Young's moduli of surface-bound liposomes by atomic force microscopy force measurements

QCM-D Investigations on Cholesterol-DNA Tethering of Liposomes to Microbubbles for Therapy

Lipid-shelled microbubbles (MBs) offer potential as theranostic agents, capable of providing both contrast enhancement in ultrasound imaging as well as a route for triggered drug release and improved localized drug delivery. A common motif in the design of such therapeutic vehicles is the attachment of the drug carrier, often in the form of liposomes, to the microbubble. Traditionally, such attachments have been based around biotin-streptavidin and maleimide-PDP chemistries. Comparatively, the use of DNA-lipid tethers offers potential advantage. First, their specificity permits the construction of more complex architectures that might include bespoke combinations of different drug-loaded liposomes and/or targeting groups, such as affimers or antibodies. Second, the use of dual-lipid tether strategies should increase the strength of the individual tethers tethering the liposomes to the bubbles. The ability of cholesterol-DNA (cDNA) tethers for conjugation of liposomes to supported lipid bilayers has previously been demonstrated. For in vivo applications, bubbles and liposomes often contain a proportion of polyethylene glycol (PEG) to promote stealth-like properties and increase lifetimes. However, the associated steric effects may hinder tethering of the drug payload. We show that while the presence of PEG reduced the tethering affinity, cDNA can still be used for the attachment of liposomes to a supported lipid bilayer (SLB) as measured via QCM-D. Importantly, we show, for the first time, that QCM-D can be used to study the tethering of microbubbles to SLBs using cDNA, signified by a decrease in the magnitude of the frequency shift compared to liposomes alone due to the reduced density of the MBs. We then replicate this tethering interaction in the bulk and observe attachment of liposomes to the shell of a central MB and hence formation of a model therapeutic microbubble.

Effect of mechanical properties of Jurkat cell on adhesion properties of Jurkat integrin and VCAM-1: An AFM study

Mechanical properties play key roles in the immune system, especially the activation, transformation and subsequent effector responses of immune cells. As transmembrane adhesion receptors, integrins mediate the adhesion events of both cells and cell-extracellular matrix (ECM). Integrin affinity would influence the crosslinking of cytoskeleton, leading to the change of elastic properties of cells. In this study, the cells were treated with F-actin destabilizing agent Cytochalasin-D (Cyt-D), fixed by Glutaraldehyde, and cultivated in hypotonic solution respectively. We used Atomic force microscopy (AFM) to quantitatively measure the elasticity of Jurkat cells and adhesion properties between integrins and vascular cell adhesion molecule-1 (VCAM-1), and immunofluorescence to study the alteration of cytoskeleton. Glutaraldehyde had a positive effect on the adhesion force and Young's modulus. However, these mechanical properties decreased in a hypotonic environment, confirming the findings of cellular physiological structure. There was no significant difference in the bond strength and elasticity of Jurkat cells treated with Cytochalasin-D, probably because of lower importance of actin in suspension cells. All the treatments in this study pose a negative effect on the adhesion probability between integrins and VCAM-1, which demonstrates the effect of structural alteration of the cytoskeleton on the conformation of integrin. Clear consistency between adhesion force of integrin/VCAM-1 bond and Young's modulus of Jurkat cells was shown. Our results further demonstrated the relationship between cytoskeleton and integrin-ligand by mechanical characteristics.

Shape stability of ellipsoidal nanomaterials prepared by physical deformation

The nonspherical shape of nanomaterials (NMs) represents a key attribute for controlling their biological behaviors. Analyzing shape stability over time represents a significant concern because nonspherical NMs are likely to rearrange into a thermodynamically more stable spherical shape. In this investigation, ellipsoidal NMs were designed by physical deformation of core/shell nanospheres composed of poly(isobutylcyanoacrylate) and chitosan or a mixture of chitosan and thiolated chitosan. After optimizing the process parameters for designing ellipsoidal NMs, the shape stability during storage was investigated for 6 months at different temperatures (4 °C, 20 °C and 40 °C). The NM shape was examined by analyzing the aspect ratio from images obtained by electron microscopy techniques. The results demonstrated the feasibility of designing shape-persistent ellipsoidal NMs by physical deformation of spherical particles.

Role of the interactions of soft hyaluronan nanomaterials with CD44 and supported bilayer membranes in the cellular uptake

Increasing valence by acting on nanomaterial morphology can enhance the ability of a ligand to specifically bind to targeted cells. Herein, we investigated cell internalization of soft hyaluronic acid (HA) nanoplatelets (NPs) that exhibit a typical hexagonal shape, flat surfaces and high aspect ratio (Γ≈12 to 20), as characterized by atomic force microscopy in hydrated conditions. Fluorescence imaging revealed that internalization of HA-NPs by a T24 tumor cell line and by macrophages was higher than native polysaccharide in a dose-dependent and time-dependent manners. The ability of HA-NPs to efficiently compete with native HA assessed using Bio-layer interferometry showed that NPs had a stronger interaction with recombinant CD44 receptor compared to native HA. The results were discussed regarding physical properties of the NPs and the implication of multivalent interactions in HA binding to CD44. Experiments conducted on supported bilayer membranes with different compositions showed that non-specific interactions of NPs with lipid membranes were negligible. Our findings provide insights into intracellular drug delivery using soft HA-NPs through receptor-mediated multivalent interactions.

Protective effect of syringic acid via restoring cells biomechanics and organelle structure in human lens epithelial cells

We have previously reported that syringic acid (SA) extracted from D. aurantiacum var. denneanum (kerr) may be used to prevent diabetic cataract (DC). However, the underlying mechanisms through which SA prevents DC in human lens epithelial cells (HLECs) remained unclear. In the present study, we employed single-molecule optics technologies, including transmission electron microscopy (TEM), atomic force microscopy (AFM), laser scanning confocal microscopy (LSCM) and Raman spectroscopy, to monitor the effect of SA on HLECs biomechanics and organelle structure in real-time. TEM suggested that SA improved the ultrastructure of HLECs with regard to nuclear chromatin condensation and reducing mitochondrial swelling and degeneration, which may aid in the maintenance of HLECs integrity in the presence of glucose. AFM revealed a reduced surface roughness and stiffness following SA treatment, suggesting an improved viscoelasticity of HELCs. Raman spectrometry and LSCM further revealed that these changes were related to a modification of cell liquidity and cytoskeletal structure by SA. Taken together, these results provide insights into the effects of SA on the biomechanics of HLECs and further strengthen the evidence for its potential use as a novel therapeutic strategy for DC prevention.

Stiffness of HIV-1 Mimicking Polymer Nanoparticles Modulates Ganglioside-Mediated Cellular Uptake and Trafficking

The monosialodihexosylganglioside, GM3, and its binding to CD169 (Siglec-1) have been indicated as key factors in the glycoprotein-independent sequestration of the human immunodeficiency virus-1 (HIV-1) in virus-containing compartments (VCCs) in myeloid cells. Here, lipid-wrapped polymer nanoparticles (NPs) are applied as a virus-mimicking model to characterize the effect of core stiffness on NP uptake and intracellular fate triggered by GM3-CD169 binding in macrophages. GM3-functionalized lipid-wrapped NPs are assembled with poly(lactic-co-glycolic) acid (PLGA) as well as with low and high molecular weight polylactic acid (PLAlMW and PLAhMW) cores. The NPs have an average diameter of 146 ± 17 nm and comparable surface properties defined by the self-assembled lipid layer. Due to differences in the glass transition temperature, the Young's modulus (E) differs substantially under physiological conditions between PLGA (E PLGA = 60 ± 32 MPa), PLAlMW (E PLA lMW = 86 ± 25 MPa), and PLAhMW (E PLA hMW = 1.41 ± 0.67 GPa) NPs. Only the stiff GM3-presenting PLAhMW NPs but not the softer PLGA or PLAlMW NPs avoid a lysosomal pathway and localize in tetraspanin (CD9)-positive compartments that resemble VCCs. These observations suggest that GM3-CD169-induced sequestration of NPs in nonlysosomal compartments is not entirely determined by ligand-receptor interactions but also depends on core stiffness.

Cross-linker-Modulated Nanogel Flexibility Correlates with Tunable Targeting to a Sterically Impeded Endothelial Marker

Deformability of injectable nanocarriers impacts rheological behavior, drug loading, and affinity target adhesion. Here, we present atomic force microscopy (AFM) and spectroscopy measurements of nanocarrier Young's moduli, tune the moduli of deformable nanocarriers with cross-linkers, and demonstrate vascular targeting behavior that correlates with Young's modulus. Homobifunctional cross-linkers were introduced into lysozyme-dextran nanogels (NGs). Single particle-scale AFM measurements determined NG moduli varying from ∼50-150 kPa for unmodified NGs or NGs with a short hydrophilic cross-linker (2,2'-(ethylenedioxy)bis(ethylamine), EOD) to ∼350 kPa for NGs modified with a longer hydrophilic cross-linker (4,9-dioxa-1,12-dodecanediamine, DODD) to ∼10 MPa for NGs modified with a longer hydrophobic cross-linker (1,12-diaminododecane, DAD). Cross-linked NGs were conjugated to antibodies for plasmalemma vesicle associated protein (PLVAP), a caveolar endothelial marker that cannot be accessed by rigid particles larger than ∼100 nm. In previous work, 150 nm NGs effectively targeted PLVAP, where rigid particles of similar diameter did not. EOD-modified NGs targeted PLVAP less effectively than unmodified NGs, but more effectively than DODD or DAD modified NGs, which both yielded low levels of targeting, resembling results previously obtained with polystyrene particles. Cross-linked NGs were also conjugated to antibodies against intracellular adhesion molecule-1 (ICAM-1), an endothelial marker accessible to large rigid particles. Cross-linked NGs and unmodified NGs targeted uniformly to ICAM-1. We thus demonstrate cross-linker modification of NGs, AFM determination of NG mechanical properties varying with cross-linker, and tuning of specific sterically constrained vascular targeting behavior in correlation with cross-linker-modified NG mechanical properties.

Characteristics and Glucose Uptake Promoting Effect of Chrysin-Loaded Phytosomes Prepared with Different Phospholipid Matrices

Chrysin-loaded phytosomes (CP) were prepared using either soya phosphatidylcholine (SPC) or egg phospholipid (EPL) by the solvent evaporation method. Different phospholipid matrices resulted in significant differences in size, mechanical property and solubility of the CP. The most stable CP was obtained with EPL at a molar ratio of 1:3 (chrysin: EPL, CEP-1:3). CEP-1:3 displayed an average size of 117 nm with uniform size distribution (polydispersity index: 0.30) and zeta potential of -31 mV. A significantly greater elastic modulus of CEP-1:3 (2.7-fold) indicated tighter packing and strong molecular bonding than those of CP prepared with SPC (CSP-1:3). X-ray diffraction and Fourier transform infrared spectroscopic analysis of CEP-1:3 confirmed molecular complexation. CEP-1:3 displayed a greater glucose uptake promoting effect than free chrysin and CSP-1:3 in muscle cells by stimulating gene expression of peroxisome proliferator-activated receptor γ and glucose transporter type 4. The results of the present study suggest that the phospholipid matrix used for the preparation of phytosomes critically influences their performance.

Acoustic deformation for the extraction of mechanical properties of lipid vesicle populations

We use an ultrasonic standing wave to simultaneously trap and deform thousands of soft lipid vesicles immersed in a liquid solution. In our device, acoustic radiation stresses comparable in magnitude to those generated in optical stretching devices are achieved over a spatial extent of more than ten acoustic wavelengths. We solve the acoustic scattering problem in the long-wavelength limit to obtain the radiation stress. The result is then combined with thin-shell elasticity theory to form expressions that relate the deformed geometry to the applied acoustic field intensity. Using observation of the deformed geometry and this model, we rapidly extract mechanical properties, such as the membrane Young's modulus, from populations of lipid vesicles.

Detailed Morphological Characterization of Nanocrystalline Active Ingredients in Solid Oral Dosage Forms Using Atomic Force Microscopy

The characterization of nanocrystalline active ingredients in multicomponent formulations for the design and manufacture of products with increased bioavailability is often challenging. The purpose of this study is to develop an atomic force microscopy (AFM) imaging method for the detailed morphological characterization of nanocrystalline active ingredients in multicomponent oral formulations. The AFM images of aprepitant and sirolimus nanoparticles in aqueous suspension show that their sizes are comparable with those measured using dynamic light scattering (DLS) analysis. The method also provides information on a wide-sized range of particles, including small particles that can often only be detected by DLS when larger particles are removed by additional filtration steps. An expected advantage of the AFM method is the ability to obtain a detailed information on particle morphology and stiffness, which allows the active pharmaceutical ingredient and excipient (titanium dioxide) particles to be distinguished. Selective imaging of particles can also be achieved by varying the surface properties of the AFM solid substrate, which allows to control the interactions between the substrate and the active pharmaceutical ingredient and excipient particles. AFM analysis in combination with other methods (e.g., DLS), should facilitate the rational development of formulations based on nanoparticles.

Calibration of colloidal probes with atomic force microscopy for micromechanical assessment

Mechanical assessment of biological materials and tissue-engineered scaffolds is increasingly focusing at lower length scale levels. Amongst other techniques, atomic force microscopy (AFM) has gained popularity as an instrument to interrogate material properties, such as the indentation modulus, at the microscale via cantilever-based indentation tests equipped with colloidal probes. Current analysis approaches of the indentation modulus from such tests require the size and shape of the colloidal probe as well as the spring constant of the cantilever. To make this technique reproducible, there still exist the challenge of proper calibration and validation of such mechanical assessment. Here, we present a method to (a) fabricate and characterize cantilevers with colloidal probes and (b) provide a guide for estimating the spring constant and the sphere diameter that should be used for a given sample to achieve the highest possible measurement sensitivity. We validated our method by testing agarose samples with indentation moduli ranging over three orders of magnitude via AFM and compared these results with bulk compression tests. Our results show that quantitative measurements of indentation modulus is achieved over three orders of magnitude ranging from 1 kPa to 1000 kPa via AFM cantilever-based microindentation experiments. Therefore, our approach could be used for quantitative micromechanical measurements without the need to perform further validation via bulk compression experiments.

Investigation on the Nanomechanics of Liposome Adsorption on Titanium Alloys: Temperature and Loading Effects

The mechanical properties of liposomes, determined by the lipid phase state at ambient temperature, have a close relationship with their physiological activities. Here, atomic force microscopy (AFM) was used to produce images and perform force measurements on titanium alloys at two adsorbed temperatures. The mechanical properties were evaluated under repeated loading and unloading, suggesting a better reversibility and resistance of gel phase liposomes. The liquid phase liposomes were irreversibly damaged during the first approach while the gel phase liposomes could bear more iterations, resulting from water flow reversibly going across the membranes. The statistical data offered strong evidence that the lipid membranes in the gel phase are robust enough to resist the tip penetration, mainly due to their orderly organization and strong hydrophobic interactions between lipid molecules. This work regarding the mechanical properties of liposomes with different phases provides guidance for future clinical applications, such as artificial joints.

3D depth profiling of the interaction between an AFM tip and fluid polymer solutions

In the atomic force microscopy (AFM) investigation of soft polymers and liquids, the tip-sample interaction is dominated by long-range van der Waals forces, capillary forces and adhesion. Furthermore, the tip can indent several tens of nanometres into the surface, and it can pull off a polymer filament from the surface. Therefore, measuring the unperturbed shape of a polymeric fluid can be challenging. Here, we study the tip-sample interaction with polystyrene droplets swollen in chloroform vapour, where we can utilize the solvent vapour concentration to adjust the specimen's mechanical properties from a stiff solid to a fluid film. With the same AFM tip, we use two different AFM force spectroscopy methods to measure three-dimensional (3D) depth profiles of the tip-sample interaction: force-distance (FD) curves and amplitude-phase-distance (APD) curves. The 3D depth profiles reconstructed from FD and APD measurements provide detailed insight into the tip-sample interaction mechanism for a fluid polymer solution. The fluid's intrinsic relaxation time, which we measure with an AFM-based step-strain experiment, is essential for understanding the tip-sample interaction mechanism. Furthermore, measuring 3D depth profiles and using APD data to reconstruct the unperturbed surface comprise a versatile methodology for obtaining accurate dimensional measurements of fluid and gel-like objects on the nanometre scale.

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.

Mechanical response of adherent giant liposomes to indentation with a conical AFM-tip

Indentation of giant liposomes with a conical indenter is described by means of a tension-based membrane model. We found that nonlinear membrane theory neglecting the impact of bending sufficiently describes the mechanical response of liposomes to indentation as measured by atomic force microscopy. Giant vesicles are gently adsorbed on glassy surfaces via avidin-biotin linkages and indented centrally using an atomic force microscope equipped with conventional sharp tips mounted on top of an inverted microscope. Force indentation curves display a nonlinear response that allows to extract pre-stress of the bilayer T0 and the area compressibility modulus KA by computing the contour of the vesicle at a given force. The values for KA of fluid membranes correspond well to what is known from micropipet suction experiments and inferred from membrane undulation monitoring. Assembly of actin shells inside the liposome considerably stiffens the vesicles resulting in significantly larger area compressibility modules. The analysis can be easily extended to different indenter geometries with rotational symmetry.

Characteristics of spermatogonial stem cells derived from neonatal porcine testis

The aim of this study was to isolate and characterise porcine spermatogonial stem cells (PSSCs). The putative porcine germline stem cells from testis were isolated successfully by an improving way of enrichment with lymphocyte separation medium (LSM). Results from RT-PCR analyses showed that PSSCs were positive for OCT4, SOX2, NANOG, PGP9.5, c-MYC, KEL4 and PRDM-14 which are multipotent stem cell markers. At the protein level, the results of immunofluorescence analyses showed that PSSCs were positive for OCT4, PGP9.5, SOX2 and CD29. We successfully differentiated these PSSCs into adipocytes and muscle cells and then defined their characteristics, including morphology, surface stem cell markers, and mechanical properties. But the experiment of teratoma formation was negative. The results indicated the PSSCs could be multipotent. Atomic force microscopy was used to characterise the morphological and mechanical properties of undifferentiated PSSCs, as well as the differentiated adipocytes and muscle cells, which could be potentially useful for distinguishing PSSCs from differentiated cells.

WAVE PROPAGATION IN LIPOSOMES

The general behavior of wave propagation in liposomes, including the effect of rotary inertia, is examined in this paper, based on a continuum cylindrical shell model. The disperse curves are obtained by solving an eigenvalue problem. The characteristics of wave propagation in liposomes are described using numerical examples. The results show that wave propagation in liposomes has a threshold critical frequency beyond which the wave speed drops dramatically and also a cut-off critical frequency below which the corresponding wave mode does not appear. The torsional wave speed is obtained for the symmetrical circumferential mode n = 0. The cut-off or threshold critical frequency decreases with the increase of liposomal radius, but the effect of radius on wave speed is not significant in the frequency region higher than the critical frequency. On the other hand, the wave number n leads to an increase in the critical frequency. For the first and second wave modes, the wave speed is insensitive to the wave number when the frequency is greater than the critical frequency. For the third wave mode in the low frequency region, the wave number leads to an increase in the wave speed. The rotary inertia has little influence on those wave modes which contain cut-off frequencies. For other wave modes, the rotary inertia results in a decrease in the wave speed in the high frequency region. This investigation may provide a useful guide in the applications of liposomes in ultrasound-based drug delivery and release.

Hydroxyl radical scavenging mechanism of human erythrocytes by quercetin-germanium (IV) complex

Quercetin is a popular flavonoid in plant foods, herbs, and dietary supplement. Germanium, a kind of trace elements, can enhance the body immunity. This study investigated the hydroxyl-radical-scavenging mechanism of the quercertin-germanium (IV) (Qu-Ge) complex to human erythrocytes, especially the effects on ultrastructure and mechanical properties of cell membrane, plasma membrane potential and intracellular free Ca(2+) concentration. Results showed that QuGe(2), a kind of the Qu-Ge complex, could reduce the oxidative damage of erythrocytes, change the cell-surface morphology, and partly recover the disruption of plasma membrane potential and intracellular free Ca(2+) level. Atomic force microscopy (AFM) was used to characterize the changes of the cell morphology, cell-membrane ultrastructure and biophysical properties at nanoscalar level. QuGe(2) has triggered the antioxidative factor to inhibit cellular damage. These results can improve the understanding of hydroxyl-radical-scavenging mechanism of human erythrocytes induced by the Qu-Ge complex, which can be potentially developed as a new antioxidant for treatment of oxidative damage.

The role of particle geometry and mechanics in the biological domain

Nanostructured particulate materials are expected to revolutionize diagnostics and the delivery of therapeutics for healthcare. To date, chemistry-derived solutions have been the major focus in the design of materials to control interactions with biological systems. Only recently has control over a new set of physical parameters, including size, shape, and rigidity, been explored to optimize the biological response and the in vivo performance of nanoengineered delivery vectors. This Review highlights the methods used to manipulate the physical properties of particles and the relevance of these physical properties to cellular and circulatory interactions. Finally, the importance of future work to synergistically tailor both physical and chemical properties of particulate materials is discussed, with the aim of improving control over particle interactions in the biological domain.

AFM studies of cellular mechanics during osteogenic differentiation of human amniotic fluid-derived stem cells

Amniotic fluid-derived stem cells (AFSCs) are becoming an important source of cells for regenerative medicine given with apparent advantages of accessibility, renewal capacity and multipotentiality. In this study, the mechanical properties of human amniotic fluid-derived stem cells (hAFSCs), such as the average Young's modulus, were determined by atomic force microscopy (3.97 ± 0.53 kPa for hAFSCs vs. 1.52 ± 0.63 kPa for fully differentiated osteoblasts). These differences in cell elasticity result primarily from differential actin cytoskeleton organization in these two cell types. Furthermore, ultrastructures, nanostructural details on the surface of cell, were visualized by atomic force microscopy (AFM). It was clearly shown that surface of osteoblasts were covered by mineralized particles, and the histogram of particles size showed that most of the particles on the surface of osteoblasts distributed from 200 to 400 nm in diameter, while the diameter of hAFSCs particles ranged from 100 to 200 nm. In contrast, there were some dips on the surface of hAFSCs, and particles were smaller than that of osteoblasts. Additionally, as osteogenic differentiation of hAFSCs progressed, more and more stress fibers were replaced by a thinner actin network which is characteristic of mature osteoblasts. These results can improve our understanding of the mechanical properties of hAFSCs during osteogenic differentiation. AFM can be used as a powerful tool for detecting ultrastructures and mechanical properties.

Determination of the mechanical properties of DOPC:DOPS liposomes using an image procession algorithm and micropipette-aspiration techniques

Quantification of the mechanical properties of liposomes is critical in helping to predict their behavior during various applications such as targeted drug delivery, response to mechanical characterization or their interactions with isolated cytoskeletal elements. A numerical implementation of the Evans aspiration technique, and an image processing algorithm for measuring deformation of spherical DOPC:DOPS liposomes is presented. Liposomes were aspirated to pressures of -10mmHg (∼-1300Pa). The area expansion and Young's moduli of the liposomes were found to be 0.067Nm⁻¹ (67±4dyn/cm) and 15±1 MPa.

Liposome characterization by quartz crystal microbalance measurements and atomic force microscopy

This chapter reviews liposome characterization by quartz crystal microbalance (QCM) measurements and atomic force microscopy (AFM). In many studies, AFM imaging is simply used to image liposomes with resolution often that does not allow morphological analysis. Although liposome size can be obtained by processing AFM images, it is found that liposomes flatten upon surface adsorption or immobilization. Liposome stability and stiffness have been characterized by using AFM imaging or AFM force measurements, although the latter method, using a microsphere attached on the AFM cantilever, seems more appropriate to limit liposome damage and to obtain more quantitative analysis, such as the Young's modulus. Investigation of liposome layers by QCM revealed that liposomes can be detected from a combined analysis of frequency and bandwidth shifts. However, QCM by itself provides only limited information on liposomes. QCM can be used to assess the presence of a layer and also to discriminate between rigid and viscoelastic ones. Liposome properties have been derived from QCM curves, but often this requires making hypotheses that are difficult to assess. AFM and QCM analyses need to be combined with other techniques to provide complementary information.

The morphological and biomechanical changes of keratocytes cultured on modified p (HEMA-MMA) hydrogel studied by AFM

The poor integration with host cornea tissue and the low mechanical properties of pHEMA hydrogel for artificial cornea remains a difficult problem to solve. A modified pHEMA hydrogel, MMA copolymerized and type-I collagen and bFGF immobilized, was previously prepared in an attempt to solve the problems. In this study, the cytotoxicity of Col/bFGF-p (HEMA-MMA) and p (HEMA-MMA) was studied by cell adhesion assay and atomic force microscopy (AFM). The results of cell adhesion assay show that the attachment of keratocytes on the modified membrane is much higher than that of the unmodified membrane. This indicates that the material after modification have better cell-material interaction. The AFM images reveal that the morphology of keratocytes cultured on different substrate is obviously different. The cell cultured on modified membrane presented a completely elongated and spindle-shape morphology. The force-distance indicates that the biomechanical of keratocytes changes significantly after culturing on different substrates. The adhesion force (2328+/-523 pN) and Young's modulus (0.51+/-0.125 kPa) of the cell cultured on modified membrane are much higher, and the stiffness (0.08+/-0.022 mN/m) is lower than those of the cell cultured on unmodified membrane. These results show that the cytotoxicity of Col/bFGF-p (HEMA-MMA) for keratocytes is much improved.

Atomic force microscopy: a tool to study the structure, dynamics and stability of liposomal drug delivery systems

Much work has been done during the past few decades to develop effective drug delivery systems (DDS), many of which are based on nanotechnology science. Liposomes are the most attractive lipid vesicles for drug delivery. The multifunctional properties of liposomes have a key role in modifying the bioavailability profile of a therapeutic agent. Different analytical techniques can be used to describe liposomes, not least applied scanning probe microscopy (SPM) techniques. Atomic force microscopy (AFM) seems to be one of the most effectively applied SPM techniques. This review article outlines the applications of AFM in evaluating the physical characteristics and stability of liposomal DDSs. Other well-known microscopy techniques used in evaluating liposome physical characteristics are also mentioned, and the contribution of AFM to evaluating liposomal stability is discussed. Among the advantages of AFM in examining the physicochemical properties of liposomal DDSs is its ability to provide morphological and metrology information on liposome properties. AFM thus appears to be a promising tool in technological characterization of liposomal DDSs.

Artesunate induced morphological and mechanical changes of Jurkat cell studied by AFM

In this study, we have used atomic force microscopy (AFM) to study the morphology and mechanical property changes of Jurkat cells exposed to different concentrations of Artesunate (ART) for 24 h at single cellular level. Cell viability and proliferation assays were performed by using the Cell Counting Kit-8. The concentration of ART, which resulted in the inhibition rate >50% was selected. The AFM images revealed that the cell membrane changed and the ultrastructure also became complex. Mechanical properties of individual cell were tracked with AFM-based force spectroscopy. The force curves revealed that when a cell was exposed to the ART, the mechanical properties changed obviously. Treated cells had a lower adhesion force of 416.8+/-37.9 pN, whereas control group had a higher adhesion force of 1064.2+/-97.0 pN. The Young's modulus decreased to nearly one-third, from control group of 0.648+/-0.037 kPa to treated group of 0.254+/-0.035 kPa and the stiffness increased to nearly 1.5 times, from control group of 1.231+/-0.084 mN/m to treated group of 1.917+/-0.137 mN/m. These results suggest that ART can inhibit the proliferation of Jurkat and induce changes in the morphological structure and mechanical properties of Jurkat cells. The high resolution and high sensitivity of AFM can be used to detect morphological and mechanical properties of cells exposed to ART. The AFM may be developed to be a useful tool for detecting the cell death and evaluating the anti-carcinogen efficacy against tumor cell. SCANNING 31: 83-89, 2009. (c) 2009 Wiley Periodicals, Inc.

AFM study of the stability of a dense affinity-bound liposome layer

Liposomes that are surface-bound to a biomaterial such as a contact lens are of interest for localized delivery of therapeutic agents, but it is not known whether such liposome layers are sufficiently robust. The stability of a dense, PEG-functionalized layer of liposomes, affinity-bound onto a multilayer coated surface, was tested under various stress conditions using colloid-probe atomic force miscroscopy (AFM). The different stress effects were generated by varying the applied normal load of the probe and the impinging fluid shear through different approach velocities and by varying the applied lateral forces by scanning under increasing force loads. The effect of applied forces (normal and lateral) was further investigated by coating the probe with a layer of albumin. The liposomes remained intact following the ramping of both protein-coated and uncoated probes under the normal and lateral loads. The low-fouling nature of these liposomes, with respect to nonspecific protein adsorption, was also demonstrated from the interaction force measurements which showed only weak adhesion from the protein layer during the contact period of the albumin-coated probe. The observed adhesive interactions were concluded to be a direct result of the applied load from the probe, during the force measurements, rather than from attraction of the protein molecules for the surface-bound liposomes. The low frictional response of the liposome layer indicated the viscoelastic nature of these molecules, which enabled liposome structure retention during the continuous load application. The demonstrated stability of the liposomes presents a system of viable and localized drug delivery in, for example, ophthalmic applications.