A strain-responsive visual indicator comprises a pair of overlapping
shutter strips (1, 2) adapted to be mounted to a substrate such that
strain in the substrate causes relative movement M between them. Each
shutter strip comprises an alternating set of windows (5) or (6) and bars
(7) or (8) and the distal strip (2) is backed by a strip (3) of a
different color exposed through its windows (6), or the distal strip can
instead be printed with a set of alternating colors in place of its
windows and bars. The effect is that under different strain conditions
different color indications from the distal strip (2) will be visible
through the windows (5) of the proximal strip (1), in accordance with the
relative positions of the two strips. The indicator can be incorporated
e.g. in the shaft of a toothbrush to give a visual warning in response to
flexure of the shaft if excessive brushing pressure is applied. Numerous
other applications and the use of different types of indicia are
indicated in the specification.