Artificial hagfish protein fibers with ultra-high and tunable stiffness

Development of keratin-based fibers fabricated by interfacial polyelectrolyte complexation for suture applications

Interfacial Polyelectrolyte Complexation (IPC) is a convenient way to produce composite, micro-scale fibers. In this paper, we report the successful development of novel keratin-based IPC fibers and also demonstrate the feasibility of using these fibers as sutures through a proof-of-concept in vivo study. Two composite fibers were produced: chitosan-keratin (CK) and keratin-keratin (KK). These fibers were evaluated for their physico-chemical, mechanical and biochemical properties. In the dry state, the CK fiber had a greater Young's modulus of about 2 GPa while the KK fiber registered a longer strain-at-break of about 100 % due to the strain-stiffening effect. Notably, the keratins were found to assemble into amyloids within the composite fibers based on Congo red staining and Wide-Angle X-Ray Scattering. Functionally, both fibers were malleable could be weaved, braided and knotted. When used as sutures to close incisional wounds in mice over 21 days, these fibers were found to elicit minimal host tissue response and were partially degraded over the duration. Interestingly, the KK fiber evoked a lower extent of immune cell response and fibrous capsule encapsulation that was comparable to commercial, non-absorbable Dafilon® sutures. This work demonstrated the possibility of producing keratin-based IPC fibers which may find practicality as medical sutures.

pH-Dependent Reversible Self-Assembly of β-Lactoglobulin-Derived Reducing Peptides

Peptide-based self-assembled nanostructures are emerging vehicles for nutrient delivery and interface engineering. The present study screened eight β-lactoglobulin (β-Lg) derived peptides and found that two reducing peptides [EQSLVCQCLV (EV-10) and VCQCLVR (VR-7)] demonstrated pH-dependent reversible fibrilization. EV-10 formed fibrils at pH 2.0 but became unordered aggregates at pH 7.0. VR-7 showed the opposite trend. Both peptides could undergo repetitive transitions between fibrils and unordered aggregates during consecutive pH-cycling. Fibrilization of both peptides was dominated by charges carried by N- and C-terminals. Both fibrils were characterized by a cross-β sheet structure where the β-sheet was arranged in an antiparallel manner. Fe3+ was reduced by Cys and EV-10 (pH 5.0 and 7.0) simultaneously upon mixing. In contrast, EV-10 fibrils released Fe3+ reducing capacity progressively, which were beneficial to long-term protection Fe2+. The EV-10 fibrils remained intact after simulated gastric digestion and finally dissociated after intestinal digestion. The results shed light on the mechanisms of fibrilization of β-Lg derived peptides. This study was beneficial to the rational design of smart pH-responsive materials for drug delivery and antioxidants for nutrients susceptible to oxidation.

Artificial structural proteins: Synthesis, assembly and material applications

Structural proteins have evolved over billions of years and offer outstanding mechanical properties, such as resilience, toughness and stiffness. Advances in modular protein engineering, polypeptide modification, and synthetic biology have led to the development of novel biomimetic structural proteins to perform in biomedical and military fields. However, the development of customized structural proteins and assemblies with superior performance remains a major challenge, due to the inherent limitations of biosynthesis, difficulty in mimicking the complexed macroscale assembly, etc. This review summarizes the approaches for the design and production of biomimetic structural proteins, and their chemical modifications for multiscale assembly. Furthermore, we discuss the function tailoring and current applications of biomimetic structural protein assemblies. A perspective of future research is to reveal how the mechanical properties are encoded in the sequences and conformations. This review, therefore, provides an important reference for the development of structural proteins-mimetics from replication of nature to even outperforming nature.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Directed Self-Assembly of Heterologously Expressed Hagfish EsTKα and EsTKγ for Functional Hydrogel

Hagfish slime proteins have long been considered useful due to their potential applications in novel green, environmental, and functional bionic materials. The two main component proteins in the slime thread of hagfish, (opt)EsTKα and (opt)EsTKγ, were used as raw materials. However, the methods available to assemble these two proteins are time- and labor-intensive. The conditions affecting protein self-assembly, such as the pH of the assembly buffer, protein concentration, and the protein addition ratio, were the subject of the present research. Through a series of tests, the self-assembly results of a variety of assembly conditions were explored. Finally, a simplified protein self-assembly method was identified that allows for simple, direct assembly of the two proteins directly. This method does not require protein purification. Under the optimal assembly conditions obtained by exploration, a new gel material was synthesized from the hagfish protein through self-assembly of the (opt)EsTKα and (opt)EsTKγ. This assembly method has the benefits of being a simple, time-saving, and efficient. The self-assembled protein gel products were verified by SDS polyacrylamide gel electrophoresis (SDS-PAGE) and contained (opt)EsTKα and (opt)EsTKγ proteins. Scanning electron microscopy (SEM) was used to investigate the self-assembled protein gel after freeze-drying, and it was observed that the self-assembled protein formed a dense, three-dimensional porous network structure, meaning that it had good water retention. Evaluation of the gel with atomic force microscopy (AFM) indicated that the surface of the protein fiber skeleton show the network-like structure and relatively smooth. Characterization by circular dichroism (CD) and Fourier transform infrared spectroscopy (FT-IR) demonstrated that the two proteins were successfully assembled, and that the assembled protein had a secondary structure dominated by α-helices. The rheological properties of the self-assembled products were tested to confirm that they were indeed hydrogel property.

Extensible and self-recoverable proteinaceous materials derived from scallop byssal thread

Biologically derived and biologically inspired fibers with outstanding mechanical properties have found attractive technical applications across diverse fields. Despite recent advances, few fibers can simultaneously possess high-extensibility and self-recovery properties especially under wet conditions. Here, we report protein-based fibers made from recombinant scallop byssal proteins with outstanding extensibility and self-recovery properties. We initially investigated the mechanical properties of the native byssal thread taken from scallop Chlamys farreri and reveal its high extensibility (327 ± 32%) that outperforms most natural biological fibers. Combining transcriptome and proteomics, we select the most abundant scallop byssal protein type 5-2 (Sbp5-2) in the thread region, and produce a recombinant protein consisting of 7 tandem repeat motifs (rTRM7) of the Sbp5-2 protein. Applying an organic solvent-enabled drawing process, we produce bio-inspired extensible rTRM7 fiber with high-extensibility (234 ± 35%) and self-recovery capability in wet condition, recapitulating the hierarchical structure and mechanical properties of the native scallop byssal thread. We further show that the mechanical properties of rTRM7 fiber are highly regulated by hydrogen bonding and intermolecular crosslinking formed through disulfide bond and metal-carboxyl coordination. With its outstanding mechanical properties, rTRM7 fiber can also be seamlessly integrated with graphene to create motion sensors and electrophysiological signal transmission electrode.

Bio-inspired materials are an intense area of study as researchers try to adapt biomaterials for other applications. Here, the authors report on the processing of protein materials derived from the byssal thread of scallops to create high-extensibility materials with self-recovery under wet conditions.

Engineering High Strength and Super-Toughness of Unfolded Structural Proteins and their Extraordinary Anti-Adhesion Performance for Abdominal Hernia Repair

The utility of unfolded structural proteins with diverse sequences offers multiple potentials to create functional biomaterials. However, it is challenging to overcome their structural defects for the development of biological fibers with a combination of high strength and high toughness. Herein, robust fibers from a recombinant unfolded protein consisting of resilin and supercharged polypeptide are fabricated via wet-spinning approaches. Particularly, the highly ordered structures induced by supramolecular complexation significantly improve the fiber's mechanical performance. In contrast to chemical fibers with high strength and low toughness (or vice versa), the present fibers demonstrate exceptional high strength and super-toughness, showing a breaking strength of ≈550 MPa and a toughness of ≈250 MJ m^-3 , respectively, surpassing many polymers and artificial protein fibers. Remarkably, the outstanding biocompatibility and superior mechanical properties allow application of the constructed fiber patches for efficient abdominal hernia repair in rat models. In stark contrast to clinical patches, there is no observed tissue adhesion by this treatment. Therefore, this work provides a new type of engineered protein material for surgical applications.

The next generation of protein super-fibres: robust recombinant production and recovery of hagfish intermediate filament proteins with fibre spinning and mechanical-structural characterizations

Native hagfish intermediate filament proteins have impressive mechanical properties. However, using these native fibres for any application is impractical, necessitating their recombinant production. In the only literature report on the proteins (denoted α and ɣ), heterologous expression levels, using E. coli, were low and no attempts were made to optimize expression, explore wet‐spinning, or spin the two proteins individually into fibres. Reported here is the high production (~8 g l⁻¹ of dry protein) of the hagfish intermediate filament proteins, with yields orders of magnitude higher (325–1000×) than previous reports. The proteins were spun into fibres individually and in their native‐like 1:1 ratio. For all fibres, the hallmark α‐helix to β‐sheet conversion occurred after draw‐processing. The native‐like 1:1 ratio fibres achieved the highest average tensile strength in this study at nearly 200 MPa with an elastic modulus of 5.7 GPa, representing the highest tensile strength reported for these proteins without chemical cross‐linking. Interestingly, the recombinant α protein achieved nearly the same mechanical properties when spun as a homopolymeric fibre. These results suggest that varying the two protein ratios beyond the natural 1:1 ratio will allow a high degree of tunability. With robust heterologous expression and purification established, optimizing fibre spinning will be accelerated compared to difficult to produce proteins such as spider silks.

From reductionism to synthesis: The case of hagfish slime

Reductionist strategies aim to understand the mechanisms of complex systems by studying individual parts and their interactions. In this review, we discuss how reductionist approaches have shed light on the structure, function, and production of a complex biomaterial - hagfish defensive slime. Hagfish slime is an extremely dilute hydrogel-like material composed of seawater, mucus, and silk-like proteins that can deploy rapidly. Despite being composed almost entirely of water, hagfish slime has remarkable physical properties, including high strength and toughness. While hagfish slime has a promising future in biomimetics, including the development of eco-friendly high-performance fibers, recreating hagfish slime in the lab has been a difficult challenge. Over the past two decades, reductionist experiments have provided a wealth of information about the individual components of hagfish slime. However, a reductionist approach provides a limited understanding because hagfish defensive slime, like most biological phenomena, is more than just the sum of its parts. We end by providing some thoughts about how the knowledge generated in the last few decades might be synthesized into a working model that can explain hagfish slime structure and function.

Self-assembled fibrinogen–fibronectin hybrid protein nanofibers with medium-sensitive stability

Hybrid protein nanofibers (hPNFs) have been identified as promising nano building blocks for numerous applications in nanomedicine and tissue engineering. We have recently reported a nature-inspired, self-assembly route to create hPNFs from human plasma proteins, i.e., albumin and hemoglobin. However, it is still unclear whether the same route can be applied to other plasma proteins and whether it is possible to control the composition of the resulting fibers. In this context, to further understand the hPNFs self-assembly mechanism and to optimize their properties, we report herein on ethanol-induced self-assembly of two different plasma proteins, i.e., fibrinogen (FG) and fibronectin (FN). We show that by varying initial protein ratios, the composition and thus the properties of the resulting hPNFs can be fine-tuned. Specifically, atomic force microscopy, hydrodynamic diameter, and zeta potential data together revealed a strong correlation of the hPNFs dimensions and surface charge to their initial protein mixing ratio. The composition-independent prompt dissolution of hPNFs in ultrapure water, in contrast to their stability in PBS, indicates that the molecular arrangement of FN and FG in hPNFs is mainly based on electrostatic interactions. Supported by experimental data we introduce a feasible mechanism that explains the interactions between FN and FG and their self-assembly to hPNFs. These findings contribute to the understanding of dual protein interactions, which can be beneficial in designing innovative biomaterials with multifaceted biological and physical characteristics.

Hybrid protein nanofibers (hPNFs) have been identified as promising nano building blocks for numerous applications in nanomedicine and tissue engineering.

Progress in Modern Marine Biomaterials Research

The growing demand for new, sophisticated, multifunctional materials has brought natural structural composites into focus, since they underwent a substantial optimization during long evolutionary selection pressure and adaptation processes. Marine biological materials are the most important sources of both inspiration for biomimetics and of raw materials for practical applications in technology and biomedicine. The use of marine natural products as multifunctional biomaterials is currently undergoing a renaissance in the modern materials science. The diversity of marine biomaterials, their forms and fields of application are highlighted in this review. We will discuss the challenges, solutions, and future directions of modern marine biomaterialogy using a thorough analysis of scientific sources over the past ten years.

Robust Biological Fibers Based on Widely Available Proteins: Facile Fabrication and Suturing Application

Lightweight and mechanically strong protein fibers are promising for many technical applications. Despite the widespread investigation of biological fibers based on spider silk and silkworm proteins, it remains a challenge to develop low-cost proteins and convenient spinning technology for the fabrication of robust biological fibers. Since there are plenty of widely available proteins in nature, it is meaningful to investigate the preparation of fibers by the proteins and explore their biomedical applications. Here, a facile microfluidic strategy is developed for the scalable construction of biological fibers via a series of easily accessible spherical and linear proteins including chicken egg, quail egg, goose egg, bovine serum albumin, milk, and collagen. It is found that the crosslinking effect in microfluidic chips and double-drawn treatment after spinning are crucial for the formation of fibers. Thus, high tensile strength and toughness are realized in the fibers, which are comparable or even higher than that of many recombinant spider silks or regenerated silkworm fibers. Moreover, the suturing applications in rat and minipig models are realized by employing the mechanically strong fibers. Therefore, this work opens a new direction for the production of biological fibers from natural sources.

Fabrication and Mechanical Properties of Engineered Protein-Based Adhesives and Fibers

Protein-based structural biomaterials are of great interest for various applications because the sequence flexibility within the proteins may result in their improved mechanical and structural integrity and tunability. As the two representative examples, protein-based adhesives and fibers have attracted tremendous attention. The typical protein adhesives, which are secreted by mussels, sandcastle worms, barnacles, and caddisfly larvae, exhibit robust underwater adhesion performance. In order to mimic the adhesion performance of these marine organisms, two main biological adhesives are presented, including genetically engineered protein-based adhesives and biomimetic chemically synthetized adhesives. Moreover, various protein-based fibers inspired by spider and silkworm proteins, collagen, elastin, and resilin are studied extensively. The achievements in synthesis and fabrication of structural biomaterials by DNA recombinant technology and chemical regeneration certainly will accelerate the explorations and applications of protein-based adhesives and fibers in wound healing, tissue regeneration, drug delivery, biosensors, and other high-tech applications. However, the mechanical properties of the biological structural materials still do not match those of natural systems. More efforts need to be devoted to the study of the interplay of the protein structure, cohesion and adhesion effects, fiber processing, and mechanical performance.

Structure and Nanomechanics of Dry and Hydrated Intermediate Filament Films and Fibers Produced from Hagfish Slime Fibers

Intermediate filaments (IFs) are known for their extensibility, flexibility, toughness, and their ability to hydrate. Using keratin-like IFs obtained from slime fibers from the invertebrate Atlantic hagfish ( Myxine glutinosa), films were produced by drop-casting and coagulation on the surface of a MgCl₂ buffer. Drop-casting produced self-supporting, smooth, and dense films rich in β-sheets (61%), whereas coagulation formed thin, porous films with a nanorough surface and a lower β-sheet content (51%). The films hydrated and swelled immediately when immersed in water and did not dissolve. X-ray diffraction showed that the β-crystallites remained stable upon hydration, that swelling presumably happens in the amorphous C-terminal tail-domains of the IFs, and that high salt conditions caused a denser network mesh size, suggesting polyelectrolyte behavior. Hydration resulted in a roughly 1000-fold decrease in apparent Young's modulus from 10⁹ to 10⁶ Pa as revealed by atomic force microscopy nanoindentation. Nanoindentation-based power-law rheology and stress-relaxation measurements indicated viscoelasticity and a soft-solid hydrogel character for hydrated films, where roughly 80% of energy is elastically stored and 20% is dissipated. By pulling coagulation films from the buffer interface, macroscopic fibers with highly aligned IF β-crystals similar to natural hagfish fibers were produced. We propose that viscoelasticity and strong hydrogen bonding interactions with the buffer interface are crucial for the production of such long biomimetic fibers with aligned β-sheets. This study demonstrates that hagfish fiber IFs can be reconstituted into functional biomimetic materials that are stiff when dry and retain the ability to hydrate to become soft and viscoelastic when in water.

Metal-Tunable Self-Assembly of Hierarchical Structure in Mussel-Inspired Peptide Films

Bottom-up control over structural hierarchy from the nanoscale through the macroscale is a critical aspect of biological materials fabrication and function, which can inspire production of advanced materials. Mussel byssal threads are a prime example of protein-based biofibers in which hierarchical organization of protein building blocks coupled via metal complexation leads to notable mechanical behaviors, such as high toughness and self-healing. Using a natural amino acid sequence from byssal thread proteins, which functions as a pH-triggered self-assembly point, we created free-standing peptide films with complex hierarchical organization across multiple length scales that can be controlled by inclusion of metal ions (Zn²⁺ and Cu²⁺) during the assembly process. Additionally, analysis of film mechanical performance indicates that metal coordination bestows up to an order of magnitude increase in material stiffness, providing a paradigm for creating tunable polymeric materials with multiscale organizational structure.