Silk fibroin nanocomposites as tissue engineering scaffolds - A systematic review

Silk Fibroin and Its Nanocomposites for Wound Care: A Comprehensive Review

For most individuals, wound healing is a highly organized, straightforward process, wherein the body transitions through different phases in a timely manner. However, there are instances where external intervention becomes necessary to support and facilitate different phases of the body's innate healing mechanism. Furthermore, in developing countries, the cost of the intervention significantly impacts access to treatment options as affordability becomes a determining factor. This is particularly true in cases of long-term wound treatment and management, such as chronic wounds and infections. Silk fibroin (SF) and its nanocomposites have emerged as promising biomaterials with potent wound-healing activity. Driven by this motivation, this Review presents a critical overview of the recent advancements in different aspects of wound care using SF and SF-based nanocomposites. In this context, we explore various formats of hemostats and assess their suitability for different bleeding situations. The subsequent sections discuss the primary causes of nonhealing wounds, i.e., prolonged inflammation and infections. Herein, different treatment strategies to achieve immunomodulatory and antibacterial properties in a wound dressing were reviewed. Despite exhibiting excellent pro-healing properties, few silk-based products reach the market. This Review concludes by highlighting the bottlenecks in translating silk-based products into the market and the prospects for the future.

Eco-friendly bio-nanocomposites: pioneering sustainable biomedical advancements in engineering

Biomedical nanocomposites, which are an upcoming breed of mischievous materials, have ushered in a new dimension in the healthcare sector. Incorporating these materials tends to boost features this component already possesses and give might to things these components could not withstand alone. The biopolymer, which carries the nanoparticles, can simultaneously improve the composite's stiffness and biological characteristics, and vice versa. This increases the options of the composite and the number of times it can be used. The bio-nanocomposites and nanoparticles enable the ecocompatibility of the medicine in their biodegradability, and they, in this way, have ecological sustainability. The outcome is the improved properties of medicine and its associated positive impact on the environment. They have broad applications in antimicrobial agents, drug carriers, tissue regeneration, wound care, dentistry, bioimaging, and bone filler, among others. The dissertation on the elements of bio-nanocomposites emphasizes production techniques, their diverse applications in medicine, match-up issues, and future-boasting prospects in the bio-nanocomposites field. Through the utilization of such materials, scientists can develop more suitable for the environment and healthy biomedical solutions, and world healthcare in this way improves as well.

MXene/Silk Fibroin Strengthened PVA-Based Eutectogel with Excellent Self-Healing Ability and Environmental Adaptability: Design, Synthesis, and Sensing Application

A superelastic self-healing eutectogel was designed and prepared using poly (vinyl alcohol) (PVA) as the bulk skeleton material, while silk fibroin (SF) and two-dimensional (2D) MXene (Ti3C2TX) as reinforcing fillers. In brief, the eutectogel possesses a high tensile strength of 7.63 MPa, and its elongation at break reached 1115.2%, higher than most reported polymers (<1000%). In addition, the eutectogel-assembled sensor has a high ionic conductivity of 0.61 S/m and a high strain sensitivity of 5.17 kPa-1. Moreover, eutectogel shows excellent self-healing ability and can achieve self-healing quickly within 10 min, while its tensile strength and elongation at break can be restored to 84.7% and 97.4% of the initial levels. Besides, a stable electrical signal can be transmitted after 200 cycles at 30% strain. Finally, the eutectogel can withstand various environmental conditions, such as atmospheric or even vacuum evaporation and low-temperature freezing, while maintaining good mechanical and sensing performances. The assembled flexible sensors based on the eutectogel demonstrate their significant application prospects in wearable devices, especially human physiological monitoring.

Review of Spider Silk Applications in Biomedical and Tissue Engineering

This review will present the latest research related to the production and application of spider silk and silk-based materials in reconstructive and regenerative medicine and tissue engineering, with a focus on musculoskeletal tissues, and including skin regeneration and tissue repair of bone and cartilage, ligaments, muscle tissue, peripheral nerves, and artificial blood vessels. Natural spider silk synthesis is reviewed, and the further recombinant production of spider silk proteins. Research insights into possible spider silk structures, like fibers (1D), coatings (2D), and 3D constructs, including porous structures, hydrogels, and organ-on-chip designs, have been reviewed considering a design of bioactive materials for smart medical implants and drug delivery systems. Silk is one of the toughest natural materials, with high strain at failure and mechanical strength. Novel biomaterials with silk fibroin can mimic the tissue structure and promote regeneration and new tissue growth. Silk proteins are important in designing tissue-on-chip or organ-on-chip technologies and micro devices for the precise engineering of artificial tissues and organs, disease modeling, and the further selection of adequate medical treatments. Recent research indicates that silk (films, hydrogels, capsules, or liposomes coated with silk proteins) has the potential to provide controlled drug release at the target destination. However, even with clear advantages, there are still challenges that need further research, including clinical trials.

Silk Fibroin Materials: Biomedical Applications and Perspectives

The golden rule in tissue engineering is the creation of a synthetic device that simulates the native tissue, thus leading to the proper restoration of its anatomical and functional integrity, avoiding the limitations related to approaches based on autografts and allografts. The emergence of synthetic biocompatible materials has led to the production of innovative scaffolds that, if combined with cells and/or bioactive molecules, can improve tissue regeneration. In the last decade, silk fibroin (SF) has gained attention as a promising biomaterial in regenerative medicine due to its enhanced bio/cytocompatibility, chemical stability, and mechanical properties. Moreover, the possibility to produce advanced medical tools such as films, fibers, hydrogels, 3D porous scaffolds, non-woven scaffolds, particles or composite materials from a raw aqueous solution emphasizes the versatility of SF. Such devices are capable of meeting the most diverse tissue needs; hence, they represent an innovative clinical solution for the treatment of bone/cartilage, the cardiovascular system, neural, skin, and pancreatic tissue regeneration, as well as for many other biomedical applications. The present narrative review encompasses topics such as (i) the most interesting features of SF-based biomaterials, bare SF's biological nature and structural features, and comprehending the related chemo-physical properties and techniques used to produce the desired formulations of SF; (ii) the different applications of SF-based biomaterials and their related composite structures, discussing their biocompatibility and effectiveness in the medical field. Particularly, applications in regenerative medicine are also analyzed herein to highlight the different therapeutic strategies applied to various body sectors.

A novel 3D printed type II silk fibroin/polycaprolactone mesh for the treatment of pelvic organ prolapse

Pelvic organ prolapse (POP) is one of the common diseases in middle-aged and elderly women, caused by weakened pelvic floor muscle ligament tissue support. Pelvic floor reconstruction with mesh implantation has been proven to be an effective treatment for POP. However, traditional non-degradable and inflexible pelvic floor implantation meshes have been associated with pain, vaginal infections, and the need for additional surgeries. In this study, novel meshes with pre-designed structures were fabricated with solution-based electrohydrodynamic printing (EHDP) technology, using a series of polycaprolactone/silk fibroin composites as bioinks. The PCL/SF mesh mechanical performances were particularly enhanced with the addition of silk II, leading it to obtain higher adaptability with soft tissue repair. The mesh containing SF showed more robust degradation performance in the in vitro degradation assay. Furthermore, biocompatibility tests conducted on mouse embryonic fibroblasts (NIH/3T3) revealed enhanced cell affinity. Finally, the biocompatibility and tissue repair properties of PCL/SF mesh were verified through the implantation of meshes in the muscle defect site of mice. The results demonstrated that the 3D printed PCL/SF mesh prepared by EHDP exhibits superior mechanical properties, biocompatibility, biodegradability, as well as ligament and muscle fiber repair ability. The novel implantable meshes are promising for curing POP.

Progress and emerging techniques for biomaterial-based derivation of mesenchymal stem cells (MSCs) from pluripotent stem cells (PSCs)

The use of mesenchymal stem cells (MSCs) for clinical purposes has skyrocketed in the past decade. Their multilineage differentiation potentials and immunomodulatory properties have facilitated the discovery of therapies for various illnesses. MSCs can be isolated from infant and adult tissue sources, which means they are easily available. However, this raises concerns because of the heterogeneity among the various MSC sources, which limits their effective use. Variabilities arise from donor- and tissue-specific differences, such as age, sex, and tissue source. Moreover, adult-sourced MSCs have limited proliferation potentials, which hinders their long-term therapeutic efficacy. These limitations of adult MSCs have prompted researchers to develop a new method for generating MSCs. Pluripotent stem cells (PSCs), such as embryonic stem cells and induced PSCs (iPSCs), can differentiate into various types of cells. Herein, a thorough review of the characteristics, functions, and clinical importance of MSCs is presented. The existing sources of MSCs, including adult- and infant-based sources, are compared. The most recent techniques for deriving MSCs from iPSCs, with a focus on biomaterial-assisted methods in both two- and three-dimensional culture systems, are listed and elaborated. Finally, several opportunities to develop improved methods for efficiently producing MSCs with the aim of advancing their various clinical applications are described.

Electrochemical Sensing of H2O2 by Employing a Flexible Fe3O4/Graphene/Carbon Cloth as Working Electrode

We report the synthesis of Fe₃O₄/graphene (Fe₃O₄/Gr) nanocomposite for highly selective and highly sensitive peroxide sensor application. The nanocomposites were produced by a modified co-precipitation method. Further, structural, chemical, and morphological characterization of the Fe₃O₄/Gr was investigated by standard characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and high-resolution TEM (HRTEM), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). The average crystal size of Fe₃O₄ nanoparticles was calculated as 14.5 nm. Moreover, nanocomposite (Fe₃O₄/Gr) was employed to fabricate the flexible electrode using polymeric carbon fiber cloth or carbon cloth (pCFC or CC) as support. The electrochemical performance of as-fabricated Fe₃O₄/Gr/CC was evaluated toward H₂O₂ with excellent electrocatalytic activity. It was found that Fe₃O₄/Gr/CC-based electrodes show a good linear range, high sensitivity, and a low detection limit for H₂O₂ detection. The linear range for the optimized sensor was found to be in the range of 10-110 μM and limit of detection was calculated as 4.79 μM with a sensitivity of 0.037 µA μM^-1 cm^-2. The cost-effective materials used in this work as compared to noble metals provide satisfactory results. As well as showing high stability, the proposed biosensor is also highly reproducible.

Nano-biotechnology in tumour and cancerous disease: A perspective review

In recent years, drug manufacturers and researchers have begun to consider the nanobiotechnology approach to improve the drug delivery system for tumour and cancer diseases. In this article, we review current strategies to improve tumour and cancer drug delivery, which mainly focuses on sustaining biocompatibility, biodistribution, and active targeting. The conventional therapy using cornerstone drugs such as fludarabine, cisplatin etoposide, and paclitaxel has its own challenges especially not being able to discriminate between tumour versus normal cells which eventually led to toxicity and side effects in the patients. In contrast to the conventional approach, nanoparticle-based drug delivery provides target-specific delivery and controlled release of the drug, which provides a better therapeutic window for treatment options by focusing on the eradication of diseased cells via active targeting and sparing normal cells via passive targeting. Additionally, treatment of tumours associated with the brain is hampered by the impermeability of the blood-brain barriers to the drugs, which eventually led to poor survival in the patients. Nanoparticle-based therapy offers superior delivery of drugs to the target by breaching the blood-brain barriers. Herein, we provide an overview of the properties of nanoparticles that are crucial for nanotechnology applications. We address the potential future applications of nanobiotechnology targeting specific or desired areas. In particular, the use of nanomaterials, biostructures, and drug delivery methods for the targeted treatment of tumours and cancer are explored.

Two Hawks with One Arrow: A Review on Bifunctional Scaffolds for Photothermal Therapy and Bone Regeneration

Despite the significant improvement in the survival rate of cancer patients, the total cure of bone cancer is still a knotty clinical challenge. Traditional surgical resectionof bone tumors is less than satisfactory, which inevitably results in bone defects and the inevitable residual tumor cells. For the purpose of realizing minimal invasiveness and local curative effects, photothermal therapy (PTT) under the irradiation of near-infrared light has made extensive progress in ablating tumors, and various photothermal therapeutic agents (PTAs) for the treatment of bone tumors have thus been reported in the past few years, has and have tended to focus on osteogenic bio-scaffolds modified with PTAs in order to break through the limitation that PTT lacks, osteogenic capacity. These so-called bifunctional scaffolds simultaneously ablate bone tumors and generate new tissues at the bone defects. This review summarizes the recent application progress of various bifunctional scaffolds and puts forward some practical constraints and future perspectives on bifunctional scaffolds for tumor therapy and bone regeneration: two hawks with one arrow.

Research progress of natural silk fibroin and the appplication for drug delivery in chemotherapies

Silk fibroin has been widely used in biological fields due to its biocompatibility, mechanical properties, biodegradability, and safety. Recently, silk fibroin as a drug carrier was developed rapidly and achieved remarkable progress in cancer treatment. The silk fibroin-based delivery system could effectively kill tumor cells without significant side effects and drug resistance. However, few studies have been reported on silk fibroin delivery systems for antitumor therapy. The advancement of silk fibroin-based drug delivery systems research and its applications in cancer therapy are highlighted in this study. The properties, applications, private opinions, and future prospects of silk fibroin carriers are discussed to understand better the development of anti-cancer drug delivery systems, which may also contribute to advancing silk fibroin innovation.

Electrospun Silk Fibroin-CNT Composite Fibers: Characterization and Application in Mediating Fibroblast Stimulation

Electrospinning is a simple, low-cost, and highly efficient technique to generate desirable nano/microfibers from polymer solutions. Silk fibroin (SF), a biopolymer found in Bombyx mori cocoons, has attracted attention for various biomedical applications. In this study, functionalized CNT was incorporated in SF to generate biocomposite fibers by electrospinning. The electrospun (E-spun) fibers were well aligned with morphology mimicking the locally oriented ECM proteins in connective tissues. While as-spun fibers dissolved in water in just two minutes, ethanol vapor post-treatment promoted β-sheet formation leading to improved fiber stability in an aqueous environment (>14 days). The addition of a minute amount of CNT effectively improved the E-spun fiber alignment and mechanical strength while retained high biocompatibility and biodegradability. The fibers’ electrical conductivity increased by 13.7 folds and 21.8 folds, respectively, in the presence of 0.1 w% and 0.2 w% CNT in SF fibers. With aligned SF-CNT 0.1 % fibers as a cell culture matrix, we found electrical stimulation effectively activated fibroblasts from patients of pelvic organ prolapse (POP), a connective tissue disorder. The stimulation boosted the fibroblasts’ productivity of collagen III (COLIII) and collagen I (COLI) by 74 folds and 58 folds, respectively, and reduced the COLI to COLIII ratio favorable for tissue repair. The developed material and method offer a simple, direct, and effective way to remedy the dysfunctional fibroblasts of patients for personalized cell therapeutic treatment of diseases and health conditions associated with collagen disorder.

Advances in Multifunctional Bioactive Coatings for Metallic Bone Implants

To fix the bone in orthopedics, it is almost always necessary to use implants. Metals provide the needed physical and mechanical properties for load-bearing applications. Although widely used as biomedical materials for the replacement of hard tissue, metallic implants still confront challenges, among which the foremost is their low biocompatibility. Some of them also suffer from excessive wear, low corrosion resistance, infections and shielding stress. To address these issues, various coatings have been applied to enhance their in vitro and in vivo performance. When merged with the beneficial properties of various bio-ceramic or polymer coatings remarkable bioactive, osteogenic, antibacterial, or biodegradable composite implants can be created. In this review, bioactive and high-performance coatings for metallic bone implants are systematically reviewed and their biocompatibility is discussed. Updates in coating materials and formulations for metallic implants, as well as their production routes, have been provided. The ways of improving the bioactive coating performance by incorporating bioactive moieties such as growth factors, osteogenic factors, immunomodulatory factors, antibiotics, or other drugs that are locally released in a controlled manner have also been addressed.

Performance of Colombian Silk Fibroin Hydrogels for Hyaline Cartilage Tissue Engineering

The development and evaluation of scaffolds play a crucial role in the engineering of hyaline cartilage tissue. This work aims to evaluate the performance of silk fibroin hydrogels fabricated from the cocoons of the Colombian hybrid in the in vitro regeneration of hyaline cartilage. The scaffolds were physicochemically characterized, and their performance was evaluated in a cellular model. The results showed that the scaffolds were rich in random coils and β-sheets in their structure and susceptible to various serine proteases with different degradation profiles. Furthermore, they showed a significant increase in ACAN, COL10A1, and COL2A1 expression compared to pellet culture alone and allowed GAG deposition. The soluble portion of the scaffold did not affect chondrogenesis. Furthermore, they promoted the increase in COL1A2, showing a slight tendency to differentiate towards fibrous cartilage. The results also showed that Colombian silk could be used as a source of biomedical devices, paving the way for sericulture to become a more diverse economic activity in emerging countries.

Silk fibroin scaffolds: A promising candidate for bone regeneration

It remains a big challenge in clinical practice to repair large-sized bone defects and many factors limit the application of autografts and allografts, The application of exogenous scaffolds is an alternate strategy for bone regeneration, among which the silk fibroin (SF) scaffold is a promising candidate. Due to the advantages of excellent biocompatibility, satisfying mechanical property, controllable biodegradability and structural adjustability, SF scaffolds exhibit great potential in bone regeneration with the help of well-designed structures, bioactive components and functional surface modification. This review will summarize the cell and tissue interaction with SF scaffolds, techniques to fabricate SF-based scaffolds and modifications of SF scaffolds to enhance osteogenesis, which will provide a deep and comprehensive insight into SF scaffolds and inspire the design and fabrication of novel SF scaffolds for superior osteogenic performance. However, there still needs more comprehensive efforts to promote better clinical translation of SF scaffolds, including more experiments in big animal models and clinical trials. Furthermore, deeper investigations are also in demand to reveal the degradation and clearing mechanisms of SF scaffolds and evaluate the influence of degradation products.

Enhanced formation of bioactive and strong silk-bioglass hybrid materials through organic-inorganic mutual molecular nucleation induction and templating

Materials based on silk fibroin (SF) are important for many biomedical applications due to their excellent biocompatibility and tunable biodegradability. However, the insufficient mechanical strength and low bioactivity of these materials have limited their applications. For silk hydrogels, slow gelation is also a crucial problem. In this work, a simple approach is developed to address these challenging problems all at once. By mixing SF solution with bioglass (BG) sol, instant gelation of silk is induced, the storage modulus of the hydrogel and the compressive modulus of the aerogel are significantly enhanced. The formation of a complex of SF and tetraethyl orthosilicate (TEOS), either through hydrogen bonding or TEOS condensation on SF, facilitated the aggregation of SF and, on the other hand, created active sites for the condensation of TEOS and BG formation on the surface of silk nanofibrils. The resultant hybrid gels have much higher capacity for biomineralization, indicating their higher bioactivity, compared with the pristine silk gels. This organic (SF)-inorganic (BG) mutual nucleation induction and templating can be used for a general approach to produce bioactive silk materials of various formats not limited to gels and may also inspire the formation of other functional protein-BG hybrid materials.

Bombyx mori cocoon as a promising pharmacological agent: A review of ethnopharmacology, chemistry, and biological activities

Silk cocoon, naturally produced by silkworms scientifically named Bombyx mori L. (Lepidoptera, Bombycidae), is one of the well-known medicinal agents with several therapeutic activities. The present study aims to review the various aspects of the silk cocoon, including chemical composition, traditional uses, biological and biotechnological activities, and toxicological issues, to provide a scientific source for scholars. For this purpose, Electronic databases including PubMed, Scopus, Google Scholar, Web of Science, and traditional literature, were searched up to December 2021. According to the historical data, silk farming is acknowledged as one of the most ancient agricultural findings. The silk is generally composed of 75–83% fibroin, 17–25% sericin, and 1–5% non-sericin components, including secondary metabolites, wax, pigments, carbohydrates, and other impurities. Flavonoids, especially quercetin and kaempferol, alkaloids, coumarin derivatives, and phenolic acids, are among the secondary metabolites isolated from the silk cocoon. In recent years the biological properties of the silk cocoon, especially its major proteins, namely fibroin and sericin, have drawn special attention. Scientific literature has investigated several pharmacological effects of the silk cocoon and its ingredients, including cardioprotective, antioxidant, anticancer, antidiabetic, antihyperlipidemia, gastroprotective, as well as ameliorated skin health activities. In addition, it has been extensively taken into consideration in drug delivery and tissue engineering study fields. Furthermore, its toxicity is in acceptable range.

Recent advances in 3D-printed polylactide and polycaprolactone-based biomaterials for tissue engineering applications

The three-dimensional printing (3DP) also known as the additive manufacturing (AM), a novel and futuristic technology that facilitates the printing of multiscale, biomimetic, intricate cytoarchitecture, function-structure hierarchy, multi-cellular tissues in the complicated micro-environment, patient-specific scaffolds, and medical devices. There is an increasing demand for developing 3D-printed products that can be utilized for organ transplantations due to the organ shortage. Nowadays, the 3DP has gained considerable interest in the tissue engineering (TE) field. Polylactide (PLA) and polycaprolactone (PCL) are exemplary biomaterials with excellent physicochemical properties and biocompatibility, which have drawn notable attraction in tissue regeneration. Herein, the recent advancements in the PLA and PCL biodegradable polymer-based composites as well as their reinforcement with hydrogels and bio-ceramics scaffolds manufactured through 3DP are systematically summarized and the applications of bone, cardiac, neural, vascularized and skin tissue regeneration are thoroughly elucidated. The interaction between implanted biodegradable polymers, in-vivo and in-vitro testing models for possible evaluation of degradation and biological properties are also illustrated. The final section of this review incorporates the current challenges and future opportunities in the 3DP of PCL- and PLA-based composites that will prove helpful for biomedical engineers to fulfill the demands of the clinical field.

Biocomposite-based nanostructured delivery systems for treatment and control of inflammatory lung diseases

Diseases related to the lungs are among the most prevalent medical problems threatening human life. The treatment options and therapeutics available for these diseases are hindered by inadequate drug concentrations at pathological sites, a dearth of cell-specific targeting and different biological barriers in the alveoli or conducting airways. Nanostructured delivery systems for lung drug delivery have been significant in addressing these issues. The strategies used include surface engineering by altering the material structure or incorporation of specific ligands to reach prespecified targets. The unique characteristics of nanoparticles, such as controlled size and distribution, surface functional groups and therapeutic release triggering capabilities, are tailored to specific requirements to overcome the major therapeutic barriers in pulmonary diseases. In the present review, the authors intend to deliver significant up-to-date research in nanostructured therapies in inflammatory lung diseases with an emphasis on biocomposite-based nanoparticles.

Graphene Oxide-Protein-Based Scaffolds for Tissue Engineering: Recent Advances and Applications

The field of tissue engineering is constantly evolving as it aims to develop bioengineered and functional tissues and organs for repair or replacement. Due to their large surface area and ability to interact with proteins and peptides, graphene oxides offer valuable physiochemical and biological features for biomedical applications and have been successfully employed for optimizing scaffold architectures for a wide range of organs, from the skin to cardiac tissue. This review critically focuses on opportunities to employ protein-graphene oxide structures either as nanocomposites or as biocomplexes and highlights the effects of carbonaceous nanostructures on protein conformation and structural stability for applications in tissue engineering and regenerative medicine. Herein, recent applications and the biological activity of nanocomposite bioconjugates are analyzed with respect to cell viability and proliferation, along with the ability of these constructs to sustain the formation of new and functional tissue. Novel strategies and approaches based on stem cell therapy, as well as the involvement of the extracellular matrix in the design of smart nanoplatforms, are discussed.

Polyvinyl Alcohol/Chitosan and Polyvinyl Alcohol/Ag@MOF Bilayer Hydrogel for Tissue Engineering Applications

In this paper, polyvinyl alcohol/Ag-Metal-organic framework (PVA/Ag@MOF) and polyvinyl alcohol/chitosan (PVA/CS) were used as the inner and outer layers to successfully prepare a bilayer composite hydrogel for tissue engineering scaffold. The performance of bilayer hydrogels was evaluated. The outer layer (PVA/CS) has a uniform pore size distribution, good water retention, biocompatibility and cell adhesion ability. The inner layer (PVA/Ag@MOF) has good antibacterial activity and poor biocompatibility. PVA, PVA/0.1%Ag@MOF, PVA/0.5%Ag@MOF, and PVA/1.0%Ag@MOF show anti-microbial activity in ascending order. However, its use as an inner layer avoids direct contact with cells and prevents infection. The cell viability of all samples was above 90%, indicating that the bilayer hydrogel was non-toxic to A549 cells. The bilayer hydrogel scaffold combines the advantages of the inner and outer layers. In summary, this new bilayer composite is an ideal lung scaffold for tissue engineering.