WORK IN PROGRESS
Exploring different possibilities to knit stretch sensors.

With Schoeller Wool

A hand knitted 3×6cm rectangle from Schoeller’s Nm 10/3, 80% PES, 20% Inox steel fiber (M869) conductive (resistive) wool. The range is between 10-5K Ohm resting and 200 Ohm when fully stretched.

Front/Back

Stretched in both directions yields very similar results

Video of sensor connected to an arduino, sending values to a Processing graph application

Video of sensor connected to a battery and an LED

Hand knitted, conductive yarn mixed with normal yarns

Hand knitted single and double rib stitch mixed with conductive yarn from Schoeller, Nm 50/2 60/40 Pes/Inox @ Euros 65.00/kg (25,000 metres/kg).
The resistance gets lower when stretched. Following is the result of simple hand stretch test.
Yellow knit: 20M ohm (laying down on the desk) - 5k ohm (stretched maximum by hand)
Green knit: 300k ohm (laying down on the desk) - 2k ohm (stretched maximum by hand)
Both of them where possible to read through Arduino analog inputs with internal pull-up. Although sometimes it does not go back to original state when released. Green knit shows much stable result.

Crochet, conductive yarn mixed with normal yarns

Mesh stitch crochet, normal yarn mixed with conductive yarn from Schoeller, Nm 50/2 60/40 Pes/Inox @ Euros 65.00/kg (25,000 metres/kg). The range is between 250-300 Ohm resting and 60-80 Ohm when fully stretched per 10 cm.
#ST3003: Double stitch crochet

Stretch test: Normal 200-230k ohm >> stretched 5-10k ohm

#ST3004: Treble Crochet with 1 stitch interval

Stretch test: Normal 120-140k ohm >> stretched 20-30k ohm

#ST3005: Double Crochet with 2 stitch interval

Stretch test: Normal 10-15k ohm >> stretched 5-8k ohm

*note: these crochet pieces does not come back fully after being stretched, so the normal state resistance differs according to how much it came back. Backing with other material to support shrinking back will help to have a stable normal state.

With Silver Plated Nylon conductive thread

Because the threads used here are intended for sewing and are much thinner, the knitting is very loose and after repeated stretching it stretches out and does not react any more. It might be able to improve this by using some regular thread to stitch the loops of knitted conductive thread to the stretchy base material at regular intervals. Forcing it back into an initial (not stretched) state.

66 Yarn 22+3ply 110 PET, from LessEMF.

117/17 2ply, from LessEMF.

From Plug and Wear

Knitted stretch sensor from www.plugandwear.com
The resistance difference is almost 1k ohm to 800 ohm. The range and original resistance differs a lot depending on how it was held and pressed at the beginning. Stretch amount is +7cm on holizontal way, + 3cm on vertical way. It was measured between right top conductive yarn and left bottom conductive yarn.