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Poly(phosphate ester)s, polyphosphonates, and polyphosphazenes are three classes of phosphorus-containing polymers that have received wide attention over the past decade for their utility in biomedicine and tissue engineering. These three families of polymers can lead to a number of subclasses of polymers with varied properties. Significant research in this area has led to niche polymers with morphologies ranging from viscous gels to amorphous microparticles for utility in drug delivery. Furthermore, the pentavalency of phosphorus offers the potential for covalent linking of the drug. The classes of polymers discussed in this review are being explored in human clinical trials for vaccine delivery as well as delivery of oncolytic and CNS therapeutics. More applications in the areas of DNA delivery and tissue engineering are also being explored.

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A number of experimental studies on the dynamic behavior of the chemostat have shown that the specific growth rate does not instantaneously adjust to changes in the concentration of limiting substrate in the chemostat following disturbances in the steady state input limiting substrate concentration or in the steady state dilution rate. Instead of an instantaneous response, as would be predicted by the Monod equation, experimental studies have shown that the specific growth rate experiences a dynamic lag in responding to the changes in the concentration of limiting substrate in the culture vessel. The observed dynamic lag has been recognized by researchers in such terms as an inertial phenomenon and as a hysteresis effect, but as yet a systems engineering approach has not been applied to the observed data. The present paper criticizes the use of the hfonod equation as a dynamic relationship and offers as an alternative a dynamic equation relating specific growth rate to the limiting substrate concentration in the chemostat. Following the development of equations, experimental methods of evaluating parameters are discussed. Dynamic responses of analog simulations (incorporating the newly derived equations) are compared with the dynamic responses predicted by the Monod equation and with the dynamic responses of experimental chemostats.

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