CELLS AND MEMBRANES - Nanoindentation with Biological Applications - page 324

Depth-sensing nanoindentation techniques have recently been
extended to characterize soft films and membranes such as polymer or
biopolymer membranes. 13 However, the limitations of the conventional
nanoindentation method still prompt for a need to develop new
techniques to characterize multi-scale freestanding membranes from the
nano-scale of cell membrane to macro-scale of tissues.
2.4. Shaft-loaded blister test
Conventional indentation is not suitable for measuring material behavior
of thin soft membranes because it is normally associated with a
“substrate backing” effect. To avoid such complication, the sample
film is clamped by two rings to form a circular diaphragm, shown as
Fig. 10-3 . Such a configuration also helps to prevent a delicate ultrathin
membrane from being damaged by the conventional mechanical grips.
An external load is then applied to the center via either a ball bearing or a
spherical capped cylindrical shaft, causing the film to deform to a conical
cusp. Simultaneous monitoring of the load, central displacement, and
the deformed profile yields the material parameters of the sample. The
testing configuration can be used to characterize membranes with
dimensions in the centimeter range. The smallest possible radius can be
down to submicron if standard lithography is to drill a through-hole in a
rigid plate while the film is clamped between two such identical plates.
The mechanical response of the sample can span multiple length-
scales depending on the ratio of the membrane thickness to radius. In a
typical loading process, as the external load increases incrementally from
null, film deformation is confined to a local volume negligibly small
comparable to the film thickness according to the Hertz indentation
contact model. Depending on the indenter dimension, the micro-scale
indentation range represents the response of single cells or even cell
membrane at the surface layer. Progressive increase in external load
subjects the entire sample to deform by plate-bending, mixed bending-
stretching transition, and ultimately pure membrane stretching,
manifesting the meso- to macro- scale behavior. While bending subjects
the film to shearing perpendicular to the sample plane, stretching puts a