There is a growing need to optimize current in vitro platforms to improve drug development and medical research. Microphysiological systems or Organ-on-Chips (OoCs) represent a paradigm shift, offering more physiologically relevant ways to study tissues, organs and associated diseases to conventional in vitro culture systems and animal models. Furthermore, OoCs enable researchers to have greater control over the cellular environment, allowing regulation of factors such as medium flow, pH, glucose, oxygen levels and barrier integrity.

The human gut is a complex ecosystem where the interplay between epithelial cells and bacteria shapes our health. The intestinal barrier, composed of tightly connected cells, serves as a barrier against harmful substances and microorganisms while facilitating the absorption of essential nutrients. Extensive research has been conducted to understand the influence of both internal and external factors on barrier function, highlighting the vital role of the intestinal epithelium in preventing disease. The human-microbial crosstalk (HuMiX) gut-on-a-chip is a microfluidic system designed to model the intestinal barrier in relation to the intestinal microbiome (Fig. 1a). The HuMiX system, first introduced by Shah et al., has contributed to unraveling insights into the therapeutic treatment of colorectal cancer (CRC) using synbiotics and uncovering the role of microbiome-derived metabolites in driving CRC development.

Figure 1: Schematic diagram of HuMiX with the integrated electrodes. For each HuMiX device, four electrode pairs were positioned on the bottom side of the HuMiX gasket (b) and four electrode pairs were placed on the PC lid (c). Scale bar = 1 cm.

Transepithelial/endothelial electrical resistance (TEER) emerges as a key player in assessing barrier integrity in OoCs. TEER serves as a reliable indicator of the permeability of barrier-forming cells such as intestinal and endothelial cells and is widely used in in vitro models. Given its non-invasive nature, robust signal-to-noise ratios, simplicity of integration, TEER is considered an invaluable tool. Integrating thin-film electrodes within the HuMiX system presented a significant challenge. The existing complexities, including compressible layers, uneven surfaces, and non-transparent materials, necessitated an innovative fabrication approach.

In this blog post, we present a groundbreaking paper that introduces a significant advancement to the HuMiX system; thin-film electrode integration. This development allows real-time TEER measurements, providing a direct and dynamic read-out on the tightness of the intestinal barrier. First, we explore the design, fabrication, and integration of these electrodes into the HuMiX platform, shedding light on how this innovation ensures compatibility with the complex design of the HuMiX platform. The unique combination of the polyimide tape and carrier foil allows for the precise positioning of the electrodes to the HuMiX channels, before entering a sterile environment (Fig. 1b and c). In addition, our design includes gaskets that conform around the edges of the polyimide tape, forming a tight seal, effectively addressing leakage issues. To gain a more comprehensive understanding of epithelial barrier function, we strategically integrated a total of 8 electrode pairs to support multiple localized four-point measurements spatially distributed across the entire cell culture channel. Data was collected at multiple frequencies using impedance spectroscopy and the outlined multiplexer system enables the collection of data from multiple positions within the HuMiX device. Through simulations assessing the electrical field, we established the feasibility of detecting local TEER data within relevant ranges.

Our research concludes in a proof-of-concept where the designed electrodes are displayed in action. We demonstrate their utility in real-time measurements of barrier formation, disruption and recovery of an epithelial cell line within the previously developed HuMiX platform. Consequently, our study lays the foundation for standardized impedance measurements over large and uneven surface areas, providing spatially resolved insights into barrier integrity.