Sustained delivery of vascular endothelial growth factor with alginate beads

The effect of vascular endothelial growth factor (VEGF) presentation within fibrin matrices on endothelial cell branching

Vascular endothelial growth factor (VEGF) has been extensively investigated to promote vascularization at damaged or diseased sites and in tissue implants. Here we are interested in determining if the manner in which VEGF is presented from a scaffold to endothelial cells influences the architecture of the blood vessels formed. We bound VEGF to nanoparticles and placed these nanoparticles inside fibrin hydrogels, which contained human umbilical vein endothelial cells (HUVECs) bound to cytodex beads. Fibroblast cells are plated on top of the fibrin gel to further mimic a physiologic environment. In addition, we used a chorioallantoic membrane (CAM) assay to determine the role of VEGF presentation on angiogenesis in vivo. We tested VEGF bound in high density and low density to study differences between growth factor presentation in heterogeneous nanodomains and homogenous distribution. VEGF covalently bound to nanoparticles at high density led to an increase in HUVEC tube branching, thickness, and total vessel network length compared to soluble VEGF. While VEGF bound electrostatically exhibited no significant difference with covalently bound VEGF in the tube formation assay, this method failed to promote host vessel infiltration into the fibrin implant on the CAM. Together our data suggest that the mode of VEGF presentation to endothelial cells influences the vessel architecture and vascularization of implants in vivo.

Immunoisolating semi-permeable membranes for cell encapsulation: focus on hydrogels

Cell-based medicine has recently emerged as a promising cure for patients suffering from various diseases and disorders that cannot be cured/treated using technologies currently available. Encapsulation within semi-permeable membranes offers transplanted cell protection from the surrounding host environment to achieve successful therapeutic function following in vivo implantation. Apart from the immunoisolation requirements, the encapsulating material must allow for cell survival and differentiation while maintaining its physico-mechanical properties throughout the required implantation period. Here we review the progress made in the development of cell encapsulation technologies from the mass transport side, highlighting the essential requirements of materials comprising immunoisolating membranes. The review will focus on hydrogels, the most common polymers used in cell encapsulation, and discuss the advantages of these materials and the challenges faced in the modification of their immunoisolating and permeability characteristics in order to optimize their function.

Vascular endothelial growth factor release from alginate microspheres under simulated physiological compressive loading and the effect on human vascular endothelial cells

Bone tissue engineering has generated promising results in bone defect repair, but is limited by the inherently poor nutrient supply to nonvascularized tissue-engineered bone grafts. In this study, we investigated the delivery of vascular endothelial growth factor (VEGF) and the effect on in vitro cultured human umbilical vein endothelial cells (HUVECs), in an attempt to provide experimental basis for promoting the vascularization of tissue-engineered bone grafts. A mechanical stimulator was developed to generate a periodic compressive load analogous to goat locomotor characteristics, simulating the mechanical stimulation applied on the fracture ends of the load-bearing bone. Poly-l-lysine-coated VEGF/alginate microspheres were combined with demineralized bone matrices into composites, and the in vitro release of VEGF from these composites was evaluated under periodic compression. The effect of the release media on HUVECs was also investigated. Compression slightly accelerated VEGF release at the early stage (<11 days) compared with noncompressed composites, although the release profiles of the two composite groups were generally similar. The released VEGF promoted HUVEC proliferation. In addition, the periodic compression applied on composites containing both HUVECs and VEGF/alginate microspheres promoted the expression of matrix metalloproteinases-2/9 in HUVECs. This study provides a model for investigating VEGF release under simulated in vivo biomechanical conditions and without the disadvantage of the rapid degradation of VEGF in in vivo investigation of VEGF release. The results also provide important guidelines for future in vivo experiments.

Facile formation of dynamic hydrogel microspheres for triggered growth factor delivery

Dynamic hydrogels have emerged as an important class of biomaterials for temporal control over growth factor delivery. In this study we formed dynamic hydrogel microspheres from protein-polymer conjugates using an aqueous two-phase suspension polymerization process. This polymerization process enabled rapid microsphere formation without the use of an organic phase, surfactants, mechanical strain or toxic radical initiators. The microspheres' size distribution was modulated by varying the protein-polymer conformation in the pre-polymer solution. Notably, the protein's ligand-induced, nanometer-scale conformational change translated to maximum hydrogel volume changes of 76±10%. The magnitude of the microspheres' volume change was tuned by varying the crosslinking time and ligand identity. After characterizing the microspheres' dynamic properties, we encapsulated two important therapeutic proteins, vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2), in the hydrogel microspheres and characterized how the microspheres' dynamic properties controlled their release. Significantly, the aqueous two-phase suspension polymerization process enabled high encapsulation efficiencies (65.8±4.8% and 79.5±3.0% for VEGF and BMP-2, respectively). Also, the microspheres' ligand-induced volume change triggered VEGF and BMP-2 release at specific, predetermined times. There are hundreds of proteins that undergo well-characterized conformational changes that could be processed into hydrogel microspheres via aqueous two-phase suspension polymerizations. Therefore, this approach could be used to form dynamic, growth-factor-releasing hydrogel microspheres that respond to a broad range of specific biochemical ligands.

Inhalable, bioresponsive microparticles for targeted drug delivery in the lungs

Objective

There is a growing interest in developing bioresponsive drug delivery systems to achieve greater control over drug release than can be achieved with the conventional diffusion controlled polymeric delivery systems. While a number of such systems have been studied for oral or parenteral delivery, little or no work has been done on bioresponsive delivery systems for inhalation. Using the raised elastase levels present at sites of lung inflammation as a proof-of-concept model, we endeavoured to develop a prototype of inhalable elastase sensitive microparticles (ESMs).

Methods

Microparticles degradable by the enzyme elastase were formed by crosslinking the polymer alginate in the presence of an elastase substrate, elastin, using Ca+2 ions and subsequent spray drying.

Key findings

The bioresponsive release of a protein cargo in the presence of elastase demonstrated the enzyme-specific degradability of the particles. The microparticles showed favorable properties such as high drug encapsulation and good powder dispersibility. Potential polymer toxicity in the lungs was assessed by impinging the microparticles on Calu-3 cell monolayers and assessing changes in transepithelial permeability and induction of cytokine release. The microparticles displayed no toxic or immunogenic effects.

Conclusions

With a manufacturing method that is amenable to scale-up, the ability to be aerosolised efficiently from a first-generation inhaler device, enzyme-specific degradability and lack of toxicity, the ESMs show significant promise as pulmonary drug carriers.

Surface functionalization of polyketal microparticles with nitrilotriacetic acid-nickel complexes for efficient protein capture and delivery

Microparticle drug delivery systems have been used for over 20 years to deliver a variety of drugs and therapeutics. However, effective microencapsulation of proteins has been limited by low encapsulation efficiencies, large required amounts of protein, and risk of protein denaturation. In this work, we have adapted a widely used immobilized metal affinity protein purification strategy to non-covalently attach proteins to the surface of microparticles. Polyketal microparticles were surface modified with nitrilotriacetic acid-nickel complexes which have a high affinity for sequential histidine tags on proteins. We demonstrate that this high affinity interaction can efficiently capture proteins from dilute solutions with little risk of protein denaturation. Proteins that bound to the Ni-NTA complex retain activity and can diffuse away from the microparticles to activate cells from a distance. In addition, this surface modification can also be used for microparticle targeting by tethering cell-specific ligands to the surface of the particles, using VE-Cadherin and endothelial cells as a model. In summary, we show that immobilized metal affinity strategies have the potential to improve targeting and protein delivery via degradable polymer microparticles.

Characteristics of rhVEGF release from topical hydrogel formulations

PURPOSE

To study recombinant human vascular endothelial growth factor (rhVEGF), the release characteristics from topical gel formulations, and its interaction with the gelling agents.

METHODS

The release kinetics were followed by quantifying rhVEGF that diffused into the receptor chamber of Franz cells. Analytical ultracentrifuge (AUC) was used to characterize the sedimentation velocity of rhVEGF experienced in the gel. The interactions were characterized by isothermal calorimetry (ITC), and rhVEGF conformation was assessed by circular dichroism (CD).

RESULTS

The fraction of protein released was linear with the square root of time. The release rate constants did not show significant change within a wide range of bulk viscosities created by different concentrations of hydroxypropyl methylcellulose (HPMC) or MC gels. Sedimentation velocity determined by AUC generated comparable sedimentation coefficients of protein in these gels. AUC and ITC revealed no significant interaction between rhVEGF and HPMC and some change on secondary structure of the protein by Far UV CD, which was not the case with carboxymethyl cellulose (CMC).

CONCLUSIONS

Microviscosity, not bulk viscosity, was the key factor for the release of rhVEGF from cellulosic gels such as HPMC. Interaction between rhVEGF and CMC resulted in slower, and reduced amount of, release from the gel.

Outgrowth endothelial cells: sources, characteristics and potential applications in tissue engineering and regenerative medicine

Endothelial progenitor cells from peripheral blood or cord blood are attracting increasing interest as a potential cell source for cellular therapies aiming to enhance the neovascularization of tissue engineered constructs or ischemic tissues. The present review focus on a specific population contained in endothelial progenitor cell cultures designated as outgrowth endothelial cells (OEC) or endothelial colony forming cells from peripheral blood or cord blood. Special attention will be paid to what is currently known in terms of the origin and the cell biological or functional characteristics of OEC. Furthermore, we will discuss current concepts, how OEC might be integrated in complex tissue engineered constructs based on biomaterial or co-cultures, with special emphasis on their potential application in bone tissue engineering and related vascularization strategies.

In situ tissue engineering of canine skull with guided bone regeneration

CONCLUSION

Calcium alginate (CA) membrane prevents excessive fibrous tissue intrusion and/or dislocation of a bone scaffold. However, CA membrane did not always accelerate cranial bone regeneration.

OBJECTIVE

We previously reported skull regeneration using a bone substitute material (BSM), which consisted of collagen-coated beta-tricalcium phosphate and autologous bone fragments, and bone marrow-derived stromal cells (BSCs). However, excessive fibrous tissue intrusion or dislocation of the BSM occasionally interrupted bone regeneration. To avoid such problems, we examined CA membrane, which is useful for guided bone regeneration (GBR), to investigate whether this material maintains the bone regenerative space.

MATERIALS AND METHODS

Bone defects (2x2 cm) were created in the skulls of 12 adult beagle dogs using the same clinical procedure. Four experimental models were tested with or without BSM plus BSCs or CA membrane. In group I, the original free bone flap was replaced at the defect. In group II, after replacing the bone flap, the defect was covered with CA membrane. In group III, BSM plus BSCs were used as a gap filler. In group IV, BSM plus BSCs and CA membrane were applied. Histological examinations were performed 3 and 6 months after the operation.

RESULTS

In groups I and II, bone regeneration was not observed but fibrous tissue intrusion was prevented in group II. Bone neogenesis was more observed in group III than in group IV at 3 months (p<0.05). At 6 months, the regenerated areas were larger than those observed at 3 months, but the differences between groups III and IV were not statistically significant.

Naturally derived materials-based cell and drug delivery systems in skin regeneration

The objective of regenerative medicine is to provide cells with a local environment of artificial extracellular matrix where they can proliferate and differentiate efficiently and therefore, induce the repair of defective tissues according to the natural healing potential of patients. For this purpose, naturally derived materials are being widely used because of their similarities to the extracellular matrix, typically good biocharacteristics and inherent cellular interaction. Also, natural polymers can be engineered to release growth factors and related agents in response to physiologic signals to imitate the natural healing process and to promote fast tissue regeneration and reduce scarring in wounds. Although synthetic materials have been used extensively in tissue engineering fields, this review illustrates the contribution of natural materials and natural materials-based protein delivery systems to regenerative medicine research, with emphasis on the application of multifunctional vehicles for cell and growth factor delivery in skin regeneration research.

Carrageenan-based hydrogels for the controlled delivery of PDGF-BB in bone tissue engineering applications

One of the major drawbacks found in most bone tissue engineering approaches developed so far consists in the lack of strategies to promote vascularisation. Some studies have addressed different issues that may enhance vascularisation in tissue engineered constructs, most of them involving the use of growth factors (GFs) that are involved in the restitution of the vascularity in a damaged zone. The use of sustained delivery systems might also play an important role in the re-establishment of angiogenesis. In this study, kappa-carrageenan, a naturally occurring polymer, was used to develop hydrogel beads with the ability to incorporate GFs with the purpose of establishing an effective angiogenesis mechanism. Some processing parameters were studied and their influence on the final bead properties was evaluated. Platelet derived growth factor (PDGF-BB) was selected as the angiogenic factor to incorporate in the developed beads, and the results demonstrate the achievement of an efficient encapsulation and controlled release profile matching those usually required for the development of a fully functional vascular network. In general, the obtained results demonstrate the potential of these systems for bone tissue engineering applications.

Functionalized poly(lactic-co-glycolic acid) enhances drug delivery and provides chemical moieties for surface engineering while preserving biocompatibility

Poly(lactic-co-glycolic acid) (PLGA) is one of the more widely used polymers for biomedical applications. Nonetheless, PLGA lacks chemical moieties that facilitate cellular interactions and surface chemistries. Furthermore, incorporation of hydrophilic molecules is often problematic. The integration of polymer functionalities would afford the opportunity to alter device characteristics, thereby enabling control over drug interactions, conjugations and cellular phenomena. In an effort to introduce amine functionalities and improve polymer versatility, we synthesized two block copolymers (PLGA-PLL 502H and PLGA-PLL 503H) composed of PLGA and poly(epsilon-carbobenzoxy-l-lysine) utilizing dicyclohexyl carbodiimide coupling. PLGA-PLL microspheres encapsulated approximately sixfold (502H) and threefold (503H) more vascular endothelial growth factor, and 41% (503H) more ciliary neurotrophic factor than their PLGA counterparts. While the amine functionalities were amenable to the delivery of large molecules and surface conjugations, they did not compromise polymer biocompatibility. With the versatile combination of properties, biocompatibility and ease of synthesis, these block copolymers have the potential for diverse utility in the fields of drug delivery and tissue engineering.

From natural bone grafts to tissue engineering therapeutics: Brainstorming on pharmaceutical formulative requirements and challenges

Tissue engineering is an emerging multidisciplinary field of investigation focused on the regeneration of diseased or injured tissues through the delivery of appropriate molecular and mechanical signals. Therefore, bone tissue engineering covers all the attempts to reestablish a normal physiology or to speed up healing of bone in all musculoskeletal disorders and injuries that are lashing modern societies. This article attempts to give a pharmaceutical perspective on the production of engineered man-made bone grafts that are described as implantable tissue engineering therapeutics, and to highlight the importance of understanding bone composition and structure, as well as osteogenesis and bone healing processes, to improve the design and development of such implants. In addition, special emphasis is given to pharmaceutical aspects that are frequently minimized, but that, instead, may be useful for formulation developments and in vitro/in vivo correlations.

Drug releasing systems in cardiovascular tissue engineering

Heart disease and atherosclerosis are the leading causes of morbidity and mortality worldwide. The lack of suitable autologous grafts has produced a need for artificial grafts; however, current artificial grafts carry significant limitations, including thrombosis, infection, limited durability and the inability to grow. Tissue engineering of blood vessels, cardiovascular structures and whole organs is a promising approach for creating replacement tissues to repair congenital defects and/or diseased tissues. In an attempt to surmount the shortcomings of artificial grafts, tissue-engineered cardiovascular graft (TECVG), constructs obtained using cultured autologous vascular cells seeded onto a synthetic biodegradable polymer scaffold, have been developed. Autologous TECVGs have the potential advantages of growth, durability, resistance to infection, and freedom from problems of rejection, thrombogenicity and donor scarcity. Moreover polymers engrafted with growth factors, cytokines, drugs have been developed allowing drug-releasing systems capable of focused and localized delivery of molecules depending on the environmental requirements and the milieu in which the scaffold is placed. A broad range of applications for compound-releasing, tissue-engineered grafts have been suggested ranging from drug delivery to gene therapy. This review will describe advances in the development of drug-delivery systems for cardiovascular applications focusing on the manufacturing techniques and on the compounds delivered by these systems to date.

VEGF-controlled release within a bone defect from alginate/chitosan/PLA-H scaffolds

VEGF and its receptors constitute the key signaling system for angiogenic activity in tissue formation, but a direct implication of the growth factor in the recruitment, survival and activity of bone forming cells has also emerged. For this reason, we developed a composite (alginate/chitosan/PLA-H) system that controls the release kinetics of incorporated VEGF to enhance neovascularization in bone healing. VEGF release kinetics and tissue distribution were determined using iodinated ((125)I) growth factor. VEGF was firstly encapsulated in alginate microspheres. To reduce the high in vitro burst release, the microspheres were included in scaffolds. Matrices were prepared with alginate (A-1, A-2), chitosan (CH-1, CH-2) or by coating the CH-1 matrix with a PLA-H (30 kDa) film (CH-1-PLA), the latter one optimally reducing the in vitro and in vivo burst effect. The VEGF in vitro release profile from CH-1-PLA was characterized by a 13% release within the first 24h followed by a constant release rate throughout 5 weeks. For VEGF released from composite scaffolds in vitro, bioactivity was maintained above 90% of the expected value. Despite the fact that the in vivo release rate was slightly faster, a good in vitro-in vivo correlation was found. The VEGF released from CH-1 and CH-1-PLA matrices implanted into the femurs of rats remained located around the implantation site with a negligible systemic exposure. These scaffolds provided a bone local GF concentration above 10 ng/g during 2 and 5 weeks, respectively, in accordance to the in vivo release kinetics. Our data show that the incorporation of VEGF into the present scaffolds allows for a controlled release rate and localization of the GF within the bone defect.

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.

Controlled delivery of VEGF via modulation of alginate microparticle ionic crosslinking

Clinical application of therapeutic angiogenesis is hampered by a lack of viable systems that demonstrate controlled, sustained release of vascular endothelial growth factor (VEGF). Alginate has emerged as a popular material for VEGF delivery; however most alginate-based systems offer limited means to control the rate of VEGF release beyond reducing the VEGF:alginate ratio to suboptimal efficiency. This study describes methods to control the release of VEGF from small (<10 microm mean diameter) alginate microparticles via the use of different ionic crosslinkers. Crosslinking with Zn(2+) versus Ca(2+) reduced VEGF diffusional release and the combination of discrete populations of either Zn(2+)- or Ca(2+)-crosslinked particles allowed for control over the sustained release profiles for VEGF. The particle preparations were non-toxic and VEGF was bioactive after release. These results demonstrate that ionic modulation of alginate crosslinking is a viable strategy for controlling release of VEGF while retaining the high protein:polymer ratio that makes alginate an attractive carrier for delivery of protein therapeutics.

Intramuscular drug transport under mechanical loading: resonance between tissue function and uptake

Dynamic architecture and motion in mechanically active target tissues can influence the pharmacokinetics of locally delivered agents. Drug transport in skeletal muscle under controlled mechanical loads was investigated. Static (0-20%) and cyclic (+/-2.5% amplitude, 0-20% mean, 1-3 Hz) strains and electrically paced isometric contractions (0.1-3 Hz, 0% strain) were applied to rat soleus incubated in 1 mM 20 kDa FITC-dextran. Dextran penetration, tissue porosity, and active force-length relationship over 0-20% strain correlated (r=0.9-1.0), and all increased 1.5-fold from baseline at 0% to a maximum at 10% (Lo), demonstrating biologic significance of Lo and impact of fiber size and distribution on function and pharmacokinetics. Overall penetration decreased but relative enhancement of penetration at Lo increased with dextran size (4-150 kDa). Penetration increased linearly (0.084 mm/Hz) with cyclic stretch, demonstrating dispersion. Penetration increased with contraction rate by 1.5-fold from baseline to a maximum at 0.5 Hz, revealing architectural modulation of dispersion. Impact of architecture and dispersion on intramuscular transport was computationally modeled. Mechanical architecture and function underlie intramuscular pharmacokinetics and act in concert to effect resonance between optimal physiologic performance and drug uptake. Therapeutic management of characteristic function in tissue targets may enable a physiologic mechanism for controlled drug transport.

Wound healing dressings and drug delivery systems: a review

The variety of wound types has resulted in a wide range of wound dressings with new products frequently introduced to target different aspects of the wound healing process. The ideal dressing should achieve rapid healing at reasonable cost with minimal inconvenience to the patient. This article offers a review of the common wound management dressings and emerging technologies for achieving improved wound healing. It also reviews many of the dressings and novel polymers used for the delivery of drugs to acute, chronic and other types of wound. These include hydrocolloids, alginates, hydrogels, polyurethane, collagen, chitosan, pectin and hyaluronic acid. There is also a brief section on the use of biological polymers as tissue engineered scaffolds and skin grafts. Pharmacological agents such as antibiotics, vitamins, minerals, growth factors and other wound healing accelerators that take active part in the healing process are discussed. Direct delivery of these agents to the wound site is desirable, particularly when systemic delivery could cause organ damage due to toxicological concerns associated with the preferred agents. This review concerns the requirement for formulations with improved properties for effective and accurate delivery of the required therapeutic agents. General formulation approaches towards achieving optimum physical properties and controlled delivery characteristics for an active wound healing dosage form are also considered briefly.

A review of the development of a vehicle for localized and controlled drug delivery for implantable biosensors

A major obstacle to the development of implantable biosensors is the foreign body response (FBR) that results from tissue trauma during implantation and the continuous presence of the implant in the body. The in vivo stability and functionality of biosensors are compromised by damage to sensor components and decreased analyte transport to the sensor. This paper summarizes research undertaken by our group since 2001 to control the FBR toward implanted sensors. Localized and sustained delivery of the anti-inflammatory drug, dexamethasone, and the angiogenic growth factor, vascular endothelial growth factor (VEGF), was utilized to inhibit inflammation as well as fibrosis and provide a stable tissue-device interface without producing systemic adverse effects. The drug-loaded polylactic-co-glycolic acid (PLGA) microspheres were embedded in a polyvinyl alcohol (PVA) hydrogel composite to fabricate a drug-eluting, permeable external coating for implantable devices. The composites were fabricated using the freeze-thaw cycle method and had mechanical properties similar to soft body tissue. Dexamethasone-loaded microsphere/hydrogel composites were able to provide anti-inflammatory protection, preventing the FBR. Moreover, concurrent release of dexamethasone with VEGF induced neoangiogenesis in addition to providing anti-inflammatory protection. Sustained release of dexamethasone is required for the entire sensor lifetime, as a delayed inflammatory response developed after depletion of the drug from the composites. These studies have shown the potential of PLGA microsphere/PVA hydrogel-based composites as drug-eluting external coatings for implantable biosensors.

The use of murine embryonic stem cells, alginate encapsulation, and rotary microgravity bioreactor in bone tissue engineering

The application of embryonic stem cells (ESCs) in bone tissue engineering and regenerative medicine requires the development of suitable bioprocesses that facilitate the integrated, reproducible, automatable production of clinically-relevant, scaleable, and integrated bioprocesses that generate sufficient cell numbers resulting in the formation of three-dimensional (3D) mineralised, bone tissue-like constructs. Previously, we have reported the enhanced differentiation of undifferentiated mESCs toward the osteogenic lineage in the absence of embryoid body formation. Herein, we present an efficient and integrated 3D bioprocess based on the encapsulation of undifferentiated mESCs within alginate hydrogels and culture in a rotary cell culture microgravity bioreactor. Specifically, for the first 3 days, encapsulated mESCs were cultured in 50% (v/v) HepG2 conditioned medium to generate a cell population with enhanced mesodermal differentiation capability followed by osteogenic differentiation using osteogenic media containing ascorbic acid, beta-glycerophosphate and dexamethasone. 3D mineralised constructs were generated that displayed the morphological, phenotypical, and molecular attributes of the osteogenic lineage, as well mechanical strength and mineralised calcium/phosphate deposition. Consequently, this bioprocess provides an efficient, automatable, scalable and functional culture system for application to bone tissue engineering in the context of macroscopic bone formation.

Controlled delivery of platelet-rich plasma-derived growth factors for bone formation

Platelet-rich plasma (PRP) represents an autologous source of growth factors essential for bone regeneration. The clinical efficacy of PRP is, however, unpredictable, and this is likely due to the inefficient and inconsistent delivery of PRP-derived growth factors. Previous investigations have shown that current methods of PRP preparation result in a premature release of the relevant bone stimulatory factors. As successful bone regeneration requires multiple factors presented in a physiologic temporal and spatial cascade, the objective of this study is to control the bioavailability of PRP-derived growth factors using a hydrogel carrier system. Specifically, the release of platelet-derived growth factor, transforming growth factor beta-1, and insulin-like growth factor from two types of alginate carriers was compared over time. The effects of the released factors on the growth and alkaline phosphatase (ALP) activity of human osteoblast-like cells were also evaluated. It was found that factor release profiles varied as function of carrier type, and binding of growth factors to the alginate matrix also modulated their release. The bioactivity of released factors was maintained in vitro and they promoted cell proliferation and ALP activity. These results demonstrate the potential of this autologous multifactor delivery system for controlling the bioavailability of PRP-derived factors. Future studies will focus on optimizing this system to increase the clinical efficacy of PRP by matching the distribution and temporal sequencing of PRP-derived factors to the bone healing cascade.

The effect of sulfation of alginate hydrogels on the specific binding and controlled release of heparin-binding proteins

To mimic the high affinity of heparin-binding proteins to heparin/heparan sulfate, the uronic acids in non-sulfated alginate were sulfated, and hydrogels of mixed alginate/alginate-sulfate were fabricated. Surface plasmon resonance analysis probed the interactions of 13 proteins with alginate-sulfate. Of these, the 10 heparin-binding proteins revealed strong binding to alginate-sulfate and heparin, but not to alginate. The equilibrium binding constants to alginate-sulfate were comparable or one order of magnitude higher than those obtained between the proteins and heparin. Only the fibroblast growth factors (FGFs) revealed higher affinity for heparin than to alginate-sulfate. Sulfation of hyaluronan, as well, resulted in strong binding of basic FGF to hyaluronan-sulfate, but not to hyaluronan. Mixed hydrogels of alginate/alginate-sulfate sustained the release of basic FGF, with the release rate being dependent on the percentage of bFGF bound to the hydrogels. In vivo, the delivery of bFGF bound to alginate/alginate-sulfate scaffolds induced the formation of twice the number of blood vessels compared to when bFGF was delivered adsorbed to the matrix and 51% of the vessels were matured, as judged by pericyte coverage of the vessels. Our results thus describe the engineering of alginate hydrogels for the spatially presentation and controlled delivery of heparin-binding proteins.

Engineering of multifunctional gels integrating highly efficient growth factor delivery with endothelial cell transplantation

Transplantation of Bcl-2-transduced human umbilical vein endothelial cells (ECs) in protein gels into the gastrocnemius muscle improves local reperfusion in immunodeficient mouse hosts with induced hind limb ischemia. We tested the hypothesis that incorporation of local, sustained growth factor delivery could enhance and accelerate this effect. Tissue engineering scaffolds often use synthetic polymers to enable controlled release of proteins, but most synthetic delivery systems have major limitations, most notably hydrophobicity and inefficient protein loading. Here, we report the development of a novel alginate-based delivery system for vascular endothelial growth factor-A(165) (VEGF) that exhibits superior loading efficiency and physical properties to previous systems in vitro. In vivo, VEGF released from alginate microparticles within protein gels was biologically active and, when combined with EC transplantation, led to increased survival of transplanted cells at 28 days. The composite graft described also improved early (14 days) tissue perfusion and late (28 days) muscle myoglobin expression, a sign of recovery from ischemia, compared with EC transplantation and VEGF delivery separately. We conclude that our improved approach to sustained VEGF delivery in tissue engineering is useful in vivo and that the integration of high efficiency protein delivery enhances the therapeutic effect of protein gel-based EC transplantation.

Stem cells and scaffolds for vascularizing engineered tissue constructs

The clinical impact of tissue engineering depends upon our ability to direct cells to form tissues with characteristic structural and mechanical properties from the molecular level up to organized tissue. Induction and creation of functional vascular networks has been one of the main goals of tissue engineering either in vitro, for the transplantation of prevascularized constructs, or in vivo, for cellular organization within the implantation site. In most cases, tissue engineering attempts to recapitulate certain aspects of normal development in order to stimulate cell differentiation and functional tissue assembly. The induction of tissue growth generally involves the use of biodegradable and bioactive materials designed, ideally, to provide a mechanical, physical, and biochemical template for tissue regeneration. Human embryonic stem cells (hESCs), derived from the inner cell mass of a developing blastocyst, are capable of differentiating into all cell types of the body. Specifically, hESCs have the capability to differentiate and form blood vessels de novo in a process called vasculogenesis. Human ESC-derived endothelial progenitor cells (EPCs) and endothelial cells have substantial potential for microvessel formation, in vitro and in vivo. Human adult EPCs are being isolated to understand the fundamental biology of how these cells are regulated as a population and to explore whether these cells can be differentiated and reimplanted as a cellular therapy in order to arrest or even reverse damaged vasculature. This chapter focuses on advances made toward the generation and engineering of functional vascular tissue, focusing on both the scaffolds - the synthetic and biopolymer materials - and the cell sources - hESCs and hEPCs.

Preparation of calcium alginate microgel beads in an electrodispersion reactor using an internal source of calcium carbonate nanoparticles

An electrodispersion reactor has been used to prepare calcium alginate (Ca-alginate) microgel beads in this study. In the electrodispersion reactor, pulsed electric fields are utilized to atomize aqueous mixtures of sodium alginate and CaCO3 nanoparticles (dispersed phase) from a nozzle into an immiscible, insulating second liquid (continuous phase) containing a soluble organic acid. This technique combines the features of the electrohydrodynamic force driven emulsion processes and externally triggered gelations in microreactors (the droplets) ultimately to yield soft gel beads. The average particle size of the Ca-alginate gels generated by this method changed from 412 +/- 90 to 10 +/- 3 microm as the applied peak voltage was increased. A diagram depicting structural information for the Ca-alginate was constructed as a function of the concentrations of sodium alginate and CaCO3 nanoparticles. From this diagram, a critical concentration of sodium alginate required for sol-gel transformation was observed. The characteristic highly porous structure of Ca-alginate particles made by this technique appears suitable for microencapsulation applications. Finally, time scale analysis was performed for the electrodispersion processes that include reactions in the microreactor droplets to provide guidelines for the future employment of this technique. This electrodispersion reactor can be used potentially in the formation of many reaction-based microencapsulation systems.

Photo-crosslinkable and biodegradable Pluronic/heparin hydrogels for local and sustained delivery of angiogenic growth factor

Photo-crosslinkable and biodegradable Pluronic/heparin composite hydrogels were fabricated for local and sustained delivery of basic fibroblast growth factor (bFGF) to induce angiogenesis. Terminally di-acrylated Pluronic F127 and vinyl group conjugated heparin were used as a mixed macromer precursor solution to prepare a photo-crosslinkable hydrogel. An aqueous solution containing the two macromers with different weight ratios was photo-crosslinked in the presence of bFGF to produce in situ formed bFGF loaded Pluronic/heparin hydrogels. Swelling, mass erosion, bFGF release characteristics of Pluronic/heparin hydrogels were thoroughly examined by varying the weight ratio of the two macromers. The incorporation of heparin in the composite hydrogel enabled the controlled release of bFGF over a one month period in a near zero order manner. The prolonged release of bFGF could be attributed to the specific interaction between bFGF and heparin in the hydrogel matrices. The released bFGF fraction from the degradable hydrogels also showed sufficient proliferation activity of human umbilical vein endothelial cell (HUVEC). When the Pluronic/heparin hydrogels were implanted in vivo, a significant extent of neo-vascularization was observed.

Hydrogels for tissue engineering and delivery of tissue-inducing substances

This manuscript presents hydrogels (HGs) from a tissue engineering perspective being especially written for those who are approaching this field by offering a concise but inclusive review of hydrogel synthesis, properties, characterization methods, and applications.

Microvessel-Like Structures from Outgrowth Endothelial Cells from Human Peripheral Blood in 2-Dimensional and 3-Dimensional Co-Cultures with Osteoblastic Lineage Cells

Tissue regeneration involves complex processes in the interaction between different cell types that control the process of neo-vascularization. In bone, osteoblasts and bone marrow stem cells provide cue elements for the proliferation of endothelial cells, differentiation of endothelial precursors, and the maturation of a vascular network. In this study, we investigated outgrowth endothelial cells (OECs), a potential source of autologous endothelial cells derived from human peripheral blood, in direct 2-dimensional (2-D) and 3-D co-culture systems with cells relevant for the regeneration of bone tissue, such as osteoblasts. In the co-cultures, OECs were evaluated in terms of their stability as an endothelial population at the single cell level using flow cytometry and their ability to establish a pre-vascular network at the light-microscopical and ultra-structural level. In co-cultures with the osteoblast cell line MG63 and with human primary osteoblasts (pOBs), OECs, in contrast to human umbilical vein endothelial cells, formed highly organized microvessel-like structures. These microvessel-like structures included the formation of a vascular lumen with tight junctional complexes at intercellular contacts of endothelial cells. In the co-culture, the formation of this vascular network was achieved in the standard growth medium for OECs. Furthermore, using a rotating culture vessel system, 3-D co-cultures consisting of OECs and pOBs were generated. Based on these observations, we conclude that OECs could provide a valuable source of autologous endothelial cells for the generation of complex tissue-engineered tissues.

In vitro models of angiogenesis

Neovascularization can be categorized into two general processes: vasculogenesis and angiogenesis. Angiogenesis is the formation of new capillaries from pre-existing vessels, requiring growth factor driven recruitment, migration, proliferation, and differentiation of endothelial cells (ECs). Complex cell-cell and cell-extracellular matrix (ECM) interactions contribute to this process, leading finally to a network of tube-like formations of endothelial cells supported by surrounding mural cells. The study of angiogenesis has broad clinical implications in the fields of peripheral and coronary vascular disease, oncology, hematology, wound healing, dermatology, and ophthalmology, among others. As such, novel, clinically relevant models of angiogenesis in vitro are crucial to the understanding of angiogenic processes. We highlight some of the advances made in the development of these models, and discuss the importance of incorporating the three-dimensional cell-matrix and EC-mural cell interactions into these in vitro assays of angiogenesis. This review also discusses our own 3-D angiogenesis assay and some of the in vitro results from our lab as they relate to therapeutic neovascularization and tissue engineering of vascular grafts.

Immobilization and bioactivity of glucose oxidase in hydrogel microspheres formulated by an emulsification–internal gelation–adsorption–polyelectrolyte coating method

The purpose of this study was to develop a novel microsphere formulation of glucose oxidase (GOX) with high drug loading, encapsulation efficiency and bioactivity. GOX was encapsulated in alginate/chitosan microspheres (ACMS) using an emulsification-internal gelation, followed by GOX adsorption and polyelectrolyte coating method. The factors influencing GOX loading, encapsulation efficiency and activity of the loaded GOX were investigated. The resultant ACMS in wet state were spherical with a mean diameter of about 138 microm. GOX loading was found to be pH dependent. High GOX loading and encapsulation efficiency were achieved when the pH of the adsorption medium was lower than the isoelectric point (pI) of GOX. GOX loading and encapsulation efficiency increased with increasing GOX concentration in the loading solution, but decreased with increasing chitosan concentration in the coating solution. The activity of loaded GOX increased and then decreased with increasing chitosan concentration. The activity of GOX in ACMS was maintained and showed sustained production of H(2)O(2) as compared to free GOX. Around 90% of the original activity of immobilized GOX remained after lyophilization and storage at -20 degrees C for a month. These results suggest that the ACMS and the fabrication method are suitable for microencapsulation of proteins like GOX.