Mechanically Flexible Sensor Measures Shear Stress

Most shear sensors rely on a micro electro-mechanical system (MEMS) fabricated on rigid silicon, so they lack the capability to measure shear stresses on curved or flexible surfaces. In October [2010], researchers at Imec’s associated lab at Ghent University announced they had invented a way to fabricate shear sensors for use on a flexible surface such as human skin. The new sensors use an optical technology embedded in a thin flexible substrate. (Imec, based in Leuven, Belgium, performs research in nano-electronics and nano-technology with a staff of over 1750 and more than 550 industrial residents and guest researchers.)

For now, the demand for compact, flexible shear sensors comes mainly from the medical community, which could use such sensors to measure the skin friction between a prosthesis and stump, for example. But researchers might also use such sensors to create a sensitive, artificial “skin” for robots.

This prototype shear sensor bends with a radius of 4 mm.

The innovative sensors rely on a process that embeds optical components into flexible substrates as thin as 50μm. One such sensor “stack” comprises a vertical-cavity surface-emitting laser (VCSEL) source and a photodiode separated by a transparent and deformable layer of silicone.

A shear force moves the position of a VCSEL light source with respect to a light sensor.

The alignment of the VCSEL and photodiode lets the photodiode capture the greatest amount of laser light when the stack experiences no shear stress. An applied shear stress causes the laser and the photodiode move relative to each other, and the photodiode captures the changes in light caused by the misalignment, as shown in the diagram above.
The imec researchers used a layer of Dow-Corning Sylgard 184 silicone material that deforms linearly to shear stress. The next work will focus on adapting the sensor design so it also will indicate the direction of shear stress.

According to an imec contact, scientists plan to deliver a paper on the sensor technology at an IEEE conference within a few weeks.  As soon as I have more details, I’ll post them either in this description or as a blog comment. –Jon Titus