Tank-treading of microcapsules in shear flow

International audienceWe investigated experimentally the deformation of soft microcapsules and the dynamics of their membrane in simple shear flows. Firstly, the tank-treading motion, i.e. the rotation of the membrane, was visualized and quantified by tracking particles included in the membrane by a new protocol. The period of membrane rotation increased quadratically with the extension of the large axis. The tracking of the distance between two close micro-particles showed membrane contraction at the tips and stretching on the sides, a specific property of soft particles such as capsules. Present experimental results are discussed in regard to previous numerical simulations. This analysis showed that the variation of the tank-treading period with the Taylor parameter (deformation) cannot be explained by purely elastic membrane models. It suggests a strong effect of membrane viscosity whose order of magnitude is determined. Secondly, two distinct shapes of sheared microcapsules were observed. For moderate deformations, the shape was a steady ellipsoid in the shear plane. For larger deformations, the capsule became asymmetric and presented a S-like shape. When the viscous shear stress increased by three orders of magnitude, the small axis decreased by 70 % whereas the large axis increased by 100% before any break-up. The inclination angle decreased from 40 • to 8 • , almost aligned with the flow direction as expected by theory/numerics on capsules and experiments/theory/numerics on drops and vesicles. Whatever the microcapsule size and the concentration of proteins, the characteristic lengths of the shape, the Taylor parameter and the inclination angle satisfy master curves versus the longest axis or the normalized shear stress or the capillary number in agreement with theory for non negligible membrane viscosity in the regime of moderate deformations. Finally, we observed that very small deviation from sphericity gave rise to swinging motion, i.e. shape oscillations, in the small deformation regime. In conclusion, this study of tank-treading motion supports the role of membrane viscosity on the dynamics of microcapsules in shear flow by both independent methods which compare experimental data with numerical results in the regime of large deformations and with the theory in the regime of moderate deformations

Compression of biocompatible liquid‐filled HSA‐alginate capsules: Determination of the membrane mechanical properties

International audienceAbstract Compression experiments between two parallel plates are performed on a series of biocompatible HSA‐alginate capsules with two different membrane thicknesses. The capsule geometry and size as well as the average membrane thickness are first measured. The compression set‐up is fitted with a sensitive force transducer that allows measurement of the compression force as a function of plate separation. The response of the capsule is analyzed by assuming different constitutive models for the membrane, where the shear and surface dilatation effects are accounted. An apparent area dilatation modulus is then computed for different values of the plate separation and required to remain constant as the capsule deformation increases. This allows identification of plausible constitutive laws for the membrane material. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 82: 207–212, 2003

Role of M2-like macrophage recruitment during angiogenic growth factor therapy

International audienceTherapeutic angiogenesis has yet to fulfill its promise for the clinical treatment of ischemic diseases. Given the impact of macrophages during pathophysiological angiogenesis, we asked whether macrophages may similarly modulate vascular responses to targeted angiogenic therapies. Mouse matrigel plug assay and rat myocardial infarction (MI) model were used to assess angiogenic therapy with either VEGF-A or FGF-2 with HGF (F+H) delivered locally via albumin-alginate microcapsules. The infiltration of classical M1-type and alternative M2-like macrophages was assessed. Clodronate was used to prevent macrophage recruitment, and the VEGFR2 blocking antibody, DC101, to prevent VEGF-A signaling. At 3 weeks after matrigel implantation, the combination therapy (F+H) led to increased total, and specifically M2-like, macrophage infiltration versus control and VEGF-A plugs, correlating with the angiogenic response. In contrast, VEGF-A preferential recruited M1-type macrophages. In agreement with a direct role of M2-like macrophages in F+H-induced vessel growth, clodronate radically decreased angiogenesis. Further, DC101 reduced F+H-induced angiogenesis, without altering macrophage infiltration, revealing macrophage-derived VEGF-A as a crucial determinant of tissue responsiveness. Similarly, increased cardiac M2-like macrophage infiltration was found following F+H therapy post-MI, with strong correlation between macrophage levels and angiogenic and arteriogenic responses. In conclusion, M2-like macrophages play a decisive role, linked to VEGF-A production, in regulation of tissue responsiveness to angiogenic therapies including the combination of F+H. Our data suggest that future attempts at therapeutic revascularization in ischemic patients might benefit from coupling targeted growth factor delivery with either direct or indirect approaches to recruit pro-angiogenic macrophages in order to maximize therapeutic angiogenic/arteriogenic responses

Characterization of the mechanical properties of cross-linked serum albumin microcapsules: effect of size and protein concentration

International audienceA microfluidic technique is used to characterize the mechanical behavior of capsules that are produced in a two-step process: first, an emulsification step to form droplets, followed by a cross-linking step to encapsulate the droplets within a thin membrane composed of cross-linked proteins. The objective is to study the influence of the capsule size and protein concentration on the membrane mechanical properties. The microcapsules are fabricated by cross-linking of human serum albumin (HSA) with concentrations from 15 to 35 % (w/v). A wide range of capsule radii (∼40–450 μm) is obtained by varying the stirring speed in the emulsification step. For each stirring speed, a low threshold value in protein concentration is found, below which no coherent capsules could be produced. The smaller the stirring speed, the lower the concentration can be. Increasing the concentration from the threshold value and considering capsules of a given size, we show that the surface shear modulus of the membrane increases with the concentration following a sigmoidal curve. The increase in mechanical resistance reveals a higher degree of cross-linking in the membrane. Varying the stirring speed, we find that the surface shear modulus strongly increases with the capsule radius: its increase is two orders of magnitude larger than the increase in size for the capsules under consideration. It demonstrates that the cross-linking reaction is a function of the emulsion size distribution and that capsules produced in batch through emulsification processes inherently have a distribution in mechanical resistance

Chimie du végétal et produits innovants à forte valeur ajoutée

International audienceCes vingt-cinq dernières années ont vu émerger la possibilité d’une société reposant sur un apport carboné biosourcé en remplacement du carbone fossile, transition souvent appelée à tort « société décarbonée ». Le territoire champardennais, terre agricole, s’est engagé dès les années 1990 dans une démarche de bioraffinerie durable en associant recherche académique et acteurs économiques et politiques.C’est dans ce contexte que s’est développée une recherche en chimie du végétal tournée vers les secteurs à moyenne ou haute valeur ajoutée en visant des produits de spécialité ou des actifs originaux et performants dans des domaines aussi variés que les matériaux, la chimie fine, l’environnement, l’agrochimie, la santé ou la cosmétique

Encapsulation of Natural Polyphenolic Compounds; a Review

Natural polyphenols are valuable compounds possessing scavenging properties towards radical oxygen species, and complexing properties towards proteins. These abilities make polyphenols interesting for the treatment of various diseases like inflammation or cancer, but also for anti-ageing purposes in cosmetic formulations, or for nutraceutical applications. Unfortunately, these properties are also responsible for a lack in long-term stability, making these natural compounds very sensitive to light and heat. Moreover, polyphenols often present a poor biodisponibility mainly due to low water solubility. Lastly, many of these molecules possess a very astringent and bitter taste, which limits their use in food or in oral medications. To circumvent these drawbacks, delivery systems have been developed, and among them, encapsulation would appear to be a promising approach. Many encapsulation methods are described in the literature, among which some have been successfully applied to plant polyphenols. In this review, after a general presentation of the large chemical family of plant polyphenols and of their main chemical and biological properties, encapsulation processes applied to polyphenols are classified into physical, physico-chemical, chemical methods, and other connected stabilization methods. After a brief description of each encapsulation process, their applications to polyphenol encapsulation for pharmaceutical, food or cosmetological purposes are presented