(ABC NEWS) -- What started out as one wig made for the daughter of a friend has become thousands of wigs made for children battling cancer, just two years later.
Holly Christensen of Palmer, Alaska, initially started the project after a friend's daughter was diagnosed with cancer. Christensen used to work as a cancer nurse and knew the treatment would be difficult on the 2-year-old girl.
"Once a cancer nurse, always a cancer nurse. It’s so heart-wrenching and life-changing to work in cancer," Christensen told ABC News in 2015.
Since then, more than 4,000 kids have received princess and superhero wigs from Magic Yarn, an all-volunteer run group with crocheters from all over the world.
Co-founder Bree Hitchcock told ABC News that since that first wig was made, the demand for the wigs -- that mimic characters from Elsa to Rapunzel -- has exploded.
"We've basically been playing catch-up ever since," Hitchcock said.
Wigs can be requested on an individual basis or in bulk by a hospital or treatment center. The wigs are free and are shipped within four weeks of the request.
"Sometimes we get a request for a wig for a child who is very ill," she said. "In that case, we'll get it to them sooner."
It's particularly rewarding, Hitchcock said, when they ship a wig to a child who is "obsessed" with the particular character.
"They take on the personality traits of that character," she said, "like strength or determination."
Hitchcock doesn't think the demand for wigs will slow down until childhood cancer rates do the same. The National Cancer Institute estimates that 15,270 children and adolescents ages 0 to 19 years will be diagnosed with cancer and 1,790 will die of the disease in the U.S. in 2017.