Modelling degradation of biodegradable polymers and their mechanical properties

Bioresorbable polymers are used for a wide range of medical applications inside the human body including fixation screws and plates for broken bones, sutures and scaffolds for tissue regeneration. Over a period of months or years, these devices degrade by hydrolysis of the ester bonds; they become fully absorbed into the body, thus removing the need for repeat surgery. The degradation pathway of these devices, including the loss of mechanical properties, is of great importance. However, the complexity of the degradation process, and the number of factors involved, means that degradation trends are not fully understood. Also, for many of the commonly used biodegradable polymers, no theoretical understanding exists for changes to mechanical properties during degradation. The devices are therefore currently designed to be over-supportive, which may inhibit the healing process due to the stress shielding effect. A general mathematical framework has been developed through several PhD projects at Leicester to model the degradation of bioresorbable polymers. This PhD thesis consists of three parts. The first part reviews the existing understanding of bioresorbable polymer degradation. In the second part, the previous models are simplified and improved. These new models are then used to reveal an in-depth understanding of the underlying mechanisms of the degradation process. The third part of this thesis focuses on understanding the change in Young’s modulus of degrading polymers. In order to do so, a novel atomistic finite element method is developed, which can simulate the mechanical response of a representative unit of a biodegradable polymer. The method is used to study the mechanical behaviour of polymer chains once chain scissions are introduced. The study leads to a concept for effective cavities for polymer chain scission, called the Effective Cavity Theory, which can be used to predict the change in Young’s modulus of a degrading polymer.