A pumpless microfluidic device driven by surface tension for pancreatic islet analysis

Pump-Free Microfluidics for Cell Concentration Analysis on Smartphones in Clinical Settings (SmartFlow): Design, Development, and Evaluation

BACKGROUND

Cell concentration in body fluid is an important factor for clinical diagnosis. The traditional method involves clinicians manually counting cells under microscopes, which is labor-intensive. Automated cell concentration estimation can be achieved using flow cytometers; however, their high cost limits accessibility. Microfluidic systems, although cheaper than flow cytometers, still require high-speed cameras and syringe pumps to drive the flow and ensure video quality. In this paper, we present SmartFlow, a low-cost solution for cell concentration estimation using smartphone-based computer vision on 3D-printed, pump-free microfluidic platforms.

OBJECTIVE

The objective was to design and fabricate microfluidic chips, coupled with clinical utilities, for cell counting and concentration analysis. We answered the following research questions (RQs): RQ1, Can gravity drive the flow within the microfluidic chips, eliminating the need for external pumps? RQ2, How does the microfluidic chip design impact video quality for cell analysis? RQ3, Can smartphone-captured videos be used to estimate cell count and concentration in microfluidic chips?

METHODS

To answer the 3 RQs, 2 experiments were conducted. In the cell flow velocity experiment, diluted sheep blood flowed through the microfluidic chips with and without a bottleneck design to answer RQ1 and RQ2, respectively. In the cell concentration analysis experiment, sheep blood diluted into 13 concentrations flowed through the microfluidic chips while videos were recorded by smartphones for the concentration measurement.

RESULTS

In the cell flow velocity experiment, we designed and fabricated 2 versions of microfluidic chips. The ANOVA test (Straight: F6, 99=6144.45, P<.001; Bottleneck: F6, 99=3475.78, P<.001) showed the height difference had a significant impact on the cell velocity, which implied gravity could drive the flow. The video sharpness analysis demonstrated that video quality followed an exponential decay with increasing height differences (video quality=100e-k×Height) and a bottleneck design could effectively preserve video quality (Straight: R2=0.95, k=4.33; Bottleneck: R2=0.91, k=0.59). Samples from the 13 cell concentrations were used for cell counting and cell concentration estimation analysis. The accuracy of cell counting (n=35, 60-second samples, R2=0.96, mean absolute error=1.10, mean squared error=2.24, root mean squared error=1.50) and cell concentration regression (n=39, 150-second samples, R2=0.99, mean absolute error=0.24, mean squared error=0.11, root mean squared error=0.33 on a logarithmic scale, mean average percentage error=0.25) were evaluated using 5-fold cross-validation by comparing the algorithmic estimation to ground truth.

CONCLUSIONS

In conclusion, we demonstrated the importance of the flow velocity in a microfluidic system, and we proposed SmartFlow, a low-cost system for computer vision-based cellular analysis. The proposed system could count the cells and estimate cell concentrations in the samples.

Unveiling islet heterogeneity using an automated microfluidic imaging system

Islets of Langerhans are a therapeutic target for diabetes and prediabetes. Measurements of therapeutic inhibitory or excitatory concentrations are often performed using large groups of islets, however, the population heterogeneity cannot be observed when examining the ensemble response. Normal islet function and islet response to therapeutic treatment can be affected by islet heterogeneity, influencing the progression of diabetes mellitus. To identify heterogeneity in an islet population, we developed a simple microfluidic device capable of delivering a stimulant to four independent chambers, allowing measurements of individual responses from a population of 20-25 islets. The device enabled accurate delivery of the same stimulant concentration to all four chambers, with an error <1% between chambers. To demonstrate the capability of this system, ensemble and individual EC/IC[Formula: see text] measurements of glucose and diazoxide were performed on murine islets. Results showed little heterogeneity of glucose EC[Formula: see text] values with all 21 islets within ± 0.6 mM of the ensemble value of 7.4 mM. In contrast, application of diazoxide to 24 islets in the presence of 20 mM glucose resulted in 37% of islets having an IC[Formula: see text] greater than 10% from the ensemble value of 10.2 μM. The simple system developed here is amenable to further studies on islet heterogeneity, and is applicable to investigate heterogeneity in other cell types.

Rocking- and diffusion-based culture of tumor spheroids-on-a-chip

Tumor spheroids are now intensively investigated toward preclinical and clinical applications, necessitating the establishment of accessible and cost-effective methods for routine operations. Without losing the advantage of organ-chip technologies, we developed a rocking system for facile formation and culture of tumor spheroids in hydrogel microwells of a suspended membrane under microfluidic conditions. While the rocking is controlled with a step motor, the microfluidic device is made of two plastic plates, allowing plugging directly syringe tubes with Luer connectors. Upon injection of the culture medium into the tubes and subsequent rocking of the chip, the medium flows back and forth in the channel underneath the membrane, ensuring a diffusion-based culture. Our results showed that such a rocking- and diffusion-based culture method significantly improved the quality of the tumor spheroids when compared to the static culture, particularly in terms of growth rate, roundness, junction formation and compactness of the spheroids. Notably, dynamically cultured tumor spheroids showed increased drug resistance, suggesting alternative assay conditions. Overall, the present method is pumpless, connectionless, and user-friendly, thereby facilitating the advancement of tumor-spheroid-based applications.

Twenty years of islet-on-a-chip: microfluidic tools for dissecting islet metabolism and function

Pancreatic islets are metabolically active micron-sized tissues responsible for controlling blood glucose through the secretion of insulin and glucagon. A loss of functional islet mass results in type 1 and 2 diabetes. Islet-on-a-chip devices are powerful microfluidic tools used to trap and study living ex vivo human and murine pancreatic islets and potentially stem cell-derived islet organoids. Devices developed over the past twenty years offer the ability to treat islets with controlled and dynamic microenvironments to mimic in vivo conditions and facilitate diabetes research. In this review, we explore the various islet-on-a-chip devices used to immobilize islets, regulate the microenvironment, and dynamically detect islet metabolism and insulin secretion. We first describe and assess the various methods used to immobilize islets including chambers, dam-walls, and hydrodynamic traps. We subsequently describe the surrounding methods used to create glucose gradients, enhance the reaggregation of dispersed islets, and control the microenvironment of stem cell-derived islet organoids. We focus on the various methods used to measure insulin secretion including capillary electrophoresis, droplet microfluidics, off-chip ELISAs, and on-chip fluorescence anisotropy immunoassays. Additionally, we delve into the various multiparametric readouts (NAD(P)H, Ca2+-activity, and O2-consumption rate) achieved primarily by adopting a microscopy-compatible optical window into the devices. By critical assessment of these advancements, we aim to inspire the development of new devices by the microfluidics community and accelerate the adoption of islet-on-a-chip devices by the wider diabetes research and clinical communities.

Animal Models in Diabetic Research-History, Presence, and Future Perspectives

Diabetes mellitus (DM) is a very serious disease, the incidence of which has been increasing worldwide. The beginning of diabetic research can be traced back to the 17th century. Since then, animals have been experimented on for diabetic research. However, the greatest development of diabetes research occurred in the second half of the last century, along with the development of laboratory techniques. Information obtained by monitoring patients and animal models led to the finding that there are several types of DM that differ significantly from each other in the causes of the onset and course of the disease. Through different types of animal models, researchers have studied the pathophysiology of all types of diabetic conditions and discovered suitable methods for therapy. Interestingly, despite the unquestionable success in understanding DM through animal models, we did not fully succeed in transferring the data obtained from animal models to human clinical research. On the contrary, we have observed that the chances of drug failure in human clinical trials are very high. In this review, we will summarize the history and presence of animal models in the research of DM over the last hundred years. Furthermore, we have summarized the new methodological approaches, such as "organ-on-chip," that have the potential to screen the newly discovered drugs for human clinical trials and advance the level of knowledge about diabetes, as well as its therapy, towards a personalized approach.

Islet-on-chip: promotion of islet health and function via encapsulation within a polymerizable fibrillar collagen scaffold

The protection and interrogation of pancreatic β-cell health and function ex vivo is a fundamental aspect of diabetes research, including mechanistic studies, evaluation of β-cell health modulators, and development and quality control of replacement β-cell populations. However, present-day islet culture formats, including traditional suspension culture as well as many recently developed microfluidic devices, suspend islets in a liquid microenvironment, disrupting mechanochemical signaling normally found in vivo and limiting β-cell viability and function in vitro. Herein, we present a novel three-dimensional (3D) microphysiological system (MPS) to extend islet health and function ex vivo by incorporating a polymerizable collagen scaffold to restore biophysical support and islet-collagen mechanobiological cues. Informed by computational models of gas and molecular transport relevant to β-cell physiology, a MPS configuration was down-selected based on simulated oxygen and nutrient delivery to collagen-encapsulated islets, and 3D-printing was applied as a readily accessible, low-cost rapid prototyping method. Recreating critical aspects of the in vivo microenvironment within the MPS via perfusion and islet-collagen interactions mitigated post-isolation ischemia and apoptosis in mouse islets over a 5-day period. In contrast, islets maintained in traditional suspension formats exhibited progressive hypoxic and apoptotic cores. Finally, dynamic glucose-stimulated insulin secretion measurements were performed on collagen-encapsulated mouse islets in the absence and presence of well-known chemical stressor thapsigargin using the MPS platform and compared to conventional protocols involving commercial perifusion machines. Overall, the MPS described here provides a user-friendly islet culture platform that not only supports long-term β-cell health and function but also enables multiparametric evaluations.

Development of electrochemical Zn2+ sensors for rapid voltammetric detection of glucose-stimulated insulin release from pancreatic β-cells

Diabetes is a chronic disease characterized by elevated blood glucose levels resulting from absent or ineffective insulin release from pancreatic β-cells. β-cell function is routinely assessed in vitro using static or dynamic glucose-stimulated insulin secretion (GSIS) assays followed by insulin quantification via time-consuming, costly enzyme-linked immunosorbent assays (ELISA). In this study, we developed a highly sensitive electrochemical sensor for zinc (Zn2+), an ion co-released with insulin, as a rapid and low-cost method for measuring dynamic insulin release. Different modifications to glassy carbon electrodes (GCE) were evaluated to develop a sensor that detects physiological Zn2+ concentrations while operating within a biological Krebs Ringer Buffer (KRB) medium (pH 7.2). Electrodeposition of bismuth and indium improved Zn2+ sensitivity and limit of detection (LOD), and a Nafion coating improved selectivity. Using anodic stripping voltammetry (ASV) with a pre-concentration time of 6 min, we achieved a LOD of 2.3 μg/L over the wide linear range of 2.5-500 μg/L Zn2+. Sensor performance improved with 10-min pre-concentration, resulting in increased sensitivity, lower LOD (0.18 μg/L), and a bilinear response over the range of 0.25-10 μg/L Zn2+. We further characterized the physicochemical properties of the Zn2+ sensor using scanning electron microscopy (SEM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Finally, we demonstrated the sensor's capability to measure Zn2+ release from glucose-stimulated INS-1 β-cells and primary mouse islets. Our results exhibited a high correlation with secreted insulin and validated the sensor's potential as a rapid alternative to conventional two-step GSIS plus ELISA methods.

Pumpless deterministic lateral displacement separation using a paper capillary wick

Deterministic lateral displacement (DLD) is a passive separation method that separates particles by hydrodynamic size. This label-free method is a promising technique for cell separation because of its high size resolution and insensitivity to flow rate. Development of capillary-driven microfluidic technologies allows microfluidic devices to be operated without any external power for fluid pumping, lowering their total cost and complexity. Herein, we develop and test a DLD-based particle and cell sorting method that is driven entirely by capillary pressure. We show microchip self-filling, flow focusing, flow stability, and capture of separated particles. We achieve separation efficiency of 92% for particle-particle separation and more than 99% efficiency for cell-particle separation. The high performance of driven flow and separation along with simplicity of the operation and setup make it a valuable candidate for point-of-care devices.

Employment of osmotic pump as a novel feeding system to operate the laminar-flow microfluidic microbial fuel cell

Laminar-flow microfluidic microbial fuel cell (LMMFC) has attracted attention due to the advantage of the liquid-liquid interface between anolyte and catholyte without the use of membrane as a separator resulting in less fabrication cost. Unlike previous studies of LMMFC using syringe pumps, this study proposes the use of osmotic pumps to feed anolyte and catholyte in the microchannel without any additional power supply. The osmotic pump was constructed with two cylindrical chambers separated by a forward osmosis membrane, with the initial draw solution concentration of 90 g l^-1 NaCl. We have, for the first time, demonstrated using the osmotic pumps to deliver both anolyte and catholyte and create co-laminar flow in LMMFC. Under the catholyte and anolyte flow rates of 18 ml/h and 40 ml/h respectively, LMMFC cultivated with Shewanella oneidensis produced the maximum power density of 87 mW m^-2 and current density of 747 mA m^-2 with the internal resistance of 1660 Ω. Further studies are warranted to develop osmotic pumps-fed LMMFC into a potential platform for portable biosensors.

Novel Pumping Methods for Microfluidic Devices: A Comprehensive Review

This review is an account of methods that use various strategies to control microfluidic flow control with high accuracy. The reviewed systems are divided into two large groups based on the way they create flow: passive systems (non-mechanical systems) and active (mechanical) systems. Each group is presented by a number of device fabrications. We try to explain the main principles of operation, and we list advantages and disadvantages of the presented systems. Mechanical systems are considered in more detail, as they are currently an area of increased interest due to their unique precision flow control and "multitasking". These systems are often applied as mini-laboratories, working autonomously without any additional operations, provided by humans, which is very important under complicated conditions. We also reviewed the integration of autonomous microfluidic systems with a smartphone or single-board computer when all data are retrieved and processed without using a personal computer. In addition, we discuss future trends and possible solutions for further development of this area of technology.

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.

Smartphone-microfluidic fluorescence imaging system for studying islet physiology

Smartphone technology has been recently applied for biomedical image acquisition and data analysis due to its high-quality imaging capability, and flexibility to customize multi-purpose apps. In this work, we developed and characterized a smartphone-microfluidic fluorescence imaging system for studying the physiology of pancreatic islets. We further evaluated the system capability by performing real-time fluorescence imaging on mouse islets labeled with either chemical fluorescence dyes or genetically encoded fluorescent protein indicators (GEFPIs). Our results showed that the system was capable of analyzing key beta-cell insulin stimulator-release coupling factors in response to various stimuli with high-resolution dynamics. Furthermore, the integration of a microfluidics allowed high-resolution detection of insulin secretion at single islet level. When compared to conventional fluorescence microscopes and macro islet perifusion apparatus, the system has the advantages of low cost, portable, and easy to operate. With all of these features, we envision that this smartphone-microfluidic fluorescence imaging system can be applied to study islet physiology and clinical applications.

Sensing Intra‐ and Extra‐Cellular Ca2+ in the Islet of Langerhans

Calcium ions (Ca2+) take part in intra‐ and inter‐cellular signaling to mediate cellular functions. Sensing this ubiquitous messenger is instrumental in disentangling the specific functions of cellular sub‐compartments and/or intercellular communications. In this review, the authors first describe intra‐ and inter‐cellular Ca2+ signaling in relation to insulin secretion from the pancreatic islets, and then outline the development of diverse sensors, for example, chemically synthesized indicators, genetically encoded proteins, and ion‐selective microelectrodes, for intra‐ and extra‐cellular sensing of Ca2+. Particular emphasis is placed on emerging approaches in this field, such as low‐affinity Ca2+ indicators and unique Ca2+‐responsive composite materials. The authors conclude by remarking on the challenges and opportunities for further developments in this field, which may facilitate a more comprehensive understanding of Ca2+ signaling within and outside the islets, and its relevance in health and disease.

A Smartphone-Fluidic Digital Imaging Analysis System for Pancreatic Islet Mass Quantification

Islet beta-cell viability, function, and mass are three decisive attributes that determine the efficacy of human islet transplantation for type 1 diabetes mellitus (T1DM) patients. Islet mass is commonly assessed manually, which often leads to error and bias. Digital imaging analysis (DIA) system has shown its potential as an alternative, but it has some associated limitations. In this study, a Smartphone-Fluidic Digital Imaging Analysis (SFDIA) System, which incorporates microfluidic techniques and Python-based video processing software, was developed for islet mass assessment. We quantified islets by tracking multiple moving islets in a microfluidic channel using the SFDIA system, and we achieved a relatively consistent result. The counts from the SFDIA and manual counting showed an average difference of 2.91 ± 1.50%. Furthermore, our software can analyze and extract key human islet mass parameters, including quantity, size, volume, IEq, morphology, and purity, which are not fully obtainable from traditional manual counting methods. Using SFDIA on a representative islet sample, we measured an average diameter of 99.88 ± 53.91 µm, an average circularity of 0.591 ± 0.133, and an average solidity of 0.853 ± 0.107. Via analysis of dithizone-stained islets using SFDIA, we found that a higher islet tissue percentage is associated with top-layer islets as opposed to middle-layer islets (0.735 ± 0.213 and 0.576 ± 0.223, respectively). Our results indicate that the SFDIA system can potentially be used as a multi-parameter islet mass assay that is superior in accuracy and consistency, when compared to conventional manual techniques.

Go with the flow: modeling unique biological flows in engineered in vitro platforms

Interest in recapitulating in vivo phenomena in vitro using organ-on-a-chip technology has grown rapidly and with it, attention to the types of fluid flow experienced in the body has followed suit. These platforms offer distinct advantages over in vivo models with regards to human relevance, cost, and control of inputs (e.g., controlled manipulation of biomechanical cues from fluid perfusion). Given the critical role biophysical forces play in several tissues and organs, it is therefore imperative that engineered in vitro platforms capture the complex, unique flow profiles experienced in the body that are intimately tied with organ function. In this review, we outline the complex and unique flow regimes experienced by three different organ systems: blood vasculature, lymphatic vasculature, and the intestinal system. We highlight current state-of-the-art platforms that strive to replicate physiological flows within engineered tissues while introducing potential limitations in current approaches.

Pumpless, modular, microphysiological systems enabling tunable perfusion for long-term cultivation of endothelialized lumens

Given the increased recognition of the importance of physiologically relevant microenvironments when designing in vitro assays, microphysiological systems (MPS) that mimic the critical function and structure of tissues and organs have gained considerable attention as alternatives to traditional experimental models. Accordingly, the field is growing rapidly, and some promising MPS are being tested for use in pharmaceutical development and toxicological testing. However, most MPS are complex and require additional infrastructure, which limits their successful translation. Here, we present a pumpless, modular MPS consisting of 1) a resistance module that controls flow rate and 2) a physiologically relevant, three-dimensional blood vessel module. Flow is provided by an attached reservoir tank that feeds fluid into the resistance channel via hydrostatic pressure. The flow rate is controlled by the height of the media in the tank and the resistance channel's dimensions. The flow from the resistance module is streamed into the blood vessel module using a liquid bridge. We utilize optical coherence tomography (OCT) to measure fluid velocity at regions of interest. The endothelial cells cultured in the MPS remain viable for up to 14 days and demonstrate the functional characteristics of the human blood vessels verified by tight junction expression and diffusion assay. Our results show that a modular MPS can simulate a functional endothelium in vitro while simplifying the operation of the MPS. The simplicity of the system allows for modifications to incorporate other microenvironmental components and to build other organ-modeling systems easily.

Engineering-inspired approaches to study β-cell function and diabetes

Strategies to mitigate the pathologies from diabetes range from simply administering insulin to prescribing complex drug/biologic regimens combined with lifestyle changes. There is a substantial effort to better understand β-cell physiology during diabetes pathogenesis as a means to develop improved therapies. The convergence of multiple fields ranging from developmental biology to microfluidic engineering has led to the development of new experimental systems to better study complex aspects of diabetes and β-cell biology. Here we discuss the available insulin-secreting cell types used in research, ranging from primary human β-cells, to cell lines, to pluripotent stem cell-derived β-like cells. Each of these sources possess inherent strengths and weaknesses pertinent to specific applications, especially in the context of engineered platforms. We then outline how insulin-expressing cells have been used in engineered platforms and how recent advances allow for better mimicry of in vivo conditions. Chief among these conditions are β-cell interactions with other endocrine organs. This facet is beginning to be thoroughly addressed by the organ-on-a-chip community, but holds enormous potential in the development of novel diabetes therapeutics. Furthermore, high throughput strategies focused on studying β-cell biology, improving β-cell differentiation, or proliferation have led to enormous contributions in the field and will no doubt be instrumental in bringing new diabetes therapeutics to the clinic.

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.

Pumpless microfluidic device with open top cell culture under oscillatory shear stress

Here we developed a 96-well plate-based pumpless microfluidic device to mimic bidirectional oscillatory shear stress experienced by osteoblasts at the endosteal niche located at the interface between bone and bone marrow. The culture device was designed to be high-throughput with 32 open top culture chambers for convenient cell seeding and staining. Mathematical modeling was used to simulate the control of oscillatory shear stress with the peak stress in the range of 0.3 to 50 mPa. Osteoblasts, cultured under oscillatory shear stress, were found to be highly viable and significantly aligned along the direction of flow. The modeling and experimental results demonstrate for the first time that cells can be cultured under controllable oscillatory shear stress in the open top culture chamber and pumpless configurations.

Surface tension measurement of normal human blood samples by pendant drop method

The surface tension of blood plays an important role not only in the birth and decompression sickness but also in other functionality of the organism. It also provides capillary action during blood flow process. In this article, a simple and low-cost device is designed and fabricated for measuring the surface tension of blood by pendant drop method. In this device, a droplet of blood is formed in a closed chamber on tip of an 18-gauge blunt needle and it is photographed by a camera in very humid conditions (RH = 99%) to minimise the evaporation. A wetted wick is provided at the bottom of the chamber for maintaining constant relative humidity in chamber. Surface tension of blood is inferred using drop shape factor method and image analysis technique at various experimental conditions. This device is validated and calibrated with surface tension measurements of water and silicone oil. Its measurements are in good agreement against data reported in literature. Post-validation, surface tensions of blood samples with and without anticoagulant of healthy persons at various temperatures (range from 20 to 40 °C) was measured. It was found that the surface tension of normal blood samples strongly correlates with blood temperature. The surface tension of female blood was remarkably different from same of male blood. However, the effect of age (21-60 year) on the surface tension was negligible for all practical purposes. Increased percentage of anticoagulant in blood increases its surface tension. This research specifies a baseline for surface tension of normal blood samples at various conditions which in turn provides new insights to pathologists in identifying various disease conditions.

A Multiplexed Microfluidic Platform toward Interrogating Endocrine Function: Simultaneous Sensing of Extracellular Ca2+ and Hormone

Extracellular Ca²⁺ ([Ca²⁺]_ex) is an important regulator of various physiological and pathological functions, including intercellular communication for synchronized cellular activities (e.g., coordinated hormone secretion from endocrine tissues). Yet it is rarely possible to concurrently quantify the dynamic changes of [Ca²⁺]_ex and related bioactive molecules with high accuracy and temporal resolution. This work aims to develop a multiplexed microfluidic platform to enable monitoring oscillatory [Ca²⁺]_ex and hormone(s) in a biomimetic environment. To this end, a low-affinity fluorescent indicator, Rhod-5N, is identified as a suitable sensor for a range of [Ca²⁺]_ex based on its demonstrated high sensitivity and selectivity to Ca²⁺ in biomedical samples, including human serum and cell culture medium. A microfluidic chip is devised to allow for the immobilization of microscale subjects (analogous to biological tissues), precise control of the perfusion gradient at sites of interest, and integration of modalities for fluorescence measurement and enzyme-linked immunosorbent assay. As this analytical system is demonstrated to be viable to quantify the dynamic changes of Ca²⁺ (0.2-2 mM) and insulin (15-150 mU L^-1) concurrently, with high temporal resolution, it has the potential to provide key insights into the essential roles of [Ca²⁺]_ex in the secretory function of endocrine tissues and to identify novel therapeutic targets for human diseases.

Genetically Encoded, Photostable Indicators to Image Dynamic Zn2+ Secretion of Pancreatic Islets

As an essential element for living organisms, zinc (Zn2+) exerts its biological functions both intracellularly and extracellularly. Previous studies have reported a number of genetically encoded Zn2+ indicators (GEZIs), which have been widely used to monitor Zn2+ in the cytosol and intracellular organelles. However, it is challenging to localize existing GEZIs to the extracellular space to detect secreted Zn2+. Herein, we report two photostable, green fluorescent protein (GFP) based indicators, ZIBG1 and ZIBG2, which respond to Zn2+ selectively and have affinities suited for detecting Zn2+ secretion from intracellular vesicles. In particular, ZIBG2 can be effectively targeted to the extracellular side of plasma membrane. We applied cell surface-localized ZIBG2 to monitor glucose-induced dynamic Zn2+ secretion from mouse insulinoma MIN6 cells and primary mouse and human pancreatic islets. Because Zn2+ is co-released with insulin from β-cells, the fluorescence of cell surface-localized ZIBG2 was shown to be a strong indicator for the functional potency of islets. Our work here has thus expanded the use of GEZIs to image dynamic Zn2+ secretion in live tissue. Because it is convenient to use genetically encoded indicators for expression over extended periods and for in vivo delivery, we envision future applications of ZIBG2 in development of induced β-cells or islets to advance cell replacement therapies for diabetes and in direct imaging of Zn2+ secretion dynamics in vivo.

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.

Gold nanoparticle amplification strategies for multiplex SPRi-based immunosensing of human pancreatic islet hormones

In this work, we demonstrate the potential use of SPRi for secretion-monitoring of pancreatic islets, small micro-organs that regulate glucose homeostasis in the body. In the islets, somatostatin works as a paracrine inhibitor of insulin and glucagon secretion. However, this inhibitory effect is lost in diabetic individuals and little is known about its contribution to the pathology of diabetes. Thus, the simultaneous detection of insulin, glucagon and somatostatin could help understand such communications. Previously, the authors introduced an SPRi biosensor to simultaneously monitor insulin, glucagon and somatostatin using an indirect competitive immunoassay. However, our sensor achieved a relatively high LOD for somatostatin detection (246 nM), the smallest of the three hormones. For this reason, the present work aimed to improve the performance of our SPRi biosensor using gold nanoparticles (GNPs) as a means of ensuring somatostatin detection from a small group of islets. Although GNP amplification is frequently reported in the literature for individual detection of analytes using SPR, studies regarding the optimal strategy in a multiplexed SPR setup are missing. Therefore, with the aim of finding the optimal GNP amplification strategies for multiplex sensing we compared three architectures: (1) GNPs immobilized on the sensor surface, (2) GNPs conjugated with primary antibodies (GNP-Ab1) and (3) GNPs conjugated with a secondary antibody (GNP-Ab2). Among these strategies an immunoassay using GNP-Ab2 conjugates was able to achieve multiplex detection of the three hormones without cross-reactivity and with 9-fold LOD improvement for insulin, 10-fold for glucagon and 200-fold for somatostatin when compared to the SPRi biosensor without GNPs. The present work denotes the first report of the simultaneous detection of such hormones with a sensitivity level comparable to ELISA assays, particularly for somatostatin.

Non-invasive marker-independent high content analysis of a microphysiological human pancreas-on-a-chip model

The increasing prevalence of diabetes, its heterogeneity, and the limited number of treatment options drive the need for physiologically relevant assay platforms with human genetic background that have the potential to improve mechanistic understanding and e\xpedite diabetes-related research and treatment. In this study, we developed an endocrine pancreas-on-a-chip model based on a tailored microfluidic platform, which enables self-guided trapping of single human pseudo-islets. Continuous, low-shear perfusion provides a physiologically relevant microenvironment especially important for modeling and monitoring of the endocrine function as well as sufficient supply with nutrients and oxygen. Human pseudo-islets, generated from the conditionally immortalized EndoC-βH3 cell line, were successfully injected by hydrostatic pressure-driven flow without altered viability. To track insulin secretion kinetics in response to glucose stimulation in a time-resolved manner, dynamic sampling of the supernatant as well as non-invasive real-time monitoring using Raman microspectroscopy was established on-chip. Dynamic sampling indicated a biphasic glucose-stimulated insulin response. Raman microspectroscopy allowed to trace glucose responsiveness in situ and to visualize different molecular structures such as lipids, mitochondria and nuclei. In-depth spectral analyses demonstrated a glucose stimulation-dependent, increased mitochondrial activity, and a switch in lipid composition of insulin secreting vesicles, supporting the high performance of our pancreas-on-a-chip model.

Toll-like receptors TLR2 and TLR4 block the replication of pancreatic β cells in diet-induced obesity

Consumption of a high-energy Western diet triggers mild adaptive β cell proliferation to compensate for peripheral insulin resistance; however, the underlying molecular mechanism remains unclear. In the present study we show that the toll-like receptors TLR2 and TLR4 inhibited the diet-induced replication of β cells in mice and humans. The combined, but not the individual, loss of TLR2 and TLR4 increased the replication of β cells, but not that of α cells, leading to enlarged β cell area and hyperinsulinemia in diet-induced obesity. Loss of TLR2 and TLR4 increased the nuclear abundance of the cell cycle regulators cyclin D2 and Cdk4 in a manner dependent on the signaling mediator Erk. These data reveal a regulatory mechanism controlling the proliferation of β cells in diet-induced obesity and suggest that selective targeting of the TLR2/TLR4 pathways may reverse β cell failure in patients with 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.

Microfluidic Disc-on-a-Chip Device for Mouse Intervertebral Disc-Pitching a Next-Generation Research Platform To Study Disc Degeneration

Low back pain is the most common cause of disability worldwide, and intervertebral disc degeneration is a major cause of low back pain. Unfortunately, discogenic low back pain is often treated with symptomatic relief interventions, as no disease-modifying medications are yet available. Both to-be-deciphered disc biology/pathology and inadequate in vitro research platform are major hurdles limiting drug discovery progress for disc degeneration. Here, we developed a microfluidic disc-on-a-chip device tailored for mouse disc organ as an in vitro research platform. We hypothesize that continuous nutrients empowered by a microfluidic device would improve biological performance of cultured mouse discs compared to those in static condition. This device permitted continuous media flow to mimic in vivo disc microenvironment. Intriguingly, mouse discs cultured on the microfluidic device exhibited much higher cell viability, better preserved structure integrity and anabolic-catabolic metabolism in both nucleus pulposus and annulus fibrosus, for up to 21 days compared to those in static culture. This first "disc-on-a-chip" device lays groundwork for future preclinical studies in a relative long-term organ culture given the chronic nature of intervertebral disc degeneration. In addition, this platform is readily transformable into a streamlined in vitro research platform to recapitulate physiological and pathophysiological microenvironment to accelerate disc research.

Stem-cell based organ-on-a-chip models for diabetes research

Diabetes mellitus (DM) ranks among the severest global health concerns of the 21st century. It encompasses a group of chronic disorders characterized by a dysregulated glucose metabolism, which arises as a consequence of progressive autoimmune destruction of pancreatic beta-cells (type 1 DM), or as a result of beta-cell dysfunction combined with systemic insulin resistance (type 2 DM). Human cohort studies have provided evidence of genetic and environmental contributions to DM; yet, these studies are mostly restricted to investigating statistical correlations between DM and certain risk factors. Mechanistic studies, on the other hand, aimed at re-creating the clinical picture of human DM in animal models. A translation to human biology is, however, often inadequate owing to significant differences between animal and human physiology, including the species-specific glucose regulation. Thus, there is an urgent need for the development of advanced human in vitro models with the potential to identify novel treatment options for DM. This review provides an overview of the technological advances in research on DM-relevant stem cells and their integration into microphysiological environments as provided by the organ-on-a-chip technology.

Pumpless platform for high-throughput dynamic multicellular culture and chemosensitivity evaluation

We report here a novel pumpless, 96-well plate-based platform for high-throughput dynamic multicellular culture and chemosensitivity evaluation. A gravity-driven flow strategy was developed to generate and sustain the flow rate of culture medium within 10% in the platform's 20 culture chambers. The ability of the platform to generate and sustain the medium flow was demonstrated by computational simulation, flow visualization, and ascertaining the previously known effect of flow-induced shear stress on the stimulated osteogenic differentiation of osteoblasts. The high-throughput utility of the platform was demonstrated by in situ cell staining and high content screening of chemosensitivity assays of multiple myeloma and osteoblast co-cultures. Endpoint characterization and data analyses for all 20 culture chambers required less than 1 hour.

Converting Adult Pancreatic Islet α Cells into β Cells by Targeting Both Dnmt1 and Arx

Insulin-producing pancreatic β cells in mice can slowly regenerate from glucagon-producing α cells in settings like β cell loss, but the basis of this conversion is unknown. Moreover, it remains unclear if this intra-islet cell conversion is relevant to diseases like type 1 diabetes (T1D). We show that the α cell regulators Aristaless-related homeobox (Arx) and DNA methyltransferase 1 (Dnmt1) maintain α cell identity in mice. Within 3 months of Dnmt1 and Arx loss, lineage tracing and single-cell RNA sequencing revealed extensive α cell conversion into progeny resembling native β cells. Physiological studies demonstrated that converted α cells acquire hallmark β cell electrophysiology and show glucose-stimulated insulin secretion. In T1D patients, subsets of glucagon-expressing cells show loss of DNMT1 and ARX and produce insulin and other β cell factors, suggesting that DNMT1 and ARX maintain α cell identity in humans. Our work reveals pathways regulated by Arx and Dnmt1 that are sufficient for achieving targeted generation of β cells from adult pancreatic α cells.