Biological materials exhibit remarkable, purpose-adapted properties that provide a source of inspiration for designing new materials to meet the requirements of future applications. For instance, marine mussels are able to attach to a broad spectrum of hard surfaces under hostile conditions. Controlling wet-adhesion of synthetic macromolecules by analogue processes promises to strongly impact materials sciences by offering advanced coatings, adhesives, and glues. The de novo design of macromolecules to mimic complex aspects of mussel adhesion still constitutes a challenge. Phage display allows material scientists to design specifically interacting molecules with tailored affinity to material surfaces. Here, we report on the integration of enzymatic processing steps into phage display biopanning to expand the biocombinatorial procedure and enable the direct selection of enzymatically activable peptide adhesion domains. Adsorption isotherms and single molecule force spectroscopy show that those de novo peptides mimic complex aspects of bioadhesion, such as enzymatic activation (by tyrosinase), the switchability from weak to strong binders, and adsorption under hostile saltwater conditions. Furthermore, peptide-poly(ethylene oxide) conjugates are synthesized to generate protective coatings, which possess anti-fouling properties and suppress irreversible interactions with blood-plasma protein cocktails. The extended phage display procedure provides a generic way to non-natural peptide adhesion domains, which not only mimic nature but also improve biological sequence sections extractable from mussel-glue proteins. The de novo peptides manage to combine several tasks in a minimal 12-mer sequence and thus pave the way to overcome major challenges of technical wet glues.
Direct force measurements by atomic force microscopy (AFM) in combination with the colloidal probe technique are widely used to determine interaction forces in colloidal systems. However, a number of limitations are still preventing a more universal applicability of this technique. Currently, one of the most significant limitations is that only particles with diameters of several micrometers can be used as probe particles. Here, we present a novel approach, based on the combination of nanofluidics and AFM (also referred to as FluidFM-technique), that allows to overcome this size limit and extend the size of suitable probe particles below diameters of 500 nanometers. Moreover, by aspiration of colloidal particles with a hollow AFM-cantilever, the immobilization process is independent of the particle's surface chemistry. Furthermore, the probe particles can be exchanged in situ. The applicability of the FluidFM-technique is demonstrated with silica particles, which are also the types of particles most often used for the preparation of colloidal probes. By comparing 'classical' colloidal probes, i.e. probes from particles irreversibly attached with glue, and various particle sizes aspirated by the FluidFM-technique, we can quantitatively evaluate the instrumental limits. Evaluation of the force profiles demonstrate that even for 500 nm silica particles the diffuse layer properties can be evaluated quantitatively. Therefore, direct force measurements on the level of particle sizes used in industrial formulations will become available in the future.
Recombinant spider silk proteins, such as eADF4(C16), can be used for various applications. Colloidal particles of eADF4(C16) show potential as drug delivery systems. Tuning the colloidal properties of suspensions of eADF4(C16) particles represents a major prerequisite for their use in pharmaceutical formulations. In this study we determined the surface properties concerning inter-particle interactions by means of electrophoretic mobility and direct force measurements. The surface charge of eADF4(C16) spider silk particles was determined as a function of ionic strength and pH, respectively. The resulting electrophoretic mobility can be described using the O'Brien and White theory and is directly related to the amino acid sequence of the protein. We determined the extension of a fuzzy protein layer protruding into the solution by direct force measurements using a colloidal probe technique. This soft layer leads to deviations in the electrophoretic mobility and is responsible for additional repulsive forces at small separation distances. These steric forces lead to a stabilization of the particle suspension at high ionic strength. † Electronic supplementary information (ESI) available. See
Poly(N-isopropyl acrylamide) (PNIPAM) hydrogels are well known for their temperature-dependent water uptake and release. Hence, they are ideal candidates for water management applications. However, efficiency and rate of water uptake and release, respectively, have to be optimized. Here, highly stable 3D PNIPAM sponges that show a sufficiently low density and high specific pore volume, required for maximizing the amount and rate of water absorption-desorption, are presented. They are prepared by a top-down approach based on freeze-drying a dispersion of short crosslinked PNIPAM fibers coated with crosslinked PNIPAM. The sponges have low densities (4.10-21.04 mg cm ), high porosities >98%, and high specific pore volumes in the range of 47-243 cm g depending on the concentration of the dispersions. The sponges absorb high amounts of water (≈7000%) at temperatures below the lower critical solution temperature (LCST) of PNIPAM and can release more than 80% of the absorbed water above the LCST in less than 2 min. Moreover, the water-swollen sponges are reversibly foldable, can be confined to different shapes, and have compressive elastic modulus below 10 Pa. Hence, these spongy materials are of interest not only for water management but also for biomedical applications, smart textiles, and catalysis.
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