Enzymatically cross-linked alginic-hyaluronic acid composite hydrogels as cell delivery vehicles

Biofabrication of engineered dento-alveolar tissue

Oral health is essential for a good overall health. Dento-alveolar conditions have a high prevalence, ranging from tooth decay periodontitis to alveolar bone resorption. However, oral tissues exhibit a limited regenerative capacity, and full recovery is challenging. Therefore, regenerative therapies for dento-alveolar tissue (e.g., alveolar bone, periodontal membrane, dentin-pulp complex) have gained much attention, and novel approaches have been proposed in recent decades. This review focuses on the cells, biomaterials and the biofabrication methods used to develop therapies for tooth root bioengineering. Examples of the techniques covered are the multitude of additive manufacturing techniques and bioprinting approaches used to create scaffolds or tissue constructs. Furthermore, biomaterials and stem cells utilized during biofabrication will also be described for different target tissues. As these new therapies gradually become a reality in the lab, the translation to the clinic is still minute, with a further need to overcome multiple challenges and broaden the clinical application of these alternatives.

Modeling a Dynamic Printability Window on Polysaccharide Blend Inks for Extrusion Bioprinting

Extrusion-based bioprinting is one of the most widespread technologies due to its affordability, wide range of processable materials, and ease of use. However, the formulation of new inks for this technique is based on time-consuming trial-and-error processes to establish the optimal ink composition and printing parameters. Here, a dynamic printability window was modeled for the assessment of the printability of polysaccharide blend inks of alginate and hyaluronic acid with the intent to build a versatile predictive tool to speed up the testing procedures. The model considers both the rheological properties of the blends (viscosity, shear thinning behavior, and viscoelasticity) and their printability (in terms of extrudability and the ability of forming a well-defined filament and detailed geometries). By imposing some conditions on the model equations, it was possible to define empirical bands in which the printability is ensured. The predictive capability of the built model was successfully verified on an untested blend of alginate and hyaluronic acid chosen to simultaneously optimize the printability index and minimize the size of the deposited filament.

The Emerging Use of ASC/Scaffold Composites for the Regeneration of Osteochondral Defects

Articular cartilage is composed of chondrocytes surrounded by a porous permeable extracellular matrix. It has a limited spontaneous healing capability post-injury which, if left untreated, can result in severe osteochondral disease. Currently, osteochondral (OC) defects are treated by bone marrow stimulation, artificial joint replacement, or transplantation of bone, cartilage, and periosteum, while autologous osteochondral transplantation is also an option; it carries the risk of donor site damage and is limited only to the treatment of small defects. Allografts may be used for larger defects; however, they have the potential to elicit an immune response. A possible alternative solution to treat osteochondral diseases involves the use of stromal/stem cells. Human adipose-derived stromal/stem cells (ASCs) can differentiate into cartilage and bone cells. The ASC can be combined with both natural and synthetic scaffolds to support cell delivery, growth, proliferation, migration, and differentiation. Combinations of both types of scaffolds along with ASCs and/or growth factors have shown promising results for the treatment of OC defects based on in vitro and in vivo experiments. Indeed, these findings have translated to several active clinical trials testing the use of ASC-scaffold composites on human subjects. The current review critically examines the literature describing ASC-scaffold composites as a potential alternative to conventional therapies for OC tissue regeneration.

Silkworm spinning inspired 3D printing towards high strength scaffold for bone regeneration

Inspired by the silkworm spinning process for production of tough cocoon, a gradient printing-assembly technique with silk fibroin (SF) and hydroxyapatite (HA) to achieve high strength scaffold for bone regeneration...

Collagen- and hyaluronic acid-based hydrogels and their biomedical applications

Hydrogels have been widely investigated in biomedical fields due to their similar physical and biochemical properties to the extracellular matrix (ECM). Collagen and hyaluronic acid (HA) are the main components of the ECM in many tissues. As a result, hydrogels prepared from collagen and HA hold inherent advantages in mimicking the structure and function of the native ECM. Numerous studies have focused on the development of collagen and HA hydrogels and their biomedical applications. In this extensive review, we provide a summary and analysis of the sources, features, and modifications of collagen and HA. Specifically, we highlight the fabrication, properties, and potential biomedical applications as well as promising commercialization of hydrogels based on these two natural polymers.

Mechanotransducive Biomimetic Systems for Chondrogenic Differentiation In Vitro

Osteoarthritis (OA) is a long-term chronic joint disease characterized by the deterioration of bones and cartilage, which results in rubbing of bones which causes joint stiffness, pain, and restriction of movement. Tissue engineering strategies for repairing damaged and diseased cartilage tissue have been widely studied with various types of stem cells, chondrocytes, and extracellular matrices being on the lead of new discoveries. The application of natural or synthetic compound-based scaffolds for the improvement of chondrogenic differentiation efficiency and cartilage tissue engineering is of great interest in regenerative medicine. However, the properties of such constructs under conditions of mechanical load, which is one of the most important factors for the successful cartilage regeneration and functioning in vivo is poorly understood. In this review, we have primarily focused on natural compounds, particularly extracellular matrix macromolecule-based scaffolds and their combinations for the chondrogenic differentiation of stem cells and chondrocytes. We also discuss different mechanical forces and compression models that are used for In Vitro studies to improve chondrogenic differentiation. Summary of provided mechanical stimulation models In Vitro reviews the current state of the cartilage tissue regeneration technologies and to the potential for more efficient application of cell- and scaffold-based technologies for osteoarthritis or other cartilage disorders.

Degradation-Dependent Protein Release from Enzyme Sensitive Injectable Glycol Chitosan Hydrogel

Glycol chitosan (GC) is a hydrophilic chitosan derivative, known for its aqueous solubility. Previously, we have demonstrated the feasibility of preparing injectable, enzymatically crosslinked hydrogels from HPP [3-(4-Hydroxyphenyl)-propionic acid (98%)]-modified GC. However, HPP-GC gels showed very slow degradation, which presents challenges as an in vivo protein delivery vehicle. This study reports the potential of acetylated HPP-GC hydrogels as a biodegradable hydrogel platform for sustained protein delivery. Enzymatic crosslinking was used to prepare injectable, biodegradable hydrogels from HPP-GC with various degrees of acetylation (DA). The acetylated polymers were characterized using Fourier transform infrared and nuclear magnetic resonance spectroscopy. Rheological methods were used to characterize the mechanical behavior of the hydrogels. In vitro degradation and protein release were performed in the presence and absence of lysozyme. In vivo degradation was studied using a mouse subcutaneous implantation model. Finally, two hydrogel formulations with distinct in vitro/in vivo degradation and in vitro protein release were evaluated in 477-SKH1-Elite mice using live animal imaging to understand in vivo protein release profiles. The lysozyme-mediated degradation of the gels was demonstrated in vitro and the degradation rate was found to be dependent on the DA of the polymers. In vivo degradation study further confirmed that gels formed from polymers with higher DA degraded faster. In vitro protein release demonstrated the feasibility to achieve lysozyme-mediated protein release from the gels and that the rate of protein release can be modulated by varying the DA. In vivo protein release study further confirmed the feasibility to achieve differential protein release by varying the DA. The feasibility to develop degradable enzymatically crosslinked GC hydrogels is demonstrated. Gels with a wide spectrum of degradation time ranging from less than a week and more than 6 weeks can be developed using this approach. The study also showed the feasibility to fine tune in vivo protein release by modulating HPP-GC acetylation. The hydrogel platform therefore holds significant promise as a protein delivery vehicle for various biomedical and regenerative engineering applications. Impact statement The study describes the feasibility to develop a novel enzyme sensitive biodegradable and injectable hydrogel, where in the in vivo degradation rate and protein release profile can be modulated over a wide range. The described hydrogel platform has the potential to develop into a clinically relevant injectable and tunable protein delivery vehicle for a wide range of biomedical applications.

Tunable and high tissue adhesive properties of injectable chitosan based hydrogels through polymer architecture modulation

Chitosan-based hydrogels have been widely used for various biomedical applications due to their versatile properties such as biocompatibility, biodegradability, muco-adhesiveness, hemostatic effect and so on. However, the inherent rigidity and brittleness of pure chitosan hydrogels are still unmanageable, which has limited their potential use in biomaterial research. In this study, we developed in situ forming chitosan/PEG hydrogels with improved mechanical properties, using the enzymatic crosslinking reaction of horseradish peroxidase (HRP). The effect of PEG on physico-chemical properties of hybrid hydrogels was thoroughly elucidated by varying the content (0-100 %), molecular weight (4, 10 and 20 kDa) and geometry (linear, 4-arm) of the PEG derivatives. The resulting hydrogels demonstrated excellent hemostatic ability and are highly biocompatible in vivo, comparable to commercially available fibrin glue. We suggest these chitosan/PEG hybrid hydrogels with tunable physicochemical and tissue adhesive properties have great potential for a wide range of biomedical applications in the near future.

Crosslinking strategies for silk fibroin hydrogels: promising biomedical materials

Due to their strong biomimetic potential, silk fibroin (SF) hydrogels are impressive candidates for tissue engineering, due to their tunable mechanical properties, biocompatibility, low immunotoxicity, controllable biodegradability, and a remarkable capacity for biomaterial modification and the realization of a specific molecular structure. The fundamental chemical and physical structure of SF allows its structure to be altered using various crosslinking strategies. The established crosslinking methods enable the formation of three-dimensional (3D) networks under physiological conditions. There are different chemical and physical crosslinking mechanisms available for the generation of SF hydrogels (SFHs). These methods, either chemical or physical, change the structure of SF and improve its mechanical stability, although each method has its advantages and disadvantages. While chemical crosslinking agents guarantee the mechanical strength of SFH through the generation of covalent bonds, they could cause some toxicity, and their usage is not compatible with a cell-friendly technology. On the other hand, physical crosslinking approaches have been implemented in the absence of chemical solvents by the induction of β-sheet conformation in the SF structure. Unfortunately, it is not easy to control the shape and properties of SFHs when using this method. The current review discusses the different crosslinking mechanisms of SFH in detail, in order to support the development of engineered SFHs for biomedical applications.

Influence of the Mechanical Environment on the Regeneration of Osteochondral Defects

Articular cartilage is a highly specialised connective tissue of diarthrodial joints which provides a smooth, lubricated surface for joint articulation and plays a crucial role in the transmission of loads. In vivo cartilage is subjected to mechanical stimuli that are essential for cartilage development and the maintenance of a chondrocytic phenotype. Cartilage damage caused by traumatic injuries, ageing, or degradative diseases leads to impaired loading resistance and progressive degeneration of both the articular cartilage and the underlying subchondral bone. Since the tissue has limited self-repairing capacity due its avascular nature, restoration of its mechanical properties is still a major challenge. Tissue engineering techniques have the potential to heal osteochondral defects using a combination of stem cells, growth factors, and biomaterials that could produce a biomechanically functional tissue, representative of native hyaline cartilage. However, current clinical approaches fail to repair full-thickness defects that include the underlying subchondral bone. Moreover, when tested in vivo, current tissue-engineered grafts show limited capacity to regenerate the damaged tissue due to poor integration with host cartilage and the failure to retain structural integrity after insertion, resulting in reduced mechanical function. The aim of this review is to examine the optimal characteristics of osteochondral scaffolds. Additionally, an overview on the latest biomaterials potentially able to replicate the natural mechanical environment of articular cartilage and their role in maintaining mechanical cues to drive chondrogenesis will be detailed, as well as the overall mechanical performance of grafts engineered using different technologies.

Biomimetic Aspects of Restorative Dentistry Biomaterials

Biomimetic has emerged as a multi-disciplinary science in several biomedical subjects in recent decades, including biomaterials and dentistry. In restorative dentistry, biomimetic approaches have been applied for a range of applications, such as restoring tooth defects using bioinspired peptides to achieve remineralization, bioactive and biomimetic biomaterials, and tissue engineering for regeneration. Advancements in the modern adhesive restorative materials, understanding of biomaterial–tissue interaction at the nano and microscale further enhanced the restorative materials’ properties (such as color, morphology, and strength) to mimic natural teeth. In addition, the tissue-engineering approaches resulted in regeneration of lost or damaged dental tissues mimicking their natural counterpart. The aim of the present article is to review various biomimetic approaches used to replace lost or damaged dental tissues using restorative biomaterials and tissue-engineering techniques. In addition, tooth structure, and various biomimetic properties of dental restorative materials and tissue-engineering scaffold materials, are discussed.

Body-Integrated, Enzyme-Triggered Degradable, Silk-Based Mechanical Sensors for Customized Health/Fitness Monitoring and In Situ Treatment

Mechanical signals such as pressure and strain reflect important psychological and physiological states of the human body. Body-integrated sensors, including skin-mounted and surgically implanted ones, allow personalized health monitoring for the general population as well as patients. However, the development of such measuring devices has been hindered by the strict requirements for human-biocompatible materials and the need for high performance sensors; most existing devices or sensors do not meet all the desired specifications. Here, a set of flexible, stretchable, wearable, implantable, and degradable mechanical sensors is reported with excellent mechanical robustness and compliance, outstanding biocompatibility, remotely-triggered degradation, and excellent sensing performance, using a conductive silk fibroin hydrogel (CSFH). They can detect multiple mechanical signals such as pressure, strain, and bending angles. Moreover, combined with a drug-loaded silk-based microneedle array, sensor-equipped devices are shown to be effective for real-time monitoring and in situ treatment of epilepsy in a rodent model. These sensors offer potential applications in custom health monitoring wearables, and in situ treatment of chronic clinical disorders.

Modulating Alginate Hydrogels for Improved Biological Performance as Cellular 3D Microenvironments

The rational choice and design of biomaterials for biomedical applications is crucial for successful in vitro and in vivo strategies, ultimately dictating their performance and potential clinical applications. Alginate, a marine-derived polysaccharide obtained from seaweeds, is one of the most widely used polymers in the biomedical field, particularly to build three dimensional (3D) systems for in vitro culture and in vivo delivery of cells. Despite their biocompatibility, alginate hydrogels often require modifications to improve their biological activity, namely via inclusion of mammalian cell-interactive domains and fine-tuning of mechanical properties. These modifications enable the addition of new features for greater versatility and control over alginate-based systems, extending the plethora of applications and procedures where they can be used. Additionally, hybrid systems based on alginate combination with other components can also be explored to improve the mimicry of extracellular microenvironments and their dynamics. This review provides an overview on alginate properties and current clinical applications, along with different strategies that have been reported to improve alginate hydrogels performance as 3D matrices and 4D dynamic systems.

Hyaluronic acid as a bioink for extrusion-based 3D printing

Biofabrication is enriching the tissue engineering field with new ways of producing structurally organized complex tissues. Among the numerous bioinks under investigation, hyaluronic acid (HA) and its derivatives stand out for their biological relevance, cytocompatibility, shear-thinning properties, and potential to fine-tune the desired properties with chemical modification. In this paper, we review the recent advances on bioinks containing HA. The available literature is presented based on subjects including the rheological properties in connection with printability, the chemical strategies for endowing HA with the desired properties, the clinical application, the most advanced preclinical studies, the advantages and limitations in comparison with similar biopolymer-based bioinks, and future perspectives.

Nanocomposite Gel as Injectable Therapeutic Scaffold: Microstructural Aspects and Bioactive Properties

The development of tissue scaffolds able to provide proper and accelerated regeneration of tissue is a main task of tissue engineering. We developed a nanocomposite gel that may be used as an injectable therapeutic scaffold. The nanocomposite gel is based on biocompatible gelling agents with embedded nanoparticles (iron oxide, silver, and hydroxyapatite) providing therapeutic properties. We have investigated the microstructure of the nanocomposite gel exposed to different substrates (porous materials and biological tissue). Here we show that the nanocomposite gel has the ability to self-reassemble mimicking the substrate morphology: exposition on porous mineral substrate caused reassembling of nanocomposite gel into 10× smaller scale structure; exposition to a section of humerus cortical bone decreased the microstructure scale more than twice (to ≤3 μm). The reassembling happens through a transitional layer which exists near the phase separation boundary. Our results impact the knowledge of gels explaining their abundance in biological organisms from the microstructural point of view. The results of our biological experiments showed that the nanocomposite gel may find diverse applications in the biomedical field.

Preparation of methyl alginate and its application in acidified milk drinks

A series of methyl alginate with different degree of esterification (DE) were prepared with low cost, and its application in acidified milk drinks (AMDs) was investigated. The gel strength, molecular weight, solution apparent viscosity and optical rotation value of methyl alginate all decreased with the increase of DE. Methyl alginate with DE equal or larger than 60.7% was more effective in stabilizing AMDs than high-methoxyl pectin (HMP) under the same conditions, and the higher the DE, the better its stabilizing ability. The methyl alginate had better synergistic action for stabilizing AMDs with propylene glycol alginate (PGA) than HMP. The reason for this could be attributed to that methyl alginate and PGA had the same main chain structure, and the same levorotatory optical activity. The results indicated that the methyl alginate had a potential industry application prospect for stabilizing AMDs as an alternative to HMP.

Horseradish peroxidase-catalyzed hydrogelation for biomedical applications

Hydrogels catalyzed by horseradish peroxidase (HRP) serve as an efficient and effective platform for biomedical applications due to their mild reaction conditions for cells, fast and adjustable gelation rate in physiological conditions, and an abundance of substrates as water-soluble biocompatible polymers.

Enzymatically crosslinked silk-hyaluronic acid hydrogels

In this study, silk fibroin and hyaluronic acid (HA) were enzymatically crosslinked to form biocompatible composite hydrogels with tunable mechanical properties similar to that of native tissues. The formation of di-tyrosine crosslinks between silk fibroin proteins via horseradish peroxidase has resulted in a highly elastic hydrogel but exhibits time-dependent stiffening related to silk self-assembly and crystallization. Utilizing the same method of crosslinking, tyramine-substituted HA forms hydrophilic and bioactive hydrogels that tend to have limited mechanics and degrade rapidly. To address the limitations of these singular component scaffolds, HA was covalently crosslinked with silk, forming a composite hydrogel that exhibited both mechanical integrity and hydrophilicity. The composite hydrogels were assessed using unconfined compression and infrared spectroscopy to reveal of the physical properties over time in relation to polymer concentration. In addition, the hydrogels were characterized by enzymatic degradation and for cytotoxicity. Results showed that increasing HA concentration, decreased gelation time, increased degradation rate, and reduced changes that were observed over time in mechanics, water retention, and crystallization. These hydrogel composites provide a biologically relevant system with controllable temporal stiffening and elasticity, thus offering enhanced tunable scaffolds for short or long term applications in tissue engineering.

Crosslinking method of hyaluronic-based hydrogel for biomedical applications

In the field of tissue engineering, there is a need for advancement beyond conventional scaffolds and preformed hydrogels. Injectable hydrogels have gained wider admiration among researchers as they can be used in minimally invasive surgical procedures. Injectable gels completely fill the defect area and have good permeability and hence are promising biomaterials. The technique can be effectively applied to deliver a wide range of bioactive agents, such as drugs, proteins, growth factors, and even living cells. Hyaluronic acid is a promising candidate for the tissue engineering field because of its unique physicochemical and biological properties. Thus, this review provides an overview of various methods of chemical and physical crosslinking using different linkers that have been investigated to develop the mechanical properties, biodegradation, and biocompatibility of hyaluronic acid as an injectable hydrogel in cell scaffolds, drug delivery systems, and wound healing applications.

In situ forming gelatin hydrogels by dual-enzymatic cross-linking for enhanced tissue adhesiveness

In situ forming hydrogels show promise as therapeutic implants and carriers in a wide range of biomedical applications.

Biodegradable Polymers and Stem Cells for Bioprinting

It is imperative to develop organ manufacturing technologies based on the high organ failure mortality and serious donor shortage problems. As an emerging and promising technology, bioprinting has attracted more and more attention with its super precision, easy reproduction, fast manipulation and advantages in many hot research areas, such as tissue engineering, organ manufacturing, and drug screening. Basically, bioprinting technology consists of inkjet bioprinting, laser-based bioprinting and extrusion-based bioprinting techniques. Biodegradable polymers and stem cells are common printing inks. In the printed constructs, biodegradable polymers are usually used as support scaffolds, while stem cells can be engaged to differentiate into different cell/tissue types. The integration of biodegradable polymers and stem cells with the bioprinting techniques has provided huge opportunities for modern science and technologies, including tissue repair, organ transplantation and energy metabolism.

Evaluation of enzymatically crosslinked injectable glycol chitosan hydrogel

Enzymatically cross-linkable phenol-conjugated glycol chitosan was prepared by reacting glycol chitosan with 3-(4-hydroxyphenyl)propionic acid (HPP). The chemical modification was confirmed by FTIR, ¹H-NMR and UV spectroscopy. Glycol chitosan hydrogels (HPP-GC) with or without rhBMP-2 were prepared by the oxidative coupling of the substituted phenol groups in the presence of hydrogen peroxide and horse radish peroxidase. Rheological characterization demonstrated the feasibility of developing hydrogels with varying storage moduli by changing the polymer concentration. The gel presented a microporous structure with pore sizes ranging from 50-350 μm. The good viability of encapsulated 7F2 osteoblasts indicated non-toxicity of the gelation conditions. In vitro release of rhBMP-2 in phosphate buffer solution showed ∼11% release in 360 h. The ability of the hydrogel to maintain the in vivo bioactivity of rhBMP-2 was evaluated in a bilateral critical size calvarial bone defect model in Col3.6 transgenic fluorescent reporter mice. The presence of fluorescent green osteoblast cells with overlying red alizarin complexone and yellow stain indicating osteoclast TRAP activity confirmed active cell-mediated mineralization and remodelling process at the implantation site. The complete closure of the defect site at 4 and 8 weeks post implantation demonstrated the potent osteoinductivity of the rhBMP-2 containing gel.

Enzymatic regulation of functional vascular networks using gelatin hydrogels

To manufacture tissue engineering-based functional tissues, scaffold materials that can be sufficiently vascularized to mimic the functionality and complexity of native tissues are needed. Currently, vascular network bioengineering is largely carried out using natural hydrogels as embedding scaffolds, but most natural hydrogels have poor mechanical stability and durability, factors that critically limit their widespread use. In this study, we examined the suitability of gelatin-phenolic hydroxyl (gelatin-Ph) hydrogels that can be enzymatically crosslinked, allowing tuning of the storage modulus and the proteolytic degradation rate, for use as injectable hydrogels to support the human progenitor cell-based formation of a stable and mature vascular network. Porcine gelatin-Ph hydrogels were found to be cytocompatible with human blood-derived endothelial colony-forming cells and white adipose tissue-derived mesenchymal stem cells, resulting in >87% viability, and cell proliferation and spreading could be modulated by using hydrogels with different proteolytic degradability and stiffness. In addition, gelatin was extracted from mouse dermis and murine gelatin-Ph hydrogels were prepared. Importantly, implantation of human cell-laden porcine or murine gelatin-Ph hydrogels into immunodeficient mice resulted in the rapid formation of functional anastomoses between the bioengineered human vascular network and the mouse vasculature. Furthermore, the degree of enzymatic crosslinking of the gelatin-Ph hydrogels could be used to modulate cell behavior and the extent of vascular network formation in vivo. Our report details a technique for the synthesis of gelatin-Ph hydrogels from allogeneic or xenogeneic dermal skin and suggests that these hydrogels can be used for biomedical applications that require the formation of microvascular networks, including the development of complex engineered tissues.

Drug release from enzyme-mediated in situ-forming hydrogel based on gum tragacanth–tyramine conjugate

In the present study, injectable hydrogels based on gum tragacanth–tyramine conjugate were prepared by enzymatic oxidation of tyramine radicals in the presence of hydrogen peroxide. Then, in vitro release of bovine serum albumin and insulin as model protein drugs from this polymeric network was investigated. Also, to improve the properties of this hydrogel, a blended hydrogel composed of tyramine-conjugated gelatin and tyramine-conjugated tragacanth was prepared. Experimental results showed that the gelation time ranged from 3 to 28 s depending on the polymer and enzyme concentrations. Results of morphological investigation of hydrogels indicated that the average pore size of hydrogels varied from 120 to 160 µm. Swelling degree of hydrogels and the rate of drug release decreased by increasing of hydrogen peroxide and polymer concentrations. The release profile of drug from hydrogels followed Higuchi and Fickian diffusion mechanism. Finally, it was shown that the swelling characteristics and drug release behavior of this polymeric network could be improved by blending it with tyramine-conjugated gelatin.

Highly tunable elastomeric silk biomaterials

Elastomeric, fully degradable and biocompatible biomaterials are rare, with current options presenting significant limitations in terms of ease of functionalization and tunable mechanical and degradation properties. We report a new method for covalently crosslinking tyrosine residues in silk proteins, via horseradish peroxidase and hydrogen peroxide, to generate highly elastic hydrogels with tunable properties. The tunable mechanical properties, gelation kinetics and swelling properties of these new protein polymers, in addition to their ability to withstand shear strains on the order of 100%, compressive strains greater than 70% and display stiffness between 200 - 10,000 Pa, covering a significant portion of the properties of native soft tissues. Molecular weight and solvent composition allowed control of material mechanical properties over several orders of magnitude while maintaining high resilience and resistance to fatigue. Encapsulation of human bone marrow derived mesenchymal stem cells (hMSC) showed long term survival and exhibited cell-matrix interactions reflective of both silk concentration and gelation conditions. Further biocompatibility of these materials were demonstrated with in vivo evaluation. These new protein-based elastomeric and degradable hydrogels represent an exciting new biomaterials option, with a unique combination of properties, for tissue engineering and regenerative medicine.

Recombinant human lactoferrin as a biomaterial for bone tissue engineering: mechanism of antiapoptotic and osteogenic activity

Lactoferrin is a bioactive globular protein with unique properties towards musculo-skeletal cells and anabolic to bone in vivo. Even though the potent anti-apoptotic and osteogenic activity of lactoferrin has been reported, the mechanism of action has not been fully elucidated. The study demonstrates that the anti-apoptotic effect of rhLF towards MC3T3 pre-osteoblast cells is mediated by Wnt5a/PKA pathway and the stabilization of β-catenin by rhLF is dependent on PKA/LRP6 signaling pathway. The study also investigates the feasibility of developing rhLF as a biomaterial for cell delivery. The injectable rhLF cell delivery vehicles are prepared by enzymatic crosslinking of tyramine-modified rhLF in the presence of hydrogen peroxide and horseradish peroxidase. The modified rhLF shows bioactivity similar to unmodified rhLF. The rhLF gels support encapsulated MC3T3 cell viability, proliferation, and differentiation, as well as phosphorylation of signaling proteins. In conclusion, the study demonstrates the involvement of Wnt5a, LRP6, and PKA signaling in rhLF-mediated bioactivity towards MC3T3 cells and the feasibility of developing an injectable cell delivery vehicle from rhLF.

Biomaterials in tooth tissue engineering: a review

Biomaterials play a crucial role in the field of tissue engineering. They are utilized for fabricating frameworks known as scaffolds, matrices or constructs which are interconnected porous structures that establish a cellular microenvironment required for optimal tissue regeneration. Several natural and synthetic biomaterials have been utilized for fabrication of tissue engineering scaffolds. Amongst different biomaterials, polymers are the most extensively experimented and employed materials. They can be tailored to provide good interconnected porosity, large surface area, adequate mechanical strengths, varying surface characterization and different geometries required for tissue regeneration. A single type of material may however not meet all the requirements. Selection of two or more biomaterials, optimization of their physical, chemical and mechanical properties and advanced fabrication techniques are required to obtain scaffold designs intended for their final application. Current focus is aimed at designing biomaterials such that they will replicate the local extra cellular environment of the native organ and enable cell-cell and cell-scaffold interactions at micro level required for functional tissue regeneration. This article provides an insight into the different biomaterials available and the emerging use of nano engineering principles for the construction of bioactive scaffolds in tooth regeneration.

Effects of cell density and biomacromolecule addition on the flow behavior of concentrated mesenchymal cell suspensions

With the rapidly growing interest in the use of mesenchymal stromal cells (MSCs) for cell therapy and regenerative medicine applications, either alone as an injected suspension, or dispersed within injectable hydrogel delivery systems, greater understanding of the structure-function-property characteristics of suspensions of adhesion-dependent mesenchymal cells is required. In this paper, we present the results of an experimental study into the flow behavior of concentrated suspensions of living cells of mesenchymal origin (fibroblasts) over a wide range of cell concentrations, with and without the addition of hyaluronic acid (HA), a commonly utilized biomolecule in injectable hydrogel formulations. We characterize the change in the shear viscosity as a function of shear stress and shear rate for cell volume fractions varying from 20 to 60%. We show that high volume fraction suspensions of living mesenchymal cells, known to be capable of homotypic interactions, exhibit highly complex but reproducible rheological footprints, including yield stress, shear thinning and shear-induced fracture behaviors. We show that with the addition of HA, we can significantly modify and tailor the rheology of these cell suspensions at all volume fractions. Using FACS and confocal imaging, we show that the observed effect of HA addition is due to a significantly modulation in the formation of cellular aggregates in these suspensions, and thus the resultant volume spanning network. Considering the aggregates as fractal structures, we show that by taking into account the changes in volume fractions with shear, we are able to plot a master curve for the range of conditions investigated and extract from it the average adhesion force between individual cells, across a population of millions of cells. The outcomes of this study not only provide new insight into the complexity of the flow behaviors of concentrated, adhesive mesenchymal cell suspensions, and their sensitivity to associative biomacromolecule addition, but also a novel, rapid method by which to measure the average adhesion force between individual cells, and the impacts of biomacromolecules on this important parameter.