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Abstract
Cathodic Voltage Controlled Electrical Stimulation (CVCES) is a novel electrochemical technique for treatment and prevention of orthopedic implant-associated infections. The infection is treated by utilizing the metallic implant as a cathode and application of a negative voltage. Promising results have been reported from the previously conducted in vitro and in vivo studies, and various strains of bacteria have been eradicated with and without the use of antibiotics. Before clinical trials can be conducted, it is important to understand the underlying mechanism behind CVCES and develop a reliable dose-planning approach. Computational modeling can serve as a powerful tool to recognize the electrochemical changes that take place adjacent to the implant upon application of CVCES. This thesis uses various orthopedic implant metals as cathode and models the electrochemical reactions that take place on their surface. Oxygen reduction reaction and hydrogen evolution reaction are the considered electrochemical reactions on the cathode, and oxygen and chlorine evolution are considered on the anode. Transport processes in the models consider diffusion and electrophoretic migration of dissolved species, while convection is neglected. The models simulate electric potential profiles, current profiles, and concentration profiles of the involved species over a period of time. Mathematical models, utilizing different sets of electrolyte and electrode configurations have been developed and analyzed in this thesis. Commercial software COMSOL Multiphysics has been used to design the computational models to predict the effects of CVCES on the surrounding environment.
A complementary computational and experimental study is shown with a planar titanium coupon to evaluate the microenvironment changes associated with CVCES in physiological saline. A rapid alkalization was observed in the vicinity of cathode. The pH and current profiles obtained from the theoretical models have been in satisfactory agreement with the experimentally measured values, thus establishing the validity of the models.
Hydrogen embrittlement is a possible detrimental side effect when metallic implant is subjected to a cathodic voltage as a result of hydrogen generation on its surface. An experimental and computational evolution of hydrogen embrittlement of cathodically stimulated titanium is shown in this thesis. Titanium samples, subjected to 24 hours of -1.8 V of CVCES, have been mechanically tested and showed no signs of hydrogen embrittlement. Modeling work has shown accumulation of hydrogen on the working electrode surface beyond its saturation concentration.
Computational models of cathodically stimulated human sized, clinically relevant knee implant components have been developed and effects of implant’s form factor on the surrounding electrochemical environment has been analyzed. Further, knee joint models have been designed by incorporating bicarbonate buffer system with physiological saline as electrolyte to approximate the buffer capacity of the tissue. The introduction of buffer system in the models made the ionic diffusion sluggish and restricted the alkaline pH front. As a part of future clinical application study of CVCES, simulations were performed with different implant compositions and counter electrode configurations and viability of these variations were evaluated.
CVCES modifies the electrochemical microenvironment next to the metallic substrate and that leads to disruption and destruction of the biofilm attached to its surface. Experiments have been performed showing live cell images of biofilm disruption, and the mathematical models show the changes in pH at the corresponding time points, thus relating the electrochemical modifications in the physiological conditions inside the biofilms to the biocidal effects of CVCES. This study helps towards understanding the antimicrobial mechanism behind CVCES.
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