Probing viscoelastic properties of single cells through their entire thickness

Cells are composed of a number of membrane barriers and fluids of differing linear and nonlinear elasticities and viscosities. Nanomechanical measurements through the entire thickness of a cell can be difficult to perform, given that most available mechanical probes (e.g. atomic force microscope tips) are tapered or limited in length. We have recently demonstrated a simple method of growing Ag-Ga nanoneedles (100 nm diameter, 7-70 microns long) onto AFM tips at room temperature. The constant diameter needle gives clearly measurable force-distance curves when the 29 micron long nanoneedle is forced longitudinally into a red blood cell. Because the needle is long we observe a force response all the way through the cell down to the substrate beneath. A critical force of 175 nN breaks the outer cell membrane, 75 nN at 278 nm beyond the first membrane breaks an inner membrane, and at 1,200 nm the needle goes into hard contact with the glass substrate. Note that the needle is rigid, being well below its buckling limit of 2,000 nN. Within an electron microscope we have inserted a 200 nm diameter nanoneedle at oblique angles over 2.5 microns into yeast cells without it breaking, and much greater distances appear possible.