br Limitations Though the use of organ or

Limitations Though the use of organ- or tissue-on-a-chip devices shows a great deal of promise for future applications, it is important to remember that these are relatively new technologies and require further advancements prior to widespread use. It is first necessary to characterize these devices with drugs that have well studies ADMET properties, and to validate the relevance to clinical efficacy and toxicity. Additionally, the devices that are currently being used in the lab are labor intensive to create, and require specialized training to maintain. There are many technical challenges with fabrication processes, issues with bubbles and flow perturbations that can destroy cell cultures, and issues with cell contamination in porous microfluidic devices. The use of a rocker platform or a gravity-driven flow can aid in the elimination of bubbles for some microfluidic platforms, however this solution does not address unidirectional shear flow (Sung et al., 2010; Esch et al., 2015). Additionally, consistent cell seeding can be difficult to achieve within complex channel designs, and the simplified ECMs commonly used in culturing can lead to matrix degradation or contraction. To create a more complex ECM, some groups have used Matrigel, however this ECM is derived from tumor rxr receptor and is highly dissimilar to normal cell ECM and can have a high batch-to-batch variance. In addition to ECM, cell media does not reflect in vivo context, and can also impact cell phenotype (Kolbe et al., 2011). In order for these technologies to transition to industrial and clinical use, it is necessary to create “user friendly”, robust and scalable testing systems. In addition to concerns with fabrication and cell maintenance, there is a consensus among all researchers that no in vitro culture will ever completely represent the complexity of whole animal systems. The feedback mechanisms of extensive interrelationships and crosstalk from multiple cell types and dynamics that modulate physiological processes are currently very difficult to recapitulate in vitro. Additionally, adaptive immune responses, and complex system-level behaviors of the endocrine, skeletal and nervous system have not yet been investigated (Astashkina et al., 2012). Finally, there will always be an issue of systematic, “off target” toxicity. In vitro studies often involve just a few cell or tissue types at most, but often toxicity can crop up in areas of the body that weren\'t necessarily a “target” of the drug being introduced. As a result of these limitations, in vivo animal studies retain the advantage of possibly uncovering such off target responses. However, with differences in animal and human physiology, such results are often not predictive of human response. Using such animal studies to suggest which organ compartments need to be added to the in vitro human model should be valuable. Chip devices need to make many advancements before modeling full tissue or organ level functions, but will have many exciting applications for future drug discovery, and disease modeling and treatment.
Outstanding Questions In addition to the limitations mentioned above, there are a few outstanding questions that must be addressed prior to the widespread use of lab-on-a-chip devices. The first, and most concerning is: Can these devices really mimic organ level functions and interactions? Macro-scale architecture, and micro-scale spatial heterogeneity found in organoids and tissue sections can be very difficult to recapitulate in microfluidic channels, but play large roles in many physiological functions. Additionally, how can we account for the function of organs regulated by humoral, neurogenic, and metabolic factors? With existing organ on a chip technology, these factors can only be fully accounted for in whole-body, in vivo animal models. Once these questions have been addressed, microfluidic-based chip technologies have the potential for extensive breakthroughs in drug discovery, disease modeling and furthering our understanding of organ and tissue interactions within the human body. This novel modeling technology has many interesting applications that are just beginning to be explored. For example, disease modeling has already been highlighted, but further along that route is understanding cancer formation and metastasis. It is currently understood that cancer growth and spread is not only affected by the surrounding chemical environment, but also the surrounding ECM and mechanical factors such as shear stress which can easily be manipulated in microfluidics (Chivukula et al., 2015). Additionally, the study of many rare diseases may be possible. These diseases often have small sample populations for study and therefore suffer from a low availability for in vivo studies. If replicated in vitro, it may be possible to study a much larger population size and gain insight into some of the mechanisms of these diseases. Another interesting application is for stratified medicine—developing a drug for a particular set of the population, rather than a “one size fits all” approach to medical treatments (Trusheim et al., 2007). For example, treatments could be developed based on the presence or absence of particular biomarkers. High throughput testing methods, like the use of microfluidic in vitro models, will assist greatly in the development of these stratified treatments at the pharmaceutical development and testing stages. Additionally, these technologies have applications in predicting appropriate drug doses or administration regimens to achieve desired effects prior to clinical dosing tests. Finally, the study of nervous, endocrine, sensory, and reproductive systems for which we currently lack dynamic models will likely be of focus in the near future.