From micro-total-analysis-system (MTAS) to neurons-on-chip – the application range is broad and has given major new insights into the spread of the disease on a patient level as well as on a molecular scale. Most important findings were, that the size of the protein aggregates found in patients and in in-vitro assays is related to their toxicity. Especially the larger aggregated protein assemblies (oligomers) are considered as the damage causing species as described in an interview with the biotech company Fluidic Analytics¹.

This fact is the main driver in the research field for the development of protein sizing technologies of higher precision and specificity. In particular – since a single misfolded protein can corrupt others and spread the disease – we, therefore, seek methods to detect these specimens on a single-molecule level. Microfluidics has provided powerful tools for protein misfolding disease research: Microdroplet devices, diffusional sizing devices and electrophoresis-on-chip have proven invaluable for the study of protein aggregates (See Fig.1 A-C). However, to truly reach MTAS and analyze protein solutions on the single-molecule level, the nanofluidic regime opens up a totally new set of applications which are challenging to achieve or not possible at all with conventional microfluidics.

Fig. 1: Illustration of micro-, and nanofluidic chip designs used in protein misfolding disease research (A) Microdroplet generators are used to separate single cells (FACS) or confine protein into independent experiments in micron-sized droplets; (B) Diffusional-sizing devices use the spread of a sample injected in the centre of a microfluidics channel and its developing diffusion profile to estimate its hydrodynamic radius; (C) Electrophoresis on chip allows to measure the charge of molecules flowing through a perpendicular applied electric field; (A*) Smaller channel cross-sections allow the generation of nanodroplets; (B*) Diffusional sizing can be facilitated by observing single molecules diffusing in nanochannels between two reservoirs. If channels widths reach the size of proteins also filtration becomes possible; (C*) Electrophoretic trapping in nanofunnels can be used to concentrate and capture charged molecules in solution (e.g. DNA);

The new chip designs that arise from smaller channel widths are similar to microfluidic chip layouts but allow to significantly decrease the needed sample concentrations to the femtomolar range and the usage of new physics by reaching the nanoscale (See Fig.1 (A*)-(C*)). Nanodroplet maker chips (A*) produce even smaller independent nanoreactors for massive parallelization of drug testing or drug encapsulation. Nanochannels (B*) in between two microfluidic reservoirs can be used to measure single proteins in solution as they propagate from one reservoir to the other. This allows to measure the sizes of protein monomers (approx. 0.5 nm) up to macromolecular assemblies such as oligomers (2-10 nm), exosomes (40-100 nm) or viruses (80-120 nm) in solution, without permanent surface immobilization and a relatively simple chip design. Nanofunnels (C*) concentrate charged proteins and short stranded DNA (e.g. µDNA) in solution by application of an electric field, which facilitates an easier detection of these with optical setups. Especially for the analysis of protein solutions, nanochannels have the major advantage of eased experimental setup and conduction of experiments. Microfluidic diffusional sizing devices need external machinery such as multiple-precision syringe pumps, optical microscopes, high-voltage power supplies and standardized flow measurement protocols to allow for reliable protein sizing measurements. With a nanochannel device, a single 50 µl injection manually pipetted into a chip mounted on top of a commercial microscope can be used to evaluate e.g. antibody binding events with the highest precision at femtomolar concentrations. This decreases the total sample consumption by orders of magnitude, increases throughput and eases the training of staff for conduction of single-molecule experiments drastically. However, the availability of nanofluidic devices is currently limited to research laboratories and companies² with expensive clean-room facilities or researchers with expertise in nanofabrication³– where high prototyping costs put additional constraints.

A way to circumvent this matter is soft lithography⁴, which allows a cost-effective fabrication of disposable nanofluidic devices from a single master wafer. Recently, we demonstrated the scalable integration of nanofluidic functionalities into existing microfluidic designs using two-photon lithography as an effective alternative fabrication method in comparison to conventional electron beam lithography⁵. Commercial 2-photon lithography systems (e.g. Nanoscribe’s Quantum X and Photonic Professional GT2) are available and provide a sophisticated way to produce nanofluidic master wafers, but open-source systems can also be found in the community, and provide an even more cost-effective nanolithography solution for master fabrication and soft lithographic chip imprinting without cleanroom facilities.

Commercialization potential in nanofluidics, therefore, lies within three sectors: Firstly, in the application of nanofluidic devices for biotechnological diagnostics and antibody development e.g. antibody testing to specific targets such as viruses, exosomes or protein complexes in solution at minimal sample consumption. Secondly, in the cost-effective fabrication and distribution of nanofluidic devices tailored to customer’s needs using fast and flexible fabrication techniques and thirdly in the key-knowledge transfer of chip architecture and physical effects happening on the nanoscale (similar to PCB design parameters known from the electronics industry) which can be exchanged by consulting services. However, for all of this to be economically relevant, the fabrication and prototyping need to be realized in a cheap and scalable manner without expensive cleanroom facility maintenance costs and at faster design-to-device delivery times than conventional chip industry allows. The combination of two-photon lithography with soft lithography, therefore, provides an effective integration of nanofluidics into existing microfluidic designs and paves the way for a broad implementation of nanofluidic chips for various applications related to neurodegenerative disease research and cancer diagnostics.

Neuron image photocredit: Oliver Vanderpoorten and Colin Hockings

References

<sup>1. Ruairi J MacKenzie, „Size Matters: Diffusion Technique Sorts Out Pathological Proteins”, NNR, https://www.technologynetworks.com/neuroscience/blog/size-matters-diffusion-technique-sorts-out-pathological-proteins-322669

2. Wunsch, B., Smith, J., Gifford, S. et al. Nanoscale lateral displacement arrays for the separation of exosomes and colloids down to 20 nm. Nature Nanotech 11, 936–940 (2016). https://doi.org/10.1038/nnano.2016.134

3. Levin, S., Fritzsche, J., Nilsson, S. et al. A nanofluidic device for parallel single nanoparticle catalysis in solution. Nat Commun 10, 4426 (2019). https://doi.org/10.1038/s41467-019-12458-1

4.Qin, D., Xia, Y. & Whitesides, G. Soft lithography for micro- and nanoscale patterning. Nat Protoc 5, 491–502 (2010). https://doi.org/10.1038/nprot.2009.234.

5. Vanderpoorten, O., Peter, Q., Challa, P.K. et al. Scalable integration of nano-, and microfluidics with hybrid two-photon lithography. Microsyst Nanoeng 5, 40 (2019). https://doi.org/10.1038/s41378-019-0080-3.</sup>

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Oliver Vanderpoorten

Oliver Vanderpoorten is part of the Centre for Misfolding Disease at the University of Cambridge where he conducts research on nanolithography and nanoscopy methods. He holds two master degrees in engineering subjects which give him an applied attitude towards biotechnological applications: He holds a Master in Electrical and Microsystems Engineering from OTH Regensburg (2015) and a second Master’s degree in Sensor Technologies from the University of Cambridge (2016). During his PhD in the EPSRC funded Sensor CDT he explored the landscape of nanofluidic chips for various applications related to life sciences and protein misfolding diseases and developed 2-photon lithography as fabrication method for nanofluidic devices. In his free time he’s interested in entrepreneurship, sailing, painting and spends his time with outreach to explain science to the public in an informal way (https://youtu.be/TPJPYvumErM?t=100).

The post Bridging the micro to the nanoscale – how to facilitate the transition to nanofluidic devices as new standard in life sciences appeared first on The MicroFluidic Circle.

Microfluidics advances have hovered on the horizon as a “new way” for some time. The creation and development of new drugs can cost millions, yet many end up being recalled for toxicity, or just plain not as effective in humans as they are in lab animals.

The authors of a 2019 statistical analysis, “Impact of organ-on-a-chip technology on pharmaceutical R&D costs,” describe how high drug prices are driven by the huge expense of creating new product. They report that 60-75 percent of new drugs that succeed in non-human phases fail in later phases. “Better predictive models are needed,” they concluded, in something of an understatement.

Better predictive models include microfluidic platforms called organs-on-a-chip. These platforms are engineered to improve on the prognostic capacity of animal or in vitro models — which too often inaccurately simulate human physiology. As controlled microenvironments with vasculature perfusion that mimic the structure and function of human tissue, organ-a-chip technology has the potential to decrease the conventional roadblock of extensive development time frames, and their choking costs.

Although actual expenditures of private pharmaceutical companies are not made public, the statistical analysts estimate the potential of microfluidics to reduce R&D costs at 10-25 percent. The authors granularly analyzed some development costs by phase. They concluded that organ-on-a-chip technology could significantly reduce R&D costs by reducing the length of the early-stage research process.

The analysts note that, “Experts believed that the technology will help to make quicker and more precise decisions” during initial stages of research. One analyst said that if organ-on-chip was capable of identifying appropriate biomarkers, it would become the “Holy Grail” of biotechnology.

The authors do not pretend change arrives astride a fast horse. “The extent to which organ-on-a-chip can evolve in terms of predictability and applicability to the human biophysiology is yet to be seen,” they write. “Challenges of automation, parallelization, standardization and ease of use remain.”

Concerns driving the current conversation among experts focus on the magnitude of transformation. The tradition of utilizing in vitro and animal models is deeply ingrained and rooted, to state one obvious funding barrier. Results anticipated by innovators and early adopters can “take much longer to materialize when meeting the skepticism of the late majority and laggards.”

Not only are microfluidic devices pushing for legitimacy in the minds of some, but the very methods used to create them are often never-before-seen tools butting up against history as well. 3D bioprinting comes to mind.

There are pre-publication reports concerning vascularized micro-tissues for many major organs. For example, micro-tissued pancreatic islets have potential for insulin regulation and insulinomas. Liver buds are being researched. There has been progress in bioprinting vascularized thyroid glands.

In addition, genome engineering techniques may create genetic disease models using microfluidic platforms. Personalized medicine applications may include drug screening for patient-specific tumors.

In their study titled “Long-term Expanding Human Airway Organoids for Disease Modeling,” lung specialists wrote that “human airway organoids represent versatile models for the in vitro study of hereditary, malignant, and infectious pulmonary disease.”

The world certainly does have an “infectious pulmonary disease” on board.

Can regulatory shibboleths and personal proclivities accommodate the pace of innovation? Despite the complexity of research, the degree of potential is climbing. According to a recent Research and Markets report, microfluidics technology is replicating many functions of traditional healthcare, including clinical diagnostics, point-of-care diagnostics and drug delivery: “The global microfluidics market size is projected to reach USD 44.0 billion by 2025 from USD 15.7 billion in 2020.”

One research team recently described why “frontier technological tools by which infections are studied and new drugs and vaccines are tested” include “microfluidic chambers for the culture of organoids.” Given they are Milan, Italy-based, this team comprises COVID-19 experts.

The pandemic-weary world awaits “frontier tools.”

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Kathy Jean Schultz

Kathy Jean Schultz is a freelance medical science writer who focuses on medical innovations. She earned a Master’s Degree in Research Methodology from Hofstra University, and a Master’s Degree in Psychology from Long Island University. She is a member of the National Association of Science Writers, and the Association of Health Care Journalists. Her articles about organoids include "Would you trust a 3-D printed mini organ to test your drugs?" and "Stem cells not only slow disease, they come with their own safety test".

• http://kathyjeanschultz.pressfolios.com/

The post Microfluidics and Approval Bottlenecks in a Pandemic appeared first on The MicroFluidic Circle.

The most obvious application for these inertially focused microfluidic devices is for cell sorting or cell isolation, and the heart of these devices is one or several spiral channels. Working with a small sample volume is their most significant advantage. The other benefit is the high throughput analysis. My colleagues and I used a cascaded microfluidic device consisting of two spiral channels and one zigzag channel, for the continuous, fast, and high-throughput size-based isolation of cancer tumor cells (CTC) from blood cells. The platform simultaneously separates four different types of cells, namely, human lung cancer cells (A549), human breast cancer cells (MCF-7), red blood cells (RBCs), and white blood cells (WBCs) based on their size differences.

Often only a more particle-enriched solution is desired, rather than full particle separation, when performing a particular experiment. For example, in patients with leukaemia, blood biopsy cannot be used because the counts of leukaemia blast cells in the blood is very low. By using a cascade of four spirals, an enriched sample of blast cells has been obtained, while red blood cells and healthy white blood cells were directed to the waste reservoir.

Although blood biopsy for leukaemia patients, which usually need a bone marrow biopsy, might be less invasive, in general, taking blood is categorized as intrusive. Therefore using a urine sample to detect prostate cancer is more compatible with the comfort of the patients. That is the reason that a group of researchers have developed a one spiral microfluidic chip capable of isolating prostate cancer cells from the urine.

These devices, alongside other microfluidic devices, can easily find a place in point-of-care applications.

However, inertial microfluidic focusing devices can have other interesting applications. For example, they have been used to separate and purify Tetraselmis suecica (lipid-rich microalgae) cultures from Phaeodactylum tricornutum (invasive diatom). Using the same device and in a similar study, yeasts (Saccharomyces pastorianus and Saccharomyces cerevisiae) have been separated from beer spoilage microorganisms (Lactobacillus brevis and Pediococcus damnosus).

As it is clear from these examples, Inertial microfluidic focusing devices have presented a high potential for high throughput, low cost and time-saving applications.  However, the fluid dynamics is not as simple as it is outlined earlier here. Many other parameters such as fluid density, particles elasticity, centrifugal forces, microparticles density, etc can add to the complexity of the system. Therefore finding the streams of individual microparticles and designing the outlet system will become cumbersome.

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Azarmidokht Gholamipour-Shirazi

Azar is a research fellow in the School of Metallurgy and Materials, University of Birmingham (UK). She received her PhD from the University of Lille (France). During her PhD and postdoc research, she has used microfluidic devices for a wide range of applications. Her publications are available here. In her free time, you might find her hiking or trail walking.

• http://orcid.org/0000-0001-6420-5986

The post Applications of inertial microfluidic focusing (with the focus on commercial products ) appeared first on The MicroFluidic Circle.

A team of scientists from various institutions, including the U.S. Department of Energy, investigated the mechanics: how do these bacteria transport electrons across cell walls? They probed how Listeria bacteria residing in the human intestine transport electrons — in the form of tiny currents — through their cell wall.

The findings were just one example of bacteria in the human gut that regularly produce electricity. It seems that hundreds of other bacteria may be employing similar, but so-far-unidentified processes. A wide range of bacteria types might be conducting a similar transport of electrons outside their cell wall. But findings suggested that Listeria appeared to use a method that is different from other electricity-producing bacteria.

Does this mean these bacteria could predictably produce electricity outside of their normal environment? The researchers noted that these findings point toward the potential creation of batteries from microbes.

The traditional medical focus has been understanding how bacteria infect — or maintain — a healthy gut. But the findings about electrons have spawned new questions: Is it possible that microbe-derived batteries could generate electricity in large waste treatment plants? Could batteries built from microbes feasibly generate electricity on a grand scale?

“While scientists have found bacteria that produce electricity in exotic environments like mines and the bottoms of lakes, researchers have missed a source closer to home: the human gut,” the researchers noted.

Recent advances outline the potential for microfluidic engineering of microbiomes’ microbial interactions. Microfluidic chips have the capacity to fuel the automated assembly and analysis of microbial communities.

In addition, novel tools provide a framework to evaluate how metabolic networks drive biological processes. There are new platforms for designing microbiomes with specific properties, and to describe what principles govern their interactions. A goal is identifying processes to be manipulated and monitored, including the prediction of metabolic flux throughout interacting networks.

Mapping gut bacteria types in the human microbiome remains incomplete, due to its wide range of bacterial diversity. Technical roadblocks have included figuring out how to isolate single-bacteria. But a new microfluidic-emulsion device has created microdroplets to capture a single bacterium. Single-bacteria isolation could reveal which properties yield the most insight about infection in humans. Additional studies have described microfluidic approaches to creating realistic in-vitro models of the human intestinal microbiome.

It’s been known for a long time that the body produces electricity. Electrocardiograms measure the heart’s electricity. Electroencephalography measures electrical activity in the brain. Brushing of very dry human hair generates static electricity — with results ranging from being a nuisance, to pain. But the idea that it might be possible to capture enough body-generated electricity for outside-the-body uses, is new.

The human microbiome contains communities of microbes, bacteria, viruses and fungi that function together. Animals have microbiomes controlling their health or sickness too. Soil, water and rocks also contain microbiomes that mediate chemical changes.

Within a microbiome, microbes move around and fuel each other in complicated ways. How we eat, live and use medicine impacts how microbes will interact with each other — working together to maintain health, or to cause disease. Some human microbes cause sickness, while others sustain life.

What’s next for the concept of microbe-produced batteries? Investors are more often mulling partnerships for cutting-edge innovations, from Artificial Intelligence to genetics. A recent Pharmacy and Therapeutics Journal report, “The Human Microbiome Market,” confirms that 260 microbiome therapeutics are currently being evaluated in different stages of development. “The microbiome-based medical products market can be expected to witness substantial growth over the coming decade,” according to the report. “The concept of microbiome-based therapeutics has generated significant enthusiasm within the medical science community, defining a new frontier in the field of medicine.”

A new frontier indeed. These particular findings address two compelling issues: medical advances, and also urgent energy-production problems.

This unique pairing would seem to check a lot of boxes for investors.

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Kathy Jean Schultz

Kathy Jean Schultz is a freelance medical science writer who focuses on medical innovations. She earned a Master’s Degree in Research Methodology from Hofstra University, and a Master’s Degree in Psychology from Long Island University. She is a member of the National Association of Science Writers, and the Association of Health Care Journalists. Her articles about organoids include "Would you trust a 3-D printed mini organ to test your drugs?" and "Stem cells not only slow disease, they come with their own safety test".

• http://kathyjeanschultz.pressfolios.com/

The post Generating Electricity by Making Batteries from Human Microbes? Microfluidics Hold the Key appeared first on The MicroFluidic Circle.

All the current lung-on-a-chip devices are aiming to recapitulate the complexity of the lungs to some extent to offer an advanced cell-culture model for drug discovery purposes. They usually consist of three components: (1) an air channel where epithelial cells can be grown and transferred later to air-liquid interface, (2) a “blood” channel that serves as the vascular part of the chip, and (3) a semi-permeable membrane to separate the air and fluidic channels. Together,  all these new features can offer an in vitro model that has these advantages over more conventional in vitro models: (1)emulating the complexity of the lungs to some extent for instance by the integration of a stretchable membrane, (2)adding the respiratory breathing motions to the model either by stretching or the deflection of a thin membrane, (3) having “blood” and airflow in the apical and basal sides which can induce shear stress on cells and present a more realistic model than the other static in vitro models, and (4) the feasibility of using an extracellular matrix (ECM) based material as the membrane or added to the polymeric membrane. In some models, two sides of channels have been included to stimulate breathing by generating cyclic pneumatic pressure. This is helpful when the focus of the model is alveolar. Otherwise, a cyclic flow in the lower channel would be sufficient to mimic the physiological shear stress on cells. In addition, some biochemical factors such as proteins (collagen and fibronectin)or growth factors can be added to the chip (coated on both sides of the membrane) to recapitulate the cellular microenvironment.Polydimethylsiloxane(PDMS) is the most common material in the fabrication of these lung-on-a-chip devices since PDMS is optically transparent, biocompatible, easy to use, and flexible. Lung-on-a-chip devices aim to provide a tool for studying a drug. Besides, these devices can be used to simulate a lung disease such as inflammation, asthma, lung cancer, pulmonary fibrosis, or lung injuries and simulate the lung cells/tissue interactions at a “realistic” scale by controlling their microenvironment.

To begin with, we should mention the device designed and developed by Huh et al. in which mechanical strain was applied to stretch a porous PDMS membrane. They showed that the mechanical strain facilitated the uptake and transfer of nanoparticles by epithelial and endothelial. These results were confirmed by observing the similar effects in the whole mouse lung. The same research group later successfully modeled a human disease model-on-a-chip for pulmonary edema. This group led by DE Ingnercan be considered as one of the most dynamic teams in the field that has introduced various inventions not only to lung-on-a-chip devices but also to other organ-on-a-chip devices. In another study, they developed a small airway-on-a-chip and lined the airway with epithelial cells from patients suffering from chronic obstructive pulmonary disease. Using viral and bacterial infections, they could model the disease in their device. Moreover, they fabricated a microfluidic chip containing embedded electrodes for measuring trans-epithelial electrical resistance (TEER), which is used to monitor and quantify the integrity of cultured epithelial under statistic conditions. Later, they developed a lung alveolus-on-a-chip to model pulmonary thrombosis by co-culturing epithelium and endothelium. In another interesting work, KL Sellgren et al. designed a chip for culturing primary endothelial, epithelial, and lung fibroblasts to model the human airway. Polytetrafluoroethylene (PTFE) and polyester (PET) membranes were integrated between different layers of the chip. This work showed the feasibility of culturing primary airway epithelial cells with lung fibroblast and endothelium while providing appropriate compartmentalization in the chip. Such a triple co-culture with the capability of perfusion cannot be provided by the current commercial in vitro models. Other researchers also tried to integrate electrodes in a lung-on-a-chip device for monitoring the electrochemical and mechanical changing at the lung alveolar interface. Their device was also made of PDMS and mimicked the mechanical strain in alveolar by bulging a porous PDMS-based membrane. The advantage of this model is to create a 3D cyclic strain applied in all directions. An array of suspended gels was used instead of a porous membrane as a barrier between airway epithelial cells and airway smooth muscle cells in another work. There, the gel was a mixture of type I collagen and Matrigel which could enhance cell adhesion and growth. Xu et al.introduced an exciting approach to combine other organs in a lung-on-a-chip for studying lung cancer metastasis. This device had an upstream, lung, and three downstream organs that may be affected by lung cancer metastasis. These are only some of the works done in the field of lung-on-a-chip and more papers with details can be found in a recent review paper published in Biomicrofluics.

In the last few years, several organ-on-a-chip start-up companies have been founded, aiming to introduce a new platform for the drug discovery industry. Some of these companies offer devices at “body-on-chip” scales. Hesperos, Tissuse, CnBio, DRAPER are examples of these companies that are trying to provide a more complex model for assessing the pharmacokinetics and pharmacodynamic of new drugs on the human body than the conventional in vitro cell culture. Indeed, these models cannot completely mimic the complexity of the human body in the absorption, distributions, metabolism, and elimination of these new drugs but they are at least one step ahead of the available models. Some of the companies took another approach to construct an organ-on-a-chip that has a simulated tissue interface. Dr. Donald Ingber at Wyss Institute is one of the pioneers in this area and the founder of Emulate Inc. They offer a range of organ-on-a-chips including lung-on-a-chip or airway-on-a-chip. The strength of their technology is the ease of use for users as they developed several control modules. These modules can be used to control and monitor individual chips — another lung-on-a-chip company, Alveolix, founded by Dr. Olivier Guenatfor simulating the alveolar barrier. Not having vascular perfusion may be considered as a weakness, but it simplified the operation of the chip and would enable the users to operate multiple chips at the same time. Emulate and Alveolix are the most well-known companies in the area of lung-on-a-chip but there are more companies in the field. NORTIS, Quorum Technologies (Artery-on-a-chip Vessel), MIMETAS, SYnVIVO, 4DESIGN BIOSCIENCES, and AIM BIOTECH are good examples of these companies which have focused their efforts to bring some of the complexity of the human tissues to their in vitro models.

Regardless of all these advancements, there are still some issues that need to be addressed to introduce a more biomimetic model. For instance, the cell-to-liquid ratio should be improved to avoid the dilution of secreted proteins, metabolites, and factors. Fabricating smaller channels or growing cells in 3D may help to solve this problem. The thickness of membranes is typically around 10 µm which is much thicker than the basement membrane of blood vessels (300 – 400 nm). Molding hydrogel with integrated microchannels or 3D printing of hydrogels is a possible solution to tackle this challenge. The current models for alveolar can be improved by recapitulating the 3D shape of lungs air sacs. Instead of stretching the membrane that separates apical and basal sides, the membrane can be inflated by flowing air to the air channels.

To sum up, lung-on-a-chip and airway-on-a-chip are only one piece of the puzzle for developing a reliable platform in a drug discovery journey. As the future organs-on-a-chip or body-on-a-chip is supposed to be a comprehensive platform for testing a new drug, multiple chips should be combined to cover all aspects of human physiology. An interesting approach is to design a chip that mimics the human lungs at a smaller scale while having airways, bronchioles, and alveoli at the same time and show how various epithelial cells may interact with each other or which type is more prone to a specific disease or a drug. Realizing such a design can be very challenging based on the current technologies but 3D printing and bioprinting can bridge this gas and facilitate the progress toward a 3D lung model.

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Mohammadhossein Dabaghi

Mohammadhossein Dabaghi is currently a postdoctoral fellow at the department of medicine at McMaster University. He completed his Ph.D. at McMaster University majoring in Biomedical Engineering and worked on new microfabrication technologies to develop an Artificial Placenta device for preterm neonates with respiratory failure. He earned a master’s degree in Biomedical/Chemical Engineering (2014), and a BS in Chemical Engineering from Sharif University of Technology, Iran (2012). His research interests focus on lung-on-a-chips, biosensors for organ-on-a-chips, and bioprinting.

• https://www.researchgate.net/profile/Mohammadhossein_Dabaghi2

The post The Promise of Lung-on-a-Chip Devices appeared first on The MicroFluidic Circle.

While healthcare access has increased globally in the last three decades¹, at least half the world still lacks access to essential healthcare services². For the ones that do have access, the high costs can often push them towards extreme poverty, bankruptcies, and homelessness. While there is a direct correlation between income equality and access to healthcare, another major reason that access to healthcare remains elusive to half the world population is because of the multiple dimensions that constitute healthcare – awareness and access to information about healthcare, access to health services such as hospital, ambulance, diagnostics, medicines, trained doctors and nurses, life-saving treatments, follow-ups among several others. Covering most or all of these aspects simultaneously at a location can often be a challenging task.

Diagnostics and Microfluidics

Next to health awareness and immunizations, diagnostics are among the front liners in health services. The majority of the downstream healthcare decisions (and therefore costs) are dependent on timely and accurate diagnosis. As our understandings of human physiology and pathology have evolved, so have the diagnostic tools. Unlike lab testing which involved collecting samples from patients and transporting them to a distant laboratory and waiting to learn the results, point-of-care (POC) diagnostics have made medical testing possible at patient’s location in a much-reduced timeframe thus expediting medical decisions.

Microfluidic technologies have played a critical role in the rapid evolution of POC devices in the health industry. They offer numerous advantages: lower costs, reduced sample volumes, faster turn-around times, user-friendliness, device portability and high-throughput screening(HTS). Low costs and portability make it possible to adapt them in settings with limited or non-existent healthcare infrastructure. User-friendliness, easier result interpretations (such as through colorimetric assays) make it an attractive option in settings with limited or no access to trained health care providers.

Several companies have now successfully introduced microfluidic POC testing devices into the market. Several products have now made it to pharmacy shelves such as pregnancy tests and glucose monitoring kits. The global market for microfluidics was valued at 8.28 billion in 2017 and is expected to reach up to USD 27.91 billion by 2023³. While the majority of the market for microfluidic devices is currently in developed countries, it is the developing countries where it can serve as a major game-changer. While cancers and cardiac diseases are the top killers in developed countries such as the US, infectious diseases, such as malaria, HIV/AIDS, and tuberculosis, are the leading cause of death in several developing nations. Current diagnostic procedures for these involve benchtop tests and assays with heavy reliability on instruments and reagents and are often time-consuming. With the focus shifting towards microfluidics, there have been several recent attempts to introduce microfluidics in developing nations. Several proof-of-concept studies have indeed shown that microfluidic POC devices can be a route to provide reliable and faster diagnostic services in these areas. Chin et al used a microfluidic device to detect HIV and syphilis in the Rwandan population using blood volumes as little as 1 μl and a turn-around time of 20 minutes⁴. Hugo et al. demonstrated the potential of a centrifugal microfluidic platform for POC diagnostic applications in South Africa⁵. Taylor et al showed the potential of PCR-on-chip device for malaria diagnosis among patient samples from Uganda⁶. Diagnostics For All, a non-profit started by Dr. George Whitesides and his group at Harvard, uses paper microfluidics for development of POC diagnostics for developing countries. Field studies have been conducted in Vietnam and Kenya⁷. Commercial companies that have microfluidic POC devices for infectious diseases include Alere, Trinity Biotech, and IMMY that use lateral flow immunoassay strips for the detection of diseases such as malaria, meningitis, filariasis, HIV, flu, and Legionnaire’s Disease. Alere, a market leader in this field, partners with several non-profits for the distribution of malaria and HIV testing POC kits in developing nations⁸.

Conclusion and future perspective

While enough studies and pilot projects have demonstrated the vast potential of microfluidics in the diagnosis of infectious diseases in a developing country, the target groups are way bigger than current reach of the microfluidic research and market. This calls for an action plan involving several stakeholders, such as the World Health Organization, governments, administrators, private companies, non-profits, and locals, to work in concert and collaborations. While mass-production will undoubtedly help meet the demands and lower the cost at the manufacturer and consumer ends, affordability will always remain a huge bottleneck in the widespread use of these devices in developing countries. Current efforts and successes have only been possible due to the involvement of non-profits. This issue further calls for bridging the gap between research and policy. Governments can play major roles here by introducing schemes and subsidies to increase the reach of the products.

There are innovations required on multiple fronts to tackle the issue of introducing microfluidic POC diagnostic devices to such a massive fraction of the world population. There is a need to ensure that material used for making these devices is readily available, safe, light-weight and well-suited for mass-production. The final devices also need to be handy and easy to transport. Technologies such as lab-on-a-drone⁹ need to be further advanced to ensure easy and wide distribution of these devices to remote areas. While the devices need to be user-friendly and must require minimal training, there should be provisions to store results and data for future use by integrating with everyday devices such as cell phones. Devices should also include multiple tests within the same setup to ensure high-throughput screening among populations plagued by multiple infections.

While the current issue at hand is focused on microfluidic-based affordable diagnostics for developing countries, microfluidics can play important roles even in health services downstream of diagnosis. Microfluidic organ-on-a-chip and human-on-a-chip platforms are promising technologies to advance precision medicines. Several drugs are known to have variable effects on different populations owing to genetic differences. Testing established drugs on organ-on-chip platforms based on cells from a certain sub-population can provide useful results in terms of predicting the safety of the introduction of drugs to that population. Furthermore, these platforms have also been touted as the future of novel drug development and toxicity testing.

While the field of microfluidics has now been around for 30 years, the technology still remains heavily confined to academia and basic research. Institutions and companies should now steer this field into practical real-world applications – providing healthcare access around the world being a major one due to the overwhelming size of the population that lacks basic healthcare. The low cost, low sample volumes, portability, reliability and faster turn-around times make it the most promising candidate for a tool that can bring diagnostics from bench to bedside.

References

<sup>1. https://medicalxpress.com/news/2018-05-global-healthcare-access-quality-.html

2. https://www.who.int/news-room/detail/13-12-2017-world-bank-and-who-half-the-world-lacks-access-to-essential-health-services-100-million-still-pushed-into-extreme-poverty-because-of-health-expenses

3. https://www.marketsandmarkets.com/Market-Reports/microfluidics-market-1305.html

4. Chin, C. D. et al. Microfluidics-based diagnostics of infectious diseases in the developing world. Nat. Med. 17, 1015–1019 (2011).

5. Hugo, S., Land, K., Madou, M. & Kido, H. A centrifugal microfluidic platform for point-of-care diagnostic applications. S. Afr. J. Sci. 110, (2014).

6. Taylor, B. J. et al. A lab-on-chip for malaria diagnosis and surveillance. Malar. J. 13, 179 (2014).

7. http://dfa.org/

8. https://www.alere.com/en/home/about/corporate-responsibility.html

9. Priye, A. et al. Lab-on-a-Drone: Toward Pinpoint Deployment of Smartphone-Enabled Nucleic Acid-Based Diagnostics for Mobile Health Care. Anal. Chem. 88, 4651–4660 (2016)</sup>

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Vardhman Kumar

Vardhman is a Ph.D. candidate in the Department of Biomedical Engineering at Duke University. His work in Prof. Shyni Varghese’s lab involves the development of microfluidic platforms for understanding the biophysics of development and diseases.

• https://scholars.duke.edu/person/vardhman.kumar

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1. How do we cost-effectively and accurately screen potential sepsis pharmaceuticals?
2. Can we diagnose sepsis patients earlier to permit corrective intervention before irreversible disease progression?

We believe microfluidics can help answer these questions.

In the pursuit of developing a single, manufacturable system to answer our outstanding questions in sepsis research, we add an additional question to the problem: how can the field of microfluidics overcome the implicit challenges faced when bridging basic science research and clinical application? It is clear that communication between the research lab and the clinic (a field often referred to as translational biomedical science) can have an immediate impact on the health care system and in the context of sepsis, this increased dialogue has left a profound mark on the disease.

Microfluidics and Sepsis Research

Since the rise of conventional polydimethylsiloxane (PDMS) based microfluidic devices in the late 1990s and early 2000s, production of microphysiological systems (MPS) for a broad range of applications has increased, while the cost associated with these devices has declined. This large-scale development means that MPS are no longer primarily restricted to the labs of engineers and are now reaching the hands of knowledgeable biologists. From whole organ mimetics to blood-brain barrier models, microfluidic devices open the door to highly controlled studies on human cell lines and associated clinical samples. What is lost in physiological accuracy, is gained in simplicity and reproducibility, allowing for often complex or even unimaginable studies (when performed in vivo) at the lab bench. These points are emphasized in the context of sepsis research due to the following:

• Previous studies in murine (a.k.a. mouse and rat) models of sepsis have highlighted important physiological differences between these experimental species and what really occurs in humans, particularly in the pathologic immune response defining the disease³.
• Mass failure of immune-modulating drugs for the treatment of sepsis and the large financial burden associated with those shortcomings means labs are acutely aware of the need for cost-effective ways to screen for future therapeutics

For these reasons, microfluidics is a very attractive tool for sepsis research. In Dr. James McGrath’s lab at the University of Rochester, we utilize microfluidic devices to mimic the inflamed microvasculature related to sepsis pathophysiology and investigate the relationship between white blood cells (neutrophils) and the endothelial cells that line the vascular wall (Movie 1)⁴. The systems used herein, and others alike, allow for the highly controlled and meticulous study of the immune response in sepsis, all aimed at developing a better understanding of the disease, with the future hope of a cure. In doing so, we can maintain a significantly lower price point compared to studies performed in murine models.

Across the field, ‘lab-on-a-chip’ microfluidic systems are being exploited for drug discovery in all aspects of human disease because of their inherent controllable properties and low cost. These efforts were reviewed previously in Nature Drug Discovery⁵.

Movie 1. In our lab, microfluidic devices are assembled in a layer-by-layer fashion, allowing for human endothelial cell culture on a silicon nanomembrane (green/yellow chip). The resulting microvascular mimetics facilitate a live microscopy study of the white blood cell-lead immune response. The final frame depicts the nanoporous membrane utilized as the endothelial cell culture support.

Microfluidics in the Clinic

With low production costs, facile use, and increased throughput, microfluidic devices have left the confines of basic science research and entered the mainstream of clinical translation. Receiving major attention in a January 2019 issue of National Geographic, ‘lab-on-a-chip’ devices are highlighted as ‘the future of medicine’, with emphasis on the potential for personalized medicine. By incorporating patient cells and fluid samples to a specialized device, personalized medicine can permit patient-specific therapy and guided pharmaceuticals. Additionally, the requirement of minute sample volumes allows for increased experimentation/diagnostics on often precious patient samples or expensive reagents. These benefits have already proven impactful to the sepsis community: In a 2018 issue of Nature Biomedical Engineering, Ellett and colleagues describe a microfluidic platform that analyzes patient blood cell (neutrophil) motility to accurately diagnoses sepsis⁶. The device used immediately showcases the benefits of microfluidics in the clinic, as the diagnosis is performed using a single drop of blood, eliminating the need for large volume venipuncture draws or post-draw processing. In our lab, we are working on translating our MPS, namely, a stem cell-based mimetic of the blood-brain barrier, to act as an early-stage sepsis diagnostic. In addition to diagnostics, microfluidics has been suggested as an avenue to drugless sepsis treatment. Since the 1990s, it has been hypothesized that membrane-based, low footprint microfluidic devices may facilitate a process termed extracorporeal blood purification; a procedure in which harmful inflammatory factors are continuously filtered out of the blood, halting the maladaptive immune response. Given the applications presented here, the potential for impactful microfluidic technologies in the clinic is clear.

Bridging the Gap

As interest peaks in the use of microfluidics in the clinic, the question remains: how do we translate often meticulous device manufacturing to large scale production? This fabrication dilemma is most likely the demise of many microfluidic technologies developed in the lab. What is routine at a small scale in the cleanroom, may not be feasible at the necessary volumes to launch microfluidic devices into the clinic. Because of these concerns, many labs and clinics are less willing to adopt microfluidics in their work.

So how do we change this environment? Based on our recent experience, we suggest the following:

• Design devices that may be used for a variety of applications (simplicity and functionality is key)
• Work closely with microfluidics companies to adjust your designs for higher volume production goals
• Involve other labs that work on similar products to share the benefits of these platforms
• Talk to clinicians that would handle these devices to understand what problems are most important to them

In conclusion, microfluidics has the ability to open the door to truly translational science. In a single device, one can study the disease mechanism, develop targeted drugs, and diagnose patients. These multidisciplinary platforms effectively narrow the gap between basic science research and the clinic, paving the way to a future with a direct line of communication, from the patient straight to the lab.

References

<sup>1. Singer, M., Deutschman, C. S., Seymour, C. & et al. The third international consensus definitions for sepsis and septic shock (sepsis-3). JAMA315, 801-810, doi:10.1001/jama.2016.0287 (2016).

2. Gotts, J. E. & Matthay, M. A. Sepsis: pathophysiology and clinical management. BMJ353, i1585, doi:10.1136/bmj.i1585 (2016).

3. Rittirsch, D., Hoesel, L. M. & Ward, P. A. The disconnect between animal models of sepsis and human sepsis. J Leukoc Biol81, 137-143, doi:10.1189/jlb.0806542 (2007).

4. Salminen, A. T. et al. Ultrathin Dual-Scale Nano- and Microporous Membranes for Vascular Transmigration Models. Small15, e1804111, doi:10.1002/smll.201804111 (2019).

5. Dittrich, P. S. & Manz, A. Lab-on-a-chip: microfluidics in drug discovery. Nat Rev Drug Discov5, 210-218, doi:10.1038/nrd1985 (2006).

6. Ellett, F. et al. Diagnosis of sepsis from a drop of blood by measurement of spontaneous neutrophil motility in a microfluidic assay. Nature Biomedical Engineering2, 207-214, doi:10.1038/s41551-018-0208-z (2018).</sup>

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Alec Salminen

Alec Salminen is a Ph.D. candidate in the Department of Biomedical Engineering at the University of Rochester. His work, under Dr. James McGrath, involves the fabrication of silicon nanomembranes and microfluidic mimetics of the microvasculature to study the innate immune response in sepsis. Additionally, Alec works with a microfluidic manufacturer to fabricate devices for a variety of applications, including an early-stage sepsis diagnostic.

• https://www.researchgate.net/profile/Alec_Salminen

Elysia Masters, a Ph.D. candidate in the Department of Biomedical Engineering at the University of Rochester, aided in the writing of this post. More information on her work can be found below.

ResearchGate | LinkedIn

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For example, the density of metabolites in sweat — along with its ease of collection from skin pores — make it a useful biofluidic candidate for analysis. One recent study looked at how microchannels and micro reservoirs, pre-filled with fluorescent probes that react to target analytes in sweat, can perform quantitative analysis. To capture sweat, fluorometric sensing modalities were integrated into a skin-interfaced microfluidic system that was paired with a smartphone-based imaging module. This method yielded an accurate measurement of biomarkers in sweat.

In another recent study, researchers developed wearable sensors to monitor biomolecule levels by combining continuous fluid sampling with in-situ analysis. Depending upon the target biomolecule, the particular assay was interchangeable.

The microfluidic device featured a droplet-flow method for timing, and a micropump to produce nanolitre-sized droplets. Biomarker variations within fluids, over time, yield insight into tissue physiology and may help to create personalized treatments.

The study’s palm-sized sensor autonomously detected deviations from steady-state level.“We demonstrate how the sensor can track perturbed glucose and lactate levels in dermal tissue with results in close agreement with standard off-line analysis and consistent with changes in peripheral blood levels,” the authors wrote.

Biomarker concentrations fluctuate continuously, as does chemical signalling. The capacity for continuous measurement of these dynamics has significant implications.

Many current point-of-care devices are single-measurement tools. The use of microfluidics for continuous monitoring has been strained where microfluidic systems rely upon bulky laboratory equipment such as syringe pumps and microscopes — impractical as wearable devices. But recent advances address this.

For example, the linear nature of microscale flow has required many external control devices. Another recent study, by an international research team, highlights the design of networks with a nonlinear relation between flow rate and its applied pressure. This relation can be harnessed to switch the direction of internal flows by manipulating the input and output pressures.

Using rigid polymer channels to carry water, the investigators showed that these networks demonstrate a fluid version of Braess’s Paradox: closing an intermediate channel resulted in a higher rather than lower, total flow rate. These findings are scalable and can implement flow routing with multiple switches. Practical applications can encompass built-in control mechanisms in microfluidic networks, furthering the creation of portable systems — such as wearable healthcare technologies

These new findings seem to have clear advantages, yet final shepherding of new findings toward commercialization remains the most challenging step. A new device can fail clinically, or it can run out of funding, miscalculate the market, or collide with regulations, according to Georgia Tech benchtop-to-bedside expert Tiffany Wilson.

“Find out about clinical workflow and how health care operates, then maybe decide not to pursue the prototype you had planned, but work on a new one instead,” she warns. “It generally doesn’t work to take what was built in the lab and make the same thing with medical-grade materials, and unfortunately, many researchers don’t realize this until it’s too late.”

And, she notes, “Words matter. For example, if I want to market my new catheter as ‘pain-free,’ the FDA may want me to conduct an expensive clinical trial, but if I take that same catheter and market it as ‘low friction,’ which is why it’s pain-free, then I can demonstrate that with simple bench tests.”

The variety of viewpoints should not be underestimated. “Many stakeholders need their questions answered,” Wilson said. The clinician is only a part of the equation. The hospital supply chain may not be able to handle it. Regulators may not approve it.

“Also, know your competition,” Wilson advised. “Are you more competitive than the current standard of care?”

Yes, we are, microfluidics pioneers can now confirm.

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Kathy Jean Schultz

Kathy Jean Schultz is a freelance medical science writer who focuses on medical innovations. She earned a Master’s Degree in Research Methodology from Hofstra University, and a Master’s Degree in Psychology from Long Island University. She is a member of the National Association of Science Writers, and the Association of Health Care Journalists. Her articles about organoids include "Would you trust a 3-D printed mini organ to test your drugs?" and "Stem cells not only slow disease, they come with their own safety test".

• http://kathyjeanschultz.pressfolios.com/

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This story illustrates how I feel about much of the experimental work that I do and what I see others do in academic and clinical labs. There is an existing infrastructure sometimes far below the cutting edge of science and technology that “works” well enough to test your hypothesis and publish the paper you need for a better job where you can make others deal with the same inconvenient infrastructure. There are many logical reasons for sticking with what works, but, as in the case of the new roundabout, there is a point where superior technology becomes economical.

The tools used to develop therapies for diabetes, in all its forms, also need an upgrade. As with most biomedical research, two complementary platforms are generally used. On the one hand, cultured cells are an inexpensive resource for early-stage high-throughput studies. On the other hand, animals provide a complex, physiological environment to more accurately predict what is happening in the human body. However, the limitations of existing animal and cell culture models are not addressed by simply using both. Microfluidic platforms have natural advantages over both traditional cell culture and animal models. There are of course limitations here as well and these pain points are opportunities for commercial development. The advantages and current limitations of using microfluidic models of human tissues and organs are listed below to encourage their commercial development and adoption for diabetes research.

The Advantages We Need to Exploit

• Organ Modularity and Isolation: When experimenting on animals, it is often challenging to deduce the molecular mechanisms of disease traits due to the complexity of interactions between different organs, tissues, and cells. One can delete or overexpress a gene to demonstrate the respective necessity and sufficiency of that gene for the trait, but in which organ is the expression of that gene important? Tissue-specific genetic manipulation helps address this issue, but it remains challenging for diseases like diabetes where multiple genes operating in multiple organs contribute to pathogenesis. Moreover, many disease-associated genes and cell types are vital (i.e. lethal when deleted) necessitating alternative approaches. Engineered organ models can be connected to build systems such as the “body-on-a-chip” platforms discussed in a previous blog post¹.Microfluidics aid in the logistical difficulty of combining multiple cell types, tissues, and organs as a synthetic vasculature to promote nutrient delivery and waste removal. Relevant organ, tissue, or cellular components can be added as needed and connected in a manner that enables sampling of the inputs and outputs for each component of the system.

• Controlled Human Genetic Background: Gene networks that are most relevant to diabetes such as those involved in immunity and metabolism are also the most evolutionarily distinct between mice, the predominant animal model for diabetes², and humans³. In this regard, an experimental system with human cells is advantageous over mice. Multiple tissues derived from a single pluripotent stem cell line can be used for a completely isogenic model if desired. Alternatively, different genetic backgrounds can be combined to simulate transplantation, chimerism, or other experimental conditions. For example, combining five different organ models made from mutant and control cells from the same stem cell line is simpler than developing constructs for five separate genes each with distinct tissue-specific promoters. In addition, similar to the above point about vital genes, it is also conceivable that a tissue of interest lacks an established tissue-specific gene promoter.

• Data Quality, Automation and Cost Reduction: The temporal resolution of experimental data is often limited by the minimum sample volumes of well-based assays and the capacity of the human user. Microfluidics is naturally amenable to automated sample collection, reagent mixing, and measurement acquisition, reducing user error and manual effort. Microfluidics also have the inherent potential for cost efficiency due to the small amounts of reagents required⁴. Huge cost savings are possible if the supporting instrumentation is made to be simple and self-contained. By minimizing the time, error, effort, and expense of data collection, microfluidics simultaneously enable superior temporal resolution and data quality.

The Limitations We Need to Address

• Not quite in vivo…yet: Although genetic divergence favours a platform with a human genetic background for diabetes research, conventional cell culture falls woefully short of recapitulating important tissue-specific hallmarks of diabetes as well as the systemic nature of the disease. This is the primary criticism and limitation of cell culture and thus a major opportunity for microfluidic organ models. In my opinion, the key moving forward for diabetes research is less about making perfect and interconnected replicas of every organ in the body (which very well could take forever), and more about meeting a set of design criteria that cover the key features and phenotypes underlying specific hypotheses to be tested. The latter is achievable in our lifetime and can close the gap between patients and existing preclinical models.

• Not quite user-friendly …yet: A broadly accessible platform cannot become mainstream without industry’s help and probably its leadership, too. Academic labs simply cannot manufacture enough products with the necessary quality control for all the parties that stand to benefit by using it. Industry involvement will also be needed to achieve the right balance of standardization and flexibility. Utility for a broad range of applications will help the broad adoption of the technology. Finally, product designs need to integrate controls for tissue culture and functional readouts that allow operation without extensive training or expertise.

Conclusion

Diabetes research presents an exciting market opportunity for the development of microfluidic organ models and systems. The powerful advantages afforded by microfluidics are much needed to complement (and perhaps eventually replace) traditional cell culture and animal testing. If designed appropriately for non-expert users and with the right biological questions in mind, these platforms will be adopted and more importantly will help pave the new and improved road to a cure.

References

<sup>1. Organ On Chips: Questions To Address Before They Can Move Into Mainstream Applications. URL: https://ufluidix.com/circle/organ-on-chips-questions-to-address-before-they-can-move-into-mainstream-applications/

2. King A. The use of animal models in diabetes research. British Journal of Pharmacology. 166(3) (2012).

3. Yue F, Cheng Y, Breschi A, et al. A comparative encyclopedia of DNA elements in the mouse genome. Nature. 515(7527) (2014).

4. https://www.fluigent.com/microfluidic-expertise/what-is-microfluidic/microfluidic-definitions-and-advantages/</sup>

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Benjamin D. Pope

Ben Pope works with a group of scientists and engineers who aim to cure diabetes. His current research leverages cellular engineering and genomics to elucidate the molecular bases of human nutrition. Originally trained in molecular biology, he identified DNA elements that define structural and functional chromosome units by both chromosome engineering in embryonic stem cells and computational analysis of genomic datasets. He has since cross-trained in bioengineering and developed a microfluidic chip for continuous sensing of insulin secreted by ex vivo human islets described in a previous blog post.

• https://scholar.harvard.edu/pope

The post Developing Diabetes Therapies with Microfluidic Organ Models appeared first on The MicroFluidic Circle.

In general, PDMS belongs to the silicone family with some unique features which have turned PDMS to the most desirable material in microfluidics. Transparency, biocompatibility, flexibility (Young’s elastic modulus of ~ 1 – 3 MPa), high gas-permeability, low dielectric constant, low surface tension, and low solubility are some of PDMS characteristics.

The PDMS strength stems from its capability in soft lithography. First, PDMS base monomer is thoroughly mixed by its curing agent, then this prepolymer is degassed to remove all air bubbles, and finally, it is ready to be poured on the mold. The mold can be made by conventional methods such as photolithography or newer technologies such as 3D printing. Regardless of the mold type, PDMS can replicate features on the mold from macroscale to nanoscale. After curing and peeling off the PDMS replica from the mold, the PDMS part should be sealed by a flat surface. Here, another advantage of PDMS fabrication comes to play. There are a variety of techniques to seal a PDMS replica such as conformal contact, physical bonding, vacuum bonding, oxygen plasma bonding, corona surface activation, flame bonding, wet-bonding, adhesive bonding, and to name a few. Some of these methods are reversible, and some are irreversible which can be chosen based on applications. In most of these techniques, there is no need to use a chemical or solvent to achieve the sealing. The chemical- or solvent-free bonding of PDMS to PDMS substrates or other substrates eliminates any chance of chemical contamination to tested samples.

PDMS soft lithography allows researchers to fabricate devices with multilayers of PDMS. This process is called “sandwiching,” meaning that several layers of PDMS replica can be stacked on top of each other to build a more complex geometry. In sandwiching, other components such as membranes (porous or non-porous) can be added between layers to create the desired device. There are different ways to bond these membranes to PDMS: (1) membranes first can be treated by a silane molecule (such as 3-aminopropyltriethoxysilane), and then both treated membrane and PDMS are exposed to oxygen plasma and be bonded, (2) some adhesive (double-sided tapes or adhesives) may be used to attach membranes to PDMS if no high pressure is sought, and (3) silicon dioxide (SiO₂) can be coated on a membrane by sputtering and be bonded to PDMS using oxygen plasma.

There are a lot of strategies to tailor the bulk or surface properties of PDMS. This allows researchers to modify their PDMS devices based on their needs. For tailoring bulk properties of PDMS, these methods can be used: (1) changing the ratio of base and curing agent, (2) playing with curing conditions (temperature and time), (3) adding other molecules to PDMS, and (4) adding fillers such as SiO₂ to PDMS. Playing with these parameters can impact the bulk properties of PDMS such as elasticity, transparency, photothermal effect, refractive index, electrical conductivity, and to name a few. The review paper which was written by P. Wolf et al., comprehensively reviews all different techniques for PDMS bulk or surface modification. In most scenarios, surface modification of PDMS is the area of interest in microfluidics. The most common way is to use oxygen or air plasma, corona discharge, and ultraviolet light or ozone exposure to introduce silanol (Si-OH) groups to the surface of PDMS at the cost of methyl groups (Si–CH₃). This results in forming a thin layer of few nanometers on the surface (it may contain some cracks as well) and decreasing water contact angle to lower than 5°. When a long-lasting surface modification strategy is desired, the surface of PDMS can be tailored by polydopamine (PDA), various polyethylene glycol (PEG) derivatives, or silane-based molecules. Hydrophobicity or hydrophilicity of PDMS surfaces can be easily rendered using one of these methods. There is a broad range of strategies and techniques to tailor PDMS properties in the literature which can be considered as one of the advantages of PDMS over other materials for microfabrication.

Tailoring the bulk properties of PDMS to improve its conductivity has become a topic of research in flexible and stretchable electronics. Different fillers such as carbon nanotubes, graphite, silver particles, nanowires, and gold nanotubes have been added to PDMS to reach higher conductivity. As PDA is conductive, it has been coated on PDMS to integrate electrochemical sensors for various applications.

With the origin of organ-on-a-chips and microfluidic cell culture platforms, PDMS has gained more attractions from researchers from other fields. As a result, the surface of PDMS has been coated with PDA, gelatine, collagen, or fibronectin to enhance the adhesion, proliferation, and the growth of cells inside a chip. Besides, the surface of PDMS first can be modified by PEG or a silane molecule or PDAas linker, and then a bioactive molecule can be attached to the linker for achieving specific applications such as differentiation or detection of a secreted biomolecule by cells.

PDMS is permeable to gases such as oxygen and carbon dioxide. This led to the origin of microfluidics blood oxygenators in which gas exchange between blood and air is needed.PDMS membranes can be easily fabricated by spin-coating wet PDMS on a flat substrate such as a wafer. The properties of the PDMS membrane can be tuned by changing the speed of the spinner, curing agent ratio, and curing temperature. PDMS membranes as thin as ~ 1 µm can be produced without observing pinhole defects. When the permeability should be avoided, the thickness of PDMS can be increased over 1 mm.

In a nutshell, PDMS has shown its potential in the realization of so many applications in microfluidics. Microfabrication with PDMS enables us to fabricate devices with simple designs to devices with very complex features. PDMS has been extensively studied, and this provides fantastic resources for everyone to optimize PDMS properties based on their needs.

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Mohammadhossein Dabaghi

Mohammadhossein Dabaghi is currently a postdoctoral fellow at the department of medicine at McMaster University. He completed his Ph.D. at McMaster University majoring in Biomedical Engineering and worked on new microfabrication technologies to develop an Artificial Placenta device for preterm neonates with respiratory failure. He earned a master’s degree in Biomedical/Chemical Engineering (2014), and a BS in Chemical Engineering from Sharif University of Technology, Iran (2012). His research interests focus on lung-on-a-chips, biosensors for organ-on-a-chips, and bioprinting.

• https://www.researchgate.net/profile/Mohammadhossein_Dabaghi2

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