David Merida

Detection of atomic scale changes in the free volume void size of three-dimensional colorectal cancer cell culture using positron annihilation lifetime spectroscopy.

Positron annihilation lifetime spectroscopy (PALS) provides a direct measurement of the free volume void sizes in polymers and biological systems. This free volume is critical in explaining and understanding physical and mechanical properties of polymers. Moreover, PALS has been recently proposed as a potential tool in detecting cancer at early stages, probing the differences in the subnanometer scale free volume voids between cancerous/healthy skin samples of the same patient. Despite several investigations on free volume in complex cancerous tissues, no positron annihilation studies of living cancer cell cultures have been reported. We demonstrate that PALS can be applied to the study in human living 3D cell cultures. The technique is also capable to detect atomic scale changes in the size of the free volume voids due to the biological responses to TGF-β. PALS may be developed to characterize the effect of different culture conditions in the free volume voids of cells grown in vitro.

Ceramide increases free volume voids in DPPC membranes

Positron annihilation lifetime spectroscopy (PALS) can measure changes in local free volume voids in lipid bilayers. PALS has been applied, together with differential scanning calorimetry (DSC) and molecular dynamics (MD) simulations, to study free volume voids in DPPC and DPPC : ceramide (85 : 15 mol : mol) model membranes in the 20–60 °C range. The free volume void average size clearly increases with the gel–fluid phase transition of the lipid, or lipid mixture. Ceramide increases void size at all temperatures, particularly in the range causing the gel–fluid transition of the mixture. A parallel study of PALS and calorimetric data indicates that, for the complex thermotropic transition of the DPPC–ceramide mixture, PALS is detecting the transition of the DPPC component, while calorimetry changes indicate mainly the melting of the ceramide-enriched domains. Molecular dynamics calculations provide a clear distinction between ceramide-rich and poor domains, and show that the voids are predominantly located near the membrane nodal plane. The ceramide-induced increase in void volume size occurs as well at temperatures when both phospholipid and ceramide are in the fluid state, indicating that the effect is not the result of phospholipid–ceramide domain coexistence. The above observations may be related to hitherto unexplained properties of ceramide, such as the increase in membrane permeability, and the induction of transmembrane (flip-flop) lipid motion.

Cholesterol–Ceramide Interactions in Phospholipid and Sphingolipid Bilayers As Observed by Positron Annihilation Lifetime Spectroscopy and Molecular Dynamics Simulations

Free volume voids in lipid bilayers can be measured by positron annihilation lifetime spectroscopy (PALS). This technique has been applied, together with differential scanning calorimetry and molecular dynamics (MD) simulations, to study the effects of cholesterol (Chol) and ceramide (Cer) on free volume voids in sphingomyelin (SM) or dipalmitoylphosphatidylcholine (DPPC) bilayers. Binary lipid samples with Chol were studied (DPPC:Chol 60:40, SM:Chol 60:40 mol ratio), and no phase transition was detected in the 20-60 °C range, in agreement with calorimetric data. Chol-driven liquid-ordered phase showed an intermediate free volume void size as compared to gel and fluid phases. For SM and SM:Cer (85:15 mol:mol) model membranes measured in the 20-60 °C range the gel-to-fluid phase transition could be observed with a related increase in free volume, which was more pronounced for the SM:Cer sample. MD simulations suggest a hitherto unsuspected lipid tilting in SM:Cer bilayers but not in pure SM. Ternary samples of DPPC:Cer:Chol (54:23:23) and SM:Cer:Chol (54:23:23) were measured, and a clear pattern of free volume increase was observed in the 20-60 °C because of the gel-to-fluid transition. Interestingly, MD simulations showed a tendency of Cer to change its distribution along the membrane to make room for Chol in ternary mixtures. The results suggest that the gel phase formed in these ternary mixtures is stabilized by Chol-Cer interactions.

Sub-nanoscale free volume and local elastic modulus of chitosan–carbon nanotube biomimetic nanocomposite scaffold-materials

Future progress in materials for tissue engineering and 3D cell cultures applications requires control of two key physical properties: nanoscale mechanical properties and mass transport. These requirements remain uncontrolled partly due to a lack of physical parameters and quantitative measurements. Using chitosan scaffolds as a model system in close-to-physiological conditions and a combination of experimental techniques and theory, we link structure with local nanomechanical properties. Additionally we introduce a parameter, the free volume, to predict variations in transport properties. By fabricating nanocomposites with single walled carbon nanotubes (SWNTs) we are able to test our approach: incorporation of acid-treated, soluble, ∼80 nm SWNTs in a chitosan matrix leads to a 2 fold increase in mean local elastic modulus and a decrease of 3% of the free volume available for oxygen diffusion. Inclusion of hydrophobic, ∼800 nm SWNTs leads to a 100 fold increase of elastic modulus and doubles the voids percentage available for the transport of glucose.

Positron Annihilation Spectroscopy Study of NiMnGa Modulated and Non-Modulated Martensitic Phases

We have studied by means of positron lifetime measurements the role that vacancy type defects play in the martensitic transformation in magnetic Ni-Mn-Ga shape memory alloys. The measurements presented in this work have been performed in two ternary alloys transforming to modulate and non-modulated martensitic phases. Positron experiments have been realized at room temperature after subsequent isochronal heating at different temperatures up to a maximum temperature of 600°C. Positron results show a large variation of the average lifetime value with the isochronal annealing temperature in non-modulated samples. However, the response in the modulated samples is quite different. These results obtained in both samples are discussed in term of different type of positrons trapping defects and their evolution with the annealing temperature. They have also been compared with Differential Scanning Calorimetry experiments performed in the same samples. The work states the clear influence of the vacancy concentration on the martensitic transformations of these alloys.

Defects structure characterization of NiMnGa alloys by PALS

We have studied the behaviour of defects in off-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in six ternary alloys. The studied samples cover a large composition range. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures up to a maximum temperature of 700°C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in three of these samples, with significant differences between them. In the other three, the response is quite different. The results are discussed in terms of different types of positron trapping defects and their evolution with the annealing temperature. The present work shows a high dependence of recovery behaviour with composition in NiMnGa ferromagnetic shape memory alloys.