Spider minor ampullate silk protein nanoparticles: an effective protein delivery system capable of enhancing systemic immune responses

Spider silk proteins (spidroins) are particularly attractive due to their excellent biocompatibility. Spider can produce up to seven different types of spidroins, each with unique properties and functions. Spider minor ampullate silk protein (MiSp) might be particularly interesting for biomedical applications, as the constituent silk is mechanically strong and does not super-contract in water, attributed to its amino acid composition. In this study, we evaluate the potential of recombinant nanoparticles derived from Araneus ventricosus MiSp as a protein delivery carrier. The MiSp-based nanoparticles were able to serve as an effective delivery system, achieving nearly 100% efficiency in loading the model protein lysozyme, and displayed a sustained release profile at physiological pH. These nanoparticles could significantly improve the delivery efficacy of the model proteins through different administration routes. Furthermore, nanoparticles loaded with model protein antigen lysozyme after subcutaneous or intramuscular administration could enhance antigen-specific immune responses in mouse models, through a mechanism involving antigen-depot effects at the injection site, long-term antigen persistence, and efficient uptake by dendritic cells as well as internalization by lymph nodes. These findings highlight the transnational potential of MiSp-based nanoparticle system for protein drug and vaccine delivery.

MMPs-responsive silk spheres for controlled drug release within tumor microenvironment

Silk is a biocompatible and biodegradable material that enables the formation of various morphological forms, including nanospheres. The functionalization of bioengineered silk makes it possible to produce particles with specific properties. In addition to tumor cells, the tumor microenvironment (TME) includes stromal, immune, endothelial cells, signaling molecules, and the extracellular matrix (ECM). Matrix metalloproteinases (MMPs) are overexpressed in TME. We investigated bioengineered spider silks functionalized with MMP-responsive peptides to obtain targeted drug release from spheres within TME. Soluble silks MS12.2MS1, MS12.9MS1, and MS22.9MS2 and the corresponding silk spheres carrying MMP-2 or MMP-2/9 responsive peptides were produced, loaded with doxorubicin (Dox), and analyzed for their susceptibility to MMP-2/9 digestion. Although all variants of functionalized silks and spheres were specifically degraded by MMP-2/9, the MS22.9MS2 nanospheres showed the highest levels of degradation and release of Dox after enzyme treatment. Moreover, functionalized spheres were degraded in the presence of cancer cells releasing MMP-2/9. In the 2D and 3D spheroid cancer models, the MMP-2/9-responsive substrate was degraded and released from spheres when loaded into MS22.9MS2 particles but not into the control MS2 spheres. The present study demonstrated that a silk-based MMP-responsive delivery system could be used for controlled drug release within the tumor microenvironment.

Improving solubility of poorly water-soluble drugs by protein-based strategy: A review

Poorly water-soluble drugs are frequently encountered and present a most challengeable difficulty in pharmaceutical development. Poor solubility of drugs can lead to suboptimal bioavailability and therapeutic efficiency. Increasing efforts have been contributed to improve the solubility of poorly water-soluble drugs for better pharmacokinetics and pharmacodynamics. Among various solubility enhancement technologies, protein-based strategy to address poorly water-soluble drugs issues has special interests for natural advantages including versatile interactions between proteins and hydrophobic drugs, biocompatibility, biodegradation, and metabolization of proteins. The protein-drug formulations could be formed by covalent conjugations or noncovalent interactions to facilitate solubility of poorly water-soluble drugs. This review is to summarize the advances using proteins including plant proteins, mammalian proteins, and recombinant proteins, to enhance water solubility of poorly water-soluble drugs.

Design of Recombinant Spider Silk Proteins for Cell Type Specific Binding

Cytophilic (cell-adhesive) materials are very important for tissue engineering and regenerative medicine. However, for engineering hierarchically organized tissue structures comprising different cell types, cell-specific attachment and guidance are decisive. In this context, materials made of recombinant spider silk proteins are promising scaffolds, since they exhibit high biocompatibility, biodegradability, and the underlying proteins can be genetically functionalized. Here, previously established spider silk variants based on the engineered Araneus diadematus fibroin 4 (eADF4(C16)) are genetically modified with cell adhesive peptide sequences from extracellular matrix proteins, including IKVAV, YIGSR, QHREDGS, and KGD. Interestingly, eADF4(C16)-KGD as one of 18 tested variants is cell-selective for C2C12 mouse myoblasts, one out of 11 tested cell lines. Co-culturing with B50 rat neuronal cells confirms the cell-specificity of eADF4(C16)-KGD material surfaces for C2C12 mouse myoblast adhesion.

Bioengineering of spider silks for the production of biomedical materials

Spider silks are well known for their extraordinary mechanical properties. This characteristic is a result of the interplay of composition, structure and self-assembly of spider silk proteins (spidroins). Advances in synthetic biology have enabled the design and production of spidroins with the aim of biomimicking the structure-property-function relationships of spider silks. Although in nature only fibers are formed from spidroins, in vitro, scientists can explore non-natural morphologies including nanofibrils, particles, capsules, hydrogels, films or foams. The versatility of spidroins, along with their biocompatible and biodegradable nature, also placed them as leading-edge biological macromolecules for improved drug delivery and various biomedical applications. Accordingly, in this review, we highlight the relationship between the molecular structure of spider silk and its mechanical properties and aims to provide a critical summary of recent progress in research employing recombinantly produced bioengineered spidroins for the production of innovative bio-derived structural materials.

Impacts of Blended Bombyx mori Silk Fibroin and Recombinant Spider Silk Fibroin Hydrogels on Cell Growth

Binary-blended hydrogels fabricated from Bombyx mori silk fibroin (SF) and recombinant spider silk protein eADF4(C16) were developed and investigated concerning gelation and cellular interactions in vitro. With an increasing concentration of eADF4(C16), the gelation time of SF was shortened from typically one week to less than 48 h depending on the blending ratio. The biological tests with primary cells and two cell lines revealed that the cells cannot adhere and preferably formed cell aggregates on eADF4(C16) hydrogels, due to the polyanionic properties of eADF4(C16). Mixing SF in the blends ameliorated the cellular activities, as the proliferation of L929 fibroblasts and SaOS-2 osteoblast-like cells increased with an increase of SF content. The blended SF:eADF4(C16) hydrogels attained the advantages as well as overcame the limitations of each individual material, underlining the utilization of the hydrogels in several biomedical applications.

Self-Assembled Silk Fibroin-Based Aggregates for Delivery of Camptothecin

A water-soluble hydrolysate of silk fibroin (SF) (~30 kDa) was esterified with tocopherol, ergocalciferol, and testosterone to form SF aggregates for the controlled delivery of the anticancer drug camptothecin (CPT). Elemental analysis and ¹H NMR spectroscopy showed a degree of substitution (DS) on SF of 0.4 to 3.8 mol %. Yields of 58 to 71% on vitamins- and testosterone-grafted SF conjugates were achieved. CPT was efficiently incorporated into the lipophilic core of SF aggregates using a dialysis-precipitation method, achieving drug contents of 6.3-8.5 wt %. FTIR spectra and DSC thermograms showed that tocopherol- and testosterone-grafted SF conjugates predominantly adopted a β-sheet conformation. After the esterification of tyrosine residues on SF chains with the vitamin or testosterone, the hydrodynamic diameters almost doubled or tripled that of SF. The zeta potential values after esterification increased to about -30 mV, which favors the stability of aggregates in aqueous medium. Controlled and almost quantitative release of CPT was achieved after 6 days in PBS at 37 °C, with almost linear release during the first 8 h. MCF-7 cancer cells exhibited good uptake of CPT-loaded SF aggregates after 6 h, causing cell death and cell cycle arrest in the G2/M phase. Substantial uptake of the CPT-loaded aggregates into MCF-7 spheroids was shown after 3 days. Furthermore, all CPT-loaded SF aggregates demonstrated superior toxicity to MCF-7 spheroids compared with parent CPT. Blank SF aggregates induced no hemolysis at pH 6.2 and 7.4, while CPT-loaded SF aggregates provoked hemolysis at pH 6.2 but not at pH 7.4. In contrast, parent CPT caused hemolysis at both pH tested. Therefore, CPT-loaded SF aggregates are promising candidates for chemotherapy.

Systemic and Local Silk-Based Drug Delivery Systems for Cancer Therapy

For years, surgery, radiotherapy, and chemotherapy have been the gold standards to treat cancer, although continuing research has sought a more effective approach. While advances can be seen in the development of anticancer drugs, the tools that can improve their delivery remain a challenge. As anticancer drugs can affect the entire body, the control of their distribution is desirable to prevent systemic toxicity. The application of a suitable drug delivery platform may resolve this problem. Among other materials, silks offer many advantageous properties, including biodegradability, biocompatibility, and the possibility of obtaining a variety of morphological structures. These characteristics allow the exploration of silk for biomedical applications and as a platform for drug delivery. We have reviewed silk structures that can be used for local and systemic drug delivery for use in cancer therapy. After a short description of the most studied silks, we discuss the advantages of using silk for drug delivery. The tables summarize the descriptions of silk structures for the local and systemic transport of anticancer drugs. The most popular techniques for silk particle preparation are presented. Further prospects for using silk as a drug carrier are considered. The application of various silk biomaterials can improve cancer treatment by the controllable delivery of chemotherapeutics, immunotherapeutics, photosensitizers, hormones, nucleotherapeutics, targeted therapeutics (e.g., kinase inhibitors), and inorganic nanoparticles, among others.

Artificial spider silk supports and guides neurite extension in vitro

Surgical intervention with the use of autografts is considered the gold standard to treat peripheral nerve injuries. However, a biomaterial that supports and guides nerve growth would be an attractive alternative to overcome problems with limited availability, morbidity at the site of harvest, and nerve mismatches related to autografts. Native spider silk is a promising material for construction of nerve guidance conduit (NGC), as it enables regeneration of cm-long nerve injuries in sheep, but regulatory requirements for medical devices demand synthetic materials. Here, we use a recombinant spider silk protein (NT2RepCT) and a functionalized variant carrying a peptide derived from vitronectin (VN-NT2RepCT) as substrates for nerve growth support and neurite extension, using a dorsal root ganglion cell line, ND7/23. Two-dimensional coatings were benchmarked against poly-d-lysine and recombinant laminins. Both spider silk coatings performed as the control substrates with regards to proliferation, survival, and neurite growth. Furthermore, NT2RepCT and VN-NT2RepCT spun into continuous fibers in a biomimetic spinning set-up support cell survival, neurite growth, and guidance to an even larger extent than native spider silk. Thus, artificial spider silk is a promising biomaterial for development of NGCs.

Progress in the application of spider silk protein in medicine

Spider silk protein has attracted much attention on account of its excellent mechanical properties, biodegradability, and biocompatibility. As the main protein component of spider silk, spidroin plays important role in spider spinning under natural circumstances and biomaterial application in medicine as well. Compare to the native spidroin which has a large molecular weight (>300 kDa) with highly repeat glycine and polyalanine regions, the recombinant spidroin was maintained the core amino motifs and much easier to collect. Here, we reviewed the application of recombinant spider silk protein eADF4(C16), major ampullate spidroin (MaSp), minor ampullate spidroin (MiSp), and the derivatives of recombinant spider silk protein in drug delivery system. Moreover, we also reviewed the application of spider silk protein in the field of alternative materials, repairing materials, wound dressing, surgical sutures along with advances in recombinant spider silk protein.

Hyperthermia treatment of cancer cells by the application of targeted silk/iron oxide composite spheres

Magnetic iron oxide nanoparticles (IONPs) are one of the most extensively studied materials for theranostic applications. IONPs can be used for magnetic resonance imaging (MRI), delivery of therapeutics, and hyperthermia treatment. Silk is a biocompatible material and can be used for biomedical applications. Previously, we produced spheres made of H2.1MS1 bioengineered silk that specifically carried a drug to the Her2-overexpressing cancer cells. To confer biocompatibility and targeting properties to IONPs, we blended these particles with bioengineered spider silks. Three bioengineered silks (MS1Fe1, MS1Fe2, and MS1Fe1Fe2) functionalized with the adhesion peptides F1 and F2, were constructed and investigated to form the composite spheres with IONPs carrying a positive or negative charge. Due to its highest IONP content, MS1Fe1 silk was used to produce spheres from the H2.1MS1:MS1Fe silk blend to obtain a carrier with cell-targeting properties. Composite H2.1MS1:MS1Fe1/IONP spheres made of silks blended at different ratios were obtained. Although the increased content of MS1Fe1 silk in particles resulted in an increased affinity of the spheres to IONPs, it decreased the binding of the composite particles to cancer cells. The H2.1MS1:MS1Fe1 particles prepared at a ratio of 8:2 and loaded with IONPs exhibited the ability to bind to the targeted cancer cells similar to the control spheres without IONPs. Moreover, when exposed to the alternating magnetic field, these particles generated 2.5 times higher heat. They caused an almost three times higher percentage of apoptosis in cancer cells than the control particles. The blending of silks enabled the generation of cancer-targeting spheres with a high affinity for iron oxide nanoparticles, which can be used for anti-cancer hyperthermia therapy.

Designed Spider Silk-Based Drug Carrier for Redox- or pH-Triggered Drug Release

Targeted drug delivery and controlled drug release can be obtained using specifically designed polymers as carriers. Due to their biocompatibility and biodegradability and especially the lack of an immune response, materials made of spider silk proteins are promising candidates for use in such applications. Particles made of recombinant spider silk proteins have previously been shown to be suitable drug and gene carriers as they could readily be loaded with various drug substances or biologicals, and subsequent release was observed over a defined period of time. However, the respective substances were bound non-covalently via hydrophobic or charge-charge interactions, and hence, the release of loaded substances could not be spatio-temporally controlled. Here, we present a setup of chemically modified recombinant spider silk protein eADF4 and variants thereof, combining their well-established biocompatible properties with covalent drug binding and triggered release upon changes in the pH or redox state, respectively. The usefulness of the spider silk platform technology was shown with model substances and cytostatic drugs bound to spider silk particles or films via a pH-labile hydrazine linker as one option, and the drugs could be released from the spider silk carriers upon acidification of the environment as seen, e.g., in tumorous tissues or endo/lysosomes. Sulfhydryl-bearing spider silk variants allowed model substance release if exposed to intracellular GSH (glutathione) levels as a second coupling option. The combination of non-immunogenic, nontoxic spider silk materials as drug carriers with precisely triggerable release chemistry presents a platform technology for a wide range of applications.

Bioengineered elastin- and silk-biomaterials for drug and gene delivery

Advances in medical science have led to diverse new therapeutic modalities, as well as enhanced understanding of the progression of various disease states. These findings facilitate the design and development of more customized and exquisite drug delivery systems that aim to improve therapeutic indices of drugs to treat a variety of conditions. Synthetic polymer-based drug carriers have often been the focus of such research. However, these structures suffer from challenges with heterogeneity of the starting material, limited chemical features, complex functionalization methods, and in some cases a lack of biocompatibility. Consequently, protein-based polymers have garnered much attention in recent years due to their monodisperse features, ease of production and functionalization, and biocompatibility. Genetic engineering techniques enable the advancement of protein-based drug delivery systems with finely tuned physicochemical properties, and thus an expanded level of customization unavailable with synthetic polymers. Of these genetically engineered proteins, elastin-like proteins (ELP), silk-like proteins (SLP), and silk-elastin-like proteins (SELP) provide a unique set of alternatives for designing drug delivery systems due to their inherent chemical and physical properties and ease of engineering afforded by recombinant DNA technologies. In this review we examine the advantages of genetically engineered drug delivery systems with emphasis on ELP and SLP constructions. Methods for fabrication and relevant biomedical applications will also be discussed.

Evaluation of silk-based bioink during pre and post 3D bioprinting: A review

During past few decades, the demand for the replacement of damaged organs is increasing consistently. This is due to the advancement in tissue engineering, which opens the possibility of regeneration of damaged organs or tissues into functional parts with the help of 3D bioprinting. Bioprinting technology presents an excellent potential to develop complex structures with precise control over cell suspension and structure. A brief description of different types of 3D bioprinting techniques, including inkjet-based, laser-based, and extrusion-based bioprinting is presented here. Due to innate advantageous features like tunable biodegradability, biocompatibility, elasticity and mechanical robustness, silk has carved a niche in the realm of tissue engineering. In this review article, the focus is to highlight the possible approach of exploring silk as bioink for fabrication of bioprinted implants using 3D bioprinting. This review discusses different type of degumming, dissolution techniques for extraction of proteins from different sources of silk. Different recently reported 3D bioprinting techniques suitable for silk-based bioink are further elaborated. Postprinting characterization of resultant scaffolds are also describe here. However, there is an astounding progress in 3D bioprinting technology, still there is a need to develop further the current bioprinting technology to make it suitable for generation of heterogeneous tissue construct. The possibility of utilizing the adhesive property of sericin to consider it as bioink is elaborated.

Surface Modification of Spider Silk Particles to Direct Biomolecular Corona Formation

In recent years, spider silk-based materials have attracted attention because of their biocompatibility, processability, and biodegradability. For their potential use in biomaterial applications, i.e., as drug delivery systems and implant coatings for tissue regeneration, it is vital to understand the interactions between the silk biomaterial surface and the biological environment. Like most polymeric carrier systems, spider silk material surfaces can adsorb proteins when in contact with blood, resulting in the formation of a biomolecular corona. Here, we assessed the effect of surface net charge of materials made of recombinant spider silk on the biomolecular corona composition. In-depth proteomic analysis of the biomolecular corona revealed that positively charged spider silk materials surfaces interacted predominantly with fibrinogen-based proteins. This fibrinogen enrichment correlated with blood clotting observed for both positively charged spider silk films and particles. In contrast, negative surface charges prevented blood clotting. Genetic engineering allows the fine-tuning of surface properties of the spider silk particles providing a whole set of recombinant spider silk proteins with different charges or peptide tags to be used for, for example, drug delivery or cell docking, and several of these were analyzed concerning the composition of their biomolecular corona. Taken together this study demonstrates how the surface net charge of recombinant spider silk surfaces affects the composition of the biomolecular corona, which in turn affects macroscopic effects such as fibrin formation and blood clotting.

Recent advances in polymeric materials for the delivery of RNA therapeutics

Introduction: The delivery of nucleic acid therapeutics through non-viral carriers face multiple biological barriers that reduce their therapeutic efficiency. Despite great progress, there remains a significant technological gap that continues to limit clinical translation of these nanocarriers. A number of polymeric materials are being exploited to efficiently deliver nucleic acids and achieve therapeutic effects. Areas covered: We discuss the recent advances in the polymeric materials for the delivery of nucleic acid therapeutics. We examine the use of common polymer architectures and highlight the challenges that exist for their development from bench side to clinic. We also provide an overview of the most notable improvements made to circumvent such challenges, including structural modification and stimuli-responsive approaches, for safe and effective nucleic acid delivery. Expert opinion: It has become apparent that a universal carrier that follows 'one-size' fits all model cannot be expected for delivery of all nucleic acid therapeutics. Carriers need to be designed to exhibit sensitivity and specificity toward individual targets diseases/indications, and relevant subcellular compartments, each of which possess their own unique challenges. The ability to devise synthetic methods that control the molecular architecture enables the future development that allow for the construction of 'intelligent' designs.

Cellular uptake, intracellular distribution and degradation of Her2-targeting silk nanospheres

BACKGROUND

The development of nanocarrier technology has attracted great interest in the last decade. Biodegradable spheres made of functionalized silk have considerable potential to be used as drug delivery systems for cancer treatment. A targeting ligand displayed at the surface of a carrier, with a specific affinity towards a particular receptor, can further enhance the accumulation and uptake of nanoparticles at the site of a tumor.

MATERIALS AND METHODS

The hybrid constructs were obtained by adding a Her2-binding peptide (H2.1) to MS1 and MS2 bioengineered silks based on the MaSp1 and MaSp2 proteins from N. clavipes, respectively. The H2.1MS1 and H2.1MS2 proteins were blended at a weight ratio of 8:2. Stable silk particles were formed by mixing a soluble protein with potassium phosphate using a micromixing technique. We used specific inhibitors of endocytosis to determine the cellular uptake pathway of the silk nanoparticles in human Her2-positive breast cancer cells. The subcellular distribution of silk particles was investigated by evaluating the signal colocalization with organelle-specific tracker. Moreover, lysosomal and exosomal inhibitors were implemented to evaluate their impact on the silk spheres behavior and degradation.

RESULTS

The functionalized spheres were specifically taken up by Her2-positive cancer cells. Silk particles facilitated the entry into cells through both the clathrin- and caveola-dependent pathways of endocytosis. Upon entering the cells, the particles accumulated in the lysosomes, where intracellular degradation occurred.

CONCLUSIONS

The present study demonstrated directly that the lysosomal function was essential for silk-based carrier elimination. The degradation of the carrier is of great importance to develop an optimal drug delivery system.

Composite spheres made of bioengineered spider silk and iron oxide nanoparticles for theranostics applications

Bioengineered spider silk is a biomaterial that has exquisite mechanical properties, biocompatibility, and biodegradability. Iron oxide nanoparticles can be applied for the detection and analysis of biomolecules, target drug delivery, as MRI contrast agents and as therapeutic agents for hyperthermia-based cancer treatments. In this study, we investigated three bioengineered silks, MS1, MS2 and EMS2, and their potential to form a composite material with magnetic iron oxide nanoparticles (IONPs). The presence of IONPs did not impede the self-assembly properties of MS1, MS2, and EMS2 silks, and spheres formed. The EMS2 spheres had the highest content of IONPs, and the presence of magnetite IONPs in these carriers was confirmed by several methods such as SEM, EDXS, SQUID, MIP-OES and zeta potential measurement. The interaction of EMS2 and IONPs did not modify the superparamagnetic properties of the IONPs, but it influenced the secondary structure of the spheres. The composite particles exhibited a more than two-fold higher loading efficiency for doxorubicin than the plain EMS2 spheres. For both the EMS2 and EMS2/IONP spheres, the drug revealed a pH-dependent release profile with advantageous kinetics for carriers made of the composite material. The composite spheres can be potentially applied for a combined cancer treatment via hyperthermia and drug delivery.

A green method for the production of an efficient bioimaging nanotool

The possibility of exploring basic biological phenomena requires the development of new and efficient bio-imaging tools. These should ideally combine the feasibility of production (potentially through the use of green chemistry) together with high targeting efficiency, low cytotoxicity, and optimal contrast characteristics. In this work, we developed nanovesicles based on cardanol, a natural and renewable byproduct of the cashew industry, and a fluorescent reporter was encapsulated in them through an environment-friendly synthesis method. In vitro investigations demonstrated that the cardanol nanovesicles are efficiently taken-up by both professional and non-professional phagocytic cells, which have been modeled in our approach by macrophages and HeLa cells, respectively. Co-localization studies show high affinity of the nanovesicles towards the cell plasma membrane. Moreover, metabolic assays confirmed that these nanostructures are biocompatible in a specific concentration range, and do not promote inflammation response in human macrophages. Stability studies carried out at different temperatures showed that the nanovesicles are stable at both 37 °C and 20 °C, while the formation of aggregates occurs when the nanodispersion is incubated at 4 °C. The results demonstrate the high potential of fluorescent cardanol nanovesicles as a green bioimaging tool, especially for investigating cell membrane dynamics.

Nanostructured, Self-Assembled Spider Silk Materials for Biomedical Applications

The extraordinary mechanical properties of spider silk fibers result from the interplay of composition, structure and self-assembly of spider silk proteins (spidroins). Genetic approaches enabled the biotechnological production of recombinant spidroins which have been employed to unravel the self-assembly and spinning process. Various processing conditions allowed to explore non-natural morphologies including nanofibrils, particles, capsules, hydrogels, films or foams. Recombinant spider silk proteins and materials made thereof can be utilized for biomedical applications, such as drug delivery, tissue engineering or 3D-biomanufacturing.

The Biomedical Use of Silk: Past, Present, Future

Humans have long appreciated silk for its lustrous appeal and remarkable physical properties, yet as the mysteries of silk are unraveled, it becomes clear that this outstanding biopolymer is more than a high-tech fiber. This progress report provides a critical but detailed insight into the biomedical use of silk. This journey begins with a historical perspective of silk and its uses, including the long-standing desire to reverse engineer silk. Selected silk structure-function relationships are then examined to appreciate past and current silk challenges. From this, biocompatibility and biodegradation are reviewed with a specific focus of silk performance in humans. The current clinical uses of silk (e.g., sutures, surgical meshes, and fabrics) are discussed, as well as clinical trials (e.g., wound healing, tissue engineering) and emerging biomedical applications of silk across selected formats, such as silk solution, films, scaffolds, electrospun materials, hydrogels, and particles. The journey finishes with a look at the roadmap of next-generation recombinant silks, especially the development pipeline of this new industry for clinical use.

Biomedical Applications of Recombinant Silk-Based Materials

Silk is mostly known as a luxurious textile, which originates from silkworms first cultivated in China. A deeper look into the variety of silk reveals that it can be used for much more, in nature and by humanity. For medical purposes, natural silks were recognized early as a potential biomaterial for surgical threads or wound dressings; however, as biomedical engineering advances, the demand for high-performance, naturally derived biomaterials becomes more pressing and stringent. A common problem of natural materials is their large batch-to-batch variation, the quantity available, their potentially high immunogenicity, and their fast biodegradation. Some of these common problems also apply to silk; therefore, recombinant approaches for producing silk proteins have been developed. There are several research groups which study and utilize various recombinantly produced silk proteins, and many of these have also investigated their products for biomedical applications. This review gives a critical overview over of the results for applications of recombinant silk proteins in biomedical engineering.

Recombinant Spider Silk Hydrogels for Sustained Release of Biologicals

Therapeutic biologics (i.e., proteins) have been widely recognized for the treatment, prevention, and cure of a variety of human diseases and syndromes. However, design of novel protein-delivery systems to achieve a nontoxic, constant, and efficient delivery with minimal doses of therapeutic biologics is still challenging. Here, recombinant spider silk-based materials are employed as a delivery system for the administration of therapeutic biologicals. Hydrogels made of the recombinant spider silk protein eADF4(C16) were used to encapsulate the model biologicals BSA, HRP, and LYS by direct loading or through diffusion, and their release was studied. Release of model biologicals from eADF4(C16) hydrogels is in part dependent on the electrostatic interaction between the biological and the recombinant spider silk protein variant used. In addition, tailoring the pore sizes of eADF4(C16) hydrogels strongly influenced the release kinetics. In a second approach, a particles-in-hydrogel system was used, showing a prolonged release in comparison with that of plain hydrogels (from days to week). The particle-enforced spider silk hydrogels are injectable and can be 3D printed. These initial studies indicate the potential of recombinant spider silk proteins to design novel injectable hydrogels that are suitable for delivering therapeutic biologics.

Silk nanofibril self-assembly versus electrospinning

Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface-to-volume ratio. Therefore, we introduce here self-assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self-assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk-based dopes, used in self-assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self-assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non-native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under:  Biology-Inspired Nanomaterials > Peptide-Based Structures  Nanotechnology Approaches to Biology > Nanoscale Systems in Biology  Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Tailor-made spider-eggcase-silk spheres for efficient lysosomal drug delivery

Spider silks are attractive biopolymers due to their excellent mechanical properties and biomimetic potential. To optimize the electrostatic interaction for lysosomal drug delivery, a spider-eggcase-silk protein was genetically engineered using 5× His Tag with a tailor-made isoelectric point of 4.8. By a facile HFIP-on-oil method, silk spheres were assembled as rapidly as 10 s. After the post-treatment of ethanol, silk spheres were determined with an improved compressive modulus by AFM indentation. Under incubation of silk spheres in a Doxorubicin solution, a maximum of 35% loading and average of 30% loading efficiency were determined. In the cytotoxicity experiment, silk spheres exhibited intrinsic biocompatibility and showed good control of the loaded drug in the neutral PBS solution. Significantly, by 96 h, the accumulative drug release at pH 4.5 was approximately 4.5-fold higher than that at pH 7.4. By conducting the platelet adhesion and hemolysis assay, Doxorubicin-loaded silk spheres exhibited good hemocompatibility. To further demonstrate this release behavior, within 24 h, Doxorubicin-loaded silk spheres were efficiently delivered to lysosomes and then released the payload to the nuclei of Hela cells.

Recombinant spider-eggcase-silk spheres are facilely prepared as drug carriers with a tailor-made isoelectric point specifically for lysosomal delivery.

Silk Materials Functionalized via Genetic Engineering for Biomedical Applications

The great mechanical properties, biocompatibility and biodegradability of silk-based materials make them applicable to the biomedical field. Genetic engineering enables the construction of synthetic equivalents of natural silks. Knowledge about the relationship between the structure and function of silk proteins enables the design of bioengineered silks that can serve as the foundation of new biomaterials. Furthermore, in order to better address the needs of modern biomedicine, genetic engineering can be used to obtain silk-based materials with new functionalities. Sequences encoding new peptides or domains can be added to the sequences encoding the silk proteins. The expression of one cDNA fragment indicates that each silk molecule is related to a functional fragment. This review summarizes the proposed genetic functionalization of silk-based materials that can be potentially useful for biomedical applications.

Blending two bioengineered spider silks to develop cancer targeting spheres

Blending two bioengineered spider silks combines the most favorable characteristics of both silks and can lead to the formation of an optimal drug delivery vehicle.

Structural Insights into Water-Based Spider Silk Protein-Nanoclay Composites with Excellent Gas and Water Vapor Barrier Properties

Nature reveals a great variety of inorganic-organic composite materials exhibiting good mechanical properties, high thermal and chemical stability, and good barrier properties. One class of natural bio-nanocomposites, e.g. found in mussel shells, comprises protein matrices with layered inorganic fillers. Inspired by such natural bio-nanocomposites, the cationic recombinant spider silk protein eADF4(κ16) was processed together with the synthetic layered silicate sodium hectorite in an all-aqueous setup. Drop-casting of this bio-nanocomposite resulted in a thermally and chemically stable film reflecting a one-dimensional crystal. Surprisingly, this bio-nanocomposite coating was, though produced in an all-aqueous process, completely water insoluble. Analyzing the structural details showed a low inner free volume due to the well-oriented self-assembly/alignment of the spider silk proteins on the nanoclay surface, yielding high oxygen and water vapor barrier properties. The here demonstrated properties in combination with good biocompatibility qualify this new bio-nanocomposite to be used in packaging applications.

Cellular uptake of drug loaded spider silk particles

Medical therapies are often accompanied by not-wanted side-effects or, even worse, targeted cells can develop drug resistance leading to an ineffective treatment. Here, it was shown that drugs can be efficiently delivered into and released within cells when spider silk particles were used as a carrier.