Cao, L. et al. Proc. Natl. Acad. Sci. USA advance online publication (23 April 2012).

Cells in vivo exist within a complex and dynamic biophysical milieu. Forces exerted by cells on the surrounding extracellular matrix (ECM) and vice versa are thought to play an important role in normal function and disease. However, there are few methods to monitor ECM strain, particularly within tissue. Cao et al. report using phage display to identify peptides that can discriminate between relaxed and strained forms of fibronectin. They screened a phage-display library for peptides that bound fibronectin subjected to varying amounts of mechanical strain on PDMS surfaces in vitro and identified two such discriminatory probes that had complementary specificity. Fluorescently labeled versions of the phages—or of peptides derived from them—distinguished between relaxed and strained fibronectin in vitro and showed different binding patterns in ex vivo mammalian tissue.