Organ-level physiology is recapitulated in vitro by culturing cells in perfused, microfluidic devices.

Microfluidic organs-on-chips

Microengineered cell culture systems are becoming sufficiently sophisticated that they can recapitulate many of the phenomena observed in tissues and organisms. Here, Huh and colleagues discuss the advances made in these 'organs-on-chips' and how they could be used in drug development, including target identification and validation, toxicity screening and stratified medicine.

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Organs-on-chips at the frontiers of drug discovery

https://orbilu.uni.lu/bitstream/10993/21149/1/Advantages%20and%20challenges%20of%20microfluidic%20cell%20culture%20in%20polydimethylsiloxane%20devices.pdf

Advantages and challenges of microfluidic cell culture in polydimethylsiloxane devices.

Microfluidics consist of microfabricated structures for liquid handling, with cross-sections in the 1–500 μm range, and small volume capacity (fL-nL) Capillary tubes connected with fittings,1 although utilizing small volumes, are not considered microfluidics for the purposes of this paper since they are not microfabricated Likewise, millifluidic systems, made by conventional machining tools, are excluded due to their larger feature sizes (>500 μm)

Though micromachined systems for gas chromatography were introduced in the 1970’s,2 the field of microfluidics did not gain much traction until the 1990’s3 Silicon and glass were the original materials used, but then the focus shifted to include polymer substrates, and in particular, polydimethylsiloxane (PDMS) Since then the field has grown to encompass a wide variety of materials and applications The successful demonstration of electrophoresis and electroosmotic pumping in a microfluidic device provided a nonmechanical method for both fluid control and separation4 Laser induced fluorescence (LIF) enabled sensitive detection of fluorophores or fluorescently labeled molecules The expanded availability of low-cost printing allowed for cheaper and quicker mask fabrication for use in soft lithography5 Commercial microfluidic systems are now available from Abbott, Agilent, Caliper, Dolomite, Micralyne, Microfluidic Chip Shop, Micrux Technologies and Waters, as a few prominent examples For a more thorough description of the history of microfluidics, we refer the reader to a number of comprehensive, specialized reviews,3, 6–11 as well as a more general 2006 review12

The field of microfluidics offers many advantages compared to carrying out processes through bulk solution chemistry, the first of which relates to a lesson taught to every first-year chemistry student Simply stated, diffusion is slow! Thus, the smaller the distance required for interaction, the faster it will be Smaller channel dimensions also lead to smaller sample volumes (fL-nL), which can reduce the amount of sample or reagents required for testing and analysis Reduced dimensions can also lead to portable devices to enable on-site testing (provided the associated hardware is similarly portable) Finally, integration of multiple processes (like labeling, purification, separation and detection) in a microfluidic device can be highly enabling for many applications

Microelectromechanical systems (MEMS) contain integrated electrical and mechanical parts that create a sensor or system Applications of MEMS are ubiquitous, including automobiles, phones, video games and medical and biological sensors13 Micro-total analysis systems, also known as labs-on-a-chip, are the chemical analogue of MEMS, as integrated microfluidic devices that are capable of automating multiple processes relevant to laboratory sciences For example, a typical lab-on-a-chip system might selectively purify a complex mixture (through filtering, antibody capture, etc), then separate target components and detect them

Microfluidic devices consist of a core of common components Areas defined by empty space, such as reservoirs (wells), chambers and microchannels, are central to microfluidic systems Positive features, created by areas of solid material, add increased functionality to a chip and can consist of membranes, monoliths, pneumatic controls, beams and pillars Given the ubiquitous nature of negative components, and microchannels in particular, we focus here on a few of their properties Microfluidic channels have small overall volumes, laminar flow and a large surface-to-volume ratio Dimensions of a typical separation channel in microchip electrophoresis (μCE) are: 50 μm width, 15 μm height and 5 cm length for a volume of 375 nL Flow in these devices is normally nonturbulent due to low Reynolds numbers For example, water flowing at 20°C in the above channel at 1 μL/min (222 cm/s) results in a Reynolds number of ~05, where <2000 is laminar flow Since flow is nonturbulent, mixing is normally diffusion-limited Small channel sizes also have a high surface-to-volume ratio, leading to different characteristics from what are commonly found in bulk volumes The material surface can be used to manipulate fluid movement (such as by electroosmotic flow, EOF) and surface interactions For a solution in contact with a charged surface, a double layer of charge is created as oppositely charged ions are attracted to the surface charges This electrical double layer consists of an inner rigid or Stern Layer and an outer diffuse layer An electrostatic potential known as the zeta potential is formed, with the magnitude of the potential decreasing as distance from the surface increases The electrical double layer is the basis for EOF, wherein an applied voltage causes the loosely bound diffuse layer to move towards an electrode, dragging the bulk solution along Charges on the exposed surface also exert a greater influence on the fluid in a channel as its size decreases Larger surface-to-volume ratios are more prone to nonspecific adsorption and surface fouling In particular, non-charged and hydrophobic microdevice surfaces can cause proteins in solution to denature and stick

We focus our review on advances in microfluidic systems since 2008 In doing this, we occasionally must cover foundational work in microfluidics that is considerably less recent We do not focus on chemical synthesis applications of microfluidics although it is an expanding area, nor do we delve into lithography, device fabrication or production costs Our specific emphasis herein is on four areas within microfluidics: properties and applications of commonly used materials, basic functions, integration, and selected applications For each of these four topics we provide a concluding section on opportunities for future development, and at the end of this review, we offer general conclusions and prospective for future work in the field Due to the considerable scope of the field of microfluidics, we limit our discussion to selected examples from each area, but cite in-depth reviews for the reader to turn to for further information about specific topics We also refer the reader to recent comprehensive reviews on advances in lab-on-a-chip systems by Arora et al10 and Kovarik et al14

/pdf/advances-in-microfluidic-materials-functions-integration-and-27mmao3n1o.pdf

Advances in microfluidic materials, functions, integration, and applications.

The past decades have witnessed significant efforts toward the development of three-dimensional (3D) cell cultures as systems that better mimic in vivo physiology. Today, 3D cell cultures are emerging, not only as a new tool in early drug discovery but also as potential therapeutics to treat disease. In this review, we assess leading 3D cell culture technologies and their impact on drug discovery, including spheroids, organoids, scaffolds, hydrogels, organs-on-chips, and 3D bioprinting. We also discuss the implementation of these technologies in compound identification, screening, and development, ranging from disease modeling to assessment of efficacy and safety profiles.

https://journals.sagepub.com/doi/pdf/10.1177/1087057117696795

Three-Dimensional Cell Cultures in Drug Discovery and Development.

To avoid interspecies extrapolation in toxicokinetics and drug development, it is convenient to directly develop human data. In that case, exposure dose should pose null or negligible risk to the exposed individual, but still be sufficiently high to allow quantification. We propose to reduce the dose received by human volunteers during exposure, and to compensate for loss of information by exposing the same volunteers to a nontoxic agent. This method was applied to develop 1,3-butadiene (BD) exposure protocols for humans. To study the potential of such a procedure, we worked with simulated data. Three exposure times (20, 10, and 5 minutes) and four exposure concentrations (2, 1, 0.8, and 0.5 ppm) were used to define 12 inhalation exposure scenarios for BD. Isoflurane was used as a probe, with simulated exposure of 20 subjects to 20 ppm isoflurane during 15 minutes. Isoflurane or BD-exhaled air concentrations were supposed to be measured 10 times. A three-compartment physiological toxicokinetic model was used to jointly describe BD and isoflurane data. For each subject, BD data were analyzed, in a Bayesian framework, either alone or together with the isoflurane data. The precision of BD metabolic rate constant or fraction metabolized was increased, and bias reduced, when BD and probe data were considered jointly. An exposure to 10 ppm × min BD and 300 ppm × min isoflurane gave equivalent precision and bias as a unique exposure to 40 ppm × min BD. The BD dose received by volunteers could therefore be at least quartered if BD exposure was supplemented with that of a probe.

Use of a chemical probe to increase safety for human volunteers in toxicokinetic studies.

L'evaluation de la toxicite des produits chimiques passe par une etude de leur toxicocinetique. Cette derniere vise a etablir un lien entre l'exposition a une substance toxique et la quantite de celle-ci atteignant les tissus cibles de l'action toxique. Pour ce faire, l'experimentation recueille des donnees de distribution spatiale et temporelle de la substance dans l'organisme etudie (par exemple, des mesures de concentration sanguine en differents instants). Il est ensuite possible de construire des modeles mathematiques parametres (modeles toxicocinetiques) pour caracteriser la relation entre l'exposition et la quantite interne de toxique. Ces modeles sont generalement des modeles compartimentaux et reposent frequemment sur une description physiologique de l'organisme (modele PBPK). Des traitements statistiques appropries permettent d'ajuster ces modeles a l'aide des donnees experimentales. Nous decrivons comment l'analyse bayesienne permet de prendre en compte et d'estimer l'incertitude et la variabilite inherentes aux donnees toxicocinetiques. Elle integre l'information disponible a priori sur la valeur des parametres, limitant, de ce fait, le nombre d'expositions experimentales necessaires. Le processus bayesien d'analyse de donnees comprend l'estimation des parametres (calibration), la verification et la validation des modeles. Ce processus peut ensuite etre complete par le choix entre plusieurs modeles construits a partir d'hypotheses differentes ou l'optimisation de nouveaux protocoles experimentaux. Une illustration de l'approche bayesienne est proposee pour le cas du 1,3-butadiene, substance chimique consideree comme potentiellement carcinogene pour l'homme.

https://hal-ineris.archives-ouvertes.fr/ineris-00961895/document

L’analyse statistique bayésienne de données toxicocinétiques

Estimating human exposure to pyrethroids' mixtures from biomonitoring data using physiologically based pharmacokinetic modeling.

Assessing the impacts on fetal dosimetry of the modelling of the placental transfers of xenobiotics in a pregnancy physiologically based pharmacokinetic model

Phthalates are known reproductive and developmental toxicants in animal studies which suggest that such effects could occur for humans. There are few data on exposure of pregnant women, although pregnancy is a key period relative to developmental toxicity. This study reports the first assessment of exposure to phthalates in the French pregnant population. Several phthalates metabolites (MEP, 5OH-MEHP, 5oxo-MEHP, 5cx-MEHP, 2cx-MEHP, MEHP, MnBP, MiBP, MiNP, 7oxo-Summe-MeOP, 7OH-MeOP, 7cx-MeOP, MBzBP, MnOP, MCPP, MCHP) were measured in 279 urine samples of pregnant women from ELFE pilot study. The purpose of the ELFE (Etude Longitudinale Francaise depuis l'Enfance) project is to build a nationally representative cohort of 20,000 children to be followed from birth to adulthood using a multidisciplinary approach in order to characterise the relationship between environmental exposures and the socio-economic context on health and behaviours more thoroughly. A pilot study was carried out to validate the data collection methods in October 2007 in Seine Saint Denis county and Rhone Alpes region. The measured concentrations in urine of most of the metabolites were similar to other European or American studies on pregnant women. Calculations of the daily intake using the urinary concentrations were performed for the DEP, DEHP, DnBP, DiBP and showed that median exposure is below RfD (US EPA, 2007) and TDI values (EFSA, 2005) . A simple kinetics model was used to better characterize the exposure to DEHP.

Céline Brochot

Papers

Use of a chemical probe to increase safety for human volunteers in toxicokinetic studies.

L’analyse statistique bayésienne de données toxicocinétiques

Estimating human exposure to pyrethroids' mixtures from biomonitoring data using physiologically based pharmacokinetic modeling.

Assessing the impacts on fetal dosimetry of the modelling of the placental transfers of xenobiotics in a pregnancy physiologically based pharmacokinetic model

Exposure assessment of phthalates in French pregnant women : Results of ELFE pilot study