Interfacial Adsorption of Silk Fibroin Peptides and Their Interaction with Surfactants at the Solid-Water Interface

Adsorbed protein film on pump surfaces leads to particle formation during fill-finish manufacturing

During fill-finish manufacturing, therapeutic proteins may aggregate or form subvisible particles in response to the physical stresses encountered within filling pumps. Understanding and quantitating this risk is important since filling may be the last unit operation before the patient receives their dose. We studied particle formation from lab-scale to manufacturing-scale using sensitive and robust protein formulations. Filling experiments with a ceramic rotary piston pump were integrated with a rinse-stripping method to investigate the relationship between protein adsorption and particle formation. For a sensitive protein, multilayer film formation on the piston surface correlated with high levels of subvisible particles in solution. For a robust protein formulation, adsorption and subvisible particle formation were minimal. These results support an aggregation mechanism that is initiated by adsorption to pump surfaces and propagated by mechanical and/or hydrodynamic disruption of the film. The elemental analysis confirmed that ceramic wear debris remained at trace levels and did not contribute appreciably to protein aggregation.

Interpenetrated biosurfactant-silk fibroin networks - a SANS study

Silk fibroin (SF) based hydrogels have been exploited for years for their inherent biocompatibility and favorable mechanical properties which makes them interesting for biotechnology applications. In this study we investigate silk based composite hydrogels where pH-sensitive, anionic biosurfactant assemblies (sophorolipids SL-C18 : 1 and SL-C18 : 0), are employed to improve the present properties of SF. Results suggest that the presence of SL surfactant assemblies leads to faster gelling of SF by accelerating the refolding from random coil to β-sheet as shown by infrared and UV-visible spectroscopy. Small angle neutron scattering (SANS) including contrast matching studies show that SF and SL assemblies coexist in a fibrillary network that is, in the case of SL-C18 : 0, interpenetrating. The resulting overall network structure in composite gels is slightly more affected by SL-C18 : 1 than by SL-C18 : 0, whereas the structure of both SF and surfactant assemblies remains unchanged. No disassembly of SL surfactant structures is observed, which gives a new perspective on SF-surfactant interactions. The hydrophobic effect within SF is favored in the presence of SL, leading to faster refolding of SF into β-sheet conformation. The presented composite gels, being an interpenetrating network of which one compound (SL-C18 : 0) can be tweaked by pH, open an interesting option towards improved workability and stimuli responsive mechanical properties of SF based hydrogels with possible applications in controlled cell culture and tissue engineering or drug delivery. The presented SANS analysis approach has the potential to be expanded to other protein-surfactant systems and composite hydrogels.

Efficient Regulation of the Behaviors of Silk Fibroin Hydrogel via Enzyme-Catalyzed Coupling of Hyaluronic Acid

Pure silk fibroin (SF) hydrogel exhibits poor elasticity and low water retention ability, owing to the compact crystalline structure and high content of hydrophobic amino acids. Herein, a composite double-network hydrogel of SF and tyramine-modified hyaluronic acid (mHA) was constructed, via the laccase-catalyzed coupling reactions between the phenolic hydroxyl groups from SF and mHA chains. The obtained hydrogel exhibits improved structural stability and flexibility compared to pure SF hydrogel. Meanwhile, the swelling ratio, mechanical property, drug loading, and release behaviors can be readily regulated by alcoholization, altering pH value, and ionic strength of soaking solutions. Increasing pH values promoted the swelling capacity of SF/mHA hydrogel, resulting in an efficient loading of cationic drugs and sustained release of anionic drugs as well. The addition of inorganic salts reduced electrostatic repulsion in the hydrogel scaffold, accompanying with a noticeable improvement of toughness. Furthermore, alcohol treatment induced conformation changes of fibroin protein, and the composite hydrogel achieved a higher fracture and improved elasticity. The present work provides a biological alternative to regulate the mechanical behavior, drug loading, and sustained release capacity of the SF-based hydrogel.

Interfacial Assembly Inspired by Marine Mussels and Antifouling Effects of Polypeptoids: A Neutron Reflection Study

Polypeptoid-coated surfaces and many surface-grafted hydrophilic polymer brushes have been proven efficient in antifouling-the prevention of nonspecific biomolecular adsorption and cell attachment. Protein adsorption, in particular, is known to mediate subsequent cell-surface interactions. However, the detailed antifouling mechanism of polypeptoid and other polymer brush coatings at the molecular level is not well understood. Moreover, most adsorption studies focus only on measuring a single adsorbed mass value, and few techniques are capable of characterizing the hydrated in situ layer structure of either the antifouling coating or adsorbed proteins. In this study, interfacial assembly of polypeptoid brushes with different chain lengths has been investigated in situ using neutron reflection (NR). Consistent with past simulation results, NR revealed a common two-step structure for grafted polypeptoids consisting of a dense inner region that included a mussel adhesive-inspired oligopeptide for grafting polypeptoid chains and a highly hydrated upper region with very low polymer density (molecular brush). Protein adsorption was studied with human serum albumin (HSA) and fibrinogen (FIB), two common serum proteins of different sizes but similar isoelectric points (IEPs). In contrast to controls, we observed higher resistance by grafted polypeptoid against adsorption of the larger FIB, especially for longer chain lengths. Changing the pH to close to the IEPs of the proteins, which generally promotes adsorption, also did not significantly affect the antifouling effect against FIB, which was corroborated by atomic force microscopy imaging. Moreover, NR enabled characterization of the in situ hydrated layer structures of the polypeptoids together with proteins adsorbed under selected conditions. While adsorption on bare SiO2 controls resulted in surface-induced protein denaturation, this was not observed on polypeptoids. Our current results therefore highlight the detailed in situ view that NR may provide for characterizing protein adsorption on polymer brushes as well as the excellent antifouling behavior of polypeptoids.

Influence of pH on the surface and foaming properties of aqueous silk fibroin solutions

Silk fibroin (SF) adsorbs at the air/water interface, reduces the surface tension, and forms interfacial layers suppressing bubble coalescence and stabilizing foam. Variation of pH alters the inter-molecular interactions of SF in the interfacial layers and thus interfacial network formation, dilatational visco-elasticity and foaming properties. At pH 4, around the isoelectric point, the reduced electrostatic repulsion between the SF molecules results in thicker adsorbed layers, but adsorption rate, foaming rate and foam stability are lower than at pH 3 and pH 7. At the highest pH investigated (pH 7), the small aggregate size and high protein flexibility lead to the formation of more ordered and stable viscoelastic interfacial networks, which are resistant to deformation breakage and generate homogeneous, denser and more stable foams.

Tough Photocrosslinked Silk Fibroin/Graphene Oxide Nanocomposite Hydrogels

The development of protein-based hydrogels for tissue engineering applications is often limited by their mechanical properties. Herein, we present the facile fabrication of tough regenerated silk fibroin (RSF)/graphene oxide (GO) nanocomposite hydrogels by a photochemical cross-linking method. The RSF/GO composite hydrogels demonstrated soft and adhesive properties during initial stages of photocrosslinking (<2 min), which is not observed for the pristine RSF hydrogel, and rendered a tough and nonadhesive hydrogel upon complete cross-linking (10 min). The composite hydrogels exhibited superior tensile mechanical properties, increased β-sheet content, and decreased chain mobility compared to that of the pristine RSF hydrogels. The composite hydrogels demonstrated Young's modulus as high as ∼8 MPa, which is significantly higher than native cartilage (∼1.5 MPa), and tensile toughness as high as ∼2.4 MJ/m³, which is greater than that of electroactive polymer muscles and at par with RSF/GO composite membranes fabricated by layer-by-layer assembly. Small-angle scattering study reveals the hierarchical structure of photocrosslinked RSF hydrogels to comprise randomly distributed water-poor (hydrophobic) and water-rich (hydrophilic) regions at the nanoscale, whereas water pores and channels exhibiting fractal-like characteristics at the microscale. The size of hydrophobic domain (containing β-sheets) was observed to increase slightly with GO incorporation and/or alcohol post-treatment, whereas the size of the hydrophilic domain (intersheet distance containing random coils) was observed to increase significantly, which influences/affects water uptake capacity, cross-link density, and mechanical properties of hydrogels. The presented results have implications for both fundamental understanding of the structure-property relationship of RSF-based hydrogels and their technological applications.

Real-time measurement of the crystal violet adsorption behavior and interaction process at the silica-aqueous interface by near-field evanescent wave

The interfacial adsorption and interaction of crystal violet (CV) at the silica-water interface was real-time measured based on a total-internal-reflection-induced near-field evanescent wave (TIR-NFEW). A silica optical fiber (SOF) was employed as a charged substrate for CV adsorption and as a light transmission waveguide for evanescent wave production for the investigation system. According to the change of evanescent wave intensity, the CV adsorption behavior could be real-time monitored at the silica-aqueous interface. The Langmuir adsorption model and two kinetic models were applied to obtain the related thermodynamic and kinetic data, including the adsorption equilibrium constant (Kads) of (5.9 ± 1.5) × 104 M-1 and adsorption free energy (ΔG) of -21.6 ± 0.6 kJ mol-1. Meanwhile, this method was shown to be able to isolate the elementary processes of adsorption and desorption under steady-state conditions, and gave an adsorption rate constant (ka) and desorption rate constant (kd) of 2089 ± 6.96 M min-1 and 0.35 ± 0.0012 min-1 for a 15 rpm flow rate. The surface interaction process was revealed and the adsorption mechanism proposed by a molecular orientation adsorption model with three-stage-concentration, indicating that CV first adsorbed on Si-O- sites through electrostatic attraction, then on Si-OH sites through hydrogen bonding, and lastly on the surface through van der Waals forces with different CV concentrations. This study can provide a molecular-level interpretation of CV adsorption and provides important insights into how CV adsorption can be controlled at the silica-water interface.

Real-Time Monitoring of Azo Dye Interfacial Adsorption at Silica-Water Interface by Total Internal Reflection-Induced Surface Evanescent Wave

An interface research method based on total internal reflection induced evanescent wave (TIR-EW) is developed to monitor the adsorption behavior of azo dye at the silica-water interface. The monitoring system is constructed by employing silica optical fiber (SOF) as both charged substrate for dye adsorption and light transmission waveguide for evanescent wave production. According to the change of evanescent wave intensity and followed by Beer's law, the methylene blue (MB) adsorption behavior can be real-time monitored at the silica-water interface. Langmuir adsorption model and pseudo-first-order model are applied to obtain the related thermodynamic and kinetic data. The adsorption equilibrium constant ( K_ads) and adsorption free energy (Δ G) of MB at the silica-water interface are determined to be (3.3 ± 0.5) × 10⁴ M^-1 and -25.7 ± 1.7 kJ mol^-1. Meanwhile, this method is highlighted to isolate elementary processes of adsorption and desorption under steady-state conditions, and gives adsorption rate constant ( k_a) and desorption rate constant ( k_d) of 8585 ± 19.8 min^-1 and 0.26 ± 0.0006 min^-1 for 15 r/min flow rate. The surface interaction process is revealed and adsorption mechanism is proposed, indicating MB first adsorbed on Si-O^- sites through electrostatic attraction and then on Si-OH sites through hydrogen bond with increasing MB concentrations. Our findings from this study provided molecular-level interpretation of azo dye adsorption at silica-water interface, and the results provide important insight into how MB adsorption can be controlled at the interface.