Ions and electrical current can be transported through the cells (transcellular) and through the space between adjacent cells (paracellular). The dotted lines show the transcellular pathway of ion or electrical current flow. The solid lines demonstrate the paracellular pathway of ion or current flow.

Initially, 24 hours after cell seeding transwell, TEER values are generally low, because the current passes can pass easily between the cells. Over time, the cells multiply and start covering the gaps. Finally, a confluent cellular monolayer is formed. At that point, the permeable membrane is fully covered with cells and does not allow easy passage of electrical current. This results in a high TEER value.

Cell type A allows greater amounts of current and ions to pass between the cells and yields a low TEER value. With its tighter junctions, cell monolayers of type B cells will show a higher TEER value. While both monolayers are confluent, the TEER resistance values can be markedly different based on the nature of the cells themselves.

As TEER rises, you can see the direct correlation with the cellular barrier, confluence of the tissue, and the cytotoxicity of the media tested.

WPI’s EVOM is considered as the gold standard for TEER measurement with numerous citations for current and older model WPI manufactured instruments including EVOM, EVOM2, ERS2, REMS, EVOM3, EVOM™ Manual, EVOM™ Auto and electrodes such as STX HTS, ENDOHM, STX2, and STX4. EVOM™ has been well the recognized technology for more than 30 years (See TEER measurement techniques for in vitro barrier model systems - PubMed), and it has been cited in numerous peer-reviewed publications. 

EVOM™ Manual and EVOM™ Auto are simple to use for quick readings, TEER is non-invasive and non-destructive to cell, and it gives you measurable quantitative data on the health of your barrier.

WPI now offers Corning® Permeable membrane cell culture inserts with different membrane, pore size, and plate design choices. The selection of the plate type depends on your application.  

• Polycarbonate (PC) membranes allow maximum diffusion. Hence, PC membranes are normally suitable for drug uptake and transport studies (e.g., EVM-AC-03-C3415)
• PET (Polyester or Polyethylene Terephthalate) clear membranes can allow sufficient optical transparency to see cell growth onto these membrane inserts by phase contrast microscopy (e.g., EVM-AC-03-C3460, EVM-AC-03-3470) 

Cell culture inserts can be detachable transwells available in 6 (e.g., EVM-AC-03-C3450), 12 (e.g., EVM-AC-03-C3460), and 24 (e.g., EVM-AC-03-C3460) well plate formats. These removable inserts are suitable for studies that do not require high throughput and for studies where two or more different cell types need to be grown on two sides of the membranes. The 24-well cell culture inserts can be used for automated TEER measurement using the EVOM™ Auto by moving the inserts to a companion plate.  

WPI also offers HTS (high throughput screening) plates which are available in 24 (e.g. EVM-AC-03-C3399) and 96 (e.g. EVM-AC-03-C3381) well plate formats. The HTS plates have one single assembly for apical compartment for all the wells. Apical and basolateral access zones are fixed in a consistent location on the plate. Thus, HTS plates allow for high throughput and automation. For example, HTS plates are commonly used for WPI’s EVOM™ Auto for automated TEER measurement. 

• Normally insert plates with membrane pore size of 0.4 (e.g., EVM-AC-03-C3391) and 1.0 (e.g., EVM-AC-03-C3380) µm are used for transport, permeability, and co-culture studies.
• Toxicity, Air-Liquid Interface (ALI), Tissue Engineering applications commonly use 0.4 and 3 µm (e.g., EVM-AC-03-C3385) membranes. 
• Cell migration and invasion studies require larger membrane pore size of 3, 5 (e.g. (EVM-AC-03-C3388), 8 µm (e.g., EVM-AC-03-C3384) of the cell culture inserts.  

WPI offers the most reliable Corning cell culture insert plates with different membrane types and pore size options for different applications, as well as enables the state-of-the art TEER measurement solution using WPI’s EVOM™ to verify functional tissue growth. TEER measurement is crucial for validating in vitro studies performed using permeable membrane inserts and plates. 

These are some of the popular applications for TEER measurement. However, tissue engineering, assessing epithelial/endothelial barrier function, organoid research, and drug absorptions studies are just the tip of the iceberg. TEER is used for cell culturing, disease modeling, quality control in cell-based assays and cell therapies, manufacturing quality control and more. Now, it's even possible to monitor TEER for organ on a chip systems.