Expanding the boundaries of silk sericin biomaterials in biomedical applications

Silk sericin (SS) has a long history as a by-product of the textile industry. SS has emerged as a sustainable material for biomedical engineering due to its material properties including water solubility, diverse impact on biological activities including antibacterial and antioxidant properties, and ability to promote cell adhesion and proliferation. This review addresses the origin, structure, properties, extraction, and underlying functions of this protein. An overview of the growing research studies and market evolution is presented, along with highlights of the most common fabrication matrices (hydrogels, bioinks, porous and fibrous scaffolds) and tissue engineering applications. Finally, the future trends with this protein as a multifaceted toolbox for bioengineering are explored, along with the challenges with SS. Overall, the present review can serve as a foundation for the creation of innovative biomaterials utilizing SS as a fundamental building block that hold market potential.

Water-Modulated Biomimetic Hyper-Attribute-Gel Electronic Skin for Robotics and Skin-Attachable Wearables

Electronic skin (e-skin), mimicking the physical-chemical and sensory properties of human skin, is promising to be applied as robotic skins and skin-attachable wearables with multisensory functionalities. To date, most e-skins are dedicated to sensory function development to mimic human skins in one or several aspects, yet advanced e-skin covering all the hyper-attributes (including both the sensory and physical-chemical properties) of human skins is seldom reported. Herein, a water-modulated biomimetic hyper-attribute-gel (Hygel) e-skin with reversible gel-solid transition is proposed, which exhibits all the desired skin-like physical-chemical properties (stretchability, self-healing, biocompatibility, biodegradability, weak acidity, antibacterial activities, flame retardance, and temperature adaptivity), sensory properties (pressure, temperature, humidity, strain, and contact), function reconfigurability, and evolvability. Then the Hygel e-skin is applied as an on-robot e-skin and skin-attached wearable to demonstrate its highly skin-like attributes in capturing multiple sensory information, reconfiguring desired functions, and excellent skin compatibility for real-time gesture recognition via deep learning. This Hygel e-skin may find more applications in advanced robotics and even skin-replaceable artificial skin.

Continuous Production of Highly Tuned Silk/Calcium-Based Composites: Exploring New Pathways for Skin Regeneration

Calcium plays an important role in barrier function repair and skin homeostasis. In particular, calcium phosphates (CaPs) are well established materials for biomedical engineering due to their biocompatibility. To generate biomaterials with a more complete set of biological properties, previously discarded silk sericin (SS) has been recovered and used as a template to grow CaPs. Crucial characteristics for skin applications, such as antibacterial activity, can be further enhanced by doping CaPs with cerium (Ce) ions. The effectiveness of cell attachment and growth on the materials highly depends on their morphology, particle size distribution, and chemical composition. These characteristics can be tailored through the application of oscillatory flow technology, which provides precise mixing control of the reaction medium. Thus, in the present work, CaP/SS and CaP/SS/Ce particles were fabricated for the first time using a modular oscillatory flow plate reactor (MOFPR) in a continuous mode. Furthermore, the biological behavior of both these composites and of previously produced pure CaPs was assessed using human dermal fibroblasts (HDFs). It was demonstrated that both CaP based with plate-shaped nanoparticles and CaP-SS-based composites significantly improved cell viability and proliferation over time. The results obtained represent a first step towards the reinvention of CaPs for skin engineering.

Silk-based microcarriers: current developments and future perspectives

Cell-seeded microcarriers (MCs) are currently one of the most promising topics in biotechnology. These systems are supportive structures for cell growth and expansion that allow efficient nutrient and gas transfer between the media and the attached cells. Silk proteins have been increasingly used for this purpose in the past few years due to their biocompatibility, biodegradability and non-toxicity. To date, several silk fibroin spherical MCs in combination with alginate, gelatin and calcium phosphates have been reported with very interesting outcomes. In addition, other silk-based three-dimensional structures such as microparticles with chitosan and collagen, as well as organoids, have been increasingly studied. In this study, the physicochemical and biological properties of these biomaterials, as well as the recent methodologies for their processing and for cell culture, are discussed. The potential biomedical applications are also addressed. In addition, an analysis of the future perspectives is presented, where the potential of innovative silk-based MCs processing technologies is highlighted.

Rapid regeneration of enamel-like-oriented inorganic crystals by using rotary evaporation

Enamel, the hardest tissue in the human body, has excellent mechanical properties, mainly due to its highly ordered spatial structure. Fabricating enamel-like structure is still a challenge today. In this work, a simple and highly efficient method was introduced, using the silk fibroin as a template to regulate calcium- and phosphate- supersaturated solution to regenerate enamel-like hydroxyapatite crystals on various substrates (enamel, dentin, titanium, and polyethylene) under rotary evaporation. The enamel-like zinc oxide nanorod array structure was also successfully synthesized using the aforementioned method. This strategy provides a new approach to design and fabricate mineral crystals with particular orientation coatings for materials.

The Significance and Utilisation of Biomimetic and Bioinspired Strategies in the Field of Biomedical Material Engineering: The Case of Calcium Phosphat-Protein Template Constructs

This review provides a summary of recent research on biomimetic and bioinspired strategies applied in the field of biomedical material engineering and focusing particularly on calcium phosphate-protein template constructs inspired by biomineralisation. A description of and discussion on the biomineralisation process is followed by a general summary of the application of the biomimetic and bioinspired strategies in the fields of biomedical material engineering and regenerative medicine. Particular attention is devoted to the description of individual peptides and proteins that serve as templates for the biomimetic mineralisation of calcium phosphate. Moreover, the review also presents a description of smart devices including delivery systems and constructs with specific functions. The paper concludes with a summary of and discussion on potential future developments in this field.

Hydroxyapatite/sericin composites: A simple synthesis route under near-physiological conditions of temperature and pH and preliminary study of the effect of sericin on the biomineralization process

Synthesis of hydroxyapatite (HAp) and sericin (SS) nanocomposites was carried out by a simple precipitation method performed in batch in a stirred tank reactor (ST). The reaction was achieved by mixing a solution of calcium chloride dihydrate, in which SS was dissolved, with a solution of disodium hydrogen phosphate at 37 °C. Three experimental conditions were studied by varying the concentration of SS: HAp, HAp/SS1 (0.01 g/L of SS) and HAp/SS2 (1 g/L of SS). The chemical and physical properties of the resulting HAp/SS nanocomposites were studied using several techniques (Atomic Absorption Spectrometry, Ultraviolet-Visible Spectrophotometry, Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Selected area diffraction (SAED) and Thermogravimetric analysis (TGA)). pH profile was also monitored over time for each experimental condition. The results revealed that nano single-phased HAp was formed with both rod and plate-like shape. Additionally, the particles have low crystallinity, characteristic similar to biological HAp. Regarding the influence of SS, one observed that with increasing SS concentration there is an increase in the mean particle size and the number of plate-like particles, as well as an increase in the aggregation degree and a decrease of the crystallinity. Further, the composites obtained have an inorganic/organic composition comparable to bone. Finally, in vitro cytotoxicity showed that the synthetized nanoparticles are non-toxic and cell viability is higher for HAp and HAp/SS samples when compared to a commercially available HAp. The produced materials can thus be considered suitable candidates for bone related applications.

Bioinspired mineralization of a functionalized injectable dense collagen hydrogel through silk sericin incorporation

The incorporation of silk sericin into injectable dense collagen hydrogels represents a powerful approach to mimic the biomineralization process, together with the osteogenic stimulation of seeded mesenchymal stem cells, in vitro.

Silk scaffolds in bone tissue engineering: An overview

Bone tissue plays multiple roles in our day-to-day functionality. The frequency of accidental bone damage and disorder is increasing worldwide. Moreover, as the world population continues to grow, the percentage of the elderly population continues to grow, which results in an increased number of bone degenerative diseases. This increased elderly population pushes the need for artificial bone implants that specifically employ biocompatible materials. A vast body of literature is available on the use of silk in bone tissue engineering. The current work presents an overview of this literature from materials and fabrication perspective. As silk is an easy-to-process biopolymer; this allows silk-based biomaterials to be molded into diverse forms and architectures, which further affects the degradability. This makes silk-based scaffolds suitable for treating a variety of bone reconstruction and regeneration objectives. Silk surfaces offer active sites that aid the mineralization and/or bonding of bioactive molecules that facilitate bone regeneration. Silk has also been blended with a variety of polymers and minerals to enhance its advantageous properties or introduce new ones. Several successful works, both in vitro and in vivo, have been reported using silk-based scaffolds to regenerate bone tissues or other parts of the skeletal system such as cartilage and ligament. A growing trend is observed toward the use of mineralized and nanofibrous scaffolds along with the development of technology that allows to control scaffold architecture, its biodegradability and the sustained releasing property of scaffolds. Further development of silk-based scaffolds for bone tissue engineering, taking them up to and beyond the stage of human trials, is hoped to be achieved in the near future through a cross-disciplinary coalition of tissue engineers, material scientists and manufacturing engineers.

STATEMENT OF SIGNIFICANCE

The state-of-art of silk biomaterials in bone tissue engineering, covering their wide applications as cell scaffolding matrices to micro-nano carriers for delivering bone growth factors and therapeutic molecules to diseased or damaged sites to facilitate bone regeneration, is emphasized here. The review rationalizes that the choice of silk protein as a biomaterial is not only because of its natural polymeric nature, mechanical robustness, flexibility and wide range of cell compatibility but also because of its ability to template the growth of hydroxyapatite, the chief inorganic component of bone mineral matrix, resulting in improved osteointegration. The discussion extends to the role of inorganic ions such as Si and Ca as matrix components in combination with silk to influence bone regrowth. The effect of ions or growth factor-loaded vehicle incorporation into regenerative matrix, nanotopography is also considered.

Artificial enamel induced by phase transformation of amorphous nanoparticles

Human tooth enamel has tightly packed c-axis-oriented hydroxyapatite (HAP: Ca10(PO4)6(OH)2) nanorods with high elastic modulus. Fabrication of an enamel architecture in vitro supports the repair of teeth using HAP; however, existing methods require complex and laborious steps to form an enamel-like structure. Here we present a very simple and effective technique for forming artificial enamel in near-physiological solution using a substrate composed of amorphous calcium phosphate (ACP) nanoparticles. Without any functionalized modification of the substrate surface, faint dissolution and successive phase transformation automatically induce formation of an intermediate layer of low-crystalline HAP nanoparticles, on which highly oriented HAP nanorods grow by geometrical selection. We also show that an enamel structure forms on a substrate of amorphous calcium carbonate when the surface nanoparticles react so as to form an intermediate layer similar to that in ACP. Our results demonstrate that there is a wide range of substrate choices for nanorod array formation. Contrary to current understanding, a stable surface designed in nanoscale is not essential for the growth of arranged guest crystals. Reactive amorphous nanoparticles and their transformation efficiently induce a nanorod array structure.

Processing and characterization of silk sericin from Bombyx mori and its application in biomaterials and biomedicines

Bombyx mori silk is composed of 60-80% fibroin, 15-35% sericin and 1-5% non-sericin component including wax, pigments, sugars and other impurities. For two decades, the protein-based silk fibroin was extensively used in the research and development of medical biomaterials and biomedicines. Sericin is frequently ignored and abandoned as a byproduct or waste in the processing of traditional silk fabrics, silk floss or modern silk biomaterials. However, similar to fibroin, sericin is not only a highly useful biological material, but also a lot of biological activity. Moreover, the non-sericin component present with sericin in the cocoon shell also has a strong biological activity. In this review, the extraction and recovery methods of sericin and the non-sericin component from the cocoon layer are reported, and their composition, properties and biological activity are described to produce a comprehensive report on biomedical materials and biological drugs. In addition, related problems or concerns present in the research and development of sericin are discussed, and a potential application of sericin in sustainable development is also presented.

Ca2+-induced self-assembly of Bombyx mori silk sericin into a nanofibrous network-like protein matrix for directing controlled nucleation of hydroxylapatite nano-needles

Bone biomineralization is a well-regulated protein-mediated process where hydroxylapatite (HAP) crystals are nucleated with preferred orientation within self-assembled protein matrix. Mimicking this process is a promising approach to the production of bone-like protein/mineral nanocomposites for bone repair and regeneration. Towards the goal of fabricating such nanocomposites from sericin, a protein spun by Bombyx mori (B.mori) silkworm, and bone mineral HAP, for the first time we investigated the chemical mechanism underpinning the synergistic processes of the conformational change/self-assembly of B.mori sericin ( BS ) as well as the nucleation of HAP on the resultant self-assembled BS matrix. We found that BS , rich in anionic amino acid residues, could bind Ca²⁺ ions from the HAP precursor solution through electrostatic attraction. The Ca²⁺binding drove the conformational change of BS from random coils into β-sheets and its concomitant self-assembly into interconnected nanofibrous network-like protein matrix, which initiated the nucleation and growth of HAP crystals. HAP crystals directed by the resultant self-assembled BS matrix grew preferentially along their crystallographic c-axis, leading to the formation of HAP nano-needles. The HAP nano-needles in the self-assembled BS matrix were subsequently aggregated into globules, probably driven by the hydrogen bonding between C=O groups of BS and O-H groups of HAP nano-needles. The present work sheds light on the chemical mechanisms of BS self-assembly and the controlled mineralization directed by the self-assembled matrix. We also found that the resultant nanocomposites could promote the osteogenic differentiation of human bone marrow-derived mesenchymal stem cells. Thus our work also generates a biomimetic approach to bone-like silk protein/mineral nanocomposite scaffolds that can find potential applications in bone repair and regeneration.

A novel nanocomposite for bone tissue engineering based on chitosan–silk sericin/hydroxyapatite: biomimetic synthesis and its cytocompatibility

Scheme of the formation mechanism of CS–SS/HA-s and CS–SS/HA-g nanocomposites.

Biomimetic nucleation of hydroxyapatite crystals mediated by Antheraea pernyi silk sericin promotes osteogenic differentiation of human bone marrow derived mesenchymal stem cells

Biomacromolecules have been used as templates to grow hydroxyapatite crystals (HAps) by biomineralization to fabricate mineralized materials for potential application in bone tissue engineering. Silk sericin is a protein with features desirable as a biomaterial, such as increased hydrophilicity and biodegradation. Mineralization of the silk sericin from Antheraea pernyi (A. pernyi) silkworm has rarely been reported. Here, for the first time, nucleation of HAps on A. pernyi silk sericin (AS) was attempted through a wet precipitation method and consequently the cell viability and osteogenic differentiation of BMSCs on mineralized AS were investigated. It was found that AS mediated the nucleation of HAps in the form of nanoneedles while self-assembling into β-sheet conformation, leading to the formation of a biomineralized protein based biomaterial. The cell viability assay of BMSCs showed that the mineralization of AS stimulated cell adhesion and proliferation, showing that the resultant AS biomaterial is biocompatible. The differentiation assay confirmed that the mineralized AS significantly promoted the osteogenic differentiation of BMSCs when compared to nonmineralized AS as well as other types of sericin (B. mori sericin), suggesting that the resultant mineralized AS biomaterial has potential in promoting bone formation. This result represented the first work proving the osteogenic differentiation of BMSCs directed by silk sericin. Therefore, the biomineralization of A. pernyi silk sericin coupled with seeding BMSCs on the resultant mineralized biomaterials is a useful strategy to develop the potential application of this unexplored silk sericin in the field of bone tissue engineering. This study lays the foundation for the use of A. pernyi silk sericin as a potential scaffold for tissue engineering.

Novel functionalized nano-TiO2 loading electrocatalytic membrane for oily wastewater treatment

Membrane fouling is a critical problem in membrane filtration processes for water purification. Electrocatalytic membrane reactor (ECMR) was an effective method to avoid membrane fouling and improve water quality. This study focuses on the preparation and characterization of a novel functionalized nano-TiO(2) loading electrocatalytic membrane for oily wastewater treatment. A TiO(2)/carbon membrane used in the reactor is prepared by coating TiO(2) as an electrocatalyst via a sol-gel process on a conductive microporous carbon membrane. In order to immobilize TiO(2) on the carbon membrane, the carbon membrane is first pretreated with HNO(3) to generate the oxygen-containing functional groups on its surface. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) analyses are used to evaluate the morphology and microstructure of the membranes. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements are employed to illustrate the eletrochemical activity of the TiO(2)/carbon membrane. The membrane performance is investigated by treating oily wastewater. The oil removal rate increases with a decrease in the liquid hourly space velocity (LHSV) through the ECMR. The COD removal rate was 100% with a LHSV of 7.2 h(-1) and 87.4% with a LHSV of 21.6 h(-1) during the treatment of 200 mg/L oily water. It suggests that the synergistic effect of electrocatalytic oxidation and membrane separation in the ECMR plays a key role.

Enhancing effect of glycerol on the tensile properties of Bombyx mori cocoon sericin films

An environmental physical method described herein was developed to improve the tensile properties of Bombyx mori cocoon sericin films, by using the plasticizer of glycerol, which has a nontoxic effect compared with other chemical crosslinkers. The changes in the tensile characteristics and the structure of glycerolated (0–40 wt% of glycerol) sericin films were investigated. Sericin films, both in dry and wet states, showed enhanced tensile properties, which might be regulated by the addition of different concentrations of glycerol. The introduction of glycerol results in the higher amorphous structure in sericin films as evidenced by analysis of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra, thermogravimetry (TGA) and differential scanning calorimetry (DSC) curves. Scanning Electron Microscopy (SEM) observation revealed that glycerol was homogeneously blended with sericin molecules when its content was 10 wt%, while a small amount of redundant glycerol emerged on the surface of sericin films when its content was increased to 20 wt% or higher. Our results suggest that the introduction of glycerol is a novel nontoxic strategy which can improve the mechanical features of sericin-based materials and subsequently promote the feasibility of its application in tissue engineering.

Effect of proteins on the synthesis and assembly of calcium phosphate nanomaterials

Recent developments on biomineralization and biomaterials have demonstrated that proteins play an important role in the formation of biominerals in the body, which induce or inhibit mineralization of calcium phosphate together with modulation of the mineral phase structure. Many efforts have been made to understand the mechanism of this process and mimic the exquisite structure of biominerals in biomimetic methodologies. This review is focused on recent advances in the synthesis of calcium phosphate materials by taking advantage of protein assemblies. We try to review the examples of templates based on proteins and polypeptides that have been successfully employed to manufacture calcium phosphate nanomaterials.