3.1. Chip fabrication and TEER measurement in the chip
The overall design of the GBA chip was based on our previous gut-liver chip models (Lee et al., 2017; S. Y. Lee & J. H. Sung, 2018). The GBA chip consists of two parts, gut barrier module and BBB module, which can be easily assembled and separated (Fig. 2). Each module has channels to provide fluidic flow (Shemesh et al., 2015), and porous membranes to support cell culture and reproduce the barrier function of corresponding tissues.
In a microfluidic chip, the current distribution through the cell layer determines TEER measurement accuracy and is affected by membrane area, probe design/shape, and microfluidic channel geometry (ávan der Meer, JungáKim, ávan der Helm, & den Berg, 2015; Elbrecht, Long, & Hickman, 2016). The measured TEER values of both cells in the chip showed similar values to those of transwell counterparts (Fig. 3). TEER values of Caco-2 and bEnd.3 were between 220 and 230 Ω*cm2 and approximately 20 Ω*cm2respectively, for both chip and static well cultures, consistent with previously reported values. The range of TEER values for Caco-2 was reported to be 100-200 Ω*cm2 and that of bEnd.3 ranged from 15 to 30 Ω*cm2(Booth & Kim, 2012; Chi et al., 2015; S. Y. Lee & J. H. Sung, 2018; Puscas et al., 2019).