Engineered Fibrillar Extracellular Matrices for the Study of Directed Cell Migration

The biology of cell adhesion and migration has traditionally been studied on 2D glass or plastic surfaces. While such studies have shed light on the molecular mechanisms governing these processes [1], current knowledge is limited by the dissimilarity between the flat surfaces conventionally employed and the topographically complex extracellular matrix (ECM) cells routinely navigate within the body. On ECM-coated flat surfaces, cells are presented with an unlimited expanse of adhesive ligand and can spread and migrate freely. Conversely, the availability of ligand in vivo is generally restricted to ECM structures, forcing cells to form adhesions in prescribed locations distributed through 3D space depending on the geometry and organization of the surrounding matrix [2]. These physical constraints on cell adhesion likely have profound consequences on intracellular signaling and resulting migration, and calls into question whether the mechanisms and modes of cell motility observed on flat substrates are truly reflective of the in vivo scenario [3]. The topographies of ECMs found in vivo are varied but largely fibrillar, ranging from the tightly crosslinked fibers that form the sheet-like basement membrane, to the structure of fibrin-rich clots and collagenous connective tissues. Collagen comprises approximately 25% of the human body by mass, and as such, purified collagen has served as a popular setting for the study of cell migration within a fibrillar context for many decades [4]. However, a major limitation to the use of these gels is the inability to orthogonally dictate key structural features that impact cell behavior. For example, in contrast to the large range of fiber diameters found in vivo within connective tissue resulting from hierarchical collagen assembly and multiple types of collagens [3], collagen gels are limited to fibril diameters of ∼500nm. Furthermore, recreating the structural anisotropy common to connective tissues in collagen gels is technically challenging [5]. Thus, there remains a significant need for engineered fibrillar materials that afford precise and independent control of architectural and mechanical features for application in cell biology. In this work, we develop two approaches to fabricating fibrillar ECMs in order to study cell adhesion and migration in vitro.