We thank P. Kim for assistance with the gel formulation, M. Khan for microstructure fabrication, R. S. Friedlander for assistance with confocal imaging, M. Kolle and A. Ehrlicher for technical assistance, and A. Grinthal for help with manuscript preparation. The work was supported by the US DOE under award DE-SC0005247 (experiment) and by the US NSF under award CMMI-1124839 (computational modelling).