Another idea is to replace metal with much lighter plastic composite materials – which is already happening on Boeing's 50% plastic 787 Dreamliner – but de Haan believes that by "thinking in composites" instead of simply swapping the materials into a design that was originally based on steels, further efficiencies could be gained. Different fuels including biofuels and hydrogen fuel cells are being investigated, but each have their own difficulties – biofuel, for instance, has a tendency to freeze at altitude, and there is still difficulty in compressing hydrogen to a small enough tank space to make it viable. Fuel cells are also an energy-inefficient storage medium.