A new transducer based on the evanescent field effect for high-resolution displacement and force measurements

A new transducer, using the effect of frustrated total internal reflection of electromagnetic waves, was developed for displacement and force measurem ents in muscle research. The physical theory for this effect predicts the f ormation of an evanescent electromagnetic field in the gap between two sepa rated optical surfaces. This evanescent electromagnetic field enables the t ransmission of light across the gap between the two media, an effect not co ntained in ray optics formalism. In the apparatus described here, the optic al surfaces were obtained by bevelling 2 optical fibres to angles higher th an the specific critical angle of the media used. Since the amount of trans mitted light depends strongly on the distance between the surfaces, very sm all forces are detectable if one fibre is used as a cantilever. To test the transducer a small myofibrillar bundle (20 mum diameter) was mounted isome trically on it and contraction activated by release of "caged" Ca2+. Force and displacement steps of 180-220 nN and 9-11 nm respectively were measured with resolutions of 10 nN and 288 pm, respectively. From the oretical and practical considerations, it is expected that the presented principle will be able to achieve even much higher sensitivity for the determination of fo rce (better than picoNewton) and distance (femtometre).