Production of purified alginates suitable for use in immunoisolated transplantation

Treatment of diabetes with encapsulated islets

Cell encapsulation has been proposed for the treatment of a wide variety of diseases since it allows for transplantation of cells in the absence of undesired immunosuppression. The technology has been proposed to be a solution for the treatment of diabetes since it potentially allows a mandatory minute-to-minute regulation of glucose levels without side-effects. Encapsulation is based on the principle that transplanted tissue is protected for the host immune system by a semipermeable capsule. Many different concepts of capsules have been tested. During the past two decades three major approaches of encapsulation have been studied. These include (i) intravascular macrocapsules, which are anastomosed to the vascular system as AV shunt, (ii) extravascular macrocapsules, which are mostly diffusion chambers transplanted at different sites and (iii) extravascular microcapsules transplanted in the peritoneal cavity. The advantages and pitfalls of the three approaches are discussed and compared in view of applicability in clinical islet transplantation.

Inorganic nanoporous membranes for immunoisolated cell-based drug delivery

Materials advances enabled by nanotecbnology have brought about promising approaches to improve the encapsulation mechanism for immunoisolated cell-based drug delivery. Cell-based drug delivery is a promising treatment for many diseases but has thus far achieved only limited clinical success. Treatment of insulin dependent diabetes mellitus (IDDM) by transplantation of pancreatic beta-cells represents the most anticipated application ofcell-based drug delivery technology. This review outlines the challenges involved with maintaining transplanted cell viability and discusses how inorganic nanoporous membranes may be useful in achieving clinical success.

Epo delivery by genetically engineered C2C12 myoblasts immobilized in microcapsules

ver the last half century, the use of erythropoietin (Epo) in the management of malignancies has been extensively studied. Originally viewed as the renal hormone responsible for red blood cell production, many recent in vivo and clinical approaches demonstrate that various tissues locally produce Epo in response to physical or metabolic stress. Thus, not only its circulating erythrocyte mass regulator activity but also the recently discovered nonhematological actions are being thoroughly investigated in order to fulfill the specific Epo delivery requirements for each therapeutic approach.

Direct effect of alginate purification on the survival of islets immobilized in alginate-based microcapsules

Alginate purification has been shown to decrease the host immune response to implanted alginate-based microcapsules, but the direct effect of contaminants on islet cell survival remains unknown. Wistar rat islets were immobilized in calcium alginate beads made with crude vs. purified alginate and then incubated in CMRL culture medium. Islet survival was evaluated at 1, 4, 7, 14 and 27 days post-encapsulation. Islet viability was investigated using a dual staining assay (propidium iodide and orange acridine). The islet cell necrosis and the proportion of apoptotic cells were quantified under optical microscopy and with a TUNEL assay, respectively. Islets immobilized in purified alginate were more viable, and had fewer necrotic centers, a smaller area of central necrosis and a lower number of apoptotic cells. At day 14 and 27 post-encapsulation, respectively, 48% and 23% of islets were viable with purified alginate vs. 18% and 8% with crude alginate (p<0.05). At day 14, the surface area of central necrosis and the number of necrotic islets were more important with the impure alginate (65% vs. 45% and 73% vs. 53%, respectively; p<0.05). We conclude that alginate purification improves the survival of islets that are immobilized in alginate-based microcapsules. These findings indicate that caution should be taken in the interpretation of in vivo experiments, as the results could be explained by either a direct effect on islet survival or a modification of the host reaction, or both. Moreover, it suggests that the effect on islet viability should be assessed during the development of biomaterials for cell encapsulation.

A gel-forming poly-L: -guluronic acid produced from no guluronate-rich marine algae using new hydrolysis method: test for endovascular embolization

To prepare a gel-forming poly-L-guluronic acid (Poly-G) from no guluronate-rich Laminaria japonica, a new hydrolysis method was employed with a lower HCl concentration (0.025-0.15 M) and a shorter treatment time (5 min). The Poly-Gs were set to measure purity, presence of poly-L-guluronic block, molecular weight distribution, polymer yield, viscosity, and compressive gel strength. Finally, the Poly-G was tested to embolize the renal vascular system by using a rabbit model and angiography. Optimized Poly-G could be selected with respect to wt% concentration, polymer yield, gel-forming stability, viscosity, and gel strength as an endovascular embolizing agent. Overall, 0.4-0.6% of 0.03 M-Poly-G obtained from acid treatment with 0.03 M of HCl had molecular weights greater than 80 kDa, and the best gelling capacity with an injectable viscosity (30-120 cP). It was successfully delivered into the vascular bed of a rabbit kidney and was shown angiographically to embolize the renal vascular system.

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.

Hydrogels used for cell-based drug delivery

Stem cells, progenitor cells, and lineage-committed cells are being considered as a new generation of drug depots for the sustained release of therapeutic biomolecules. Hydrogels are often used in conjunction with the therapeutic secreting cells to provide a physical barrier to protect the cells from hostile extrinsic factors. Although the hydrogels significantly improve the therapeutic efficacy of transplanted cells, there have been no successful products commercialized based on these technologies. Recently, biomaterials are increasingly designed to provide cells with both a physical barrier and an extracellular matrix to further improve the secretion of therapeutic proteins from cells. This review will discuss (1) the cell encapsulation process, (2) the immunogenicity of the encapsulating hydrogel, (3) the transport properties of the hydrogel, (4) the hydrogel mechanical properties, and will propose new strategies to improve the hydrogel and cell interaction for successful cell-based drug delivery strategies.

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.

Extraction and physicochemical characterization of Sargassum vulgare alginate from Brazil

Alginate fractions from Sargassum vulgare brown seaweed were characterized by (1)H NMR and fluorescence spectroscopy and by rheological measurements. The alginate extraction conditions were investigated. In order to carry out the structural and physicochemical characterization, samples extracted for 1 and 5h at 60 degrees C were further purified by re-precipitation with ethanol and denoted as SVLV (S. vulgare low viscosity) and SVHV (S. vulgare high viscosity), respectively. The M/G ratio values for SVLV and SVHV were 1.56 and 1.27, respectively, higher than the ratio for most Sargassum spp. alginates (0.19-0.82). The homopolymeric blocks F(GG) and F(MM) of these fractions characterized by (1)H NMR spectroscopy were 0.43 and 0.55 for SVHV and 0.36 and 0.58 for SVLV samples, respectively, these values typically being within 0.28-0.77 and 0.07-0.41, respectively. Therefore, the alginate samples from S. vulgare are much richer in mannuronic block structures than those from other Sargassum species. Values of M(w) for alginate samples were also calculated using intrinsic viscosity data. The M(w) value for SVLV (1.94 x 10(5)g/mol) was lower than that for SVHV (3.3 x 10(5)g/mol). Newtonian behavior was observed for a solution concentration as high as 0.7% for SVLV, while for SVHV the solutions behaved as a Newtonian fluid up to 0.5%. The optimal conditions for obtaining the alginates from S. vulgare were 60 degrees C and 5h extraction. Under these conditions, a more viscous alginate in higher yield was extracted from the seaweed biomass.

Preliminary investigation of calcium alginate gel as a biocompatible material for endovascular aneurysm embolization in vivo

OBJECTIVE

We sought to expand our assessment of calcium alginate as an embolic agent in an aneurysm model in swine that survived from 30 to 90 days. The objective of this study was to assess the biocompatibility and stability of calcium alginate in aneurysms in vivo.

METHODS

Ten models were created from a venous pouch sutured to the carotid artery, simulating flow to a side-wall aneurysm. Eight swine received complete embolizations, and two were less than 50% embolized to be used as controls. Alginate and calcium chloride were injected from concentric-tube microcatheters to form a mass that filled the aneurysm pouch.

RESULTS

Angiography and histology verified complete aneurysm occlusion and neck healing up to 90 days in eight swine. Both control animal aneurysms ruptured within 8 days. No animals showed evidence of downstream calcium alginate gel propagation. A minor bioactive response to the alginate gel was noted at 30 days, and fibrous tissue grew over the aneurysm orifice, sealing off the defect. No degenerative or inflammatory response was observed. At 90 days, moderate fibrous tissue surrounded the alginate. Tissue growth across the aneurysm neck remained complete and stable with no signs of neointimal growth into the parent vessel.

CONCLUSION

Calcium alginate was an effective endovascular occlusion material that filled the aneurysm and provided an effective template for tissue growth across the aneurysm neck after 30 days and up to 90 days. Complete filling of the aneurysm with calcium alginate ensures stability, biocompatibility, and optimal healing for up to 90 days in swine.

Zeta‐potentials of alginate‐PLL capsules: A predictive measure for biocompatibility?

Alginate-poly-L-lysine (PLL) microencapsulation of cells is a promising approach to prevent rejection in the absence of immunosuppression. Clinical application, however, is hampered by insufficient insight in factors influencing biocompatibility of the capsules. By now, it has been accepted that not only the chemical composition of the materials applied but also other factors contribute to bioincompatibility. The zeta-potential serves as a measure for the electrical charge of the surface and has been shown to be a predictive value for the interfacial reactions between the biomaterial and the surrounding tissue in other applications. In the present study, we have assessed the streaming potential of alginate-PLL capsules composed of either low-, intermediate-, or high-guluronic (G) alginate to calculate the zeta-potential. The zeta-potentials of the capsules were compared to the biological response against the capsules at 4 weeks after implantation in the rat. We show that high-G and low-G alginates provoke a more severe response in the rat than capsules prepared of intermediate-G alginate. This correlates with a higher zeta-potential of the high-G and low-G alginates and by a change in zeta-potential at lower pH. These lower pH-levels are common directly after implantation as the consequence of a host-response associated with mandatory surgery. Our results suggest that we should not only consider the capsule properties under physiological circumstances to explain bioincompatibility but also the capsule features during common pathophysiological situations.

Alginate-based microcapsules for immunoisolation of pancreatic islets

Transplantation of microencapsulated cells is proposed as a therapy for the treatment of a wide variety of diseases since it allows for transplantation of endocrine cells in the absence of undesired immunosuppression. The technology is based on the principle that foreign cells are protected from the host immune system by an artificial membrane. In spite of the simplicity of the concept, progress in the field of immunoisolation has been hampered for many years due to biocompatibility issues. During the last years important advances have been made in the knowledge of the characteristics and requirements capsules have to meet in order to provide optimal biocompatibility and survival of the enveloped tissue. Novel insight shows that not only the capsules material but also the enveloped cells should be hold responsible for loss of a significant portion of the immunoisolated cells and, thus, failure of the grafts on the long term. Microcapsules without cells can be produced as such that they remain free of any significant foreign body response for prolonged periods of time in both experimental animals and humans. New approaches in which newly discovered inflammatory responses are silenced bring the technology of transplantation of immunoisolated cells close to clinical application.

Expression of Human Calcitonin by Microencapsulated Recombinant Myoblasts

A novel approach to treat post-menopausal osteoporosis is proposed by engineering cell lines for the secretion of human calcitonin (hCT) and which would then be suitable for implantation in different allogeneic hosts. Mouse myoblast were transfected with the cDNA for hCT using the liposome-based gene delivery technique and clones secreting of human calcitonin were isolated. Human calcitonin expression was detected by ELISA. Western blot and immunohistochemical analyses also confirmed the plasmid, pcDNA3-hCT, had been transfected into the cells. Upon enclosure in microcapsules, which are biocompatible membranes that permit exit of therapeutic proteins but not entry of immune mediators, the myoblasts continued to secrete human calcitonin into the culture medium. These results demonstrate the feasibility of using these encapsulated recombinant myoblasts to deliver human calcitonin and the potential of allergenic gene therapy for postmenopausal osteoporosis.

Biocompatibility of alginate–poly-l-lysine microcapsules for cell therapy☆

Cell microencapsulation holds promise for the treatment of many diseases by the continuous delivery of therapeutic products. The biocompatibility of the microcapsules and their biomaterials components is a critical issue for the long-term efficacy of this technology. The objective of this paper is to provide detailed information about the principal factors affecting the biocompatibility of alginates and alginate-poly-l-lysine microcapsules, which are the most frequently employed biomaterials and encapsulation devices for cell immobilization, respectively. Some of these factors include the alginate composition and purification, the selection of the polycation, the interactions between the alginates and the polycation, the microcapsule fabrication process, the uniformity of the devices and the implantation procedure. Improved knowledge will lead to the production of standardized transplantation-grade biomaterials and biocompatible microcapsules.

Evaluation of alginate purification methods: effect on polyphenol, endotoxin, and protein contamination

Alginate, a polysaccharide extracted from brown seaweed, is widely used for the microencapsulation of islets of Langerhans, allowing their transplantation without immunosuppression. This natural polymer is known to be largely contaminated. The implantation of islets encapsulated using unpurified alginate leads to the development of fibrotic cell overgrowth around the microcapsules and normalization of the blood glucose is restricted to a very short period if it is achieved at all. Several research groups have developed their own purification method and obtained relatively good results. No comparative evaluation of the efficiencies of these methods has been published. We conducted an evaluative study of five different alginate preparations: a pharmaceutical-grade alginate in its raw state, the same alginate after purification according to three different published methods, and a commercially available purified alginate. The results showed that all purification methods reduced the amounts of known contaminants, that is, polyphenols, endotoxins, and proteins, although with varying efficiencies. Increased viscosity of alginate solutions was observed after purification of the alginates. Despite a general efficiency in decreasing contamination levels, all of the purified alginates contained relatively high residual amounts of protein contaminants. Because proteins may be immunogenic, these residual proteins may have a role in persisting microcapsule immunogenicity.

Biopolymer-based biomaterials as scaffolds for tissue engineering

Biopolymers as biomaterials and matrices in tissue engineering offer important options in control of structure, morphology and chemistry as reasonable substitutes or mimics of extracellular matrix systems. These features also provide for control of material functions such as mechanical properties in gel, fiber and porous scaffold formats. The inherent biodegradability of biopolymers is important to help regulate the rate and extent of cell and tissue remodeling in vitro or in vivo. The ability to genetically redesign these polymer systems to bioengineer appropriate features to regulate cell responses and interactions is another important feature that offers both fundamental insight into chemistry-structure-function relationships as well as direct utility as biomaterials. Biopolymer matrices for biomaterials and tissue engineering can directly influence the functional attributes of tissues formed on these materials and suggest they will continue play an increasingly important role in the field.

Physicochemical model of alginate–poly-l-lysine microcapsules defined at the micrometric/nanometric scale using ATR-FTIR, XPS, and ToF-SIMS

Alginate-poly-L-lysine-alginate (APA) microcapsules are currently being investigated as a means to immuno-isolate transplanted cells, but their biocompatibility is limited. In this study, we verified the hypothesis that poly-L-lysine (PLL), which is immunogenic when unbound, is exposed at the APA microcapsule surface. To do so, we analysed the microcapsule membrane at the micrometric/nanometric scale using attenuated total reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The results indicate that PLL and alginate molecules interact within the membrane. PLL exists in considerable amounts near the surface, contributing to the majority of the carbon within the outermost 100 Angstroms of the membrane. PLL was also detected at the true surface (the outermost monolayer) of the microcapsules. The exposure of PLL does not appear to result from defects in the outer alginate coating. This physicochemical model of APA microcapsules could explain their immunogenicity and will play an important role in the optimization of the microcapsule design.

Long-Term Expression of Erythropoietin from Myoblasts Immobilized in Biocompatible and Neovascularized Microcapsules

The present paper investigates the long-term functionality of an ex vivo gene therapy approach based on cell microencapsulation for the continuous delivery of erythropoietin (EPO) without implementation of immunosuppressive protocols. Polymer microcapsules (0.5 ml) loaded with EPO-secreting C(2)C(12) myoblasts and releasing 15,490 +/- 600 IU EPO/24 h were implanted in the peritoneum and subcutaneous tissue of syngeneic and allogeneic mice. High and constant hematocrit levels were maintained for more than 100 days in all implanted mice. Capsules retrieved from the peritoneum were free-floating or forming small capsule clusters, and we detected only a weak fibroblast outgrowth in capsules adhered to organs, whereas capsules explanted from the subcutaneous region appeared altogether as a richly vascularized structure with no signs of major host reaction. Interestingly, the functionality of capsules implanted in the allogeneic mice persisted until day 210 after implantation. These results highlight the feasibility of cell encapsulation technology for the long-term delivery of EPO independent of the method of administration and the mouse strain.

Deletion of the tissue response against alginate-pll capsules by temporary release of co-encapsulated steroids

Transplantation of encapsulated living cells is a promising approach for the treatment of a wide variety of diseases. Large-scale application of the technique, however, is hampered by inflammatory responses against the capsules. In the present study, we investigate whether tissue responses against alginate-PLL-alginate capsules can be modulated by co-encapsulation and temporary release of immunomodulating factors such as dexamethasone. Such an approach may be mandatory in order to increase the function and survival of encapsulated tissue since it has been shown that the tissue response can be caused by many, insurmountable factors. In an in vitro assay, we demonstrated an antiproliferative effect of dexamethasone-containing capsules on L929-mouse-fibroblasts. Subsequently, capsules prepared of purified alginate with or without solved dexamethasone were implanted in the peritoneal cavity of rats and retrieved one month later for histological evaluation. Most of the capsules without dexamethasone proved to be overgrown and adherent to the abdominal organs whereas with co-encapsulated dexamethasone the majority of the capsules were found freely floating in the peritoneal cavity without overgrowth. We conclude that co-encapsulation of dexamethasone has a profound effect on fibroblasts and macrophages adherence to immunoisolating capsules.

Microencapsulation of living cells in semi-permeable membranes with covalently cross-linked layers

Microencapsulation in semi-permeable membranes protects transplanted cells against immune destruction. Microcapsule strength is critical. We describe a method to microencapsulate living cells in alginate-poly-L-lysine (PLL)-alginate membranes with covalent links between adjacent layers of microcapsule membranes, while preserving the desired membrane molecular weight cut-off (MWCO) and microencapsulated cell viability. A heterobifunctional photoactivatable cross-linker, N-5-azido-2-nitrobenzoyloxysuccinimide (ANB-NOS) was used. The N-hydroxysuccinimide ester group of ANB-NOS was covalently linked to PLL. Islets of Langerhans were immobilized in alginate beads, incubated in PLL-ANB-NOS and again in alginate. Upon illumination with UVA, covalent links were created between the phenyl azide residue of ANB-NOS and alginate from both the core bead and the outer coating. Covalently linked microcapsules remained intact after 3 years in a strong alkaline buffer (pH 12), whereas standard microcapsules disappeared within 45 s in the same solution. A standardized mechanical stress broke 22-fold more standard than covalently linked microcapsules. The MWCO and microencapsulated cell viability were similar with standard and covalently linked microcapsules. These microcapsules, extremely resistant to chemical and mechanical stresses, will be useful in numerous applications.

Biocompatibility Evaluation of Different Alginates and Alginate-Based Microcapsules

Biocompatibility of biomaterials and biomaterial-based medical devices is a critical issue for the long-term function on multiple therapeutic systems. In the past few years, there has been an increasing interest in producing more biocompatible biomaterials and in developing novel assays to analyze the quality of the products. In this study, a battery of in vitro techniques to assess the biocompatibility of alginates with different compositions and purities and alginate-based microcapsules is presented. Study of the protein and polyphenol content of the alginates revealed clear differences between the nonpurified and the purified alginates. A similar behavior was observed when the mitogenic activity and the tumor necrosis factor-alphasecretion induced by the alginates were assessed. Interestingly, when the latter two techniques were adapted to evaluate the different alginate microcapsules, a correlation with the results obtained for the alginate samples was observed. These results reinforce the idea of using the full battery of assays here reported to screen alginates and alginate-based microcapsules before implantation.

Effect of poly-L-lysine coating on macrophage activation by alginate-based microcapsules: Assessment using a newin vitromethod

The characteristics of the microcapsule surface, which interacts directly with the host macrophages, may have a role in the biocompatibility of alginate-poly-L-lysine (PLL)-alginate (APA) microcapsule. The objectives of the study were: 1) to develop and validate a simple, rapid, and sensitive in vitro method for assessing microcapsule biocompatibility, based on microcapsule coincubation with macrophages and measurement, by reverse transcriptase-polymerase chain reaction, of cytokine mRNA expression, and 2) to evaluate the effect of alginate purification and PLL coating on macrophage activation. The mRNA expression of tumor necrosis factor-alpha and interleukin-1beta was significantly higher when macrophages were coincubated with beads made with nonpurified compared with purified alginate (p<0.01, p<0.05, respectively) and negative control (p<0.001) or with APA microcapsules compared with non-PLL-coated alginate beads and negative control (p<0.001). The mRNA expression of interleukin-6 differed significantly only when APA microcapsules were compared with a negative control (p<0.05). These results confirm that alginate purification improves microcapsule biocompatibility, and suggest that PLL is not completely covered and/or neutralized by the second alginate incubation and thus has a role in the host macrophage activation. The assay is sensitive to both alginate contaminants and microcapsule surface characteristics and may be a useful tool for the development of biocompatible microcapsules.

Improved survival of microencapsulated islets during in vitro culture and enhanced metabolic function following transplantation

AIMS/HYPOTHESIS

The aim of this study was to determine whether a simple alginate capsule can prolong islet survival and function during long-term tissue culture. We also wanted to observe the ability of these encapsulated islets to restore glucose responsiveness to diabetic recipients, along with the quantity of islets required to do so.

METHODS

We compared the recovery and metabolic function of encapsulated canine islets with that of non-encapsulated canine islets following 1, 2 or 3 weeks of tissue culture. These culture preparations were also transplanted into diabetic nude mice and compared for their ability to reverse diabetes. Furthermore, short-term cultured encapsulated and non-encapsulated islets were transplanted in varying numbers to determine the minimum dose required to normalise blood glucose and prolong recipient survival.

RESULTS

Islet recovery following 1, 2 and 3 weeks of tissue culture was significantly higher when islets were encapsulated. When these islets were recovered at 1, 2 and 3 weeks and transplanted into diabetic nude mice, survival at 100 days was 100% for all encapsulated groups, versus 66%, 33% and 33% respectively for the non-encapsulated islets. Additionally, substantially fewer short-term cultured islets were required to normalise blood glucose when the islets were encapsulated. Recipients of encapsulated islets also had significantly longer survival times than recipients of non-encapsulated preparations.

CONCLUSIONS/INTERPRETATION

This study demonstrates that encapsulation of islets with purified alginate improves islet survival and function in vitro and in vivo.

Sustained delivery of vascular endothelial growth factor with alginate beads

Therapeutic angiogenesis is the growth of blood vessels from a pre-existing vasculature for clinical applications such as treating myocardial and limb ischemia. Vascular endothelial growth factor (VEGF) is a potent signal transduction molecule that acts specifically on vascular endothelial cells. Encapsulation of VEGF in a polymer matrix not only protects protein against enzymatic degradation in the body, but also allows proteins to be released at a controllable rate into a localized area. In this study, VEGF was encapsulated in calcium alginate beads by the extrusion/external gelation method, and was subsequently released in PBS and in serum media. The objective was to optimize VEGF encapsulation yield and obtain VEGF release at a constant rate from alginate matrices in vitro. The incorporation of low concentrations of VEGF and NaCl can increase encapsulation yield to 97%. The rate of VEGF release from alginate beads was higher in serum than in PBS, which was due to the capacity of the serum in reducing the electrostatic interaction between alginate and VEGF. The presence of CaCl(2) in the release supernatant can shield the alginate interaction with VEGF, and a constant release rate of 6 ng/ml/day may be sustained for 14 days. These results suggest that the alginate-VEGF delivery system may be useful in the development of vascular tissue engineering and wound healing applications.

NMR properties of alginate microbeads

Alginates are a family of unbranched polysaccharides with properties that vary widely depending on their composition. In the presence of multivalent cations (frequently Ca2+), alginates form a gel. Consequently, alginates have been used to encapsulate a variety of biological materials, including cells. In this study, we present NMR relaxation and diffusion data from alginate microbeads with similar size and properties to those used in the development of a bioartificial pancreas. Our data demonstrate that the transverse relaxation time (T2) of water within the gel depends on the guluronic acid content of the alginate, whereas the longitudinal relaxation time (T1) and the apparent diffusion coefficient of water do not. Our data further suggest that the diffusion of Ca2+ ions is hindered by the presence of a poly-L-lysine layer, a layer commonly added to provide mechanical support to the beads and immunoprotection to the encapsulated cells in the event of implantation. The impact of these data on our understanding of the role of alginate gels in the development of a bioartificial pancreas is discussed.

Biocompatibility and surface structure of chemically modified immunoisolating alginate‐PLL capsules

Grafting of encapsulated living cells has the potential to cure a wide variety of diseases. Large-scale application of the technique, however, is hampered by insufficient biocompatibility of the capsules. A major factor in the biocompatibility of capsules is inadequate covering of the inflammatory poly-L-lysine (PLL) on the capsules' surface. In the present study, we investigate whether tissue responses against alginate-PLL capsules can be reduced by crosslinking the surface of the capsules with heparin or polyacrylic acid. Our transplant study in rats shows a tissue response composed of fibroblasts and macrophages on alginate-PLL-alginate and alginate-PLL-heparin capsules that was completely absent on alginate-PLL-polyacrylic acid capsules. Atomic force microscopy analyses of the capsules demonstrates that the improved biocompatibility of alginate-PLL-capsules by polyacrylic acid coating should not only be explained by a more adequate binding of PLL but also by the induction of a smoother surface. This study shows for the first time that biologic responses against capsules can be successfully deleted by chemically crosslinking biocompatible molecules on the surface of alginate-PLL capsules.

Fourier transform infrared spectroscopy studies of alginate-PLL capsules with varying compositions

Microencapsulation of cells is a promising approach to prevention of rejection in the absence of immunosuppression. Clinical application, however, is hampered by insufficient insight into the factors that influence the biocompatibility of the capsules. Capsules prepared of alginates with a high guluronic (G) acid content proved to be more adequate for clinical application since they are more stable, but, unfortunately, they are less biocompatible than capsules prepared of intermediate-G alginate. In order to get some insight into the physicochemical factors that influence the biocompatibility of capsules for the encapsulation of living cells, the chemical compositions of alginate[bond]Ca beads and alginate[bond]PLL capsules were studied by Fourier transform infrared spectroscopy. We found that during the transition of the alginate[bond]Ca beads to alginate[bond]PLL capsules, Ca connecting the alginate molecules, disappeared at the surface of both high-G and intermediate-G alginate[bond]PLL capsules. At the same time, it turned out that high-G alginate[bond]PLL capsules contained more hydrogen bonding than did intermediate[bond]G alginate capsules. Thus the well-known higher stability of high-G alginate[bond]PLL compared to intermediate-G alginate[bond]PLL capsules is not caused by a higher degree of binding to Ca of the alginate molecules but rather by the presence of more hydrogen bonds. Another observation was that after the transition from bead to capsule, high-G alginate[bond]PLL capsules contained 20% more PLL than the intermediate-G alginate[bond]PLL capsules. Finally, we show that in both high-G and intermediate-G alginate[bond]PLL capsules, the PLL exists in the alpha-helix, in the antiparallel beta-sheet, and in the random coil conformation. This study shows that FT-IR allows for successful analyses of the chemical factors essential for understanding differences in the biocompatibility of alginate[bond]PLL capsules.

Insulin-like growth factor II allows prolonged blood glucose normalization with a reduced islet cell mass transplantation

IGF-II has been reported to decrease neonatal islet cell apoptosis and in vitro adult islet cell necrosis and apoptosis, but the usefulness of IGF-II in a transplantation setting is unknown. We evaluated the effect of in vitro IGF-II incubations on microencapsulated rat islet survival both in vitro and in minimal mass transplantations into diabetic mice. After 6 d in culture, fresh examinations, histology, fluorescence microscopy, sodium 3'-[1-(phenyl-amino-carbonyl)-3,4-tetrazolium]-bis (4-methoxy-6-nitro)-benzene sulfonic acid hydrate assay, and apoptosis studies all indicated that IGF-II significantly improves islet cell viability in a dose-dependent fashion. IGF-II 100 ng/ml and 500 ng/ml induced a 51% and 83% increase of viable islets (P = 0.052, P < 0.01). A 20%, 29%, and 33% reduction of the apoptotic index was observed with 50, 100, and 500 ng/ml incubations respectively (P < 0.05; P < 0.005; P < 0.001). Ten weeks after transplantation of 150 encapsulated rat islet equivalents incubated with IGF-II 500 ng/ml, 80% of diabetic mice were normoglycemic. Without IGF-II preincubation, only 8% of the recipients remained normoglycemic with the transplantation of 150 islets and 42% with 300 islets (P < 0.05). In conclusion, IGF-II promotes islet cell survival, and allows successful transplantation using a smaller number of islets.

Association between macrophage activation and function of micro-encapsulated rat islets

AIMS/HYPOTHESIS

Survival of microencapsulated islet grafts is limited, even when inflammatory reactions against the capsules are restricted to a small portion of less than 10%.

METHODS

This study investigates both in vivo in rat recipients and in vitro whether cellular overgrowth on this minority of the capsules contributes to limitations in the functional survival of the 90% of the encapsulated islets which remain free of any cellular overgrowth.

RESULTS

In successful rat recipients of an allogenic microencapsulated islet graft we found that the vast majority of cells in the capsular overgrowth were activated ED-1 and ED-2 positive macrophages which were found in numbers of approximately 1500 per capsule. Co-culture of encapsulated islets with 1500 (nr8383) rat-macrophages per capsule showed that the activation of macrophages was caused by islet-derived bioactive factors since TNF-alpha and IL-1beta secretion by macrophages was induced by islet-containing capsules and not by empty capsules. This activation of macrophages was associated with a decrease in function of the encapsulated islets as evidenced by a quantitatively reduced (35%) insulin response in static incubation and a slower response in perifusion.

CONCLUSION/INTERPRETATION

Present research aims to design strategies for the temporary inhibition of macrophage activation since macrophages are predominantly present in the first two months after implantation. These strategies will serve as a pertinent basis for future clinical application of microencapsulated islets.

Long-term biocompatibility, chemistry, and function of microencapsulated pancreatic islets

Transplantation of encapsulated living cells is a promising approach for the treatment of a wide variety of diseases. Large-scale application of the technique, however, is hampered by insufficient biocompatibility of the capsules. In the present study, we have implemented new as well as previously reported technologies to test biocompatibility issues of immunoisolating microcapsules on the long term (i.e. 2 years) instead of usually reported short time periods. When transplanted empty, the capsules proved to be highly biocompatible not only for short periods (i.e. 1 month) but also on the long term as evidenced by the absence of any significant biological response up to 2 years after implantation in rats. The immunoprotective properties of the capsules were confirmed by prolonged survival of encapsulated islet allografts up to 200 days. The surface of the applied capsule was analyzed and provides new insight in the chemical structure of true biocompatible and immunoprotective capsules applicable for transplantation of encapsulated islets in type I diabetes.

Tissue responses against immunoisolating alginate-PLL capsules in the immediate posttransplant period

Alginate-polylysine (PLL) capsules are commonly applied for immunoisolation of living cells for the treatment of a wide variety of diseases. Large-scale application of the technique, however, is hampered by insufficient biocompatibility of the capsules with failure of the grafts as a consequence. Most studies addressing biocompatibility issues of alginate-PLL capsules have focused on the degree of overgrowth on the capsules after graft failure and not on the reaction against the capsules in the immediate posttransplant period. Therefore, capsules were implanted in the peritoneal cavity of rats and retrieved 1, 5, and 7 days later for histological examination and X-ray photoelectron spectroscopy analysis for evaluation of chemical changes at the capsule surface. After implantation, the nitrogen signal increased from 5% on day 0, to 8.6% on day 7, illustrating protein adsorption on the capsule's surface. This increase in protein content of the membrane was accompanied by an increase in the percentage of overgrown capsules from 0.5 +/- 0.3% on day 1 to 3.3 +/- 1.6% on day 7. The cellular overgrowth was composed of monocytes/macrophages, granulocytes, fibroblasts, erythrocytes, multinucleated giant cells, and basophils. This overgrowth was not statical as generally assumed but rather dynamic as illustrated by our observation that at day 1 after implantation we mainly found monocytes/macrophages and granulocytes that on later time points were substituted by fibroblasts. As the inflammatory reaction predictably interfere with survival of encapsulated cells, efforts should be made to suppress activities or recruitment of inflammatory cells. These efforts may be temporary rather than permanent because most inflammatory cells have disappeared after 2 weeks of implantation.

Tailoring the pore architecture in 3-D alginate scaffolds by controlling the freezing regime during fabrication

The pore architecture in 3-D polymeric scaffoldings plays a critical role in tissue engineering as it provides the framework for the seeded cells to organize into a functioning tissue. In the present paper, we investigate the effect of freezing regime on the pore microstructure in 3-D alginate scaffolds, fabricated by the freeze-dry method. The scaffolds have shown isotropic pore structure, when the calcium crosslinked alginate solutions were slowly frozen at -20 degrees C, in a nearly homogenous cold atmosphere; the pores were spherical and interconnected. In contrast, when the cooling process was performed in liquid nitrogen or oil bath, where a temperature gradient was formed along the freezing solution, two main regions of pore structure were noted; at the interface with the cooling medium, small spherical pores were seen and above them a region with elongated pores. The different pore shape affected the compressibility of the scaffolds, while it had no effect on albumin diffusion. Rat hepatocytes seeded within the scaffolds were arranged according to the their pore shape. In scaffolds with elongated pores, the cells were lining along the pores, thus forming lines of interacting cells. In the scaffolds with the isotropic spherical pores, the hepatocytes clustered into spheroid-like aggregates. Thus, it appears that pore shape can modulate hepatocyte morphogenesis.

Flow properties of liquid calcium alginate polymer injected through medical microcatheters for endovascular embolization

The flow properties of liquid calcium alginate injections were investigated for application in endovascular embolization. Alginate shear properties were assessed with a rheometer and a controlled injection system. The experimental results were used to model the flow properties and predict alginate's flow characteristics within various medical microcatheter delivery systems. The results suggest that alginates undergo shear-thinning effects with increasing shear. A flow comparison of 2.0 wt % alginate and a Newtonian fluid (82 cP) injected from the same microcatheter had similar flow rates at low injection pressure (100 kPa). However, at high injection pressure (2100 kPa), the alginate was injectable at a flow rate 100% higher than was the Newtonian fluid. Further analysis of injections through microcatheters resulted in a flow model for predicting viscosity changes, flow rates, and injection pressures of liquid alginate at medium-to-high shear rates. The predicted injection pressures and flow rates had an average variance of less than 15% from that of the experimental flow data. This study indicates that calcium alginate has the requisite flow properties for successful delivery to vascular lesions via endovascular injection. Possible uses of alginates include treating arteriovenous malformations (AVMs), aneurysms, blood flow to tumors, and vascular hemorrhages.

Biocompatibility of microcapsules for cell immobilization elaborated with different type of alginates

The biocompatibility of alginate-PLL-alginate (APA) microcapsules has been evaluated with respect to impurity levels. The impurity content of three different alginates (a raw high M-alginate, a raw high G-alginate and a purified high G-alginate) has been determined and the in vivo antigenic response of APA beads made with each alginate assessed. Results show that purification of the alginate not only reduces the total amount of impurities (63% less in polyphenols, 91.45% less in endotoxins and 68.5% less in protein in relation to raw high M-alginate), but also avoids an antibody response when microcapsules of this material are implanted in mice. In contrast, raw alginates produced a detectable antibody response though the differences in their impurity content. Consequently, this work revealed that purity of the alginate rather than their chemical composition, is probably of greater importance in determining microcapsule biocompatibility.

Immunoisolation techniques for islet cell transplantation

Diabetes remains a devastating disease, with tremendous cost in terms of human suffering and healthcare expenditures. The burden of diabetes is primarily related to the multiple complications, including retinopathy, nephropathy, neuropathy and cardiovascular disease that can develop as the disease progresses. It has been shown that these complications can be prevented, and in some cases, reversed by islet cell transplantation, which, until recently, had remained elusive as a viable routine treatment modality. In recent studies, islet cell transplantation has shown great promise as a viable alternative to solid pancreas transplantation. However, severe shortage of human pancreases and the need to use immunosuppressive drugs to prevent transplant rejection, remain major obstacles to routine use of islet cell transplants for the treatment of patients with Type 1 diabetes. In the attempt to overcome these barriers, many procedures have been designed to immunoisolate islet cells for transplantation. The ultimate goal in islet cell transplantation is the availability of unlimited supply of cells to be transplanted in a simple procedure performed with little or no use of immunosuppressive drugs. The development of reliable procedures to immunoisolate islets by microencapsulation prior to transplantation has a great deal of potential to accomplish this objective.

Purification of polymeric biomaterials

Employing a combined filtration and precipitation method, the endotoxin concentration in sodium alginate (SA) and sodium cellulose sulfate (SCS) was reduced to a value of 200 EU/g polymer. This is one tenth of the regulatory threshold calculated, for example, for an appropriate bioartificial pancreas that consists of approximately 420,000 encapsulated islets of Langerhans. The low endotoxin (ET) levels were maintained below this threshold during a six-month storage period. The purification procedure of the polymers did not negatively influence the final microcapsule properties. The mechanical stability of microcapsules from purified material is even slightly higher than that of microcapsules from the original polymers. A second approach to avoid endotoxin release from the device is its direct complexation during the bead or capsule formation process. The durability of endotoxin binding in binary, ternary, and quaternary complexes could be demonstrated for storage in culture medium and saline. Very low total endotoxin release from the complexes was detected after three months in culture medium and five months in saline. This complexation is primarily based on electrostatic interactions with the participating cationic components and provides additional security for the final bioartificial organ or delivery device.

A novel class of amitogenic alginate microcapsules for long-term immunoisolated transplantation

In the light of results of clinical trials with immunoisolated human parathyroid tissue Ba2+-alginate capsules were developed that meet the requirements for long-term immunoisolated transplantation of (allogeneic and xenogeneic) cells and tissue fragments. Biocompatibility of the capsules was achieved by subjecting high-M alginate extracted from freshly collected brown algae to a simple purification protocol that removes quantitatively mitogenic and cytotoxic impurities without degradation of the alginate polymers. The final ultra-high-viscosity, clinical-grade (UHV/CG) product did not evoke any (significant) foreign body reaction in BB rats or in baboons. Similarly, the very sensitive pERK assay did not reveal any mitogenic impurities. Encapsulated cells also exhibited excellent secretory properties under in vitro conditions. Despite biocompatible material, pericapsular fibrosis is also induced by imperfect capsule surfaces that can favor cell attachment and migration under the release of material traces. This material can interact with free end monomers of the alginate polymers under formation of mitogenic advanced glycation products. Smooth surfaces, and thus topographical biocompatibility of the capsules (visualized by atomic force microscopy), can be generated by appropriate crosslinking of the UHV/CG-alginate with Ba2+ and simultaneous suppression of capsule swelling by incorporation of proteins and/or perfluorocarbons (i.e., medically approved compounds with high oxygen capacity). Perfluorocarbon-loaded alginate capsules allow long-term non-invasive monitoring of the location and the oxygen supply of the transplants by using 19F-MRI. Transplantation studies in rats demonstrated that these capsules were functional over a period of more than two years.

The effects of alginate composition on encapsulated betaTC3 cells

The effects of alginate composition on the growth of murine insulinoma betaTC3 cells encapsulated in alginate/poly-L-lysine/alginate (APA) beads, and on the overall metabolic and secretory characteristics of the encapsulated cell system, were investigated for four different types of alginate. Two of the alginates used had a high guluronic acid content (73% in guluronic acid residues) with varying molecular weight, while the other two had a high mannuronic acid content (68% in mannuronic acid residues) with varying molecular weight. Each composition was tested using two different polymer concentrations. Our data show that betaTC3 cells encapsulated in alginates with a high guluronic acid content experienced a transient hindrance in their metabolic and secretory activity because of growth inhibition. Conversely, betaTC3 cells encapsulated in alginates with a high mannuronic acid content experienced a rapid increase in metabolic and secretory activity as a result of rapid cell growth. Our data also demonstrate that an increase in either molecular weight or concentration of high mannuronic acid alginates did not alter the behavior of the encapsulated betaTC3 cells. Conversely, an increase in molecular weight and concentration of high guluronic acid alginates prolonged the hindrance of glucose metabolism, insulin secretion and cell growth. These observations can be best interpreted by changes in the microstructure of the alginate matrix, i.e., interaction between the contiguous guluronic acid residues and the Ca2+ ions, as a result of the different compositions.

Calcium alginate gel: a biocompatible and mechanically stable polymer for endovascular embolization

The development and optimization of calcium alginate for potential use in endovascular occlusion was investigated by testing its in vitro and in vivo mechanical stability and biocompatibility. The compressive resistance, rheology, and polymer yield of reacted alginate, and the polymer viscosity of unreacted alginate, were assessed. Biocompatibility was tested by injecting calcium alginate into the kidney capsule of rats. The reactivity of alginates with various structures and levels of purity were compared visually and histologically. Results suggest that calcium alginate is a biocompatible and mechanically stable gel for endovascular applications. Purified alginates exhibited compressive strength of 22 kPa and above at 40% compression, with no significant loss in elasticity. Purified alginate strength was significantly higher than that of crude alginates (p < 0.08). Purified alginates also exhibited significantly lower tissue reaction than crude alginates (p < 0.05). Of the alginates tested, purified high guluronic acid alginates (PHG) exhibited optimal strength and polymer yield, increased biocompatibility, and decreased viscosity. Clinical embolization treatments may be improved with the development of stable and biocompatible polymers such as calcium alginate. Possible uses of improved endovascular polymers include treating arteriovenous malformations (AVMs), aneurysms, blood flow to tumors, and vascular hemorrhaging.

Immunocompatibility and biocompatibility of cell delivery systems

Immunoisolation therapy overcomes important disadvantages of implanting free cells. By mechanically blocking immune attacks, synthetic membranes around grafted cells should obviate the need for immunosuppression. The membrane used for encapsulation must be biocompatible and immunocompatible to the recipient and also to the encapsulated graft. The ability of the host to accept the implanted graft depends not only on the material used for encapsulation, but also on the defense reaction of the recipient, which is very individual. Such a reaction usually starts as absorption of cell-adhesive proteins, immunoglobulins, complement components, growth factors and some other proteins on the surface of the device. The absorption of proteins is difficult to avoid, but the amount and specificity of absorbed proteins can be controlled to some extent by selection and modification of the device material. If the adsorption of proteins to the surface of the implanted material is reduced, the overgrowth of the device with fibroblast-like and macrophage-like cells is also reduced. Cell adhesion at the surface of the implanted device is, in addition to the selected polymeric material, greatly influenced by the device content. Xenografts trigger a more vigorous inflammatory reaction than allografts, most probably due to the release of antigenic products from encapsulated deteriorated and dying cells which diffuse through the membrane and activate adhering immune cells. There is an evident effect of autoimmune status on the fate of the encapsulated graft. While encapsulated xenogeneic islets readily reverse streptozotocin-induced diabetes in mice, the same xenografts are short-functioning in NOD autoimmune diabetes-prone mice. Autoantibodies, to which most devices are impermeable, are not involved. Among the cytotoxic factors which are responsible for the limited survival of the encapsulated graft the most important are cytokines and perhaps some other low-molecular-weight factors released by activated macrophages at the surface of the encapsulating membrane.

Technology of mammalian cell encapsulation

Entrapment of mammalian cells in physical membranes has been practiced since the early 1950s when it was originally introduced as a basic research tool. The method has since been developed based on the promise of its therapeutic usefulness in tissue transplantation. Encapsulation physically isolates a cell mass from an outside environment and aims to maintain normal cellular physiology within a desired permeability barrier. Numerous encapsulation techniques have been developed over the years. These techniques are generally classified as microencapsulation (involving small spherical vehicles and conformally coated tissues) and macroencapsulation (involving larger flat-sheet and hollow-fiber membranes). This review is intended to summarize techniques of cell encapsulation as well as methods for evaluating the performance of encapsulated cells. The techniques reviewed include microencapsulation with polyelectrolyte complexation emphasizing alginate-polylysine capsules, thermoreversible gelation with agarose as a prototype system, interfacial precipitation and interfacial polymerization, as well as the technology of flat sheet and hollow fiber-based macroencapsulation. Four aspects of encapsulated cells that are critical for the success of the technology, namely the capsule permeability, mechanical properties, immune protection and biocompatibility, have been singled out and methods to evaluate these properties were summarized. Finally, speculations regarding future directions of cell encapsulation research and device development are included from the authors' perspective.

Cell transplantation for endocrine disorders

Current hormonal replacement therapy for endocrine disorders cannot, unfortunately, reproduce the complex metabolic interactions of hormones. The organ or cell transplantation would be a more physiological approach to the treatment of endocrine disorders. For decades, remarkable progress in organ or cell transplantation in endocrine disorders has been made, especially in recent years. But there are many limitations in the widespread application of allotransplantation because of rejection. Various methods of immunomanipulations designed to overcome rejection have been proposed, which include immunosuppression, immunomodulation and immunoisolation. The transplantation of immunoisolated cells and some clinical results of the transplants were reviewed. Also a perspective for future directions on endocrine cell transplantation was provided in this review. Human islet cell transplantation for the cure of diabetes was emphasized in this chapter and other cell transplantation for endocrine disorders was also discussed briefly, including parathyroid tissue transplantation, bioartificial thyroid transplantation and adrenal cell transplantation.

Alginate dressings in surgery and wound management: Part 3

This final article in the series looks at the functioning of seaweed-derived dressings at a cellular level, as well as the importance of choosing appropriate secondary dressings

Non-invasive evaluation of the location, the functional integrity and the oxygen supply of implants: 19F nuclear magnetic resonance imaging of perfluorocarbon-loaded Ba2+-alginate beads

19F nuclear magnetic resonance imaging (MRI) can be used as a non-invasive tool to simultaneously determine the location, the integrity and the oxygen supply of Ba2+-alginate implants. This requires that the beads (implants) are pre-loaded with the perfluorocarbon compound F-44E. Implantation of solid 19F-labelled beads into the peritoneum, below the kidney capsule or into the muscle of Wistar WU rats demonstrated that these beads could be detected by 19F-MRI for up to 18 months after implantation. This indicated that F-44E is not considerably released from the beads during implantation. The signal to noise ratio of liquid-core beads was higher by a factor of 4 than the signal to noise ratio of solid beads, but liquid-core beads were more fragile and also too large for implantation under the kidney capsule and into the intramuscular tissue. Quantitative 2-dimensional 19F-T1 maps (resolution 0.5 x 0.5 mm) could be deduced from 19F-MRI measurements. These T1-maps correlated to the local pO2-values. The partial oxygen pressure estimated in F-44E-loaded Ba2+-alginate beads showed that the oxygen supply inside the beads was very poor when they were implanted below the kidney capsule or into the peritoneal cavity. These low pO2-values obtained for the renal subcapsular site and the peritoneum may explain the failure of previous immunoisolated islet transplantation studies using these locations.

Hepatocyte behavior within three-dimensional porous alginate scaffolds

A potential approach to facilitate the performance of implanted hepatocytes is to enable their aggregation and re-expression of their differentiated function prior to implantation. Here we examined the behavior of freshly isolated rat adult hepatocytes seeded within a novel three-dimensional (3-D) scaffold based on alginate. The attractive features of this scaffold include a highly porous structure (sponge-like) with interconnecting pores, and pore sizes with diameters of 100-150 microm. Due to their hydrophilic nature, seeding hepatocytes onto the alginate sponges was efficient. DNA measurements showed that the total cell number within the sponges did not change over 2 weeks, indicating that hepatocytes do not proliferate under these culture conditions. Nearly all seeded cells maintained viability, according to the MTT assay. Within 24 h post-seeding, small clusters of viable cells, were seen scattered within the sponge. More than 90% of the seeded cells participated in the aggregation; the high efficiency is attributed to the non-adherent nature of alginate. The spheroids had smooth boundaries and by day 4 in culture reached an average diameter of 100 microm, which is at the same magnitude of the sponge pore size. The cells appeared to synthesize fibronectin which was deposited on the spheroids. No laminin or collagen type IV were detected in the deposit. The 3-D arrangement of hepatocytes within the alginate sponges promoted their functional expression; within a week the cells secreted the maximal albumin secretion rate of 60 microg albumin/10(6) cells/day. Urea secretion rate did not depend on cell aggregation and was similar to that obtained when hepatocytes were cultured on collagen type I coated dishes (100 microg/10(6) cells/day). Our studies show that alginate sponges can provide a conducive environment to facilitate the performance of cultured hepatocytes by enhancing their aggregation.