Oxygen consumption rate (OCR) is a critical parameter to assess as part of investigations of islet physiology and viability. Oxygen is the final acceptor of electrons traversing the electron transport chain (ETC), and in islets oxygen utilization associated with the ETC represents most of the total cellular OCR 
. Since the rate of the ETC reflects cellular metabolism of nutrients, the ability to generate energy by the cell, and driving forces and signals for oxidative stress and apoptosis. OCR is a window into many aspects of cellular function. Islets in particular increase ATP production as part of the mechanism mediating the secretion of insulin. Accordingly, OCR has been used for islet quality assessment in islet transplantation 
, tests of viability in the optimization of immunoisolation devices 
, studies of beta cells during differentiation from stem cells 
, and mechanistic studies of islet physiology 
Due to the importance of OCR for studies of cells of all types, there have been a number of approaches to its measurement. In general, these are broken down into two categories, flow and static systems. Use of flow systems involves measuring the difference between inflow and outflow oxygen content, whereas in the static systems the decrease in oxygen content in a closed chamber is measured over a period of time. Flow systems are able to resolve temporal changes in islet respiration in response to the presence or washout of effector agents. Two static systems that have been used for islet studies include mixed chambers that are equipped with a Clark oxygen electrode 
and multiwell plates containing an oxygen sensor at the bottom of each well (BD Oxygen Biosensor Systems 
and Seahorse Bioscience XF Analyzers 
). A major feature of the plate-based systems is the facility to carry out multiple experiments in parallel.
In general, to be able to measure the decrement in oxygen tension in bulk solution (flow or well-mixed) a relatively large number of islets must be used. A mode of real time analysis of OCR by single islets utilizes oxygen-specific sensors to measure oxygen tension in media surrounding or inside the islet 
. These measurements are reflective of, but not precisely proportional to, OCR. Nonetheless, the benefits of the single islet analysis are the resolution of intrinsic kinetic responses that are dampened by heterogeneous responses from multiple islets, the small number of islets required to obtain data and ease of characterization of the tissue under interrogation. However, the use of physical sensors has the drawbacks of being invasive, sensitive to the positioning of the sensor within the islet, and incapable of assessing more than one islet at a time. Imaging of single islets using fluorescence-based dyes allows an integrated cellular response in the parameter of interest to be tracked with minimal perturbation to the cell. Commonly imaged parameters in islets include calcium 
, mitochondrial membrane potential 
and NADH 
. However, there are no validated oxygen-sensitive fluorescent dyes that can be used to track oxygen tension in or around the islet. We therefore endeavored to develop an imaging method based on the use of platinum porphyrins, oxygen-sensitive dyes that are available in various chemical forms that widely differ in solubility and other chemical properties 
. In theory, a lipophilic dye could be used which would then situate in the lipid membranes of the islet. However, the dye could only be loaded into islets in the presence of high amounts of DMSO, which would be harmful to the islet. We therefore chose to utilize alginate hydrogels that could be used to encapsulate the islet in the presence of the dye, thereby trapping the dye in the extracellular fluid around and within the islet.
Encapsulation has been tested as a way to provide an immunoprotective barrier when transplanting islets into diabetic patients 
. Materials used in the encapsulation process such as alginate are porous, permitting the exchange of nutrients and cellular waste products to the surrounding solutions. Collectively, studies have shown that islet viability is maintained during and after the encapsulation process both in vitro and in vivo 
. Nonetheless, using traditional cell encapsulation procedures it has been difficult to make spherical shaped microcapsules in the optimal size range for islets (100–200 µm) 
Recently, microfluidic techniques have advanced cell encapsulation by using hydrogels or other biopolymers as the microcapsules 
. This approach preserves cell viability and generates a uniform set of monodisperse microencapsulated cells necessary to obtain reliable individual cell behavior from a population of cells 
. Tan and co-workers 
used microfluidic devices to generate microcapsules where the polymerization of the microcapsule surrounding Jurkat cells occurred following a T-junction that mixed the cells and the solutions catalyzing the polymerization. In this paper, we used such a T-junction to encapsulate single isolated pancreatic islets and a water-soluble, oxygen-sensitive, fluorescent dye inside a 180- µm sized alginate microcapsule. The dye situates itself during the encapsulation process in the extracellular space between the islet cells and the alginate layer, and acts as a 3-dimensional real time oxygen sensor. The alginate microcapsule-based sensor was stable, sensitive to small changes in oxygen tension, and responded to various effectors of mitochondrial metabolism in real time. Our approach can be used to screen the effects of compounds on OCR in single islets, and also could be adapted for use with other fluorescent dyes sensitive to many compounds of biological importance to islets and other cells.