This is a relatively new case humidifier from Stretto in Switzerland. It consists of a crystal bag /pouch inside a black plastic enclosure which is fixed to the inside of a guitar case by an adhesive strip. The Stretto bag is soaked in water for 10 minutes and after wiping off the excess water, is inserted into the enclosure which is then snapped closed. The one offered for guitars is actually the Stretto bag and enclosure for cellos and should maintain a humidity of 50-60% in the case for 2 to 3 weeks. When the bag is dried out, it should be re-immersed in water to be ready for re-use over a period up to 8 to 10 months - then the bag should be replaced. By being located in the case and not inside the guitar, this system offers less risk of leakage of water or gel inside the guitar body. The basic unit includes one bag plus one spare one.
Packs of 4 spare bags are available at GBP8.51 + vat = £10