Precise measurements of capsule mechanical properties using indentation

Quantitative Elasticity Mapping of Submicron Silica Hollow Particles by PeakForce QNM AFM Mode

Silica hollow spheres with a diameter of 100-300 nm and a shell thickness of 8±2 nm were synthesized using a self-templating amphiphilic polymeric precursor, i.e., poly(ethylene glycol)-substituted hyperbranched polyethoxysiloxane. Their elastic properties were addressed with a high-frequency AFM indentation method based on the PeakForce QNM (quantitative nanomechanical mapping) mode enabling simultaneous visualization of the surface morphology and high-resolution mapping of the mechanical properties. The factors affecting the accuracy of the mechanical measurements such as a local slope of the particle surface, deformation of the silica hollow particles by a solid substrate, shell thickness variation, and applied force range were analysed. The Young's modulus of the shell material was evaluated as E=26±7 GPa independent of the applied force in the elastic regime of deformations. Beyond the elastic regime, the buckling instability was observed revealing a non-linear force-deformation response with a hysteresis between the loading and unloading force-distance curves and irreversible deformation of the shell at high applied forces. Thus, it was demonstrated that PeakForce QNM mode can be used for quantitative measurements of the elastic properties of submicon-sized silica hollow particles with nano-size shell thickness, as well as for estimation of the buckling behaviour beyond the elastic regime of shell deformations.

The role of the cortex in indentation experiments of animal cells

We present a model useful for interpretation of indentation experiments on animal cells. We use finite element modeling for a thorough representation of the complex structure of an animal cell. In our model, the crucial constituent is the cell cortex-a rigid layer of cytoplasmic proteins present on the inner side of the cell membrane. It plays a vital role in the mechanical interactions between cells. The cell cortex is modeled by a three-dimensional solid to reflect its bending stiffness. This approach allows us to interpret the results of the indentation measurements and extract the mechanical properties of the individual elements of the cell structure. During the simulations, we scan a broad range of parameters such as cortex thickness and Young's modulus, cytoplasm Young's modulus, and indenter radius, which define cell properties and experimental conditions. Finally, we propose a simple closed-form formula that approximates the simulated results with satisfactory accuracy. Our formula is as easy to use as Hertz's function to extract cell properties from the measurement, yet it considers the cell's inner structure, including cell cortex, cytoplasm, and nucleus.

Effect of sodium citrate on asphaltene film at the oil-water interface

HYPOTHESIS

Sodium citrate (Na₃Cit) has been proven to improve the oil sands extraction recovery, but its mechanism is still unclear. Here we hypothesize that the presence of Na₃Cit affects the asphaltene behaviour at the oil-water interface, which enhances oil-water separation and, thereby, heavy oil recovery.

EXPERIMENTS

Na₃Cit-asphaltene interaction was first investigated on their interfacial shear rheology at one heptol-water interface. Na₃Cit-asphaltene interaction was further revealed by measuring the interaction forces between two heptol-water interfaces using the atomic force microscopy droplet technique combined with the Stokes-Reynolds-Young-Laplace (SRYL) model. Interfacial properties were further illustrated through interfacial tension, zeta potential, Langmuir trough, and FE-SEM.

FINDINGS

Na₃Cit was found to weaken the strength of the asphaltene film at the heptol-water interface. Moreover, Na₃Cit could diminish the adhesion forces observed between two asphaltene-in-heptol droplets in high salinity solutions. Besides, Na₃Cit also made the asphaltene-in-heptol droplet more negatively charged. These results collectively suggest that Na₃Cit-asphaltene interaction results in a looser and more elastic asphaltene interfacial network with the slow formation and reduces the adhesion between two interfaces, all of which are most likely the consequence of increased electrostatic repulsion between asphaltene interfacial nanoaggregates. Our study provided new understandings of Na₃Cit-asphaltene interactions at the interface.

Alginate Core-Shell Capsules for 3D Cultivation of Adipose-Derived Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are primary candidates in tissue engineering and stem cell therapies due to their intriguing regenerative and immunomodulatory potential. Their ability to self-assemble into three-dimensional (3D) aggregates further improves some of their therapeutic properties, e.g., differentiation potential, secretion of cytokines, and homing capacity after administration. However, high hydrodynamic shear forces and the resulting mechanical stresses within commercially available dynamic cultivation systems can decrease their regenerative properties. Cells embedded within a polymer matrix, however, lack cell-to-cell interactions found in their physiological environment. Here, we present a “semi scaffold-free” approach to protect the cells from high shear forces by a physical barrier, but still allow formation of a 3D structure with in vivo-like cell-to-cell contacts. We highlight a relatively simple method to create core–shell capsules by inverse gelation. The capsules consist of an outer barrier made from sodium alginate, which allows for nutrient and waste diffusion and an inner compartment for direct cell-cell interactions. Next to capsule characterization, a harvesting procedure was established and viability and proliferation of human adipose-derived MSCs were investigated. In the future, this encapsulation and cultivation technique might be used for MSC-expansion in scalable dynamic bioreactor systems, facilitating downstream procedures, such as cell harvest and differentiation into mature tissue grafts.

Stiffness of Fluid and Gel Phase Lipid Nanovesicles: Weighting the Contributions of Membrane Bending Modulus and Luminal Pressurization

The mechanical properties of biogenic membranous compartments are thought to be relevant in numerous biological processes; however, their quantitative measurement remains challenging for most of the already available force spectroscopy (FS)-based techniques. In particular, the debate on the mechanics of lipid nanovesicles and on the interpretation of their mechanical response to an applied force is still open. This is mostly due to the current lack of a unified model being able to describe the mechanical response of both gel and fluid phase lipid vesicles and to disentangle the contributions of membrane rigidity and luminal pressure. In this framework, we herein propose a simple model in which the interplay of membrane rigidity and luminal pressure to the overall vesicle stiffness is described as a series of springs; this approach allows estimating these two contributions for both gel and fluid phase liposomes. Atomic force microscopy-based FS, performed on both vesicles and supported lipid bilayers, is exploited for obtaining all the parameters involved in the model. Moreover, the use of coarse-grained full-scale molecular dynamics simulations allowed for better understanding of the differences in the mechanical responses of gel and fluid phase bilayers and supported the experimental findings. The results suggest that the pressure contribution is similar among all the probed vesicle types; however, it plays a dominant role in the mechanical response of lipid nanovesicles presenting a fluid phase membrane, while its contribution becomes comparable to the one of membrane rigidity in nanovesicles with a gel phase lipid membrane. The results presented herein offer a simple way to quantify two of the most important parameters in vesicle nanomechanics (membrane rigidity and internal pressurization), and as such represent a first step toward a currently unavailable, unified model for the mechanical response of gel and fluid phase lipid nanovesicles.

Tailoring the mechanoresponsive release from silica nanocapsules

Triggering the release of encapsulated cargos using mechanical stress acting on a nanocarrier is a strategy with potential applications from drug delivery to self-healing coatings. The mechanically triggered release of encapsulated molecules can be controlled by tuning the mechanical properties of the nanocapsules, which are strongly linked to the nanocapsule architecture. Here, silica nanocarriers were designed to tune precisely the release initiated by mechanical stress. We synthesized silica nanocapsules (SiNCs) with a finely tunable diameter and shell thickness and performed AFM nanoindentation experiments to determine the breaking force of single SiNCs. We demonstrated that it is possible to trigger the release of encapsulated payload by the application of an external mechanical force on the SiNCs. Furthermore, we successfully controlled the breaking force and the amount of released payload by tailoring the architecture of the nanocarriers, illustrating how such mechanoresponsive SiNCs could be used as responsive nanocarriers for the delivery of molecular cargos.

Organic Hollow Mesoporous Silica as a Promising Sandalwood Essential Oil Carrier

As film-forming agents, fillers and adsorbents, microplastics are often added to daily personal care products. Because of their chemical stability, they remain in the environment for thousands of years, endangering the safety of the environment and human health. Therefore, it is urgent to find an environmentally friendly substitute for microplastics. Using n-octyltrimethoxysilane (OTMS) and tetraethoxysilane (TEOS) as silicon sources, a novel, environmentally friendly, organic hollow mesoporous silica system is designed with a high loading capacity and excellent adsorption characteristics in this work. In our methodology, sandalwood essential oil (SEO) was successfully loaded into the nanoparticle cavities, and was involved in the formation of Pickering emulsion as well, with a content of up to 40% (w/w). The developed system was a stable carrier for the dispersion of SEO in water. This system can not only overcome the shortcomings of poor water solubility and volatility of sandalwood essential oil, but also act as a microplastic substitute with broad prospects in the cosmetics and personal care industry, laying a foundation for the preparation and applications of high loading capacity microcapsules in aqueous media.

Effect of quercetin on lipid membrane rigidity: assessment by atomic force microscopy and molecular dynamics simulations

Quercetin (3,3',4',5,7-pentahydroxyl-flavone) is a natural flavonoid with many valuable biological effects, but its solubility in water is low, posing major limitations in applications. Quercetin encapsulation in liposomes increases its bioavailability; the drug effect on liposome elastic properties is required for formulation development. Here, we quantify the effect of quercetin molecules on the rigidity of lipoid E80 liposomes using atomic force microscopy (AFM) and molecular dynamics (MD) simulations. AFM images show no effect of quercetin molecules on liposomes morphology and structure. However, AFM force curves suggest that quercetin softens lipid membranes; the Young modulus measured for liposomes encapsulating quercetin is smaller than that determined for blank liposomes. We then used MD simulations to interpret the effect of quercetin on membrane rigidity in terms of molecular interactions. The decrease in membrane rigidity was confirmed by the simulations, which also revealed that quercetin affects structural and dynamic properties: membrane thickness is decreased, acyl chains disorder is increased, and diffusion coefficients of lipid molecules are also increased. Such changes appear to be related to the preferential localization of quercetin within the membrane, near the interface between the hydrophobic core and polar head groups of the lipids.

Elastic moduli of lipid membranes: Reproducibility of AFM measures

Membrane elastic properties play a major role in membrane remodeling events, such as vesicle fusion and fission. They are also crucial in drug delivery by liposomes. Different experimental techniques are available to measure elastic properties. Among them, atomic force microscopy (AFM) presents the unique advantage of being directly applicable to nano-sized liposomes. Unfortunately, different AFM measures reported in the literature show little agreement among each other and are difficult to compare with measures of bending modulus obtained by other experimental techniques or by molecular simulations. In this work we determine the bending rigidity of Egg PC liposomes in terms of Young modulus via AFM measurements, using two different tip shapes and different cantilever force constants. We interpret the measures using the Hertz and Shell models, and observe a clear dependency of the Young modulus values on the tip properties and on the interpretative theory. The effect of the AFM tip shape is less important than the effect of the cantilever force constant, and the mathematical model has a major effect on the interpretation of the data. The Shell theory provides the closest agreement between AFM data and other experimental data for the membrane bending modulus. Finally, we compare the results to calculations of bending modulus from molecular dynamics simulations of membrane buckles. Simulations provide values of bending modulus consistent with literature data, but the agreement with AFM experiments is reasonable only for some specific experimental conditions.

Interfacial Properties of Chitosan in Interfacial Shear and Capsule Compression

The time-dependent behavior of surface-active adsorption layers at the oil/water interface can dictate emulsion behavior at both the micro- and macroscale. In addition, self-healing behavior of the adsorption layer may benefit emulsion stability subject to large deformation under processing or during final application. We explore the behavior of chitosan, a known hydrophilic emulsifier, which forms nanoparticle aggregates when the concentration of acetate buffer exceeds 0.3 M. We observe a Pickering adsorption layer building and strain-dependent behavior of the chitosan at the medium chain triglyceride oil/water interface. We compare this to the behavior of identical chitosan layers coated on oil droplets via atomic force microscopy colloidal probe compression in both linear and oscillatory compressions. In both interfacial shear rheometry and the capsule compression, a thick, elastic layer with strong time-dependent recovery behavior is observed, suggesting that the layer has some self-healing capabilities.

Poroelastic properties of hydrogel microparticles

Hydrogels can be formed in a number of different geometries depending upon desired function. However, due to the lack of appropriate models required to interpret experimental data, it remains unclear whether hydrogel microparticles have the same poroelastic properties as hydrogel films made with the same components. We perform numerical simulations to determine the universal force relaxation of a poroelastic hydrogel particle undergoing constant compression by a spherical probe, allowing analysis of experimental measurements of hydrogel particle material properties for the first time. In addition, we perform experimental measurements, using colloidal probe atomic force microscopy, of the force relaxation of polyacrylamide films and particles made with identical monomer and cross-linker concentrations. We fit our universal curve to the experimental data in order to extract material properties including shear modulus, Poisson's ratio and solvent diffusivity. Good agreement is found for the shear modulus and Poisson's ratio between the particles and the films. In contrast, the diffusivity of the polyacrylamide particles was found to be about half that of the films, suggesting that differences in the synthesis and homogeneity of the films and the particles play a role in determining transport and subsequent release of molecules in hydrogel particles.

Modeling atomic force microscopy and shell mechanical properties estimation of coated microbubbles

We present an extensive comparison with experimental data of our theoretical/numerical model for the static response of coated microbubbles (MBs) subject to compression from an atomic force microscope (afm). The mechanics of the MB's coating is described in the context of elastic thin shell theory. The encapsulated fluid is treated as compressible/incompressible pertaining to a gas/liquid, while the thinning of the liquid film between the MB and the afm cantilever is modeled via introduction of an interaction potential and the resulting disjoining pressure. As the external force increases, the experimental force-deformation (f-d) curves of MBs covered with polymer have an initial linear response (Reissner regime), followed by a non-linear curved downwards response (Pogorelov regime) where buckling takes place. On the other hand, the f-d curve for MBs covered with lipid monolayers initially follows the Reissner regime, but buckling is bypassed to a curved upwards regime where internal gas pressure dominates. The elastic properties, namely Young's modulus and shell thickness, for MB's covered with polymer can be estimated by combining the buckling point and the slope of the Reissner regime or the slopes of Reissner and Pogorelov regimes. Comparison of the present model with afm f-d curves for polymer shows satisfactory agreement. The area dilatation and bending moduli are shown to be the appropriate independent elastic parameters of MBs covered with phospholipid monolayers and are estimated by combination of the transition from Reissner to pressure dominated regime. Simulations and experiments in this case are in excellent agreement.

Virus-Inspired Deformable Mesoporous Nanocomposites for High Efficiency Drug Delivery

Mesoporous nanoparticles as a versatile platform for cancer theranostics have been widely used, but their cellular delivery efficiency is still far from satisfactory. Although deformability is emerging as an important parameter influencing cellular uptake enhancement, the facile synthesis of deformable mesoporous nanocomposite with adjustable mechanical property is challenging but meaningful for a deeper understanding of cellular uptake mechanisms and significantly improving cancer therapy. In this work, yolk-shell structured eccentric mesoporous organosilica (YEMO) nanocomposites with adjustable mechanical property are successfully prepared by an organosilane-assisted anisotropic self-assembly approach. The feasibility to precisely control the mechanical property of the YEMO by manipulating the structural parameters, the crosslinking degree of mesoporous framework, and the rotation rate of the reaction is demonstrated. The study of the fabrication mechanism and mechanical properties of YEMO are discussed in detail. The Young's modulus (E_Y ) of YEMO can be adjusted from 2.4 to 65 MPa. Thereby, the continuous control of the cellular uptake from ≈15% to ≈80% under the same incubation time is achieved. To further prove the higher efficiency drug delivery of YEMO with soft characteristics, the higher toxicity of the "soft" YEMO loaded with the anticancer drug doxorubicin compared to the "stiff" one is demonstrated.

Thermally coupled dark-anoxia incubation: A platform technology to induce auto-fermentation and thus cell-wall thinning in both nitrogen-replete and nitrogen-deplete Nannochloropsis slurries

Nitrogen-deprived Nannochloropsis cells invested their fixed carbon into the accumulation of triacylglycerol and cell wall cellulose (thickness of N-replete cell walls = 27.8 ± 5.8, N-deplete cell walls = 51.0 ± 10.2 nm). In this study, the effect of nitrogen depletion on the ability of the cells to weaken their own cell walls via autolysis was investigated. Autolytic cell wall thinning was achieved in both N-replete and N-deplete biomass by incubating highly concentrated slurries in darkness at 38 °C. The incubation forced cells to anaerobically ferment their intracellular cellulose and resulted in 30-40% reduction in cell wall thickness for both biomass types. This wall depletion weakened the cells and increased the extent of cell rupture by mechanical force (from 42 to 78% for N-replete biomass, from 36 to 62% for N-deplete biomass). Importantly, autolysis did not adversely impact the amino acid content of protein-rich N-replete biomass or the fatty acid content of lipid-rich N-deplete biomass.

Use of microaspiration to study the mechanical properties of polymer gel microparticles

The mechanical properties of polyacrylamide (PA) and polydimethylsiloxane (PDMS) microparticle populations have been measured using microaspiration, a recently developed experimental technique. Microaspiration is an augmented version of micropipette aspiration, in which optical microscopy data are obtained as individual soft particles pass through the tip of a micropipette. During microaspiration, the ion current passing through the pipette tip is also measured, and the synchronised optical and current data streams are used to study and quantify mechanical properties. Ion current signatures for the poroelastic PA particles were qualitatively different from those of the viscoelastic PDMS particles. For PA particles the current gradually reduced during each aspiration event, whereas for PDMS particles the current trace resembled a negative top hat function. For PA particles it was found that the maximum change in current during aspiration (ΔIh) increased with particle size. By considering the initial elastic response, a mean effective shear modulus (G') of 6.6 ± 0.2 kPa was found for aspiration of 115 PA particles of ∼10-20 μm diameter. Using a viscoelastic model to describe flow into the pipette, a mean initial effective elastic modulus (E0') of 3.5 ± 1.7 MPa was found for aspiration of 17 PDMS particles of ∼ 9-11 μm diameter. These moduli are consistent with previously reported literature values, providing initial validation of the microaspiration method.

Synthesis, Characterization, and Applications of Polymer-Silica Core-Shell Microparticle Capsules

Encapsulation is a powerful method for the targeted delivery of concentrated reagents and capture of valuable materials in dilute systems. To this end, many encapsulation schemes for specific scenarios have been devised that incorporate chemospecificity or stimulus response in terms of uptake or release. However, an encapsulation platform that enables highly tailorable surface chemistry for targeting, stimulus response, and core chemistry for capture and release of reagents remains elusive. Here, we present such a system comprising composite core-shell capsule particles of hydrophilic polymers coated with thin silica layers synthesized via straightforward one-pot syntheses. Silica is found to encapsulate a range of polymer hydrogels through a mechanism independent of the specific core chemistry. The hybrid materials possess significantly enhanced rigidity while allowing surface modification through simple yet versatile silane coupling reactions without a reduction in the functionality of the core. They are shown to have applications as diverse as recyclable catalysis and controlled delivery vehicles for agrochemicals. The successful synthesis and utilization of this catalog of materials indicate the broader capability of simple composite structures in an array of high-value applications.

Minimum information reporting in bio-nano experimental literature

Studying the interactions between nanoengineered materials and biological systems plays a vital role in the development of biological applications of nanotechnology and the improvement of our fundamental understanding of the bio-nano interface. A significant barrier to progress in this multidisciplinary area is the variability of published literature with regards to characterizations performed and experimental details reported. Here, we suggest a 'minimum information standard' for experimental literature investigating bio-nano interactions. This standard consists of specific components to be reported, divided into three categories: material characterization, biological characterization and details of experimental protocols. Our intention is for these proposed standards to improve reproducibility, increase quantitative comparisons of bio-nano materials, and facilitate meta analyses and in silico modelling.

Ultrasonically synthesized organic liquid-filled chitosan microcapsules: part 2: characterization using AFM (atomic force microscopy) and combined AFM-confocal laser scanning fluorescence microscopy

Atomic Force Microscopy (AFM) is used to measure the stiffness and Young's modulus of individual microcapsules that have a chitosan cross-linked shell encapsulating tetradecane. The oil filled microcapsules were prepared using a one pot synthesis via ultrasonic emulsification of tetradecane and crosslinking of the chitosan shell in aqueous solutions of acetic acid. The concentration of acetic acid in aqueous solutions of chitosan was varied from 0.2% to 25% v/v. The effect of acetic acid concentration and size of the individual microcapsules on the strength was probed. The deformations and forces required to rupture the microcapsules were also measured. Three dimensional deformations of microcapsules under large applied loads were obtained by the combination of Laser Scanning Confocal Microscopy (LSCM) with Atomic Force Microscopy (AFM). The stiffness, and hence the modulus, of the microcapsules was found to decrease with an increase in size with the average stiffness ranging from 82 to 111 mN m-1 and average Young's modulus ranging from 0.4 to 6.5 MPa. The forces required to rupture the microcapsules varied from 150 to 250 nN with deformations of the microcapsules up to 62 to 110% relative to their radius, respectively. Three dimensional images obtained using laser scanning confocal microscopy showed that the microcapsules retained their structure and shape after being subjected to large deformations and subsequent removal of the loads. Based on the above observations, the oil filled chitosan crosslinked microcapsules are an ideal choice for use in the food and pharmaceutical industries as they would be able to withstand the process conditions encountered.

Multiligand Metal-Phenolic Assembly from Green Tea Infusions

The synthesis of hybrid functional materials using the coordination-driven assembly of metal-phenolic networks (MPNs) is of interest in diverse areas of materials science. To date, MPN assembly has been explored as monoligand systems (i.e., containing a single type of phenolic ligand) where the phenolic components are primarily obtained from natural sources via extraction, isolation, and purification processes. Herein, we demonstrate the fabrication of MPNs from a readily available, crude phenolic source-green tea (GT) infusions. We employ our recently introduced rust-mediated continuous assembly strategy to prepare these GT MPN systems. The resulting hollow MPN capsules contain multiple phenolic ligands and have a shell thickness that can be controlled through the reaction time. These multiligand MPN systems have different properties compared to the analogous MPN systems reported previously. For example, the Young's modulus (as determined using colloidal-probe atomic force microscopy) of the GT MPN system presented herein is less than half that of MPN systems prepared using tannic acid and iron salt solutions, and the disassembly kinetics are faster (∼50%) than other, comparable MPN systems under identical disassembly conditions. Additionally, the use of rust-mediated assembly enables the formation of stable capsules under conditions where the conventional approach (i.e., using iron salt solutions) results in colloidally unstable dispersions. These differences highlight how the choice of phenolic ligand and its source, as well as the assembly protocol (e.g., using solution-based or solid-state iron sources), can be used to tune the properties of MPNs. The strategy presented herein expands the toolbox of MPN assembly while also providing new insights into the nature and robustness of metal-phenolic interfacial assembly when using solution-based or solid-state metal sources.

Preparation and application of flavor and fragrance capsules

The preparation methods and applications of flavor and fragrance capsules based on polymeric, inorganic and polymeric–inorganic wall materials are summarized.

Mechanical Properties of Membranes Composed of Gel-Phase or Fluid-Phase Phospholipids Probed on Liposomes by Atomic Force Spectroscopy

In many liposome applications, the nanomechanical properties of the membrane envelope are essential to ensure, e.g., physical stability, protection, or penetration into tissues. Of all factors, the lipid composition and its phase behavior are susceptible to tune the mechanical properties of membranes. To investigate this, small unilamellar vesicles (SUV; diameter < 200 nm), referred to as liposomes, were produced using either unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in aqueous buffer at pH 6.7. The respective melting temperatures of these phospholipids were -20 and 41 °C. X-ray diffraction analysis confirmed that at 20 °C DOPC was in the fluid phase and DPPC was in the gel phase. After adsorption of the liposomes onto flat silicon substrates, atomic force microscopy (AFM) was used to image and probe the mechanical properties of the liposome membrane. The resulting force-distance curves were treated using an analytical model based on the shell theory to yield the Young's modulus (E) and the bending rigidity (k_C) of the curved membranes. The mechanical investigation showed that DPPC membranes were much stiffer (E = 116 ± 45 MPa) than those of DOPC (E = 13 ± 9 MPa) at 20 °C. The study demonstrates that the employed methodology allows discrimination of the respective properties of gel- or fluid-phase membranes when in the shape of liposomes. It opens perspectives to map the mechanical properties of liposomes containing both fluid and gel phases or of biological systems.