The viscoelastic properties of human kidney cell lines from different tumor types (carcinoma (A-498) and adenocarcinoma (ACHN)) are compared to a non-tumorigenic cell line (RC-124). Our methodology is based on the mapping of viscoelastic properties (elasticity modulus E and apparent viscosity η) over the surface of tens of individual cells with atomic force microscopy (AFM). The viscoelastic properties are averaged over datasets as large as 15000 data points per cell line. We also propose a model to estimate the apparent viscosity of soft materials using the hysteresis observed in conventional AFM deflection-displacement curves, without any modification to the standard AFM apparatus. The comparison of the three cell lines show that the non-tumorigenic cells are less deformable and more viscous than cancerous cells, and that cancer cell lines have distinctive viscoelastic properties. In particular, we obtained that E(RC-124) > E(A-498) > E(ACHN) and η(RC-124) > η(A-498) > η(ACHN).

Living cells are constantly exchanging momentum with their surroundings. So far, there is no consensus regarding how cells respond to such external stimuli, although it reveals much about their internal structures, motility as well as the emergence of disorders. Here, we report that twelve cell lines, ranging from healthy fibroblasts to cancer cells, hold a ubiquitous double power-law viscoelastic relaxation compatible with the fractional Kelvin-Voigt viscoelastic model. Atomic Force Microscopy measurements in time domain were employed to determine the mechanical parameters, namely, the fast and slow relaxation exponents, the crossover timescale between power law regimes, and the cell stiffness. These cell-dependent quantities show strong correlation with their collective migration and invasiveness properties. Beyond that, the crossover timescale sets the fastest timescale for cells to perform their biological functions.

The deformation properties of submicrometric drug carriers can influence their tissue-penetration ability and thus the drug targeting. The aim of this study was to determine whether the oily core composition (raw oils or a dispersion of oils and solid lipid) surrounded by a polymeric wall [poly(ε-caprolactone), (PCL)] can affect the deformation properties of nanocapsules (NCs) or lipid-core nanocapsules (LNCs). Formulations were prepared as aqueous suspensions using a polymer and either a mixture of caprylic/capric triglyceride (CCT) and octyl methoxycinnamate (OMC) or a mixture of CCT, OMC and sorbitan monostearate (SM) as core components, respectively. Formulations had mean diameters close to 200 nm presenting monomodal distributions. A polysorbate 80 coating rendered ζ-potential values close to zero, acting as a steric stabilizer. Atomic force microscopy (AFM) showed, through force curves analysis, that the cantilever deflection was more pronounced for the LNCs than for the NCs. The same force applied to NC produced an indentation around twice that observed for the LNCs. The Young's modulus (E) values were 0.537 MPa (LNC) and 0.364 MPa (NC) considering conical geometry while E = 0.17 MPa (NC) and E = 0.241 (LNC) for spherical geometry. These data confirm that the LNCs are stiffer than the NCs. The rigidity of both the polymer wall and lipid core is higher for LNCs. In conclusion, LNCs presented distinct mechanical properties compared to the conventional polymeric NCs.

Nanoindentation of soft materials is a growing research field, demanding sophisticated models to extract accurate information from these materials. In this work we investigate the nanoindentation of thin soft films by sharp conical indenters using Finite Elements Modeling. Based on the work of Dimitriadis et al. [Biophys. J. 82, 2798 (2002)], we propose that load-displacement (F × δ) curves for conical indenters can be described by F = F_Hertz(δ)g[χ(δ,h)], where F_Hertz(δ) is the regular Hertz model, and g[χ(δ,h)] is a correction function that includes finite thickness effects. To test the applicability of the model, we analyze the elastic modulus of fibroblast cells as measured by Atomic Force Microscopy. The elastic modulus obtained with Hertz model is overestimated by 50% (when compared to our thickness-corrected model) in the thickest parts of the cell (3.67 μm), and by approximately 128% in the lamellipodia region (0.45 μm), illustrating the importance of the sample thickness for the evaluation of the mechanical properties.

A theoretical investigation of the combined effect of confinement geometry, valley degeneracy, crystallographic orientations, and anisotropy of the effective mass on the intraband absorption of the conduction band of $\mathrm{Si}$ nanocrystals is presented. We show that the coupling between the nanocrystal shape and arbitrary orientations of the crystalline $\mathrm{Si}$ core plays a dramatic role in the intraband optical properties through interesting changes in the electronic structure and intra- and inter-valley degeneracy. Different orientations of the crystalline $\mathrm{Si}$ also affect the shape and orientation of the orbital wave functions, thereby modifying transition rules and fine structure of the intraband absorption. The anisotropy of the effective mass is also responsible for the emergence of absorption peaks that are not present in isotropic nanocrystals. Additional intraband transitions are also induced by changes in the nanocrystal shape with respect to spherical dots. Moreover, the quantum confined Stark effect blueshifts the absorption peaks and induces the appearance of peak structures as a consequence of the relaxation of selection rules caused by the displacement of the wave functions in response to external electric fields. This effect was found to be only appreciable in large nanocrystals.