A Carbon Drug Delivery System for Lithium
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ABSTRACT Lithium was introduced into pyrolyzed phenolic resins by dissolving lithium nitrate (5, 10, and 15% by mass) in a resol precursor. Impregnated specimens were pyrolyzed at 500'C, 575'C, and 650 0 C in inert atmosphere. After pyrolysis, samples were placed in 5 ml of phosphate buffered saline solution, refrigerated at 5'C, for various Li release times. Inductively coupled plasmaatomic spectroscopy was used to analyze these solutions, which were tested every 24 hours for 5 days; another set was tested after 60 days. Samples containing 5% lithium salt, fired to 500'C, released Li+ at a lower rate than those fired at higher temperature. At the early stages of exposure to saline, samples fired at 575°C and 650'C released Li- at a higher rate, which fell to that of 500'C samples after many days. After leaching, nuclear reaction analysis using alpha radiation, with an exposure time of I hour, allowed us to analyze [Li+] and gradient up to 121im below the surface. This indicates that a smaller [Li+] remains in 650'C samples than in those fired at 575°C and 500'C. For 500'C samples, [Li÷] near the surface was lower than that for samples fired at 575°C and 650 0 C. This indicates that 500'C samples release Li+ from near the surface, whereas samples fired at higher temperature release Li+ from deep below the surface, probably because of higher permeability. Li+ release rates of samples fired at 500'C and below 650'C follow a simple diffusion law, with diffusivities between 10j" and 10-18 m2/s. Li+ rate may controlled over long time by a multilayered sprayed precursor with variable concentration. INTRODUCTION Because of the low safety threshold for poisonous drugs, and the danger and inefficiency of delivering these drugs orally, there is a need for a drug delivery system with a controlled rate at
various locations in the human body. The implanted material must be biocompatible and easily formed to a geometry dictated by its location. We have designed a Li' delivery system, using pyrolyzed resin. Release rate from resin depends on the firing temperature and fabrication technique, both of which affect permeability. This allows designing a multilayered drug delivery system with variable release rate, using either of molding or spraying technique. When gelled resol is heated to 230'C, it is converted to a fully cured phenolic resin; there is a mass loss of I1%, mostly due to condensation leading to chain and cross-link formation. Upon raising the temperature, this resin transforms directly into a carbon of specific gravity 1.45, which retains the morphology of the resin without passing through a mesomorphic stage. This form of carbon has a glass-like presence and is referred to as Glassy Polymeric Carbon (GPC) [1]. The properties of the pyrolyzed resin depend on the precursor, the manufacturing process and firing temperature [2]. The escape rate of gaseous products and aromatic molecules varies with temperature and heating rate. These products leave pores and out gassing passages, which lead to permeability, reaching a m
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