Gut-on-chip (GOC) platforms are microfluidic devices with micro-chambers and channels to support and control media volumes and flow through a living cell culture generally seeded on membranes that create 2 chambers (lower and upper). This technology mimics the physiological architecture and function as well as the pathological states of human epithelial organs. Several models reproducing human GOC have underlined their potential value as for disease and drug screening modelling.
The Genesis of GOC
Organs-on-chip (OOC) are the future of personalized medicine and pharmaceuticals research. The concept aims at reducing time and costs associated to in vivo and in vitro analyses as in preclinical screening for drug development research. However, in vivo analyses are often limited by ethical issues, and provide results that are usually non reproducible in humans. [1-3] Whereas, in vitro models are static cell cultures that do not mimic many crucial features of the living tissues, thus resulting in low-predictive models. 
An important issue of OOC preparation is the accurate reproduction from the microscopic to the macroscopic features of (a) tissue/organ district(s). The physiological key structure in organs or tissues selected could thus be better investigated, helping to define disease onset and/or development still unexplored to date. 
Given the increasing interest on drug development, discovery and screening, a focus on the mechanisms underlying drug absorption and metabolism is mandatory for the success of future drug testing in humans. And, in drug oral absorption and metabolism, understanding the physiological activity of e.g. the small intestine, with all the issues regarding intestinal epithelial transport processes and factors which affect them, is central. 
The GOC model and the microenvironment parameters
To date, GOC devices in literature are represented by micro-devices, generally adapted from e.g. lung- and heart-on-chip, to mimic the mechanical/structural properties of the intestine.
 Other studies are based on micro-engineered, biomimetic systems containing proper microfluidic channels that simulate the human intestine as an intestinal epithelial monolayer eventually combined with microbial symbionts.  However, these devices imply some disadvantages and open issues, such as the necessity to better reproduce and mimic the in vivo epithelium. The main advantages of these advanced models with respect to static cultures are in the more accurate reproduction of the in vivo environment, by controlling the microfluidics parameters, and the option to operate in continuum, thus reducing the delays and costs of research. 
However, recent microfabrication techniques (that tend to replicate plain functional units of the small intestine) still have major limitations in the inaccurate simulation of the peristalsis dynamics (membrane curvature, stretching, bending) and the lack of a sensing network in situ .
A multifunctional GOC device must detect intertwined parameters that are at the basis of various physiological processes and responses to e.g. nutrients and to drug-based treatments. In situ automated sensing of the intestinal microenvironmental parameters in real time has been projected, in a multifunctional device for continuous detection of e.g. cell adhesion epithelial-type processes. 
According to recent research that highlights the urgent need for more physiologically relevant models of human organs, GOC is presented as an integrated model with both electromechanical and sensing elements, able to reproduce/detect the key structural and functional outputs of a human gastrointestinal (GI) epithelial system.  Figure 1
Micromechanical stimulation sensors are used to mimic the mechanical processes and incorporated electrodes are able to control pH gradients, plasma membrane potentials and solute/ion transepithelial uptake or translocation.
An integrated multiparameter device for personalized medicine
GOC devices are integrating different types of electrodes for the detection in real-time of several parameters. (Fig.1) In recent studies this has been achieved with the utilization of e.g. ITO (indium tin oxide) or gold electrodes (Au) patterned by wet etching technique, located at the two lower/upper interfaces of the cell culture or near the membrane seeded with cells. [9-12] ITO and Au electrodes have shown some important results in the detection of TEER (transepithelial electrical resistance) and of transepithelial transport, in monitoring the membrane-trafficking/exchange processes and nutrient/drug fluxes. In particular, TEER is a widely used experimental readout and quality control assay, such as for measuring the formation and integrity of epithelial monolayers.  This is generally applied to cells cultured under static conditions in vitro, being an important index of intestinal barrier integrity; so, there is a necessity to translate this standard methodology into the microfluidic GOC.  The TEER values extrapolated by software and electronic support platforms are worth to measure the tight junction dynamics of cultured cell monolayers. In some works, it has been demonstrated that a stable and robust electrode set-up revealed TEER values of intestinal monolayers very closed to the traditional static experiments, providing interesting features in terms of real time monitoring. The embedded electrodes have also allowed to a real time optical investigation, given by their low fluorescence emission and higher transparency. So, this provides for qualitative assessing of cell cultures using optical and confocal microscopy. Another important advantage in addition to ITO/Au electrodes is the possibility to induce software-controlled mechanical deflection of the membrane suspended between the two fluidic chambers, both above and below the membrane, and simultaneous measurements by the embedded sensors.  This helps in reproducing local mechanical stretching and bending, comparable to an in vivo situation, monitoring the effects in continuum. 
Finally, there is growing interest in the impact of microbial symbionts on GI health. Only recently, pathogenic bacterial challenges have been developed in in vitro platforms to study how they influence the normal microbiota and intestinal epithelial cells (thus contributing to disease development), and to design possible drug therapies. Considering bacteria and the microbiome, a GOC could help elucidating the integration of GI and immune functions.  Together, all these considerations suggest that a GOC scheme that emulates the dynamic intestinal microenvironment, more faithfully could mimic intestinal function or disorders than previously described by adopting standard in vitro models.
Conclusion and outlook
The GOC strategy provides a better model of the intestine than the previous cell culture systems by dynamically recreating and controlling multiple features of the human organ/tissue. A layout of its active microenvironment as well as the controlled microfluidic environment simulated with a fluid flow may be essential for e.g. transport and absorption of nutrients and/or toxicity studies and allow “more physiological” growth of the microbiome and the possibility to study pathogenic disorders along the axis from the gut to other organs and tissue districts.
Lucia Giampetruzzi1, Amilcare Barca2, Luca Francioso1, Tiziano Verri2, Pietro Siciliano1
1Institute for Microelectronics and
Microsystems IMM-CNR, Lecce, Italy
2Applied Physiology Laboratory’,
(DiSTeBA) , University of Salento, Lecce, Italy
Institute for Microelectronics and
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