Progress in the development of nano-sized hybrid therapeutics and nano-sized drug delivery systems - "nanomedicines" - over the last decade has been remarkable. One key requirement is the knowledge of how polymers interact with the many biological barriers that they meet en route to the site of action. Current research is studying dendrimers, dendrons and dendronised polymers as these more branched architectures rapidly traffic through cells showing high levels of exocytosis or transcytosis across biological barrierslike the gastrointestinal tract. The question is why ? The focus of a new PhD project here in Cardiff is to probe the diffusion of polymeric vehicles through, and their impact on, the structure of the mucous layer that exists on many epithelial surfaces, arguably the ultimate barrier to a successful drug delivery.