Comparison of two types of alginate microcapsules on stability and biocompatibility in vitro and in vivo

Comparison of Linear Poly Ethylene Imine (LPEI) and Poly L-Lysine (PLL) in Fabrication of CHOK1 Cell-Loaded Multilayer Alginate Microcapsules

Purpose: Poly l-lysine (PLL) has been introduced as a strengthening covering layer for alginate microcapsules which are the most convenient way for cell encapsulation. Some disadvantages of PLL such as high price and low biocompatibility have prompted scientists to find better alternatives. Linear poly ethylene imine (LPEI), thanks to its highly similar structure to PLL, could be considered as a proper cost-effective alternative. In this study LPEI and PLL were compared as covering layers of cell-loaded alginate-LPEI-alginate (cALA) and alginate-PLL-alginate (cAPA) microcapsules. Methods: In addition to the physico-mechanical properties, the encapsulation efficiency, cell survival post encapsulation, cell viability, and cellular metabolic activity within the microcapsules were evaluated using trypan blue, live/dead cell staining, and MTT test, respectively. Results: Physico-mechanical evaluation of the microcapsules revealed that the cell microencapsulation process did not affect their shape, size, and mechanical stability. Although the encapsulation efficiency for cALA and cAPA was not different (P >0.05), cell survival post encapsulation was higher in cALA than in cAPA (P<0.05) which could be the reason for the higher cell viability and also cellular metabolic activity within these microcapsules in comparison to cAPA. Conclusion: Here, based on these results, ALA could be introduced as a preferable alternative to APA for cell encapsulation.

Characterization of Polyethylene Glycol-Reinforced Alginate Microcapsules for Mechanically Stable Cell Immunoisolation

Islet transplantation within mechanically stable microcapsules offers the promise of long-term diabetes reversal without chronic immunosuppression. Reinforcing the ionically gelled network of alginate (ALG) hydrogels with covalently linked polyethylene glycol (PEG) may create hybrid structures with desirable mechanical properties. This report describes the fabrication of hybrid PEG-ALG interpenetrating polymer networks and the investigation of microcapsule swelling, surface modulus, rheology, compression, and permeability. It is demonstrated that hybrid networks are more resistant to bulk swelling and compressive deformation and display improved shape recovery and long-term resilience. Interestingly, it is shown that PEG-ALG networks behave like ALG during microscale surface deformation and small amplitude shear while exhibiting similar permeability properties. The results from this report's in vitro characterization are interpreted according to viscoelastic polymer theory and provide new insight into hybrid hydrogel mechanical behavior. This new understanding of PEG-ALG mechanical performance is then linked to previous work that demonstrated the success of hybrid polymer immunoisolation devices in vivo.

The Unusual Properties of Polytetrafluoroethylene Enable Massive-Volume Vitrification of Stem Cells with Low-Concentration Cryoprotectants

Injectable stem cell-hydrogel constructs hold great potential for regenerative medicine and cell-based therapies. However, their clinical application is still challenging due to their short shelf-life at ambient temperature and the time-consuming fabrication procedure. Banking the constructs at cryogenic temperature may offer the possibility of "off-the-shelf" availability to end-users. However, ice formation during the cryopreservation process may compromise the construct quality and cell viability. Vitrification, cooling biological samples without apparent ice formation, has been explored to resolve the challenge. However, contemporary vitrification methods are limited to very small volume (up to ~0.25 ml) and/or need highly toxic and high concentration (up to ~8 M) of permeable cryoprotectants (pCPAs). Here, we show that polytetrafluoroethylene (PTFE, best known as Teflon for making non-stick cookware) capillary is flexible and unusually stable at a cryogenic temperature. By using the PTFE capillary as a flexible cryopreservation vessel together with alginate hydrogel microencapsulation and Fe3O4 nanoparticle-mediated nanowarming to suppress ice formation, massive-volume (10 ml) vitrification of cell-alginate hydrogel constructs with a low concentration (~2.5 M) of pCPA can be achieved. This may greatly facilitate the use of stem cell-based constructs for tissue regeneration and cell based therapies in the clinic.

Immunological Challenges Facing Translation of Alginate Encapsulated Porcine Islet Xenotransplantation to Human Clinical Trials

Transplantation of alginate-encapsulated islets has the potential to treat patients suffering from type I diabetes, a condition characterized by an autoimmune attack against insulin-secreting beta cells. However, there are multiple immunological challenges associated with this procedure, all of which must be adequately addressed prior to translation from trials in small animal and nonhuman primate models to human clinical trials. Principal threats to graft viability include immune-mediated destruction triggered by immunogenic alginate impurities, unfavorable polymer composition and surface characteristics, and release of membrane-permeable antigens, as well as damage associated molecular patterns (DAMPs) by the encapsulated islets themselves. The lack of standardization of significant parameters of bioencapsulation device design and manufacture (i.e., purification protocols, surface-modification grafting techniques, alginate composition modifications) between labs is yet another obstacle that must be overcome before a clinically effective and applicable protocol for encapsulating islets can be implemented. Nonetheless, substantial progress is being made, as is evident from prolonged graft survival times and improved protection from immune-mediated graft destruction reported by various research groups, but also with regard to discoveries of specific pathways involved in explaining observed outcomes. Progress in the latter is essential for a comprehensive understanding of the mechanisms responsible for the varying levels of immunogenicity of certain alginate devices. Successful translation of encapsulated islet transplantation from in vitro and animal model testing to human clinical trials hinges on application of this knowledge of the pathways and interactions which comprise immune-mediated rejection. Thus, this review not only focuses on the different factors contributing to provocation of the immune reaction by encapsulated islets, but also on the defining characteristics of the response itself.

Layer-by-layer assembled biopolymer microcapsule with separate layer cavities generated by gas-liquid microfluidic approach

In this work, a layer-by-layer (LbL) assembled biopolymer microcapsule with separate layer cavities is generated by a novel and convenient gas-liquid microfluidic approach. This approach exhibits combined advantages of microfluidic approach and LbL assembly method, and it can straightforwardly build LbL-assembled capsules in mild aqueous environments at room temperature. In particular, using this approach we can build the polyelectrolyte multilayer capsule with favorable cavities in each layer, and without the need for organic solvent, emulsifying agent, or sacrificial template. Various components (e.g., drugs, proteins, fluorescent dyes, and nanoparticles) can be respectively encapsulated in the separate layer cavities of the LbL-assembled capsules. Moreover, the encapsulated capsules present the ability as colorimetric sensors, and they also exhibit the interesting release behavior. Therefore, the LbL-assembled biopolymer capsule is a promising candidate for biomedical applications in targeted delivery, controlled release, and bio-detection.

Determination of the Mechanical Strength of Microcapsules

The promise of pancreatic islet transplantation is hindered by organ shortage, and the need for immunosuppression of transplant recipient in order to prevent rejection. Alginate microencapsulation can overcome these hurdles; however further optimization of this technique is required. Among the critical factors to be optimized is the durability of alginate microcapsules, which can be determined by their mechanical strength tests. Here we describe several simple and reliable methods to assist in assessing the mechanical strength of alginate beads.

Imaging of Hydrogel Microsphere Structure and Foreign Body Response Based on Endogenous X-Ray Phase Contrast

Transplantation of functional islets encapsulated in stable biomaterials has the potential to cure Type I diabetes. However, the success of these materials requires the ability to quantitatively evaluate their stability. Imaging techniques that enable monitoring of biomaterial performance are critical to further development in the field. X-ray phase-contrast (XPC) imaging is an emerging class of X-ray techniques that have shown significant promise for imaging biomaterial and soft tissue structures. In this study, XPC imaging techniques are shown to enable three dimensional (3D) imaging and evaluation of islet volume, alginate hydrogel structure, and local soft tissue features ex vivo. Rat islets were encapsulated in sterile ultrapurified alginate systems produced using a high-throughput microfluidic system. The encapsulated islets were implanted in omentum pouches created in a rodent model of type 1 diabetes. Microbeads were imaged with XPC imaging before implantation and as whole tissue samples after explantation from the animals. XPC microcomputed tomography (μCT) was performed with systems using tube-based and synchrotron X-ray sources. Islets could be identified within alginate beads and the islet volume was quantified in the synchrotron-based μCT volumes. Omental adipose tissue could be distinguished from inflammatory regions resulting from implanted beads in harvested samples with both XPC imaging techniques. Individual beads and the local encapsulation response were observed and quantified using quantitative measurements, which showed good agreement with histology. The 3D structure of the microbeads could be characterized with XPC imaging and failed beads could also be identified. These results point to the substantial potential of XPC imaging as a tool for imaging biomaterials in small animal models and deliver a critical step toward in vivo imaging.

Long-Term Function of Alginate-Encapsulated Islets

Human trials have demonstrated the feasibility of alginate-encapsulated islet cells for the treatment of type 1 diabetes. Encapsulated islets can be protected from the host's immune system and remain viable and functional following transplantation. However, the long-term success of these therapies requires that alginate microcapsules maintain their immunoprotective capacity and stability in vivo for sustained periods. In part, as a consequence of different encapsulation strategies, islet encapsulation studies have produced inconsistent results in regard to graft functioning time, stability, and overall metabolic benefits. Alginate composition (proportion of M- and G-blocks), alginate purity, the cross-linking ions (calcium or barium), and the presence or absence of additional polymer coating layers influence the success of cell encapsulation. This review summarizes the outcomes of long-term studies of alginate-encapsulated islet transplants in animals and humans and provides a critical discussion of the graft failure mechanisms, including issues with graft biocompatibility, transplantation site, and integrity of the encapsulated islet grafts. Strategies to improve the mechanical stability of alginate capsules and methods for monitoring graft survival and function in vivo are presented.

Hybrid hydrogels based on keratin and alginate for tissue engineering

Novel hybrid hydrogels based on alginate and keratin were successfully produced for the first time. The self-assembly properties of keratin, and its ability to mimic the extracellular matrix were combined with the excellent chemical and mechanical stability and biocompatibility of alginate to produce 2D and 3D hybrid hydrogels. These hybrid hydrogels were prepared using two different approaches: sonication, to obtain 2D hydrogels, and a pressure-driven extrusion technique to produce 3D hydrogels. All results indicated that the composition of the hydrogels had a significant effect on their physical properties, and that they can easily be tuned to obtain materials suitable for biological applications. The cell-material interaction was assessed through the use of human umbilical vein endothelial cells, and the results demonstrated that the alginate/keratin hybrid biomaterials supported cell attachment, spreading and proliferation. The results proved that such novel hybrid hydrogels might find applications as scaffolds for soft tissue regeneration.

Three-dimensional scaffolds for tissue engineering applications: role of porosity and pore size

Tissue engineering applications commonly encompass the use of three-dimensional (3D) scaffolds to provide a suitable microenvironment for the incorporation of cells or growth factors to regenerate damaged tissues or organs. These scaffolds serve to mimic the actual in vivo microenvironment where cells interact and behave according to the mechanical cues obtained from the surrounding 3D environment. Hence, the material properties of the scaffolds are vital in determining cellular response and fate. These 3D scaffolds are generally highly porous with interconnected pore networks to facilitate nutrient and oxygen diffusion and waste removal. This review focuses on the various fabrication techniques (e.g., conventional and rapid prototyping methods) that have been employed to fabricate 3D scaffolds of different pore sizes and porosity. The different pore size and porosity measurement methods will also be discussed. Scaffolds with graded porosity have also been studied for their ability to better represent the actual in vivo situation where cells are exposed to layers of different tissues with varying properties. In addition, the ability of pore size and porosity of scaffolds to direct cellular responses and alter the mechanical properties of scaffolds will be reviewed, followed by a look at nature's own scaffold, the extracellular matrix. Overall, the limitations of current scaffold fabrication approaches for tissue engineering applications and some novel and promising alternatives will be highlighted.

The reconstruction of lung alveolus-like structure in collagen-matrigel/microcapsules scaffolds in vitro

This study attempted to use collagen–Matrigel as extracellular matrix (ECM) to supply cells with three-dimensional (3D) culture condition and employ alginate-poly-l-lysine-alginate (APA) microcapsules to control the formation of alveolus-like structure in vitro. We tested mice foetal pulmonary cells (FPCs) by immunohistochemistry after 2D culture. The alveolus-like structure was reconstructed by seeding FPCs in collagen–Matrigel mixed with APA microcapsules 1.5 ml. A self-made mould was used to keep the structure from contraction. Meanwhile, it provided static stretch to the structure. After 7, 14 and 21 days of culture, the alveolus-like structure was analysed histologically and immunohistochemically, or by scanning transmission electron microscopy (TEM). We also observed these structures under inverted phase contrast microscope. The expression of pro-surfactant protein C (SpC) was detected by reverse transcription-polymerase chain reaction (RT-PCR). We obtained fibroblasts, epithelial cells and alveolar type II (AE2) cells in FPCs. In the reconstructed structure, seeding cells surrounding the APA microcapsules constructed alveolus-like structures, the size of them ranges from 200 to 300 μm. In each reconstructed lung tissue sheet, microcapsules had integrity. Pan-cytokeratin, vimentin and SpC positive cells were observed in 7- and 14-day cultured structures. TEM showed lamellar bodies of AE2 cells in the reconstructed tissues whereas RT-PCR expressed SpC gene. Primary mice FPCs could form alveolus-like structures in collagen–Matrigel/APA microcapsules engineered scaffolds, which could maintain a differentiated state of AE2 cells.

Biocompatibility and physicochemical characteristics of alginate-polycation microcapsules

There is a need for better understanding of the biocompatibility of alginate-polycation microcapsules based on their physicochemical characteristics. Microcapsules composed of alginate with 44% (IntG) or 71% (HiG) guluronate, gelled with calcium (Ca) or barium (Ba) and coated with poly-L-lysine (PLL) or poly-l-ornithine (PLO), followed by IntG alginate were compared. For microcapsules with an IntG(Ca) gel core, using PLO instead of PLL resulted in less immune cell adhesion after 2 days in C57BL/6J mice. The PLO microcapsules were also characterized by greater hydrophilicity and superior resistance to swelling and damage under osmotic stress. For microcapsules with a PLL membrane, replacing the IntG(Ca) gel core with IntG(Ba) or HiG(Ca) gel resulted in stronger immune responses (p<0.05). This was explained by poor penetration of PLL into the gel, as demonstrated by Fourier transform infrared spectroscopy analyses and membrane rupturing during osmotic swelling. X-ray photoelectron spectroscopy analyses show that all microcapsules had the same amount of polycation at their surface. Moreover, alginate coatings had non-significant effects on the biocompatibility and physicochemical properties of the microcapsules. Thus, alginate-polycation interactions for membrane formation are more important for biocompatibility than either the quantity of polycation at the surface or the alginate coating.

Making a case: nanofabrication techniques in encapsulated cell therapy for people with diabetes

A nanoporous immunoisolative case/capsule that encases/encapsulates insulin-secreting cells vastly expands the source of therapeutic cells available for grafting in people with diabetes, including cells from animal sources, stem cells, and genetically engineered cells. These encapsulated cellular grafts potentially provide an endogenous, renewable, and long-term source of insulin without the need for pharmacological immunosuppression. Micro- and nanofabrication techniques used principally in the semiconductor industry can play a positive role in encapsulated cell therapy. Many of these techniques do not have direct applicability in cell encapsulation, but can be leveraged to develop processes suitable for this application. This commentary highlights the salient features of an effective cell encapsulation system, enumerates limitations of existing encapsulation schemes, and touches upon progress in key areas of encapsulation technology; one example of how micro- and nanofabrication technology may be used to develop a more effective platform for cell encapsulation is presented. This commentary urges further exploration and expansion of techniques used traditionally in electronics and optics for cell-based therapy in people with diabetes.

Monodisperse alginate microcapsules with oil core generated from a microfluidic device

A microfluidic approach is developed to fabricate monodisperse alginate microcapsules with oil cores, which have the potential to be a brand-new type of vehicles for encapsulating, storing and/or transferring lipophilic drugs or active ingredients/chemicals. The alginate microcapsules with oil cores are generated in a microcapillary microfluidic device using monodisperse oil-in-water-in-oil (O/W/O) double emulsions as templates. All the as-prepared alginate microcapsules and the encapsulated oil droplets are satisfactorily spherical. Both the alginate microcapsule size and the thickness of alginate membrane can be easily controlled by modulating the dimensions of microfluidic device and the flow rates of solutions, because the outer diameter of the O/W/O double emulsion templates and the size of their inner oil cores can be controlled independently by adjusting the inner diameters of emulsification tubes and the flow rates of different solutions. The as-fabricated monodisperse micro-egg-like core-shell alginate microcapsules with oil cores may open new possibilities for engineering novel functional materials.

Differential dynamical effects of macromolecular crowding on an intrinsically disordered protein and a globular protein: implications for in-cell NMR spectroscopy

In-cell NMR provides a valuable means to assess how macromolecules, with concentrations up to 300 g/L in the cytoplasm, affect the structure and dynamics of proteins at atomic resolution. Here an intrinsically disordered protein, alpha-synuclein (alphaSN), and a globular protein, chymotrypsin inhibitor 2 (CI2) were examined by using in-cell NMR. High-resolution in-cell spectra of alphaSN can be obtained, but CI2 leaks from the cell and the remaining intracellular CI2 is not detectable. Even after stabilizing the cells from leakage by using alginate encapsulation, no CI2 signal is detected. From in vitro studies we conclude that this difference in detectability is the result of the differential dynamical response of disordered and ordered proteins to the changes of motion caused by the increased viscosity in cells.