Thermally Cross-Linked Oligo(poly(ethylene glycol) fumarate) Hydrogels Support Osteogenic Differentiation of Encapsulated Marrow Stromal Cells In Vitro

Injectable ionic hydrogel conductors: Advancing material design to transform cardiac pacing

Direct pacing of the mid myocardium where re-entry originates can be used to prevent ventricular arrhythmias and circumvent the need for painful defibrillation or cardiac ablation. However, there are no pacing electrodes small enough to navigate the coronary veins that cross these culprit scar regions. To address this need, we have developed an injectable ionically conductive hydrogel electrode that can fill the epicardial coronary veins and transform them into flexible electrodes. A new hydrogel chemistry based on a polyether urethane diacrylamide macromer was developed that matches myocardial stiffness and is resistant to hydrolysis. Conductivity was imparted using ionic precursor solutions with values in the range of 2-3X of native myocardium that was retained after implantation. Ionic hydrogel electrodes provided stable electrical stimuli over many cycles and across a substantial length of the cardiac vein. Rapid in situ cure was achieved with redox initiation after injection from a double-barrel syringe with a mixing head. An ex vivo porcine model was used to identify the requisite viscosity and cure rate for hydrogel retention and homogeneity. Finally, successful in vivo deployment and pacing in a porcine model demonstrated that the ionic hydrogel electrode filled the anterior interventricular vein to depths far more distal and refined than current technologies. Collectively, these studies demonstrate the potential of this injectable ionic hydrogel electrode to pace previously inaccessible mid-myocardial tissue and pave the pathway for painless defibrillation.

Biological interaction of bioactive polymeric membranes in induced bone defects in rabbit tibias

The study aimed to evaluate bone repair using three osteoinductive polymers in bone defects created in rabbit tibias. Forty-eight adult rabbits were assessed at various time points: three, seven, fourteen, and thirty days. The groups included a control group (without biomaterial), M1 (Poly L Lactide co Polycaprolactone/Polyethylene Glycol), M2 (Poly L Lactide co Polycaprolactone/Polyethylene Glycol/β-Tricalcium Phosphate), and M3 (Poly L Lactide co Polycaprolactone/Polyethylene Glycol/nano hydroxyapatite). Histomorphometric analysis was conducted to evaluate new bone formation within and around the bone defect. At 14 (p<0.05) and 30 days (p<0.05), the callus area in the membrane groups, particularly in M3, was also significantly larger than in the control group, indicating the osteoinductive potential of these biomaterials. The callus consisted of both bone and cartilaginous matrix, suggesting a robust activation of endochondral ossification. The number of osteoclast was higher in the membrane groups, especially at 14 days in the M3 group, indicating increased bone remodeling activity. The membranes were not fully absorbed by 30 days, creating a space between the defect and the periosteum. In conclusion, all three membranes showed significant chondro and osteoinductive potential, with the membrane containing nano-hydroxyapatite demonstrating the most pronounced potential.

Gelatin as It Is: History and Modernity

The data concerning the synthesis and physicochemical characteristics of one of the practically important proteins-gelatin, as well as the possibilities of its practical application, are systematized and discussed. When considering the latter, emphasis is placed on the use of gelatin in those areas of science and technology that are associated with the specifics of the spatial/molecular structure of this high-molecular compound, namely, as a binder for the silver halide photographic process, immobilized matrix systems with a nano-level organization of an immobilized substance, matrices for creating pharmaceutical/dosage forms and protein-based nanosystems. It was concluded that the use of this protein is promising in the future.

Injectable pH-responsive adhesive hydrogels for bone tissue engineering inspired by the underwater attachment strategy of marine mussels

A major challenge in tissue engineering is the development of alternatives to traditional bone autografts and allografts that can regenerate critical-sized bone defects. Here we present the design of injectable pH-responsive double-crosslinked adhesive hydrogels inspired by the molecular mechanism and environmental post-processing of marine mussel adhesive. Nine adhesive hydrogel formulations were developed through the conjugation of crosslinkable catechol functional groups (DOPA) and the synthetic oligomer oligo[poly(ethylene glycol) fumarate] (OPF), varying the DOPA content (w/w%) and molecular weight (MW) of the OPF backbone to produce formulations with a range of swelling ratios, porosities, and crosslink densities. DOPA incorporation altered the surface chemistry, mechanical properties, and surface topography of hydrogels, resulting in an increase in material stiffness, slower degradation, and enhanced pre-osteoblast cell attachment and proliferation. When injected within simulated bone defects, DOPA-mediated interfacial adhesive interactions also prevented the displacement of scaffolds, an effect that was maintained even after swelling within physiological conditions. Taken together, OPF-DOPA hydrogels represent a promising new material to enhanced tissue integration and the prevention of the post-implantation migration of scaffolds that can occur due to biomechanical loading in vivo.

The effects of shock wave stimulation of mesenchymal stem cells on proliferation, migration, and differentiation in an injectable gelatin matrix for osteogenic regeneration

The treatment of a variety of defects in bony sites could benefit from mitogenic stimulation of osteoprogenitor cells, including endogenous bone marrow-derived mesenchymal stem cells (bMSCs), and from provision of such cells with a matrix permissive of their migration, proliferation, and osteogenic differentiation. That such MSC stimulation could result from treatment with noninvasive (extracorporeal) shock waves (ESWs), and the matrix delivered by injection could enable this therapeutic approach to be employed for applications in which preformed scaffolds and growth factor therapy are difficult to deploy. The objectives of the present study were to investigate focused ESWs for their effects on proliferation, migration, and osteogenic differentiation in an injectable gelatin (Gtn) matrix capable of undergoing covalent cross-linking in vivo. Gtn was conjugated with hydroxyphenyl propionic acid (HPA) in order to enable it to be covalently cross-linked with minute amounts of horseradish peroxidase and hydrogen peroxide. The results demonstrated that 500 shocks of 0.4-mJ/mm² energy flux density resulted in a twofold greater proliferation of bMSCs in the Gtn-HPA matrix after 14 days, compared with bMSCs grown with supplementation with platelet-derived growth factor (PDGF)-BB, a known mitogen for bMSCs. Moreover, SW treatment enhanced substantially osteogenic differentiation of bMSCs. The Gtn-HPA gel was permissive of MSC migration under the chemotactic influence of the growth factor, PDGF-BB, incorporated into and released by the gel. ESW treatment had no effect on the motility of the MSCs. The findings of the study warrant further investigation of this combined treatment modality for select bony defects.

Free and hydrogel encapsulated exosome-based therapies in regenerative medicine

Over the last few decades, mesenchymal stem cells-derived exosomes (MSCs-Ex) have attracted a lot of attention as a therapeutic tool in regenerative medicine. Exosomes are extracellular vehicles (EVs) that play important roles in cell-cell communication through various processes such as stress response, senescence, angiogenesis, and cell differentiation. Success in the field of regenerative medicine sparked exploration of the potential use of exosomes as key therapeutic effectors of MSCs to promote tissue regeneration. Various approaches including direct injection, intravenous injection, intraperitoneal injection, oral administration, and hydrogel-based encapsulation have been exploited to deliver exosomes to target tissues in different disease models. Despite significant advances in exosome therapy, it is unclear which approach is more effective for administering exosomes. Herein, we critically review the emerging progress in the applications of exosomes in the form of free or association with hydrogels as therapeutic agents for applications in regenerative medicine.

Peptide-modified methacrylated glycol chitosan hydrogels as a cell-viability supporting pro-angiogenic cell delivery platform for human adipose-derived stem/stromal cells

Cell-based therapies involving the injection of adipose-derived stem/stromal cells (ASCs) within rationally designed biomaterials are a promising approach for stimulating angiogenesis. With this focus, the current work explored the effects of incorporating integrin-binding RGD or IKVAV peptides within in situ-gelling N-methacrylate glycol chitosan (MGC) hydrogels on the response of encapsulated human ASCs. Initial studies focused on hydrogel characterization to validate that the MGC, MGC-RGD, and MGC-IKVAV hydrogels had similar biomechanical properties. ASC viability following encapsulation and culture under 2% O2 was significantly impaired in the MGC-IKVAV group relative to the MGC and MGC-RGD groups. In contrast, sustained viability, along with enhanced cell spreading and metabolic activity were observed in the MGC-RGD group. Investigation of angiogenic transcription suggested that the incorporation of the peptide groups did not substantially alter the pro-angiogenic gene expression profile of the encapsulated ASCs after 7 days of culture under 2% O2. Consistent with the in vitro findings, preliminary in vivo characterization following subcutaneous implantation into NOD/SCID mice showed that ASC retention was enhanced in the MGC-RGD hydrogels relative to the MGC-IKVAV group at 14 days. Further, the encapsulated ASCs in the MGC and MGC-RGD groups promoted murine CD31⁺ endothelial cell recruitment to the peri-implant region. Overall, the results indicate that the MGC-RGD and MGC hydrogels are promising platforms for ASC delivery, and suggest that strategies that support long-term ASC viability can augment in vivo angiogenesis through paracrine mechanisms. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 571-585, 2019.

Improved in situ seeding of 3D printed scaffolds using cell-releasing hydrogels

The design of tissue engineered scaffolds based on polymerized high internal phase emulsions (polyHIPEs) has emerged as a promising bone grafting strategy. We previously reported the ability to 3D print emulsion inks to better mimic the structure and mechanical properties of native bone while precisely matching defect geometry. In the current study, redox-initiated hydrogel carriers were investigated for in situ delivery of human mesenchymal stem cells (hMSCs) utilizing the biodegradable macromer, poly(ethylene glycol)-dithiothreitol. Hydrogel carrier properties including network formation time, sol-gel fraction, and swelling ratio were modulated to achieve rapid cure without external stimuli and a target cell-release period of 5-7 days. These in situ carriers enabled improved distribution of hMSCs in 3D printed polyHIPE grafts over standard suspension seeding. Additionally, carrier-loaded polyHIPEs supported sustained cell viability and osteogenic differentiation of hMSCs post-release. In summary, these findings demonstrate the potential of this in situ curing hydrogel carrier to enhance the cell distribution and retention of hMSCs in bone grafts. Although initially focused on improving bone regeneration, the ability to encapsulate cells in a hydrogel carrier without relying on external stimuli that can be attenuated in large grafts or tissues is expected to have a wide range of applications in tissue engineering.

Transient cellular adhesion on poly(ethylene-glycol)-dimethacrylate hydrogels facilitates a novel stem cell bandage approach

We discovered a transient adhesion property in poly(ethylene glycol) dimethacrylate (PEG-DMA) hydrogels and employed it to develop a novel "stem cell bandage" model of cellular delivery. First, we cultured human mesenchymal stromal cells (MSCs) on the surface of PEG-DMA hydrogels with high amounts of arginine-glycine-aspartic acid (RGD) adhesive peptides (RGD++) or without RGD (RGD-). On day 1, MSCs underwent an initial adhesion to RGD- hydrogels that was not significantly different over 13 days (n = 6). In addition, cells appeared to be well spread by day 3. Significantly fewer cells were present on RGD- hydrogels on day 15 compared to day 9, suggesting that RGD- hydrogels allow for an initial cellular adhesion that is stable for multiple days, but transient over longer periods with a decrease by day 15. This initial adhesion is especially surprising considering that PEG-DMA does not contain any biological adhesion motifs and is almost chemically identical to poly(ethylene glycol) diacrylate (PEG-DA), which has been shown to be non-adhesive without RGD. We hypothesized that MSCs could be cultured on RGD- PEG-DMA hydrogels and then applied to a wound site to deliver cells in a novel approach that we refer to as a "stem cell bandage". RGD- donor hydrogels were successfully able to deliver MSCs to PEG-DMA acceptor hydrogels with high RGD content (RGD++) or low amounts of RGD (RGD+). Our novel "bandage" approach promoted cell delivery to these model surfaces while preventing cells from diffusing away. This stem cell delivery strategy may provide advantages over more common stem cell delivery approaches such as direct injections or encapsulation and thus may be valuable as an alternative tissue engineering approach.

Injectable OPF/graphene oxide hydrogels provide mechanical support and enhance cell electrical signaling after implantation into myocardial infarct

After myocardial infarction (MI), the scar tissue contributes to ventricular dysfunction by electrically uncoupling viable cardiomyocytes in the infarct region. Injection of a conductive hydrogel could not only provide mechanical support to the infarcted region, but also synchronize contraction and restore ventricular function by electrically connecting isolated cardiomyocytes to intact tissue.

Methods: We created a conductive hydrogel by introducing graphene oxide (GO) nanoparticles into oligo(poly(ethylene glycol) fumarate) (OPF) hydrogels. The hydrogels were characterized by AFM and electrochemistry workstation. A rat model of myocardial infarction was used to investigate the ability of OPF/GO to improve cardiac electrical propagation in the injured heart in vivo. Echocardiography (ECHO) was used to evaluate heart function 4 weeks after MI. Ca²⁺ imaging was used to visualize beating cardiomyocytes (CMs). Immunofluorescence staining was used to visualize the expression of cardiac-specific markers.

Results: OPF/GO hydrogels had semiconductive properties that were lacking in pure OPF. In addition, the incorporation of GO into OPF hydrogels could improve cell attachment in vitro. Injection of OPF/GO 4 weeks after myocardial infarction in rats enhanced the Ca²⁺ signal conduction of cardiomyocytes in the infarcted region in comparison with PBS or OPF alone. Moreover, the injection of OPF/GO hydrogel into the infarct region enhanced the generation of cytoskeletal structure and intercalated disc assembly. Echocardiography analysis showed improvement in load-dependent ejection fraction/fractional shortening of heart function 4 weeks after injection.

Conclusions: We prepared a conductive hydrogel (OPF/GO) that provide mechanical support and biological conduction in vitro and in vivo. We found that injected OPF/GO hydrogels can provide mechanical support and electric connection between healthy myocardium and the cardiomyocytes in the scar via activating the canonical Wnt signal pathway, thus upregulating the generation of Cx43 and gap junction associated proteins. Injection of OPF/GO hydrogel maintained better heart function after myocardial infarction than the injection of a nonconductive polymer.

Branched poly (lactic acid) microparticles for enhancing the 5-aminolevulinic acid phototoxicity

An innovative microcarrier based on a carboxy-enriched and branched polylactic acid derivative was developed to enhance the in vitro phototoxicity of the photosensitizer and prodrug 5-aminolevulinic. Microparticles, prepared by double emulsion technique and loaded with the prodrug were carefully characterized and the effect of the polymer structure on the chemical, physical and biological properties of the final product was evaluated. Results showed that microparticles have a spherical shape and ability to allocate up to 30 μg of the photosensitizer per mg of carrier despite their difference in solubility. Release studies performed in various simulated physiological conditions demonstrate the influence of the branched structure and the presence of the additional carboxylic groups on the release rate and the possibility to modulate it. In vitro assays conducted on human epithelial adenocarcinoma cells proved the not cytotoxicity of the carriers in a wide range of concentrations. The hemocompatibility and surface proteins adsorption were evaluated at different microparticles concentrations to evaluate the safety and estimate the possible microparticles residential time in the bloodstream. The advantages, of loading 5-aminolevulinic acid in the prepared carrier has been deeply described in terms of enhanced phototoxicity, compared to the free 5-aminolevulinic acid formulation after irradiation with light at 635 nm. The obtained results demonstrate the advantages of the prepared derivative compared to the linear polylactide for future application in photodynamic therapy based on the photosensitizer 5-aminolevulinic acid.

Mechanically resilient injectable scaffolds for intramuscular stem cell delivery and cytokine release

A promising strategy for treating peripheral ischemia involves the delivery of stem cells to promote angiogenesis through paracrine signaling. Treatment success depends on cell localization, retention, and survival within the mechanically dynamic intramuscular (IM) environment. Herein we describe an injectable, in situ-gelling hydrogel for the IM delivery of adipose-derived stem/stromal cells (ASCs), specifically designed to withstand the dynamic loading conditions of the lower limb and facilitate cytokine release from encapsulated cells. Copolymers of poly(trimethylene carbonate)-b-poly(ethylene glycol)-b-poly(trimethylene carbonate) diacrylate were used to modulate the properties of methacrylated glycol chitosan hydrogels crosslinked by thermally-initiated polymerization using ammonium persulfate and N,N,N',N'-tetramethylethylenediamine. The scaffolds had an ultimate compressive strain over 75% and maintained mechanical properties during compressive fatigue testing at physiological levels. Rapid crosslinking (<3 min) was achieved at low initiator concentration (5 mM). Following injection and crosslinking within the scaffolds, human ASCs demonstrated high viability (>90%) over two weeks in culture under both normoxia and hypoxia. Release of angiogenic and chemotactic cytokines was enhanced from encapsulated cells under sustained hypoxia, in comparison to normoxic and tissue culture polystyrene controls. When delivered by IM injection in a mouse model of hindlimb ischemia, human ASCs were well retained in the scaffold over 28 days and significantly increased the IM vascular density compared to untreated controls.

Composite Bioscaffolds Incorporating Decellularized ECM as a Cell-Instructive Component Within Hydrogels as In Vitro Models and Cell Delivery Systems

Decellularized tissues represent promising biomaterials, which harness the innate capacity of the tissue-specific extracellular matrix (ECM) to direct cell functions including stem cell proliferation and lineage-specific differentiation. However, bioscaffolds derived exclusively from decellularized ECM offer limited versatility in terms of tuning biomechanical properties, as well as cell-cell and cell-ECM interactions that are important mediators of the cellular response. As an alternative approach, in the current chapter we describe methods for incorporating cryo-milled decellularized tissues as a cell-instructive component within a hydrogel carrier designed to crosslink under mild conditions. This composite strategy can enable in situ cell encapsulation with high cell viability, allowing efficient seeding with a homogeneous distribution of cells and ECM. Detailed protocols are provided for the effective decellularization of human adipose tissue and porcine auricular cartilage, as well as the cryo-milling process used to generate the ECM particles. Further, we describe methods for synthesizing methacrylated chondroitin sulphate (MCS) and for performing UV-initiated and thermally induced crosslinking to form hydrogel carriers for adipose and cartilage regeneration. The hydrogel composites offer great flexibility, and the hydrogel phase, ECM source, particle size, cell type(s) and seeding density can be tuned to promote the desired cellular response. Overall, these systems represent promising platforms for the development of tissue-specific 3-D in vitro cell culture models and in vivo cell delivery systems.

Anatomic Mesenchymal Stem Cell-Based Engineered Cartilage Constructs for Biologic Total Joint Replacement

Cartilage has a poor healing response, and few viable options exist for repair of extensive damage. Hyaluronic acid (HA) hydrogels seeded with mesenchymal stem cells (MSCs) polymerized through UV crosslinking can generate functional tissue, but this crosslinking is not compatible with indirect rapid prototyping utilizing opaque anatomic molds. Methacrylate-modified polymers can also be chemically crosslinked in a cytocompatible manner using ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine (TEMED). The objectives of this study were to (1) compare APS/TEMED crosslinking with UV crosslinking in terms of functional maturation of MSC-seeded HA hydrogels; (2) generate an anatomic mold of a complex joint surface through rapid prototyping; and (3) grow anatomic MSC-seeded HA hydrogel constructs using this alternative crosslinking method. Juvenile bovine MSCs were suspended in methacrylated HA (MeHA) and crosslinked either through UV polymerization or chemically with APS/TEMED to generate cylindrical constructs. Minipig porcine femoral heads were imaged using microCT, and anatomic negative molds were generated by three-dimensional printing using fused deposition modeling. Molded HA constructs were produced using the APS/TEMED method. All constructs were cultured for up to 12 weeks in a chemically defined medium supplemented with TGF-β3 and characterized by mechanical testing, biochemical assays, and histologic analysis. Both UV- and APS/TEMED-polymerized constructs showed increasing mechanical properties and robust proteoglycan and collagen deposition over time. At 12 weeks, APS/TEMED-polymerized constructs had higher equilibrium and dynamic moduli than UV-polymerized constructs, with no differences in proteoglycan or collagen content. Molded HA constructs retained their hemispherical shape in culture and demonstrated increasing mechanical properties and proteoglycan and collagen deposition, especially at the edges compared to the center of these larger constructs. Immunohistochemistry showed abundant collagen type II staining and little collagen type I staining. APS/TEMED crosslinking can be used to produce MSC-seeded HA-based neocartilage and can be used in combination with rapid prototyping techniques to generate anatomic MSC-seeded HA constructs for use in filling large and anatomically complex chondral defects or for biologic joint replacement.

Hydrogels as Extracellular Matrix Analogs

The extracellular matrix (ECM) is the non-cellular component of tissue that provides physical scaffolding to cells. Emerging studies have shown that beyond structural support, the ECM provides tissue-specific biochemical and biophysical cues that are required for tissue morphogenesis and homeostasis. Hydrogel-based platforms have played a key role in advancing our knowledge of the role of ECM in regulating various cellular functions. Synthetic hydrogels allow for tunable biofunctionality, as their material properties can be tailored to mimic those of native tissues. This review discusses current advances in the design of hydrogels with defined physical and chemical properties. We also highlight research findings that demonstrate the impact of matrix properties on directing stem cell fate, such as self-renewal and differentiation. Recent and future efforts towards understanding cell-material interactions will not only advance our basic understanding, but will also help design tissue-specific matrices and delivery systems to transplant stem cells and control their response in vivo.

Evaluation of hydrogel composing of Pluronic F127 and carboxymethyl hexanoyl chitosan as injectable scaffold for tissue engineering applications

This study demonstrated a novel hydrogel system composing of Pluronic F127, carboxymethyl hexanoyl chitosan (CA) and glutaraldehyde (GA) for encapsulating fibroblasts (L-929). The thermal behavior of the hydrogel was evaluated using TGA, the swelling behavior of the hydrogel was evaluated in Dulbecco's Modified Eagle's medium (DMEM), and the mechanical properties were determined through dynamic mechanical analysis. Cells were encapsulated by simple mixing, and the viability of encapsulated cells was determined using alamar blue cell viability assay and the cells morphology was examined using fluorescent imaging. The results indicated that the Tgel of this system was around 30°C, where sol-gel transformation occurred within 90s. Although the addition of CA and GA reduced the shear moduli slightly, the F127/CA/GA gel was able to remain in gelling state in the medium for more than 1 month. In vitro cell culture study revealed that F-127/CA/GA hydrogels were non-cytotoxic. Moreover, the viability of encapsulated L929 was 106% after incubation for 5 days. Based on these results, these F127/CA/GA hydrogels can be used to encapsulate cells for tissue engineering applications.

Biomedical Applications of Biodegradable Polyesters

The focus in the field of biomedical engineering has shifted in recent years to biodegradable polymers and, in particular, polyesters. Dozens of polyester-based medical devices are commercially available, and every year more are introduced to the market. The mechanical performance and wide range of biodegradation properties of this class of polymers allow for high degrees of selectivity for targeted clinical applications. Recent research endeavors to expand the application of polymers have been driven by a need to target the general hydrophobic nature of polyesters and their limited cell motif sites. This review provides a comprehensive investigation into advanced strategies to modify polyesters and their clinical potential for future biomedical applications.

Fabrication of cell-laden macroporous biodegradable hydrogels with tunable porosities and pore sizes

In this work, we investigated a cytocompatible particulate leaching method for the fabrication of cell-laden macroporous hydrogels. We used dehydrated and uncrosslinked gelatin microspheres as leachable porogens to create macroporous oligo(poly(ethylene glycol) fumarate) hydrogels. Varying gelatin content and size resulted in a wide range of porosities and pore sizes, respectively. Encapsulated mesenchymal stem cells (MSCs) exhibited high viability immediately following the fabrication process, and culture of cell-laden hydrogels revealed improved cell viability with increasing porosity. Additionally, the osteogenic potential of the encapsulated MSCs was evaluated over 16 days. Overall, this study presents a robust method for the preparation of cell-laden macroporous hydrogels with desired porosity and pore size for tissue engineering applications.

Potential use of alginate beads as a chondrocyte delivery vehicle and stepwise dissolving porogen in a hydrogel scaffold for cartilage tissue engineering

A chondroitin sulfate (CS)–alginate bead composite gel (CS–ABG) is developed, and which exhibits superiority to aid cartilage regeneration.

Synthetic biodegradable hydrogel delivery of demineralized bone matrix for bone augmentation in a rat model

There exists a strong clinical need for a more capable and robust method to achieve bone augmentation, and a system with fine-tuned delivery of demineralized bone matrix (DBM) has the potential to meet that need. As such, the objective of the present study was to investigate a synthetic biodegradable hydrogel for the delivery of DBM for bone augmentation in a rat model. Oligo(poly(ethylene glycol) fumarate) (OPF) constructs were designed and fabricated by varying the content of rat-derived DBM particles (either 1:3, 1:1 or 3:1 DBM:OPF weight ratio on a dry basis) and using two DBM particle size ranges (50-150 or 150-250 μm). The physical properties of the constructs and the bioactivity of the DBM were evaluated. Selected formulations (1:1 and 3:1 with 50-150 μm DBM) were evaluated in vivo compared to an empty control to investigate the effect of DBM dose and construct properties on bone augmentation. Overall, 3:1 constructs with higher DBM content achieved the greatest volume of bone augmentation, exceeding 1:1 constructs and empty implants by 3- and 5-fold, respectively. As such, we have established that a synthetic, biodegradable hydrogel can function as a carrier for DBM, and that the volume of bone augmentation achieved by the constructs correlates directly to the DBM dose.

Osteochondral tissue regeneration through polymeric delivery of DNA encoding for the SOX trio and RUNX2

Native osteochondral repair is often inadequate owing to the inherent properties of the tissue, and current clinical repair strategies can result in healing with a limited lifespan and donor site morbidity. This work investigates the use of polymeric gene therapy to address this problem by delivering DNA encoding for transcription factors complexed with the branched poly(ethylenimine)-hyaluronic acid (bPEI-HA) delivery vector via a porous oligo[poly(ethylene glycol) fumarate] hydrogel scaffold. To evaluate the potential of this approach, a bilayered scaffold mimicking native osteochondral tissue organization was loaded with DNA/bPEI-HA complexes. Next, bilayered implants either unloaded or loaded in a spatial fashion with bPEI-HA and DNA encoding for either Runt-related transcription factor 2 (RUNX2) or SRY (sex determining region Y)-box 5, 6, and 9 (the SOX trio), to generate bone and cartilage tissues respectively, were fabricated and implanted in a rat osteochondral defect. At 6weeks post-implantation, micro-computed tomography analysis and histological scoring were performed on the explants to evaluate the quality and quantity of tissue repair in each group. The incorporation of DNA encoding for RUNX2 in the bone layer of these scaffolds significantly increased bone growth. Additionally, a spatially loaded combination of RUNX2 and SOX trio DNA loading significantly improved healing relative to empty hydrogels or either factor alone. Finally, the results of this study suggest that subchondral bone formation is necessary for correct cartilage healing.

Dual growth factor delivery from bilayered, biodegradable hydrogel composites for spatially-guided osteochondral tissue repair

The present work investigated the use of biodegradable hydrogel composite scaffolds, based on the macromer oligo(poly(ethylene glycol) fumarate) (OPF), to deliver growth factors for the repair of osteochondral tissue in a rabbit model. In particular, bilayered OPF composites were used to mimic the structural layers of the osteochondral unit, and insulin-like growth factor-1 (IGF-1) and bone morphogenetic protein-2 (BMP-2) were loaded into gelatin microparticles and embedded within the OPF hydrogel matrix in a spatially controlled manner. Three different scaffold formulations were implanted in a medial femoral condyle osteochondral defect: 1) IGF-1 in the chondral layer, 2) BMP-2 in the subchondral layer, and 3) IGF-1 and BMP-2 in their respective separate layers. The quantity and quality of osteochondral repair was evaluated at 6 and 12 weeks with histological scoring and micro-computed tomography (micro-CT). While histological scoring results at 6 weeks showed no differences between experimental groups, micro-CT analysis revealed that the delivery of BMP-2 alone increased the number of bony trabecular islets formed, an indication of early bone formation, over that of IGF-1 delivery alone. At 12 weeks post-implantation, minimal differences were detected between the three groups for cartilage repair. However, the dual delivery of IGF-1 and BMP-2 had a higher proportion of subchondral bone repair, greater bone growth at the defect margins, and lower bone specific surface than the single delivery of IGF-1. These results suggest that the delivery of BMP-2 enhances subchondral bone formation and that, while the dual delivery of IGF-1 and BMP-2 in separate layers does not improve cartilage repair under the conditions studied, they may synergistically enhance the degree of subchondral bone formation. Overall, bilayered OPF hydrogel composites demonstrate potential as spatially-guided, multiple growth factor release vehicles for osteochondral tissue repair.

Gelatin carriers for drug and cell delivery in tissue engineering

The ability of gelatin to form complexes with different drugs has been investigated for controlled release applications. Gelatin parameters, such as crosslinking density and isoelectric point, have been tuned in order to optimize gelatin degradation and drug delivery kinetics. In recent years, focus has shifted away from the use of gelatin in isolation toward the modification of gelatin with functional groups and the fabrication of material composites with embedded gelatin carriers. In this review, we highlight some of the latest work being performed in these areas and comment on trends in the field. Specifically, we discuss gelatin modifications for immune system evasion, drug stabilization, and targeted delivery, as well as gelatin composite systems based on ceramics, naturally-occurring polymers, and synthetic polymers.

Injectable redox-polymerized methylcellulose hydrogels as potential soft tissue filler materials

There is a significant clinical need for long-lasting, injectable materials for soft tissue reconstruction. Methylcellulose (MC) is an FDA-approved polysaccharide derivative of cellulose that is inexpensive, renewable, and biocompatible, and may serve as an alternative to existing synthetic and natural fillers. In this study, MC was modified with functional methacrylate groups and polymerized using a redox-initiation system to produce hydrogels with tunable properties. By varying the percent methacrylation and macromer concentration, the equilibrium moduli of the hydrogels were found to range between 1.29 ± 0.46 and 12.8 ± 2.94 kPa, on par with human adipose tissue, and also displayed an inverse relationship to the swelling properties. Rheological analyses determined gelation onset and completion to be in accordance with the ISO standard for injectable materials. Cellulase enzymatic treatment resulted in complete degradation of the hydrogels by 48 h, presenting the possibility of minimally invasive removal of the materials in the event of malposition or host reaction. In addition, co-culture experiments with human dermal fibroblasts showed the gels to be cytocompatible based on DNA measurements and Live/Dead staining. Taken together, these redox-polymerized MC hydrogels may be of use for a wide range of clinical indications requiring soft tissue augmentation.

Cell encapsulation via microtechnologies

The encapsulation of living cells in a variety of soft polymers or hydrogels is important, particularly, for the rehabilitation of functional tissues capable of repairing or replacing damaged organs. Cellular encapsulation segregates cells from the surrounding tissue to protect the implanted cell from the recipient's immune system after transplantation. Diverse hydrogel membranes have been popularly used as encapsulating materials and permit the diffusion of gas, nutrients, wastes and therapeutic products smoothly. This review describes a variety of methods that have been developed to achieve cellular encapsulation using microscale platform. Microtechnologies have been adopted to precisely control the encapsulated cell number, size and shape of a cell-laden polymer structure. We provide a brief overview of recent microtechnology-based cell encapsulation methods, with a detailed description of the relevant processes. Finally, we discuss the current challenges and future directions likely to be taken by cell microencapsulation approaches toward tissue engineering and cell therapy applications.

Injection of composite with bone marrow-derived mesenchymal stem cells and a novel synthetic hydrogel after myocardial infarction: a protective role in left ventricle function

A number of studies have shown that the transplantation of bone marrow-derived mesenchymal stem cells (BMSCs) into the thinned infarct wall improves regional wall motion. In this study, we hypothesized that the injection of biomaterials and MSCs into the infarcted myocardium can preserve left ventricular (LV) function. To test this hypothesis, anterior acute myocardial infarction (AMI) was induced in 34 rabbits and BMSCs with hydrogel composite were prepared. One week after inducing AMI, 28 of the 34 rabbits were divided into four groups (Groups A-D; three rabbits were used for bone marrow aspiration, and three rabbits died) and all received an epicardial injection. Group A received BMSCs with hydrogel composite marked by 5-bromodeoxyuridine (BrdU); Group B received BMSCs only marked by BrdU; Group C received hydrogel only marked by BrdU; and Group D was the control group, which received fetal bovine serum. Echocardiography was performed before AMI was induced, 1 week after AMI, and 4 weeks after the epicardial injection. The results were compared with those before AMI, and the rabbits of all the four groups had significantly larger LV end-diastolic diameter (LVDd), thinner anterior wall (AW), lower LV ejection fraction (LVEF), lower VS and VE (p<0.05) 1 week after AMI, which were more significant in Group A (p<0.05). Compared with 1 week after AMI, Group A and Group B had significantly smaller LVDd, thicker AW, larger LVEF, larger VS and VE (p<0.05) 4 weeks after the epicardial injection, which were more significant in Group A (p<0.05); however, there was no significant difference in Group C and Group D. These results suggested that BMSCs with hydrogel composite could serve as an injectable biomaterial that prevents LV remodeling and dilation, and improves local systolic and diastolic function after AMI.

Development of injectable organic/inorganic colloidal composite gels made of self-assembling gelatin nanospheres and calcium phosphate nanocrystals

Colloidal gels are a particularly attractive class of hydrogels for applications in regenerative medicine, and allow for a "bottom-up" fabrication of multi-functional biomaterials by employing micro- or nanoscale particles as building blocks to assemble into shape-specific bulk scaffolds. So far, however, the synthesis of colloidal composite gels composed of both organic and inorganic particles has hardly been investigated. The current study has focused on the development of injectable colloidal organic-inorganic composite gels using calcium phosphate (CaP) nanoparticles and gelatin (Gel) nanospheres as building blocks. These novel Gel-CaP colloidal composite gels exhibited a strongly enhanced gel elasticity, shear-thinning and self-healing behavior, and gel stability at high ionic strengths, while chemical - potentially cytotoxic - functionalizations were not necessary to introduce sufficiently strong cohesive interactions. Moreover, it was shown in vitro that osteoconductive CaP nanoparticles can be used as an additional tool to reduce the degradation rate of otherwise fast-degradable gelatin nanospheres and fine-tune the control over the release of growth factors. Finally, it was shown that these colloidal composite gels support attachment, spreading and proliferation of cultured stem cells. Based on these results, it can be concluded that proof-of-principle has been obtained for the design of novel advanced composite materials made of nanoscale particulate building blocks which exhibit great potential for use in regenerative medicine.

A factorial analysis of the combined effects of hydrogel fabrication parameters on the in vitro swelling and degradation of oligo(poly(ethylene glycol) fumarate) hydrogels

In this study, a full factorial approach was used to investigate the effects of poly(ethylene glycol) (PEG) molecular weight (MW; 10,000 vs. 35,000 nominal MW), crosslinker-to-macromer carbon-carbon double bond ratio (DBR; 40 vs. 60), crosslinker type (PEG-diacrylate (PEGDA) vs. N,N'-methylene bisacrylamide (MB)), crosslinking extent of incorporated gelatin microparticles (low vs. high), and incubation medium composition (with or without collagenase) on the swelling and degradation characteristics of oligo[(poly(ethylene glycol) fumarate)] (OPF) hydrogel composites as indicated by the swelling ratio and the percentage of mass remaining, respectively. Each factor consisted of two levels, which were selected based on previous in vitro and in vivo studies utilizing these hydrogels for various tissue engineering applications. Fractional factorial analyses of the main effects indicated that the mean swelling ratio and the mean percentage of mass remaining of OPF composite hydrogels were significantly affected by every factor. In particular, increasing the PEG chain MW of OPF macromers significantly increased the mean swelling ratio and decreased the mean percentage of mass remaining by 5.7 ± 0.3 and 17.2 ± 0.6%, respectively. However, changing the crosslinker from MB to PEGDA reduced the mean swelling ratio and increased the mean percentage of mass remaining of OPF composite hydrogels by 4.9 ± 0.2 and 9.4 ± 0.9%, respectively. Additionally, it was found that the swelling characteristics of hydrogels fabricated with higher PEG chain MW or with MB were more sensitive to increases in DBR. Collectively, the main and cross effects observed between factors enables informed tuning of the swelling and degradation properties of OPF-based hydrogels for various tissue engineering applications. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 3477-3487, 2014.

Preparation of cell-encapsulation devices in confined microenvironment

The entrapment of cells into hydrogel microdevice in form of microparticles or microfibers is one of the most appealing and useful tools for cell-based therapy and tissue engineering. Cell encapsulation procedures allow the immunoisolation of cells from the surrounding environment, after their transplantation and the maintenance of the normal cellular physiology. Factors affecting the efficacy of microdevices, which include size, size distribution, morphology, and porosity are all highly dependent on the method of preparation. In this respect, microfluidic based methods offer a promising strategy to fabricate highly uniform and morphologically controlled microdevices with tunable chemical and mechanical properties. In the current review, various cell microencapsulation procedures, based on a microfluidics, are critically analyzed with a special focus on the effect of the procedure on the morphology, viability and functions of the embedded cells. Moreover, a brief introduction about the optimal characteristics of microdevice intended for cell encapsulation, together with the currently used materials for the production is reported. A further challenging application of microfluidics for the development of "living microchip" is also presented. Finally, the limitations, challenging and future work on the microfluidic approach are also discussed.

Comparison of photopolymerizable thiol-ene PEG and acrylate-based PEG hydrogels for cartilage development

When designing hydrogels for tissue regeneration, differences in polymerization mechanism and network structure have the potential to impact cellular behavior. Poly(ethylene glycol) hydrogels were formed by free-radical photopolymerization of acrylates (chain-growth) or thiol-norbornenes (step-growth) to investigate the impact of hydrogel system (polymerization mechanism and network structure) on the development of engineered tissue. Bovine chondrocytes were encapsulated in hydrogels and cultured under free swelling or dynamic compressive loading. In the acrylate system immediately after encapsulation chondrocytes exhibited high levels of intracellular ROS concomitant with a reduction in hydrogel compressive modulus and higher variability in cell deformation upon compressive strain; findings that were not observed in the thiol-norbornene system. Long-term the quantity of sulfated glycosaminoglycans and total collagen was greater in the acrylate system, but the quality resembled that of hypertrophic cartilage with positive staining for aggrecan, collagens I, II, and X and collagen catabolism. The thiol-norbornene system led to hyaline-like cartilage production especially under mechanical loading with positive staining for aggrecan and collagen II and minimal staining for collagens I and X and collagen catabolism. Findings from this study confirm that the polymerization mechanism and network structure have long-term effects on the quality of engineered cartilage, especially under mechanical loading.

The influence of matrix degradation and functionality on cell survival and morphogenesis in PEG-based hydrogels

Two norbornene-functionalized PEG macromers are synthesized to render hydrogels with different hydrolytic degradability. Dithiol-containing linkers such as dithiothreitol or biscysteine-containing peptides are used to control proteolytic degradability. The influence of thiol-ene gel degradability on cell survival and morphogenesis in 3D is assessed using hMSCs and pancreatic MIN6 β cells. The initial cell viability can be negatively affected in highly crosslinked thiol-ene hydrogels. When cells are encapsulated in thiol-ene gels lacking cell-adhesive motifs, their survival and proliferation are promoted in more hydrolytically labile hydrogels. The degree of 3D cell spreading in encapsulated hMSCs is enhanced when the matrices are immobilized with cell-adhesive motifs.

Bone morphogenetic protein-2 release from composite hydrogels of oligo(poly(ethylene glycol) fumarate) and gelatin

PURPOSE

Hydrogel composites of oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles (GMs) were investigated as carriers of bone morphogenetic protein-2 (BMP-2) for bone tissue engineering applications.

METHODS

Hydrogel composites with different physical characteristics were prepared by changing the amount and type (acidic vs. basic) of gelatin incorporated in the OPF bulk phase. Composites with differing physical properties (degradation, swelling, and mechanical properties) and differing BMP-2 loading phase were investigated to determine the effect of these factors on BMP-2 release profiles over 28 days.

RESULTS

Overall, higher gelatin amount increased the degradation and swelling of composites, and acidic GMs further increased the degradation and swelling and reduced the compressive modulus of the composites. The most significant factor affecting the release of BMP-2 from composites was the loading phase of the growth factor: GM loading reduced the burst release, increased BMP-2 release during the later phases of the experiment, and increased the cumulative release in faster degrading samples.

CONCLUSIONS

The results indicate that the physical properties and the BMP-2 release kinetics of hydrogel composites can be controlled by adjusting multiple parameters at the time of the hydrogel composite fabrication.

Comparison of cell-loading methods in hydrogel systems

Bone regenerative medicine, based on the combined use of cells and scaffolds, represents a promising strategy in bone regeneration. Hydrogels have attracted huge interests for application as a scaffold for minimally invasive surgery. Collagen and oligo(poly(ethylene glycol)fumarate) (OPF) hydrogels are the representatives of two main categories of hydrogels, that is, natural- and synthetic-based hydrogels. With these the optimal cell-loading (i.e., cell distribution inside the hydrogels) method was assessed. The cell behavior of both bone marrow- and adipose tissue-derived mesenchymal stem cells (BM- and AT-MSCs) in three loading methods, which are dispersed (i.e., homogeneous cell encapsulation, D), sandwich (i.e., cells located in between two hydrogel layers, S), and spheroid (i.e., cell pellets encapsulation, Sp) loading in two hydrogel systems (i.e., collagen and OPF), was compared. The results suggested that the cell behavior was influenced by the hydrogel type, meaning cells cultured in collagen hydrogels had higher proliferation and osteogenic differentiation capacity than in OPF hydrogels. In addition, AT-MSCs exhibited higher proliferation and osteogenic properties compared to BM-MSCs. However, no difference was observed for mineralization among the three loading methods, which did not approve the hypothesis that S and Sp loading would increase osteogenic capacity compared to D loading. In conclusion, D and Sp loading represents two promising cell loading methods for injectable bone substitute materials that allow application of minimally invasive surgery for cell-based regenerative treatment.

Development of 3D hydrogel culture systems with on-demand cell separation

Recently there has been an increased interest in the effects of paracrine signaling between groups of cells, particularly in the context of better understanding how stem cells contribute to tissue repair. Most current 3D co-culture methods lack the ability to effectively separate two cell populations after the culture period, which is important for simultaneously analyzing the reciprocal effects of each cell type on the other. Here, we detail the development of a 3D hydrogel co-culture system that allows us to culture different cell types for up to 7 days and subsequently separate and isolate the different cell populations using enzyme-sensitive glues. Separable 3D co-culture laminates were prepared by laminating PEG-based hydrogels with enzyme-degradable hydrogel adhesives. Encapsulated cell populations exhibited good segregation with well-defined interfaces. Furthermore, constructs can be separated on-demand upon addition of the appropriate enzyme, while cell viability remains high throughout the culture period, even after laminate separation. This platform offers great potential for a variety of basic cell signaling studies as the incorporation of an enzyme-sensitive adhesive interface allows the on-demand separation of individual cell populations for immediate analysis or further culture to examine persistence of co-culture effects and paracrine signaling on cell populations.

Oligo[poly(ethylene glycol)fumarate] hydrogel enhances osteochondral repair in porcine femoral condyle defects

BACKGROUND

Management of osteochondritis dissecans remains a challenge. Use of oligo[poly(ethylene glycol)fumarate] (OPF) hydrogel scaffold alone has been reported in osteochondral defect repair in small animal models. However, preclinical evaluation of usage of this scaffold alone as a treatment strategy is limited.

QUESTIONS/PURPOSES

We therefore (1) determined in vitro pore size and mechanical stiffness of freeze-dried and rehydrated freeze-dried OPF hydrogels, respectively; (2) assessed in vivo gross defect filling percentage and histologic findings in defects implanted with rehydrated freeze-dried hydrogels for 2 and 4 months in a porcine model; (3) analyzed highly magnified histologic sections for different types of cartilage repair tissues, subchondral bone, and scaffold; and (4) assessed neotissue filling percentage, cartilage phenotype, and Wakitani scores.

METHODS

We measured pore size of freeze-dried OPF hydrogel scaffolds and mechanical stiffness of fresh and rehydrated forms. Twenty-four osteochondral defects from 12 eight-month-old micropigs were equally divided into scaffold and control (no scaffold) groups. Gross and histologic examination, one-way ANOVA, and one-way Mann-Whitney U test were performed at 2 and 4 months postoperatively.

RESULTS

Pore sizes ranged from 20 to 433 μm in diameter. Rehydrated freeze-dried scaffolds had mechanical stiffness of 1 MPa. The scaffold itself increased percentage of neotissue filling at both 2 and 4 months to 58% and 54%, respectively, with hyaline cartilage making up 39% of neotissue at 4 months.

CONCLUSIONS

Rehydrated freeze-dried OPF hydrogel can enhance formation of hyaline-fibrocartilaginous mixed repair tissue of osteochondral defects in a porcine model.

CLINICAL RELEVANCE

Rehydrated freeze-dried OPF hydrogel alone implanted into cartilage defects is insufficient to generate a homogeneously hyaline cartilage repair tissue, but its spacer effect can be enhanced by other tissue-regenerating mediators.

Subcutaneous tissue response and osteogenic performance of calcium phosphate nanoparticle-enriched hydrogels in the tibial medullary cavity of guinea pigs

In the current study, oligo(poly(ethylene glycol) fumarate) (OPF)-based hydrogels were tested for the first time as injectable bone substitute materials. The primary feature of the material design was the incorporation of calcium phosphate (CaP) nanoparticles within the polymeric matrix in order to compare the soft tissue response and bone-forming capacity of plain OPF hydrogels with CaP-enriched OPF hydrogel composites. To that end, pre-set scaffolds were implanted subcutaneously, whereas flowable polymeric precursor solutions were injected in a tibial ablation model in guinea pigs. After 8 weeks of implantation, histological and histomorphometrical evaluation of the subcutaneous scaffolds confirmed the biocompatibility of both types of hydrogels. Nevertheless, OPF hydrogels presented a loose structure, massive cellular infiltration and extensive material degradation compared to OPF-CaP hydrogels that were more compact. Microcomputed tomography and histological and histomorphometrical analyses showed comparable amounts of new trabecular bone in all tibias and some material remnants in the medial and distal regions. Particularly, highly calcified areas were observed in the distal region of OPF-CaP-treated tibias, which indicate a heterogeneous distribution of the mineral phase throughout the hydrogel matrix. This phenomenon can be attributed to either hindered gelation under highly perfused in vivo conditions or a faster degradation rate of the polymeric hydrogel matrix compared to the nanostructured mineral phase, resulting in loss of entrapment of the CaP nanoparticles and subsequent sedimentation.

Injectable biodegradable hydrogels for embryonic stem cell transplantation: improved cardiac remodelling and function of myocardial infarction

In this study, an injectable, biodegradable hydrogel composite of oligo[poly(ethylene glycol) fumarate] (OPF) was investigated as a carrier of mouse embryonic stem cells (mESCs) for the treatment of myocardial infarction (MI). The OPF hydrogels were used to encapsulate mESCs. The cell differentiation in vitro over 14 days was determined via immunohistochemical examination. Then, mESCs encapsulated in OPF hydrogels were injected into the LV wall of a rat MI model. Detailed histological analysis and echocardiography were used to determine the structural and functional consequences after 4 weeks of transplantation. With ascorbic acid induction, mESCs could differentiate into cardiomyocytes and other cell types in all three lineages in the OPF hydrogel. After transplantation, both the 24-hr cell retention and 4-week graft size were significantly greater in the OPF + ESC group than that of the PBS + ESC group (P < 0.01). Four weeks after transplantation, OPF hydrogel alone significantly reduced the infarct size and collagen deposition and improved the cardiac function. The heart function and revascularization improved significantly, while the infarct size and fibrotic area decreased significantly in the OPF + ESC group compared with that of the PBS + ESC, OPF and PBS groups (P < 0.01). All treatments had significantly reduced MMP2 and MMP9 protein levels compared to the PBS control group, and the OPF + ESC group decreased most by Western blotting. Transplanted mESCs expressed cardiovascular markers. This study suggests the potential of a method for heart regeneration involving OPF hydrogels for stem cell encapsulation and transplantation.

Building bridges: leveraging interdisciplinary collaborations in the development of biomaterials to meet clinical needs

Our laboratory at Rice University has forged numerous collaborations with clinicians and basic scientists over the years to advance the development of novel biomaterials and the modification of existing materials to meet clinical needs. This review highlights collaborative advances in biomaterials research from our laboratory in the areas of scaffold development, drug delivery, and gene therapy, especially as related to applications in bone and cartilage tissue engineering.

Foamed oligo(poly(ethylene glycol)fumarate) hydrogels as versatile prefabricated scaffolds for tissue engineering

Radically cross-linked hydrogels are frequently used as cell carriers due to their excellent biocompatibility and their tissue-like mechanical properties. Through frequent investigation, PEG-based polymers such as oligo(poly(ethylene glycol)fumarate [OPF] have proven to be especially suitable as cell carriers by encapsulating cells during hydrogel formation. In some cases, NaCl or biodegradable gelatin microparticles were added prior to cross-linking in order to provide space for the proliferating cells, which would otherwise stay embedded in the hydrogel matrix. However, all of these immediate cross-linking procedures involve time consuming sample preparation and sterilization directly before cell culture and often show notable swelling after their preparation. In this study, ready to use OPF-hydrogel scaffolds were prepared by gas foaming, freeze drying, individual packing into bags and subsequent γ-sterilization. The scaffolds could be stored and used "off-the-shelf" without any need for further processing prior to cell culture. Thus the handling was simplified and the sterility of the cell carrier was assured. Further improvement of the gel system was achieved using a two component injectable system, which may be used for homogenous injection molding in order to create individually shaped three dimensional scaffolds. In order to evaluate the suitability of the scaffolds for tissue engineering, constructs were seeded with juvenile bovine chondrocytes and cultured for 28 days. Cross-sections of the respective constructs showed an intense and homogenous red staining of GAG with safranin O, indicating a homogenous cell distribution within the scaffolds and the production of substantial amounts of GAG-rich matrix.

Utilizing microfluidics to synthesize polyethylene glycol microbeads for Förster resonance energy transfer based glucose sensing

Here, we utilize microfluidic droplet technology to generate photopolymerizeable polyethylene glycol (PEG) hydrogel microbeads incorporating a fluorescence-based glucose bioassay. A microfluidic T-junction and multiphase flow of fluorescein isothiocyanate dextran, tetramethyl rhodamine isothiocyanate concanavalin A, and PEG in water were used to generate microdroplets in a continuous stream of hexadecane. The microdroplets were photopolymerized mid-stream with ultraviolet light exposure to form PEG microbeads and were collected at the outlet for further analysis. Devices were prototyped in PDMS and generated highly monodisperse 72 ± 2 μm sized microbeads (measured after transfer into aqueous phase) at a continuous flow rate between 0.04 ml/h-0.06 ml/h. Scanning electron microscopy analysis was conducted to analyze and confirm microbead integrity and surface morphology. Glucose sensing was carried out using a Förster resonance energy transfer (FRET) based assay. A proportional fluorescence intensity increase was measured within a 1-10 mM glucose concentration range. Microfluidically synthesized microbeads encapsulating sensing biomolecules offer a quick and low cost method to generate monodisperse biosensors for a variety of applications including cell cultures systems, tissue engineering, etc.

Synthesis of oligo(poly(ethylene glycol) fumarate)

This protocol describes the synthesis of oligo(poly(ethylene glycol) fumarate) (OPF; 1-35 kDa; a polymer useful for tissue engineering applications) by a one-pot reaction of poly(ethylene glycol) (PEG) and fumaryl chloride. The procedure involves three parts: dichloromethane and PEG are first dried; the reaction step follows, in which fumaryl chloride and triethylamine are added dropwise to a solution of PEG in dichloromethane; and finally, the product solution is filtered to remove by-product salt, and the OPF product is twice crystallized, washed and dried under vacuum. The reaction is affected by the molecular weight of PEG and reactant molar ratio. The OPF product is cross-linked by radical polymerization by either a thermally induced or ultraviolet-induced radical initiator, and the physical properties of the OPF oligomer and resulting cross-linked hydrogel are easily tailored by varying PEG molecular weight. OPF hydrogels are injectable, they polymerize in situ and they undergo biodegradation by hydrolysis of ester bonds. The expected time required to complete this protocol is 6 d.

Development of hydrogels and biomimetic regulators as tissue engineering scaffolds

This paper reviews major research and development issues relating to hydrogels as scaffolds for tissue engineering, the article starts with a brief introduction of tissue engineering and hydrogels as extracellular matrix mimics, followed by a description of the various types of hydrogels and preparation methods, before a discussion of the physical and chemical properties that are important to their application. There follows a short comment on the trends of future research and development. Throughout the discussion there is an emphasis on the genetic understanding of bone tissue engineering application.