Molecularly designed alginate hydrogels susceptible to local proteolysis as three-dimensional cellular microenvironments

Biocompatible Coatings from Smart Biopolymer Nanoparticles for Enzymatically Induced Drug Release

Nanoparticles can be used as a smart drug delivery system, when they release the drug only upon degradation by specific enzymes. A method to create such responsive materials is the formation of hydrogel nanoparticles, which have enzymatically degradable crosslinkers. Such hydrogel nanoparticles were prepared by ionotropic gelation sodium alginate with lysine-rich peptide sequences-either α-poly-L-lysine (PLL) or the aggrecanase-labile sequence KKKK-GRD-ARGSV↓NITEGE-DRG-KKKK. The nanoparticle suspensions obtained were analyzed by means of dynamic light scattering and nanoparticle tracking analysis. Degradation experiments carried out with the nanoparticles in suspension revealed enzyme-induced lability. Drugs present in the polymer solution during the ionotropic gelation can be encapsulated in the nanoparticles. Drug loading was investigated for interferon-β (IFN-β) as a model, using a bioluminescence assay with MX2Luc2 cells. The encapsulation efficiency for IFN-β was found to be approximately 25%. The nanoparticles suspension can be used to spray-coat titanium alloys (Ti-6Al-4V) as a common implant material. The coatings were proven by ellipsometry, reflection-absorption infrared spectroscopy, and X-ray photoelectron spectroscopy. An enzyme-responsive decrease in layer thickness is observed due to the degradation of the coatings. The Alg/peptide coatings were cytocompatible for human gingival fibroblasts (HGFIB), which was investigated by CellTiterBlue and lactate dehydrogenase (LDH) assay. However, HGFIBs showed poor adhesion and proliferation on the Alg/peptide coatings, but these could be improved by modification of the alginate with a RGD-peptide sequence. The smart drug release system presented can be further tailored to have the right release kinetics and cell adhesion properties.

Injectable collagen/RGD systems for bone tissue engineering applications

Imbalance crosslink density and polymer concentration gradient is formed within the traditional alginate hydrogel using calcium chloride as a crosslinking agent in external gelation for instantaneously process. In this studying, type I collagen (Col I) blended calcium salt form of poly(γ-glutamic acid) (γCaPGA) was mixing with RGD-modified alginate with convenient gelation process and suitable for practical use. The hydrophilicity of the resulting hydrogels was evaluated through swelling tests, water retention capacity tests, and water vapor permeation tests. Mineralization was qualitatively evaluated by alizarin red dyeing at day 14, verifying the deposition of calcium. The in vitro osteogenic differentiation is monitored by determining the early and late osteocalcin (OCN) and osteopontin (OPN) markers with MG63 cells. Obtained results demonstrated that no extremely changes in mechanical properties. After 14 days of culture, hydrogels significantly stimulated OCN/OPN gene expressions and MG63 cell proliferation. Unusually, γCaPGA with RGD-modified alginate appeared better calcium deposition in 14 days than the other. However, addition of Col I can counterpoise RGD effect in blood coagulation and platelet adhesion made the hydrogel more flexibility and selectively in use. This studying provided that non-covalently crosslinked hydrogel by γCaPGA with alginate can be upgrading by RGD and Col I in water uptake capability, obviously effective for MG63 cells and are remarkably biocompatible and exhibited no cytotoxicity. Moreover, results also displayed the injectable process without complicated procedure, have high cost/performance ratio and have great potential for bone regeneration.

A New High Throughput Screening Platform for Cell Encapsulation in Alginate Hydrogel Shows Improved Hepatocyte Functions by Mesenchymal Stromal Cells Co-encapsulation

Hepatocyte transplantation has emerged as an alternative to liver transplant for liver disease. Hepatocytes encapsulated in alginate microbeads have been proposed for the treatment of acute liver failure, as they are able to provide hepatic functions while the liver regenerates. Furthermore, they do not require immunosuppression, as the alginate protects the hepatocytes from the recipient's immune cells. Mesenchymal stromal cells are very attractive candidates for regenerative medicine, being able to differentiate into cells of the mesenchymal lineages and having extensive proliferative ability. When co-cultured with hepatocytes in two-dimensional cultures, they exert a trophic role, drastically improving hepatocytes survival and functions. In this study we aimed to (i) devise a high throughput system (HTS) to allow testing of a variety of different parameters for cell encapsulation and (ii) using this HTS, investigate whether mesenchymal stromal cells could have beneficial effects on the hepatocytes when co-encapsulated in alginate microbeads. Using our HTS platform, we observed some improvement of hepatocyte behavior with MSCs, subsequently confirmed in the low throughput analysis of cell function in alginate microbeads. Therefore, our study shows that mesenchymal stromal cells may be a good option to improve the function of hepatocytes microbeads. Furthermore, the platform developed may be used for HTS studies on cell encapsulation, in which several conditions (e.g., number of cells, combinations of cells, alginate modifications) could be easily compared at the same time.

Gellan Gum Hydrogels with Enzyme-Sensitive Biodegradation and Endothelial Cell Biorecognition Sites

The survival of a biomaterial or tissue engineered construct is mainly hampered by the deficient microcirculation in its core, and limited nutrients and oxygen availability to the implanted or colonizing host cells. Aiming to address these issues, we herein propose bioresponsive gellan gum (GG) hydrogels that are biodegradable by metalloproteinase 1 (MMP-1) and enable endothelial cells adhesion and proliferation. GG is chemically functionalized with divinyl sulfone (DVS) and then biofunctionalized with thiol cell-adhesive peptides (T1 or C16) to confer GG endothelial cell biorecognition cues. Biodegradable hydrogels are then formed by Michael type addition of GGDVS or/and peptide-functionalized GGDVS with a dithiol peptide crosslinker sensitive to MMP-1. The mechanical properties (6 to 5580 Pa), swelling (17 to 11), MMP-1-driven degradation (up to 70%), and molecules diffusion coefficients of hydrogels are tuned by increasing the polymer amount and crosslinking density. Human umbilical cord vein endothelial cells depict a polarized elongated morphology when encapsulated within T1-containing hydrogels, in contrast to the round morphology observed in C16-containing hydrogels. Cell organization is favored as early as 1 d of cell culture within the T1-modified hydrogels with higher concentration of peptide, while cell proliferation is higher in T1-modified hydrogels with higher modulus. In conclusion, biodegradable and bioresponsive GGDVS hydrogels are promising endothelial cell responsive materials that can be used for vascularization strategies.

Nano-biphasic calcium phosphate/polyvinyl alcohol composites with enhanced bioactivity for bone repair via low-temperature three-dimensional printing and loading with platelet-rich fibrin

Background and aim

As a newly emerging three-dimensional (3D) printing technology, low-temperature robocasting can be used to fabricate geometrically complex ceramic scaffolds at low temperatures. Here, we aimed to fabricate 3D printed ceramic scaffolds composed of nano-biphasic calcium phosphate (BCP), polyvinyl alcohol (PVA), and platelet-rich fibrin (PRF) at a low temperature without the addition of toxic chemicals.

Methods

Corresponding nonprinted scaffolds were prepared using a freeze-drying method. Compared with the nonprinted scaffolds, the printed scaffolds had specific shapes and well-connected internal structures.

Results

The incorporation of PRF enabled both the sustained release of bioactive factors from the scaffolds and improved biocompatibility and biological activity toward bone marrow-derived mesenchymal stem cells (BMSCs) in vitro. Additionally, the printed BCP/PVA/PRF scaffolds promoted significantly better BMSC adhesion, proliferation, and osteogenic differentiation in vitro than the printed BCP/PVA scaffolds. In vivo, the printed BCP/PVA/PRF scaffolds induced a greater extent of appropriate bone formation than the printed BCP/PVA scaffolds and nonprinted scaffolds in a critical-size segmental bone defect model in rabbits.

Conclusion

These experiments indicate that low-temperature robocasting could potentially be used to fabricate 3D printed BCP/PVA/PRF scaffolds with desired shapes and internal structures and incorporated bioactive factors to enhance the repair of segmental bone defects.

Blocking of matrix metalloproteinases-13 responsive peptide in poly(urethane urea) for potential cartilage tissue engineering applications

The matching of scaffold degradation rate with neotissue growth is required for tissue engineering applications. Timely provision of proper spaces especially for cartilage tissue engineering plays a pivotal role in chondrocyte cluster formation. In this study, poly(urethane urea) was synthesized using conventional two-stage method by extending the isocyanate group terminated prepolymers with different amounts of GPLGLWARK peptide, which responses the degrading induced by matrix metalloproteinase 13, the main proteinase for cartilage matrix degradation. The Fourier transform infrared spectrometer with the attenuated total reflection and 1H nuclear magnetic resonance spectra revealed that the peptides were introduced to poly(urethane urea) according to the characteristic absorption bands of the peptide and the newly formed urea bonds. The ultraviolet-visible spectroscopy spectra showed that the weight percentages of the peptide in the three poly(urethane urea) were 25%, 32%, and 35%. Atomic force microscopy images revealed that phase separation occurred in all poly(urethane urea) samples and became increasingly apparent with increasing amount of peptides introduced. Mechanical tests showed that the poly(urethane urea) strength increased with increasing amount of peptides in poly(urethane urea). Poly(urethane urea) proteolysis in matrix metalloproteinase 13 solution was more rapid than hydrolysis in aqueous buffer, and proteolysis rate was dependent on the amount of peptides in poly(urethane urea). Cell proliferation on the material surface in vitro displayed nontoxicity for all synthesized poly(urethane urea). In vivo subcutaneous implantation evaluation revealed the presence of local foreign body reactions triggered by poly(urethane urea) but was not due to peptide in poly(urethane urea). Moreover, the synthesized poly(urethane urea) with significant phase separation did not degrade under the matrix metalloproteinase 13 free subcutaneous environment, but poly(urethane urea) with minimal phase separation was degraded by attacking of the enzymes adsorbed on the hydrophobic surface through non-specific adsorption.

3D Culture of Mesenchymal Stem Cells in Alginate Hydrogels

Three-dimensional (3D) cell culture systems have gained increasing interest among the scientific community, as they are more biologically relevant than traditional two-dimensional (2D) monolayer cultures. Alginate hydrogels can be formed under cytocompatibility conditions, being among the most widely used cell-entrapment 3D matrices. They recapitulate key structural features of the natural extracellular matrix and can be bio-functionalized with bioactive moieties, such as peptides, to specifically modulate cell behavior. Moreover, alginate viscoelastic properties can be tuned to match those of different types of native tissues. Ionic alginate hydrogels are transparent, allowing routine monitoring of entrapped cells, and crosslinking can be reverted using chelating agents for easy cell recovery. In this chapter, we describe some key steps to establish and characterize 3D cultures of mesenchymal stem cells using alginate hydrogels.

Multicellular tumor invasion and plasticity in biomimetic materials

Cancer cell invasion through the extracellular matrix is associated with metastatic spread and therapeutic resistance. In carcinomas, the detachment and dissemination of individual cells has been associated with an epithelial-mesenchymal transition, but tumors can also invade using collective, multicellular phenotypes. This malignant tumor progression is also associated with alignment and stiffening of the surrounding extracellular matrix. Historically, tumor invasion has been investigated using 2D monolayer culture, small animal models or patient histology. These assays have been complemented by the use of natural biomaterials such as reconstituted basement membrane and collagen I. More recently, engineered materials with well-defined physical, chemical and biomolecular properties have enabled more controlled microenvironments. In this review, we highlight recent developments in multicellular tumor invasion based on microfabricated structures or hydrogels. We emphasize the role of interfacial geometries, biomaterial stiffness, matrix remodeling, and co-culture models. Finally, we discuss future directions for the field, particularly integration with precision measurements of biomaterial properties and single cell heterogeneity, standardization and scale-up of these platforms, as well as integration with patient-derived samples.

Injectable hybrid system for strontium local delivery promotes bone regeneration in a rat critical-sized defect model

Strontium (Sr) has been described as having beneficial influence in bone strength and architecture. However, negative systemic effects have been reported on oral administration of Sr ranelate, leading to strict restrictions in clinical application. We hypothesized that local delivery of Sr improves osteogenesis without eliciting detrimental side effects. Therefore, the in vivo response to an injectable Sr-hybrid system composed of RGD-alginate hydrogel cross-linked in situ with Sr and reinforced with Sr-doped hydroxyapatite microspheres, was investigated. The system was injected in a critical-sized bone defect model and compared to a similar Sr-free material. Micro-CT results show a trend towards higher new bone formed in Sr-hybrid group and major histological differences were observed between groups. Higher cell invasion was detected at the center of the defect of Sr-hybrid group after 15 days with earlier bone formation. Higher material degradation with increase of collagen fibers and bone formation in the center of the defect after 60 days was observed as opposed to bone formation restricted to the periphery of the defect in the control. These histological findings support the evidence of an improved response with the Sr enriched material. Importantly, no alterations were observed in the Sr levels in systemic organs or serum.

Ablation of Y1 receptor impairs osteoclast bone-resorbing activity

Y₁ receptor (Y₁R)-signalling pathway plays a pivotal role in the regulation of bone metabolism. The lack of Y₁R-signalling stimulates bone mass accretion that has been mainly attributed to Y₁R disruption from bone-forming cells. Still, the involvement of Y₁R-signalling in the control of bone-resorbing cells remained to be explored. Therefore, in this study we assessed the role of Y₁R deficiency in osteoclast formation and resorption activity. Here we demonstrate that Y₁R germline deletion (Y₁R^−/−) led to increased formation of highly multinucleated (n > 8) osteoclasts and enhanced surface area, possibly due to monocyte chemoattractant protein-1 (MCP-1) overexpression regulated by RANKL-signalling. Interestingly, functional studies revealed that these giant Y₁R^−/− multinucleated cells produce poorly demineralized eroded pits, which were associated to reduce expression of osteoclast matrix degradation markers, such as tartrate-resistant acid phosphatase-5b (TRAcP5b), matrix metalloproteinase-9 (MMP-9) and cathepsin-K (CTSK). Tridimensional (3D) morphologic analyses of resorption pits, using an in-house developed quantitative computational tool (BonePit), showed that Y₁R^−/− resorption pits displayed a marked reduction in surface area, volume and depth. Together, these data demonstrates that the lack of Y₁Rs stimulates the formation of larger multinucleated osteoclasts in vitro with reduced bone-resorbing activity, unveiling a novel therapeutic option for osteoclastic bone diseases based on Y₁R-signalling ablation.

A 3D in vitro model to explore the inter-conversion between epithelial and mesenchymal states during EMT and its reversion

Epithelial-to-mesenchymal transitions (EMT) are strongly implicated in cancer dissemination. Intermediate states, arising from inter-conversion between epithelial (E) and mesenchymal (M) states, are characterized by phenotypic heterogeneity combining E and M features and increased plasticity. Hybrid EMT states are highly relevant in metastatic contexts, but have been largely neglected, partially due to the lack of physiologically-relevant 3D platforms to study them. Here we propose a new in vitro model, combining mammary E cells with a bioengineered 3D matrix, to explore phenotypic and functional properties of cells in transition between E and M states. Optimized alginate-based 3D matrices provided adequate 3D microenvironments, where normal epithelial morphogenesis was recapitulated, with formation of acini-like structures, similar to those found in native mammary tissue. TGFβ1-driven EMT in 3D could be successfully promoted, generating M-like cells. TGFβ1 removal resulted in phenotypic switching to an intermediate state (RE cells), a hybrid cell population expressing both E and M markers at gene/protein levels. RE cells exhibited increased proliferative/clonogenic activity, as compared to M cells, being able to form large colonies containing cells with front-back polarity, suggesting a more aggressive phenotype. Our 3D model provides a powerful tool to investigate the role of the microenvironment on metastable EMT stages.

Establishment of feeder-free culture system for human induced pluripotent stem cell on DAS nanocrystalline graphene

The maintenance of undifferentiated human pluripotent stem cells (hPSC) under xeno-free condition requires the use of human feeder cells or extracellular matrix (ECM) coating. However, human-derived sources may cause human pathogen contamination by viral or non-viral agents to the patients. Here we demonstrate feeder-free and xeno-free culture system for hPSC expansion using diffusion assisted synthesis-grown nanocrystalline graphene (DAS-NG), a synthetic non-biological nanomaterial which completely rule out the concern of human pathogen contamination. DAS-NG exhibited advanced biocompatibilities including surface nanoroughness, oxygen containing functional groups and hydrophilicity. hPSC cultured on DAS-NG could maintain pluripotency in vitro and in vivo, and especially cell adhesion-related gene expression profile was comparable to those of cultured on feeders, while hPSC cultured without DAS-NG differentiated spontaneously with high expression of somatic cell-enriched adhesion genes. This feeder-free and xeno-free culture method using DAS-NG will facilitate the generation of clinical-grade hPSC.

Strategies to Maximize the Potential of Marine Biomaterials as a Platform for Cell Therapy

Marine biopolymers have been explored as a promising cell therapy system for efficient cell delivery and tissue engineering. However, the marine biomaterial-based systems themselves have exhibited limited performance in terms of maintenance of cell viability and functions, promotion of cell proliferation and differentiation as well as cell delivery efficiency. Thus, numerous novel strategies have been devised to improve cell therapy outcomes. The strategies include optimization of physical and biochemical properties, provision of stimuli-responsive functions, and design of platforms for efficient cell delivery and tissue engineering. These approaches have demonstrated substantial improvement of therapeutic outcomes in a variety of research settings. In this review, therefore, research progress made with marine biomaterials as a platform for cell therapy is reported along with current research directions to further advance cell therapies as a tool to cure incurable diseases.

Bioengineered cell-instructive 3D matrices as vehicles for cellular therapies

The delivery of cells, namely of stem cells, from biomaterial-based vehicles has been used to promote and accelerate functional tissue regeneration. Carrier materials can transport and localize cells into target sites, increase their survival and engraftment, and provide a supportive matrix to assist cellular assembly into a newly formed tissue. Hydrogels are amongst the most widely used carrier materials, as they can entrap and protect cells within a real three-dimensional (3D) microenvironment, and intrinsically present many structural properties of the natural extracellular matrix (ECM). To recreate important features of the native ECM, hydrogels may be bio-functionalized with specific cell-instructive moieties for improved biomimetic properties. The biophysical properties of hydrogels can also be tuned, increasing their versatility, as it is currently recognized that stem cells respond to their biomechanical environment. Here, an overview of some strategies commonly employed in the preparation of biomimetic matrices is presented, using alginate as a model hydrogel.

Efficient functionalization of alginate biomaterials

Peptide coupled alginates obtained by chemical functionalization of alginates are commonly used as scaffold materials for cells in regenerative medicine and tissue engineering. We here present an alternative to the commonly used carbodiimide chemistry, using partial periodate oxidation followed by reductive amination. High and precise degrees of substitution were obtained with high reproducibility, and without formation of by-products. A protocol was established using l-Tyrosine methyl ester as a model compound and the non-toxic pic-BH3 as the reducing agent. DOSY was used to indirectly verify covalent binding and the structure of the product was further elucidated using NMR spectroscopy. The coupling efficiency was to some extent dependent on alginate composition, being most efficient on mannuronan. Three different bioactive peptide sequences (GRGDYP, GRGDSP and KHIFSDDSSE) were coupled to 8% periodate oxidized alginate resulting in degrees of substitution between 3.9 and 6.9%. Cell adhesion studies of mouse myoblasts (C2C12) and human dental stem cells (RP89) to gels containing various amounts of GRGDSP coupled alginate demonstrated the bioactivity of the material where RP89 cells needed higher peptide concentrations to adhere.

Culture of hESC-derived pancreatic progenitors in alginate-based scaffolds

The effect of alginate-based scaffolds with added basement membrane proteins on the in vitro development of hESC-derived pancreatic progenitors was investigated. Cell clusters were encapsulated in scaffolds containing the basement membrane proteins collagen IV, laminin, fibronectin, or extracellular matrix-derived peptides, and maintained in culture for up to 46 days. The cells remained viable throughout the experiment with no signs of central necrosis. Whereas nonencapsulated cells aggregated into larger clusters, some of which showed signs of morphological changes and tissue organization, the alginate matrix stabilized the cluster size and displayed more homogeneous cell morphologies, allowing culture for long periods of time. For all conditions tested, a stable or declining expression of insulin and PDX1 and an increase in glucagon and somatostatin over time indicated a progressive reduction in beta cell-related gene expression. Alginate scaffolds can provide a chemically defined, xeno-free and easily scalable alternative for culture of pancreatic progenitors. Although no increase in insulin and PDX1 gene expression after alginate-immobilized cell culture was seen in this study, further optimization of the matrix physicochemical and biological properties and of the medium composition may still be a relevant strategy to promote the stabilization or maturation of stem cell-derived beta cells.

Finding and tracing human MSC in 3D microenvironments with the photoconvertible protein Dendra2

Mesenchymal Stem/Stromal Cells (MSC) are a promising cell type for cell-based therapies - from tissue regeneration to treatment of autoimmune diseases - due to their capacity to migrate to damaged tissues, to differentiate in different lineages and to their immunomodulatory and paracrine properties. Here, a simple and reliable imaging technique was developed to study MSC dynamical behavior in natural and bioengineered 3D matrices. Human MSC were transfected to express a fluorescent photoswitchable protein, Dendra2, which was used to highlight and follow the same group of cells for more than seven days, even if removed from the microscope to the incubator. This strategy provided reliable tracking in 3D microenvironments with different properties, including the hydrogels Matrigel and alginate as well as chitosan porous scaffolds. Comparison of cells mobility within matrices with tuned physicochemical properties revealed that MSC embedded in Matrigel migrated 64% more with 5.2 mg protein/mL than with 9.6 mg/mL and that MSC embedded in RGD-alginate migrated 51% faster with 1% polymer concentration than in 2% RGD-alginate. This platform thus provides a straightforward approach to characterize MSC dynamics in 3D and has applications in the field of stem cell biology and for the development of biomaterials for tissue regeneration.

3D Cell Culture in Alginate Hydrogels

This review compiles information regarding the use of alginate, and in particular alginate hydrogels, in culturing cells in 3D. Knowledge of alginate chemical structure and functionality are shown to be important parameters in design of alginate-based matrices for cell culture. Gel elasticity as well as hydrogel stability can be impacted by the type of alginate used, its concentration, the choice of gelation technique (ionic or covalent), and divalent cation chosen as the gel inducing ion. The use of peptide-coupled alginate can control cell-matrix interactions. Gelation of alginate with concomitant immobilization of cells can take various forms. Droplets or beads have been utilized since the 1980s for immobilizing cells. Newer matrices such as macroporous scaffolds are now entering the 3D cell culture product market. Finally, delayed gelling, injectable, alginate systems show utility in the translation of in vitro cell culture to in vivo tissue engineering applications. Alginate has a history and a future in 3D cell culture. Historically, cells were encapsulated in alginate droplets cross-linked with calcium for the development of artificial organs. Now, several commercial products based on alginate are being used as 3D cell culture systems that also demonstrate the possibility of replacing or regenerating tissue.

Opportunities for multicomponent hybrid hydrogels in biomedical applications

Hydrogels provide mechanical support and a hydrated environment that offer good cytocompatibility and controlled release of molecules, and myriad hydrogels thus have been studied for biomedical applications. In the past few decades, research in these areas has shifted increasingly to multicomponent hydrogels that better capture the multifunctional nature of native biological environments and that offer opportunities to selectively tailor materials properties. This review summarizes recent approaches aimed at producing multicomponent hydrogels, with descriptions of contemporary chemical and physical approaches for forming networks, and of the use of both synthetic and biologically derived molecules to impart desired properties. Specific multicomponent materials with enhanced mechanical properties are presented, as well as materials in which multiple biological functions are imparted for applications in tissue engineering, cancer treatment, and gene therapies. The progress in the field suggests significant promise for these approaches in the development of biomedically relevant materials.

Biofunctionalized pectin hydrogels as 3D cellular microenvironments

Pectin hydrogels were prepared by internal ionotropic gelation and explored as MSC delivery vehicles.

Neonatal Human Dermal Fibroblasts Immobilized in RGD–Alginate Induce Angiogenesis

Promoting angiogenesis in a damaged tissue is a major challenge for tissue regeneration. Recent findings in tissue engineering suggest that fibroblasts (FBs) play an important role in orchestrating the angiogenic process. Fibroblasts maintain the structural integrity of connective tissue by continuously secreting growth factors and extracellular matrix precursors, which are essential for endothelial cell (EC) adhesion and spreading, thus playing a crucial role in angiogenesis. We hypothesized that FBs immobilized in alginate gels grafted with the RGD peptidic sequence could influence the recruitment of ECs to improve vascularization. In this work, the modulation of immobilized human FBs within the 3D synthetic extracellular matrix was assessed. Experiments using cocultures of ECs and FBs in indirect contact as well as angiogenic assays were performed to assess the influence of FBs immobilized in RGD–alginate in ECs' viability, stabilization, sprouting, and assembly into capillary-like structures. This study demonstrates the ability of FBs immobilized within RGD–alginate microspheres to modulate and support capillary-like structures' assembly. These findings indicate that the microenvironment created by these stromal cells in the scaffold modulates capillary morphogenesis, thus stimulating angiogenesis in situ and can potentially be used in regenerative medicine in clinical scenarios where vascularization is essential.

Matrix-driven formation of mesenchymal stem cell-extracellular matrix microtissues on soft alginate hydrogels

Mesenchymal stem cells (MSCs) can be made to rearrange into microtissues in response to specific matrix cues, a process that depends on a balance between cell-matrix and cell-cell interactions. The effect of such cues, and especially their interplay, is still not fully understood, particularly in three-dimensional (3-D) systems. Here, the behaviour of human MSCs cultured within hydrogel matrices with tailored stiffness and composition was evaluated. MSC aggregation occurred only in more compliant matrices (G'≤ 120 Pa), when compared to stiffer ones, both in the presence and in the absence of matrix-bound arginine-glycine-aspartic acid cell-adhesion ligands (RGD; 0, 100 and 200 μM). Fibronectin assembly stabilized cell-cell contacts within aggregates, even in non-adhesive matrices. However, MSCs were able to substantially contract the artificial matrix only when RGD was present. Moreover, compliant matrices facilitated cell proliferation and provided an environment conducive for MSC osteogenic differentiation, even without RGD. Cell interactions with the original matrix became less important as time progressed, while the de novo-produced extracellular matrix became a more critical determinant of cell fate. These data provide further insights into the mechanisms by which MSCs sense their microenvironment to organize into tissues, and provide new clues to the design of cell-instructive 3-D matrices.

Application of cell encapsulation for controlled delivery of biological therapeutics

Cell microencapsulation technology is likely to have an increasingly important role in new approaches rather than the classical and pioneering organ replacement. Apart from becoming a tool for protein and morphogen release and long-term drug delivery, it is becoming a new three-dimensional platform for stem cell research. Recent progress in the field has resulted in biodegradable scaffolds that are able to retain and release the cell content in different anatomical locations. Additional advances include the use biomimetic scaffolds that provide greater control over material-cell interactions and the development of more precise encapsulated cell-tracking systems. This review summarises the state of the art of cell microencapsulation and discusses the main directions and challenges of this field towards the controlled delivery of biological therapeutics.

Injectable alginate hydrogels for cell delivery in tissue engineering

Alginate hydrogels are extremely versatile and adaptable biomaterials, with great potential for use in biomedical applications. Their extracellular matrix-like features have been key factors for their choice as vehicles for cell delivery strategies aimed at tissue regeneration. A variety of strategies to decorate them with biofunctional moieties and to modulate their biophysical properties have been developed recently, which further allow their tailoring to the desired application. Additionally, their potential use as injectable materials offers several advantages over preformed scaffold-based approaches, namely: easy incorporation of therapeutic agents, such as cells, under mild conditions; minimally invasive local delivery; and high contourability, which is essential for filling in irregular defects. Alginate hydrogels have already been explored as cell delivery systems to enhance regeneration in different tissues and organs. Here, the in vitro and in vivo potential of injectable alginate hydrogels to deliver cells in a targeted fashion is reviewed. In each example, the selected crosslinking approach, the cell type, the target tissue and the main findings of the study are highlighted.

Implanted neonatal human dermal fibroblasts influence the recruitment of endothelial cells in mice

The vascularization of new tissue within a reasonable time is a crucial prerequisite for the success of different cell- and material-based strategies. Considering that angiogenesis is a multi-step process involving humoral and cellular regulatory components, only in vivo assays provide the adequate information about vessel formation and the recruitment of endothelial cells. The present study aimed to investigate if neonatal human dermal fibroblasts could influence in vivo neovascularization. Results obtained showed that fibroblasts were able to recruit endothelial cells to vascularize the implanted matrix, which was further colonized by murine functional blood vessels after one week. The vessels exhibited higher levels of hemoglobin, compared with the control matrix, implanted without fibroblasts, in which no vessel formation could be observed. No significant differences were detected in systemic inflammation. The presence of vessels originated from the host vasculature suggested that host vascular response was involved, which constitutes a fundamental aspect in the process of neovascularization. Fibroblasts implanted within matrigel increased the presence of endothelial cells with positive staining for CD31 and for CD34 and the production of collagen influencing the angiogenic process and promoting the formation of microvessels. New strategies in tissue engineering could be delineated with improved angiogenesis using neonatal fibroblasts.

Effect of cell density on mesenchymal stem cells aggregation in RGD-alginate 3D matrices under osteoinductive conditions

Cellular activities in 3D are differentially affected by several matrix-intrinsic and extrinsic factors. This study highlights the relevance of optimizing initial cell densities when establishing 3D cultures for specific applications. Independently of the entrapping density, MSCs cultured within RGD-alginate hydrogels showed steady-state levels of metabolic activity and were in a nearly non-proliferative state, but recovered "normal" activity levels when retrieved from 3D matrices and re-cultured as monolayers. Importantly, high-densities promoted the establishment of cell-cell contacts with formation of multicellular clusters stabilized by endogenous ECM, and also stimulated MSCs osteogenic differentiation. These MSC-ECM microtissues may be used as building blocks for tissue engineering.

Sensory neurons and osteoblasts: close partners in a microfluidic platform

We presented a microfluidic-based coculture system as a new tool to be explored for modeling biological processes and pharmacological screening concerning peripheral tissues innervation.

Injectable MMP-sensitive alginate hydrogels as hMSC delivery systems

Hydrogels with the potential to provide minimally invasive cell delivery represent a powerful tool for tissue-regeneration therapies. In this context, entrapped cells should be able to escape the matrix becoming more available to actively participate in the healing process. Here, we analyzed the performance of proteolytically degradable alginate hydrogels as vehicles for human mesenchymal stem cells (hMSC) transplantation. Alginate was modified with the matrix metalloproteinase (MMP)-sensitive peptide Pro-Val-Gly-Leu-Iso-Gly (PVGLIG), which did not promote dendritic cell maturation in vitro, neither free nor conjugated to alginate chains, indicating low immunogenicity. hMSC were entrapped within MMP-sensitive and MMP-insensitive alginate hydrogels, both containing cell-adhesion RGD peptides. Softer (2 wt % alginate) and stiffer (4 wt % alginate) matrices were tested. When embedded in a Matrigel layer, hMSC-laden MMP-sensitive alginate hydrogels promoted more extensive outward cell migration and invasion into the tissue mimic. In vivo, after 4 weeks of subcutaneous implantation in a xenograft mouse model, hMSC-laden MMP-sensitive alginate hydrogels showed higher degradation and host tissue invasion than their MMP-insensitive equivalents. In both cases, softer matrices degraded faster than stiffer ones. The transplanted hMSC were able to produce their own collagenous extracellular matrix, and were located not only inside the hydrogels, but also outside, integrated in the host tissue. In summary, injectable MMP-sensitive alginate hydrogels can act as localized depots of cells and confer protection to transplanted cells while facilitating tissue regeneration.

In situ gelation for cell immobilization and culture in alginate foam scaffolds

Essential cellular functions are often lost under culture in traditional two-dimensional (2D) systems. Therefore, biologically more realistic three-dimensional (3D) cell culture systems are needed that provide mechanical and biochemical cues which may otherwise be unavailable in 2D. For the present study, an alginate-based hydrogel system was used in which cells in an alginate solution were seeded onto dried alginate foams. A uniform distribution of NIH:3T3 and NHIK 3025 cells entrapped within the foam was achieved by in situ gelation induced by calcium ions integrated in the foam. The seeding efficiency of the cells was about 100% for cells added in a seeding solution containing 0.1-1.0% alginate compared with 18% when seeded without alginate. The NHIK 3025 cells were allowed to proliferate and form multi-cellular structures inside the transparent gel that were later vital stained and evaluated by confocal microscopy. Gels were de-gelled at different time points to isolate the multi-cellular structures and to determine the spheroid growth rate. It was also demonstrated that the mechanical properties of the gel could largely be varied through selection of type and concentration of the applied alginate and by immersing the already gelled disks in solutions providing additional gel-forming ions. Cells can efficiently be incorporated into the gel, and single cells and multi-cellular structures that may be formed inside can be retrieved without influencing cell viability or contaminating the sample with enzymes. The data show that the current system may overcome some limitations of current 3D scaffolds such as cell retrieval and in situ cell staining and imaging.

Functionalization of biomaterials with small osteoinductive moieties

Human mesenchymal stem cells (MSCs) are currently recognized as a powerful cell source for regenerative medicine, notably for their capacity to differentiate into multiple cell types. The combination of MSCs with biomaterials functionalized with instructive cues can be used as a strategy to direct specific lineage commitment, and can thus improve the therapeutic efficacy of these cells. In terms of biomaterial design, one common approach is the functionalization of materials with ligands capable of directly binding to cell receptors and trigger specific differentiation signaling pathways. Other strategies focus on the use of moieties that have an indirect effect, acting, for example, as sequesters of bioactive ligands present in the extracellular milieu that, in turn, will interact with cells. Compared with complex biomolecules, the use of simple compounds, such as chemical moieties and peptides, and other small molecules can be advantageous by leading to less expensive and easily tunable biomaterial formulations. This review describes different strategies that have been used to promote substrate-mediated guidance of osteogenic differentiation of immature osteoblasts, osteoprogenitors and MSCs, through chemically conjugated small moieties, both in two- and three-dimensional set-ups. In each case, the selected moiety, the coupling strategy and the main findings of the study were highlighted. The latest advances and future perspectives in the field are also discussed.

Oxidized alginate hydrogels as niche environments for corneal epithelial cells

Chemical and biochemical modification of hydrogels is one strategy to create physiological constructs that maintain cell function. The aim of this study was to apply oxidised alginate hydrogels as a basis for development of a biomimetic niche for limbal epithelial stem cells that may be applied to treating corneal dysfunction. The stem phenotype of bovine limbal epithelial cells (LEC) and the viability of corneal epithelial cells (CEC) were examined in oxidised alginate gels containing collagen IV over a 3-day culture period. Oxidation increased cell viability (P ≤ 0.05) and this improved further with addition of collagen IV (P ≤ 0.01). Oxidised gels presented larger internal pores (diameter: 0.2-0.8 µm) than unmodified gels (pore diameter: 0.05-0.1 µm) and were significantly less stiff (P ≤ 0.001), indicating that an increase in pore size and a decrease in stiffness contributed to improved cell viability. The diffusion of collagen IV from oxidised alginate gels was similar to that of unmodified gels suggesting that oxidation may not affect the retention of extracellular matrix proteins in alginate gels. These data demonstrate that oxidised alginate gels containing corneal extracellular matrix proteins can influence corneal epithelial cell function in a manner that may impact beneficially on corneal wound healing therapy.