Design of a polyurethane membrane for the encapsulation of islets of Langerhans

A Case for Material Stiffness as a Design Parameter in Encapsulated Islet Transplantation

Diabetes is a disease that plagues over 463 million people globally. Approximately 40 million of these patients have type 1 diabetes mellitus (T1DM), and the global incidence is increasing by up to 5% per year. T1DM is where the body's immune system attacks the pancreas, specifically the pancreatic beta cells, with antibodies to prevent insulin production. Although current treatments such as exogenous insulin injections have been successful, exorbitant insulin costs and meticulous administration present the need for alternative long-term solutions to glucose dysregulation caused by diabetes. Encapsulated islet transplantation (EIT) is a tissue-engineered solution to diabetes. Donor islets are encapsulated in a semipermeable hydrogel, allowing the diffusion of oxygen, glucose, and insulin but preventing leukocyte infiltration and antibody access to the transplanted cells. Although successful in small animal models, EIT is still far from commercial use owing to necessary long-term systemic immunosuppressants and consistent immune rejection. Most published research has focused on tailoring the characteristics of the capsule material to promote clinical viability. However, most studies have been limited in scope to biochemical changes. Current mechanobiology studies on the effect of substrate stiffness on the function of leukocytes, especially macrophages-primary foreign body response (FBR) orchestrators, show promise in tailoring a favorable response to tissue-engineered therapies such as EIT. In this review, we explore strategies to improve the clinical viability of EIT. A brief overview of the immune system, the FBR, and current biochemical approaches will be elucidated throughout this exploration. Furthermore, an argument for using substrate stiffness as a capsule design parameter to increase EIT efficacy and clinical viability will be posed.

3D-printed polyurethane immunoisolation bags with controlled pore architecture for macroencapsulation of islet clusters encapsulated in alginate gel

Diabetes mellitus is a fast-growing chronic metabolic condition caused by insulin deficiency or resistance, leading to lifelong insulin use. It has become one of the world's most difficult non-communicable diseases. The goal of this study was to view the effectiveness of the combined method of macro- and microencapsulation for islet transplantation. The process of 3D printing is used to make macroencapsulation bags with regulated diffusion properties thanks to the emerging small pored channels. The ink used to manufacture 3D-printed bags with controlled specifications was polyurethane solution (13% w/v). Swelling experiments revealed that there was very little swelling and that the membrane maintained its structural stability. Alginate beads (made from 5% w/v solution) were used to microencapsulate islet cell clusters. Direct contact assay was used to confirm in vitro cytocompatibility. The insulin release from the encapsulated rabbit islets was confirmed using a glucose challenge assay. When challenged with 20 mM glucose on day 7, the encapsulated islet cells released insulin at a rate of 9.72 ± 0.65 mU/L, which was identical to the RIN-5F islet cell line control, confirming the functioning of the encapsulated islets. After 21 days of culture, the islets were shown to be viable utilizing a live-dead assay. As a result, our work demonstrates that 3D printing for macroencapsulating cells, as well as microencapsulation with alginates, is a viable scale-up technology with great potential in the field of pancreatic islet transplantation.

A Zwitterionic Polyurethane Nanoporous Device with Low Foreign-Body Response for Islet Encapsulation

Encapsulation of insulin-producing cells is a promising strategy for treatment of type 1 diabetes. However, engineering an encapsulation device that is both safe (i.e., no cell escape and no breakage) and functional (i.e., low foreign-body response (FBR) and high mass transfer) remains a challenge. Here, a family of zwitterionic polyurethanes (ZPU) with sulfobetaine groups in the polymer backbone is developed, which are fabricated into encapsulation devices with tunable nanoporous structures via electrospinning. The ZPU encapsulation device is hydrophilic and fouling-resistant, exhibits robust mechanical properties, and prevents cell escape while still allowing efficient mass transfer. The ZPU device also induces a much lower FBR or cellular overgrowth upon intraperitoneal implantation in C57BL/6 mice for up to 6 months compared to devices made of similar polyurethane without the zwitterionic modification. The therapeutic potential of the ZPU device is shown for islet encapsulation and diabetes correction in mice for ≈3 months is demonstrated. As a proof of concept, the scalability and retrievability of the ZPU device in pigs and dogs are further demonstrated. Collectively, these attributes make ZPU devices attractive candidates for cell encapsulation therapies.

Leachable Poly(Trimethylene Carbonate)/CaCO3 Composites for Additive Manufacturing of Microporous Vascular Structures

The aim of this work was to fabricate microporous poly(trimethylene carbonate) (PTMC) vascular structures by stereolithography (SLA) for applications in tissue engineering and organ models. Leachable CaCO₃ particles with an average size of 0.56 μm were used as porogens. Composites of photocrosslinkable PTMC and CaCO₃ particles were cast on glass plates, crosslinked by ultraviolet light treatment and leached in watery HCl solutions. In order to obtain interconnected pore structures, the PTMC/CaCO₃ composites had to contain at least 30 vol % CaCO₃. Leached PTMC films had porosities ranging from 33% to 71% and a pore size of around 0.5 μm. The mechanical properties of the microporous PTMC films matched with those of natural blood vessels. Resins based on PTMC/CaCO₃ composites with 45 vol % CaCO₃ particles were formulated and successfully used to build vascular structures of various shapes and sizes by SLA. The intrinsic permeabilities of the microporous PTMC films and vascular structures were at least one order of magnitude higher than reported for the extracellular matrix, indicating no mass transfer limitations in the case of cell seeding.

Evaluation of an Immunoisolation Membrane Formed by Incorporating a Polyvinyl Alcohol Hydrogel within a Microporous Filter Support

An immunoisolation membrane formed by incorporating a high water content polyvinyl alcohol (PVA) hydrogel into a microporous polyether sulfone (PES) filter has been investigated in this study. The PVA hydrogel is formed in situ within the filter pores via glutaraldehyde (GA) crosslinking under acidic conditions. The tortuous nature of the microporous filter pores securely anchors the embedded hydrogel to provide excellent structural integrity. The high void fraction of the PES filter support (>80%) and high water content of the PVA hydrogel (>85% water by weight) allow excellent solute transport rates, while an appropriate level of glutaraldehyde crosslinking supplies the required molecular size selectivity. In vitro permeability measurements made with solutes covering a wide range of molecular sizes demonstrate high transport rates for small nutrient molecules with rapidly diminishing permeabilities above a molecular weight of approximately 1,000 Dalton. Implantation experiments show that the membrane properties are not deleteriously affected by prolonged in vivo exposure or common sterilization techniques. Thus, this hybrid hydrogel/filter membrane system offers a promising approach to the immunoisolation of implanted cells.

Membranes to achieve immunoprotection of transplanted islets

Transplantation of islet or beta cells is seen as the cure for type 1 diabetes since it allows physiological regulation of blood glucose levels without requiring any compliance from the patients. In order to circumvent the use of immunosuppressive drugs (and their side effects), semipermeable membranes have been developed to encapsulate and immunoprotect transplanted cells. This review presents the historical developments of immunoisolation and provides an update on the current research in this field. A particular emphasis is laid on the fabrication, characterization and performance of membranes developed for immunoisolation applications.

Design of a microporous controlled delivery system for theophylline tablets

The aim of present work was to develop a microporous-controlled delivery system for theophylline via coating a blend of PCL and PEG on the surface of tablets, where PCL was the major component of film coating material and PEG was acted as a leachable pore-forming agent when contacting with an aqueous medium. The influences of the type of solvent, the amount of PEG, and the thickness of films on the mechanical and thermal properties of coating films and drug release performance were investigated. The DSC thermograms and FTIR spectra indicated both PCL and PEG remained independently in the blended films. The mechanical data showed a decrease tendency as increase in the amount of PEG in the blends due to highly crystalline character of PEG. Slower evaporation rate of acetone than dichloromethane enhanced phase separation between PCL and PEG during film formation, and resulted in the pore size in films prepared from acetone larger than from dichloromethane. The release rate of coated tablets were increased by increasing the amount of pore-forming agent, and the corresponding values from tablets coated in dichloromethane were less than in acetone. Much denser structure and smaller pore size of films formed from dichloromethane contributed to this result. The release of drug from tablets coated in acetone showed a profile more close to a zero-order constant release profile. The penetration of water into drug core played an important role in influencing drug release pattern.

Nonporous polyurethane membranes as islet immunoisolation matrices--biocompatibility studies

Novel elastomeric nonporous polyurethane membranes were synthesised with differing hard segment contents for evaluation as possible islet encapsulation matrices. Physico-chemical properties of these membranes were reported earlier by authors and have been found suitable for immunoisolation. In the present study, membranes were evaluated for their in vitro biocompatibility. Membranes T1, T4, T5 and T6 did not show toxicity in direct cell contact study towards L929 fibroblasts. However, T2 and T3 were found cytotoxic and were excluded from further testing. NIH3T3 cells when exposed to leach out products of T4, T5 and T6 showed no cytotoxicity, while T1 decreased cellular viability as confirmed by MTT assay. T4 and T5 alone were seen to be compatible with mouse islets while T6 was incompatible to the mouse islets. Digital image analysis (DIA) studies showed intact morphology of islets cultured on the T4 and T5 with viability (88.4 and 91% respectively) comparable to islets on tissue culture polystyrene (TCPS) control. Islets on T4 and T5 also retained their functionality, as judged by insulin secretion in response to in vitro glucose challenge (16.0 mM). These studies point out the crucial role of surface free energy and hydrophilicity in deciding compatibility of polyurethane membranes with islets of Langerhans. Studies indicate that polyurethane membranes T4 and T5 could be potential candidates for islet immunoisolation.

Biohybrid artificial pancreas based on macrocapsule device

On the basis of an overview of the current methodologies used for biohybrid artificial pancreas (BAP) and a critical analysis of the problems in designing the BAP devices, especially macrocapsule systems, a new type of refillable BAP is proposed. The following highlights the unique features of this design: (1) use of a thermally reversible synthetic hydrogel made of N-isopropylacrylamide based copolymer as an extracellular matrix facilitates the recharge of the encapsulated islets whenever necessary; (2) use of a pouch system composed of an inert processable immunoprotective membrane with appropriate mechanical, chemical and transportation properties makes it easy to fabricate the BAP device; (3) introduction of an oxygen carrying polymer ensures an adequate supply of oxygen to maintain high viability and function of the islets; (4) incorporation of biospecific polymers within the matrix to stimulate insulin secretion from islets may decrease the number of islets required, consequently resulting in reduced implant volume. The design concept and technology may also be utilized to deliver cells to treat other hormone deficiency syndromes. This paper also discusses the future development of BAPs.

Materials for immunoisolated cell transplantation

Encapsulated cell therapy provides site-specific continuous delivery of cell-synthesized molecules. Cell encapsulation therapy is based on the concept of immunoisolation. Foreign cells are surrounded with a semi-permeable membrane prior to transplantation to shield them from the host's natural defense system. This membrane is selectively permeable to transport of nutrients and therapeutic agents but relatively impermeable to larger molecules and cells of the hosts' immune system. Most encapsulation devices also utilize an internal matrix to keep cells suspended within the capsule. Proper choice of materials and materials processing techniques to formulate membrane and matrix components is essential to the success of these devices. A successful encapsulation device recreates the natural three-dimensional tissue environment that supports cell function and maintains cell viability. This review summarizes recent developments in materials development for cell encapsulation devices and highlights some ongoing challenges faced by those in the field.

Permeability assessment of capsules for islet transplantation

Despite considerable progress in the development of immunoisolation devices, the optimal permeability of such devices is not known. This limitation stems partly from deficits in knowledge about which molecules should be allowed to traverse the semipermeable membrane and which molecules should be excluded, and also partly from experimental obstacles that have prevented a systematic study of permeability. To determine the optimal permeability of immunoisolation devices, we have created a series of microcapsules (800 microM diameter) that span a broad range of molecular exclusion limits yet are identical in wall thickness and chemical composition. Capsule permeability was precisely defined by two complementary methods--size exclusion chromatography (SEC) and a newly developed methodology to assess permeability of biologically relevant proteins. The entry of interleukin-1 beta-125I was significantly delayed, but not prevented, when the capsule exclusion limit was decreased from 230 kD to 3.2 kD, as determined by SEC with dextran standards. The influx of IgG was as predicted, based on the viscosity radius R eta of IgG and the capsule exclusion limit defined by SEC. Glucose-stimulated insulin secretion by encapsulated pancreatic islets did not differ as capsule permeability was decreased from a molecular exclusion limit of 230 kD to 120 kD. These studies should assist in the design of immunoisolation devices by defining the permeability optimal for cell function and also should be applicable to any cell type or immunoisolation device.

Permeability and biocompatibility of a new hydrogel used for encapsulation of hepatocytes

A new high-water-content (83%) and highly permeable anionic polyelectrolyte hydrogel was obtained by phase inversion of a polymer solution containing 6% polyacrylonitrile-sodium methallylsulphonate, 91% dimethylsulphoxide and 3% physiological saline solution. Hydrogel-based hollow fibres (HFs) were fabricated with a co-extrusion apparatus in collaboration with Hospal (France). HFs have an internal diameter of 800 microns and a wall thickness of 100 microns. Experimental results demonstrated that hydrogel-based HFs were permeable to albumin (mol. wt 69,000) and human immunoglobulin G (150,000), but were impermeable to immunoglobulins A (170,000) and M (900,000) after 24 h of diffusion. In vitro, the viability of isolated rat hepatocytes injected into the HFs was 64 +/- 6% after 10 d versus 30 +/- 5% for hepatocytes cultured in Petri dishes (P = 0.0001). Under these conditions, the amount of albumin released by encapsulated hepatocytes was 12 +/- 3 micrograms/24 h/10(6) cells at day 10, whereas at that time no albumin was released by hepatocytes cultured in Petri dishes. In vivo, histological study of hydrogel HFs implanted up to 6 wk in the peritoneum of rats revealed a low inflammatory tissue reaction without giant multinucleate cells in the foreign tissue, which decreased after the third week. The survival rate of encapsulated hepatocytes was over 85% 45 d after transplantation in the peritoneum of syngeneic Lewis rats. Therefore, this hydrogel demonstrates highly favourable properties for encapsulation of hepatocytes with regard to its biocompatibility, permeability and ability to maintain hepatocytes in a functional state for prolonged periods.

Influence of corona surface treatment on the properties of an artificial membrane used for Langerhans islets encapsulation: permeability and biocompatibility studies

An artificial membrane (AN69 Hospal) suitable for pancreatic islets encapsulation was submitted to a physicochemical treatment (corona discharge) to improve its insulin permeability. This effect depends on the duration of the electrical discharge (expressed as the speed of a conveyor belt) and the distance between the electrodes and the membrane. Among the various treatments tested, the most efficient (distance of 5 cm and a speed of 2 cm s-1) produced a three-fold increase in insulin diffusion. This improvement persisted after a protein-coating test which mimics in vivo conditions. At 1 y after the peritoneal implantation, the corona-treated membrane remained biocompatible. Thus, corona discharge treatment may serve to optimize the properties of artificial membranes used for pancreatic islets encapsulation.