Hydrogen sulfide-releasing silk fibroin scaffold for bone tissue engineering

Hydrogen Sulfide Delivery to Enhance Bone Tissue Engineering Cell Survival

Though crucial for natural bone healing, local calcium ion (Ca2+) and phosphate ion (Pi) concentrations can exceed the cytotoxic limit leading to mitochondrial overload, oxidative stress, and cell death. For bone tissue engineering applications, H2S can be employed as a cytoprotective molecule to enhance mesenchymal stem cell (MSC) tolerance to cytotoxic Ca2+/Pi concentrations. Varied concentrations of sodium hydrogen sulfide (NaSH), a fast-releasing H2S donor, were applied to assess the influence of H2S on MSC proliferation. The results suggested a toxicity limit of 4 mM for NaSH and that 1 mM of NaSH could improve cell proliferation and differentiation in the presence of cytotoxic levels of Ca2+ (32 mM) and/or Pi (16 mM). To controllably deliver H2S over time, a novel donor molecule (thioglutamic acid-GluSH) was synthesized and evaluated for its H2S release profile. Excitingly, GluSH successfully maintained cytoprotective level of H2S over 7 days. Furthermore, MSCs exposed to cytotoxic Ca2+/Pi concentrations in the presence of GluSH were able to thrive and differentiate into osteoblasts. These findings suggest that the incorporation of a sustained H2S donor such as GluSH into CaP-based bone graft substitutes can facilitate considerable cytoprotection, making it an attractive option for complex bone regenerative engineering applications.

Nanocomposite Methacrylated Silk Fibroin-Based Scaffolds for Bone Tissue Engineering

The treatment of bone defects is a clinical challenge. Bone tissue engineering is gaining interest as an alternative to current treatments, with the development of 3D porous structures (scaffolds) helpful in promoting bone regeneration by ensuring temporary functional support. In this work, methacrylated silk fibroin (SilMA) sponges were investigated as scaffolds for bone tissue engineering by exploiting the combination of physical (induced by NaCl salt during particulate leaching) and chemical crosslinking (induced by UV-light exposure) techniques. A biomimetic approach was adopted to better simulate the extracellular matrix of the bone by introducing either natural (mussel shell-derived) or synthetic-origin hydroxyapatite nanoparticles into the SilMA sponges. The obtained materials were characterized in terms of pore size, water absorption capability and mechanical properties to understand both the effect of the inclusion of the two different types of nanoparticles and the effect of the photocrosslinking. Moreover, the SilMA sponges were tested for their bioactivity and suitability for bone tissue engineering purposes by using osteosarcoma cells, studying their metabolism by an AlamarBlue assay and their morphology by scanning electron microscopy. Results indicate that photocrosslinking helps in obtaining more regular structures with bimodal pore size distributions and in enhancing the stability of the constructs in water. Moreover, the addition of naturally derived hydroxyapatite was observed to be more effective at activating osteosarcoma cell metabolism than synthetic hydroxyapatite, showing a statistically significant difference in the AlamarBlue measurement on day 7 after seeding. The methacrylated silk fibroin/hydroxyapatite nanocomposite sponges developed in this work were found to be promising tools for targeting bone regeneration with a sustainable approach.

Infection Microenvironment-Responsive Coating on Titanium Surfaces for On-Demand Release of Therapeutic Gas and Antibiotic

Aseptic loosening and bacterial infection pose significant challenges in the clinical application of titanium (Ti) orthopedic implants, which are primarily caused by insufficient osseointegration and bacterial contamination. To address these issues, a responsive coating on Ti surface is constructed, which achieves enhanced osseointegration and infection elimination by on-demand release of therapeutic gas hydrogen sulfide (H2S) and antibiotic. TiO2 nanotubes (TNT) are anodized on the Ti surface to enhance its bioactivity and serve as reservoirs for the antibiotic. An infection microenvironment-responsive macromolecular H2S donor layer is coated on top of TNT to inhibit premature leakage of antibiotic. This layer exhibits a sustained release of low-dosage H2S, which is capable of promoting the osteogenic differentiation and migration of cells. Moreover, the compactness of the macromolecular H2S donor layer could be broken by bacterial invasion, leading to rapid antibiotic release thus preventing infection. In vitro antibacterial experiments validates significant antibacterial activity of the coating against both Gram-negative (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus). Crucially, this coating effectively suppresses implant-associated infection with 98.7% antibacterial efficiency in a rat femoral bone defect model, mitigates inflammation at the defect site and promotes osseointegration of the Ti orthopedic implant.

Gas Therapy: Generating, Delivery, and Biomedical Applications

Oxygen (O2 ), nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2 S), and hydrogen (H2 ) with direct effects, and carbon dioxide (CO2 ) with complementary effects on the condition of various diseases are known as therapeutic gases. The targeted delivery and in situ generation of these therapeutic gases with controllable release at the site of disease has attracted attention to avoid the risk of gas poisoning and improve their performance in treating various diseases such as cancer therapy, cardiovascular therapy, bone tissue engineering, and wound healing. Stimuli-responsive gas-generating sources and delivery systems based on biomaterials that enable on-demand and controllable release are promising approaches for precise gas therapy. This work highlights current advances in the design and development of new approaches and systems to generate and deliver therapeutic gases at the site of disease with on-demand release behavior. The performance of the delivered gases in various biomedical applications is then discussed.

Advanced Applications of Silk-Based Hydrogels for Tissue Engineering: A Short Review

Silk has been consistently popular throughout human history due to its enigmatic properties. Today, it continues to be widely utilized as a polymer, having first been introduced to the textile industry. Furthermore, the health sector has also integrated silk. The Bombyx mori silk fibroin (SF) holds the record for being the most sustainable, functional, biocompatible, and easily produced type among all available SF sources. SF is a biopolymer approved by the FDA due to its high biocompatibility. It is versatile and can be used in various fields, as it is non-toxic and has no allergenic effects. Additionally, it enhances cell adhesion, adaptation, and proliferation. The use of SF has increased due to the rapid advancement in tissue engineering. This review comprises an introduction to SF and an assessment of the relevant literature using various methods and techniques to enhance the tissue engineering of SF-based hydrogels. Consequently, the function of SF in skin tissue engineering, wound repair, bone tissue engineering, cartilage tissue engineering, and drug delivery systems is therefore analysed. The potential future applications of this functional biopolymer for biomedical engineering are also explored.

Investigation of osteogenesis and angiogenesis in perfusion bioreactors using improved multi-layer PCL-nHA-nZnO electrospun scaffolds

PURPOSE

Bone tissue engineering aims to create a three-dimensional, matured, angiogenic scaffold with a suitable thickness that resembles a natural bone matrix. On the other hand, electrospun fibers, which researchers have considered due to their good biomimetic properties, are considered 2D structures. Due to the highly interwoven network and small pore size, achieving the desired thickness for bone lesions has always been challenging. In bone tissue engineering, bioreactors are crucial for achieving initial tissue maturity and introducing certain signals as flow parameters for differentiation.

METHODS

In the present study, Human bone marrow mesenchymal stem cells (hBMSCs) and human umbilical vein endothelial cells (HUVECs) were co-cultured in a perfusion bioreactor on treated (improved pore size by gelatin sacrification and subsequent ultrasonication) 5-layer polycaprolactone-nano hydroxyapatite-nano zinc oxide (T-PHZ) scaffolds to investigate osteogenesis and angiogenesis simultaneously. The flow parameters and stresses on the cells were studied using two patterns of parallel and vertical scaffolds relative to the flow of the culture medium. In dynamic vertical flow (DVF), the culture medium flows perpendicular to the scaffolds, and in dynamic parallel flow (DPF), the culture medium flows parallel to the scaffolds. In all evaluations, static samples (S) served as the control group.

RESULTS

Live/dead, and MTT assays demonstrated the biocompatibility of the 5-layer scaffolds and the suitability of the bioreactor's functional conditions. ALP activity, EDAX analysis, and calcium content measurements exhibited greater osteogenesis for T-PHZ scaffolds in DVF conditions. Calcium content increased by a factor of 2.2, 1.8, and 1.6 during days 7 to 14 of culture under DVF, DPF and S conditions, respectively. After 21 days of co-culturing, an immunohistochemistry (IHC) test was performed to investigate angiogenesis and osteogenesis. Five antibodies were investigated in DVF, CD31, VEGFA, and VEGFR2 for angiogenesis, osteocalcin, and RUNX2 for osteogenesis. Compressive stress applied in DVF mode has increased osteogenic activity compared to DPF.

CONCLUSION

The results indicated the development of ideal systems for osteogenesis and angiogenesis on the treated multilayer electrospun scaffolds in the perfusion bioreactor.

Evaluation of new robust silk fibroin hydrogels for posterior scleral reinforcement in rabbits

Background: Currently, there is no ideal material available for posterior scleral reinforcement (PSR) to prevent the progression of high myopia. In this study, we investigated robust regenerated silk fibroin (RSF) hydrogels as potential grafts for PSR in animal experiments to evaluate their safety and biological reactions. Methods: PSR surgery was performed on the right eye of twenty-eight adult New Zealand white rabbits, with the left eye serving as a self-control. Ten rabbits were observed for 3 months, while 18 rabbits were observed for 6 months. The rabbits were evaluated using intraocular pressure (IOP), anterior segment and fundus photography, A- and B-ultrasound, optical coherence tomography (OCT), histology, and biomechanical tests. Results: No complications such as significant IOP fluctuation, anterior chamber inflammation, vitreous opacity, retinal lesion, infection, or material exposure were observed. Furthermore, no evidence of pathological changes in the optic nerve and retina, or structural abnormalities on OCT, were found. The RSF grafts were appropriately located at the posterior sclera and enclosed in fibrous capsules. The scleral thickness and collagen fiber content of the treated eyes increased after surgery. The ultimate stress of the reinforced sclera increased by 30.7%, and the elastic modulus increased by 33.0% compared to those of the control eyes at 6 months after surgery. Conclusion: Robust RSF hydrogels exhibited good biocompatibility and promoted the formation of fibrous capsules at the posterior sclera in vivo. The biomechanical properties of the reinforced sclera were strengthened. These findings suggest that RSF hydrogel is a potential material for PSR.

Intelligent polymeric hydrogen sulfide delivery systems for therapeutic applications

Hydrogen sulfide (H₂S) plays an important role in regulating various pathological processes such as protecting mammalian cell from harmful injuries, promoting tissue regeneration, and regulating the process of various diseases caused by physiological disorders. Studies have revealed that the physiological effects of H₂S are highly associated with its concentrations. At relatively low concentration, H₂S shows beneficial functions. However, long-time and high-dose donation of H₂S would inhibit regular biological process, resulting in cell dysfunction and apoptosis. To regulate the dosage of H₂S delivery for precision medicine, H₂S delivery systems with intelligent characteristics were developed and a variety of biocompatibility polymers have been utilized to establish intelligent polymeric H₂S delivery systems, with the abilities to specifically target the lesions, smartly respond to pathological microenvironments, as well as real-timely monitor H₂S delivery and lesion conditions by incorporating imaging-capable moieties. In this review, we focus on the design, preparation, and therapeutic applications of intelligent polymeric H₂S delivery systems in cardiovascular therapy, inflammatory therapy, tissue regenerative therapy, cancer therapy and bacteria-associated therapy. Strategies for precise H₂S therapies especially imaging-guided H₂S theranostics are highlighted. Since H₂S donors with stimuli-responsive characters are vital components for establishing intelligent H₂S delivery systems, the development of H₂S donors is also briefly introduced.

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H₂S is an endogenous gasotransmitter that plays important role in regulating various physiological and pathological pathways.

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Controlled H₂S delivery is vital since the therapeutic effects of H₂S are highly associated with its concentrations.

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Intelligent polymeric H₂S delivery systems possess specific targeting, stimuli responsive and imaging guided capabilities, representing a strategic option for next generation of therapies.

Gasotransmitter-Induced Therapeutic Angiogenesis: A Biomaterial Prospective

Gasotransmitters such as NO, H₂S, and CO have emerged as key players in the regulation of various pathophysiological functions, prompting the development of gas therapy for various pathogeneses. Deficient production of gasotransmitters has been linked to various diseases such as hypertension, endothelial dysfunction, myocardial infarction, ischemia, and impaired wound healing, as they are involved in the regulatory action of angiogenesis. A better understanding of the regulatory mechanisms has given new hope to address the vascular impairment caused by the breakthroughs in gasotransmitters as therapeutics. However, the unstable nature and poor target specificity of gas donors limit the full efficacy of drugs. In this regard, biomaterials that possess excellent biocompatibility and porosity are ideal drug carriers to deliver the gas transmitters in a tunable manner for therapeutic angiogenesis. This review article provides a comprehensive discussion of biomaterial-based gasotransmitter delivery approaches for therapeutic angiogenesis. The critical role of gasotransmitters in modulating angiogenesis during tissue repair as well as their challenges and future directions are demonstrated.

On-demand therapeutic delivery of hydrogen sulfide aided by biomolecules

Hydrogen sulfide (H₂S), known as the third gasotransmitter, exerts various physiological functions including cardiac protection, angiogenesis, anti-inflammatory, and anti-cancer capability. Given its promising therapeutic potential as well as severe perniciousness if improper use, the sustained and tunable H₂S delivery systems are highly required for H₂S-based gas therapy with enhanced bioactivity and reduced side effects. To this end, a series of stimuli-responsive compounds capable of releasing H₂S (termed H₂S donors) have been designed over the past two decades to mimic the endogenous generation of H₂S and elucidate the biological functions. Further to improve the stability of H₂S donors and achieve the targeted delivery, various delivery systems have been constructed. In this review, we focus on the recent advances of an emerging subset, biomolecular-based H₂S delivery systems, which combine H₂S donors with biomolecular vectors including polysaccharide, peptide, and protein. We demonstrated their basic structures, building strategies, and therapeutic applications respectively to unfold their inherent merits endued by biomolecules including biocompatibility, biodegradability as well as expansibility. The varied development potentials of biomolecular-based H₂S delivery systems based on their specific properties are also discussed. At the end, brief future outlooks and upcoming challenges are presented as well.

Toughening Polyamidoamine Hydrogels through Covalent Grafting of Short Silk Fibers

Linear amphoteric polyamidoamines (PAAs) are usually water-soluble, biodegradable and biocompatible. Crosslinked PAAs form in water hydrogels, retaining most of the favorable properties of their linear counterparts. The hydrogels prepared by the radical post-polymerization of the oligo-α,ω-bisacrylamido-terminated PAA called AGMA1, obtained by the polyaddition of 4-aminobutylguanidine (agmatine) with 2,2-bis(acrylamido)acetic acid, exhibit excellent cell-adhesion properties both in vitro and in vivo. However, due to their low mechanical strength, AGMA1 hydrogels cannot be sewn to biological tissues and need to be reinforced with fibrous materials. In this work, short silk fibers gave excellent results in this sense, proving capable of establishing covalent bonds with the PAA matrix, thanks to their lysine content, which provided amino groups capable of reacting with the terminal acrylamide groups of the AGMA1 precursor in the final crosslinking phase. Morphological analyses demonstrated that the AGMA1 matrix was intimately interconnected and adherent to the silk fibers, with neither visible holes nor empty volumes. The silk/H-AGMA1 composites were still reversibly swellable in water. In the swollen state, they could be sewn and showed no detachment between fibers and matrix and exhibited significantly improved mechanical properties compared with the plain hydrogels, particularly as regards their Young's modulus and elongation at break.

Reactive sulfur and selenium species in the regulation of bone homeostasis

Reactive oxygen species (ROS) are important modulators of physiological signaling and play important roles in bone tissue regulation. Both reactive sulfur species (RSS) and reactive selenium species (RSeS) are involved in ROS signaling, and recent work suggests RSS and RSeS involvement in the regulation of bone homeostasis. For example, RSS can promote osteogenic differentiation and decrease osteoclast activity and differentiation, and the antioxidant activity of RSeS play crucial roles in balancing bone remodeling. Here, we outline current research progress on the application of RSS and RSeS in bone disease and regeneration. Focusing on these investigations, we highlight different methods, tools, and sources of RSS and RSeS, and we also highlight future opportunities for delivery of RSS and RSeS in biological environments relating to bone.

Chitosan-Based Scaffolds for Facilitated Endogenous Bone Re-Generation

Facilitated endogenous tissue engineering, as a facile and effective strategy, is emerging for use in bone tissue regeneration. However, the development of bioactive scaffolds with excellent osteo-inductivity to recruit endogenous stem cells homing and differentiation towards lesion areas remains an urgent problem. Chitosan (CS), with versatile qualities including good biocompatibility, biodegradability, and tunable physicochemical and biological properties is undergoing vigorously development in the field of bone repair. Based on this, the review focus on recent advances in chitosan-based scaffolds for facilitated endogenous bone regeneration. Initially, we introduced and compared the facilitated endogenous tissue engineering with traditional tissue engineering. Subsequently, the various CS-based bone repair scaffolds and their fabrication methods were briefly explored. Furthermore, the functional design of CS-based scaffolds in bone endogenous regeneration including biomolecular loading, inorganic nanomaterials hybridization, and physical stimulation was highlighted and discussed. Finally, the major challenges and further research directions of CS-based scaffolds were also elaborated. We hope that this review will provide valuable reference for further bone repair research in the future.

Hemoglobin I from Lucina pectinata on Collagen Scaffold: A Prospective Hydrogen Sulfide Scavenger

Hydrogen sulfide (H2S), independently of being a toxic gas with a characteristic smell of rotten eggs, is a crucial signaling molecule with significant physiological functions. Given the rapid diffusivity of the gas, it is a challenge to develop robust sensors and biomarkers to quantify free or bound H2S. In addition, there is the need to further develop a robust biosystem to efficiently trap or scavenge H2S from different producing environments. The work presented here uses recombinant met-aquo rHbI (rHbI-H2O) immobilization techniques on collagen to determine its ability to bind H2S due to its high affinity ( $1.24\times {10}^8$  M-1). The hemeprotein will function as a scavenger on this scaffold system. UV-Vis absorption and UV-Vis diffuse reflectance (%R) spectroscopy of rHbI-H2O and rHbI-sulfide (rHbI-H2S) complex in solution and collagen scaffold demonstrated that the heme chromophore retains its reactivity and properties. UV-Vis diffuse reflectance measurements, transformed using the Kubelka-Munk function (K-M function), show a linear correlation ( $R_2=0.9987$ and 0.9916) of rHbI-H2O and rHbI-H2S within concentrations from 1 μM to 35 μM for derivatives. The extraordinary affinity of rHbI-H2O for H2S suggests recombinant met-aquo HbI in a collagen scaffold is an excellent scavenger moiety for hydrogen sulfide. These findings give insight into H2S trapping using the rHbI-H2O-collagen scaffold, where the rHbI-H2S concentration can be determined. Future pathways are to work toward the development of a met-aquo rHbI collagen solution capable of being printed as single drops on polymer, cotton or chromatographic paper. Upon exposure of these matrixes to H2S, the rHbI-H2S complex is formed and its concentration determined using UV-Vis diffuse reflectance technique.

Implantation of injectable SF hydrogel with sustained hydrogen sulfide delivery reduces neuronal pyroptosis and enhances functional recovery after severe intracerebral hemorrhage

Hydrogen sulfide (H₂S), an important endogenous signaling molecule, plays an important neuroprotective role in the central nervous system. However, there is no ideal delivery material or method involving the sustained and controlled release of H₂S for clinical application in brain diseases. Silk fibroin (SF)-based hydrogels have become a potentially promising strategy for local, controlled, sustained drug release in the treatment of various disorders. Here, we show a silk fibroin (SF)-based hydrogel with sustained H₂S delivery (H₂S@SF hydrogel) is effective in treating brain injury through stereotactic orthotopic injection in a severe intracerebral hemorrhage (ICH) mouse model. In this study, we observed H₂S@SF hydrogel sustained H₂S release in vitro and in vivo. The physicochemical properties of H₂S@SF hydrogel were studied using FE-SEM, Raman spectroscopy and Rheological analysis. Treatment with H₂S@SF hydrogel attenuated brain edema, reduced hemorrhage volume and improved the recovery of neurological deficits after severe ICH following stereotactic orthotopic injection. Double immunofluorescent staining also revealed that H₂S@SF hydrogel may reduce cell pyroptosis in the striatum, cortex and hippocampus. However, when using endogenous H₂S production inhibitor AOAA, H₂S@SF hydrogel could not suppress ICH-induced cell pyroptosis. Hence, the therapeutic effect of the H₂S@SF hydrogel may be partly the result of the slow-release of H₂S and/or the effect of the SF hydrogel on the production of endogenous H₂S. Altogether, the results exhibit promising attributes of injectable silk fibroin hydrogel and the utility of H₂S-loaded injectable SF hydrogel as an alternative biomaterial toward brain injury treatment for clinical application.

A Review of 3D Printed Bone Implants

3D printing, that is, additive manufacturing, has solved many major problems in general manufacturing, such as three-dimensional tissue structure, microenvironment control difficulty, product production efficiency and repeatability, etc., improved the manufacturing speed and precision of personalized bone implants, and provided a lot of support for curing patients with bone injuries. The application of 3D printing technology in the medical field is gradually extensive, especially in orthopedics. The purpose of this review is to provide a report on the related achievements of bone implants based on 3D printing technology in recent years, including materials, molding methods, optimization of implant structure and performance, etc., in order to point out the existing shortcomings of 3D printing bone implants, promote the development of all aspects of bone implants, and make a prospect of 4D printing, hoping to provide some reference for the subsequent research of 3D printing bone implants.

Perfused Platforms to Mimic Bone Microenvironment at the Macro/Milli/Microscale: Pros and Cons

As life expectancy increases, the population experiences progressive ageing. Ageing, in turn, is connected to an increase in bone-related diseases (i.e., osteoporosis and increased risk of fractures). Hence, the search for new approaches to study the occurrence of bone-related diseases and to develop new drugs for their prevention and treatment becomes more pressing. However, to date, a reliable in vitro model that can fully recapitulate the characteristics of bone tissue, either in physiological or altered conditions, is not available. Indeed, current methods for modelling normal and pathological bone are poor predictors of treatment outcomes in humans, as they fail to mimic the in vivo cellular microenvironment and tissue complexity. Bone, in fact, is a dynamic network including differently specialized cells and the extracellular matrix, constantly subjected to external and internal stimuli. To this regard, perfused vascularized models are a novel field of investigation that can offer a new technological approach to overcome the limitations of traditional cell culture methods. It allows the combination of perfusion, mechanical and biochemical stimuli, biological cues, biomaterials (mimicking the extracellular matrix of bone), and multiple cell types. This review will discuss macro, milli, and microscale perfused devices designed to model bone structure and microenvironment, focusing on the role of perfusion and encompassing different degrees of complexity. These devices are a very first, though promising, step for the development of 3D in vitro platforms for preclinical screening of novel anabolic or anti-catabolic therapeutic approaches to improve bone health.

Bone Scaffolds: An Incorporation of Biomaterials, Cells, and Biofactors

Large injuries to bones are still one of the most challenging musculoskeletal problems. Tissue engineering can combine stem cells, scaffold biomaterials, and biofactors to aid in resolving this complication. Therefore, this review aims to provide information on the recent advances made to utilize the potential of biomaterials for making bone scaffolds and the assisted stem cell therapy and use of biofactors for bone tissue engineering. The requirements and different types of biomaterials used for making scaffolds are reviewed. Furthermore, the importance of stem cells and biofactors (growth factors and extracellular vesicles) in bone regeneration and their use in bone scaffolds and the key findings are discussed. Lastly, some of the main obstacles in bone tissue engineering and future trends are highlighted.

JK-2 loaded electrospun membrane for promoting bone regeneration

Hydrogen sulfide (H₂S) has been as an essential gasotransmitter and a potential therapeutic approach for several biomedical treatments such as cardiovascular disorders, hypertension, and other diseases. The endogenous and exogenous H₂S also plays a crucial role in the bone anabolic process and a protective mechanism in cell signalling. In this study, we have utilized two types of polymers, polycaprolactone (PCL) and gelatin (Gel), for the fabrication of JK-2 (H₂S donor) loaded nanofibrous scaffold via electrospinning process for bone healing and bone tissue engineering. Comparing the PCL/Gel and PCL/Gel-JK-2 scaffolds, the latter demonstrated enhanced cell adhesion and proliferation capabilities. Furthermore, both experimental scaffolds have been subjected to an in vivo experiment for 4 and 8 weeks in a bone-defect model of a rabbit to determine their biological responses under physiological conditions. There was an obvious increase in bone regeneration in the PCL/Gel-JK-2 group compared to the control and PCL/Gel groups. These results indicate the use of PCL/Gel scaffolds loaded with JK-2 should be considered for possible bone regeneration.

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

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

Biomaterial-Assisted Regenerative Medicine

This review aims to show case recent regenerative medicine based on biomaterial technologies. Regenerative medicine has arousing substantial interest throughout the world, with “The enhancement of cell activity” one of the essential concepts for the development of regenerative medicine. For example, drug research on drug screening is an important field of regenerative medicine, with the purpose of efficient evaluation of drug effects. It is crucial to enhance cell activity in the body for drug research because the difference in cell condition between in vitro and in vivo leads to a gap in drug evaluation. Biomaterial technology is essential for the further development of regenerative medicine because biomaterials effectively support cell culture or cell transplantation with high cell viability or activity. For example, biomaterial-based cell culture and drug screening could obtain information similar to preclinical or clinical studies. In the case of in vivo studies, biomaterials can assist cell activity, such as natural healing potential, leading to efficient tissue repair of damaged tissue. Therefore, regenerative medicine combined with biomaterials has been noted. For the research of biomaterial-based regenerative medicine, the research objective of regenerative medicine should link to the properties of the biomaterial used in the study. This review introduces regenerative medicine with biomaterial.

A novel and selective silk fibroin fragmentation method

In the tissue-engineering field silk fibroin can be tailored to the target applications by modifying its secondary structure and molecular weight, and functionalizing the molecule with specific active groups linked to the amino acid side chains. To better tune the silk fibroin molecular weight and structural properties, we propose the creation of a lower molecular weight fibroin-derived material through a selective and tunable enzymatic attack on the fibroin chain. Cleavage at specific amino acid sites leads to precise silk fibroin fragmentation and, thus, lower molecular weight materials whose length and properties can be tuned with the enzyme concentration. The cleavage increased the presence of free amino groups, hence reactivity, and aqueous solutions of the resulting polymer remained stable for up to seven days. Films of fragmented fibroin were prepared and characterized, demonstrating that the fragmentation did not affect β-sheet formation after methanol treatment, but differences were detected after the water-vapor annealing process, confirmed by structural and thermal analyses. The adopted fragmentation method is fast, controllable and precise, allowing the creation of a silk-derived material class that is stable in water, with a tunable molecular weight and secondary structure rearrangements, and is thus a versatile tool for the further tunability and modulation of bioengineered constructs.

Endothelium as a Source and Target of H2S to Improve Its Trophism and Function

The vascular endothelium consists of a single layer of squamous endothelial cells (ECs) lining the inner surface of blood vessels. Nowadays, it is no longer considered as a simple barrier between the blood and vessel wall, but a central hub to control blood flow homeostasis and fulfill tissue metabolic demands by furnishing oxygen and nutrients. The endothelium regulates the proper functioning of vessels and microcirculation, in terms of tone control, blood fluidity, and fine tuning of inflammatory and redox reactions within the vessel wall and in surrounding tissues. This multiplicity of effects is due to the ability of ECs to produce, process, and release key modulators. Among these, gasotransmitters such as nitric oxide (NO) and hydrogen sulfide (H₂S) are very active molecules constitutively produced by endotheliocytes for the maintenance and control of vascular physiological functions, while their impairment is responsible for endothelial dysfunction and cardiovascular disorders such as hypertension, atherosclerosis, and impaired wound healing and vascularization due to diabetes, infections, and ischemia. Upregulation of H₂S producing enzymes and administration of H₂S donors can be considered as innovative therapeutic approaches to improve EC biology and function, to revert endothelial dysfunction or to prevent cardiovascular disease progression. This review will focus on the beneficial autocrine/paracrine properties of H₂S on ECs and the state of the art on H₂S potentiating drugs and tools.

Association of Hydrogen Sulfide with Femoral Bone Mineral Density in Osteoporosis Patients: A Preliminary Study

Background

Accumulated evidence has suggested that hydrogen sulfide (H₂S) has a role in bone formation and bone tissue regeneration. However, it is unknown whether the H₂S content is associated with bone mineral density (BMD) in patients with osteopenia/osteoporosis.

Material/Methods

In the present study, we aimed to explore the changes of serum H₂S in osteopenia and osteoporosis patients. We analyzed femur expression of cystathionine β synthase (CBS), cystathionine γ lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST), which are key enzymes for generating H₂S.

Results

Sixteen (16%) patients had osteopenia, 9 (9%) had osteoporosis, and 75 (75%) had normal BMD. In comparison with patients with normal BMD (controls), the serum levels of H₂S were unexpectedly increased in patients with osteopenia and osteoporosis. This increase was much higher in patients with osteoporosis than in those with osteopenia. Serum H₂S levels were negatively correlated with femoral BMD, but not lumbar BMD. Interestingly, the expression of CBS and CSE were downregulated in femur tissues in patients with osteoporosis, whereas the expression of 3-MST remained unchanged. Serum phosphorus levels, alkaline phosphatase, hemoglobin, and triglycerides were found to be closely associated with CBS and CSE scores in femur tissues.

Conclusions

Serum H₂S levels and femur CBS and CSE expression may be involved in osteoporosis pathogenesis.

Preparation and characterization of a soluble eggshell membrane/agarose composite scaffold with possible applications in cartilage regeneration

Articular hyaline cartilage is an extremely hydrated, not vascularized tissue with a low-cell density. The damage of this tissue can occur after injuries or gradual stress and tears (osteoarthritis), minor damages can be self-healed in several weeks, but major injuries may eventually require surgery. In fact, in this case, because of nature of the cartilage (the absence of cells and vascularization) it is difficult to expect its natural regeneration in a reasonable amount of time. In recent years, cell therapy, in which cells are directly transplanted, has attracted attention. In this study, a scaffold for implanting chondrocytes was prepared. The scaffold was made as a sponge using the eggshell membrane and agarose. The eggshell membrane is structurally similar to the extracellular matrix and nontoxic due to its many collagen components and has good biocompatibility and biodegradability. However, scaffolds made of collagen only has poor mechanical properties. For this reason, the disulfide bond of collagen extracted from the insoluble eggshell membrane was cut, converted into water-soluble, and then mixed with agarose to prepare a scaffold. Agarose is capable of controlling mechanical properties, has excellent biocompatibility, and is suitable for forming a hydrogel having a three-dimensional porosity. The scaffold was examined for Fourier-transform infrared, mechanical properties, biodegradability, and biocompatibility. In in vitro experiment, cytotoxicity, cell proliferation, and messenger RNA expression were investigated. The study demonstrated that the agarose/eggshell membrane scaffold can be used for chondrocyte transplantation.

Bone-like ceramic scaffolds designed with bioinspired porosity induce a different stem cell response

Biomaterial science increasingly seeks more biomimetic scaffolds that functionally augment the native bone tissue. In this paper, a new concept of a structural scaffold design is presented where the physiological multi-scale architecture is fully incorporated in a single-scaffold solution. Hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) bioceramic scaffolds with different bioinspired porosity, mimicking the spongy and cortical bone tissue, were studied. In vitro experiments, looking at the mesenchymal stem cells behaviour, were conducted in a perfusion bioreactor that mimics the physiological conditions in terms of interstitial fluid flow and associated induced shear stress. All the biomaterials enhanced cell adhesion and cell viability. Cortical bone scaffolds, with an aligned architecture, induced an overexpression of several late stage genes involved in the process of osteogenic differentiation compared to the spongy bone scaffolds. This study reveals the exciting prospect of bioinspired porous designed ceramic scaffolds that combines both cortical and cancellous bone in a single ceramic bone graft. It is prospected that dual core shell scaffold could significantly modulate osteogenic processes, once implanted in patients, rapidly forming mature bone tissue at the tissue interface, followed by subsequent bone maturation in the inner spongy structure.

Hydrogen Sulfide: a Novel Immunoinflammatory Regulator in Rheumatoid Arthritis

Hydrogen sulfide (H₂S), an endogenous, gaseous, signaling transmitter, has been shown to have vasodilative, anti-oxidative, anti-inflammatory, and cytoprotective activities. Increasing evidence also indicates that H₂S can suppress the production of inflammatory mediators by immune cells, for example, T cells and macrophages. Inflammation is closely related to an immune response in several diseases such as rheumatoid arthritis (RA), multiple sclerosis (MS), systemic lupus erythematosus (SLE), and cancer. Considering these biological effects of H₂S, a potential role in the treatment of immune-related RA is being exploited. In the present review, we will provide an overview of the therapeutic potential of H₂S in RA treatment.

Cytocompatibility of Modified Silk Fibroin with Glycidyl Methacrylate for Tissue Engineering and Biomedical Applications

Hydrogel with chemical modification has been used for 3D printing in the biomedical field of cell and tissue-based regeneration because it provides a good cellular microenvironment and mechanical supportive ability. As a scaffold and a matrix, hydrogel itself has to be modified chemically and physically to form a β-sheet crosslinking structure for the strength of the biomaterials. These chemical modifications could affect the biological damage done to encapsulated cells or surrounding tissues due to unreacted chemical residues. Biological assessment, including assessment of the cytocompatibility of hydrogel in clinical trials, must involve testing with cytotoxicity, irritation, and sensitization. Here, we modified silk fibroin and glycidyl methacrylate (Silk-GMA) and evaluated the physical characterizations, residual chemical detection, and the biological effect of residual GMA depending on dialysis periods. Silk-GMA depending on each dialysis period had a typical β-sheet structure in the characterization analysis and residual GMA decreased from dialysis day 1. Moreover, cell proliferation and viability rate gradually increased; additionally, necrotic and apoptotic cells decreased from dialysis day 2. These results indicate that the dialysis periods during chemical modification of natural polymer are important for removing unreacted chemical residues and for the potential application of the manufacturing standardization for chemically modified hydrogel for the clinical transplantation for tissue engineering and biomedical applications.

Three-dimensional silk fibroin scaffolds incorporated with graphene for bone regeneration

Porous three-dimensional (3D) silk fibroin (SF) scaffolds were widely applied for bone regeneration and showed excellent biocompatibility and biodegradability. Recently graphene was developed for bone scaffolds due to its osteogenic properties. Thus, we combine the SF and graphene to improve the osteogenic properties of SF scaffolds. In our study, we explored the incorporation of SF scaffolds with graphene to develop osteogenic scaffolds capable of accelerating bone formation. The 3D SF scaffolds were fabricated with different contents of graphene (0, 0.5, and 2%). Fluorescence images showed that the graphene nanosheets were homogeneously dispersed in the SF scaffolds. The addition of graphene affected the microarchitecture of the scaffolds. The G/SF scaffolds were cocultured with rat bone marrow-derived mesenchymal stem cells (rBMSCs) for 21 days. The cell morphology and cell proliferation study suggested that 0 and 0.5% G/SF scaffolds displayed good cell proliferation. In addition, immunofluorescent staining (e.g., osteonectin, osteopontin, and osteocalcin) and ALP activities indicated that the osteogenic properties was more actively exhibited on 0.5% G/SF scaffolds compared with the other groups. Our results indicated that SF scaffolds incorporated with graphene could be an appropriate scaffold for bone tissue engineering.

Delivery of Salvianolic Acid B for Efficient Osteogenesis and Angiogenesis from Silk Fibroin Combined with Graphene Oxide

The efficiency of drugs often hinges on drug carriers. To effectively transport therapeutic plant molecules, drug delivery carriers should be able to carry large doses of therapeutic drugs, enable their sustained release, and maintain their biological activity. Here, graphene oxide (GO) is demonstrated to be a valid carrier for delivering therapeutic plant molecules. Salvianolic acid B (SB), which contains a large number of hydroxyl groups, bound to the carboxyl groups of GO by self-assembly. Silk fibroin (SF) substrates were combined with functionalized GO through the freeze-drying method. SF/GO scaffolds could be loaded with large doses of SB, maintain the biological activity of SB while continuously releasing SB, and significantly promote the osteogenic differentiation of rat bone marrow mesenchymal stem cells (rBMSCs). SF/GO/SB also dramatically enhanced endothelial cell (EA-hy9.26) migration and tubulogenesis in vitro. Eight weeks after implantation of SF/GO/SB scaffolds in a rat cranial defect model, the defect area showed more new bone and angiogenesis than that following SF and SF/GO scaffold implantation. Therefore, GO is an effective sustained-release carrier for therapeutic plant molecules, such as SB, which can repair bone defects by promoting osteogenic differentiation and angiogenesis.

Fabrication of a Highly Conductive Silk Knitted Composite Scaffold by Two-Step Electrostatic Self-Assembly for Potential Peripheral Nerve Regeneration

Peripheral nerve injury is a common serious disease, and the electrical conductivity of nerve scaffolds is of special significance for nerve regeneration. Here, a highly conductive silk knitted composite scaffold was prepared by utilizing hydrogen bonding and electrostatic adsorption between silk amino, graphene (RGO), and polyaniline (PANI). Compared to traditional in situ polymerization of aniline (ANI), the surface of the RGO/PANI/silk conductive knitted scaffold prepared by two-step electrostatic self-assembly had more uniform PANI particles and lower resistance; when GO was 1 g/L and ANI was 0.4, 0.6, or 0.8 mol/L, the RGO/PANI/silk scaffold had better electrical properties when the conductivity was between 0.62 × 10^-3 and 1.72 × 10^-3 S/cm. The scaffolds had good conductive stability under different physical stresses and good mechanical properties, wherein ultimately the strength, elongation at break, and Young's modulus ranges were 28.07-34.97 MPa, 105.91-109.85%, and 10.2-12.48 MPa, respectively, and so they provided good support. Conductive scaffolds had ordered loops, fiber structure, and large pore sizes between 40 and 70 μm. In summary, RGO/PANI/silk scaffold with good conductivity, pore size distribution, mechanical properties, thermal properties had potential applications in the field of peripheral nerve regeneration.

Hydrogen Sulfide as Potential Regulatory Gasotransmitter in Arthritic Diseases

The social and economic impact of chronic inflammatory diseases, such as arthritis, explains the growing interest of the research in this field. The antioxidant and anti-inflammatory properties of the endogenous gasotransmitter hydrogen sulfide (H₂S) were recently demonstrated in the context of different inflammatory diseases. In particular, H₂S is able to suppress the production of pro-inflammatory mediations by lymphocytes and innate immunity cells. Considering these biological effects of H₂S, a potential role in the treatment of inflammatory arthritis, such as rheumatoid arthritis (RA), can be postulated. However, despite the growing interest in H₂S, more evidence is needed to understand the pathophysiology and the potential of H₂S as a therapeutic agent. Within this review, we provide an overview on H₂S biological effects, on its role in immune-mediated inflammatory diseases, on H₂S releasing drugs, and on systems of tissue repair and regeneration that are currently under investigation for potential therapeutic applications in arthritic diseases.

Chitosan Derivatives and Their Application in Biomedicine

Chitosan is a product of the deacetylation of chitin, which is widely found in nature. Chitosan is insoluble in water and most organic solvents, which seriously limits both its application scope and applicable fields. However, chitosan contains active functional groups that are liable to chemical reactions; thus, chitosan derivatives can be obtained through the chemical modification of chitosan. The modification of chitosan has been an important aspect of chitosan research, showing a better solubility, pH-sensitive targeting, an increased number of delivery systems, etc. This review summarizes the modification of chitosan by acylation, carboxylation, alkylation, and quaternization in order to improve the water solubility, pH sensitivity, and the targeting of chitosan derivatives. The applications of chitosan derivatives in the antibacterial, sustained slowly release, targeting, and delivery system fields are also described. Chitosan derivatives will have a large impact and show potential in biomedicine for the development of drugs in future.

Biological Effects of Morpholin-4-Ium 4 Methoxyphenyl (Morpholino) Phosphinodithioate and Other Phosphorothioate-Based Hydrogen Sulfide Donors

Significance: Hydrogen sulfide (H₂S) is regarded as the third gasotransmitter along with nitric oxide and carbon monoxide. Extensive studies have demonstrated a variety of biological roles for H₂S in neurophysiology, cardiovascular disease, endocrine regulation, and other physiological and pathological processes. Recent Advances: Novel H₂S donors have proved useful in understanding the biological functions of H₂S, with morpholin-4-ium 4 methoxyphenyl (morpholino) phosphinodithioate (GYY4137) being one of the most common pharmacological tools used. One advantage of GYY4137 over sulfide salts is its ability to release H₂S in a slow and sustained manner akin to endogenous H₂S production, rather than the delivery of H₂S as a single concentrated burst. Critical Issues: Here, we summarize recent progress made in the characterization of the biological activities and pharmacological effects of GYY4137 in a range of in vitro and in vivo systems. Recent developments in the structural modification of GYY4137 to generate new compounds and their biological effects are also discussed. Future Directions: Slow-releasing H₂S donor, GYY4137, and other phosphorothioate-based H₂S donors are potent tools to study the biological functions of H₂S. Despite recent progress, more work needs to be performed on these new compounds to unravel the mechanisms behind H₂S release and pace of its discharge, as well as to define the effects of by-products of donors after H₂S liberation. This will not only lead to better in-depth understanding of the biological effects of H₂S but will also shed light on the future development of a new class of therapeutic agents with potential to treat a wide range of human diseases.

Hydrogen Sulfide in Bone Tissue Regeneration and Repair: State of the Art and New Perspectives

The importance of hydrogen sulfide (H₂S) in the regulation of multiple physiological functions has been clearly recognized in the over 20 years since it was first identified as a novel gasotransmitter. In bone tissue H₂S exerts a cytoprotective effect and promotes bone formation. Just recently, the scientific community has begun to appreciate its role as a therapeutic agent in bone pathologies. Pharmacological administration of H₂S achieved encouraging results in preclinical studies in the treatment of systemic bone diseases, such as osteoporosis; however, a local delivery of H₂S at sites of bone damage may provide additional opportunities of treatment. Here, we highlight how H₂S stimulates multiple signaling pathways involved in various stages of the processes of bone repair. Moreover, we discuss how material science and chemistry have recently developed biomaterials and H₂S-donors with improved features, laying the ground for the development of H₂S-releasing devices for bone regenerative medicine. This review is intended to give a state-of-the-art description of the pro-regenerative properties of H₂S, with a focus on bone tissue, and to discuss the potential of H₂S-releasing scaffolds as a support for bone repair.