the invention provides a solution to the problem of delivering molecules in a physiologically relevant manner to direct cell fate . for example , a notch ligand molecule is presented in a microenvironment that mimics the environment encountered in vivo . accordingly , the invention features a cell delivery vehicle comprising a biocompatible hydrogel or polymer and a composition that binds to a notch receptor and methods of directing cell fate , e . g ., stem cell differentiation , using such compositions .