Deciphering the mechanism of protein interaction with silk fibroin for drug delivery systems

Silk Nanocarrier with Tunable Size to Improve Transdermal Capacity for Hydrophilic and Hydrophobic Drugs

Transdermal drug delivery is an attractive option for multiple disease therapies as it reduces adverse reactions and improves patient compliance. Water-dispersible β-sheet rich silk nanofiber carriers have hydrophobic properties that benefit transdermal delivery but still show inferior transdermal capacities. Thus, hydrophobic silk nanofibers were fabricated to fine-tune their size and endow them with desirable transdermal delivery capacities. Silk nanocarrier length was shortened from 2000 nm to approximately 40 nm after ultrasonic treatment. In vitro human skin and in vivo animal studies revealed different transdermal behaviors for silk nanocarriers at different nanosizes. Silk nanocarriers passed slowly through the corneum without destroying the corneum structure. Improved transdermal capacity was achieved for smaller size carriers. Both hydrophilic and hydrophobic drugs could be loaded onto silk nanocarriers, suggesting a promising future for different disease therapies. No cytotoxicity and skin irritation were identified for silk nanocarriers, which strengthened their superiority as transdermal carriers. Therefore, silk nanocarriers <100 nm may promote the percutaneous absorption of active cargos for disease therapy and cosmetic applications.

Biopolymeric Prodrug Systems as Potential Antineoplastic Therapy

Nowadays, cancer represents a major public health issue, a substantial economic issue, and a burden for society. Limited by numerous disadvantages, conventional chemotherapy is being replaced by new strategies targeting tumor cells. In this context, therapies based on biopolymer prodrug systems represent a promising alternative for improving the pharmacokinetic and pharmacologic properties of drugs and reducing their toxicity. The polymer-directed enzyme prodrug therapy is based on tumor cell targeting and release of the drug using polymer–drug and polymer–enzyme conjugates. In addition, current trends are oriented towards natural sources. They are biocompatible, biodegradable, and represent a valuable and renewable source. Therefore, numerous antitumor molecules have been conjugated with natural polymers. The present manuscript highlights the latest research focused on polymer–drug conjugates containing natural polymers such as chitosan, hyaluronic acid, dextran, pullulan, silk fibroin, heparin, and polysaccharides from Auricularia auricula.

Micro- and Nanopatterned Silk Substrates for Antifouling Applications

A major problem of current biomedical implants is the bacterial colonization and subsequent biofilm formation, which seriously affects their functioning and can lead to serious post-surgical complications. Intensive efforts have been directed toward the development of novel technologies that can prevent bacterial colonization while requiring minimal antibiotics doses. To this end, biocompatible materials with intrinsic antifouling capabilities are in high demand. Silk fibroin, widely employed in biotechnology, represents an interesting candidate. Here, we employ a soft-lithography approach to realize micro- and nanostructured silk fibroin substrates, with different geometries. We show that patterned silk film substrates support mammal cells (HEK-293) adhesion and proliferation, and at the same time, they intrinsically display remarkable antifouling properties. We employ Escherichia coli as representative Gram-negative bacteria, and we observe an up to 66% decrease in the number of bacteria that adhere to patterned silk surfaces as compared to control, flat silk samples. The mechanism leading to the inhibition of biofilm formation critically depends on the microstructure geometry, involving both a steric and a hydrophobic effect. We also couple silk fibroin patterned films to a biocompatible, optically responsive organic semiconductor, and we verify that the antifouling properties are very well preserved. The technology described here is of interest for the next generation of biomedical implants, involving the use of materials with enhanced antibacterial capability, easy processability, high biocompatibility, and prompt availability for coupling with photoimaging and photodetection techniques.

Recent trends in protein and peptide-based biomaterials for advanced drug delivery

Engineering protein and peptide-based materials for drug delivery applications has gained momentum due to their biochemical and biophysical properties over synthetic materials, including biocompatibility, ease of synthesis and purification, tunability, scalability, and lack of toxicity. These biomolecules have been used to develop a host of drug delivery platforms, such as peptide- and protein-drug conjugates, injectable particles, and drug depots to deliver small molecule drugs, therapeutic proteins, and nucleic acids. In this review, we discuss progress in engineering the architecture and biological functions of peptide-based biomaterials -naturally derived, chemically synthesized and recombinant- with a focus on the molecular features that modulate their structure-function relationships for drug delivery.

Silk Fibroin for CpG Oligodeoxynucleotide Delivery

CpG oligodeoxynucleotides (ODNs) have attracted increasing attention as immunotherapeutic agents. However, efficient transfection of CpG ODNs into the immune cells remains a big challenge. In this study, for the first time, we reported that silk fibroin (SF) could function as an efficient carrier for CpG ODNs. A novel strategy was developed to prepare SF-CpG ODNs nanoparticles (NPs) based on self-assembly of SF. The as-prepared SF-CpG NPs were spherical in shape and were uniformly dispersed. SF-CpG NPs exhibited good stability and biocompatibility. SF-CpG NPs possessed significantly enhanced (7 folds) cellular uptake compared with CpG ODNs. Release of CpG ODNs from SF-CpG NPs was accelerated in environment-mimicking TLR9-localized endo/lysosome. SF-CpG NPs stimulated about four folds higher levels of immune cytokines and nitric oxide compared with CpG ODNs. Our results suggested that SF notably improved the CpG ODNs delivery. SF-CpG NPs have strong potential in immunotherapy.

Regenerated Antheraea pernyi Silk Fibroin/Poly(N-isopropylacrylamide) Thermosensitive Composite Hydrogel with Improved Mechanical Strength

At present, Antheraea pernyi silk fibroin (ASF) has attracted research efforts to investigate it as a raw material for fabrication of biomedical devices because of its superior cytocompatibility. Nevertheless, native ASF is not easily processed into a hydrogel without any crosslinking agent, and a single hydrogel shows poor mechanical properties. In this paper, a series of ASF/poly (N-isopropylacrylamide) (PNIPAAm) composite hydrogels with different ASF contents were manufactured by a simple in situ polymerization method without any crosslinking agent. Meanwhile, the structures, morphologies and thermal properties of composite hydrogels were investigated by XRD, FTIR, SEM, DSC and TGA, respectively. The results indicate that the secondary structure of silk in the composite hydrogel can be controlled by changing the ASF content and the thermal stability of composite hydrogels is enhanced with an increase in crystalline structure. The composite hydrogels showed similar lower critical solution temperatures (LCST) at about 32 °C, which matched well with the LCST of PNIPAAm. Finally, the obtained thermosensitive composite hydrogels exhibited enhanced mechanical properties, which can be tuned by varying the content of ASF. This strategy to prepare an ASF-based responsive composite hydrogel with enhanced mechanical properties represents a valuable route for developing the fields of ASF, and, furthermore, their attractive applications can meet the needs of different biomaterial fields.

A Review of the Emerging Role of Silk for the Treatment of the Eye

Silk is a remarkable biopolymer with a long history of medical use. Silk fabrications have a robust track record for load-bearing applications, including surgical threads and meshes, which are clinically approved for use in humans. The progression of top-down and bottom-up engineering approaches using silk as the basis of a drug delivery or cell-loaded matrix helped to re-ignite interest in this ancient material. This review comprehensively summarises the current applications of silk for tissue engineering and drug delivery, with specific reference to the eye. Additionally, the review also covers emerging trends for the use of silk as a biologically active biopolymer for the treatment of eye disorders. The review concludes with future capabilities of silk to contribute to advanced, electronically-enhanced ocular drug delivery concepts.

The method of purifying bioengineered spider silk determines the silk sphere properties

Bioengineered spider silks are a biomaterial with great potential for applications in biomedicine. They are biocompatible,biodegradable and can self-assemble into films, hydrogels, scaffolds, fibers, capsules and spheres. A novel, tag-free, bioengineered spider silk named MS2(9x) was constructed. It is a 9-mer of the consensus motif derived from MaSp2–the spidroin of Nephila clavipes dragline silk. Thermal and acidic extraction methods were used to purify MS2(9x). Both purification protocols gave a similar quantity and quality of soluble silk; however, they differed in the secondary structure and zeta potential value. Spheres made of these purified variants differed with regard to critical features such as particle size, morphology, zeta potential and drug loading. Independent of the purification method, neither variant of the MS2(9x) spheres was cytotoxic, which confirmed that both methods can be used for biomedical applications. However, this study highlights the impact that the applied purification method has on the further biomaterial properties.

Silk-based blood stabilization for diagnostics

Advanced personalized medical diagnostics depend on the availability of high-quality biological samples. These are typically biofluids, such as blood, saliva, or urine; and their collection and storage is critical to obtain reliable results. Without proper temperature regulation, protein biomarkers in particular can degrade rapidly in blood samples, an effect that ultimately compromises the quality and reliability of laboratory tests. Here, we present the use of silk fibroin as a solid matrix to encapsulate blood analytes, protecting them from thermally induced damage that could be encountered during nonrefrigerated transportation or freeze-thaw cycles. Blood samples are recovered by simple dissolution of the silk matrix in water. This process is demonstrated to be compatible with a number of immunoassays and provides enhanced sample preservation in comparison with traditional air-drying paper approaches. Additional processing can remediate interactions with conformational structures of the silk protein to further enhance blood stabilization and recovery. This approach can provide expanded utility for remote collection of blood and other biospecimens empowering new modalities of temperature-independent remote diagnostics.

Silk-based stabilization of biomacromolecules

Silk fibroin is a high molecular weight amphiphilic protein that self-assembles into robust biomaterials with remarkable properties including stabilization of biologicals and tunable release kinetics correlated to processing conditions. Cells, antibiotics,monoclonal antibodies and peptides, among other biologics, have been encapsulated in silk using various processing approaches and material formats. The mechanistic basis for the entrapment and stabilization features, along with insights into the modulation of release of the entrained compounds from silks will be reviewed with a focus on stabilization of bioactive molecules.

Design of Silk-Vaterite Microsphere Systems as Drug Carriers with pH-responsive Release Behavior

Improving the therapeutic efficacy of chemotherapy remains a key goal for cancer therapy. Various passive and active targeting strategies have been developed to facilitate drug release targeted to cancer lesions, but actively designing tunable drug release behavior for these needs remains a challenge. As a step towards this need, silk-vaterite microspheres were fabricated and utilized as carriers to tune drug release. Doxorubicin (DOX) was loaded on the microspheres with high efficiency and the release behavior was regulated by tuning the microspheres via thermal processing. In vitro cell inhibition results showed that the drug-loaded microspheres had different cytotoxic efficiencies depending on the DOX release rates. Better efficacy at lower drug doses suggests options to optimize anticancer effects while minimizing toxic side effects. The tunable drug release capacity combined with the inherent passive targeting property of vaterite-based carriers based on pH sensitivity suggests a promising system for enhanced efficacy of chemotherapy.

Enhanced the Negative Charges of Antheraea pernyi Silk Fibroin by Methylglyoxal Modification

Antheraea pernyi silk fibroin has favorable biocompatibility, good bioactivity and controllable biodegradability, meeting the basic requirements of controlled drug release carriers. Enhancing the negative charge of silk fibroin could further increase the encapsulation and loading efficiency of positively charged drugs. In this study, Antheraea pernyi silk fibroin was chemically modified by methylglyoxal in aqueous solution. The electric charge properties of Antheraea pernyi silk fibroin were examined to characterize the modification, the results indicated that the isoelectric point of Antheraea pernyi silk fibroin decreased from 4.5 to 3.9, and the zeta potential reduced from-11.7 mV to-12.8 mV. Amino acid analysis and 1H-NMR spectra showed that arginine residue of Antheraea pernyi silk fibroin side chain was modified by methylglyoxal for enhancing negative charge of silk fibroin. These results suggested that methylglyoxal-modified Antheraea pernyi silk fibroin could be considered as a potential starting material in loading positively charged drugs.

Controlling silk fibroin microspheres via molecular weight distribution

Silk fibroin (SF) microspheres were produced by salting out SF solution via the addition of potassium phosphate buffer solution (K2HPO4-KH2PO4). The morphology, size and polydispersity of SF microspheres were adjusted by changing the molecular weight (MW) distribution and concentration of SF, as well as the ionic strength and pH of the buffer solution. Changing the conditions under which the SF fiber dissolved in the Lithium Boride (LiBr) solution resulted in altering the MW distribution of SF solution. Under optimal salting-out conditions (ionic strength>0.7 M and pH>7) and using a smaller and narrower SF MW distribution, SF microspheres with smoother shapes and more uniform sizes were produced. Meanwhile, the size and polydispersity of the microspheres increased when the SF concentration was increased from 0.25 mg/mL to 20 mg/mL. The improved SF microspheres, obtained by altering the distribution of molecular weight, have potential in drug and gene delivery applications.