Engineering challenges for instrumenting and controlling integrated organ-on-chip systems

Abstract

The sophistication and success of recently reported microfabricated organs-on-chips and human organ constructs have made it possible to design scaled and interconnected organ systems that may significantly augment the current drug development pipeline and lead to advances in systems biology. Physiologically realistic live microHuman (μHu) and milliHuman (mHu) systems operating for weeks to months present exciting and important engineering challenges such as determining the appropriate size for each organ to ensure appropriate relative organ functional activity, achieving appropriate cell density, providing the requisite universal perfusion media, sensing the breadth of physiological responses, and maintaining stable control of the entire system, while maintaining fluid scaling that consists of ~5 mL for the mHu and ~5 μL for the μHu. We believe that successful mHu and μHu systems for drug development and systems biology will require low-volume microdevices that support chemical signaling, microfabricated pumps, valves and microformulators, automated optical microscopy, electrochemical sensors for rapid metabolic assessment, ion mobility-mass spectrometry for real-time molecular analysis, advanced bioinformatics, and machine learning algorithms for automated model inference and integrated electronic control. Toward this goal, we are building functional prototype components and are working toward top-down system integration.

Fig. 1

Schematic representation of how multiple OoC devices might be interconnected to form an instrumented mHu system.

Fig. 2

Conceptual drawing of an organ-chip cartridge that includes a PC and µCA for both the microvascular and interstitial compartments of a single OoC or HoC. On the left are the inputs to the organ, and on the right are outputs, including an online MS if desired. The vertical arrows access the secondary space of the organ (interstitial, bronchial, biliary, urinary, etc.). Interconnected cartridges would comprise an instrumented mHu or µHu system, either in series as shown for selected organs or in parallel as in Fig. 1.

Fig. 3

Dynamic electrochemical measurements in microbioreactors. (a) Core carbon metabolism and agents that affect it. (b) Modified Cytosensor™ sensor head for measuring glucose, lactate, and oxygen concentrations for ~10⁵ cells in a ~3µL chamber [37]. (c) Screen-printed electrode for the same measurements. Adapted from [38]. (d) Demonstration of how dynamic metabolic measurements can discriminate between different biological toxins. Adapted from [39].

Fig. 4

Ion mobility-mass spectrometry (IM-MS). (a) Configuration of an IM-MS with GC or ultraperformance liquid chromatography (UPLC) preseparation. (b) Use of gas-phase electrophoresis for rapid separation of molecules by their collision cross-section in low-pressure inert gas. (c) Conceptual ordering of molecules in 2-D IM-MS conformational space. The vertical axis is relative on drift time (µs) or collision cross-section (molecular surface area, Ų) and the horizontal axis is mass-to-charge ratio (m/z). (d) Empirical data demonstrating chemical class specific ordering by a single pass through an IM-MS. (e) 4-D dataset representing ca. 100 000 molecular signals. The entire dataset represents one time point sample from the microfluidic device. Each voxel encodes molecule-specific data of 1) UPLC retention time, 2) IM cross section, 3) MS mass-to-charge, and 4) relative abundance. These data occupy only 10⁻⁶ of the instrument phase space. (f) Self-organizing IM-MS heat maps of metabolites in biological system state space. This type of analysis is also ideal for time-series measurements. Panels (a) and (b) are adapted with kind permission from Springer Science+Business Media: Anal. Bioanal. Chem., Biomolecular structural separations by ion mobility-mass spectrometry, 391, 2008, 906, L. S. Fenn and J. A. McLean, Fig. 1(a) and (b). Panel (c) is adapted with kind permission from Springer Science+Business Media: Anal. Bioanal. Chem., Biomolecular structural separations by ion mobility-mass spectrometry, 391, 2008, 906, L. S. Fenn and J. A. McLean, Fig. 2(a). Panel (d) is adapted with kind permission from Springer Science+Business Media: Anal. Bioanal. Chem., Characterizing ion mobility-mass spectrometry conformation space for the analysis of complex biological samples, 394, 2009, 235, L. S. Fenn, M. Kliman, A. Mahsut, S. R. Zhao, J. A. McLean, Fig. 1(a).

Fig. 5

Conceptual drawing of how two single-pole, seven-throw (1P-7 T) valves and an on-chip peristaltic pump could provide the requisite PC functions in Fig. 2. The valve configuration shown has perfusate from the arterial supply perfusing Organ N and then exiting into the venous return. Other configurations of the valve on the left are used for adding media to make up any that is removed for sampling or flushing, loading cells (with the effluent going to waste), accepting fluid from an upstream organ, local recirculation, infusing a drug, dye, or marker into the single organ, or adding deionized (DI) water to adjust osmolarity after evaporation of water out of the device. The valve on the right is for venous return (as configured), sending fluid to a downstream organ, waste, recirculations, the µCA, an external autosampler, or an online MS. A second PC might be required to prepare cells in any secondary spaces, for example, to establish a confluent alveolar epithelium in a lung.

Fig. 6

Conceptual drawing of how 1P-4 T and 4P-3 T valves and another on-chip peristaltic pump could provide the requisite µCA functions in Fig. 2. The valve configuration shown would allow calibration of the electrochemical sensor array with one of three standard solutions (configuration shown) or sensor washing without affecting the organ. Other modes allow the organ effluent to bypass the washed sensor to minimize biofouling (Store), or the delivery of organ fluid to the calibrated electrochemical sensor array (Measure). In this design, all of the µCA output is sent to waste rather than to venous return.