Cell culture and animal model systems are essential for preclinical drug testing. However, the lack of complexity in conventional cell cultures, together with the time-consuming nature of animal studies, limit their predictability of clinical response and exacerbate the high cost of drug discovery and development. Microphysiological systems (MPS), which include OOC technology [

], are defined as complex, multicellular (and now multi-organ), in vitro culture systems that mimic the physiological properties of a specific organ or tissue that govern function [

]. Several pivotal MPS studies have expanded our understanding of biological systems with respect to disease treatment and prevention, and give promise to OOC technology that bridges the gap between in vitro experiments and animal studies [

,

]. A landmark 2010 study by Huh and colleagues described an OOC model of the alveolar–capillary interface of the human lung that demonstrated lung physiology in vitro [

]. More than a decade following this foundational work, a study by Ronaldson-Bourchard and colleagues demonstrated how multiple organs can be integrated to study disease progression and drug toxicities systemically [

], highlighting the exciting potential of OOC models in drug development. We describe here advances in OOC technology over the past decade by spotlighting the work of two key opinion leaders, Donald Ingber [

] and Gordana Vunjak-Novakovic [

], in a ‘then and now’ synopsis. These studies add layers of complexity to OOC technology in faithfully recapitulating structure and function both at the organ level, by incorporating biophysics/motion, and at the systems level by interconnecting multiple organs through recirculating vascular flow (Figure 1).

An explosion of OOC designs across academic labs and spin-off companies [

] occurred during the 2010s, prompting efforts to translate a proof-of-principle idea into a drug development reality. OOC technologies matured from recapitulating single organs to linking multiple organ tissues [

] to recreate a human ‘body-on-a-chip’, which was accelerated by Defense Advanced Research Projects Agency (DARPA) and National Institutes of Health (NIH) funding initiatives awarded to several pioneers in the field (including Ingber). In 2022, researchers led by Vunjak-Novakovic published a multi-tissue chip comprising human heart, liver, bone, and skin organs connected by vascular flow [

]. Each tissue niche maintained its unique environment, with supporting stromal cells, a selectively permeable endothelial barrier, and vascular flow containing circulating immune cells, cytokines, and extracellular vesicles. Ronaldson-Bouchard and coworkers showed that their multi-tissue chip engineered from human iPSCs supported crosstalk between organs by detecting heart tissue-derived exosomes in other organs and CD14⁺ monocytes that infiltrated the injured tissue. Importantly, the authors showed that selective infiltration of CD14⁺ cells to the injured organ only occurred when the endothelial barrier was intact. Furthermore, proteomic analyses revealed that the multi-tissue chip with a functional endothelial barrier most closely matched human tissue. The authors then performed pharmacokinetic/pharmacodynamic (PK/PD) studies of doxorubicin treatment and analyzed toxicity. They reported that miR-1 (a known biomarker for predicting doxorubicin-induced cardiotoxicity) was significantly upregulated in the multi-tissue chip. In addition, the expression of early miRNA biomarkers of doxorubicin cardiomyopathy in the multi-tissue chip most closely matched the patient data, as compared to each tissue individually or the multi-tissue chip without the endothelial barrier, highlighting the potential of these systems for improved testing of therapies and identification of early biomarkers. It is important to note that the significance of the endothelial barrier in recapitulating physiological tissue responses was not fully realized in 2010. Biological and engineering advances over a decade have resulted in an appreciation for designing systems today that maintain media and culture environments for proper tissue maturation to reproduce in vivo functional responses.

Back in 2010, Huh and colleagues discussed the possibility that biomimetic microsystems could replace animal testing, which was an ambitious prospect at the time. Although there has been significant progress toward this goal, considerable work remains for the OOC field to transform the drug discovery pipeline, including improvements in areas such as cell sourcing, platform fabrication, integration across organs, orthogonal assay development, and cross-comparison of animal, tissue chip, and human data. These factors need to be validated and standardized, together with clear establishment of the context of use, in partnership with regulatory bodies [

]. In December 2022 a major step toward dramatically reducing animal testing occurred with the passing of the FDA Modernization Act 2.0. This legislation eliminates the federal mandate of animal testing for any new FDA-approved drugs, which has been in effect since 1938, and comes with financial support for an FDA-wide New Alternative Methods Program. Several organizations (e.g., the US Health and Environmental Sciences Institute and the UK National Centre for Replacement, Reduction, and Refinement) have been working with the FDA to expand the use of MPS for drug testing. In addition, companies such as Emulate Inc., which started out of the group of Don Ingber shortly after the 2010 article, raised considerable venture capital funding to commercialize OOC technology and demonstrate translational relevance [

]. We foresee that the next decade of OOCs will break the mold of traditional drug testing and improve assessments of drug efficacy and safety.