Density gradients at hydrogel interfaces for enhanced cell penetration.

We report that stiffness gradients facilitate infiltration of cells through otherwise cell-impermeable hydrogel interfaces. By enabling the separation of hydrogel manufacturing and cell seeding, and by improving cell colonization of additively manufactured hydrogel elements, interfacial density gradients present a promising strategy to progress in the creation of 3D tissue models.

Self-assembly of enantiopure domains: the case of indigo on Cu(111).

The adsorption of indigo molecules on Cu(111) was investigated by low temperature (5 K) scanning tunneling microscopy from the isolated single molecule regime to one monolayer. Structural optimization and image calculations demonstrate that the molecules are in a physisorbed state. Because of the reduced symmetry at the surface, single molecules acquire a chiral character upon adsorption leading to a two-dimensional (2D) chirality. They adopt two adsorption configurations, related by a mirror symmetry of the substrate, each with a distinct molecular orientation. Consequently, the 2D chirality is expressed by the orientation of the molecule. For higher coverage, molecules self-assemble by hydrogen bonding in nearly homochiral molecular chains, whose orientation is determined by the orientation taken by the isolated molecules. When the coverage approaches one monolayer, these chains pack into domains. Finally, the completion of the monolayer induces the expulsion of the molecules of the wrong chirality that are still in these domains, leading to perfect resolution in enantiopure domains.

Mapping van der Waals forces with frequency modulation dynamic force microscopy.

Nanometre-size gold clusters supported on MoS(2)(0001) are investigated by means of ultrahigh-vacuum frequency modulation dynamic force microscopy. Topography and frequency shift images are simultaneously obtained using the average tunnelling current to regulate the tip-substrate distance. Two families of clusters are observed, giving different frequency shift images. While the topographic and frequency shift profiles have similar shapes on small clusters (size [Formula: see text] nm), they are quite different near the top of large clusters (size [Formula: see text] nm): the topographic profile is rounded, but the frequency shift profile exhibits rather steep edges and a depression near the centre of the island. It is demonstrated that these differences result from the finite range of van der Waals forces. On small islands, the frequency shift is dominated by the interaction of the tip with the substrate. On large islands, it is dominated by the interaction with the island. The particular observed shape results from the geometry of the island. These interpretations are comforted by analytical and numerical calculations. In particular, the characteristic shape of the frequency shift profiles on large islands can be reproduced by introducing realistic parameters and considering only the contribution of van der Waals forces.

Atomic and macroscopic reaction rates of a surface-catalyzed reaction

The catalytic oxidation of carbon monoxide (CO) on a platinum (111) surface was studied by scanning tunneling microscopy. The adsorbed oxygen atoms and CO molecules were imaged with atomic resolution, and their reactions to carbon dioxide (CO2) were monitored as functions of time. The results allowed the formulation of a rate law that takes the distribution of the reactants in separate domains into account. From temperature-dependent measurements, the kinetic parameters were obtained. Their values agree well with data from macroscopic measurements. In this way, a kinetic description of a chemical reaction was achieved that is based solely on the statistics of the underlying atomic processes.