Jaroslav Lukes

PDMS substrate stiffness affects the morphology and growth profiles of cancerous prostate and melanoma cells.

A deep understanding of the interaction between cancerous cells and surfaces is particularly important for the design of lab-on-chip devices involving the use of polydimethylsiloxane (PDMS). In our studies, the effect of PDMS substrate stiffness on mechanical properties of cancerous cells was investigated in conditions where the PDMS substrate is not covered with any of extracellular matrix proteins. Two human prostate cancer (Du145 and PC-3) and two melanoma (WM115 and WM266-4) cell lines were cultured on two groups of PDMS substrates that were characterized by distinct stiffness, i.e. 0.75 ± 0.06 MPa and 2.92 ± 0.12 MPa. The results showed the strong effect on cellular behavior and morphology. The detailed analysis of chemical and physical properties of substrates revealed that cellular behavior occurs only due to substrate elasticity.

Influence of diamond and graphite bonds on mechanical properties of DLC thin films

Mechanical properties of diamond-like carbon thin films with various ratios of sp3/sp2 bonds were studied. The films were prepared in argon atmosphere (0.25 Pa) by laser deposition method for laser energy densities from 4 Jcm−2 to 14 Jcm−2. The sp2 and sp3 bonds were calculated by X-ray photoelectron spectroscopy. Films contained sp3 bonds up to 70 %. Surface properties as roughness and atomic force microscopy topology were measured. Hardness (and reduced Youngs modulus) were determined by nanoindentation and reached of 30 GPa (203 GPa). Films adhesion was studied using scratch test and was up to 12 N for biomedical alloy (titanium substrates – Ti-6Al-4V). Relations among deposition conditions and measured properties are presented.

Elastic Properties of Human Osteon and Osteonal Lamella Computed by a Bidirectional Micromechanical Model and Validated by Nanoindentation

Knowledge of the anisotropic elastic properties of osteon and osteonal lamellae provides a better understanding of various pathophysiological conditions, such as aging, osteoporosis, osteoarthritis, and other degenerative diseases. For this reason, it is important to investigate and understand the elasticity of cortical bone. We created a bidirectional micromechanical model based on inverse homogenization for predicting the elastic properties of osteon and osteonal lamellae of cortical bone. The shape, the dimensions, and the curvature of osteon and osteonal lamellae are described by appropriately chosen curvilinear coordinate systems, so that the model operates close to the real morphology of these bone components. The model was used to calculate nine orthotropic elastic constants of osteonal lamellae. The input values have the elastic properties of a single osteon. We also expressed the dependence of the elastic properties of the lamellae on the angle of orientation. To validate the model, we performed nanoindentation tests on several osteonal lamellae. We compared the experimental results with the calculated results, and there was good agreement between them. The inverted model was used to calculate the elastic properties of a single osteon, where the input values are the elastic constants of osteonal lamellae. These calculations reveal that the model can be used in both directions of homogenization, i.e., direct homogenization and also inverse homogenization. The model described here can provide either the unknown elastic properties of a single lamella from the known elastic properties at the level of a single osteon, or the unknown elastic properties of a single osteon from the known elastic properties at the level of a single lamella.

Nanoindentation of intervertebral disc tissues localised by SHG imaging

J. Šepitka*, J. Lukeš, L. Staněk, E. Filová, Z. Burdı́ková and J. Řeznı́ček Department of Mechanics, Biomechanics and Mechatronics, Faculty of Mechanical Engineering, Czech Technical University in Prague, Technicka 4, Prague 16607, Czech Republic; General Faculty Hospital, Institute of pathology, Charles University, Prague, Czech Republic; Institute of Experimental Medicine of the Academy of Sciences of the Czech Republic, Prague, Czech Republic; Department of Biomathematics, Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic

Micromechanical Properties of Biocompatible Materials for Bone Tissue Engineering Produced by Direct 3D Printing

Bone implants in form of artificial scaffolds manufactured from poly-lactic acid (PLA) represent an attractive alternative to traditional surgical treatments of defective bones (i.e. autografts and allografts). In this work factors influencing biocompatibility and primary stability of implants manufactured from PLA using direct 3D printing were assessed using nanoindentation. For this reason bulk sample of the PLA material and a printed object were subjected to nanomechanical measurement. Quasi-static nanoindentation was employed to identify elastic modulus and hardness distribution on surface and within volume of the samples. Moreover mechanical properties along scanning direction and interlayer characteristics were also assessed. Gradients in mechanical properties have been identified within volume of the material, within the printing layers and at contact between individual layers.

Inspection of Post Impact Fatigue Damage in Carbon Fibre Composite Using Modulus Mapping Technique

This study is focused on inspection of damage extent induced into C/PPS composite material by fatigue and impact loading. Initial damage to specimens was induced by drop-weight out-of-plane impact damage. Several levels of damage states (intact specimen, fatigued and impacted specimen, ruptured specimen) were inspected using modulus mapping (MM) technique. Quantification of the damage level was based on comparison of results from MM obtained in distinct locations on the specimens. Regions of interest were selected in order to determine magnitude of damage after impact and to assess remaining loading capabilities of the material. For this purpose, material maps provided information about location where matrix had been inflicted by the damage. Results show that impact loading has no measurable influence on mechanical properties of the matrix. However, gradient in mechanical properties was detected in the vicinity of crack. Results were validated using quasi-static nanoindentation and constant strain rate continuous measurement that showed depth profile of mechanical properties.

Compression tests of a living cell: a contact detection problem

Alterations in the mechanical properties of cells are associated with cellular diversification and diseases. Assessment of mechanical properties of living cells was mostly associated with atomic force microscopy (AFM) technique. However, the contact conditions of AFM tip with cell’s membrane are very complex (Vichare et al. 2012). Advantageously, very well-defined geometry of flat-end probe used for nanoindentation can simplify the contact problem into compression of an inflated spherical membrane by two rigid parallel planes (Arfsten et al. 2008;Nadler 2010). From our previous experiments, we used the following cell because of their long viability and good adhesion: COS-1 cells (ATCC code: CRL-1650) were derived from African green monkey kidney; the cells grow attached to the base (adherent) and have the morphology of fibroblasts (ATCC, VA, USA). The aims of this study are as follows: (1) determination of the contact between flat punch tip and single cell and (2) to identify membrane bursting critical load.

Examination of the Microrheology of Intervertebral Disc by Nanoindentation

Micro- and Nanoindentation technique has become a standard method for material testing of mechanical properties recently. Especially, indentation method with Oliver and Pharr theory for an analysis of isotropic elastic materials is very strait forward and become a part of every indenter’s software. Some multidirectional indentation analyses have been successfully applied on bone tissues such as cortical osteons and trabecular lamellae to identify their local elastic orthotropic or transverse isotropic properties. Present nanoindenters make possible an acquisition of creep and relaxation data, for that a model of creep or relaxation function can be found in the literature and the viscoelastic material parameters are derived.

Nanoindentation of porcine bone lamellae

Representative volume element (RVE) is widely used in the modelling of a bone. Single bone lamella creating the trabeculae in trabecular bone or the interstitial and osteonal lamellae within cortical bone is suggested to be the proper RVE. Once the lamella’s material constants are determined, the stiffness or compliance matrix is given in case of elastic material, the RVEs can be mathematically assembled in macroscopic anisotropic model (Mares 2006). Nanoindentation with in situ scanning of topography allows the placement of indents in a single lamella of few microns width (Hengsberger et al. 2002). Analysis of indentation of isotropic elastic material by Oliver and Pharr theory is a standard method of indenter software for determination of the hardness and the elastic modulus of samples. Swadener and Pharr (2001) have proposed the technique and solution of an indentation problem of an anisotropic material. We used fabric tensor theory (Zysset and Curnier 1995) to build up the calibration stiffness tensors in order to solve the Swadener and Pharr problem backwards: know the directional indentation moduli and derive the anisotropic stiffness tensor. We validated the procedure on mineralised turkey leg tendons with well-oriented collagen fibres in longitudinal direction (Lukes et al. 2009). This composition is mostly considered to be the bone lamella architecture.

Mapping of Local Changes of Mechanical Properties in Trabecular Interconnections

This paper deals with evaluation of mechanical properties of human trabeculae in the interconnection area. Local changes in the trabecular connections were evaluated using both quasi-static nanoindenation and modulus mapping technique. Connecting point of two trabeculae was revealed by precise grinding and polishing. A rectangular region in the interconnection was selected and inspected by modulus mapping procedure. Moreover several quasi-static indentation measurements using cube-corner indenter were performed along distinct lamellae. The obtained elastic properties were then compared with the values of the rod-like trabeculae. The comparison does not indicate significant differences in elastic properties between the trabecular rods and interconnections.