Biocompatible controlled release polymers for delivery of polypeptides and growth factors

Biomimetic gradient scaffolds for the tissue engineering and regeneration of rotator cuff enthesis

Rotator cuff tear is one of the most common musculoskeletal disorders, which often results in recurrent shoulder pain and limited movement. Enthesis is a structurally complex and functionally critical interface connecting tendon and bone that plays an essential role in maintaining integrity of the shoulder joint. Despite the availability of advanced surgical procedures for rotator cuff repair, there is a high rate of failure following surgery due to suboptimal enthesis healing and regeneration. Novel strategies based on tissue engineering are gaining popularity in improving tendon-bone interface (TBI) regeneration. Through incorporating physical and biochemical cues into scaffold design which mimics the structure and composition of native enthesis is advantageous to guide specific differentiation of seeding cells and facilitate the formation of functional tissues. In this review, we summarize the current state of research in enthesis tissue engineering highlighting the development and application of biomimetic scaffolds that replicate the gradient TBI. We also discuss the latest techniques for fabricating potential translatable scaffolds such as 3D bioprinting and microfluidic device. While preclinical studies have demonstrated encouraging results of biomimetic gradient scaffolds, the translation of these findings into clinical applications necessitates a comprehensive understanding of their safety and long-term efficacy.

Gene delivery of hypoxia-inducible VEGF targeting collagen effectively improves cardiac function after myocardial infarction

Vascular endothelial growth factor (VEGF) plays important roles in improvement of cardiac function following myocardial infarction (MI). However, the lack of a steerable delivery system of VEGF targeting the infarcted myocardium reduces the therapeutic efficacy and safety. Here, we constructed a series of lentiviral vector systems which could express a fusion protein consisted of a collagen-binding domain (CBD) and hVEGF (CBDhVEGF), under the control of 5HRE-hCMVmp (5HRE), the hypoxia-inducible promoter consists of five copies of the hypoxia-responsive element (HRE) and a human cytomegalovirus minimal promoter (hCMVmp). We demonstrated that 5HRE has the comparable ability to strongly drive CBDhVEGF under hypoxic condition as the ubiquitous CMV promoter, but it can hardly drive target gene under normoxic condition. 5HRE-drived CBDhVEGF specifically bound to type I collagen and significantly promoted the viability of HUVEC cells. Moreover, after injection of lentivirus into heart of mouse with MI, CBDhVEGF was mainly retained in infarcted myocardium where containing rich collagen and significantly improved angiogenesis and cardiac function when compared with hVEGF. Moreover, CBDhVEGF mediated by lentivirus has little leakage from infarcted zone into blood than hVEGF. Taken together, our results indicate that 5HRE-CBDhVEGF lentiviral vector system could improve cardiac function in the collagen-targeting and hypoxia-inducible manners.

Neuroprotective Effects of Encapsulated CNTF-Producing Cells in a Rodent Model of Huntington's Disease are Dependent on the Proximity of the Implant to the Lesioned Striatum

Huntington's disease (HD) is a devastating genetic disorder with no effective treatments for preventing or lessening the underlying neuronal degeneration. Intracerebral delivery of CNTF in animal models of HD has shown considerable promise as a means of protecting striatal neurons that would otherwise be destined to die. The present study examines whether the neuroprotective effects of CNTF require that the delivery be immediately proximal to the lesion site or whether protective effects can be exerted when the delivery site is more distal to the site of injury. Encapsulated CNTF-producing cells were implanted into the lateral ventricle either ipsilateral or contralateral to an intrastriatal quinolinic acid (QA) injection. A robust neuroprotective effect was observed only in those animals receiving CNTF implants ipsilateral to the QA injection. In these animals, the loss of striatal ChAT and GAD activity as well as the behavioral impairments that resulted from QA were completely prevented. In contrast, no neurochemical or behavioral benefits were produced by implants of CNTF-producing cells in the contralateral ventricle. These data continue to support the use of cellular delivery of CNTF for HD but caution that delivery directly to the striatum may be needed if any clinical benefits are to be seen.

Polymers for Induction of Revascularization in the Rat Fascial Flap: Application of Vascular Endothelial Growth Factor and Pancreatic Islet Cells

One of the major obstacles in transplanting avascular tissue or metabolically active cells for ischemic diseases is the loss of transplanted cells due to lack of oxygen and nutrients in the early posttransplantation period. Biodegradable polymeric tissue engineering scaffolds and hydrogels have a potential to incorporate cells or cellular organoids such as islets of Langerhans and growth factors. In this study, we tested the efficiency of two types of polymeric materials to carry recombinant human vascular endothelial growth factor (rhVEGF) or pancreatic tumor cell lines, namely Ins-1 and AR42J, for the induction of new vessels. Chitosan hydrogel fibers with micropores were prepared and molded into a cylinder construct (5 mm φ 8 mm height). Macroporous PLGA scaffolds with a pore size of 250–400 μm were prepared and cut into cylinders (6 mm φ 3 mm height). Both chitosan and PLGA constructs were loaded with rhVEGF (3 μg) or seeded with the cell lines (5 × 105 cells and 3 × 105 cells/construct, respectively, for AR42J and INS-1 cells), and transplanted into the fascial flaps of Wistar rats. At distinct time points up to 4 weeks postimplantation, polymers were explanted, fixed, and vessel density was counted on sections stained with anti-Factor-VIII antibody. Additionally, the kinetics of rhVEGF release from PLGA microspheres (φ of 50–80 μm) was determined using VEGF Elisa. Endogenous VEGF release from pancreatic rat cell lines was also determined. Light microscopy study was performed on H&E-stained paraffin sections of the islet-polymer samples. The vascular density of rhVEGF-loaded chitosan constructs was increased fourfold 2 weeks after subcutaneous transplantation compared with rhVEGF-unloaded controls (465 ± 144 vs. 104 ± 80 vessels per mm2, p < 0.05). Protein leakage occurred, but was not observed after 2 weeks. Higher insulin content was detected in rat islet grafts transplanted following VEGF application. More than 50% of total rhVEGF was released on the first day of in vitro culture of PLGA microspheres. rhVEGF secretion had another, but smaller, peak on the third day followed by a constant release. By comparison, endogeneous VEGF secretion of pancreatic tumor cells was measured within a 3-day culture period. Biodegradable polymer scaffolds and hydrogels may have potential use as solid supports to induce angiogenesis for pancreatic cell transplantation.

Angiogenic Effects of Collagen/Mesoporous Nanoparticle Composite Scaffold Delivering VEGF165

Vascularization is a key issue for the success of tissue engineering to repair damaged tissue. In this study, we report a composite scaffold delivering angiogenic factor for this purpose. Vascular endothelial growth factor (VEGF) was loaded on mesoporous silica nanoparticle (MSN), which was then incorporated within a type I collagen sponge, to produce collagen/MSN/VEGF (CMV) scaffold. The CMV composite scaffold could release VEGF sustainably over the test period of 28 days. The release of VEGF improved the cell proliferation. Moreover, the in vivo angiogenesis of the scaffold, as studied by the chick chorioallantoic membrane (CAM) model, showed that the VEGF-releasing scaffold induced significantly increased number of blood vessel complexes when compared with VEGF-free scaffold. The composite scaffold showed good biocompatibility, as examined in rat subcutaneous tissue. These results demonstrate that the CMV scaffold with VEGF-releasing capacity can be potentially used to stimulate angiogenesis and tissue repair.

In situ Surface Tailoring with Zwitterionic Carboxybetaine Moieties on Self-Assembled Thin Film for Antifouling Biointerfaces

A novel biointerface bearing zwitterionic carboxybetaine moieties was developed for effective resistance to nonspecific adsorption of proteins and blood cells. Self-assembled thin films (SAFs) of (N,N-dimethylaminopropyl) trimethoxysilane were formed as mattress layers by either vapor or solution deposition. Subsequently, the tertiary amine head groups on SAFs were reacted with β-propiolactone to give zwitterionic carboxybetaine moieties via in situ synthesis. The optimal reaction time of 8 h for both preparation methods was verified by static contact angle measurements. According to the X-ray photoelectron spectroscopy, 67.3% of amine groups on SAFs prepared from the vapor deposition was converted to the zwitterionic structures after reaction of β-propiolactone. The antifouling properties of the zwitterionic biointerfaces were quantitatively evaluated in the presence of protein solutions using a quartz crystal microbalance with dissipation, showing a great improvement by factors of 6.5 and 20.2 from tertiary amine SAFs and bare SiO₂ surfaces, respectively. More importantly, the zwitterionic SAFs were brought to contact with undiluted human blood in chaotic-mixer microfluidic systems; the results present their capability to effectively repel blood cell adhesion. Accordingly, in this work, development of carboxybetaine SAFs offers a facile yet effective strategy to fabricate biocompatible biointerfaces for a variety of potential applications in surface coatings for medical devices.

Physical non-viral gene delivery methods for tissue engineering

The integration of gene therapy into tissue engineering to control differentiation and direct tissue formation is not a new concept; however, successful delivery of nucleic acids into primary cells, progenitor cells, and stem cells has proven exceptionally challenging. Viral vectors are generally highly effective at delivering nucleic acids to a variety of cell populations, both dividing and non-dividing, yet these viral vectors are marred by significant safety concerns. Non-viral vectors are preferred for gene therapy, despite lower transfection efficiencies, and possess many customizable attributes that are desirable for tissue engineering applications. However, there is no single non-viral gene delivery strategy that "fits-all" cell types and tissues. Thus, there is a compelling opportunity to examine different non-viral vectors, especially physical vectors, and compare their relative degrees of success. This review examines the advantages and disadvantages of physical non-viral methods (i.e., microinjection, ballistic gene delivery, electroporation, sonoporation, laser irradiation, magnetofection, and electric field-induced molecular vibration), with particular attention given to electroporation because of its versatility, with further special emphasis on Nucleofection™. In addition, attributes of cellular character that can be used to improve differentiation strategies are examined for tissue engineering applications. Ultimately, electroporation exhibits a high transfection efficiency in many cell types, which is highly desirable for tissue engineering applications, but electroporation and other physical non-viral gene delivery methods are still limited by poor cell viability. Overcoming the challenge of poor cell viability in highly efficient physical non-viral techniques is the key to using gene delivery to enhance tissue engineering applications.

Polymer-coated mesoporous silica nanoparticles for the controlled release of macromolecules

With the goal of achieving constant release of large biological molecules over an extended period of time we focused on hybrid inorganic/organic nanoparticles. We synthesized poly(ethylene glycol) (PEG)-coated mesoporous silica nanoparticles (MSNs) with incorporated trypsin inhibitor (TI), a model protein molecule for growth factors. Due to the goal of incorporating large protein molecules the pore size of the as-synthesized MSNs was expanded by a hydrothermal treatment prior to TI incorporation. In vitro release from the MSNs without the thin polymer film shows an initial burst followed by continuous release. In the case of polymer-coated MSNs the initial burst release was completely suppressed and approximate zero order release was achieved for 4 weeks.

Temporal and spatial control over soluble protein signaling for musculoskeletal tissue engineering

Orthopedic tissue engineering strategies have developed rapidly in response to large and growing clinical needs. However, current clinical methods for replacement of natural tissue function have significant limitations, and pragmatic challenges have hindered clinical use of emerging tissue engineering approaches. In addition, current methods are not yet capable of achieving complex spatial and temporal regulation of soluble signaling (e.g. growth factor signaling), which may be required for complex, functional tissue regeneration. We have begun to develop a series of new medical devices, which are designed to temporally and spatially regulate growth factor and cytokine signaling during tissue regeneration. The initial goal of these studies is to regulate the behavior of multipotent stem cells, and to promote formation of clinically relevant tissue interfaces (e.g. bone-tendon interfaces). The ultimate goal is to further understand and recapitulate the complex processes that lead to functional musculoskeletal development and regeneration.

Bio-printing of collagen and VEGF-releasing fibrin gel scaffolds for neural stem cell culture

Time-released delivery of soluble growth factors (GFs) in engineered hydrogel tissue constructs promotes the migration and proliferation of embedded cells, which is an important factor for designing scaffolds that ultimately aim for neural tissue regeneration. We report a tissue engineering technique to print murine neural stem cells (C17.2), collagen hydrogel, and GF (vascular endothelial growth factor: VEGF)-releasing fibrin gel to construct an artificial neural tissue. We examined the morphological changes of the printed C17.2 cells embedded in the collagen and its migration toward the fibrin gel. The cells showed high viability (92.89+/-2.32%) after printing, which was equivalent to that of manually-plated cells. C17.2 cells printed within 1mm from the border of VEGF-releasing fibrin gel showed GF-induced changes in their morphology. The cells printed in this range also migrated toward the fibrin gel, with the total migration distance of 102.4+/-76.1microm over 3days. The cells in the control samples (fibrin without the VEGF or VEGF printed directly in collagen) neither proliferated nor migrated. The results demonstrated that bio-printing of VEGF-containing fibrin gel supported sustained release of the GF in the collagen scaffold. The presented method can be gainfully used in the development of three-dimensional (3D) artificial tissue assays and neural tissue regeneration applications.

Novel factor-loaded polyphosphazene matrices: potential for driving angiogenesis

Currently employed bone tissue engineered scaffolds often lack the potential for vascularization, which may be enhanced through the incorporation of and regulated release of angiogenic factors. For this reason, the objective here was to fabricate and characterize protein-loaded amino acid ester polyphosphazene (Pphos)-based scaffolds and evaluate the novel sintering method used for protein incorporation, a method which will ultimately allow for the incorporation of proangiogenic agents. To test the hypothesis, Pphos and their composite microspheres with nanocrystalline hydroxyapatite (Pphos-HAp) were fabricated via the emulsion solvent evaporation method. Next, bovine serum albumin (BSA)-containing microsphere matrices were created using a novel solvent-non-solvent approach for protein loading. The resulting protein (BSA) loaded circular porous microsphere based scaffolds were characterized for morphology, porosity, protein structure, protein distribution and subsequent protein release pattern. Scanning electron microscopy revealed porous microsphere scaffolds with a smooth surface and sufficient level of sintering, illustrated by fusion of adjacent microspheres. The porosity measured for the poly(ethyl phenylalanato:glycinato)phosphazene (PNPhGly) and poly(ethyl phenylalanato:glycinato)phosphazene-hydroxyapatite (PNPhGly-HAp) scaffolds were 23 +/- 0.11% and 18 +/- 4.02%, respectively, and within the range of trabecular bone. Circular dichroism confirmed an intact secondary protein structure for BSA following the solvent sintering method used for loading and confocal microscopy verified that FITC-BSA was successfully entrapped both between adjacent microspheres and within the surface of the microspheres while sintering. For both Pphos and their composite microsphere scaffolds, BSA was released at a steady rate over a 21 day time period, following a zero order release profile. HAp particles in the composite scaffolds served to improve the release profile pattern, underscoring the potential of HAp for growth factor delivery. Moreover, the results of this work suggest that the solvent-non-solvent technique for protein loading is an optimal one that will allow for future development of angiogenic factor-loaded Pphos matrices with the capacity to invoke neovascularization.

The effect of coencapsulation of bovine insulin with cyclodextrins in ethylcellulose microcapsules

Polymeric microcapsules have been widely investigated for protein delivery. Common problems include: low stability, low encapsulation efficiency, lack of uniformity, and burst release. Cyclodextrins (CDs) are known to enhance stability and solubility of proteins in solution. This research examines the effect of alpha-, beta-, and gamma-CDs on: (1) stability, (2) encapsulation, and (3) release of insulin from ethylcellulose microcapsules. All CDs improved thermal stability of insulin by lowering the enthalpy of unfolding by 16-52%. alpha- and gamma-CDs also increased the encapsulation efficiency of insulin and improved uniformity of the microcapsule formulations. Two mathematical models were proposed to account for insulin release and consisted of multiple zero order and first order input processes, and a single first order output process. All CDs decreased the initial burst release of insulin by up to 30%. This research demonstrates the potential for CDs to improve stability, uniformity, and encapsulation of proteins in microcapsule formulations.

Fibrin gel-immobilized VEGF and bFGF efficiently stimulate angiogenesis in the AV loop model

The modulation of angiogenic processes in matrices is of great interest in tissue engineering. We assessed the angiogenic effects of fibrin-immobilized VEGF and bFGF in an arteriovenous loop (AVL) model in 22 AVLs created between the femoral artery and vein in rats. The loops were placed in isolation chambers and were embedded in 500 microL fibrin gel (FG) (group A) or in 500 microL FG loaded with 0.1 ng/microL VEGF and 0.1 ng/microL bFGF (group B). After two and four weeks specimens were explanted and investigated using histological, morphometrical, and ultramorphological [scanning electron microscope (SEM) of vascular corrosion replicas] techniques. In both groups, the AVL induced formation of densely vascularized connective tissue with differentiated and functional vessels inside the fibrin matrix. VEGF and bFGF induced significantly higher absolute and relative vascular density and a faster resorption of the fibrin matrix. SEM analysis in both groups revealed characteristics of an immature vascular bed, with a higher vascular density in group B. VEGF and bFGF efficiently stimulated sprouting of blood vessels in the AVL model. The implantation of vascular carriers into given growth factor-loaded matrix volumes may eventually allow efficient generation of axially vascularized, tissue-engineered composites.

A biodegradable copolymer for the slow release of growth hormone expedites scarring in diabetic rats

In many diseases wound healing is impaired. This study was designed to establish whether the healing process in diabetes could be improved using a site-specific polymer delivery system containing hGH. The system was first optimized in in vitro experiments performed on cultured fibroblasts taken from healthy and diabetic rats and then tested in an incisional wound model created in the diabetic Wistar rat. In the in vitro experiments using cultured fibroblasts, cell viability, growth, and proliferation were determined, along with polymer degradation, hormone release rates and the expression of TGFbeta1 in the culture medium. For the in vivo experiments, polymer discs with/without GH were inserted through 3 cm incisions made on the backs of the animals. Wound specimens were obtained 7 and 30 days after surgery to evaluate inflammatory/apoptotic cells, metalloprotease expression and neoangiogenesis using microscopy and immunohistochemical techniques. The local administration of GH using a polymer delivery system did not affect the normal wound healing process. Conversely, when used in diabetic animals, epidermal and dermal repair was expedited. Our findings indicate that GH induces cell proliferation, enhances CD4(+) infiltration; increases extracellular matrix protein deposition; stimulates angiogenesis; and diminishes apoptosis at the diabetic wound site. These effects give rise to a comparable wound healing process to that observed in healthy animals.

Improved Neovascularization of PEGT/PBT Copolymer Matrices in Response to Surface Modification by Biomimetic Coating

PEGT/PBT (polyethylene glycol terephthalate/polybutylene terephthalate) copolymer matrices with three different surface coatings [calcium-phosphate (Ca-P), collagen, and gas plasma] were placed into dorsal skinfold chambers of 24 balb/c mice. Untreated PEGT/PBT matrices served as the controls. The basal surfaces of the implants directly contacted the striated skin muscle. Neovascularization of the implants was analyzed by intravital fluorescence microscopy. Microcirculatory observations were performed in the surrounding skin muscle, at the border zone of the implant, and in the center of the implant. The functional vessel density (FVD; mm/mm2), as the length of perfused microvessels per observation area, was measured by computer-assisted analysis. The FVD served as the parameter of neovascularization. At the end of the protocol, histological observation of hematoxylin/eosin-standard-stained sections was performed by light microscopy. The FVD in the center of the implant on day 8 was only observed in gas-plasma-coated (8.8 +/- 10.2 mm/mm2) and Ca-P-coated implants (0.8 +/- 2.0 mm/mm2). None of the other groups showed perfused microvessels in the center of the implant on day 8 (p < 0.05). The FVD values in the center of the gas-plasma-coated and the Ca-P-coated implants were 20.7 +/- 8.2 and 19.2 +/- 15.5 mm/mm2 as compared with 7.1 +/- 17.4 and 7.7 +/- 5.9 mm/mm2 for collagen-coated and untreated implants on day 16. The histological examination confirmed the profound microvascular ingrowth into the matrix pores of the gas-plasma-treated and the Ca-P-coated copolymer matrices in the center of the implants. The study showed that the ingrowth of microvessels into PEGT/PBT matrices can be accelerated by Ca-P coating and gas plasma treatment in the dorsal skinfold chamber in mice.

Angiogenesis-like activity of endothelial cells co-cultured with VEGF-producing smooth muscle cells

A number of strategies have been investigated to improve therapeutic vascularization of ischemic and bioengineered tissues. In these studies, we genetically modified vascular smooth muscle cells (VSMC) to promote endothelial cell proliferation, migration, and formation of microvascular networks. VSMCs were virally transduced to produce vascular endothelial growth factor (VEGF), which acts as a chemoattractant and mitogen of endothelial cells (EC). VSMCs transduced with VEGF(165) cDNA produced significant levels of the protein (2-4 ng/10(5) cell/day). The proliferation of ECs increased after exposure to VEGF-transfected SMCs or their conditioned media. The chemotactic response of ECs to the VEGF-producing cells was explored in two in vitro systems, the modified Boyden chamber assay and a 2-D fence-style migration assay, and both demonstrated increased migration of ECs in response to VEGF-transfected cells. Furthermore, endothelial cells seeded on top of the VEGF-transfected SMCs formed capillary-like structures. These results suggest that VSMCs genetically modified to produce VEGF could be a potential delivery mechanism to enhance endothelial cell migration and subsequent capillary formation, which in turn could improve vascularization of ischemic or regenerating tissue. Furthermore, this system could potentially be used as an in vitro test bed for evaluation of novel angiogenic and anti-angiogenic compounds.

Bioluminescent imaging: Emerging technology for non-invasive imaging of bone tissue engineering

Bone tissue engineering is a multidisciplinary research area in which new strategies are developed to treat patients with large bone defects as occurring during e.g. hip revisions, upon trauma or in spinal fusions. In vivo evaluation of bone formation in animal models is highly relevant for graft evaluation but is time-consuming, invasive and difficult to quantify. As a consequence, most in vivo studies ignore the dynamic nature of bone regeneration and the molecular processes underlying it. In vivo bioluminescent imaging (BLI) is a relatively young research field with great potential that may overcome these problems. BLI encompasses non-invasive imaging of luciferase gene activity using cooled charge coupled device cameras in luciferase transgenic animals or in grafted, luciferase transgenic cells. The imaging procedure is technically simple and quantifiable. Because luciferase expression can be put under the control of tissue-specific regulatory elements, BLI allows non-invasive imaging of processes highly relevant to bone tissue engineering like differentiation, apoptosis, vasculogenesis and inflammation. In this review, we describe the basic principle of BLI and discuss transgenic animals and constructs currently available for application in bone tissue engineering. Furthermore, we reflect on technical developments that will make BLI even more promising for future application in bone tissue engineering research.

Angiogenesis with biomaterial-based drug- and cell-delivery systems

Angiogenesis, the formation of new blood vessels from existing ones, is an important event in several biological processes, including wound healing. It plays a key role in determining the final functionality and integration of any implanted medical device. In addition, angiogenesis is a required event for organ development and has been accepted as a rate-limiting step in engineering tissue replacements. Besides these regenerative processes, uncontrolled angiogenesis is also involved in a number of pathologies, including tumor growth and metastases. Like angiogenesis, biomaterials also play a role in wound healing after medical device implantation and in tissue engineering. Interactions between the device biomaterials and host tissue will factor into the final device integration. Additionally, tissue-engineering strategies utilize biomaterials to a great extent because the paradigm of tissue engineering involves the use of cells, growth factors and scaffolding matrices in order to regenerate or replace tissue. Since almost all tissues are three-dimensional, the biomaterial scaffold plays an integral role in the paradigm. This review will emphasize the influence of biomaterials on angiogenesis as it applies to medical device implantation, tissue engineering and therapies for pathological angiogenesis.

Biodegradable polymer with collagen microsponge serves as a new bioengineered cardiovascular prosthesis

OBJECTIVE

Biodegradable materials with autologous cell seeding have attracted much interest as potential cardiovascular grafts. However, pretreatment of these materials requires a complicated and invasive procedure that carries the risk of infection. To avoid these problems, we sought to develop a biodegradable graft material containing collagen microsponge that would permit the regeneration of autologous vessel tissue. The ability of this material to accelerate in situ cellularization with autologous endothelial and smooth muscle cells was tested with and without precellularization.

METHODS

Poly(lactic-co-glycolic acid) as a biodegradable scaffold was compounded with collagen microsponge to form a vascular patch material. These poly(lactic-co-glycolic acid)-collagen patches with (n = 10) or without (n = 10) autologous vessel cellularization were used to patch the canine pulmonary artery trunk. Histologic and biochemical assessments were performed 2 and 6 months after the implantation.

RESULTS

There was no thrombus formation in either group, and the poly(lactic-co-glycolic acid) scaffold was almost completely absorbed in both groups. Histologic results showed the formation of an endothelial cell monolayer, a parallel alignment of smooth muscle cells, and reconstructed vessel wall with elastin and collagen fibers. The cellular and extracellular components in the patch had increased to levels similar to those in native tissue at 6 months.

CONCLUSIONS

The poly(lactic-co-glycolic acid)-collagen microsponge patch with and without precellularization showed good histologic findings and durability. This patch shows promise as a bioengineered material for promoting in situ cellularization and the regeneration of autologous tissue in cardiovascular surgery.

Delivery of a growth factor fusion protein having collagen-binding activity to wound tissues

Recently, we established a collagen-binding growth factor consisting of epidermal growth factor and the fibronectin collagen-binding domain (FNCBD-EGF). FNCBD-EGF is a biologically active fusion protein that could stably bind to collagen materials, and exert its growth factor activity even after collagen binding. In this study, we investigated the concept that FNCBD moiety with high collagen affinity may enhance the effective local concentration of EGF at the site of administration in the following tissues: skin wounds, catheter-injured arteries, and hind limb muscles. In an animal model of impaired wound healing, application of FNCBD-EGF in combination with collagen gel induced granulation tissue formation in the wounds due to its sustained retention. In the injured artery, infused FNCBD-EGF remained bound to collagen exposed on the injured tissues even after blood circulation was restored. Injection of the fusion protein into the hind limbs revealed that our delivery system was effective for direct administration to muscular tissue.

Controlled-release of IGF-I and TGF-beta1 in a photopolymerizing hydrogel for cartilage tissue engineering

Photopolymerizing hydrogel systems provide a method to encapsulate cells and implant materials in a minimally invasive manner. Controlled release of growth factors in the hydrogels may enhance the ability to engineer tissues. IGF-I and TGF-beta1 were loaded in PLGA microspheres using a double emulsion technique. 125 ng and 200 pg of active IGF-I and TGF-beta, respectively, as measured by ELISA, were released over 15 days. The growth factor containing microspheres were photoencapsulated with bovine articular chondrocytes in PEO-based hydrogels and incubated in vitro for two weeks. Statistically significant changes in glycosaminoglycan (GAG) production compared to control gels either without microspheres or with blank spheres were observed after a 14 day incubation with IGF-I and IGF-I/TGF-beta microspheres combined, with a maximum density of 8.41+/-2.5% wet weight GAG. Total collagen density was low and decreased with the IGF-I/TGF-beta microspheres after two weeks incubation, but otherwise remained unchanged in all other experimental groups. Cell content increased 10-fold to 0.18+/-0.056 x 10(6) cells/mg wet weight and extracellular matrix (ECM) staining by H&E increased in hydrogels with IGF-I/TGF-beta microspheres. In conclusion, photoencapsulation of microspheres in PEO-based hydrogels provides a method to deliver molecules such as growth factors in porous hydrogel systems.

Extensive in vivo angiogenesis following controlled release of human vascular endothelial cell growth factor: implications for tissue engineering and wound healing

Vascular endothelial cell growth factor (VEGF) has strong stimulating effects on vascularization. Though very potent, VEGF is rapidly degraded due to its short half-life and when administrated by uncontrolled and nonspecific methods; however, its systemic administration in large doses can cause harmful side effects. Controlled release technology would allow delivering desired levels of bioactive VEGF within extended periods and permit examination of the in vivo effects of the compound in a broader way. The objective of this study was to determine the in vitro release behavior of VEGF from calcium alginate microspheres and the potency of this controlled release system in promoting localized neovascularization at the subcutaneous site of the rat model. In vitro release of human VEGF165 (2 and 4 microg/cm3 microsphere) was studied for 3 weeks under static conditions at 25 degrees C, and daily hormone release was measured using a competitive enzyme immunoassay. Following an uncontrolled release within the first 4 days, a quite constant zero-order VEGF release of 50 to 90 and 70 to 120 ng/day was achieved from 2 and 4 microg/cm3 polymer loaded microspheres respectively. In vivo angiogenesis was studied for a period of 8 weeks and evaluated using immunoperoidase staining and histopathological measurements. In vivo studies with rats (n = 24) showed a considerable level of capillary network formation at the epigastric groin fascia of VEGF microsphere-implanted rats starting from the first week. The most extensive neovascularization was observed in the group with 3 week postimplanted 4 microg VEGF containing microspheres; this level of vascularization was quite similar after 8 weeks. While the control group showed no evidence of angiogenesis, the difference in VEGF-induced neovascularization is statistically significant (p < 0.03). Immunostaining of the specimens showed a strong relationship between the release of human VEGF and neovascularization. The controlled VEGF release system described here promotes vigorous angiogenesis and has applicability for tissue engineering and wound healing studies.

Development of poly-(D,L-lactide--coglycolide) microsphere formulations containing recombinant human vascular endothelial growth factor to promote local angiogenesis

Although preclinical animal studies have demonstrated the utility of recombinant human vascular endothelial growth factor (rhVEGF) in promoting neovascularization in regions of ischemia, rhVEGF systemic administration did not provide clinical benefit to patients in recent placebo-controlled Phase II clinical trials. The amount of rhVEGF localized in the ischemic region after systemic administration is minimal and does not persist for more than 1 day. A greater persistence of rhVEGF at the region of ischemia may provide an increased angiogenesis with the eventual formation of patent blood vessels to restore nourishment to the tissues. We sought to develop a formulation of rhVEGF in poly(D,L-lactide--co-glycolide) (PLG) microspheres that would provide a continuous local delivery of intact protein. A stable formulation of rhVEGF for encapsulation contained a small amount of a stabilizing sugar, trehalose. Addition of excess trehalose increased the rate of release from the PLG. In addition, PLG with free acid end groups appeared to retard the initial release of rhVEGF by associating with it through ionic interactions at the positively charged heparin binding domain. rhVEGF was released continuously for 21 days with a very low (less than 10%) initial burst. The released rhVEGF aggregated and hydrolyzed over time and lost heparin affinity but not receptor affinity. The compression molding of rhVEGF PLG microspheres into disks yielded formulations with a low initial release and a lag of 10 days followed by complete release. The PLG microsphere formulations were assessed in the corneal implant model of angiogenesis and generated a dose-dependent angiogenic response. These formulations were also administered intravitreally and subretinally, generating local neovascularization comparable to the human disease states, vitroretinopathy and age-related macular degeneration, respectively. The rhVEGF PLG formulations may increase local angiogenesis without systemic side effects and may also be useful in the development of ocular disease models.

Growth factor delivery for tissue engineering

A tissue-engineered implant is a biologic-biomaterial combination in which some component of tissue has been combined with a biomaterial to create a device for the restoration or modification of tissue or organ function. Specific growth factors, released from a delivery device or from co-transplanted cells, would aid in the induction of host parenchymal cell infiltration and improve engraftment of co-delivered cells for more efficient tissue regeneration or ameliorate disease states. The characteristic properties of growth factors are described to provide a biological basis for their use in tissue engineered devices. The principles of polymeric device development for therapeutic growth factor delivery in the context of tissue engineering are outlined. A review of experimental evidence illustrates examples of growth factor delivery from devices such as microparticles, scaffolds, and encapsulated cells, for their use in the application areas of musculoskeletal tissue, neural tissue, and hepatic tissue.

Epidermal and fibroblast growth factors enhance fibrovascular integration of porous polyethylene implants

PURPOSE

Porous implants used in functional and aesthetic reconstruction of the orbit, face, and cranium are less likely to develop complications after they become biointegrated. We investigated whether the administration of exogenous growth factors could increase the rate of implant integration.

METHODS

High-density porous polyethylene cubes were placed in dorsal paraspinal muscles of rabbits, and daily transcutaneous injections of saline, epidermal growth factor, or basic fibroblast growth factor were administered directly over the cubes for 10 days. At serial time points up to 10 weeks, cubes were explanted and the fibroblasts present at the center of the cubes were counted.

RESULTS

Injections of epidermal growth factor and basic fibroblast growth factor increased the rate at which fibroblasts accumulated in porous polyethylene implants and decreased the time required to achieve a maximal rate of cellular accumulation within the cubes. At 4 weeks, when all cell populations had attained a linear rate of accumulation, cubes previously injected with saline, epidermal growth factor, or basic fibroblast growth factor contained an average of 10, 40, and 80 cells per 0.0156 mm2, at their centers, respectively.

CONCLUSIONS

Enhancement of the rate of biointegration of porous polyethylene cubes in rabbits is achievable by repeated, transcutaneous administration of exogenous growth factors.

Cell delivery to the central nervous system

A dysfunctional central nervous system (CNS) resulting from neurological disorders and diseases impacts all of humanity. The outcome presents a staggering health care issue with a tremendous potential for developing interventive therapies. The delivery of therapeutic molecules to the CNS has been hampered by the presence of the blood-brain barrier (BBB). To circumvent this barrier, putative therapeutic molecules have been delivered to the CNS by such methods as pumps/osmotic pumps, osmotic opening of the BBB, sustained polymer release systems and cell delivery via site-specific transplantation of cells. This review presents an overview of some of the CNS delivery technologies with special emphasis on transplantation of cells with and without the use of polymer encapsulation technology.

Tissue-engineered ligament: cells, matrix, and growth factors

Current treatment modalities for anterior cruciate ligament (ACL) tears rely on the use of grafts for reconstruction. Treatment can be divided into three categories: autografts, allografts, and synthetic graft replacements. The varied success rates and associated advantages and disadvantages of each method have resulted in controversy as to the best treatment for ACL injuries.

Growth factor delivery for tissue engineering

A tissue-engineered implant is a biologic-biomaterial combination in which some component of tissue has been combined with a biomaterial to create a device for the restoration or modification of tissue or organ function. Specific growth factors, released from a delivery device or from co-transplanted cells, would aid in the induction of host parenchymal cell infiltration and improve engraftment of co-delivered cells for more efficient tissue regeneration or ameliorate disease states. The characteristic properties of growth factors are described to provide a biological basis for their use in tissue engineered devices. The principles of polymeric device development for therapeutic growth factor delivery in the context of tissue engineering are outlined. A review of experimental evidence illustrates examples of growth factor delivery from devices such as microparticles, scaffolds, and encapsulated cells, for their use in the application areas of musculoskeletal tissue, neural tissue, and hepatic tissue.

Mechanisms controlling diffusion and release of model proteins through and from partially esterified hyaluronic acid membranes

The effects of polymer percent esterification and protein molecular weight on the diffusion of two model proteins, deoxyribonuclease (DNase) and ribonuclease A (RNase A), through and from partially esterified hyaluronic acid membranes were compared. The permeability of the polymer membranes was inversely related to the degree of polymer esterification and the molecular weight of the protein. Transport rates of proteins through the membranes decreased dramatically over narrow ranges of polymer esterification. As expected, the apparent diffusivity of the larger protein in the polymer matrix was more sensitive to changes in membrane hydration than that of the smaller protein. These observations demonstrated the dependence of the mobility of large molecular weight proteins on polymer hydration and chain relaxation. The relationship between protein diffusion through and release from the modified hyaluronate matrices was also investigated using RNase A as a model. The release profiles from fully esterified membranes showed lag behavior and varied with protein load and hyaluronate hydrolysis rates, while release from less esterified membranes was rapid and independent of polymer esterification or hydrolysis. Potential applications of modified hyaluronate matrices in the controlled delivery of proteins are discussed.

Variation in the diffusion and release of ribonuclease through and from esterified hyaluronic acid membranes: effect of changes in matrix characteristics

50 h) were detected for the transport and release of a model protein (ribonuclease A) compared with that for the translucent region which showed no lag time. The results highlight the importance of carefully controlling matrix formation to ensure reproducible transport and release characteristics from polymer matrices.

Adenovirus-mediated gene therapy of osteoblasts in vitro and in vivo

Modulation of biological pathways governing osteogenesis may accelerate osseous regeneration and reduce the incidence of complications associated with fracture healing. Transforming growth factor beta1 (TGF-beta1) is a potent growth factor implicated in the regulation of osteogenesis and fracture repair. The use of recombinant proteins, however, has significant disadvantages and has limited the clinical utility of these molecules. Targeted gene therapy using adenovirus vectors is a technique that may circumvent difficulties associated with growth factor delivery. In this study, we investigate the efficacy of replication-deficient adenoviruses containing the human TGF-beta1 and the bacterial lacZ genes in transfecting osteoblasts in vitro and osseous tissues in vivo. We demonstrate that adenovirus-mediated gene therapy efficiently transfects osteoblasts in vitro with the TGF-beta1 virus causing a marked up-regulation in TGF-beta1 mRNA expression even 7 days after transfection. Increased TGF-beta1 mRNA expression was efficiently translated into protein production and resulted in approximately a 46-fold increase in TGF-beta1 synthesis as compared with control cells (vehicle- or B-galactosidase-transfected). Moreover, virally produced TGF-beta1 was functionally active and regulated the expression of collagen IalphaI (5-fold increase) and the vascular endothelial growth factor (2.5-fold increase). Using an adenovirus vector encoding the Escherichia coli LacZ gene, we demonstrated that adenovirus-mediated gene transfer efficiently transfects osteoblasts and osteocytes in vivo and that transfection can be performed by a simple percutaneous injection. Finally, we show that delivery of the hTGF-beta1 gene to osseous tissues in vivo results in significant changes in the epiphyseal plate primarily as a result of increased thickness of the provisional calcification zone.

Polypeptide growth factors: targeted delivery systems

Growth factors are becoming extremely valuable tools in our attempts to understand the mechanisms that modulate cellular activities. Their targeting to appropriate cells and maintaining adequate pharmacological levels becomes essential, particularly in view of the different effects that these compounds have on various cells and the dose dependence of their response. Within this context, this review focuses primarily on the delivery of growth factors involved in the processes of wound healing and tissue repair.

Cellular response to transforming growth factor-beta1 and basic fibroblast growth factor depends on release kinetics and extracellular matrix interactions

The extracellular matrix plays an important role in growth factor biology, serving as a potential platform for rapid growth factor mobilization or a sink for concentrated sequestration. We now demonstrate that when a growth factor binds reversibly to the matrix, its effects are augmented by this interaction, and when the factor is absorbed irreversibly to the extracellular matrix, it becomes sequestered. These findings call into question the notion that all growth factors are best presented to cells and tissues in a sustained and controlled fashion. In our studies, we examined basic fibroblast growth factor (bFGF) and transforming growth factor-beta1 (TGF-beta1) release kinetics from synthetically fabricated microsphere devices and naturally synthesized extracellular matrix. While the sustained release of bFGF was up to 3.0-fold more potent at increasing vascular endothelial and smooth muscle cell proliferation than bolus administration, the reverse was true for TGF-beta1. A bolus of TGF-beta1 inhibited vascular cells up to 3.8-fold more efficiently than the same amount of TGF-beta1 if control-released. Both growth factors bound to the extracellular matrix, but only bFGF was released in a controlled fashion (2.8%/day). Contact with the extracellular matrix and subsequent release enhanced bFGF activity such that it was 86% more effective at increasing smooth muscle cell numbers than equal amounts of growth factor diluted from frozen stock. TGF-beta1 remained tightly adherent. The small amount of TGF-beta1 released from the extracellular matrix was approximately 30% less effective than bolus administration at inhibiting vascular endothelial and smooth muscle cell growth. Sustained growth factor release may be the preferable mode of administration, but only when a similar mode of metabolism is utilized endogenously.

Polylactic acid (PLA) root replica in ridge maintenance after loss of a vertically fractured incisor

A periodontally affected tooth was prepared for a special treatment: Calcium hydroxide was introduced into the apical half of the root canal whereas its cervical part was filled with glass ionomer cement. The tooth was shortened subgingivally. After 6 weeks of epithelization over the residual root a palatal full-thickness flap was mobilized. The root was carefully extracted and chairside copy-milled from the biodegradable polylactic acid (PLA) material. The PLA-replica was implanted immediately into the socket and the flap was sutured. Aim of the treatment was to prevent the ridge collapse of the extraction area. Ridge height could be preserved during the 21 months of observation. With time the radiographic density of the cancellous bone increased in the implanted area, indicating that a PLA-replica is replaced by host's bone tissue.

Controlled release of endothelial cell growth factor from chitosan-albumin microspheres for localized angiogenesis: in vitro and in vivo studies

Endothelial cell growth factor (ECGF) stimulates vascularization, however its relatively short half-life requires this angiogenic factor to be frequently administrated by non-specific and uncontrolled methods. This work describes the use of biocompatible chitosan, a polysaccharide having structural similarity to glycosaminoglycans, -albumin microspheres, as well as its fiber form, as a potential delivery system for the controlled and localized release of ECGF. Chitosan-albumin microspheres (400-600 microns) and fibers, formed in 0.5 M sodium hydroxide-methanol solution were incubated with ECGF. In vitro release was performed in PBS at 37 degrees C, under constant stirring. In vivo experiments were realized by implanting ECGF loaded matrices subcutaneously into rat groin fascia. After an initial ECGF burst of 1.32-1.62 mg (22-27%) within the first 2 hours, a daily release of 120-420 micrograms (2-7%) during the first, and 60-240 micrograms (1-4%) during the second week was observed from M(r) 70.000, 750.000, and 2,000.000 chitosan containing microspheres of 6 mg/ml loading. ECGF release rate of < 30 micrograms (0.5%)/day was maintained during the third week of experiments. By the increase in ECGF loading (12 mg/ml polymer), while the amount of release increased, percent release decreased. Chitosan-albumin fibers gave a ECGF release rate nearly similar to microspheres, and in vivo studies demonstrated a high degree of neovascularization for both types of implants, starting from 7 day-post implantation. Control animals that received ECGF injection did not show any significant neovascularization, after same period of time.

Vascular effects of sustained-release fibroblast growth factors

Since the half-life of most angiogenic growth factors is several hours or less, sustained-release delivery would be optimal for their future clinical use. Two fibroblast growth factors, basic fibroblast growth factor (bFGF) and endothelial cell growth factor (ECGF), were delivered in two sustained-released modalities (poloxamer 407 and a gelatin sponge [Gelfoam]) to attempt to increase soft tissue vascularity. In vitro bioactivity of ECGF-poloxamer formulations was also tested on endothelial cell cultures. Among vascular-compromised skin flaps in rabbits, ECGF-poloxamer (N = 26), bFGF-poloxamer (N = 5), ECGF-poloxamer (N = 9, irradiated), and bFGF-Gelfoam flaps (N = 22) did not demonstrate significant differences in viability and vascularity compared to controls (p > .05). Irradiation had a detrimental effect on both flap vascularity and viability (p = .02). Future efforts for sustained delivery of angiogenic proteins are critical in order to make them clinically useful in wound healing.

In vitro slow release profile of endothelial cell growth factor immobilized within calcium alginate microbeads

Although a variety of angiogenic growth factors have been isolated, its appropriate in vivo delivery remains problematic due to nonspecific, uncontrolled delivery by conventional methods. We have investigated calcium alginate microbeads as a vehicle for the controlled slow-release of endothelial cell growth factor (ECGF). Three different microbead compositions, dependent on ECGF amount and alginate percentage were studied. Microbeads were incubated in a 1.5% calcium chloride solution and release of ECGF into solution was measured spectrophotometrically at specific timepoints. Our results show release rate and amount released after the first 2 hours are dependent on initial quick delivery of ECGF in the first 2 hours after which a sustained controlled release occurred for 4-5 days. Beyond this point, release at a slower rate was noted for at least approximately 2 weeks. Calcium alginate microbeads demonstrated a controlled and predictable rate of release and that the amount of ECGF delivered can be varied by varying the initial concentration of ECGF in the microbeads. Based on these observations we conclude that calcium alginate microbeads are a convenient and practical vehicle for sustained ECGF delivery.

Potential of polymer microencapsulation technology for vaccine innovation

Biodegradable polymer microspheres or microcapsules developed over the past decade for reliable, preprogrammed release of contraceptive steroids have significant potential for adaptation to antigen release for immunization. In addition, polymeric encapsulation of antigens could prevent the acid and enzymatic degradation that has been a barrier to the development of oral vaccines. This review summarizes the published experience with microencapsulated hormones and antigens, describes the process of microsphere production, discusses the strengths and weaknesses of this approach to immunization, and outlines the gaps in knowledge. Microsphere technology has the potential benefits of reducing the number of inoculations, enhancing the immune response via both parenteral and oral vaccination routes, and in reducing the total antigen dose required to achieve immune protection.