A model of ultrasound-enhanced diffusion in a biofilm

Ultrasound enhanced diffusion in hydrogels: An experimental and non-equilibrium molecular dynamics study

Focused ultrasound has experimentally been found to enhance the diffusion of nanoparticles; our aim with this work is to study this effect closer using both experiments and non-equilibrium molecular dynamics. Measurements from single particle tracking of 40 nm polystyrene nanoparticles in an agarose hydrogel with and without focused ultrasound are presented and compared with a previous experimental study using 100 nm polystyrene nanoparticles. In both cases, we observed an increase in the mean square displacement during focused ultrasound treatment. We developed a coarse-grained non-equilibrium molecular dynamics model with an implicit solvent to investigate the increase in the mean square displacement and its frequency and amplitude dependencies. This model consists of polymer fibers and two sizes of nanoparticles, and the effect of the focused ultrasound was modeled as an external oscillating force field. A comparison between the simulation and experimental results shows similar mean square displacement trends, suggesting that the particle velocity is a significant contributor to the observed ultrasound-enhanced mean square displacement. The resulting diffusion coefficients from the model are compared to the diffusion equation for a two-time continuous time random walk. The model is found to have the same frequency dependency. At lower particle velocity amplitude values, the model has a quadratic relation with the particle velocity amplitude as described by the two-time continuous time random walk derived diffusion equation, but at higher amplitudes, the model deviates, and its diffusion coefficient reaches the non-hindered diffusion coefficient. This observation suggests that at higher ultrasound intensities in hydrogels, the non-hindered diffusion coefficient can be used.

A Highly Efficacious Electrical Biofilm Treatment System for Combating Chronic Wound Bacterial Infections

Biofilm infection has a high prevalence in chronic wounds and can delay wound healing. Current treatment using debridement and antibiotic administration imposes a significant burden on patients and healthcare systems. To address their limitations, a highly efficacious electrical antibiofilm treatment system is described in this paper. This system uses high-intensity current (75 mA cm-2 ) to completely debride biofilm above the wound surface and enhance antibiotic delivery into biofilm-infected wounds simultaneously. Combining these two effects, this system uses short treatments (≤2 h) to reduce bacterial count of methicillin-resistant S. aureus (MRSA) biofilm-infected ex vivo skin wounds from 1010 to 105.2 colony-forming units (CFU) g-1 . Taking advantage of the hydrogel ionic circuit design, this system enhances the in vivo safety of high-intensity current application compared to conventional devices. The in vivo antibiofilm efficacy of the system is tested using a diabetic mouse-based wound infection model. MRSA biofilm bacterial count decreases from 109.0 to 104.6 CFU g-1 at 1 day post-treatment and to 103.3 CFU g-1 at 7 days post-treatment, both of which are below the clinical threshold for infection. Overall, this novel technology provides a quick, safe, yet highly efficacious treatment to chronic wound biofilm infections.

Effect of ultrasound amplitude and frequency on nanoparticle diffusion in an agarose hydrogel

Exposure of nanoparticles in a porous medium, such as a hydrogel, to low-intensity ultrasound has been observed to dramatically enhance particle penetration rate. Enhancement of nanoparticle penetration is a key issue affecting applications such as biofilm mitigation and targeted drug delivery in human tissue. The current study used fluorescent imaging to obtain detailed experimental measurements of the effect of ultrasound amplitude and frequency on diffusion of nanoparticles of different diameters in an agarose hydrogel, which is often used as a simulant for biofilms and biological tissues. We demonstrate that the acoustic enhancement occurs via the phenomenon of oscillatory diffusion, in which a combination of an oscillatory flow together with random hindering of the particles by interaction with hydrogel proteins induces a stochastic random walk of the particles. The measured variation of acoustic diffusion coefficients with amplitude and frequency were used to validate a previous statistical theory of oscillatory diffusion based on the continuous time random walk approach.

Measurement of ultrasound-enhanced diffusion coefficient of nanoparticles in an agarose hydrogel

An experimental study has been performed to measure the effect of ultrasound on nanoparticle diffusion in an agarose hydrogel. Agarose hydrogel is often used as a simulant for biofilms and certain biological tissues, such as muscle and brain tissue. The work was motivated by recent experiments indicating that ultrasonic excitation of moderate intensity can significantly enhance nanoparticle diffusion in a hydrogel. The objective of the current study was to obtain detailed measurements of the effect of ultrasound on nanoparticle diffusion in comparison to the molecular diffusion in the absence of acoustic excitation. Experiments were conducted with 1 MHz ultrasound waves and nanoparticle diameters of 20 and 100 nm, using fluorescent imaging to measure particle concentration distribution. Under ultrasound exposure, the experiments yield estimates for both acoustic diffusion coefficients as well as acoustic streaming velocity within the hydrogel. Measured values of acoustic streaming velocity were on the order of 0.1 μm/s, which agree well with a theoretical estimate. Measured values of the acoustic diffusion coefficient were found to be 74% larger than the molecular diffusion coefficient of the nanoparticles for 20 nm particles and 133% larger than the molecular diffusion coefficient for 100 nm particles.

Theory for acoustic streaming in soft porous matter and its applications to ultrasound-enhanced convective delivery

This paper develops theory for bulk acoustic streaming in soft porous materials, with applications to biological tissue. The principal results of this paper are: (i) streaming equations for such porous media, which show interestingly significant differences from those that describe streaming in pure fluids; (ii) the Green functions obtained for these equations in isotropic, infinite media; and (iii) approximate evaluation of the sources in the streaming equations from acoustic wave forms often used, and the streaming velocities and particle trajectories resulting therefrom. People are now investigating acoustic enhancement of delivery of therapeutics such as drug molecules or other particulates, introduced directly into cellular tissue. A comparison of the predictions of the theory in this paper to available data is made and shown to be surprisingly good. Some macroscale effects of the ultrastructure of the tissue that are not contained in the current paper are pointed out for future studies.

The synergistic bactericidal effect of vancomycin on UTMD treated biofilm involves damage to bacterial cells and enhancement of metabolic activities

In this study, the synergistic effect of vancomycin, a cell wall synthesis inhibitor, and ultrasound-targeted microbubble destruction (UTMD), on cell viability of Staphylococcus epidermidis, embedded in biofilm, was investigated. Biofilms are the leading causes of antibiotic-resistant bacterial infections of medical implants and prosthetics worldwide. The antibiotic-resistant nature of biofilm-embedded pathogens poses a critical challenge to the medical community. Previously, studies have demonstrated the efficacy of using ultrasound waves and UTMD in circumventing this problem. However, the mechanism(s) underlying this phenomenon was not clear. Here, the present study showed that both ultrasound and UTMD damaged the cell wall structure of S. epidermidis, and floccules and fragments from damaged cells were observed on transmission electron microscope micrograph. However, the cell membrane integrity was not seriously affected by treatments, and the treatment increased the metabolic activity levels of the dormant biofilm-embedded bacteria, detected by confocal laser scanning microscope and flow cytometry, which could make them susceptible to the effect of the antibiotic. Thus, the biological mechanism underlying the efficacy of the combined treatment involving UTMD and vancomycin in the case of S. epidermidis biofilm was dissected, which may be utilized for further investigations on other biofilm pathogens before clinical use.