Mechanical strain controls endothelial patterning during angiogenic sprouting.

Cyclic strain is known to affect endothelial cell phenotype, but its effects on neovascular pattern formation remain poorly understood. To examine how cyclic strain affects angiogenesis, we designed a stretchable, polydimethylsiloxane (PDMS)-based multi-well system that supports a 3D cell culture model of angiogenesis, consisting of endothelial cells coated onto microcarrier beads embedded in a fibrin gel with a supporting monolayer of smooth muscle cells atop the gel. Calibration of the integrated system showed a linear relationship between applied strain and strain within the fibrin gel. Capillaries formed in unstrained conditions grew radially outward, while 3D constructs subjected to 10% cyclic strain at 0.7 Hz sprouted in a direction parallel to the applied strain. Removal of the tissue from the strain stimulus eliminated directional sprouting. To better understand this directional biasing, the strain field surrounding a microcarrier bead was modeled computationally, showing local strain anisotropy surrounding a microcarrier. Confocal reflection microscopy revealed only modest fiber alignment in regions of the gel close to microcarriers, with no evidence of alignment further away. Together, these data showed that externally applied cyclic strain can spatially pattern capillaries in a 3D culture, and suggests a means to control pattern formation in engineered tissues.