are transported in the electrolyte. When the conductivity of the electrolyte is high and the redox reactions are relatively sluggish, this length scale parameter is comparable to the thickness of the positive electrode or larger than it. In that case lithium insertion into/extraction from storage particles occurs everywhere within an active zone of the positive electrode, but with the rates least at the current collector of the positive electrode. If the conductivity of the electrolyte is poor and the redox reactions rapid, the length scale for the solution is small compared to the thickness of the positive electrode and insertion into/extraction from storage particles occurs only in a narrow slice of the positive electrode. This slice moved along the positive electrode and separates a region of it that is completely filled/empty from a region of it that has not yet gained or lost any of its lithium. In all cases there will usually be a region of the positive electrode near the separator that is completely filled during discharge and completely empty during battery charging. Our results also give outcomes from which the internal resistance of the battery can be estimated.