Thin film assembly of spider silk-like block copolymers.

We report the self-assembly of monolayers of spider silk-like block copolymers. Langmuir isotherms were obtained for a series of bioengineered variants of the spider silks, and stable monolayers were generated. Langmuir-Blodgett films were prepared by transferring the monolayers onto silica substrates and were subsequently analyzed by atomic force microscopy (AFM). Static contact angle measurements were performed to characterize interactions across the interface (thin film, water, air), and molecular modeling was used to predict 3D conformation of spider silk-like block copolymers. The influence of molecular architecture and volume fraction of the proteins on the self-assembly process was assessed. At high surface pressure, spider silk-like block copolymers with minimal hydrophobic block (f(A) = 12%) formed oblate structures, whereas block copolymer with a 6-fold larger hydrophobic domain (f(A) = 46%) formed prolate structures. The varied morphologies obtained with increased hydrophobicity offer new options for biomaterials for coatings and related options. The design and use of bioengineered protein block copolymers assembled at air-water interfaces provides a promising approach to compare 2D microstructures and molecular architectures of these amphiphiles, leading to more rationale designs for a range of nanoengineered biomaterial needs as well as providing a basis of comparison to more traditional synthetic block copolymer systems.

Protein-based block copolymers.

Advances in genetic engineering have led to the synthesis of protein-based block copolymers with control of chemistry and molecular weight, resulting in unique physical and biological properties. The benefits from incorporating peptide blocks into copolymer designs arise from the fundamental properties of proteins to adopt ordered conformations and to undergo self-assembly, providing control over structure formation at various length scales when compared to conventional block copolymers. This review covers the synthesis, structure, assembly, properties, and applications of protein-based block copolymers.

Role of polyalanine domains in beta-sheet formation in spider silk block copolymers.

Genetically engineered spider silk-like block copolymers were studied to determine the influence of polyalanine domain size on secondary structure. The role of polyalanine block distribution on beta-sheet formation was explored using FT-IR and WAXS. The number of polyalanine blocks had a direct effect on the formation of crystalline beta-sheets, reflected in the change in crystallinity index as the blocks of polyalanines increased. WAXS analysis confirmed the crystalline nature of the sample with the largest number of polyalanine blocks. This approach provides a platform for further exploration of the role of specific amino acid chemistries in regulating the assembly of beta-sheet secondary structures, leading to options to regulate material properties through manipulation of this key component in spider silks.

Self-assembly of genetically engineered spider silk block copolymers.

The design, construction, and preliminary characterization of a novel family of spider silk-like block copolymers are described. The design was based on the assembly of individual spider silk modules, in particular, polyalanine (A) and glycine-rich (B) blocks, that display different phase behavior in aqueous solution. Spider silk was chosen as a model for these block copolymer studies based on its extraordinary material properties, such as toughness, biocompatibility, and biodegradability. Trends in spider silk-like block copolymer secondary structure and assembly behavior into specific material morphologies were determined as a function of the number of hydrophobic blocks, the presence of a hydrophilic purification tag and solvent effects. Structures and morphologies were assessed by Fourier transform infrared spectroscopy and scanning electron microscopy. In terms of structure, beta-sheet content increased with an increase in the number of polyalanine blocks, and the purification tag had significant impact on the secondary structure. In terms of morphology, spheres, rod-like structures, bowl-shaped micelles, and giant compound micelles were observed and the morphologies were linked with the size of the hydrophobic block, the presence of the purification tag, and the solvent environment. This study provides a basis for future designs of smart biomaterials based on spider silk chemistries, with controlled structure-architecture-function relationships.

Spider silks and their applications.

Spider silks are characterized by remarkable diversity in their chemistry, structure and functions, ranging from orb web construction to adhesives and cocoons. These unique materials have prompted efforts to explore potential applications of spider silk equivalent to those of silkworm silks, which have undergone 5,000 years of domestication and have a variety of uses, from textiles to biomedical materials. Recent progress in genetic engineering of spider silks and the development of new chimeric spider silks with enhanced functions and specific characteristics have advanced spider silk technologies. Further progress in yields of expressed spider-silk proteins, in the control of self-assembly processes and in the selective exploration of material applications is anticipated in the future. The unique features of spider silks, the progress and challenges in the cloning and expression of these silks, environmentally triggered silk assembly and disassembly and the formation of fibers, films and novel chimeric composite materials from genetically engineered spider silks will be reviewed.

Collagen Structural Hierarchy and Susceptibility to Degradation by Ultraviolet Radiation.

Collagen type I is the most abundant extracellular matrix protein in the human body, providing the basis for tissue structure and directing cellular functions. Collagen has complex structural hierarchy, organized at different length scales, including the characteristic triple helical feature. In the present study, the relationship between collagen structure (native vs. denatured) and sensitivity to UV radiation was assessed, with a focus on changes in primary structure, changes in conformation, microstructure and material properties. A brief review of free radical reactions involved in collagen degradation is also provided as a mechanistic basis for the changes observed in the study. Structural and functional changes in the collagens were related to the initial conformation (native vs. denatured) and the energy of irradiation. These changes were tracked using SDS-PAGE to assess molecular weight, Fourier transform infrared (FTIR) spectroscopy to study changes in the secondary structure, and atomic force microscopy (AFM) to characterize changes in mechanical properties. The results correlate differences in sensitivity to irradiation with initial collagen structural state: collagen in native conformation vs. heat-treated (denatured) collagen. Changes in collagen were found at all levels of the hierarchical structural organization. In general, the native collagen triple helix is most sensitive to UV-254nm radiation. The triple helix delays single chain degradation. The loss of the triple helix in collagen is accompanied by hydrogen abstraction through free radical mechanisms. The results received suggest that the effects of electromagnetic radiation on biologically relevant extracellular matrices (collagen in the present study) are important to assess in the context of the state of collagen structure. The results have implications in tissue remodeling, wound repair and disease progression.

Nanolayer biomaterial coatings of silk fibroin for controlled release.

An all-aqueous, stepwise deposition process with silk fibroin protein for the assembly of nanoscale layered controlled release coatings was exploited. Model compounds, Rhodamine B, Even Blue and Azoalbumin, representing small molecule drugs and therapeutically relevant proteins were incorporated in the nanocoating process and their loading and release behavior was quantified. In addition, the structure and morphology of the coatings were characterized. Release studies in vitro showed that control of beta-sheet crystal content and the multilayer structure of the silk coatings correlated with the release properties of the incorporated compounds. In particular, higher crystallinity and a thicker silk capping layer suppressed the initial burst of release and prolonged the duration of release. These novel coatings and deposition approach provide a unique option to regulate structure and morphology, and thus release kinetics. The results also suggest these systems as a promising framework for surface engineering of biomaterials and medical devices to regulate the release of drugs, when considered with the all-aqueous process involved, the conformal nature of the coatings, the robust material properties of silk fibroin, and the degradability and biocompatibility of this family of protein.