Mechanical properties of soft materials are of great interest. Studying mechanical properties is determined as collecting and processing data to obtain information about material rigidity, toughness, etc. in both static and dynamic (viscoelastic response) regimes. For example with biological cells, the study of cell mechanics is of great importance not only for bioengineering, but also for human health, since changes in the rigidity of tissues is implicated in the pathogenesis of many diseases. Atomic Force Microscopy (AFM) has been used to study cell rigidity for more than a decade having been disclosed by Binning in U.S. Pat. No. 4,724,318 issued on Feb. 9, 1988 (now Re. 33387 issued on Oct. 17, 1990) and hereby incorporated by reference. The main problems of this approach are in the nature of both objects of study (for example, biological cells) and the technique (AFM) itself. Cells, or any other soft heterogeneous surfaces being intrinsically variable and inhomogeneous objects, require a large number of measurements to attain statistically robust results. The AFM related problem is as follows. The time required for such data acquisition can be too long to be practical. During a long time of measurements, the characteristics of the test objects can change, which makes the process of measurement meaningless.
The problem of getting dynamical (for example, viscoelastic) data for soft samples, in particular, biological cells, is very time consuming in nature. While the technique is relevant to other samples, we will discuss biological cells as a representative example. Keeping cells viable and steady under the AFM for a long time is simply impractical. To date, one of the most detailed studies, Park et al, 1 was done by collecting force curves only in about three points per cell for the total statistics of about ten cells per cell line (three cell lines were analyzed). As one observes from Berdyyva et al 2, to make a statistical conclusion on a cell line, one would be expected to collect at least several hundreds of the force curves (16×16 force curves in the example of reference 2) per cell region (for example, three different regions were identify in the case of epithelial cell) with simultaneous collection of the surface topography of information. Knowledge of sample topography is paramount here because the surface must be flat to be processed with the existing models. The fastest way to collect both force and the surface topography is the use of the well-known force-volume mode of the AFM operation. The 1 Hz data collection used in Park et al 1 would require requires 10-30 minutes (depending on topology of the surface) in the force-volume mode to collect the data with a single frequency. With collecting frequency data in the range of 50-300 Hz 1, one would need at least 10 frequency points (better 20 or more). So, the time required to obtain data per cell should be in the range between 5 hours (16×16 pixels, 3 regions, 10 minutes per FV scan, and 10 points) to 30 hours (16×16 pixels, 3 regions, 30 min, 20 points) for a single cell. While doing measurements for several hours per cell is theoretically feasible, in practice this would require too much work to collect reasonable statistics (at least ˜10 cells). Cancer cells, for example, will require even more cells to measure due to their intrinsically high variability. Moreover, those cells are much more sensitive to the environment, and consequently, it is harder to maintain their viability over extended period of time.
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