From Molecules to Cells: Imaging Soft Samples with the Atomic Force Microscope

Abstract

Since its invention a few years ago, the atomic force microscope has become one of the most widely used near-field microscopes. Surfaces of hard sample are imaged routinely with atomic resolution. Soft samples, however, remain challenging. An overview is presented on the application of atomic force microscopy to organic samples ranging from thin ordered films at molecular resolution to living cells. Fundamental mechanisms of the image formation are discussed, and novel imaging modes are introduced that exploit different aspects of the tip-sample interaction for local measurements of the micromechanical properties of the sample. As examples, images of Langmuir-Blodgett films, which map the local viscoelasticity as well as the friction coefficient, are presented.

Atomic force microscopy – looking at mechanosensors on the cell surface

“…Soon after its invention, AFM became a valuable tool for imaging cells (Butt et al, 1990;Radmacher et al, 1992). However, AFM imaging of single cells requires their firm attachment to a surface, which is not always a simple task.…”

“…However, AFM imaging of single cells requires their firm attachment to a surface, which is not always a simple task. A straightforward approach is to exploit the ability of animal cells to spread and adhere to solid supports (Radmacher et al, 1992). Coating the substrate with adhesion proteins might be used to enhance immobilization, and this method has made it possible to observe, for example, actin filament dynamics beneath the plasma membrane of glial cells (Henderson et al, 1992).…”

Numerical study of the hydrodynamic drag force in atomic force microscopy measurements undertaken in fluids

“…One of the major advantages of AFM is the ability to undertake measurements of specimens in fluid environments. This advantage is particularly valuable because it allows undertaking investigations of biological samples in their natural, physiological environment [4][5][6][7][8][9][10][11][12]. The AFM force measurements on soft samples in fluid environments are affected by a hydrodynamic drag force, which results from the viscous friction of the cantilever with the surrounding fluid.…”

Contribution of cellular contractility to spatial and temporal variations in cellular stiffness

“…SPM can also be used to evaluate mechanical properties, because its probe is in physical contact with the samples during measurement. To obtain cellular stiffness with SPM, two methods have been proposed, namely, a force modulation mode [22,23] and a force mapping mode [24]. According to experiments performed by using these methods, the local stiffness of fibroblasts is not homogeneous on the cellular surface, but varies largely from point to point [25,26].…”