Silk fibroin/collagen protein hybrid cell-encapsulating hydrogels with tunable gelation and improved physical and biological properties

Bioengineered Silk Protein-Based 3D In Vitro Models for Tissue Engineering and Drug Development: From Silk Matrix Properties to Biomedical Applications

3D in vitro model has emerged as a valuable tool for studying tissue development, drug screening, and disease modeling. 3D systems can accurately replicate tissue microstructures and physiological features, mirroring the in vivo microenvironment departing from conventional 2D cell cultures. Various 3D in vitro models utilizing biomacromolecules like collagen and synthetic polymers have been developed to meet diverse research needs and address the complex challenges of contemporary research. Silk proteins, bearing structural and functional similarities to collagen, have been increasingly employed to construct advanced 3D in vitro systems, surpassing the limitations of 2D cultures. This review examines silk proteins' composition, structure, properties, and functions, elucidating their role in 3D in vitro models. Furthermore, recent advances in biomedical applications involving silk-based organoid models are discussed. In particular, the unique physiological attributes of silk matrix constituents in in vitro tissue constructs are highlighted, providing a meticulous evaluation of their importance. Additionally, it outlines the current research hurdles and complexities while contemplating future avenues, thereby paving the way for developing complex and biomimetic silk protein-based microtissues.

Silk fibroin/vitreous humor hydrogel scaffold modified by a carbodiimide crosslinker for wound healing

Natural-derived biomaterials can be used as substrates for the growth, proliferation, and differentiation of cells. In this study, bovine vitreous humor as a biological material was cross-linked to silk fibroin with different concentration ratios to design a suitable substrate for corneal tissue regeneration. The cross-linked samples were evaluated with different analyses such as structural, physical (optical, swelling, and degradation), mechanical, and biological (viability, cell adhesion) assays. The results showed that all samples had excellent transparency, especially those with higher silk fibroin content. Increasing the ratio of vitreous humor to silk fibroin decreased mechanical strength and increased swelling and degradation, respectively. There was no significant difference in the toxicity of the samples, and with the increase in vitreous humor ratio, adhesion and cell proliferation increased. Generally, silk fibroin with vitreous humor can provide desirable characteristics as a transparent film for corneal wound healing.

Harnessing the potential of hydrogels for advanced therapeutic applications: current achievements and future directions

The applications of hydrogels have expanded significantly due to their versatile, highly tunable properties and breakthroughs in biomaterial technologies. In this review, we cover the major achievements and the potential of hydrogels in therapeutic applications, focusing primarily on two areas: emerging cell-based therapies and promising non-cell therapeutic modalities. Within the context of cell therapy, we discuss the capacity of hydrogels to overcome the existing translational challenges faced by mainstream cell therapy paradigms, provide a detailed discussion on the advantages and principal design considerations of hydrogels for boosting the efficacy of cell therapy, as well as list specific examples of their applications in different disease scenarios. We then explore the potential of hydrogels in drug delivery, physical intervention therapies, and other non-cell therapeutic areas (e.g., bioadhesives, artificial tissues, and biosensors), emphasizing their utility beyond mere delivery vehicles. Additionally, we complement our discussion on the latest progress and challenges in the clinical application of hydrogels and outline future research directions, particularly in terms of integration with advanced biomanufacturing technologies. This review aims to present a comprehensive view and critical insights into the design and selection of hydrogels for both cell therapy and non-cell therapies, tailored to meet the therapeutic requirements of diverse diseases and situations.

Advances in enhancing the mechanical properties of biopolymer hydrogels via multi-strategic approaches

The limited mechanical properties of biopolymer-based hydrogels have hindered their widespread applications in biomedicine and tissue engineering. In recent years, researchers have shown significant interest in developing novel approaches to enhance the mechanical performance of hydrogels. This review focuses on key strategies for enhancing mechanical properties of hydrogels, including dual-crosslinking, double networks, and nanocomposite hydrogels, with a comprehensive analysis of their underlying mechanisms, benefits, and limitations. It also introduces the classic application scenarios of biopolymer-based hydrogels and the direction of future research efforts, including wound dressings and tissue engineering based on 3D bioprinting. This review is expected to deepen the understanding of the structure-mechanical performance-function relationship of hydrogels and guide the further study of their biomedical applications.

Functionally graded hydrogels with opposing biochemical cues for osteochondral tissue engineering

Osteochondral tissue (OC) repair remains a significant challenge in the field of musculoskeletal tissue engineering. OC tissue displays a gradient structure characterized by variations in both cell types and extracellular matrix components, from cartilage to the subchondral bone. These functional gradients observed in the native tissue have been replicated to engineer OC tissuein vitro. While diverse fabrication methods have been employed to create these microenvironments, emulating the natural gradients and effective regeneration of the tissue continues to present a significant challenge. In this study, we present the design and development of CMC-silk interpenetrating (IPN) hydrogel with opposing dual biochemical gradients similar to native tissue with the aim to regenerate the complete OC unit. The gradients of biochemical cues were generated using an in-house-built extrusion system. Firstly, we fabricated a hydrogel that exhibits a smooth transition of sulfated carboxymethyl cellulose (sCMC) and TGF-β1 (SCT gradient hydrogel) from the upper to the lower region of the IPN hydrogel to regenerate the cartilage layer. Secondly, a hydrogel with a hydroxyapatite (HAp) gradient (HAp gradient hydrogel) from the lower to the upper region was fabricated to facilitate the regeneration of the subchondral bone layer. Subsequently, we developed a dual biochemical gradient hydrogel with a smooth transition of sCMC + TGF-β1 and HAp gradients in opposing directions, along with a blend of both biochemical cues in the middle. The results showed that the dual biochemical gradient hydrogels with biochemical cues corresponding to the three zones (i.e. cartilage, interface and bone) of the OC tissue led to differentiation of bone-marrow-derived mesenchymal stem cells to zone-specific lineages, thereby demonstrating their efficacy in directing the fate of progenitor cells. In summary, our study provided a simple and innovative method for incorporating gradients of biochemical cues into hydrogels. The gradients of biochemical cues spatially guided the differentiation of stem cells and facilitated tissue growth, which would eventually lead to the regeneration of the entire OC tissue with a smooth transition from cartilage (soft) to bone (hard) tissues. This promising approach is translatable and has the potential to generate numerous biochemical and biophysical gradients for regeneration of other interface tissues, such as tendon-to-muscle and ligament-to-bone.

Synthesis of Alginate/Collagen Bioink for Bioprinting Respiratory Tissue Models

Synthesis of bioinks for bioprinting of respiratory tissue requires considerations related to immunogenicity, mechanical properties, printability, and cellular compatibility. Biomaterials can be tailored to provide the appropriate combination of these properties through the synergy of materials with individual pros and cons. Sodium alginate, a water-soluble polymer derived from seaweed, is a cheap yet printable biomaterial with good structural properties; however, it lacks physiological relevance and cell binding sites. Collagen, a common component in the extra cellular matrix of many tissues, is expensive and lacks printability; however, it is highly biocompatible and exhibits sites for cellular binding. This paper presents our study on the synthesis of bioinks from alginate and collagen for use in bioprinting respiratory tissue models. Bioinks were synthesized from 40 mg/mL (4%) alginate and 3 mg/mL (0.3%) collagen in varying ratios (1:0, 4:1, 3:1, 2:1, and 1:1); then examined in terms of rheological properties, printability, compressive, and tensile properties and cellular compatibility. The results illustrate that the ratio of alginate to collagen has a profound impact on bioink performance and that, among the examined ratios, the 3:1 ratio is the most appropriate for use in bioprinting respiratory tissue scaffolds.

An injectable photocuring silk fibroin-based hydrogel for constructing an antioxidant microenvironment for skin repair

Skin has a protein microenvironment dominated by functional collagen fibers, while oxidative stress caused by injury can greatly slow down the progress of wound healing. Here, methacrylated dopamine was incorporated into methacrylated silk fibroin molecule chains to develop an injectable hydrogel with photocuring properties for constructing an antioxidant skin protein microenvironment. This silk fibroin-based hydrogel (SF-g-SDA) showed good tensile and adhesion properties for adapting to the wound shape and skin movement, exhibited stable mechanical properties, good biodegradability and cytocompatibility, and promoted cell adhesion and vascularization in vitro. In addition, its phenolic hydroxyl-mediated antioxidant properties effectively protected cells from damage caused by oxidative stress and supported normal cellular life activities. In animal experiments, SF-g-SDA achieved better skin repair effects in comparison to commercial Tegaderm™ in vivo, showing its ability to accelerate wound healing, improve collagen deposition and alignment in newly fabricated tissues, and promote neovascularization and hair follicle formation. These experimental results indicated that the SF-g-SDA hydrogel is a promising wound dressing.

Collagen fibril-like injectable hydrogels from self-assembled nanoparticles for promoting wound healing

Soft hydrogels are excellent candidate materials for repairing various tissue defects, yet the mechanical strength, anti-swelling properties, and biocompatibility of many soft hydrogels need to be improved. Herein, inspired by the nanostructure of collagen fibrils, we developed a strategy toward achieving a soft but tough, anti-swelling nanofibrillar hydrogel by combining the self-assembly and chemical crosslinking of nanoparticles. Specifically, the collagen fibril-like injectable hydrogel was subtly designed and fabricated by self-assembling methylacrylyl hydroxypropyl chitosan (HM) with laponite (LAP) to form nanoparticles, followed by the inter-nanoparticle bonding through photo-crosslinking. The assembly mechanism of nanoparticles was elucidated by both experimental and simulation techniques. Due to the unique structure of the crosslinked nanoparticles, the nanocomposite hydrogels exhibited low stiffness (G'< 2 kPa), high compressive strength (709 kPa), and anti-swelling (swelling ratio of 1.07 in PBS) properties. Additionally, by harnessing the photo-crosslinking ability of the nanoparticles, the nanocomposite hydrogels were processed as microgels, which can be three-dimensionally (3D) printed into complex shapes. Furthermore, we demonstrated that these nanocomposite hydrogels are highly biocompatible, biodegradability, and can effectively promote fibroblast migration and accelerate blood vessel formation during wound healing. This work presents a promising approach to develop biomimetic, nanofibrillar soft hydrogels for regenerative medicine applications.

Development of a bioactive silk fibroin bilayer scaffold for wound healing and scar inhibition

In cases of deep skin defects, spontaneous tissue regeneration and excessive collagen deposition lead to hyperplastic scars. Conventional remedial action after scar formation is limited with a high recurrence rate. In this study, we designed a new artificial skin bilayer using silk fibroin nanofibers films (SNF) as the epidermis, and silk fibroin (SF) / hyaluronic acid (HA) scaffold as the dermal layer. The regenerated SF film was used as a binder to form a functional SNF-SF-HA bilayer scaffold. The bilayer scaffold showed high porosity, hydrophilicity, and strength, and retained its shape over 30 days in PBS. In vitro, human umbilical vein endothelial cells were seeded into the bilayer scaffold and showed superior cell viability. In vivo analyses using the rabbit ear hypertrophic scar (HS) model indicated that the bilayer scaffold not only supported the reconstruction of new tissue, but also inhibited scar formation. The scaffold possibly achieved scar inhabitation by reducing wound contraction, weakening inflammatory reactions, and regulating collagen deposition and type conversion, which was partly observed through the downregulation of type I collagen, transforming growth factor-β, and α-smooth muscle actin. This study describes a new strategy to expand the application of silk-based biomaterials for the treatment of hyperplastic skin scars.

Construction of ultrasonically treated collagen/silk fibroin composite scaffolds to induce cartilage regeneration

A novel tissue-specific functional tissue engineering scaffold for cartilage repair should have a three-dimensional structure, good biosafety and biological activity, and should be able to promote cartilage tissue regeneration. This study aimed to determine the effect of ultrasound-treated collagen/silk fibroin (Col/SF) composite scaffolds with good mechanical properties and high biological activity on cartilage repair. The characteristics of the scaffolds with different Col/SF ratios (7:3, 8:2, and 9:1) were determined by scanning electron microscopy, Fourier-transform infrared spectroscopy, and porosity, water absorption, and compression tests. In vitro evaluations revealed the biocompatibility of the Col/SF scaffolds. Results suggested that the optimal ratio of Col/SF composite scaffolds was 7:3. The Col/SF scaffolds induced adipose-derived stem cells to undergo chondrogenic differentiation under chondrogenic culture conditions. The efficiency of Col/SF scaffolds for cartilage regeneration applications was further evaluated using an in vivo model of full-thickness articular cartilage defects in New Zealand rabbits. The Col/SF scaffolds effectively promoted osteochondral regeneration as evidenced by macroscopic, histological, and immunohistochemical evaluation. The study demonstrates that ultrasound-treated Col/SF scaffolds show great potential for repairing cartilage defects.

A comprehensive review of silk-fibroin hydrogels for cell and drug delivery applications in tissue engineering and regenerative medicine

Hydrogel scaffolds hold great promise for developing novel treatment strategies in the field of regenerative medicine. Within this context, silk fibroin (SF) has proven to be a versatile material for a wide range of tissue engineering applications owing to its structural and functional properties. In the present review, we report on the design and fabrication of different forms of SF-based scaffolds for tissue regeneration applications, particularly for skin, bone, and neural tissues. In particular, SF hydrogels have emerged as delivery systems for a wide range of bio-actives. Given the growing interest in the field, this review has a primary focus on the fabrication, characterization, and properties of SF hydrogels. We also discuss their potential for the delivery of drugs, stem cells, genes, peptides, and growth factors, including future directions in the field of SF hydrogel scaffolds.

Recent advance in bioactive hydrogels for repairing spinal cord injury: material design, biofunctional regulation, and applications

Functional hydrogels show potential application in repairing spinal cord injury (SCI) due to their unique chemical, physical, and biological properties and functions. In this comprehensive review, we present recent advance in the material design, functional regulation, and SCI repair applications of bioactive hydrogels. Different from previously released reviews on hydrogels and three-dimensional scaffolds for the SCI repair, this work focuses on the strategies for material design and biologically functional regulation of hydrogels, specifically aiming to show how these significant efforts can promoting the repairing performance of SCI. We demonstrate various methods and techniques for the fabrication of bioactive hydrogels with the biological components such as DNA, proteins, peptides, biomass polysaccharides, and biopolymers to obtain unique biological properties of hydrogels, including the cell biocompatibility, self-healing, anti-bacterial activity, injectability, bio-adhesion, bio-degradation, and other multi-functions for repairing SCI. The functional regulation of bioactive hydrogels with drugs/growth factors, polymers, nanoparticles, one-dimensional materials, and two-dimensional materials for highly effective treating SCI are introduced and discussed in detail. This work shows new viewpoints and ideas on the design and synthesis of bioactive hydrogels with the state-of-the-art knowledges of materials science and nanotechnology, and will bridge the connection of materials science and biomedicine, and further inspire clinical potential of bioactive hydrogels in biomedical fields.

Quantitative T2 mapping monitoring the maturation of engineered elastic cartilage in a rabbit model

BACKGROUND

Cartilage tissue engineering provides a promising approach to reconstruct craniofacial defects, and a noninvasive method is needed to assess its effectiveness. Although magnetic resonance imaging (MRI) has been used to evaluate articular cartilage in vivo, few studies focused on its feasibility in monitoring engineered elastic cartilage (EC).

METHODS

Auricular cartilage, silk fibroin (SF) scaffold, and EC consisting of rabbit auricular chondrocytes and SF scaffold were transplanted subcutaneously into the rabbit back. In eight weeks after transplantation, grafts were imaged by MRI using PROSET, PDW VISTA SPAIR, 3D T2 VISTA, 2D MIXED T2 Multislice, and SAG TE multiecho sequences, followed by histological examination and biochemical analysis. Statistical analyses were performed to identify the association between T2 values and biochemical indicator values of EC.

RESULTS

In vivo imaging shows that 2D MIXED T2 Multislice sequence (T2 mapping) clearly distinguished the native cartilage, engineered cartilage and fibrous tissue. T2 values showed high correlations with cartilage-specific biochemical parameters at different time points, especially the elastic cartilage specific protein elastin (ELN, r= -0.939, P < 0.001).

CONCLUSION

Quantitative T2 mapping can effectively detect the in vivo maturity of engineered elastic cartilage after subcutaneously transplantation. This study would promote the clinical application of MRI T2 mapping in monitoring engineered elastic cartilage in the repair of craniofacial defects.

Delivery of Induced Neural Stem Cells Through Mechano-Tuned Silk-Collagen Hydrogels for the Recovery of Contused Spinal Cord in Rats

Neural stem cells (NSC) have tremendous potential for therapeutic regeneration of diseased or traumatized neural tissues, including injured spinal cord. However, transplanted NSC suffer from low cell survival and uncontrolled differentiation, limiting in vivo efficacy. Here, this issue is tackled by delivery through silk-collagen protein hydrogels that are stiffness-matched, stress-relaxing, and shear-thinning. The mechanically-tuned hydrogels protect NSC reprogrammed from fibroblasts (iNSC) initially from injection shear-stress, and enhance long-term survival over 12 weeks. Hydrogel-iNSC treatment alleviates neural inflammation, with reduced inflammatory cells and lesions than NSC-only. The iNSC migrate from the hydrogel into surrounding tissues, secrete up-regulated neurotrophic factors, and differentiate into neural cell subtypes, forming synapses. More serotonergic axons are observed in the lesion cavity, and locomotor functions are improved in hydrogel-iNSC than in iNSC-only. This study highlights the ability of mechanically-tuned protein hydrogels to protect iNSC from the injection stress and severe inflammatory environment, allowing them to differentiate and function to recover the injured spinal cord.

Human Albumin-Based Hydrogels for Their Potential Xeno-Free Microneedle Applications

Nowadays, hydrogels-based microneedles (MNs) have attracted a great interest owing to their outstanding qualities for biomedical applications. For the fabrication of hydrogels-based microneedles as tissue engineering scaffolds and drug delivery carriers, various biomaterials have been tested. They are required to feature tunable physiochemical properties, biodegradability, biocompatibility, nonimmunogenicity, high drug loading capacity, and sustained drug release. Among biomaterials, human proteins are the most ideal biomaterials for fabrication of hydrogels-based MNs; however, they are mechanically weak and poorly processible. To the best of the knowledge, there are no reports of xeno-free human protein-based MNs so far. Here, human albumin-based hydrogels and microneedles for tissue engineering and drug delivery by using relatively new processible human serum albumin methacryloyl (HSAMA) are engineered. The resultant HSAMA hydrogels display tunable mechanical properties, biodegradability, and good biocompatibility. Moreover, the xeno-free HSAMA microneedles display a sustained drug release profile and significant mechanical strength to penetrate the model skin. In vitro, they also show good biocompatibility and anticancer efficacy. Sustainable processible human albumin-based biomaterials may be employed as a xeno-free platform in vivo for tissue engineering and drug delivery in clinical trials in the future.

Biodegradable and Non-Biodegradable Biomaterials and Their Effect on Cell Differentiation

Biomaterials for tissue scaffolds are key components in modern tissue engineering and regenerative medicine. Targeted reconstructive therapies require a proper choice of biomaterial and an adequate choice of cells to be seeded on it. The introduction of stem cells, and the transdifferentiation procedures, into regenerative medicine opened a new era and created new challenges for modern biomaterials. They must not only fulfill the mechanical functions of a scaffold for implanted cells and represent the expected mechanical strength of the artificial tissue, but furthermore, they should also assure their survival and, if possible, affect their desired way of differentiation. This paper aims to review how modern biomaterials, including synthetic (i.e., polylactic acid, polyurethane, polyvinyl alcohol, polyethylene terephthalate, ceramics) and natural (i.e., silk fibroin, decellularized scaffolds), both non-biodegradable and biodegradable, could influence (tissue) stem cells fate, regulate and direct their differentiation into desired target somatic cells.

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.

3D-Printed Hybrid Collagen/GelMA Hydrogels for Tissue Engineering Applications

Bioprinting is an emerging technology involved in the fabrication of three-dimensional tissue constructs for the repair and regeneration of various tissues and organs. Collagen, a natural protein found abundantly in the extracellular matrix of several tissues, can be extracted from collagen-rich tissues of animals such as sheep, cows, rats, pigs, horses, birds, and marine animals. However, due to the poor printability of collagen bioinks, biocompatible collagen scaffolds that mimic the extracellular matrix (ECM) are difficult to fabricate using bioprinting techniques. Gelatin methacrylate (GelMA), a semi-synthetic polymer with tunable physical and chemical properties, has been found to be a promising biomaterial in various bioprinting applications. The printability of collagen can be improved by combining it with semi-synthetic polymers such as GelMA to develop hybrid hydrogels. Such hybrid hydrogels printed have also been identified to have enhanced mechanical properties. Hybrid GelMA meshes have not previously been prepared with collagen from ovine sources. This study provides a novel comparison between the properties of hybrid meshes with ovine skin and bovine hide collagen. GelMA (8% w/v) was integrated with three different concentrations (0.5%, 1%, and 2%) of bovine and ovine collagen forming hybrid hydrogels inks that were printed into meshes with enhanced properties. The maximum percentage of collagen suitable for integration with GelMA, forming hybrid hydrogels with a stable degradation rate was 1%. The water-soluble nature of ovine collagen promoted faster degradation of the hybrid meshes, although the structural crosslinking was identified to be higher than bovine hybrid meshes. The 1% bovine collagen hybrid meshes stood out in terms of their stable degradation rates.

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.

Ancient fibrous biomaterials from silkworm protein fibroin and spider silk blends: Biomechanical patterns

Silkworm silk protein fibroin and spider silk spidroin are known biocompatible and natural biodegradable polymers in biomedical applications. The presence of β-sheets in silk fibroin and spider spidroin conformation improves their mechanical properties. The strength and toughness of pure recombinant silkworm fibroin and spidroin are relatively low due to reduced molecular weight. Hence, blending is the foremost approach of recent studies to optimize silk fibroin and spidroin's mechanical properties. As summarised in the present review, numerous research investigations evaluate the blending of natural and synthetic polymers. The effects of blending silk fibroin and spidroin with natural and synthetic polymers on the mechanical properties are discussed in this review article. Indeed, combining natural and synthetic polymers with silk fibroin and spidroin changes their conformation and structure, fine-tuning the blends' mechanical properties. STATEMENT OF SIGNIFICANCE: Silkworm and spider silk proteins (silk fibroin and spidroin) are biocompatible and biodegradable natural polymers having different types of biomedical applications. Their mechanical and biological properties may be tuned through various strategies such as blending, conjugating and cross-linking. Blending is the most common method to modify fibroin and spidroin properties on demand, this review article aims to categorize and evaluate the effects of blending fibroin and spidroin with different natural and synthetic polymers. Increased polarity and hydrophilicity end to hydrogen bonding triggered conformational change in fibroin and spidroin blends. The effect of polarity and hydrophilicity of the blending compound is discussed and categorized to a combinatorial, synergistic and indirect impacts. This outlook guides us to choose the blending compounds mindfully as this mixing affects the biochemical and biophysical characteristics of the biomaterials.

Development of Biomimetic Hepatic Lobule-Like Constructs on Silk-Collagen Composite Scaffolds for Liver Tissue Engineering

Constructing an engineered hepatic lobule-mimetic model is challenging owing to complicated lobular architecture and crucial hepatic functionality. Our previous study has demonstrated the feasibility of using silk fibroin (SF) scaffolds as functional templates for engineering hepatic lobule-like constructs. But the unsatisfactory chemical and physical performances of the SF-only scaffold and the inherent defect in the functional activity of the carcinoma-derived seeding cells remain to be addressed to satisfy the downstream application demand. In this study, SF-collagen I (SFC) composite scaffolds with improved physical and chemical properties were fabricated, and their utilization for bioengineering a more hepatic lobule-like construct was explored using the immortalized human hepatocyte-derived liver progenitor-like cells (iHepLPCs) and endothelial cells incorporated in the dynamic culture system. The SFC scaffolds prepared through the directional lyophilization process showed radially aligned porous structures with increased swelling ratio and porosity, ameliorative mechanical stiffness that resembled the normal liver matrix more closely, and improved biocompatibility. The iHepLPCs displayed a hepatic plate-like distribution and differentiated into matured hepatocytes with improved hepatic function in vitro and in vivo. Moreover, hepatocyte–endothelial cell interphase arrangement was generated in the co-culture compartment with improved polarity, bile capillary formation, and enhanced liver functions compared with the monocultures. Thus, a more biomimetic hepatic lobule-like model was established and could provide a valuable and robust platform for various applications, including bioartificial liver and drug screening.

A sonication-induced silk-collagen hydrogel for functional cartilage regeneration

Cartilage tissue has limited self-regeneration capacity and current treatment methods often result in fibrocartilage formation. Although collagen has shown the ability to induce chondrogenesis of mesenchymal stem cells (MSCs) and regenerate hyaline cartilage, the application of a pure collagen hydrogel is inherently limited by its fast degradation, poor mechanical properties and excessive cell-mediated shrinkage. To overcome this challenge, we developed a sonication-induced silk-collagen composite hydrogel (COL + SF(S)) and investigated its physicochemical and biological properties compared with a collagen hydrogel (COL) and a non-sonicated silk-collagen composite hydrogel (COL + SF(NS)). The results showed that the sonication treatment of silk fibroin induced antiparallel β-sheet formation and a stronger negative charge on the silk fibroin molecule, which resulted in improved mechanical properties of the COL + SF(S) hydrogel. The COL + SF(S) hydrogel exhibited superior stability during cell culture and promoted the gene expression of SOX9 at the early stage and sulfated glycosaminoglycan (sGAG) deposition without any exogenous growth factor. Moreover, the cartilage regeneration capacity of the COL + SF(S) group was evaluated in rabbit knee defects. The COL + SF(S) group exhibited well-integrated articular hyaline cartilage closely resembling native articular cartilage after 6 months. Overall, the COL + SF(S) hydrogel holds great potential as a scaffold material to regenerate functional hyaline cartilage.

One-Step FRESH Bioprinting of Low-Viscosity Silk Fibroin Inks

Silk fibroin (SF) is an attractive material for composing bioinks suitable for three-dimensional (3D) bioprinting. However, the low viscosity of SF solutions obtained through common dissolution methods limits 3D-bioprinting applications without the addition of thickeners or partial gelation beforehand. Here, we report a method of 3D bioprinting low-viscosity SF solutions without additives. We combined a method of freeform reversible embedding of suspended hydrogels, known as the FRESH method, with horseradish peroxidase-catalyzed cross-linking. Using this method, we successfully fabricated 3D SF hydrogel constructs from low-viscosity SF ink (10% w/w, 50 mPa s at 1 s^-1 shear rate), which does not yield 3D constructs when printed onto a plate in air. Studies using mouse fibroblasts confirmed that the printing process was cell-friendly. Additionally, cells enclosed in printed SF hydrogel constructs maintained > 90% viability for 11 days of culture. These results demonstrate that the 3D bioprinting technique developed in this study enables new 3D bioprinting applications using SF inks and thus has a great potential to contribute to tissue engineering and regenerative medicine.

Stem Cell-Laden Hydrogel-Based 3D Bioprinting for Bone and Cartilage Tissue Engineering

Tremendous advances in tissue engineering and regenerative medicine have revealed the potential of fabricating biomaterials to solve the dilemma of bone and articular defects by promoting osteochondral and cartilage regeneration. Three-dimensional (3D) bioprinting is an innovative fabrication technology to precisely distribute the cell-laden bioink for the construction of artificial tissues, demonstrating great prospect in bone and joint construction areas. With well controllable printability, biocompatibility, biodegradability, and mechanical properties, hydrogels have been emerging as an attractive 3D bioprinting material, which provides a favorable biomimetic microenvironment for cell adhesion, orientation, migration, proliferation, and differentiation. Stem cell-based therapy has been known as a promising approach in regenerative medicine; however, limitations arise from the uncontrollable proliferation, migration, and differentiation of the stem cells and fortunately could be improved after stem cells were encapsulated in the hydrogel. In this review, our focus was centered on the characterization and application of stem cell-laden hydrogel-based 3D bioprinting for bone and cartilage tissue engineering. We not only highlighted the effect of various kinds of hydrogels, stem cells, inorganic particles, and growth factors on chondrogenesis and osteogenesis but also outlined the relationship between biophysical properties like biocompatibility, biodegradability, osteoinductivity, and the regeneration of bone and cartilage. This study was invented to discuss the challenge we have been encountering, the recent progress we have achieved, and the future perspective we have proposed for in this field.

Hydrogels: Properties and Applications in Biomedicine

Hydrogels are crosslinked polymer chains with three-dimensional (3D) network structures, which can absorb relatively large amounts of fluid. Because of the high water content, soft structure, and porosity of hydrogels, they closely resemble living tissues. Research in recent years shows that hydrogels have been applied in various fields, such as agriculture, biomaterials, the food industry, drug delivery, tissue engineering, and regenerative medicine. Along with the underlying technology improvements of hydrogel development, hydrogels can be expected to be applied in more fields. Although not all hydrogels have good biodegradability and biocompatibility, such as synthetic hydrogels (polyvinyl alcohol, polyacrylamide, polyethylene glycol hydrogels, etc.), their biodegradability and biocompatibility can be adjusted by modification of their functional group or incorporation of natural polymers. Hence, scientists are still interested in the biomedical applications of hydrogels due to their creative adjustability for different uses. In this review, we first introduce the basic information of hydrogels, such as structure, classification, and synthesis. Then, we further describe the recent applications of hydrogels in 3D cell cultures, drug delivery, wound dressing, and tissue engineering.

Photocurable antimicrobial silk‐based hydrogels for corneal repair

Corneal transplantation is the current gold standard treatment to restore visual acuity to patients with severe corneal diseases and injuries. Due to severe donor tissue shortage, efforts to develop a corneal equivalent have been made but the challenge remains unmet. Another issue of concern in ocular surgery is the difficult instillation and fast drainage of antibiotic ocular eye drops as bacterial infections can jeopardize implant success by delaying or impairing tissue healing. In this study, we developed antimicrobial silk‐based hydrogels that have the potential to be photoactivated in situ, fully adapting to the corneal injury shape. Gentamicin‐loaded methacrylated‐silk (SilkMA) hydrogels were prepared within minutes using low UV intensity (3 mW/cm²). SilkMA gels provided a Young's modulus between 21 and 79 kPa together with a light transmittance spectrum and water content (83%–90%) similar to the human cornea. Polymer concentration (15%–25%) was found to offer a tool for tailoring the physical properties of the hydrogels. We confirmed that the methacrylation did not affect the material's in vitro degradation and biocompatibility by observing fibroblast adhesion and proliferation. Importantly, agar diffusion tests showed that the synthesized hydrogels were able to inhibit Staphylococcus aureus and Pseudomonas aeruginosa growth for 72 h. These characteristics along with their injectability and viscoelasticity demonstrate the potential of SilkMA hydrogels to be applied in several soft tissue engineering fields. As such, for the first time we demonstrate the potential of photocurable antimicrobial SilkMA hydrogels as a novel biomaterial to facilitate corneal regeneration.

Dynamic Protein and Polypeptide Hydrogels Based on Schiff Base Co-assembly for Biomedicine

Stimuli-responsive hydrogels are promising building blocks for biomedical devices, attributable to their excellent hydrophilicity, biocompatibility, and dynamic responsiveness to temperature, light, pH, and water content. Although hydrogels find interesting applications...

Cellular modulation by the mechanical cues from biomaterials for tissue engineering

Mechanical cues from the extracellular matrix (ECM) microenvironment are known to be significant in modulating the fate of stem cells to guide developmental processes and maintain bodily homeostasis. Tissue engineering has provided a promising approach to the repair or regeneration of damaged tissues. Scaffolds are fundamental in cell-based regenerative therapies. Developing artificial ECM that mimics the mechanical properties of native ECM would greatly help to guide cell functions and thus promote tissue regeneration. In this review, we introduce various mechanical cues provided by the ECM including elasticity, viscoelasticity, topography, and external stimuli, and their effects on cell behaviours. Meanwhile, we discuss the underlying principles and strategies to develop natural or synthetic biomaterials with different mechanical properties for cellular modulation, and explore the mechanism by which the mechanical cues from biomaterials regulate cell function toward tissue regeneration. We also discuss the challenges in multimodal mechanical modulation of cell behaviours and the interplay between mechanical cues and other microenvironmental factors.

Methacrylated Silk Fibroin Hydrogels: pH as a Tool to Control Functionality

The last decade has witnessed significant progress in the development of photosensitive polymers for in situ polymerization and 3D printing applications. Light-mediated sol-gel transitions have immense potential for tissue engineering applications as cell-laden materials can be crosslinked within minutes under mild environmental conditions. Silk fibroin (SF) is extensively explored in regenerative medicine applications due to its ease of modification and exceptional mechanical properties along with cytocompatibility. To efficiently design SF materials, the in vivo assembly of SF proteins must be considered. During SF biosynthesis, changes in pH, water content, and metal ion concentrations throughout the silkworm gland divisions drive the transition from liquid silk to its fiber form. Herein, we study the effect of the glycidyl-methacrylate-modified SF (SilkMA) solution pH on the properties and secondary structure of SilkMA hydrogels by testing formulations prepared at pH 5, 7, and 8. Our results demonstrate an influence of the prepolymer solution pH on the hydrogel rheological properties, compressive modulus, optical transmittance, and network swellability. The hydrogel pH did not affect the in vitro viability and morphology of human dermal fibroblasts. This work demonstrates the utility of the solution pH to tailor the SilkMA conformational structure development toward utility and function and shows the need to strictly control the pH to reduce batch-to-batch variability and ensure reproducibility.

Physical and Bioactive Properties of Glycosaminoglycan Hydrogels Modulated by Polymer Design Parameters and Polymer Ratio

Glycosaminoglycans (GAGs), such as hyaluronic acid (HA) and chondroitin sulfate (CS), have seen widespread adoption as components of tissue engineering scaffolds because of their potent bioactive properties and ease of chemical modification. However, modification of the biopolymers will impair biological recognition of the GAG and reduce the bioactive properties of the material. In this work, we studied how the degree of thiolation of HA and CS, along with other key hydrogel design parameters, affected the physical and bioactive properties of the bulk hydrogel. Although properties, such as the HA molecular weight, did not have a major effect, increasing the degree of thiolation of both HA and CS decreased their biorecognition in experimental analogues for cell/matrix remodeling and binding. Furthermore, combining HA and CS into dual polymer network hydrogels also modulated the physical and bioactive properties, as seen with differences in gel stiffness, degradation rate, and encapsulated cell viability.

Mechanics of 3D Cell-Hydrogel Interactions: Experiments, Models, and Mechanisms

Hydrogels are highly water-swollen molecular networks that are ideal platforms to create tissue mimetics owing to their vast and tunable properties. As such, hydrogels are promising cell-delivery vehicles for applications in tissue engineering and have also emerged as an important base for ex vivo models to study healthy and pathophysiological events in a carefully controlled three-dimensional environment. Cells are readily encapsulated in hydrogels resulting in a plethora of biochemical and mechanical communication mechanisms, which recapitulates the natural cell and extracellular matrix interaction in tissues. These interactions are complex, with multiple events that are invariably coupled and spanning multiple length and time scales. To study and identify the underlying mechanisms involved, an integrated experimental and computational approach is ideally needed. This review discusses the state of our knowledge on cell-hydrogel interactions, with a focus on mechanics and transport, and in this context, highlights recent advancements in experiments, mathematical and computational modeling. The review begins with a background on the thermodynamics and physics fundamentals that govern hydrogel mechanics and transport. The review focuses on two main classes of hydrogels, described as semiflexible polymer networks that represent physically cross-linked fibrous hydrogels and flexible polymer networks representing the chemically cross-linked synthetic and natural hydrogels. In this review, we highlight five main cell-hydrogel interactions that involve key cellular functions related to communication, mechanosensing, migration, growth, and tissue deposition and elaboration. For each of these cellular functions, recent experiments and the most up to date modeling strategies are discussed and then followed by a summary of how to tune hydrogel properties to achieve a desired functional cellular outcome. We conclude with a summary linking these advancements and make the case for the need to integrate experiments and modeling to advance our fundamental understanding of cell-matrix interactions that will ultimately help identify new therapeutic approaches and enable successful tissue engineering.

Colonization versus encapsulation in cell-laden materials design: porosity and process biocompatibility determine cellularization pathways

Seeding materials with living cells has been-and still is-one of the most promising approaches to reproduce the complexity and the functionality of living matter. The strategies to associate living cells with materials are limited to cell encapsulation and colonization, however, the requirements for these two approaches have been seldom discussed systematically. Here we propose a simple two-dimensional map based on materials' pore size and the cytocompatibility of their fabrication process to draw, for the first time, a guide to building cellularized materials. We believe this approach may serve as a straightforward guideline to design new, more relevant materials, able to seize the complexity and the function of biological materials. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Human Umbilical Cord Mesenchymal Stem Cell Derived Exosomes Delivered Using Silk Fibroin and Sericin Composite Hydrogel Promote Wound Healing

Recent studies have shown that the hydrogels formed by composite biomaterials are better choice than hydrogels formed by single biomaterial for tissue repair. We explored the feasibility of the composite hydrogel formed by silk fibroin (SF) and silk sericin (SS) in tissue repair for the excellent mechanical properties of SF, and cell adhesion and biocompatible properties of SS. In our study, the SF SS hydrogel was formed by SF and SS protein with separate extraction method (LiBr dissolution for SF and hot alkaline water dissolution for SS), while SF-SS hydrogel was formed by SF and SS protein using simultaneous extraction method (LiBr dissolution for SF and SS protein). The effects of the two composite hydrogels on the release of inflammatory cytokines from macrophages and the wound were analyzed. Moreover, two hydrogels were used to encapsulate and deliver human umbilical cord mesenchymal stem cell derived exosomes (UMSC-Exo). Both SF SS and SF-SS hydrogels promoted wound healing, angiogenesis, and reduced inflammation and TNF-α secretion by macrophages. These beneficial effects were more significant in the experimental group treated by UMSC-Exo encapsulated in SF-SS hydrogel. Our study found that SF-SS hydrogel could be used as an excellent alternative to deliver exosomes for tissue repair.

Designed protein- and peptide-based hydrogels for biomedical sciences

Proteins are fundamentally the most important macromolecules for biochemical, mechanical, and structural functions in living organisms. Therefore, they provide us with diverse structural building blocks for constructing various types of biomaterials, including an important class of such materials, hydrogels. Since natural peptides and proteins are biocompatible and biodegradable, they have features advantageous for their use as the building blocks of hydrogels for biomedical applications. They display constitutional and mechanical similarities with the native extracellular matrix (ECM), and can be easily bio-functionalized via genetic and chemical engineering with features such as bio-recognition, specific stimulus-reactivity, and controlled degradation. This review aims to give an overview of hydrogels made up of recombinant proteins or synthetic peptides as the structural elements building the polymer network. A wide variety of hydrogels composed of protein or peptide building blocks with different origins and compositions - including β-hairpin peptides, α-helical coiled coil peptides, elastin-like peptides, silk fibroin, and resilin - have been designed to date. In this review, the structures and characteristics of these natural proteins and peptides, with each of their gelation mechanisms, and the physical, chemical, and mechanical properties as well as biocompatibility of the resulting hydrogels are described. In addition, this review discusses the potential of using protein- or peptide-based hydrogels in the field of biomedical sciences, especially tissue engineering.

Silk-Reinforced Collagen Hydrogels with Raised Multiscale Stiffness for Mesenchymal Cells 3D Culture

Type I collagen hydrogels are of high interest in tissue engineering. With the evolution of 3D bioprinting technologies, a high number of collagen-based scaffolds have been reported for the development of 3D cell cultures. A recent proposal was to mix collagen with silk fibroin derived from Bombyx mori silkworm. Nevertheless, due to the difficulties in the preparation and the characteristics of the protein, several problems such as phase separation and collagen denaturation appear during the procedure. Therefore, the common solution is to diminish the concentration of collagen although in that way the most biologically relevant component is reduced. In this study, we present a new, simple, and effective method to develop a collagen-silk hybrid hydrogel with high collagen concentration and with increased stiffness approaching that of natural tissues, which could be of high interest for the development of cardiac patches for myocardial regeneration and for preconditioning of mesenchymal stem cells (MSCs) to improve their therapeutic potential. Sericin in the silk was preserved by using a physical solubilizing procedure that results in a preserved fibrous structure of type I collagen, as shown by ultrastructural imaging. The macro- and micromechanical properties of the hybrid hydrogels measured by tensile stretch and atomic force microscopy, respectively, showed a more than twofold stiffening than the collagen-only hydrogels. Rheological measurements showed improved printability properties for the developed biomaterial. The suitability of the hydrogels for 3D cell culture was assessed by 3D bioprinting bone marrow-derived MSCs cultured within the scaffolds. The result was a biomaterial with improved printability characteristics that better resembled the mechanical properties of natural soft tissues while preserving biocompatibility owing to the high concentration of collagen. Impact statement In this study, we report the development of silk microfiber-reinforced type I collagen hydrogels for 3D bioprinting and cell culture. In contrast with previously reported studies, a novel physical method allowed the preservation of the silk sericin protein. Hydrogels were stable, showed no phase separation between the biomaterials, and they presented improved printability. An increase between two- and threefold of the multiscale stiffness of the scaffolds was achieved with no need of using additional crosslinkers or complex methods, which could be of high relevance for cardiac patches development and for preconditioning mesenchymal stem cells (MSCs) for therapeutic applications. We demonstrate that bone marrow-derived MSCs can be effectively bioprinted and 3D cultured within the stiffened structures.

Utilization of GelMA with phosphate glass fibers for glial cell alignment

Glial cell alignment in tissue engineered constructs is essential for achieving functional outcomes in neural recovery. While gelatin methacrylate (GelMA) hydrogel offers superior biocompatibility along with permissive structure and tailorable mechanical properties, phosphate glass fibers (PGFs) can provide physical cues for directionality of neural growth. Aligned PGFs were fabricated by a melt quenching and fiber drawing method and utilized with synthesized GelMA hydrogel. The mechanical properties of GelMA and biocompatibility of the GelMA-PGFs composite were investigated in vitro using rat glial cells. GelMA with 86% methacrylation degree were photo-crosslinked using 0.1%wt photo-initiator (PI). Photocrosslinking under UV exposure for 60 s was used to produce hydrogels (GelMA-60). PGFs were introduced into the GelMA before crosslinking. Storage modulus and loss modulus of GelMA-60 was 24.73 ± 2.52 and 1.08 ± 0.23 kN/m² , respectively. Increased cell alignment was observed in GelMA-PGFs compared with GelMA hydrogel alone. These findings suggest GelMA-PGFs can provide glial cells with physical cues necessary to achieve cell alignment. This approach could further be used to achieve glial cell alignment in bioengineered constructs designed to bridge damaged nerve tissue.

Sustained Photosynthesis and Oxygen Generation of Microalgae-Embedded Silk Fibroin Hydrogels

Microalgae immobilized in hydrogels offer advantages over those cultured in suspension culture in terms of carbon fixation and oxygen emission. However, alginate as a commonly used hydrogel for microalgal immobilization encounters problems with mechanical strength and stability. To address this limitation, silk fibroin (silk) hydrogels prepared by ultrasonication were utilized to host microalgae when mixed with the presonicated protein solution prior to its gelation. The gelation time, stability, and light transmission of these silk gels were evaluated, and a silk concentration of 4% w/v and a gel thickness of 1 mm provided mechanical strength and stability during algal culture in comparison to alginate hydrogels. Furthermore, silk hydrogels with algal cell densities of 7.6 × 10⁵ and 7.8 × 10⁷ cells/mL had better stability than those with a lower cell density (3.2 × 10³ cells/mL), likely due to cell confinement and impact on proliferation. The silk hydrogels with microalgae at a high density generated 6.13 mg/L of oxygen continuously for 7 days. An oxygen-generating device was fabricated by coating the surface of a dialysis tube with a thin layer of the microalgae-embedded silk hydrogel, where the microalgal cells were nourished with culture medium prefilled in the dialysis tube. When suspended in a sealed flask filled with CO₂ gas, the system continuously produced oxygen (151 mL) for at least 60 days, with an oxygen production efficiency 6 times that of microalgal suspension culture controls. This microalgae embedding and cultivation technique could have potential utility in air purification, tissue repair, and other applications due to the efficient and sustained generation of oxygen.

Hydrogels of engineered bacterial fimbriae can finely tune 2D human cell culture

Demand continues to grow for biomimetic materials able to create well-defined environments for modulating the behaviour of living cells in culture. Here, we describe hydrogels based upon the polymeric bacterial fimbriae protein capsular antigen fragment 1 (Caf1) that presents tunable biological properties for enhanced tissue cell culture applications. We demonstrate how Caf1 hydrogels can regulate cellular functions such as spreading, proliferation and matrix deposition of human dermal fibroblast cells (hDFBs). Caf1 hydrogels exploring a range of mechanical properties were prepared using copolymers featuring controlled compositions of inert wild-type Caf1 subunits and a mutant subunit displaying the RGDS peptide motif. The hydrogels showed excellent cytocompatibility with hDFBs and the ability to modulate both cell morphology and matrix deposition. Interestingly, Caf1 hydrogels displaying faster stress relaxation were demonstrated to show the highest metabolic activities of growing cells in comparison with other Caf1 hydrogel formulations. The stiffest Caf1 hydrogel impacted cellular morphology, inducing alignment of the cells. This work is significant as it clearly indicates that Caf1-based hydrogels offer tuneable biochemical and mechanical substrates conditions suitable for cell culture applications.

Graphene Integrated Hydrogels Based Biomaterials in Photothermal Biomedicine

Recently, photothermal therapy (PTT) has emerged as one of the most promising biomedical strategies for different areas in the biomedical field owing to its superior advantages, such as being noninvasive, target-specific and having fewer side effects. Graphene-based hydrogels (GGels), which have excellent mechanical and optical properties, high light-to-heat conversion efficiency and good biocompatibility, have been intensively exploited as potential photothermal conversion materials. This comprehensive review summarizes the current development of graphene-integrated hydrogel composites and their application in photothermal biomedicine. The latest advances in the synthesis strategies, unique properties and potential applications of photothermal-responsive GGel nanocomposites in biomedical fields are introduced in detail. This review aims to provide a better understanding of the current progress in GGel material fabrication, photothermal properties and potential PTT-based biomedical applications, thereby aiding in more research efforts to facilitate the further advancement of photothermal biomedicine.

Pore size modulates in vitro osteogenesis of bone marrow mesenchymal stem cells in fibronectin/gelatin coated silk fibroin scaffolds

Porous scaffolds have been widely used for bone tissue engineering (BTE), and the pore structure of scaffolds plays an important role in osteogenesis. Silk fibroin (SF) is a favorable biomaterial for BTE due to its excellent mechanical property, biocompatibility, and biodegradation, but the lack of cell attachment sites in SF chemical structure resulted in poor cell-material interactions. In this study, SF scaffolds were coated with fibronectin/gelatin (Fn/G) to improve cell adhesion. Furthermore, the effect of pore size in Fn/G coated SF scaffolds on osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) were investigated in vitro. Scaffolds with average pore diameters of 384.52, 275.23, and 173.8 μm were prepared by salt leaching method, labelled as Large, Medium, and Small group. Porcine BMSCs were seeded on scaffolds and cultured in osteogenic medium for 21 days to evaluate cell proliferation, alkaline phosphatase (ALP) activity, calcium deposition, gene expression of osteogenic markers, and histological performance. The results showed Fn/G coating effectively improved cell adhesion on SF scaffolds. Cell metabolic rate in each group increased significantly with time, but there was no statistical difference at each time point among the three groups. On day 21, ALP/DNA and calcium/DNA in the Small group were significantly higher than those in the Large group. Among the three pore sizes, the Small group showed higher mRNA expression of COl I on day 7, OPN on day 14, and OCN on day 21. Immunohistochemical staining on day 21 showed that Col I and OCN in Small group were more highly expressed. In conclusion, the Fn/G coated SF scaffolds with a mean pore diameter of 173.8 μm was optimal for osteogenic differentiation of BMSC in vitro.

3D Cell Culture: Recent Development in Materials with Tunable Stiffness

It is widely accepted that three-dimensional cell culture systems simulate physiological conditions better than traditional 2D systems. Although extracellular matrix components strongly modulate cell behavior, several studies underlined the importance of mechanosensing in the control of different cell functions such as growth, proliferation, differentiation, and migration. Human tissues are characterized by different degrees of stiffness, and various pathologies (e.g., tumor or fibrosis) cause changes in the mechanical properties through the alteration of the extracellular matrix structure. Additionally, these modifications have an impact on disease progression and on therapy response. Hence, the development of platforms whose stiffness could be modulated may improve our knowledge of cell behavior under different mechanical stress stimuli. In this review, we have analyzed the mechanical diversity of healthy and diseased tissues, and we have summarized recently developed materials with a wide range of stiffness.

Construction of a Silk Fibroin/Polyethylene Glycol Double Network Hydrogel with Co-Culture of HUVECs and UCMSCs for a Functional Vascular Network

The success of complex tissue and internal organ reconstruction relies principally on the fabrication of a 3D vascular network, which guarantees the delivery of oxygen and nutrients in addition to the disposal of waste. In this study, a rapidly forming cell-encapsulated double network (DN) hydrogel is constructed by an ultrasonically activated silk fibroin network and bioorthogonal-mediated polyethylene glycol network. This DN hydrogel can be solidified within 10 s, and its mechanical property gradually increases to ∼20 kPa after 30 min. This work also demonstrates that coencapsulation of human umbilical vein endothelial cells (HUVECs) and umbilical cord-derived mesenchymal stem cells (UCMSCs) into the DN hydrogel can facilitate the formation of more mature vessels and complete the capillary network in comparison with the hydrogels encapsulated with a single cell type both in vitro and in vivo. Taking together, the DN hydrogel, combined with coencapsulation of HUVECs and UCMSCs, represents a strategy for the construction of a functional vascular network.