2.1 Development of the multi-organ microphysiological system (MO–MPS)
We developed a multi-organ microphysiological system (MO–MPS) integrated with a liver part having metabolic functions based on pharmacokinetics and a lung cancer part as a drug target. The bloodin vivo is dispensed to the organs by the heart after assimilating the oxygen from the lung. The blood flow ratio between liver, heart, and lung is 1:3.3:3.3(Fig. 1A).(Takahashi, (1989)) A stirrer-based micropump, which we had developed previously, was integrated for medium perfusion on the MO–MPS.(Nakayama et al., (2014)) The flow ratio was controlled by the resistance of a bypass-channel installed onto the MO–MPS to be similar in vivo (Fig. 1B). The design of the bypass channel was optimized using ANSYS 17.1 (ANSYS Inc., USA), a fluidic simulation software by the finite element method (Fig. 1C). The dimensions of the chambers of the lung and the liver cancer parts were 15 mm2 and 50 mm2, respectively. The height of the channel and cell culture chambers were 380 µm and 770 µm, respectively.
The MO–MPS was fabricated by conventional photolithography and soft lithography methods.(McDonald and Whitesides, (2002)) A chromium mask for photolithography, which was used to fabricate microstructure, was designed using a computer-aided design software (AutoCAD, Autodesk, USA) and fabricated using a micro-pattern generator (μPG101, Heidelberg instruments Inc., Germany). A negative photoresist (SU-8 2010, MicroChem Corp., USA) was spin-coated onto a silicon wafer. After baking, the microchannel pattern was formed on the substrate using ultraviolet lamp. The stepped structures of the microchannels and the cell culture chambers were fabricated similarly. The substrate was used as a master mold for polydimethysiloxane (PDMS, SILPOT 184, Dow Corning Toray, Japan), the material of the MO–MPS. A 10:1 mixture of PDMS and a polymerization agent was poured onto the master mold and heat cured in an oven at 75 ˚C for 2 h. The cured PDMS was peeled from the master mold to produce a PDMS chip as to which the channel pattern was transferred. The surface of the PDMS was coated with CYTOP (CTL-107MK, AGC, Japan) to spin the stirrer bar smoothly. A 7% CYTOP solution was casted onto the micropump chamber and baked at 65 ℃ for 30 min and 100 ℃ for 60 min. The PDMS surfaces were activated using a plasma cleaner (PDC-32G, Harrick Plasma, USA). After setting the stirrer bar into the chamber, the MO–MPS was assembled to permanently bond the PDMS chips (Fig. 1D).