Time-scale biodegradation

We should also note that microorganisms have water-filled pores (porins) through their exterior membranes (Fig. 17.13) which permit the passive entrance of small hydrophilic substances (Madigan et al., 2000). In studies of enteric bacteria, passive glucose uptake exhibited transmembrane uptake time scales of less than a millisecond (Nikaido, 1979). Thus, the rate of passive uptake of small, hydrophilic molecules (< 500 molecular mass units) via membrane pores of bacteria is not likely to cause them to avoid biodegradation for prolonged times. 

Since the copolyamide contained two different amide bonds, both of which were similar, but not identical to a peptide bond, it was hoped that the extracellular enzymes of the bacteria or fungi would cleave the polymer to small fragments that could be utilized by the microoganisms as a food. In a fast screening technique, a culture of bacteria and/or fungi was fed a standard amount of hydrolyzed casein and the amount and rate of carbon dioxide liberated was monitored. In order to determine whether a material was biodegradable on the time scale used, the material was added to a similar flask and any increase in the rate or amount of carbon dioxide over the control was noted (5,6h... 

It is desirable to reflect natural biodegradation diversity, but a complete set of all possible pathways can result in too many choices, a problem known as a combinatorial explosion . For example, if there were ten metabohtes at each stage of prediction and no convergence of metabolites, the munber of metabolites would increase by an exponent of ten at each step. The resultant thousands of pathways would be beyond hiunan evaluation on a reasonable time scale. To deal with this problem, it is necessary to further guide users by assigning priorities to every rule that governs each predicted reaction. 

The strategy for the tissue engineering is to select a polymer with the best biocompatibility, extend its time-scale of biodegradation, increase its mechanical strength and convert it into nano-fiber stracture resembling the stracmre and properties of natural tissue. 

In order to perform its function well, after implantation a scaffold must have the best achievable biocompatibility, it should also have a microstructure resembling that of natural tissue (for cell seeding, ingrowth and proliferation), adequate mechanical strength and biodegradability matching the time-scale of tissue regeneration. 

However, although it seems that Biomax sufficiently disintegrates under composting conditions [100], the final metabolisation of the material in a reasonable time scale is still under discussion. The producer itself admits that Biomax in the current formulation (June 2000) does not degrade fast enough to meet the accepted standards and thus, will improve the material with regard to its biodegradability [101]. 

Table 4. Time-scale for biodegradation of peroxidised polyolefins...

Carbon is retained in the soil during oxo-biodegradation in a form accessible to growing plants, rather than by being eliminated to the environment as carbon dioxide as is the case with hydro-biodegradable polymers (e.g. pure cellulose and starch). The time-scale for complete oxo-bioassimilation of the synthetic polyolefins is very similar to that for their natural analogues such as c/5(polyisoprene) and related plant exudates and lignocellulose, the structural material of plants.. 

Fig. 2. Typical time scales for biodegradation in different environments [11]...

This standard describes the biodegradation, in a laboratory-scale composter, of cellulose at 58° 2° C. No extent or time-scale are quoted for pass or fail but cellulose, which gives 75.3% CO2 formation at 50°C is quoted at a positive reference from which it must be assumed that the polymer should biodegrade at a similar rate to cellulose. 

This analysis indicates that the rate of mass transfer is the rate-limiting step for each soil type. This explains why the desorption and mineralization profiles were qualitatively similar. The difference in time scales may likely be due to the experimental design the mineralization experiment only detected complete mineralization and did not detect the initial biodegradation to intermediate metabolites while the desorption experiment measured all phenanthrene that desorbed. The bioavailability number analysis shows that for each soil, the desorption resistant fraction has a bioavailability number that is several orders of magnitude lower than that of the labile fraction. This implies that although a... 

Biodegradable Hydrocarbon Polymers an Environmentally Acceptable Solution... 231 Table 2 Time scale for the bioassimilation of aged oxo-biodegradable polyethylene [6]... 

Each of these processes requires a different time-scale for biodegradation (Fig. 6) [19]. 

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