The atomic force microscope: A tool for science and industry

Force Sensing on Cells and Tissues by Atomic Force Microscopy

Biosensors are aimed at detecting tiny physical and chemical stimuli in biological systems. Physical forces are ubiquitous, being implied in all cellular processes, including cell adhesion, migration, and differentiation. Given the strong interplay between cells and their microenvironment, the extracellular matrix (ECM) and the structural and mechanical properties of the ECM play an important role in the transmission of external stimuli to single cells within the tissue. Vice versa, cells themselves also use self-generated forces to probe the biophysical properties of the ECM. ECM mechanics influence cell fate, regulate tissue development, and show peculiar features in health and disease conditions of living organisms. Force sensing in biological systems is therefore crucial to dissecting and understanding complex biological processes, such as mechanotransduction. Atomic Force Microscopy (AFM), which can both sense and apply forces at the nanoscale, with sub-nanonewton sensitivity, represents an enabling technology and a crucial experimental tool in biophysics and mechanobiology. In this work, we report on the application of AFM to the study of biomechanical fingerprints of different components of biological systems, such as the ECM, the whole cell, and cellular components, such as the nucleus, lamellipodia and the glycocalyx. We show that physical observables such as the (spatially resolved) Young's Modulus (YM) of elasticity of ECMs or cells, and the effective thickness and stiffness of the glycocalyx, can be quantitatively characterized by AFM. Their modification can be correlated to changes in the microenvironment, physio-pathological conditions, or gene regulation.

Physics Comes to the Aid of Medicine-Clinically-Relevant Microorganisms through the Eyes of Atomic Force Microscope

Despite the hope that was raised with the implementation of antibiotics to the treatment of infections in medical practice, the initial enthusiasm has substantially faded due to increasing drug resistance in pathogenic microorganisms. Therefore, there is a need for novel analytical and diagnostic methods in order to extend our knowledge regarding the mode of action of the conventional and novel antimicrobial agents from a perspective of single microbial cells as well as their communities growing in infected sites, i.e., biofilms. In recent years, atomic force microscopy (AFM) has been mostly used to study different aspects of the pathophysiology of noninfectious conditions with attempts to characterize morphological and rheological properties of tissues, individual mammalian cells as well as their organelles and extracellular matrix, and cells’ mechanical changes upon exposure to different stimuli. At the same time, an ever-growing number of studies have demonstrated AFM as a valuable approach in studying microorganisms in regard to changes in their morphology and nanomechanical properties, e.g., stiffness in response to antimicrobial treatment or interaction with a substrate as well as the mechanisms behind their virulence. This review summarizes recent developments and the authors’ point of view on AFM-based evaluation of microorganisms’ response to applied antimicrobial treatment within a group of selected bacteria, fungi, and viruses. The AFM potential in development of modern diagnostic and therapeutic methods for combating of infections caused by drug-resistant bacterial strains is also discussed.

Large colloidal probes for atomic force microscopy: Fabrication and calibration issues

Atomic force microscopy (AFM) is a powerful tool to investigate interaction forces at the micro and nanoscale. Cantilever stiffness, dimensions and geometry of the tip can be chosen according to the requirements of the specific application, in terms of spatial resolution and force sensitivity. Colloidal probes (CPs), obtained by attaching a spherical particle to a tipless (TL) cantilever, offer several advantages for accurate force measurements: tunable and well-characterisable radius; higher averaging capabilities (at the expense of spatial resolution) and sensitivity to weak interactions; a well-defined interaction geometry (sphere on flat), which allows accurate and reliable data fitting by means of analytical models. The dynamics of standard AFM probes has been widely investigated, and protocols have been developed for the calibration of the cantilever spring constant. Nevertheless, the dynamics of CPs, and in particular of large CPs, with radius well above 10 μm and mass comparable, or larger, than the cantilever mass, is at present still poorly characterized. Here we describe the fabrication and calibration of (large) CPs. We describe and discuss the peculiar dynamical behaviour of CPs, and present an alternative protocol for the accurate calibration of the spring constant.

Integration of atomic force microscopy and a microfluidic liquid cell for aqueous imaging and force spectroscopy

We have designed and built a microfluidic liquid cell capable of high-resolution atomic force microscope (AFM) imaging and force spectroscopy. The liquid cell was assembled from three molded poly(dimethylsiloxane) (PDMS) pieces and integrated with commercially purchased probes. The AFM probe was embedded within the assembly such that the cantilever and tip protrude into the microfluidic channel. This channel is defined by the PDMS assembly on the top, a PDMS gasket on all four sides, and the sample substrate on the bottom, forming a liquid-tight seal. Our design features a low volume fluidic channel on the order of 50 nl, which is a reduction of over 3-5 orders of magnitude compared to several commercial liquid cells. This device facilitates testing at high shear rates and laminar flow conditions coupled with full AFM functionality in microfluidic aqueous environments, including execution of both force displacement curves and high resolution imaging.

NMR characterization of hydration and thermal stress in tomato fruit cuticles

In its natural environment, the plant cuticle, which is composed of the biopolymer cutin and a mixture of surface and embedded cuticular waxes, experiences a wide variety of temperatures and hydration states. Consequently, a complete understanding of cuticular function requires study of its thermal and mechanical properties as a function of hydration. Herein, we report the results of a comprehensive 13C nuclear magnetic resonance (NMR) relaxation study of hydrated tomato fruit cuticle. Cross-polarization and direct-polarization experiments serve to measure the solid-like and liquid-like components, respectively, of hydrated cuticle. Localized, high-frequency motions are probed by T1(C) spin relaxation measurements, whereas T1rho(H) and T1rho(C) experiments reflect low-frequency, lower amplitude polymer-chain motions. In addition, variable-temperature measurements of T1(C) and T1rho(C) for dry tomato cuticles are used to evaluate the impact of temperature stress. Results of these experiments are interpreted in terms of changes occurring in individual polymer motions of the cutin/wax components of tomato cuticle and in the interaction of these components within intact cuticle, both of which are expected to influence the functional integrity of this protective plant covering.

Past, present and future of atomic force microscopy in life sciences and medicine

To introduce this special issue of the Journal of Molecular Recognition dedicated to the applications of atomic force microscopy (AFM) in life sciences, this paper presents a short summary of the history of AFM in biology. Based on contributions from the first international conference of AFM in biological sciences and medicine (AFM BioMed Barcelona, 19-21 April 2007), we present and discuss recent progress made using AFM for studying cells and cellular interactions, probing single molecules, imaging biosurfaces at high resolution and investigating model membranes and their interactions. Future prospects in these different fields are also highlighted.

Measuring the Size Dependence of Young's Modulus Using Force Modulation Atomic Force Microscopy†

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.

The influence of water on the nanomechanical behavior of the plant biopolyester cutin as studied by AFM and solid-state NMR

Atomic force microscopy and solid-state nuclear magnetic resonance have been used to investigate the effect of water absorption on the nanoscale elastic properties of the biopolyester, cutin, isolated from tomato fruit cuticle. Changes in the humidity and temperature at which fruits are grown or stored can affect the plant surface (cuticle) and modify its susceptibility to pathogenic attack by altering the cuticle's rheological properties. In this work, atomic force microscopy measurements of the surface mechanical properties of isolated plant cutin have been made as a first step to probing the impact of water uptake from the environment on surface flexibility. A dramatic decrease in surface elastic modulus (from approximately 32 to approximately 6 MPa) accompanies increases in water content as small as 2 wt %. Complementary solid-state nuclear magnetic resonance measurements reveal enhanced local mobility of the acyl chain segments with increasing water content, even at molecular sites remote from the covalent cross-links that are likely to play a crucial role in cutin's elastic properties.

Characterization of lacrymal component accumulation on worn soft contact lens surfaces by atomic force microscopy

The purpose of this study was to investigate lacrymal component accumulation on a soft contact lens (SCL) surface after various periods of continuous wear, using the recently developed atomic force microscopy (AFM). AFM allowed high resolution images of unworn and worn SCL, and presented two main advantages. 1. The SCL are analysed under nearly physiological conditions without being dried or destroyed. So the same SCL was analysed at various times during a long wearing period. To identify the deposited tear proteins, a qualitative analysis of solubilized deposit by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) on 4-15% gradient minigels was performed as well. We present typical images which emphasize the importance of the coating by lacrymal components. AFM analysis of worn SCL showed the deposition on the surface of a uniform lacrymal component coating (named deposit type I) with a progressive accumulation of numerous discrete granules (named deposit type II). SDS-PAGE of extracted deposits revealed the main tear proteins as: IgA, lactoferrin, tear lipocalin and lysozyme and the unknown protein of molecular weight 30,000. There is no clear difference in the protein patterns of the two types of deposits. Furthermore, a particular mode of use of AFM is described to illustrate the potential of this technique as a local tool for measuring protein coating thickness. Thus, for analysis of protein deposits on SCL surfaces, SDS-PAGE on minigels and AFM were easy and rapid to perform. When associated, these two techniques could find use in a wide range of worn SCL evaluation and most generally in biocompatibility evaluation studies.

Streptavidin binding observed with an atomic force microscope

An atomic force microscope (AFM) was used to investigate a specific recognition reaction: the binding of streptavidin to a biotinylated lipid bilayer. Prior to the recognition reaction, the phase coexistence of the lipid bilayer was clearly observed: fluid domains were lower than the crystalline domains. After introducing to the bilayer a very dilute solution of streptavidin to give a final concentration of approximately 0.5 microM, the recognition reaction was imaged in real time. Several hours later, we observed a contrast reversal, i.e., the previously lower fluid domains grew so much in height that they became higher than the crystalline domains. We found that the streptavidin molecules bound almost exclusively to the biotin in the fluid domain (less than 0.25% coverage of the crystalline domains). The apparent structure of the few streptavidin molecules bound to the crystalline domain of the bilayer is shown to depend on the applied force. Finally, in a 2-dimensional quasi-crystal in which the streptavidin molecules were compressed at the air-water interface molecular resolution was achieved.

Tip-radius-induced artifacts in AFM images of protamine-complexed DNA fibers

Isolated DNA fibers complexed with protamine (the chromosomal protein that packages DNA in mammalian sperm) have been produced by partially decondensing the highly compacted mouse sperm chromatin particle on a glass coverslip. These DNA fibers were then scanned with the atomic force microscope (AFM). While the smallest of the fibers appear in AFM images as ribbon-like structures 250-350 A wide and 10-25 A high, experiments indicate that these images are the result of a convolution of the imaging-tip's shape with the object's actual shape. In such convolutions the height of the object is affected only by the compressibility of the object, while the width is affected in addition by the sharpness of the tip. Images of polyamidoamine particles also appear to show this artifact. We have also deduced the tip's radius of curvature from images of sharp steps and attempt to demonstrate the artifacts associated with a relatively large imaging tip.

Imaging cells with the atomic force microscope

Different types of cells have been imaged with the atomic force microscope. The morphology of the archaebacterium Halobacterium halobium in its dry state was revealed. On a leaf of the small Indian tree Lagerstroemia subcostata a stoma was imaged. The lower side of a water lily leaf was imaged in water showing features down to 12 nm. Finally, fixed red and white blood cells were imaged in buffer showing features down to 8 nm. The images demonstrate that atomic force microscopy can provide high-resolution images of cell surfaces under physiological conditions.