Integrated perfusion and separation systems for entrainment of insulin secretion from islets of Langerhans

A vascularized 3D model of the human pancreatic islet forex vivostudy of immune cell-islet interaction

Insulin is an essential regulator of blood glucose homeostasis that is produced exclusively byβcells within the pancreatic islets of healthy individuals. In those affected by diabetes, immune inflammation, damage, and destruction of isletβcells leads to insulin deficiency and hyperglycemia. Current efforts to understand the mechanisms underlyingβcell damage in diabetes rely onin vitro-cultured cadaveric islets. However, isolation of these islets involves removal of crucial matrix and vasculature that supports islets in the intact pancreas. Unsurprisingly, these islets demonstrate reduced functionality over time in standard culture conditions, thereby limiting their value for understanding native islet biology. Leveraging a novel, vascularized micro-organ (VMO) approach, we have recapitulated elements of the native pancreas by incorporating isolated human islets within a three-dimensional matrix nourished by living, perfusable blood vessels. Importantly, these islets show long-term viability and maintain robust glucose-stimulated insulin responses. Furthermore, vessel-mediated delivery of immune cells to these tissues provides a model to assess islet-immune cell interactions and subsequent islet killing-key steps in type 1 diabetes pathogenesis. Together, these results establish the islet-VMO as a novel,ex vivoplatform for studying human islet biology in both health and disease.

Relations between Glucose and d-Amino Acids in the Modulation of Biochemical and Functional Properties of Rodent Islets of Langerhans

The cell-to-cell signaling role of d-amino acids (d-AAs) in the mammalian endocrine system, particularly in the islets of Langerhans, has drawn growing interest for their potential involvement in modulating glucose metabolism. Previous studies found colocalization of serine racemase [produces d-serine (d-Ser)] and d-alanine (d-Ala) within insulin-secreting beta cells and d-aspartate (d-Asp) within glucagon-secreting alpha cells. Expressed in the islets, functional N-methyl-d-aspartate receptors are involved in the modulation of glucose-stimulated insulin secretion and have binding sites for several d-AAs. However, knowledge of the regulation of d-AA levels in the islets during glucose stimulation as well as the response of islets to different levels of extracellular d-AAs is limited. In this study, we determined the intracellular and extracellular levels of d-Ser, d-Ala, and d-Asp in cultures of isolated rodent islets exposed to different levels of extracellular glucose. We found that the intracellular levels of the enantiomers demonstrated large variability and, in general, were not affected by extracellular glucose levels. However, significantly lower levels of extracellular d-Ser and d-Ala were observed in the islet media supplemented with 20 mM concentration of glucose compared to the control condition utilizing 3 mM glucose. Glucose-induced oscillations of intracellular free calcium concentration ([Ca2+]i), a proxy for insulin secretion, were modulated by the exogenous application of d-Ser and d-Ala but not by their l-stereoisomers. Our results provide new insights into the roles of d-AAs in the biochemistry and function of pancreatic islets.

Bone/cartilage organoid on-chip: Construction strategy and application

The necessity of disease models for bone/cartilage related disorders is well-recognized, but the barrier between ex-vivo cell culture, animal models and the real human body has been pending for decades. The organoid-on-a-chip technique showed opportunity to revolutionize basic research and drug screening for diseases like osteoporosis and arthritis. The bone/cartilage organoid on-chip (BCoC) system is a novel platform of multi-tissue which faithfully emulate the essential elements, biologic functions and pathophysiological response under real circumstances. In this review, we propose the concept of BCoC platform, summarize the basic modules and current efforts to orchestrate them on a single microfluidic system. Current disease models, unsolved problems and future challenging are also discussed, the aim should be a deeper understanding of diseases, and ultimate realization of generic ex-vivo tools for further therapeutic strategies of pathological conditions.

Automated cellular stimulation with integrated pneumatic valves and fluidic capacitors

Microfluidic technologies have proven to be a reliable tool in profiling dynamic insulin secretion from islets of Langerhans. Most of these systems rely on external pressure sources to induce flow, leading to difficulties moving to more elaborate systems. To reduce complexity, a microfluidic system was developed that used a single vacuum source at the outlet to drive fluidic transport of immunoassay reagents and stimulation solutions throughout the device. A downside to this approach is the lack of flow control over the reagents delivered to the islet chamber. To address this challenge, 4-layer pneumatic valves were integrated into the perfusion lines to automate and control the delivery of stimulants; however, it was found that as the valves closed, spikes in the flow would lead to abnormal insulin secretion profiles. Fluidic capacitors were then incorporated after the valves and found to remove the spikes. The combination of the valves and capacitors resulted in automated collection of insulin secretion profiles from single murine islets that were similar to those previously reported in the literature. In the future, these integrated fluidic components may enable more complex channel designs to be used with a relatively simple flow control solution.

Coordination of pancreatic islet rhythmic activity by delayed negative feedback

Secretion of insulin from the pancreas is pulsatile, driven by intrinsic oscillations within individual islets of Langerhans. The secretions are coordinated among the many islets distributed throughout the pancreas producing a synchronized rhythm in vivo that is essential for maintaining normal glucose levels. One hypothesized mechanism for the coordination of islet activity is negative feedback, whereby sequestration of glucose in response to elevated insulin leads to a reduction in the blood glucose level that is sensed by the islet population. This global signal of glucose then coordinates the individual islets. In this study, we tested how this coordination mechanism is affected by time delays in the negative feedback, using a microfluidic system to monitor Ca2+ levels in a small population of islets and implementing glucose control through a negative feedback system. We found that islet synchronization occurs even with time delays in the feedback of up to 7 min. We also found that a second, slower closed-loop oscillation period is produced during delayed feedback in which islet oscillations are clustered into episodes. The period of this second oscillatory mode increases with the time delay and appears to be a second stable behavior that coexists with the faster synchronized oscillation. The general conclusion is that islet coordination through negative feedback is a viable means of islet coordination that is robust to delays in the timing of the feedback, and could complement other potential coordination mechanisms such as entrainment by pancreatic ganglia.NEW & NOTEWORTHY Insulin secretion from islets of Langerhans is rhythmic, and these rhythms are coordinated to produce oscillatory plasma insulin levels. Using a combination of microfluidics and computational modeling, we demonstrate that coordination can occur through negative feedback of the type provided by the liver, even if that feedback is delayed by several minutes. We also demonstrate that a second, slower, mode of oscillations can occur when feedback is delayed where faster oscillations are grouped into episodes.

Pancreatic islet organoids-on-a-chip: how far have we gone?

Diabetes mellitus (DM) is a disease caused by dysfunction or disruption of pancreatic islets. The advent and development of microfluidic organoids-on-a-chip platforms have facilitated reproduce of complex and dynamic environment for tissue or organ development and complex disease processes. For the research and treatment of DM, the platforms have been widely used to investigate the physiology and pathophysiology of islets. In this review, we first highlight how pancreatic islet organoids-on-a-chip have improved the reproducibility of stem cell differentiation and organoid culture. We further discuss the efficiency of microfluidics in the functional evaluation of pancreatic islet organoids, such as single-islet-sensitivity detection, long-term real-time monitoring, and automatic glucose adjustment to provide relevant stimulation. Then, we present the applications of islet-on-a-chip technology in disease modeling, drug screening and cell replacement therapy. Finally, we summarize the development and challenges of islet-on-a-chip and discuss the prospects of future research.

The Foundation for Engineering a Pancreatic Islet Niche

Progress in diabetes research is hindered, in part, by deficiencies in current experimental systems to accurately model human pathophysiology and/or predict clinical outcomes. Engineering human-centric platforms that more closely mimic in vivo physiology, however, requires thoughtful and informed design. Summarizing our contemporary understanding of the unique and critical features of the pancreatic islet can inform engineering design criteria. Furthermore, a broad understanding of conventional experimental practices and their current advantages and limitations ensures that new models address key gaps. Improving beyond traditional cell culture, emerging platforms are combining diabetes-relevant cells within three-dimensional niches containing dynamic matrices and controlled fluidic flow. While highly promising, islet-on-a-chip prototypes must evolve their utility, adaptability, and adoptability to ensure broad and reproducible use. Here we propose a roadmap for engineers to craft biorelevant and accessible diabetes models. Concurrently, we seek to inspire biologists to leverage such tools to ask complex and nuanced questions. The progenies of such diabetes models should ultimately enable investigators to translate ambitious research expeditions from benchtop to the clinic.

Alternative Brain Slice-on-a-Chip for Organotypic Culture and Effective Fluorescence Injection Testing

Mouse brain slices are one of the most common models to study brain development and functioning, increasing the number of study models that integrate microfluidic systems for hippocampal slice cultures. This report presents an alternative brain slice-on-a-chip, integrating an injection system inside the chip to dispense a fluorescent dye for long-term monitoring. Hippocampal slices have been cultured inside these chips, observing fluorescence signals from living cells, maintaining the cytoarchitecture of the slices. Having fluorescence images of biological samples inside the chip demonstrates the effectiveness of the staining process using the injection method avoiding leaks or biological contamination. The technology developed in this study presents a significant improvement in the local administration of reagents within a brain slice-on-a-chip system, which could be a suitable option for organotypic cultures in a microfluidic chip acting as a highly effective bioreactor.

A microfluidic system for monitoring glucagon secretion from human pancreatic islets of Langerhans

Glucagon is a 29-amino acid peptide released from α-cells within pancreatic islets of Langerhans to help raise blood glucose levels. While a plethora of methodologies have been developed for quantitative measurement of insulin released from islets, such methods are not well developed for glucagon despite its importance in blood sugar regulation. In this work, a simple yet robust microfluidic device was developed for holding human pancreatic islets and perfuse them with glucose. The perfusate was collected into 2 min fractions and glucagon quantified using a homogeneous time-resolved Förster resonance energy transfer (TR-FRET) sandwich immunoassay. Simulation of fluid flow within the microfluidic device indicated the device produced low amounts of shear stress on islets, and characterization of the flow with standard glucagon solutions revealed response times within 2 fractions (<4 min). Results with human islets from multiple donors demonstrated either a "burst" of glucagon or a "sustained" glucagon release across the entire period of stimulation. The simplicity, yet robustness, of the device and method is expected to appeal to a number of researchers examining pancreatic islet physiology.

A Microfluidic Hanging-Drop-Based Islet Perifusion System for Studying Glucose-Stimulated Insulin Secretion From Multiple Individual Pancreatic Islets

Islet perifusion systems can be used to monitor the highly dynamic insulin release of pancreatic islets in glucose-stimulated insulin secretion (GSIS) assays. Here, we present a new generation of the microfluidic hanging-drop-based islet perifusion platform that was developed to study the alterations in insulin secretion dynamics from single pancreatic islet microtissues at high temporal resolution. The platform was completely redesigned to increase experimental throughput and to reduce operational complexity. The experimental throughput was increased fourfold by implementing a network of interconnected hanging drops, which allows for performing GSIS assays with four individual islet microtissues in parallel with a sampling interval of 30 s. We introduced a self-regulating drop-height mechanism that enables continuous flow and maintains a constant liquid volume in the chip, which enables simple and robust operation. Upon glucose stimulation, reproducible biphasic insulin release was simultaneously observed from all islets in the system. The measured insulin concentrations showed low sample-to-sample variation as a consequence of precise liquid handling with stable drop volumes, equal flow rates in the channels, and accurately controlled sampling volumes in all four drops. The presented device will be a valuable tool in islet and diabetes research for studying dynamic insulin secretion from individual pancreatic islets.

A microfluidic bubble perfusion device for brain slice culture

Ex vivo brain slice cultures are utilized as analytical models for studying neurophysiology. Common approaches to maintaining slice cultures include roller tube and membrane interface techniques. The rise of organ-on-chip technologies has demonstrated the value of microfluidic perfusion culture systems for sampling and analysis of complex biology under well-controlled in vitro or ex vivo conditions. A number of approaches to microfluidic brain slice culture have been developed, however these typically involve complex design, fabrication, or operational parameters in order to meet the high oxygen demands of brain slices. Here, we present proof-of-principle for a novel approach to microfluidic brain slice culture. In this system, which we term a microfluidic bubble perfusion device, principles of droplet microfluidics were employed to generate droplets of perfusion media dispersed between bubbles of carbogen gas, and brain tissue slices were perfused with the resulting monodispersed droplets and bubbles. The challenge of tissue immobilization in the flow system was addressed using a two-part cytocompatible carbohydrate-based tissue adhesive. Best practices are discussed for perfusion chamber designs that maintain segmented flow throughout the course of perfusion. Control of droplet and bubble volumes was possible across the range of ca. 4-15 μL, bubble generation frequency was well controlled in the range ca. 1-7 bubbles per min, and bubble duty cycle was well controlled across the range ca. 20-80%. Murine hypothalamic tissue slices containing the suprachiasmatic nuclei were successfully maintained for durations of 8-10 hours, with tissue remaining viable for the duration of perfusion as assessed by Ca²⁺ imaging and propidium iodide (PI) staining.

Building Biomimetic Potency Tests for Islet Transplantation

Diabetes is a disease of insulin insufficiency, requiring many to rely on exogenous insulin with constant monitoring to avoid a fatal outcome. Islet transplantation is a recent therapy that can provide insulin independence, but the procedure is still limited by both the availability of human islets and reliable tests to assess their function. While stem cell technologies are poised to fill the shortage of transplantable cells, better methods are still needed for predicting transplantation outcome. To ensure islet quality, we propose that the next generation of islet potency tests should be biomimetic systems that match glucose stimulation dynamics and cell microenvironmental preferences and rapidly assess conditional and continuous insulin secretion with minimal manual handing. Here, we review the current approaches for islet potency testing and outline technologies and methods that can be used to arrive at a more predictive potency test that tracks islet secretory capacity in a relevant context. With the development of potency tests that can report on islet secretion dynamics in a context relevant to their intended function, islet transplantation can expand into a more widely accessible and reliable treatment option for individuals with diabetes.

Profiling Glucose-Stimulated and M3 Receptor-Activated Insulin Secretion Dynamics from Islets of Langerhans Using an Extended-Lifetime Fluorescence Dye

Pulsatile insulin from pancreatic islets is crucial for glucose homeostasis, but the mechanism behind coordinated pulsatility is still under investigation. One hypothesis suggests that cholinergic stimulation of islets by pancreatic ganglia resets these endocrine units, producing synchronization. Previously, it was shown that intracellular Ca²⁺ oscillations within islets can be entrained by pulses of a cholinergic agonist, carbachol (CCh). Although these proxy measurements of Ca²⁺ provided insight into the synchronization mechanism, measurement of insulin output would be more direct evidence. To this end, a fluorescence anisotropy competitive immunoassay for online insulin detection from single and grouped islets in a microfluidic system was developed using a piezoelectric pressure-driven fluid delivery system and a squaraine rotaxane fluorophore, SeTau-647, as the fluorescent label for insulin. Due to SeTau-647 having a longer lifetime and higher brightness compared to the previously used Cy5 fluorophore, a 45% increase in the anisotropy range was observed with enhanced signal-to-noise ratio (S/N) of the measurements. This new system was tested by measuring glucose-stimulated insulin secretion from single and groups of murine and human islets. Distinct islet entrainment of groups of murine islets by pulses of CCh was also observed, providing further evidence for the hypothesis that pulsatile output from the ganglia can synchronize islet behavior. We expect that this relatively straightforward, homogeneous assay can be widely used for examining not only insulin secretion but other secreted factors from different tissues.

A review on microfluidics manipulation of the extracellular chemical microenvironment and its emerging application to cell analysis

Spatiotemporal manipulation of extracellular chemical environments with simultaneous monitoring of cellular responses plays an essential role in exploring fundamental biological processes and expands our understanding of underlying mechanisms. Despite the rapid progress and promising successes in manipulation strategies, many challenges remain due to the small size of cells and the rapid diffusion of chemical molecules. Fortunately, emerging microfluidic technology has become a powerful approach for precisely controlling the extracellular chemical microenvironment, which benefits from its integration capacity, automation, and high-throughput capability, as well as its high resolution down to submicron. Here, we summarize recent advances in microfluidics manipulation of the extracellular chemical microenvironment, including the following aspects: i) Spatial manipulation of chemical microenvironments realized by convection flow-, diffusion-, and droplet-based microfluidics, and surface chemical modification; ii) Temporal manipulation of chemical microenvironments enabled by flow switching/shifting, moving/flowing cells across laminar flows, integrated microvalves/pumps, and droplet manipulation; iii) Spatiotemporal manipulation of chemical microenvironments implemented by a coupling strategy and open-space microfluidics; and iv) High-throughput manipulation of chemical microenvironments. Finally, we briefly present typical applications of the above-mentioned technical advances in cell-based analyses including cell migration, cell signaling, cell differentiation, multicellular analysis, and drug screening. We further discuss the future improvement of microfluidics manipulation of extracellular chemical microenvironments to fulfill the needs of biological and biomedical research and applications.

Rapid lipolytic oscillations in ex vivo adipose tissue explants revealed through microfluidic droplet sampling at high temporal resolution

Our understanding of adipose tissue biology has steadily evolved. While structural and energy storage functionalities have been in the forefront, a key endocrine role for adipocytes was revealed only over the last few decades. In contrast to the wealth of information on dynamic function of other endocrine tissues, few studies have focused on dynamic adipose tissue function or on tool development toward that end. Here, we apply our unique droplet-based microfluidic devices to culture, perfuse, and sample secretions from primary murine epididymal white adipose tissue (eWAT), and from predifferentiated clusters of 3T3-L1 adipocytes. Through automated control, oil-segmented aqueous droplets (∼2.6 nL) were sampled from tissue or cells at 3.5 second temporal resolution (including sample and reference droplets), with integrated enzyme assays enabling real-time quantification of glycerol (down to 1.9 fmol per droplet). This high resolution revealed previously unreported oscillations in secreted glycerol at frequencies of 0.2 to 2.0 min^-1 (∼30-300 s periods) present in the primary tissue but not in clustered cells. Low-level bursts (∼50 fmol) released in basal conditions were contrasted with larger bursts (∼300 fmol) during stimulation. Further, both fold changes and burst magnitudes were decreased in eWAT of aged and obese mice. These results, combined with immunostaining and photobleaching analyses, suggest that gap-junctional coupling or nerve cell innervation within the intact ex vivo tissue explants play important roles in this apparent tissue-level, lipolytic synchronization. High-resolution, quantitative sampling by droplet microfluidics thus permitted unique biological information to be observed, giving an analytical framework poised for future studies of dynamic oscillatory function of adipose and other tissues.

In Vitro Platform for Studying Human Insulin Release Dynamics of Single Pancreatic Islet Microtissues at High Resolution

Insulin is released from pancreatic islets in a biphasic and pulsatile manner in response to elevated glucose levels. This highly dynamic insulin release can be studied in vitro with islet perifusion assays. Herein, a novel platform to perform glucose-stimulated insulin secretion (GSIS) assays with single islets is presented for studying the dynamics of insulin release at high temporal resolution. A standardized human islet model is developed and a microfluidic hanging-drop-based perifusion system is engineered, which facilitates rapid glucose switching, minimal sample dilution, low analyte dispersion, and short sampling intervals. Human islet microtissues feature robust and long-term glucose responsiveness and demonstrate reproducible dynamic GSIS with a prominent first phase and a sustained, pulsatile second phase. Perifusion of single islet microtissues produces a higher peak secretion rate, higher secretion during the first and second phases of insulin release, as well as more defined pulsations during the second phase in comparison to perifusion of pooled islets. The developed platform enables to study compound effects on both phases of insulin secretion as shown with two classes of insulin secretagogs. It provides a new tool for studying physiologically relevant dynamic insulin secretion at comparably low sample-to-sample variation and high temporal resolution.

A Versatile Model of Microfluidic Perifusion System for the Evaluation of C-Peptide Secretion Profiles: Comparison Between Human Pancreatic Islets and HLSC-Derived Islet-Like Structures

A robust and easy-to-use tool for the ex vivo dynamic evaluation of pancreatic islet (PI) function is essential for further development of novel cell-based therapeutic approaches to treating diabetes. Here, we developed four different glucose perifusion protocols (GPPs) in a microfluidic perifusion system (MPS), based entirely on commercially available components. After validation, the GPPs were used to evaluate C-peptide secretion profiles of PIs derived from different donors (healthy, obese, and type 2 diabetic) and from human liver stem-cell-derived islet-like structures (HLSC-ILS). Using this device, we demonstrated that PIs derived from healthy donors displayed a physiological C-peptide secretion profile as characterized by the response to (a) different glucose concentrations, (b) consecutive pulses of high-glucose concentrations, (c) a glucose threshold ranging from 5–8 mM, and (d) a constant high-glucose perifusion in a biphasic manner. Moreover, we were able to detect a dysregulated secretion profile in PIs derived from both obese and type 2 diabetes mellitus (T2DM) donors. Finally, we also evaluated the kinetic secretion profiles of HLSC-ILS, demonstrating that, nonetheless, with a lower amplitude of secretion compared to PI derived from healthy donors, they were already glucose-responsive on day seven post-differentiation. In conclusion, we have provided evidence that our MPS is a versatile device and may represent a valuable tool to study insulin-producing cells in vitro.

A microfluidic platform integrating pressure-driven and electroosmotic-driven flow with inline filters for affinity separations

Pancreatic islets of Langerhans release glucagon to maintain blood glucose levels, and release of this peptide is dysregulated in diabetes mellitus. Although the importance of proper secretion of this peptide has been shown, no measurement of its release at the single islet level has been reported. In previous work, a non-competitive assay for glucagon was developed with a 6 pM limit of detection, low enough to measure from a single islet. To incorporate this method in an online assay, a microfluidic system with several distinct features was developed. To maintain appropriate flow rates in the presence of the high concentration of salt that was required for the assay, a piezo-actuated pressure transducer with in-line flow sensors was used to drive sample flow through 80 × 50 μm (width × depth) channels, while electroosmotic flow was used to gate the sample away from 15 × 5 μm separation channel. Flow rates tested with this system were 50 - 200 nL min^-1 with relative standard deviations (RSDs) ranging from 1 - 4 %. Use of the pressure-driven flow was found to increase the amount of clogs in the system, so a method to incorporate in-line filters into the channels was developed. A total of 4 low resistance, in-line microfabricated filters were evaluated, with all designs prolonging the operation time of the microfluidic device to more than 4 hours without clogs observed. Use of this system enabled highly reproducible injections (3-6% RSD). During initial incorporation of the noncompetitive assay for glucagon, it was determined that Joule heating was problematic and temperature measurements revealed the separation channel increased to more than 50°C during operation. A 3D-printed manifold was used to hold a Peltier cooler in place on the microfluidic device which produced a 2.6-fold improvement in the amount of the noncovalent glucagon complex that was detected compared to without cooling. These features are expected to be useful for not only long-term monitoring of the glucagon release from islets of Langerhans, but has the potential to be applied to a number of other microfluidic separation-based assays as well.

Synchronized stimulation and continuous insulin sensing in a microfluidic human Islet on a Chip designed for scalable manufacturing

Pancreatic β cell function is compromised in diabetes and is typically assessed by measuring insulin secretion during glucose stimulation. Traditionally, measurement of glucose-stimulated insulin secretion involves manual liquid handling, heterogeneous stimulus delivery, and enzyme-linked immunosorbent assays that require large numbers of islets and processing time. Though microfluidic devices have been developed to address some of these limitations, traditional methods for islet testing remain the most common due to the learning curve for adopting microfluidic devices and the incompatibility of most device materials with large-scale manufacturing. We designed and built a thermoplastic, microfluidic-based Islet on a Chip compatible with commercial fabrication methods, that automates islet loading, stimulation, and insulin sensing. Inspired by the perfusion of native islets by designated arterioles and capillaries, the chip delivers synchronized glucose pulses to islets positioned in parallel channels. By flowing suspensions of human cadaveric islets onto the chip, we confirmed automatic capture of islets. Fluorescent glucose tracking demonstrated that stimulus delivery was synchronized within a two-minute window independent of the presence or size of captured islets. Insulin secretion was continuously sensed by an automated, on-chip immunoassay and quantified by fluorescence anisotropy. By integrating scalable manufacturing materials, on-line, continuous insulin measurement, and precise spatiotemporal stimulation into an easy-to-use design, the Islet on a Chip should accelerate efforts to study and develop effective treatments for diabetes.

3-D physiomimetic extracellular matrix hydrogels provide a supportive microenvironment for rodent and human islet culture

Organ-on-a-chip platforms serve as cost-efficient testbeds for screening pharmaceutical agents, mimicking natural physiology, and studying disease. In the field of diabetes, the development of an islet-on-a-chip platform would have broad implications in understanding disease pathology and discovering potential therapies. Islet microphysiological systems are limited, however, by their poor cell survival and function in culture. A key factor that has been implicated in this decline is the disruption of islet-matrix interactions following isolation. Herein, we sought to recapitulate the in vivo peri-islet niche using decellularized extracellular matrix (ECM) hydrogels. Sourcing from porcine bladder, lung, and pancreas tissues, 3-D ECM hydrogels were generated, characterized, and validated using both rodent and human pancreatic islets. Optimized decellularization protocols resulted in hydrogels with distinctive viscoelastic properties that correlated to their matrix composition. The in situ 3-D encapsulation of human or rat islets within ECM hydrogels resulted in improved functional stability over standard culture conditions. Islet composition and morphology were also altered, with enhanced retention of islet-resident endothelial cells and the formation of cord-like structures or sprouts emerging from the islet spheroid. These supportive 3-D physiomimetic ECM hydrogels can be leveraged within microfluidic platforms for the long-term culture of islets.

Synchronization of pancreatic islets by periodic or non-periodic muscarinic agonist pulse trains

Pulsatile insulin secretion into the portal vein from the many pancreatic islets of Langerhans is critical for efficient glucose homeostasis. The islets are themselves endogenous oscillators, but since they are not physically coupled it is not obvious how their oscillations are synchronized across the pancreas. It has been proposed that synchronization of islets is achieved through periodic activity of intrapancreatic ganglia, and indeed there are data supporting this proposal. Postganglionic nerves are cholinergic, and their product, acetylcholine, can influence islet β-cells through actions on M₃ muscarinic receptors which are coupled to G_q type G-proteins. In addition, the neurons secrete several peptide hormones that act on β-cell receptors. The data supporting synchronization via intrapancreatic ganglia are, however, limited. In particular, it has not been shown that trains of muscarinic pulses are effective at synchronizing islets in vitro. Also, if as has been suggested, there is a ganglionic pacemaker driving islets to a preferred frequency, no neural circuitry for this pacemaker has been identified. In this study, both points are addressed using a microfluidic system that allows for the pulsed application of the muscarinic agonist carbachol. We find that murine islets are entrained and synchronized over a wide range of frequencies when the carbachol pulsing is periodic, adding support to the hypothesis that ganglia can synchronize islets in vivo. We also find that islet synchronization is very effective even if the carbachol pulses are applied at random times. This suggests that a neural pacemaker is not needed; all that is required is that islets receive occasional coordinated input from postganglionic neurons. The endogenous rhythmic activity of the islets then sets the frequency of the islet population rhythm, while the input from ganglia acts only to keep the islet oscillators in phase.

Nanotechnology in cell replacement therapies for type 1 diabetes

Islet transplantation is a promising long-term, compliance-free, complication-preventing treatment for type 1 diabetes. However, islet transplantation is currently limited to a narrow set of patients due to the shortage of donor islets and side effects from immunosuppression. Encapsulating cells in an immunoisolating membrane can allow for their transplantation without the need for immunosuppression. Alternatively, "open" systems may improve islet health and function by allowing vascular ingrowth at clinically attractive sites. Many processes that enable graft success in both approaches occur at the nanoscale level-in this review we thus consider nanotechnology in cell replacement therapies for type 1 diabetes. A variety of biomaterial-based strategies at the nanometer range have emerged to promote immune-isolation or modulation, proangiogenic, or insulinotropic effects. Additionally, coating islets with nano-thin polymer films has burgeoned as an islet protection modality. Materials approaches that utilize nanoscale features manipulate biology at the molecular scale, offering unique solutions to the enduring challenges of islet transplantation.

Gene expression patterns in synchronized islet populations

In vivo levels of insulin are oscillatory with a period of ~5-10 minutes, indicating that the islets of Langerhans within the pancreas are synchronized. While the synchronizing factors are still under investigation, one result of this behavior is expected to be coordinated and oscillatory intracellular factors, such as intracellular Ca²⁺ levels, throughout the islet population. In other cell types, oscillatory intracellular signals, like intracellular Ca²⁺, have been shown to affect specific gene expression. To test how the gene expression landscape may differ between a synchronized islet population with its reproducible intracellular oscillations and an unsynchronized islet population with heterogeneous oscillations, gene set enrichment analysis (GSEA) was used to compare an islet population that had been synchronized using a glucose wave with a 5-min period, and an unsynchronized islet population. In the population exposed to the glucose wave, 58/62 islets showed synchronization as evidenced by coordinated intracellular Ca²⁺ oscillations with an average oscillation period of 5.1 min, while in the unsynchronized population 29/62 islets showed slow oscillations with an average period of 5.2 min. The synchronized islets also had a significantly smaller drift of their oscillation period during the experiment as compared to the unsynchronized population. GSEA indicated that the synchronized population had reduced expression of gene sets related to protein translation, protein turnover, energy expenditure, and insulin synthesis, while those that were related to maintenance of cell morphology were increased.

3D-Models of Insulin-Producing β-Cells: from Primary Islet Cells to Stem Cell-Derived Islets

There is a need for physiologically relevant assay platforms to provide functionally relevant models of diabetes, to accelerate the discovery of new treatment options and boost developments in drug discovery. In this review, we compare several 3D-strategies that have been used to increase the functional relevance of ex vivo human primary pancreatic islets and developments into the generation of stem cell derived pancreatic beta-cells (β-cells). Special attention will be given to recent approaches combining the use of extracellular matrix (ECM) scaffolds with pancreatic molecular memory, which can be used to improve yield and functionality of in vitro stem cell-derived pancreatic models. The ultimate goal is to develop scalable cell-based platforms for diabetes research and drug screening. This article will critically assess key aspects related to in vitro pancreatic 3D-ECM models and highlight the most promising approaches for future research.

Automated microfluidic droplet sampling with integrated, mix-and-read immunoassays to resolve endocrine tissue secretion dynamics

A fully automated droplet generation and analysis device based on pressure driven push-up valves for precise pumping of fluid and volumetric metering has been developed for high resolution hormone secretion sampling and measurement. The device consists of a 3D-printer templated reservoir for single cells or single tissue culturing, a Y-shaped channel for reagents and sample mixing, a T-junction channel for droplet formation, a reference channel to overcome drifts in fluorescence signal, and a long droplet storage channel allowing incubation for homogeneous immunoassays. The droplets were made by alternating peristaltic pumping of aqueous and oil phases. Device operation was automated, giving precise control over several droplet parameters such as size, oil spacing, and ratio of sample and reference droplets. By integrating an antibody-oligonucleotide based homogeneous immunoassay on-chip, high resolution temporal sampling into droplets was combined with separation-free quantification of insulin secretion from single islets of Langerhans using direct optical readout from the droplets. Quantitative assays of glucose-stimulated insulin secretion were demonstrated at 15 second temporal resolution while detecting as low as 10 amol per droplet, revealing fast insulin oscillations that mirror well-known intracellular calcium signals. This droplet sampling and direct optical analysis approach effectively digitizes the secretory time record from cells into droplets, and the system should be generalizable to a variety of cells and tissue types.

Microfluidic-enabled quantitative measurements of insulin release dynamics from single islets of Langerhans in response to 5-palmitic acid hydroxy stearic acid

Proper release of insulin from pancreatic islets of Langerhans is essential for maintaining glucose homeostasis. For full efficacy, both the pattern and the amount of hormone release are critical. It is therefore important to understand how insulin levels are secreted from single islets in both a quantitative fashion and in a manner that resolves temporal dynamics. In this study, we describe a microfluidic analytical system that can both quantitatively monitor insulin secretion from single islets while simultaneously maintaining high temporal sampling to resolve dynamics of release. We have applied this system to determine the acute and chronic effects of a recently-identified lipid, 5-palmitic acid hydroxy stearic acid (5-PAHSA), which is a member of the fatty acid hydroxy fatty acid class of lipids that are upregulated in healthy individuals. Chronic incubation (48 h) with 5-PAHSA significantly increased glucose-stimulated insulin secretion (GSIS) in murine islets compared to chronic incubation without the lipid or in the presence of palmitic acid (PA). The studies were continued in human islets from both healthy donors and donors diagnosed with type 2 diabetes mellitus (T2DM). Total amounts of GSIS were not only augmented in islets that were chronically incubated with 5-PAHSA, but the dynamic insulin release profiles also improved as noted by more pronounced insulin oscillations. With this quantitative microfluidic system, we have corroborated the anti-diabetic effects of 5-PAHSA by demonstrating improved islet function after chronic incubation with this lipid via improved oscillatory dynamics along with higher basal and peak release rates.

Advancement of analytical modes in a multichannel, microfluidic droplet-based sample chopper employing phase-locked detection

In this work, we expand upon our recently developed droplet-based sample chopping concepts by introducing a multiplexed fluidic micro-chopper device (μChopper). Six aqueous input channels were integrated with a single oil input, and each of these seven channels was controlled by a pneumatic valve for automated sampling through software control. This improved design, while maintaining high precision in valve-based droplet generation at bandwidths of 0.03 to 0.05 Hz, enabled a variety of analytical modes to be employed on-chip compared to previous devices limited to sample/reference alternations. The device was analytically validated for real-time, continuous calibration with a single sample and five standards; multiplexed analysis during calibration using a mixed mode; and standard addition through spiking of six sample droplets with varying amounts of standard. Finally, the standard addition mode was applied to protein quantification in human serum samples using on-chip, homogeneous fluorescence immunoassays. Ultimately, with only ~1.2 μL of total analyzed solution volume- representing 100-fold and 75-fold reductions in reagent and serum volumes, respectively-we were able to generate full, six-point standard addition curves in only 1.5 min, and results correlated well with those from standard plate-reader equipment. This work thus exploited microfluidic valves for both their automation and droplet phase-locking capabilities, resulting in a micro-analytical tool capable of complex analytical interrogation modes on sub-microliter sample volumes while also leveraging drastic noise rejection via lock-in detection. The multichannel μChopper device should prove particularly useful in analyzing precious biological samples or for dynamic analyses at small volume scales.

A 3D microfluidic perfusion system made from glass for multiparametric analysis of stimulus-secretioncoupling in pancreatic islets

Microfluidic perfusion systems (MPS) are well suited to perform multiparametric measurements with small amounts of tissue to function as an Organ on Chip device (OOC). Such microphysiolgical characterization is particularly valuable in research on the stimulus-secretion-coupling of pancreatic islets. Pancreatic islets are fully functional competent mini-organs, which serve as fuel sensors and transduce metabolic activity into rates of hormone secretion. To enable the simultaneous measurement of fluorescence and oxygen consumption we designed a microfluidic perfusion system from borosilicate glass by 3D femtosecond laser ablation. Retention of islets was accomplished by a plain well design. The characteristics of flow and shear force in the microchannels and wells were simulated and compared with the measured exchange of the perfusion media. Distribution of latex beads, MIN6 cell pseudo islets and isolated mouse islets in the MPS was characterized in dependence of flow rate and well depth. Overall, the observations suggested that a sufficient retention of the islets at low shear stress, together with sufficient exchange of test medium, was achieved at a well depth of 300 μm and perfusion rates between 40 and 240 μl/min. This enabled multiparametric measurement of oxygen consumption, NAD(P)H autofluorescence, cytosolic Ca²⁺ concentration, and insulin secretion by isolated mouse islets. After appropriate correction for different lag times, kinetics of these processes could be compared. Such measurements permit a more precise insight into metabolic changes underlying the regulation of insulin secretion. Thus, rapid prototyping using laser ablation enables flexible adaption of borosilicate MPS designs to different demands of biomedical research.

Microphysiological Analysis Platform of Pancreatic Islet β-Cell Spheroids

The hallmarks of diabetics are insufficient secretion of insulin and dysregulation of glucagon. It is critical to understand release mechanisms of insulin, glucagon, and other hormones from the islets of Langerhans. In spite of remarkable advancements in diabetes research and practice, robust and reproducible models that can measure pancreatic β-cell function are lacking. Here, a microphysiological analysis platform (MAP) that allows the uniform 3D spheroid formation of pancreatic β-cell islets, large-scale morphological phenotyping, and gene expression mapping of chronic glycemia and lipidemia development is reported. The MAP enables the scaffold-free formation of densely packed β-cell spheroids (i.e., multiple array of 110 bioreactors) surrounded with a perfusion flow network inspired by physiologically relevant microenvironment. The MAP permits dynamic perturbations on the β-cell spheroids and the precise controls of glycemia and lipidemia, which allow us to confirm that cellular apoptosis in the β-cell spheroid under hyperglycemia and hyperlipidemia is mostly dependent to a reactive oxygen species-induced caspase-mediated pathway. The β-cells' MAP might provide a potential new map in the pathophysiological mechanisms of β cells.

A microfluidic in-line ELISA for measuring secreted protein under perfusion

Recent progress in the development of microfluidic microphysiological systems such as 'organs-on-chips' and microfabricated cell culture is geared to simulate organ-level physiology. These tissue models leverage microengineering technologies that provide capabilities of presenting cultured cells with input signals in a more physiologically relevant context such as perfused flow. Proteins that are secreted from cells have important information about the health of the cells. Techniques to quantify cellular proteins include mass spectrometry to ELISA (enzyme-linked immunosorbent assay). Although our capability to perturb the cells in the microphysiological systems with varying inputs is well established, we lack the tools to monitor in-line the cellular responses. User intervention for sample collection and off-site is cumbersome, causes delays in obtaining results, and is especially expensive because of collection, storage, and offline processing of the samples, and in many case, technically impractical to carry out because of limitated sample volumes. To address these shortcomings, we report the development of an ELISA that is carried out in-line under perfusion within a microfluidic device. Using this assay, we measured the albumin secreted from perfused hepatocytes without and under stimulation by IL-6. Since the method is based on a sandwich ELISA, we envision broad application of this technology to not just organs-on-chips but also to characterizing the temporal release and measurement of soluble factors and response to drugs.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

Demonstration of Interposed Modular Hydrogel Sheet for Multicellular Analysis in a Microfluidic Assembly Platform

Hydrogel sheets have emerged as a promising biomaterial scaffold for the encapsulation and transfer of multicellular structures. Although the improvement of the chemical interactions and the design of micro-scaled geometry have contributed to the development of multipurpose hydrogel scaffolds, the application of hydrogel sheets to assess multicellular structures is still challenging. To expand the technical applicability of hydrogel sheets, we here demonstrate that a single layer of the hydrogel sheet can be integrated as an interposed module in a microfluidic device for multicellular analysis. As a cell culture unit, encapsulated pancreatic insulinoma (MIN6) cells in the hydrogel sheet were labeled and examined via multiple microchannels. After obtaining simultaneously multi-labeled cells in the hydrogel sheet that had been incorporated into the microfluidic device, each modular hydrogel sheet was also recoverable and re-cultured without any distortion. The modular hydrogel sheet can be simply manipulated and conserved as a multicellular module in a three-dimensional (3D) in vitro culture platform. Using the modular concept of hydrogel sheets capable of cell culture and/or assay, an integrated multicellular analysis in the microfluidic device is expected to improve accessibility, scalability, and practicality for end users.

3D-templated, fully automated microfluidic input/output multiplexer for endocrine tissue culture and secretion sampling

A fully automated, 16-channel microfluidic input/output multiplexer (μMUX) has been developed for interfacing to primary cells and to improve understanding of the dynamics of endocrine tissue function. The device utilizes pressure driven push-up valves for precise manipulation of nutrient input and hormone output dynamics, allowing time resolved interrogation of the cells. The ability to alternate any of the 16 channels from input to output, and vice versa, provides for high experimental flexibility without the need to alter microchannel designs. 3D-printed interface templates were custom designed to sculpt the above-channel polydimethylsiloxane (PDMS) in microdevices, creating millimeter scale reservoirs and confinement chambers to interface primary murine islets and adipose tissue explants to the μMUX sampling channels. This μMUX device and control system was first programmed for dynamic studies of pancreatic islet function to collect ∼90 minute insulin secretion profiles from groups of ∼10 islets. The automated system was also operated in temporal stimulation and cell imaging mode. Adipose tissue explants were exposed to a temporal mimic of post-prandial insulin and glucose levels, while simultaneous switching between labeled and unlabeled free fatty acid permitted fluorescent imaging of fatty acid uptake dynamics in real time over a ∼2.5 hour period. Application with varying stimulation and sampling modes on multiple murine tissue types highlights the inherent flexibility of this novel, 3D-templated μMUX device. The tissue culture reservoirs and μMUX control components presented herein should be adaptable as individual modules in other microfluidic systems, such as organ-on-a-chip devices, and should be translatable to different tissues such as liver, heart, skeletal muscle, and others.

Noncompetitive affinity assays of glucagon and amylin using mirror-image aptamers as affinity probes

The ability to detect picomolar concentrations of glucagon and amylin using fluorescently labeled mirror-image aptamers, so-called Spiegelmers, is demonstrated. Spiegelmers rival the specificity of antibodies and overcome the problem of biostability of natural aptamers in a biological matrix. Using Spiegelmers as affinity probes, noncompetitive capillary electrophoresis affinity assays of glucagon and murine amylin were developed and optimized. The detection limit for glucagon was 6 pM and for amylin was 40 pM. Glucagon-like peptide-1 and -2 did not interfere with the glucagon assay, while the amylin assay showed cross-reactivity to calcitonin gene related peptide. The developed assays were combined with a competitive immunoassay for insulin to measure glucagon, amylin, and insulin secretion from batches of islets after incubation with different glucose concentrations. The development of these assays is an important step towards incorporation into an online measurement system for monitoring dynamic secretion from single islets.

Online fluorescence anisotropy immunoassay for monitoring insulin secretion from islets of Langerhans

Insulin secretion from islets of Langerhans is a dynamic process that is essential for maintaining glucose homeostasis. The ability to measure dynamic changes in insulin levels upon glucose stimulation from single islets will allow testing of therapeutics and investigating mechanisms of defective secretion observed in metabolic diseases. Most approaches to date for measurement of rapid changes in insulin levels rely on separations, making the assays difficult to translate to non-specialist laboratories. To enable rapid measurements of secretion dynamics from a single islet in a manner that will be more suitable for transfer to non-specialized laboratories, a microfluidic online fluorescence anisotropy immunoassay was developed. A single islet was housed inside a microfluidic chamber and stimulated with varying glucose levels from a gravity-based perfusion system. The total effluent of the islet chamber containing the islet secretions was mixed with gravity-driven solutions of insulin antibody and Cy5-labeled insulin. After mixing was complete, a linearly polarized 635 nm laser was used to excite the immunoassay mixture and the emission was split into parallel and perpendicular components for determination of anisotropy. Key factors for reproducible anisotropy measurements, including temperature homogeneity and flow rate stability were optimized, which resulted in a 4 nM limit of detection for insulin with <1% RSD of anisotropy values. The capability of this system for measuring insulin secretion from single islets was shown by stimulating an islet with varying glucose levels. As the entire analysis is performed optically, this system should be readily transferable to other laboratories.

Culture and Sampling of Primary Adipose Tissue in Practical Microfluidic Systems

Microfluidic culture of primary adipose tissue allows for reduced sample and reagent volumes as well as constant media perfusion of the cells. By continuously flowing media over the tissue, microfluidic sampling systems can more accurately mimic vascular flow in vivo. Quantitative measurements can be performed on or off chip to provide time-resolved secretion data, furthering insight into the dynamics of the function of adipose tissue. Buoyancy resulting from the large lipid storage capacity in this tissue presents a unique challenge for culture, and it is important to account for this buoyancy during microdevice design. Herein, we describe approaches for microfluidic device fabrication that utilize 3D-printed interface templating to help counteract cell buoyancy. We apply such methods to the culture of both isolated, dispersed primary adipocytes and epididymal adipose explants. To facilitate more widespread adoption of the methodology, the devices presented here are designed for user-friendly operation. Only handheld syringes are needed to control flow, and devices are inexpensive and disposable.

Dual Detection System for Simultaneous Measurement of Intracellular Fluorescent Markers and Cellular Secretion

Glucose-stimulated insulin secretion from pancreatic β-cells within islets of Langerhans plays a critical role in maintaining glucose homeostasis. Although this process is essential for maintaining euglycemia, the underlying intracellular mechanisms that control it are still unclear. To allow simultaneous correlation between intracellular signal transduction events and extracellular secretion, an analytical system was developed that integrates fluorescence imaging of intracellular probes with high-speed automated insulin immunoassays. As a demonstration of the system, intracellular [Ca²⁺] ([Ca²⁺]_i) was measured by imaging Fura-2 fluorescence simultaneously with insulin secretion from islets exposed to elevated glucose levels. Both [Ca²⁺]_i and insulin were oscillatory during application of 10 mM glucose with temporal and quantitative profiles similar to what has been observed elsewhere. In previous work, sinusoidal glucose levels have been used to test the entrainment of islets while monitoring either [Ca²⁺]_i or insulin levels; using this newly developed system, we show unambiguously that oscillations of both [Ca²⁺]_i and insulin release are entrained to oscillatory glucose levels and that the temporal correlation of these are maintained throughout the experiment. It is expected that the developed analytical system can be expanded to investigate a number of other intracellular messengers in islets or other stimulus-secretion pathways in different cells.

The quest to make fully functional human pancreatic beta cells from embryonic stem cells: climbing a mountain in the clouds

The production of fully functional insulin-secreting cells to treat diabetes is a major goal of regenerative medicine. In this article, I review progress towards this goal over the last 15 years from the perspective of a beta cell biologist. I describe the current state-of-the-art, and speculate on the general approaches that will be required to identify and achieve our ultimate goal of producing functional beta cells. The need for deeper phenotyping of heterogeneous cultures of stem cell derived islet-like cells in parallel with a better understanding of the heterogeneity of the target cell type(s) is emphasised. This deep phenotyping should include high-throughput single-cell analysis, as well as comprehensive 'omics technologies to provide unbiased characterisation of cell products and human beta cells. There are justified calls for more detailed and well-powered studies of primary human pancreatic beta cell physiology, and I propose online databases of standardised human beta cell responses to physiological stimuli, including both functional and metabolomic/proteomic/transcriptomic profiles. With a concerted, community-wide effort, including both basic and applied scientists, beta cell replacement will become a clinical reality for patients with diabetes.

Macro-to-micro interfacing to microfluidic channels using 3D-printed templates: application to time-resolved secretion sampling of endocrine tissue

Employing 3D-printed templates for macro-to-micro interfacing, a passively operated polydimethysiloxane (PDMS) microfluidic device was designed for time-resolved secretion sampling from primary murine islets and epidiymal white adipose tissue explants. Interfacing in similar devices is typically accomplished through manually punched or drilled fluidic reservoirs. We previously introduced the concept of using hand fabricated polymer inserts to template cell culture and sampling reservoirs into PDMS devices, allowing rapid stimulation and sampling of endocrine tissue. However, fabrication of the fluidic reservoirs was time consuming, tedious, and was prone to errors during device curing. Here, we have implemented computer-aided design and 3D printing to circumvent these fabrication obstacles. In addition to rapid prototyping and design iteration advantages, the ability to match these 3D-printed interface templates with channel patterns is highly beneficial. By digitizing the template fabrication process, more robust components can be produced with reduced fabrication variability. Herein, 3D-printed templates were used for sculpting millimetre-scale reservoirs into the above-channel, bulk PDMS in passively-operated, eight-channel devices designed for time-resolved secretion sampling of murine tissue. Devices were proven functional by temporally assaying glucose-stimulated insulin secretion from <10 pancreatic islets and glycerol secretion from 2 mm adipose tissue explants, suggesting that 3D-printed interface templates could be applicable to a variety of cells and tissue types. More generally, this work validates desktop 3D printers as versatile interfacing tools in microfluidic laboratories.

Multiplexing Fluorescence Anisotropy Using Frequency Encoding

In this report, a method to multiplex fluorescence anisotropy measurements is described using frequency encoding. As a demonstration of the method, simultaneous competitive immunoassays for insulin and glucagon were performed by measuring the ratio of bound and free Cy5-insulin and FITC-glucagon in the presence of their respective antibodies. A vertically polarized 635 nm laser was pulsed at 73 Hz and used to excite Cy5-insulin, while a vertically polarized 488 nm laser pulsed at 137 Hz excited FITC-glucagon. The total emission was split into parallel and perpendicular polarizations and collected onto separate photomultiplier tubes. The signals from each channel were demodulated using a fast Fourier transform, resolving the contributions from each fluorophore. Anisotropy calculations were carried out using the magnitude of the peaks in the frequency domain. The method produced the expected shape of the calibration curves with limits of detection of 0.6 and 5 nM for insulin and glucagon, respectively. This methodology could readily be expanded to other biological systems and further multiplexed to monitor increased numbers of analytes.

Microfluidic Devices for the Measurement of Cellular Secretion

The release of chemical information from cells and tissues holds the key to understanding cellular behavior and dysfunction. The development of methodologies that can measure cellular secretion in a time-dependent fashion is therefore essential. Often these measurements are made difficult by the high-salt conditions of the cellular environment, the presence of numerous other secreted factors, and the small mass samples that are produced when frequent sampling is used to resolve secretory dynamics. In this review, the methods that we have developed for measuring hormone release from islets of Langerhans are dissected to illustrate the practical difficulties of studying cellular secretions. Other methods from the literature are presented that provide alternative approaches to particularly challenging areas of monitoring cellular secretion. The examples presented in this review serve as case studies and should be adaptable to other cell types and systems for unique applications.

Effects of molecular confinement and crowding on horseradish peroxidase kinetics using a nanofluidic gradient mixer

The effects of molecular confinement and crowding on enzyme kinetics were studied at length scales and under conditions similar to those found in biological cells. These experiments were carried out using a nanofluidic network of channels constituting a nanofluidic gradient mixer, providing the basis for measuring multiple experimental conditions simultaneously. The 100 nm × 40 μm nanochannels were wet etched directly into borosilicate glass, then annealed and characterized with fluorescein emission prior to kinetic measurements. The nanofluidic gradient mixer was then used to measure the kinetics of the conversion of the horseradish peroxidase (HRP)-catalyzed conversion of non-fluorescent Amplex Red (AR) to the fluorescent product resorufin in the presence of hydrogen peroxide (H2O2). The design of the gradient mixer allows reaction kinetics to be studied under multiple (five) unique solution compositions in a single experiment. To characterize the efficiency of the device the effects of confinement on HRP-catalyzed AR conversion kinetics were studied by varying the starting ratio of AR : H2O2. Equimolar concentrations of Amplex Red and H2O2 yielded the highest reaction rates followed by 2 : 1, 1 : 2, 5 : 1, and finally 1 : 5 [AR] : [H2O2]. Under all conditions, initial reaction velocities were decreased by excess H2O2. Crowding effects on kinetics were studied by increasing solution viscosity in the nanochannels in the range 1.0-1.6 cP with sucrose. Increasing the solution viscosities in these confined geometries decreases the initial reaction velocity at the highest concentration from 3.79 μM min(-1) at 1.00 cP to 0.192 μM min(-1) at 1.59 cP. Variations in reaction velocity are interpreted in the context of models for HRP catalysis and for molecular crowding.

Microfluidic Device for the Measurement of Amino Acid Secretion Dynamics from Murine and Human Islets of Langerhans

Islets of Langerhans are the regulators of in vivo blood glucose levels through the secretion of endocrine hormones. Amino acids, released from various cells within islets or from intrapancreatic neurons, are hypothesized to further adjust hormone secretions. In contrast to the well-accepted mechanism of glucose-stimulated insulin secretion, several questions remain as to the function of amino acids in the regulation of hormone release from islets. To understand the autocrine and paracrine roles that amino acids play in islet physiology, a microfluidic system was developed to perform online monitoring of the secretion profiles of amino acids from 2-5 islets. The device contained an islet chamber with the ability to perfuse stimulants and an amino acid measurement system with derivatization and electrophoretic separation integrated on a single microchip. The setup was optimized to allow -15 kV to be applied to the device for high efficiency and rapid separations of derivatized amino acids. The compositions of the derivatization and separation buffers were optimized to prevent precipitations in the channels, which allowed continuous monitoring of secretion for over 2 h. With this method, 10 amino acids were resolved with limits of detection ranging from 1 to 20 nM. When murine islets were perfused with 3 mM glucose, the secretion rates of 9 amino acids were measured and ranged from 30 to 400 fmol islet(-1) min(-1). As the glucose concentration was increased to 20 mM, the dynamic changes of amino acids were monitored. The biological relevance of the amino acid secretions was verified using 2,4-dinitrophenol as an inhibitor of the proton motive force. The microfluidic system was also used to measure dynamic changes of amino acid release from human islets, which showed different release profiles compared to their murine counterparts.

Marching along to an Offbeat Drum: Entrainment of Synthetic Gene Oscillators by a Noisy Stimulus

Modulation of biological oscillations by stimuli lies at the root of many phenomena, including maintenance of circadian rhythms, propagation of neural signals, and somitogenesis. While it is well established that regular periodic modulation can entrain an oscillator, an aperiodic (noisy) modulation can also robustly entrain oscillations. This latter scenario may describe, for instance, the effect of irregular weather patterns on circadian rhythms, or why irregular neural stimuli can still reliably transmit information. A synthetic gene oscillator approach has already proven to be useful in understanding the entrainment of biological oscillators by periodic signaling, mimicking the entrainment of a number of noisy oscillating systems. We similarly seek to use synthetic biology as a platform to understand how aperiodic signals can strongly correlate the behavior of cells. This study should lead to a deeper understanding of how fluctuations in our environment and even within our body may promote substantial synchrony among our cells. Specifically, we investigate experimentally and theoretically the entrainment of a synthetic gene oscillator in E. coli by a noisy stimulus. This phenomenon was experimentally studied and verified by a combination of microfluidics and microscopy using the real synthetic circuit. Stochastic simulation of an associated model further supports that the synthetic gene oscillator can be strongly entrained by aperiodic signals, especially telegraph noise. Finally, widespread applicability of aperiodic entrainment beyond the synthetic gene oscillator is supported by results derived from both a model for a natural oscillator in D. discoideum and a model for predator-prey oscillations.