Biomimetic design of artificial micro-vasculatures for tissue engineering

Over the last decade, highly innovative micro-fabrication techniques have been developed that are set to revolutionise the biomedical industry. Fabrication processes, such as photolithography, wet and dry etching, moulding, embossing and lamination, have been developed for a range of biocompatible and biodegradable polymeric materials. One area where these fabrication techniques could play a significant role is in the development of artificial micro-vasculatures for the creation of tissue samples for drug screening and clinical applications. Despite the enormous technological advances in the field of tissue engineering, one of the major challenges is the creation of miniaturised fluid distribution networks to transport nutrients and waste products, in order to sustain the viability of the culture. In recent years, there has been considerable interest in the development of microfluidic manifolds that mimic the hierarchical vascular and parenchymal networks found in nature. This article provides an overview of microfluidic tissue constructs, and also reviews the hydrodynamic scaling laws that underpin the fluid mechanics of vascular systems. It shows how Murray's law, which governs the optimum ratio between the diameters of the parent and daughter branches in biological networks, can be used to design the microfluidic channels in artificial vasculatures. It is shown that it is possible to introduce precise control over the shear stress or residence time in a hierarchical network, in order to aid cell adhesion and enhance the diffusion of nutrients and waste products. Finally, the paper describes the hydrodynamic extensions that are necessary in order to apply Murray's law to the rectangular channels that are often employed in artificial micro-vasculatures.