Biocompatibility oxidation degradation

Niosomes In order to circumvent some of the limitations encountered with liposomes, such as their chemical instability, the cost and purity of the natural phospholipids, and oxidative degradation of the phospholipids, niosomes have been developed. Niosomes are nonionic surfactant vesicles which exhibit the same bilay-ered structures as liposomes. Their advantages over liposomes include improved chemical stability and low production costs. Moreover, niosomes are biocompatible, biodegradable, and nonimmunogenic [215], They were also shown to increase the ocular bioavailability of hydrophilic drugs significantly more than liposomes. This is due to the fact that the surfactants in the niosomes act as penetrations enhancers and remove the mucous layer from the ocular surface [209]. 

The bulk properties of polyolefin(s) (PO) can be tailored via catalyst design. PO are important polymers which are derived from relatively inexpensive feed stocks and are generally hydrophobic in nature. PO require surface modification to improve certain properties such as adhesion, wettability, printability and biocompatibility. Upon exposure to light or heat, PO usually exhibit an induction period, during which minimal oxygen uptake or changes in physical properties are observed [3]. PO are subject to thermal and oxidative degradation and cannot be used in practical applications, such as automotive parts, unless they are protected with efficient antioxidants [4]. 

As previously mentioned, degradable microspheres have gained attention as promising delivery vehicles for steroids in postmenopausal therapy. Copolymers of CL and d,l-LA were used to prepare microspheres for prolonged release of progesterone and [5-estradiol. The system offered a constant release for up to 40 days in vitro and 70 days in vivo [226]. Similarly, PCL copolymers have been considered useful for androgen replacement therapy in the treatment of aging men with a testosterone deficiency. Micelles of PCL-block-poly(ethylene oxide) released dihydrotestosterone in a controlled fashion over 30 days. The biocompatibility was confirmed in vitro in a HeLa cell culture [227]. 

Polyethylene oxide) (PEO) is a semicrystalline water-soluble polymer [64, 65], with a crystallinity that is very sensitive to the thermal history of the sample, making this property interesting as an indicator of degradation. Because it is biodegradable and biocompatible, PEO is a good candidate for environmental and medical applications [66-68]. The mechanisms of thermo- and photo-oxidation of PEO have already been investigated [69, 70] on the basis of IR identification of the oxidation products and are summarized in Scheme 10.1. 

PU and silicone rubber are biocompatible materials which are commonly used in a variety of medical applications [53], e.g., as a raw material for central venous catheters and tracheotomy tubes. Although these materials are biocompatible, the side effects which occur during clinical use include inflammation, infection and biofilm formation and growth. This in turn initiates the degradation of the material, e.g., previous studies have proven that the degradation of PU catheters is caused by either oxidation or hydrolysis of the material [54]. The degradation of silicone rubber is a hydrolysis phenomenon [55], which could be catalysed by an acidic environment. 

As final product or as-used medical devices are generally made available in the sterilized state, the effect of sterilization and sterilization technique must be considered. Steam sterilization, radiation sterilization, or ethylene oxide sterilization may lead to modifications in the surface and bulk properties of the material and this may have a potential impact on the biocompatibility. Radiation sterilization may aosslink or degrade polymers leading to property changes. Ethylene oxide residuals are known to have an adverse reaction on in vitro toxicity tests. For these reasons, characterization of the final product following sterilization is necessary. 

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