Progress and Trends in Artificial Silk Spinning: A Systematic Review

Molecular organization of fibroin heavy chain and mechanism of fibre formation in Bombyx mori

Fibroins' transition from liquid to solid is fundamental to spinning and underpins the impressive native properties of silk. Herein, we establish a fibroin heavy chain fold for the Silk-I polymorph, which could be relevant for other similar proteins, and explains mechanistically the liquid-to-solid transition of this silk, driven by pH reduction and flow stress. Combining spectroscopy and modelling we propose that the liquid Silk-I fibroin heavy chain (FibH) from the silkworm, Bombyx mori, adopts a newly reported β-solenoid structure. Similarly, using rheology we propose that FibH N-terminal domain (NTD) templates reversible higher-order oligomerization driven by pH reduction. Our integrated approach bridges the gap in understanding FibH structure and provides insight into the spatial and temporal hierarchical self-assembly across length scales. Our findings elucidate the complex rheological behaviour of Silk-I, solutions and gels, and the observed liquid crystalline textures within the silk gland. We also find that the NTD undergoes hydrolysis during standard regeneration, explaining key differences between native and regenerated silk feedstocks. In general, in this study we emphasize the unique characteristics of native and native-like silks, offering a fresh perspective on our fundamental understanding of silk-fibre production and applications.

Regenerated Fiber's Ideal Target: Comparable to Natural Fiber

The toughness of silk naturally obtained from spiders and silkworms exceeds that of all other natural and man-made fibers. These insects transform aqueous protein feedstocks into mechanically specialized materials, which represents an engineering phenomenon that has developed over millions of years of natural evolution. Silkworms have become a new research hotspot due to the difficulties in collecting spider silk and other challenges. According to continuous research on the natural spinning process of the silkworm, it is possible to divide the main aspects of bionic spinning into two main segments: the solvent and behavior. This work focuses on the various methods currently used for the spinning of artificial silk fibers to replicate natural silk fibers, providing new insights based on changes in the fiber properties and production processes over time.

The ratio of fibroin to sericin in the middle silk gland of Bombyx mori and its correlation with the extensional behavior of the silk dope

Understanding how native silk spinning occurs is crucial for designing artificial spinning systems. One often overlooked factor in Bombyx mori is the secretion of sericin proteins. Herein, we investigate the variation in amino acid content at different locations in the middle silk gland (MSG) of B. mori. This variation corresponds to an increase in sericin content when moving towards the anterior region of the MSG, while the posterior region predominantly contains fibroin. We estimate the mass ratio of sericin to fibroin to be ~25/75 wt% in the anterior MSG, depending on the fitting method. Then, we demonstrate that the improvement in the extensional behavior of the silk dope in the MSG correlates with the increase in sericin content. The addition of sericin may decrease the viscosity of the silk dope, a factor associated with an increase in the spinnability of silk. We further discuss whether this effect could also result from other known physicochemical changes within the MSG.

Replicating shear-mediated self-assembly of spider silk through microfluidics

The development of artificial spider silk with properties similar to native silk has been a challenging task in materials science. In this study, we use a microfluidic device to create continuous fibers based on recombinant MaSp2 spidroin. The strategy incorporates ion-induced liquid-liquid phase separation, pH-driven fibrillation, and shear-dependent induction of β-sheet formation. We find that a threshold shear stress of approximately 72 Pa is required for fiber formation, and that β-sheet formation is dependent on the presence of polyalanine blocks in the repetitive sequence. The MaSp2 fiber formed has a β-sheet content (29.2%) comparable to that of native dragline with a shear stress requirement of 111 Pa. Interestingly, the polyalanine blocks have limited influence on the occurrence of liquid-liquid phase separation and hierarchical structure. These results offer insights into the shear-induced crystallization and sequence-structure relationship of spider silk and have significant implications for the rational design of artificially spun fibers.

Rapid production of chimeric silkworm/spider silk with improved mechanical properties by infection of nonpermissive Bombyx mori with recombinant AcMNPV harboring native-size of spidroin genes

Spider silks with excellent mechanical properties attract more attention from scientists worldwide, and the dragline silk that serves as the framework of the spider's web is considered one of the strongest fibers. However, it is unfeasible for large-scale production of spider silk due to its highly territorial, cannibalistic, predatory, and solitary behavior. Herein, to alleviate some of these problems and explore aneasy way to produce spider fibers, we constructed recombinant baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) simultaneously expressing Trichonephila clavipes native ampullate spidroin 2 (MaSp-G) and spidroin 1 (MaSp-C) driven by the promoters of silkworm fibroin genes, to infect the nonpermissive Bombyx mori larvae at the fifth instar. MaSp-G and MaSp-C were co-expressed in the posterior silk glands (PSGs) of infected silkworms and successfully secreted into the lumen of the silk gland for fibroin globule assembly. The integration of MaSp-G and MaSp-C into silkworm silk fibers significantly improved the mechanical properties of these chimeric silk fibers, especially the strength and extensibility, which may be caused by the increment of β-sheet in the chimeric silkworm/spider silk fiber. These results demonstrated that silkworms could be developed as the nonpermissive heterologous host for the mass production of chimeric silkworm/spider silk fibers via the recombinant baculovirus AcMNPV.

Real-Time Monitoring of the Structural Transition of Bombyx mori Liquid Silk under Pressure by Solid-State NMR

Silk fibroin is stored in the silk glands of Bombyx mori silkworms as a condensed aqueous solution called liquid silk. It is converted into silk fibers at the silkworm's spinnerets under mechanical forces including shear stress and pressure. However, the detailed mechanism of the structural transition of liquid silk to silk fibers under pressure is not well understood. Magic angle spinning (MAS) in solid-state nuclear magnetic resonance (NMR) can exert pressure on liquid samples in a quantitative manner. In this study, solid-state NMR was used to quantitatively analyze the impact of pressure on the structural transition of liquid silk. A combination of 13C DD-MAS and CP-MAS NMR measurements enabled the conformation and dynamics of the crystalline region of the silk fibroin (both before (Silk Ip) and after (Silk IIp) the structural transition) to be detected in real time with atomic resolution. Spectral analyses proposed that the pressure-induced structural transition from Silk Ip to Silk IIp proceeds by a two-step autocatalytic reaction mechanism. The first reaction step is a nucleation step in which Silk Ip transforms to single lamellar Silk IIp, and the second is a growth step in which the single lamellar Silk IIp acts as a catalyst that reacts with Silk Ip molecules to further form Silk IIp molecules, resulting in stacked lamellar Silk IIp. Furthermore, the rate constant in the second step shows a significant pressure dependence, with an increase in pressure accelerating the formation of large stacked lamellar Silk IIp.

Pulling and analyzing silk fibers from aqueous solution using a robotic device

Spiders, silkworms, and many other animals can spin silk with exceptional properties. However, artificially spun fibers often fall short of their natural counterparts partly due sub-optimal production methods. A variety of methods, such as wet-, dry-, and biomimetic spinning have been used. The methods are based on extrusion, whereas natural spinning also involves pulling. Another shortcoming is that there is a lack feedback control during extension. Here we demonstrate a robotic fiber pulling device that enables controlled pulling of silk fibers and in situ measurement of extensional forces during the pulling and tensile testing of the pulled fibers. The pulling device was used to study two types of silk-one recombinant spider silk (a structural variant of ADF3) and one regenerated silk fibroin. Also, dextran-a branched polysaccharide-was used as a reference material for the procedure due to its straightforward preparation and storage. No post-treatments were applied. The pulled regenerated silk fibroin fibers achieved high tensile strength in comparison to similar extrusion-based methods. The mechanical properties of the recombinant spider silk fibers seemed to be affected by the liquid-liquid phase separation of the silk proteins.

Embedding Living Cells with a Mechanically Reinforced and Functionally Programmable Hydrogel Fiber Platform

Living materials represent a new frontier in functional material design, integrating synthetic biology tools to endow materials with programmable, dynamic, and life-like characteristics. However, a major challenge in creating living materials is balancing the tradeoff between structural stability, mechanical performance, and functional programmability. To address this challenge, a sheath-core living hydrogel fiber platform that synergistically integrates living bacteria with hydrogel fibers to achieve both functional diversity and structural and mechanical robustness is proposed. In the design, microfluidic spinning is used to produce hydrogel fiber, which offers advantages in both structural and functional designability due to their hierarchical porous architectures that can be tailored and their mechanical performance that can be enhanced through a variety of post-processing approaches. By introducing living bacteria, the platform is endowed with programmable functionality and life-like capabilities. This work reconstructs the genetic circuits of living bacteria to express chromoproteins and fluorescent proteins as two prototypes that enable the coloration of living fibers and sensing water pollutants by monitoring the amount of fluorescent protein expressed. Altogether, this study establishes a structure-property-function optimized living hydrogel fiber platform, providing a new tool for accelerating the practical applications of the emerging living material systems.

A brief review on the mechanisms and approaches of silk spinning-inspired biofabrication

Silk spinning, observed in spiders and insects, exhibits a remarkable biological source of inspiration for advanced polymer fabrications. Because of the systems design, silk spinning represents a holistic and circular approach to sustainable polymer fabrication, characterized by renewable resources, ambient and aqueous processing conditions, and fully recyclable "wastes." Also, silk spinning results in structures that are characterized by the combination of monolithic proteinaceous composition and mechanical strength, as well as demonstrate tunable degradation profiles and minimal immunogenicity, thus making it a viable alternative to most synthetic polymers for the development of advanced biomedical devices. However, the fundamental mechanisms of silk spinning remain incompletely understood, thus impeding the efforts to harness the advantageous properties of silk spinning. Here, we present a concise and timely review of several essential features of silk spinning, including the molecular designs of silk proteins and the solvent cues along the spinning apparatus. The solvent cues, including salt ions, pH, and water content, are suggested to direct the hierarchical assembly of silk proteins and thus play a central role in silk spinning. We also discuss several hypotheses on the roles of solvent cues to provide a relatively comprehensive analysis and to identify the current knowledge gap. We then review the state-of-the-art bioinspired fabrications with silk proteins, including fiber spinning and additive approaches/three-dimensional (3D) printing. An emphasis throughout the article is placed on the universal characteristics of silk spinning developed through millions of years of individual evolution pathways in spiders and silkworms. This review serves as a stepping stone for future research endeavors, facilitating the in vitro recapitulation of silk spinning and advancing the field of bioinspired polymer fabrication.

Protein fibers with self-recoverable mechanical properties via dynamic imine chemistry

The manipulation of internal interactions at the molecular level within biological fibers is of particular importance but challenging, severely limiting their tunability in macroscopic performances and applications. It thus becomes imperative to explore new approaches to enhance biological fibers' stability and environmental tolerance and to impart them with diverse functionalities, such as mechanical recoverability and stimulus-triggered responses. Herein, we develop a dynamic imine fiber chemistry (DIFC) approach to engineer molecular interactions to fabricate strong and tough protein fibers with recoverability and actuating behaviors. The resulting DIF fibers exhibit extraordinary mechanical performances, outperforming many recombinant silks and synthetic polymer fibers. Remarkably, impaired DIF fibers caused by fatigue or strong acid treatment are quickly recovered in water directed by the DIFC strategy. Reproducible mechanical performance is thus observed. The DIF fibers also exhibit exotic mechanical stability at extreme temperatures (e.g., -196 °C and 150 °C). When triggered by humidity, the DIFC endows the protein fibers with diverse actuation behaviors, such as self-folding, self-stretching, and self-contracting. Therefore, the established DIFC represents an alternative strategy to strengthen biological fibers and may pave the way for their high-tech applications.

Continuous Wet Spinning of Regenerated Silk Fibers from Spinning Dopes Containing 4% Fibroin Protein

The wet spinning of fibers from regenerated silk fibroin has long been a research goal. Due to the degradation of the molecular structure of the fibroin protein during the preparation of the regenerated silk fibroin solution, fibroin concentrations with at least 10% protein content are required to achieve sufficient viscosity for wet spinning. In this study, a spinning dope formulation of regenerated silk fibroin is presented that shows a rheological behavior similar to that of native silk fibroin isolated from the glands of B. mori silkworm larvae. In addition, we present a wet-spinning process that enables, for the first time, the continuous wet spinning of regenerated silk fibroin with only 4% fibroin protein content into an endless fiber. Furthermore, the tensile strength of these wet-spun regenerated silk fibroin fibers per percentage of fibroin is higher than that of all continuous spinning approaches applied to regenerated and native silk fibroin published so far.

Biomimetic polymer fibers-function by design

Biomimicry applies the fundamental principles of natural materials, processes, and structures to technological applications. This review presents the two strategies of biomimicry-bottom-up and top-down approaches, using biomimetic polymer fibers and suitable spinning techniques as examples. The bottom-up biomimicry approach helps to acquire fundamental knowledge on biological systems, which can then be leveraged for technological advancements. Within this context, we discuss the spinning of silk and collagen fibers due to their unique natural mechanical properties. To achieve successful biomimicry, it is imperative to carefully adjust the spinning solution and processing parameters. On the other hand, top-down biomimicry aims to solve technological problems by seeking solutions from natural role models. This approach will be illustrated using examples such as spider webs, animal hair, and tissue structures. To contextualize biomimicking approaches in practical applications, this review will give an overview of biomimetic filter technologies, textiles, and tissue engineering.

Instantaneous Formation of Silk Protein Aerosols and Fibers with a Portable Spray Device Under Ambient Conditions

A variety of artificial silk spinning approaches have been attempted to mimic the natural spinning process found in silkworms and spiders, yet instantaneous silk fiber formation with hierarchical structure under physiological and ambient conditions without post-treatment procedures remains unaddressed. Here, we report a new strategy to fabricate silk protein-based aerosols and silk fibers instantaneously (< 1 s) in situ using a simple, portable, spray device, avoiding complicated and costly advanced manufacturing techniques. The key to success is the instantaneous conformational transition of silk fibroin from random coil to β-sheet right before spraying by mixing silk and polyethylene glycol (PEG) solutions in the spray device, allowing aerosols and silk fibers to be sprayed in situ, with further control achieved via the molecular weight of silk. The spinning process of the spray device is based on the use of green solvents, i.e., all steps of instant conformational transition of silk fibroin are carried out in aqueous conditions or with buffers at ambient conditions, in combination with shear and elongational flow caused by the hydraulic pressure generated in the spray container. The system supports a portable and user-friendly system that could be used for drug delivery carriers, wound coating materials and rapid silk fiber conformal coatings on surfaces.

Kinetic Analysis Reveals the Role of Secondary Nucleation in Regenerated Silk Fibroin Self-Assembly

Silk proteins obtained from the Bombyx mori silkworm have been extensively studied due to their remarkable mechanical properties. One of the major structural components of this complex material is silk fibroin, which can be isolated and processed further in vitro to form artificial functional materials. Due to the excellent biocompatibility and rich self-assembly behavior, there has been sustained interest in such materials formed through the assembly of regenerated silk fibroin feedstocks. The molecular mechanisms by which the soluble regenerated fibroin molecules self-assemble into protein nanofibrils remain, however, largely unknown. Here, we use the framework of chemical kinetics to connect macroscopic measurements of regenerated silk fibroin self-assembly to the underlying microscopic mechanisms. Our results reveal that the aggregation of regenerated silk fibroin is dominated by a nonclassical secondary nucleation processes, where the formation of new fibrils is catalyzed by the existing aggregates in an autocatalytic manner. Such secondary nucleation pathways were originally discovered in the context of polymerization of disease-associated proteins, but the present results demonstrate that this pathway can also occur in functional assembly. Furthermore, our results show that shear flow induces the formation of nuclei, which subsequently accelerate the process of aggregation through an autocatalytic amplification driven by the secondary nucleation pathway. Taken together, these results allow us to identify the parameters governing the kinetics of regenerated silk fibroin self-assembly and expand our current understanding of the spinning of bioinspired protein-based fibers, which have a wide range of applications in materials science.

Wild Silkworm Cocoon Waste Conversion into Tough Regenerated Silk Fibers by Solution Spinning

Wild silkworm silk fibers have garnered attention owing to their softness, natural color, lightweight, and excellent mechanical properties. Because most wild silkworm cocoons obtained are pierced or dirty after the eclosion process, it is difficult to reel the long filament from the pierced cocoons to use as textile materials. Therefore, damaged wild silkworm cocoons are typically removed during the industrial process. Artificial silk spinning has been developed to transform domesticated silkworm silk solutions into regenerated silk fibers. However, regenerated fibers derived from wild silkworm silk have not been reported. Here, we produced regenerated silk fibers using a dry-wet spinning method using a dope solution derived from wild silkworm silk cocoon wastes. These regenerated silk fibers have thick and uniform diameters, unlike native silk fibers, contributing to their usefulness for sterilization and handling in medical applications. Moreover, they exhibited the same level of mechanical strength as their native counterparts. The molecular orientation and crystallinity of the regenerated silk fibers were adjustable by the drawing process, enabling the realization of their various tensile properties. This study promotes the utilization of unused protein resources to produce mechanically stable and tough silk-based fibers.

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.

Biological Materials Processing: Time-Tested Tricks for Sustainable Fiber Fabrication

There is an urgent need to improve the sustainability of the materials we produce and use. Here, we explore what humans can learn from nature about how to sustainably fabricate polymeric fibers with excellent material properties by reviewing the physical and chemical aspects of materials processing distilled from diverse model systems, including spider silk, mussel byssus, velvet worm slime, hagfish slime, and mistletoe viscin. We identify common and divergent strategies, highlighting the potential for bioinspired design and technology transfer. Despite the diversity of the biopolymeric fibers surveyed, we identify several common strategies across multiple systems, including: (1) use of stimuli-responsive biomolecular building blocks, (2) use of concentrated fluid precursor phases (e.g., coacervates and liquid crystals) stored under controlled chemical conditions, and (3) use of chemical (pH, salt concentration, redox chemistry) and physical (mechanical shear, extensional flow) stimuli to trigger the transition from fluid precursor to solid material. Importantly, because these materials largely form and function outside of the body of the organisms, these principles can more easily be transferred for bioinspired design in synthetic systems. We end the review by discussing ongoing efforts and challenges to mimic biological model systems, with a particular focus on artificial spider silks and mussel-inspired materials.

As Fiber Meets with AIE: Opening a Wonderland for Smart Flexible Materials

Aggregation-induced emission luminogens (AIEgens) have recently been developed at a tremendous pace in the area of organic luminescent materials by virtue of their superior properties. However, the practical applications of AIEgens still face the challenge of transforming AIEgens from molecules into materials. Till now, many AIEgens have been integrated into fiber, endowing the fiber with prominent fluorescence and/or photosensitizing capacities. AIEgens and fiber complement each other for making progress in flexible smart materials, in which the utilization of AIEgens creates new application possibilities for fiber, and the fiber provides an excellent carrier for AIEgens towards realizing the conversion from molecule to materials and an ideal platform to research the aggregate state of AIEgens in mesoscale and macroscale. This review begins with a brief summary of the recent advances related to some typical AIEgens with various functions and the technology for the fabrication of AIEgen-functionalized fiber. The most representative applications are then highlighted by focusing on energy conversion, personal protective equipment, biomedical, sensor, and fluorescence-related fields. Finally, the challenges, opportunities, and tendencies in future development are discussed in detail. This review hopes to inspire innovation in AIEgens and fiber from the view of mesoscale and macroscale.

Complete gene sequence and mechanical property of the fourth type of major ampullate silk protein

Spiders spin a great diversity of silk types for daily survival and reproduction. Of the six orb-weaver silk types, the dragline silk forming orb web frame attracts the most attention because of its extremely high tensile strength and toughness. So far, four types of major ampullate silk proteins (MaSp1-4) that make up dragline silk have been identified. These MaSp types have diversified amino acid motifs that underlie the impressive mechanical property of dragline silk by forming particular structures. Existing knowledge of MaSp4 proteins is fragmented, making it difficult to illuminate the structure and function of MaSp4. Here, we report the full-length MaSp4 gene with 11,334 bp from the orb-weaving spider Araneus ventricosus. Removing the only intron, the spliced complete transcript of MaSp4 gene is 6897 bp and encodes 2298 amino acids. Analysis of the primary structure of A. ventricosus MaSp4 protein reveals the repetitive region lacks poly-A and GGX motifs but has the unique GPGPQ motifs. Quantitative real-time PCR analyses show high levels of MaSp4 mRNA were detected in major ampullate gland. Structural characterization using CD- and FTIR sepctroscopy reveals a mainly α-helical solution conformation and a very high β-turn content within fibers. Collectively, our new findings provide complete template for recombinant silk protein with specific properties and support that the GPGPQ motif found in MaSp4 could increase flexibility in dragline silk by packing in more β-turns, expanding the repertoire of sequences known to form β-turn that is available for artificial chimeric silk fibers. STATEMENT OF SIGNIFICANCE: Dragline silk forming orb web frame attracts the most attention because of its extremely high tensile strength and toughness. So far, four types of major ampullate silk proteins (MaSp1-4) that make up dragline silk have been identified. Existing knowledge of MaSp4 proteins is fragmented, making it difficult to illuminate the structure and function of MaSp4. Here, we report the full-length MaSp4 gene from the orb-weaving spider Araneus ventricosus. We further identify the sequence, structure, and mechanical property of MaSp4 protein, providing a new insight into the structure-funtion relationships associated with MaSp4. Collectively, our new findings provide complete template for recombinant silk protein with specific properties and support that the GPGPQ motif found in MaSp4 could increase flexibility in dragline silk by packing in more β-turns, expanding the repertoire of sequences known to form β-turn that is available for artificial chimeric silk fibers.

Recombinant Spider Silk Fiber with High Dimensional Stability in Water and Its NMR Characterization

Spider dragline silk has unique characteristics of strength and extensibility, including supercontraction. When we use it as a biomaterial or material for textiles, it is important to suppress the effect of water on the fiber by as much as possible in order to maintain dimensional stability. In order to produce spider silk with a highly hydrophobic character, based on the sequence of ADF-3 silk, we produced recombinant silk (RSSP(VLI)) where all QQ sequences were replaced by VL, while single Q was replaced by I. The artificial RSSP(VLI) fiber was prepared using formic acid as the spinning solvent and methanol as the coagulant solvent. The dimensional stability and water absorption experiments of the fiber were performed for eight kinds of silk fiber. RSSP(VLI) fiber showed high dimensional stability, which is suitable for textiles. A remarkable decrease in the motion of the fiber in water was made evident by ¹³C solid-state NMR. This study using ¹³C solid-state NMR is the first trial to put spider silk to practical use and provide information regarding the molecular design of new recombinant spider silk materials with high dimensional stability in water, allowing recombinant spider silk proteins to be used in next-generation biomaterials and materials for textiles.

Understanding the Interfacial Adhesion between Natural Silk and Polycaprolactone for Fabrication of Continuous Silk Biocomposites

The poor interfacial adhesion between silk fiber and polyester species remains a critical problem for the optimal mechanical performance of silk-reinforced polyester composites. Here, we investigated in quantitative terms the interfacial properties between natural silk fibers and polycaprolactone (PCL) at nano-, micro-, and macroscales and fabricated continuous silk-PCL composite filaments by melt extrusion and drawing processing of PCL melt at 100, 120, and 140 °C. Bombyx mori (Bm) silk, Antheraea pernyi (Ap) silk, and polyamide6 (PA6) fiber were compared to the composite with PCL. The Ap silk exhibited the highest surface energy, the best wettability, and the largest interfacial shear strength (IFSS) with PCL. The silk-PCL composite from the 120 °C melt processing displayed the highest tensile modulus, implying an optimal temperature for interfacial adhesion. The Raman imaging technique revealed in detail the nature of the physical fusion of the interface phase in these silk- and polyamide-reinforced PCL composites. This work is intended to lay a foundation for the design and processing of robust composites from continuous silk fibers and bioresorbable polyesters for potential structural biomaterials.

Conservation of Three-Dimensional Structure of Lepidoptera and Trichoptera L-Fibroins for 290 Million Years

The divergence of sister orders Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) from a silk-spinning ancestor occurred around 290 million years ago. Trichoptera larvae are mainly aquatic, and Lepidoptera larvae are almost entirely terrestrial—distinct habitats that required molecular adaptation of their silk for deployment in water and air, respectively. The major protein components of their silks are heavy chain and light chain fibroins. In an effort to identify molecular changes in L-fibroins that may have contributed to the divergent use of silk in water and air, we used the ColabFold implementation of AlphaFold2 to predict three-dimensional structures of L-fibroins from both orders. A comparison of the structures revealed that despite the ancient divergence, profoundly different habitats, and low sequence conservation, a novel 10-helix core structure was strongly conserved in L-fibroins from both orders. Previously known intra- and intermolecular disulfide linkages were accurately predicted. Structural variations outside of the core may represent molecular changes that contributed to the evolution of insect silks adapted to water or air. The distributions of electrostatic potential, for example, were not conserved and present distinct order-specific surfaces for potential interactions with or modulation by external factors. Additionally, the interactions of L-fibroins with the H-fibroin C-termini are different for these orders; lepidopteran L-fibroins have N-terminal insertions that are not present in trichopteran L-fibroins, which form an unstructured ribbon in isolation but become part of an intermolecular β-sheet when folded with their corresponding H-fibroin C-termini. The results are an example of protein structure prediction from deep sequence data of understudied proteins made possible by AlphaFold2.

The properties of native Trichonephila dragline silk and its biomedical applications

Spider silk has fascinated mankind for millennia, but it is only in recent decades that scientific research has begun to unravel all its characteristics and applications. The uniqueness of spider silk resides in its versatility, in which a combination of high strength and extensibility results in extraordinary toughness, superior to almost all natural and man-made fibers. Dragline silk consists of proteins with highly repetitive amino acid sequences, which have been correlated with specific secondary structures responsible for its physical properties. The native fiber also shows high cytocompatibility coupled with low immunogenicity, making it a promising natural biomaterial for numerous biomedical applications. Recently, novel technologies have enabled new insights into the material and biomedical properties of silk. Due to the increasing interest in spider silk, as well as the desire to produce synthetic alternatives, we present an update on the current knowledge of silk fibers produced by the spider genus Trichonephila.

Spidroin N-terminal domain forms amyloid-like fibril based hydrogels and provides a protein immobilization platform

Recombinant spider silk proteins (spidroins) have multiple potential applications in development of novel biomaterials, but their multimodal and aggregation-prone nature have complicated production and straightforward applications. Here, we report that recombinant miniature spidroins, and importantly also the N-terminal domain (NT) on its own, rapidly form self-supporting and transparent hydrogels at 37 °C. The gelation is caused by NT α-helix to β-sheet conversion and formation of amyloid-like fibrils, and fusion proteins composed of NT and green fluorescent protein or purine nucleoside phosphorylase form hydrogels with intact functions of the fusion moieties. Our findings demonstrate that recombinant NT and fusion proteins give high expression yields and bestow attractive properties to hydrogels, e.g., transparency, cross-linker free gelation and straightforward immobilization of active proteins at high density.

Recombinant spider silks are of interest but the multimodal and aggregation-prone nature of them is a limitation. Here, the authors report on a miniature spidroin based on the N-terminal domain which forms a hydrogel at 37 °C which allows for ease of production and fusion protein modification to generate functional biomaterials.

Recent advances in bioprinting using silk protein-based bioinks

3D printing has experienced swift growth for biological applications in the field of regenerative medicine and tissue engineering. Essential features of bioprinting include determining the appropriate bioink, printing speed mechanics, and print resolution while also maintaining cytocompatibility. However, the scarcity of bioinks that provide printing and print properties and cell support remains a limitation. Silk Fibroin (SF) displays exceptional features and versatility for inks and shows the potential to print complex structures with tunable mechanical properties, degradation rates, and cytocompatibility. Here we summarize recent advances and needs with the use of SF protein from Bombyx mori silkworm as a bioink, including crosslinking methods for extrusion bioprinting using SF and the maintenance of cell viability during and post bioprinting. Additionally, we discuss how encapsulated cells within these SF-based 3D bioprinted constructs are differentiated into various lineages such as skin, cartilage, and bone to expedite tissue regeneration. We then shift the focus towards SF-based 3D printing applications, including magnetically decorated hydrogels, in situ bioprinting, and a next-generation 4D bioprinting approach. Future perspectives on improvements in printing strategies and the use of multicomponent bioinks to improve print fidelity are also discussed.

Formylation of Recombinant Spider Silk in Formic Acid and Wet Spinning Studied Using Nuclear Magnetic Resonance and Infrared Spectroscopies

We reported wet spinning of recombinant spider silk protein (RSSP) and formylation of RSSP in formic acid (FA). First, FA was selected as the spinning solvent and the detailed spinning condition was determined. Next, the mechanical property was compared between the RSSP fiber spun after allowing the spinning solution dissolved in FA to stand for 2 days and the fiber spun immediately after being dissolved in FA for 4 h. The tensile strength of the former fiber was lower than the strength of the latter fiber. This difference can be explained by the difference in the degree of formylation as follows. FA is a known formylating agent, although most researchers who prepared silk fiber by wet spinning with FA have not pointed out about formylation. The formylation of the Ser OH group was confirmed by ¹³C solution nuclear magnetic resonance (NMR), and the time course of formylation of the RSSP film prepared from the FA solution was tracked by Fourier transform infrared spectroscopy. The ¹³C solid-state NMR spectra were also compared between two kinds of the formylated RSSP fibers and indicated that the packing state was tighter for the latter fiber than the former one, which could explain higher tensile strength of the latter fiber in the dry state. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated that the RSSP sample decomposed gradually with storage time in FA and the decomposition has begun partly even at 2 h after dissolution in FA. The decomposition by formylation seems to have no significant effect on the backbone structure of the RSSP fiber, although the packing of the fiber becomes loose as a whole. Finally, preliminary trial of deformylation of the formylated RSSP fiber was performed.

Engineered Spider Silk Proteins for Biomimetic Spinning of Fibers with Toughness Equal to Dragline Silks

Spider silk is the toughest fiber found in nature, and bulk production of artificial spider silk that matches its mechanical properties remains elusive. Development of miniature spider silk proteins (mini-spidroins) has made large-scale fiber production economically feasible, but the fibers' mechanical properties are inferior to native silk. The spider silk fiber's tensile strength is conferred by poly-alanine stretches that are zipped together by tight side chain packing in β-sheet crystals. Spidroins are secreted so they must be void of long stretches of hydrophobic residues, since such segments get inserted into the endoplasmic reticulum membrane. At the same time, hydrophobic residues have high β-strand propensity and can mediate tight inter-β-sheet interactions, features that are attractive for generation of strong artificial silks. Protein production in prokaryotes can circumvent biological laws that spiders, being eukaryotic organisms, must obey, and the authors thus design mini-spidroins that are predicted to more avidly form stronger β-sheets than the wildtype protein. Biomimetic spinning of the engineered mini-spidroins indeed results in fibers with increased tensile strength and two fiber types display toughness equal to native dragline silks. Bioreactor expression and purification result in a protein yield of ≈9 g L^-1 which is in line with requirements for economically feasible bulk scale production.

Complexity of Spider Dragline Silk

The tiny spider makes dragline silk fibers with unbeatable toughness, all under the most innocuous conditions. Scientists have persistently tried to emulate its natural silk spinning process using recombinant proteins with a view toward creating a new wave of smart materials, yet most efforts have fallen short of attaining the native fiber’s excellent mechanical properties. One reason for these shortcomings may be that artificial spider silk systems tend to be overly simplified and may not sufficiently take into account the true complexity of the underlying protein sequences and of the multidimensional aspects of the natural self-assembly process that give rise to the hierarchically structured fibers. Here, we discuss recent findings regarding the material constituents of spider dragline silk, including novel spidroin subtypes, nonspidroin proteins, and possible involvement of post-translational modifications, which together suggest a complexity that transcends the two-component MaSp1/MaSp2 system. We subsequently consider insights into the spidroin domain functions, structures, and overall mechanisms for the rapid transition from disordered soluble protein into a highly organized fiber, including the possibility of viewing spider silk self-assembly through a framework relevant to biomolecular condensates. Finally, we consider the concept of “biomimetics” as it applies to artificial spider silk production with a focus on key practical aspects of design and evaluation that may hopefully inform efforts to more closely reproduce the remarkable structure and function of the native silk fiber using artificial methods.

Recombinant Spider Silk: Promises and Bottlenecks

Spider silk threads have exceptional mechanical properties such as toughness, elasticity and low density, which reach maximum values compared to other fibre materials. They are superior even compared to Kevlar and steel. These extraordinary properties stem from long length and specific protein structures. Spider silk proteins can consist of more than 20,000 amino acids. Polypeptide stretches account for more than 90% of the whole protein, and these domains can be repeated more than a hundred times. Each repeat unit has a specific function resulting in the final properties of the silk. These properties make them attractive for innovative material development for medical or technical products as well as cosmetics. However, with livestock breeding of spiders it is not possible to reach high volumes of silk due to the cannibalistic behaviour of these animals. In order to obtain spider silk proteins (spidroins) on a large scale, recombinant production is attempted in various expression systems such as plants, bacteria, yeasts, insects, silkworms, mammalian cells and animals. For viable large-scale production, cost-effective and efficient production systems are needed. This review describes the different types of spider silk, their proteins and structures and discusses the production of these difficult-to-express proteins in different host organisms with an emphasis on plant systems.

Silk-based bioinspired structural and functional materials

Natural biological materials provide a rich source of inspiration for building high-performance materials with extensive applications. By mimicking their chemical compositions and hierarchical architectures, the past decades have witnessed the rapid development of bioinspired materials. As a very promising biosourced raw material, silk is drawing increasing attention due to excellent mechanical properties, favorable versatility, and good biocompatibility. In this review, we provide an overview of the recent progress in silk-based bioinspired structural and functional materials. We first give a brief introduction of silk, covering its sources, features, extraction, and forms. We then summarize the preparation and application of silk-based materials mimicking four typical biological materials including bone, nacre, skin, and polar bear hair. Finally, we discuss the current challenges and future prospects of this field.

Functionalized Fiber-Based Strain Sensors: Pathway to Next-Generation Wearable Electronics

Wearable strain sensors are arousing increasing research interests in recent years on account of their potentials in motion detection, personal and public healthcare, future entertainment, man-machine interaction, artificial intelligence, and so forth. Much research has focused on fiber-based sensors due to the appealing performance of fibers, including processing flexibility, wearing comfortability, outstanding lifetime and serviceability, low-cost and large-scale capacity. Herein, we review the latest advances in functionalization and device fabrication of fiber materials toward applications in fiber-based wearable strain sensors. We describe the approaches for preparing conductive fibers such as spinning, surface modification, and structural transformation. We also introduce the fabrication and sensing mechanisms of state-of-the-art sensors and analyze their merits and demerits. The applications toward motion detection, healthcare, man-machine interaction, future entertainment, and multifunctional sensing are summarized with typical examples. We finally critically analyze tough challenges and future remarks of fiber-based strain sensors, aiming to implement them in real applications.

Programmable Knot Microfibers from Piezoelectric Microfluidics

Microfibers have demonstrated significant application values in a large number of areas. Current efforts focus on developing new technologies to prepare microfibers with controllable morphological and structural features to enhance their functions. Here, a piezoelectric microfluidic platform is presented for consecutive spinning of functional microfibers with programmable spindle-knots. In this platform, a jet of a pregel-solution flowing in the channel can be subjected to a programmable piezoelectric signal and vibrates synchronously. Following a rapid polymerization of the wavy jet, microfibers with corresponding morphologies can be generated, including uniform, gradient, and symmetrical knots. Such a unique knot structure contributes to a water-collection mechanism. Thus, it has been observed that microfibers with programmed knots enable even more flexible droplet handling and active water transport. In addition, by constructing higher-order knot fiber networks, practical applications including spray reaction, lab-on-a-chip vapor detection, etc., can also be demonstrated. it is believed that this platform opens a new avenue for fiber spinning, and the programmable microfibers would be highly applicable in chemical, biomedical, and environmental areas.

Electrospun Silk Fibroin Scaffolds for Tissue Regeneration: Chemical, Structural, and Toxicological Implications of the Formic Acid-Silk Fibroin Interaction

The dissolution of Bombyx mori silk fibroin (SF) films in formic acid (FA) for the preparation of electrospinning dopes is widely exploited to produce electrospun SF scaffolds. The SILKBridge^® nerve conduit is an example of medical device having in its wall structure an electrospun component produced from an FA spinning dope. Though highly volatile, residual FA remains trapped into the bulk of the SF nanofibers. The purpose of this work is to investigate the type and strength of the interaction between FA and SF in electrospun mats, to quantify its amount and to evaluate its possible toxicological impact on human health. The presence of residual FA in SF mats was detected by FTIR and Raman spectroscopy (new carbonyl peak at about 1,725 cm⁻¹) and by solid state NMR, which revealed a new carbonyl signal at about 164.3 ppm, attributed to FA by isotopic ¹³C substitution. Changes occurred also in the spectral ranges of hydroxylated amino acids (Ser and Thr), demonstrating that FA interacted with SF by forming formyl esters. The total amount of FA was determined by HS-GC/MS analysis and accounted for 247 ± 20 μmol/g. The greatest part was present as formyl ester, a small part (about 3%) as free FA. Approximately 17% of the 1,500 μmol/g of hydroxy amino acids (Ser and Thr) theoretically available were involved in the formation of formyl esters. Treatment with alkali (Na₂CO₃) succeeded to remove the greatest part of FA, but not all. Alkali-treated electrospun SF mats underwent morphological, physical, and mechanical changes. The average diameter of the fibers increased from about 440 nm to about 480 nm, the mat shrunk, became stiffer (the modulus increased from about 5.5 MPa to about 7 MPa), and lost elasticity (the strain decreased from about 1 mm/mm to about 0.8 mm/mm). Biocompatibility studies with human adult dermal fibroblasts did not show significant difference in cell proliferation (313 ± 18 and 309 ± 23 cells/mm² for untreated and alkali-treated SF mat, respectively) and metabolic activity. An in-depth evaluation of the possible toxicological impact of residual FA was made using the SILKBridge^® nerve conduit as case study, following the provisions of the ISO 10993-1 standard. The Potential Patient Daily Intake, calculated from the total amount of FA determined by HS-GC/MS, was 2.4 mg/day and the Tolerable Exposure level was set to 35.4 mg/day. This allowed to obtain a value of the Margin of Safety of 15, indicating that the amount of FA left on SF mats after electrospinning does not raise concerns for human health.

An Image-Analysis-Based Method for the Prediction of Recombinant Protein Fiber Tensile Strength

Silk fibers derived from the cocoon of silk moths and the wide range of silks produced by spiders exhibit an array of features, such as extraordinary tensile strength, elasticity, and adhesive properties. The functional features and mechanical properties can be derived from the structural composition and organization of the silk fibers. Artificial recombinant protein fibers based on engineered spider silk proteins have been successfully made previously and represent a promising way towards the large-scale production of fibers with predesigned features. However, for the production and use of protein fibers, there is a need for reliable objective quality control procedures that could be automated and that do not destroy the fibers in the process. Furthermore, there is still a lack of understanding the specifics of how the structural composition and organization relate to the ultimate function of silk-like fibers. In this study, we develop a new method for the categorization of protein fibers that enabled a highly accurate prediction of fiber tensile strength. Based on the use of a common light microscope equipped with polarizers together with image analysis for the precise determination of fiber morphology and optical properties, this represents an easy-to-use, objective non-destructive quality control process for protein fiber manufacturing and provides further insights into the link between the supramolecular organization and mechanical functionality of protein fibers.

Presence of β-Turn Structure in Recombinant Spider Silk Dissolved in Formic Acid Revealed with NMR

Spider dragline silk is a biopolymer with excellent mechanical properties. The development of recombinant spider silk protein (RSP)-based materials with these properties is desirable. Formic acid (FA) is a spinning solvent for regenerated Bombyx mori silk fiber with excellent mechanical properties. To use FA as a spinning solvent for RSP with the sequence of major ampullate spider silk protein from Araneus diadematus, we determined the conformation of RSP in FA using solution NMR to determine the role of FA as a spinning solvent. We assigned ¹H, ¹³C, and ¹⁵N chemical shifts to 32-residue repetitive sequences, including polyAla and Gly-rich regions of RSP. Chemical shift evaluation revealed that RSP is in mainly random coil conformation with partially type II β-turn structure in the Gly-Pro-Gly-X motifs of the Gly-rich region in FA, which was confirmed by the ¹⁵N NOE data. In addition, formylation at the Ser OH groups occurred in FA. Furthermore, we evaluated the conformation of the as-cast film of RSP dissolved in FA using solid-state NMR and found that β-sheet structure was predominantly formed.

Diverse silk and silk-like proteins derived from terrestrial and marine organisms and their applications

Organisms develop unique systems in a given environment. In the process of adaptation, they employ materials in a clever way, which has inspired mankind extensively. Understanding the behavior and material properties of living organisms provides a way to emulate these natural systems and engineer various materials. Silk is a material that has been with human for over 5000 years, and the success of mass production of silkworm silk has realized its applications to medical, pharmaceutical, optical, and even electronic fields. Spider silk, which was characterized later, has expanded the application sectors to textile and military materials based on its tough mechanical properties. Because silk proteins are main components of these materials and there are abundant creatures producing silks that have not been studied, the introduction of new silk proteins would be a breakthrough of engineering materials to open innovative industry fields. Therefore, in this review, we present diverse silk and silk-like proteins and how they are utilized with respect to organism's survival. Here, the range of organisms are not constrained to silkworms and spiders but expanded to other insects, and even marine creatures which produce silk-like proteins that are not observed in terrestrial silks. This viewpoint broadening of silk and silk-like proteins would suggest diverse targets of engineering to design promising silk-based materials. STATEMENT OF SIGNIFICANCE: Silk has been developed as a biomedical material due to unique mechanical and chemical properties. For decades, silks from various silkworm and spider species have been intensively studied. More recently, other silk and silk-like proteins with different sequences and structures have been reported, not only limited to terrestrial organisms (honeybee, green lacewing, caddisfly, and ant), but also from marine creatures (mussel, squid, sea anemone, and pearl oyster). Nevertheless, there has hardly been well-organized literature on silks from such organisms. Regarding the relationship among sequence-structure-properties, this review addresses how silks have been utilized with respect to organism's survival. Finally, this information aims to improve the understanding of diverse silk and silk-like proteins which can offer a significant interest to engineering fields.

Nanocomposite and bio-nanocomposite polymeric materials/membranes development in energy and medical sector: A review

Nanocomposite and bio-nanocomposite polymer materials/membranes have fascinated prominent attention in the energy as well as the medical sector. Their composites make them appropriate choices for various applications in the medical, energy and industrial sectors. Composite materials are subject of interest in the polymer industry. Different kinds of fillers, such as cellulose-based fillers, carbon black, clay nanomaterials, glass fibers, ceramic nanomaterial, carbon quantum dots, talc and many others have been incorporated into polymers to improve the quality of the final product. These results are dependent on a variety of factors; however, nanoparticle dispersion and distribution are major obstacles to fully using nanocomposites/bio-nanocomposites materials/membranes in various applications. This review examines the various nanocomposite and bio-nanocomposite materials applications in the energy and medical sector. The review also covers the variety of ways for increasing nanocomposite and bio-nanocomposite materials features, each with its own set of applications. Recent researches on composite materials have shown that polymeric nanocomposites and bio-nanocomposites are promising materials that have been intensively explored for many applications that include electronics, environmental remediation, energy, sensing (biosensor) and energy storage devices among other applications. In this review, we studied various nanocomposite and bio-nanocomposite materials, their controlling parameters to develop the product and examine their features and applications in the fields of energy and the medical sector.

Moderate conformational transition promotes the formation of a self-reinforced highly oriented silk fibroin network structure

A highly oriented molecular network structure (HOMNS) is a common and favorable design in natural and regenerated silks to achieve self-reinforcement of the material. However, the fundamental issues related to the formation of the HOMNS in silk fibroin materials and its influence on mechanical performance have not yet been addressed. By combining experimental characterization and molecular dynamics simulation, this work revealed that moderate conformational transition of silk fibroin promoted the formation of a low-density crosslinking molecular network among proteins. Such a molecular network is beneficial to further rearrangement of amorphous proteins in subsequent processing to form HOMNS. Here, a structure was confirmed that can strengthen the materials several times compared with the same material without HOMNS. These investigations improved the in-depth understanding of the fundamental questions related to the silk fibroin assembly, revealed their crucial structural remodeling, and paved the way for new fabrication strategies of mechanical-enhanced silk fibroin materials.

Immune Response to Silk Sericin-Fibroin Composites: Potential Immunogenic Elements and Alternatives for Immunomodulation

The unique properties of silk proteins (SPs), particularly silk sericin (SS) and silk fibroin (SF), have attracted attention in the design of scaffolds for tissue engineering over the past decades. Since SF has good mechanical properties, while SS displays bioactivity, scaffolds combining both proteins should exhibit complementary properties enhancing the potential of these materials. Unfortunately, SS-SF composites can generate chronic immune responses and their immunogenic element is not completely clear. The potential of SS-SF composites in tissue engineering, elements which may contribute to their immunogenicity, and alternatives for their preparation and design, to modulate the immune response and take advantage of their useful properties, are discussed in this review. It is known that SS can enhance β-sheet formation in SF, which may act as hydrophobic regions with a strong affinity for adsorption proteins inducing the chronic recruitment of inflammatory cells. Therefore, tailoring the exposure of hydrophobic regions at the scaffold surface should represent a viable strategy to modulate the immune response. This can be achieved by coating SS-SF composites with SS or other hydrophilic polymers, to take advantage of their antibiofouling properties. Research is still needed to realize the full potential of these composites for tissue engineering.

Cutaneous Regeneration Mechanism of β-Sheet Silk Fibroin in a Rat Burn Wound Healing Model

Therapeutic dressings to enhance burn wound repair and regeneration are required. Silk fibroin (SF), a natural protein, induces cell migration and serves as a biomaterial in various dressings. SF dressings usually contain α-helices and β-sheets. The former has been confirmed to improve cell proliferation and migration, but the wound healing effect and related mechanisms of β-sheet SF remain unclear. We investigated the effects of β-sheet SF in vivo and in vitro. Alcohol-treated α-helix SF transformed into the β-sheet form, which promoted granulation formation and re-epithelialization when applied as lyophilized SF dressing (LSFD) in a rat burn model. Our in vitro results showed that β-sheet SF increased human dermal fibroblast (HDF) migration and promoted the expression of extracellular matrix (ECM) proteins (fibronectin and type III collagen), matrix metalloproteinase-12, and the cell adhesion molecule, integrin β1, in rat granulation tissue and HDFs. This confirms the role of crosstalk between integrin β1 and ECM proteins in cell migration. In summary, we demonstrated that β-sheet SF facilitates tissue regeneration by modulating cell adhesion molecules in dermal fibroblasts. LSFD could find clinical application for burn wound regeneration. Moreover, β-sheet SF could be combined with anti-inflammatory materials, growth factors, or antibiotics to develop novel dressings.

Macroscopic Assembly of Sericin toward Self-Healable Silk

Silk contains an adhesive glycoprotein, silk sericin, in which silk fibroins can be enfolded and chemically stabilized. Silk sericin is gaining importance as the material for the creation of functional bioscaffolds. However, the assembly of silk sericin is generally limited to the blend of polymers or proteins due to its inherent poor mechanical strength. Here, we report a simple macroscopic controlled assembly of silk sericin fibers based on their secondary structure via wet-spinning. In addition, plasticization of silk sericin using glycerol immobilized with glutaraldehyde was found to induce dimensional stability, affording stable linear fibers with self-adhesion. Furthermore, cyclo-phenylalanine nanowires were incorporated into the silk sericin dope for a practical demonstration of their potential in artificial silk production with superstructure formation. The physicochemical characteristics of the spun fibers have also been elucidated using Fourier-transform infrared spectroscopy, electron microscopy, tensile test, differential scanning calorimetry, and 2D X-ray diffraction.

The Food-Materials Nexus: Next Generation Bioplastics and Advanced Materials from Agri-Food Residues

The most recent strategies available for upcycling agri-food losses and waste (FLW) into functional bioplastics and advanced materials are reviewed and the valorization of food residuals are put in perspective, adding to the water-food-energy nexus. Low value or underutilized biomass, biocolloids, water-soluble biopolymers, polymerizable monomers, and nutrients are introduced as feasible building blocks for biotechnological conversion into bioplastics. The latter are demonstrated for their incorporation in multifunctional packaging, biomedical devices, sensors, actuators, and energy conversion and storage devices, contributing to the valorization efforts within the future circular bioeconomy. Strategies are introduced to effectively synthesize, deconstruct and reassemble or engineer FLW-derived monomeric, polymeric, and colloidal building blocks. Multifunctional bioplastics are introduced considering the structural, chemical, physical as well as the accessibility of FLW precursors. Processing techniques are analyzed within the fields of polymer chemistry and physics. The prospects of FLW streams and biomass surplus, considering their availability, interactions with water and thermal stability, are critically discussed in a near-future scenario that is expected to lead to next-generation bioplastics and advanced materials.

Biocompatible Micron-Scale Silk Fibers Fabricated by Microfluidic Wet Spinning

For successful material deployment in tissue engineering, the material itself, its mechanical properties, and the microscopic geometry of the product are of particular interest. While silk is a widely applied protein-based tissue engineering material with strong mechanical properties, the size and shape of artificially spun silk fibers are limited by existing processes. This study adjusts a microfluidic spinneret to manufacture micron-sized wet-spun fibers with three different materials enabling diverse geometries for tissue engineering applications. The spinneret is direct laser written (DLW) inside a microfluidic polydimethylsiloxane (PDMS) chip using two-photon lithography, applying a novel surface treatment that enables a tight print-channel sealing. Alginate, polyacrylonitrile, and silk fibers with diameters down to 1 µm are spun, while the spinneret geometry controls the shape of the silk fiber, and the spinning process tailors the mechanical property. Cell-cultivation experiments affirm bio-compatibility and showcase an interplay between the cell-sized fibers and cells. The presented spinning process pushes the boundaries of fiber fabrication toward smaller diameters and more complex shapes with increased surface-to-volume ratio and will substantially contribute to future tailored tissue engineering materials for healthcare applications.