In oral bioavailability studies, evaluation of the absorption and transport of drugs and food components across the intestinal barrier is crucial. Advances in the field of organ-on-a-chip technology have resulted in a dynamic gut-on-a-chip model that better mimics the in vivo microenvironment of the intestine. Despite a few recent integration attempts, ensuring a biologically relevant microenvironment while coupling with a fully online detection system still represents a major challenge. Herein, we designed an online technique to measure drug permeability and analyse unknown product formation across an intestinal epithelial layer of Caco-2 and HT29-MTX cells cultured on a flow-through Transwell system, while ensuring the quality and relevance of the biological model. Chip-based ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) was coupled to the dynamic Transwell system via a series of switching valves, thus allowing alternating measurements of the apical and basolateral sides of the in vitro model. Two trap columns were integrated for online sample pre-treatment and compatibility enhancement. Temporal analysis of the intestinal permeability was successfully demonstrated using verapamil as a model drug and ergotamine epimers as a model for natural toxins present in foods. Evidence was obtained that our newly developed dynamic system provided reliable results versus classical static in vitro models, and moreover, for the first time, epimer-specific transport is shown for ergotamine. Finally, initial experiments with the drug granisetron suggest that metabolic activity can be studied as well, thus highlighting the versatility of the bio-integrated online analysis system developed.

A novel, integrated, in vitro gastrointestinal (GI) system is presented to study oral bioavailability parameters of small molecules. Three compartments were combined into one hyphenated, flow-through set-up. In the first compartment, a compound was exposed dynamically to enzymatic digestion in three consecutive microreactors, mimicking the processes of the mouth, stomach, and intestine. The resulting solution (chyme) continued to the second compartment, a flow-through barrier model of the intestinal epithelium allowing absorption of the compound and metabolites thereof. The composition of the effluents from the barrier model were analysed either offline by electrospray-ionisation-mass spectrometry (ESI–MS), or online in the final compartment using chip-based ESI–MS. Two model drugs, omeprazole and verapamil, were used to test the integrated model. Omeprazole was shown to be broken down upon treatment with gastric acid, but reached the cell barrier unharmed when introduced to the system in a manner emulating an enteric-coated formulation. In contrast, verapamil was unaffected by digestion. Finally, a reduced uptake of verapamil was observed when verapamil was introduced to the system dissolved in apple juice, a simple food matrix. It is envisaged that this integrated, compartmentalised GI system has potential for enabling future research in the fields of pharmacology, toxicology, and nutrition.

Organs-on-a-chip are used for unravelling biological processes like, homeostasis, metabolism and responses to stimuli. Monitoring the microenvironment is crucial for establishing relevant biological organ-ona-chip models. Online and in situ analysis of organ-on-a-chip systems allows for automated and realtime analysis of biological processes.  Biological integrity needs to be preserved when interfacing organ-on-a-chip models with sensors and high-end instruments.

Organ-on-a-chip technology is used to study biological processes that involve multiple cell types and temporal changes like, homeostasis, metabolism of compounds and responses to chemical triggers. Main benefits of organon-a-chip systems include: improved mimicking of the in vivo situation, easy manipulation of the microenvironment and low reagent consumption. Exploiting the unique dynamic aspects of organ-on-a-chip technology, such as liquid flow, automated online measurement of parameters by sensors or online coupling to analytical equipment becomes feasible. Apart from the challenge to detect drug uptake and chemical changes in real-time with high resolution at the microscale, the biggest challenge, is the detection of the analyte of interest in cell culture medium, as this contains high amounts of salts, sugars and proteins required by the living cells. In this review online and in situ analytical techniques integrated with organ-on-a-chip devices are discussed with special emphasis on maintaining the biological relevance, achieving analytical compatibility, system integration and final applicability.