Directing specific, complex cell behaviors, such as differentiation, in response to biomaterials for regenerative medicine applications is, at present, a mostly unrealized goal. To date, current technological advances have been inspired by the reductionist point of view, focused on developing simple and merely adequate environments that facilitate simple cellular adhesion. However, even if extracellular matrix (ECM)-derived peptides, such as Arg-Gly-Asp (RGD), have largely demonstrated their utility in supporting cell adhesion, their lack of biological specificity is simply not optimal for controlling more integrated processes, such as cell differentiation. These more complex cellular processes require specific integrin-signaling scaffolds and presumably synergistic integrin and growth factor-receptor signaling. This article will introduce some current efforts to engineer ECM variants that incorporate additional levels of complexity for directing greater integrin specificity and synergistic ECM growth factor signaling toward directing cell phenotype.Keywords cell phenotype; differentiation; extracellular matrix; integrins; protein engineering; regenerative medicineThe ability to direct specific cell behavior in biomaterials, in particular differentiation, would be of particular use for regenerative medicine applications, but until now this has been a mostly unfulfilled goal in the fields of biomaterials and regenerative medicine [1,2]. Current technological advances have been inspired by the reductionist point of view and have focused on developing simple and merely adequate environments that facilitate simple cell processes, such as cellular adhesion. For example, the integrin-binding trimeric peptide motif, Arg-GlyAsp (RGD), derived from the extracellular matrix (ECM) protein fibronectin (FN) has been abundantly tested and used as an 'ECM mimetic' in the context of synthetic biomaterials [3]. However, even if ECM-derived peptides have largely demonstrated their utility, such as supporting cell adhesion, the lack of biological specificity of many of these peptide motifs is simply not optimal for controlling more integrated processes, such as differentiation [4]. These complex cellular processes require more specific integrin-signaling scaffolds, such as those naturally presented in the native ECM. However, the modification of materials with full-length ECM proteins, whilst displaying the molecule in its native form, presents significant complications in working with macromolecules that present complex biology. For example, the ECM protein FN is capable of binding at least 20 distinct integrins [5]. In addition, FN, as well as many other structural ECM proteins, are also capable of binding a significant number of polypeptide growth factors (GFs) [6][7][8]. For example, active TGF-β has been shown to
