Sustained Growth Factor Delivery Research Articles

Local and Sustained Vascular Endothelial Growth Factor Delivery for Angiogenesis Using an Injectable System

We hypothesize that a microsphere/hydrogel combination system could be useful for the local and sustained delivery of recombinant human vascular endothelial growth factor (rhVEGF) to enhance angiogenesis in vivo. Poly(D,L-lactide-co-glycolide) (PLGA) microspheres containing rhVEGF were loaded into alginate gels by ionic cross-linking. The rhVEGF release from the system was monitored and bioactivity was tested in vitro. The combination system was subcutaneously injected into mice using a syringe, and new blood vessel formation was evaluated. Sustained rhVEGF release from the combination system was observed for 3 weeks, and the released rhVEGF remained bioactive. Endothelial cell proliferation was significantly enhanced when cells were cultured with the rhVEGF-releasing combination system in vitro. When the combination system was implanted, the granulation tissue layer was thicker with more newly formed blood vessels than that with a single dose VEGF injection. The rhVEGF release was controlled by varying relative portions of microspheres and hydrogels in combination delivery systems, which efficiently promoted new blood vessel formation in vivo. This combination system could be a promising delivery vehicle for therapeutic angiogenesis.

Local application of hepatocyte growth factor using gelatin hydrogels attenuates noise-induced hearing loss in guinea pigs

Conclusion: Local application of hepatocyte growth factor using biodegradable gelatin hydrogels attenuates noise-induced hearing loss in guinea pigs. Objectives: To develop an inner ear drug delivery system using gelatin hydrogels that is capable of a sustained delivery of growth factors to the cochlea. We examined the efficacy of the local application of gelatin hydrogels containing hepatocyte growth factor (HGF) in protecting cochlear hair cells from noise-induced damage. Materials and methods: A piece of gelatin hydrogel previously immersed in either HGF or saline was placed on the round window membrane of a guinea pig 1 h after noise exposure (4 kHz octave band noise at 120 dB sound pressure level for 3 h). Auditory function was monitored using auditory brainstem responses (ABRs), and the loss of hair cells was evaluated quantitatively. Results: Local HGF treatment significantly reduced the noise exposure-caused ABR threshold shifts and the loss of outer hair cells in the basal portion of the cochleae.

Fabrication and characterization of a novel microparticle with gyrus-patterned surface and growth factor delivery for cartilage tissue engineering

Microparticles can serve as substrates for cell amplification and deliver the expanded cells to the site of the defect. It was hypothesized that a novel microparticle combined of sustained and localized delivery of proliferative growth factors and gyrus-patterned surface would influence the cell behaviours of adherence and expansion on the microparticle in the present study. To test the hypothesis, gelatin particles with diameter ranging from 280 to 350 µm were fabricated and were modified by cryogenic freeze-drying treatment and basic fibroblast growth factor (bFGF) incorporation. The results of in vitro chondrocyte culture illustrated that cells could proliferate more obviously on the microparticles with bFGF addition, but no correlation between attachment rate and bFGF was observed. On the other hand, microparticles with gyrus-patterned surface demonstrated the highest cell attachment rate and higher rate of cell growth, in particular on bFGF combined ones. It seems to be a promising candidate as a chondrocyte microparticle and could be the potential application in cartilage tissue engineering.

Immobilization of Growth Factors on Collagen Scaffolds Mediated by Polyanionic Collagen Mimetic Peptides and Its Effect on Endothelial Cell Morphogenesis

Angiogenesis, a morphogenic event endothelial cells (ECs) undergo in response to 3-D environmental triggers, is critical to the survival and ultimate functional capacity of engineered tissue constructs. Here we present a new collagen mimetic peptide (CMP) architecture consisting of multiple anionic charges at the peptide's N-terminus designed to attract growth factors by charge-charge interactions and bind to collagen by CMP-collagen interaction. The anionic CMPs exhibited specific binding affinity to type I collagen substrates while attracting vascular endothelial growth factors (VEGFs), which led to enhanced morphological features of ECs, indicative of tubulogenesis. The results show that these new CMPs could be used to direct proliferation and differentiation of cells in collagen scaffolds by localization and sustained delivery of growth factors and other morphogens.

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.

PDGF‐BB released in tendon repair using a novel delivery system promotes cell proliferation and collagen remodeling

The purpose of this study was to promote fibroblast proliferation and collagen remodeling in flexor tendon repair through sustained delivery of platelet derived growth factor (PDGF-BB). The release kinetics of PDGF-BB from a novel fibrin matrix delivery system was initially evaluated in vitro. After the in vivo degradation rate of the fibrin matrix was determined using fluorescently tagged fibrin, PDGF-BB was delivered to the site of flexor tendon repair in vivo in a canine model. The effect of PDGF-BB on intrasynovial tendon healing was studied using histology-based assays (cell density, proliferation, and type I collagen expression) and by measuring total DNA levels and reducible collagen crosslink levels. The fibrin matrix delivery system provided sustained release of PDGF-BB in vitro at a rate modulated by the ratio of heparin to growth factor. In vivo, the fibrin matrix remained at the repair site for more than 10 days. Delivery of PDGF-BB led to a qualitative increase in cell density, cell proliferation, and type I collagen mRNA expression. PDGF-BB also led to statistically significant increases in total DNA (20% increase at 7 days, 18% increase at 14 days) and reducible collagen crosslinks (30% increase at 7 days). Sustained delivery of growth factors may be achieved using a novel fibrin-based delivery system. PDGF-BB delivery increased cell proliferation and matrix remodeling and thus may accelerate flexor tendon healing.

Perspectives On: Local and Sustained Delivery of Angiogenic Growth Factors

This review emphasizes the role of angiogenesis in tissue engineering, introduces various angiogenic growth factors, and highlights current status of delivery systems for angiogenic growth factors using natural and synthetic biomaterials. A short overview of angiogenic growth factors is presented, followed by the introduction of emerging strategies for designing smart delivery carriers.

The osteoinductive properties of Nell-1 in a rat spinal fusion model

Recombinant growth factors bone morphogenetic protein-2 (BMP-2) and BMP-7 are currently approved for human use but are associated with various adverse effects including ectopic bone formation and local inflammatory reaction. The development of alternative growth factors may help minimize the adverse effects of current osteoinductive therapeutics. Nell-1 (Nel-like molecule-1; Nel [a protein strongly expressed in neural tissue encoding epidermal growth factor like domain]) is a novel secretory molecule that appears to act more specifically on osteoblasts than the BMPs, which can act on multiple cell types. From a molecular point of view, Nell-1 is directly regulated by runt-related transcription factor 2 (Runx2/Cbfa1), a master regulatory gene controlling bone formation. Previous studies showed that Nell-1 accelerates osteogenic differentiation in vitro and calvarial bone formation in vivo. We hypothesize that Nell-1 may also effectively form bone in spinal fusion. Our primary aim was to assess if direct adenoviral gene delivery with Nell-1 in a demineralized bone matrix (DBM) carrier can improve spinal fusion in a rat model. Because adenoviral vectors allow for sustained growth factor delivery, they were used for initial feasibility testing before protein studies. Two groups of 20 athymic rats underwent posterolateral intertransverse process spinal fusion at L4-L5 with implanted DBM carrier containing either adenovirus coding for Nell-1 (AdNell-1) or control, Lac Z (AdLacZ). No cells were implanted. The 20 rats were sacrificed at 6 weeks for evaluation of spinal fusion. All animals underwent Faxitron radiographs at 2, 4, and 6 weeks, manual spine palpation at 6 weeks, and high-resolution micro computerized tomography (microCT) at 6 weeks. Spinal fusion rate was assessed by: 1) 6-week Faxitron images; 2) manual palpation by three independent observers; 3) microCT; and 4) histology. New bone formation was assessed by hematoxylin-eosin and Masson trichrome staining on decalcified, coronally sectioned spine segments. All differences achieved statistical significance. After 6 weeks, direct application of adenoviral Nell-1 in a DBM carrier achieved significantly higher rates of spinal fusion over Lac Z controls: 60% Nell-1 versus 20% Lac Z by manual palpation and 70% Nell-1 versus 20% Lac Z by microCT and histology. Histological assessment of bone quality and maturity revealed more mature, higher quality bone in all the Nell-1 treated specimens relative to Lac Z at 6 weeks. Spinal fusion is more accurately assessed by microCT and histology than manual palpation. Direct application of adenoviral Nell-1 in a DBM carrier achieved significantly higher rates of spinal fusion over Lac Z controls at 6 weeks. Direct application of adenoviral Nell-1 in a DBM carrier also achieved significantly higher rates of spinal fusion over other reports in the literature using direct adenoviral BMP application. Direct application of adenoviral BMP in an allograft carrier achieved 8% fusion for BMP-2 and 16% fusion for BMP-7 at 8 weeks. These results indicate that Nell-1 may be a potent osteoinductive molecule. In addition, the regulation of Nell-1 by the master bone regulatory gene, Runx2 suggests that Nell-1 may exert its effects more specifically in osteoblastic cells than BMPs which affect multiple cell types. Overall, Nell-1 may fulfil a current need for an osteoinductive factor.

Sustained delivery of growth factors from methylidene malonate 2.1.2-based polymers

The incorporation of growth factors into new methylidene malonate 2.1.2-based biocompatible polymeric blends of oligomers and polymers to improve their stability and controlled release was investigated. Five growth factors were used in this study: FGF2, PDGF, TGF- β, NGF and GM-CSF. Formulation in poly(methylidene malonate 2.1.2) blends was achieved by a four-step optimized process, using different oligomers/polymers ratios. Once dried, formulations could be subsequently stored at 4 or 20 °C or immediately subjected to degradation in conditioned cell culture medium. Toxicity of blends and their degradation products were evaluated in several cell lines with MTT. Bioactivity and biospecificity of the formulated growth factors were investigated using MTT and immunohistochemical staining. Combined ELISA and crystal violet colorimetric assays were performed to analyze growth factors release. Limited toxicities were observed for unloaded poly(methylidene malonate 2.1.2) blends. Once optimized, growth factors formulations did not reveal lower bioactivities or loss of biospecificity. Moreover, a sustained release over a 21-day period with more than 90% of preserved bioactivity was reached. To conclude, dual growth factor delivery was made possible by the mean of poly(methylidene malonate 2.1.2) blends. These studies demonstrate the ability of methylidene malonate 2.1.2-based polymeric blends for the delivery of growth factors.

Hematopoietic Progenitor Cell Mobilization in Mice by Sustained Delivery of Granulocyte Colony-Stimulating Factor

The objective of these studies was to determine the effect of sustained delivery of growth factors (GFs) on hematopoietic progenitor cells (HPCs) in mice. In these studies, granulocyte colony-stimulating factor (G-CSF) was administered using the poloxamer-based matrix, ProGelz (PG) and G-CSF, and pharmacokinetics (PKs) and HPC mobilization was assessed. A single injection of G-CSF formulated in PG (17% poloxamer-407 and 5% hydroxypropyl methylcellulose [HPMC]) administered to BALB/c mice mobilized HPC significantly more rapidly to the spleen, but not the blood, than multiple injections of saline-formulated G-CSF. Two days after a single injection of PG G-CSF, the frequency of colony-forming unit-culture (CFU-c) in the spleen was increased 289-fold compared with an 8-fold increase after 2 days of twice-daily injections of saline-formulated G-CSF. Indeed, 4 days of twice-daily G-CSF injections were required to achieve the same level of HPC mobilization. In contrast, a similar mobilization of HPC to the blood was observed between PG and saline-formulated G-CSF. The mechanism for the accelerated and increased mobilization to the spleen by the PG-formulation of G-CSF is due, in part, to its increased bioavailability (>1.5-fold), T(max) (6-fold), and prolonged elimination (Tbeta) half-life (>3-fold) as compared with a saline formulation. In addition, we observed a more rapid trafficking of the PG G-CSF to the marrow, which could also facilitate mobilization.

Synthetic Biocompatible Cyclodextrin-Based Constructs for Local Gene Delivery to Improve Cutaneous Wound Healing

The localized, sustained delivery of growth factors for wound healing therapy is actively being explored by gene transfer to the wound site. Biocompatible matrices such as bovine collagen have demonstrated usefulness in sustaining gene therapy vectors that express growth factors in local sites for tissue repair. Here, new synthetic biocompatible materials are prepared and shown to deliver a protein to cultured cells via the use of an adenoviral delivery vector. The synthetic construct consists of a linear, beta-cyclodextrin-containing polymer and an adamantane-based cross-linking polymer. When the two polymers are combined, they create an extended network by the formation of inclusion complexes between the cyclodextrins and adamantanes. The properties of the network are altered by controlling the polymer molecular weights and the number of adamantanes on the cross-linking polymer, and these modifications and others such as replacement of the beta-cyclodextrin (host) and adamantane (guest) with other cyclodextrins (hosts such as alpha, gamma, and substituted members) and inclusion complex forming molecules (guests) provide the ability to rationally design network characteristics. Fibroblasts exposed to these synthetic constructs show proliferation rates and migration patterns similar to those obtained with collagen. Gene delivery (green fluorescent protein) to fibroblasts via the inclusion of adenoviral vectors in the synthetic construct is equivalent to levels observed with collagen. These in vitro results suggest that the synthetic constructs are suitable for in vivo tissue repair applications.

Delivery of osteoinductive growth factors from degradable PEG hydrogels influences osteoblast differentiation and mineralization.

Degradable poly(ethylene glycol) (PEG) hydrogels with varying mass loss profiles were investigated to assess their applicability as delivery vehicles for osteoinductive growth factors in bone tissue engineering. Protein release is readily controlled by changes in both the structure (i.e., macromer concentration) and chemistry (i.e., number of degradable units) of the starting macromer. In vitro studies indicate an increase in total protein levels, alkaline phosphatase, and mineralization by osteoblasts cultured in the presence of osteoinductive growth factors compared to cells cultured with standard media. When growth factors are delivered from a 25 wt% hydrogel, a significant increase in both alkaline phosphatase and mineralization was seen after 3 weeks of culture over growth factor delivery in a bolus fashion. Additionally, gene expression levels of both osteocalcin and type I collagen were higher at early timepoints when growth factors were released from hydrogels. These results indicate that growth factors remain active after photoencapsulation, that the sustained delivery of growth factors alters various markers of osteoblastic differentiation, and that these networks could be useful as delivery vehicles for growth factors in bone tissue engineering. Finally, ectopic bone formation was present in subcutaneous rat tissue after implantation of hydrogel networks loaded with osteoinductive growth factors.

Gene transfer and expression of platelet-derived growth factors modulate periodontal cellular activity.

Platelet-derived growth factor (PDGF) is a potent stimulator of wound healing. PDGF gene therapy may promote greater periodontal regeneration than local protein application, due to sustained growth factor delivery to the target tissue. This investigation tested the ability of recombinant adenoviruses (rAds) encoding PDGF-A or PDGF-1308 (a PDGF-A dominant-negative mutant that disrupts endogenous PDGF bioactivity) to affect cells derived from the periodontium. Osteoblasts, periodontal ligament fibroblasts, and gingival fibroblasts were transduced with rAds, and gene expression, DNA synthesis, and cell proliferation were evaluated. The results revealed strong message for the PDGF-A gene for 7 days following gene delivery. Ad2/PDGF-A enhanced the mitogenic and proliferative response in all cell types, while Ad2/PDGF-1308 potently inhibited mitogenesis and proliferation. In conclusion, Ad2/PDGF can effectively transduce cells derived from the periodontium and promote biological activity equivalent to PDGF-AA. These studies support the potential use of gene therapy for sustained PDGF release in periodontal tissues.

Bioabsorbable polymer scaffolds for tissue engineering capable of sustained growth factor delivery

Engineering new tissues utilizing cell transplantation on biodegradable polymer matrices is an attractive approach to treat patients suffering from the loss or dysfunction of a number of tissues and organs. The matrices must maintain structural integrity during the process of tissue formation, and promote the vascularization of the developing tissue. A number of molecules (angiogenic factors) have been identified that promote the formation of new vascular beds from endothelial cells present within tissues, and the localized, controlled delivery of these factors from a matrix may allow an enhanced vascularization of engineered tissues. We have developed a gas foaming polymer processing approach that allows the fabrication of three-dimensional porous matrices from bioabsorbable materials (e.g., copolymers of lactide and glycolide [PLG]) without the use of organic solvents or high temperatures. The effects of several processing parameters (e.g., gas type, polymer composition and molecular weight) on the process were studied. Several gases (CO 2, N 2, He) were utilized in the fabrication process, but only CO 2 resulted in the formation of highly porous, structurally intact matrices. Crystalline polymers (polylactide and polyglycolide) did not form porous matrices, while amorphous copolymers (50:50, 75:25, and 85:15 ratio of lactide:glycolide) foamed to yield matrices with porosity up to 95%. The mechanical properties of matrices were also regulated by the choice of PLG composition and molecular weight. Angiogenic factors (e.g., vascular endothelial growth factor) were subsequently incorporated into matrices during the fabrication process, and released in a controlled manner. Importantly, the released growth factor retains over 90% of its bioactivity. In summary, a promising system for the incorporation and delivery of angiogenic factors from three-dimensional, biodegradable polymer matrices has been developed, and the fabrication process allows incorporation under mild conditions.

Modulation of the biologic activity of the rabbit intervertebral disc by gene therapy: an in vivo study of adenovirus-mediated transfer of the human transforming growth factor beta 1 encoding gene.

In vivo studies using a rabbit model to determine the biologic effects of direct, adenovirus-mediated transfer of a therapeutic gene to the intervertebral disc. 1) To deliver an exogenous therapeutic gene to rabbit lumbar intervertebral discs in vivo, 2) to quantify the resulting amount of gene expression, and 3) to determine the effect on the biologic activity of the discs. Although growth factors such as transforming growth factor beta 1 appear to have promising therapeutic properties, there currently is no practical method for sustained delivery of exogenous growth factors to the disc for the management of certain chronic types of disease (e.g., disc degeneration). A possible solution is to modify the disc cells genetically through gene transfer such that the cells manufacture the desired growth factors endogenously on a continuous basis. Saline, with or without virus, was injected directly into lumbar discs of 22 skeletally mature female New Zealand white rabbits. Group 1 (n = 11) received the adenovirus construct Ad/CMV-hTGF beta 1 containing the therapeutic human transforming growth factor beta 1-encoding gene. Group 2 (n = 6) received adenovirus containing the luciferase marker gene. Group 3 (n = 5) received saline only. The rabbits were killed 1 week after injection. Immunohistochemical staining for human transforming growth factor beta 1 was performed on the disc tissues of one rabbit from Group 1. Nucleus pulposus tissues from the remaining rabbits were cultured in serumless medium. Bioassays were performed to determine human transforming growth factor beta 1 production and proteoglycan synthesis. Discs injected with Ad/CMV-hTGF beta 1 exhibited extensive and intense positive immunostaining for transforming growth factor beta 1. The nucleus pulposus tissues from the discs injected with Ad/CMV-hTGF beta 1 exhibited a 30-fold increase in active transforming growth factor beta 1 production, and a 5-fold increase in total (active + latent) transforming growth factor beta 1 production over that from intact control discs (P < 0.05). Furthermore, these tissues exhibited a 100% increase in proteoglycan synthesis compared with intact control tissue, which was statistically significant (P < 0.05). The results of this study suggest that the intervertebral disc is an appropriate site for adenovirus-mediated transfer of exogenous genes and subsequent production of therapeutic growth factors. Gene therapy therefore may have useful applications for study of the basic science of the intervertebral disc and for clinical management of degenerative disc disease.

Effects of Growth Factor Presence on Mineralization of Porous Poly(Lactide-Co-Glycolide) Scaffolds In Vitro

AbstractBasic design requirements of scaffolds for bone tissue engineering applications include biocompatibility, temporally controlled degradability, osteoconductivity, mechanical integrity, and mass transport capabilities. A recent study has attempted to meet design criteria via the growth of a carbonated apatite (bone-like) mineral on the inner pore surfaces of a highly porous 85:15 poly(lactide-co-glycolide) scaffold using a biomimetic process. It has also been recently demonstrated that the mineralization strategy can be combined with sustained growth factor delivery to induce vascular tissue ingrowth. The present study examines the effect of protein presence on the mineralization process by measuring the amount of protein incorporated into the scaffold during the process of mineral formation. Surprisingly, vascular endothelial growth factor incorporates more readily into control scaffolds than into scaffolds subjected to mineralization treatment. This finding suggests that there is little incorporation of protein into the growing mineral film, and offers insight into the mechanism for sustained drug delivery from the mineralized scaffolds.

Adenovirus-mediated gene transfer to nucleus pulposus cells. Implications for the treatment of intervertebral disc degeneration.

In vitro and in vivo studies using a rabbit model were performed to determine the feasibility of adenovirus-mediated gene transfer to the intervertebral disc. This study was conducted to determine whether it is possible to transfer genes to cells within the intervertebral disc by direct injection of an adenovirus and to determine the duration of gene expression obtained by this method. Although growth factors have the potential to stimulate the regeneration of nucleus pulposus, sustained delivery of growth factors to a degenerated disc is clinically unfeasible with present technology. Novel approaches such as gene transfer should be investigated as possible solutions to this problem. The lacZ marker gene was used to evaluate gene delivery to cells within intervertebral discs. For the in vitro study, cell cultures were established from the nucleus pulposus tissue of New Zealand white rabbits and infected with an adenovirus encoding the lacZ gene (Ad-lacZ). For the in vivo study, the anterior aspects of lumbar intervertebral discs were surgically exposed, and Ad-lacZ in saline solution was directly injected into the nucleus pulposus. An equal volume of saline only was injected into control discs. Expression of the transferred gene was detected by staining with 5-bromo-4-chloro-3-indolyl-beta-galactosidase (X-Gal). The in vitro experiments confirmed that nucleus pulposus cells were efficiently transduced by an adenoviral vector carrying the lacZ gene. In vivo injection of Ad-lacZ into the nucleus pulposus resulted in the transduction of a considerable number of cells. Marker gene expression in vivo persisted at an apparently undiminished level for at least 12 weeks. No staining was noted in control discs. The results show the feasibility of adenovirus-mediated gene transfer to the intervertebral disc. Expression of the marker gene persisted at least 12 weeks in vivo. This successful demonstration of exogenous gene transfer to the disc and sustained, long-term expression suggests that the adenoviral vector may be suitable for delivery of appropriate genes to the disc for the treatment of spinal disorders.

Polymer science for macroencapsulation of cells for central nervous system transplantation

The goal of encapsulated cell therapy research is to develop implants containing living xenogeneic cells to treat serious and disabling human conditions. The enabling concept is straightforward: cells or small clusters of tissue are surrounded by a selective membrane barrier which admits oxygen and required metabolites, releases bioactive cell secretions but restricts the transport of the larger cytotoxic agents of the body's immune defense system. Use of a selective membrane both eliminates the need for chronic immunosuppression in the host and allows cells to be obtained from non-human sources, thus avoiding the cell-sourcing constraints which have limited the clinical application of general successful investigative trials of unencapsulated cell transplantation for chronic pain, Parkinson's disease, and type I diabetes. Target applications for encapsulated cell therapy include these same disorders as well as other disabilities caused by loss of secretory cell function which cannot be adequately treated by current organ transplantation or drug therapies and conditions potentially capable of responding to local sustained delivery of growth factors and other biologic response modifiers. Several types of device configurations are possible. Here we focus on easily retrieved, non-vascularized, macrocapsules. Such devices have four basic components: a hollow fiber or flat sheet membrane (usually thermoplastic based), cells (primary or dividing), and extracellular matrix (natural or synthetic) to promote cell viability and function, and other device components such as seals, tethers and radio-opaque markers. Choice of membrane and extracellular matrix polymers as well as issues surrounding implantation and biocompatibility evaluation are complex, inter-related, and ultimately driven by implantation site and delivery requirements. Cross species immunoisolated cell therapy has been validated small and large animal models of chronic pain, Parkinson's disease, and type 1 diabetes and is under active investigation by several groups in animal models of Huntington's, Hemophilia, Alzheimer's, ALS, and other CNS disorders.