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Gecko‐Inspired Surfaces: A Path to Strong and Reversible Dry Adhesives

The amazing adhesion of gecko pads to almost any kind of surfaces has inspired a very active research direction over the last decade: the investigation of how geckos achieve this feat and how this knowledge can be turned into new strategies to reversibly join surfaces. This article reviews the fabrication approaches used so far for the creation of micro- and nanostructured fibrillar surfaces with adhesive properties. In the light of the pertinent contact mechanics, the adhesive properties are presented and discussed. The decisive design parameters are fiber radius and aspect ratio, tilt angle, hierarchical arrangement and the effect of the backing layer. Also first responsive systems that allow thermal switching between nonadhesive and adhesive states are described. These structures show a high potential of application, providing the remaining issues of robustness, reliability, and large-area manufacture can be solved.

Advances in Colloidal Assembly : The Design of Structure and Hierarchy in Two and Three Dimensions

This Review highlights the large number of methods to exploit colloidal assembly of comparably simple particles with nano- to micrometer dimensions in order to access complex structural hierarchies from nanoscopic over microscopic to macroscopic dimensions

Multivalent H-bonds for self-healing hydrogels

UPy is used as a reversible and dynamic crosslinker to prepare hydrogels that are injectable and undergo rapid self-healing in response to damage.

Phototriggering of Cell Adhesion by Caged Cyclic RGD Peptides

The controlled and selective adhesion of cells to surfaces is an important issue in cell biology and tissue engineering. Different strategies have been reported in which thermally, photochemically, and electrochemically responsive surfaces and materials are used to manipulate cell adhesion. A more generic approach that would be suitable for any system, independent of its chemical constitution, would be advantageous. Such a strategy could not rely on material properties; instead, the molecular interactions involved in cell attachment must be controlled directly. The design of a strategy to trigger the attachment event needs to consider the sensitivity of cells to most triggering sources (electric fields, chemical stimuli, pressure, and temperature jumps). Light of wavelength above 320 nm appears to be a convenient trigger, as its interaction with biomolecular species is negligible. Light-controlled cellular attachment requires the development of photosensitive molecules able to mediate cellular adhesion and whose activity changes upon irradiation. For this study, we selected the RGD cell-adhesive peptide, well known to promote integrin-mediated cell adhesion, and modified it by introducing a photolabile caging group on the carboxylic acid side chain of the aspartic acid residue (Scheme 1). The presence of the caging group may cause steric hindrance, conformational constraint, or changes in the charge distribution of the peptide and thus prevent recognition of the peptide by the integrins. Light irradiation releases the cage from the peptide structure and restores the activity of the peptide to enable in situ site and temporal control of cell attachment. Cell-repellent surfaces modified with the caged peptide (“off” state) can become cell-adhesive (“on” state) upon irradiation with light of the appropriate wavelength and intensity. The selection of the caging position requires previous knowledge of the structural characteristics of the RGD– integrin binding site. In the particular case of the pentapeptide cyclo(-Arg-Gly-Asp-d-Phe-Val-) (cyclo(RGDfK)), a very active and selective ligand of integrin aVb3, [7] it has been shown that the binding site involves two divalent cations, and that the aspartate unit acts as a ligand for one of them. Therefore, we decided to introduce the caging group at this position. It is also known that the amino acid in the fifth position (Lys) does not have significant influence on the activity of the peptide. The free amine group of the Lys residue has been used as anchoring position through which the peptide can be coupled to surfaces. 3-(4,5-Dimethoxy-2-nitrophenyl)-2-butyl ester (DMNPB) was selected as the photolabile caging group (lmax = 346 nm, emax = 4100m 1 cm ). The caged Asp derivative DMNPBAsp-Fmoc (Fmoc= 9-fluorenylmethoxycarbonyl) and the caged peptide cyclo[RGD(DMNPB)fK] were obtained and characterized as described in the Supporting Information. Their UV spectra are shown in Figure 1. The photolytic properties of the caged peptide in solution were then determined quantitatively. Upon exposure for 2 h to light of wavelength 364 nm in neutral buffered solution, up to 70% of cyclo[RGD(DMNPB)fK] disappeared, and up to 93% of the photolytic reaction product obtained was Scheme 1. Chemical structure of cyclo[RGD(DMNPB)fK] (DMNPB in red) attached to the surface through the TEG linker (green). The caging group is released upon irradiation at 351 nm.

Light-triggered in vivo activation of adhesive peptides regulates cell adhesion, inflammation and vascularization of biomaterials

Materials engineered to elicit targeted cellular responses in regenerative medicine must display bioligands with precise spatial and temporal control. Although materials with temporally regulated presentation of bioadhesive ligands using external triggers, such as light and electric fields, have been recently realized for cells in culture, the impact of in vivo temporal ligand presentation on cell-material responses is unknown. Here, we present a general strategy to temporally and spatially control the in vivo presentation of bioligands using cell adhesive peptides with a protecting group that can be easily removed via transdermal light exposure to render the peptide fully active. We demonstrate that non-invasive, transdermal time-regulated activation of cell-adhesive RGD peptide on implanted biomaterials regulates in vivo cell adhesion, inflammation, fibrous encapsulation, and vascularization of the material. This work shows that triggered in vivo presentation of bioligands can be harnessed to direct tissue reparative responses associated with implanted biomaterials.

Wavelength-Selective Caged Surfaces: How Many Functional Levels Are Possible?

The possibility of wavelength-selective cleavage of seven photolabile caging groups from different families has been studied. Amine-, thiol-, and carboxylic-terminated organosilanes were caged with o-nitrobenzyl (NVOC, NPPOC), benzoin (BNZ), (coumarin-4-yl)methyl (DEACM), 7-nitroindoline (DNI, BNI), and p-hydroxyphenacyl (pHP) derivatives. Caged surfaces modified with the different chromophores were prepared, and their photosensitivity at selected wavelengths was quantified. Different pairs, trios, and quartets of chromophore combinations with wavelength-selective photoresponse were identified. Our results show, for the first time, the possibility of generating surfaces with up to four different and independently addressable functional levels. In addition, this manuscript presents the first systematic comparison of the photolytic properties of different photolabile groups under different irradiation conditions.

Triggered Cell Release from Materials Using Bioadhesive Photocleavable Linkers

The ability to trigger or turn “on” or “off” material properties with external stimuli in order to control biological responses is critically important to biotechnological and biomedical applications. One such application is the use of light to trigger cell adhesion to synthetic materials by controlling the presentation of the bioadhesive arginine-glycine-aspartic acid (RGD) oligopeptide. Successful strategies for photoactivation of cell adhesion include direct modifi cation of the chemical structure of the RGD peptide with photoresponsive molecules, [ 1 ]

Superhydrophilic and superhydrophobic nanostructured surfaces via plasma treatment

Polyethylene terephthalate (PET) films have been structured with isolated nanofibrils and fibril bundles using oxidative plasma treatments with increasing etching ratios. The transition from fibrils to bundles was smooth and it was associated with a significant reduction in the overall top area fraction and with the development of a second organisation level at a larger length scale. This increased complexity was reflected in the surface properties. The surfaces with two-level substructures showed superhydrophilic and superhydrophobic properties depending on the surface chemistry. These properties were preserved during prolonged storage and resisted moderate mechanical stress. By combining different contact angle and drop impact measurements, the optimum surface design and plasma processing parameters for maximizing stability of the superhydrophobic or superhydrophilic properties of the PET films were identified.

Colloidal assemblies on patterned silane layers

The site-selective assembly of colloidal polymer particles onto laterally patterned silane layers was studied as a model system for the object assembly process at mesoscale dimensions. The structured silane monolayers on silicon oxide substrates were fabricated by a combination of liquid- and gas-phase deposition of different trialkoxysilanes with a photolithographic patterning technique. By using this method various types of surface functionalizations such as regions with amino functions next to areas of the bare silica surface or positively charged regions of a quaternary ammonium silane surrounded by a hydrophobic octadecylsilane film could be obtained. Furthermore, a triethoxysilane with a photoprotected amino group was synthesized, which allowed direct photopatterning after monolayer preparation, leading to free NH2 groups at the irradiated regions. The different silane monolayer patterns were used to study the surface assembly behavior of carboxylated methacrylate particles by optical and scanning electron microscopy. In dependence of the assembly conditions (different surface functionalizations, pH, and drying conditions), a selective preference of the particles for a specific surface type versus others was found. Site-specific colloid adsorption could be observed also on the photosensitive silane layers after local deprotection with light. From the photosensitive silane and positively charged ammonium silane, molecularly mixed monolayers were prepared, which allowed particle adsorption and photoactivation within the same monolayer as shown by fluorescence labeling.

Antibacterial Strategies from the Sea: Polymer-Bound Cl-Catechols for Prevention of Biofilm Formation

Inspired by the amino acid 2-chloro-4,5-dihydroxyphenylalanine (Cl-DOPA), present in the composition of the proteinaceous glue of the sandcastle worm Phragmatopoma californica, a simple strategy is presented to confer antifouling properties to polymer surfaces using (but not releasing) a bioinspired biocide. Cl-Dopamine is used to functionalize polymer materials and hydrogel films easily, to prevent biofilm formation on them.