Scientific publications

We explore different electrode microstructures and the associated implications on the electrochemical stability of activated carbon/lithium titanate (Li4Ti5O12, LTO) composite electrodes by incrementally increasing the LTO content. At low LTO concentrations, the electrochemical stability is progressively improved with respect to neat activated carbon based electrodes. This trend is abruptly changed for high LTO concentrations (72 mass%) as the electrolyte starts to decompose unexpectedly far below the electrochemical stability boundaries of the single materials. We attribute this to a loss of electrical percolation and local degradation spots caused by peculiarities of the carbon distribution: Initially the sub-micrometer-sized LTO solely occupies spaces between the large, micrometer-sized activated carbon. With increasing LTO content the activated carbon particles get separated in an insulating LTO matrix. Electrochemical stability can be reestablished with electronic conduction paths of well distributed sub-micrometer-sized carbon black particles. By this way, cell degradation can be reduced and the cycle life of cells with high LTO concentration is prolonged from 10 to >36,000 cycles. Finally, we propose a simple method to distinguish cell fading caused by electrolyte decomposition from cell fading caused by poor electrical percolation.

Advanced medical devices, treatments and therapies demand an understanding of the role of interfacial properties on the cellular response. This is particularly important in the emerging fields of cell therapies and tissue regeneration. In this study, we evaluate the role of surface nanotopography on the fate of human dental pulp derived stem cells (hDPSC). These stem cells have attracted interest because of their capacity to differentiate to a range of useful lineages but are relatively easy to isolate. We generated and utilized density gradients of gold nanoparticles which allowed us to examine, on a single substrate, the influence of nanofeature density and size on stem cell behavior. We found that hDPSC adhered in greater numbers and proliferated faster on the sections of the gradients with higher density of nanotopography features. Furthermore, greater surface nanotopography density directed the differentiation of hDPSC to osteogenic lineages. This study demonstrates that carefully tuned surface nanotopography can be used to manipulate and guide the proliferation and differentiation of these cells. The outcomes of this study can be important in the rational design of culture substrates and vehicles for cell therapies, tissue engineering constructs and the next generation of biomedical devices where control over the growth of different tissues is required.

In this study, we propose a novel estimate of listening effort using electroencephalographic data. This method is a translation of our past findings, gained from the evoked electroencephalographic activity, to the oscillatory EEG activity. To test this technique, electroencephalographic data from experienced hearing aid users with moderate hearing loss were recorded, wearing hearing aids. The investigated hearing aid settings were: a directional microphone combined with a noise reduction algorithm in a medium and a strong setting, the noise reduction setting turned off, and a setting using omnidirectional microphones without any noise reduction. The results suggest that the electroencephalographic estimate of listening effort seems to be a useful tool to map the exerted effort of the participants. In addition, the results indicate that a directional processing mode can reduce the listening effort in multitalker listening situations.

Spherosilicates and polyhedral oligomeric silsesquioxanes represent unique well-defined rigid building blocks for molecular and hybrid materials. Drawbacks in their synthesis are often low yields and the restricted presence of functional groups either based on incomplete transformation of all corners or the reactivity of the functional groups. Particularly amine-functionalization reveals some synthetic challenges. In this study we report the synthesis of a new class of octafunctionalized hydrogen bond forming spherosilicates via a facile route based on octabromo alkyl functionalized cubic spherosilicates. Four different alkyl chain lengths, namely C4, C5, C6 and C11, were realized starting from [small omega]-alkenylbromides via hydrosilylation of Q8M8H. Using sodium azide in a mixture of acetonitrile : DMF = 10 : 1, the octaazide was obtained quantitatively and could be rapidly transformed in an octaamine cube via catalytic hydrogenation over Pd/C in absolute ethanol. The following reaction to hydrogen bond forming spherosilicates was performed in situ by adding propyl isocyanate. All transformations proceed quantitatively at the eight corners of the cube, which was evidenced by NMR spectroscopy and ESI-MS measurements. The Q8-target compound can be separated after each reaction step over simple chemical workup while no cage rearrangement was observed. The structures were confirmed using 1H, 13C, 29Si-NMR, FT-IR, elemental analysis and ESI-MS. The method opens a high yield route (overall isolated yield 83-88%) for structural building blocks in hybrid materials.

Micropatterned polymer surfaces that operate at various temperatures are required for emerging technical applications such as handling of objects or space debris. As the mechanical properties of polymers can vary significantly with temperature, adhesion performance can exhibit large variability. In the present paper, we experimentally study temperature effects on the adhesion of micropatterned adhesives (pillar length 20 µm, aspect ratios 0.4 and 2) made from three different polymers, i.e., polydimethylsiloxane (PDMS), perfluoropolyether dimethacrylate (PFPEdma), and polyurethane (PU-ht). PU specimens showed the highest pull-off stresses of about 57 kPa at 60 °C, i.e., more than twice the value of unpatterned control samples. The work of separation similarly showed a maximum at that temperature, which was identified as the glass transition temperature, T g . PDMS and PFPEdma specimens were tested above their T g . As a result, the adhesion properties decreased monotonically (about 50% for both materials) for temperature elevation from 20 to 120 °C. Overall, the results obtained in our study indicate that the operating temperature related to the glass transition temperature should be considered as a significant parameter for assessing the adhesion performance of micropatterned adhesives and in the technical design of adhesion devices.

The potential of a new design of adhesive microstructures in the micrometer range for enhanced dry adhesion is investigated. Using a two-photon lithography system, complex 3D master structures of funnel-shaped microstructures are fabricated for replication into poly(ethylene glycol) dimethacrylate polymer. The diameter, the flap thickness, and the opening angle of the structures are varied systematically. The adhesion of single structures is characterized using a triboindenter system equipped with a flat diamond punch. The pull-off stresses obtained reaches values up to 5.6 MPa, which is higher than any values reported in literature for artificial dry adhesives. Experimental and numerical results suggest a characteristic attachment mechanism that leads to intimate contact formation from the edges toward the center of the structures. van der Waals interactions most likely dominate the adhesion, while contributions by suction or capillarity play only a minor role. Funnel-shaped microstructures are a promising concept for strong and reversible adhesives, applicable in novel pick and place handling systems or wall-walking robots.

In this work, we propose three amendments to Persson’s contact mechanics theory, the most important one being a modification of the way in which the stress distribution broadens with increasing resolution of random roughness features. The three adjustable coefficients of our treatment are gauged on existing reference data and tested against results of the contact mechanics challenge and a new set of data for adhesive slabs of finite width. Although the coefficients turn out to be of order unity, their problem-specific tuning is required to achieve highly accurate results, such as an essentially perfect dependence of contact area on load for non-adhesive, self-affine solids. Despite an overall convincing agreement between theory and full simulations, we find it to be intrinsically impossible to make the theory reflect the exact asymptotics of the stress distribution at small and large stresses. In addition, we find that the transition from small to large contact happens too abruptly with decreasing thickness of the elastic slab.

Bio-inspired fibrillar surfaces with reversible adhesion to stiff substrates have been thoroughly investigated over the last decade. In this paper we propose a novel composite fibril consisting of a soft tip layer and stiffer stalk with differently shaped interfaces (flat vs. curved) between them. A tensile stress is applied remotely on the free end of the fibril whose other end adheres to a rigid substrate. The stress distributions and the resulting adhesion of such structures were numerically investigated under plane strain ( 2 D ) and axisymmetric ( 3 D ) conditions. The stress intensities were evaluated for different combinations of layer thickness and Young’s moduli. The adhesion strength values were found to increase for thinner layers and larger modulus ratio; these trends are also reflected in selected experimental results. The results of this paper provide a new strategy for optimizing adhesion strength of fibrillar surfaces.

The benefits of synthetic fibrillar dry adhesives for temporary and reversible attachment to hard objects with smooth surfaces have been successfully demonstrated in previous studies. However, surface roughness induces a dramatic reduction in pull-off stresses and necessarily requires revised design concepts. Toward this aim, we introduce cylindrical two-phase single pillars, which are composed of a mechanically stiff stalk and a soft tip layer. Adhesion to smooth and rough substrates is shown to exceed that of conventional pillar structures. The adhesion characteristics can be tuned by varying the thickness of the soft tip layer, the ratio of the Young’s moduli and the curvature of the interface between the two phases. For rough substrates, adhesion values similar to those obtained on smooth substrates were achieved. Our concept of composite pillars overcomes current practical limitations caused by surface roughness and opens up fields of application where roughness is omnipresent.