Date of Award

May 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Dacheng Ren

Second Advisor

Anthony Garza

Keywords

Biofilm, Electrostimulation, Implant, Wireless

Subject Categories

Engineering

Abstract

Bacterial biofilms can form on medical implants and cause serious device-associated infections that are incurable by conventional antibiotics because of high-level tolerance to antimicrobials. Common strategies for controlling device-associated infections, such as coating with antimicrobials and modification of surface properties, can reduce or delay biofilm formation, but the inhibitory effect can be overcome by bacteria over time, and eradicating mature biofilms remains challenging. Direct currents (DCs) have been shown to have bactericidal effects and synergy with conventional antibiotics in concurrent treatment has been demonstrated for killing biofilm cells. However, these systems require a direct connection between electrodes and a power source, which requires skin-piercing wiring for current delivery. This is an invasive process that causes discomfort and can lead to secondary infections. In this study, we developed a new method to achieve DC treatment wirelessly towards the non-invasive control of device-associated infections. Pseudomonas aeruginosa PAO1 and Staphylococcus aureus ALC2085 were used as model organisms to investigate the killing efficacy of wirelessly delivered DC.

In the proof-of-concept experiments, we demonstrate that antibiotic tolerant biofilm cells can be effectively eradicated by electromagnetically induced DC from a remote power source. For example, the number of viable P. aeruginosa and S. aureus biofilm cells was reduced by approximately 3.4 and 2 logs, respectively, after treatment with 60 µA/cm2 of wirelessly delivered DC using stainless steel electrodes for 6 hours. DC generated with graphite-based TGONTM electrodes was also effective especially against S. aureus. For example, the viability of P. aeruginosa and S. aureus biofilm was reduced by 1.4 and 2.5 logs, respectively, after treatment with the 30 µA/cm2 of wirelessly delivered DC for 3 hours. Synergy in biofilm killing was also observed between lower level DC and antimicrobials (tobramycin and chlorhexidine for P. aeruginosa and S. aureus, respectively). These conditions were found safe to both human lung epithelial cells and mouse fibroblast cells. Additionally, the viability of S. aureus and Streptococcus mutans biofilms on the denture material were also reduced by 5 and 4 logs, respectively, by the concurrent treatment with the 28 µA/cm2 of DC and 50 µg/mL chlorhexidine for 1 hour.

To further evaluate the potential of this technology, we engineered a prototype device after comparing different device designs with varying shapes, electrode layouts, and electrode materials. The prototype device based on the selected design demonstrated 1.0 and 2.6 logs of killing of P. aeruginosa and S. aureus biofilms, respectively, with 6 µA/cm2 of wirelessly delivered DC for 6 h using an ex vivo model with porcine skin. Further in vivo test using a rabbit model showed that the prototype device inserted under the dermis tissue killed S. aureus biofilm cells by 65 % in vivo when receiving a magnetic field from outside of the body to generate DC. No tissue damage was found according to the histological analysis.

The killing mechanism of DC treatment was investigated in this study by comparing the killing effects of different electrochemical products. The results show that DC treatment using TGON electrodes killed bacterial cells by generating hypochlorite from the anode; while the DC treatment using stainless steel electrodes induced Fenton reaction and generated free radicals that have potent bactericidal effects.

In summary, the findings from this study indicate that wirelessly delivered DC has promising anti-biofilm effects on bacterial pathogens, both in vitro and in vivo. To our best knowledge, this is the first study to report the bactericidal activity of wirelessly delivered DC treatment. With the capability to kill bacterial biofilm without using a directly connected power source, this platform technology has potential applications for noninvasive treatment of biofilm infections associated with orthopedic, cochlear and other implanted medical devices.

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