Soft-tissue augmentation with injectable alginate and syngeneic fibroblasts

High-temperature emulsification coupled with low-temperature gelation for fabrication of agarose microsphere implants with well-controlled size for skin tissue enhancement

Soft tissue deficiencies profoundly impact the daily lives, and mental well-being of patients. Microspheres facilitate collagen synthesis by establishing a conducive environment for fibroblast growth. Existing synthetic polymer microspheres are typically prepared through emulsification and cross-linking at normal temperatures. However, residues of cross-linking agents can adversely affect biocompatibility, thereby limiting their biomedical applications. Agarose has garnered significant attention owing to its biodegradability and excellent biocompatibility. We have for the first time developed high-temperature emulsification coupled with low-temperature gelation for fabrication of agarose microsphere implants with well-controlled size for skin tissue enhancement. The agarose microspheres exhibited favorable sphericity and dispersion, possessing a uniform particle size with an average diameter of 37.24 μm. Furthermore, the microspheres demonstrated commendable injectability and biodegradability. Additionally, the implants displayed remarkable biocompatibility, effectively promoting the proliferation of human foreskin fibroblast-1 (HFF-1) cells. The microspheres exhibited no systemic toxicity and induced no hemolytic or thermogenic reactions. In photoaged mice skin models, the agarose microspheres augmented dermal density, and enhanced skin elasticity. The microspheres showed the capacity to stimulate the regeneration of collagen fibers. The agarose microspheres offer a novel avenue for soft tissue filling and hold significance in the field of tissue engineering and skin regeneration.

Bovine pulp extracellular matrix hydrogel for regenerative endodontic applications: in vitro characterization and in vivo analysis in a necrotic tooth model

BACKGROUND

Regenerative endodontic procedures (REPs) offer the promise of restoring vitality and function to a previously necrotic and infected tooth. However, the nature of regenerated tissues following REPs remains unpredictable and uncontrollable. Decellularized extracellular matrix scaffolds have gained recent attention as scaffolds for regenerative endodontics.

OBJECTIVES

Preparation and characterization of a bovine dental pulp-derived extracellular matrix (P-ECM) hydrogel for regenerative endodontic applications. Biocompatibility and regenerative capacity of the prepared scaffold were evaluated in vivo in a canine animal model.

METHODS

Fifteen freshly extracted bovine molar teeth were used to prepare P-ECM hydrogels following approval of the institutional review board of the faculty of dentistry, Alexandria University. Decellularization and lyophilization of the extracted pulp tissues, DNA quantification and histological examination of decellularized P-ECM were done. P-ECM hydrogel was prepared by digestion of decellularized pulps. Prepared scaffolds were evaluated for protein content and release as well as release of VEGF, bFGF, TGF-β1 and BMP2 using ELISA. Rabbit dental pulp stem cells' (rDPSCs) viability in response to P-ECM hydrogels was performed. Finally, proof-of-concept of the regenerative capacity of P-ECM scaffolds was assessed in an infected mature canine tooth model following REPs versus blood clot (BC), injectable platelet-rich fibrin (i-PRF) or hyaluronic acid (HA). Statistical analysis was done using independent t test, the Friedman test and chi-square tests (p value ≤ 0.05).

RESULTS

DNA was found to be below the cut-off point (50 ng/mg tissue). Histological evaluation revealed absence of nuclei, retention of glycosaminoglycans (GAGs) and collagen content, respectively. P-ECM hydrogel had a total protein content of (493.12 µg/µl) and protein release was detected up to 14 days. P-ECM hydrogel also retained VEGF, bFGF, TGF-β1 and BMP2. P-ECM hydrogel maintained the viability of rDPSCs as compared to cells cultured under control conditions. P-ECM hydrogel triggered more organized tissues compared to BC, i-PRF and HA when used in REPs for necrotic mature teeth in dogs. Periapical inflammation was significantly less in HA and P-ECM groups compared to blood-derived scaffolds.

CONCLUSION

Bovine dental pulp-derived extracellular matrix (P-ECM) hydrogel scaffold retained its bioactive properties and demonstrated a promising potential in regenerative endodontic procedures compared to conventional blood-derived scaffolds.

The histologic reaction and permanence of hyaluronic acid gel, calcium hydroxylapatite microspheres, and extracellular matrix bio gel

BACKGROUND

The filling materials on the beauty market can be classified into three types: natural biological materials, synthetic polymer materials, and composites containing bioactive substances. However, comparative experimental data is lacking to compare their biological responses and permanence.

AIMS

The main object of this study was to evaluate the biological response of these three types of fillers to provide a theoretical basis for clinical application.

METHODS

Six-week-old female mice were injected subcutaneously with hyaluronic acid (HA) gel, calcium hydroxylapatite (CaHA) microspheres, and extracellular matrix (ECM) bio gel to observe the body reaction and permanence. At 1, 4, 8, and 16 weeks, the test sites were excised and analyzed by histopathology and proteomics.

RESULTS

Extracellular matrix had a minimal foreign body response. HA had a good volume effect at the early stage but the volume retention rate was lower than CaHA in the long term. CaHA could stimulate neo-collagen formation.

CONCLUSION

This study has proven the effectiveness and safety of these fillers and could provide clinical guidance for the plastic surgeon.

The Regenerative Potential of bFGF in Dental Pulp Repair and Regeneration

Regenerative endodontic therapy intends to induce the host’s natural wound-healing process, which can restore the vitality, immunity, and sensitivity of the inflammatory or necrotic pulp tissue destroyed by infection or trauma. Myriads of growth factors are critical in the processes of pulp repair and regeneration. Among the key regulatory factors are the fibroblast growth factors, which have turned out to be the master regulators of both organogenesis and tissue homeostasis. Fibroblast growth factors, a family composed of 22 polypeptides, have been used in tissue repair and regeneration settings, in conditions as diverse as burns, ulcers, bone-related diseases, and spinal cord injuries. Meanwhile, in dentistry, the basic fibroblast growth factor is the most frequently investigated. Thereby, the aim of this review is 2-fold: 1) foremost, to explore the underlying mechanisms of the bFGF in dental pulp repair and regeneration and 2) in addition, to shed light on the potential therapeutic strategies of the bFGF in dental pulp–related clinical applications.

Flexible Adipose-Vascular Tissue Assembly Using Combinational 3D Printing for Volume-Stable Soft Tissue Reconstruction

A new concept, assembling cell-laden tissue modules, is for the first time proposed for soft tissue engineering. Adipose-vascular tissue modules composed of a synthetic polymer-based substructure and customized bioinks using planar 3D cell printing are engineered. Such tissue modules are systematically assembled into a synthetic polymer-based module holder fabricated with rotational 3D printing, resulting in the development of a flexible and volumetric tissue assembly. Whereas most of the previous studies about the construction of adipose tissue are limited to hypoxia, poor vascularization, rapid resorption, and mismatch in mechanical properties, it is aimed to realize the construction of nonhypoxic, flexible, and volume-stable tissue assembly in this study. The significance of engineered tissue assembly is proven through various in vitro and in vivo evaluations. In particular, stable volume and remarkable neovascularization/adipogenesis are observed in the implanted assembly over four weeks. Interestingly, the size of newly formed lipid droplets and the remodeled morphology in the assembly are comparable to those in native adipose tissue. As far as it is known, this work is a first report suggesting a cell printing-based tissue assembly for functional reconstruction of soft tissue.

Effect of bFGF and fibroblasts combined with hyaluronic acid-based hydrogels on soft tissue augmentation: an experimental study in rats

Background

Hyaluronic acid (HA) has been applied as a primary biomaterial for temporary soft tissue augmentation and as a carrier for cells and the delivery of growth factors to promote tissue regeneration. Although HA derivatives are the most versatile soft tissue fillers on the market, they are resorbed early, within 3 to 12 months. To overcome their short duration, they can be combined with cells or growth factors. The purpose of this study was to investigate the stimulating effects of human fibroblasts and basic fibroblast growth factors (bFGF) on collagen synthesis during soft tissue augmentation by HA hydrogels and to compare these with the effects of a commercial HA derivative (Restylane®).

Methods

The hydrogel group included four conditions. The first condition consisted of hydrogel (H) alone as a negative control, and the other three conditions were bFGF-containing hydrogel (HB), human fibroblast-containing hydrogel (HF), and human fibroblast/bFGF-containing hydrogel (HBF). In the Restylane® group (HGF), the hydrogel was replaced with Restylane® (R, RB, RF, RBF). The gels were implanted subdermally into the back of each nude mouse at four separate sites. Twelve nude mice were used for the hydrogel (n = 6) and Restylane® groups (n = 6). The specimens were harvested 8 weeks after implantation and assessed histomorphometrically, and collagen synthesis was evaluated by RT-PCR.

Results

The hydrogel group showed good biocompatibility with the surrounding tissues and stimulated the formation of a fibrous matrix. HBF and HF showed significantly higher soft tissue synthesis compared to H (p < 0.05), and human collagen type I was well expressed in HB, HF, and HBF; HBF showed the strongest expression. The Restylane® filler was surrounded by a fibrous capsule without any soft tissue infiltration from the neighboring tissue, and collagen synthesis within the Restylane® filler could not be observed, even though no inflammatory reactions were observed.

Conclusion

This study revealed that HA-based hydrogel alone or hydrogel combined with fibroblasts and/or bFGF can be effectively used for soft tissue augmentation.

Hyaluronic acid in tobacco-exposed rats. Inflammatory reaction, and duration of effect1

PURPOSE

To evaluate the hyaluronic acid (HA) inflammatory reaction, fibroblasts, fibrosis and duration of effect in the dorsal region of tobacco-exposed rats.

METHODS

Ten Wistar rats were divided into two groups: tobacco-exposed-group (TEG;n=5) and air-control-group (CG;n=5). The TEG animals were tobacco-exposed twice a day, 30-minutes/session, during 60 days. After this period, all animals received 0.1 mL HA subcutaneous injection in the dorsal area. The volume of HA was measured immediately after HA injection and weekly using a hand-caliper in nine weeks. After this period, all the animals were euthanized, and a specimen of was collected to evaluate inflammatory cells, fibroblasts, and fibrosis by HE.

RESULTS

This study showed a higher inflammatory reaction in TEG than CG: inflammatory cell-count (CG: 1.07±0.9; TEG: 8.61±0.36, p<0.001); fibroblast count (CG: 2.92±0.17; TEG: 19.14±0.62, p<0.001), and fibrosis quantification (CG: 2.0; TEG: 3.75, p<0.001). The analysis of the HA volume in nine weeks in the dorsal region did not show a difference between groups (p=0.39).

CONCLUSIONS

This study suggested that the HA injection in the TEG caused an increase in inflammatory cell count, fibroblast, and fibrosis quantification when compared to the CG. There was no difference in the duration of effect of HA between the groups.

Spheroid Culture System Confers Differentiated Transcriptome Profile and Functional Advantage to 3T3-L1 Adipocytes

This study highlights functional differences between 2-D monolayer and 3-D spheroid 3T3-L1 adipocyte culture models and explores the underlying genomic mechanisms responsible for the different phenotypes present. The spheroids showed higher triglyceride accumulation than the monolayer culture and further increase with larger spheroid size. Whole transcriptome analysis indicated significant differential expression of genes related to adipogenesis, including adipocytokine signaling, fatty acid metabolism, and PPAR-γ signaling. Spheroids also showed downregulation of matrix metalloproteinases (MMPs), integrin, actin-cytoskeleton associated genes, and Rho/GTPase3 expression relative to 2-D monolayer, indicating suppression of the Rho-ROCK pathway and thereby promoting adipogenic differentiation. When exposed to linoleic acid (500 μM) and TNF-α (125 ng/mL) to promote chronic adiposity, linoleic acid treatment resulted in increased intracellular triglycerides and subsequent TNF-α treatment resulted in significantly altered adipocytokine signaling, fatty acid metabolism, and PPAR signaling, in addition to upregulation of multiple MMPs in spheroids vs. monolayer. Overall, 3-D spheroids showed enhanced adipogenic phenotype as indicated by triglyceride synthesis and transcriptome changes while retaining sensitivity to a pro-inflammatory stimulus. The 3-D spheroid culture thus may provide a simple, convenient, and sensitive in vitro model to study adipocyte response to metabolic stresses relevant to clinical pathologies.

Three-dimensional macroporous materials for tissue engineering of craniofacial bone

Repair of critical-size defects caused by trauma, removal of a tumour, or congenital abnormalities is a challenge in the craniomaxillofacial region because of the limitations associated with treatment. We have reviewed research papers and updated information relevant to the various types of macroporous scaffolds. We have included papers on several biomaterials and their use in various craniofacial defects such as mandibular, calvarial, and others, as well as the latest technological developments such as 3-dimensional printed scaffolds. We selected all papers about scaffolds, stem cells, and growth factors for review. Initial selection was by review of titles and abstracts, and the full texts of potentially suitable articles were then assessed. Methods of tissue engineering for repair of critical-size defects in the craniofacial bones seem to be viable options for surgical treatment in the future. Macroporous scaffolds with interconnected pores are of great value in regeneration of bone in the craniofacial region. In recent years, various natural or synthetic materials, or both, have been developed, on which macroporous scaffolds can be based. In this review we present a review on the various types of three-dimensional macroporous scaffolds that have been developed in recent years, and evaluate their potential for regeneration of craniofacial bone.

Autologous Adipose-Derived Tissue Matrix Part II: Implantation Biology

BACKGROUND

In part 1 of this study it was shown that liposuctioned fat could be a sufficient source of autologous collagen for use as a filler or in reconstruction. The collagen composition in liposuctioned fat was shown to form a cross-linked helical matrix composed of types II, III, and IV. Additionally, viable adipocytes and fibroblasts among other cells were found.

OBJECTIVES

The purpose of this research was to study the biology of this matrix after subsequent implantation compared to Juvederm (Allergan, Parsippany, NJ) common soft tissue filler.

METHODS

Fat was obtained from individuals undergoing routine liposuction and was processed by a two-step process to obtain a connective tissue matrix. The matrix was then cryo-frozen for a minimum of 4 weeks after which it was thawed and implanted in 46 nude mice. Juvederm Ultra was used as the control article and the animals followed for one year.

RESULTS

Liposuctioned fat was obtained from 10 individuals and processed as previously described. Mice were harvested at 3, 6, 9, and 12 months and histology obtained. There were no adverse effects from either article and the bio-reactivity rating was 0. The implanted collagen compared favorably to Juvederm at all stages and was found to be replaced by new collagen and fat.

CONCLUSIONS

A collagen matrix with viable cells for autologous use can be obtained from liposuctioned fat which has been processed and cryo-frozen. The material lasts at least one year and is slowly replaced by new collagenand fat.

LEVEL OF EVIDENCE

5.

Nanostructured injectable cell microcarriers for tissue regeneration

Biodegradable polymer microspheres have emerged as cell carriers for the regeneration and repair of irregularly shaped tissue defects due to their injectability, controllable biodegradability and capacity for drug incorporation and release. Notably, recent advances in nanotechnology allowed the manipulation of the physical and chemical properties of the microspheres at the nanoscale, creating nanostructured microspheres mimicking the composition and/or structure of natural extracellular matrix. These nanostructured microspheres, including nanocomposite microspheres and nanofibrous microspheres, have been employed as cell carriers for tissue regeneration. They enhance cell attachment and proliferation, promote positive cell-carrier interactions and facilitate stem cell differentiation for target tissue regeneration. This review highlights the recent advances in nanostructured microspheres that are employed as injectable, biomimetic and cell-instructive cell carriers.

Sodium Deoxycholate Hydrogels: Effects of Modifications on Gelation, Drug Release, and Nanotemplating

In the present study, sodium deoxycholate (NaDC) was used to produce gelation of tris(hydroxymethyl)amino-methane (TRIS) solutions above, below, and near the pKa of NaDC, respectively, which yielded a neutral gelator, a charged gelator, and a mixture of each. Impacts of ionic interactions on gel formation were studied in detail and showed that pH can be used to modify many hydrogel properties including sol-gel temperature, crystallinity, and mechanical strength. Several formulations yielded a unique rheological finding of two stable regions of elastic modulus. The release of a small molecule has been investigated under different hydrogel conditions and at variable shear rate, suggesting utility as a drug-delivery vehicle. It was also observed that pH modification of the hydrogels affected nanoparticle formation. Nanoparticles derived from a Group of Uniform Materials Based on Organic Salts (nanoGUMBOS), specifically cyanine-based NIR dyes, were templated within the hydrogel network for potential applications in tissue imaging. These nanoGUMBOS were found to be size-tunable, although material-dependent. Further understanding of NaDC/TRIS gelation has broadened the tunability and multidimensional applications of these tailored hydrogel systems.

A First Step in De Novo Synthesis of a Living Pulp Tissue Replacement Using Dental Pulp MSCs and Tissue Growth Factors, Encapsulated within a Bioinspired Alginate Hydrogel

INTRODUCTION

A living, self-supporting pulp tissue replacement in vitro and for transplantation is an attractive yet unmet bioengineering challenge. Our aim is to create 3-dimensional alginate-based microenvironments that replicate the shape of gutta-percha and comprise key elements for the proliferation of progenitor cells and the release of growth factors.

METHODS

An RGD-bearing alginate framework was used to encapsulate dental pulp stem cells and human umbilical vein endothelial cells in a ratio of 1:1. The alginate hydrogel also retained and delivered 2 key growth factors, vascular endothelial growth factor-121 and fibroblast growth factor, in a sufficient amount to induce proliferation. A method was then devised to replicate the shape of gutta-percha using RGD alginate within a custom-made mold of thermoresponsive N-isopropylacrylamide. Plugs of alginate containing different permutations of growth factor-based encapsulates were tested and evaluated for viability, proliferation, and release kinetics between 1 and 14 days.

RESULTS

According to scanning electron microscopic and confocal microscopic observations, the encapsulated human endothelial cells and dental pulp stem cell distribution were frequent and extensive throughout the length of the construct. There were also high levels of viability in all test environments. Furthermore, cell proliferation was higher in the growth factor-based groups. Growth factor release kinetics also showed significant differences between them. Interestingly, the combination of vascular endothelial growth factor and fibroblast growth factor synergize to significantly up-regulate cell proliferation.

CONCLUSIONS

RGD-alginate scaffolds can be fabricated into shapes to fill the pulp space by simple templating. The addition of dual growth factors to cocultures of stem cells within RGD-alginate scaffolds led to the creation of microenvironments that significantly enhance the proliferation of dental pulp stem cell/human umbilical vein endothelial cell combinations.

Using bimodal MRI/fluorescence imaging to identify host angiogenic response to implants

Therapies that promote angiogenesis have been successfully applied using various combinations of proangiogenic factors together with a biodegradable delivery vehicle. In this study we used bimodal noninvasive monitoring to show that the host response to a proangiogenic biomaterial can be drastically affected by the mode of implantation and the surface area-to-volume ratio of the implant material. Fluorescence/MRI probes were covalently conjugated to VEGF-bearing biodegradable PEG-fibrinogen hydrogel implants and used to document the in vivo degradation and liberation of bioactive constituents in an s.c. rat implantation model. The hydrogel biodegradation and angiogenic host response with three types of VEGF-bearing implant configurations were compared: preformed cylindrical plugs, preformed injectable microbeads, and hydrogel precursor, injected and polymerized in situ. Although all three were made with identical amounts of precursor constituents, the MRI data revealed that in situ polymerized hydrogels were fully degraded within 2 wk; microbead degradation was more moderate, and plugs degraded significantly more slowly than the other configurations. The presence of hydrogel degradation products containing the fluorescent label in the surrounding tissues revealed a distinct biphasic release profile for each type of implant configuration. The purported in vivo VEGF release profile from the microbeads resulted in highly vascularized s.c. tissue containing up to 16-fold more capillaries in comparison with controls. These findings demonstrate that the configuration of an implant can play an important role not only in the degradation and resorption properties of the materials, but also in consequent host angiogenic response.

Adipose tissue regeneration in vivo using micronized acellular allogenic dermis as an injectable scaffold

Over the past few years, the clinical use of injectable artificial materials in plastic surgery has increased. In addition, autologous lipoimplantation is being performed for volume replacement of soft tissue for aesthetic purposes. In this study, acellular allogenic dermis was utilized as a scaffold for the culturing of preadipocytes, confirming the possibility of three-dimensional proliferation of progenitor cells, the eventual differentiation of stromal cells in adipose tissue into the adipocytes, and the in vivo implantation of such adipocytes to form fat tissue. Preadipocytes, recently called ASCs (adipose tissue-derived stromal/stem cells), were cultured in acellular allogenic dermis, successfully attached to the dermal particles in a three-dimensional structure, and proliferated, differentiated, and eventually formed a cluster. For the in vivo implantation, four groups were formed: the first group was cultured within the dermal scaffold for 24 h before implantation (24-h preconditioned group), the second group was induced for differentiation for 10 days before implantation (10-day preconditioned group), the third group was implanted immediately after cell propagation (nonpreconditioned group), and the control group was implanted with only dermal scaffold. In vivo implanted preadipocytes showed great differentiation into adipocytes within the dermal scaffolds. Also, the 10-day preconditioned group showed a greater volume of fat tissue compared to the 24-h preconditioned group. From these results, we confirmed that after a three-dimensional culture in acellular allogenic dermis, implanted preadipocytes formed a greater amount of fat tissue and that this could be a possible effective method for future soft tissue restoration.

Dual-stage growth factor release within 3D protein-engineered hydrogel niches promotes adipogenesis

Engineered biomimetic microenvironments from hydrogels are an emerging strategy to achieve lineage-specific differentiation in vitro. In addition to recapitulating critical matrix cues found in the native three-dimensional (3D) niche, the hydrogel can also be designed to deliver soluble factors that are present within the native inductive microenvironment. We demonstrate a versatile materials approach for the dual-stage delivery of multiple soluble factors within a 3D hydrogel to induce adipogenesis. We use a Mixing-Induced Two-Component Hydrogel (MITCH) embedded with alginate microgels to deliver two pro-adipogenic soluble factors, fibroblast growth factor 1 (FGF-1) and bone morphogenetic protein 4 (BMP-4) with two distinct delivery profiles. We show that dual-stage delivery of FGF-1 and BMP-4 to human adipose-derived stromal cells (hADSCs) significantly increases lipid accumulation compared with the simultaneous delivery of both growth factors together. Furthermore, dual-stage growth factor delivery within a 3D hydrogel resulted in substantially more lipid accumulation compared to identical delivery profiles in 2D cultures. Gene expression analysis shows upregulation of key adipogenic markers indicative of brown-like adipocytes. These data suggest that dual-stage release of FGF-1 and BMP-4 within 3D microenvironments can promote the in vitro development of mature adipocytes.

A surface-tethered spheroid model for functional evaluation of 3T3-L1 adipocytes

In order to effectively treat obesity, it must be better understood at the cellular level with respect to metabolic state and environmental stress. However, current two-dimensional (2D) in vitro cell culture methods do not represent the in vivo adipose tissue appropriately due to the absence of complex architecture and cellular signaling. Conversely, 3D in vitro cultures have been reported to have optimal results mimicking the adipose tissue in vivo. The main aim of this study was to examine the efficacy of a novel conjugate of a genetically engineered polymer, elastin-like polypeptide (ELP) and a synthetic polymer, polyethyleneimine (PEI), toward creating a 3D preadipocyte culture system. We then used this 3D culture model to study the preadipocyte differentiation and adipocyte maintenance processes when subjected to various dosages of nutritionally relevant free fatty acids with respect to total DNA and protein content, cell viability, and intracellular triglyceride accumulation. Our results showed that 3T3-L1 preadipocytes cultured on the ELP-PEI surface formed 3D spheroids within 72 h, whereas the cells cultured on unmodified tissue culture polystyrene surfaces remained in monolayer configuration. Significant statistical differences were discovered between the 3D spheroid and 2D monolayer culture with respect to the DNA and protein content, fatty acid consumption, and triglyceride accumulation, indicating differences in cellular response. Results indicated that the 3D culture may be a more sensitive modeling technique for in vitro adipocyte culture and provides a platform for future evaluation of 3D in vitro adipocyte function.

Adipose tissue engineering with cells in engineered matrices

Tissue engineering has shown promise for the development of constructs to facilitate large volume soft tissue augmentation in reconstructive and cosmetic plastic surgery. This article reviews the key progress to date in the field of adipose tissue engineering. In order to effectively design a soft tissue substitute, it is critical to understand the native tissue environment and function. As such, the basic physiology of adipose tissue is described and the process of adipogenesis is discussed. In this article, we have focused on tissue engineering using a cell-seeded scaffold approach, where engineered extracellular matrix substitutes are seeded with exogenous cells that may contribute to the regenerative response. The strengths and limitations of each of the possible cell sources for adipose tissue engineering, including adipose-derived stem cells, are detailed. We briefly highlight some of the results from the major studies to date, involving a range of synthetic and naturally derived scaffolds. While these studies have shown that adipose tissue regeneration is possible, more research is required to develop optimized constructs that will facilitate safe, predictable and long-term augmentation in clinical applications.

New perspectives in cell delivery systems for tissue regeneration: natural-derived injectable hydrogels

Natural polymers, because of their biocompatibility, availability, and physico-chemical properties have been the materials of choice for the fabrication of injectable hydrogels for regenerative medicine. In particular, they are appealing materials for delivery systems and provide sustained and controlled release of drugs, proteins, gene, cells, and other active biomolecules immobilized.In this work, the use of hydrogels obtained from natural source polymers as cell delivery systems is discussed. These materials were investigated for the repair of cartilage, bone, adipose tissue, intervertebral disc, neural, and cardiac tissue. Papers from the last ten years were considered, with a particular focus on the advances of the last five years. A critical discussion is centered on new perspectives and challenges in the regeneration of specific tissues, with the aim of highlighting the limits of current systems and possible future advancements.

Adipogenic differentiation of scaffold-bound human adipose tissue-derived stem cells (hASC) for soft tissue engineering

Adipose tissue engineering, instead of tissue substitution, often uses autologous adipose tissue-derived stem cells (hASC). These cells are known to improve graft integration and to support neovascularization of scaffolds when seeded onto biomaterials. In this study we thought to engineer adipose tissue using scaffold-bound hASC, since they can be differentiated into the adipocyte cell lineage and used for soft tissue regeneration. We show here by microscopy and gene expression of the peroxysome proliferator-activated receptor gene (PPARγ2) that hASC growing on polypropylene fibrous scaffolds as well as on three-dimensional nonwoven scaffolds can be turned into adipose tissue within 19 days. Freshly isolated hASC displayed a higher differentiation potential than hASC cultured for eight passages. In addition, we proved a modified alginate microcapsule to directly induce adipogenic differentiation of incorporated hASC. The results may help to improve long-term success of adipose tissue regeneration, especially for large-scale soft tissue defects, and support the development of cell-scaffold combinations which can be shaped individually and directly induce the adipogenic differentiation of incorporated hASC at the site of implantation.

Layer-shaped alginate hydrogels enhance the biological performance of human adipose-derived stem cells

BACKGROUND

The reconstruction of adipose tissue defects is often challenged by the complications that may occur following plastic and reconstructive surgery, including donor-site morbidity, implant migration and foreign body reaction. To overcome these problems, adipose tissue engineering (ATE) using stem cell-based regeneration strategies has been widely explored in the last years. Mounting evidence has shown that adipose-derived stem cells (ADSCs) represent a promising cell source for ATE. In the context of a small number of reports concerning adipose tissue regeneration using three-dimensional (3-D) systems, the present study was designed to evaluate the biological performance of a novel alginate matrix that incorporates human ADSCs (hADSCs).

RESULTS

Culture-expanded cells isolated from the stromal vascular fraction (SVF), corresponding to the third passage which showed the expression of mesenchymal stem cell (MSC) markers, were used in the 3-D culture systems. The latter represented a calcium alginate hydrogel, obtained by the diffusion of calcium gluconate (CGH matrix), and shaped as discoid-thin layer. For comparative purposes, a similar hADSC-laden alginate hydrogel cross-linked with calcium chloride was considered as reference hydrogel (RH matrix). Both hydrogels showed a porous structure under scanning electron microscopy (SEM) and the hADSCs embedded displayed normal spherical morphologies, some of them showing signs of mitosis. More than 85% of the entrapped cells survived throughout the incubation period of 7 days. The percentage of viable cells was significantly higher within CGH matrix at 2 days post-seeding, and approximately similar within both hydrogels after 7 days of culture. Moreover, both alginate-based hydrogels stimulated cell proliferation. The number of hADSC within hydrogels has increased during the incubation period of 7 days and was higher in the case of CGH matrix. Cells grown under adipogenic conditions for 21 days showed that both analyzed 3-D culture systems support adipogenic differentiation in terms of neutral lipid accumulation and perillipin expression. Furthermore, the cells encapsulated in CGH matrix displayed a more differentiated phenotype.

CONCLUSIONS

The results of this study suggest that both CGH and RH matrices successfully support the survival and adipogenesis of hADSC. An enhancement of biological performance was detected in the case of CGH matrix, suggesting its promising application in ATE.

Injectable biomaterials for adipose tissue engineering

Adipose tissue engineering has recently gained significant attention from materials scientists as a result of the exponential growth of soft tissue filler procedures being performed within the clinic. While several injectable materials are currently being marketed for filling subcutaneous voids, they often face limited longevity due to rapid resorption. Their inability to encourage natural adipose formation or ingrowth necessitates repeated injections for a prolonged effect and thus classifies them as temporary fillers. As a result, a significant need for injectable materials that not only act as fillers but also promote in vivo adipogenesis is beginning to be realized. This paper will discuss the advantages and disadvantages of commercially available soft tissue fillers. It will then summarize the current state of research using injectable synthetic materials, biopolymers and extracellular matrix-derived materials for adipose tissue engineering. Furthermore, the successful attributes observed across each of these materials will be outlined along with a discussion of the current difficulties and future directions for adipose tissue engineering.

Tissue augmentation with fibrin sealant and cultured fibroblasts: a preliminary study

BACKGROUND

Nonoperative subdermal tissue augmentation is one of the most frequently performed procedures in plastic surgery and dermatological practice. Many products, from biological to synthetic filler substances, are currently available. However, none has achieved ideal clinical efficacy, especially regarding volume maintenance and longevity. We examined the use of fibrin sealant as a biological and fully degradable matrix for dermal augmentation in combination with precultured human fibroblasts and hyaluronic acid gel (HYAFF).

METHODS

Four implant preparations were studied: fibrin glue only (F); 1% HYAFF mixed in fibrin glue (FH); 1.8 × 10(6) cells/ml of fibrin glue (FC); and 1% HYAFF and 1.8 × 10(6) cells/ml of fibrin glue (FHC). Each mouse was given two separate subcutaneous injections of implant material. At 1, 3, and 6 weeks two mice from each group were sacrificed, such that there was an n = 4 for each implant group at each time point. The mice were grossly examined for implant retention and the implants were evaluated by means of immunohistochemistry for fibrosis, integration into surrounding tissue, presence of elastin, and blood vessel infiltration.

RESULTS

Only the implants in the cell-containing groups, FC and FHC, remained after 6 weeks. Moreover, with the exception of a mild inflammatory response, no adverse affects of the cell-seeded implants were noted.

CONCLUSION

Presence of fibroblasts increases implant durability. Further studies should evaluate the ideal hyaluronic acid and fibroblast concentration for long-term longevity.

Injectable hydrogel scaffold from decellularized human lipoaspirate

Soft tissue fillers are rapidly gaining popularity for aesthetic improvements or repair of adipose tissue deficits. Several injectable biopolymers have been investigated for this purpose, but often show rapid resorption or limited adipogenesis and do not mimic the native adipose extracellular matrix (ECM). We have generated an injectable adipose matrix scaffold by efficiently removing both the cellular and lipid contents of human lipoaspirate. The decellularized material retained the complex composition of peptides and glycosaminoglycans found in native adipose ECM. This matrix can be further processed by solubilizing the extracted ECM to generate a thermally responsive hydrogel that self-assembles upon subcutaneous injection. This hydrogel also supports the growth and survival of patient matched adipose-derived stem cells in vitro. The development of an injectable hydrogel from human lipoaspirate represents a minimally invasive option for adipose tissue engineering in terms of both the collection of source material and delivery of the scaffold.

Gelatinized copper-capillary alginate gel functions as an injectable tissue scaffolding system for stem cell transplants

In severe hypoxic-ischemic brain injury, cellular components such as neurons and astrocytes are injured or destroyed along with the supporting extracellular matrix. This presents a challenge to the field of regenerative medicine since the lack of extracellular matrix and supporting structures makes the transplant milieu inhospitable to the transplanted cells. A potential solution to this problem is the use of a biomaterial to provide the extracellular components needed to keep cells localized in cystic brain regions, allowing the cells to form connections and repair lost brain tissue. Ideally, this biomaterial would be combined with stem cells, which have been proven to have therapeutic potentials, and could be delivered via an injection. To study this approach, we derived a hydrogel biomaterial tissue scaffold from oligomeric gelatin and copper-capillary alginate gel (GCCAG). We then demonstrated that our multipotent astrocytic stem cells (MASCs) could be maintained in GCCAG scaffolds for up to 2 weeks in vitro and that the cells retained their multipotency. We next performed a pilot transplant study in which GCCAG was mixed with MASCs and injected into the brain of a neonatal rat pup. After a week in vivo, our results showed that: the GCCAG biomaterial did not cause a significant reactive gliosis; viable cells were retained within the injected scaffolds; and some delivered cells migrated into the surrounding brain tissue. Therefore, GCCAG tissue scaffolds are a promising, novel injectable system for transplantation of stem cells to the brain.

Modular injectable matrices based on alginate solution/microsphere mixtures that gel in situ and co-deliver immunomodulatory factors

Biocompatible polymer solutions that can crosslink in situ following injection to form stable hydrogels are of interest as depots for sustained delivery of therapeutic factors or cells, and as scaffolds for regenerative medicine. Here, injectable self-gelling alginate formulations obtained by mixing alginate microspheres (as calcium reservoirs) with soluble alginate solutions were characterized for potential use in immunotherapy. Rapid redistribution of calcium ions from microspheres into the surrounding alginate solution led to crosslinking and formation of stable hydrogels. The mechanical properties of the resulting gels correlated with the concentration of calcium-reservoir microspheres added to the solution. Soluble factors such as the cytokine interleukin-2 were readily incorporated into self-gelling alginate matrices by simply mixing them with the formulation prior to gelation. Using alginate microspheres as modular components, strategies for binding immunostimulatory CpG oligonucleotides onto the surface of microspheres were also demonstrated. When injected subcutaneously in the flanks of mice, self-gelling alginate formed soft macroporous gels supporting cellular infiltration and allowing ready access to microspheres carrying therapeutic factors embedded in the matrix. This in situ gelling formulation may thus be useful for stimulating immune cells at desired locales, such as solid tumors or infection sites, as well as for other soft tissue regeneration applications.

Bioengineering strategies to generate vascularized soft tissue grafts with sustained shape

Tissue engineering offers the possibility for soft tissue reconstruction and augmentation without autologous grafting or conventional synthetic materials. Two critical challenges have been addressed in a number of recent studies: a biology challenge of angiogenesis and an engineering challenge of shape maintenance. These two challenges are inter-related and are effectively addressed by integrated bioengineering strategies. Recently, several integrated bioengineering strategies have been applied to improve bioengineered adipose tissue grafts, including internalized microchannels, delivery of angiogenic growth factors, tailored biomaterials and transplantation of precursor cells with continuing differentiation potential. Bioengineered soft tissue grafts are only clinically meaningful if they are vascularized, maintain shape and dimensions, and remodel with the host. Ongoing studies have begun to demonstrate the feasibility towards an ultimate goal to generate vascularized soft tissue grafts that maintain anatomically desirable shape and dimensions.

pH-Responsive microgel dispersions for repairing damaged load-bearing soft tissue

An important challenge for colloid scientists is to design injectable dispersions that provide structural support for damaged soft tissue and enable regeneration of tissue over the longer term. In this article we highlight a new area of research that aims to produce pH-responsive microgel dispersions that restore the mechanical properties of damaged, load-bearing, soft tissue. Chronic back pain due to degeneration of the intervertebral disc (IVD) is a major health problem and is the primary potential application for the work discussed. pH-Responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the incorporated ionic co-monomer. The work considered here involves microgel particles containing MAA (methacrylic acid). The particles show pronounced pH-triggered swelling. The concentrated microgel dispersions change from a fluid to a gel at pH values greater than ca. 6.2, which is within the physiological pH range. The rheological properties are pH-dependent and can be adjusted using particle composition or concentration. Degenerated IVDs containing injected, gelled, microgel dispersions show improved mechanical properties. The disc height under biomechanically meaningful loads can be restored to values observed in non-degenerated IVDs. We also discuss the steps required to provide a minimally invasive injectable microgel system for restoring both the IVD mechanical properties and regenerating tissue in vivo. The approach discussed should also be suitable for other soft tissue types in the body.

Study of pH-responsive microgels containing methacrylic acid: effects of particle composition and added calcium

pH-responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the ionic monomer incorporated within the particles. In recent work from our group, it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of pH-responsive microgel dispersions (Saunders, J. M.; Tong, T.; LeMaitre, C.; Freemont, A. J.; Saunders, B. R. Soft Matter 2007, 3, 486). These dispersions change from a fluid to a gel with increasing pH. The present work investigates the pH-dependent properties of dispersions of microgel particles containing MAA (methacrylic acid) and also the effects of added Ca2+. Two microgels are discussed: microgel A is poly(EA/MAA/AM) (EA and AM are ethyl acrylate and allyl methacrylate), and microgel B is poly(EA/MAA/BDDA) (butanediol diacrylate). The pH-dependent particle properties investigated include hydrodynamic diameters and electrophoretic mobilities. The critical coagulation concentrations (CCC) of dilute dispersions and the elastic modulus (G') of concentrated, gelled microgel dispersions were also investigated. In the absence of added Ca2+, the particle swelling and G' were smallest and largest, respectively, for microgel A. The changes in hydrodynamic diameter and mobility with pH were explained in terms of a core-shell swelling mechanism. Added Ca2+ was found to significantly decrease the CCCs, extents of particle swelling, and magnitude of the electrophoretic mobility. This was attributed to the ionic cross-linking of neighboring RCOO- groups by Ca2+. It is suggested that the formation of ionic cross-links is inefficient within the microgel particles because of the presence of covalent cross-links that oppose the large-scale conformational rearrangement of neighboring RCOO- groups. The effect of Ca2+ on the properties of the gelled dispersions is important from the viewpoint of potential application in vivo. Rheological studies of the gelled microgel dispersions showed that added Ca2+ did not have a specific influence on G'. The differences observed in the presence of Ca2+ were attributed to ionic strength effects (screening). The key parameter that controls G' of the gelled microgel dispersions is pH. The results from this work suggest that the elasticity of the gels would be slightly reduced in vivo as a consequence of the high ionic strength present.

Vascularized adipose tissue grafts from human mesenchymal stem cells with bioactive cues and microchannel conduits

Vascularization is critical to the survival of engineered tissues. This study combined biophysical and bioactive approaches to induce neovascularization in vivo. Further, we tested the effects of engineered vascularization on adipose tissue grafts. Hydrogel cylinders were fabricated from poly(ethylene glycol) diacrylate (PEG) in four configurations: PEG alone, PEG with basic fibroblast growth factor (bFGF), microchanneled PEG, or both bFGF-adsorbed and microchanneled PEG. In vivo implantation revealed no neovascularization in PEG, but substantial angiogenesis in bFGF-adsorbed and/or microchanneled PEG. The infiltrating host tissue consisted of erythrocyte-filled blood vessels lined by endothelial cells, and immunolocalized to vascular endothelial growth factor (VEGF). Human mesenchymal stem cells were differentiated into adipogenic cells, and encapsulated in PEG with both microchanneled and adsorbed bFGF. Upon in vivo implantation subcutaneously in immunodeficient mice, oil red O positive adipose tissue was present and interspersed with interstitial fibrous (IF) capsules. VEGF was immunolocalized in the IF capsules surrounding the engineered adipose tissue. These findings suggest that bioactive cues and/or microchannels promote the genesis of vascularized tissue phenotypes such as the tested adipose tissue grafts. Especially, engineered microchannels may provide a generic approach for modifying existing biomaterials by providing conduits for vascularization and/or diffusion.

The role of biological extracellular matrix scaffolds in vascularized three-dimensional tissue growthin vivo

An in vivo murine vascularized chamber model has been shown to generate spontaneous angiogenesis and new tissue formation. This experiment aimed to assess the effects of common biological scaffolds on tissue growth in this model. Either laminin-1, type I collagen, fibrin glue, hyaluronan, or sea sponge was inserted into silicone chambers containing the epigastric artery and vein, one end was sealed with adipose tissue and the other with bone wax, then incubated subcutaneously. After 2, 4, or 6 weeks, tissue from chambers containing collagen I, fibrin glue, hyaluronan, or no added scaffold (control) had small amounts of vascularized connective tissue. Chambers containing sea sponge had moderate connective tissue growth together with a mild "foreign body" inflammatory response. Chambers containing laminin-1, at a concentration 10-fold lower than its concentration in Matrigel, resulted in a moderate adipogenic response. In summary, (1) biological hydrogels are resorbed and gradually replaced by vascularized connective tissue; (2) sponge-like matrices with large pores support connective tissue growth within the pores and become encapsulated with granulation tissue; (3) laminin-containing scaffolds facilitate adipogenesis. It is concluded that the nature and chemical composition of the scaffold exerts a significant influence on the amount and type of tissue generated in this in vivo chamber model.