User-defined local stimulation of live tissue through a movable microfluidic port

Atmospheric Pressure Enhanced Self-Sealing Rotation-SlipChip with Programmable Concentration Gradient Generation for Microbiological Applications

In microbiological research, traditional methods for bacterial screening and antibiotic susceptibility testing are resource-intensive. Microfluidics offers an efficient alternative with rapid results and minimal sample consumption, but the demand for cost-effective, user-friendly platforms persists in communities and hospitals. Inspired by the Magdeburg hemispheres, the strategy adapts to local conditions, leveraging omnipresent atmospheric pressure for self-sealing of Rotation-SlipChip (RSC) equipped with a 3D circular Christmas tree-like microfluidic concentration gradient generator. This innovative approach provides an accessible and adaptable platform for microbiological research and testing in diverse settings. The RSC can avoid leakage concerns during multiple concentration gradient generation, chip-rotating, and final long-term incubation reaction (≥24 h). Furtherly, RSC subtypes adapted to different reactions can be fabricated in less than 15 min with cost less than $1, the result can be read through designated observational windows by naked-eye. Moreover, the RSC demonstrates its capability for evaluating bacterial biomarker activity, enabling the rapid assessment of β-galactosidase concentration and enzyme activity within 30 min, and the limit of detection can be reduced by 10-fold. It also rapidly determines the minimum antibiotic inhibitory concentration and antibiotic combined medications results within 4 h. Overall, these low-cost and user-friendly RSC make them invaluable tools in determinations at previously impractical environment.

Sustained delivery of focal ischemia coupled to real-time neurochemical sensing in brain slices

Local stimulation of tissue can occur naturally in events like immune-mediated inflammation and focal ischemic injuries in brain and is confined to specific regions within tissue, occurring on various timescales. Making chemical measurements at the exact site of stimulation with current technologies is difficult yet important for understanding tissue response. We have developed a microfluidic device capable of local stimulation of brain slices with minimal lateral spread over time and submillimeter, tunable spatial resolution. This device is compatible with electrochemical measurements to monitor signaling at the site of stimulation over time. The PDMS-based device is three layers and contains a culture well, channel layer, and exit port layer for the channels. Channels with exit ports straddling the stimulus channels and ports were specifically fabricated to focus the stimulus over time. We demonstrated that the device is compatible with fast-scan cyclic voltammetry (FSCV) recording of neurotransmitter release. Localized hypoxia in tissue was verified using Image-iT Green Hypoxia Reagent and coupling this device with FSCV enabled measurement of local dopamine changes at the site of focal ischemia for the first time. This work provides a significant advance in knowledge of local neurochemical fluctuations during sustained tissue injury. Overall, the unique capabilities of the device to deliver sustained localized stimulation combined with real-time sensing provide an innovative platform to answer significant biological questions about how tissues respond at the site of controlled, localized injury and chemical stimulation.

Rapid Fabrication by Digital Light Processing 3D Printing of a SlipChip with Movable Ports for Local Delivery to Ex Vivo Organ Cultures

SlipChips are two-part microfluidic devices that can be reconfigured to change fluidic pathways for a wide range of functions, including tissue stimulation. Currently, fabrication of these devices at the prototype stage requires a skilled microfluidic technician, e.g., for wet etching or alignment steps. In most cases, SlipChip functionality requires an optically clear, smooth, and flat surface that is fluorophilic and hydrophobic. Here, we tested digital light processing (DLP) 3D printing, which is rapid, reproducible, and easily shared, as a solution for fabrication of SlipChips at the prototype stage. As a case study, we sought to fabricate a SlipChip intended for local delivery to live tissue slices through a movable microfluidic port. The device was comprised of two multi-layer components: an enclosed channel with a delivery port and a culture chamber for tissue slices with a permeable support. Once the design was optimized, we demonstrated its function by locally delivering a chemical probe to slices of hydrogel and to living tissue with up to 120 µm spatial resolution. By establishing the design principles for 3D printing of SlipChip devices, this work will enhance the ability to rapidly prototype such devices at mid-scale levels of production.

Modeling Immunity In Vitro: Slices, Chips, and Engineered Tissues

Modeling immunity in vitro has the potential to be a powerful tool for investigating fundamental biological questions, informing therapeutics and vaccines, and providing new insight into disease progression. There are two major elements to immunity that are necessary to model: primary immune tissues and peripheral tissues with immune components. Here, we systematically review progress made along three strategies to modeling immunity: ex vivo cultures, which preserve native tissue structure; microfluidic devices, which constitute a versatile approach to providing physiologically relevant fluid flow and environmental control; and engineered tissues, which provide precise control of the 3D microenvironment and biophysical cues. While many models focus on disease modeling, more primary immune tissue models are necessary to advance the field. Moving forward, we anticipate that the expansion of patient-specific models may inform why immunity varies from patient to patient and allow for the rapid comprehension and treatment of emerging diseases, such as coronavirus disease 2019.

Selective Fluorination of the Surface of Polymeric Materials after Stereolithography 3D Printing

With the microfluidics community embracing 3D resin printing as a rapid fabrication method, controlling surface chemistry has emerged as a new challenge. Fluorination of 3D-printed surfaces is highly desirable in many applications due to chemical inertness, low friction coefficients, antifouling properties, and the potential for selective hydrophobic patterning. Despite sporadic reports, silanization methods have not been optimized for covalent bonding with polymeric resins. As a case study, we tested the silanization of a commercially available (meth)acrylate-based resin (BV-007A) with a fluoroalkyl trichlorosilane. Interestingly, plasma oxidation was unnecessary for silanization of this resin and indeed was ineffective. Solvent-based deposition in a fluorinated oil (FC-40) generated significantly higher contact angles than deposition in ethanol or gas-phase deposition, yielding hydrophobic surfaces with contact angle >110° under optimized conditions. Attenuated total reflectance-Fourier transform infrared spectroscopy indicated that the increase in the contact angle correlated with consumption of a carbonyl moiety, suggesting covalent bonding of silane without plasma oxidation. Consistent with a covalent bond, silanization was resistant to mechanical damage and hydrolysis in methanol and was stable over long-term storage. When tested on a suite of photocrosslinkable resins, this silanization protocol generated highly hydrophobic surfaces (contact angle > 110°) on three resins and moderate hydrophobicity (90-100°) on the remainder. Selective patterning of hydrophobic regions in an open 3D-printed microchannel was possible in combination with simple masking techniques. Thus, this facile fluorination strategy is expected to be applicable for resin-printed materials in a variety of contexts including micropatterning and multiphase microfluidics.

Microphysiological models of the central nervous system with fluid flow

There are over 1,000 described neurological and neurodegenerative disorders affecting nearly 100 million Americans - roughly one third of the U.S. population. Collectively, treatment of neurological conditions is estimated to cost $800 billion every year. Lowering this societal burden will require developing better model systems in which to study these diverse disorders. Microphysiological systems are promising tools for modeling healthy and diseased neural tissues to study mechanisms and treatment of neuropathology. One major benefit of microphysiological systems is the ability to incorporate biophysical forces, namely the forces derived from biological fluid flow. Fluid flow in the central nervous system (CNS) is a complex but important element of physiology, and pathologies as diverse as traumatic or ischemic injury, cancer, neurodegenerative disease, and natural aging have all been found to alter flow pathways. In this review, we summarize recent advances in three-dimensional microphysiological systems for studying the biology and therapy of CNS disorders and highlight the ability and growing need to incorporate biological fluid flow in these miniaturized model systems.

Acute Lymph Node Slices Are a Functional Model System to Study Immunity Ex Vivo

The lymph node is a highly organized and dynamic structure that is critical for facilitating the intercellular interactions that constitute adaptive immunity. Most ex vivo studies of the lymph node begin by reducing it to a cell suspension, thus losing the spatial organization, or fixing it, thus losing the ability to make repeated measurements. Live murine lymph node tissue slices offer the potential to retain spatial complexity and dynamic accessibility, but their viability, level of immune activation, and retention of antigen-specific functions have not been validated. Here we systematically characterized live murine lymph node slices as a platform to study immunity. Live lymph node slices maintained the expected spatial organization and cell populations while reflecting the 3D spatial complexity of the organ. Slices collected under optimized conditions were comparable to cell suspensions in terms of both 24-h viability and inflammation. Slices responded to T cell receptor cross-linking with increased surface marker expression and cytokine secretion, in some cases more strongly than matched lymphocyte cultures. Furthermore, slices processed protein antigens, and slices from vaccinated animals responded to ex vivo challenge with antigen-specific cytokine secretion. In summary, lymph node slices provide a versatile platform to investigate immune functions in spatially organized tissue, enabling well-defined stimulation, time-course analysis, and parallel read-outs.

Labelling primary immune cells using bright blue fluorescent nanoparticles

Tracking cell movements is an important aspect of many biological studies. Reagents for cell tracking must not alter the biological state of the cell and must be bright enough to be visualized above background autofluorescence, a particular concern when imaging in tissue. Currently there are few reagents compatible with standard UV excitation filter sets (e.g. DAPI) that fulfill those requirements, despite the development of many dyes optimized for violet excitation (405 nm). A family of boron-based fluorescent dyes, difluoroboron β-diketonates, has previously served as bio-imaging reagents with UV excitation, offering high quantum yields and wide excitation peaks. In this study, we investigated the use of one such dye as a potential cell tracking reagent. A library of difluoroboron dibenzoylmethane (BF2dbm) conjugates were synthesized with biocompatible polymers including: poly(l-lactic acid) (PLLA), poly(ε-caprolactone) (PCL), and block copolymers with poly(ethylene glycol) (PEG). Dye-polymer conjugates were fabricated into nanoparticles, which were stable for a week at 37 °C in water and cell culture media, but quickly aggregated in saline. Nanoparticles were used to label primary splenocytes; phagocytic cell types were more effectively labelled. Labelling with nanoparticles did not affect cellular viability, nor basic immune responses. Labelled cells were more easily distinguished when imaged on a live tissue background than those labelled with a commercially available UV-excitable cytoplasmic labelling reagent. The high efficiency in terms of both fluorescence and cellular labelling may allow these nanoparticles to act as a short-term cell labelling strategy while wide excitation peaks offer utility across imaging and analysis platforms.

Application of an open-chamber multi-channel microfluidic device to test chemotherapy drugs

The use of precision medicine for chemotherapy requires the individualization of the therapeutic regimen for each patient. This approach improves treatment efficacy and reduces the probability of administering ineffective drugs. To ensure accurate decision-making in a timely manner, anticancer drug efficacy tests must be performed within a short timeframe using a small number of cancer cells. These requirements can be satisfied via microfluidics-based drug screening platforms, which are composed of complex fluidic channels and closed systems. Owing to their complexity, skilled manipulation is required. In this study, we developed a microfluidic platform, to accurately perform multiple drug efficacy tests using a small number of cells, which can be conducted via simple manipulation. As it is a small, open-chamber system, a minimal number of cells could be loaded through simple pipetting. Furthermore, the extracellular matrix gel inside the chamber provides an in vivo-like environment that enables the localized delivery of the drugs to spontaneously diffuse from the channels underneath the chamber without a pump, thereby efficiently and robustly testing the efficacy and resistance of multiple drugs. We demonstrated that this platform enabled the rapid and facile testing of multiple drugs using a small number of cells (~ 10,000) over a short period of time (~ 2 days). These results provide the possibility of using this powerful platform for selecting therapeutic medication, developing new drugs, and delivering personalized medicine to patients.

Significant Unresolved Questions and Opportunities for Bioengineering in Understanding and Treating COVID-19 Disease Progression

COVID-19 is a disease that manifests itself in a multitude of ways across a wide range of tissues. Many factors are involved, and though impressive strides have been made in studying this novel disease in a very short time, there is still a great deal that is unknown about how the virus functions. Clinical data has been crucial for providing information on COVID-19 progression and determining risk factors. However, the mechanisms leading to the multi-tissue pathology are yet to be fully established. Although insights from SARS-CoV-1 and MERS-CoV have been valuable, it is clear that SARS-CoV-2 is different and merits its own extensive studies. In this review, we highlight unresolved questions surrounding this virus including the temporal immune dynamics, infection of non-pulmonary tissue, early life exposure, and the role of circadian rhythms. Risk factors such as sex and exposure to pollutants are also explored followed by a discussion of ways in which bioengineering approaches can be employed to help understand COVID-19. The use of sophisticated in vitro models can be employed to interrogate intercellular interactions and also to tease apart effects of the virus itself from the resulting immune response. Additionally, spatiotemporal information can be gleaned from these models to learn more about the dynamics of the virus and COVID-19 progression. Application of advanced tissue and organ system models into COVID-19 research can result in more nuanced insight into the mechanisms underlying this condition and elucidate strategies to combat its effects.

Monitoring tissue-level remodelling during inflammatory arthritis using a three-dimensional synovium-on-a-chip with non-invasive light scattering biosensing

Rheumatoid arthritis is a chronic, systemic joint disease in which an autoimmune response translates into an inflammatory attack resulting in joint damage, disability and decreased quality of life. Despite recent introduction of therapeutic agents such as anti-TNFα, even the best current therapies fail to achieve disease remission in most arthritis patients. Therefore, research into the mechanisms governing the destructive inflammatory process in rheumatoid arthritis is of great importance and may reveal novel strategies for the therapeutic interventions. To gain deeper insight into its pathogensis, we have developed for the first time a three-dimensional synovium-on-a-chip system in order to monitor the onset and progression of inflammatory synovial tissue responses. In our study, patient-derived primary synovial organoids are cultivated on a single chip platform containing embedded organic-photodetector arrays for over a week in the absence and presence of tumor-necrosis-factor. Using a label-free and non-invasive optical light-scatter biosensing strategy inflammation-induced 3D tissue-level architectural changes were already detected after two days. We demonstrate that the integration of complex human synovial organ cultures in a lab-on-a-chip provides reproducible and reliable information on how systemic stress factors affect synovial tissue architectures.

Microfluidics for interrogating live intact tissues

The intricate microarchitecture of tissues - the "tissue microenvironment" - is a strong determinant of tissue function. Microfluidics offers an invaluable tool to precisely stimulate, manipulate, and analyze the tissue microenvironment in live tissues and engineer mass transport around and into small tissue volumes. Such control is critical in clinical studies, especially where tissue samples are scarce, in analytical sensors, where testing smaller amounts of analytes results in faster, more portable sensors, and in biological experiments, where accurate control of the cellular microenvironment is needed. Microfluidics also provides inexpensive multiplexing strategies to address the pressing need to test large quantities of drugs and reagents on a single biopsy specimen, increasing testing accuracy, relevance, and speed while reducing overall diagnostic cost. Here, we review the use of microfluidics to study the physiology and pathophysiology of intact live tissues at sub-millimeter scales. We categorize uses as either in vitro studies - where a piece of an organism must be excised and introduced into the microfluidic device - or in vivo studies - where whole organisms are small enough to be introduced into microchannels or where a microfluidic device is interfaced with a live tissue surface (e.g. the skin or inside an internal organ or tumor) that forms part of an animal larger than the device. These microfluidic systems promise to deliver functional measurements obtained directly on intact tissue - such as the response of tissue to drugs or the analysis of tissue secretions - that cannot be obtained otherwise.

Two-way communication between ex vivo tissues on a microfluidic chip: application to tumor-lymph node interaction

Experimentally accessible tools to replicate the complex biological events of in vivo organs offer the potential to reveal mechanisms of disease and potential routes to therapy. In particular, models of inter-organ communication are emerging as the next essential step towards creating a body-on-a-chip, and may be particularly useful for poorly understood processes such as tumor immunity. In this paper, we report the first multi-compartment microfluidic chip that continuously recirculates a small volume of media through two ex vivo tissue samples to support inter-organ cross-talk via secreted factors. To test on-chip communication, protein release and capture were quantified using well-defined artificial tissue samples and model proteins. Proteins released by one sample were transferred to the downstream reservoir and detectable in the downstream sample. Next, the chip was applied to model the communication between a tumor and a lymph node, to test whether on-chip dual-organ culture could recreate key features of tumor-induced immune suppression. Slices of murine lymph node were co-cultured with tumor or healthy tissue on-chip with recirculating media, then tested for their ability to respond to T cell stimulation. Interestingly, lymph node slices co-cultured with tumor slices appeared more immunosuppressed than those co-cultured with healthy tissue, suggesting that the chip may successfully model some features of tumor-immune interaction. In conclusion, this new microfluidic system provides on-chip co-culture of pairs of tissue slices under continuous recirculating flow, and has the potential to model complex inter-organ communication ex vivo with full experimental accessibility of the tissues and their media.