Abstract: A resonant sensor for determining structural property changes, in particular for detecting the presence of chemical or biological species, comprises a structure (2) mounted to be capable of resonating and having a cyclically symmetrical configuration with two independent degenerative modes of vibration of a common natural frequency (f), and means (24,26,28,30,32,34,36,38) for exciting the structure (2) to resonate according to the two degenerative modes, regions (8,12,16,20) of the structure (2) being modified such that, on changes in the structural properties of the modified regions (8,12,16,20), for example by the addition or subtraction of mass, the natural frequencies (f1, f2) of the two modes of vibration become different, the difference in frequencies (?f) being proportional to the change in structural properties.

Abstract: A bilayer of polymer film arranged in a manner to create respective flow paths for a first and second gaseous or liquid fluid, wherein the ratio of the surface area of polymer film adapted to contact and thereby isolate both fluids to the total matrix volume is in excess of 700 m.sup.2 /m.sup.3, processes for the preparation thereof, heat exchangers comprising such bilayers and uses thereof.