M. Vert
Research Center for Artificial Biopolymers, UMR CNRS 5473, University Montpellier 1, Faculty of Pharmacy, 15 Avenue Charles Flahault, BP 14491, F-34093 Montpellier Cedex 05, France
Keywords: Degradable polymers, Biodegradable polymers, Aliphatic polyesters, PLA polymers, Functional polyesters
For the last sixty years synthetic polymeric materials have grown progressively up to forming one of the most attractive domains in materials science. This success is due primarily to their low cost, their reproducibility and their resistance to physical ageing and biological attacks. However, the resistance of synthetic polymers to the action of living systems is becoming more and more problematic in several domains where they are used for a limited period of time before becoming wastes. It is the case in surgery, in pharmacology, in agriculture and in the environment as well. In these domains, time-resistant polymeric wastes are less and less acceptable. From this viewpoint, sutures, bone fracture fixation devices, mulch films and packagings are philosophically comparable. Basically any artificial polymeric device of time-limited use should be eliminated and ideally biorecycled after use. To achieve such a goal, Mother Nature has set up outstanding processes based on enzymatic digestion that requires living cells to be active. One can take advantage of the natural machinery to biodegrade artificial polymers as it is the case for bacterial poly(β-hydroxy acid)-type aliphatic polyesters (PHA), poly(ε-caprolactone) (PCL) or starch-based blends. However, there are limitations. For instance, PHA and PCL biodegrades outdoor but are almost biostable in the human body. Polymers that can be degraded via non-enzymatic chemical routes should be easier to control. Poly(β-hydroxy acid)-type polyesters came up that degrade in both the environment and in an animal body. However, chemistry driven cleavages of polymer chains are dependent on chemical and physico-chemical and physical factors, such as the size, for instance. Moreover, other parameters have to be taken into account if one wants a system to be of practical interest. In particular, degradable or biodegradable polymers have to be first materials, i.e. matter that is doing something useful for human activities. However the biorecycling requirement limits very much the possibility of property adjustment with respect to copolymerization and formulation presently exploited to make polymers and polymeric devices. Last but not least, turning a synthetic polymer that is not a dramatic pollutant to a batch of degradation low molar mass by-products may not be innocent. In attempts to include these remarks into a strategy to design aliphatic polyesters that could cover the specifications of different applications, we have been investigating the so-called artificial biopolymers, i.e polymers made to generate metabolites upon degradation, for many years. Lactic acid-based aliphatic polyesters are the archetype of bioresorbable or biorecyclable polymers issued from this strategy but they have their own limits, one of them being the sensitivity to heat. Another one is the absence of functionalization that closes the route to applications requiring degradable hydrogels or water soluble polymers. Attempts to generate hydrophilic aliphatic polyesters will be presented .
