In vitro drug release studies from organoclay/poly(dimethyl siloxane) nanocomposite matrices.

Silicone elastomers are versatile biomaterials and have been used for fabrication of drug release systems, usually incorporating lipophilic drugs. However, attempts have been made to extend the use of these biomaterials to the delivery of hydrophilic drugs. Furthermore, the need to improve mechanical properties of silicones led, among others, to the incorporation of organoclay nanoparticles and, therefore, has introduced some new parameters to be investigated regarding their effect on the release profile. In this work, the delivery of 2-methyl-5-nitroimidazole-1-ethanol (metronidazole) from nanocomposites with silicone matrix based on condensation cured elastomers with different molecular weights was investigated in various surrounding liquids. The results showed that incorporation of organic modified montmorillonite (OMMT) generally decreases the drug release rate and restricts the initial burst effect. Interestingly, OMMT concentrations of 2 phr in low MW silicone systems seem to enhance drug release and, independently of interpretation, it might indicate a route for the adjustment of diffusivity through the nanoclay concentration. Maximum drug release rates can rather be achieved with low MW PDMS than with the higher MW elastomers.

The effect of accelerated ageing on performance properties of addition type silicone biomaterials.

The UV-protection provided to addition type silicone elastomers by various colorants, such as conventional dry earth pigments, as well as the so called "functional or reactive" pigments, was investigated. Moreover, the effect of a UV light absorber and a silica filler was also explored. Under the experimental parameters of this work, the exposure of silicone to UV radiation resulted in some changes of the IR absorbance, thermal decomposition after 400 degrees C, T(g) and tensile properties, whereas the storage modulus of samples was not affected. The obtained spectroscopic data, as well as the results of TGA and storage modulus, were interpreted by assuming that chain scission takes place during aging, whereas the improvement of tensile strength allows the hypothesis of a post-curing process, initiated by UV radiation. Therefore, the increase of T(g) could partly be due to the above reason and, furthermore, to the contribution of a rearrangement of chain fragments within the free volume of the elastomeric material. Regarding the evaluation of various coloring agents used in this work, the obtained results show that dry pigments are more sensitive to accelerated ageing conditions in comparison with functional liquid pigments. Moreover, the hydrophobic character of silicone matrix is enhanced, with the addition of this type pigments because of the vinyl functional silanes groups present in their chemical structure. Finally, it should be noted that the incorporation of silica nanofiller did not seem to prevent the silicone elastomer from degradation upon UV irradiation, but showed a significant reinforcing effect.

Release of propranolol and diclofenac from low Mw DL-poly(lactic acid).

The controlled release of two drugs, i.e. the sodium salt of diclofenac and propranolol was studied, by using low molecular weight D,L-Poly(lactic acid) as a matrix. Tablets of the above polymer containing those drugs were immersed into buffers with various pH values and delivery was recorded as a function of time, via UV-spectroscopy. The results showed that the polymer is appropriate for such biomedical applications, as generally, it ensures complete drug delivery within 45-60 days, which is acceptable for most cases. On the other hand, the release rate depends on many parameters including the interactions among drug, matrix and the surrounding liquid, which adds complexity to the process and requires careful investigation for proper design of a controlled release system.

Fracture force, deflection at fracture, and toughness of repaired denture resin subjected to microwave polymerization or reinforced with wire or glass fiber.

STATEMENT OF PROBLEM: The ultimate goal of denture repair is to restore the denture's original strength and avoid further fracture. The best materials and methods for repair have not been conclusively determined. PURPOSE: This study investigated the fracture force, deflection at fracture, and toughness of a heat-polymerized denture base material repaired with heat-polymerized resin, autopolymerized resin alone, or autopolymerized resin with glass fiber or wire reinforcement. MATERIAL AND METHODS: Eight groups were evaluated: 6 with autopolymerized resin repairs, 1 with heat-polymerized resin repairs, and a control group of intact specimens. The 6 autopolymerized resin groups included 1 group with no reinforcement, 1 treated with microwave irradiation after polymerization, 2 with monolayer or multilayer glass fiber reinforcement, and 2 with round or braided wire reinforcement. Each group consisted of 12 specimens. The experimental specimens were cut, and a 3-mm butt joint gap was repaired as indicated by the group assignment. A 3-point bending test was used to determine the fracture force, deflection at fracture, and toughness of the specimens. The data were analyzed with 1-way analysis of variance and the Tukey post-hoc test (alpha=.05). RESULTS: The fracture force (28.4 to 73.4 N), deflection (1.6 to 3.8 mm), and toughness (0.02 to 0.13 J) values for all repaired groups were significantly lower than those for the control group (82.79 N, 4.4 mm, and 0.16 J, respectively), with one exception: the mean fracture force of specimens reinforced with round wire (102.9 N). Failure mode was always adhesive, meaning that fracture occurred between the denture base and repair resin. CONCLUSION: Among the repair treatments tested, the most effective was microwave-irradiated, autopolymerized resin reinforced with round wire or monolayer glass fiber ribbon.

Xanthan gum as a carrier for controlled release of drugs.

Systems based on xanthan gum matrix containing 1%, 2% and 5% salicylic acid were prepared and studied as controlled release devices. Swelling of the matrix in distilled water and buffer solutions showed that the ionic strength of the liquid has a strong effect on the sorptive properties of the matrix. From the release experiments, conducted in distilled water at 37 +/- 0.5 degrees C, it was found that the drug delivery process was accomplished within the first 10 hours after immersion and salicylic acid was always released via a non-Fickian transport. The phenomenon can be described by a release exponent (n) in the area of 0.77 independently of the initial concentration of salicylic acid in the xanthan matrix. These results can be interpreted taking into consideration the dimensional and physical changes of the polymeric matrix during swelling.

Swelling properties of various polymers used in controlled release systems.

The effect of powder packing and porosity of specimens on the swelling properties of polymeric materials was studied, in various swelling liquids, such as distilled water and 0.1 N hydrochloric acid solution. Capsules, tablets and films of hydroxypropyl methylcellulose, poly(ethylene oxide) and sodium alginate were prepared, and their weight uptake after immersion into the above solutions was recorded as a function of time, in order to assess the swelling process. Measurements of some characteristics of the as-received powders were also performed in an attempt to classify the specimens prepared according to their porosity. Within the experimental conditions of this work, it was shown that the porosity of polymeric specimens is a dominant factor that controls their swelling behaviour. Increased porosity leads to fast initial rates of weight uptake and high extent of equilibrium swelling. On the other hand, dissolution and possible degradation of polymers susceptible to acid hydrolysis results in some variations from the above mentioned behaviour. With respect to the application in controlled release systems, the overall delivery rate from a polymeric specimen is expected to be a function of both swelling and disintegration characteristics of a specimen, and therefore, the weight uptake can be considered a measure of the release only in the case of polymers with low water solubility and increased stability to hydrolysis.

Swelling properties of various polymers used in controlled release systems.

The effect of powder packing and porosity of specimens on the swelling properties of polymeric materials was studied, in various swelling liquids, such as distilled water and 0.1 N hydrochloric acid solution. Capsules, tablets and films of hydroxypropyl methylcellulose, poly(ethylene oxide) and sodium alginate were prepared and their weight uptake after immersion into the above solutions was recorded as a function of time, in order to assess the swelling process. Measurements of some characteristics of the as received powders were also performed as an attempt to classify the specimens prepared according to their porosity. Within the experimental conditions of this work, it was shown that the porosity of polymeric specimens is a dominant factor that controls their swelling behaviour. Increased porosity leads to fast initial rates of weight uptake and high extent of equilibrium swelling. On the other hand, dissolution and possible degradation of polymers susceptible to acid hydrolysis, results in some variations from the above-mentioned behaviour. With respect to the application in controlled release systems, the overall delivery rate from a polymeric specimen is expected to be a function of both swelling and disintegration characteristics of a specimen and, therefore, the weight uptake can be considered a measure of the release only in the case of polymers with low water solubility and increased stability to hydrolysis.

Polymers for use in controlled release systems: the effect of surfactants on their swelling properties.

The effect of an ampholytic surfactant on the swelling properties of polymeric materials was studied, using various swelling liquids. Tablets were prepared consisting of hydroxypropyl methylcellulose, poly(oxyethylene) and sodium alginate. Tego betain was the non-ionic surfactant used as an additive in a series of samples made of the above polymers. Those tablets were immersed in distilled water, phosphate buffer and 0.1 N HCl, and their weight uptake was recorded as a function of time, in order to assess the swelling process. Measurements of the contact angle of the above systems were also carried out for estimating their wetting properties. The results of this study showed a selectivity among polymers, surfactant and surrounding liquid. Clearly, an enhancement of the swelling capacity of hydroxypropyl methylcellulose tablets due to the surfactant was recorded. An unclear effect was observed in the case of poly(oxyethylene), whereas for sodium alginate, the dominant factor is its water solubility that controls swelling behaviour.

Physical properties of a silicone prosthetic elastomer stored in simulated skin secretions.

STATEMENT OF PROBLEM: Facial prostheses worn over an extended time are exposed to various environmental factors, including sebaceous oils (sebum) and perspiration. PURPOSE: This study investigated the physical properties of tensile strength and modulus, elongation, tear strength, hardness, weight, and color change, of a silicone facial elastomer after immersion for 6 months in simulated sebum and perspiration at 37 degrees C. MATERIAL AND METHODS: Specimens made of Episil silicone elastomer were immersed in simulated alkaline or acidic perspiration as well as in sebum. Tensile and tear tests were conducted according to ISO specifications no. 37 and 39, respectively, in a Monsanto testing machine. Shore A hardness measurements were run according to ASTM D 2240. Weight changes were followed at 5, 15, 30, and 180 days, and color changes were determined in the CIE LAB system using a tristimulus colorimeter. RESULTS: An improvement of mechanical properties for specimens immersed in acidic perspiration was attributed to facilitation of the propagation of cross-linking reaction during aging of the silicone samples. Some weight increase was observed for the specimens immersed into the aqueous solution, whereas for those immersed in sebum, weight loss was recorded, probably because of extraction of some compounds. In this latter case, the color change was lower than that corresponding to simulated perspiration. CONCLUSION: The silicone specimens aged for a period, which simulates 1.5 years of clinical service, showed minimal changes with respect to the properties studied.

Properties of maxillofacial silicone elastomers reinforced with silica powder.

Compounds of a silicone elastomer suitable for preparing maxillofacial prostheses, reinforced with various amounts of silica powder, have been studied for their mechanical response and wetting properties in terms of contact angle. Tensile strength and elongation at break showed an increase with increasing silica volume fraction up to 35%, whereas the Young modulus displayed small dependence on the silica content and the resistance to tear increased continuously with filler volume fraction (Vf). The wetting properties assessed via the contact angle, seemed to degrade with increasing silica Vf, but a dependence on the elastomer network density has also been recorded.