A very nice technology for this is called the Rubbery Ruler from Unimelb
Limited (U. of Melbourne, AU).  Try:

http://www.ph.unimelb.edu.au/inventions/rubberyruler/

The technology is a capacitive sensor (C changes with stretching of a device)
and is claimed to have been used to monitor body movement is space, fruit
growth and others.  We tried to arrange a license to manufacture and sell the
sensors, but Unimelb's extreme unreasonable expectations forced us to abandon
that project (funny.....patents are MANY years old and no commercial
products????).  I still think the technology offers promise for many niche
applications and maybe some mainstream ones.  It may work for you.  We did
much research into manufacturing processes and applications for this
technology. If you have any questions, let me know. Good-Luck.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel: 716-328-5850
Fax: 716-328-1144
http://www.frontiernet.net/~rmi/
***** Designing Electronics for Research and Industry *****