Micro and Nanosystems - Volume 5, Issue 3, 2013

Throughout the past four decades, many inroads have been made in islet transplantation, so much so that this treatment is now seen as a viable therapeutic option for an increasing number of Type 1 diabetic patients. Islet transplantation not only provides initial insulin independence for selected Type 1 diabetic patients but also delivers long-term graft function and symptomatic benefit in terms of avoidance of severe hypoglycemia. However, despite many recent successes in this therapy, research and development is needed to improve this procedure further, with the ultimate aim of providing long-term insulin independence without the requirement for immunosuppressive medication. There are still numerous hurdles that must be overcome to ensure the accomplishment of this goal. Such challenges include, but are not solely limited to, the inadequate supply of insulin producing cells, the inability to fully determine islet function prior to and following transplantation, and the resultant immune mediated rejection of donor islets post transplantation. Years after transplantation, some patients present with a yet to be understood steady decline in islet graft function. The future development of islet transplantation hinges on the ability to understand and overcome these obstacles. Micro and nano technologies are seen as playing a vital role in addressing these challenges for accomplishing the ultimate goal of offering a functional cure for diabetes.

Encapsulation of islets is a promising strategy to deliver a cell therapy for treatment of type 1 diabetes without the need for anti-rejection drugs. The most common form of encapsulation is the microcapsule, which is made from sodium alginate, and is made robust by brief exposure to either Ca2+ or Ba2+. Transplantation of encapsulated human islets into diabetic immunodeficient mice results in rapid normalization of blood glucose levels. The number of encapsulated islets required to achieve normoglycemia is the same as the number of non-encapsulated islets. The minimal islet mass is 2000-3000 islet equivalents (IEQ), but this number can be reduced to 750-1000 IEQ if the encapsulated islets are pretreated overnight with 100µM desferrioxamine prior to transplantation. Transplantation of encapsulated islets into recipients with type 1 diabetes has been performed by several groups, with long term function of the grafts and a reduction of insulin requirements, but not insulin-independence, being demonstrated. A major issue that needs to be addressed to improve efficacy, is the prevention of pericapsular fibrotic overgrowth. This can lead to islet necrosis, as the pores that permit passage of nutrients and oxygen are obstructed. This overgrowth is caused by an inflammatory response to the encapsulation material and/or to antigens that leak through the pores. The peritoneal cavity seems not to be the best site for implantation, because of the large number of macrophages. Better survival is achieved in animals when encapsulated cells are grafted subcutaneously, but the low oxygen levels here limit the function of β cells since they thrive best at higher oxygen levels. An alternative to using microcapsules is the macrocapsule, but as yet there have been no human studies with islets examining its benefits. Reasons for continuing to pursue the use of encapsulated islets in the clinic are twofold. Firstly, they offer a potential means of overcoming the need for subcutaneous insulin administration without the need for anti-rejection drugs. Secondly, they lay the foundation for other forms of encapsulated allogeneic cell therapies, namely with stem cells or cells derived from them.

This minireview focuses on recent developments in the field of islet microencapsulation as one of the strategies for immunoprotection of transplanted insulin-producing cells for future diabetes treatment. A growing number of peerreviewed publications and reviews in the last few years demonstrate both the progress and strong motivation to identify safe and functional biomaterials. This minireview is primarily based on a literature survey covering the last few years, with the main section devoted to the discussion of various microcapsule types currently under development. The information on microcapsules is complemented by a brief discussion of two additional current strategies used for immunoprotection, namely macroencapsulation and conformal coating, to capture the overall situation in the field of islet encapsulation and transplantation. This is followed by briefly commenting on the characterization of encapsulation devices. In conclusion, the past and very recent achievements (1) reveal that islet microencapsulation remains a vital strategy to protect islet cells and (2) allow for proposing the next steps to bring this strategy closer to clinics.

The whole blood model is a powerful method to determine the immediate inflammatory reactions towards foreign objects in general. This review focuses on the use of a lepirudin based whole blood model for evaluating microspheres relevant in cell transplantation applications. This whole blood model can be regarded as a holistic model with readouts from cross-talks between leukocytes, complement, most of the coagulation components and fibrinolysis. A major advantage of this model is the possibility of evaluating a panel of different microspheres under identical conditions, and also the possibility of comparing reaction patterns between species. This model is a valuable tool for gaining a mechanistic understanding by selected readouts (as complement and coagulation activation products, cytokines, cell-surface receptors, protein adsorption, cell-attachment), and by use of inflammatory blocking agents (inhibitors). The whole blood model is put in the context of today’s knowledge about inflammatory systems, discussed according to biocompatibility and biotolerability terms and finally discussed according to its ability to predict the outcome of transplanted microspheres in an in vivo environment.

Microencapsulated islet transplantation has been investigated for over three decades in an attempt to eliminate the hostile effects of long-term immunosuppressant usage and improve islet engraftment in patients suffering from Type 1 Diabetes Mellitus (T1DM). Once feasibility of the islet encapsulation process was confirmed, the biocompatibility and survival of microencapsulated islets were directly tested in different animal models. Such animal models include: immunodeficient rodents (nude and NOD-SCID mice), immunocompetent rodents, autoimmune diabetic rodents (NOD mice and BB rats), and large animals (dogs, pigs, and non-human primates). In general, the majority of microcapsules tested in vivo was found to be biocompatible with the host and retained their function. This was especially apparent in small animal models. However, greater variability and reduced biocompatibility was observed in larger animal models. Since the first attempts to introduce this methodology to the clinical field, different devices and several implementation techniques have been proposed in order to support this technology. To date islet microencapsulation in the absence of immunosuppressive medication has not yet successfully reached the clinical setting. The purpose of this review is to summarize the transplant outcomes of different microencapsulation systems and transplant models, to reveal the major obstacles that each has presented, and to propose better avenues of investigation into future studies, which may lead to a successful clinical application for this technology.

The technique of microencapsulation was introduced nearly eighty years ago and its potential application in cell therapy was described almost fifty years later. The technique offers a wide range of applications in both industrial products and personalized medicine. The potential application of the technique in pancreatic islet immunoisolation prior to transplantation was first described in 1980 with the resulting construct termed as bioartificial pancreas. Since that first description of microencapsulated islet technology, interest has waxed and waned because of the conflicting data that have been generated in a variety of experimental and clinical studies. Routine clinical application has been impaired because of a number of factors including the need to optimize the technique, determination of the optimum site of transplantation, and the scarcity of appropriate scaled up devices for high throughput fabrication of the construct to be used in large animals and human studies. In this review article, we discuss the microencapsulation procedure and its various applications, and current impediments to routine use of this technology in the biomedical setting especially in encapsulated islet transplantation. In particular, we highlight the role of microfluidics in microencapsulation in many areas of industrial and medical applications. In medical applications, we have highlighted developments in the bioartificial pancreas and the outstanding issues that need to be resolved to make this technology a viable treatment option for individuals afflicted with Type 1 diabetes.

Hypoxia is a prevalent factor regulating numerous biological processes; and of specific interest here, hypoxia is a complication in clinical procedures such as islet transplantation. Recently, temporal oscillations between hypoxia and normoxia, termed intermittent hypoxia (IH), have shown “preconditioning” benefits to pancreatic islets [1-3]. In islet physiology, effects of static and transient hypoxia have not been well-understood or studied, primarily due to a lack of appropriate tools to control islet microenvironments. Addressing this gap of knowledge, this review will discuss the following points: 1) Discussion of the physiological and pathophysiological states of islet oxygenation will illustrate a significant diffusion limitation imposed on isolated islets. 2) Brief survey of current islet hypoxia studies will uncover the need for a microscaled oxygen modulation technique. 3) Novel microfluidic methods will be introduced that can manipulate oxygen microenvironments. 4) A technique based on microfluidics will be shown to precondition islets via intermittent hypoxia. The view of hypoxia as a key parameter in islet and transplant physiology is expanding. Our message here is that microscaled tools and techniques have developed to a level capable of experimentally addressing this increasingly important view on hypoxia.

This review outlines the current status of microfluidic devices used to study the physiology and pathophysiology of pancreatic islets of Langerhans, mainly focusing on the design features and specialized applications of existing microfluidic devices, as well as their advantages and limitations. This review then briefly concludes by describing future perspectives on ways to improve and implement microfluidic technology for islet study.

Pancreatic islet transplantation is a promising cure for restoring normoglycemia in diabetic patients. A noninvasive, in vivo approach that could monitor survival of islet grafts is critically needed in clinic. Here we provide a concise overview of the most recent advances in magnetic resonance (MR) imaging of islet transplantation with emphasis on 1) 1H MRI tracking of transplanted islets, 2) 19F MR imaging of islet grafts, 3) theranostic agents for islet transplantation. Based on the progress in this field during the past decade, we envision new avenues for future investigation and clinical research that are emphasized in this review.

To improve the stability of electrophoresis separation by adding samples at one time, two PMMA microfluidic chips with different configurations were made by hot embossing/bonding method. The reason of bad peak repetition during continuous electrophoresis separation in the double-T microfluidic chip has been analyzed, and a Y-T combination structure has been put forward. Based on the result of the computational fluid dynamics (CFD) simulation, the cause of bad peak repetition was found, and it is that the buffer was pushed back to sample reservoir by the flow field. A fruitful method for achieving good peak repetition with a Y-T combination structure has been developed by our research group. The structure was optimized via the CFD simulation. A sequence electrophoresis separation experiment using vitamin B2 and borax buffer was successfully put into practice. Depending on laser-induced fluorescence (LIF) detection, the relative standard deviations (TRSD) of peak height were 5% (n=6).