Light-Induced Switching of Surfaces at Wetting Transitions through Photoisomerization of Polymer Monolayers

We report on a method to generate surfaces whose wettability can be reversibly switched between a superhydrophobic and Wenzel state or a Wenzel and superwetting state just by a short UV or VIS irradiation. To achieve this, we generate a silicon surface with a nanoscale roughness (“black silicon”) and attach a polymer monolayer to it. The polymer contains a fluorinated azobenzene moiety which can be switched between the <i>cis</i> and <i>trans</i> state depending on the wavelength of the light used during illumination. The surface energy of the polymer coating is carefully adjusted to the energy value which separates distinct wetting regimes of the nanorough surface. This coupling of light induced switching to a transition of the wetting regimes can cause changes in the water contact angle as high as Δθ = 140° in the advancing CA or more than 175° in the receding CA even when the surface energy is changed only in a rather small range. Short irradiation times with UV or VIS light are enough to change the roll-off angle from <5° to no roll off at all and back. We discuss the requirements necessary so that large changes in the contact angle occur during photoswitching processes on rough surfaces

Molting Materials: Restoring Superhydrophobicity after Severe Damage via Snakeskin-like Shedding

The nanostructures that are required to generate superhydrophobic surfaces are always sensitive to shear and are easily damaged, especially by scratching with sharp objects. As a result of this destruction, the water repellency will be lost. We introduce a novel approach to restoring the original surface properties after mechanical damage. In this approach, the damaged layer is shed like the skin of a snake. This is demonstrated with a three-layer stack as a proof-of-principle system: when the original, superhydrophobic surface layer is damaged, this leads to the dissolution of a sacrificial layer below it. Thus, the damaged layer is shed, a new unscathed surface is uncovered, and superhydrophobicity can easily be restored after a short washing

Time-Resolved Analysis of Biological Reactions Based on Heterogeneous Assays in Liquid Plugs of Nanoliter Volume

In this article, we present a concept which uses liquid plugs as reaction volumes for heterogeneous assay reactions to facilitate time-resolved analysis of biomolecular reactions. For this purpose, the reaction is first compartmentalized to a train of many identical plugs. Therefore, we established a simple fluidic setup build from off-the-shelf available tubing and connectors. It permits reliable formation of plugs and successive dosing of further assay reagents to these compartments (plug volume <5% CV). The time course of the reaction is obtained by routing the plugs successively through a detector. Thereby, the arrival time of a given plug at the detector represents the reaction time of the overall reaction at that moment. Thus, each analyzed plug represents a discrete state of the overall reaction. With this approach, we can achieve a temporal resolution as small as one second, which hardly can be met by conventional analytical methods for analysis of endogenous biological compounds. For analysis of the content of the plugs, we developed a method which allows for heterogeneous assays in two-phase flow. For this purpose, functionalized superparamagnetic beads are enclosed in the plugs for specific binding of the assay product. Purification from supernatant species is achieved by transferring the beads with bound analyte across the phase boundary between aqueous plugs and water-immiscible carrier fluid. We demonstrate this assay principle exemplarily for a sandwich immunoassay (cytokine IL-8). Time-resolved analysis is validated by monitoring a cell-free in vitro expression reaction (<i>turboGFP</i>) in plugs and conventionally in bulk solution. We show that our approach allows for analyzing the entire course of a reaction in a single run. It permits kinetic studies of biological processes with significantly reduced experimental effort and consumption of costly reagents

“Grafting Through”: Mechanistic Aspects of Radical Polymerization Reactions with Surface-Attached Monomers

In this paper, we investigate the influence of selected reaction parameters on the formation of surface-attached polymer monolayers. The process is based on the use of self-assembled monolayers containing a polymerizable group and the performance of a bulk free radical polymerization reaction (“grafting through polymerization”). To this, methacryl moieties were immobilized on silica gel surfaces via a silane linker. During the polymerization reaction in a conventional way, free polymer is formed in solution. However, every now and then during chain growth also surface-attached monomers become integrated in the polymer chains, leading instantaneously to covalent linking of the growing polymer molecules to the surfaces. As more and more polymer chains become attached, this leads to the formation of a surface-attached polymer layer on the silica surface. Various sets of polymerization reactions were performed and the influence of a variation of temperature, reaction time and concentration of monomer, initiator, and immobilized monomer onto the layer formation are investigated. We propose a model of the layer formation process and the grafting-through process is compared to grafting-to and grafting-from techniques

Binding of Functionalized Polymers to Surface-Attached Polymer Networks Containing Reactive Groups

To study diffusion and binding of polymers into surface-attached networks containing reactive groups, surface-attached polymer networks bound to oxidized silicon surfaces are generated, which contain succinimide ester groups. The surface-attached polymer layers are brought into contact with poly­(ethylene glycol)­s (PEG), which carry terminal amine end groups and which have systematically varied molecular weights. The coupling reaction between the active ester groups in the polymer networks and the amine groups in the incoming chains are studied by ellipsometry, surface plasmon spectroscopy, AFM, and Fourier transform infrared spectroscopy (FTIR). The degree of functionalization of the reactive layers by the PEG-NH₂ depends strongly on the cross-link density of the network, the active ester content, and the molecular weight of the amine-terminated polymer. A model for the attachment reaction is proposed which suggests that the incoming polymer chains bind only at the outer periphery of the network in a narrow penetration zone. According to this model, when the incoming polymers are rather short, penetration into the layer and binding are prohibited by the high segment density and the anisotropic stretching of the surface-attached networks (“entropic shielding”). For incoming chains with a higher molecular weight and/or networks with a small mesh sizes, size exclusion effects determine diffusion and binding

Low Ice Adhesion on Nano-Textured Superhydrophobic Surfaces under Supersaturated Conditions

Ice adhesion on superhydrophobic surfaces can significantly increase in humid environments because of frost nucleation within the textures. Here, we studied frost formation and ice adhesion on superhydrophobic surfaces with various surface morphologies using direct microscale imaging combined with macroscale adhesion tests. Whereas ice adhesion increases on microtextured surfaces, a 15-fold decrease is observed on nanotextured surfaces. This reduction is because of the inhibition of frost formation within the nanofeatures and the stabilization of vapor pockets. Such “Cassie ice”-promoting textures can be used in the design of anti-icing surfaces

Functional Cryogel Microstructures Prepared by Light-Induced Cross-Linking of a Photoreactive Copolymer

A novel, highly efficient method for the preparation of functional, microstructured and surface-attached cryogels is described. Photoinduced C,H-insertion reactions are used to generate cryogels in a single, rapid photo-cross-linking process. To this end, solutions containing both a photoreactive copolymer and the (bio)­molecules to be immobilized are placed on a polymeric substrate followed by freezing and a short UV exposure. This strategy combines photolithography and cryogel formation allowing for a simultaneous generation and (bio)­functionalization of cryogels in a single reaction step. To demonstrate the potential of the generated materials for bioanalytical applications, we successfully prepared DNA and protein cryogel microarrays

Ultralow Friction Induced by Tribochemical Reactions: A Novel Mechanism of Lubrication on Steel Surfaces

The tribological properties of two steel surfaces rubbing against each other are measured while they are in contact with 1,3-diketones of varying structure. Such systems show after a short running-in period ultralow friction properties with a coefficient of friction of as low as μ = 0.005. It is suggested that the extremely favorable friction properties are caused by a tribochemical reaction between the 1,3-diketones and the steel surfaces, leading to formation of a chelated iron–diketo complex. The influence of temperature and the molecular structure of the 1,3 diketo-lubricants onto the friction properties of the system is elucidated under both static and dynamic conditions. With progression of the tribochemical reaction, the sliding surfaces become very conformal and smooth, so that the pressure is greatly reduced and further wear is strongly reduced. All iron particles potentially generated by wear during the initial running-in period are completely dissolved through complex formation. It is proposed that the tribochemical polishing reaction causes a transition from boundary lubrication to fluid lubrication

1,3-Diketone Fluids and Their Complexes with Iron

Tribological experiments with 1,3-diketone fluids in contact with iron surfaces show ultralow friction, which was suggested to be connected to the formation of iron complexes. In order to support this assumption, we calculate infrared and optical spectra of various substituted 1,3-diketones and their iron complexes using gradient-corrected density functional theory (DFT). The description of the complexes requires the application of the DFT+U scheme for a correct prediction of the high spin state on the central iron atom. With this approach, we obtain excellent agreement between experiment and simulation in infrared and optical spectra, allowing for the determination of 1,3-diketone tautomeric forms. The match in the spectra of the complex strongly supports the assumption of iron complex formation by these lubricants

Analysis of Calcium Transients and Uniaxial Contraction Force in Single Human Embryonic Stem Cell-Derived Cardiomyocytes on Microstructured Elastic Substrate with Spatially Controlled Surface Chemistries

The mechanical activity of cardiomyocytes is the result of a process called excitation–contraction coupling (ECC). A membrane depolarization wave induces a transient cytosolic calcium concentration increase that triggers activation of calcium-sensitive contractile proteins, leading to cell contraction and force generation. An experimental setup capable of acquiring simultaneously all ECC features would have an enormous impact on cardiac drug development and disease study. In this work, we develop a microengineered elastomeric substrate with tailor-made surface chemistry to measure simultaneously the uniaxial contraction force and the calcium transients generated by single human cardiomyocytes <i>in vitro</i>. Microreplication followed by photocuring is used to generate an array consisting of elastomeric micropillars. A second photochemical process is employed to spatially control the surface chemistry of the elastomeric pillar. As result, human embryonic stem cell-derived cardiomyocytes (hESC-CMs) can be confined in rectangular cell-adhesive areas, which induce cell elongation and promote suspended cell anchoring between two adjacent micropillars. In this end-to-end conformation, confocal fluorescence microscopy allows simultaneous detection of calcium transients and micropillar deflection induced by a single-cell uniaxial contraction force. Computational finite elements modeling (FEM) and 3D reconstruction of the cell–pillar interface allow force quantification. The platform is used to follow calcium dynamics and contraction force evolution in hESC-CMs cultures over the course of several weeks. Our results show how a biomaterial-based platform can be a versatile tool for <i>in vitro</i> assaying of cardiac functional properties of single-cell human cardiomyocytes, with applications in both <i>in vitro</i> developmental studies and drug screening on cardiac cultures