Model for an encapsulated microbubble using transient network theory

Encapsulated microbubbles (EMBs) are widely used to enhance contrast in ultrasound sonography and are finding increasing use in biomedical therapies such as drug/gene delivery and tissue ablation. EMBs consist of a gas core surrounded by a shell made of protein, polymer, or lipid. We present a novel model for an EMB based on a statistically-based continuum theory for the encapsulating material using transient networks. The use of transient network theory provides a general framework that allows a variety of viscoelastic shell materials to be simulated, including a purely elastic solid or a viscous fluid. The model permits local stress in the shell to be calculated, and can be readily extended to modeling nonspherical EMB deformations. The model accurately reproduces the experimentally-measured radial response of an ultrasonically-driven, spherical, lipid-coated microbubble, and provides a better fit than that given by common spherical EMB models by Marmottant et al., Chatterjee and Sarkar, and Hoff. Extensions of the model to nonspherical EMB oscillations are discussed.