Müller's professor Helene Haeusler originally proposed the assignment to create suitable toys to use in a therapeutic setting for children with physical and occupational disabilities. Müller spent years perfecting the series through testing, research and internships at institutions. The fine-tuning of different tactile elements such as jute and vinyl inspired the marketing slogan, "Therapeutic toys -- coarse but cute," to accompany their 1967 début at the Leipzig Trade Fair. The toys were a breakthrough for the field and have been lauded by pediatric orthopedic and neuropsychiatry clinics throughout Europe and Japan.
The toys were awarded the "Good Design" distinction by the Office of Industrial Design in East Germany in the 1970s. Because of the unique nature of the toys, Müller was able to persevere throughout the 20th century, running a remarkably successful business despite trade regulations imposed on East Germany. After the collapse of the Berlin Wall she regained rights for her designs and continued to hand make a limited number each year.