Degradable times

Starting with the results of GPC analysis, two approaches have been successfully applied to the problem of polymer degradation, using either the differential or the integral expression of the kinetics equations a) in the first technique, a number of GPC traces are recorded at successive degradation times or degradation yields. Each MWD is divided in a number of... 

Figure 1 shows the results of those experiments. It is important to recognize that in this experiment the polymer was present in a form affording high surface area when compared to other studies where rods, pellets, plates, fibers, and so forth were used. It is often quite difficult to compare exact degradation times from various independent studies due to differences in implant surface area and... 

At present there is no reason evident why poly(N-acylhydroxy-proline esters) should not be suitable for the formation of microcapsules or microspheres as well. For microencapsulated drug fonmula-tions the longer degradation times of poly(N-acylhydroxyproline esters) as compared to poly (lactic acid) could again be a distinctive advantage for long-term applications. 

The product yield of LDPE and HDPE for the same degradation time is in the order of ZSM-5 catalyst > RFCC catalyst > thermal, and the cumulative yield of liquid product from LDPE was greater than that from HDPE. 

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

Figure 20 Raman scattering from PVC thermally degraded at 80°C. The excitation laser was 514 nm the degradation time in minutes is indicated above each spectrum. Reproduced form Everall [51]. , with kind permission of Springer Science and Business Media. 

LC-CC chromatograms featured an additional peak in the degraded samples that increased with increasing degradation time, and could be attributed to different functionality as a consequence of hydrolytic degradation ... 

Fig. 9 Chromatograms obtained for the two monitored transitions for available OH-hexa-BDE standards are showed on the top. Below is showed the peak found for a degradation time of 12 h...

Treatment with the fungus strongly increased the toxicity of the samples. First, embryos exposed to the non-diluted treated samples, at a degradation time of 12 h and 7 days, exhibited almost a 100% of mortality and only 2 of the 24 embryos... 

The synthesis of poly(anhydride-co-amide)s (Table VII) of various chemistries was pursued by Hartmann and Schulz (1989) as a means of improving biocompatibility and extending the degradation times of polyanhydrides. This work also contains calorimetry data on the thermal transitions and spectroscopic characterization. 

Zygourakis (1990 Zygourakis and Markenscoff, 1996) developed a discretized model in which cells are assigned a degradation time, upon exposure to solvent, based on their identity as either drug, polymer, solvent, or void. The initial distribution of cells can be modeled after the microstructure of the polymer matrix and multiple phases are explicitly accounted for. The solution is found numerically. 

Biological and abiotic degradation times of selected organotins... 

Degradation rate of chlorpyrifos in abiotic substrates varies, ranging from about 1 week in seawater (50% degradation) to more than 24 weeks in soils under conditions of dryness, low temperatures, reduced microbial activity, and low organic content. Intermediate degradation rates reported have been 3.4 weeks for sediments and 7.6 weeks for distilled water. In biological samples, degradation time is comparatively short — usually less than 9 h in fishes and probably the same in birds and invertebrates. 

If degradation is due to sequential or synergistic mechanisms, then a simple extrapolation may not be reasonable. For example, in accelerating the rate of oxidation of polyethylene it may be necessary to identify the induction time and subsequent degradation time separately and to produce Arrhenius diagrams for each. The total time to failure is the sum of the two times. Figure 9.3 shows an instance of where unknowing extrapolation of the short-term results in tests on polyaramid fibres could have led to overestimates of lifetime and premature failure. 

Other less obvious factors are also considered while choosing a suture. For instance, the age and health of the patient. An older person in poor health generally requires a longer period to heal, thus for absorbate sutures, the required degradation time needs to be increased. Conversely, for an active child, while the degradation time may be less, the suture may be exposed to more frequent sudden stresses and the body tissues are generally thinner. The presence of immunodeficiency is also an important factor since they are more susceptible... 

Degradation times are normally stated in terms of half-life.9 Measured results vary substantially, depending on experimental technique, but in wet soil at warm (>20°C) temperatures both TNT and 2,4-DNT can show a half-life of one day or less. At temperatures just above freezing, this increases to a couple of weeks while at temperatures below freezing, the half-life extends to years. 

Bioremediation using Biodrain is not possible for compounds resistant to biodegradation. Much longer degradation times are required for compounds such as polychlorinated biphenyls (PCBs) and polynuclear aromatics (PNAs) 3 to 7 years may be required for highly resistant contaminants. Bioremediation is also limited by below-freezing temperatures and free aqueous metals concentrations. Metals can be extracted or immobilized prior to biotreatment. Biodrain cannot be installed in rock or some landfill situations unless holes are drilled first. Current installation limits are approximately 40 ft. 

Table E6.6.1 Nondimensional degradation times, k ty, required for various levels of degradation in four systems...

Table 8.1 Biological and Abiotic Degradation Times of Selected Organotins... 

Hydrogel degradation time was controlled by the molecular weight obtained and the crosslink density of the material. 

The degradation rate can be controlled using acidic and basic excipients acidic excipients increase the degradation rates and facilitate a zero-order release rate over a 2-week period (Sparer et al. 1984). Basic additives increase the degradation time of the polymers and create a polymer that degrades specifically at the surface (Heller 1985). By careful choice of the excipient added, the degradation rate can be closely controlled. No experiments have shown the use of these polymers with proteins or peptides. This is not, however, indicative of the fact that these polymers are not compatible with proteins or peptides, but they are probably not the most appropriate polymeric carrier for oral delivery of biomacromolecules. 

---