Engineering challenges in the development of an encapsulated cell system for treatment of type 1 diabetes

Decellularized scaffold of cryopreserved rat kidney retains its recellularization potential

The multi-cellular nature of renal tissue makes it the most challenging organ for regeneration. Therefore, till date whole organ transplantations remain the definitive treatment for the end stage renal disease (ESRD). The shortage of available organs for the transplantation has, thus, remained a major concern as well as an unsolved problem. In this regard generation of whole organ scaffold through decellularization followed by regeneration of the whole organ by recellularization is being viewed as a potential alternative for generating functional tissues. Despite its growing interest, the optimal processing to achieve functional organ still remains unsolved. The biggest challenge remains is the time line for obtaining kidney. Keeping these facts in mind, we have assessed the effects of cryostorage (3 months) on renal tissue architecture and its potential for decellularization and recellularization in comparison to the freshly isolated kidneys. The light microscopy exploiting different microscopic stains as well as immuno-histochemistry and Scanning electron microscopy (SEM) demonstrated that ECM framework is well retained following kidney cryopreservation. The strength of these structures was reinforced by calculating mechanical stress which confirmed the similarity between the freshly isolated and cryopreserved tissue. The recellularization of these bio-scaffolds, with mesenchymal stem cells quickly repopulated the decellularized structures irrespective of the kidneys status, i.e. freshly isolated or the cryopreserved. The growth pattern employing mesenchymal stem cells demonstrated their equivalent recellularization potential. Based on these observations, it may be concluded that cryopreserved kidneys can be exploited as scaffolds for future development of functional organ.

Mathematical modeling of cryoprotectant addition and removal for the cryopreservation of engineered or natural tissues

Long-term storage of natural tissues or tissue-engineered constructs is critical to allow off-the-shelf availability. Vitrification is a method of cryopreservation that eliminates ice formation, as ice may be detrimental to the function of natural or bioartificial tissues. In order to achieve the vitreous state, high concentrations of CPAs must be added and later removed. The high concentrations may be deleterious to cells as the CPAs are cytotoxic and single-step addition or removal will result in excessive osmotic excursions and cell death. A previously described mathematical model accounting for the mass transfer of CPAs through the sample matrix and cell membrane was expanded to incorporate heat transfer and CPA cytotoxicity. Simulations were performed for two systems, an encapsulated system of insulin-secreting cells and articular cartilage, each with different transport properties, geometry and size. Cytotoxicity and mass transfer are dependent on temperature, with a higher temperature allowing more rapid mass transfer but also causing increased cytotoxicity. The effects of temperature are exacerbated for articular cartilage, which has larger dimensions and slower mass transport through the matrix. Simulations indicate that addition and removal at 4°C is preferable to 25°C, as cell death is higher at 25°C due to increased cytotoxicity in spite of the faster mass transport. Additionally, the model indicates that less cytotoxic CPAs, especially at high temperature, would significantly improve the cryopreservation outcome. Overall, the mathematical model allows the design of addition and removal protocols that insure CPA equilibration throughout the sample while still minimizing CPA exposure and maximizing cell survival.

Development and optimization of a dual-photoinitiator, emulsion-based technique for rapid generation of cell-laden hydrogel microspheres

A growing number of clinical trials explore the use of cell-based therapies for the treatment of disease and restoration of damaged tissue; however, limited cell survival and engraftment remains a significant challenge. As the field continues to progress, microencapsulation strategies are proving to be a valuable tool for protecting and supporting these cell therapies while preserving minimally invasive delivery. This work presents a novel, dual-photoinitiator technique for encapsulation of cells within hydrogel microspheres. A desktop vortexer was used to generate an emulsion of poly(ethylene glycol) diacrylate (PEGDA) or PEGDA-based precursor solution in mineral oil. Through an optimized combination of photoinitiators added to both the aqueous and the oil phase, rapid gelation of the suspended polymer droplets was achieved. The photoinitiator combination provided superior cross-linking consistency and greater particle yield, and required lower overall initiator concentrations compared with a single initiator system. When cells were combined with the precursor solution, these benefits translated to excellent microencapsulation yield with 60-80% viability for the tested cell types. It was further shown that the scaffold material could be modified with cell-adhesive peptides to be used as surface-seeded microcarriers, or additionally with enzymatically degradable sequences to support three-dimensional spreading, migration and long-term culture of encapsulated cells. Three cell lines relevant to neural stem cell therapies are demonstrated here, but this technology is adaptable, scalable and easy to implement with standard laboratory equipment, making it a useful tool for advancing the next generation of cell-based therapeutics.

Effect of alginate composition and purity on alginate microspheres

BACKGROUND

Alginate is commonly used to microencapsulate islets in experiments with islet allografts and xenografts for the treatment of Type I diabetes. The purpose of the present study is to determine the effects of alginate composition and purity on the morphology and size of microspheres.

METHODS

Microcapsules produced with the impure alginate types, medium-viscosity high-guluronic acid (IMVG), low-viscosity high-G (ILVG), low-viscosity high-mannuronic acid (ILVM) and medium-viscosity high-M (IMVM) were compared with one another and others generated with a highly purified LVM (HPLVM) alginate. Droplets of 1.5% alginate from an air-syringe pump were gelled in 1.1% CaCl2 solution. While leaving the alginate pressure and needle recess constant, the air-jacket pressure was varied between 9.5-10.5 PPSI to enhance stable microcapsule generation and different batches of microbeads were made from each alginate type.

RESULTS

The sizes of the high-guluronic acid alginate microbeads were consistently bigger than those of the corresponding high-mannuronic acid alginate beads at all air-jacket settings. At the optimal air-jacket pressure of 9.0 PPSI, the mean+SD diameter of the IMVG microbeads was 780 + 20 microm, while that of IMVM was 607 + 44 microm (p < 0.0001, n=30). Similarly, the mean ILVG microbead diameter was 816+28 microm compared to 656+26 microm for ILVM capsules (p<0.0001, n=30). Less polymorphism was found with the HPLVM microspheres than with the ILVM microbeads.

CONCLUSION

Highly purified high-mannuronic acid alginate will provide smaller, spherical microcapsules suitable for islet cell transplantation.

Photosynthetic oxygen generator for bioartificial pancreas

Immunoisolation of pancreatic islets interrupts their vascular connections and results in severe cell hypoxia and dysfunction. This process is believed to be the major obstacle to a successful cure of diabetes by implantation of bioartificial pancreas. Here we describe a new technology for microalga-based, photosynthetic oxygen supply to encapsulated islets, in which a thermophylic strain of the unicellular alga Chlorella was used as a natural photosynthetic oxygen generator. Following determinations of the optimal number of alga cells required for compensation of islet respiration, an appropriate number of islets and algae were co-encapsulated in alginate and perifused with oxygen-free medium at increasing glucose concentrations. No insulin response to glucose was obtained in islets alone, or upon inactivation of photosynthesis by darkness. However, under illumination, photosynthetic- dependent oxygen generation induced higher glucose-stimulated insulin response when compared to normoxic perifusion. Such photosynthetic oxygen generation may have a potential application in development of various bioartificial tissues, in particular the endocrine pancreas.

Tissue-culture surfaces with mixtures of aminated and fluorinated functional groups. Part 2. Growth and function of transgenic rat insulinoma cells (betaG I/17)

Interactions of transplantable cells with synthetic polymers can influence the function of biohybrid artificial organs. This study explored growth and secretion of human insulin by betaG I/17 cells cultured on surfaces bearing diamine groups (N2), trifluoropropyl groups (F3) and mixtures of the two. Cells cultured on high-F3 and high-N2 surfaces spread well, grew rapidly and produced >1.8 mol lactate per mol glucose consumed, closely resembling cells grown on the permissive control, glass. On one mixed surface, with a molar ratio of 33 N2 groups:67 F3 groups, cells had a lower lactate/glucose ratio, adopted a rounded form, grew slowly and were quick to form emergent aggregates, similar to cultures on the inhibitory control, untreated polystyrene. Cultures on surfaces with higher F3 content secreted the most insulin and, in the case of the highest-F3 surface, showed improved responsiveness to secretagogues. Hormone secretion was roughly 50% greater when cells were grown on F3 surfaces conditioned by earlier cultures of betaG I/17. Incubation of conditioned surfaces with high concentrations of a polyclonal anti-laminin serum prior to re-plating partially abolished this improvement in secretory function. Polymers bearing trifluoropropyl groups appear to be attractive candidates for use in the artificial endocrine pancreas. Surface coatings that include laminin might promote function of transgenic insulinoma cells in vitro and in vivo.