François Paquet-Mercier

Structure of silk by raman spectromicroscopy: From the spinning glands to the fibers †

Raman spectroscopy has long been proved to be a useful tool to study the conformation of protein‐based materials such as silk. Thanks to recent developments, linearly polarized Raman spectromicroscopy has appeared very efficient to characterize the molecular structure of native single silk fibers and spinning dopes because it can provide information relative to the protein secondary structure, molecular orientation, and amino acid composition. This review will describe recent advances in the study of the structure of silk by Raman spectromicroscopy. A particular emphasis is put on the spider dragline and silkworm cocoon threads, other fibers spun by orb‐weaving spiders, the spinning dope contained in their silk glands and the effect of mechanical deformation. Taken together, the results of the literature show that Raman spectromicroscopy is particularly efficient to investigate all aspects of silk structure and production. The data provided can lead to a better understanding of the structure of the silk dope, transformations occurring during the spinning process, and structure and mechanical properties of native fibers.

Evidence by infrared spectroscopy of the presence of two types of β-sheets in major ampullate spider silk and silkworm silk

The major ampullate (MA) silk of spider is known to be composed of oriented β-sheet nanocrystals dispersed within an amorphous matrix. The presence of an interphase has also been proposed, but it has not been reported for the fibroin of the silkworm Bombyx mori (B. mori). To obtain quantitative information regarding this third phase, the deuteration of B. mori silk and Nephila clavipes MA silk has been probed by attenuated total reflection infrared spectroscopy. The spectral decomposition of the amide II region has allowed determination of the level of orientation and content of the different secondary structures. The data reveal that, in addition to the amorphous domains, part of the β-sheets is deuterated upon immersion in D2O for both silks. The D2O-inaccessible β-sheets are associated with crystallites, while the interphase is composed of D2O-accessible ones. It is found that the former β-sheets are slightly more oriented along the fiber axis than the latter ones, which suggests that the interphase β-sheets are located at both ends of the crystals. The total β-sheet content is similar for B. mori silk (50 ± 4%) and MA silk (46 ± 4%). However, 27 ± 3% of the β-sheets of MA silk are D2O-accessible compared to 8 ± 3% for B. mori silk. These data suggest that around 5 amino acids for B. mori silk and 9 amino acids for MA silk would be involved in the interphase β-sheets. The higher amount of interphase β-sheets for MA silk is believed to contribute to its higher toughness.

Native spider silk as a biological optical fiber

In this study, we demonstrate the use of eco-friendly native spider silk as an efficient optical fiber in air, highly bent fibers, and physiological liquid. We also integrated the silk filament in a photonic chip made of polymer microstructures fabricated by UV lithography. The molding process is non-destructive for silk and leads to an efficient micro-optical coupling between silk and synthetic optical structures. These optical performances combined with the unique biocompatibility, bioresorbability, flexibility, and tensile strength of silk filaments pave the way for new applications in biological media and for original biophotonic purposes.

A Microfluidic Bioreactor with in Situ SERS Imaging for the Study of Controlled Flow Patterns of Biofilm Precursor Materials

A microfluidic bioreactor with an easy to fabricate nano-plasmonic surface is demonstrated for studies of biofilms and their precursor materials via Surface Enhanced Raman Spectroscopy (SERS). The system uses a novel design to induce sheath flow confinement of a sodium citrate biofilm precursor stream against the SERS imaging surface to measure spatial variations in the concentration profile. The unoptimised SERS enhancement was approximately 2.5 × 104, thereby improving data acquisition time, reducing laser power requirements and enabling a citrate detection limit of 0.1 mM, which was well below the concentrations used in biofilm nutrient solutions. The flow confinement was observed by both optical microscopy and SERS imaging with good complementarity. We demonstrate the new bioreactor by growing flow-templated biofilms on the microchannel wall. This work opens the way for in situ spectral imaging of biofilms and their biochemical environment under dynamic flow conditions.

Hydrodynamic Effects on Biofilms at the Biointerface Using a Microfluidic Electrochemical Cell: Case Study of Pseudomonas sp.

The anchoring biofilm layer is expected to exhibit a different response to environmental stresses than for portions in the bulk, due to the protection from other strata and the proximity to the attachment surface. The effect of hydrodynamic stress on surface-adhered biofilm layers was tested using a specially designed microfluidic bio flow cell with an embedded three-electrode detection system. In situ electrochemical impedance spectroscopy (EIS) measurements of biocapacitance and bioresistance of Pseudomonas sp. biofilms were conducted during the growth phase and under different shear flow conditions with verification by other surface sensitive techniques. Distinct, but reversible changes to the amount of biofilm and its structure at the attachment surface were observed during the application of elevated shear stress. In contrast, regular microscopy revealed permanent distortion to the biofilm bulk, in the form of streamers and ripples. Following the application of extreme shear stresses, complete removal of significant portions of biofilm outer layers occurred, but this did not change the measured quantity of biofilm at the electrode attachment surface. The structure of the remaining biofilm, however, appeared to be modified and susceptible to further changes following application of shear stress directly to the unprotected biofilm layers at the attachment surface.

Structure and Mechanical Properties of Spider Silk Films at the Air–Water Interface

The kinetics of adsorption of solubilized spider major ampullate (MA) silk fibers at the air-water interface and the molecular structure and mechanical properties of the interfacial films formed have been studied using various physical techniques. The data show that Nephila clavipes MA proteins progressively adsorb at the interface and ultimately form a highly cohesive thin film. In situ infrared spectroscopy shows that as soon as they reach the interface the proteins predominantly form β sheets. The protein secondary structure does not change significantly as the film grows, and the amount of β sheet is the same as that of the natural fiber. This suggests that the final β-sheet content is mainly dictated by the primary structure and not by the underlying formation process. The measure of the shear elastic constant at low strain reveals a very strong, viscous, cohesive assembly. The β sheets seem to form cross-links dispersed within an intermolecular network, thus probably playing a major role in the film strength. More importantly, the molecular weight seems to be a crucial factor because interfacial films made from the natural proteins are ~7 times stronger and ~3 times more viscous than those obtained previously with shorter recombinant proteins. Brewster angle microscopy at the air-water interface and transmission electron microscopy of transferred films have revealed a homogeneous organization on the micrometer scale. The images suggest that the structural assembly at the air-water interface leads to the formation of macroscopically solid and highly cohesive networks. Overall, the results suggest that natural spider silk proteins, although sharing similarities with recombinant proteins, have the particular ability to self-assemble into ordered materials with exceptional mechanical properties.

Effect of Mechanical Deformation on the Structure of Regenerated Bombyx mori Silk Fibroin Films as Revealed Using Raman and Infrared Spectroscopy

To better understand the effect of mechanical stress during the spinning of silk, the protein orientation and conformation of Bombyx mori regenerated silk fibroin (RSF) films have been studied as a function of deformation in a static mode or in real time by tensile-Raman experiments and polarization modulation infrared linear dichroism (PM-IRLD), respectively. The data show that either for step-by-step or continuous stretching, elongation induces the progressive formation of β-sheets that align along the drawing axis, in particular above a draw ratio of 2. The formation of β-sheets begins before their alignment during a continuous drawing. Unordered chains were, however, never found to be oriented, which explains the very low level of orientation of the amorphous phase of the natural fiber. Stress-perturbed unordered chains readily convert into β-sheets, the strain-induced transformation following a two-state process. The final level of orientation and β-sheet content are lower than those found in the native fiber, indicating that various parameters have to be optimized in order to implement a spinning process as efficient as the natural one. Finally, during the stress relaxation period in a step-by-step drawing, there is essentially no change of the content and orientation of the β-sheets, suggesting that only unordered structures tend to reorganize.

Through thick and thin: a microfluidic approach for continuous measurements of biofilm viscosity and the effect of ionic strength

Continuous, non-intrusive measurements of time-varying viscosity of Pseudomonas sp. biofilms are made using a microfluidic method that combines video tracking with a semi-empirical viscous flow model. The approach uses measured velocity and height of tracked biofilm segments, which move under the constant laminar flow of a nutrient solution. Following a low viscosity growth stage, rapid thickening was observed. During this stage, viscosity increased by over an order of magnitude in less than ten hours. The technique was also demonstrated as a promising platform for parallel experiments by subjecting multiple biofilm-laden microchannels to nutrient solutions containing NaCl in the range of 0 to 34 mM. Preliminary data suggest a strong relationship between ionic strength and biofilm properties, such as average viscosity and rapid thickening onset time. The technique opens the way for a combinatorial approach to study the response of biofilm viscosity under well-controlled physical, chemical and biological growth conditions.

Development and calibration of a microfluidic biofilm growth cell with flow-templating and multi-modal characterization.

We report the development of a microfluidic flow-templating platform with multi-modal characterization for studies of biofilms and their precursor materials. A key feature is a special three inlet flow-template compartment, which confines and controls the location of biofilm growth against a template wall. Characterization compartments include Raman imaging to study the localization of the nutrient solutions, optical microscopy to quantify biofilm biomass and localization, and cyclic voltammetry for flow velocity measurements. Each compartment is tested and then utilized to make preliminary measurements.

Optical Propagation and Integration of Pristine Major Ampullate Spider Silk Fibers

Propagation properties of pristine silkworm are presented. The 5 microns-diameter fibers have been also integrated with polymer microstructures leading to successful optical coupling. These results pave the way for biophotonics applications.