Raphael Zahn

Engineering the Extracellular Environment: Strategies for Building 2D and 3D Cellular Structures

Cell fate is regulated by extracellular environmental signals. Receptor specific interaction of the cell with proteins, glycans, soluble factors as well as neighboring cells can steer cells towards proliferation, differentiation, apoptosis or migration. In this review, approaches to build cellular structures by engineering aspects of the extracellular environment are described. These methods include non-specific modifications to control the wettability and stiffness of surfaces using self-assembled monolayers (SAMs) and polyelectrolyte multilayers (PEMs) as well as methods where the temporal activation and spatial distribution of adhesion ligands is controlled. Building on these techniques, construction of two-dimensional cell sheets using temperature sensitive polymers or electrochemical dissolution is described together with current applications of these grafts in the clinical arena. Finally, methods to pattern cells in three-dimensions as well as to functionalize the 3D environment with biologic motifs take us one step closer to being able to engineer multicellular tissues and organs.

Ion-induced cell sheet detachment from standard cell culture surfaces coated with polyelectrolytes

Polyelectrolyte multilayers (PEMs), formed by alternating layer-by-layer deposition of polyanions and polycations, are an ideal substrate for controlling cellular adhesion and behavior. In the present study we propose a simple mechanism for the controlled detachment of C(2)C(12) myoblasts cell sheets from PEMs consisting of poly(l-lysine) and hyaluronic acid with a topmost layer of fibronectin. The multilayers were deposited on two standard cell culture surfaces: glass and polystyrene. Adding a low concentration of nontoxic ferrocyanide to the cell culture medium resulted in erosion of the polyelectrolyte multilayer and rapid detachment of viable cell sheets. Additional Quartz Crystal Microbalance and Atomic Force Microscopy measurements indicated that the detached cells retained their extracellular matrix and that no polyelectrolyte molecules remained bound to the cell sheets. The dissolution of polyelectrolyte multilayers by multivalent ions is a promising approach to cell sheet engineering that could potentially be used for regenerative medicine.

Nanoplasmonic sensing of metal–halide complex formation and the electric double layer capacitor

Many nanotechnological devices are based on implementing electrochemistry with plasmonic nanostructures, but these systems are challenging to understand. We present a detailed study of the influence of electrochemical potentials on plasmon resonances, in the absence of surface coatings and redox active molecules, by synchronized voltammetry and spectroscopy. The experiments are performed on gold nanodisks and nanohole arrays in thin gold films, which are fabricated by improved methods. New insights are provided by high resolution spectroscopy and variable scan rates. Furthermore, we introduce new analytical models in order to understand the spectral changes quantitatively. In contrast to most previous literature, we find that the plasmonic signal is caused almost entirely by the formation of ionic complexes on the metal surface, most likely gold chloride in this study. The refractometric sensing effect from the ions in the electric double layer can be fully neglected, and the charging of the metal gives a surprisingly small effect for these systems. Our conclusions are consistent for both localized nanoparticle plasmons and propagating surface plasmons. We consider the results in this work especially important in the context of combined electrochemical and optical sensors.

Norovirus GII.4 Virus-like Particles Recognize Galactosylceramides in Domains of Planar Supported Lipid Bilayers

A sticky situation: Domain-dependent recognition of the glycosphingolipid galactosylceramide by norovirus-like particles (see picture; red/yellow) is shown using supported lipid bilayers (purple) as model membranes. Optimal ligand presentation is found to promote strong binding to GalCer. This presentation can be found at the edges of the glycosphingolipid-enriched domains (green) and binding is repressed in the absence of these domains.

Effect of polyelectrolyte interdiffusion on electron transport in redox-active polyelectrolyte multilayers

We report that polyelectrolyte interdiffusion has a strong influence on the speed of electron transport in redox-active polyelectrolyte multilayers. For interdiffusive polyelectrolyte multilayers, the measured electron diffusion constants were consistently two orders of magnitude larger than the ones for systems where interdiffusion was not possible. This finding was verified for different polyelectrolyte multilayer systems that either allowed or prevented polyelectrolyte interdiffusion. The electron diffusion constants were deduced from cyclic voltammograms at various scanning speeds using the Randles–Sevcik equation for irreversible redox processes. The increased speed of electron transport in interdiffusive multilayers is explained by short-range movement of the redox-sites, allowing fast electron exchange through physical contact of neighboring redox-sites.

From nanodroplets to continuous films: how the morphology of polyelectrolyte multilayers depends on the dielectric permittivity and the surface charge of the supporting substrate

Using atomic force microscopy, we investigated how the morphology of layer-by-layer deposited polyelectrolyte multilayers is influenced by the physical properties of the supporting substrate. The surface coverage of the assembly and its topography were found to be dependent on the dielectric permittivity of the substrate and the strength of the electrostatic interactions between polyanions and polycations. For poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS), a strongly interacting polyelectrolyte couple, no dependency of the surface morphology on the physical properties of the underlying substrate was observed. In contrast, the weakly interacting pair poly(L-lysine)/hyaluronic acid (PLL/HA) formed rapidly continuous, flat layers on substrates of low dielectric permittivity and inhomogeneous droplet-films on substrates of high dielectric permittivity. Variations in the dielectric permittivity account for changes in the image charges that are induced in the substrate. These changes influence the balance between repulsive electrostatic forces (and image forces) and attractive van der Waals interactions, and thus cause the differences in surface morphology. Differences in surface charge did not influence the morphology of the polyelectrolyte multilayers, but higher surface charge resulted in more polymeric material adsorbed on the surface. A comparison between (PLL/HA) multilayers with and without an initial layer of poly(ethyleneimine) (PEI) supports this hypothesis.

Tuning the electrochemical swelling of polyelectrolyte multilayers toward nanoactuation.

We discuss physicochemical determinants of electrochemical polyelectrolyte multilayer swelling that are relevant to actuator usage. We used electrochemical quartz crystal microbalance with dissipation monitoring (EC-QCM-D) and cyclic voltammetry to compare the electrochemical swelling of two types of ferrocyanide-containing polyelectrolyte multilayers, poly(l-glutamic acid)/poly(allylamine hydrochloride) (PGA/PAH), and carboxymethyl cellulose/poly(diallyldimethylammonium chloride) (CMC/PDDA). We showed that ferrocyanide oxidation causes the swelling of PGA/PAH multilayers whereas it results in the contraction of CMC/PDDA multilayers. This behavior can be attributed to the presence of a positive and a negative Donnan potential in the case of PGA/PAH and CMC/PDDA multilayers, respectively. Using multilayers consisting of PGA and poly(allylamine) ferrocene (PGA/PAH-FC), we applied EC-QCM-D and demonstrated potentiostatic thickness control with nanometer precision and showed that the multilayers thickness depends linearly on the applied potential within a certain potential range.

Topological and network analysis of lithium ion battery components: the importance of pore space connectivity for cell operation

The structure of lithium ion battery components, such as electrodes and separators, are commonly characterised in terms of their porosity and tortuosity. The ratio of these values gives the effective transport of lithium ions in the electrolyte-filled pore spaces, which can be used to determine the ionic resistivity and corresponding voltage losses. Here, we show that these microstructural characteristics are not sufficient. Analysis of tomographic data of commercial separators reveals that different polyolefin separators have similar porosity and through-plane tortuosity, which, in the homogenised picture of lithium ion cell operation, would imply that these different separators exhibit similar performance. However, numerical diffusion simulations indicate that this is not the case. We demonstrate that the extent to which lithium ion concentration gradients are induced or smoothed by the separator structure is linked to pore space connectivity, a parameter that can be determined by topological or network based analysis of separators. These findings enable us to propose how to design separator microstructures that are safer and accommodate fast charge and discharge.