Extensional flows occur widely in important biological functions of complex fluids, e.g. blood circulation, respiratory and gastrointestinal mucin flows and the synovial fluid in the joints. Extensional flows can significantly stretch the long-chain molecules present in such fluids, resulting in dramatic increases in flow resistance that are not quantified by standard rheological tests, yet are vital for full characterization of fluid samples. We propose to test the potential of a novel microfluidic chip for performing microfluidic extensional rheology measurements. The chip is based on the classic cross-slot design but has an optimized form so as to provide a pure planar extensional flow of constant strain rate along the exit channels. The novel geometry will be validated through a combination of state-of-the-art experimental techniques performed on model dilute polymer solutions in comparison with numerical simulations using viscoelastic fluid flow models. Subsequently we will use the device to study the extensional flow behaviour of model biofluid analogues based on solutions of well-characterized hyaluronic acid and mucin samples. These long-chain molecules are found in fluids such as synovial fluid and mucus, the flow properties of which can be altered by e.g. over or under expression or degradation of molecular chain length. Such altered rheology of body fluids is associated with conditions ranging from mucositis to cystic fibrosis and arthritis. There will be several outcomes of major importance from this work. Firstly we will be producing a state-of-the-art microfluidic extensional rheometer for testing miniature fluid samples. Second, we will be gaining much needed insight into the development of diseases and leading the way to improved therapeutics and formulation of prosthetic fluids. From a more fundamental perspective, we will be obtaining benchmark data on model biofluids for comparison with future work on healthy and diseased physiological fluids.