Samuel Lörcher

Fluorescent Protein Senses and Reports Mechanical Damage in Glass-Fiber-Reinforced Polymer Composites

Yellow fluorescent protein (YFP) is used as a mechanoresponsive layer at the fiber/resin interface in glass-fiber-reinforced composites. The protein loses its fluorescence when subjected to mechanical stress. Within the material, it reports interfacial shear debonding and barely visible impact damage by a transition from a fluorescent to a non-fluorescent state.

Mechanical unfolding of a fluorescent protein enables self-reporting of damage in carbon-fibre-reinforced composites

Carbon-fibre-reinforced polymer composites with an enhanced yellow fluorescent protein (eYFP) at the interface of fibres and resin were prepared. The protein was immobilized on the carbon fibres by physisorption and by covalent conjugation, respectively. The immobilized eYFP fluoresced on the carbon fibres, in contrast to non-protein fluorophores that were fully quenched by the carbon surface. The fibres were embedded into epoxy resin, and the eYFP remained fluorescent within the composite material. Micromechanical tests demonstrated that the interfacial shear strength of the material was not altered by the presence of the protein. Immunostaining of single fibre specimen revealed that eYFP loses its fluorescence in response to pull-out of fibres from resin droplets. The protein was able to detect barely visible impact damage such as fibre–resin debonding and fibre fractures by loss of its fluorescence. Therefore, it acts as a molecular force and stress/strain sensor at the fibre–resin interface and renders the composite self-sensing and self-reporting of microscopic damage. The mechanoresponsive effect of the eYFP did not depend on the type of eYFP immobilization. Fibres with the physisorbed protein gave similar results as fibres to which the protein was conjugated via covalent linkers. The results show that fluorescent proteins are compatible with carbon fibre composites. Such mechanophores could therefore be implemented as a safety feature into composites to assure material integrity and thereby prevent catastrophic material failure.

Tailoring the mass distribution and functional group density of dimethylsiloxane-based films by thermal evaporation

The tailoring of molecular weight distribution and the functional group density of vinyl-terminated polydimethylsiloxane (PDMS) by molecular beam deposition is demonstrated herein. Thermally evaporated PDMS and its residue are characterized using gel permeation chromatography and nuclear magnetic resonance. Thermal fragmentation of vinyl groups occurs for evaporation temperatures above 487 K (214 °C). At a background pressure of 10−6 mbar, the maximum molecular weight distribution is adjusted from (700 ± 100) g/mol to (6100 ± 100) g/mol with a polydispersity index of 1.06 ± 0.02. The content of vinyl-termination per repeating unit of PDMS is tailored from (2.8 ± 0.2)% to (5.6 ± 0.1)%. Molecular weights of vinyl-terminated PDMS evaporated at temperatures above 388 K (115 °C) correspond to those attributed to trimethyl-terminated PDMS. Side groups of linear PDMS dominate intermolecular interactions and vapor pressure.

Combining polymers with the functionality of proteins : new concepts for atom transfer radical polymerization, nanoreactors and damage self-reporting materials

Proteins are macromolecules with a great diversity of functions. By combining these biomolecules with polymers, exciting opportunities for new concepts in polymer sciences arise. This highlight exemplifies the aforementioned with current research results of our group. We review our discovery that the proteins horseradish peroxidase and hemoglobin possess ATRPase activity, i.e. they catalyze atom transfer radical polymerizations. Moreover, a permeabilization method for polymersomes is presented, where the photo-reaction of an α-hydroxyalkylphenone with block copolymer vesicles yields enzyme-containing nanoreactors. A further intriguing possibility to obtain functional nanoreactors is to enclose a polymerization catalyst into the thermosome, a protein cage from the family of chaperonins. Last but not least, fluorescent proteins are discussed as mechanoresponsive molecular sensors that report microdamages within fiber-reinforced composite materials.

Leakage current, self-clearing and actuation efficiency of nanometer-thin, low-voltage dielectric elastomer transducers tailored by thermal evaporation

The low-voltage operation is the key challenge for dielectric elastomer transducers (DET) to enter the application field of medically approved actuators or sensors, such as artificial muscles or skin. Recently, it has been successfully shown that the reduction of the elastomer film thickness to a few hundred nanometers allows for the DEA operation reaching 6 % strain using only a few volts. Molecular beam deposition (MBD) enables us to tailor elastomer films with low defect level. Combined with in situ spectroscopic ellipsometry, MBD is a unique method to reliably deposit polydimethylsiloxane (PDMS) thin films with true nanometer precision. The homogenous cross-linking of the PDMS film has been in situ realized by curing through ultraviolet (UV) radiation during deposition. We present the successful tailoring of the elastomer membrane’s elastic modulus down to a few hundreds of kPa by varying the UV-irradiation density. Atomic force microscopy (AFM) nano-indentation reveals homogeneously polymerized membranes. An adhesion layer of thiol-functionalized PDMS is applied to localize gold particles of the electrode layer to prevent diffusion into the nanometer-thin elastomer film and to reduce the leakage current. The understanding of leakage currents of such nanometer-thin elastomer films is crucial to preserve the unique actuation efficiency for DETs in low-voltage operation. Leakage currents are determined for a 200 nm-thin DEA as low as 10-3 A/m2 at applied electric fields of about 80 V/μm just before local breakdown events occur. Known as self-clearing, the vaporization of local defects enables to regain the functionality of the DET with subsequent reduced leakage current. AFM is utilized for the characterization of these DET low-voltage nanostructures regarding their vertical strain and actuation efficiency. A strain-to-voltage-squared (s/V2) ratio of 755 %/kV2 for a single-layer 500 nm-thin DEA is acquired - by far the highest reported (s/V2)-value for thin-film DEAs. A two-layer DET nanostructure is compared to a single layer DET with doubled elastomer film thickness to evaluate the repeatedly discussed stiffening electrode effect. This occurs when DET nanostructures are stacked above hundreds of times, the major challenge remaining to realize biomimetic DET with forces and compliance close to the natural muscles.