Effect of ambient humidity on the strength of the adhesion force of single yeast cell inside environmental-SEM

Evaluating Young's Modulus of Single Yeast Cells Based on Compression Using an Atomic Force Microscope with a Flat Tip

In this research, atomic force microscopy (AFM) with a flat tip cantilever is utilized to measure Young's modulus of a whole yeast cell (Saccharomyces cerevisiae BY4741). The results acquired from AFM are similar to those obtained using a microfluidic chip compression system. The mechanical properties of single yeast cells are important parameters which can be examined using AFM. Conventional studies apply AFM with a sharp cantilever tip to indent the cell and measure the force-indentation curve, from which Young's modulus can be calculated. However, sharp tips introduce problems because the shape variation can lead to a different result and cannot represent the stiffness of the whole cell. It can lead to a lack of broader meaning when evaluating Young's modulus of yeast cells. In this report, we confirm the differences in results obtained when measuring the compression of a poly(dimethylsiloxane) bead using a commercial sharp tip versus a unique flat tip. The flat tip effectively avoids tip-derived errors, so we use this method to compress whole yeast cells and generate a force–deformation curve. We believe our proposed method is effective for evaluating Young's modulus of whole yeast cells.

Correlation of Surface Morphology and Interfacial Adhesive Behavior between Cellulose Surfaces: Quantitative Measurements in Peak-Force Mode with the Colloidal Probe Technique

A better understanding of cellulose-cellulose interactions is needed in applications such as paper making and all-cellulose composites. To date, cellulose-cellulose studies have been chemistry-oriented. In these studies, the sample surfaces have been modified with different chemicals and then tested under an atomic force microscope (AFM) using a colloidal probe (CP). Studies of cellulose-cellulose interaction based on sample morphology and mechanical properties have been rare as a result of the complex surface structure and the soft texture of the cellulose. The current surface interaction models, such as the Johnson-Kendall-Roberts (JKR) model in which the studied bodies are assumed to have smooth surfaces, can no longer fully reveal the interfacial behavior between two cellulose surfaces. Therefore, we propose a new type of contact model for rough-rough interaction by dividing the surface contacts into primary and secondary levels. The main idea of the new model is to take into account local individual contact details between rough surfaces. The model considers the effect of the surface topography by including the asperities and valleys on a cellulose sphere used as the colloidal probe in imaging the topography of a cellulose membrane (CM). In addition, the correlation between the surface morphology and adhesion is studied. To verify the importance of including the effect of the surface roughness in contact analysis and validate our hypothesis on the correlation between the surface morphology and adhesion, an extensive set of experiments was performed. In the experiments, a combination of the AFM peak-force mode (PFM) and the CP technique was employed to acquire a massive amount of information on cellulose-cellulose interactions by measuring the adhesion among six CSs of different sizes and a CM.

3D investigation of dynamic behavior and sensitivity analysis of the parameters of spherical biological particles in the first phase of AFM-based manipulations with the consideration of humidity effect

The imaging and manipulation tools being the same in an AFM has necessitated the modeling and simulation of the AFM-based manipulation processes. In earlier studies, the dynamic behavior of biological particles in the course of manipulation has been modeled and simulated two-dimensionally. Now, with the advancements made in the modeling techniques, a 3D model of the manipulation of biological particles is more accurate than its 2D counterpart. In this paper, the effect of humidity has been taken into consideration in the three-dimensional modeling of the manipulation. By employing this model, the equations for the motion modes of particles (sliding, rolling, and spinning) at the onset of movement have been derived and the critical force magnitude has been obtained. In order to reduce the potential damage to the manipulated biological particle, the maximum radius of the tip has been determined. The effective parameters in this process have been extracted by performing sensitivity analysis using the Sobol method. In comparison to the results obtained for a dry environment, the results obtained by simulating the manipulation of a yeast particle in a wet environment shows that the critical force for the onset of particle movement diminishes by considering the moisture effect (high humidity levels). The parameters influencing the magnitude of the critical force include the particle radius, particle material, surface energy of the chosen substrate, amount of preload and the contact angle. Also, the results of the performed sensitivity analysis indicate a very high influence of particle radius on the critical manipulation force and a very low impact of cantilever width on the critical force.

Application of Environmental Scanning Electron Microscope-Nanomanipulation System on Spheroplast Yeast Cells Surface Observation

The preparation and observations of spheroplast W303 cells are described with Environmental Scanning Electron Microscope (ESEM). The spheroplasting conversion was successfully confirmed qualitatively, by the evaluation of the morphological change between the normal W303 cells and the spheroplast W303 cells, and quantitatively, by determining the spheroplast conversion percentage based on the OD₈₀₀ absorbance data. From the optical microscope observations as expected, the normal cells had an oval shape whereas spheroplast cells resemble a spherical shape. This was also confirmed under four different mediums, that is, yeast peptone-dextrose (YPD), sterile water, sorbitol-EDTA-sodium citrate buffer (SCE), and sorbitol-Tris-Hcl-CaCl₂ (CaS). It was also observed that the SCE and CaS mediums had a higher number of spheroplast cells as compared to the YPD and sterile water mediums. The OD₈₀₀ absorbance data also showed that the whole W303 cells were fully converted to the spheroplast cells after about 15 minutes. The observations of the normal and the spheroplast W303 cells were then performed under an environmental scanning electron microscope (ESEM). The normal cells showed a smooth cell surface whereas the spheroplast cells had a bleb-like surface after the loss of its integrity when removing the cell wall.

Evaluation of Chemical Interactions between Small Molecules in the Gas Phase Using Chemical Force Microscopy

Chemical force microscopy analyzes the interactions between various chemical/biochemical moieties in situ. In this work we examined force-distance curves and lateral force to measure the interaction between modified AFM tips and differently functionalized molecular monolayers. Especially for the measurements in gas phase, we investigated the effect of humidity on the analysis of force-distance curves and the images in lateral force mode. Flat chemical patterns composed of different functional groups were made through micro-contact printing and lateral force mode provided more resolved analysis of the chemical patterns. From the images of 1-octadecanethiol/11-mercapto-1-undecanoic acid patterns, the amine group functionalized tip brought out higher contrast of the patterns than an intact silicon nitride tip owing to the additional chemical interaction between carboxyl and amine groups. For more complex chemical interactions, relative chemical affinities toward specific peptides were assessed on the pattern of 1-octadecanethiol/phenyl-terminated alkanethiol. The lateral image of chemical force microscopy reflected specific preference of a peptide to phenyl group as well as the hydrophobic interaction.

Bending spring rate investigation of nanopipette for cell injection

Bending of nanopipette tips during cell penetration is a major cause of cell injection failure. However, the flexural rigidity of nanopipettes is little known due to their irregular structure. In this paper, we report a quantitative method to estimate the flexural rigidity of a nanopipette by investigating its bending spring rate. First nanopipettes with a tip size of 300 nm are fabricated from various glass tubes by laser pulling followed by focused ion beam (FIB) milling. Then the bending spring rate of the nanopipettes is investigated inside a scanning electron microscope (SEM). Finally, a yeast cell penetration test is performed on these nanopipettes, which have different bending spring rates. The results show that nanopipettes with a higher bending spring rate have better cell penetration capability, which confirms that the bending spring rate may well reflect the flexural rigidity of a nanopipette. This method provides a quantitative parameter for characterizing the mechanical property of a nanopipette that can be potentially taken as a standard specification in the future. This general method can also be used to estimate other one-dimensional structures for cell injection, which will greatly benefit basic cell biology research and clinical applications.

Improved Laser Manipulation for On-chip Fabricated Microstructures Based on Solution Replacement and Its Application in Single Cell Analysis

In this paper, we present the fabrication and assembly of microstructures inside a microfluidic device based on a photocrosslinkable resin and optical tweezers. We also report a method of solution replacement inside the microfluidic channel in order to improve the manipulation performance and apply the assembled microstructures for single cell cultivation. By the illumination of patterned ultraviolet (UV) through a microscope, microstructures of arbitrary shape were fabricated by the photocrosslinkable resin inside a microfluidic channel. Based on the microfluidic channel with both glass and polydimethylsiloxane (PDMS) surfaces, immovable and movable microstructures were fabricated and manipulated. The microstructures were fabricated at the desired places and manipulated by the optical tweezers. A rotational microstructure including a microgear and a rotation axis was assembled and rotated in demonstrating this technique. The improved laser manipulation of microstructures was achieved based on the on-chip solution replacement method. The manipulation speed of the microstructures increased when the viscosity of the solvent decreased. The movement efficiency of the fabricated microstructures inside the lower viscosity solvent was evaluated and compared with those microstructures inside the former high viscosity solvent. A novel cell cage was fabricated and the cultivation of a single yeast cell ( w303) was demonstrated in the cell cage, inside the microfluidic device.

Examination of the transition of cultured neuronal cells from submerged to exposed using an environmental scanning electron microscope (ESEM)

Relatively few studies of fully hydrated live or fixed cultured animal cells viewed by environmental scanning electron microscopy (ESEM) have been published. In some cases there may have been some drying out of the cells. In this study the interface between water and cells is imaged as water is carefully evaporated to expose cells. Technical difficulties associated with the process, including inadvertent rewetting of cells are described. Suggestions are made for optimising operating parameters for viewing fully hydrated cultured cells by ESEM. The prospects for viewing live cultured cells are discussed.

Single cell stiffness measurement at various humidity conditions by nanomanipulation of a nano-needle

This paper presents a method for single cell stiffness measurement based on a nano-needle and nanomanipulation. The nano-needle with a buffering beam was fabricated from an atomic force microscope cantilever by the focused ion beam etching technique. Wild type yeast cells (W303) were prepared and placed on the sample stage inside an environmental scanning electron microscope (ESEM) chamber. The nanomanipulator actuated the nano-needle to press against a single yeast cell. As a result, the deformation of the cell and nano-needle was observed by the ESEM system in real-time. Finally, the stiffness of the single cell was determined based on this deformation information. To reveal the relationship between the cell stiffness and the environmental humidity conditions, the cell stiffness was measured at three different humidity conditions, i.e. 40, 70 and 100%, respectively. The results show that the stiffness of a single cell is reduced with increasing humidity.

Theoretical models for surface forces and adhesion and their measurement using atomic force microscopy

The increasing importance of studies on soft matter and their impact on new technologies, including those associated with nanotechnology, has brought intermolecular and surface forces to the forefront of physics and materials science, for these are the prevailing forces in micro and nanosystems. With experimental methods such as the atomic force spectroscopy (AFS), it is now possible to measure these forces accurately, in addition to providing information on local material properties such as elasticity, hardness and adhesion. This review provides the theoretical and experimental background of afs, adhesion forces, intermolecular interactions and surface forces in air, vacuum and in solution.

Evaluation of the correlation between focal adhesion kinase phosphorylation and cell adhesion force using "DEP" technology

Dielectrophoresis (DEP) is the phenomenon in which a particle, such as a living cell, is polarized and moved by electrical gravity in a non-uniform electric field. In the present study, the DEP force is utilized to act on the cells to induce spatial movement for investigating the correlation between the cell adhesion force and activation level of focal adhesion kinase (FAK). The DEP force produced by the non-uniform electric field was used to measure the cell adhesion force of ECV304 cells, on type 1 collagen (COL1)- and fibronectin (FN)-coated polydimethylsiloxane (PDMS) membranes. For COL1-coating, ECV304 cells revealed weak and variable adhesion force (0.343–0.760 nN) in the first eight hours of incubation. Interestingly, the cell adhesion force of ECV304 at two and five hours of cultivation was significantly high and matched their FAK activation level. In comparison, ECV304 on FN-coated membrane had higher and more stable cell adhesion force (0.577–2.053 nN). FN coating intensified the cell adhesion force of ECV304 with culture time and similar outcome was present on the activation level of FAK. Therefore, this study demonstrated a relationship between cell adhesion force and FAK activation level that was dependant on the choice of the extracellular matrix (ECM) component. Subsequently, two tyrosine kinase inhibitors (AG18 and genistein) and one PI3K inhibitor (LY294002) were applied to study the influence of protein phosphorylation on the cell adhesion force. FAK plays an important role on cell attachment and DEP force measurement is a useful technique for studying cell adhesion.