AFM nanoindentation reveals decrease of elastic modulus of lipid bilayers near freezing point of water

Cell lipid membranes are the primary site of irreversible injury during freezing/thawing and cryopreservation of cells, but the underlying causes remain unknown. Here, we probe the effect of cooling from 20 °C to 0 °C on the structure and mechanical properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers using atomic force microscopy (AFM) imaging and AFM-based nanoindentation in a liquid environment. The Young's modulus of elasticity (E) at each temperature for DPPC was obtained at different ionic strengths. Both at 20 mM and 150 mM NaCl, E of DPPC bilayers increases exponentially -as expected-as the temperature is lowered between 20 °C and 5 °C, but at 0 °C E drops from the values measured at 5 °C. Our results support the hypothesis that mechanical weakening of the bilayer at 0 °C  is produced by  structural changes at the lipid-fluid interface.