Date of Award

Fall 2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical Engineering

First Advisor

Yuri Lvov

Abstract

Encapsulation of antimicrobial agents (simple antiseptics and more specific antibiotics) within micro-scale and nano-scale containers may provide prolonged and more evenly distributed drug release. One of such containers proposed at Louisiana Tech is natural halloysite clay nanotubes. Halloysite is an aluminosilicate tube with a length of approximately 1 µm, outer diameter of approximately 50 nm, and internal lumen of 15 nm. The chemical composition of halloysite is similar to more common clay–kaolinite, and it can be described as rolled sheets of kaolinite. Halloysite, loaded with drugs inside its lumen, has shown aqueous release of the loaded chemicals over 10-20 hours and, with formation of the tube-end stoppers, for days. Typical loading efficiency for drugs is 5-10 wt%. An attractive aspect of halloysite nanotubes is an economically viable, green and biocompatible material. However, the size of halloysite clay tube is not in the safe size range(less than 1 µm) for injection.

In this study, halloysite tubes were used as nanocontainers loaded with antiseptics and antibiotics, and the controlled release of these drugs was monitored. The popular antiseptic brilliant green was loaded into halloysite at 5 wt% and provided sustained release over 24 hours. The release profile was fitted with an exponential first-order curve time rate of approximately 3.5 hours. Measurement of antibacterial efficiency of the brilliant green tubule nano-formulation on S. aureus bacterial culture demonstrated that the antibacterial action was extended up to 72 hours (as compared with 4-5 hours activity for non-encapsulated brilliant green). In order to further decrease the release rate, tube-ending stoppers were developed. The formation of benzotriazole-copper coating on halloysite nanotubes provided additional encapsulation and slowed release of the loaded substances to 100-200 hours.

Silver ions have strong antimicrobial efficiency, and we used halloysite as a silver carrier. First, we developed template synthesis of 15 nm diameter silver nanorods inside the tube lumen and, second, spherical 5-10 nm diameter silver nanoparticles on the nanotube external surface. The composites of silver nanorods encased in halloysite tubes with the polymer paint were prepared, and the composite coating enhanced antimicrobial activity and increased the tensile strength. An additional advantage was that the coating containing 5 wt% silver-loaded halloysite did not change color after light exposure, contrary to the sample prepared with unshelled silver nanoparticles (so, we described protective core-shell effect). Halloysite nanotube templates have an additional potential for scalable manufacturing of ceramic encapsulated metal nanorods for composite materials.

In the next stage of this research project, we studied loading and slow release of two antibiotics (ciprofloxacin and gentamicin) from halloysite nanotubes. First, we demonstrated enhanced efficiency of antibiotics loaded in these clay nanotubes for multidrug-resistant gangrene bacteria (longer time action). Afterwards, we developed doping of bone cements (polymethylmethacrylate and tricalcium phosphates) with 5-8 wt% of antibiotics loaded halloysite. This approach allowed us synergetic improvement of adhesivity and strength of the bone composites in combination with longer time (200 hours) antibiotics release. This is the time typically needed for after surgery treatment of implant-bone conjunctions (bone glue).

Orthopedic grade polymethylmethacrylate (PMMA) bone cement, admixed with prophylactic antibiotics (e.g. gentamicin), is widely used in hip and knee replacement surgery. For cement loaded with antibiotics, there is a critical need to improve its structural integrity and to control antibiotic release. In particular, preventing reaction of the drug with the cement during its polymerization is important. The resulting composite material has a significantly larger material strength and adhesion on cow bone surface and provides sustained release of antibiotics loaded within the halloysite lumen. Besides, the addition of halloysite significantly (ca. 20 ° C) reduces the PMMA polymerization temperature. When added directly, many common antibiotics have carbonyl groups that react with monomers or decompose during free radical polymerization. This construct allows separation of antibiotics from other components of the cement and hence provides a larger choice of antibiotics. In future work, antibiotics combinations in the nanotubes could be mixed in the cement with enhanced antibacterial action for a customized, sustained, "a la carte" multiple drug delivery. Halloysite nanotubes have no cytotoxic effects placing halloysite on par with other well-known materials, such as bioactive glass, silica and hydroxyapatite.

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