Subphase exchange experiments with the pendant drop technique

The bile salt/phospholipid ratio determines the extent of in vitro intestinal lipolysis of triglycerides: Interfacial and emulsion studies

This study focused on the protein-stabilised triglyceride (TG)/water interfaces and oil-in-water emulsions, and explored the influence of varying molar ratios of bile salts (BSs) and phospholipids (PLs) on the intestinal lipolysis of TGs. The presence of these two major groups of biosurfactants delivered with human bile to the physiological environment of intestinal digestion was replicated in our experiments by using mixtures of individual BSs and PLs under in vitro small intestinal lipolysis conditions. Conducted initially, retrospective analysis of available scientific literature revealed that an average molar ratio of 9:4 for BSs to PLs (BS/PL) can be considered physiological in the postprandial adult human small intestine. Our experimental data showed that combining BSs and PLs synergistically enhanced interfacial activity, substantially reducing oil-water interfacial tension (IFT) during interfacial lipolysis experiments with pancreatic lipase, especially at the BS/PL-9:4 ratio. Other BS/PL molar proportions (BS/PL-6.5:6.5 and BS/PL-4:9) and an equimolar amount of BSs (BS-13) followed in IFT reduction efficiency, while using PLs alone as biosurfactants was the least efficient. In the following emulsion lipolysis experiments, BS/PL-9:4 outperformed other BS/PL mixtures in terms of enhancing the TG digestion extent. The degree of TG conversion and the desorption efficiency of interfacial material post-lipolysis correlated directly with the BS/PL ratio, decreasing as the PL proportion increased. In conclusion, this study highlights the crucial role of biliary PLs, alongside BSs, in replicating the physiological function of bile in intestinal lipolysis of emulsified TGs. Our results showed different contributions of PLs and BSs to lipolysis, strongly suggesting that any future in vitro studies aiming to simulate the human digestion conditions should take into account the impact of biliary PLs - not just BSs - to accurately mimic the physiological role of bile in intestinal lipolysis. This is particularly crucial given the fact that existing in vitro digestion protocols typically focus solely on applying specific concentrations and/or compositions of BSs to simulate the action of human bile during intestinal digestion, while overlooking the presence and concentration of biliary PLs under physiological gut conditions.

Biomaterials based on hyaluronic acid, collagen and peptides for three-dimensional cell culture and their application in stem cell differentiation

In recent decades, three-dimensional (3D) cell culture technologies have been developed rapidly in the field of tissue engineering and regeneration, and have shown unique advantages and great prospects in the differentiation of stem cells. Herein, the article reviews the progress and advantages of 3D cell culture technologies in the field of stem cell differentiation. Firstly, 3D cell culture technologies are divided into two main categories: scaffoldless and scaffolds. Secondly, the effects of hydrogels scaffolds and porous scaffolds on stem cell differentiation in the scaffold category were mainly reviewed. Among them, hydrogels scaffolds are divided into natural hydrogels and synthetic hydrogels. Natural materials include polysaccharides, proteins, and their derivatives, focusing on hyaluronic acid, collagen and polypeptides. Synthetic materials mainly include polyethylene glycol (PEG), polyacrylic acid (PAA), polyvinyl alcohol (PVA), etc. In addition, since the preparation techniques have a large impact on the properties of porous scaffolds, several techniques for preparing porous scaffolds based on different macromolecular materials are reviewed. Finally, the future prospects and challenges of 3D cell culture in the field of stem cell differentiation are reviewed. This review will provide a useful guideline for the selection of materials and techniques for 3D cell culture in stem cell differentiation.

Interfacial Dynamics of Adsorption Layers as Supports for Biomedical Research and Diagnostics

The input of chemical and physical sciences to life sciences is increasingly important. Surface science as a complex multidisciplinary research area provides many relevant practical tools to support research in medicine. The tensiometry and surface rheology of human biological liquids as diagnostic tools have been very successfully applied. Additionally, for the characterization of pulmonary surfactants, this methodology is essential to deepen the insights into the functionality of the lungs and for the most efficient administration of certain drugs. Problems in ophthalmology can be addressed using surface science methods, such as the stability of the wetting films and the development of artificial tears. The serious problem of obesity is fast-developing in many industrial countries and must be better understood, while therapies for its treatment must also be developed. Finally, the application of fullerenes as a suitable system for detecting cancer in humans is discussed.

Hyaluronic acid and human/bovine serum albumin shelled nanocapsules: Interaction with mucins and in vitro digestibility of interfacial films

Liquid lipid nanocapsules are oil droplets surrounded by a protective shell, which enable high load and allow controlled delivery of lipophilic compounds. However, their use in food formulations requires analysing their digestibility and interaction with mucin. Here, serum albumins and hyaluronic acid shelled olive oil nanocapsules are analysed to discern differences between human and bovine variants, the latter usually used as model system. Interfacial interaction of albumins and hyaluronic acid reveals that human albumin presents limited conformational changes upon adsorption, which increase by complexation with the polysaccharide present at the interface. The latter also promotes hydrophobic interactions with mucin, especially at pH 3 and protects albumin interfacial layer under in vitro gastric digestion. The interfacial unfolding induced in human albumin by hyaluronic acid facilitates in vitro lipolysis while its limited conformational changes provide the largest protection against in vitro lipolysis.

Interfacial protein-protein displacement at fluid interfaces

Protein blends are used to stabilise many traditional and emerging emulsion products, resulting in complex, non-equilibrated interfacial structures. The interface composition just after emulsification is dependent on the competitive adsorption between proteins. Over time, non-adsorbed proteins are capable of displacing the initially adsorbed ones. Such rearrangements are important to consider, since the integrity of the interfacial film could be compromised after partial displacement, which may result in the physical destabilisation of emulsions. In the present review, we critically describe various experimental techniques to assess the interfacial composition, properties and mechanisms of protein displacement. The type of information that can be obtained from the different techniques is described, from which we comment on their suitability for displacement studies. Comparative studies between model interfaces and emulsions allow for evaluating the impact of minor components and the different fluid dynamics during interface formation. We extensively discuss available mechanistic physical models that describe interfacial properties and the dynamics of complex mixed systems, with a focus on protein in-plane and bulk-interface interactions. The potential of Brownian dynamic simulations to describe the parameters that govern interfacial displacement is also addressed. This review thus provides ample information for characterising the interfacial properties over time in protein blend-stabilised emulsions, based on both experimental and modelling approaches.

Experimental techniques to study protein-surfactant interactions: New insights into competitive adsorptions via drop subphase and interface exchange

Protein surfactant (PS) interactions is an essential topic for many fundamental and technological applications such as life science, nanobiotechnology processes, food industry, biodiesel production and drug delivery systems. Several experimental techniques and data analysis approaches have been developed to characterize PS interactions in bulk and at interfaces. However, to evaluate the mechanisms and the level of interactions quantitatively, e.g., PS ratio in complexes, their stability in bulk, and reversibility of their interfacial adsorption, new experimental techniques and protocols are still needed, especially with relevance for in-situ biological conditions. The available standard techniques can provide us with the basic understanding of interactions mainly under static conditions and far from physiological criteria. However, detailed measurements at complex interfaces can be formidable due to the sophisticated tools required to carefully probe nanometric phenomena at interfaces without disturbing the adsorbed layer. Tensiometry-based techniques such as drop profile analysis tensiometry (PAT) have been among the most powerful methods for characterizing protein's and surfactant's adsorption layers at interfaces via measuring equilibrium and dynamic interfacial tension and dilational rheology analysis. PAT provides us with insightful data such as kinetics and isotherms of adsorption and related surface activity parameters. However, the data analysis and interpretation can be challenging for mixed protein-surfactant solutions via standard PAT experimental protocols. The combination of a coaxial double capillary (micro flow exchange system) with drop profile analysis tensiometry (CDC-PAT) is a promising tool to provide valuable results under different competitive adsorption/desorption conditions via novel experimental protocols. CDC-PAT provides unique experimental protocols to exchange the droplet subphase in a continuous dynamic mode during the in-situ analysis of the corresponding interfacial adsorbed layer. The contribution of diffusion/convection mechanisms on the kinetics of the adsorption/desorption processes can also be investigated using CDC-PAT. Here, firstly, we review the commonly available techniques for characterizing protein-surfactant interactions in the bulk phase and at interfaces. Secondly, we give an overview for applications of the coaxial double capillary PAT setup for investigations of mixed protein-surfactant adsorbed layers and address recently developed protocols and analysis procedures. Exploring the competitive sequential adsorption of proteins and surfactants and the reversibility of pre-adsorbed layers via the subphase exchange are the particular experiments we can perform using CDC-PAT. Also the sequential and simultaneous competitive adsorption/desorption processes of some ionic and nonionic surfactants (SDS, CTAB, DTAB, and Triton) and proteins (bovine serum albumin (BSA), lysozyme, and lipase) using CDC-PAT are discussed. Last but not least, the fabrication of micro-nanocomposite layers and membranes are additional applications of CDC-PAT discussed in this work.

Upper digestion fate of citrus pectin-stabilized emulsion: An interfacial behavior perspective

Citrus pectin can serve as a naturally digestion-resistant emulsifier, although how it achieves this effect is still unknown. In this study, the upper digestion fate of an emulsion stabilized by different concentrations of citrus pectin, and changes in its interfacial properties during digestion, were investigated. Emulsions stabilized by high-concentration citrus pectin (3 %) were relatively stable during digestion and had a lower free fatty acid (FFA) release rate than emulsions stabilized by low-concentration citrus pectin (1 %). At the low concentration, the citrus pectin interface had a thin absorbing layer and was largely replaced by bile salts, while at high concentration the citrus pectin interface possessed a uniform and thick adsorbing layer that resisted the replacement of bile salts and enabled lipase adsorption. This study has improved our understanding of the digestion of emulsion from the interface and will be useful for designing emulsion-based functional foods that can achieve targeted release.

Investigating the role of hyaluronic acid in improving curcumin bioaccessibility from nanoemulsions

A major challenge in delivering curcumin effectively to the gut is its low solubility. One interesting approach to increase curcumin bioaccessibility is its emulsification. Here, we present curcumin-loaded liquid lipid nanocapsules (LLNs), obtained through olive oil emulsification, in which LLNs are coated by a protective shell composed of Bovine Serum Albumin (BSA) and hyaluronic acid (HA). Bioaccessibility of curcumin is evaluated following a standard in vitro digestion protocol. The presence of HA in the shell increases the amount of curcumin retained in the LLNs after in vitro gastric digestion from ~25% to ~85%. This protective effect occurs when HA binds to BSA in the shell. Moreover, this binding appears to be reinforced under gastric conditions, hence evidencing the crucial role of interfacial composition in protecting encapsulated curcumin. Interfacial engineering of nanoemulsions provides a route to improve the bioaccessibility of encapsulated curcumin at different stages in the gut.

Microgels at interfaces, from mickering emulsions to flat interfaces and back

In this review, we cover the topic of p(NIPAM) based microgels at interfaces, revisiting classical studies in light of the newest ones. In particular, we focus on their use as emulsifiers in the so-called mickering emulsions, i.e. Pickering emulsion stabilized by soft particles. Given the complexity of the experimental characterization and simulation of these soft particles at interfaces, the review is structured in progressive complexity levels, until we reach the highly interesting and promising responsiveness to stimuli of mickering emulsions. We start from the lowest level of complexity, the current understanding of the behavior of single microgels confined at a flat interface. Then, we discuss their collective behavior upon crowding, their responsiveness at interfaces, and their macroscopic properties as microgel films. Once we have the necessary characterization tools, we proceed to discuss the complex and convoluted picture of responsive mickering emulsions. The way is rough, with current controversial and contradicting studies, but it holds promising results as well. We state open questions worth of being tackled by the Soft Matter community, and we conclude that it is worth the trouble of continuing after the master theory of microgel interfacial activity, as it will pave the way to widely adopt responsive mickering emulsions as the worthy Pickering emulsion successors.

Effect of Hyaluronic Acid and Pluronic-F68 on the Surface Properties of Foam as a Delivery System for Polidocanol in Sclerotherapy

The use of foams to deliver bioactive agents and drugs is increasing in pharmaceutics. One example is the use of foam as a delivery system for polidocanol (POL) in sclerotherapy, with the addition of bioactive compounds to improve the delivery system being a current subject of study. This work shows the influence of two bioactive additives on the structure and stability of POL foam: hyaluronic acid (HA) and Pluronic-F68 (F68). HA is a natural non-surface-active biopolymer present in the extracellular matrix while F68 is a surface-active poloxamer that is biocompatible with plasma-derived fluids. Both additives increase the bulk viscosity of the sample, improving foam stability. However, HA doubled and F68 quadruplicated the foam half lifetime of POL. HA reduced the size and polydispersity of the bubble size distribution and increased the surface elasticity with respect to POL. Both facts have a positive impact in terms of foam stability. F68 also altered bubble structure and increased surface elasticity, again contributing to the enhancement of foam stability. The surface characterization of these systems is important, as in foam sclerotherapy it is crucial to assure the presence of POL at the surface of the bubbles in order to deliver the sclerosant agent in the target vein.

Real-time measurements of milk fat globule membrane modulation during simulated intestinal digestion using electron paramagnetic resonance spectroscopy

Milk Fat Globules with their unique interfacial structure and membrane composition are a key nutritional source for mammalian infants, however, there is a limited understanding of the dynamics of fat digestion in these structures. Lipid digestion is an interfacial process involving interactions of enzymes and bile salts with the interface of suspended lipid droplets in an aqueous environment. In this study, we have developed an electron paramagnetic resonance spectroscopy approach to evaluate real time dynamics of milk fat globules interfacial structure during simulated intestinal digestion. To measure these dynamics, natural milk fat globule membrane was labeled with EPR-active probe, partitioning of EPR probes into MFGs membrane was validated using saturation-recovery measurements and calculation of the depth parameter Φ. After validation, the selected spin probe was used to evaluate the membrane's fluidity as a measure of the interface's modulation in the presence of bile salts and pancreatic lipase. Independently, bile salts were found to have a rigidifying effect on the spin probed MFGM, while pancreatic lipase resulted in an increase in membrane fluidity. When combined, the effect of lipase appears to be diminished in the presence of bile salts. These results indicate the efficacy of EPR in providing an insight into small time scale molecular dynamics of phospholipid interfaces in milk fat globules. Understanding interfacial dynamics of naturally occurring complex structures can significantly aid in understanding the role of interfacial composition and structural complexity in delivery of nutrients during digestion.

Interaction of surfactant and protein at the O/W interface and its effect on colloidal and biological properties of polymeric nanocarriers

HYPOTHESIS

The use of polymer-based surfactants in the double-emulsion (water/oil/water, W/O/W) solvent-evaporation technique is becoming a widespread strategy for preparing biocompatible and biodegradable polymeric nanoparticles (NPs) loaded with biomolecules of interest in biomedicine, or biotechnology. This approach enhances the stability of the NPs, reduces their size and recognition by the mononuclear phagocytic system, and protects the encapsulated biomolecule against losing biological activity. Different protocols to add the surfactant during the synthesis lead to different NP colloidal properties and biological activity.

EXPERIMENTS

We develop an in vitro model to mimic the first step of the W/O/W NP synthesis method, which enables us to analyze the surfactant-biomolecule interaction at the O/W interface. We compare the interfacial properties when the surfactant is added from the aqueous or the organic phase, and the effect of pH of the biomolecule solution. We work with a widely used biocompatible surfactant (Pluronic F68), and lysozyme, reported as a protein model.

FINDINGS

The surfactant, when added from the water phase, displaces the protein from the interface, hence protecting the biomolecule. This could explain the improved colloidal stability of NPs, and the higher biological activity of the lysozyme released from nanoparticles found with the counterpart preparation.

Thermoresponsive microgels at the air-water interface: the impact of the swelling state on interfacial conformation

Poly(N-vinylcaprolactam) (PVCL) is a new temperature-responsive type of polymer microgel with improved biocompatibility as compared to more commonly used poly(N-isopropylacrylamide) (PNIPAM). Both polymers swell at low temperatures and collapse at high ones, showing a volume phase transition temperature (VPTT) around the physiological temperature. Exploring the interfacial characteristics of thermoresponsive microgels is important due to their potential application in emulsion based systems with tailored stabilities and controlled degradation profiles. In this work, we study the properties of charged PVCL particles at the air-water interface by a combination of adsorption, dilatational rheology and Langmuir monolayers. Although PVCL particles adsorb spontaneously at the air-water interface in both, swollen and collapsed conformations, the interfacial properties show significant differences depending on the swelling state. In particular, the total amount of adsorbed microgels and the rigidity of the monolayer increase as the temperature increases above the VPTT, which is connected to the more compact morphology of the microgels in this regime. Dilatational rheology data show the formation of a very loose adsorbed layer with low cohesivity. In addition, collapsed microgels yield a continuous increase of the surface pressure, whereas swollen microgels show a phase transition at intermediate compressions caused by the deformation of the loose external polymer shell of the particles. We also provide a qualitative interpretation for the surface pressure behavior in terms of microgel-microgel effective pair potentials, and correlate our experimental findings to recent rescaling models that take into account the importance of the internal polymer degrees of freedom in the rearrangement of the conformation of the microgel particles at the interface.

Effect of cross-linker glutaraldehyde on gastric digestion of emulsified albumin

Human serum albumin (HSA) has been shown to be an ideal protein for nanoparticle preparation. These are usually prepared by using cross linker agents such as glutaraldehyde (GAD). Liquid lipid nanocapsules (LLN) constitute a new generation of nanoparticles more biocompatible and versatile for oral delivery of lipophylic drugs. The first barrier that an orally administered formulation must cross is the gastrointestinal tract. Hence, it is crucial to address the impact of gastrointestinal digestion on these structures in order to achieve an optimal formulation. This study evaluates the effect of gastric digestion on HSA emulsions structured with GAD as a model substrate for the preparation of LLN. This is done by SDS-PAGE, emulsion microstructure, and interfacial tension techniques. Our results demonstrate that the cross- linking procedure with GAD strongly inhibits pepsin digestion by formation of inter- and/or intramolecular covalent bonds between substrate amino acids. Emulsification of HSA also protects from gastric digestion probably by the orientation of the HSA molecule, which exposes the majority of pepsin cleaving sites preferably to the hydrophobic part of the oil-water interface. In this emulsified HSA, cross-linking with GAD at the interface promotes structural modifications on the HSA interfacial layer, restricting the access of pepsin to cleavage sites. We identify interfacial aspects underlying enzymatic hydrolysis of the protein. Assuring that HSA-GAD structures resist passage through the gastric compartment is crucial is important towards the rational design of oral delivery systems and the first step to get the complete digestion profile.

Comparison of the effect of ethylene and hexene-1 co-monomers on the composition, microstructure, rheology, thermal and mechanical behaviour of randomized polypropylene hetero-phasic block co-polymers

Ethylene and hexene-1 co-monomers were applied to isotactic polypropylene reactor alloys to compare the effect of the co-monomer type on their interfacial interaction and impact strength.

Natural Inhibitors of Lipase: Examining Lipolysis in a Single Droplet

Inhibition of lipase activity is one of the approaches to reduced fat intake with nutritional prevention promoting healthier diet. The food industry is very interested in the use of natural extracts, hence reducing the side effects of commercial drugs inhibiting lipolysis. In this work we propose a novel methodology to rapidly assess lipolysis/inhibition in a single droplet by interfacial tension and dilatational elasticity. The evolution of the interfacial tension of lipase in simplified duodenal fluid in the absence and that in the presence of the pharmaceutical drug Xenical are the negative (5 ± 1 mN/m) and positive (9 ± 1 mN/m) controls of the inhibition of lipolysis, respectively. Then, we correlate the inhibition with the reduction of the interfacial activity of lipase and further identify the mode of action of the inhibition based on dilatational response (conformational changes induced in the molecule/blocking of adsorption sites). This work provides new insight into the lipase inhibition mechanism and a rapid methodology to identify the potential of new natural inhibitors.

Improved digestibility of β-lactoglobulin by pulsed light processing: a dilatational and shear study

Modifying the protein conformation appears to improve the digestibility of proteins in the battle against allergies. However, it is important not to lose the protein functionality in the process. Light pulse technology has been recently tested as an efficient non-thermal process which alters the conformation of proteins while improving their functionality as stabilizers. Also, in order to rationally design emulsion based food products with specific digestion profiles, we need to understand how interfacial composition influences the digestion of coated interfaces. This study has been designed to investigate the effects of pulsed light (PL) treatment on the gastrointestinal digestion of protein covered interfaces. We have used a combination of dilatational and shear rheology which highlights inter and intra-molecular interactions providing new molecular details on protein digestibility. The in vitro digestion model analyses sequentially pepsinolysis, trypsinolysis and lipolysis of β-lactoglobulin (BLG) and pulsed light treated β-lactoglobulin (PL-BLG). The results show that the PL-treatment seems to facilitate digestibility of the protein network, especially regarding trypsinolysis. Firstly, PL treatment just barely enhances the enzymatic degradation of BLG by pepsin, which dilutes and weakens the interfacial layer, due to increased hydrophobicity of the protein owing to PL-treatment. Secondly, PL treatment importantly modifies the susceptibility of BLG to trypsin hydrolysis. While it dilutes the interfacial layer in all cases, it strengthens the BLG and weakens the PL-BLG interfacial layer. Finally, this weakening appears to slightly facilitate lipolysis as evidenced by the results obtained upon addition of lipase and bile salts (BS). This research allows identification of the interfacial mechanisms affecting enzymatic hydrolysis of proteins and lipolysis, which demonstrates an improved digestibility of PL-BLG. The fact that PL treatment did not affect the functionality of the protein makes it a valuable alternative for tailoring novel food matrices with improved functional properties such as decreased digestibility, controlled energy intake and low allergenicity.