Water desorption in Kelvin-probe force microscopy: a generic model.

Nanoparticles or similar, nanoscale objects such as proteins or biological fibrils usually have to be deposited from aqueous suspension onto a solid support surface for further characterization by atomic force microscopy (AFM) and related methods such as Kelvin-probe force microscopy (KFM). Here we show, on the examples of functionalized nanoparticles and collagen fibrils, that water desorption after sample preparation affects their electrostatic potential determined by KFM in a predictable manner. We explain this effect with a simple, analytical model based on the capacitance of the partially dielectric-filled tip-sample system. We also propose practical measures to avoid false interpretation of electrical AFM-based experiments. As the phenomenon is very generic it may have significant implications in the application of AFM to nanoparticles and other nanostructures including biological ones.

Assessing the Potential for Drug-Nanoparticle Surface Interactions To Improve Drug Penetration into the Skin.

There is continued debate as to how nanomaterials enhance the passive diffusion of drugs through the skin. This study examined if drug-nanoparticle surface interactions, which occurred during topical application, had the capability to enhance percutaneous penetration. Atomic force microscopy force adhesion measurements were used to demonstrate that a model drug, tetracaine, strongly adsorbed to the surface of a negatively charged carboxyl-modified polystyrene nanoparticle (NanoPSCOOH) through both its methyl and amine functionalities (up to a 6- and 16-fold greater adhesion force respectively compared with the CH3-CH3 control). These drug-particle adhesion forces were significantly reduced (p < 0.05) to values that were lower than the CH3-CH3 control measurements when tetracaine interacted with a silica nanoparticle (NanoSiO2). This reduction in adhesion was attributed to the lower surface charge of the NanoSiO2 (ca. -23 mV) compared to the NanoPSCOOH (ca. -40 mV), which diminished the electrostatic interactions between positive amine of tetracaine and the negative particle. Mixing NanoPSCOOH with tetracaine on the skin retarded percutaneous drug penetration compared to the control (tetracaine saturated solution without nanoparticles), but the NanoSiO2, which still adsorbed the tetracaine, produced a 3.6-fold enhancement in percutaneous penetration compared to the same control. These data demonstrated the capability of moderate nanoparticle surface interactions that occurred within the application vehicle to promote drug percutaneous penetration.

Signal reversal in Kelvin-probe force microscopy.

Kelvin-probe force microscopy is a measurement mode of atomic force microscopy, which is used to quantitatively map the electrical surface potential of a sample. Inadequate hardware and electronic design can lead to signal cross talk and, in consequence, false results. Here, we show that certain cross talk artifacts not only do manifest themselves in additional noise, reduced resolution, or an offset of the measured surface potential but can also lead to an inverted signal scale and, crucially, cannot be diagnosed with a known reference signal. We show experimental data on an electrically homogeneous sample, describe a method to detect the artifact, and propose simple remedies, which should be well within the reach of most research and industrial laboratories.

Back to basics: the development of a simple, homogenous, two-component dry-powder inhaler formulation for the delivery of budesonide using miscible vinyl polymers.

It was hypothesised that formulating a dry-powder inhaler (DPI) using a refined, smooth grade of lactose, without fines and a polymer coated drug microparticle should produce an homogeneous formulation in which aerosolization behaviour could be modified. Hence, the aim of this study was to develop a simple two component polymer coated-budesonide/lactose blend in which the drug microparticle adhesive forces could be optimised by modifying the drug coating in order to improve aerosolization from a DPI. Budesonide microparticles (1.83 +/- 0.03 microm) were coated with the vinyl polymers by adsorption and then spray-dried. The drug was blended with three different types of lactose, checked for uniformity of mixing and loaded into Pulvinal devices. The median volume particle size of all but one of the polymer coated microparticles remained below 4 microm after spray-drying and the content uniformity for all the blends >96%. Coating the budesonide with 0.01% poly(vinyl alcohol) increased the fine particle fraction (FPF) in the next generation impactor (NGI) from 29.1 +/- 0.7% to 52.8 +/- 1.0% and reduced the force of adhesion from 410 +/- 182 to 241 +/- 82 nN with smooth lactose. This illustrates that vinyl polymers could effectively modify adhesive interactions without the need for ternary components such as fines.

Investigating the ability of nanoparticle-loaded hydroxypropyl methylcellulose and xanthan gum gels to enhance drug penetration into the skin.

Nanoparticle-loaded topical formulations can disrupt drug aggregation through controlled drug-nanoparticle interactions to enhance topical drug delivery. However, the complex relationship between the drug, nanoparticle and formulation vehicle requires further understanding. The aim of this study was to use nanoparticle-loaded hydroxypropyl methylcellulose (HPMC) and xanthan gum gels to probe how the drug, nanoparticle and formulation vehicle interactions influenced the delivery of an aggregated drug into the skin. Tetracaine was chosen as a model drug. It was loaded into HPMC and xanthan gum gels, and it was presented to porcine skin using infinite and finite dosing protocols. Gel infinite doses showed no important differences in tetracaine skin permeation rate, but HPMC gel finite doses delivered the drug more efficiently (46.99±7.96µg/cm2/h) compared to the xanthan gum (1.16±0.14µg/cm2/h). Finite doses of the nanoparticle-loaded HPMC gel generated a 10-fold increase in drug flux (109.95±28.63µg/cm2/h) compared to the equivalent xanthan gum system (14.19±2.27µg/cm2/h). Rheology measurements suggested that the differences in the gels ability to administer the drug into the skin were not a consequence of gel-nanoparticle interactions rather, they were a consequence of the dehydration mediated diffusional restriction imparted on the drug by xanthan gum compared to the viscosity independent interactions of HPMC with the drug.

Investigating the influence of drug aggregation on the percutaneous penetration rate of tetracaine when applying low doses of the agent topically to the skin.

Understanding the molecular aggregation of therapeutic agents is particularly important when applying low doses of a drug to the surface of the skin. The aim of this study was to understand how the concentration of a drug influenced its molecular aggregation and its subsequent percutaneous penetration after topical application. A model drug tetracaine was shown to form a series of different aggregates across the µM (fluorescence spectroscopy) to mM (light scattering analysis) concentration range. The aggregate formation process was sensitive to the pH of the vehicle in which the drug was dissolved (pH 4, critical aggregation concentration (CAC) - 11 µM; pH 8, CAC - 7 µM) and it appeared to have an impact upon the drug's percutaneous penetration. At pH 4, increasing the concentration of the drug in the donor solution decreased the skin permeability coefficient (Kp) of tetracaine (13.7 ± 4.3 × 10(-3)cm/h to 0.06 ± 0.02 × 10(-3)cm/h), whilst at pH 8, it increased the Kp (29.9 ± 9.9 × 10(-3)cm/h to 75.1 ± 41.7 × 10(-3)cm/h). These data trends were reproduced in a silicone membrane and this supported the notion that the more polar aggregates formed at pH 4 acted to decrease the proportion of species available to pass through the skin, whilst the more hydrophobic aggregates formed in pH 8 increased the membrane diffusing species.

The NanoBeamBalance: a passive, tensile-test device for the atomic force microscope.

An add-on device is presented, which significantly expands the force measurement capabilities of the atomic force microscope (AFM). The device consists of a completely passive mechanism, which translates the vertical motion of the AFM tip in force measurements into a horizontal motion of two sample support pads. The advantage is that it is much easier to deposit microscopic samples from suspension onto flat surfaces than to attach them reliably between tip and a surface. The working-principle and the design of the device is comprehensively described and demonstrated on the example of collagen fibres with a diameter of a few µm. Well-defined tensile measurements in longitudinal direction were performed, showing that the tensile stiffness of collagen fibres from rat tail tendon decreases by a factor of 5 when rehydrated from a dried sample and slowly increases upon cross-linking with glutaraldehyde.

Nanoscale scraping and dissection of collagen fibrils.

The main function of collagen is mechanical, hence there is a fundamental scientific interest in experimentally investigating the mechanical and structural properties of collagen fibrils on the nanometre scale. Here, we present a novel atomic force microscopy (AFM) based scraping technique that can dissect the outer layer of a biological specimen. Applied to individual collagen fibrils, the technique was successfully used to expose the fibril core and reveal the presence of a D-banding-like structure. AFM nanoindentation measurements of fibril shell and core indicated no significant differences in mechanical properties such as stiffness (reduced modulus), hardness, adhesion and adhesion work. This suggests that collagen fibrils are mechanically homogeneous structures. The scraping technique can be applied to other biological specimens, as demonstrated on the example of bacteria.