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Joachim P Spatz

Publications and source records attributed to Joachim P Spatz.

15 recordsLinked to original sources
PubMed2006

Cellular unbinding forces of initial adhesion processes on nanopatterned surfaces probed with magnetic tweezers.

To study the dependence of unbinding forces on the distance of molecularly defined and integrin specific c(-RGDfK-) ligand patches in initial cellular adhesion processes, we developed a magnetic tweezers setup for applying vertical forces of up to 200 pN to rat embryonic fibroblasts. The ligand patch distance is controlled with a hexagonally close packed pattern of biofunctionalized gold nanoparticles prepared by block-copolymer micelle nanolithography. Each gold nanoparticle potentially activates up to one alpha(v)beta(3)-integrin. The distances between the gold nanoparticles determine the separation of individual integrins and thus the assembly of integrin clusters. The results show an increase in cellular unbinding forces from approximately 6 to more than 200 pN for a decreasing ligand distance of 145 to 58 nm after 5 min of cell adhesion. Furthermore, we observe a strong dependence on adhesion time during the first 10 min of cell surface contact suggesting an active, cooperative cell response that is controlled by the spacing between individually activated integrins.

Animals↗
PubMed2006

Tuning surface energies with nanopatterned substrates.

A novel approach to varying the surface energy of biofunctional substrates has been developed, where surface energies are controlled by utilizing tunable nanopatterned substrates. In this study we functionalized the nanopattern with streptavidin, providing an adhesive interface for biotinylated probes. To obtain the surface energies, we applied the Johnson-Kendall-Roberts model to the adhesion-induced deformation of elastic beads. The results reveal a linear relationship between surface energy and ligand density, demonstrating the capability of this technique to adjust surface energy.

Adsorption↗
PubMed2005

Lateral spacing of integrin ligands influences cell spreading and focal adhesion assembly.

Cell-extracellular matrix (cell-ECM) interactions mediated by integrin receptors are essential for providing positional and environmental information necessary for many cell functions, such as proliferation, differentiation and survival. In vitro studies on cell adhesion to randomly adsorbed molecules on substrates have been limited to sub-micrometer patches, thus preventing the detailed study of structural arrangement of integrins and their ligands. In this article, we illustrate the role of the distance between integrin ligands, namely the RGD (arginine-glycine-aspartate) sequence present in ECM proteins, in the control of cell adhesion. By using substrates, which carry cyclic RGD peptides arranged in highly defined nanopatterns, we investigated the dynamics of cell spreading and the molecular composition of adhesion sites in relation to a fixed spacing between the peptides on the surface. Our novel approach for in vitro studies on cell adhesion indicates that not only the composition, but also the spatial organization of the extracellular environment is important in regulating cell-ECM interactions.

Animals↗
PubMed2005

Symmetry dependence of holograms for optical trapping.

No iterative algorithm is necessary to calculate holograms for most holographic optical trapping patterns. Instead, holograms may be produced by a simple extension of the prisms-and-lenses method. This formulaic approach yields the same diffraction efficiency as iterative algorithms for any asymmetric or symmetric but nonperiodic pattern of points while requiring less calculation time. A slight spatial disordering of periodic patterns significantly reduces intensity variations between the different traps without extra calculation costs. Eliminating laborious hologram calculations should greatly facilitate interactive holographic trapping.

Algorithms↗
PubMed2005

A theoretical description of elastic pillar substrates in biophysical experiments.

Arrays of elastic pillars are used in biophysical experiments as sensors for traction forces. The evaluation of the forces can be complicated if they are coupled to the pillar displacements over large distances. This is the case if many of the pillars are interconnected by elastic linkages as, for example, in fiber networks that are grown on top of pillars. To calculate the traction forces in such a network, we developed a set of nonlinear inhomogeneous equations relating the forces in the linking elements to the resulting pillar deflections. We chose a homogeneous, activated two-dimensional network of cytoskeletal actin filaments to illustrate that a pillar substrate is generally not a force sensor but a force-gradient sensor. In homogeneous networks the forces acting along the filaments can be approximated by analyzing only pillar deflections in the edge zones of the substrate and by integration over the corresponding force gradients.

Actin Cytoskeleton↗
PubMed2005

Ultrathin coatings from isocyanate-terminated star PEG prepolymers: layer formation and characterization.

In this study we present the preparation of thin and ultrathin coatings from six-arm star-shaped isocyanate-terminated prepolymers on amino-functionalized silicon wafers. The backbone of the stars is a statistical copolymer of ethylene oxide and propylene oxide in the ratio 80:20 (Star PEG). Film preparation by spin coating from aqueous THF resulted in a variety of film morphologies that are determined by the water content of the solvent. Water is indispensable for activation of the isocyanate-terminated stars in solution and for proper cross-linking of the coatings on the substrate. This cross-linking results in a dense network of PEG chains on the substrate linked via urea groups with a mesh size of the network that corresponds to the arm length of the stars. Layer thickness variations between 3 and 500 nm revealed a strong dependence of the contact angle with water on the layer thickness which is explained by the chemical composition of the coatings. Due to the high functionality of the star-shaped prepolymers, free amino groups remain in the films that were detected by fluorescence microscopy after reaction with 4-chloro-7-nitrobenzofurazan (NBF). To test the system for the ability to prevent unspecific interaction with proteins, adsorption of fluorescence-labeled avidin was examined with fluorescence microscopy. For layer thicknesses between 3 and 50 nm, no protein adsorption could be detected.

Journal Article↗
PubMed2005

Mechanical response analysis and power generation by single-cell stretching.

To harvest useful information about cell response due to mechanical perturbations under physiological conditions, a cantilever-based technique was designed, which allowed precise application of arbitrary forces or deformation histories on a single cell in vitro. Essential requirements for these investigations are a mechanism for applying an automated cell force and an induced-deformation detection system based on fiber-optical force sensing and closed loop control. The required mechanical stability of the setup can persist for several hours since mechanical drifts due to thermal gradients can be eliminated sufficiently (these gradients are caused by local heating of the cell observation chamber to 37 degrees C). During mechanical characterization, the cell is visualized with an optical microscope, which enables the simultaneous observation of cell shape and intracellular morphological changes. Either the cell elongation is observed as a reaction against a constant load or the cell force is measured as a response to constant deformation. Passive viscoelastic deformation and active cell response can be discriminated. The active power generated during contraction is in the range of Pmax= 10(-16) Watts, which corresponds to 2500 ATP molecules s(-1) at 10 k(B)T/molecule. The ratio of contractive to dissipative power is estimated to be in the range of 10(-2). The highest forces supported by the cell suggest that about 10(4) molecular motors must be involved in contraction. This indicates an energy-conversion efficiency of approximately 0.5. Our findings propose that, in addition to the recruitment of cell-contractile elements upon mechanical stimulation, the cell cytoskeleton becomes increasingly crosslinked in response to a mechanical pull. Quantitative stress-strain data, such as those presented here, may be employed to test physical models that describe cellular responses to mechanical stimuli.

3T3 Cells↗
PubMed2005

Microtubule gliding and cross-linked microtubule networks on micropillar interfaces.

We combined biochemical and topographical patterning to achieve motor-driven microtubule gliding on top of microfabricated pillar arrays with limited and controllable surface interactions of gliding microtubules. Kinesins immobilized on pillar heads pushed microtubules from the top of one micropillar to the next bridging up to 20 mum deep gaps filled with buffer solution. Distances of more than 10 mum were by-passed, and microtubule buckling was occasionally observed. The velocity distributions of microtubules gliding on poly(dimethylsiloxane) (PDMS) pillars, on flat PDMS, and on glass were found to be different, most likely due to topological and/or chemical differences between the substrates. We also used pillar arrays to suspend cross-linked microtubule networks, whose structural characteristics were governed by the topographical characteristics of the pillar pattern. These experiments open new possibilities to study the dynamics and the self-organization of motor/microtubule networks in defined topologically structured environments.

Adsorption↗
PubMed2004

Activation of integrin function by nanopatterned adhesive interfaces.

To study the function behind the molecular arrangement of single integrins in cell adhesion, we designed a hexagonally close-packed rigid template of cell-adhesive gold nanodots coated with cyclic RGDfK peptide by using block-copolymer micelle nanolithography. The diameter of the adhesive dots is < 8 nm, which allows the binding of one integrin per dot. These dots are positioned with high precision at 28, 58, 73, and 85 nm spacing at interfaces. A separation of > or = 73 nm between the adhesive dots results in limited cell attachment and spreading, and dramatically reduces the formation of focal adhesion and actin stress fibers. We attribute these cellular responses to restricted integrin clustering rather than insufficient number of ligand molecules in the cell-matrix interface since "micro-nanopatterned" substrates consisting of alternating fields with dense and no nanodots do support cell adhesion. We propose that the range between 58-73 nm is a universal length scale for integrin clustering and activation, since these properties are shared by a variety of cultured cells.

Cell Adhesion↗
PubMed2004

Cell shape normalization, dendrite orientation, and melanin production of normal and genetically altered (haploinsufficient NF1)-melanocytes by microstructured substrate interactions.

Little is known about how functional regulation failure in genetically altered cells is influenced by topographical confinement of cells, a situation often present in tissues in vivo. We used cultured melanocytes derived from human skin samples as a model system for such investigations. Normal melanocytes have a very well defined shape consisting of a cell body and two dendrites arranged 180 degrees relative to each other. In contrast, neurofibromin 1-melanocytes (NF1-melanocytes) have up to a 50% reduction of neurofibromin 1, which results in an altered morphology that can be easily measured. NF1-melanocytes deviate from the defined structure of normal melanocytes by forming more than two dendrites per cell. We show that morphology consequences of genetically altered melanocytes can be canceled if cells interact with substrates microstructured by stripes that apply mechanophysical signals in the form of physical topography. The strength of the mechanophysical signal was varied systematically by increasing the height of the microstructures. Melanocytes respond to surface topographical features that are larger than 50 nm and have lateral confinements smaller 4 microns. The response of normal and NF1-melanocytes to different topographies was analyzed quantitatively by determining density distributions for the number of dendrites per cell, the angles between dendrites, and the orientation imprinted in the substrate. The synthesis of melanin, a pigment produced by melanocytes, differs in the case of genetically altered NF1- and normal melanocytes. In both cases, the interaction with microstripes enhanced melanin production significantly. This enhanced melanin production is speculated to be caused by the mechanical stabilization of the dendrites by substrate guidance.

Cell Size↗
PubMed2003

Genome function and nuclear architecture: from gene expression to nanoscience.

Biophysical, chemical, and nanoscience approaches to the study of nuclear structure and activity have been developing recently and hold considerable promise. A selection of fundamental problems in genome organization and function are reviewed and discussed in the context of these new perspectives and approaches. Advancing these concepts will require coordinated networks of physicists, chemists, and materials scientists collaborating with cell, developmental, and genome biologists.

Animals↗
PubMed2002

Fluorescence of dyes adsorbed on highly organized, nanostructured gold surfaces.

It is shown that fluorescent dyes can be adsorbed selectively on gold nanoparticles which are immobilized on a glass substrate and that the fluorescence originating from the adsorbed dyes exhibits significantly less quenching when compared to dyes adsorbed on bulk gold. Self-assembled monolayers of lissamine sulfide molecules have been studied both on bulk gold and on glass surfaces bearing gold nanoparticles. Gold nanoparticles have been arranged in ordered, two-dimensional patterns, with periodicity in the microm range and used as substrate for the fluorescent dyes. Optical resolution of the fluorescence originating from the pattern has been achieved with laser-scanning confocal microscopy.

Journal Article↗