Abstract: The present invention relates to an etched optical fiber as force transducer with feedback control, with a force range of 1-108 pN and a displacement range of 10-105 nm with a spatial resolution of the order of tens of nanometers are accessible with the instrument. The sample deformation (evolution) can be imaged simultaneously with rheological measurements. The device is used to perform extension rheology of polymer melts, silk, or other bio-fluids at microscales, measure mechanical responses of living cells such as neurons, muscle cells, etc. It is also used as a passive probe for microscale force measurements and to investigate properties of active suspensions such as bacterial baths.