Integrated electrochemical measurement of endothelial permeability in a 3D hydrogel-based microfluidic vascular model

Mimicking the physiological or pathophysiological barrier function of endothelial and epithelial cells is an essential consideration in organ-on-a-chip models of numerous tissues including the vascular system, lungs, gut and blood-brain barrier. Recent models have furthermore incorporated 3D extracellular matrix hydrogels to recapitulate the composition and cell-matrix interactions found in the native microenvironment. Assessment of barrier function in these 3D organ-on-a-chip models, however, is typically limited to diffusive permeability measurements that are exclusively fluorescence-based. In this work, an on-chip electrochemical method to measure endothelial permeability in a 3D hydrogel-based vascular model was developed that replaces the ubiquitous fluorescent tracer with an electroactive one. Unlike the traditional fluorescent-based method, this electrochemical method eliminates the need for bulky, costly and complex optical instrumentation that require measurements to be performed outside of the incubator. A 3D extracellular matrix gel-based microfluidic model was first developed that incorporates capillary pressure barrier microstructures. Micromilling of thermoplastics was used to fabricate these microstructures in a rapid, moldless fashion. As a proof-of-concept demonstration, the permeability of endothelial cells cultured on hydrogels was electrochemically measured after being subject to perfusion conditions, and following exposure to known permeability mediators. In summary, the electrochemical permeability assay possesses both the benefits of on-chip integration and robustness of the traditional fluorescence-based assay while also enabling the measurement of barrier function in an organ-on-a-chip incorporating 3D culture conditions.